Prepared in cooperation with the National Park Service, U.S. Fish and Wildlife Service, 

Bureau of Reclamation, Nevada Department of Wildlife, Southern Nevada Water Authority, 

University of Nevada, Reno, and University of Nevada, Las Vegas

A Synthesis of Aquatic Science for Management of 

Lakes Mead and Mohave

U.S. Department of the Interior

U.S. Geological Survey

Circular 1381

A Synthesis of Aquatic Science for 

Management of Lakes Mead and 

Mohave

Edited by Michael R. Rosen, U.S. Geological Survey; Kent Turner, National 

Park Service; Steven L. Goodbred, U.S. Geological Survey; and Jennell M. 

Miller, University of Nevada, Las Vegas

Prepared in cooperation with the National Park Service, U.S. Fish and 

Wildlife Service, Bureau of Reclamation, Nevada Department of Wildlife, 

Southern Nevada Water Authority, University of Nevada, Reno, and 

University of Nevada, Las Vegas

Circular 1381

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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Although this report is in the public domain, permission must be secured from the individual copyright owners 

to reproduce any copyrighted materials contained within this report.

Suggested citation:

Rosen, M.R., Turner, K., Goodbred, S.L., and Miller, J.M., eds., 2012, A synthesis of aquatic science for management 

of Lakes Mead and Mohave: U.S. Geological Survey Circular 1381, 162 p. 

Library of Congress Cataloging-in-Publication Data

Rosen, Michael R., Turner, Kent, Goodbred, Steven L., and Miller, Jennell M.

A synthesis of aquatic science for management of Lakes Mead and Mohave / edited by Michael R. Rosen ...[et al.].

p. cm. –(U.S. Geological Survey Circular ; 1381)

Includes bibliographic references.

1. Lakes—Lake Mead National Recreation Area (Ariz. and Nev.)—Management. 2. Natural resources—Lake Mead 

National Recreation Area (Ariz. and Nev.)—Management. 3. Mead, Lake (Ariz. and Nev.). 4. Mohave, Lake (Ariz. and 

Nev.) I. Rosen, Michael R., 1961– II. Geological Survey (U.S.) III. Series: U.S. Geological Survey Circular 1381 

GB1622.S96 2013

628.1'120979159—dc23

2012050148

ISBN 978-1-4113-3527-1

ii
Foreword iii

Foreword

Lake Mead provides many significant benefits that have made the modern development of 

the southwestern United States possible. The lake also provides important aquatic habitat 

for a wide variety of wildlife including endangered species, and a diversity of world-class 

water based recreational opportunities for more than 8 million visitors annually. It is one 

of the most extensively used and intensively monitored reservoirs in the United States. 

The largest reservoir by volume in the United States, it supplies critical storage of water 

supplies for more than 25 million people in three western states (California, Arizona, 

and Nevada). Storage within Lake Mead supplies drinking water and the hydropower to 

provide electricity for major cities including Las Vegas, Phoenix, Los Angeles, Tucson, and 

San Diego, and irrigation of greater than 2.5 million acres of croplands.

Due to the importance of Lake Mead, multiple agencies are actively involved in its 

monitoring and research. These agencies have a long history of collaboration in the 

assessment of water quality, water-dependent resources, and ecosystem health. In 

2004, the National Park Service obtained funds from the Southern Nevada Public Lands 

Management Act to enhance this partnership and expand monitoring and research efforts 

to increase the overall understanding of Lake Mead and Lake Mohave. Participating 

agencies included the National Park Service, Southern Nevada Water Authority, U.S. 

Geological Survey, Nevada Department of Wildlife, Bureau of Reclamation, U.S. Fish and 

Wildlife Service, University of Nevada, Las Vegas, and University of Nevada, Reno.

Results of these important efforts have been presented in Lake Mead Science Symposia 

conducted in 2009 and 2012. The relationships forged by the collaboration led to the 

development in 2012 of the Lake Mead Ecosystem Monitoring (LaMEM) Work Group, 

which has formalized the partnership and documented an interagency purpose and 

mission statement with common objectives for protection of Lake Mead and Lake 

Mohave water quality and water-dependent resources. This Circular has been developed 

to summarize the state of the knowledge related to the interests and objectives of the 

LaMEM Work Group, to inform management and the public of current lake conditions, and 

identify future needs for monitoring and research. It is hoped that this report will provide 

a framework for continued long-term investigations and analysis of the environmental 

health of Lakes Mead and Mohave.

William H. Werkheiser

Associate Director for Water
Preface

The purpose of this Circular is to provide a synthesis of published information and a 

summary of technical findings and associated implications that may affect natural 

resource management of Lake Mead National Recreation Area (LMNRA). Synthesized 

information and summarized findings should lead to a better public understanding of the 

natural resources of Lakes Mead and Mohave, and the issues related to maintaining their 

resources into the future.

Acknowledgments

This project could not have been completed without the cooperation of all the federal, 

state, and local agencies, universities, and stakeholders involved in research and 

management of the Lakes Mead and Mohave. Special thanks to Mark Anderson (National 

Park Service), John Reuter (University of California, Davis), Daniel Bright (U.S. Geological 

Survey), and Roger Buehrer (Southern Nevada Water Authority) for their detailed 

reviews and suggestions that improved the clarity and presentation of this Circular. 

The descriptions of river operations and the water budget were improved greatly by 

the comments of Becky J. Blasius, Daniel A. Bunk, Christopher R. Cutler, Jeannette E. 

Davis, Janet Kirsch, W. Paul Miller, Shana G. Tighi, and Bruce E. Williams of the Bureau 

of Reclamation, Lower Colorado Region. Mark Sappington (National Park Service) is 

acknowledged for maps and for conceiving of and creating infographics in figs. 2-3, 2-4, 

and 2-6; Warren Turkett (Southern Nevada Water Authority) is acknowledged for his 

contribution of time to figure development within Chapter 4. Shawn L. Gerstenberger 

(University of Nevada, Las Vegas) and Bryan Moore (National Park Service) also are 

thanked for their contributions to this effort.

iv
Contents v

Contents

Foreword ............................................................................................. iii

Preface ............................................................................................. iv

Acknowledgments ....................................................................................... iv

 Chapter 1.—Introduction and Summary of Findings

By Kent Turner, Michael R. Rosen, Steven L. Goodbred, and Jennell M. Miller... 1

Chapter 2.—Environmental Setting of Lake Mead National Recreation Area

By Kent Turner, Michael R. Rosen, G. Chris Holdren, Steven L. Goodbred, and

David C. Twichell ........................................................................ 7

Chapter 3.—Hydrology and Management of Lakes Mead and Mohave within the

Colorado River Basin 

By G. Chris Holdren, Todd Tietjen, Kent Turner, and Jennell M. Miller ............ 23

Chapter 4.—Lake Water Quality 

By Todd Tietjen, G. Chris Holdren, Michael R. Rosen, Ronald J. Veley, 

Michael J. Moran, Brett Vanderford, Wai Hing Wong, and Douglas D. Drury ... 35

Chapter 5.—Wildlife and Biological Resources

By Sudeep Chandra, Scott R. Abella, Brandon A. Albrecht, Joseph G. Barnes,

E. Cayenne Engel, Steven L. Goodbred, Paul B. Holden, Ron B. Kegerries, 

Jef R. Jaeger, Erik Orsak, Michael R. Rosen, John Sjöberg, and 

Wai Hing Wong ........................................................................... 69

Chapter 6.—Threats and Stressors to the Health of the Ecosystems of Lakes Mead 

and Mohave

 By Michael R. Rosen, Steven L. Goodbred, Wai Hing Wong, Reynaldo Patiño, 

Kent Turner, Craig J. Palmer, and Peggy Roefer ....................................105

Chapter 7.—Management Implications of the Science

By Kent Turner, Steven L. Goodbred, Michael R. Rosen, and Jennell M. Miller...139

Glossary .............................................................................................157

Appendix A.—List of 96 species compiled from observations made during monthly aquatic 

bird surveys on Lakes Mead and Mohave over a 5-year period (2004–2009) ......161
Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Energy

kilowatthour (kWh) 3,600,000 joule (J)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Elevation, as used in this report, refers to distance above Power House Datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L).

vi
Chapter 1

Introduction and Summary of Findings 1

Chapter

Lakes Mead and Mohave, which are 

the centerpieces of Lake Mead National 

Recreation Area (LMNRA), provide 

many significant benefits that have 

made the modern development of the 

Southwestern United States possible. 

Lake Mead is the largest reservoir by 

volume in the nation and it supplies 

critical storage of water supplies for 

more than 25 million people in three 

Western States (California, Arizona, 

and Nevada). Storage within Lake 

Mead supplies drinking water and 

the hydropower to provide electricity 

for major cities including Las Vegas, 

Phoenix, Los Angeles, Tucson, and 

San Diego, and irrigation of more than 

2.5 million acres of croplands (National 

Park Service, 2010). Lake Mead is 

arguably the most important reservoir 

in the nation because of its size and the 

services it delivers to the Western United 

States (Holdren and Turner, 2010).

By Kent Turner1; Michael R. Rosen2, Steven L. 

Goodbred2

, and Jennell M. Miller3

Introduction and 

Summary of Findings

1

National Park Service

2

U.S. Geological Survey

3

University of Nevada, Las Vegas

Boat harbor in Boulder Basin, Lake Mead. Photograph by National Park Service.
2 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

While these reservoirs and others have modified the 

original, free-flowing Colorado River ecosystem, Lakes Mead 

and Mohave still provide important habitat for a variety of 

fish and wildlife species. The lakes provide critical habitat 

for populations of the federally listed endangered razorback 

sucker (Xyrauchen texanus) (Chapter 5), as well as critical 

habitat for the federally listed endangered bonytail chub (Gila 

elegans). In addition, the lakes support large populations of 

non-native sportfish including smallmouth bass (Micropterus 

dolomieu), largemouth bass (Micropterus salmoides), striped 

bass (Morone saxatilis), and channel catfish (Ictalurus 

punctatus). LMNRA also is a regionally important habitat for 

many birds, with 92 documented species of water-dependent 

birds (appendix A). Significant populations of peregrine 

falcons (Falco peregrinus) are present, and more than 

30 eyries were documented in 2010.

Razorback sucker (Xyrauchen texanus) caught and released for survey of razorback 

sucker populations. Photograph by National Park Service.

Lakes Mead and Mohave (fig. 1-1) provide a diversity of 

world-class, water-based recreational opportunities for more 

than 8 million visitors annually. Established as the nation’s 

first National Recreation Area in 1964, LMNRA is managed 

by the National Park Service to meet legislative mandates to 

provide high-quality, water-based recreation in a manner that 

Juvenile peregrine falcon (Falco peregrinus) feeding on prey along the shoreline of Lake 

Mead. Photograph by Joseph G. Barnes, University of Nevada, Las Vegas.

preserves unimpaired the area’s natural and cultural resources 

for the enjoyment of future generations. Key objectives 

within those mandates include maintaining safe water for 

body-contact recreation, maintaining the aesthetic quality of 

the recreational setting, and insuring water quality to support 

healthy populations of fish and wildlife.

Given these benefits and uses, multiple Federal, State, 

and local agencies have an obvious interest in the overall 

water quality and environmental health of Lakes Mead and 

Mohave. A primary catalyst for this interest has been related to 

Las Vegas Wash, the main surface drainageway for Las Vegas 

Valley, which conveys water from four wastewater-treatment 

facilities. These facilities return more than 190 million gal/d 

of treated sewage effluent to Boulder Basin of Lake Mead. As 

a result, Lake Mead is one of the most intensively monitored 

reservoirs in the United States. Boulder Basin in particular 

has been extensively monitored by agencies with waterquality management responsibilities on Lake Mead at the 

Federal, State, and local levels. Moreover, that interest has 

generated numerous interagency partnerships and forums for 

the development of mutual water-quality and environmental 

health objectives, and desired water-resource monitoring and 

research programs. Ongoing partnerships include the Lake 

Mead Water Quality Forum facilitated by the Nevada Division 

of Environmental Protection; the Lake Mead Ecosystem 

Monitoring Work Group composed of members of Federal, 
Chapter 1

Introduction and Summary of Findings 3

nvtac11-4177_fig01-01

95

95

215

95

93

93

93

93

15

15

NEVADA

NEVADA ARIZONA

MEAD

River

Colorado River

HOOVER DAM

Willow Beach

National Fish 

Hatchery (USFWS)

DAVIS DAM

Las Vegas

Henderson

Boulder City

Lake Mead

Fish Hatchery

(NDOW)

Alfred Merritt

Smith Water 

Treatment 

Facility (SNWA)

LAKE

Muddy

Logandale

Overton

Searchlight

Laughlin

Gregg Basin

Overton Arm

Callville Bay

Temple Basin Virgin Basin

Las Vegas Bay

Boulder

Basin

Pearce 

Ferry

Boulder Canyon

Black 

Canyon

Virgin 

Canyon

Iceberg

Canyon

Virgin River

Wash

LasVegas

LAKE

MOHAVE

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Areas of population

Grand Canyon National Park

Lake Mead National

 Recreation Area

115°0'W 114°30'W 114°0'W

36°30'N

36°0'N

35°30'N

Base from National Park Service and U.S. Geological Survey digital data,1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983

Nevada

Arizona

Study Area

Figure 1-1. Lake Mead National Recreation Area (LMNRA) showing important features of the landscape. NDOW, 

Nevada Department of Wildlife; SNWA, Southern Nevada Water Authority; USFWS, U.S. Fish and Wildlife Service.
4 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

State, and local agencies and organizations concerned with 

understanding and protecting the ecosystems of Lakes Mead 

and Mohave; and the Las Vegas Valley Watershed Advisory 

Committee comprising water and wastewater agencies in 

southern Nevada working to enhance watershed management 

of Lake Mead and its southern Nevada tributaries. 

Data collection and monitoring efforts supported by 

past partnerships have resulted in the recognition of new, 

emerging water-quality and environmental health issues 

within Lakes Mead and Mohave. In addition to population 

growth within Las Vegas Valley, urbanization has increased 

rapidly over the last 30 years along the Interstate-15 corridor 

from Overton, Nevada, through St. George, Utah, and within 

the tributary watersheds of the Virgin and Muddy Rivers. 

With population growth comes the likelihood of increased 

wastewater discharge from urbanized watersheds and the 

potential for increased nutrient loading and associated 

changes in algal production; contaminants such as industrial 

byproducts, volatile organic compounds, pesticides and 

heavy metals; endocrine disrupting compounds related to 

pharmaceuticals and personal care products; and human and 

wildlife pathogens. As urbanization expands in watersheds 

near LMNRA, it also increases the potential for non-point 

sources of environmental contaminants, such as herbicides 

and pesticides applied to golf courses, lawns, and power 

line rights-of-ways, as well as nutrients, such as nitrogen 

and phosphorous applied to lawns and golf courses. These 

potential water-quality effects from increased urbanization 

in tributary watersheds may be exacerbated by lowering lake 

levels caused by variations in climate or increased water 

demands, which reduces the amount of water available to 

dilute environmental contaminants. In addition, lower lake 

levels, as well as higher water temperatures resulting from 

climate change or increasingly larger shallow water areas, 

may result in changes to water circulation and resident biota, 

and potentially change the ecosystems of both lakes. Invasive

species, such as quagga mussels (Dreissena rostriformis 

bugensis), which were discovered in Lake Mead in 2007, also 

have the potential to produce significant ecosystem changes 

that will affect water quality (National Park Service, 2010). 

In recognition of these emerging threats to Lake Mead 

water quality, LMNRA obtained funding through the Southern 

Nevada Public Land Management Act in 2004 to build upon 

existing partnerships and enhance the understanding of water 

quality and natural resources throughout Lakes Mead and 

Mohave. This partnership program, coordinated by LMNRA, 

led to a number of research and monitoring products, 

including two Lake Mead Science Symposia, in January 

2009 (http://www.lakemeadsymposium.org) and January 

2012 (http://www.nvwra.org/presentations); and in 2010, the 

interagency development of a LMNRA Long Term Aquatic 

Resources Monitoring and Research Plan for Lakes Mead and 

Mohave (National Park Service, 2010). The plan documents 

six strategic objectives to maintain the quality of water 

within Lakes Mead and Mohave to support productive sports 

fisheries, healthy populations of native fish, aquatic-dependent 

wildlife, and aquatic and shoreline vegetation; extraordinary 

water-based recreation; and regional and community 

municipal and industrial uses including domestic water 

supply (National Park Service, 2010). The plan also outlines 

monitoring and research activities related to five ecosystem 

categories, including water quality and limnology; fish and 

aquatic biota; sediments; birds; and riparian vegetation, 

and suggests monitoring and research for three ecosystem 

stressors, including contaminants, invasive species, and 

climate change. A summary of key findings and management 

implications for these ecosystem categories and stressors are 

provided in table 1-1 and in more detail in table 7-1.

This Circular includes seven chapters. Chapter 2

introduces the environmental setting and characteristics of 

Lakes Mead and Mohave and provides a brief management 

context of the lakes within the Colorado River system as 

well as overviews of the geological bedrock and sediment 

accumulations of the lakes. Chapter 3 contains summaries of 

the operational and hydrologic characteristics of Lakes Mead 

and Mohave. Chapter 4 provides information on water quality, 

including discussion on the monitoring of contaminants and 

sediments within the reservoirs. Chapter 5 describes aquatic 

biota and wildlife, including food-web dynamics, plankton, 

invertebrates, fish, aquatic birds, and aquatic vegetation. 

Chapter 6 outlines threats and stressors to the health of 

Lake Mead aquatic ecosystems that include a range of 

environmental contaminants, invasive species, and climate 

change. Chapter 7 provides a summary of overall findings 

and a more detailed discussion on associated management 

implications, additional research, and monitoring needs.
Chapter 1

Introduction and Summary of Findings 5

Table 1-1. Summary of key scientific findings, management implications, and data and information needs for each of the ecosystem 

categories and stressors identified in Lake Mead National Recreational Area’s Aquatic Resources Plan.

Resource 

component

and related goals

Scientific findings Management implications Recommendations for data

or information needs

Water quality 

and

limnology

Basic water-quality parameters are within 

good ranges of State standards and EPA 

lake criteria. Potential problems with 

nutrient balance, algae, and dissolved 

oxygen can occur at times and in some 

areas of Lake Mead.

Recent Lake Mead-wide scope 

of monitoring has provided solid 

baseline to characterize water quality. 

More information is needed for

Lake Mohave. Tributary inflows 

provide the highest productivity, but 

also the greatest potential to cause 

nutrient related issues. 

Maintaining existing (2012) level 

of Lake Mead-wide monitoring of 

physical and biological parameters 

essential to assess trends and evaluate 

conditions. Monitoring is the 

foundation to assess impacts from 

quagga mussels, urbanization within 

watersheds, and potential climate 

change impacts.

Fish and 

aquatic biota

Sport fish populations are sufficient to 

support important recreational fishery. 

Native fish within Lake Mohave are 

declining. Lake Mead native fish 

populations are small but important 

because they are self-recruiting. 

Zooplankton composition may be 

influenced by quagga mussels but no 

significant changes noted to date.

Sport fish populations appear stable 

and have reached a balance with 

reservoir operations over the past 

20 years. Quagga mussels and the 

introduced gizzard shad (Dorosoma 

cepedianum), and wastewater 

treatment technologies may impact 

the balance. It is important to monitor 

status of spawning and use areas and 

population dynamics for razorback 

suckers.

Annual sport fish and shad population 

monitoring provides baseline to 

assess impacts of quagga mussels, 

nutrient cycling, and climate change.

Native fish population monitoring 

is critical for assessing trends and 

evaluating management. Need to 

better understand contaminant effects 

on native fish and wildlife.

Sediment Sediment deposition in Lake Mead 

prior to creation of Lake Powell 

was significant; the rate has greatly 

slowed since Glen Canyon Dam was 

completed. Low concentrations of legacy

pesticides and some emerging organics 

are present in sediments mostly in Las 

Vegas Bay, which appears to trap many 

contaminants. Lake Mohave has very 

little sediment accumulation.

Sediment deposition may act as a 

sink for low levels of contaminants. 

Re-suspension of contaminants could 

occur with water-level fluctuations 

or increases in storm intensities 

New delta deposits from lowering 

lake levels provide bird habitat and 

potential new riparian habitats.

Better understanding is needed of 

the relationship of contaminants in 

sediments to food-web transfers. 

Characterize transport of sediments

and potential re-suspension and flux 

of contaminants at the sediment-water 

interface. Monitor delta deposition 

in response to lowering lake levels to 

assess habitat potential and alteration 

of reservoir hydrology.

Birds Lakes Mead and Mohave provide 

important migration and wintering 

habitat. Trends include increasing 

numbers of wintering bald eagles 

(Haliaeetus leucocephalus) and 

nesting peregrine falcons. Lake Mead 

fluctuations have produced a variety of 

shorebird habitats. Songbird habitats are 

limited. Contaminants documented in 

birds and eggs in Las Vegas Wash.

An understanding of habitats created 

at different water levels and different 

rates of water-level change, in 

relationship to aquatic and shorebird 

use is important to understand the

continued role of Lake Mead in 

regional conservation. Understanding 

pathways of contaminants within the 

food web and to bird reproduction is 

needed to assess risks to population 

health.

Monitoring of population dynamics 

and relationship to available habitats 

needed to assess response to low 

water and evaluate bird responses to 

ecosystem changes. Monitoring

of potential contaminant impacts 

to bird populations is warranted. 

Research of potential impacts of 

quagga mussels on bird health 

is needed.

Riparian and 

aquatic 

vegetation

Lake Mead riparian vegetation is mostly 

limited to tributary deltas. Lower lake 

levels resulted in new deltas at tributary 

confluences. Lake Mohave is ringed with 

shoreline riparian habitats, mostly nonnative tamarisk. Mesquite groves line 

much of Lake Mohave’s upper riparian 

fringe.

New deltas provide potential for 

riparian habitats. Newly exposed 

shoreline habitats have potential to 

spread non-native species. The nearshore band of riparian habitat of Lake

Mohave requires active management. 

Quagga mussel infestation may alter 

growth of aquatic vegetation.

Early detection monitoring needed 

for aquatic invasive vegetation and 

for littoral and aquatic vegetation. 

Vegetation and community inventory

and monitoring at Virgin River and 

Colorado River deltas is needed to 

assess restoration potential.
6 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Chapter 1 References

Holdren, G.C., and Turner, K., 2010, Characteristics of Lake 

Mead, Arizona–Nevada: Lake and Reservoir Management, 

v. 26, no. 4, p. 230-239.

National Park Service, 2010, Long-term limnological and 

aquatic resources monitoring and research plan for Lakes 

Mead and Mohave: Boulder City, Nev., Lake Mead 

National Recreation Area and University of Nevada, Las 

Vegas, 94 p., accessed August 17, 2012, at http://www.nps.

gov/lake/naturescience/loader.cfm?csModule=security/

getfile&%3bpageid=431205.

Resource 

component and

related goals

Scientific findings Management implications Recommendations for data

or information needs

Contaminants Legacy contaminants declining due

to regulations and mitigation.

Emerging contaminants, including

endocrine disrupting compounds

present in low concentrations,

especially near Las Vegas Wash.

Biomarkers of endocrine disruption

documented in common carp 

(Cyprinus carpio).

Emerging contaminants of concern

not seen at levels currently known to

pose a threat to human health, but

have been documented to cause

a number of health effects to

individual fish. Contaminants pose

risk to fish and wildlife.

Continued monitoring of legacy

contaminants and inventory and

monitoring of emerging contaminants

of concern in the water column is 

needed. Greatest new need is for 

information related to population 

level impacts to fish and wildlife and 

documentation of movement of 

contaminants through the food web.

Invasive species Quagga mussels have become

the dominant benthic organism in 

vast areas of the lakes. Quagga 

mussels are reproducing in lakes 

year around, with juvenile veliger 

larvae a significant proportion of

zooplankton at certain times of the 

year.

Quagga mussels have potential to alter

water quality and nutrient cycling,

plankton composition, and foodweb dynamics. They can degrade

recreational setting. Quagga mussels

are a significant threat to ecosystems 

of Lakes Mead and Mohave.

Interagency quagga mussel 

monitoring plan has provided quality 

baseline of their population.

Existing adult and veliger larvae 

monitoring should continue. 

Additional work is needed to

comprehensively assess ecosystem 

impacts and food-web dynamics.

Climate change Climate models developed for the

Colorado River watershed indicate

probability of decline in watershed

snowpack and thus reduced water

availability. Models point to

increased potential for summer

thunderstorms and flash floods.

Models indicate high probability for 

longer periods of low water levels in 

Lake Mead. This would alter water 

circulation patterns, nutrient cycling, 

and food-web dynamics. Higher water 

surface temperatures could raise 

productivity, and also raise the risk of 

pathogenic organisms to thrive. 

Information needed for Lakes Mead 

and Mohave relates to potential 

impacts of low flows, lower water 

levels, increased air temperatures, 

and increased water temperatures on 

limnology, ecosystems, fish and 

wildlife, and recreation and potential 

pathogens. 

Table 1-1. Summary of key scientific findings, management implications, and data and information needs for each of the ecosystem 

categories and stressors identified in Lake Mead National Recreational Area’s Aquatic Resources Plan.—Continued
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 7

2

Chapter

Lakes Mead and Mohave provide opportunities for millions of regional, national, and international 

visitors to enjoy a wide array of water-based recreation in a spectacular desert setting. The national 

significance of the site’s recreational opportunities and scientific values led to its designation as the 

nation’s first National Recreation Area in 1964. The stark contrast of the deep blue lakes with spacious 

open water basins against a backdrop of mountain and canyon scenery creates a diversity of landscapes 

inviting recreation from the active to the contemplative (Maxon, 2009). The quality of the setting as a 

backdrop for the recreational experience has resulted in designation of approximately 200,000 acres of 

lands surrounding the lakes as wilderness (National Park Service, 2005). 

By Kent Turner1, Michael R. Rosen2, 

G. Chris Holdren3

, Steven L. Goodbred2, and 

David C. Twichell2

Environmental Setting 

of Lake Mead National 

Recreation Area

Panorama of Boulder Basin, Lake Mead. Photograph by National Park Service.

1

National Park Service

2

U.S. Geological Survey

3

Bureau of Reclamation
8 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Desert bighorn sheep (Ovis canadensis nelsoni) near Willow Beach, 

Arizona. Photograph by Phillip Cummingham, U.S. Fish and Wildlife 

Service.

Coyote (Canis latrans) looking for food along Overton Arm. 

Photograph by Michael R. Rosen, U.S. Geological Survey.

Davis Dam, near Bullhead City, Ariz., which created Lake Mohave, rises 

approximately 140 ft (42.7 m) above the level of the Colorado River. It is a zoned 

earthfill structure with concrete spillway, intake structure, and power plant. 

Photograph by Michael R. Rosen, U.S. Geological Survey.

LMNRA is part of a land of rugged beauty 

where the Colorado Plateau geologic province 

transitions to the Basin and Range, and where 

influences of three of the four North American 

deserts converge (Houk, 1997; Rohde, 1999). 

The variety of exposed geology and topography, 

coupled with abundant water resources in a desert

land, create a diversity of wildlife habitats. 

Visitors may observe desert bighorn sheep (Ovis canadensis 

nelsoni) feeding along the shoreline ridges, be surprised by 

coyote (Canis latrans) drinking from the shoreline, and then 

be drawn to waterbirds feeding on fish (Maxon, 2009). 

Given the multitude of societal needs met by Lakes 

Mead and Mohave, and their importance to water and 

wildlife conservation, numerous Federal, State, and local 

agencies have interests in direct management and providing 

scientific information for management decisions for the 

lakes. These entities include the Bureau of Reclamation, 

National Park Service, Southern Nevada Water Authority, 

Nevada Department of Wildlife, Arizona Game and Fish 

Department, U.S. Fish and Wildlife Service, U.S. Geological 

Survey, and wastewater reclamation districts who discharge 

into Las Vegas Wash, representing Clark County and the 

cities of Henderson, Las Vegas, and North Las Vegas. 

These agencies coordinate management and exchange 

scientific information necessary to address the interests 

of consumptive uses and domestic water supplies, fish 

and wildlife conservation, and recreation. The framework 

for water management is established by operations of the 

Hoover and Davis Dams, hydrology and water supply of the 

Colorado River, and contributions from Las Vegas Wash, 

Muddy River, and Virgin River.
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 9

View of Hoover Dam from the new Mike O’Callaghan–Pat Tillman Memorial Bridge 

(Colorado River Bridge). Photograph by Michael R. Rosen, U.S. Geological Survey.

View of Lake Mead at Echo Bay (Overton Arm). Photograph by Michael R. Rosen, U.S. 

Geological Survey.

Lake Mead was formed by the completion of Hoover 

Dam in 1935 (Bureau of Reclamation, 2008). The dam 

and lake behind it were intended to provide flood control 

for the Colorado River, hydroelectric power for the nation, 

and a reliable water supply for agriculture and human 

consumption. In addition to these benefits, the lake and 

surrounding areas also have provided numerous recreational 

opportunities for humans and additional habitat for aquatic 

and terrestrial wildlife. 

Lakes Mead and Mohave contain more than 140 mi 

(225.3 km) of former river channels, a combined 225,000 

surface acres, and a wide range of water depths and 

geomorphic configurations (fig. 2-1; table 2-1). Lake Mead 

extends from Hoover Dam to Pearce Ferry at full pool and 

contains four large subbasins: Boulder, Virgin, Temple, and 

Gregg; four narrow canyons: Black, Boulder, Virgin, and 

Iceberg; and the 30-mi long Overton Arm, which extends 

from the Virgin and Muddy Rivers to the Virgin Basin. The 

Colorado River supplies 97 percent of the inflow into Lake 

Mead (Las Vegas Valley Watershed Advisory Committee, 

2009). Other tributaries to Lake Mead include the Virgin 

River, Muddy River, and Las Vegas Wash.

Lake Mohave, which is Lake Mead’s downstream 

neighbor (fig. 2-1), was created in 1951 by the construction 

of Davis Dam to stabilize flows from Hoover Dam and 

to help provide required water deliveries to Mexico. The 

Colorado River below Lake Mohave also provides water 

for Laughlin, Nevada, as well as up to 251 megawatts of 

hydroelectric power from Davis Dam. Lake Mohave extends 

approximately 67 mi (107.8 km) along the valley from 

Hoover Dam to Davis Dam, and is both narrow and shallow 

compared to Lake Mead (table 2-1; National Park Service, 

2010). Lake Mohave also is ecologically important because 

it is home to the largest existing population of endangered 

razorback sucker (Xyrauchen texanus).
10 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

nvtac11-4177_fig02-01

95

95

215

95

93

93

93

93

15

15

NEVADA

NEVADA ARIZONA

River

Colorado River

HOOVER DAM

Willow Beach

National Fish 

Hatchery (USFWS)

DAVIS DAM

Las Vegas

Logandale

Overton

Henderson

Lake Mead

Fish Hatchery

(NDOW)

Alfred Merritt

Smith Water 

Treatment 

Facility (SNWA)

Muddy

Searchlight

Laughlin

Gregg Basin

Overton Arm

Temple Basin Virgin Basin

Las Vegas Bay

Boulder

Basin

Pearce 

Ferry

Boulder Canyon

Black 

Canyon

Virgin 

Canyon

Iceberg

Canyon

Virgin River

Wash

LasVegas

Callville Bay

Boulder 

City

MEAD

LAKE

LAKE

MOHAVE

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Areas of population

Grand Canyon National Park

Lake Mead National

 Recreation Area

115°0'W 114°30'W 114°0'W

36°30'N

36°0'N

35°30'N

Base from National Park Service and U.S. Geological Survey digital data,1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983

Nevada

Arizona

Study Area

Figure 2-1. Lake Mead National Recreation Area (LMNRA) showing important locations and basins in the 

lakes. NDOW, Nevada Department of Wildlife; SNWA, Southern Nevada Water Authority; USFWS, U.S. Fish 

and Wildlife Service.
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 11

Table 2-1. Characteristics of Lakes Mead and Mohave.

[Abbreviations: acre-ft, acre-foot; ft, foot; km, kilometer; km2, square kilometer; m, meter; m3, cubic meter; mi, mile; yr, year]

Lake characteristics

(full pool)

1

Lake Mead Lake Mohave

Value Value

Surface area 157,418 acres (637 km2) 2 28.084×103 acres (114 km2)

Volume 28.8×106 acre-ft (3.55×1010 m3) 3 1.8×106 acre-ft (2.22×109 m3)

Mean depth 182 ft (55.5 m) 4 85 ft (25.9 m)

Maximum depth 532 ft (162 m) 4 165 ft (50.3 m)

Watershed area 5167×103 mi2 (433×103 km2) 6 168×103 mi2 (435×103 km2)

Mean inflow 10.9×106 acre-ft/yr (1.34×1010 m3/yr) 9.6×106 acre-ft/yr (1.18×1010 m3/yr)

Hydraulic residence time 72.6 yr 760 days

Shoreline length 2759 mi (1,221 km) 2309 mi (497 km)

Watershed area to lake surface area ratio 681:1 3,813:1

1

Lake Mead values from Holdren and Turner (2010) except where noted.

2

Computed from the U.S. Geological Survey, National Hydrographic Dataset flood polygon for Lakes Mead and Mohave.

3

Taken from the Lake Mohave area-capacity tables (Available Capacity Table and Area-Capacity Curves for Lake Mohave, Bureau of Reclamation, October 1, 

1949). Includes 8,530 acre-ft in dead storage below 533-ft elevation.

4

Computed from fathometer readings taken by the U.S. Geological Survey in April 2002 (Lake Mohave Geophysical Survey 2002: GIS Data Release, 

available in Cross and others (2005). Readings were adjusted for vessel draft (1 m) and lake elevation at the time of survey (642 ft). Calculations were based on 

95,216 fathometer readings taken from Willow Beach to Davis Dam.

5

Watershed area is from the headwaters of the Colorado River to Hoover Dam.

6

Watershed area is from the headwaters of the Colorado River to Davis Dam.

7

Calculated from average inflow and lake volume. Actual residence time depends on reservoir operations for Lakes Mead and Powell.

Boating and water skiing are popular recreational activities at Lake Mead. 

Photograph by National Park Service.

Use of Water in Lakes Mead and Mohave

As mentioned above, providing flood control for 

the Colorado River, irrigation and domestic uses, and the 

development of hydropower were the primary authorized 

purposes within the Boulder Canyon Project Act of 1928 that 

legislated construction of Hoover Dam (Bureau of Reclamation, 

2005). Annual water deliveries from storage provided by Lake 

Mead helps to irrigate more than 2.5 million acres of land, with 

more than 80 percent of the allocations released through Hoover 

Dam utilized for agriculture within the States of Arizona and 

California. Hoover Dam’s facilities feature 17 turbines, of which 

1 is rated at 86,000 horsepower, 1 at 100,000 horsepower, and the 

remaining 15 at 178,000 horsepower each. The dam’s maximum 

hydropower generation is more than 2 gigawatts, and annual 

output exceeds 4 billion kilowatt hours, which is delivered by 

the Western Area Power Administration within the States of 

California, Arizona, and Nevada (Bureau of Reclamation, 2005). 

The electricity generated from Hoover Dam in an average year 

is enough to meet the annual usage of nearly 1.4 million homes 

(Bureau of Reclamation, 2009). 
12 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Minor flooding in the Virgin and Muddy Rivers in April 2010 

carries muddy water to Overton Arm. Photograph by Jorge 

Arufe, U.S. Geological Survey.

Among Lake Mead’s authorized primary purposes is operation to 

maintain flows to ensure efficient and timely delivery of Colorado River 

allocations for irrigation and domestic purposes to Arizona, Nevada, and 

California (Lower Basin States), as well as to meet treaty obligations 

to Mexico. These allocations include 300,000 acre-ft for Nevada, 

2.8 million acre-ft (maf) for Arizona, 4.4 maf for California, and 1.5 maf 

for Mexico (Bureau of Reclamation, 2008). The prolonged drought of 

the 2000s has resulted in the Colorado River Basin States and the Bureau 

of Reclamation developing criteria for water allocations during specific 

low reservoir conditions (Bureau of Reclamation, 2007). These and other 

guidelines also have provided mechanisms for the Lower Basin States 

and Mexico to store currently unused allocations of water in Lake Mead 

for use in future years (Bureau of Reclamation, 2007). The potential 

for new water storage has highlighted some interagency concerns over 

degradation of this stored water by local stream inputs. These possible 

changes have renewed regional interest in maintaining the existing high 

water quality of Lake Mead.

Lake Mead plays a key role in the entire storage capacity and 

operational framework of the Colorado River (see Chapter 3). The lake 

has enough capacity (28.5 maf) to hold the entire flow of the Colorado 

River for 2 years of average annual flow. Lakes Mead and Powell together 

provide approximately 85 percent of the total storage capacity on the 

Colorado River, and currently are operated in close coordination to meet 

the annual delivery requirements of the river (Bureau of Reclamation, 

2007). Lake Mohave is operated to provide steady flows to meet 

downstream requirements including treaty obligations to Mexico.

Annual visitation to LMNRA exceeds 8 million people. More than 

60 percent of all visitors to the recreation area use some type of motorized 

watercraft; peak day use on the water between Lakes Mead and Mohave 

can exceed 5,000 boats (National Park Service, 2002). Lakes Mead and 

Mohave together provide in excess of 250,000 angler days annually 

(National Park Service, 2010). This visitation includes large numbers of 

residents of Nevada, Arizona, and California because LMNRA provides 

outstanding water-based recreational opportunities and amenities that 

contribute to improving the quality of life for the region. In general, 

recreational activities on Lakes Mead and Mohave include boating (both 

Lakes Mead and Mohave 

Characteristics and Tributary Sources

Lake Mead covers 157,420 acres 

(637.1 km2) and extends 65 mi (104.6 km) from 

the inflowing Colorado River to Hoover Dam at 

full pool. The lake has a maximum water depth 

of 519 ft (158.2 m), an average depth of 183 ft 

(55.8 m), and holds more than 28 maf (greater 

than 35.5 billion m3) of water at full capacity. 

Key characteristics of Lake Mead were recently 

updated by Holdren and Turner (2010) and are 

summarized in table 2-1.

From upstream to downstream, Lake 

Mead’s four large, deep, but connected basins 

along the historical Colorado River channel are: 

Gregg, Temple, Virgin, and Boulder (fig. 2-1). 

These four basins are ecologically distinct from 

one another because the waters within them 

retain the properties of their sources (LaBounty 

and Burns, 2005). Four narrow canyons (Iceberg, 

Virgin, Boulder, and Black) and the nearly 33-mi 

(53.1-m) long Overton Arm, which extends south 

from the Virgin and Muddy Rivers to the Virgin 

Basin, are other important features of the lake. 

When full, it takes water an average of 2.6 years 

to travel through the lake.

Families visit Lake Mead for water recreation, including kayaking. 

Photograph by Jennell M. Miller, University of Nevada, Las Vegas.

power boating and 

paddlecraft), swimming, 

fishing, scuba diving, 

picnicking, and shoreline 

camping (National Park 

Service, 2002). More than 

$45 million annually are 

directly generated from 

concession sales on the 

lakes, with an additional 

estimation of greater than 

$250 million annually 

added to the regional 

economy from recreation 

expenditures (Duffield 

and others, 2007).
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 13

Lake Mohave covers approximately 

30,000 acres (121.4 km2) extending 67 mi 

(107.8 km) from Hoover Dam to Davis Dam. The 

lake has a maximum depth of 165 ft (50.3 m) and 

stores more than 1.8 maf (more than 2 billion m3) 

of water at full capacity (table 2-1). Lake 

Mohave is long and narrow. There is only one 

large basin near the center of the lake, and it is 

4 mi (6.4 km) across at its widest point (fig. 2-1). 

On average, it takes water about 60 days to travel 

through Lake Mohave when it is full. Davis 

Dam is operated as a regulation dam to hold the 

water released from Hoover Dam for smooth 

deliveries to meet downstream requirements in 

an efficient manner. As a result, the water level in 

Lake Mohave fluctuates between approximately 

elevation 630 and 645 ft (192.0 and 196.6 m) on 

an annual basis, and within predictable cycles. 

The availability of water within the 

Colorado River system depends primarily upon 

the amount of annual snowmelt and rainfall 

received on the western slope of the Rocky 

Mountains in Colorado, which is the source 

of the Colorado River. Total inflows to Lake 

Mead averaged about 10.9 maf/yr (13.4 billion 

m3/yr) between 1935 and 2001 (Ferrari, 2008). 

Flows decreased from 1999 through 2010 as the 

entire Colorado River Basin experienced drought 

conditions. More recent (1999–2010) Colorado 

River inflows average 8.23 maf/yr (10.1 billion 

m3/yr) with additional inflow contributed by the 

lake’s other tributaries. Inflows to Lake Mead 

are determined by releases from Glen Canyon 

Dam, as determined by annual operating plans 

and guidelines to meet operational requirements 

established through the framework of law, policy 

and guidelines collectively known as the “Law of 

the River.” A smaller percentage of water comes 

from tributaries and washes along the river 

between Glen Canyon Dam and Lake Mead. 

In addition to inflow from the Colorado River 

mainstream, Lake Mead receives water from the 

Virgin and Muddy Rivers and from Las Vegas 

Wash (fig. 2-2). These three inputs combined, 

however, provide only 3 percent of the total 

input to Lake Mead (see U.S. Geological Survey 

National Water Information System web site at 

http://waterdata.usgs.gov/nwis). Only during 

periods of flooding do these tributaries contribute 

a larger proportion of the overall flow (up to 

55,000 ft3/s), and then only for short periods of 

time (generally hours to a few days). Diversions 

Figure 2-2. Discharge at the primary river gaging stations measuring 

discharge to Lake Mead for 2010. Left axis scales are for the Colorado River and 

right axis scales are for all other rivers. 

Expansion of the Las Vegas Metropolitan area occurred rapidly 

from 1970 to 2008. Photograph by Michael R. Rosen, U.S. 

Geological Survey.

Gregg Basin and Temple Basin are fed by the mainstream of the 

Colorado River. At current (2012) lake levels, the Colorado River enters 

Lake Mead at the northern end of the Gregg Basin, nearly 60 mi (96.6 km) 

upstream of Hoover Dam (fig. 2-1). The Virgin and Muddy Rivers flow into 

the Overton Arm, and then travel 25 mi (40.2 km) to merge with Colorado 

River water in the Virgin Basin. The combined flows from the upper end of 

Lake Mead enter the east end of Boulder Basin at the Narrows. Las Vegas 

Wash enters Las Vegas Bay at the west end of Lake Mead’s Boulder Basin.

for irrigation upstream 

of Lake Mead limit 

flows in the Virgin and 

Muddy Rivers during 

the summer months. 

The annual inflow via 

Las Vegas Wash has 

increased over the past 

30 years as a result of 

the rapid population 

growth in Las Vegas. 

Average flows in Las 

Vegas Wash have more 

than doubled during this 

period. 

 Colorado River

 Las Vegas Wash

 Virgin River

 Muddy River

EXPLANATION

Jan. Feb. Mar. Apr. May June July Aug.Sept. Oct. Nov. Dec. Jan.

200

250

300

350

400

450

500

550

Cubic meters per second 

2010

8,000

10,000

12,000

14,000

16,000

18,000

20,000

Cubic feet per second

Colorado River Other Rivers

0

10

20

30

40

50

60

70

80

 Cubic meters per second

0

500

1,000

1,500

2,000

2,500

3,000

Cubic feet per second
14 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The average annual precipitation at Lake 

Mead, based on data from several weather stations 

around the lake, is only 5.74 in/yr (0.146 m/yr). 

Because of the large size of Lake Mead, direct 

precipitation on the lake surface would contribute 

75,500 acre-ft/yr (9.31×107 m3/yr) at full pool, 

or slightly more than 6 in. (15.2 cm) of lake 

elevation. This is less than 1 percent of the total 

tributary inflow, but more than the average annual 

inflow from the Muddy River (Holdren and Turner 

2010).

The Colorado River upstream of Lake Mead 

drains an area of 149,000 mi2 (386,000.0 km2) 

including parts of Colorado, Wyoming, New 

Mexico, Utah, Nevada, and Arizona. The Overton 

Arm collects the combined drainages of the 

Virgin and Muddy Rivers. These watersheds 

have a combined area of more than 12,030 mi2

(31,157.6 km2). Most of the land in the Colorado, 

Virgin, and Muddy River watersheds is either 

rangeland or forest, but the cities of St. George, 

Utah, and Mesquite, Nev., are undergoing rapid 

development that may affect both the quality of 

water in Lake Mead and the quantity of water 

delivered to the lake in the future.

The total drainage area for the Las Vegas 

Wash watershed is 2,193 mi2 (5,679.8 km2). 

Drainage in Las Vegas Wash includes non-point 

surface and groundwater discharges, non-point 

runoff from the Las Vegas metropolitan area, and 

treated wastewater from the cities of Las Vegas, 

Henderson, North Las Vegas, and Clark County 

municipal wastewater treatment facilities. The 

Las Vegas Wash watershed is predominantly 

non-developed scrub lands of the Mojave 

Desert, but the rapidly expanding urban Las 

Vegas metropolitan area covers nearly 450 mi2

(1,165.5 km2; 2010 U.S. Census Bureau data, 

http://www.census.gov).

Several basins within the Lake Mead 

watershed do not contribute runoff to the lake. 

These include 3,959 mi2 (10,253.8 km2) in 

the Great Divide basin in Wyoming and 697 

mi2 (1,805.2 km2) on the Colorado Plateau 

in the Colorado River watershed, 3,780 mi2

(9,790.1 km2) in the White and Meadow Valley 

Wash subbasins of the Muddy River watershed, 

which is more than one-half of the total area of 

that watershed, and 607 mi2 (1,572.1 km2) in the 

Las Vegas Wash watershed.

Outflows from Lake Mead include water 

releases downstream through Hoover Dam, 

withdrawals for drinking water for southern 

Nevada, and evaporation. Nearly all water entering 

Lake Mohave comes through releases of Colorado 

River water through Hoover Dam. 

Geology below Lake Mead 

Lake Mead lies within the Basin and Range 

Geologic Province of the Southwestern United 

States, and the shape of the lake is controlled by 

the complex terrain that characterizes this province 

(fig. 2-1). The result of this geology is that the lake 

is divided into five broad basins (Gregg Basin to 

the east; Temple Basin, Virgin Basin, and Overton 

Arm in the central part of the lake, and Boulder 

Basin to the west) separated by two narrow gorges 

where the lake cuts through mountain ranges 

(Virgin and Boulder Canyons).

Three different lake-floor substrates were 

identified on the basis of mapping completed 

prior to formation of the lake (Longwell, 1936) 

and geophysical data (Twichell and others, 

2005): rock outcrops, alluvial deposits (material 

that has been eroded from the rocks by water 

and accumulated on hill slopes as sediments), 

and post-impoundment sediment deposits. The 

rock outcrops and alluvial deposits predate the 

lake, while the post-impoundment sediment has 

accumulated since completion of Hoover Dam. 

Areas of rock outcrop composed of Precambrian to 

Tertiary-age igneous, volcanic, and metamorphic 

rocks make up the flanks of the narrow gorges and 

Tertiary-age sandstones flank parts of Overton 

Arm, Virgin Basin, and Boulder Basin (Longwell, 

1960). Alluvial deposits are Quaternary age (less 

than 1 million years old) (Longwell, 1960) and 

occupy large parts of the flanks of Overton Arm, 

Virgin Basin, and Boulder Basin (fig. 2-3). The 

post-impoundment sediment fills the axial (central) 

valley of the pre-impoundment Colorado River as 

well as the floors of tributary valleys.
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 15

Sediment Accumulation in Lake Mead

Sediment accumulation in Lake Mead has 

been extensively studied. The earliest studies 

preceded construction of the Hoover Dam and 

involved mapping the geology and topography 

of the region to be flooded by the reservoir 

(Longwell, 1936, 1960). Shortly after completion 

of the dam annual, bathymetric surveys were 

conducted along the pre-impoundment Colorado 

River channel to monitor sedimentation 

(Gould, 1960), cores were collected to assess 

the composition of the post-impoundment 

sediment (Gould, 1960) and suspended sediment 

concentrations in the Colorado River (Alexander 

and others, 1997) and Lake Mead (Gould, 1951; 

Howard, 1960) were measured to determine 

the volume of sediment brought to the lake 

and its distribution within the lake. Additional 

bathymetric surveys were completed in 1964 

(prior to construction of the Glen Canyon Dam 

upriver of this reservoir; Lara and Sanders, 

1970) and 2001 (Ferrari, 2008). A detailed 

geophysical survey of the entire lake, the results 

of which were used to map sediment distribution 

and thickness, was completed between 1999 

and 2001 (Twichell and others 1999, 2003, 

2005, 2009).

Figure 2-3. Interpreted lake-floor geology. Four units were mapped: outcrops of Tertiary and older rocks, Quaternary alluvial deposits, 

thick post-impoundment sediment deposits, and thin post-impoundment sediment deposits. These are represented in the map as rock, 

alluvial deposits, thick sediment, and thin sediment, respectively. An oversized version (11×17) of this figure is available for download at 

http://pubs.usgs.gov/circ/1381. 

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

No information

(about 8,000 acres or 9%*)

Alluvial deposits

(about 40,000 acres or 44%*) Rock 

(about 18,000 acres or 20%*)

Thick sediment

(about 20,000 acres or 22%*) 

Thin sediment

(about 4,000 acres or 5%*)

Areas of exposed rock outcrop 

which have not been covered 

by sediment (or only have a 

minimal sediment cover) since 

the lake formed.

Silt and sand more than 

1 meter thick that has 

accumulated primarily on the 

floors of the original river 

valleys since impoundment. 

Silt and sand less than 1 meter 

thick that has accumulated 

since impoundment.

Unconsolidated sand and 

gravel that was deposited 

primarily as alluvial fans prior 

to the formation of the lake. 

Shallow bays and other regions 

that have not been surveyed. 

*Note: Calculations and mapping are 

based on a lake elevation of 1,100 feet.

Surveys were conducted between 1999 

and 2001 by the U.S. Geological Survey in 

collaboration with the Bureau of 

Reclamation, University of Nevada Las 

Vegas, and the National Park Service. 

LAKE AT

1,100 FEET

ELEVATION

Echo Bay

Temple Bar

South

Cove

LAKE AT

FULL POOL

(1,220 FEET

ELEVATION)

ColoradoRive r

Virgin River

Muddy River

B L A C K M O U N T A I N S

HI L L E R M O U N TA I N S

Boulder Harbor 

Hemenway

Harbor 

Callville Bay

Las Vegas

Bay Las Vegas Wash

HOOVER DAM 

Overton Beach

CIRCULAR 1381 FIGURE 2.3—Interpreted lake-floor geology. Four units were mapped: outcrops of Tertiary and older rocks, 

Quaternary alluvial deposits, thick post-impoundment sediment deposits, and thin post-impoundment sediment deposits. 

These are represented in the map as rock, alluvial deposits, thick sediment, and thin sediment, respectively.

Rosen, M.R., Turner, K., Goodbred, S.L., and Miller, J.M., eds., 2012, 

A synthesis of aquatic science for management of Lakes Mead and Mohave
16 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Figure 2-4. Sedimentation patterns in Lake Mead have contributed to declines in the capacity of the reservoir to hold water; however, 

with the completion of Glen Canyon Dam, the rate of sediment accumulation has slowed greatly. An oversized version (11×17) of this 

figure is available for download at http://pubs.usgs.gov/circ/1381. 

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

Sediment thickness,

 in feet

0 - 2

2 - 5

5 - 10

10 - 25

25 - 50

50 - 75

75 - 100

100 - 150

150 - 200

200 - 250

250 - 300

EXPLANATION

CIRCULAR 1381 FIGURE 2.4—Sedimentation patterns in Lake Mead have contributed to declines

in the capacity of the reservoir to hold water; however, with the completion of 

Glen Canyon Dam, the rate of sediment accumulation has slowed greatly

Rosen, M.R., Turner, K., Goodbred, S.L., and Miller, J.M., eds., 2012, 

A synthesis of aquatic science for management of Lakes Mead and Mohave

Echo Bay

Temple Bar

South

Cove

LAKE AT

FULL POOL

(1,220 FEET

ELEVATION)

Boulder Harbor 

Hemenway

Harbor HOOVER DAM 

Callville Bay

Surveys were conducted between 

1999 and 2001 by the U.S. Geological 

Survey in collaboration with the 

Bureau of Reclamation, University 

of Nevada Las Vegas, and the 

National Park Service. 

Las Vegas

Bay

Muddy River

Virgin River

Colorado Rive

L

r

as Vegas Wash

Overton Beach

LAKE MEAD SEDIMENTATION STATISTICS

 Reservoir Rate of Sediment 

 Year Capacity Sedimentation Volume

 (acre-feet @ 1,220 feet) (acre-feet per year) (acre-feet)

 1935

 1948 

 1963 

 2001

Notes:

Increase in reservoir capacity and decrease in sediment volume due to significant 

compaction of sediments in lower elevation portions of the reservoir.

Decreased rate of sedimentation due to closure of Glen Canyon Dam in 1963.

*

**

137,000

97,429

88,200

< 10,000

30,944,400

29,878,000

28,321,300

28,543,420

0

1,066,400

2,623,100

* ** 2,400,980 *

Sediment accumulation has been significant. Lara 

and Sanders (1970) reported that 1,425,900 acre-ft 

(1.7588×109 m3) of sediment accumulated in Lake 

Mead between 1935 and 1948–49. An additional 

1,293,100 acre-ft (1.5950×109 m3) accumulated 

between 1948–49 and 1963–64, for a total of 

2,716,900 acre-ft (3.3512×109 m3), or approximately 

12 percent of the original lake volume. The completion 

of Glen Canyon Dam greatly slowed the amount of 

sediment transport to Lake Mead and was estimated 

to increase the life of Lake Mead by 500 years (Lara 

and Sanders, 1970). The Ferrari survey confirmed the 

interception of most sediment by Lake Powell and 

found the volume of Lake Mead actually increased by 

219,150 acre-ft (2.7032×108 m3) between 1963–64 and 

2001 (Ferrari, 2008). The increase was attributed to 

the reduction in sediment inflow and the consolidation 

(compaction) of previous sediment deposits.

Post-impoundment sediment extends the entire 

length of the lake from the mouths of the Colorado 

River and its tributaries to the Hoover Dam. This 

sediment is thickest along the original path of the 

Colorado River and thinner along the floors of the 

former Virgin River and Las Vegas Wash (fig. 2-4). 

At the eastern end of the lake, off the mouth of the 

Colorado River, these sediments are almost 279 ft 

(85.0 m) thick, thinning to about 82.0 ft (25.0 m) 

behind Hoover Dam. Sediment filling the floors of 

tributary valleys is mostly less than 3.3-ft (1.0-m) 

thick except in the deltas off the mouths of the 

tributary rivers, where sediment can reach thicknesses 

of 33 ft (10.0 m; Twichell and others, 2001, 2003). 
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 17

Clay at the top of a lake core from Las Vegas Bay. Photograph by 

Michael R. Rosen, U.S. Geological Survey.

Figure 2-5. Four bathymetric profiles along the Colorado River channel through Lake Mead showing changes in sedimentation 

(modified from Ferrari, 2008).

Elevation, in meters

0 20 40 60 80 100 120

Distance from Hoover Dam, in kilometers

200

300

400

140 160 180

West East

2001 lake elevation

Hoover 

Dam

Extent of geophysical survey

Boulder

Basin

Virgin

Basin

Temple

Basin

Gregg

Basin

Boulder

Canyon

Virgin Canyon

2001

1963

1948

1945

1940

1937

Elevation is referenced to the U.S.Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

A sandy delta at the mouth of the Colorado 

River advanced rapidly into the lake during the 

first 13 years after the dam was completed, slowed 

during the next 15 years, and after construction of 

the Glen Canyon Dam upstream in 1965 nearly 

stopped advancing (fig. 2-5). These surveys show 

that sediment also accumulated along the remainder 

of the profile at rates that decreased throughout the 

history of the lake (Smith and others, 1960; Ferrari, 

2008). The regional geophysical mapping shows 

that post-impoundment sediments are found only 

along the floors of the deepest parts of the lake, 

namely, above the river valleys that drained the area 

prior to impoundment, which suggests that sediment 

has been distributed throughout the lake primarily 

by near-bottom flows. The early studies demonstrated that the Colorado River water with its heavy 

suspended sediment load, which was denser than the water mass in Lake Mead, sank upon entering the lake 

and flowed along the full length of the lake floor, following the deepest path available (Gould, 1951, 1960). 

As these flows, known as density flows or turbidity currents, traveled away from the river mouth across 

decreasing lake-floor slopes, they slowed and sediment settled out of suspension. Experiments indicate that 

the coarsest sediment in turbidity currents settles out at faster speeds (because larger grains require more 

turbulence to keep them in suspension) and the finer sediment travels farther because it takes longer to settle 

from suspension (Middleton and Southard, 1984). Cores from the floor of Lake Mead showed repeated 

sedimentary units (beds) of fine sand or coarse silt that grade upward to clay, which are typically the type of 

deposits that result from the passage of turbidity currents (Twichell and others, 2005). 
18 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Because of the well documented history of 

water discharge and suspended sediment loads of 

the Colorado River (Alexander and others, 1997), 

the rates at which turbidity currents travel the length 

of the lake (Gould, 1951), and the distribution and 

composition of sediments on the lake floor (Twichell 

and others, 2005), Lake Mead has proven to be a 

unique shallow-water natural laboratory for the study 

of turbidity currents, a process that most commonly 

is studied in more inaccessible, deep-sea settings 

(Bouma and Stone, 2000, and references therein).

Geology below Lake Mohave

Lake Mohave was created with the completion

of Davis Dam in 1950 and impoundment of the 

Colorado River below Hoover Dam. This region of 

the Colorado River Valley lies between the Black 

Mountains to the east and the Eldorado and Newberry 

Mountains to the west (fig. 2-6). Metamorphic and 

coarse-grained igneous rock makes up most of the 

exposed bedrock of these mountains. The northern 

section of the reservoir is constrained by the steep 

volcanic walls of Black Canyon. Below Black Canyon, 

Lake Mohave gradually widens with alluvial deposits 

bounding this section of the lake.

Farther south, Lake Mohave is constricted by a 

local protrusion of volcanic rock at Painted Canyon 

(Cross and others, 2005). The lake widens again to 

the south, where it lies within a wider, more gently 

sloping alluvial basin, reaching its greatest width 

in the central part of this basin. Lake Mohave is 

constricted once again still farther to the south with 

increasing slope of the alluvial basin and is bounded 

by the steep slopes of the Newberry Mountains to the 

west and alluvium to the east. Davis Dam, constructed 

within a narrow gorge cut into Precambrian igneous 

rock, marks the southern end of the lake.

Sediment Accumulation in Lake Mohave 

The floor of Lake Mohave is characterized by 

pre-impoundment features, including undulating sand 

deposits in the former river channel, rock outcrops 

along steep cliffs adjacent to the river channel, treelined floodplains adjacent to the river channel in the 

wider basins, and alluvial fans at the mouths of washes, 

which fringe most of the central part of the lake. In 

contrast to Lake Mead, remarkably little sediment has 

accumulated in Lake Mohave since its impoundment in 

1950 (fig. 2-6).

Virtually all sediment transported by the Colorado 

River has been trapped in Lake Mead or in other 

upstream reservoirs, such as Lake Powell. The small 

amount of fine-grained sediment that has accumulated 

in Lake Mohave tends to occur in the deepest parts 

of the lake within sheltered areas along the edges of 

the drowned Colorado River channel. Other postimpoundment deposits include debris flows at the 

mouths of washes that are probably associated with 

flash floods and landslides along the base of steep cliffs 

in the northern section of the lake, which appear to be 

the result of cliff collapse. One notable debris flow at 

the mouth of Eldorado Canyon is the result of a large 

flash flood that moved through the wash in 1975.
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 19

nvtac11-4177_fig02-06

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

HOOVER DAM 

DAVIS DAM 

B L A C K M O U N T A I N S

BLACK CANYON

E L D O R A D O M O U N T A I N S

N E W B E R R Y

M O U N T A I N S

RESOURCES

Lake Mohave Geophysical Survey 2002: GIS Data Release

U.S. Geological Survey Open-File Report 2004-1247

Reconnaissance Geology Between Lake Mead and 

Davis Dam Arizona-Nevada

U.S. Geological Survey Professional Paper 374-E, p. E1-E51

Surficial Geology and Analysis of Post-Impoundment 

Sediment of Lake Mohave; Interpretation of Sidescan 

Sonar and Seismic-Reflection Data

U.S. Geological Survey Open-File Report 2004-1256

Willow Beach

Cottonwood

Cove

Katherine

Sand

(about 4,500 acres or 16%*)

Pre-impoundment water-borne 

deposits found in the riverbed 

and wind-borne deposits found 

in low-lying areas adjacent to 

the riverbed.

No Information

(about 9,700 acres or 35%*)

Shallow bays and other regions 

that have not been surveyed. 

Rock (about 2,000 acres or 7%*)

Rock outcrops and 

semi-consolidated sediments 

found along the margins of the 

lake especially where the lake 

abuts a mountain range.

Alluvial deposits

(about 3,200 acres or 11%*)

Includes pre-impoundment unconsolidated 

sand and gravel deposits that fringe most of 

the central part of the lake along with 

post-impoundment debris flows found 

mostly at the mouths of washes and 

landslides found at the base of cliffs.

Floodplain

(about 7,000 acres or 25%*)

Pre-impoundment fine-grained 

deposits associated with 

low-lying flat areas on either 

side of the relict Colorado River 

channel.

Mud

(about 600 acres or 1%*)

Fine-grained post-impoundment 

sediment deposits found along the 

edges of the channel and often 

downstream of promontories. 

River bank

(about 600 acres or 1%*)

Steep slopes generally of unconsolidated 

sediment found along the edge of the 

pre-impoundment river channel.

Gravel/

Cobble/

Boulder

(about 600 acres or 1%*)

Coarse-grained pre-impoundment 

deposits found in the Colorado River 

channel. 

PAINTED

CANYON

*Note: Calculations and mapping are based on a lake elevation of 647 feet.

Surveys were conducted in 2002 by the U.S. Geological Survey in 

collaboration with the Bureau of Reclamation, University of Nevada 

Las Vegas, and the National Park Service. 

ELDORADO

CANYON

CIRCULAR 1381

FIGURE 2.6—Little sediment has accumulated in Lake Mohave due to 

upstream dams and reservoirs such as Lake Powell and Lake Mead. 

Post-impoundment lake muds cover only 1 percent of the lake floor.

Rosen, M.R., Turner, K., Goodbred, S.L., and Miller, J.M., eds., 2012, 

A synthesis of aquatic science for management of Lakes Mead and Mohave

Figure 2-6. Little sediment 

has accumulated in Lake 

Mohave due to upstream 

dams and reservoirs such as 

Lake Powell and Lake Mead. 

Post-impoundment lake 

muds cover only 1 percent of 

the lake floor. An oversized 

version (11×17) of this figure 

is available for download 

at http://pubs.usgs.gov/

circ/1381.
20 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Chapter 2 References

Alexander, R.B., Slack, J.R., Ludtke, A.S., Fitzgerald, K.K., 

and Schertz, T.L., 1997, Data from selected U.S. Geological 

Survey national stream water-quality monitoring networks 

(WQN): U.S. Geological Survey Digital Data Series DDS37, 1 CD-ROM.

Bouma, A.H., and Stone, C.G., eds., 2000, Fine-grained 

turbidite systems: SEPM Special Publication no. 68, 342 p.

Bureau of Reclamation, 2005, Hoover Dam and Powerplant: 

Bureau of Reclamation web site, accessed August 17, 2012, 

at http://www.usbr.gov/lc/region/pao/brochures/hoover.

html.

Bureau of Reclamation, 2007, Colorado River interim 

guidelines for lower basin shortages and coordinated 

operations for Lake Powell and Lake Mead—Final 

Environmental Impact Statement: Boulder City, Nev., 

Bureau of Reclamation, accessed August 17, 2012, at http://

www.usbr.gov/lc/region/programs/strategies/FEIS/index.

html.

Bureau of Reclamation, 2008, Hoover Dam—Frequently 

asked questions and answers: Bureau of Reclamation web 

site accessed August 17, 2012, at http://www.usbr.gov/lc/

hooverdam/faqs/lakefaqs.html.

Bureau of Reclamation, 2009, The Lower Colorado Region—

An overview: Bureau of Reclamation, Lower Colorado 

Region, accessed August 17, 2012, at http://www.usbr.gov/

lc/region/09overview.pdf.

Cross, V.A., Foster, D.S., and Twichell, D.C., 2005, Lake 

Mohave Geophysical Survey 2002—GIS Data Release: 

U.S. Geological Survey Open-File Report 2004–1247. 

(Also available on CD-ROM and at http://pubs.er.usgs.gov/

publication/ofr20041247.)

Duffield, J., Neher, C., and Patterson, D., 2007, Economic 

values of National Park System resources within the 

Colorado River watershed—Phase II—Missoula, Montana: 

Report to National Park Service by University of Montana, 

on file at Lake Mead National Recreation Area, Boulder 

City, Nev., 80 p.

Ferrari, R.L., 2008, 2001 Lake Mead sedimentation survey: 

Bureau of Reclamation report, Technical Service Center, 

Denver, Colo., 212 p., accessed August 17, 2012, at http://

www.usbr.gov/pmts/sediment/projects/ReservoirSurveys/

index.html.

Gould, H.R., 1951, Some quantitative aspects of Lake Mead 

turbidity currents: Society of Economic Paleontologists and 

Mineralogists, Special Publication 2, p. 34–52.

Gould, H.R., 1960, Character of the accumulated sediment, 

in Smith, W.O., Vetter, C.P., Cummings, G.B., and others, 

eds., 1960, Comprehensive survey of sedimentation in Lake 

Mead, 1948–49: U.S. Geological Survey Professional Paper 

295, p. 149–186. (Also available at http://pubs.er.usgs.gov/

publication/pp295.)

Holdren, G.C., and Turner, K., 2010, Characteristics of Lake 

Mead, Arizona–Nevada: Lake and Reservoir Management, 

v. 26, no. 4, p. 230–239.

Houk, R., 1997, Lake Mead National Recreation Area: 

Southwest Parks and Monuments Association, Tucson, 

Ariz., 15 p.

Howard, C.S., 1960, Character of the inflowing water, in

Smith, W.O., Vetter, C.P., Cummings, G.B., and others, eds., 

Comprehensive survey of sedimentation in Lake Mead, 

1948–49: U.S. Geological Survey Professional Paper 295, 

p. 103–113. (Also available at http://pubs.er.usgs.gov/

publication/pp295.)

LaBounty J.F., and Burns, N.M., 2005, Characterization of 

Boulder Basin, Lake Mead, Nevada-Arizona, USA–Based 

on analysis of 34 limnological parameters: Lake and 

Reservoir Management, v. 21, p. 277–307.

Lara, J.M., and Sanders, J.I., 1970, The 1963–64 Lake Mead 

study: Denver, Colo., Bureau of Reclamation, Report RECOCE-70-21, 172 p.

Las Vegas Valley Watershed Advisory Committee, 2009, 

2009 regional water quality plan, accessed March 2, 2011, 

at http://www.snwa.com/assets/pdf/wq_lvvwac_regional_

wqplan.pdf.

Longwell, C.R., 1936, Geology of the Boulder Reservoir 

floor: Geological Society of America Bulletin, v. 47, p. 

1,393–1,476.

Longwell, C.R., 1960, Geologic setting of Lake Mead, in

Smith, W.O., Vetter, C.P., Cummings, G.B., and others, eds., 

Comprehensive survey of sedimentation in Lake Mead, 

1948–49: U.S. Geological Survey Professional Paper 295, 

p. 11-20. (Also available at http://pubs.er.usgs.gov/

publication/pp295.)
Chapter 2

Environmental Setting of Lake Mead National Recreation Area 21

Maxon, J.C., 2009, Lake Mead and Hoover Dam—The story 

behind the scenery: Wickenburg, Ariz., KC Publications, 

48 p.

Middleton, G.V., and Southard, J.B., 1984, Mechanics of 

sediment movement: SEPM notes for short course 3, 

p. 8-1–8-34.

National Park Service, 2002, The Lake Mead National 

Recreation Area lake management plan and final 

environmental impact statement: National Park Service web 

site, accessed August 17, 2012, at http://www.nps.gov/lake/

parkmgmt/park-management-plans.htm.

National Park Service, 2005, Lake Mead National Recreation 

Area strategic plan, 2001–2005: National Park Service, 72 

p., accessed August 17, 2012, at http://www.nps.gov/lake/

parkmgmt/upload/ACFB12.doc.

National Park Service, 2010, Long-term limnological 

and aquatic resources monitoring and research plan 

for Lakes Mead and Mohave National Park Service, 

93 p., accessed August 17, 2012, at http://www.nps.

gov/lake/naturescience/loader.cfm?csModule=security/

getfile&%3bpageid=431205.

Rohde, K.M., 1999, In pictures Lake Mead—The continuing 

story: Las Vegas, Nev., KC Publications, 48 p.

Smith, W.O., Vetter, C.P., Cummings, G.B., and others, eds., 

1960, Comprehensive survey of sedimentation in Lake 

Mead, 1948–1949: U.S. Geological Survey Professional 

Paper 295, 254 p. (Also available at http://pubs.er.usgs.gov/

publication/pp295.)

Twichell, D.C., and Cross, V.A., 2009, Surficial geology of 

the floor of Lake Mead (Arizona and Nevada) as defined by 

sidescan-sonar imagery, lake-floor topography, and postimpoundment sediment thickness: U.S. Geological Survey 

Open-File Report 2009–1150. (Also available at http://pubs.

er.usgs.gov/publication/ofr20091150.)

Twichell, D.C., Cross V.A., and Belew, S.D., 2003, Mapping 

the floor of Lake Mead (Nevada and Arizona)—Preliminary 

discussion and GIS data release: U.S. Geological Survey 

Open-File Report 2003–320. (Also available at http://pubs.

er.usgs.gov/publication/ofr03320.)

Twichell, D.C., Cross, V.A., Hanson, A.D., Buck, B.J., Zybala, 

J.G., and Rudin, M.R., 2005, Seismic architecture and 

lithofacies of turbidites in Lake Mead (Arizona and Nevada, 

U.S.A.), an analogue for topographically complex basins: 

Journal of Sedimentary Research, v. 75, p. 134–148.

Twichell, D.C., Cross, V.A., Rudin, M.R., and Parolski, 

K.F., 1999, Surficial geology and distribution of postimpoundment sediment of the western part of Lake Mead 

based on a sidescan sonar and high-resolution seismicreflection survey: U.S. Geological Survey Open-File Report 

99–581, 27 p. (Also available at http://pubs.er.usgs.gov/

publication/ofr99581.)

Twichell, D.C., Cross, V.A., Rudin, M.J., Parolski, K.F., and 

Rendigs, R.R., 2001, Surficial geology and distribution of 

post-impoundment sediment in Las Vegas Bay, Lake Mead: 

U.S. Geological Survey Open-File Report 2001–70, 

27 p. (Also available at http://pubs.er.usgs.gov/publication/

ofr0170.)
22 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

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Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 23

Chapter

3

Rafting on Lake Mohave near Willow Beach. Photograph by Michael R. Rosen, U.S. Geological Survey.

Colorado River Basin Hydrology and 

River Management 

The Colorado River Basin covers 

parts of seven States: Colorado, 

Wyoming, Utah, New Mexico, Nevada, 

Arizona, and California; at 1,450 mi 

(2,333.5 km) in length, the Colorado 

River is the seventh longest river in the 

United States (fig. 3-1). The Bureau 

of Reclamation has the responsibility 

for management of this system, in 

coordination with the seven basin 

States, within a complex framework of 

law, regulations, compact, treaty, and 

policies often referred to collectively 

as the “Law of the River.” Lake Mead 

is a critical component of the overall 

Colorado River management, providing 

the capacity to store almost 2 years of 

the average runoff of the river. 

By G. Chris Holdren1, Todd Tietjen2, Kent Turner3, and 

Jennell M. Miller4 

Hydrology and 

Management of Lakes 

Mead and Mohave 

within the Colorado 

River Basin

1

Bureau of Reclamation

2

Southern Nevada Water Authority

3

National Park Service

4

University of Nevada, Las Vegas
24 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

nvtac11-4177_fig03-01

NAVAJO DAM

HOOVER DAM

PARKER DAM

CRYSTAL DAM

BLUE

MESA DAM

DAVIS 

DAM

GLEN CANYON DAM

MORROW

POINT DAM

FLAMING GORGE DAM

FONTENELLE DAM

ROOSEVELT DAM

IMPERIAL DAM

Lake

Mead

Lake

Powell

Great Salt Lake

Flaming

Gorge

Reservoir

Theodore

Roosevelt

Lake

GOLFO DE

CALIFORNIA

PACIFIC

OCEAN

Salt River

Lees Ferry 

gaging station

Gila River

 River

 Williams River

 River

Green River

 River

Gunnison River

Duchesne River

River

San Pedro River

Rio Colorado

La Cienega

De Santa Clara

WYOMING IDAHO

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

MEXICO

USA

CALIFORNIA

San Juan

Virgin River

Colorado

Little Colorado Riv er

Verde River

Santa Cruz River

Gila

R i ver

Green River

Yam pa River

White Rive r

Colorado Rive r

San Rafael Colorado River

Dolores

Bill

Agua Fria River

118° 116° 114° 112° 110° 108° 106° 104°

42°

40°

38°

36°

34°

32°

0 30 60 90 120 150 MILES

0 50 100 150 KILOMETERS

Base from National Park Service and U.S. Geological Survey digital data,1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983 

CA

NV UT CO

AZ NM

WY

Figure 3-1. Colorado River Basin showing areas of the Upper and Lower Basins including Lakes Mead (upstream from) and 

Mohave (downstream from Hoover Dam). Lee Ferry is located at the boundary of the Upper and Lower Basins on the Colorado River, 

approximately 12 mi (19.3 km) southwest of Page, Arizona.
Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 25

Through a series of compacts and treaty obligations, the 

water rights to the Colorado River have been apportioned 

or allocated to the seven basin States, along with a treaty 

obligation for water availability for Mexico. Within these legal 

requirements, in the United States the river is managed as two 

areas: the Upper Basin (Division) States of Colorado, New 

Mexico, Utah, and Wyoming; and the Lower Basin (Division) 

States of Arizona, California, and Nevada (fig. 3-1). The 

dividing line along the river for Upper and Lower Basin is set 

as Lee Ferry, Arizona, below Lake Powell.

The overall Colorado River allocations total 16.5 maf/yr. 

The overall allocation is set and managed as 7.5 maf to 

the Upper Basin States; 7.5 maf to the Lower Basin States; 

and 1.5 maf to Mexico. The Upper Basin is responsible for not 

depleting the flows of the Colorado River so all downstream 

allocations can be provided. Within the seven basin States and 

Mexico, annual consumptive use of the water ranges from 

13 to 14.5 maf. Total system storage available within the more 

than 20 dams and reservoirs of the Colorado River Basin 

is 60 maf. Nearly 85 percent of this total available storage, 

or 50.2 maf, is stored in Lake Powell (24.3 maf) and Lake 

Mead (26.1 maf). Based on measurements of inflow into the 

Lake Powell region over the past 100 years, the approximate 

average annual “natural” inflow into the Colorado River 

within the Upper Basin has been 15 maf (fig. 3-2). Average 

annual inflow in the lower basin below Lake Powell has been 

approximately 1.3 maf. Because inflows depend on the amount 

of snowfall in the western Rocky Mountains and precipitation 

patterns for the side tributary inflows, the annual inflows are 

highly variable from year to year.

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

0

5

10

15

20

25

 Long-term average

 Centralized 10-year moving average

Annual flow, in million acre-feet

Year EXPLANATION Provisional data; subject to change 

Estimated values

Figure 3-2. Colorado River computed natural flows at the U.S. Geological Survey gaging station at Lees Ferry, Arizona. Computed 

flows were provided by the Bureau of Reclamation. The term “natural” in this context refers to the absence of human development (for 

example, from depletion and regulation). “Natural Flow” is a recomputed streamflow that begins with the historical stream-gage record, 

then adds depletions and adds or subtracts impacts from reservoir regulation. The long-term average is each subsequent year being 

averaged to the previous years. The centralized 10-year moving average is the average of the 5 years before and 5 years after each 

point on the line.
26 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The long-term average natural flow in the Colorado River 

at Lees Ferry, Ariz., from 1906 to 2011 is 15.0 maf (fig. 3-2; 

flows for water years 2009–10 are estimated). The period 

from water year 2000 to 2010 was the driest 11-year period 

in the 100-year historical record for the Colorado River Basin 

(average annual flow of 12.1 maf), and the period from water 

years 1999 to 2010 has been the second driest 12-year period 

(12.5 maf) within that record. 

Water-surface elevations in Lake Mead reflect the inflowoutflow regime as well as water use and demand; however, 

large differences in elevation are ultimately a reflection of 

drought years. Lake Mead began filling in early 1935, and 

filled to an elevation greater than 1,200 ft (365.8 m) above 

Power House Datum by mid-1941 (fig. 3-3). Between the 

1940s and early 1960s, inflows into Lake Mead reflected 

the natural hydrologic variability of the Colorado River, and 

fluctuations in its elevation were approximately 19 ft (5.8 m). 

Construction of Glen Canyon Dam was completed in 1963, 

impounding Lake Powell upstream of Lake Mead. As Lake 

Powell initially filled, inflows to Lake Mead were reduced and 

its elevation declined more than 100 ft (30.5 m). After Lake 

Powell reached its minimum elevation for supplying power in 

1964, both lakes then gradually began to fill at approximately 

the same rate during the spring runoff period of 1965. 

Lake Mead’s full pool elevation first reached in 1941 

was not reached again until 1983. Seasonal fluctuations in the 

surface level of Lake Mead due to variations in inflows from 

the Colorado River were reduced by the buffering capacity of 

Lake Powell. 

Lake Mead filled to full capacity (elevation greater than 

1,221.4 ft [372.3 m]) in July 1983 and the spillway gates 

operated for the first time for flood-control purposes. The 

maximum-recorded elevation of 1,225.83 ft (373.63 m) was 

reached during this flood period. Lake Mead surface elevations 

declined through the late 1980s and increased again to higher 

than 1,215 ft (370.3 m) in 1999. Since that time, an extended 

drought has resulted in a decline in the elevation of Lake 

Mead by more than 130 ft (39.6 m). Historically, the elevation 

of the lake has declined this much only during the extreme 

drought of the 1950s, and during the filling of Lake Powell 

in the mid-1960s. In November 2010, the water level of Lake 

Mead reached just less than 1,082 ft (329.8 m), its lowest 

elevation since it was first filled in the late 1930s. Hydrologic 

conditions (for example, and in particular, the magnitude 

of the snowpack in the Colorado River Basin) improved in 

2011, and the April–July 2011 inflow into Lake Powell was 

the highest inflow since 1984, the ninth highest inflow in the 

period of record from 1908 to 2011 (source: Colorado Basin 

River Forecast Center; http://www.cbrfc.noaa.gov). 

Concern over the drought and water-level declines within 

Lake Mead and Lake Powell led the Bureau of Reclamation 

to coordinate river operations among the seven basin States in 

2007, and to develop the Colorado River Interim Guidelines 

for Lower Basin Shortages and the Coordinated Operations 

for Lake Powell and Lake Mead (Bureau of Reclamation, 

Figure 3-3. Lake Mead End-of-Month Elevations since 

completion of Hoover Dam, Bureau of Reclamation.

1930 1940 1950 1960 1970 1980 1990 2000 2010

1,000

1,020

1,040

1,060

1,080

1,100

1,120

1,140

1,160

1,180

1,200

1,220

1,240

304

310

316

322

328

334

340

347

353

359

365

371

377

Lake level, in feet above datum

Lake level, in meters above datum

Year

Spillway crest 1,221 feet

Elevation is referenced to the U.S.Geological Survey datum, adjustment of 1912, 

locally known as "Power House Datum.”

Jet flow gate testing at Hoover Dam, June 1998. Photograph by Bureau of Reclamation.
Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 27

2007). These guidelines established criteria for the protection 

of certain critical water levels within Lake Mead and Lake 

Powell for water supply, benchmarks based on Lake Mead 

elevations when the availability of water would be low enough 

to require official determination of a water shortage in the 

lower basin, and specified reductions in annual water-use 

volumes to Lower Basin States during shortage years. 

Most of Nevada’s share of Colorado River water is 

withdrawn directly from Lake Mead by the Southern Nevada 

Water Authority (SNWA) through its intakes on the western 

side of Boulder Basin. These withdrawals meet requirements 

for Las Vegas Valley, but Nevada does have additional 

diversions downstream of Davis Dam. Annual withdrawals 

by SNWA to provide municipal water for Las Vegas Valley 

are currently about 450,000 acre-ft/yr (5.5×108 m3/yr), about 

50 percent more than Nevada’s 300,000 acre-ft/yr allocation 

of Colorado River water. SNWA is given return flow credits 

for water returned to Lake Mead via Las Vegas Wash, most 

of which is highly treated wastewater. The return flow credits 

help comprise the total Southern Nevada community use and 

are required to enable the additional withdrawals beyond its 

allocation.

Discharge from Hoover Dam occurs at elevations of 895 

and 1,045 ft (272.8 and 318.5 m) above Power House Datum. 

Both outlets are in the hypolimnion (Chapter 4) at full pool 

but the upper outlet is near the bottom of the epilimnion at 

2012 lake levels. The annual discharge can exceed 9.0 maf 

(see Lake Mead Water Budget). 

Lake Mead Water Budget

A water budget can describe the net balance of Lake 

Mead’s water inputs, outflows, and losses over a given year. In 

practice, the water budget for Lake Mead is highly regulated 

and depends primarily upon releases from Lake Powell 

through Glen Canyon Dam and downstream releases through 

Hoover Dam. River operations are controlled by the Bureau 

of Reclamation to meet legal requirements and contractual 

demands for drinking, municipal, industrial, and irrigation 

water. The operational model prescribing inflows to Lake 

Mead and outflows to Lake Mohave and the lower Colorado 

River evaluates (1) releases from Lake Powell to the Colorado 

River; (2) downstream releases for the Central Arizona 

Project, Metropolitan Water District of Southern California, 

and other Lower Basin water contractors; (3) consumption by 

the Las Vegas metropolitan area through water deliveries from 

SNWA; (4) required deliveries under treaty with Mexico; (5) 

inflows from other tributaries such as the Virgin and Muddy 

Rivers and the Las Vegas Wash; (6) bank storage; and (7) 

evaporative losses.

The approximate annual minimum release from Lake 

Powell since the reservoir initially filled to capacity in 

1980, referred to operationally as the “minimum objective 

release,” is 8.23 maf. Additional tributary inflows below 

Lake Powell average approximately 0.7 maf, thus providing 

a total average operational inflow into Lake Mead of 9.0 maf 

(table 3-1). Approximate annual outflows to meet downstream 

requirements are approximately 9.6 maf. Evaporation rates in 

Table 3-1. Lake Mead water budget. 

[Data from Bureau of Reclamation. Abbreviations: maf, million acre-feet]

Lake Mead Water Budget

Given current average water demands over the past 10 years in the Lower Basin and Mexico, and a minimum 

objective release from Lake Powell (8.23 maf]), Lake Mead storage is reduced on average by about 1.2 maf1

each year.

Approximate annual inflow into Lake Mead

 (8.23 maf release from Lake Powell plus average intervening 

 flows between Lake Powell and Lake Mead)

 9.0 maf

Approximate annual outflow from Lake Mead

 (Lower Basin apportionments to States and Mexico Treaty

 allocation plus downstream regulation including side inflows, 

 evapotranspiration, transmission losses, etc.)

-9.6 maf

Approximate annual Lake Mead evaporation loss -0.6 maf

Water balance 1-1.2 maf

1

Equivalent of about 12 feet in elevation at Lake Mead.
28 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

the Mojave Desert around Lake Mead are among the highest 

in the United States (Farnsworth and Thompson, 1982; 

Westenburg and others, 2006). Thus at full pool, evaporative 

losses from the lake surface are estimated to be approximately 

0.6 maf, almost 7 percent of the average annual inflow, and 

result in the removal of approximately 6 vertical ft (1.8 m) of 

water from the lake.

When the annual release volume from Lake Powell 

totals 8.23 maf, these values of inflow and outflow (including 

evaporation) indicate that the overall average yearly Lake 

Mead water budget operates at a 1.2 maf deficit, which is 

equal to approximately 12 ft (3.7 m) of lake elevation. This 

actuality means that in years of normal to slightly below 

normal snowpack in the headwaters of the Colorado River 

and standard operations, the water level in Lake Mead will 

decline by 12 ft (3.7 m) from the previous year’s high water 

level. Significant increases in the water level of Lake Mead 

will require either several years of above average snowpack 

and precipitation within the watershed or lower consumptive 

use by Basin States. The overall water balance generally has 

been maintained since creation of Lake Mead through the 

overall storage capacity of the Colorado River system and the 

ability to store large volumes during wet years, and the fact 

that until recently, many Basin States were not using all of 

their water allocations. As climate models now predict longer 

periods of reduced snowpack within the Colorado River Basin, 

and as water demands and use approach full allocations, it is 

forecasted that the future will bring correspondingly longer 

periods of low water elevation and greater fluctuations in 

water levels than have been seen to date (Barnett and Pierce, 

2009).

Lake Mead Tributaries

All tributaries entering Lake Mead at times carry heavy 

sediment loads and have higher nutrient concentrations than 

the receiving waters of the lake. Other characteristics of the 

inflows, primarily temperature and dissolved solids, determine 

their impact on lake productivity and water quality (fig. 3-4). 

The specific conductance of water, or its ability to conduct 

electrical current (see Chapter 4), provides an estimate of 

the concentration of total dissolved solids present in water. 

Specific conductance, which differs among the Lake Mead 

tributaries (Holdren and Turner, 2010), is a traceable feature 

useful in tracking the path of one water source as it flows into 

another. 

The distribution of dissolved solids, nutrients, and other 

chemicals of interest in Lake Mead is determined by vertical 

and horizontal mixing processes, which are heavily affected 

by stratification during the summer months and by wind 

and water currents in all seasons (Baker and others, 1977). 

Stratification is the process by which deep lakes separate into 

a warm surface layer and a colder, denser layer in deeper 

water. The river inflows will enter the lake’s water column at 

the level at which their density, as determined by temperature 

and the concentrations of dissolved and suspended solids, is 

equal to the density of the receiving water (see Chapter 4 for 

a more detailed description of stratification). In Lake Mead, 

on an approximately every-other-year cycle, complete (top 

to bottom) mixing extends through late autumn, winter, early 

spring, and stratification extends from spring into autumn 

(LaBounty and Burns, 2005). 

The Colorado River enters the eastern end of Lake Mead 

at the upper end of Gregg Basin at current (2011) lake levels. 

The river is colder and less salty than Lake Mead; because 

cold water is denser than warm water, the river water typically 

flows along the bottom of the lake, deep into the water column 

of Gregg Basin. This phenomenon limits nutrient availability 

and productivity in the upper levels of the lake (Paulson and 

Baker, 1983). The low conductivity water from the Colorado 

River that enters Lake Mead can often be detected all the way 

to Hoover Dam, more than 60 mi (96.6 km) downstream.

The Virgin and Muddy Rivers also are colder than Lake 

Mead, but unlike the Colorado River, they both have higher 

concentrations of total dissolved solids than Lake Mead. Both 

rivers enter Lake Mead at the northern end of the Overton 

Arm and also flow along the bottom of the lake. Unlike Gregg 

Basin, the upper end of the Overton Arm is relatively shallow 

and the river inflows typically mix with Lake Mead water. 

During periods of heavy storm runoff, the high conductivity 

signature of the Virgin and Muddy Rivers can be observed at 

significant distances downstream from the rivers’ entry to the 

lake. Under these infrequent conditions, the high conductivity 

water can sometimes be traced all the way to Hoover Dam. 

Agricultural land uses along these rivers upstream of Lake 

Mead mitigate the impacts of the Virgin and Muddy Rivers 

on lake productivity, because during the summer months, 

almost the entire flow of both rivers is diverted for agriculture, 

thereby reducing nutrient inputs and algal productivity in the 

lake (Holdren and Turner, 2010).
Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 29

nvtac11-4177_fig03-04

95

95

215

95

93

93

93

93

15

15

NEVADA

ARIZONA

Study Area

115°W 114°30'W 114°W

36°30'N

36°N

35°30'N

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Grand Canyon 

National Park

Lake Mead National 

Recreation Area

Inflow

Outflow

Evaporation

NEVADA ARIZONA

The Colorado River 

contributes 96.9% of the 

flow to Lake Mead. 

Loss through evaporation

across Lake Mead can be as 

much as 10% at full pool. 

Discharges through 

Hoover Dam vary and, 

at times, exceed inflow.

Precipitation falling on 

Lake Mead’s surface 

contributes <1% of the flow. 

The Virgin River 

contributes 0.8% of 

the flow into Lake Mead. 

The Muddy River 

contributes 0.1% of 

the flow into Lake Mead. 

Las Vegas Wash provides 

2.2% of the flow into 

Lake Mead.

SNWA withdraws 450,000 

acre-ft/yr (about 4% of the 

average inflow) for 

municipal water.

66 ̊F 2,831 μS/cm

67 ̊F 2,955 μS/cm

71 ̊F 2,219 μS/cm

59 ̊F 876 μS/cm

River

Colorado River

HOOVER 

DAM

Willow Beach

National Fish 

Hatchery (USFWS)

DAVIS DAM

Las Vegas

Henderson

Alfred Merritt

Smith Water 

Treatment 

Facility (SNWA)

Muddy

Searchlight

Laughlin

Virgin River

Wash

LasVegas

Callville Bay

Base from National Park Service and U.S. Geological Survey digital data,1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983

Logandale

Overton

Boulder 

City

MEAD

LAKE

LAKE

MOHAVE

Figure 3-4. Relative contributions of inflows to Lake Mead and typical temperature and specific conductance 

values for these flows. Specific conductance (SC) is shown in microsiemens per centimeter (μS/cm). SNWA, 

Southern Nevada Water Authority; USFWS, U.S. Fish and Wildlife Service.
30 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

1,000 ft Lower SNWA Intake 

1,115 ft 13.16 maf

(50% of Live 

Capacity) 

895 ft Dead Pool Elevation 

Lake Mead Capacity 

1,219.6 ft 26.1 maf 

Dead Pool (2.5 maf) 

Inactive Pool (7.7 maf) 

Not to scale 

105 ft 

As of July 15, 2012 

1,145 ft 

65 ft 1,075 ft 

Shortage Conditions 

1,050 ft 

Surplus Conditions 

Normal or ICS Surplus Conditions 

Minimum Power Pool 

16.2 maf 

Elevation is referenced to the U.S.Geological Survey datum, 

adjustment of 1912, locally known as "Power House Datum.”

Figure 3-5. Bucket diagram depicting key elevation criteria under 

the 2007 Interim Guidelines (Bureau of Reclamation, 2007). Reservoir 

contents listed in the diagram refer to active storage above the dead 

pool elevation of 895 ft (272.7 m). Dead pool refers to lowest release 

capacity facilities within Hoover Dam, below which elevation additional 

releases are not possible. ICS, Intentionally Created Surplus; SNWA, 

Southern Nevada Water Authority.

Las Vegas Wash drains the Las Vegas Valley and flows into Boulder Basin at the western end 

of Lake Mead. Las Vegas Wash is saltier than Lake Mead, but its temperature relative to the lake 

varies throughout the year. Las Vegas Wash typically enters Lake Mead as an underflow at the 

bottom of the lake in the winter; an interflow in the middle of the water column in the summer and 

autumn, when it flows along the top of the temperature contrast (thermocline) between warmer 

surface water and colder bottom water; and as an overflow at the water’s surface in the spring. This 

flow regime can provide high nutrient concentrations to the lower part of the water column in the 

spring and summer and promote algal growth (LaBounty and Burns, 2005). As a result, Boulder 

Basin, and particularly Las Vegas Bay, is one area of the lake with high productivity.

Lake Mead Operations

The Bureau of Reclamation operates Hoover 

Dam to provide flood control and to meet downstream 

demands as stated within the policies and guidelines 

comprising the “Law of the River.” The Colorado River 

system operational requirements include the reservoir 

regulation of Lake Mohave and Lake Havasu (located 

downstream of Lake Mohave, outside LMNRA) to 

provide efficient deliveries of downstream users. Water 

is released from Lake Mead within standard operational 

guidelines to adapt to overall system gains and losses 

(for example, side inflows, evapotranspiration, and 

transmission losses) downstream of Hoover Dam.

Under the Colorado River Interim Guidelines 

(Bureau of Reclamation, 2007), water deliveries can 

be larger or smaller than under standard operational 

guidelines to adjust to surplus or shortage conditions 

based upon water availability and key reservoir levels 

within Lake Mead and Lake Powell. The Interim 

Guidelines provide a prescriptive methodology for 

determining the annual releases from Lake Powell 

and Lake Mead throughout the full range of reservoir 

operations, including periods of low reservoir levels 

(figs. 3-5 and 3-6).

Las Vegas Wash flowing toward Lake Mead. Photograph by Michael R. Rosen, U.S. Geological Survey.
Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 31

Figure 3-6. Diagram of key criteria elevations for Lake Powell and Lake Mead under 2007 Interim Guidelines. ICS, Intentionally 

Created Surplus.

Lake Powell

Elevation

(feet)

Operation According 

to the Interim Guidelines

Live Storage

(maf)1

3,700 Equalization Tier

Equalize, avoid spills 

or release 8.23 maf

24.3

3,636–3,666

(2008–2026) Upper Elevation 

Balancing Tier3

Release 8.23 maf; 

If Lake Mead < 1,075 feet, 

balance contents with 

a min/max release of 

7.0 and 9.0 maf

15.5–19.3

(2008–2026)

3,575

Mid-Elevation 

Release Tier

Release 7.48 maf; 

If Lake Mead < 1,025 feet, 

release 8.23 maf

9.5

3,525

Lower Elevation 

Balancing Tier

Balance contents with 

a min/max release of 

7.0 and 9.5 maf

5.9

3,490 4.0

3,370 0

Diagram not to scale

1

 Acronym for million acre-feet.

2

 This elevation is shown as approximate as it is determined each year by considering several factors including Lake Powell and Lake Mead storage, 

projected Upper Basin and Lower Basin demands, and an assumed inflow.

3

 Subject to April adjustments which may result in a release according to the Equalization Tier.

4

 Of which 2.48 maf is apportioned to Arizona, 4.4 maf to California, and 0.287 maf to Nevada.

5

 Of which 2.40 maf is apportioned to Arizona, 4.4 maf to California, and 0.283 maf to Nevada.

6

 Of which 2.32 maf is apportioned to Arizona, 4.4 maf to California, and 0.280 maf to Nevada.

7

 Whenever Lake Mead is below elevation 1,025 feet, the Secretary of the Interior shall consider whether hydrologic conditions together with 

anticipated deliveries to the Lower Division States and Mexico is likely to cause the elevation at Lake Mead to fall below 1,000 feet. Such consideration, 

in consultation with the Basin States, may result in the undertaking of further measures, consistent with applicable Federal law. 

Lake Mead

Elevation

(feet)

Operation According 

to the Interim Guidelines

Live Storage

(maf)1

1,220 Flood Control Surplus or 

Quantified Surplus Condition

Deliver > 7.5 maf

25.9

1,200

(approx.)2

Domestic Surplus or 

ICS Surplus Condition

Deliver > 7.5 maf

22.9

(approx.)2

1,145

1,105

Normal or 

ICS Surplus Condition

Deliver ≥ 7.5 maf

15.9

11.9

1,075

Shortage Condition

Deliver 7.1672

 maf

9.4

1,050

Shortage Condition

Deliver 7.0835

 maf

7.5

1,025

1,000

Shortage Condition

Deliver 7.06

 maf 

Further measures may 

be undertaken7

5.8

4.3

895 0

Lake Powell and Lake Mead Operational Diagrams and Current Conditions

The Interim Guidelines recognize that low reservoir 

levels at Lake Mead have the potential to affect the following 

facilities and resources: power generation at Hoover Dam; 

pumping by SNWA at the drinking water intakes; boat 

launching, recreational facilities, and access within Lake 

Mead; water quality (for both Lake Mead and below Hoover 

Dam); and riparian and aquatic species in and near Lakes 

Mead and Mohave. Criteria were established to provide for 

flexibility of operations in times of potential water surplus 

and periods of water shortages, while protecting, to the 

extent possible, facilities and resources potentially affected at 

various water levels.
32 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Use of Lake Mead water for its multiple purposes is 

constrained by several structural conditions. The top of 

Hoover Dam is at elevation 1,232 ft (375.5 m). The SNWA 

drinking water intakes for Las Vegas and Southern Nevada 

will operate down to lake levels of 1,000 ft (304.8 m). The 

minimum pool or elevation to support power generation 

within Hoover Dam is 1,050 ft (320.4 m). Within the 

operational criteria, an exclusive flood control pool is 

established between approximately 1,220 and 1,229 ft (371.7 

and 374.6 m) elevation, meaning that space will be reserved 

to accommodate periods of high flow and upstream releases 

to provide additional flood protection within the system. This 

nearly 10-ft (3.0-m) zone contains approximately 1.5 million 

acre-ft (maf) of exclusive flood control space above elevation 

1,220 ft (371.7 m). Lake Mead water elevations within this 

zone also are recognized as surplus conditions, meaning that 

additional water can be released to the Lower Basin States of 

California, Arizona, and Nevada, and Mexico. 

Key operational criteria for both Lake Powell and Lake 

Mead were established within the 2007 Interim Guidelines 

(fig. 3-6). The diagram reflects the intent within the criteria 

to balance or equalize storage capacities within Lake Powell 

and Lake Mead within a framework of recognizing important 

water elevations on each lake that are related to its operations.

The operational guidelines for the reservoirs are driven 

by hydrology and projected reservoir conditions of Lake 

Powell and Lake Mead. The diagram presents various 

elevations on Lake Powell that trigger such equalization 

or balancing releases. In general, Lake Powell will release 

8.23 maf or more of water downstream above elevation 

3,575 ft (1,089.7 m), or 39 percent of its capacity. Between 

elevations 3,525 and 3,575 ft (1,074.4 and 1,089.7 m), Lake 

Powell will release 7.48 maf, unless Lake Mead’s elevation 

is projected to be 1,025 ft (312.4 m) or below, in which case 

Lake Powell will release 8.23 maf. Below elevation 3,525 ft 

(1,074.4 m), or 24 percent of capacity, releases from Lake 

Powell are coordinated with the water level in Lake Mead and 

range between 7.0 and 9.5 maf. 

For Lake Mead, the most significant elevation within 

the Interim Guidelines is 1,075 ft (327.7 m), or 34 percent 

of capacity. Lake Mead will generally release 7.5 maf 

annually for United States consumptive use above watersurface elevation 1,075 ft (327.5 m), and the Guidelines 

include mechanisms for the Lower Basin States to develop 

or store surplus water when the surface elevation is above 

1,075 ft (327.5 m). At elevations 1,075 ft (327.5 m) or below, 

the shortage conditions are triggered, which provide that 

United States consumptive uses can decrease to 7.2 maf 

for Lake Mead elevations from 1,075 to 1,050 ft (327.7 to 

320.0 m; between 34 and 27 percent of capacity); to 7.1 maf 

for elevations from 1,050 to 1,025 ft (320.0 to 312.4 m; 

between 27 and 21 percent of capacity); and 7.0 maf at lake 

elevations below 1,025 ft (312.4 m). Reductions in water 

deliveries to United States water users in the Lower Basin, 

such that depletions are less than 7.5 maf, affect the States of 

Nevada and Arizona until Lake Mead’s surface declines to 

1,025 ft (312.4 m). Below 1,025 ft (312.4 m), the Secretary 

of the Interior will consult the Basin States to determine 

the allocation of water. The coordinated operations of Lake 

Powell, Lake Mead, and the Lower Basin are much more 

complex than can be described here; detailed information is 

available in the document, “Colorado River Interim Guidelines 

for Lower Basin Shortages and the Coordinated Operations for 

Lake Powell and Lake Mead” (Bureau of Reclamation, 2007). 

Lake Mohave Operations

Lake Mohave is operated as a regulation dam, or to 

hold releases from the Hoover Dam for efficient deliveries 

to meet downstream needs. As a result, Lake Mohave is 

operated within a consistent operational pattern that is tied to 

the predictable cycles of the downstream water needs. The 

surface elevation of the lake is not subject to wide variations, 

and typically is within a range of about 15 ft (4.6 m) or less 

(fig. 3-7). The annual maximum levels of approximately 645 ft 

(196.6 m) typically are reached in April or May and minimum 

levels of approximately 630 ft (192.0 m) are observed 

in September or October following summer releases for 

irrigation (fig. 3-8).

Lake Mohave is operated in support of the Lower 

Colorado River Multi-Species Conservation Program’s 

implementation of the Razorback Sucker Replacement 

Program (see Chapter 5). To support razorback sucker 

(Xyrauchen texanus) populations and habitat, the lake’s water 

level is held steady during the winter and spring months 

during the fish spawning season and raised to a higher level 

during the spring and summer months to maintain water 

quality in the backwater pools used for raising razorback 

suckers for return to the main lake system. The lake is drawn 

down in the autumn months to accommodate the harvesting of 

razorback suckers from lakeside rearing coves for their return 

to the lake. 

The lake also is held higher during the spring and 

summer months to help meet higher downstream water 

demands during peak agricultural use. In addition, the lake 

is drawn down in the autumn months to enable it to capture 

runoff from autumn and winter precipitation events.
Chapter 3

Hydrology and Management of Lakes Mead and Mohave within the Colorado River Basin 33

nvtac11-4177_fig03-07

625 

630 

635 

640 

645 

650 

Elevation, in feet

Lake Mohave Operational Constraints 

Guide Curve 

Maximum operational elevation 646.5 feet

Stay above 640 feet

Stay above 638 feet Stay below 635 feet

for 10 days 

February–April

drawdown constraint:

Not to exceed 2 feet in any

10-day period 

EXPLANATION

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Elevation is referenced to the U.S.Geological Survey datum, adjustment of 1912, locally known as "Power House Datum”.

Figure 3-7. Lake Mohave Operational Constraints, Bureau of Reclamation.

Year

620 

625 

630 

635 

640 

645 

650 

1950 1959 1969 1979 1989 1999 2009 

Elevation, in feet 

Elevation is referenced to the U.S.Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Figure 3-8. Lake Mohave average monthly surface elevations from January 1, 1950 to 

December 31, 2010.
34 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Chapter 3 References

Baker, J.R., Deacon, J.E., Burke, T.A., Egdorf, S.S., Paulson, 

L.J., and Tew, R.W., 1977, Limnological aspects of Lake 

Mead, Nevada-Arizona: U.S. Department of the Interior, 

Bureau of Reclamation Report REC-ERC-77-9.

Barnett, T.P., and Pierce, D.W., 2009, Sustainable water 

deliveries from the Colorado River in a changing climate: 

Proceedings of National Academy of Sciences, v. 106, 

p. 7,334–7,338.

Bureau of Reclamation, 2007, Final Environmental Impact 

Statement–Colorado River Interim Guidelines for Lower 

Basin Shortages and Coordinated Operations for Lake 

Powell and Lake Mead, November 2007: Bureau of 

Reclamation web site, accessed August 17, 2012, at http://

www.usbr.gov/lc/region/programs/strategies/FEIS/index.

html.

Farnsworth, R.E., and Thompson, E.S., 1982, Mean monthly, 

seasonal, and annual pan evaporation for the United 

States: National Oceanic and Atmospheric Administration 

Technical Report NWS 34, 85 p.

Holdren, G.C., and Turner, K., 2010, Characteristics of Lake 

Mead, Arizona-Nevada: Lake and Reservoir Management, 

v. 26, p. 230–239.

LaBounty J.F., and Burns, N.M., 2005, Characterization of 

Boulder Basin, Lake Mead, Nevada-Arizona, USA–Based 

on analysis of 34 limnological parameters: Lake and 

Reservoir Management, v. 21, p. 277–307.

Paulson, L.J., and Baker, J.R., 1983 [revised 1984], The 

limnology in reservoirs on the Colorado River: University 

of Nevada, Las Vegas, Department of Biological Sciences, 

Lake Mead Limnological Research Center, Las Vegas 

Technical Report No. 11, 269 p.

Westenburg, C.L., DeMeo, G.A., and Tanko, D.J., 2006, 

Evaporation from Lake Mead, Arizona and Nevada, 1997–

99: U.S. Geological Survey Scientific Investigations Report 

2006–5252, 24 p. (Also available at http://pubs.er.usgs.gov/

publication/sir20065252.)
Chapter 4

Lake Water Quality 35

1

 Southern Nevada Water Authority

2

 Bureau of Reclamation

3

 U.S. Geological Survey

4

 University of Nevada, Las Vegas

5

 Clark County Water Reclamation District

Given the importance of the availability and quality of water in Lake Mead, it has become one of the 

most intensely sampled and studied bodies of water in the United States. As a result, data are available 

from sampling stations across the lake (fig. 4-1 and see U.S. Geological Survey Automated Water-Quality 

Platforms) to provide information on past and current (2012) water-quality conditions and on invasive 

species that influence—and are affected by—water quality. Water quality in Lakes Mead and Mohave 

generally exceeds standards set by the State of Nevada to protect water supplies for public uses: drinking 

water, aquatic ecosystem health, recreation, or agricultural irrigation. In comparison to other reservoirs 

studied by the U.S. Environmental Protection Agency (USEPA) for a national lake assessment (U.S. 

Environmental Protection Agency, 2010), Lake Mead is well within the highest or ‘good’ category for 

recreation and aquatic health (see U.S. Environmental Protection Agency National Lakes Assessment 

and Lake Mead for more details). While a small part of the lake, particularly Las Vegas Bay, is locally 

influenced by runoff from urbanized tributaries such as Las Vegas Wash, contaminant loading in the 

lake as a whole is low compared to other reservoirs in the nation, which are influenced by runoff from 

more heavily urbanized watersheds (Rosen and Van Metre, 2010). 

Chapter

4

By Todd Tietjen1, G. Chris Holdren2, Michael R. 

Rosen3

, Ronald J. Veley3, Michael J. Moran3, Brett 

Vanderford1

, Wai Hing Wong4, and Douglas D. Drury5

Lake Water Quality

Telephone Cove, Lake Mohave, on a busy day. Photograph by National Park Service.
nvtac11-4177_fig04-01

CR360.7

CR346.4

LVB4.95

LVB7.3

CR355.75

CR342.5

VR9.4

VR6.0

VR2.0

CR380.0 CR390.0

CR394.0

CR395.0

CRLM_A

VR/MRLM

VR25.1

VR18.0

VR13.0

Echo Bay

LWLVB

LVB4.15

LVB3.5

EXPLANATION

VR18.0 Lake Mead sampling sites

0

0 5 KILOMETERS 

5 10 MILES

10

36 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The quantity of Colorado River water entering 

Lakes Mead and Mohave greatly exceeds the smaller 

contributions of tributary inflow (Chapter 3), and 

the greatest volume of water entering Lake Mead 

is from a high-quality source. The Colorado River 

is fed by snowmelt from the western Rockies that 

drain through a largely rural part of the country, 

which minimizes the impact of human activities. 

Figure 4.1. Map of Lake Mead showing primary sites for water-quality data collection. Sites are named for their 

locations along the main stem of the Colorado River (CR); along the flow of the Virgin River (VR); within Las Vegas Bay 

(LVB); or for their proximity to Las Vegas Wash inflow (LWLVB). Satellite imagery courtesy of NASA. Image processing 

provided by Julia Barsi, Landsat Project Science Office, NASA Goddard Space Flight Center. Data provided by the USGS 

EROS Data Center.

Furthermore, most sediments settle from Colorado River 

water as it passes through Lake Powell before entering 

Lake Mead (Covay and Beck, 2001). Information on 

water quality is summarized in this chapter in terms 

of basic water-quality characteristics; nutrients and 

productivity; organic and inorganic chemicals and 

compounds in water; organic and inorganic chemicals and 

compounds in sediment; and effects of invasive species 

on water quality.
Chapter 4

Lake Water Quality 37

U.S. Environmental Protection Agency 

National Lakes Assessment and Lake Mead

Lake Mead was included in the first U.S. 

Environmental Protection Agency (USEPA) National 

Lakes Assessment (U.S. Environmental Protection Agency, 

2009, 2010), a baseline study of the condition of lakes 

in the United States during 2007. Using the USEPA 

report and additional information from more extensive 

Lake Mead sampling, Lake Mead water quality can be 

evaluated in relation to other lakes across the nation. For 

its report, the USEPA categorized lakes into regional 

clusters based on their locations within the previously 

defined EPA level-III ecoregions (U.S. Environmental 

Protection Agency, 2003), which placed Lake Mead in 

the Xeric ecoregion. Data were available to compare 

conditions in Lake Mead to those of other lakes in 

the Xeric ecoregion with respect to six parameters: 

concentrations of total nitrogen, total phosphorus, 

chlorophyll a, and dissolved oxygen; turbidity; and an 

aggregate measure of the lakes’ suitability for recreation. 

USEPA used two separate evaluation schemes depending 

on the parameter of interest: (1) good/fair/poor relative to 

conditions found in reference lakes, or (2) undisturbed/

impacted/highly disturbed compared to selected 

reference (least disturbed) lakes. For the parameters 

available for Lake Mead, total nitrogen, total phosphorus, 

chlorophyll a, and turbidity values were rated using 

the good/fair/poor scale; dissolved oxygen was rated 

using the undisturbed/impacted/highly disturbed scale. 

The aggregate measure of lake suitability for recreation 

takes into account three separate parameters: the 

concentration of chlorophyll a, the abundance of the algal 

toxin microcystin, and the abundance of cyanobacteria 

(blue-green algae). Each of these parameters was rated 

according to a scale of low/moderate/high risk according 

to their concentrations.

To capture conditions in different basins within Lake 

Mead, data were included from sampling sites in Boulder 

Basin, the Virgin Basin, and the upper Colorado River 

Arm (figs. 1-1 and 4-1). For the suitability-for-recreation 

parameters, the Southern Nevada Water Authority’s 

drinking water intake system site was the only location 

where extensive microcystin measurements had been 

made. These data were compared to those at the lone Lake 

Mead location that was included in the National Lake 

Assessment (U.S. Environmental Protection Agency, 2009, 

2010; fig. 4-2). For the comparison, values computed for 

each site are averages of values for all samples collected 

in 2010 (including those at all sample depths, where 

appropriate) except for the cyanobacterial abundance 

data. For cyanobacteria abundance, the average value 

was computed from samples collected in 2009, which 

was the last year for which a complete set of data of this 

type is available. Samples were collected approximately 

weekly in Boulder Basin and monthly in Virgin Basin 

and the Colorado River Arm.

Water quality in Lake Mead compared favorably 

to the parameters set forth in the USEPA National 

Lake Assessment and its Technical Appendix (U.S. 

Environmental Protection Agency, 2009, 2010). For 

the selected locations, Lake Mead scored in the highest 

categories for most of the individual parameters: ‘good’ 

for total phosphorus and chlorophyll a concentrations 

and ‘reference/undisturbed’ for dissolved oxygen. 

Total nitrogen values were in the ‘good’ range for the 

Virgin Basin and the upper Colorado Arm and at the 

low end of the ‘fair’ range for Boulder Basin. Boulder 

Basin values are elevated by nitrogen loading from Las 

Vegas Wash (LaBounty and Horn, 1997). Turbidity 

values for Boulder Basin were in the ‘good’ range 

while the Virgin Basin and upper Colorado River Arm 

samples were at the low end of the ‘fair’ range. Boulder 

Basin benefited from the long settling time of water 

entering from the most significant tributary inflows. 

The Colorado River inflow raised the turbidity in the 

upper Colorado River Arm, and turbidity values for the 

Virgin Basin were likely elevated by a combination of 

loading through Overton Arm, wind-driven resuspension 

of sediments, and the production of calcium carbonate 

particles in the water column. These high-quality scores, 

including those values in the ‘fair’ range, represent 

high water quality. They reflect the large volume of 

Colorado River water from the undisturbed upstream 

environment within Grand Canyon National Park 

traveling through the lake and the settling and sediment 

deposition that occurs within Lakes Mead and Powell. 

However, Lake Mead’s scores would not have been as 

high if the selected sampling locations had been closer 

to the tributary inputs; samples from those locations 

would have been more indicative of water quality 

of the inflowing streams. As water from tributary 

inflow slows in the reservoir, most of the nutrients 

(nitrogen and phosphorus), sediments (turbidity), and 

algae (chlorophyll a) sink to the bottom. While this 

phenomenon can reduce the oxygen concentrations 

temporarily at the bottom of the water column, the 

overall impact is improved water quality throughout 

Lake Mead.
nvtac11-4177_sidbar_fig04-02

Total Nitrogen,

 in micrograms per liter

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

2,400

2,600

2,800

3,000

Total Phosphorus,

in micrograms per liter

0

20

40

60

80

100

120

140

160

180

200

Chlorophyll a,

in micrograms per liter

0

10

20

30

40

0

5

10

15

20

25

30

Dissolved Oxygen,

in milligrams per liter

0

2

4

6

8

10

Chlorophyll a,

in micrograms

per liter

Cyanobacteria Abundance

×1000 cells per liter

Microcystin,

in micrograms

Poor / High risk per liter

Fair / Moderate risk

Good / Low risk

EXPLANATION

0 20 40 60 80 100 120

Turbidity,

in Nephelometric Turbidity Units

38 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Figure 4-2. Lake Mead water-quality conditions compared to U.S. Environmental Protection Agency (2009, 

2010) indicators. Boulder Basin (diamonds), the Virgin Basin (circles), and the upper Colorado River Arm 

(squares). Most areas sampled at Lake Mead resulted in a USEPA designation of “good” condition.
Chapter 4

Lake Water Quality 39

Basic Water-Quality Characteristics

Physicochemical Description of 

Lake Mead

Information about the physicochemical 

characteristics of water in a lake, including 

temperature, dissolved oxygen, pH, and 

specific conductance are important to the 

understanding of the circulation of water in the 

lake. These data also can help in the evaluation 

of the level and timing of biological activity. 

Physicochemical data have been collected 

Three parameters constitute the USEPA’s suitability-for-recreation assessment 

to result in classifications of ‘low,’ ‘moderate,’ or ‘high’ risk of exposure to algal 

toxin. Lake Mead water quality posed a low risk of exposure to algal toxins based on 

chlorophyll a and microcystin concentrations, and cyanobacterial abundance values were 

at the low end of the ‘moderate’ risk-of-exposure range. Interpretation of these results, 

however, is less straightforward than for the other water-quality parameters. Primarily, 

chlorophyll a concentrations are low in the open waters of the lake, so the samplinglocation data underestimated algal cells where recreation is likely to occur. Secondly, 

although cyanobacterial abundance rated in the ‘moderate’ exposure to risk range, 

cyanobacteria actually present very little risk at Lake Mead for a variety of reasons. 

The Lake Mead environment favors extremely small cyanobacteria species, most of 

which are not the toxic, bloom-forming species (Microcystis sp.) that are detrimental to 

recreational uses of the lake. While Microcystis sp. and other nuisance cyanobacteria are 

found in Lake Mead, they simply do not represent a significant proportion of the overall 

population. Thus, the contribution of non-nuisance species, which is substantial, results 

in an aggregate cyanobacteria abundance value that overestimates risk. Overall, water 

quality in Lake Mead compared favorably to the findings in the USEPA National Lake 

Assessment and suggests that it is equal to or better than that in many other lakes in the 

Western United States. 

at Lake Mead for many years at many 

locations (for example, see LaBounty 

and Horn, 1997; LaBounty and Burns, 

2005, 2007; Holdren and Turner, 2010) 

using standard manual methods. In more 

recent years, however, data collection 

at several locations has been automated 

(see U.S. Geological Survey Automated 

Water-Quality Platform) to obtain these 

data at a higher frequency for enhanced 

understanding of the lake and to provide 

information for numerical reservoir models. 
40 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

U.S. Geological Survey Automated 

Water-Quality Platforms

By Ronald J. Veley and Michael J. Moran

Between October 2004 and September 2009, the 

U.S. Geological Survey collected near-continuous 

water-quality physicochemical data at Lake Mead as 

part of a larger lake-wide monitoring 

study involving a variety of water 

resource agencies (Veley and Moran, 

2012). One objective of this effort was to 

provide natural resource managers with 

basic water-quality data profiles from 

locations throughout the lake (fig. 4-3). 

Water-quality stations on Lake Mead 

were located at shallow-water sites in 

Las Vegas Bay and Overton Arm, and at 

Shallow water automated water-quality data-collection platform in Las Vegas 

Bay. Photograph by Michael R. Rosen, U.S. Geological Survey.

deep-water sites in Boulder Basin (near Sentinel Island), 

Virgin Basin, and Temple Basin. The shallow-water sites 

were located in waters less than 66-ft (20.0-m) deep, 

while the deep-water sites were in waters deeper than 

197 ft (60.0 m). At each station, an automatic-profiling 

system was used to collect near-continuous water-quality 

data. The systems used a water-quality sonde with 

sensors that measured water temperature, 

specific conductance, dissolved-oxygen 

concentration, pH, turbidity, and depth. 

The sonde starting collecting data near the 

top of the lake surface and continued until 

it nearly reached, but did not contact, the 

lake bottom. During the study, the sondes 

collected data every 6 hours beginning just 

after midnight of each day. 

15

95

93

93

15

Base from U.S. Geological Survey digital data, 1:100,000, 1979–1982

Shaded-relief base from 30-meter Digital Elevation Mode

Universal Transverse Mercator projection, Zone 11. North American Datum 1927

Wash 

Las VegasLas Vegas

0 5 10 15

0 5 10 15 20 KILOMETERS

20 MILES

115°00' 114°00'

36°00'

36°30'

Colorado

River

Lake Mead

National

Recreation

Area

Virgin River

Overton Arm

Muddy River

NEVADA

ARIZONA

Lake

Mead Las Vegas Las Vegas Bay

Boulder 

Basin

Virgin

Basin

Temple

Basin

Gregg

Basin Sentinel

Island

Callville Bay

HOOVER 

DAM

Water-quality station

EXPLANATION

Nevada

Arizona

Study Area

Figure 4-3. Locations of U.S. Geological Survey automated water-quality stations on Lake Mead, 

Nevada and Arizona.
Chapter 4

Lake Water Quality 41

Temperature and Thermal 

Stratification

 Water temperatures (fig. 4-4) in Lake 

Mead often follow the pattern that typifies warm 

monomictic lakes. Monomictic lakes are lakes 

that have a thermal stratification of the water 

column for most of the year and a single period 

of complete mixing. Occasionally, however, Lake 

Mead departs slightly from the warm monomictic 

pattern. On an average of 1 out of every 2 years, 

extensive but incomplete mixing occurs; the 

depth of the lake and complex interactions with 

the Colorado River are sufficient in these years.

Temperatures in Lake Mead’s water 

column are usually 52–54°F (11–12°C) during 

the winter months (December–February). In 

years with complete mixing, water temperature 

is usually 54°F (12°C) throughout the water 

column. During years with incomplete mixing, 

temperature in the top one-half of the water 

column usually remains at 54°F (12°C) while the 

bottom of the water column cools to 52°F (11°C). 

Beginning in March, the lake’s near-surface 

waters typically warm at different rates in two 

general areas: near inflowing tributaries and in 

the open waters that are distant from the shores 

and the influences of the tributaries (fig. 4-5). The 

tributaries are shallower than the lake and tend 

to warm faster than the lake’s deeper open-water 

water areas. Therefore, the initial warming of 

the water column commonly is detected near the 

tributaries. Subsequently, as days get longer and 

air temperatures increase, the temperature of the 

open waters of the lake also begin to increase. In 

Lake Mead, stratification is usually first detected 

in late April or early May. During this period, 

stratification begins in the shallower bays and in 

the top 32.8 ft (10.0 m) of the water column in 

open-water areas. By the end of May, the surface 

waters warm to 64°F (18°C), and by early July 

the near-surface waters reach their maximum 

temperatures of 82–86°F (28–30°C). 

10

12

14

16

18

20

22

24

26

28

30

700

750

800

850

900

950

1,000

1,050

1,100

1,150

Elevation

Feet

213

229

244

259

274

290

305

320

335

351

Meters

Temperature, 

in degrees Celsius

EXPLANATION

Month, January 2009 to December 2010 

Data from Bureau of Reclamation, used with permission 

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Sampling point

Figure 4-4. Water temperature profile for Lake Mead, at sampling site CR346 in Boulder Basin, January 2009–

December 2010 (see fig. 4-1 for location map). Notice that the largest temperature differences between the lake 

surface and the lake bottom occur during the late summer and early autumn.
nvtac-4177_fig04-5

Average

Maximum

Minimum

Intakes 1 and 2

Proposed intake 3

Lake stage

Lake bottom

Average

Maximum

Minimum

Lake stage

Lake bottom

EXPLANATION

EXPLANATION

Average

Maximum

Minimum

Lake stage

Lake bottom

EXPLANATION

Temperature, in degrees Celsius

5 1510 2520 30

325

Las Vegas Bay (Site 3)

January 2007

Elevation, in meters

330

335

340

345

350

10 20 30 35 15 25

250

275

300

325

350

10 20 30 15 25 35

Sentinel Island station

September 2007

Overton Arm station

July 2006

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as “Power House Datum.”

Epilimnion

Metalimnion

325

330

335

340

345

350

A. B. C.

42 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

A mid-July profile through the Lake Mead 

water column would reveal temperatures greater 

than 77°F (25°C) in approximately the top 32.8 

ft (10.0 m) of the water column, while between 

depths of 32.8 and 98.4 ft (10.0 and 30.0 m) 

temperatures would decrease to about 59°F 

(15°C; fig. 4-4). The thermocline, the region of 

the water column with the largest temperature 

change, typically is located between depths of 

65.6 and 98.4 ft (20.0 and 30.0 m). Below depths 

of 98.4 ft (30.0 m), the temperature gradually 

decreases to 54°F (12°C) for the remainder of 

the water column. From mid-July through late 

autumn, the warm, near-surface waters move 

gradually deeper, reaching 114.8 ft (35.0 m) 

in August and 131.2 ft (40.0 m) in September. 

Water temperatures from the surface to a depth 

of about 65.6 ft (20.0 m) generally remain 

warmer than 77°F (25°C) through the middle 

of September, decreasing to approximately 

68°F (20°C) in October and to 64°F (18°C) in 

November. Stratification of the water column 

typically ends in late November and early 

December as surface temperatures decrease to 

61–64°F (16–18°C) and the total temperature 

difference in the water column decreases to 

39–43°F (4–6°C). Finally, water typically cools 

again to 52–54°F (11–12°C) in December, with 

the actual timing dependent on air temperatures 

and the amount of wind-driven mixing that 

occurs.

The stratification of the water column 

into distinct horizontal layers of water at 

different temperatures can have important 

impacts on the ecology of a water body. The 

epilimnion (upper-most layer) has the greatest 

exposure to light and often has the most active 

phytoplankton population. A great abundance 

of phytoplankton, in turn, produces the 

most favorable conditions for zooplankton, 

which are a food source for some fish. The 

epilimnion also frequently has the highest 

concentrations of dissolved oxygen, which 

is a condition for survival for many higher 

organisms. Oxygen presence also influences the 

chemical environment: where sufficiently high 

concentrations of oxygen occur (such as in the 

epilimnion), nutrients and other chemicals can 

precipitate out of the water in forms unavailable 

to organisms. In the hypolimnion (lowest 

Figure 4-5. Automated temperature profiles and thermal stratification at Lake Mead. Automated 

temperature profiles for (A) USGS Las Vegas Bay (Site 3) station for January 2007; (B) USGS Sentinel Island 

station for September 2007; (C) USGS Overton Arm station for July 2006. In (B), potential inflow from Las 

Vegas Wash and the Colorado River are shown in the epilimnion and metalimnion, respectively. (From Veley 

and Moran, 2012. See U.S. Geological Survey Automated Water-Quality Platforms for location map.) 
Chapter 4

Lake Water Quality 43

layer), oxygen concentrations typically are lower 

because the metalimnion (middle layer) can 

serve as a barrier between the upper and lower 

layers, limiting the transfer of oxygen from above. 

Any organic material decay in the hypolimnion 

occurs through a process that consumes oxygen, 

further reducing oxygen concentrations. In 

the hypolimnion, if oxygen concentrations are 

sufficiently low, nutrients and other chemicals 

can be released from precipitated forms back into 

solution, where they are bioavailable to aquatic 

organisms and have the greatest potential to 

influence the ecosystem.

Dissolved Oxygen

Several gases, such as oxygen, nitrogen, 

and carbon dioxide, are dissolved in the water of 

streams and lakes. Dissolved oxygen (DO) is essential for aquatic life with the exception of certain 

types of bacteria, and therefore is important to 

monitor. The degree to which water is saturated 

with oxygen is determined by oxygen concentration, water temperature, and atmospheric pressure. 

Expressing oxygen concentrations in terms of percent saturation takes into account the ability of the 

water to hold oxygen. The warmer the water, the 

less oxygen it can hold. Within Lake Mead, DO 

concentrations generally are high enough (more 

than 50 percent saturation) to meet the requirements of aquatic organisms and support a thriving 

biological community.

Lake Mead’s seasonal pattern of thermal 

stratification significantly influences DO 

conditions in the lake (fig. 4-6). During the autumn 

and winter mixing period, oxygen concentrations 

generally are 90 percent of saturation or higher 

as the water column is at equilibrium with the 

atmosphere. As the water column begins to 

stratify during the spring and summer, it can 

become supersaturated with oxygen at the surface 

during the day as algal photosynthetic oxygen 

production increases with warmer temperatures 

and nutrient inputs. This supersaturation, driven 

by nutrient loading to the surface waters, can 

persist through the early summer. In some years, 

the algae that produce areas of DO supersaturation 

accumulate in the metalimnion at the density 

barrier (thermocline) established during thermal 

stratification. When dead algal material collects 

and decomposes in this layer, oxygen is consumed, 

which can produce a metalimnetic oxygen 

minimum that can persist up to 3 or 4 months. 

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

EXPLANATION

100

110

120

 90

 80

 70

 60

 50

 40

 30

Dissolved Oxygen

percent saturation

213

229

244

259

274

290

305

320

335

351

Meters

700

750

800

850

900

950

1,000

1,050

1,100

1,150

Elevation

Month, January 2009 to December 2010

Feet

Data from Bureau of Reclamation, used with permission 

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Sampling point

Figure 4-6. Dissolved oxygen (DO) profile for Lake Mead, at sampling site CR346.4 in Boulder Basin, January 2009–December 

2010 (see fig. 4-1 for location map). Notice that the highest oxygen saturation percentages are near the surface of the lake.
Increasing

acidity

Increasing

alkalinity

Neutral

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Battery acid

Acid

rain

Lemon juice

Fish reproduction

affected

Vinegar

Milk

Ammonia

Lye

Baking soda,

sea water

Milk of

Magnesia

Adult fish die

Normal range

precipitation

Normal range

of stream water

pH scale

Courtesy of Environment Canada http:www.ns.ec.gc.ca

The pH of Lake Mead 

generally is between 

7.8 and 8.2

44 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Following peak algal production in spring, 

oxygen conditions in the water column are 

generally influenced by stratification. In the 

epilimnion, where surface water is exposed to 

atmospheric oxygen, DO saturation remains 

higher than 80 percent. In the hypolimnion, 

seasonal restriction from atmospheric oxygen 

results in a gradual decrease in DO saturation 

to approximately 50 percent by the end of the 

summer stratified period. In years when the water 

column mixes completely, oxygen is replenished 

throughout all thermal layers, whereas during 

years of incomplete mixing, oxygen at the bottom 

of the water column is usually replenished 

by underflowing Colorado River waters. The 

introduction of cold, dense Colorado River 

into the hypolimnion of Lake Mead during 

the autumn and winter can be important in 

maintaining oxygen concentrations greater than 

4 mg/L until the next complete mixing of the 

water column.

DO concentrations decrease below critical 

levels only near the tributary inflows, and these 

low levels remain for only a matter of weeks. 

Near the confluence of the Las Vegas Wash and 

Lake Mead, oxygen dynamics are similar to 

those in other reservoirs with tributary inflow. 

In this area (Las Vegas Bay), the phytoplankton 

community increases algal biomass using 

nutrients contributed by Las Vegas Wash. When 

algal cells die, they begin to release nutrients 

as they settle through the water column and are 

deposited on the lake floor. On the lake floor, 

at the water-sediment interface, dead algae 

and other organic matter contributed by Las 

Vegas Wash inflow continue to decompose, 

decreasing oxygen in the vicinity. During 

autumn and winter, when mixing of the water 

column occurs, hypolimnetic waters depleted of 

oxygen mix with the upper layers. As a result, 

oxygen concentrations briefly decrease below 

50 percent saturation in Las Vegas Bay near Las 

Vegas Wash. After this period, however, oxygen 

concentrations at greater depths are typically 

only slightly lower than those in the upper water 

column.

pH

The physicochemical parameter pH is a measure of a solution’s 

acidity (fig. 4-7). Acidic solutions have a pH less than 7 and basic 

solutions have a pH greater than 7; the pH of a neutral solution is around 

7. Because the pH scale is logarithmic, there is a 10-fold difference in 

acidity between solutions that differ by 1-pH value. The pH of Lake 

Mead water generally is between 7.8 and 8.2, with slightly higher values 

in the near-surface waters and slightly lower values in the bottom part of 

the stratified water column (fig. 4-8). A carbonate buffering system—the 

equilibrium of carbon dioxide, water, and carbonic acid, bicarbonate, 

and carbonate ions—is responsible for maintaining this range in pH. The 

activity of phytoplankton can account for small, local differences in pH 

values in the lake. 

Figure 4-7. pH scale showing examples of liquids with different pH values.
nvtac11-4177_fig04-08

700

750

800

850

900

950

1,000

1,050

1,100

1,150

Elevation

Feet

Month, January 2009 to December 2010 

Data from Bureau of Reclamation, used with permission 

213

229

244

259

274

290

305

320

335

351

Meters

7.4

7.6

7.8

8.0

8.2

8.4

8.6

8.8

9.0

pH units

EXPLANATION

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Sampling point

Chapter 4

Lake Water Quality 45

During the spring and summer growing 

season, the pH of surface waters typically 

increases because free carbon dioxide (CO2) is 

used by phytoplankton during photosynthesis

and, as a result, there is less CO2 and less 

carbonic acid (carbon dioxide and water) in the 

water. Later in the growing season, pH in the 

hypolimnion tends to decrease as acidifying 

CO2 is produced by decomposition of organic 

matter in the sediment. Following the mixing 

of the water column in autumn and winter, 

pH throughout the water column shows little 

change until the following spring. However, the 

constant chemical equilibrium of the carbonate 

buffering system prevents large changes in pH 

and generally limits the importance of pH as a 

factor influencing water quality in Lake Mead 

regardless of phytoplankton (or other) inputs.

Specific Conductance

Specific conductance (SC) is a measure of the 

ability of water to conduct electricity and it is 

proportional to the amount of dissolved chemicals 

in water. Distilled or deionized water has a SC of 

at least 1 microsiemens per centimeter (μS/cm) 

because it contains very small amounts of dissolved 

chemicals. By contrast, seawater, which contains 

a large amount of dissolved chemicals, has an 

SC of approximately 50,000 μS/cm. The specific 

conductance of the water in Lake Mead is controlled 

by a set of interrelated factors—the higher SC values 

of inflow from tributaries other than the Colorado 

River, the lower SC values of the Colorado River, 

and the evaporation of surface waters combined with 

water column stratification.

Figure 4-8. pH profile for Lake Mead, at sampling site CR346.4 in Boulder Basin, January 2009–

December 2010 (see fig. 4-1 for location map). Notice that the highest pH measurements are near 

the surface of the lake.
nvtac11-4177_fig04-09

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

Specific conductance,

in microsiemens per centimeter

4 5 6 7

800

850

900

950

1,000

1,050

1,100

1,150

243

258

273

289

304

319

334

349

Meters

LVB 4.15LVB 4.95LVB 7.3

Distance from mouth of Las Vegas Wash, kilometers

LVB 3.5

Feet

4 5 6 7

800

850

900

950

1,000

1,050

1,100

1,150

243

258

273

289

304

319

334

349

Meters

LVB 4.15LVB 4.95LVB 7.3

Elevation

LVB 3.5

Feet

November 2009

July 2009

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

800

850

900

950

1,000

1,050

1,100

1,150

243

258

273

289

304

319

334

349

Meters

LVB 3.5LVB 4.15LVB 4.95LVB 7.3

Feet

March 2009

EXPLANATION

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Sampling point

46 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Figure 4-9. Seasonal differences in 

inflow patterns of Las Vegas Wash 

water into Las Vegas Bay, shown 

through specific conductance (SC) 

values from the mouth of Las Vegas 

Wash to 7 mi (11.3 km) within Las 

Vegas Bay across all depths. 
Chapter 4

Lake Water Quality 47

Surface flow in Las Vegas Wash consists 

primarily of water released from wastewatertreatment facilities. The SC of this water 

generally is twice as high as that of the 

downgradient surface waters in Boulder 

Basin (LaBounty and Horn, 1997). When the 

temperatures of Las Vegas Wash and Lake 

Mead are equivalent (during early spring and 

early autumn), the difference in the SC values 

is the primary control on the position of the 

tributary inflow into the lake (fig. 4-9). Because 

of their similar densities, water in Las Vegas 

Wash travels as an interflow, or through an 

intermediate layer, within the lake. During these 

periods, this interflow from Las Vegas Wash 

may travel relatively far into Boulder Basin, 

but increased SC is most evident closest to the 

embayments where the interflow enters the lake. 

At other times of the year, however, temperature 

differences between Las Vegas Wash water and 

lake water can result in different inflow patterns 

(fig. 4-9): 

1. During the late spring and summer, water in Las 

Vegas Wash has a higher SC than Lake Mead 

water but is warmer, and therefore less dense 

than water in the lake. As a result, the lower 

density water in the Las Vegas Wash typically 

flows across the surface of the lake, increasing 

SC in the upper layer of the lake.

2. During late autumn and winter, water in 

Las Vegas Wash has a higher SC and lower 

temperature than Lake Mead, and therefore is 

denser than water in the lake. Because of the 

higher density, water from Las Vegas Wash 

sinks and flows along the sediment/water 

interface, increasing SC in this layer. 

Analysis of trends in water-quality parameters 

collected at the automated Lake Mead stations 

showed that SC values decreased from 2005 to 

2009 at all stations except for Las Vegas Bay (see 

U.S. Geological Survey Automated Water-Quality 

Platforms; Veley and Moran, 2012). The upward 

trend in SC values at Las Vegas Bay (fig. 4-10) 

is thought to be a consequence of declining lake 

Specific conductance, in microsiemens per centimeter

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

2,000

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

February

March

April

2004 2005 2006 2007 2008 2009

Las Vegas Bay (Site 3) at 3 meters

Overton Arm at 1 meter

EXPLANATION

12-month moving average

Figure 4-10. Monthly average values of specific conductance (SC) in Lake Mead from October 2004 

to September 2009. SC averages from the U.S. Geological Survey’s Las Vegas Bay and Overton Arm 

water-quality stations on Lake Mead at depths of 10 ft (3.0 m) and 3 ft (1.0 m), respectively (Veley and 

Moran, 2012). 
48 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

levels during this period. As lake levels declined, 

a greater portion of inflow is contributed by 

Las Vegas Wash with higher SC. A similar 

upward trend in SC was not observed at the 

other shallow-water station in the Overton Arm, 

which receives water of lower SC levels from 

the Muddy and Virgin Rivers. The decreasing 

SC values at the deep-water stations are thought 

to be a result of waters with lower SC values 

entering the lake from the Colorado River. 

Overall, the large volume of Colorado 

River water that enters Lake Mead influences 

SC more heavily than does inflow from the 

other tributaries. Between 1992 and 2005, SC 

in Boulder Basin cycled between relatively low 

to high in response to the volume of Colorado 

River water being released from Lake Powell 

(LaBounty and Burns, 2005). Years with higher 

lake-surface elevation correspond to lower SC 

values. Because the temperature of the Colorado 

River inflow is cooler (and therefore denser) 

than Lake Mead water, periods of extensive, 

long-term underflow brings low SC conditions 

to the bottom of the lake. This pattern continued 

from 2005 to 2010, showing an SC decrease also 

supported by the linkage to releases from Lake 

Powell that LaBounty and Burns (2005) reported. 

As Lake Powell releases water of lower SC into 

the Colorado River that flows into Lake Mead 

downstream, the average SC of Lake Mead’s 

water column will similarly decrease.

Finally, seasonal stratification of the water 

column also produces conditions that increase 

SC locally through the process of evaporation 

(fig. 4-11). Once the water column becomes 

stratified, the surface waters continue to 

warm and are prevented from mixing with the 

remainder of the water column. As this water 

warms, evaporation rates increase and leave 

behind salts that accumulate as water is lost to 

the atmosphere.

Secchi Depth

The clarity or transparency of natural waters 

varies widely and can be measured in several 

different ways. A simple and inexpensive tool 

that has been used to measure water clarity 

for more than 100 years is the 8-in. (20.0-cm) 

black and white Secchi disk (Wernand, 2010). 

A Secchi measurement is the average of the 

depths at which the disk visually disappears 

upon repeatedly lowering it into the water and 

reappears upon raising it. Secchi data collected 

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

Month, January 2009 to December 2010

Data from Bureau of Reclamation, used with permission 

850

900

950

1,000

1,050

1,100

Specific Conductance

in microsiemens per centimeter

EXPLANATION

800

900

1,000

1,100

244

274

305

335

Meters

Elevation

Feet

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Sampling point

Figure 4-11. Specific conductance (SC) profile for Lake Mead, at sampling site CR346.4 in Boulder Basin, January 2009–

December 2010 (see fig. 4-1 for location map). Higher values occur near the air-water interface during summer when the 

lake is stratified and evaporation removes water and concentrates dissolved salts near the surface of the lake. 
nvtac11-4177_fig04-10

1999 2001 2003 2005 2007 2009 2011 2012

2

4

6

8

10

12

14

16

18

20

Secchi depth, in feet below lake surface

Secchi depth, in meters below lake surface

Year

Average monthly Secchi depth readings

Linear fit of data

12-month (yearly) smooth data

EXPLANATION

7

13

20

26

33

39

46

52

59

66

Chapter 4

Lake Water Quality 49

over time can reflect changes in the concentration 

of suspended sediments or algal abundance but 

do not quantify the amount of particles present. 

Nonetheless, water clarity is an important 

property of water. For Lake Mead, water clarity 

in open waters generally has been increasing 

over the past decade (fig. 4-12; LaBounty, 2008). 

Closer to the tributaries, however, water clarity 

generally is lower due to higher algal production 

and biomass and sediment load (turbidity) 

contained in tributary inflow. 

The values presented for both Secchi depth 

(fig. 4-12) and chlorophyll a concentrations, 

discussed later in the chapter, reflect changes in 

water quality in recent years. The measurements 

are consistent with an improvement from high 

water quality values to extremely high water 

quality values. 

Figure 4-12. Average monthly Secchi depth for Boulder Basin, Lake Mead, 2000–2009. The overall trend shows 

increases in water clarity over the past decade. Values greater than 3 ft (1.0 m) indicate very clear water. 

A Secchi disk used for 

measuring water clarity in 

lakes. Photograph by Michael R. 

Rosen, U.S. Geological Survey.
50 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Nutrients and Productivity

Phosphorus

Phosphorus is an essential nutrient used by 

all organisms for the basic processes of life. Its 

natural source is the weathering and leaching 

of phosphate-rich geological formations. 

Phosphorus concentrations in the open waters of 

Lake Mead are low, falling in the lower quartile 

of the “good” range in the USEPA National 

Lakes Assessment. However, phosphorus is used 

extensively in fertilizer and other chemicals, 

and is a component of wastewater, so it can 

be found at higher concentrations near human 

activity. Past evaluations of nutrients in Lake 

Mead have focused on phosphorus as the 

primary limiting algal nutrient from tributary 

inflow. A recent nutrient-budget study (Flow 

Science Inc., 2011) determined that while further 

refinements are needed to accurately describe 

tributary contributions of phosphorus to the lake, 

conclusions can be made. Most of the phosphorus 

entering the lake, an estimated 34,400 lb/d 

(15,600 kg/d), is bound to sediment particles in 

the inflow of the Colorado River. Approximately 

32,000 lbs (14,900 kg) of this phosphorus 

quickly settles to the lake bottom, so that most 

of the nutrient is unavailable for use by algae in 

downgradient areas of lake. A different form of 

phosphorus, dissolved phosphorus, enters the 

lake primarily through the Colorado River and 

Las Vegas Wash at loads of 280 lb/d (127.0 kg/d) 

and 134 lb/d (60.8 kg/d), respectively. Las 

Vegas Wash receives highly treated wastewater 

effluent from Las Vegas Valley and is the primary 

source of dissolved phosphorus during dryweather conditions (fig. 4-13A). Even though the 

dissolved phosphorus loads are approximately 

100 times smaller than the total phosphorus load, 

dissolved phosphorus is more readily available 

and more rapidly used by algae. In comparison 

to the Colorado River and Las Vegas Wash, 

the combined contributions of phosphorus to 

Lake Mead by the Muddy and Virgin Rivers 

are approximately 220 lb/d (99.7 kg/d) of total 

phosphorus and 31 lb/d (14.0 kg/d) of dissolved 

phosphorus.

Advanced wastewater treatment beginning 

in 1994 reduced total and dissolved phosphorus 

inputs into Boulder Basin via Las Vegas Wash. 

However, as the Las Vegas Valley population 

grew throughout the 1990s and early 2000s, 

phosphorus concentrations increased in 

wastewater. Wastewater facilities enhanced 

treatment for phosphorus removal in 2002 

and 2005 (LaBounty and Burns, 2007), and 

successfully reduced phosphorus input. By 2009, 

total phosphorus concentrations in Las Vegas 

Bay and Boulder Basin typically were less than 

10 μg/L (fg. 4-13A) and dissolved reactive 

phosphorus concentrations typically were less 

than 2 μg/L. Total phosphorus load was reduced 

by as much as 98 percent from the concentrations 

of the 1970s and 1980s.

Nitrogen

All organisms require the nutrient nitrogen 

to live and grow; it is a building block of cellular 

proteins. In Lake Mead, total concentrations 

of nitrogen have remained stable from 1999 to 

2010 at approximately 1,000 μg/L; with most 

of the nitrogen in the form of dissolved nitrite 

plus nitrate. Although wastewater contributes 

significant levels of nitrogen to Lake Mead’s 

Boulder Basin and Las Vegas Bay (fig. 4-13B), 

Colorado River water also showed moderately 

high average total nitrogen concentrations 

(516 μg/L) between 2001 and 2009 (Holdren and 

Turner, 2010). 

Organic Matter

Organic matter is important in all aquatic 

systems, storing and providing carbon as a 

source of energy and affecting many features 

of a water body’s ecology. New biomass is 

produced from available carbon. A measurement 

of total organic carbon (TOC) includes many 

components, including visible particles from 

decaying organisms and dissolved molecules 

of carbon-containing compounds. For many 

lakes and reservoirs, most of the organic matter 

present is not produced within the system but 

from land-derived sources (for example, leaves 

and tree litter) entering from the watershed 
nvtac11-4177_fig04-13A

1,050

1,100

Elevation

Feet

320

335

Meters

Total Phosphorus,

in milligrams per liter

EXPLANATION

Month, January 2009 to December 2010 

Data from Bureau of Reclamation, used with permission 

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

1.00E-3

0.02

0.03

0.05

0.06

0.08

0.09

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

A.

Sampling point

Jan. Mar. May July Sept. Nov. Jan. Mar. May July Sept. Nov. Dec.

1,020

1,040

1,060

1,080

1,100

1,120

311

317

323

329

335

341

Elevation

Feet Meters EXPLANATION

7

6

5

4

3

2

1

0

Total Nitrogen,

 in milligrams per liter

B.

Sampling point

Month, January 2009 to December 2010 

Data from Bureau of Reclamation, used with permission 

Elevation is referenced to the U.S. Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Chapter 4

Lake Water Quality 51

or inflowing rivers (Bade and others, 2007, and references 

therein). Lake Mead represents an unusual case in that it is 

large and its tributary inputs are few (that is, three or four 

locations depending on water level). Dominating inflow to 

Lake Mead is the Colorado River, which travels through the 

arid landscape of the Grand Canyon with minimal organic 

matter inputs; approximately 300 mi (482.8 km) of upstream 

watershed is protected from industry and development 

within Grand Canyon National Park. Moreover, sediments 

settle and are deposited behind Glen Canyon Dam at Lake 

Powell, effectively removing associated organic matter 

from the water above the Grand Canyon. As a result, TOC 

concentrations in Lake Mead have never been high following 

the closing of Glen Canyon Dam, and varying for example 

from approximately 3 mg/L in 2000 to about 4 mg/L in 2004 

(Roefer and others, 2005). 

Figure 4-13. Changes in the concentrations of total phosphorus (A) and total nitrogen (B) in the water column of Lake Mead near 

Las Vegas Wash in Las Vegas Bay, January 2009–December 2010. 
nvtac11-4177_fig04-14

0 

1 

2 

3 

4 

5 

6 

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 

Trophic level

Chlorophyll

Secchi depth

Phosphorus

EXPLANATION

Trophic level index

Year

Trophic level

Figure 4-14. Burns Trophic Level Index (TLI) averaged 

scores for Lake Mead, 2000–2010, from three locations within 

Boulder Basin, including sampling sites LVB6.7 and CR346.4 

(see fig. 4-1 for location map). The TLI score is calculated 

using the annual chlorophyll Trophic Level value; annual 

Secchi Trophic Level value; and annual total phosphorus 

Trophic Level value. The TLI trend has been downward from 

2000 to 2010, indicating that water quality is improving. 

52 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Productivity

In aquatic ecosystems, primary productivity is the 

growth of new biomass by primary producers (phytoplankton; 

Chapter 5), which form the base of the food web. Through the 

process of photosynthesis, phytoplankton convert dissolved 

inorganic carbon in the water into organic compounds. In 

Lake Mead, productivity is a complex process due to the 

influences of inflowing tributaries and the characteristics of 

the somewhat confined larger basins that are relatively far 

away from tributary inflow. In the large, deep basins of Lake 

Mead, productivity is generally quite low (oligotrophic) and 

has been decreasing during recent years. Nearer to inflowing 

rivers, productivity, in general, increases to mesotrophic

levels due to the introduction of two components: nutrients 

(nitrogen and phosphorus) and organic matter. Nutrients 

stimulate the production of algal growth. Areas of relatively 

greater primary production generally have higher populations 

of invertebrates, which provide food for the fishery and 

other food-web components. However, too much primary 

production (eutrophic) decreases water quality (evidenced 

by lower clarity, wide ranges in DO and pH) and, therefore, 

degrades conditions for fish and other wildlife. 

Productivity of Boulder Basin has been evaluated using 

the Burns Trophic Level Index (TLI; LaBounty and Burns, 

2005). This index does not provide a quantitative measure of 

productivity, but simply attempts to classify trends towards 

improving or degrading water quality based on the trophic

state of lakes (Burns and others, 1999). To this end, TLI 

combines information on nutrients (nitrogen and phosphorus), 

algal biomass (chlorophyll a), and water clarity (Secchi 

depth), standard water-quality parameters that also are 

described in this chapter. For TLI calculations in Lake Mead, 

however, the nitrogen component was not included; although 

nitrogen concentrations generally are high, it is the low 

concentrations of phosphorus that control primary production 

in this system (Paulson and Baker, 1983; LaBounty and Horn, 

1997; Du, 2002; LaBounty and Burns, 2005; LaBounty, 

2008; Holdren and Turner, 2010). As with other indicators of 

water quality, the Boulder Basin’s TLI scores reflect efforts 

in wastewater treatment enhancement during the past decade. 

The lower the TLI value, the higher the water quality. Basinwide, the evaluation (fig. 4-14) showed that TLI scores 

have improved during the past decade, with the 2001–2004 

period categorized as mesotrophic (TLI value is between 3 

and 4) and the 2004–2008 period categorized as oligotrophic 

(TLI value is between 2 and 3). TLI indices computed for 

sampling locations near Las Vegas Wash inflow have reflected 

an eutrophic status at some times of the year, but this level 

of productivity declines rapidly as the inflow is diluted by 

Boulder Basin.

Chlorophyll a

Chlorophyll a, one of the pigments involved in 

photosynthesis, is an indicator of water quality and is 

frequently used as a measure of algal biomass. Concentrations 

of chlorophyll a are usually the highest near a reservoir’s 

tributaries, which contribute nutrients. In Boulder Basin and 

Las Vegas Bay, elevated chlorophyll a concentrations near 

tributary inflows are particularly enhanced by the supply 

of phosphorus from Las Vegas Wash. A major algal bloom 

occurred in 2001 (see Nutrients and Algae in the Las Vegas 

Bay and Boulder Basin of Lake Mead and Chapter 6), with 

peak chlorophyll a concentrations greater than 1,000 μg/L in 

Boulder Basin (fig. 4-15). The bloom consisted primarily of 

the green algal species Pyramiclamys disecta, which is not 

known to produce or release toxins or any other compounds 

that affect drinking water taste or odor. 

The 2001 algal bloom in Lake Mead has been attributed 

to the effects of a combination of factors: lowered lake levels, 

exposed tributary delta areas, spring rains, nutrient loading, 

the position of inflows in the water column, and construction 

activities (LaBounty and Burns, 2005). The ultimate 

driving factor, however, appears to have been the increased 

phosphorus concentrations at the surface of the water column 

during the spring warming of the lake, which allowed the algal 

species to sustain a high level of productivity. 
nvtac11-4177_fig04-15

1999 2001 2003 2005 2007 2009 2011 2012

0

5

10

15

400

800

1,200

Year

Chlorophyll a, in micrograms per liter

Chapter 4

Lake Water Quality 53

Since 2001, improvements in wastewater treatment have 

resulted in significant reductions in phosphorus concentrations 

in Las Vegas Bay and Boulder Basin (LaBounty and Burns, 

2007). Simultaneously, chlorophyll a concentrations have 

decreased in these areas. Following the 2001 bloom, peak 

concentrations of chlorophyll a in Las Vegas Bay typically 

have been less than 30 μg/L and, in Boulder Basin, less 

than 5 μg/L in most years. However, continued lowering of 

lake-surface elevations during drought conditions from 2000 

to 2011 resulted in the extension of Las Vegas Wash waters 

farther into Las Vegas Bay than would have been expected at 

higher lake levels. This condition resulted in an apparent shift 

of elevated chlorophyll a concentrations farther into Las Vegas 

Bay, although their position relative to the Las Vegas WashLas Vegas Bay interface remained fairly constant. 

Figure 4-15. Chlorophyll a concentrations in Lake Mead’s Boulder Basin since 2000, measured at sampling site 

CR346.4 (see fig. 4-1 for location map). Note the spike in values in 2001 during the green algal bloom. Note the break 

in chlorophyll a values on the y-axis. 
54 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Nutrients and Algae in Las Vegas Bay and 

Boulder Basin of Lake Mead 

By Wai Hing Wong and Douglas D. Drury

Within Lake Mead, the water within Las Vegas Bay 

has the highest concentration of nutrients. Both nitrogen 

and phosphorus are discharged in wastewater effluent 

from the Las Vegas metropolitan area by Clark County 

Water Reclamation District (CCWRD), and the Cities of 

Henderson, Las Vegas, and North Las Vegas. As a result, 

among all basins of Lake Mead, Las Vegas Bay has the 

highest production of algae (LaBounty and Burns, 2005). 

In Las Vegas Bay and Boulder Basin, other humancaused and natural changes also may contribute to 

chlorophyll concentrations. However, neither the severe 

drought that began in 2000 (Holdren and Turner, 2010) 

nor the 2007 quagga mussel (Dreissena rostriformis 

bugensis) invasion (Wong and others, in press) have had 

the impact that can be attributed to the anthropogenic 

nutrient loading into Las Vegas Wash from inflows of 

wastewater and occasional stormwater. Because of low 

phosphorus concentrations, lower daily mass loadings 

of phosphorus, and daily wastewater flows being 

discharged, wastewater can be described as a persistent 

contributor of the phosphorus needed to support algal 

growth. Because storms are intermittent events and 

stormwater has higher phosphorus concentrations 

and higher daily mass loadings, stormwater can be 

considered to be an acute contributor to algal growth. 

Chlorophyll a

Chlorophyll a water-quality standards for Las 

Vegas Bay were established in 1988. The wastewater 

dischargers have been collecting chlorophyll a data in 

Las Vegas Bay for more than 20 years (Clark County 

Water Reclamation District and others, 2012a, 2012b). 

Figure 4-16. Seasonal averages of chlorophyll a concentrations in Lake Mead. Sampling site LWLVB 1.85 

is 1.85 mi (3.0 km) from the Las Vegas Bay/Las Vegas Wash interface (see fig. 4-1 for location map). Both the 

winter season (November–February) and the permit compliance season (March–October) average 

chlorophyll a concentrations are shown.

0 

5 

10 

15 

20 

25 

30 

35 

40 

1/92 1/93 1/94 1/95 1/96 1/97 1/98 1/99 1/00 1/01 1/02 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 

Winter season, November–February 1992–2012

Permit compliance season, March–October 1992–2012 

Seasonal average chlorophyll a, in micrograms per liter (μg/L)

EXPLANATION

Sampling site LWLVB 1.85

Date

Average concentration of chlorophyll a, in μg/L
Chapter 4

Lake Water Quality 55

Chlorophyll a concentrations greater than 300 μg/L 

have been observed numerous times over the years 

in individual samples. The highest chlorophyll a

concentration in any sample collected after the 2005 

treatment plant optimizations was 26 μg/L in August 

2007; the average chlorophyll a concentrations in 

Las Vegas Bay have been decreasing in recent years 

(fig. 4-16).

Phosphorus

In 1978, the load of total phosphorus being 

discharged by the City of Las Vegas and CCWRD’s 

treatment plants was estimated to be greater than 

2,800 lb/d (1,270.0 kg/d). The minimum monthly 

phosphorus discharged over the last 50+ years was 

130 lb/d (59.0 kg/d) in December 1981, just after 

the City of Las Vegas and CCWRD started up their 

advanced phosphorus-removal wastewater-treatment 

plants. In March 1994, a Total Maximum Daily Load 

(TMDL) of 334 lb/d (151.5 kg/d) of total phosphorus 

was established for Lake Mead. The wastewater 

dischargers have been calculating monthly average 

data for total phosphorus loads from the effluent of the 

wastewater treatment plants for more than 20 years 

(Clark County Water Reclamation District and others, 

2012b; fig. 4-17). The phosphorus contribution from 

wastewater effluent has been steadily decreasing even 

though population has been increasing over this time.

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 

Dischargers effluent 

Seasonal average total phosphorus, in pounds per day (lb/d)

Year

EXPLANATION

Winter season, November–February 1992–2012

Permit compliance season, March–

October 1992–2011 

Average concentration of total phosphorus, in lb/d

Figure 4-17. Average total phosphorus load in effluent discharged by wastewater treatment facilities at 

Lake Mead. Both the winter season (November–February) and the permit-compliance season (March–

October) monthly phosphorus mass loading are plotted. 
0 

10 

20 

30 

40 

50 

60 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 

Chlorophyll-a concentration, in micrograms per liter 

Total phosphorus, in pounds per day 

Total phosphorus

Chlorophyll a

EXPLANATION

Year

56 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Algal Bloom of 2001

In 2001, a large green algae bloom 

occurred across the entire Boulder 

Basin. Chlorophyll a concentrations 

in excess of 300 μg/L were measured 

in individual samples. The 2001 algal 

bloom primarily was due to a large load 

of phosphorus that entered the surface 

of Boulder Basin and remained in the 

euphotic zone (LaBounty and Burns, 

2005). Algal blooms also were apparent 

downstream in Lake Havasu and as 

far as a reservoir in San Diego County, 

Calif. and in canals of the Metropolitan 

Water District of Southern California. A 

Lake Mead-focused Algae Task Force 

was formed and is ongoing; members 

of the task force include managers and 

scientists from the City of Henderson, 

City of Las Vegas, City of North Las 

Vegas, CCWRD, National Park Service, 

Nevada Department of Wildlife, Nevada 

Division of Environmental Protection, 

Southern Nevada Water Authority, and 

the University of Nevada, Las Vegas.

Wastewater Discharge Actions

Following the 2001 algal bloom, 

the dischargers voluntarily agreed 

to remove phosphorus year round. 

The City of Henderson and CCWRD 

were able to implement year-round 

phosphorus removal by November 

2001. The City of Las Vegas achieved 

year-round phosphorus removal 

in November 2002. By 2005, the 

dischargers had optimized their 

treatment plants to remove even more 

phosphorus. The decreased phosphorus 

loadings from the dischargers after 2001 

are readily apparent in figure 4-17. No 

significant algal blooms have occurred 

since 2001. 

Chlorophyll a concentrations were reduced in Las Vegas Bay 

after the dischargers reduced their phosphorus loadings in 2001 

(fig. 4-16). For example, the amount of phosphorus discharged 

to Las Vegas Wash before 2002 was about 400 lb/d (181.4 kg/d); 

this amount was reduced to about 200 lb/d (90.7 kg/d) after 2005. 

Accordingly, the annual chlorophyll a concentration at the Las 

Vegas Bay monitoring station LWLVB 1.85 has been reduced 

from 14 to 2.4 μg/L since 2005 (fig. 4-18). The lowest average 

annual phosphorus load discharged to Las Vegas Wash was about 

170 lb/d (77.1 kg/d) in 2010. The wastewater treatment efforts have 

significantly improved the water quality in the Boulder Basin of 

Lake Mead. 

The concentrations of phosphorus in effluent from wastewatertreatment facilities increased during the winter season in 2005–

2006 and 2008–2009 (fig. 4-17). These increases were due to the 

shutdown of nutrient removal processes at the City of Las Vegas 

for construction to ensure more reliable wintertime (and otherwise) 

phosphorus removal in the future. Phosphorus loading also 

increased starting in summer 2011 to early 2012, when the City of 

North Las Vegas wastewater-treatment plant came on line in June 

2011. The North Las Vegas plant experienced start-up difficulties 

that lasted until March 2012. Clearly, achieving low phosphorus 

concentrations remains a challenge for the wastewater dischargers.

Figure 4-18. Annual discharge of total phosphorus from Las 

Vegas Wash to Las Vegas Bay and chlorophyll a concentration 

near Las Vegas Wash from 1992 to 2011.
Chapter 4

Lake Water Quality 57

Organic and Inorganic Chemicals and 

Compounds in Water

By Michael R. Rosen

Organic Compounds in the Lake Mead 

Water Column Sampled by Passive 

Samplers 

Traditional (grab) sampling for analysis of 

organic compounds in water provides a snapshot 

of the compounds detectable at a given moment 

in time. However, most organic compounds are 

present at such low concentrations that they are 

not detected in samples collected by traditional 

sampling techniques, and analytical results do 

not represent the concentrations that aquatic 

organisms accumulate over time within their 

tissues. Semipermeable membrane devices 

(SPMDs) and polar organic chemical integrative 

samplers (POCIS) capture organic chemicals 

at low concentrations over a period of time 

(Alvarez and others, 2004; Huckins and others, 

2006). SPMDs are designed to collect organic 

compounds passively. The types of compounds 

that SPMDs collect have low solubility in 

water (hydrophobic) and are likely to attach 

to particles, sediment, and fat (lipids). In this 

way, an SPMD serves as a synthetic fish that 

is used to estimate the amount of chemicals 

that can accumulate in actual fish tissues over 

a period of time. Hydrophobic compounds 

sampled by SPMDs include organochlorine 

pesticides (dichlorodiphenyltrichloroethane, 

DDT; hexachlorocyclohexane, HCH; and their 

breakdown products), polychlorinated biphenyls 

(PCBs), and certain volatile organic compounds 

(VOCs). POCIS detect organic compounds 

that are likely to remain dissolved in water 

(hydrophilic) and that could be present in the 

blood stream or organs of animals. Hydrophilic 

compounds sampled by POCIS include 

pharmaceuticals, personal care products, and 

certain pesticides.

SPMD samplers have been deployed in 

Las Vegas Wash and Lake Mead at various 

times since 1995 (Bevans and others, 1996; 

Covay and Leiker, 1998; Goodbred and 

others, 2007; Leiker and others, 2009; Rosen 

and others, 2010), and POCIS samplers since 

2006 (Rosen and others, 2010). Compounds 

detected at low concentrations by these samplers 

include organochlorine pesticides (DDT and its 

breakdown products), personal care products 

(for example, triclosan, an antibacterial agent 

used in many soaps), tonalide and galaxolide 

(fragrances), caffeine, PCBs, and VOCs 

(Goodbred and others, 2007; Rosen and others, 

2010). The highest concentrations of most 

organic compounds have been detected in Las 

Vegas Wash and Las Vegas Bay, but compounds 

also have been detected in SPMD and POCIS 

samplers as far out in Boulder Basin as Sentinel 

Island (fig. 4-19). Organic compounds in 

these waters come primarily from Las Vegas 

Wash tributary inflow, a mixture of urban 

runoff, shallow groundwater flow, and treated 

wastewater discharge. 

Deployment of SPMD and POCIS containers in Las Vegas Bay for 

vertical organic contaminant profiling. Photograph by Michael R. 

Rosen, U.S. Geological Survey.
nvtac11-4177_fig04-19

115° 114°30' 114°

36°

36°30'

0 5 10 20 MILES

0 5 10 20 KILOMETERS

Las Vegas

City of Las Vegas 

wastewater treatment plants

Clark County 

wastewater 

treatment

plants

Henderson 

wastewater 

treatment plants

Basic management

incorporated industrial site

Lake Mead

National Recreation

Area

Boulder

Basin

HOOVER

DAM

Colorado

River

Virgin

Basin

Overton Arm

Colorado

River

Muddy

River

Virgi n River

NEVADA

ARIZONA

Base from National Park Service and U.S. Geological Survey digital data, 1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983

MEAD

LAKE

Overton

Las Vegas Wash

Outlet

5.24

19.57

445.18

25.42

64.24

3.21

0.6

2.10

19.82

0.24

0.26

0.22

1.45

0.26

Gregg Basin

Sentinel

Island

Nevada

Arizona

Study Area

0.26

0.22

EXPLANATION

Semipermeable membrane

Polar organic chemical 

integrated samplers

(micrograms per POCIS)

58 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Figure 4-19. Distribution of the total concentration of organic compounds in Lake Mead sampled by semipermeable 

membrane devices (SPMDs; top value) and polar organic chemical integrative samplers (POCIS; bottom value). SPMD 

concentrations are shown in nanograms per liter (ng/L); POCIS concentrations are in micrograms (μg) per POCIS sampler. The 

estimated concentration of 50 μg per POCIS for tris (2-butoxyethyl) phosphate was not included in the total for the Las Vegas 

Valley Drive (LVVD) site. Boulder Basin (BB); Las Vegas Bay (LVB); Overton Arm (OA); Willow Beach (WB). Modified from Rosen 

and others (2010).
20

18

16

14

12

10

8

6

4

2

0

10 20 40 60 80 100

A.

0 0.1 0.2 0.3 0.4

B.

Minimum reporting level

Galaxolide, in nanograms per liter Pyrene, in nanograms per liter

Depth below lake surface, in meters

LVB-1

LVB-2

EXPLANATION

Chapter 4

Lake Water Quality 59

The organochlorine pesticide DDT has 

been banned in the United States since 1972. In 

southern Nevada, DDT was manufactured at a 

plant near Las Vegas Wash and, after the plant 

closed in the early 1970s, the disposal methods 

used for this product led to its transport into Las 

Vegas Wash by stormwater runoff (http://ndep.

nv.gov/bmi/index.htm). As a result, DDT has 

been routinely detected in Las Vegas Wash and 

Las Vegas Bay, although its concentrations, based 

on sediment analyses, have been decreasing 

primarily due to post-production clean-up efforts 

(see section, “Organic and Inorganic Chemicals 

and Compounds in Sediment”). The breakdown 

products of DDT do not completely degrade, 

however, and may persist in the environment for 

many years.

In Las Vegas Bay, sets of passive samplers

have been deployed in a vertical series to 

determine whether chemicals in Las Vegas 

Wash water enter Lake Mead near the surface or 

near the bottom of the lake. In March 2006, the 

resulting vertical chemical profile (fig. 4-20A) 

showed that water with higher concentrations 

of organic compounds from Las Vegas Wash 

entered along the bottom of the lake as underflow 

(Rosen and others, 2010). Entry of the Las 

Vegas Wash plume as underflow positions it to 

more directly affect bottom-feeding fish and 

bottom-dwelling organisms. Other chemicals, 

such as volatile organic compounds (VOCs) that 

are byproducts of gasoline combustion, were 

more highly concentrated near the surface of the 

lake (fig. 4-20B). This finding suggests that the 

source of VOCs occurred at the lake surface (a 

consequence of motorized boat traffic) rather 

than in Las Vegas Wash.

Figure 4-20. Galaxolide (A), a fragrance found in many personal care products, is present 

at elevated concentrations at the bottom of a vertical profile obtained in Las Vegas Bay, 

indicating that the source of organic compounds is Las Vegas Wash water entering the 

lake as underflow at this time of year. Pyrene (B), formed from the incomplete burning of 

gasoline, shows higher concentrations at the top of the profile, indicating that the source 

of volatile organic compounds is at the surface of the lake.
60 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Volatile Organic Compounds Related to Boating

 From 2004 through 2006, SMPDs were used to collect 

samples to investigate the distribution of gasoline-derived 

VOCs in Lakes Mead and Mohave (Lico and Johnson, 2007). 

Most of these compounds are toxic to varying degrees. 

Because of this toxicity, they also are a potential source of 

environmental pollution and pose a health hazard. VOCs 

known as BTEX compounds (benzene, toluene, ethylbenzene, 

and xylenes) and PAH compounds (polycyclic aromatic 

hydrocarbon compounds), which are produced during 

combustion of gasoline, were detected at every site sampled. 

During this period, the concentrations of BTEX and PAH 

compounds increased as the boating season progressed and 

decreased to less-than-detectable levels during the winter, 

when few boats were on the water. Moreover, concentrations 

of boat-related organic compounds were highest at sampling 

points near marinas or popular launching areas. These findings 

indicate that motorized watercraft are the major source of 

BTEX and PAH organic compounds to the lakes. The gasoline 

additive methyl tert-butyl ether (MTBE) also was detected 

during the 2004 sampling, but concentrations decreased to less 

than the detection level during the latter part of the study, most 

likely due to the removal of MTBE from gasoline purchased 

in California. 

Studies by Lico and Johnson (2007) at Lakes Mead and 

Mohave, and by Lico (2004) at Lake Tahoe (Calif. and Nev.), 

showed that two-stroke gasoline engines can release up to 

40 percent of their fuel into water bodies and are a major 

source of gasoline-derived organic compounds. In response to 

these and other studies, the National Park Service is phasing 

out the use of two-stroke engines that do not use direct 

injection within LMNRA by 2013 (http://www.nps.gov/lake/

parkmgmt/twostroke.htm). 

Inorganic Compounds in Water

Inorganic chemicals and compounds are substances 

that do not contain carbon-hydrogen bonds as a fundamental 

component of their molecule. Inorganics occur naturally in 

the environment or can be manufactured. Many are soluble 

in water and have the potential to trigger health concerns if 

the USEPA standard is exceeded. Because of this, they are 

potential drinking water contaminants. This subsection focuses 

on inorganic perchlorate, selenium, and metals. 

Perchlorate

Derived from inorganic salts, perchlorate (ClO4) is a 

naturally occurring and manufactured oxidizer that has been 

used (primarily as ammonium perchlorate) as a component in 

fireworks and solid rocket fuel. Perchlorate has been produced 

in Las Vegas Valley since the early 1950s at industrial plants 

near and upgradient of Las Vegas Wash. Leakage and transport 

of perchlorate to Las Vegas Wash through shallow aquifers has 

occurred from two manufacturing plants that were operated 

until the late 1990s (Urbansky, 1998; Boralessa, 2001; Sellars 

and others, 2007), and efforts to remove the perchlorate 

from groundwater and surface water have been underway 

for many years. Because exposure to perchlorate may create 

adverse health effects by disrupting the ability of the thyroid 

gland to produce hormones needed for normal growth and 

development (see Chapter 6), the USEPA recently (2011) 

determined that perchlorate meets the criteria for regulation 

under the Safe Drinking Water Act. Formal regulation of 

perchlorate by USEPA initiates a process to develop and 

establish a national primary drinking water regulation, which 

is a legally enforceable standard that applies to public drinking 

water supplies (U.S. Environmental Protection Agency, 2011).

Perchlorate concentrations in Lake Mead generally 

have been stable since 2005 with values of 0–3 μg/L for 

open water areas, where they are influenced by seasonal 

stratification of the water column. As a result, perchlorate 

tends to be concentrated within the uppermost layers of the 

epilimnion, reaching values of approximately 3 μg/L, whereas 

concentrations in the deep layers of the hypolimnion remain 

lower. When the water column mixes during the winter and 

autumn, perchlorate concentrations equalize throughout 

the water column at 0–3 μg/L. In Las Vegas Wash, at sites 

nearest Las Vegas Bay, however, perchlorate concentrations 

historically have been higher than in the remainder of the lake, 

decreasing from approximately 300 μg/L in 2003 to nearly 

90 μg/L in 2007 (Ryan, 2008).

Selenium

Selenium is a naturally occurring metalloid that is 

found globally in organic-rich marine sedimentary shale, 

including many geologic formations in the Western 

United States and commonly in southern Nevada. At low 

concentrations, selenium is an essential element for the health 

of animals (including humans) and some plants; at elevated 

concentrations, however, it is toxic and the threshold between 

providing a benefit and toxicity is narrow (Brown and Shrift, 

1982; National Research Council, 2005). Furthermore, 

in the aquatic environment, selenium has the potential to 

bioaccumulate in zooplankton and benthic invertebrates, 

biomagnifying as it reaches top-level predators (Presser and 

Luoma, 2010, and references therein; Chapter 6). In so doing, 

selenium has been found to negatively affect the reproductive 

health of aquatic biota and to cause deformities in birds (Seiler 

and others, 2003). Hamilton and others (2002) summarized the 

risks posed by selenium in the broader Colorado River Basin 
Chapter 4

Lake Water Quality 61

to the endangered razorback sucker (Xyrauchen texanus), 

reporting that selenium in sediment, water, and biota may 

adversely affect reproduction of this species.

Low selenium concentrations in Lake Mead itself 

resulted in a focus on concentrations and loading of 

the element in Las Vegas Wash. Monitoring selenium 

concentrations in Las Vegas Wash is important because 

soils in upgradient areas in Las Vegas Valley are known to 

contain selenium concentrations, which may leach into the 

shallow groundwater that enters the Wash and flows into 

Lake Mead (Zhou and others, 2004). Typical concentrations 

of selenium in the upper Las Vegas Wash exceed the USEPA 

criterion for protection of wildlife of 5.0 μg/L; however, 

the increased water volumes provided by the wastewater 

reclamation plants along Las Vegas Wash dilute these 

concentrations. Ryan and Zhou (2010) reported average 

selenium concentrations in the lower Las Vegas Wash area 

near the historical confluence with Lake Mead at Las Vegas 

Bay to be 3.3 μg/L, which results in an annual loading to the 

reservoir of 1,890 lbs (857.3 kg) of selenium. These values 

were in general agreement with those reported by Zhou 

and others (2004) when average selenium concentrations 

near the Las Vegas Wash confluence with the lake were 

2.85 μg/L and the reported annual load was 1,426 lbs 

(646.8 kg) of selenium.

Metals

Because elevated levels of metals can cause serious 

health problems, acceptable concentrations of metals in 

drinking water are strictly limited by State and Federal law. 

Sampling for metals in the water of Lake Mead has been 

limited, with the greatest emphasis being associated with 

sites within Las Vegas Wash (table 4-1) and the location of 

the drinking water intakes of the Southern Nevada Water 

Authority (SNWA) (table 4-2). 

Water quality test results for the Southern Nevada 

Water System (2010; table 4-1) are taken from SNWA 2010 

Water Quality Report. This report is required by the Safe 

Drinking Water Act and provides water customers with 

water-quality information about their drinking water. The 

report, which is updated each year, compares water test 

results to drinking-water standards and, in the 2010 Water 

Quality Report, all values were well below the Maximum 

Contaminant Level established by USEPA.

Table 4-1. Concentrations of metals in water in Las Vegas Wash. 

[Average (2003–07) metal concentrations in Las Vegas Wash (LW0.8) near the 

interface with Lake Mead compared to U.S. Environmental Protection Agency 

(USEPA) maximum contaminant levels for drinking water. Source for USEPA 

maximum contaminant level is http://water.epa.gov/drink/contaminants/index.

cfm. Abbreviations: μg/L, micrograms per liter]

Metal Concentration 

(μg/L)

USEPA maximum

contaminant level

(μg/L)

Aluminum1 304 50–200

Arsenic 9.0 10 

Barium 63 2,000

Chromium 1.2 100

Copper 4.5 1,300

Iron1 310 300

Manganese1 52 50

Lead 0.92 15

Nickel 8.7 None

Selenium 3.3 50

Zinc1 36 5,000

1

Secondary standard.

Table 4-2. Concentrations of metals in water at the Southern 

Nevada Water Authority’s drinking-water intakes in Lake Mead. 

[Average 2009 metal concentrations from the Southern Nevada Water 

Authority’s Water Quality Report for entry point (Lake Mead drinking water 

intakes) monitoring compared to U.S. Environmental Protection Agency 

(USEPA) maximum contaminant levels for drinking water. Source for USEPA 

maximum contaminant level, in micrograms per liter, is http://water.epa.gov/

drink/contaminants/index.cfm. Abbreviations: μg/L, micrograms per liter]

Metal Concentration 

(μg/L)

USEPA maximum 

contaminant level

(μg/L)

Arsenic 1.7 10

Barium 100 2,000

Selenium 2.0 50

Uranium 4.6 30
62 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Las Vegas Wash data were collected for 

the Las Vegas Wash Coordination Committee 

to support their efforts to monitor and improve 

water-quality conditions in the Wash. As 

mentioned previously, the water in Las Vegas 

Wash is a mixture of urban runoff, shallow 

groundwater discharge, stormwater, and 

reclaimed wastewater effluent. It is expected that 

the concentrations of metals in Lake Mead would 

be significantly lower following dilution of Las 

Vegas Wash water with the much larger volume 

of lake water. In Las Vegas Wash, the average 

concentrations for metals generally were below 

the USEPA’s Criteria Maximum Concentration, 

Criterion Continuous Concentration, or Criteria 

Recommendation for Priority Pollutants for the 

protection of aquatic organisms (for details, 

see http://water.epa.gov/scitech/swguidance/

standards/current/index.cfm).

Historically, the risk of impact to Lake 

Mead by metals was considered to be low due to 

their generally low concentrations in Las Vegas 

Wash, the only significant tributary subject to 

urban and industrial influences. However, future 

investigations of concentrations in sediment 

entering the lake would be warranted to establish 

and expand baseline conditions to better evaluate 

future conditions.

Contaminants of Emerging Concern

By Michael R. Rosen and Brett Vanderford 

Human-related sources of contamination 

that can affect the quality of water resources 

and aquatic ecosystems also are monitored and 

studied. Contaminants of emerging concern 

(CECs) include pharmaceuticals, personal care 

products, plasticizers, and other compounds 

disposed of into the environment by households 

and industries. Although these compounds 

commonly are removed by conventional 

wastewater-treatment processes to levels below 

detection, CECs typically are not completely 

removed prior to effluent discharge. Several 

studies have documented low levels of CECs 

in Lake Mead, typically in the nanograms 

per liter (part-per-trillion) range (Chapter 6; 

Daughton and Ternes, 1999; Osemwengie and 

Gerstenberger, 2004; Goodbred and others, 2007; 

Leiker and others, 2009; Benotti and others, 

2010; Rosen and others, 2010). Sparse data exist 

on the long-term bioaccumulation of CECs in 

Lake Mead and other reservoirs throughout the 

world and, as a result, the potential ecosystem 

and human health effects of these compounds at 

low concentration remain largely unknown. 

Studies on the fate and transport of CECs 

at Lake Mead began in 1996 when Bevans and 

others (1996) published the first report of the 

occurrence of endocrine disruption in common 

carp (Cyprinus carpio). Carp collected from Las 

Vegas Wash, the primary source of CECs from 

treated wastewater discharge, and Las Vegas Bay 

were found to have significantly different levels 

of plasma steroid hormone and vitellogenin 

compared to carp collected from a reference site 

upstream in Callville Bay (fig. 1-1); however, the 

cause of the endocrine disruption was unknown. 

Subsequent work used toxicity identification and 

evaluation methodology to screen Lake Mead for 

estrogenic compounds. Snyder and others (2001) 

provided a link between endocrine disruption 

and natural and synthetic hormones present 

in Las Vegas Wash, and their work along with 

other research, served as an impetus for studies 

to better understand possible impacts of CECs 

on aquatic organisms, such as the endangered 

razorback sucker, in Lake Mead. 

Organic and Inorganic Chemicals and 

Compounds in Sediment

By Michael R. Rosen

Sediment-core analyses provide information 

about how chemical inputs to a lake have 

changed with time. In 1998, multiple sediment 

cores were collected from several Lake Mead 

locations (Las Vegas Bay, Boulder and Virgin 

Basins, and Overton Arm) to determine spatial 

differences in chemical inputs and temporal 

changes in selected constituents starting from 

the completion of the Hoover Dam in the mid1930s (Covay and Beck, 2001; Rosen and Van 

Metre, 2010). Results of this study were then 

compared to those of similar studies for other 

lakes in the United States that also are influenced 

by urban watersheds; these comparisons indicate 

that concentrations of inorganic and organic 

chemicals are relatively low in Lake Mead. 

Chemical concentrations vary among basins, 

however, and even within each basin no single 
nvtac11-4177_fig04-21

1930

1940

1950

1960

1970

1980

1990

2000

0 10 20 30 40 50 60

Dichlorodiphenyldichloroethylene (DDE), in micrograms per kilogram

Calendar years

Construction of reservoir

Discontinued use of most dichlorodiphenyltrichloroethane in the U.S.

LVB Shallow

LVB Deep

Overton

Virgin Basin

EXPLANATION

Collecting a sediment core from Las Vegas Bay. Photograph by Michael R. Rosen, U.S. 

Geological Survey.

Chapter 4

Lake Water Quality 63

value can be assumed to typify conditions within the lake 

(Rosen and Van Metre, 2010). For example, concentrations of 

DDE (a breakdown product of DDT) were higher in Las Vegas 

Bay cores, particularly in the 1980s, when DDT was produced 

at a facility located along Las Vegas Wash, but concentrations 

decreased after cleanup efforts were implemented (fig. 4-21). 

Mercury also was present in sediment cores from Lake Mead; 

mercury concentrations were twice as high in the Virgin 

Basin core as in the Las Vegas Bay cores (but these are still 

relatively low concentrations compared to those in other urban 

lakes). Relative to other Lake Mead basins tested, Overton 

Arm cores had the lowest concentrations of most chemicals, 

except organic carbon.

In the Las Vegas Bay core, concentrations of the organic 

compound tetrachlorodibenzo-p-dioxin (TCDD) have steadily 

increased since the completion of Hoover Dam and appear 

to correlate with population growth in Clark County, which 

includes the Las Vegas metropolitan area (fig. 4-22). The cause 

of this steady increase in TCDD has not been determined; 

however, Rosen and Van Metre (2010) attributed the increase 

to the effects of increased urbanization and an increase in 

the area of impervious surfaces that may have allowed more 

TCDD to flow into Las Vegas Wash. TCDD is produced 

by different types of waste-burning incinerators, including 

backyard burn-barrels, and is very toxic; however, waste 

incineration by methods other than burn-barrels has declined 

in the United States since the 1980s. TCDD also may be a 

breakdown product of triclosan, an antibacterial agent used 

in deodorants, toothpastes, and other personal-care products 

(Buth and others, 2009), and therefore a likely cause for 

increased concentrations of TCDD over time consistent with 

increased use of triclosan-containing products by a growing 

population.

Elements such as manganese and lead in sediment 

cores also showed differing spatial and temporal changes 

in concentrations across Lake Mead. Manganese and lead 

concentrations were higher in Las Vegas Bay than in other 

portions of Boulder Basin, the Overton Arm, and Virgin 

Basin, particularly during the 1960s and early 1970s, but for 

different reasons. Manganese concentrations were high due to 

erosion of waste sediment from a manganese mine near Las 

Vegas Wash. The mine closed in the 1960s, but erosion of the 

sediments occurred until the 1980s when sediment-control 

structures were constructed to reduced sediment erosion into 

Las Vegas Wash (Rosen and Van Metre, 2010). 

Figure 4-21. Concentrations of DDE (a breakdown 

product of DDT) in sediment cores from Lake Mead’s 

Las Vegas Bay (LVB), Virgin Basin, and Overton 

Arm. DDE was not detected in Virgin Basin and 

Overton Arm cores indicating Las Vegas Wash was 

the source. Concentrations peaked in the early 

1980s when DDT was entering the Wash from waste 

ponds near the Wash, but after cleanup of the site 

began, concentrations decreased. Modified from 

Rosen and Van Metre (2010).
nvtac11-4177_fig04-22

1930

1940

1950

1960

1970

1980

1990

2000

0 10 20 30 40 50

Total tetrachlorodibenzo-p-dioxins, in picogram per gram

Calendar years

Construction of reservoir

0 500 1,000 1,500 2,000

People, in thousands

LVB shallow

LVB-deep

Clark County population

EXPLANATION

<0.82

64 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Lead concentrations were elevated in Las Vegas Bay 

due to the addition of lead to gasoline up until 1973; after this 

time, the addition of lead was phased out of gasoline until its 

use was banned in 1996 (http://www.epa.gov/history/topics/

lead/02.htm). Sediment cores show high concentrations of 

lead only in Las Vegas Bay sediments because motorized 

traffic is sparse in areas outside of the Las Vegas Bay area 

and near tributaries to other basins in Lake Mead. Peak lead 

concentrations in Las Vegas Bay sediments correlates with the 

greatest use of lead additives in gasoline (fig. 4-23), which has 

decreased since the 1970s. Many other trace elements showed 

decreasing concentrations in the Virgin Basin core after the 

1960s. This may have been a consequence of sediment being 

trapped behind Glen Canyon Dam at Lake Powell (Rosen and 

Van Metre, 2010), which began filling in 1963. New sediment 

cores were taken in Las Vegas Bay in 2007 to assess whether 

increases or decreases in some compounds are occurring 

and whether new compounds can be detected in the cores. In 

addition, the cores are being analyzed to see if concentrations 

of those compounds are lower in the outer part of the bay than 

in the area closest to Las Vegas Wash. 

Figure 4-22. Concentrations of total tetrachlorodibenzop-dioxin (TCDD) in sediments of Las Vegas Bay (LVB), 

Lake Mead. TCDD concentrations in Las Vegas Bay 

sediments appear to correlate with population growth in 

Clark County. Concentrations of TCDD in a core taken in 

Virgin Basin sediments were less than the detection limit 

for the length of the core.

nvtac11-4177_fig04-23

ND

1930

1940

1950

1960

1970

1980

1990

2000

0 20 40 60 80 100 120

Calendar years

Construction of reservoir

LVB Shallow

Overton Arm

Virgin Basin

EXPLANATION

Lead, in parts per million

Figure 4-23. Lead concentrations in sediment cores from 

Las Vegas Bay (LVB), Lake Mead. Lead concentrations 

were elevated when lead was used as an additive in 

gasoline. Concentrations decreased after the use of lead 

in gasoline was phased out beginning in 1973. Virgin 

Basin and Overton Arm cores do not show this increase 

because urban areas were too far away. ND = no data.
Chapter 4

Lake Water Quality 65

Effects of Invasive Species on 

Water Quality

Invasive species can have significant effects on water 

quality; and changes in water quality caused by invasive 

species can encourage the growth of other undesirable species 

(Hecky and others, 2004). One example of this relationship 

involves quagga mussels (Chapter 6), an invasive species 

that is now thriving in Lake Mead. Large numbers of quagga 

mussels in a lake can rapidly consume an extraordinary 

amount of algal cells in the water column while releasing 

nitrogen and phosphorus back to the water as waste. Under 

the right conditions, the remaining algae could use these 

nutrients to create an unsightly algal bloom that can cause 

taste and odor problems. Quagga mussels potentially can 

enhance the level of microcystin toxin in a lake because they 

do not consume many cyanobacteria, leaving this group to 

proliferate. Under the right conditions and in other water 

bodies, dreissenid mussels appear to have enhanced the 

growth of cyanobacteria and their toxin producers (Knoll 

and others, 2008). These problems have not occurred in Lake 

Mead, and are not expected, but have occurred elsewhere 

(Hecky and others, 2004) and therefore the possibility cannot 

be eliminated.

Quagga mussels also represent a future potential threat to 

DO concentrations. If Lake Mead’s quagga mussel population 

increases, oxygen demand at the bottom of the lake also 

will increase; in time, oxygen deficiencies could become 

more common in deep, poorly mixed locations. (In shallow 

regions of the lake where active mixing infuses oxygen, 

such depletions would not present a significant problem.) 

Areas of low oxygen caused by quagga mussels could 

compound the quagga mussel problem. At sufficiently low DO 

concentrations, quagga mussels and other organisms will die, 

and bacterial decomposition of dead individuals would further 

reduce oxygen concentrations. Once lake-floor sediments are 

covered by oxygen-free (anoxic) water, phosphorus stored in 

sediment can be released back to the water column (Böstrom 

and others, 1982; Nürnberg, 1988). Following seasonal 

mixing, this newly available phosphorus source could then 

stimulate algal production, reinforcing the entire cycle. At 

the time this report was published (2012), the quagga mussel 

population was not large enough in Lake Mead to affect the 

ecosystem so broadly.

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from Las Vegas Bay and semipermeable membrane devices 

from Las Vegas Wash, Nevada: Science of the Total 

Environment, v. 407, p. 2,102–2,114.

Lico, M.S., 2004, Gasoline-related organics in Lake Tahoe 

before and after prohibition of carbureted two-stroke 

engines: Lake and Reservoir Management, v. 20, 

p. 164–174.
Chapter 4

Lake Water Quality 67

Lico, M.S., and Johnson, B.T., 2007, Gasoline-related 

compounds in Lakes Mead and Mohave, Nevada, 2004–06: 

U.S. Geological Survey Scientific Investigations Report 

2007–5144, 38 p. (Also available at http://pubs.er.usgs.gov/

publication/sir20075144.)

National Research Council Committee on Minerals and Toxic 

Substances in Diets and Water for Animals, 2005, Mineral 

tolerance of animals (2d revised ed.): National Academies 

Press, Washington, D.C., 496 p.

Nürnberg, G.K., 1988, The prediction of phosphorus release 

rates from total and reductant-soluble phosphorus in anoxic 

lake sediments: Canadian Journal of Fisheries Aquatic 

Science, v. 45, p. 453–462.

Osemwengie, L.I., and Gerstenberger, S.L. 2004, Levels of 

synthetic musk compounds in municipal wastewater for 

potential estimation of biota exposure in receiving waters: 

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Paulson L.J., and Baker, J.R., 1983, The limnology in 

reservoirs on the Colorado River: University of Nevada, 

Las Vegas, Lake Mead Limnological Research Center, Las 

Vegas, 269 p.

Presser, T.S., and Luoma, S.N., 2010, A methodology 

for ecosystem-scale modeling of selenium: Integrated 

Environmental Assessment and Management, v. 6, no. 4, 

p. 685–710.

Roefer, P., Zikmund, K., Maher, T., and LaBounty, J., 2005, 

Effects of the drought on water quality in Lake Mead: 

Southwest Hydrology, v. 4, p. 26–27.

Rosen, M.R., Alvarez, D.A., Goodbred, S.L., Leiker, 

T.J., and Patiño, R., 2010, Sources and distribution of 

organic compounds using passive samplers in Lake Mead 

National Recreation Area, Nevada and Arizona, and their 

implications for potential effects on aquatic biota: Journal of 

Environmental Quality, v. 39, p. 1,161–1,172.

Rosen, M.R., and Van Metre, P.C., 2010, Assessment of 

multiple sources of anthropogenic and natural chemical 

inputs to a morphologically complex basin, Lake Mead, 

USA: Palaeogeography, Palaeoclimatology, Palaeoecology, 

v. 294, p. 30–43.

Ryan, R., 2008, Las Vegas Wash mainstream water quality 

report, 2003–2007: Las Vegas, Nev., prepared for Las Vegas 

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org/assets/pdf/resources_wqresearch_mainstreamfinal.pdf.

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Pugh, B., and Robb, J., 2007, Perchlorate—Environmental 

problems and solutions: Boca Raton, Fla., CRC Press, 

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68 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

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bugensis) on chlorophyll and water clarity in Lakes Mead 

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and Schloesser, D.W., eds., Quagga and zebra mussels—

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Exposition, Austin, Tex., January 11–14, 2004, p. 1–13.
Chapter

Wildlife and 

Biological Resources5

By Sudeep Chandra1, Scott R. Abella2, 

Brandon A. Albrecht3

, Joseph G. Barnes2, 

E. Cayenne Engel2

, Steven L. Goodbred4, 

Paul B. Holden3

, Ron B. Kegerries3, 

Jef R. Jaeger2

, Erik Orsak5, Michael R. Rosen4, 

Jon Sjöberg6

, and Wai Hing Wong2

Lake Mohave near Willow Beach. Photography by Michael R. Rosen, U.S. Geological Survey.

Chapter 5

Wildlife and Biological Resources 69

1

 University of Nevada, Reno

2

 University of Nevada, Las Vegas

3

 BIO-WEST, Inc.

4

 U.S. Geological Survey

5

 U.S. Fish and Wildlife Service

6

 Nevada Department of Wildlife

The creation of Lakes Mead and Mohave drastically changed habitats originally found along their 

region of the historical Colorado River. While still continuing to provide habitat conditions that 

support a rich diversity of species within the water, along shorelines, and in adjacent drainage areas, 

the reservoirs contain organisms that are both native and non-native to the Colorado River drainage 

(fig. 5-1). The diversity of species within these lakes continues to change with time due to changing 

habitat conditions, the invasion of non-native species, and extirpations of native species. From the 

bottom of the food web to the top predators, all organisms within the ecosystem are interconnected in 

food webs or food-chain networks. As non-native invasive species continue to be introduced into the 

lakes, alterations to the food web, species competition, and species predation likely will continue to 

change the ecosystem and populations of native organisms. Following an overview of the food web, 

this chapter summarizes information on aquatic and aquatic-dependent wildlife at Lakes Mead and 

Mohave and their relationships within the food web from members of lower trophic levels to the highest: 

phytoplankton, invertebrates, including zooplankton, and macroinvertebrates; fishes; and birds. 

The following sections describe the biological diversity, limiting factors, and ecological functions of 

these groups in Lake Mead, and to a lesser extent, in Lake Mohave. 
Figure 5-1. Conceptual illustration of the Lake Mead food web. 

70 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Overview of the Food Web

By Sudeep Chandra

Understanding a lake’s food web—or how each member 

gets its energy in the form of food—is an important aspect 

of ecological research. This knowledge can help clarify 

population dynamics and how nutrients are recycled (and 

contaminants accumulate) within an ecosystem. The term 

“food-web coupling” describes energy or food movement 

from one habitat, such as open water, to another, such as 

lake bottom, which occurs frequently in freshwater systems 

where nutrient fluxes are common. Many species live and 

interact within the Lake Mead system (fig. 5-1). Aquatic 

macrophytes and algae, which produce food from the sun’s 

energy, are at the base of the Lake Mead food web. Algae 

can be found in two habitats—at the bottom of the lake 

(benthic zone) and in open water. Plants and algae living in 

the benthic environment support invertebrates, amphibians, 

and bottom-feeding fishes, such as common carp (Cyprinus 

carpio) and the endangered razorback sucker (Xyrauchen 

texanus). Some benthic-supported organisms (for example, 

crayfish or razorback sucker) are omnivorous; they eat a 

variety of smaller benthic invertebrates in addition to plants 

and benthic algae. In the open water, phytoplankton support 

a variety of microorganisms, including bacteria, protists, and 

tiny invertebrate animals known as zooplankton. Zooplankton 

are crucial in supplying nutrients to juvenile and smaller fish 

species. Lake Mead’s bass fishery, in turn, is largely supported 

by small fish species, such as shad (Dorosoma sp.) and 

bluegill (Lepomis macrochirus). Omnivorous aquatic birds 

such as ducks eat aquatic plants and invertebrates. Grebes feed 

on both fish and large benthic invertebrates, and piscivorous 

predatory birds such as bald eagle (Haliaeetus leucocephalus) 

eat shad, bass, and carp. 

Within the food web, the strength of relationships can 

increase or decrease over time depending on the nutrients 

delivered to the lake as well as changes in the population 

structure of community members. The introduction of invasive 

species such as quagga mussel (Dreissena rostriformis 

bugensis), which feed on plankton and concentrate nutrients in 

the benthic environment, can drastically alter the connections 

and coupling across lake habitats, resulting in unpredictable 

disruptions to the food web and lake fishery. 

bald eagle, osprey, peregrine falcon

amphibians

crayfish

razorback sucker

mayflies, chironomids

New Zealand mudsnail

quagga mussel, Asian clam

aquatic plants

and algae

avocets, grebes, coots

bass

largemouth and striped

ZOOPLANKTON

copepods

cladocerans

ostracods

shad

threadfin and gizzard

BENTHIC INVERTEBRATES

common carp

lake bottom

lake surface

phytoplankton

protists

bacteria

bluegill

AQUATIC BIRDS

AQUATICALLY ASSOCIATED RAPTORS
Coots (Fulica americana) swimming through a green algal bloom in 

Boulder Basin in March 2001. Photograph by National Park Service.

Chapter 5

Wildlife and Biological Resources 71

Algae

By Wai Hing Wong and Michael R. Rosen

Algae are microscopic plants that form the base of the 

aquatic food web. Free-floating algae in the water column 

known as phytoplankton are an important source of food 

(energy) for zooplankton (microscopic, invertebrate animals 

that float in the water), some fish, and aquatic birds in the 

Lake Mead ecosystem. Similar to plants on land, most algae 

utilize the sun’s energy to grow through a process called 

photosynthesis. Phytoplankton can be categorized into 

different groups such as green algae, cyanobacteria that also 

are known as blue-green algae, diatoms, golden-brown algae, 

and dinoflagellates.

The population of phytoplankton in a lake is controlled 

by many environmental and chemical factors. Besides light 

and carbon dioxide used during photosynthesis, phytoplankton 

also require nutrients such as nitrogen and phosphorus to 

grow. Because nitrogen and phosphorus must be present at a 

certain ratio (the Redfield ratio; Redfield, 1934) for optimal 

algae growth, if either or both of these nutrients are absent 

or in low supply, algal growth will be limited. Cold winter 

temperatures also can inhibit algal growth. Reductions in 

algae populations can limit zooplankton growth and, in turn, 

reduce subsequent food availability for fish. A “phosphoruslimited” lake is one in which little phosphorus but abundant 

nitrogen are present in the water. Lake Mead generally is 

phosphorus-limited (Paulson and Baker, 1983; LaBounty and 

Horn, 1997; Du, 2002; LaBounty and Burns, 2005; LaBounty, 

2008; and Holdren and Turner, 2010). Thus, when excess 

phosphorus is transported to the lake from the Colorado River 

or other tributaries, becomes concentrated in an area through 

the excretions of organisms, or is added to the lake from 

wastewater (effluent), algae can grow rapidly. With enough 

nutrients and light, algae can grow quickly within a short 

period of time, resulting in a bloom that changes the color of a 

lake from blue to the bright green of photosynthesizing algae. 

In 2001, conditions at Lake Mead’s Boulder Basin 

resulted in a large algal bloom (LaBounty and Burns, 2005; 

Chapter 4) of green algae (Pyramichlamys dissecta). The 

bloom started in March and persisted through September; it 

was visible throughout Boulder Basin, with concentrations 

of algae (measured as chlorophyll a) peaking at less than 

200 mg/m3 in the middle and outer basins (LaBounty and 

Burns, 2005). The bloom formed a surface scum and unsightly 

conditions that affected recreation, visual enjoyment of the 

lake, and reduced light penetration into the lake. When algae 

die, microbes decompose them in a process that consumes 

dissolved oxygen; a large algal die-off following a bloom, 

therefore, can result in large increases in decomposing 

microbes and a further reduction of dissolved oxygen 

content of the water. The resulting low oxygen levels can 

kill invertebrate and fish species; however, the 2001 Lake 

Mead algal bloom did not cause fish kills. Nonetheless, in 

response to this significant algal bloom, the wastewatertreatment facilities that discharge into Las Vegas Wash 

enhanced their phosphorus removal, thereby greatly reducing 

its concentrations in the lake and the potential for a repeat 

bloom on the scale of the 2001 event (see Chapter 4 for 

information about the relationship between nutrients and algal 

productivity). 

Photomicrograph of green algae (Pyramichlamys sp.); 640x 

magnification. Photograph by Ann St. Amand, Ph.D., PhycoTech, Inc.

Photomicrograph of a diatom (Asterionella formosa); 400x 

magnification. Photograph by Ann St. Amand, Ph.D., PhycoTech, Inc. 
Photomicrograph of blue-green algae (Microcystis aeruginosa); 200x 

magnification. Photograph by Ann St. Amand, Ph.D., PhycoTech, Inc. 

Photomicrograph of Daphnia pulex, a zooplankton of the order 

Cladocera. 35x magnification; 500-micrometer scale bar. Photograph 

by Ted Rosati and John Beaver, BSA Environmental Services.

72 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Smaller algal blooms also have occurred in 

marinas and bays of Lakes Mead and Mohave 

depending on nutrient and temperature conditions 

in those environments. A brief bloom of small 

blue-green algae (cyanobacteria) occurred in the 

middle of Las Vegas Bay in 2003 (LaBounty 

and Burns, 2005). Of all areas in Lake Mead, 

Las Vegas Bay has the highest potential for 

algal growth due to the supply of nutrients from 

wastewater outflow and stormwater runoff 

from the Las Vegas metropolitan area into Las 

Vegas Wash and Lake Mead. Across Boulder 

Basin, populations of different algal groups 

generally peak throughout the warmer months 

of the year: green algae in May, diatoms in June, 

golden-brown algae in July, dinoflagellates in 

August, and blue-green algae from October to 

November (LaBounty and Burns, 2005). Among 

the thousands of algal species present in lakes 

worldwide, most are harmless and typically 

beneficial at moderate concentrations; some 

species, such as cyanobacteria, however, can 

create harmful toxins given the right conditions. 

Invertebrates

By Wai Hing Wong and Sudeep Chandra 

Zooplankton

Zooplankton are a broad group of mostly 

microscopic, invertebrate, aquatic animals. They 

are mostly free-floating and free-swimming but 

also live in the bottom sediments. Different types 

of zooplankton vary in size—microzooplankton 

are less than 200 μm, mesozooplankton are 

between 200 and 2,000 μm, and others can be 

larger than 0.8 in. (20 mm). Zooplankton feed 

on phytoplankton and other zooplankton, and, 

in turn, are consumed by other invertebrates, 

birds, and fish. Zooplankton are the main food 

of threadfin shad, which are the key source of 

food for game fish in Lakes Mead and Mohave, 

such as largemouth bass (Micropterus salmoides) 

and striped bass (Morone saxatilis). Thus, 

monitoring zooplankton abundance is critical 

to the sportfishery on both lakes (LaBounty and 

Burns, 2005). 
0

5

10 

15 

20 

25 

30

Abundance, in individuals per liter

Copepods

Cladocerans

Rotifers

Veligers 

EXPLANATION

Jan. 02

June 02

Nov. 02

Apr. 03

Sept. 03

Feb. 04

July 04

Dec. 04

May 05

Oct. 05

Mar. 06

Aug. 06

Jan. 07

June 07

Nov. 07

Apr. 08

Sept. 08

Figure 5-2. Abundance of major zooplankton groups in Lake Mead from 2002 

to 2009 (modified from Wong and others, 2010). Different groups are important in 

different years and at different times of the year.

Chapter 5

Wildlife and Biological Resources 73

Free-swimming zooplankton community types can 

be classified by the lake location in which they reside, 

commonly littoral (near shore) and limnetic (open water) 

zones. Littoral zooplankton live only in shallow water near 

the shoreline among weeds and other vegetated habitats. 

Limnetic zooplankton live predominantly in open water but 

also can be present in the littoral zone. Because the density 

of aquatic vegetation is one of the major influences on the 

diversity and abundance of littoral zooplankton, a lake, such as 

Lake Mead, that generally lacks abundant aquatic vegetation 

is dominated by limnetic zooplankton. Littoral species rarely 

exceeded 2 percent of the monthly total zooplankton densities 

collected in Lake Mead in 1984-1985 regardless of location, 

and littoral zooplankton species diversity was low (Sollberger 

and Paulson, 1992). Limnetic zooplankton abundance can be 

controlled by wind mixing, light, pH levels, dissolved oxygen, 

water temperature, and other environmental factors, but in 

Lake Mead, zooplankton abundance and species composition 

appear to be largely controlled by fish abundance and 

predation (Sollberger and Paulson, 1992). Vertical movement 

of zooplankton can change over the course of the day, with 

some species coming up to surface waters at night to feed and 

moving back down to deeper water during daylight hours.

Overall, a diverse zooplankton community exists 

in Lake Mead, with 27 limnetic and15 littoral species 

present in surveys conducted in 1984–1985 (Sollberger and 

Paulson, 1992). The major types of zooplankton observed 

in Lake Mead are copepods, cladocerans, ostracods, and 

rotifers (fig. 5-2). Similar species were found in 1996–1997 

collections (Mueller and Horn, 1999). Although species 

diversity appears to be greater in the later survey, the later 

study surveyed a greater portion of the lake than did the first 

survey. In zooplankton surveys made from 2000 to 2004, 

more than 70 species were identified in Boulder Basin alone 

(LaBounty and Burns, 2005) indicating that species diversity 

in Lake Mead is rich. Zooplankton abundance increased 

from 2000 to 2004 in Boulder Basin, with a large spike in 

population in 2003. Increases such as these lead to increases 

in the abundance of sportfish available. However, when 

populations of zooplankton-eating fish exceed the zooplankton 

supply, zooplankton abundance can crash. Such a crash, which 

is part of a natural cycle in Lake Mead, occurred at the end of 

2003 (LaBounty and Burns, 2005).
74 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Some organisms have lifecycles that include 

a temporary planktonic phase. Since 2007, 

free-swimming veligers (larval quagga mussels) 

have been part of the Lake Mead and Mohave 

zooplankton community (Beaver and others, 

2010; Wong and others, 2010). While studies 

from other areas infested by quagga mussels 

have shown that this species, in general, has the 

potential to substantially alter the zooplankton 

composition, Lake Mead monitoring to date 

shows that neither the abundance of different 

zooplankton taxa nor their seasonal patterns have 

changed significantly since quagga mussels have 

been established in the lake (Beaver and others, 

2010; Wong and others, 2010).

Macroinvertebrates

Many different types of invertebrate 

species live in the benthic (lake bottom) or open 

water environment of Lake Mead. The benthic 

invertebrate community of the lake consists 

of approximately 90 species belonging to 10 

phyla: Annelida, Arthropoda, Bryozoa, Cnidaria, 

Entoprocta, Mollusca, Nemotoda, Nemertea, 

Platyhelminthes, and Porifera (Melancon, 

1977; Peck and others, 1987). The numbers and 

densities of invertebrates can change with depth, 

depending on the type of organism (Peck and 

others, 1987). Generally, invertebrate densities 

are higher in deltas receiving inflows from the 

Colorado River, Virgin River, Muddy River, and 

Las Vegas Wash than they are in downstream 

locations. The abundance of benthic organisms 

also changes by season depending on the food 

supply, temperature, and predation by fishes. 

For example, the density of the Asian clam 

(Corbicula fluminea) often abruptly declines 

from April to July (Peck and others, 1987), 

possibly due to predation of the young-ofthe-year, which predominate in Asian clam 

populations following the summer 

spawning season. 

Photomicrograph of a polychaete, a class of invertebrate 

worm found in Lake Mead in the phylum Annelida. 

Approximate length: 15 mm. Photograph by Annie Caires, 

University of Nevada, Reno.

Photomicrograph of an oligochaete, a class of 

invertebrate worm found in Lake Mead in the phylum 

Annelida. Approximate length: 20 mm. Photograph by 

Luke Tiano, University of Nevada, Reno.

Photomicrograph of an ostracod, a class of invertebrate 

crustacean found in Lake Mead in the phylum 

Arthropoda. Approximate length: 1–2 mm. Photograph by 

Luke Tiano, University of Nevada, Reno.
0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

Chironomidae density, in number per square meter

5 15 25 45 75 125 0 

200 

400 

600 

800 

1,000 

1,600 

1,800 

2,000 

2,200 

Oligochaetae density, in number per square meter

Depth range, in meters 

0 

50 

100 

150 

200 

250 

300 

C. fluminea density, in number per square meter

5 15 25 45 75 125 

0 

5,000 

6,000 

7,000 

8,000 

9,000 

Other taxa density, in number per square meter 1,000 

2,000 

3,000 

4,000 

A. B. 

C. D. 

1986

2008 

EXPLANATION

Figure 5-3. Changes in overall abundances of Lake Mead macroinvertebrates between 1986 and 2008. Temporal 

comparison of animal densities by depth distribution on the bottom of the lake for (A) Chironomidae, (B) Corbicula, 

(C) Oligochaetae, and (D) other taxa. (Modified from Wittmann and others, 2010).

Chapter 5

Wildlife and Biological Resources 75

Many native invertebrate species are endemic to the 

unique tributary streams and spring environments at Lakes 

Mead and Mohave, and there is concern that non-native 

species will feed on or compete with these native species. 

The red swamp crayfish (Procambarus clarkii) is of particular 

concern—a non-native large-bodied invertebrate that now 

lives in the lakes, principally in the pockets of cattails and 

emergent vegetation surrounding the edges of the lakes or 

in Las Vegas Wash and other tributaries that flow into Lake 

Mead. Red swamp crayfish eat plants, other invertebrates, 

small fish, and dead fish of any size (Leavitt and others, 1989). 

While few studies on the ecology of the crayfish have been 

done in Lake Mead, there is concern about its invasive spread 

upriver and into neighboring springs where native fishes, 

amphibians, and benthic invertebrates reside. Although novel 

trophic interactions can develop in which both black bass 

and striped bass select crayfish as a food source where they 

co-exist, fish predation is often unable to eradicate the species.

Although there have been changes in the benthic 

invertebrate community of Lake Mead over the last 30 years 

(Wittmann and others, 2010), it is unclear whether these 

changes are due to declining lake levels, decreases in algal 

levels, or the introduction of non-native invertebrate species. 

Since the 1980s, the benthic community has been a mixture 

of native and non-native species; comparing data from 

1986 and 2008, abundances of Corbicula and chironomids 

generally have decreased, Oligochaeta densities have not 

changed significantly, and other taxa have increased in average 

density (Umek and others, 2010; Wittmann and others, 2010; 

fig. 5-3). The most dramatic changes were related to two new 

non-native species, the quagga mussel (Chapter 6), which 

were first found in Lake Mead in 2007, and the New Zealand 
76 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

mud snail (Potamopyrgus antipodarum), which arrived 

shortly after in 2008 (LaBounty and Roefer, 2007; Davis and 

Moeltner, 2010; Wong and Gerstenberger, 2011). Two years 

after quagga mussels were found in Boulder Basin, they had 

spread throughout the lake (Wittmann and others, 2010; Wong 

and others, 2011). Currently, this mussel, which has colonized 

rocks and hard surfaces, dominates the benthic community 

covering it with their shells and also establishing populations 

in soft sediments at depths greater than 328 ft (100.0 m; 

Wittmann and others, 2010; Wong and others, 2011).

Invertebrates as a Food Source and Invertebrate 

Feeding Strategies

Whether in benthic or open-water habitat, invertebrates 

play an important role in the ecosystems of Lakes Mead and 

Mohave. Both benthic invertebrates and zooplankton are 

major food sources for fish in these ecosystems. Analyses 

of fish-stomach contents have shown that largemouth bass, 

bluegill (Lepomis macrochirus), green sunfish (Lepomis 

cyanellus), and channel catfish (Ictalurus punctatus) rely to 

some degree on benthic invertebrates either seasonally or 

year round (Deacon and others, 1972). In contrast, threadfin 

shad (Dorosoma petenense) feed primarily on open-water 

cladocerans or copepods (Loomis and others, 2011). 

Moreover, diets of particular fishes also can differ in the 

various basins around the lake (Umek and others, 2010). In 

Las Vegas Bay and Overton Arm, 80–92 percent of the diet 

of fish includes benthic resources. The diet of top predatory 

fishes, such as striped bass and largemouth bass, includes 

primarily the intermediate consumers, such as threadfin shad, 

bluegill, and green sunfish in differing amounts depending 

on their availability in each basin. For example, predatory 

fish from Overton Arm likely use primarily threadfin shad for 

food and energy while those from Las Vegas Bay incorporate 

greater numbers of available invertebrates and other fishes into 

their diets, and utilize shad only when available (Umek and 

others, 2010). Additionally, the diets of benthic invertebrates 

and zooplankton play an important functional role in the lake 

ecosystem. Nematodes, a benthic invertebrate that lives in the 

lake-floor sediment, for example, can be parasitic, feeding on 

other invertebrates, plants, or fishes depending on the species 

and life stage. Daphnia, a type of cladoceran zooplankton, eats 

algae by filtering phytoplankton particles into its mouth.

Fishes

By Sudeep Chandra, Jon Sjöberg, Steven L. Goodbred, and Erik Orsak

Lakes Mead and Mohave are home to at least 15 different 

fish species (fig. 5-4; table 5-1). In addition to the 15 species, 

a hybrid between common carp and goldfish (Carassius 

auratus) has been documented in the Overton Arm of Lake 

Mead (Goodbred and others, 2013). Although many of these 

fishes were introduced and are not native to the Colorado 

River drainage, native fishes endemic to this region still persist 

in small numbers. Introduced sportfish species support an 

important recreational fishery, and many of these introduced 

species are important food resources for aquatic birds. The 

lake’s top predators, black bass and striped bass, have received 

considerable attention because they are an important economic 

resource for the region (Martin and others, 1982). Some nonnative fish species, such as channel catfish and carp, were 

likely present in the Colorado River prior to the creation of 

Lake Mead, but largemouth bass were introduced into the 

reservoir shortly after its completion in 1935, and threadfin 

shad were introduced 19 years later as a forage species to 

provide increased food resources for the game fish (Allan 

and Roden, 1978). 
Chapter 5

Wildlife and Biological Resources 77

Razorback sucker (Xyrauchen texanus) (Native)

Endemic to Colorado River Basin. Federally listed as endangered. Maximum 

size 36 in. (0.9 m), 13 lbs (5.9 kg), with a hardened cartilaginous dorsal ridge 

behind head and large fleshy mouth. Historically found in middle and lower 

elevation rivers, tributaries, and flood-plain habitats. Presently found in 

small numbers in rivers and reservoirs. Warm water species that reproduces 

and grows best at 54–64°F (12–18°C). Matures at 1–3 years of age and 

lives to 44 years. Young feed on zooplankton (cladocerans, copepods, and 

rotifers), juveniles consume algae and bottom ooze, and adults eat immature 

mayflies (Baetidae), stoneflies (Plecoptera, Protonemoura), and midges 

(Chironomidae), and algae and detritus (U.S. Fish and Wildlife Service, 1998). 

Two separate populations found in Lake Mead National Recreation Area: 

one in Lake Mohave and one in Lake Mead. Recently found spawning at 

the Colorado River inflow area to Lake Mead (Albrecht and others, 2010a, 

2010b). The Lake Mead population appears to be the only one to reproduce 

successfully in the lower Colorado River Basin.

Common carp (Cyprinus carpio) (Non-native)

Introduced into warm rivers, streams, ponds, and reservoirs of the Colorado 

River Basin. Maximum size 48 in. (1.2 m), 100 lbs (45.4 kg), with large scales, 

mouth barbells, and serrated dorsal spine. Matures at 2–4 years of age 

and lives to 20 years. Native to Asia. Imported to United States in mid to 

late 1800s and stocked into lower Colorado River in the late 19th century 

(Mueller, 2005). Found throughout Lake Mead National Recreation Area and 

are especially abundant in marinas. Spawns May to June at 64–86°F (18–

30°C) with optimum of 73°F (23°C). Eats variety of foods, including algae, 

seeds, and other plant matter and invertebrates. Efficient at finding and 

vacuuming small fish and eggs from substrate. Recently found to hybridize 

with goldfish in Overton Arm of Lake Mead (Goodbred and others, 2013).

Flannelmouth sucker (Catostomus latipinnis) (Native)

Indigenous to Colorado River Basin. Maximum size 30 in. (0.8 m), 

4 lbs (1.8 kg), with prominent fleshy mouth. Occurs in most middle and 

lower elevation rivers and large tributaries. Warm-water species that 

reproduces at 48–64°F (9–18°C; Weiss and others, 1998). Matures at 

3–4 years of age and lives to 20 years. Eats primarily bottom vegetation, 

benthic invertebrates, algae, organic detritus, and seeds. Although 

common in the Grand Canyon upstream, it is found only rarely in Lake 

Mead and not found in Lake Mohave.

Bonytail chub (Gila elegans) (Native)

Endemic to Colorado River Basin. Federally listed as endangered. Maximum 

size 24 in. (0.6 m) with fine scales a streamlined body, and very narrow 

caudal peduncle. Generally prefer backwaters with rocky or muddy bottoms 

and flowing pools, although they have been reported in swiftly moving water 

and feeds on surface. Spawning has been observed during May where 

eggs are laid randomly over the bottom, and no parental care occurs. Young 

bonytail chubs typically eat aquatic plants, while adults feed mostly on 

small fish, algae, plant debris, and terrestrial insects. In Lake Mead National 

Recreation Area, only a few adult individuals remain in Lake Mohave, 

although larger numbers of stocked bonytail chub survive in locations 

downstream (U.S. Fish and Wildlife Service, 2002a).

Figure 5-4. Selected native and non-native fish species that occur in Lakes Mead and Mohave. See table 5-1 for complete list. 

Illustrations by Joseph R. Tomelleri.
78 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Colorado pikeminnow (Ptychocheilus lucius) (Native)

Endemic to Colorado River Basin. Federally listed as endangered. 

Maximum size historically up to 6-ft (1.8 m) long and weighing more 

than 100 lbs (45.4 kg) although fish found now only grow up to 24 in. 

(0.6 m) and between 4 and 9 lbs (1.8 and 4.1 kg). It has an elongated 

body, a cone-shaped and somewhat flattened head forming nearly a 

quarter of the body length. Their usual habitat is the backwaters of 

the turbulent and turbid streams in the Colorado River system. Young 

pikeminnows eat cladocerans, copepods, and chironomid larvae, then 

shift to insects at around 4 in. (10.2 cm), gradually eating more fish 

as they mature. Once they achieve a length of about 1 ft (30.5 cm), 

they feed almost entirely upon fish. Natural populations survive only 

in the Upper Basin and are not currently found in Lake Mead National 

Recreation Area (U.S. Fish and Wildlife Service, 2002b).

Bluegill (Lepomis macrochirus) (Non-native) 

Originally found only east of the Rocky Mountains but now widely 

distributed throughout North America from transplanting and stocking 

as a popular sportfish. Maximum size up to 16 in. (40.6 cm) and rarely 

weighs more than 4 lbs (1.8 kg). They are characterized by a deep, 

flattened, laterally compressed body with a terminal mouth, and 

ctenoid scales. They can be found in shallow waters in lakes and in 

slow-moving areas of streams; they prefer water with many aquatic 

plants and debris for protection and feeding. Spawning starts in 

May when the male builds a nest and peaks at water temperatures 

of 67–80°F (19–27°C). Young bluegill diet consists of rotifers and 

water fleas. The adult diet consists of aquatic insect larvae (mayflies, 

caddisflies, dragonflies), but also can include crayfish, leeches, snails, 

and other small fish. They are located in shallow bays and coves with 

aquatic plants and cover throughout Lake Mead National 

Recreation Area. 

Channel catfish (Ictalurus punctatus) (Non-native)

Native to central drainages in southern Canada, the United States, 

and northern Mexico. Maximum size up to 50 in. (1.3 m) and weighs 

more than 50 lbs (22.7 kg). Has long barbels around the mouth used 

to locate food, a deeply forked tail, spines on dorsal and pectoral fins 

and unlike most fish has no scales. They inhabit lakes and larger rivers 

that have clean bottoms of sand and gravel. During spawning, eggs 

are deposited in a nest below undercut banks or under logs or stones 

and guarded by the male for some time after eggs hatch. Has a varied 

diet including fish, insects, and crustaceans. Like most catfish, they 

chiefly feed at night. Channel catfish were stocked into the Colorado 

River in the areas of Lakes Mead and Mohave as early as 1895 and 

were common before establishment of the reservoirs. 

Figure 5-4.—Continued.
Chapter 5

Wildlife and Biological Resources 79

Largemouth bass (Micropterus salmoides) (Non-native)

Originally distributed from southeastern Canada through the Great 

Lakes, and south in the Mississippi Valley to Mexico and Florida, 

and up the Atlantic coast as far north as Maryland. Maximum size 

to almost 30 in. (0.8 m) and more than 20 lbs (9.1 kg). Lives an 

average of 16 years. They are a heavy body fish where the lower 

jaw extends beyond the upper jaw, and have both a spiny and soft 

ray dorsal fin. Preferred habitat is shallow water less than 20 ft 

(6.1 m) with aquatic vegetation and other cover. Spawning starts at 

62–65°F (17–18°C) after the male makes a nest within 8 ft (2.4 m) 

of shore in shallow water. Adhesive eggs are attached to the nest 

after being fertilized by male. The nest is guarded by the male 

until shortly after the eggs have hatched. Juvenile fish consume 

mostly small baitfish, amphipods, small shrimp, and insects. Adults 

consume smaller fish (bluegill), snails, crayfish, frogs, snakes, 

salamanders, however very large fish will eat bats, small water 

birds, mammals, and baby alligators. Stocked in Lake Mead in 1935 

where a very productive sportfishery has been established. They 

also are present in Lake Mohave.

Striped bass (Morone saxatalis) (Non-native)

Native to the Atlantic coast of North America from the St. 

Lawrence River into the Gulf of Mexico to approximately Louisiana. 

They are normally anadromous fish that migrate between fresh 

and salt water although fish are stocked in large lakes that prevent 

access to the ocean. Maximum size is more than 6 ft (1.8 m) in 

length and 125 lbs (56.7 kg). Matures at 2–3 years and lives up to 

30 years. It is a deep body fish with a long head, pointed snout, 

projecting lower jaw, and a spiny and soft ray dorsal fin. In large 

landlocked lakes, they are highly pelagic preferring deeper water 

in autumn and winter then coming to the surface in spring and 

summer to find schools of forage fish. They consume fish and a 

variety of invertebrates. Stocked in Lake Mead in 1969 to establish 

a sportfishery. As spawning does not occur in all landlocked lakes, 

fish were stocked to maintain populations. However, in 1973, 

reproduction was documented in Lake Mead so further stocking 

was not continued (Wilde and Paulson, 1989) and striped bass are 

now the most abundant sportfish in both Lake Mead and 

Lake Mohave. 

Threadfin shad (Dorosoma petenense) (Non-native)

Found along the Gulf Coast from Florida to Texas and northward 

into the Mississippi valley to Tennessee and southern Arkansas 

and Oklahoma. Maximum length a little more than 8 in. (20.3 cm) 

but most are much smaller. Short lived species, normally less than 

3 years. They are pelagic in large lakes and reservoirs forming large 

schools to feed. It has a typical herring body with an elongated 

dorsal ray, silvery color, large eye, and large deciduous scales. Can 

mature in less than 1 year. Spawns at water temperatures above 

60F (16C) over plants and other objects or under logs and brush. 

The young and adults feed on a variety of planktonic organisms and 

organic debris. Stocked in Lake Mead and Lake Mohave in 1954 to 

provide forage for sportfish, threadfin shad have been the primary 

forage species supporting the striped bass fishery.

Figure 5-4.—Continued.
80 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Largemouth bass have been a focus of anglers since 

the inception of the Lake Mead sportfishery. Beginning in 

the early 1940s, largemouth bass were reported as being 

thin and in poor condition. This condition continued into the 

1950s, when it was determined that a new forage species for 

largemouth bass was needed, resulting in the introduction 

of threadfin shad in 1954. Although initial results indicated 

improved conditions, some largemouth bass populations, 

particularly those in littoral zones of lake, did not have access 

to the large pelagic schools of threadfin shad. The completion 

of Glen Canyon Dam in the 1960s and subsequent reduction 

in sediment and nutrient loads and changes to reservoir 

storage patterns led to significant decreases in threadfin 

shad (fig. 5-5) and the largemouth bass fishery. Post-dam 

phosphorus loads—a key limiting nutrient needed for algal 

growth and food-web production—were reduced by more than 

90 percent in the upper basins of Lake Mead (Morgensen and 

Padilla, 1982; Evans and Paulson, 1983). Reduced upper basin 

sediment and nutrient loads, along with improved wastewatertreatment methodologies to remove nutrients from water 

discharge into Las Vegas Wash, caused a reoligotrophication 

of the lake, a condition of reduced productivity and increased 

clarity (Peck and others, 1987; Ney, 1996). Moreover, a 

change in reservoir storage patterns at this time appears to 

have enabled successful largemouth bass spawning in the 

spring and early summer months.

Table 5-1. List of fishes found in Lakes Mead and Mohave and their scientific and common names. 

[Native fishes to the Colorado River Watershed are listed, along with the first establishment date of non-native species, if 

known]

Family name, 

scientific name Common name

Native?

(first establishment 

date of non-native 

fish, if known)

Lake

Mead

Lake 

Mohave

Clupeidae

Dorosoma petenense Threadfin shad No–1954 

Dorosoma cepedianum Gizzard shad No–2007 

Salmonidae

Oncorhynchus mykiss Rainbow trout No–1969  

Catostomidae

Catostomus latipinnis Flannelmouth sucker Yes 

Xyrauchen texanus Razorback sucker Yes  

Cyprinidae

Cyprinus carpio Common carp No  

Gila elegans Bonytail chub Yes 

Ictaluridae

Ictalurus punctatus Channel catfish No  

Serranidae

Morone saxatilis Striped bass No–1969  

Centrarchidae

Micropterus dolomieu Smallmouth bass No  

Micropterus salmoides Largemouth bass No–1935  

Lepomis cyanellus Green sunfish No  

Lepomis macrochirus Bluegill No  

Pomoxis nigromaculatus Black crappie No 

Cichlidae

Oreochromis aureus Blue tilapia No 
nvtac11-4177_fig05-05

0 

50 

100 

150 

200 

250 

300 

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 

Average peak production shad per hundred cubic meters

Year

EXPLANATION

Overton Arm

Boulder Basin 

Chapter 5

Wildlife and Biological Resources 81

Figure 5-5. Lake Mead shad peak production densities (Nevada Department of Wildlife, 2011a, 2011b).

In an attempt to create a more sustainable sport fishery 

at Lake Mead, a cool-water sportfish, striped bass, and 

several cold-water salmonid species, such as rainbow trout 

(Onchorhynchus mykiss) and brown trout (Salmo trutta), and 

silver salmon (Oncorhynchus kisutch), were introduced in 

1969. Although it was initially presumed that striped bass 

would not be able to reproduce in Lake Mead, within 10 

years, striped bass dominated the fishery and had significantly 

reduced the pelagic biomass of threadfin shad. Due to 

decreased availability of threadfin shad, fish condition was 

reduced in some largemouth bass and striped bass populations 

(figs. 5-6 and 5-7). Moreover, evidence indicated that striped 

bass were negatively affecting the recently introduced 

salmonid fishery and contributing to the severe decline of 

the long established black crappie (Pomoxis nigromaculatus) 

fishery. By 1983, trout stocking was discontinued due to poor 

long-term returns and other demands on production capability 

in fish hatcheries. Although rainbow trout stocking was begun 

again in the 1990s, the dominance of the fishery by abundant 

striped bass has limited the survival and persistence of stocked 

trout. As a result, the trout fishery at Lake Mead has been 

managed more recently as a winter-period fishery for shore 

anglers, with no survival of trout to larger sizes documented 

in recent years.
nvtac11-4177_fig05-06

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

4 

4.5 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

1992 1993 

1994 

1995 

1996 

1997 

1998 

1999 

2000 

2001 

2002 

2003 

2004 

2005 

2006 

2007 

2008 

2009 

2010 

2011 

Lakewide larval shad abundance (shad trawls) and striped bass abundance (fall gillnet CPUE)

Larval shad abundance, in number per 100 cubic meters

Striped bass abundance (fish/net night)

Lakewide average shad production

STB CPUE fall gill nets (fish/net night) 

EXPLANATION

Years

Figure 5-6. Comparison of larval threadfin shad density (peak spawning period, 

lakewide average) and striped bass (STB) abundance (autumn CPUE), Lake Mead 

1992–2011 (NDOW data). CPUE=catch per unit effort.

Figure 5-7. Fish condition. (A) striped bass in poor condition; (B) striped bass 

(Morone saxatilis) in good condition. Photograph by Nevada Department of Wildlife.

82 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Chapter 5

Wildlife and Biological Resources 83

During the 2000s, a more stable and predictable 

balance developed between striped bass and threadfin shad 

populations, possibly due to more constant nutrient loads from 

the Virgin and Muddy Rivers and Las Vegas Wash. There 

appears to be a typical cycle of 3–4 years duration within 

which striped bass numbers and, to a lesser extent, condition 

factors, decline rapidly following large-scale depletion of the 

threadfin shad forage base, followed by a rapid rebuilding 

of striped bass numbers after the quick recovery of the prey 

(Nevada Department of Wildlife, 2011a). Additionally, there is 

an annual cycle of condition factors in striped bass related to 

seasonal thermal separation from the prey base; large striped 

bass are unable to enter warmer surface waters where shad 

are abundant. The current management strategy for striped 

bass is an attempt to manipulate the structure of the lake-wide 

population by encouraging anglers to harvest the large number 

of available fish in the smaller size ranges through increased 

possession limits. Increased harvest of 12–15 in. (30–38 mm), 

primarily 1- and 2-year-old fish, would decrease the impact 

on young-of-the-year shad, thus making more of the current 

shad production available to larger striped bass when they 

can feed upon them in late summer and early autumn. Ideally, 

this should result in improved condition factors in larger fish. 

Currently, abundant, young cohorts of striped bass in Lake 

Mead are under-utilized because of many years of declining 

angler use, but allowing increased take of small striped bass 

should increase total harvest, and increase the interest in 

fishing and angler use on the lake. 

In contrast to striped bass, reduced numbers of 

largemouth bass in Lake Mead that were reported during 

the 2000s are likely due to changes in habitat from lower 

reservoir water levels. Lower water levels reduce the number 

of shallow coves, which have aquatic vegetation and cover 

that are preferred spawning habitat for largemouth bass. 

Smallmouth bass (Micropterus dolomieu), introduced from 

an unknown source and first observed in Lake Mead in 1999, 

have expanded bass fishing opportunities considerably in Lake 

Mead and now make up more than 40 percent of the annual 

black bass (largemouth and smallmouth) catch. Smallmouth 

bass have somewhat different habitat preferences for spawning 

than do largemouth bass, and their success and available 

habitat also might be related to lower reservoir elevations 

during this period, which provided more rocky, well 

washed shorelines.

Young threadfin shad abundance can vary considerably 

over a 2–3-year cycle, because of changes in predation as 

sportfish abundance varies (Nevada Department of Wildlife, 

2011a, 2011b), and because of differences in nutrient inputs 

to the lake. Although Las Vegas Bay in the Boulder Basin 

typically is the most productive area of the reservoir for 

shad production because of constant and abundant nutrient 

inputs from Las Vegas Wash, changes in lake inputs, such as 

flood flows from the Virgin and Muddy Rivers, can increase 

shad abundance in Overton Arm to equal or exceed that of 

Boulder Basin in some years. In 1987, a 4-year program 

was conducted to evaluate large-scale fertilization as a 

potential management tool for enhancing forage and game 

fish populations (Vaux and others, 1995). From 1987 to 

1989, six treatments of liquid ammonium polyphosphate 

between 5,000 and 20,000 gal were applied to a 10,000-ha 

area in northern Overton Arm. Monitoring suggested both 

forage fish and zooplankton were positively correlated with 

increases in chlorophyll but no significant improvements 

were demonstrated in the fisheries. 

Gizzard shad (Dorosoma cepedianum), a species native 

to the Mississippi River system that were first observed in 

Lake Mead in 2007, further complicates the dynamics of 

the Lake Mead fishery. Gizzard shad, thought to have come 

from an illegal introduction in the upper Colorado River 

watershed, rapidly dominated the pelagic forage base for 

sportfish, and by 2010 comprised more than 40 percent of the 

catch in lakewide sportfish surveys (Nevada Department of 

Wildlife, 2011a). Although abundant young gizzard shad have 

so far been a benefit to sportfish species through increased 

forage availability, particularly for striped bass, gizzard shad 

grow to a much larger adult size than threadfin shad and can 

become too large for most striped bass to consume. For this 

reason, the long-term effect of the gizzard shad introduction 

on the sport fishery is still unknown (Nevada Department of 

Wildlife, 2011a).

Although similar in many respects to Lake Mead, 

the Lake Mohave sport fishery does exhibit some distinct 

differences. Because Lake Mohave is operated primarily to 

regulate water released from Hoover Dam for downstream 

water users, rather than for flood control and long-term 

water storage, it is much more stable and typically fluctuates 

only several meters in elevation during the year and from 

year to year. Because of this, the reservoir does not have the 

ability to develop shoreline vegetation during low storage 

conditions that would then be flooded by high water, limiting 

the availability of cover for some fish species. Unlike Lake 

Mead, Lake Mohave lacks an abundant prey species, such as 

threadfin shad. The reservoir is relatively shallow and narrow, 

lacking the extensive protected coves typical of Lake Mead, 

and its upper 32 mi (51.5 km) are within the river-like Black 

Canyon reach influenced at its upper end by cooler water 

discharge from Hoover Dam.

Although some sportfish including channel catfish and 

smallmouth bass likely inhabited this reach of the Colorado 

River before the closure of Davis Dam in 1950, Lake 

Mohave was primarily managed as a fishery for rainbow 

trout for many years with the introduction of both trout and 

threadfin shad after the reservoir’s creation. Influenced by 
84 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

cool-water inflows from Lake Mead, the Black 

Canyon area in particular was renowned as a 

trophy-quality trout fishery, with largemouth 

bass contributing a major component of the 

fishery in warmer, downstream reaches (Allan 

and Roden, 1978). This situation all changed in 

1983, when surface-water discharge from Lake 

Mead through the Hoover Dam spillways was 

thought to have introduced striped bass eggs 

and young fish into the reservoir. Striped bass 

rapidly became the dominant sportfish in the 

reservoir, and although rainbow trout remain 

a majority of the angler catch in upper Black 

Canyon, the trout fishery now consists almost 

entirely of small, recently stocked fish (Nevada 

Department of Wildlife, 2011b). Most of the 

striped bass harvest is smaller 1–2 lb fish, but 

the reservoir also is known for its trophy striper 

fishing, producing the State record striped bass at 

63 lbs (28.6 kg) in 2001, although large striped 

bass more than 20 lbs (9.1 kg) are increasingly 

rare. One additional effect of the striped bass 

introduction has been the virtual elimination of 

threadfin shad as a food resource, with records 

of that species in Lake Mohave virtually absent 

since the late 1980s. Largemouth bass remain an 

important component of the fishery, primarily in 

downstream reaches below Black Canyon.

Species of Special Concern

By Erik Orsak

With the completion of Hoover Dam in 

1932 and Davis Dam in 1950, and the subsequent 

formation of Lakes Mead and Mohave, the 

Colorado River was forever altered from a 

free-flowing, seasonally warm, and connected 

river into two large reservoirs containing 

relatively still, cold, and isolated aquatic systems. 

Although these reservoirs provide riparian and 

aquatic habitats that are now home to a diverse 

community of wildlife (including migratory 

birds, introduced sportfish, and reptiles), 

native fishes of the Colorado River have been 

negatively affected by this change. Native species 

are now found in low numbers warranting special 

protective status, and in some cases are listed 

as endangered under the Federal Endangered 

Species Act of 1973.

Of the four large-bodied endemic fish that 

were once common in the lower Colorado River, 

Lakes Mead and Mohave are home to small 

populations of just two native fish belonging 

to the sucker family, the federally endangered 

razorback sucker and the flannelmouth sucker 

(Catostomus latipinnis; Mueller and Marsh, 

2002). Two other native fish from the minnow 

family were once common in the Colorado 

River system: the Colorado pikeminnow 

(Ptychocheilus lucius), which is believed 

extirpated from the river’s lower basin, and the 

bonytail chub (Gila elegans) remains only as a 

few adult individuals in Lake Mohave, although 

larger numbers of stocked bonytail chub survive 

in locations downstream (U.S. Fish and Wildlife 

Service, 2002a, 2002b). These native fishes 

once numbered in the tens of thousands and 

evolved in a very large range of flows that were 

characteristic of the free-flowing Colorado River 

before the dams were built and the reservoirs 

formed. The pre-dam Colorado River discharged 

large volumes of water from spring snowmelt and 

runoff or from summer monsoon thunderstorms. 

At other times of the year, the river’s discharge 

could be greatly reduced by lack of precipitation, 

especially during prolonged drought periods. In 

particular, the razorback sucker has life history 

characteristics well adapted to the ever-changing 

hydrologic conditions of the historical river, 

including a life span believed to exceed 50 years, 

and the capability to produce tens of thousands 

of eggs annually (Hamman, 1985; McCarthy and 

Minckley, 1987; Minckley and Marsh, 2009). 

However, the populations of all these native 

fishes have steadily declined over the last onehalf of the 20th century despite conservation 

efforts. Predation by non-native fish and loss of 

suitable habitats are believed to be the primary 

causes for the population declines in the lower 

Colorado River Basin (Minckley, 1983; Minckley 

and others, 1991). 

Lake Mead contains one of the few 

populations of the razorback sucker in the 

Colorado River Basin, where consistent natural 

recruitment from larval fish to adulthood has 

been documented. Adults of this fish are known 

to spawn in at least four locations within presentday Lake Mead: the inner Las Vegas Bay on 

the western end of Boulder Basin, two areas 
Chapter 5

Wildlife and Biological Resources 85

of Overton Arm, and the inflow area of Gregg 

Basin where the Colorado River enters Lake 

Mead from the Grand Canyon. The reasons why 

razorback suckers can successfully survive to 

adulthood in Lake Mead, unlike other Colorado 

River reservoirs, is still unclear and the subject of 

ongoing research. The adult population in Lake 

Mead remains small, however, likely consisting 

of fewer than 800 fish (Kegerries and others, 

2009a; Albrecht and others, 2010c). 

Adult razorback suckers also are present 

throughout much of Lake Mohave, where they 

use shallow gravel benches along coves in the 

middle and upper parts of the reservoir for 

spawning in February through April each year, as 

evidenced by the presence of thousands of young 

larval fish. Unlike in Lake Mead, successful 

spawning in Lake Mohave has not resulted in 

natural recruitment, and survival of wild fish 

to adulthood has not been documented in this 

lake for many years. As recently as the 1980s, 

Lake Mohave was thought to have a population 

of 50,000 or more adult razorback suckers, but 

those fish have been lost to old age. Despite 

an aggressive campaign by State and Federal 

biologists to capture larval razorback suckers in 

Lake Mohave and grow them in rearing ponds 

to sizes sufficient to avoid non-native predators, 

such as bass, the population has declined to less 

than 2,500 adults (Schooley and Marsh, 2007; 

Kesner and others, 2008).

The flannelmouth sucker primarily is a 

fish that exists in flowing waters, and although 

common in the Grand Canyon upstream, it is 

found only rarely in most of Lake Mead. Recent 

monitoring in 2009–2010, however, resulted in 

the capture of 52 flannelmouth in the Colorado 

River inflow area, and 5 flannelmouth were 

captured in the vicinity of the Virgin and Muddy 

River inflow areas (Albrecht and others, 2010a, 

2010b, 2010c). Historically, flannelmouth suckers 

were uncommon in the area of Lake Mohave and 

downstream reaches of the lower Colorado River 

(Minckley, 1973), although a population does 

persist in the river below Lake Mohave (Best and 

Lantow, 2010).

In both Lakes Mead and Mohave, natural 

reproduction rates are not sufficient to meet 

goals established by the U.S. Fish and Wildlife 

Service for recovery of the native species without 

continued active management by conservation 

agencies. For many years, the Bureau of 

Reclamation has coordinated interagency 

monitoring, research, and conservation programs 

for the razorback sucker on Lakes Mead 

and Mohave, following guidelines outlined 

in the Lower Colorado River Multi-Species 

Conservation Program (Bureau of Reclamation, 

2011). These programs are implemented 

primarily through two interagency groups: the 

Native Fish Work Group for Lake Mohave 

and the Interagency Lake Mead Work Group. 

Within these groups, the Federal Government 

leads efforts on developing recovery plans that 

provide a road map with detailed, site-specific, 

management actions for private, Federal, and 

State cooperation in conserving listed species 

and their ecosystems. Specific areas of focus 

include recovery and conservation activities 

for the razorback sucker in both reservoirs 

including research, monitoring to track the status 

of populations, and the stocking of large fish 

grown from wild-caught larvae to maintain adult 

populations and preserve the genetic integrity of 

the remaining small wild populations. However, 

State agencies actively manage endangered 

species in the lakes. In addition, State agencies 

manage both native and non-native game species, 

including popular recreational fish like striped 

bass, which are top predators in the reservoir’s 

food web and are known to consume razorback 

suckers. In fact, razorback suckers in Lakes 

Mead and Mohave are prone to predation by 

non-native species due to increases in the number 

of predator species and clarity of water since 

their original evolution in the turbid waters of 

the Colorado River that provided some cover and 

protection from predation (Mueller and Marsh, 

2002). For these reasons, managing the reservoirs 

and fisheries for both non-native sportfish and 

native fish is one of many challenges facing 

natural resource agencies responsible for 

managing and mitigating competing interests for 

aquatic resources. 
Figure 5-8. How native fish are studied at Lake Mead National Recreation Area. (A) First, a razorback sucker is implanted with 

a sonic transmitter. (B) Scientists then listen for the signal from the transmitter with a hydrophone to help locate 

spawning areas (C) that might change from year to year. Images courtesy of BIO-WEST, Inc., used with permission. 

Full pool (1,225 feet above mean sea level)

2010 lake level (1,103 feet above 

 mean sea level)

Las Vegas Bay spawning sites

Larval razorback sucker found

No larval razorback sucker found

EXPLANATION

0

0 1 KILOMETER 

1 MILE 

2007–2009

spawning site

2006

spawning site

1996–2005

spawning site

Las Vegas

Wash

Gypsum Wash Government Wash

2010

spawning site

Las Vegas

Bay

The Cliffs

86 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Understanding the Ecology of Rare and Sensitive Native Fishes

By Paul B. Holden, Brandon A. Albrecht, Ron B. Kegerries, and Erik Orsak

To better understand the ecology of endangered 

razorback suckers in Lake Mead, scientists have 

deployed sonic transmitters in fish to study their 

movement (fig. 5-8). Data from the transmitters indicate 

that razorback suckers occur in the areas of Las Vegas 

Bay, Echo Bay, and the inflows of the Muddy, Virgin, 

and Colorado Rivers, locations that have relatively high 

turbidity and vegetation that are essential to survival of 

young fish. Population estimates indicate that a total of 

700–1,000 wild razorback suckers live in Lake Mead 

based on the 60–80 fish caught among all Lake Mead 

locations sampled each year (Shattuck and others, 

2011). However, growth rates for razorback suckers in 

these areas of Lake Mead are substantially higher than 

other populations of razorback sucker in the Colorado 

River Basin, indicating that Lake Mead has a younger 
Chapter 5

Wildlife and Biological Resources 87

population, because growth rates slow substantially in 

older fish (Kegerries and others, 2009b). In fact, Lake 

Mead is the only large reservoir in which recruitment 

of naturally spawned fish into breeding adults has been 

documented in recent years. To learn about recruitment 

of razorback suckers, it was necessary for scientists 

to determine the age of a fish at the time of capture. 

However, the most common technique for determining 

the age of fish could not be used in this study. In the 

common technique, the fish is killed and a bone, called 

an otolith, is removed from the inner ear. The otolith 

is then sliced and examined microscopically to count 

growth rings similarly to the way trees are aged. To 

avoid harming endangered fish, scientists developed the 

technique of removing a portion of a fin and counting 

the rings on the fin spine to determine the fish’s age. 

Using this method, recruitment data can be compared 

to a number of different environmental conditions that 

could affect fish populations. For example, recruitment 

at Lake Mead appears to be influenced by changing 

lake levels, with pulses of young fish occurring at both 

high, and more recently, at low water levels (Shattuck 

and others, 2011). Because Lake Mead has the highest 

known recruitment levels of wild razorback suckers, 

it also may have the largest populations in existence 

today; however, present day populations represent 

only a small fraction of historical numbers, estimated 

to have once been in the millions throughout the 

Colorado River before Hoover Dam was constructed. 

Although these native fish appear to be reproducing 

in Lake Mead, a number of environmental conditions 

should be monitored that have the potential to adversely 

affect fish health and survival, including habitat loss 

from lake level changes, alterations to food resources 

from invasive species such as quagga mussels, and 

contaminant loading to the lake from Las Vegas Wash 

and other tributaries influenced by human activities.

Amphibians 

By Jef R. Jaeger and Jon Sjöberg

In general, the fluctuating shorelines of Lakes Mead and 

Mohave are not ideal habitats for most amphibians, but frogs 

and toads do occur along the lakes and in tributary springs 

and streams. Along the lake shorelines, the Woodhouse’s 

toad (Bufo [Anazyrus] woodhousii) can often be observed, 

particularly in wetlands and wet sandy areas near major 

inflows. This common toad is widespread across the 

Southwest and can grow to be rather large and plump, with 

the main body (snout to vent) reaching lengths of 4.5 in. 

(114 mm). In the region of Lake Mead, Woodhouse’s toad 

appears to have hybridized with and displaced a similar 

species, the Arizona toad (B. [A.] microscaphus; Bradford 

and others, 2005). Another toad, the Great Plains toad (B. [A.] 

cognatus) no longer occurs in the region following the loss of 

its historical habitats along the river due to the formations of 

Lakes Mead and Mohave (Bradford and others, 2005).

Tributary inflow areas along the lakes also are occupied 

by a non-native species, the American bullfrog (Rana 

[Lithobates] catesbeiana). This frog is predominantly aquatic 

and prefers more lentic (slack water) habitats. Adults are 

easily recognized by their very large sizes, with body lengths 

commonly greater than 6 in. (152 mm), and their distinctive, 

low pitched, rumbling drone or bellow. Bullfrogs are known 

predators of native frogs and toads, and are considered a threat 

to the conservation of some species. 

Woodhouse’s toad (Bufo [Anazyrus] woodhousii). Photograph by 

National Park Service.
88 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The red-spotted toad (B. [A.] punctatus) 

is a small native species broadly distributed 

throughout the region, where it generally occurs 

in springs and streams within rocky canyons 

(Bradford and others, 2003). During the spring, 

the often very red-spotted males fill the night air 

with their trilling calls, and are quite common 

along the warm springs within Black Canyon 

along Lake Mohave. 

Although not found on the shorelines of 

Lakes Mead and Mohave, a Pacific treefrog or 

chorus frog (in the genus Pseudacris) occurs in 

tributary areas. The treefrog found in LMNRA 

appears to be a variant of a more southern 

species, the Baja California treefrog (Pseudacris 

hypochondriaca; Recuero and others, 2006). 

This small, variable colored frog once occurred 

locally along the historical Colorado River prior 

to the formation of the reservoirs (Banta, 1961), 

and it is still common along the floodplain of the 

Muddy (Bradford and others, 2005) and Virgin 

Rivers. The canyon treefrog (Hyla arenicolor) 

also has a wide distribution, with its northwestern 

limit on the eastern edge of the Lake Mead area. 

This mainly gray-colored treefrog is abundant 

within the western Grand Canyon, where it can 

often be found around pools in rocky springs 

and streams.

Red-spotted toad (Bufo [A.] punctatus). Photograph by Gary 

Nafis, used with permission.

LMNRA also is home to a regional endemic 

species, the relict leopard frog (R. [L.] onca). 

Once occurring along the historical Colorado 

River in the areas now covered by Lakes Mead 

and Mohave, and in the basins of the Virgin and 

Muddy Rivers as far as southern Utah, natural 

populations of this frog are now limited to a 

few spring and stream habitats in Black Canyon 

and in the region of Overton Arm of Lake 

Mead (Jaeger and others, 2001; Bradford and 

others, 2004). Although the relict leopard frog 

was once thought to be extinct, it has persisted 

despite losses of suitable habitat and isolation of 

populations. As a result, the relict leopard frog is 

the subject of a multi-agency conservation effort 

(Relict Leopard Frog Conservation Team, 2005), 

which, so far, has been successful at establishing 

additional populations within the region and 

maintaining a few remaining wild populations.

A small adult relict leopard frog (Rana [L.] onca). Photograph by Jef R. Jaeger, 

University of Nevada, Las Vegas.
Chapter 5

Wildlife and Biological Resources 89

Riparian Vegetation

By Scott R. Abella and E. Cayenne Engel

Studies of historical photographs and non-impounded 

portions of the contemporary Colorado River and its 

tributaries indicate that pre-Hoover Dam vegetation along the 

Colorado River varied spatially according to factors such as 

geomorphology and temporally owing to flooding regimes 

(Webb, 1996). The geomorphic setting of the river—such 

as whether the river was passing through a steep canyon, 

intersected tributaries or alluvial fans, or contained flatter areas 

along the river bed—influenced the growing environment 

for plants and seed deposition (Bowers and others, 1997). 

Floods periodically scoured soil and vegetation from some 

locations and deposited sediment and seed in others. Historical 

photographs suggest that not all, or even most, of the river 

corridor was heavily vegetated with riparian vegetation 

because of the scouring action of the water and the desert 

climate (fig. 5-9). Historical riparian vegetation is not as well 

documented along the present-day Lake Mead corridor as it 

was farther northeast along the Grand Canyon (Webb, 1996), 

but is believed to have included:

• riparian forests containing species such as honey 

mesquite (Prosopis glandulosa), screwbean mesquite 

(Prosopis pubescens), and Goodding’s willow (Salix

gooddingii) in protected canyon areas, flatter 

bottomlands, and tributaries including the 

Las Vegas Wash;

• moist and drier marshes with shrubby or herbaceous 

wetland species such as rushes (Juncus species); 

• various transitional communities to the uplands; and

• vegetation such as creosote bush (Larrea tridentata) 

typifying upland communities growing to the river.

Figure 5-9. A view of Black Canyon looking downstream in 1871 (left) and 2009 (right), illustrating 

changes to riparian habitat that occurred along the river after the completion of Hoover Dam and 

subsequent changes to natural surface-water discharge. The summer water discharge from Hoover 

Dam submerges the sandy and rocky bar visible at a lower river stage in the foreground of the 1871 

photograph. The rock to which the boat is tied in 1871 can be seen in both photographs. Left photograph 

by Timothy O’Sullivan/Library of Congress (September 23, 1871). Right photograph by Gary A. Reese, 

Logan Simpson Design (August 17, 2009).
90 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

A.

C. D.

B.

Current vegetation along Lake Mead also is a function of 

factors like geomorphology and the coarseness of the soil along 

the shoreline, and declining lake levels (fig. 5-10). A variety of 

plant communities occupy the shoreline, with references such 

as the U.S. Natural Resources Conservation Service PLANTS 

Database (http://plants.usda.gov) providing photographs 

and general information on the species mentioned here. 

Riparian forest communities typified by Goodding’s willow 

occur in locales like the confluence of the Muddy River with 

Lake Mead, southeast of Overton (Busch and Smith, 1995); 

mesquite and catclaw acacia (Acacia greggii) in steep shoreline 

areas and flatter areas like washes entering the lake; Fremont 

cottonwood (Populus fremontii) along Las Vegas Wash 

entering Lake Mead (Stave, 2001); marshy areas with species 

including common reed (Phragmites australis) or rushes 

(Patten and others, 2008); arrowweed (Pluchea sericea), big 

Figure 5-10. Examples of riparian vegetation around Lake Mead: (A) native vegetation along a shoreline 

near Grand Wash, Lake Mead, consisting primarily of sedges, cattails, and willows; (B), a rocky shoreline 

along Stewarts Point, Lake Mead, where rough terrain in part hinders plant establishment aside from patchy 

individuals of the exotic species saltcedar; (C) saltcedar forms dense thickets at the confluence of the Virgin 

River with Lake Mead; and (D) Boulder Beach, containing primarily bare soil and monocultures of saltcedar. 

Photographs (C) and (D) both represent locations where native riparian vegetation could be established 

through management activities. Photographs by: (A) Carrie Norman, LMNRA, 2008; (B) E. Cayenne Engel, 

University of Nevada, Las Vegas, 2010; (C) and (D) Joseph G. Barnes, University of Nevada, Las Vegas, 2009 

and 2010, respectively. 

saltbush (Atriplex lentiformis), or other species comprising 

moist shrubby communities (Busch and Smith, 1995); 

upland vegetation like creosote bush (Larrea tridentata) 

present down to near-shoreline areas; and monocultures of 

the exotic tree saltcedar (Tamarix ramosissima) and related 

species (Walker and others, 2006). 

The riparian communities that currently exist around 

Lake Mead are young; their development coincides with the 

completion of the Hoover Dam in 1935 and the subsequent 

flooding of the river corridor to full capacity within 5 

years. Therefore, the existing Colorado River vegetation 

was submerged without time to adapt or “move up” the 

hillsides with the rapidly increasing water depth as the 

lake filled. While existing riparian vegetation was quickly 

submerged, there were seed sources for riparian vegetation 

in the extensive network of springs along the river 
Chapter 5

Wildlife and Biological Resources 91

corridor. These springs harbored the cattails, rushes, sedges, 

arrowweeds, willows, and mesquites (among other species) 

that provided seed sources to the newly established shoreline. 

However, like most plant successions (progressions of plant 

communities through time) in the stressful environment 

of the Mojave Desert (Abella, 2010), plant colonization 

along the shoreline is relatively slow and dependent on 

seed availability and suitable habitat. Riparian vegetation 

becomes established most prolifically along gentle slopes 

leading to the shoreline, and most of the shoreline along the 

former river corridor is steep and unsuitable for riparian plant 

establishment. Additionally, it takes time for the viable seeds 

to travel down the washes from their host sites at the larger 

springs and for suitable germination conditions to coincide 

with seed availability. While some riparian communities 

have developed and prosper around the shoreline, most of 

the vegetation along the water’s edge consists of species 

characteristic of dry uplands.

Change continues to be the rule rather than the exception 

in current Lake Mead shoreline vegetation. As reported for 

areas in the Grand Canyon where marshes have increased 

in number and extent along the regulated river reach that is 

less subject to scouring floods (Stevens and others, 1995), 

riparian vegetation could have increased along Lake Mead 

above historical amounts in some areas, while decreasing 

in others. The current extended period of decreased lake 

levels and corresponding increase in land area along Lake 

Mead’s shoreline is alarming from a reservoir-waterstorage perspective, but enlarges the area available for 

plant colonization. From 1998 to 2010, the elevation of the 

lake declined from near its full-pool elevation of 1,220 ft 

(371.9 m) to a low of 1,083 ft (330.1 m) in 2010 (Holdren and 

Turner, 2010). This drawdown exposed more than 25,000 ha 

of formerly submerged land. While this new land is available 

for plant colonization, plant establishment is complicated by 

increasing distance to water and by fluctuating water levels 

that can inundate establishing plants.

A great deal of uncertainty surrounds plant colonization 

and succession along the newly exposed shoreline. 

Succession generally is slower in deserts compared to that 

in moister regions, with newly exposed surfaces (such as 

the shoreline) sometimes requiring centuries to millennia 

to resemble old desert communities (Abella, 2010). 

Additionally, soil properties such as pH can be altered 

from those of typical desert soils through invasion of the 

non-native tree saltcedar (Walker and others, 2006). It seems 

likely that dynamic zones of vegetation will form based on 

distance from the fluctuating shoreline.

Invasion by non-native species is changing the look and 

function of landscapes across the world, including along the 

Lake Mead shoreline. For example, saltcedar has invaded 

riparian areas throughout Western North America (Busch and 

Smith, 1995). The species establishes prolifically and forms 

dense thickets that alter the landscape around it and may 

prevent establishment of native species within areas that could 

be suitable native species habitat. While saltcedar is the most 

prolific invasive species around the Lake Mead shoreline, 

several other problematic species include (but are not limited 

to) crimson fountaingrass (Pennisetum setaceum) and Sahara 

mustard (Brassica tourneforti). Left unmanaged, these species 

also may limit establishment of native riparian species that 

could function as productive wildlife habitat and provide other 

ecosystem services such as stabilizing soils and providing 

shade for recreation along the lake.

Vegetation management along the Lake Mead shoreline 

by the National Park Service has been ongoing and is planned 

for the future. Since the early 2000s, surveys for non-native 

plants have been made along the shoreline, commonly by boat, 

to detect infestations early enough to forestall invasions both 

along the shoreline and in surrounding drainages and uplands 

(Abella and others, 2009). While the scale of invasion by 

saltcedar is daunting, the National Park Service has effectively 

treated some saltcedar monocultures by cutting (burning also 

can be effective) followed by immediate herbicide application 

to the stump. Removing saltcedar has increased native plant 

cover in the Mojave Desert, although colonization by native 

plants often is slow (Harms and Hiebert, 2006). A saltcedar 

biological control agent, the tamarisk leaf beetle (Diorhabda 

carinulata), was approved for release in 2001 and has been 

moving south along the Colorado River. As of 2010, the beetle 

had reached the lower Virgin River north of Lake Mead, 

defoliating saltcedar (Tamarisk Coalition, Grand Junction, 

Colo., http://www.tamariskcoalition.org). On shoreline areas 

where little vegetation has colonized or where saltcedar is 

dying, techniques such as planting native riparian vegetation or 

treating soils to facilitate plant colonization could be attempted 

to hasten plant succession (Harms and Hiebert, 2006). From 

a vegetation standpoint, the newly exposed shoreline affords 

opportunities for increasing the extent of both riparian and 

upland vegetation.
92 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Riparian Vegetation Management

Many challenges exist to maintaining 

habitat quality along the Lake Mead shoreline, 

but several measures and management actions 

can limit degradation and improve habitat. 

Invasion and Persistence of 

Non-Native Species

Non-native plant species can out-compete 

native species and alter soil properties. These 

plants can be managed by surveying for new 

infestations and treating invaders (for example, 

through hand pulling or herbicide) and 

establishing native vegetation to occupy sites 

(Abella and others, 2009).

Unvegetated Shoreline Soils

Unvegetated soils are subject to 

erosion and provide minimal habitat value. 

Revegetation by planting greenhouse-grown 

seedlings or seeding native species can provide 

plant cover on exposed soil (Abella and 

Newton, 2009).

Lack of Propagules for Native Plants

A lack of seeds in the seed bank or 

absence of dispersal from nearby sites can slow 

plant colonization of the shoreline (Abella, 

2010). Seeding, planting greenhouse-grown 

seedlings, or transplanting individual plants can 

help hasten plant establishment (fig. 5-11).

Other Issues

Many other issues affect management 

of the shoreline. For example, an important 

objective of the National Park Service is to 

protect existing stands of riparian vegetation, 

such as mesquite woodlands from unauthorized 

cutting and unnatural fire through law 

enforcement, education, and other measures. 

Figure 5-11. Students and staff with the 

Environmental Science Program (University of 

Nevada, Las Vegas) planting greenhouse-grown 

seedlings of native perennial plants. The project 

was designed to help evaluate factors that 

limit plant establishment along newly exposed 

shoreline of Lake Mead. The wire cages are 

designed to protect the plants from being eaten 

by animals. The inset photograph (bottom 

right) shows a mature desert marigold (Baileya 

multiradiata). Main photograph by Sylvia Tran 

(November 2011); inset photograph by Scott R. 

Abella (April 2012), University of Nevada, 

Las Vegas. 
Chapter 5

Wildlife and Biological Resources 93

Littoral Vegetation

By E. Cayenne Engel and Scott R. Abella

In lake ecosystems, the littoral zone may 

be defined by site and management needs. 

Some working definitions of a lake littoral zone 

include: (a) the portion of a lake less than a 

certain depth, (b) the region that extends from 

the shoreline to the depth where sufficient light 

for plant growth reaches the substrate (known 

as the compensation level—the depth at which 

light intensity supports sufficient photosynthesis 

to compensate for respiratory energy losses; 

Lampert and Sommer, [1997]), or simply (c) 

the shallow shoreline region occupied by rooted 

vegetation (Brewer, 1994). 

In some lakes, including many areas of 

Lake Mead, littoral zones typically are narrow 

horizontally because of steep shorelines, as 

opposed to lakes with gently sloping shorelines 

that commonly contain broader littoral zones. 

In the case of Lake Mead, there generally 

are relatively narrow regions where the lake 

shoreline is shallow enough to sustain littoral 

vegetation. The scarcity of littoral vegetation is 

common among dam-created reservoirs owing 

to the shoreline structure, water turbidity, and 

fluctuating levels. Only in reservoirs with clear 

water and stable levels do well-developed 

macrophytic communities (aquatic plants 

growing in or near water) generally develop 

(Kimmel and others, 1990). Lake Mead’s 

fluctuating water levels leave little opportunity 

for plant species that require submersion in water 

for their subsistence. Additionally, within Lake 

Mead, the compensation depth varies drastically 

from approximately 6–36 ft (1.8–11.0 m), 

depending on the time of year and phytoplankton 

activity in a given area (Acki, 1975).

Many of the same anthropogenic 

changes and limitations that affect riparian 

plant colonization (see Riparian Vegetation 

Management) of Lake Mead also influence 

littoral communities. Water levels in the 

Colorado River Valley rose quickly (within 5 

years to maximum depth after damming), not 

allowing time for the naturally established littoral 

vegetation to adapt, completely submerging 

the existing plant communities. Therefore, 

any littoral vegetation currently established 

is essentially a form of primary succession 

along the lake edge, with these species largely 

becoming established in places they have not 

occupied in recent history. The exceptions are 

species that may be present along well-developed 

springs that emanate from hillsides and flow to 

the lake’s edge, with plants around the springs 

serving as seed sources. 

Cove surveys by scuba divers within 

Las Vegas Bay—more than 50 years since the 

Hoover Dam was completed—indicated that 

most near-shore areas were devoid of aquatic 

vegetation in winter and exhibited only patchy 

vegetation in summer and autumn, although 

never deeper than 16.4 ft (5.0 m; Jennifer Haley, 

written commun., in Sollberger, 1987). In areas 

of Lake Mead that do support vegetation, littoral 

vegetation can consist of grasses (Phragmites 

australis), rushes (Juncus species), sedges 

(Eleocharis rostellata, Scirpus americana), 

cattails (Typha domingensis), forbs such as 

yerba mansa (Anemopsis californica) and water 

pimpernel (Samolus parviflorus), and aquatic 

species like sago pondweed (Potomogeton 

pectinatus) that may be able to disperse from 

springs to the lakeshore via washes. However, 

the distances between springs and the shoreline 

are commonly great [averaging more than 2 

mi (3.2 km) in LMNRA and increasing as lake 

levels decline], which might create challenges to 

seed dispersal. Other challenges to development 

of littoral vegetation in Lake Mead might 

include increasing susceptibility to changes in 

lake chemistry with the introduction of humanmade products, toxic algae, and eutrophication

(Lieberman, 1995; LaBounty and Burns, 2005). 
Spraying herbicide on invasive fountaingrass (Pennisetum setaceum) near the Lake Mead 

shoreline during exotic plant surveying and management activities conducted by the ‘Weed 

Sentry’ program of the University of Nevada, Las Vegas in collaboration with the National 

Park Service (Abella and others, 2009). Photograph by Carrie Norman, National Park Service.

Southern cattail (Typha domingensis), an example of a common, 

native species that inhabits the littoral zone around Lake Mead. 

Photograph by Mitchell Urban, National Park Service.

94 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Documentation of littoral vegetation along 

the Lake Mead shoreline is limited. Uncertainly 

exists regarding potential threats to native littoral 

vegetation communities. Possible threats include 

exotic plants, changes in water quality including 

toxic algae blooms, and the invasive quagga 

mussel (Chapter 6). Potential management 

actions to enhance or maintain native habitat of 

the littoral zone include:

• Attempting to establish native vegetation 

that serves as habitat for wildlife of 

key management concern (for example, 

relict leopard frog [Bradford and others, 

2004] along with native fish and sportfish 

species). Vegetation structure is known to 

be important for a variety of nest-building 

fish, like largemouth bass, providing the 

substrate within which or on which eggs 

are laid and protecting eggs from wave 

action and erosion (Kimmel and others, 

1990). 

• Preventing establishment of invasive 

colonizers such as the exotic species 

fountaingrass (Pennisetum setaceum) 

that was detected during weed surveys 

along the Lake Mead shoreline (Abella 

and others, 2009). These species may 

outcompete native species and clog 

waterways and coves that are otherwise 

useful for human recreation.

Invasive Plant Species

Exotic Plant Species within and near the Littoral 

Zone of Lake Mead

Many exotic plant species inhabit the littoral zone of Lake 

Mead. Below are three key examples.

1. Eurasian watermilfoil (Myriophyllum spicatum)

Eurasian watermilfoil is a submerged plant that adversely impacts 

aquatic ecosystems by forming dense canopies over the surface 

of a lake that are capable of shading out native vegetation and 

degrading habitat for fish and other wildlife. Dense Eurasian 

watermilfoil mats alter water quality by raising pH, decreasing 

oxygen, increasing temperature, and increasing phosphorus and 

nitrogen loadings (Smith and Barko, 1990; Madsen and 

others, 1991).

2. Giant reed (Arundo donax)

Giant reed is a grass that can grow taller than 20 ft (6.1 m) 

in height and become established via floating root and stem 

fragments. The species provides poor wildlife habitat, and, once 

established, often forms pure stands that outcompete native 

vegetation (Bell, 1997).

3. Fountaingrass (Pennisetum setaceum)

Fountain grass may establish in areas that are not waterlogged, 

but this densely clumping and readily propagating grass can 

outcompete other species, clog shorelines, and limit establishment 

of native species (Cronk and Fuller, 1995).
American avocets (Recurvirostra americana) in breeding plumage on 

Lake Mead. Photograph by Joseph G. Barnes, University of Nevada, 

Las Vegas.

Chapter 5

Wildlife and Biological Resources 95

Aquatic Birds

By Joseph G. Barnes and Jef R. Jaeger 

The nature of the lower Colorado River, 

and the aquatic bird species the river supports, 

has changed dramatically since the creation of 

dams and associated impoundments (Rosenberg 

and others, 1991). The reservoirs of Lakes Mead 

and Mohave have altered more than 200 mi 

(322.0 km) of river. These manmade ecosystems 

have been further modified by intentional and 

unintentional introductions of non-native fish, 

other aquatic organisms, and plants, which 

have undoubtedly altered food resources for 

many aquatic birds. Lakes Mead and Mohave 

are situated in a low elevation trough along 

the eastern edge of the Mojave Desert, where 

summer temperatures are extreme, but winters 

tend to be relatively mild, with freezing 

temperatures uncommon (see Chapter 2). As a 

result, these lakes are now important stopover 

habitat and wintering grounds for many 

aquatic birds migrating along the Pacific and 

Intermountain Flyways (Brown and others, 2000).

Information on aquatic birds summarized 

in this section is based on an inventory and 

monitoring project conducted at Lakes Mead and 

Mohave from spring of 2004 through summer 

2009 (Barnes and Jaeger, 2011). Several bays, 

coves, and other sites known to attract aquatic 

birds were monitored during monthly surveys. 

Occasional surveys were conducted at other sites, 

and incidental observations of rare or unusual 

species or of large congregations of aquatic birds 

during migrations were recorded. 

The sites systematically monitored on the 

two lakes are quite different. Sediment and 

nutrient inflows greatly influence the survey sites 

on Lake Mead, particularly the nutrient-rich, 

shallow bay at the confluence of the Virgin and 

Muddy Rivers and the outflow from Las Vegas 

Wash, where treated effluent from the Las Vegas 

metropolitan area enters Las Vegas Bay (see 

Chapters 3 and 4 for more details). The Grand 

Wash Impoundment also was monitored—a 

currently isolated segment of Lake Mead near 

the entry point of the Colorado River. The Grand 

Wash Impoundment receives only occasional 

pulses of floodwaters coming down the Wash, 

but an algal bloom was observed in late 2008, 

indicative of high levels of nutrients. In contrast, 

White-faced ibis (Plegadis chihi) over Lake Mead. Photograph by 

Joseph G. Barnes, University of Nevada, Las Vegas.
Figure 5-12. Photographs looking north from the western side of the Muddy River basin, near the river 

outflow into Lake Mead, showing changing conditions over time resulting from fluctuations in the water 

level of Lake Mead. Photographs were taken during (A) January 2005, (B) June 2007, and (C) July 2007. 

Photographs by Joseph G. Barnes, University of Nevada, Las Vegas.

96 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

the more open bays monitored on Lake Mohave 

(Arizona and Nevada Bays) receive little nutrient 

inflows from surrounding lands, whereas Black 

Canyon receives cold, clear water emerging from 

the base of Hoover Dam. To further complicate 

site comparisons, the nature and scope of the 

shallow water sites on Lake Mead varied greatly 

over the course of the study as the lake level 

declined and fluctuated, while the level of Lake 

Mohave was kept within a tighter range with a 

more predictable seasonal pattern (see Chapter 2

for more detailed information on changing lake 

levels and dam operations along the river).

The inventory documented 92 species of 

aquatic birds, excluding the ruddy shelduck 

(Tadorna ferruginea), which likely escaped 

from regional captivity. Additionally, four other 

species strongly associated with Lakes Mead and 

Mohave were identified: the belted kingfisher 

(Megaceryle alcyon), bald eagle (Haliaeetus 

leucocephalus), and osprey (Pandion haliaetus), 

which rely on aquatic prey, and the peregrine 

falcon (Falco peregrinus), which on these lakes 

feeds extensively on aquatic birds (appendix A). 

In terms of observed species richness (the 

number of different species), the diversity of 

aquatic birds was higher on Lake Mead than 

on Lake Mohave. At Lake Mead, 89 species 

were observed, of which 34 were unique to the 

lake. In contrast, at Lake Mohave, 59 species 

were observed, of which only 4 were unique to 

the lake. Three of the four unique species were 

represented by no more than two individuals. 

Many species of shorebirds were documented 

only on Lake Mead, reflecting the ephemeral 

mudflats and open beaches that formed in areas 

along this lake at times when the lake level 

declined (fig. 5-12). Other species unique to 

Lake Mead included several marsh birds and 

herbivorous waterfowl seen in marshy habitats 

formed at the mouths of the major 

tributary inflows.
Chapter 5

Wildlife and Biological Resources 97

Although many of the same aquatic bird 

species occur at both lakes, species evenness 

(which accounts for the relative abundance 

among species) of aquatic birds was higher at 

sites around Lake Mead than at Lake Mohave. 

Overall, the American coot (Fulica americana), a 

migratory herbivore, is the most abundant species 

regularly observed on both lakes, occurring in 

great numbers during winter months. On Lake 

Mohave, this species alone accounted for more 

than 77 percent of the birds observed during 

monthly surveys. On Lake Mead, however, 

several other species representing several feeding 

guilds (Paszkowski and Tonn, 2006) also are 

quite abundant (fig. 5-13), particularly the 

diving carnivores, Clark’s and western grebes 

(Aechmophorus clarkii and A. occidentalis). 

Abundances on either lake, however, can 

be greatly affected by short-term congregations 

of certain other species, such as the eared grebe 

(Podiceps nigricollis), American white pelican 

(Pelecanus erythrorhynchos), snow goose (Chen 

caerulescens), and ring-billed/California gulls 

(Larus delawarensis and L. californicus; these 

two species are difficult to distinguish under 

field conditions). For example, during spring 

migration, eared grebes often form the largest 

such congregations, with one raft on Lake Mead 

in April 2007 estimated at more than 16,000 

birds. Overall seasonal abundance of aquatic 

birds on these lakes generally reflects relatively 

predictable patterns associated with winter 

residency or migratory stopovers in spring and 

autumn (fig. 5-14). Abundances of wintering 

birds typically peak from November through 

January, with the pattern driven to a great extent 

by the presence of the American Coot. Not all 

individuals of wintering species, however, remain 

on these lakes, and many individuals appear to 

stop only briefly during migration. Stopovers 

by migrating species are somewhat less well 

defined, but abundances of many migrants peak 

in March and April and again in September and 

October. During the hot summer months, the 

abundance of aquatic birds is quite low, with only 

a subset of species remaining on these lakes. 

nvtac11-4177_fig05-13

American Coot 

Clark's/Western Grebe 

Green-winged Teal 

Ruddy Duck 

Ring-billed/California Gull 

Eared Grebe 

Least/Western Sandpiper 

Northern Shoveler 

American Avocet 

American White Pelican 

79 Other Species 

American Coot 

Mallard 

Common Goldeneye 

Ring-billed/California Gull 

Double-crested Cormorant 

Lesser/Greater Scaup 

Eared Grebe 

Common Merganser 

Pied-billed Grebe 

Great Blue Heron 

50 Other Species 

Lake Mohave

LakeMead

EXPLANATION

EXPLANATION

17% 

17% 

7% 

5% 5% 5% 4% 

4% 

4% 3% 

29% 

77% 

4% 

3% 

3% 2% 

2% 1% 

1% 

1% 

1% 

5% 

Figure 5-13. Percentages of the 10 most common species 

or species-groups of aquatic birds on each lake derived 

from observations at sites monitored monthly from March 

2004 through August 2009 on Lakes Mead and Mohave. The 

identified species-pairs are difficult to distinguish in the field 

and were often recorded into these combined categories.
nvtac11-4177_fig05-12

0 

200 

400 

600 

800 

1,000 

1,200 

Jan. Feb. Mar. Apr. May 

Month (2004-2009)

Average total aquatic birds recorded

Jun July Aug. Sep. Oct. Nov. Dec. 

Lake Mead

Lake Mohave 

EXPLANATION

Figure 5-14. Monthly averages of total aquatic birds 

(on y-axis; see appendix A for species list) recorded per 

survey at sites monitored monthly on Lakes Mead and 

Mohave from March 2004 through August 2009. 

Snowy plover (Charadrius nivosus). Photograph by Joseph G. Barnes, University of 

Nevada, Las Vegas.

98 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Changes in the surface-water level of Lake Mead 

significantly alter the availability and composition of habitats 

for aquatic birds. Unlike most areas around the lake, the 

river mouths on Lake Mead and many of the coves and 

bays are shallow and have gradual slopes. For these sites, 

shifts in lake level expose or inundate large areas, some 

of which are covered by sediments. In recent years, the 

fluctuations in lake level tended to create temporary mudflats 

and open beaches that generally favored shorebirds (for 

example, sandpipers, plovers), of which many are species of 

conservation concern. For example, successful breeding by 

snowy plovers (Charadrius nivosus) was documented at Lake 

Mead in 2007 and 2008, undoubtedly resulting from declines 

in surface-water level at that time, which uncovered large 

areas of favorable habitat. The Pacific Coast population of 

snowy plover is federally listed as Threatened, and its nearest 

previously known breeding colonies are more than 300 mi 

(482.8 km) away from the breeding sites at Lake Mead. 

Mudflats on the lake are particularly ephemeral, and 

during extended periods of lake-surface-level declines, 

vegetation, such as non-native salt cedar, quickly encroaches 

on exposed sediments, rendering these areas increasingly 

unsuitable for shorebirds. The shallow topography of the 

shoreline in areas where mudflats occur, however, allows for 

quick inundation when waters rise. In recent years, variation 

in lake level has at times inundated large areas of emergent 

vegetation creating shallow-water conditions (fig. 5-12) 

advantageous to many types of waterfowl, particularly 

herbivores like the green-winged teal (Anas crecca). 

For aquatic birds, some aspects of migration strategies 

may vary, but many species rely on foraging opportunities 

along flyways to maintain energy reserves. Consequently, 

migratory pathways generally follow seasonally reliable 

resources (Shuford and others, 2002; Skagen, 2006). At Lake 

Mead, the highly variable nature of shoreline and shallowwater habitats is a result of unstable resource conditions 

from year to year. However, the impact of large changes in 

available resources on migrating or wintering bird populations 

at Lake Mead has not been studied. Because Lakes Mead and 

Mohave are part of a complex of several reservoirs extending 

from Lake Powell southward along the lower Colorado 

River into its delta at the Gulf of California in Mexico, any 

comprehensive assessment would require understanding 

resource availability and use along the entire river system. 

Moreover, a resource assessment for the river system below 

Lake Mead should include potential future climate effects on 

snowfall patterns, water availability, and associated changes in 

habitat and food resources for aquatic birds. 

In addition to potential impacts from climate, aquatic bird 

species at Lakes Mead and Mohave also will be affected by 

anthropogenic-induced change, particularly from non-native 

invasive species and contaminants contained in tributary 

inflow. For example, quagga mussels (Dreissena bugensis; 

Chapter 6) have recently invaded both lakes. Quagga mussels 

may have substantial impacts on aquatic bird species, 

particularly those that feed on invertebrates. On the lower 

Great Lakes and elsewhere, omnivorous diving ducks have 

altered migration patterns in response to quagga mussel and 

related zebra mussel invasions (Wormington and Leach, 

1992; Petrie and Schummer, 2002). During the last year of 

monitoring on Lake Mohave, increases in two omnivorous 

diving ducks known to feed on quagga mussels were observed, 

the common goldeneye (Bucephala clangula) and lesser scaup 

(Aythya affinis). Moreover, starting in 2008, white-winged 

scoters (Melanitta fusca), another omnivorous diving duck, 

also began showing up regularly on this lake.
Chapter 5

Wildlife and Biological Resources 99

Studies have indicated that contaminants in water may 

bioaccumulate in quagga and zebra mussels (Mills and others, 

1993; Link, 2010; Mueting and Gerstenberger, 2010). Birds 

that feed on these mussels may gain weight and have an 

acceptable looking body condition, but contaminant loads 

have been hypothesized to reduce survival and reproductive 

success (Austin and others, 2000). What impact quagga 

mussels will have on aquatic birds at Lakes Mead and 

Mohave is not clear. In general, contaminant levels in water 

and sediments vary across these lakes (see Chapter 4), and 

potential future increases in wastewater effluent from Las 

Aquatically Associated Raptors

By Joseph G. Barnes and Jef R. Jaeger

Osprey, bald eagles, and peregrine falcons 

occur at Lakes Mead and Mohave, where they prey 

primarily on aquatically associated species. Osprey 

feed almost exclusively on fish, while bald eagles 

are more opportunistic foragers, relying on fish, but 

also preying on aquatic birds, small mammals, and 

scavenging carrion. Bald eagles have been observed on 

numerous occasions preying on aquatic birds, primarily 

waterfowl, which may be an important portion of their 

diet while on these lakes. 

 Observations of osprey and bald eagles during 

monthly aquatic bird monitoring show that both species 

can be found on the lakes throughout the year (Barnes 

and Jaeger, 2011). However, peak occurrence for osprey 

was in September and October, which is consistent with 

autumn migration as osprey pass through the region 

heading towards winter grounds farther south (Martell 

and others, 2001). In contrast, bald eagles appear to 

winter regionally, occurring in large numbers starting 

in November or early December and departing by late 

February or early March. Annual winter counts in early 

January by survey crews covering both lakes in a single 

day indicate increasing numbers of bald eagles in recent 

years, with 163 counted in 2011, more than 60 percent 

of which were subadults (Fletcher and Jaeger, 2011). 

The migration patterns of bald eagles using Lakes 

Mead and Mohave have not been studied, but limited 

tracking information from elsewhere indicate that some 

birds wintering in California pass through Lakes Mead 

and Mohave areas during migration. Juvenile birds 

migrating from a breeding population farther south in 

Arizona also may use these lakes. 

Vegas Wash into Las Vegas Bay and Lake Mead may present 

an accumulating impact regardless of the pathway into food 

chains. Since changes in endocrine and reproductive health 

of fish in these areas have been documented (Bevans and 

others, 1996; Patiño and others, 2003), there is a concern for 

exposure to both Clark’s and western grebes, migratory birds 

that feed on fish within or near Las Vegas Bay. However, 

higher food-chain, non-migratory species, such as peregrine 

falcons that remain on Lakes Mead and Mohave for longer 

periods, are of greater concern. 

Bald eagle (Haliaeetus leucocephalus) and common raven (Corvus 

corax) on the shoreline of Lake Mead. Photograph by Joseph M. 

Hutcheson.

Peregrine falcons are bird-eating specialists less 

tightly associated with water, but along Lakes Mead 

and Mohave peregrines have been documented to 

prey on 29 species of aquatic birds, accounting for 

37 percent of their diet by item and 77 percent of diet 

by mass (Barnes, 2011). Peregrines are year-round 

residents at these lakes and their numbers have grown 

since a breeding pair was documented on Lake Mead 

in 1985. By 2010, 33 nesting territories were occupied 

within LMNRA (Barnes, 2011), with the majority of 

these located near lakeshores and densities highest 

along steep-walled canyons overlooking lake waters. 
100 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

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By Michael R. Rosen1

, Steven L. Goodbred1, 

Wai Hing Wong2

, Reynaldo Patiño1,3, Kent 

Turner4

, Craig J. Palmer2, and Peggy Roefer5

Chapter

Threats and Stressors 

to the Health of the 

Ecosystems of Lakes 

Mead and Mohave

Lake Mead at sunrise. Photograph by Michael R. Rosen, U.S. Geological Survey.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 105

Ecosystem impacts from visitor activities or natural environmental change are important concerns in 

all units of the National Park system. Possible impacts to aquatic ecosystems at Lake Mead National 

Recreation Area (LMNRA) are of particular concern because of the designation of Lakes Mead and 

Mohave as critical habitat for the federally listed endangered razorback sucker (Xyrauchen texanus), the 

significance of the sport fishery, and the regional importance of its habitats to more than 90 documented 

species of waterbirds. Potential threats to shoreline habitats are of concern not only for their ecosystem 

values but also for maintaining the recreational setting. Many areas adjacent to the shorelines of Lakes 

Mead and Mohave are designated wilderness areas. 

For purposes of this document, stressors are any chemical, biological, or physical agent that has 

a detrimental effect on aquatic ecosystems at the organism, population, or community level. Humanmade stressors at Lakes Mead and Mohave include direct effects of recreation on the lakes, like boating 

and fishing, as well as indirect effects of activities away from the lakes, such as growing population 

and increasing urbanization. Common natural environmental stressors include extended changes in 

climate (precipitation or temperature), or the erosion, transport, and loading of chemical constituents in 

rocks and sediments to aquatic environments. Human activity also can exacerbate natural stressors in a 

variety of ways.

1

 U.S. Geological Survey

2

 University of Nevada, Las Vegas

3

 Texas Tech University

4

 National Park Service

5

 Southern Nevada Water Authority
Sampling at Lake Mead. Photograph by Michael R. Rosen, 

U.S. Geological Survey.

106 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Past studies suggested that inorganic and 

organic chemicals are major environmental 

stressors at LMRNA (for example, Bevans 

and others, 1996; Hamilton and others, 2002; 

Goodbred and others, 2007). Sources of these 

chemicals include inputs from the urbanized 

tributary Las Vegas Wash, transport of naturally 

occurring selenium from rocks and soil in Las 

Vegas Valley to Lake Mead (Hamilton and 

others, 2002; Cizdziel and Zhou, 2005), or 

organic compounds from fuels and oils used 

in recreational watercraft on Lakes Mead and 

Mohave (Lico and Johnson, 2007). Other 

past or potential stressors on the aquatic 

ecosystem include algal blooms, invasive 

species [such as quagga mussels (Dreissena 

rostriformis bugensis), New Zealand mudsnails 

(Potamopyrgus antipodarum), and Asian clams 

(Corbicula fluminea)], pathogens, viruses, and 

parasites, effects of population growth, and 

changes in climate. Most information on effects 

of stressors and the health of aquatic ecosystems 

at LMNRA comes from fish studies completed 

during the 1990s and 2000s; information is 

sparse on the effects of stressors on other levels 

of the food web.

Inorganic Chemicals 

Inorganic chemicals (those without 

biologically produced carbon in them) that enter 

LMNRA come from both human and natural 

sources (Rosen and Van Metre, 2010). Mercury 

and perchlorate are the focus of this subsection 

as the two main inorganic chemicals of concern 

at LMNRA (see Chapter 4). Nearly 87 percent 

of mercury present in the environment today is 

due to human activities (U.S. Environmental 

Protection Agency, 2001b), including mining 

and mineral extraction. In the last century, 

the environmental burdens of mercury have 

increased dramatically due to extensive use of 

coal fired electrical power plants worldwide 

(Shimshack and others, 2007). Perchlorate 

was manufactured in Henderson, Nev., at the 

BMI Complex as a component of rocket fuel 

from 1945 to 1998. Although possible, natural 

contamination of water bodies by both mercury 

and perchlorate typically is of less concern.

Coal-fired power plants can release mercury 

into the air, which then can fall into water 

bodies as inorganic mercury that is soluble in 

water. However, in or near sediments or shallow 

wetland areas, bacteria convert the inorganic 

mercury to the organic, more toxic methyl 

mercury, which is less soluble and tends to 

concentrate in organic matter and biota. Mercury 

concentrations in sediment in LMNRA are 

relatively low compared to those in other lakes 

nationally as well as in other lakes in Nevada. 

However, mercury concentrations in sediment 

and fish are not uniformly distributed throughout 

Lakes Mead and Mohave (Kramer, 2009; Rosen 

and Van Metre, 2010), indicating different 

loadings and sources. 
Sediment

fraction

Soluble

fraction

Uptake and

biomagnification

Atmospheric

deposition Volatilization

Nonpoint

source runoff

Decomposition Mineralization

Plant uptake

Sedimentation

Sorption

Municipal and

industrial wastewater

Groundwater

aquifer pumping

Transport to

groundwater

Remobilization and

deposition

Drinking water

withdrawal

Figure 6-1. Sources and pathways of organic compounds in water and aquatic ecosystems. Household organic 

compounds disposed down drains are transported to municipal and wastewater-treatment plants (point sources) that 

can then be discharged into aquatic ecosystems if they are not completely removed during the treatment process. Other 

compounds may enter the ecosystem from the land surface (non-point sources) in a watershed, such as the spraying 

of pesticides on agricultural fields or golf courses. Removal of organic compounds from water in an aquatic ecosystem 

can occur from pumping for irrigation or water supplies, being attached to and buried in sediment, taken up by aquatic 

organisms, or microbial degradation.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 107

Mercury is of concern because it biomagnifies (fig. 6-1) 

up the food web; that is, animals at the top of the food web 

will have higher concentrations of mercury than algae or 

invertebrates lower in the food web. Biomagnification occurs 

because animals and plants, once exposed, have very little 

ability to eliminate mercury. As a result, animals that eat 

more, are bigger, and live longer will have higher mercury 

concentrations. For example, the recently (2007) discovered 

quagga mussels at LMNRA have the ability to filter large 

quantities of water and, through that process, bioaccumulate 

significant concentrations of mercury (Mueting and 

Gerstenberger, 2010). As quagga mussels accumulate mercury 

from the water column, other biota, such as fish and diving 

ducks that consume the mussels, also would accumulate 

mercury and continue the biomagnification process up the 

food web to concentrations high enough to cause health 

effects in top predators (Hogan and others, 2007). Mercury 

concentrations in quagga mussels soon after they became 

established at Lakes Mead and Mohave averaged 0.031 μg/g 

dry weight (DW) and 0.43 μg/g DW, respectively (Mueting 

and Gerstenberger, 2010). These concentrations are well below 

mercury concentrations in quagga mussels in Lakes Erie and 

Ontario but comparable to concentrations in these mussels in 

the Niagara River, which flows northward from Lake Erie to 

Lake Ontario. 
108 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Higher on the food web, mercury 

concentrations in sportfish studied by Cizdziel 

and others (2002, 2003), Kramer (2009), and 

Kramer and Gerstenberger (2010) varied by 

species and location. For example, mercury 

concentrations in muscle samples of blue 

tilapia (Oreochromis aurea) were less than the 

detection limit of the analytical method (0.01 

μg/g), but mercury concentrations were highest 

in striped bass (Morone saxatilis) of all fish 

species—an average concentration of 0.15 μg/g 

(Cizdziel and others, 2002, 2003). The low 

mercury concentrations in blue tilapia are likely 

a consequence of their position lower on the food 

web than striped bass and their vegetarian diet, 

whereas striped bass are predators that eat higher 

on the food web, live longer, and accumulate 

more mercury from the smaller fish they eat. 

In a more recent study of 221 fish samples 

collected in 2007 and 2008 at Lake Mead, the 

mean concentrations of mercury detected were 

less than the U.S. Environmental Protection 

Agency’s tissue residue criterion of 0.3 μg 

methylmercury per gram wet weight (ww) fish 

tissue (Kramer and Gerstenberger, 2010). This 

tissue residue value represents the concentration 

of methylmercury in freshwater and estuarine 

fish and shellfish that should not be exceeded 

to protect consumers in the general population 

(U.S. Environmental Protection Agency, 2001a). 

Results of the study included 10 samples 

(less than 5 percent) with concentrations that 

exceeded U.S. Environmental Protection Agency 

(USEPA) tissue residue criterion (Kramer and 

Gerstenberger, 2010). However, in spite of 

these generally low methylmercury values in 

Lake Mead fish, the State of Nevada has issued 

statewide fish consumption advisories (table 6-1) 

to protect public health related to this widespread 

contaminant for the majority of large water 

bodies in the State. The advisories include those 

for several of the common sportfish species in 

Lakes Mead and Mohave (fig. 5-1, table 5-1; 

http://ndow.org/fish/health/#southern). 

Perchlorate has been used to treat thyroid disorders in humans for 

almost 50 years, but in the environment, the salt can be harmful to an 

animal’s endocrine system (Kendall and Smith, 2006). Few research 

findings on the effects of perchlorate on fish health in Lake Mead have been 

published, but a study by Snyder and others (2002) indicated that there 

might be some effects on thyroid hormones in male carp in Las Vegas Bay. 

Further evidence of the possible effects of perchlorate on fish health was 

demonstrated in a controlled experiment completed on goldfish (Carassius 

auratus) (Crouch, 2003), which are in the same family as common 

carp (Cyprinus carpio). In this experiment, male and female goldfish 

were exposed to perchlorate under laboratory conditions. Results of the 

experiment indicated increased thyroid activity (hyperthyroidism) in female 

goldfish at perchlorate concentrations of 1.2 mg/L, and increased thyroid 

activity in male goldfish at significantly higher concentrations of 31 mg/L. 

However, the concentrations used in this laboratory experiment were higher 

than most concentrations measured in Las Vegas Wash, and many times 

Table 6-1. Nevada Department of Wildlife Fish Consumption 

Advisory, based on methylmercury.

[An adult meal size is considered 8 ounces of fish meat, about the size of two 

decks of cards. Children should eat smaller, age-appropriate amounts]

Location Fish species Maximum number of 

meals per month

Lake Mead

Boulder Basin Common carp 8

Channel catfish 12

Largemouth bass 16

Striped bass 4

Tilapia Unrestricted, >16

Colorado River Inflow Arm Channel catfish 8

Largemouth bass 16

Striped bass 4

Tilapia Unrestricted, >16

Overton Arm Channel catfish 4

Largemouth bass 4

Smallmouth bass 4

Striped bass 4

Lake Mohave

Common carp 12

Channel catfish 8

Largemouth bass 4

Striped bass 4
Treatment facility for bioremediation of perchlorate in Henderson, 

Nevada. Photograph provided by Nevada Division of Environmental 

Protection, Bureau of Corrective Actions, Special Projects Branch. 

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 109

higher than concentrations measured in Las 

Vegas Bay (see Chapter 4). Although perchlorate 

concentrations in Las Vegas Wash, the main 

source of perchlorate to Lake Mead, were higher 

in the past, cleanup and bioremediation since 

1997 has effectively lowered concentrations by 

90 percent. Current concentrations of perchlorate 

in Lake Mead range from 0.06 mg/L at the Las 

Vegas Wash inflow to Lake Mead, to 0.002 mg/L 

11 mi (17.7 km) south of Hoover Dam (see 

Chapter 4).

The fate and transport of metals to LMNRA 

also are of concern, whether from natural 

sources (erosion of weathered rock) or from past 

and present human uses of metals in industry 

(see Chapter 4). Many metals are essential for 

life at low concentrations but become toxic at 

elevated concentrations. Although elements 

such as selenium, copper, iron, chromium, and 

zinc are essential micronutrients for fish and 

invertebrates, other elements, such as arsenic, 

cadmium, mercury, and lead are not considered 

essential for life functions but are known to 

be toxic even at low concentrations when 

ingested over a long period. Moreover, inorganic 

chemicals such as cadmium may act as endocrine 

disrupting compounds at low concentrations. 

Arsenic, cadmium, mercury, and lead have been 

detected in common carp from Gregg Basin 

(near the inflow of the Colorado River to Lake 

Mead) and at Willow Beach on the Colorado 

River downstream of Hoover Dam (Hinck 

and others, 2006). Concentrations of these 

non-essential trace elements in fish from both 

locations were less than the toxicity benchmarks 

for reproduction, growth, and survival (Arsenic, 

5.4 μg/g; Cadmium, 0.12μg/g; mercury, 

4.47μg/g; lead, 0.4μg/g; Jarvinen and Ankley, 

1999). Another study (Patiño and others, 2012) 

in which 63 elements were analyzed in samples 

of common carp from Las Vegas Wash and Bay, 

Overton Arm, and Willow Beach, found a negative 

association between fish health and reproductive 

condition, and levels of certain trace metals (silver, 

arsenic, barium, mercury, iron, selenium, and 

zinc). Fish from Las Vegas Wash generally had the 

highest metal concentrations and the lowest fish 

health and reproductive biomarkers of the four 

sites studied. Las Vegas Bay had the next lowest 

fish health followed by Willow Beach. Overton 

Arm, which was used as a reference site, had the 

best fish health and lowest metal concentrations in 

fish compared to Las Vegas Wash, Las Vegas Bay, 

and Willow Beach.

Selenium is an essential trace element, and 

small amounts are required to meet dietary needs, 

but at elevated concentrations can cause adverse 

health effects in fish and birds, ranging from 

reduced embryo viability and egg hatchability in 

mild cases, to embryo deformities in severe cases 

(Seiler and others, 2003). Selenium concentrations 
110 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

in fish from Gregg Basin and Willow Beach (Hinck and 

others, 2006) exceeded the 1.0 μg/g wet weight (ww) that is 

commonly recommended to prevent toxicity (Hamilton, 2004). 

Humans typically are not at risk from such levels of selenium, 

but wildlife may experience adverse health effects at fairly 

low levels due to biomagnification within the food chain. Even 

if selenium concentrations in water are only slightly elevated, 

organisms at the bottom of the food chain, such as plankton, 

can consume and concentrate selenium in their diet. Through 

the bioaccumulation process, top predators at LMNRA, such 

as bald eagles or striped bass, tend to be exposed to higher 

selenium levels in their diet and may be at greater risk for 

selenium poisoning (Seiler and others, 2003). 

USEPA is currently revising the criterion for the 

concentration of selenium in water that is considered safe 

for aquatic life, also known as the water-quality standard 

for selenium; the existing standard is a concentration of 5 

μg/L. There is some debate about historical selenium levels 

in the Southwest prior to human settlement in the 1800s, but 

most experts believe natural background levels would have 

averaged 1 μg/L or less in the Colorado River Basin (Seiler 

and others, 2003). As the Western United States was settled 

and human activities, such as farming and mining increased, 

selenium concentrations throughout the Colorado River 

and LMNRA became elevated above background, with an 

average concentration from approximately 2 to 4 μg/L (Seiler 

and others, 2003). The large urban areas of Las Vegas, with 

residential lawns, landscaping, and golf courses that require 

routine watering, contribute selenium to LMNRA in the form 

of irrigation runoff. 

With a population of 1.8 million residents in 2012, the 

watering of lawns throughout the Las Vegas Valley serves 

to mobilize selenium from soil at an accelerated rate far 

exceeding concentrations that would otherwise be removed by 

natural rainfall. Leached selenium from the soil is mobilized 

in the shallow groundwater system, ultimately draining to Las 

Vegas Wash and Lake Mead. Stormwater channels typically 

flow year-round in Las Vegas Valley with irrigation-landscape 

source water that can contain selenium at concentrations of 

three to five times the USEPA standard, averaging between 

15 and 25 μg/L (Shanahan and Zhou, 2011). When urban 

flows in stormwater channels reach Las Vegas Wash, selenium 

concentrations are diluted by the higher volume treated 

discharge from wastewater facilities along the Wash. For 

example, selenium concentrations for the mainstem Wash 

below the Clark County Wastewater Reclamation Facility 

typically are less than USEPA standards, averaging between 

3 and 4 μg/L (Ryan and Zhou, 2010). 

Although selenium concentrations (average 2−4 μg/L) 

exceeded background levels throughout the Colorado 

River, including Lake Mead, this concentration is still less 

than the current USEPA water-quality standard of 5 μg/L. 

Some species of migratory birds found along selenium-rich 

tributaries flowing to Lake Mead may experience lower 

reproductive success, but the lower selenium concentrations 

in LMNRA are unlikely to pose significant risk to most 

aquatic life (Hamilton, 2004). However, some species, such 

as the endangered razorback sucker, may be particularly 

sensitive to selenium or particularly vulnerable due to their 

rare or imperiled status. A study of razorback suckers in the 

Upper Colorado River basin by Hamilton and others (2002) 

indicated that selenium concentrations greater than 4.6 μg/g 

in razorback sucker food sources can adversely affect their 

reproductive success. The effect of selenium on reproduction 

of razorback suckers in LMNRA, where highest selenium 

concentrations in Lake Mohave fish was 1.70 μg/g and in Lake 

Mead was 2.19 μg/g (Hinck and others, 2006), is unknown 

and is an area of current research. The health effects of 

chronic, long-term exposures to relatively low concentrations 

of selenium, however, are particularly difficult to detect, 

even for scientists who regularly monitor wildlife health. 

Understanding the effects of long-term exposure to selenium 

can be challenging because (1) selenium toxicity in wildlife is 

often difficult to detect in early life stages (for example, larval 

fish or newly hatched birds), the most vulnerable segment 

of the population, because effects, including mortality and 

deformities, require very intensive monitoring studies; and 

(2) it is difficult to separate the effects of low-level selenium 

toxicity from the other environmental stressors that may cause 

similar health effects, such as low weight gain or a general 

failure of young to thrive. 

Irrigation of a golf course in the Las Vegas Valley. Photograph by Jennell M. Miller, 

University of Nevada, Las Vegas.
Figure 6-2. Illustration of different radionuclides 

(isotopes) formed naturally or from atmospheric testing. 

7

Be = beryllium-7; 14C = carbon-14; CO2 = carbon dioxide; 

137Cs = cesium-137; N = nitrogen; 210Pb = lead-210; 222Rn = 

radon-222; 234Th = thorium-234; 238U = uranium-238.

tac11-4177_fig06-02

222Rn

210Pb

7

Be

14C

137Cs

238U 234Th

N CO2

Sources of short-lived isotopes

Figure 6-3. Concentrations of cesium-137 (137Cs) in an Overton Arm sediment core. 137Cs plotted versus depth (A) 

and date (B) in the sediment, and (C) for 1950–1966 shown with estimated 137Cs deposition for the continental USA from 

individual nuclear weapons test at the Nevada National Security Site. These plots show the sediment records individual 

pulses of 137Cs from tests that deposited high concentrations of 137Cs. Modified from Rosen and Van Metre (2010). 

Test 

series

Global

1963 peak

0

10

20

30

40

50

60

70

80

0.0 0.2 0.4 0.6 0.8

Depth, in centimeters

1930

1940

1950

1960

1970

1980

1990

2000

0.0 0.2 0.4 0.6 0.8 0.0 0.4 0.8

1950

1952

1954

1956

1958

1960

1962

1964

1966

0 200 400

Estimated 137Cs deposited 

(1012 Becquerel)

Overton Arm core 137Cs, in picocuries per gram Year

A. B. C.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 111

Radionuclides from Aboveground 

Nuclear Testing

Above ground nuclear tests were performed 

at the Nevada National Security Site (formerly the 

Nevada Test Site) and other locations in Nevada and the 

United States in the 1950s and 1960s, resulting in the 

distribution of very low levels of radioactive material 

(radionuclides) throughout the Northern Hemisphere 

(fig. 6-2). Radionuclides are isotopes of elements that 

have an unstable nucleus and emit either gamma rays 

and (or) subatomic particles. Some radionuclides decay 

quickly (within seconds or minutes) and some decay 

over periods of years or even millions of years. Because 

of these differences in decay rates, radionuclides 

formed by nuclear testing, or naturally formed by 

the sun, may still be found in small amounts in the 

environment even though above ground testing was 

banned in 1963.

Research has shown that many radionuclides, 

particularly 137Cs, deposited in soils in the arid 

Southwest typically are tightly bound to clays and silts 

(Foster and Haksonson, 1985). Analyses of 137Cs have 

been used to date sediment in studies around the world 

because of its wide distribution, known rate of decay 

(losing half of its radioactivity about every 30 years), 

and known time of deposit from above ground testing 

(see Rosen and Van Metre, 2010). Moreover, because 

Lake Mead is relatively close [less than 100 mi (160.9 

km)]) to the Nevada National Security Site, 137Cs 

concentrations from individual nuclear tests appear 

as discrete peaks in some sediment cores obtained 

from the bed of Lake Mead (see fig. 6-3). A 1995 study 

by Rudin and others (1997) sampled sediment from the 

lower mile of Las Vegas Wash for radionuclides. They 

found 137Cs and other radionuclides that come from 

atmospheric bomb testing at low levels, and additional 

testing and monitoring for radionuclides by the Southern 

Nevada Water Authority have confirmed that drinking 

water for the Las Vegas metropolitan area typically 

contains concentrations of radionuclides below maximum 

contaminant levels developed by the U.S. Environmental 

Protection Agency.
Hoover Dam transformer bank, Units A1 and A2, showing 

A-phase transformer with the new heat exchange unit installed. 

Photograph by Bureau of Reclamation. 

112 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Organic Compounds

Organic compounds contain carbon (C), 

and many of these chemicals in the environment 

are derived from human activity (fig. 6-1), 

although some occur naturally, such as polycyclic 

aromatic hydrocarbons (PAHs) deposited from 

forest fires. Many organic chemicals used in 

agriculture are designed to be toxic to a variety 

of pests and weeds; some of these [such as 

dichlorodiphenyltrichloroethane (DDT)], 

along with some industrial chemicals (such 

as polychlorinated biphenyls, or PCBs), have 

been banned in the United States. Other organic 

chemicals are used in the home as personal 

care products, such as medicine, fragrances, 

and antibacterial soaps. Because many personal 

care products cannot yet be totally removed by 

municipal wastewater treatment, these products 

can be transported to streams and lakes.

Detecting organic chemicals in the 

environment commonly is difficult because 

of the specialized instruments needed and 

because of the vast number of these chemicals 

present in the environment. Typically, organic 

chemicals occur at low concentrations, and 

instruments are needed to detect these chemicals 

at concentrations of about 1 ng/L, which is 

equivalent to about the volume of 1 drop from an 

eyedropper in an Olympic-sized swimming pool. 

Even when organic chemicals are detected, it 

may be difficult to distinguish a specific organic 

compound from the thousands of compounds that 

currently exist. 

In spite of these difficulties, organic 

chemicals have been detected and identified 

in many streams within the United States and 

around the world. For example, a national 

survey documented the presence of low levels 

of organic compounds from treated wastewater 

effluent in 139 targeted streams across the United 

States, including the Las Vegas Wash (Kolpin 

and others, 2002). One or more chemicals 

were detected in samples from 80 percent of 

the streams sampled, and 82 of 95 chemicals 

analyzed were detected at least once. Mixtures 

of these chemicals were common; 50 percent of 

the streams had 7 or more, and 34 percent had 10 

or more compounds detected. Generally, these 

organic compounds were detected at very low 

concentrations (in most cases, less than 1 μg/L). 

Although streams were selected for this study on 

the basis of their susceptibility to contamination 

from wastewater sources (downstream of intense 

urbanization or livestock production), the study 

shows that organic compounds are common in 

aquatic environments. 
Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 113

At Lake Mead, many organic compounds 

in the water come from human activities on 

the lake, such as motorized watercraft use, and 

from the tributary inflow of Las Vegas Wash. 

These compounds, especially water-soluble 

organic compounds, also can be transported 

downstream to Lake Mohave. In addition, 

present and past activities from the building and 

operation of Hoover Dam probably contribute 

some organic compounds to Lake Mohave, 

such as PCBs that were historically used in 

electric transformers. The operation and fueling 

of watercraft within LMNRA can introduce 

other organic compounds into the water, either 

directly as spills of unburned fuel, or indirectly 

as PAHs in engine exhaust (Lico and Johnson, 

2007). Some of these compounds are toxic to 

aquatic biota, are carcinogens, or are thought to 

be endocrine disruptors. Even though a group of 

these chemicals present in gasoline (BTEX) can 

occur at high concentrations during peak periods 

of watercraft use in LNMRA (Chapter 4; Lico 

and Johnson, 2007), they typically do not persist 

in water because they are volatile, move from 

water to air easily, and also can be degraded by 

bacteria. This is illustrated by low or less-thandetection levels of BTEX compounds during 

the non-boating season in all samples collected 

during March 2006. In spite of the fact that 

BTEX compounds have some toxicity and are 

present at high concentrations in LMNRA at 

certain times of the year, samples collected in 

Callville Bay at Lake Mead and at two locations 

on Lake Mohave in 2004–2006 during peak 

watercraft use had low concentrations of the 

chemicals compared to controls (Lico and 

Johnson, 2007). 

The sources of other manmade organic 

chemicals in LMNRA, including small areas of 

agricultural fields, golf courses, landscaped areas, 

industry, and wastewater, are derived primarily 

from Las Vegas Wash. The chemicals from these 

sources are very diverse and have a wide range of 

properties, including environmental persistence 

in water and sediment, toxicity, and unknown 

fate and transport in LMNRA. Las Vegas Wash 

is the main drainageway for natural and urban 

discharge from Las Vegas valley, as well as for 

treated wastewater effluent. Additionally, areas 

adjacent to and upgradient of the Wash have 

historically been used for production of organic 

chemicals including pesticides, such as DDT, 

and endosulfan, as well as industrial chemicals 

like PCBs and dioxins. There are 483 chemicals 

either known or suspected to be associated with 

the BMI Complex (fig. 6-4) in Henderson, Nev. 

(Sahu, 2006). As a result, concentrations of some 

legacy compounds (compounds that are not 

currently used), such as DDT and PCBs, were 

elevated enough in the 1980s to be considered 

potential threats to the health of downgradient 

aquatic ecosystems in Lake Mead. Although the 

concentrations of both compounds in water at 

Lake Mead have decreased since environmental 

regulations banned their use in the 1970s (DDT, 

1972; PCBs, 1979) DDT concentrations in Lake 

Mead were not lowered as quickly due to erosion 

of contaminated sediment in Las Vegas Wash 

below the facilities where DDT was produced 

(Chapter 4; Rosen and Van Metre, 2010). 

Better containment of waste piles since 1980 

have helped to reduce DDT and its breakdown 

products in lake sediments, but these compounds 

are still detected in fish and water in Las Vegas 

Bay (Bevans and others, 1996; Goodbred and 

others, 2007; Rosen and others, 2010). In spite of 

source reductions, some organic compounds are 

found in Las Vegas Wash in concentrations that 

are of concern for fish and wildlife (Advanced 

Concepts and Technologies International, 2011). 
114 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Figure 6-4. Historical aerial photograph of BMI Complex, Henderson, Nevada (1979). Source: Nevada Division of 

Environmental Protection web site, http://ndep.nv.gov/bmi/photos/1979.htm.
Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 115

The transport of organic chemicals from 

Las Vegas Wash into Las Vegas Bay and 

farther downstream in Lake Mead depends on 

the physical and chemical properties of these 

compounds, and on lake hydrology. Physical 

and chemical properties, for example, are 

important factors in determining the degree to 

which an organic compound will persist in the 

environment and, consequently, the exposure 

time of aquatic organisms to a particular 

compound. Some organic chemicals degrade 

quickly in the environment (minutes, hours) 

whereas others, such as DDT, degrade much 

more slowly and may persist for decades. Lake 

hydrology also influences how organic chemicals 

are distributed within the lake; for example, 

in Las Vegas Bay, organic compounds may be 

transported near the bottom of the lake or “float” 

near the surface depending on temperature and 

density differences of water in Las Vegas Wash 

and Las Vegas Bay, and on stratification of the 

water column (LaBounty and Burns, 2005). 

Changes in water density and depths of chemical 

transport from Las Vegas Wash to Las Vegas 

Bay follow a seasonal pattern (Chapter 4) that 

effects the distribution of organic chemicals 

and the exposure of aquatic organisms to these 

chemicals. 

More than 100 organic compounds have 

been detected in various media within LMNRA, 

such as water, sediment, and fish (Bevans and 

others, 1996; Covay and Beck, 2001; Snyder 

and others, 2001; Boyd and Furlong, 2002; 

Osemwengie and Gerstenberger, 2004; Hinck 

and others, 2006; Goodbred and others, 2007; 

Lico and Johnson, 2007; Marr, 2007; Rosen 

and others, 2010; Alvarez and others, 2012). 

In 1995, organic compounds detected in water 

from Las Vegas Bay and Wash and Callville Bay 

included organochlorine compounds, PAHs, 

and phthalates (Bevans and others, 1996). 

More organochlorine compounds were detected 

in water from Las Vegas Wash and at higher 

concentrations compared to Las Vegas Bay. The 

lowest concentrations and number of detections 

were observed at Callville Bay. Organochlorine 

compounds found included hexachlorobenzene, 

dimethyl tetrachloroterephthalate (DCPA), 

several components of chlordane, degradation 

products of DDT, and PCBs. Dioxins also were 

detected in water at higher concentrations in 

Las Vegas Wash compared to Las Vegas Bay 

with the lowest concentrations in Callville Bay 

(Bevans and others, 1996). In a more recent 

study, Alvarez and others (2012) detected 41 

organic contaminants in water from Las Vegas 

Bay, including PAHs, pesticides, Polybrominated 

diphenyl ethers (PDBEs), and PCBs, and 

concluded that the source of some of these 

compounds was flux from the lake bottom 

sediment. In 1998, Covay and Beck (2001) 

analyzed sediment samples from four sites in 

Lake Mead. In Las Vegas Bay, they detected 48 

organic contaminants in the inner bay, and 57 in 

the outer bay; they also detected 31 contaminants 

in Virgin Basin and 26 in Overton Arm. They 

detected PAHs, PCBs, organochlorine pesticides, 

and dioxins and furans; concentrations of the 

latter two groups were higher in Las Vegas 

Bay than in Overton Arm and Virgin Basin, 

suggesting that Las Vegas Wash was one source 

of these compounds. Alvarez and others (2012) 

detected 21 compounds in Las Vegas Bay 

sediment, including PAHs, PCBs, a PDBE, and 

organochlorine pesticides. 
116 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The highest concentrations of organic chemicals detected 

in fish sampled from May 1999 to May 2000 occurred in Las 

Vegas Bay, the area of Lake Mead that receives tributary inflow 

of treated wastewater and urban runoff from Las Vegas Wash 

(fig. 6-5); six times lower concentrations were detected in 

Overton Arm. This pattern of detections of hydrophobic organic 

contaminants in fish from Lake Mead was similar to patterns 

observed in sediment (see Chapter 4).

The number of organic contaminants detected in fish 

also was greater in Las Vegas Bay (33) than in Overton Arm 

(20); many of these contaminants are known or suspected 

endocrine disrupting compounds (Goodbred and others, 2007). 

In addition, the average concentrations of PCBs in seven fish 

sampled from Las Vegas Bay were greater than 0.156 mg/ kg; 

all concentrations were greater than the 0.11mg/kg wet weight 

(ww) value calculated by New York State to protect piscivorous 

wildlife (Newell and others, 1987). In May 1999, fish PCB 

concentrations of 0.026 μg/kg in Overton Arm were well below 

this value. An earlier study of organic compounds in Lake 

Las Vegas Bay Overton Arm

0

100

200

300

400

500

600

700

Sum of organic contaminants in whole body fish tissue,

in micrograms per kilogram

Figure 6-5. Comparison of the total 

mass of organic chemicals in fish in Las 

Vegas Bay and Overton Arm. Inputs from 

Las Vegas Wash contribute to the higher 

mass of organic chemicals for Las Vegas 

Bay fish (modified from Goodbred and 

others, 2007).

Mead fish by Bevans and others (1996) showed the highest 

number of detected oganochlorine compounds in Las Vegas 

Wash (18) compared to 17 in Las Vegas Bay and only 9 in 

Callville Bay. The highest concentrations of all detected 

organochlorines were detected in fish from either Las Vegas 

Bay or Las Vegas Wash. The data from Bevans and others 

(1996) showed that concentrations exceeded the 0.11 mg/kg 

value in 100 percent of the fish (6) from Las Vegas Wash and 

in 66 percent of the fish (4) from Las Vegas Bay, but in none 

of the fish (6) from Callville Bay. An organic contaminant 

study in fish from Lake Mohave (Marr, 2007) showed the 

presence of DDT metabolites, PDBEs, PCBs, chlordane, 

DCPA, HCB, and octachlorostyrene. All concentrations 

of PCBs in fish tissue were below the 0.11 mg/kg value 

although there also were PCBs detected in plasma of both 

razorback suckers and common carp at levels up to 

4 ng/mL.

The latest study (Intertox, 2008) in LMNRA showed 

that PCBs and chlordane collected in 2005 were detected 

only in fish from Las Vegas Wash and Las Vegas Bay and 

not at a reference site, Pahranagat National Wildlife Refuge, 

which is about 90 mi (144.8 km) north of Las Vegas. 

Twenty-five percent (8 of 33) of the fish in Las Vegas Wash/

Valley and Las Vegas Bay had a PCB concentration that 

exceeded the 0.11 mg/kg wet weight criterion to protect 

piscivorous wildlife. DDT breakdown product concentrations 

were highest in fish from Las Vegas Bay and lowest in fish 

from Pahranagat National Wildlife Refuge. The results of 

all these studies of organochlorine contaminants in fish from 

LMNRA suggest that the most significant threat is from 

PCBs, which are continuing to decrease over time but still 

might be of concern for piscivorous wildlife like bald eagles, 

which are at the top of the food chain.

Within the past couple of decades, a group of 

organic chemicals that previously were not considered 

an environmental concern have been detected in streams 

and lakes around the world, including LMNRA. These 

chemicals are collectively known as emerging contaminants, 

and many of them are part of a larger group of organic and 

inorganic chemicals often referred to as endocrine disrupting 

compounds (EDCs) because of potential effects from these 

chemicals on the endocrine system of fish and aquatic 

animals (see “Endocrine System”). Endocrine disruptors 

include legacy compounds such as DDT and PCBs, 

emerging contaminants, as well as metals like cadmium. 
Figure 6-6. Endocrine disrupting compounds mimic the response of normal cells and can (1) mimic normal 

responses and minimally affect the organism, (2) create abnormal responses by the organism’s endocrine system, 

which affect the organism, or (3) block the response of the receptor, which also can be detrimental to the organism.

Normal

hormone

source

Mimic

hormone

source

Hormone

Normal receptors

target cells

Normal

hormone response

Abnormal

hormone response

No response

blocked receptor

1 2 3

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 117

Endocrine disruption occurs through alterations 

in the production or metabolism of hormones, 

the delivery of hormones in the bloodstream, or 

through direct actions on the tissues regulated by 

hormones. Synthetic chemicals known to cause 

endocrine disruption include diethylstilbestrol 

(DES), ethinylestradiol, dioxins, PCBs, DDT/DDE, 

perchlorate, flame retardants, and some heavy metals. 

A great deal of attention has been placed on endocrine 

disruptors with estrogenic (feminizing) activity. 

The strongest evidence of endocrine disruption 

in fish from field studies suggest that exposure to 

steroidal estrogens (including estradiol, estrone, and 

ethinylestradiol) is the major cause of that disruption, 

with alkylphenols (breakdown products of chemicals 

found in detergents and plastics) contributing 

occasionally (Sumpter and Johnson, 2005). 

Endocrine System

What is the endocrine system?

The endocrine system is composed of glands and 

tissues such as the hypothalamus, pituitary, thyroid, 

adrenal, thymus, pancreas, ovaries, and testes. These 

glands and tissues produce and release hormones into the 

bloodstream that travel to different parts of the body to 

control development, growth, reproduction, and behavior. 

In some animal species, hormones also determine 

whether individuals become male or female during their 

early development.

What is an endocrine disruptor?

An endocrine disruptor is a natural or synthetic 

chemical that, when ingested by an organism, mimics, 

modifies, or blocks the actions of hormones and disrupts 

normal physiology (fig. 6-6).
nvtac11-4177_fig06-07

10 μm

Spermatozoa

Oocytes

Figure 6-7. Intersexed ovary from a female common carp 

(Cyprinus carpio), Willow Beach (Hinck and others, 2007).

118 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The first evidence of altered endocrine systems in 

LMNRA was male common carp in Las Vegas Wash and Las 

Vegas Bay that were found to have reduced male hormone 

levels and the presence of a female egg yolk precursor, 

compared to male fish from a reference site (Bevans and 

others, 1996). Subsequently, water sampled from Las Vegas 

Wash and Bay contained detectable levels of estrogenicity

due to the presence of a natural female hormone (estradiol) 

and the artificial female hormone used in birth control 

(ethinyl estradiol; Snyder and others, 1999). These findings 

suggested that exposure to estrogenic compounds in Lake 

Mead might explain, at least partially, the ‘feminization’ of 

male carp. In addition, an ecological risk assessment (Linder 

and Little, 2009) indicated that estradiol at a concentration of 

0.1 ng/L or greater in LMNRA posed a risk to more sensitive 

aquatic life. Although estrogenic compounds were present at 

some locations in LMNRA, and were suspected endocrine 

disruptors, a number of other organic chemicals also were 

present that may have been endocrine disruptors and involved 

in altering the endocrine systems of aquatic organisms and 

associated fish health.

More recent information from environmental studies at 

LMNRA suggest that EDCs likely include a relatively broad 

group of organic chemicals, including emerging compounds, 

such as triclosan, a commonly used antimicrobial (Leiker 

and others, 2009), several types of fragrances like galaxolide 

(Osemwengie and Gerstenberger, 2004; Rosen and others, 

2010), and pharmaceuticals (Boyd and Furlong, 2002; 

Benotti and others, 2010). Several environmental studies 

have documented altered health and endocrine effects in 

some aquatic biota at Lakes Mead and Mohave, including 

reduced testicular growth in male carp from Las Vegas Bay 

(Bevans and others, 1996; Patiño and others, 2003; Goodbred 

and others, 2007), and a number of abnormalities in male 

carp at Willow Beach on Lake Mohave, such as lesions, liver 

and kidney abnormalities, intersexed ovaries (fig. 6-7), and 

testicular abnormalities (Hinck and others, 2007; Patiño and 

others, 2009).

Why is knowledge of endocrine 

disruption important?

At concentrations observed in the environment, 

many synthetic chemicals cause endocrine 

disruption in laboratory animals. Some field 

studies also have implicated endocrine disruption 

as a factor contributing to the impaired health of 

fish and wildlife populations. A national study 

of 139 streams that were considered susceptible 

to contamination from intense urbanization or 

livestock production identified 33 streams that 

contained hormone-based endocrine disruptors and 

46 streams that contained pharmaceutically based 

disruptors (Kolpin and others, 2002). For further 

information on endocrine disruption, see Tulane 

University’s web site: http://e.hormone.tulane.

edu/learning/endocrine-disrupting-chemicals.html. 

Other useful web sites are those of the National 

Institute of Environmental Health Sciences (http://

www.niehs.nih.gov/health/topics/agents/endocrine/

index.cfm) and U.S. Environmental Protection 

Authority (http://www.epa.gov/endo.)

Although intersex fish have been documented at Willow 

Beach, a recent study by Patiño and others (2011) found 

no intersex common carp in Lake Mead in Las Vegas Bay, 

nor in Overton Arm or Las Vegas Wash. The occurrence of 

intersex fish in other areas of LMNRA is uncertain because 

many of the earlier endocrine and reproductive studies at 

Lake Mead did not sample gonadal tissue, a requirement to 

assess intersex. In addition, other species like largemouth bass 

(Micropterus salmoides) (Hinck and others, 2009), which are 

sensitive to developing gonadal intersex characteristics, have 

not been thoroughly assessed yet in LMNRA. Another issue 

to consider in future monitoring of endocrine and reproductive 

health using common carp as a model is to ensure no hybrids 
Male hybrid 

Female hybrid 

Common Carp 

Figure 6-8. Hybrid common carp (Cyprinus carpio) 

and goldfish (Carassius auratus) (top two fish), and 

common carp (bottom fish) from Overton Arm in 

Lake Mead. Note differences in shape and color. 

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 119

of common carp and goldfish are included in the 

analysis. Hybrids look similar (fig. 6-8) and they 

are more susceptible to gonadal tumors and have 

different endocrine profiles that could bias the 

results (Goodbred and others, 2013).

In addition to documenting intersex gonads, 

assessing sperm quality and the presence of 

vitellogenin (precursor of egg yolk normally 

found in female fish) are other ways to measure 

endocrine and reproductive effects of organic 

contaminants on male fish. Many organic 

compounds from wastewater are EDCs because 

they bind to estrogen receptors (Nishihara and 

others, 2000). Evidence to date has suggested 

that exposure to potent steroidal estrogens are 

the primary cause of endocrine disruption in fish, 

particularly the feminization of males (Sumpter 

and Johnson, 2005). Feminization can occur 

after exposure to estrogenic or anti-estrogenic 

compounds, causing alterations in sex steroid 

hormone profiles, sperm quality, and secondary 

sex characteristics (Kime, 1998). Complicating 

the interpretation of biomarkers from field studies 

is the fact that fish are exposed to multiple 

EDCs simultaneously with other chemicals that 

might interact, resulting in unknown synergisms 

(combined effects) or antagonisms (canceling 

effects) and modes of action (Ropero and others, 

2006). Because synthetic sex steroid hormones, 

like ethinyl estradiol (EE2) (the active ingredient 

in oral contraceptives), can be present at 

extremely low concentrations (ng/L) (Ying and 

others, 2002), novel and sensitive methods are 

being developed to detect such environmental 

estrogenic compounds (Snyder and others, 1999, 

2001). Xenobiotic pollutants, including EDCs, 

may disrupt reproductive endocrine function 

by interacting with the hypothalamus-pituitarygonadal axis, as well as between the endocrine 

and immune systems (Arcand-Hoy and Benson, 

1998). Pollutants can affect gamete (egg or 

sperm) development indirectly via disturbance 

of the natural hormonal environment, but if such 

pollutants have hormonal activity themselves, 

they also can directly affect the local hormonal 

environment in which the gamete develops 

(Kime and Nash, 1999). Some environmental 

contaminants can be toxic to the gamete itself. 

Sperm quality is the measure of the ability of 

sperm to successfully fertilize an egg (Rurangwa 

and others, 2004). The quality of sperm is 

a major contributing factor to successful 

production of fish larvae (Kime, 1998) and can 

be affected by endocrine disruption (Kime and 

Nash, 1999). The combination of multiple assays 

on sperm is a better predictor of male fertility 

than any individual test (Jenkins, 2000). 

In 1999, environmental studies in LMNRA 

began using multiple assays of sperm quality to 

assess endocrine and reproductive health of fish. 
nvtac11-4177_fig06-09

0

20 × 109

40 × 109

60 × 109

80 × 109

100 × 109

120 × 109

Las Vegas 

Wash 

Las Vegas 

Bay 

Willow Beach Overton Arm 

Sperm per milliliter of Milt 

2007 

Figure 6-9. Sperm counts from male common carp 

(Cyprinus carpio) in LMNRA show the reference site 

Overton Arm has the highest counts and is statistically 

different. From Jenkins and others (2009).

0 

20 

40 

60 

80 

100 

120 

Havasu + Cibola Mohave 

Percent Sperm Viability

Figure 6-10. Mean sperm viability from male 

common carp collected from downstream sites 

(Lake Havasu and Cibola National Wildlife Refuge) 

were statistically higher than those from the 

upstream site (Lake Mohave). From Jenkins and 

Goodbred (2005). 

120 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Tests included sperm counts, viability, motility, and stage of 

maturation. In Lake Mead, results suggest that sperm quality 

generally is low at sites with high concentrations of organic 

chemicals, such as Las Vegas Bay (fig. 6-9; Goodbred and 

others, 2007; Jenkins and others, 2009). Over a year of 

sampling bimonthly from May 1999–May 2000, sperm 

counts, motility, and percent mature sperm were lower in 

male common carp from Las Vegas Bay than those factors in 

fish from Overton Arm (Goodbred and others, 2007). In Lake 

Mohave, Jenkins and Goodbred (2005) found sperm viability 

reduced by 18 percent compared to fish from areas farther 

downstream in the Colorado River (fig. 6-10). Lower sperm 

quality effectively reduces the ability of male fish to fertilize 

eggs. Ratios of 1,500 sperm per egg are needed to ensure 

good fertilization rates in the catfish (Clarias batrachus)

(Rurangwa and others, 1998). It has been suggested that 

male fish in the wild closely control the sperm/egg ratio to 

achieve the minimum for full fertilization (Warner, 1997). If 

this same ratio is maintained by using milt from a fish that 

has 18 percent reduced sperm viability, the resulting ratio 

of 1,230 viable sperm per egg in fish from Lake Mohave 

would likely result in a reduced fertilization rate (Jenkins 

and Goodbred, 2005). This type of effect could be greater in 

Lake Mead, where reduced mean sperm counts at Las Vegas 

Bay and Wash are at least 30 percent lower than Overton 

Arm (fig. 6-9). 

A summary of selected organic chemicals detected 

in LMNRA is presented in table 6-2. Results of studies in 

LMNRA indicate that the endocrine system and reproduction 

in aquatic biota have been altered by some of these 

chemicals, particularly at Las Vegas Bay and Willow Beach 

(Bevans and others, 1996; Patiño and others, 2003; Jenkins 

and Goodbred, 2005; Hinck and others, 2006; Goodbred and 

others, 2007; Patiño and others, 2009). Legacy compounds 

like DDE have accumulated through the food chain to high 

concentrations in bird eggs near Las Vegas Wash and might 

be causing some effects like eggshell thinning. PCBs also are 

present at fairly high concentration at both Las Vegas Bay 

and Willow Beach and may be affecting reproduction and 

health in aquatic biota. However, many manmade organic 

compounds detected in LMNRA are present at very low 

concentrations and have little or no effect on aquatic biota. 

For example, caffeine has been detected in Lake Mead at 

higher concentrations in samples collected in August than 

in March, reflecting higher recreational use in the summer 

(Boyd and Furlong, 2002), but effects identified in fish are 

minimal (Brinley, 1934). The highest concentrations of 

BTEX compounds detected in LMNRA also showed almost 

negligible toxicity to fish (Lico and Johnson, 2007). 
Table 6-2. Selected organic compounds found in Lake Mead National Recreation Area.

[Abbreviations: WWTP, Wastewater treatment plant effluent; LVW, Las Vegas Wash; LV-H, Surface and subsurface inflow to Las Vegas Wash from Las Vegas 

and Henderson areas; HDT, transformers from Hoover Dam; NA, not analyzed]

Chemicals Use Potential effects Sources

Maximum concentrations

(parts per billion)

Lake Mead Lake Mohave

Legacy compounds

BTEX Components of gasoline Toxicity, cancer Marinas, boats 110.8

(in water)

123.5

(in water)

DDE

(breakdown product

of DDT)

Mosquito control Eggshell thinning

in birds

LVW, LV-H-pesticide 26,920

(in bird egg)

3110

(in fish)

PAHs By-product of burning

fossil fuel, found

in oil, coal, and tar

Cancer, genetic

mutations

LV-H - boat engines 4290

(in sediment)

1288

(in sediment)

PBDEs Flame retardants Endocrine disruption WWTP - clothing, 

old computers 

5834

(in fish)

632

(in fish)

PCBs

(banned in 1979)

Coolant and

insulating fluid

Neurotoxicity,

endocrine disruption

LVW, HDT - old

electric transformers 

51,390

(in fish)

31,600

(in fish)

Emerging compounds

Caffeine Stimulant Increased heart rate

and blood pressure

WWTP-coffee, tea,

energy drinks

70.138

(in water)

NA

Ethinyl estradiol Oral contraception Feminization of

male fish

WWTP, LVW - 

birth control pills

80.0004

(in water)

NA

Galaxolide Fragrance Weakly estrogenic WWTP-LVW -

perfumes, soaps 

52,876

(in fish)

NA

Triclosan

(including breakdown products)

Antimicrobial Alternations in

thyroid hormones

WWTP-LVW -

toothpaste, handsoaps 

519,105

(in fish)

NA

1

Lico and Johnson, 2007.

2

Advanced Concepts and Technologies International, 2010.

3

Hinck and others, 2006.

4

Bevans and others, 1996.

5

Goodbred and others, 2007.

6

Marr, 2007.

7

Boyd and Furlong, 2002.

8

Snyder and others, 1999.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 121
122 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The emerging compounds that are either not currently 

regulated in wastewater discharges or that do not have waterquality standards pose the most interesting challenge. Even 

though present at extremely low concentrations (table 6-2), 

compounds like ethinyl estradiol are very potent endocrine 

disputing compounds in fish and should be monitored in 

LMNRA to assess potential effects on aquatic biota. Other 

emerging compounds, like the fragrance galaxolide, are 

actually bioaccumulating in fish through the food chain 

and/ or water to quite high levels and, although galaxolide 

is not nearly as estrogenic as ethinyl estradiol, it could still 

cause adverse endocrine and reproductive effects (Schnell and 

others, 2009). The compound with the highest concentration 

in fish of any organic chemical detected in LMNRA was 

methyl triclosan—a degradation product of the antimicrobial 

triclosan (table 6-2), which disrupts thyroid hormones 

(Leiker and others, 2009). Thyroid hormones in common 

carp from Lake Mohave were lower than those in fish from 

other sites in the lower Colorado and could indicate effects 

of organic contaminants, but cooler water temperatures also 

might be a factor (Marr, 2007). A recent study in Lake Mead 

that analyzed thyroid hormones in fish is currently being 

completed and will provide some insight if this is an issue, 

especially because concentrations of triclosan can be high in 

fish (Goodbred and others, 2007). 

Organic chemicals detected in LNMRA have a wide 

diversity of sources and properties and are present in water, 

sediment, and aquatic biota at varying concentrations. 

However, one factor that may potentially increase the effects 

of organic chemicals is the extended drought in southern 

Nevada that has caused lower lake-surface levels in Lake 

Mead, resulting in a decrease in dilution of contaminants 

within the lake (Benotti and others, 2010). This effect may 

be a greater problem downstream of major sources of organic 

chemicals, such as Las Vegas Bay. As a result, alterations to 

endocrine systems, reproduction, and fish health in LMNRA 

might become more significant in the future if drought 

conditions are sustained, causing possible increases in organic 

chemical concentrations. If the drought continues much 

longer, monitoring of selected manmade organic chemicals to 

document any changes in concentrations seems warranted.

Algal Blooms

An increase in abundance of algae is one of the most 

common, readily visible effects of nutrient loading into 

aquatic ecosystems (Reckhow and Chapra, 1983; Carpenter 

and others, 1998) that can be a stressor on the system. With 

the exception of inner Las Vegas Bay, which is considered 

nutrient rich (or eutrophic at certain times of the year) due 

to its relatively high content of nutrients from wastewater, 

most regions of Lake Mead have normal levels of nutrients 

(mildly mesotrophic to oligotrophic; Chapter 4). The typical 

mesotrophic-to-oligotrophic conditions in the lake generally 

support fairly clear lake conditions rather than high levels 

of algal growth (primary productivity; Lieberman, 1995; 

LaBounty and Horn, 1997). A major algal bloom in Boulder 

Basin in 2001 (LaBounty and Burns, 2005; Chapter 4), 

however, indicated that Lake Mead has the potential to 

support very high algae growth when certain conditions 

are met. Although algae are essential to food-web function, 

an algal bloom can be a stressor primarily because of 

decreased dissolved oxygen (DO) levels that can result. 

When algae die and sink to the lake bottom, bacteria feed 

on the dead algae, using oxygen in the process. This activity 

can decrease concentrations of DO in the water column, 

especially in the hypolimnion, when the lake is stratified. In 

some circumstances, DO can be depleted in bottom waters, 

creating a condition known as hypoxia, which has occurred 

across relatively large aquatic ecosystems, such as parts of 

the Gulf of Mexico below the Mississippi River (Dale and 

others, 2010). Certain species of algae also can stress aquatic 

ecosystems by producing toxins that can be harmful during 

large blooms (Landsberg, 2002). In LMNRA, some blue-green 

algae (cyanobacteria) exist that have the potential to produce 

cyanotoxins. In other lakes, mussels, such as quagga mussels, 

have caused cyanobacterial blooms following colonization 

(Higgins and Van Zanden, 2010, and references therein). 

Cyanotoxins can be highly toxic to animals and have caused 

mortalities around the world (Stewart and others, 2008). 

Although it is unlikely that cyanobacterial toxins could cause 

significant effects in LMNRA (Chapter 4), continued research 

and monitoring of algae toxins and cyanobacteria populations 

would be prudent if quagga mussel populations continue to 

increase in Lakes Mead and Mohave.
Photograph of quagga mussel (Dreissena rostriformis bugensis) (top 

left), Asian clam (Corbicula fluminea) (top right), and New Zealand 

mudsnail (Potamopyrgus antipodarum) (bottom center) collected 

from Lake Mead, Nevada-Arizona. Photograph by Scott Rainville, 

University of Nevada, Las Vegas.

Photomicrograph of golden algae (Prymnesium parvum); 1250x 

magnification. Photograph by Ann St. Amand, Ph.D., PhycoTech, Inc.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 123

Golden alga (Prymnesium parvum) is a small organism 

tolerant of a wide range of salinity and temperature 

environments and produces toxins capable of killing gilled 

aquatic organisms like fish (Edvardsen and Imai, 2006; 

Southard and others, 2010). The first recorded incidence of 

a golden algal bloom in inland waters of the United States 

was in 1985 in the Pecos River, Texas (James and de la Cruz, 

1989). Since then, golden algae have been linked to killing 

millions of fish of different species in Texas, but records of 

blooms also have been found in many other States, including 

Arizona and Nevada (Sager and others, 2008; Southard and 

others, 2010). In Lake Mead, samples from Boulder Basin 

indicated the presence of low densities of golden alga cells as 

early as 2001 (LaBounty and Burns, 2005). A unique aspect 

of golden algal blooms in inland (typically brackish) waters is 

that they tend to occur when water temperatures are seasonally 

low (Sager and others, 2008). 

Invasive Species 

By Wai Hing Wong

Invasive species are any non-native species whose 

introduction creates or is likely to cause environmental or 

economic harm, or harm to human health. For example, 

invasive species often cause damage to the native ecosystem 

or increase industrial costs, such as maintaining cooling 

water intakes at power plants (see details below; Charles 

and Dukes, 2007). The rapid introduction of most invasive 

species is caused by human activities, whether intentional or 

unintentional, such as recreational boating, cargo shipping, 

and aquaculture (Nentwig, 2007). Quagga mussels, Asian 

clams, and New Zealand mudsnails are three major aquatic 

invasive species found in Lake Mead. These species all belong 

to the Mollusca phylum of invertebrate animals that include 

both freshwater and marine mussels, clams, oysters, and 

snails. Among these, quagga mussels are the most destructive 

because they attach in vast numbers to hard surfaces 

(biofouling), such as drinking-water intakes (fig. 6-11). The 

other effect of quagga mussels is an alteration of the food 

chain and ecosystem by collectively filtering huge amounts of 

water and removing large amounts of plankton for food, which 

then becomes unavailable for other aquatic biota like larval fish.

However, for the first time in Nevada, a toxic golden alga 

bloom was reported in Lake Las Vegas in the winter of 2010 

(Weber and Janik, 2010). This event raised concern about 

the potential for toxic blooms to spread into LMNRA and 

become yet another stressor to aquatic biota. Unfortunately, 

the environmental factors responsible for golden algal blooms 

or how and when they produce toxins are not fully understood 

(Edvardsen and Imai, 2006; Sager and others, 2008), and as a 

result, it may be difficult to manage blooms if they do occur or 

to try to prevent them from spreading to LMNRA.
The quagga mussel and another related 

dreissenid species, such as the zebra mussel 

(Dreissena polymorpha pallas), are native to 

Europe and were accidentally introduced into 

the Laurentian Great Lakes in North America 

during the 1980s (Nalepa and Schloesser, 1993). 

Currently, quagga mussels have been detected in 

only about 50 lakes outside of the Great Lakes 

(U.S. Geological Survey, 2012). Quagga mussels 

were first detected in the Boulder Basin of Lake 

Mead (Nevada and Arizona, USA) in January 

2007. The discovery of quagga mussels was 

of considerable interest because it was the first 

confirmed introduction of a dreissenid species in 

the Western United States (LaBounty and Roefer, 

ac11-4177_fig06-11

A. B.

C. D.

Figure 6-11. Examples of quagga mussel fouling at Lake Mead, Nevada-Arizona. (A) Water intake; (B) 

boat hull exterior; (C) dam gate; and (D) a portion of a sandal (Wong and others, 2011). Photographs by: 

(A) and (B) Bryan Moore, National Park Service; (C) Dave Arend, Bureau of Reclamation; and (D) by 

Wai Hing Wong, University of Nevada, Las Vegas.

2007; Stokstad, 2007). Since 2002, LMNRA has 

had policies in place for inspecting large boats 

arriving from east of the Rocky Mountains via 

transport company haul permits. From 2003 

to 2007, 6 of the 54 inspected boats entering 

LMNRA had invasive mussels on their hulls and 

were quarantined and cleaned prior to launch 

(Hickey, 2010). Most scientists studying the issue 

believe that quagga mussels were introduced into 

Lake Mead by recreational boats from the Great 

Lakes region, because adult quagga mussels are 

resistant to drying out for long periods (multiple 

days) and can tightly close their shell when out 

of water.

124 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
The number of young plankton-like quagga mussels 

(which can float or swim and are called veligers) increased 

five times in Boulder Basin of Lake Mead, from 0.9 

individuals/L in 2007 to 4.5 individuals/L in 2008. In 

contrast, the numbers of adult quagga mussels counted 

from 2007 to 2009 increased more than 14 times (624 to 

8,925 individuals/ m2) in rocky areas, and increased more than 

41 times (80 to 3,350 individuals/m2) in sandy and muddy 

areas (Loomis and others, 2011; Wong and others, in press). 

Generally, there are more quagga mussels on hard substrates 

than in soft sediments (Wittmann and others, 2010; Loomis 

and others, 2011). The presence and spread of the invasive 

quagga mussel at LMNRA may be influencing lake clarity 

and food-chain dynamics. From 2007 to 2009, water clarity 

has increased significantly (13 percent) in Boulder Basin 

of Lake Mead (Chapter 3). The increase in lake clarity was 

due primarily to a decline in algae evidenced by declining 

measurements of chlorophyll (45 percent), a pigment used in 

algae for photosynthesis (Wong and others, 2011). However, 

it is unclear how much of the change in lake clarity can be 

attributed to quagga mussels, as new processing systems 

for wastewater-treatment plants along Las Vegas Wash has 

significantly reduced the amounts of phosphorus entering 

Lake Mead. Because phosphorus is the limiting nutrient for 

algae growth in Lake Mead, any reductions of phosphorus also 

reduce algae growth, decreasing chlorophyll and improving 

water clarity. It is interesting that even with a significant 

increase in the number of quagga mussels, no detectable 

changes in composition and abundances to the lower levels of 

the food chain at Lake Mead have been documented (Beaver 

and others, 2010; Wong and others, 2010). Moreover, for 

higher levels of the food chain, no detectable differences in 

threadfin shad abundance is apparent; however, for gizzard 

shad, an invasive species first found in Lake Mead in 2007, 

abundances are increasing (Loomis and others, 2011). 

The presence of quagga mussels in Lake Mead may 

cause a number of potential ecosystems changes (fig. 6-12; 

Wong and others, in press). For example, quagga mussels 

filter plankton and nutrients from the middle of the water 

column and deposit their fecal pellets on the bottom of the 

lake, shifting energy from the pelagic (swimming) community 

to the benthic (bottom) community. Because of this shift in 

energy, fish, such as common carp, that feed on the bottom 

will benefit, while species, such as threadfin shad, that eat 

algae and zooplankton will be negatively affected. Quagga 

mussels also will reduce suspended solids in the water column, 

providing a general increase in water clarity (Wong and others, 

2003; Binding and others, 2007; Wong and others, 2011); as a 

result, shallow areas in Lake Mead may have improved habitat 

for both rooted aquatic plants and benthic algae. Additionally, 

the large population of quagga mussels currently (2012) in the 

lake may cause an increase in dissolved inorganic phosphorus 

and nitrogen (Wong and others, 2011). With substantial 

populations of mussels on the bottom of Lake Mead, the 

concentration of dissolved oxygen in the hypolimnion 

(Chapter 4) can be significantly reduced, especially during 

times of stratification (Caraco and others, 2006). 

(quagga mussels and native species)

Quagga Mussels and the Ecosystem 

Phytoplankton 

Benthic Sediment Quality 

Excretion and

Fecal Pellets 

Dissolved Oxygen

Fish 

Suspended Solids 

Water Clarity Aquatic Plants 

Benthic Invertebrates 

Shells

in spawning habitat 

Dissolved Inorganic 

Phosphorus

Dissolved Inorganic 

Nitrogen

and

Aquatic Birds

more plants provide cover for fish

Live mussels and the process of 

decomposing dead mussels consume 

oxygen in the benthic environment

certain fish

 eat quagga mussels

Microzooplankton 

Figure 6-12. Simplified conceptual diagram of the potential 

ecological impacts of quagga mussels on the Lake Mead 

ecosystem (modified from Wong and others, in press). 

Enhancing/increasing effects (green) and lowering/decreasing 

effects (brown) are shown. Solid and dashed lines represent 

direct and indirect quagga mussel impacts, respectively. The 

wider the line, the greater the expected impact. 

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 125
The New Zealand mudsnail was first discovered in 

Lake Mead in 2008 (Davis and Moeltner, 2010); however, 

information is sparse on its distribution throughout LMNRA. 

Although a small aquatic snail—the shell is typically 0.20 by 

0.47 in. (5 by 12 mm)—it is hardy and robust, and likely why 

it has successfully invaded lakes and streams in Australia, 

Asia, Europe, and North America. In the United States, the 

New Zealand mudsnail was originally found in Idaho’s Snake 

River in 1987, and currently (2012) is found throughout 

the Western United States (Benson, 2011); in some western 

streams, the New Zealand mudsnail occurs at densities greater 

than 0.5 million/m2. Because the Western United States is 

well known for its world-class trout and salmon fishing, 

there is concern that the mudsnails may impact the food 

chain for native fish, in addition to changing the physical 

characteristics of the streams (Benson, 2011). Due to the lack 

of New Zealand mudsnail studies at Lake Mead, however, the 

potential environmental impacts of this species in LMNRA are 

unknown. 

Asian clams, which are native to Southeast Asia, 

successfully invaded North American waters at the beginning 

of the 20th century. They are currently found in 36 States 

within the United States, as well as in northern and central 

Mexico (McMahon and Bogan, 2001). Asian clams live in 

soft sediment and are harvested in some locations for food. 

Similar to quagga mussels, Asian clams are bivalve mollusks 

that feed by filtering small food particles from the water. 

As a result of filter feeding, Asian clams can reduce algal 

loads, suspended-sediment particles, and some nutrients like 

phosphates, as well as increase water clarity (Karatayev and 

others, 2003). However, Asian clams also cause bioturbation

of the lake bottom, disturbing other animals and plants by 

moving sediments to dislodge food or directly consuming 

benthic fauna (Karatayev and others, 2003). Moreover, 

because of the high density of clams in some areas or on some 

water intake structures, Asian clams have caused significant 

damage to some industries; for clogged water intakes in 1986, 

damage was estimated at $1 billion (Isom, 1986). The massive 

clogging of intake structures by Asian clams happens because 

the clams have a very high reproductive rate (greater than 

68,000 pediveligers per adult per year) and adult numbers 

can exceed 2,000 individuals/m2. Asian clams were first 

discovered at Lake Mead in 1959 (Counts, 1991) and have 

occurred at densities as high as 100 individuals/m2 in the mid1980s (Melancon, 1977; Peck and others, 1987), significantly 

less than densities found in other areas of the United States. 

Current population densities have been documented at less 

than 50 individuals/m2 in Lake Mead (Wittmann and others, 

2010), indicating that Asian clams seem to be declining, but 

reasons for the decline are unknown. 

Human Health, Pathogens, and Suitability for 

Recreation

By Kent Turner, Craig Palmer, and Peggy Roefer

All bodies of water, including those within LMNRA, 

have the potential to be contaminated by pathogenic 

bacteria and other disease-causing organisms due to fecal 

contamination by waterfowl, warm-blooded animals, and 

humans, even if the body of water is considered pristine. 

Water managers worldwide are concerned over the potential 

for human illness from microbial organisms, algal toxins, or 

other contaminants. Agencies responsible for water-quality 

standards and management at Lakes Mead and Mohave 

share this concern as the lakes are used for a wide variety of 

recreational opportunities that involve full-body contact with 

the water, including swimming, waterskiing, and personal 

watercraft use. The most common type of swimming related 

illness is gastroenteritis, an inflammation of the stomach 

and the intestines that can cause symptoms like vomiting, 

headaches, and fever (U.S. Environmental Protection Agency, 

2010).

The potential for occurrence of illness-causing organisms 

can be increased by pollutants entering the water, either 

on a watershed scale through influences of nearby cities 

and communities, or on a more local scale through, for 

example, fecal material from the recreating public or from 

animals living in or near the water. Conditions that can 

lead to increased numbers of disease-causing organisms 

also can be triggered by warmer water temperatures, lower 

water levels, shallow and stagnant water, and in some cases, 

re-suspension of bacteria from sediments caused by wave 

action. Additionally, appreciable rainfall and runoff within an 

urbanized tributary can temporarily increase the potential for 

pathogenic bacteria in a lake. 

Potential pathogenic bacteria and organisms usually 

occur in small numbers, which makes them difficult to isolate 

and monitor. For that reason, water managers typically use 

other bacteria as indicators of fecal contamination to assess 

a lake’s potential to harbor pathogens and cause illness for 

recreational water users. For example, Escherichia coli is 

a type of coliform bacteria that is found in the feces of 

humans and other warm-blooded animals. U.S. Environmental 

Protection Agency (2011a) recommends using E. coli as an 

indicator bacteria for health risk from full-body contact in 

recreational waters (http://water.epa.gov/type/oceb/beaches/).

Lake Mead receives treated wastewater effluent and 

urban runoff from Las Vegas Valley via Las Vegas Wash. A 

discharge permit from the Nevada Division of Environmental 

126 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Protection (NDEP) is required for wastewater entering 

Las Vegas Wash. The urban runoff within the Wash is 

managed within regulations set by NDEP and the USEPA. 

The wastewater discharged into the Wash is treated and 

disinfected, and bacterial concentrations are normally well 

below established limits for body-contact recreation (even 

though Las Vegas Wash is not designated as a body-contact 

recreational water by NDEP). Indicator bacterial counts within 

Las Vegas Wash can be temporarily elevated above the limits 

recommended for body contact recreation, however, during 

periods of higher urban run-off or other inputs into the Wash 

(Southern Nevada Water Authority, 2011). 

Because of the importance of Lake Mead as a recreation 

area and drinking-water source, NDEP has placed weekly to 

monthly monitoring requirements for indicator bacteria at a 

number of locations on Las Vegas Wash and Lake Mead. That 

monitoring indicates that surface-water quality consistently 

meets body-contact recreational standards. These standards are 

met even within the inner Las Vegas Bay, the area of the lake 

most impacted by the water quality of the Wash (LaBounty, 

2005; Holdren and others, 2008). For example, during 2000–

2004, there was never an occurrence when concentrations 

of either fecal coliforms or E. coli in the middle and outer 

portions of Las Vegas Bay exceeded full body contact 

standards (LaBounty and others, 2003; LaBounty, 2005). In 

addition, a review was conducted of 2000–2010 data from 

routine Lake Mead bacteriological water-quality monitoring 

at 29 sites for fecal coliforms, fecal Streptococcus, and E. coli, 

with more than 3,738 samples collected (Palmer and others, 

2012). Results of the monitoring showed only 0.9 percent of 

samples contained E. coli concentrations that exceeded the 

limit set by USEPA for full-body contact in recreational waters 

(U.S. Environmental Protection Agency, 2011a). In addition, 

only 1.5 percent of samples exceeded the acceptable full-body 

contact limit for fecal Streptococcus, and 3.8 percent of the 

samples exceeded the acceptable limit for fecal coliform.

The USEPA National Lake Assessment (U.S. 

Environmental Protection Agency, 2010) collected and 

reviewed data for a number of ecosystem and human health 

indicators to characterize the state of the nation’s lakes overall, 

and provide a context for evaluation of individual lakes 

(see U.S. Environmental Protection Agency National Lakes 

Assessment and Lake Mead in Chapter 4). These studies found 

that Lake Mead was well within the USEPA National Lake 

Assessment criteria for classification within the best or “good” 

range for suitability for recreation, as related to human health. 

USEPA’s indicators of suitability for recreation included 

chlorophyll a, cyanobacteria (related to potential of algal 

toxins), and microcystin (related to algal toxins). 

As an additional evaluation of potential lake-wide 

human health issues, LMNRA, in partnership with Southern 

Nevada Water Authority, has monitored a number of highuse recreational coves and beaches on Lakes Mead and 

Mohave for 9 years. Water samples were collected by the 

NPS between May and September at nine high-use recreation 

sites from 2003 to 2010. An additional sampling site (Placer 

Cove) was added in 2008. Analyses of 655 water samples 

for concentrations of fecal indicator bacteria showed that 

fecal coliforms were present in concentrations exceeding the 

USEPA acceptable limit for full-body contact in recreational 

waters in 3.1 percent of the samples (U.S. Environmental 

Protection Agency, 2011a). Enterococcus concentrations 

exceeded the acceptable limit in 6.0 percent of 496 samples 

analyzed and fecal Streptococcus concentrations exceeded 

the acceptable limit in 5.2 percent of 649 samples analyzed. 

E. coli concentrations exceeded the USEPA acceptable 

limit for full-body contact recreation in only 0.6 percent of 

the samples. Throughout the 9-year study, Six Mile Coves 

(fig. 6-13), Placer Cove, and Box Car Cove were identified 

as those with the highest single-occurrence frequency of 

unacceptable levels of indicator bacteria.

In addition to bacteria, water managers at LMNRA also 

are concerned with a number of viruses and protozoans that 

can occur in any body of water and cause human illness, 

particularly two protozoan parasites, Cryptosporidium and 

Giardia. Cryptosporidium is commonly found in lakes 

and rivers, especially when the water is contaminated with 

sewage and animal waste. Giardia is a parasite that lives in 

the intestine of infected humans or animals and is found on 

surfaces or in soil, food, or water that has been contaminated 

with the feces from infected humans or animals. Both of 

these intestinal parasites can be spread by wildlife and are 

now widely distributed throughout the Western United 

States (U.S. Environmental Protection Agency, 2011b). 

The Southern Nevada Water Authority routinely tests for 

both of these organisms as a part of its drinking-water 

monitoring requirements. Monitoring over the past 10 years 

has detected very low numbers (1–2) of Giardia cysts and 

Cryptosporidium oocysts, with most of detected organisms 

being Giardia. During 2010, neither organism was detected 

at the drinking-water intakes within Lake Mead (Southern 

Nevada Water Authority, 2011).

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 127
nvtac11-4177_fig06-13

95

95

215

95

93

93

93

15

15

NEVADA

NEVADA ARIZONA

River

Colorado River

HOOVER DAM

Willow Beach

National Fish 

Hatchery (USFWS)

DAVIS DAM

Las Vegas

Henderson

Boulder 

City

Lake Mead

Fish Hatchery

(NDOW)

Alfred Merritt

Smith Water 

Treatment 

Facility (SNWA)

Muddy

Logandale

Overton

Searchlight

Laughlin

Gregg Basin

Overton Arm

Temple Basin

Virgin Basin

Las Vegas Bay

Boulder

Basin

Pearce 

Ferry

Boulder Canyon

Black 

Canyon

Virgin 

Canyon

Iceberg

Canyon

Virgin River

Wash

LasVegas

Base from National Park Service and U.S. Geological Survey digital data,1:675,000

Universal Transverse Mercator projection, Zone 11

North American Datum 1983 

Callville Bay

LAKE

MOHAVE

MEAD

LAKE

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Grand Canyon National Park

Lake Mead National

 Recreation Area

Sample sites

Middle Point Cove

Tea Kettle Cove

James Bay

Sandy Cove

Boxcar Cove

Boulder Beach

Hemenway Harbor

Placer Cove

Six Mile Coves 

Telephone Cove 

Arizona, South 

115°0'W 114°30'W 114°0'W

36°30'N

36°0'N

35°30'N

Nevada

Arizona

Study Area

Figure 6-13. Map of bacteriological cove-monitoring sites at Lake Mead National Recreation Area. 

NDOW, Nevada Department of Wildlife; SNWA, Southern Nevada Water Authority; USFWS, U.S. Fish and 

Wildlife Service.

128 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Effects of Climate Change

Observations and studies have shown that many natural 

systems are being affected by regional climate changes, 

particularly temperature increases, and that these changes 

likely will affect the hydrological cycle, with associated 

impacts to water resources (Brekke and others, 2009). For 

example, drought in the Southwestern United States during 

2000 to 2004, a consequence of both reduced precipitation and 

a series of the hottest years on record, resulted in streamflows 

that were lower than those during the Dust Bowl of the 1930s 

or the drought of the 1950s (Andreadis and Lettenmaier, 

2006). Drought conditions caused by below average winter 

snow accumulation in the Rocky Mountains have periodically 

reduced surface-water levels and associated storage volumes 

in Lake Mead (fig. 6-14). Some of the reduced surface 

levels in the lake during historical regional droughts likely 

were caused by increased water demand for agriculture and 

municipal use (irrigating crops and watering lawns, etc). Water 

Drought in the Southwestern United States over the past 10 years 

has led to dramatic lowering of Lake Mead. The top of the white 

layer is where the lake level was in 1999. Photograph by Michael R. 

Rosen, U.S. Geological Survey.

Drought

Filling of

Lake Powell 

Drought

1930 1940 1950 1960 1970 1980 1990 2000 2010

1,000

1,020

1,040

1,060

1,080

1,100

1,120

1,140

1,160

1,180

1,200

1,220

1,240

304

310

316

322

328

334

340

347

353

359

365

371

377

Lake level, feet above datum

Lake level, meters above datum

Year

Elevation is referenced to the U.S.Geological Survey datum, adjustment of 1912, locally known as "Power House Datum.”

Figure 6-14. Lake Mead water levels show low levels during drought periods, 1930–2010. 

levels also were lower in Lake Mead when Lake Powell (a 

large reservoir upstream of Lake Mead) was being filled in 

the mid-1980s.

Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 129
Simulations made with models of future climate in the 

Southwest indicate that water levels in Lake Mead will be 

affected by increased evaporation as air temperatures increase, 

but also by over-allocation of water from the Colorado River 

(Barnett and Pierce, 2008, 2009). In fact, temperatures in 

the Colorado River Basin are projected to increase by 5–6°F 

(2–3 °C) in the 21st century, with slightly larger increases in 

the upper Basin (Bureau of Reclamation, 2011). Additionally, 

projections of precipitation indicate that the 30-year average 

in 2070–2099 will be drier in the Southwestern United States 

than in 1950–1979 (Bureau of Reclamation, 2011). Moreover, 

the models considered to be most accurate predict that flow 

in the Colorado River basin will be reduced by 5–20 percent 

from current levels (Ray and others, 2009). As a result of 

potential, future increases in air temperature and evaporation, 

and reduced precipitation and flow in the Colorado River 

Basin, water managers will be faced with difficult choices on 

how water in the Colorado River is utilized to prevent water 

shortages at Lake Mead and in the Southwest (Ackerman and 

Stanton, 2011). 

Climate models for the Southwestern United States 

and the Colorado River Basin also have been applied to 

predictions of potential future changes in water quality at 

LMNRA. Most climate models forecast changes in rainfall 

patterns, including greater probabilities for higher intensity 

rainfall, flash floods, and storm events (Brekke and others, 

2009). These events increase the potential for transfer of 

nutrients and pollutants, including pathogenic bacteria, to 

Lake Mead. Monitoring of indicator organisms has shown 

that unacceptable levels of indicator bacteria generally are 

associated with periods immediately following a major rainfall 

or storm event (Southern Nevada Water Authority, 2011). 

Moreover, forecasts of increased air and water temperature 

and evaporation, reduced precipitation, and resulting declines 

in surface-water levels at Lake Mead (Brekke and others, 

2009) could possibly cause sediment to become exposed in 

certain areas, and increase the concentrations of resuspended 

sediment in the lake. As a result of these potential changes, 

some areas of the lake could become more conducive to local 

increases in algae and microorganisms that could possibly 

create toxic or infectious conditions. Additionally, warmer 

water temperatures have the potential to increase the spread of 

some disease-causing organisms. For example, the extremely 

rare amoeba Naeglaria fowlerii has been documented as the 

cause of three deaths within the Southern United States in 

2011 (Centers for Disease Control and Prevention, 2012). 

And although rare, this amoeba is distributed worldwide, and 

generally lives in warm, shallow, stagnant water (Centers for 

Disease Control and Prevention, 2012). 

Currently (2012), most of the research on climate 

influences and water availability at LMNRA is focused on 

human population effects, but no research has been initiated 

at LMNRA to show the potential effects of changing climate 

on ecosystem stressors or aquatic biota. For example, lowered 

surface-water levels likely will eliminate some shallowlake areas and cause loss of habitat. Razorback suckers 

traditionally have used very specific and relatively shallow 

areas in Las Vegas Bay to spawn, such as Blackbird Point; 

however, effects of lowered lake levels on razorback sucker 

reproduction is unknown. Additionally, other studies have 

shown that the concentrations of some organic contaminants 

have increased over time, perhaps due to lower lake levels, 

but the influence of this trend on aquatic organisms also is 

unknown (Benotti and others, 2010). The potential, future 

influence of increased water temperature on the occurrence of 

algal blooms and associated, reduced oxygen levels in the lake 

(Poff and others, 2002) is another climate-induced ecosystem 

concern at LMNRA. 

Population Growth

The population of Clark County, and particularly 

the greater metropolitan Las Vegas area, has grown at an 

exponential rate since the 1940s (fig. 4-22). The 2010 census 

documented the population in Clark County at 1.95 million 

people, a 38 percent increase since 2000, for an average 

increase of 3.8 percent per year. After the economic downturn 

(recession) in 2008, however, population growth of Clark 

County has been quite variable, with increases of 1.0 percent 

growth in 2009 and 1.5 percent growth in 2010, but a decrease 

of 3.4 percent in 2011 (http://www.clarkcountynv.gov/

Depts/comprehensive_planning/demographics/Documents/

Stormwater flows in Las Vegas Wash (2008). Photograph by Las 

Vegas Wash Coordination Committee.

130 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
HistoricalCCLVVAveragePopGrowthRate.xls). The cities of 

Mesquite, Nev., and St. George, Utah, both along the Virgin 

River north of Lake Mead, also grew rapidly between 1990 

and 2010. Due to this growth, plans have been developed to 

construct a pipeline from Lake Powell to St. George, Utah, for 

delivery of more than 80,000 acre-ft of water annually, enough 

water for an additional 400,000 people. 

Increases in population often are ecosystem stressors 

because of the increased water use and need for wastewater 

treatment, and also because of increased chemical use in 

households (personal-care products), and to maintain lawns 

and golf courses (fertilizers and pesticides). All of these 

contaminants may potentially enter LMNRA through treated 

wastewater discharge to tributary rivers, such as Las Vegas 

Wash or the Virgin River (see Chapters 1 and 2). 

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Chapter 6

Threats and Stressors to the Health of the Ecosystems of Lakes Mead and Mohave 137
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138 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Chapter 7

Management Implications of the Science 139

Lake Mead, particularly its Boulder Basin, is one of the most intensively monitored reservoirs in the 

United States. With its importance to societal needs and ecosystem benefits, interest in water quality 

and water resources of Lake Mead will remain high. A number of agencies have authorities and 

management interests in Lake Mead and maintain individual agency monitoring programs. These 

programs were enhanced on an interagency basis from 2004 to 2012 to facilitate intensive monitoring 

in all major basins of the lake. Recognition that increasing stressors and influences in individual basins 

can affect water quality throughout Lake Mead and gave rise to an even stronger effort towards the 

development of holistic and effective interagency approaches. 

In 2010, agency monitoring programs were used to develop a management plan for waterdependent resources at Lake Mead National Recreation Area (LMNRA). The Long-Term Limnological 

and Aquatic Resource Monitoring and Research Plan for Lakes Mead and Mohave (the Plan; National 

Park Service, 2010) documented key management questions to be addressed through monitoring 

and research, and identified interagency strategic objectives for water quality and water-dependent 

resources. Moreover, the Plan provides a framework for summarizing water quality and water resource 

information in five resource categories: water quality and limnology; fish and aquatic biota; sediments; 

birds; and riparian vegetation. The Plan also addresses three stressors to lake resources: contaminants, 

invasive species, and climate change. For each of these topics, the current (2012) state of knowledge 

is summarized for LMNRA (table 7-1), including key scientific questions and findings, management 

implications, and information needs. A more detailed discussion for each topic follows.

1

 National Park Service

2

 U.S. Geological Survey

3

 University of Nevada, Las Vegas

Chapter

7 Management 

Implications of 

the Science

By Kent Turner1, Steven L. Goodbred2, 

Michael R. Rosen2

, and Jennell M. Miller3

View of Boulder Basin, Lake Mead from Fortification Hill. Photograph by National Park Service.
Table 7-1. Key scientific findings, management implications, and recommendations.

Resource component

and related goals Scientific findings Management implications Recommendations for data or information needs

Water quality and

limnology

Basic water quality parameters are

considered well within good ranges

compared to both Nevada state 

standards and USEPA National 

Lake Assessment criteria. Potential 

problems with nutrient balance, algae, 

and dissolved oxygen can occur at 

times and in some areas of Lake Mead. 

High quality Colorado River water is 

detectable as underflow all the way to 

Hoover Dam, driving base hydrology 

and mixing. 

Recent (2004–2012) intense and

Lake Mead-wide scope of monitoring

have provided a much better 

understanding of the hydrology and

water quality; more information is 

needed for Lake Mohave. Highest 

productivity exists near tributary 

inflow areas; these areas also have 

greatest potential for early detection 

of nutrient related issues. 

Maintaining existing (2012) level 

of lake-wide monitoring of physical 

and biological parameters of water 

quality is essential to assess trends 

and evaluate conditions. Monitoring 

is foundational to assessing impacts 

from quagga mussels (Dreissena 

rostriformis bugensis), continued 

urbanization within watersheds, and 

potential climate change impacts. 

Establishment of baseline monitoring 

for Lake Mohave is critically needed. 

More intensive information at depth, 

adjacent to quagga mussel beds, 

is needed to understand dissolved 

oxygen and nutrient cycling impacts 

of quagga mussels.

Fish and aquatic biota Sport fish populations are sufficient in

size and individual fish condition to

support an important recreational

fishery. Native fish populations

within Lake Mohave are declining,

even with intensive management.

Lake Mead razorback sucker 

(Xyrauchen texanus) populations are 

small but important as a unique selfrecruiting population. Zooplankton 

composition may be influenced by 

quagga mussels but no significant 

changes noted to date.

Productivity and fish populations for

sport fishery appear stable and in 

balance with the last 20 years of 

reservoir operations. Quagga mussels 

and the introduced gizzard shad 

(Dorosoma cepedianum), as well as 

wastewater treatment technologies 

may impact that balance. It is 

important to continue to monitor 

status of spawning and use areas for 

razorback suckers, particularly for the 

Lake Mead population, and continue 

Lake Mohave razorback sucker 

management and augmentation 

activities.

Need to continue annual adult sport

fish and threadfin shad (Dorosoma 

petenense) population monitoring 

now led by Nevada Department of 

Wildlife and Arizona Game and 

Fish Department as key baseline to 

assess impacts of quagga mussels, 

nutrient cycling and balance, 

and climate change. Native fish 

population monitoring prescribed 

by conservation and recovery plans 

is critical for assessing trends and 

evaluating management. Key need 

is to understand water contaminant 

effects on native fish and other 

wildlife populations. 

Sediment Sediment deposition in Lake Mead

prior to creation of Lake Powell was

significant; rate has greatly

slowed since Lake Powell. Low

concentrations of legacy pesticides

and some organics are present in

sediments, which appear to trap many

contaminants so they can’t reach

overlying lake waters.

Deltas and sediment deposition may

act as a sink for low levels of many

contaminants. Re-suspension of

contaminants could occur with water

level fluctuations or increases in

storm intensities. New delta deposits

from lowering lake levels are good

bird habitat for some species and

potential new riparian habitats.

Better understanding of the 

relationship of contaminants in 

sediments to food web is needed. 

Characterize transport of sediments 

and potential re-suspension of 

contaminants related to lowering 

water levels. Monitoring delta 

deposition and geomorphology in 

response to lowering lake levels and 

development of potential new habitats 

and alteration of reservoir hydrology.

140 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Resource component

and related goals Scientific findings Management implications Recommendations for data or information needs

Birds Lakes Mead and Mohave provide 

important stopover habitat and 

wintering grounds for many aquatic 

birds along the Pacific Flyway. 

Trends include increasing numbers 

of wintering bald eagles and nesting 

peregrine falcons. Fluctuating 

water levels on Lake Mead have 

produced a variety of shorebird 

habitats and associated populations. 

Riparian habitats for songbirds are 

limited. Contaminant accumulations 

documented in birds and eggs in Las 

Vegas Wash; impacts to reproduction 

are not clear. 

While aquatic bird habitats are 

often ephemeral, recent levels of 

fluctuation highlight challenges

and opportunities created by large 

and often rapid changes in lake 

elevation. An understanding

of habitats created at different water 

levels and different rates of water 

level change in relationship to 

aquatic and shorebird use is 

important to understand the 

continued role of Lake Mead 

in regional conservation. 

Understanding pathways of 

contaminants within the food web 

and to bird reproduction is needed

to assess risks to population health.

Continued monitoring of population 

dynamics and relationship to 

available habitats needed to assess 

response to low water and evaluate

bird responses to changes in 

lake ecosystems. Monitoring of 

contaminants and studies of the

potential impacts to bird populations 

warranted. Research on potential 

impacts of quagga mussels on 

bird health needed, including 

impacts from bioaccumulation of 

contaminants, altered food-web 

dynamics, or avian pathogens 

induced by limnological effects of 

quagga mussels. 

Riparian and aquatic

vegetation

Lake Mead riparian vegetation is 

mostly limited to tributary deltas. 

Declining lake levels have exposed 

shoreline habitats; however, they 

present potential for invasive 

plants. Extensive deltas formed at 

confluence of the Virgin, Muddy, 

and Colorado Rivers. Lake Mohave 

is ringed with shoreline riparian 

habitats; mostly, however, non-native

tamarisk. Mesquite groves line much 

of Lake Mohave’s upper riparian 

fringe maintained by the consistent 

lake levels. Data for littoral and 

aquatic vegetation is limited.

New delta areas provide potential 

for riparian habitats with native 

species as a part of the composition. 

Newly exposed shoreline habitats

have potential to spread non-native 

species. The near-shore band of 

riparian habitat of Lake Mohave 

requires active management, to 

ensure that the pockets of native 

vegetation remain, and to manage 

non-native tamarisk for recreation

and habitat objectives. There is 

concern on the Lower Colorado 

River over spread of non-native

aquatic vegetation; and potential 

effect of quagga mussel infestation 

on growth of littoral and aquatic 

vegetation. 

Shoreline monitoring for invasive 

vegetation is needed for early 

detection and to guide management. 

This should include inventory for 

littoral and aquatic vegetation, due 

to concerns over spread potential of 

non-native aquatics, and potential 

quagga mussel impacts. Vegetation 

and community inventory and 

monitoring at Virgin River and 

Colorado River deltas is needed to 

inform management related to these 

habitats as a part of overall regional 

habitats, and to assess restoration 

potential.

Contaminants Concentrations of legacy 

contaminants such as perchlorate and 

pesticides are declining due to

regulations and mitigation. 

Emerging contaminants, including 

endocrine disrupting compounds 

well monitored and present in low 

concentrations, especially near 

Las Vegas Wash. Biomarkers of 

endocrine disruption documented in 

common carp (Cyprinus carpio) in 

several studies.

Emerging contaminants of concern 

are not seen at levels currently 

known to pose a problem to 

human health for drinking water 

or recreating public, but have been 

documented to cause a number of 

health effects to individual fish. 

Water in LMNRA is highly suitable 

for recreation and is not uniquely 

contaminated compared to other 

water bodies influenced by urban 

watersheds, but such contaminants 

pose risk to fish and wildlife.

Continued monitoring of legacy 

contaminants and inventory 

and monitoring of emerging 

contaminants of concern in the 

water column is needed. Greatest 

new need is for information related 

to population level of impacts from 

contaminants that have previously 

been documented in individual fish, 

particularly to native and sport fish.

Table 7-1. Key scientific findings, management implications, and recommendations.—Continued

Chapter 7

Management Implications of the Science 141
Table 7-1. Key scientific findings, management implications, and recommendations.—Continued

Resource component

and related goals Scientific findings Management implications Recommendations for data or information needs

Invasive species Quagga mussels have become the 

dominant benthic organism in vast 

areas of the lakes, with densities

greater in areas with rocky bottoms. 

Quagga mussels are reproducing 

in lakes year around, with juvenile 

veliger larvae a significant proportion 

of zooplankton at certain times of the 

year.

Quagga mussels have potential 

to alter water quality and nutrient 

cycling, plankton composition, 

and a food-web dynamics. Quagga 

mussels can degrade recreational 

setting, although they increase 

water clarity. Quagga mussels are a 

significant threat to ecosystems of 

Lakes Mead and Mohave.

Interagency quagga mussel 

monitoring plan has provided quality 

baseline of quagga population. 

Existing adult and veliger larvae 

monitoring should continue. 

Additional work is needed to

comprehensively assess ecosystem 

impacts and food-web dynamics.

Climate change Climate models developed for 

the Southwestern U.S. and for the 

Colorado River watershed indicate

probability of decline in watershed 

snowpack and thus reduced water 

availability. Models point to 

increased potential for summer 

thunderstorms and flash floods.

Models indicate high probability 

for longer periods of low water 

levels in Lake Mead. This would 

alter water circulation patterns, 

nutrient cycling, and potentially 

food-web dynamics. Potentially 

higher surface-water temperatures 

could raise productivity, as well 

as influence human pathogens. 

Enhanced thunderstorms and floods 

have potential to carry additional 

contaminant loading. 

Monitoring and research for climate 

change and impacts on water 

availability within the watershed are 

expected to continue on interagency 

basis given societal needs within 

the Colorado River watershed. 

Information needed for Lakes Mead 

and Mohave related to potential 

impacts of low flows, lower water 

levels, increased air temperatures, 

and increased water temperatures 

on limnology, ecosystems, fish and 

wildlife, recreation, and potential 

pathogens.

142 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave
Enjoyment of Lakes Mead and Mohave starts at a young age. 

Photograph by Jennell M. Miller, University of Nevada, Las Vegas.

Chapter 7

Management Implications of the Science 143

Water Quality and Limnology

Key Questions

• What is the status and trends of physical and chemical 

water-quality parameters (for example, conductivity, 

dissolved oxygen, nutrients, temperature, transparency, 

pH, and water levels)?

• What is the status and trends of biological waterquality parameters (for example, plankton and 

chlorophyll a)?

• What is the status and trends of contaminants in the 

water column?

Key Scientific Findings

Based on standard limnological trophic indices, the 

U.S. Environmental Protection Agency’s (USEPA) National 

Lake Assessment standard, State standards, and comparisons 

with a number of large recreational lakes around the country, 

Lakes Mead and Mohave generally are of high quality 

for recreational uses (see Chapter 4). The water surpasses 

guidelines and standards of quality in support of the beneficial 

uses of body-contact recreation, fish and wildlife populations, 

and as a source of drinking water. Measures of limnological 

characteristics and water-quality parameters are well within 

ranges considered “good” within the USEPA National Lakes 

Assessment (U.S. Environmental Protection Agency, 2010) 

and criteria for categorizing overall lake water quality (see 

Chapter 4). Trends in the quality of water in Lake Mead 

reported by SNWA for 2004–2009 show that bacteria, 

chlorophyll a, and algal levels are well within State standards 

protective of body contact recreation and aesthetics. Clarity, 

as measured in Secchi depth, has increased such that typical 

readings exceed 30 ft (9.1 m) and maximum readings exceed 

60 ft (18.3 m), values approaching those measured in Lake 

Tahoe. Reductions in chlorophyll a and increases in water 

clarity most likely result from enhancements in phosphorus 

removal achieved by the wastewater reclamation districts, 

in part as a response to a significant algal bloom in 2001, 

although recent quagga mussel introduction may play a role 

(see Chapters 4 and 6). Recent (2007–2011) 5-year trends for 

most limnological measurements are within ranges observed 

over the past 20 years, indicating a general stability in water 

quality. However, dissolved oxygen concentrations decreased 

in the tributary confluence areas of Overton Arm (Muddy 

and Virgin Rivers) and Gregg Basin (Colorado River) as lake 

levels declined from 1999 through 2010 and nutrient inputs 

from tributaries into relatively shallow lake areas increased. 

The dissolved oxygen concentrations in these areas rose in 

2011 as lake elevations rose due to significant increases in 

flows from the Colorado River. Most standard water-quality 

parameters and limnological characteristics classify the lake 

as slightly oligotrophic, and are within ranges to support 

interagency objectives for wildlife and fisheries, domestic 

water supply, and recreation. 

Instruments used to monitor water quality on a near-continuous 

basis in Lake Mead. Photograph by National Park Service.
144 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Management Implications

The basic hydrology and limnology of Lakes Mead and 

Mohave are well understood, in part owing to the large volume 

of data from past monitoring efforts, particularly for Lake 

Mead. This understanding has been invaluable in assessing 

impacts of potential contaminants and assisting agencies 

in developing water-management facilities and programs. 

Increasing urbanization within the tributary watersheds of the 

Virgin and Muddy Rivers as well as Las Vegas Wash, along 

with emerging threats such as quagga mussels, endocrine 

disrupting compounds (EDCs), and potential for lower waterlevel conditions associated with climate change, necessitates 

ongoing monitoring to maintain the existing high-quality 

water. 

For example, quagga mussels in other lake systems 

have been implicated in increases in the abundances of 

cyanobacteria (blue-green algae) species that can produce 

toxins (cyanotoxins). Although cyanobacteria cell counts 

in Lake Mead are sometimes within the USEPA’s National 

Lake Assessment “moderate risk exposure” level, the species 

identified generally are not species with potential to create 

toxins (see Chapter 4). Analyses of a small number of 

individual samples on Lake Mohave since 2007 have shown 

brief periods of higher-than-usual cyanobacterial cell counts. 

Quagga mussels in other systems also have been found to 

decrease dissolved oxygen concentrations in lake-bottom 

areas, particularly during periods of lake stratification. The 

improved treatment of wastewater discharged into Las Vegas 

Wash, however, has resulted in higher concentrations of 

dissolved oxygen in Las Vegas Bay offsetting potential quagga 

mussel-mediated dissolved oxygen reductions in Boulder 

Basin. 

Lower lake levels could alter basic hydrology and watercolumn mixing of Lake Mead, and thereby alter algal growth 

and composition, sediment distribution, and dissolved oxygen 

levels. Declining or significantly lower lake levels could result 

in the creation or growth of deltas at the confluences of Las 

Vegas Wash, the Virgin and Muddy Rivers, and Colorado 

River. Newly cut and exposed sediments and deltas have the 

potential to re-suspend contaminants bound within sediments 

to the open-water column. Reductions in dissolved oxygen 

concentrations were noted at times in the Muddy and Virgin 

Rivers and Overton Arm during extended low-flow conditions. 

To date, these periods of lower dissolved oxygen have not 

resulted in documented issues to fish or ecosystem health.

Contaminant inputs to Las Vegas Wash are the most 

significant influences to water quality in Lake Mead. While it 

appears that organic contaminants and compounds regarded 

as emerging contaminants of concern (for example, personal 

care products, pharmaceuticals, and EDCs; see Chapter 6) 

are not known to cause significant human health issues for 

Lakes Mead and Mohave, biomarkers of the effects of such 

contaminants (for example, intersex fish, reproductive tissue 

and organ impairments, and tumors; see Chapter 6) have been 

documented in common carp (Cyprinus carpio) from both 

lakes. Common carp were assessed for these biomarkers as 

surrogates for potential impacts to reproduction and population 

dynamics for razorback suckers (Xyrauchen texanus) as well 

as for recreational sportfish.

Increasing urbanization along the Muddy River and 

Virgin River tributaries has the potential to lower water 

quality in Lake Mead. Potential outcomes include algal 

blooms, higher salinity, or higher concentrations of emerging 

contaminants. Upgradient urban growth also may alter 

currents and mixing patterns of Lake Mead due to changes in 

the magnitude and timing of tributary discharge and inflow. 

The cooler and oxygen-rich inflow waters of the Colorado 

River are transported as an underflow all the way to the 

Hoover Dam (see Chapter 4), which influences the overall 

hydrology, water quality, and ecology of the lake. 

Data and Information Needs

The capability to characterize the current limnology 

and water quality of Lake Mead is a result of more than 20 

years of regulatory monitoring in Las Vegas Wash and was 

enhanced by lake-wide monitoring efforts from 2004 to 

2012. Ongoing lake-wide, long-term monitoring is needed to 

assess potential future resource changes such as increasing 

urbanization along tributaries or effects of changing climate. 

Longer-term datasets for important monitoring stations should 

include both physicochemical parameters of water quality, 

such as conductivity, dissolved oxygen, nutrients (nitrogen 

and phosphorus), temperature, transparency, pH, and water 

levels, and also the biological parameters of plankton and 

chlorophyll a. These datasets are essential for separating 

the affects from anthropogenic and natural influences; and 

evaluating statistically significant seasonal, annual, or decadal 

trends. Although a comprehensive baseline water-quality 

monitoring network has been established and operated on 

Lake Mead, no such monitoring has been conducted on Lake 

Mohave so that its historical water-quality data are sparse. 

Lake Mohave is a key recreational resource, serving more than 

2 million visitors each year. It also provides critical habitat for 

razorback sucker. The shallower water of Lake Mohave may 

react differently than Lake Mead to impacts from increased 

nutrient inputs, contaminants, or climate change. It is 

important to establish baseline water-quality monitoring for 

Lake Mohave. 

In addition to a core set of monitoring stations for both 

Lakes Mead and Mohave, lake-wide longer term monitoring 

and research should focus on priority information needs, such 

as potential issues related to periods of lowered water levels, 
Chapter 7

Management Implications of the Science 145

invasive quagga mussels, or the relation of either of these two 

issues to nutrient loads and food-web dynamics. For example, 

monitoring is needed to enhance understanding of the 

influence of lowered water levels or invasive quagga mussels 

on zooplankton and cyanobacteria, or dissolved oxygen 

concentrations with depth.

Existing monitoring of inorganic contaminants, such 

as perchlorate and selenium, are likely to continue based on 

regulatory requirements. Coordination of this monitoring 

with efforts to reduce inputs of selenium and perchlorate 

to Las Vegas Wash will be protective of Lake Mead. 

Monitoring of emerging contaminants of concern is not a 

regulatory requirement, but is needed to better understand 

potential, future impacts to ecosystem health in LMNRA 

and downstream in the Lower Colorado River. Moreover, 

monitoring for EDCs at least at current levels can enhance 

the overall understanding of effects of these compounds and 

benefit regional and national regulatory agencies responsible 

for aquatic ecosystems influenced by anthropogenic sources of 

inflow. 

Fish and Aquatic Biota

Key Questions

• What is the status and trends of the forage base?

• What is the status and trends in abundance and health 

of sportfish?

• What are the distributions, reproduction rates, and 

recruitment levels of native, non-native, and invasive 

fish?

• What is the biological, chemical, and physical 

condition of razorback sucker spawning and rearing 

habitat? Does improved water quality support recovery 

of razorback suckers? 

Key Scientific Findings

Given the size of the lakes, and their location along the 

original Colorado River channel, Lakes Mead and Mohave 

are important water resources for regionally and nationally 

significant wildlife populations. Although the lakes continue 

to provide habitat conditions that support many pre-reservoir 

plants and animals, the completion of the reservoirs has 

forever altered the Colorado River ecosystem by destroying 

original and providing new habitats. 

Lakes Mead and Mohave are classified as moderately 

productive in terms of the basic sources of food that support 

fish and wildlife (see Chapter 4). Plankton, algae, zooplankton, 

and benthic invertebrates provide base productivity (see 

Chapter 5). The tributary inflow areas of Las Vegas Wash 

and the Muddy and Virgin Rivers (Overton Arm) provide the 

greatest base productivity, due to their higher nutrient inputs.

Phytoplankton forms the base of the aquatic food web 

and, in Lake Mead, has greatest productivity in nutrient-rich 

inflow areas downgradient of Las Vegas Wash and at the 

confluence of the Muddy and Virgin Rivers. Historically, 

Las Vegas Bay has had periods of increased nutrient 

concentrations that produced algal blooms. In response to 

an algal bloom in 2001, wastewater-treatment plants have 

significantly reduced the amounts of phosphorus released 

into Las Vegas Wash over the past decade. The reduced 

phosphorus has reduced alga concentrations in Boulder Basin 

(as measured by chlorophyll a) from the high levels measured 

in 2001. Although the base algal concentrations declined from 

2001 to 2010, angler success, fish-body-condition factors, and 

catch rates during fishery monitoring show that algal levels 

have remained supportive of an active recreational sport 

fishery.

Fishing is one of most popular pastimes at Lake Mead National 

Recreation Area. Photograph by National Park Service.
146 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

The water column in Lake Mead supports more than 70 

zooplankton species, including copepods, cladocerans, rotifers, 

and ostracods (Chapter 5). The zooplankton community 

is an important food source for fisheries, particularly for 

threadfin shad (Dorosoma petenense), the primary forage 

fish. Studies of other large lakes with longer-standing quagga 

mussel infestations show a negative impact on the overall 

composition and abundance of other zooplankton. However, 

current data (2007–2011) for Lakes Mead and Mohave 

indicate that although quagga mussel veligers can periodically 

be a dominant species of zooplankton, the overall biomass and 

composition of other zooplankton remain sufficient for fish 

forage species to support the fishery.

Comprehensive surveys at Lake Mead since 1986 show 

a change in the benthic invertebrate community over the past 

30 years, but it is not clear whether such changes result from 

lowering lake levels, decreases in algal levels, invasion by new 

non-native invertebrates, or other factors. For example, since 

the 1980s, densities of Asian clams (Corbicula fluminea) and 

chironomids generally have decreased, oligochaeta densities 

have not changed significantly, and other taxa have increased 

in average density (Chapter 5). The most dramatic changes 

have been the introductions of the invasive quagga mussel and 

New Zealand mudsnail (Potamopyrgus antipodarum).

Lakes Mead and Mohave are home to at least 15 different 

species of fish. These include the native flannel mouth sucker 

(Catostomus latipinnis), razorback sucker, and bonytail chub 

(Gila elegans). The razorback sucker and bonytail chub are 

Federally listed as endangered and Lakes Mead and Mohave 

are listed as critical habitat for the razorback sucker. Lakes 

Mead and Mohave support a significant recreational sport 

fishery, with the primary sportfish including striped bass 

(Morone saxatilis), largemouth bass (Micropterus salmoides), 

and smallmouth bass (Micropterus dolomieu). Striped 

bass numbers and condition factors fluctuate based on the 

availability of their primary forage species, threadfin shad. 

While numbers of largemouth bass have declined in recent 

years, the arrival of smallmouth bass, which now comprises 

40 percent of the black bass catch, has increased opportunities 

for black bass anglers. Complicating the Lake Mead fishery 

management has been the non-intentional introductions of 

tilapia over the past 15 years, and gizzard shad (Dorosoma 

cepedianum) since 2007. Tilapia have become fairly 

established as a lesser-sought sportfish, and so far have not 

been shown to have negative impacts to the overall fishery. 

Although abundant young gizzard shad have so far provided 

an additional forage fish, their long-term impact is unknown.

As recently as the 1980s, Lake Mohave was thought to 

have a population of 50,000 or more adult razorback suckers, 

but most of these fish have been lost to old age. Although 

razorback spawning has been documented and continues in 

Lake Mohave, this has not resulted in natural recruitment 

or survival of naturally spawning fish to adults. Despite 

campaigns to capture larval razorback suckers and grow 

them to a size sufficient to avoid non-native predators such as 

striped bass when released, the population in Lake Mohave 

has declined to less than 2,500 adults. Moreover, the bonytail 

chub is believed extirpated from the Lower Colorado River, 

and is known to survive only as a handful of adults within 

Lake Mohave.

The Lake Mead razorback sucker population also is 

small, with only 80 fish captured at all monitoring stations in 

2008–2009. However, the razorback sucker population at Lake 

Mead is significant because it is one of the few populations on 

the Colorado River that continues to have recruitment solely 

from naturally spawning adults. Like the razorback sucker, 

the flannel mouth sucker has been captured in small numbers 

during monitoring surveys conducted at the mouth of the 

Colorado River inflow (52 captures in 2010) and the Virgin 

and Muddy River inflows (5 captures in 2009–2010).

Management Implications

The current status of plankton, zooplankton, and 

invertebrate base productivity appears well within ranges 

to support existing populations of native fish and an active 

recreational sport fishery in LMNRA. However, a variety 

of influences on base productivity will require continued 

monitoring and management. For example, within the 

phosphorus-limited system of Lake Mead, increases in 

concentrations of phosphorus or alterations in the mixing of 

the tributary inflows with seasonal availability of nutrients 

can have an immediate effect on overall algal production. 

Continued monitoring of overall nutrient loading and algal 

production is necessary to assess success in meeting waterquality objectives for recreational experience, sport fishing, 

and drinking water.

Quagga mussels have the potential to alter the food-web 

dynamics of Lakes Mead and Mohave, including changes to 

native fish and sportfish. Similar to changes seen in the Great 

Lakes fishery, quagga mussels have the potential to impact 

zooplankton populations and forage fish productivity. Gizzard 

shad also have the potential to influence the overall forage 

base, including threadfin shad, and alter food-web dynamics 

at Lake Mead. Although threadfin shad populations fluctuate 

in periodic cycles, they appear to have reached an equilibrium 

that adequately supports the current sport fishery.

Biomarkers of impacts to fish from emerging 

contaminants of concern and EDCs have been documented 

at LMNRA, including biomarkers with the potential to 

impact reproductive fitness. Although the impacts appear to 

be greatest in Las Vegas Bay of Lake Mead, biomarkers of 

masculinization of female common carp (Chapter 6) have been 

documented in Lake Mohave downstream of the Hoover Dam. 
Sediment sample collected from Las Vegas Bay. Photograph by 

Michael R. Rosen, U.S. Geological Survey.

Chapter 7

Management Implications of the Science 147

Data and Information Needs

Current (2012) monitoring of zooplankton, threadfin 

shad, and other forage fish is essential to provide information 

on status and trend of the base productivity and forage 

base. Continued monitoring of adult fish populations also is 

important to document fish population dynamics. Trend data 

on the food base and adult sportfish are necessary in assessing 

food-web dynamics, and assessing issues of nutrient balance, 

quagga mussel impacts, and contaminants towards meeting 

objectives of healthy fish and wildlife populations.

For native fish, information on population dynamics, 

spawning areas and habitats, and forage base and food sources 

is needed. This information supports management related to 

issues of nutrient loading and transport, as well as potential 

impacts from quagga mussels and gizzard shad.

Research is needed on the population effects of 

contaminant-related biomarkers for native and sportfish. 

Continued monitoring of emerging contaminants of concern 

also is needed, with improved understanding of contaminant 

fate and transport. More information is needed on how 

contaminants are affecting not only native fish and sportfish, 

but also aquatic birds and other wildlife. Baseline conditions 

for EDCs should be established in the Muddy and Virgin River 

watersheds prior to potential increases in urban populations 

and associated increases in wastewater effluent and urban 

runoff to the Overton Arm of Lake Mead. 

Sediments

Key Questions

• What is the status and trend of resuspension and 

transport of contaminants and nutrients from 

sediments?

• What is the status and trend of sediment delivery at 

tributaries?

Key Scientific Findings

 Sediment cores and survey maps show that large 

volumes of sediment have accumulated on the floor of Lake 

Mead since the closure of Hoover Dam, but that most of 

the sediments were deposited prior to the completion of the 

upstream Glen Canyon Dam in 1965 (Chapter 3). The early 

deposition of sediments was significant, with approximately 

2.7 million acre-ft of sediment, or 12 percent of the original 

lake volume, filled by the 1965 closing of Glen Canyon 

Dam. Glen Canyon Dam greatly reduced sedimentation rates 

and was estimated to have increased the life of Lake Mead 

by 500 years. The thickest sediments are along the original 

Colorado River channel, where they are as much as 279-ft 

(85.0-m) thick at the upper end of the lake near the entry of the 

Colorado River, thinning to about 82-ft (25.0-m) thick at the 

base of Hoover Dam (Chapter 3). Sedimentation at the entry 

of the Colorado River to Lake Mead has formed thick delta 

deposits that advanced rapidly into the lake during the first 

13 years of its formation, then advanced at a slower rate from 

1950 to 1965, and has nearly stopped since 1965 (Chapter 3; 

Twichell and others, 2005). Sediment and geologic maps of 

the lake bottom also have been used to help identify potential 

differences in quagga mussel colonization around the lakes 

because quagga mussel densities are greater within hard rock 

substrate areas than in areas of soft sediments (Chapter 5; 

Wong and others, 2011).

Sediment inflow at the main stem of a river or at tributary 

areas may bind contaminants during deposition and, for certain 

contaminants, may provide a tool for evaluating sediment 

deposition rates and depositional history. For example, several 

sediment cores were taken in Las Vegas Bay, Boulder and 

Virgin Basins, and Overton Arm to determine changes in 

sedimentation rates and chemical inputs from the completion 

of Hoover Dam to 1998, when the cores were collected 

(Chapter 3; Rosen and Van Metre, 2010). Results from those 

studies also showed that compared to other lakes in the United 

States influenced by urban discharge, the concentrations of 

inorganic and organic chemical and compounds in Lake Mead 

are relatively low (Chapters 4 and 6).

Tributary deltas moved into the lake during a period of 

declining lake levels from 1999 through 2010. Subsequent 

incision by tributary streams have eroded delta sediments, 

periodically suspending and transporting sediments and bound 

contaminants into Lake Mead.
148 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Management Implications

The erosion of sediment deposits at tributaries has the 

potential to resuspend contaminants previously bound within 

the sediments. For many contaminants, the concentrations 

within sediments are low, and resuspension of the sediments 

would not likely create significant increases in overall 

contaminant concentrations. However, for some contaminants, 

such as dichlorodiphenyltrichloroethane (DDT), for 

which significant reductions have been made in upstream 

concentrations in recent years, resuspension of sediments 

could increase concentrations in the lake. 

Migrating and expanding sediment deltas at tributary 

inflow areas as a consequence of lower lake levels have 

created extensive potential new wildlife habitats. Declining 

lake levels, however, also have potential to create additional 

areas of shallow water, which can result in increases in water 

temperature and encourage algal growth, as well as alter 

overall lake mixing and hydrology. Differing bottom materials, 

such as either hard rock surfaces or deposits of soft sediments, 

create different habitats for fish and wildlife, including 

differing population potentials for the invasive quagga mussel. 

Data and Information Needs

Periodic collection and analysis of sediment cores in 

previously sampled areas will provide information on status 

and trends to indicate if rates of contaminant inputs are 

changing, or if additional pollutants may be accumulating. 

More information is needed on potential for resuspension and 

transport of contaminants and sediments, particularly in light 

of projections of higher probabilities for lower lake levels. 

As tributary delta areas are important ecologically, and shift 

in relation to lake levels, information on status and trend of 

sediment delivery and accumulation at tributaries, and impacts 

to habitats and water mixing is necessary. Finer resolution 

sediment maps will be helpful in assessing food-web dynamics 

and wildlife shifts related to invasive quagga mussel.

Birds

Key Questions

• What is the distribution, species composition, and 

abundance of shorebirds, wading birds, waterfowl, and 

other classes of birds? What is their status and trends?

• Which bird species spend significant amounts of their 

life history locally or otherwise could be classified as 

resident species?

• What is the status and trend of shorebird habitat? What 

are the conditions of foraging/nesting sites? What type 

and degree of disturbance is present?

Key Scientific Findings

Lakes Mead and Mohave provide important stopover 

habitat and wintering grounds for many aquatic birds along 

the Pacific and Intermountain Flyways of Western North 

America. During recent, multi-year monitoring, 92 species 

of aquatic birds were documented, along with four additional 

species strongly associated with these lakes [belted kingfisher, 

(Megaceryle alcyon), osprey (Pandion haliaetus), bald eagles 

(Haliaeetus leucocephalus), and peregrine falcon (Falco 

peregrinus)]. The most common species was the American 

Coot (Fulica americana), a migratory herbivore that winters in 

large numbers in these lakes. 

Lake Mead has a greater diversity of aquatic bird species 

than Lake Mohave, with about a third of the species observed 

only on Lake Mead. This greater species diversity results 

primarily from the nutrient-rich and delta-forming inflows of 

the Muddy, Virgin, and Colorado Rivers, and Las Vegas Wash. 

Recent (2004–2009) lake-wide aquatic bird surveys found 

that the fluctuating water level of Lake Mead also produced a 

diversity of dynamic habitats, including temporary mudflats 

and open beaches preferred by many shorebirds (Chapter 5). 

Such ephemeral habitats have attracted breeding by snowy 

plovers (Charadrius nivosus), a species considered threatened 

along the Pacific Coast. In contrast, Lake Mohave maintains 

a more regular surface-water level and has limited tributary 

inflow and nutrient input. Moreover, other than periodic floods 

from local drainages, source waters to Lake Mohave are cold, 

clear waters released from Lake Mead; the lesser diversity 

of avian habitats and tributary sources results in lower bird 

species diversity for Lake Mohave.

Lakes Mead and Mohave are home to osprey, bald eagles, 

and peregrine falcon that take advantage of aquatic prey. 

Osprey and bald eagles can be observed in any season, but use 

shifts seasonally, particularly for bald eagles, which typically 

Common loon (Gavia immer) on the water in Lake Mead. Photograph 

by Joseph G. Barnes, University of Nevada, Las Vegas.
Chapter 7

Management Implications of the Science 149

are present in higher numbers during non-breeding winter 

months. Bald eagle numbers have trended upwards over the 

past decade, with 163 counted during a 1-day winter survey 

in the 2011. Peregrines falcon numbers also have grown 

dramatically since the first breeding pair was noted in 1985, 

and in 2010, 33 nesting territories were known to be occupied 

in areas around the lakes.

Riparian habitats that support songbird populations are 

limited along the shores of Lakes Mead and Mohave. Of 

conservation importance, surveys for the Federally endangered 

southwestern willow flycatcher (Empidonax traillii extimus) 

conducted through the Lower Colorado River Multi-Species 

Conservation Program have found low numbers of migrating 

flycatchers along Lake Mohave, but none along Lake Mead. 

Nesting habitat for southwestern willow flycatchers occurs 

along the Virgin River adjacent to LMNRA, and tributary 

delta areas of Lake Mead have potential to become new 

habitats, particularly where declining lake levels have exposed 

new riparian areas near tributary inflows of the Virgin and 

Muddy Rivers.

Shorebird habitat has been relatively abundant on 

Lake Mead during the past several years due to declining 

and fluctuating water levels that have produced mudflats 

and beaches. The extent and conditions of these habitats 

are always variable, becoming overgrown with emergent 

vegetation through time or easily inundated when waters rise. 

Slowly rising water levels that inundate terrestrial vegetation 

establish shallow-water conditions favorable to many species 

of waterfowl.

Management Implications 

Fluctuating water levels are a historical component of 

managing Lake Mead, although not the extreme declines 

that have occurred over the past decade. Research aimed at 

understanding the influence of large and often rapid changes 

in lake elevation on aquatic and shorebird habitats and use will 

assist managers in assessing the potential of Lake Mead in 

regional strategies for conservation of aquatic birds. 

Some preliminary evidence suggests that omnivorous 

diving ducks at Lakes Mead and Mohave may be changing 

their migration patterns in response to the availability of 

quagga mussels, similar to patterns observed at the Great 

Lakes (Chapter 5). However, additional monitoring is 

necessary to determine if changes in occurrence of these 

species is due to the use of quagga mussels as a food source 

or the influence of other dynamic changes occurring at these 

lakes. More importantly, quagga mussels can bioaccumulate 

contaminants that may negatively affect birds that use them 

as a food source. For example, selenium has been found to 

bioaccumulate in quagga mussels in the Great Lakes (Rutzke 

and others, 2000). Past tributary inflow from Las Vegas Wash 

has contained elevated selenium concentrations, giving rise 

to similar concerns at Lake Mead on the bioaccumulation of 

selenium in quagga mussels and subsequent potential impacts 

to aquatic birds within LMNRA.

Data and Information Needs

The initial 5-year inventory of aquatic birds (2004–2009) 

documented habitat use, species, and species abundance 

(Chapter 5). Continued monitoring is needed on the status and 

trends of species composition, distribution, and abundance 

of shorebirds, waterfowl, and other aquatic birds. For 

conservation management purposes, such monitoring should 

indicate which species spend a significant amount of their life 

history within LMNRA and are nesting residents. Related to 

conservation management, in light of potential for continued 

decline and fluctuations in water levels, inventories are needed 

to assess status and trend of shorebird habitat and forage and 

nesting sites of other aquatic bird species.

Research on potential impacts of quagga mussels on 

bird health may be warranted, particularly if quagga mussels 

become a food source for large numbers of birds. Monitoring 

has shown that general contaminant loading associated 

with Las Vegas Wash has resulted in the accumulation 

of contaminants in birds and bird eggs, but impacts to 

reproduction were not clear. Such findings demonstrate 

the need for monitoring of contaminants and studies of the 

potential impacts to bird populations and other species.

Riparian and Aquatic Vegetation

Key Questions

• What are the trends in distribution, connectivity, and 

abundance of riparian vegetation (native and nonnative)?

• Is riparian vegetation maintained or restored to a 

condition that supports key riparian functions?

Key Scientific Findings

Aside from deltas at tributary inflow areas, and a few 

small areas containing shoreline springs, the vast majority 

of the shoreline surrounding Lake Mead does not provide 

substantial areas of riparian habitat. More than a decade 

of lowered lake levels from 1999 through 2011 exposed 

additional delta areas at Las Vegas Wash, Virgin River, 

and Muddy River tributaries, and at the main stream of the 

Colorado River. Newly exposed deltas provide significant 

acreages of mud and silt flats, and new acreages of plant 
150 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

habitat. Although a large part of these new delta habitat areas quickly 

fill with non-native plants, particularly saltcedar tamarisk (Tamarix 

ramosissima), the potential for native riparian habitat or mixed habitat 

exists. Newly exposed shoreline areas distant from tributaries and deltas 

serve as corridors for spread of non-native vegetation.

With its predictable lake level fluctuation range of 15 ft/ yr (4.6 m/yr), 

Lake Mohave provides more stability for potential formation of shoreline 

riparian habitat. The majority of the shoreline habitat at Lake Mohave 

consists of the non-native saltcedar tamarisk, with a few areas where 

topography and soils present better conditions for the native Goodings 

Willow (Salix gooddingii); as a result, native recruitment of Goodings 

Willow currently (2012) is minimal. However, Lake Mohave does support 

substantial mesquite groves that line much of the upper riparian habitat 

fringe created by the more consistent lake levels. These groves contain two 

native mesquite species; screwbean mesquite (Prosopis pubescens) and 

honey mesquite (P. glandulosa).

Littoral or shallow zone aquatic vegetation has not been well studied. 

A limited number of surveys have outlined the species that comprise the 

near-shore habitat zone, primarily grass-like plants such as the non-native 

phragmites, and native rushes, sedges, and cattails. Formation of extensive 

areas of littoral vegetation has been limited.

Management Implications 

The main threats to riparian vegetation at Lakes Mead and Mohave 

are invasion by non-native species such as tamarisk, and impacts that occur 

from extended periods of water-level change. For tamarisk-dominated 

shorelines, the presence of the tamarisk leaf beetle (Diorhabda carinulata) 

released as a biological control will likely change species composition and 

densities within the current riparian vegetation stands. Over the long-term, 

this management technique may potentially 

increase native vegetation and enhance 

habitat; ongoing monitoring and assessing 

changes within riparian habitats as tamarisk is 

defoliated, and developing restoration plans 

as the beetle spreads along shoreline tamarisk 

stands will be important for understanding 

health and diversity of native vegetation. The 

screwbean and honey mesquite groves along 

the upper fringes of the riparian habitat have 

potential to benefit from the tamarisk beetle 

defoliations and expand their cover.

Treatment and removal of invasive 

exotics and re-planting of areas with native 

vegetation, particularly in newly exposed areas 

or areas of tamarisk leaf beetle defoliation, 

will increase the regional availability of highquality riparian habitats. Moreover, protection 

of existing native vegetation from harvesting 

and firewood collecting would be useful to 

allow regrowth.

Data and Information Needs

Littoral vegetation areas and aquatic 

vegetation have received minimal inventory 

or monitoring to date. Inventories are needed 

of species that comprise the near-shore habitat 

zone, such as the non-native phragmites, 

and native rushes, sedges, and cattails. 

These areas are threatened by a series of 

invasive aquatic plants, many of which would 

have ecological and recreational impacts. 

Monitoring is needed to better understand 

changes caused by declining lake levels on 

(1) the trends in distribution, connectivity, 

and abundance of riparian vegetation, (2) the 

potential establishment of non-native species 

in riparian habitats near the tributaries of 

Las Vegas Wash and the Virgin and Muddy 

Rivers, and (3) the limited formation of 

shoreline littoral vegetation. Additional 

monitoring and management for prevention 

of non-native vegetation from accidental 

transport by recreational boating also would 

be beneficial to limit future and current nonnative species establishing in the lakes and 

their riparian zones. Concurrent with ongoing 

monitoring, research should be performed to 

assess whether riparian vegetation has been 

maintained or restored to a condition that 

supports key riparian functions.

Cove with shoreline riparian vegetation. Photograph by National Park Service.
Chapter 7

Management Implications of the Science 151

Contaminants

Key Questions

• What is the status and trends of contaminants in the 

water column [for example, emerging contaminants, 

endocrine disruptors, VOCs, radionuclides, priority 

pollutants (USEPA and State), and pathogens]?

• What contaminants are present in native and non-native 

fish tissues and to what extent is fish health impaired?

• Which contaminants, if any, pose a risk to human 

health?

Key Scientific Findings

Las Vegas Wash is the most significant contributor 

of contaminants to Lake Mead. Contaminants entering 

through the Wash have included inorganic chemicals, such 

as perchlorate, selenium, and other metals; legacy organic 

chemicals, such as DDT, that was once manufactured in Las 

Vegas Valley; currently used organic chemicals that include 

pesticides and PAHs; emerging contaminants of concern, 

such as personal care products and EDCs; and periodic high 

bacteria loads, particularly after storm events. Sediments may 

bind certain contaminants during deposition, and thus, remove 

them from active biological processes. However, sediments 

can be re-suspended by storms or by re-exposed delta deposits 

at tributaries due to declining water levels, and through 

these processes may be periodically transported and interact 

with the water column. Moreover, a number of factors exist 

that may alter concentrations of these contaminants—some 

contaminants may bioaccumulate within the food chain, some 

may be influenced by invasive species such as quagga mussel, 

and some contaminant concentrations may change in response 

to regional and global climate trends that affect water levels in 

the lakes. 

The concentrations of the chief inorganic contaminants of 

concern, perchlorate and selenium, are within current (2012) 

established guidelines for the protection of health and wildlife. 

The concentrations of perchlorate have decreased more 

than 90 percent during the past 10 years due to a mitigation 

program at its manufacturing location on the former BMI site 

in Henderson, Nevada. Selenium concentrations have not been 

an issue for the open waters of Lake Mead, but are of concern 

for birds in Las Vegas Wash. Mercury levels in fish tissues and 

sediment are relatively low compared to those in other lakes in 

Nevada and nationally. The mean concentration of mercury in 

the tissues of fish sampled from Lake Mead are less than the 

USEPA concentrations recommended to protect human health; 

however, individual fish samples did have concentrations that 

exceeded USEPA recommended for mercury, and the Nevada 

Department of Wildlife has published fish consumption 

advisories for Lakes Mead and Mohave (Chapter 6). Certain 

trace metals within the water column are monitored through 

regulatory programs, and concentrations are all within Safe 

Drinking Water Act protection criteria (Chapter 4).

Organic contaminants, including legacy chemicals, such 

as DDT and other pesticides, and emerging contaminants of 

concern, such as personal care products and pharmaceuticals, 

have been well studied both within Las Vegas Wash and 

Boulder Basin (Chapter 6). Although these compounds 

represent a threat to water quality and ecosystem health, 

more information is needed to assess impacts and develop 

mitigation plans. Individual fish evaluated in Las Vegas 

Bay, Overton Arm, and Willow Beach in Lake Mohave have 

biomarkers indicating health and endocrine effects from 

compounds in the water column. In male common carp, 

these biomarkers include reduced male hormone levels, the 

presence of egg-yolk precursor, reduced testicular growth and 

sperm quality, lesions, and liver and kidney abnormalities 

(Chapter 6). In female common carp, biomarkers that showed 

health and endocrine effects include an intersexed ovary at Passive samplers used to monitor organic contaminants in the 

water. Photograph by Michael R. Rosen, U.S. Geological Survey.
152 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Willow Beach and skewed ratios of sex steroid hormones at 

Las Vegas Wash (Chapter 6). Many organic contaminants 

present within the water column are monitored by Southern 

Nevada Water Authority, and concentrations are less than the 

Safe Drinking Water Act protection criteria (see Chapter 4). 

Volatile organic compounds (VOCs) related to gasoline and 

boating traffic at LMNRA are not present in the open water 

column at concentrations of immediate concern. Current 

(2012) VOC concentrations should decrease even further as 

LMNRA has adopted regulations to phase out the use of twostroke engines on the lakes and meet USEPA fuel-efficient 

engine standards by 2013. 

Management Implications

The water-quality and contaminant issues related to Las 

Vegas Wash are the most significant contaminant influences 

to ecosystem health in Lake Mead, particularly in Las Vegas 

Bay and Boulder Basin. Reducing contaminant loads from Las 

Vegas Wash is important in protecting the overall water quality 

of Lake Mead. As a result, interagency partnerships for the 

protection and management of water quality along Las Vegas 

Wash, including the Lake Mead Water Quality Forum and the 

Las Vegas Valley Watershed Advisory Committee, have been 

formulated to coordinate and implement monitoring, research, 

and mitigation. The efforts of these partnerships have resulted 

in significant improvements along Las Vegas Wash over the 

past decade in water quality, floodwater management, and 

restoration of riparian vegetation and wildlife habitat. 

Increasing urbanization along the Muddy River and 

Virgin River tributaries has the potential to create additional 

sources of contaminants to Lake Mead through future 

wastewater-treatment discharges or urban runoff contaminant 

loads. Additionally, increased discharge volumes to these 

rivers and ultimately Overton Arm may alter hydrological and 

mixing patterns of Lake Mead and influence the transport and 

dispersal of contaminants in the lake.

Although it appears that organic chemicals regarded as 

emerging contaminants of concern (personal care products, 

pharmaceuticals, EDCs) are not known to be significant 

human health issues for Lakes Mead and Mohave, biomarkers 

indicative of poor fish health have been documented in carp 

from both lakes. The extent and nature of population-level 

impacts of these compounds is not yet known.

Data and Information Needs

Current monitoring (2012) in Las Vegas Wash and Lake 

Mead should continue for status and trends of contaminants 

in the water column [VOCs, radionuclides, priority pollutants 

(USEPA and State), and pathogens]. In Lake Mohave, 

additional information is needed to understand which 

contaminants might be causing endocrine and reproductive 

effects in fish from Willow Beach.

Existing programs for monitoring inorganic 

contaminants, such as perchlorate and selenium are likely 

to continue, based on regulatory requirements. Most of the 

existing monitoring is performed within Las Vegas Wash and 

at its confluence with Las Vegas Bay. More information may 

be needed on the fate and distribution of these compounds in 

Lake Mead.

Extensive monitoring for emerging contaminants of 

concern is not a regulatory requirement, but is an issue for 

ecosystem health within Lakes Mead and Mohave, as well as 

for downstream users of the Lower Colorado River. Continued 

monitoring of emerging contaminants in the water column 

and sediments would contribute to an improved understanding 

of contaminant fate and transport. Research is needed on 

the population effects of contaminant-related biomarkers for 

native fish and sportfish. More information is needed on how 

contaminants are impacting not only native fish and sportfish, 

but also aquatic birds and other wildlife.

Pathogens are monitored in Las Vegas Wash and the 

Boulder Basin of Lake Mead. This monitoring is conducted 

in compliance with regulatory requirements for wastewater 

discharges and for the drinking-water intake on the western 

side of Boulder Basin. The National Park Service currently 

monitors bacteria at selected high-use areas of Lakes Mead 

and Mohave during peak use periods. Additional monitoring is 

needed to assess which contaminants or pathogens, if any, may 

pose a risk to human health. 
Chapter 7

Management Implications of the Science 153

Invasive Species

Key Questions

• What are the trends in abundance and distribution of 

aquatic invasive species (for example, quagga mussel, 

Asian clam, New Zealand mudsnail)?

• What are the potential impacts of invasive species on 

nutrients? 

• What are potential impacts of invasive species on lake 

ecosystems?

Key Scientific Findings

The unintentional introduction of the invasive quagga 

mussel into Lakes Mead and Mohave has been the most 

significant change to the LMNRA ecosystem within the past 

30 years. The quagga mussel infestation has been pervasive, 

with rapid expansion into all basins of Lakes Mead and 

Mohave and an order of magnitude increase in total population 

over the 5-year period since the first discovery in Lake 

Mead in 2007. Densities of quagga mussels are greatest on 

hard surfaces and substrates, with lower densities in areas 

of soft sediments. Quagga mussels have been detected to 

depths of 355 ft (108.2 m), but the greatest densities of adult 

mussels occur between 30 and 40 ft (9.1 and 12.1 m) deep. 

The densities in shallower zones are thought to be related 

to availability of food sources and higher temperature in the 

water above the thermocline.

Quagga mussel veligers (floating larval forms) are 

now a significant component of Lake Mead’s zooplankton 

community. Veliger abundance in Boulder Basin increased 

from 0.9 individuals/L in 2007 to 4.5 individuals/L in 2008. 

At some times of the year, quagga mussel veligers can be the 

greatest single species in the zooplankton composition by 

numbers (40 percent), although not by weight. In 2010, the 

overall numbers of other zooplankton species in Lakes 

Mead and Mohave were sufficient to maintain a healthy 

sport fishery. 

The impacts of quagga mussels on ecosystem functions, 

including nutrient cycling, has not been documented within 

Lakes Mead or Mohave. However, impacts resulting from 

dreissenid mussels have been well documented in the Great 

Lakes and may serve as a template for possible ecosystemlevel change within LMNRA. In the Great Lakes, quagga 

mussels have increased water clarity and altered foodweb dynamics by removing suspended particles, such as 

phytoplankton, debris, silt, and micro-zooplankton from the 

water column and increasing nutrient loading on the lake 

bottom. Quagga mussels also can affect other organisms by 

direct colonization or by indirect competition for food and 

space. Moreover, quagga mussels can affect nutrient dynamics 

as they primarily use particulate nutrients for food and 

excrete waste nutrients either in the dissolved form, such as 

ammonium, or in the particulate form (fecal pellets). 

The New Zealand mudsnail was first found in 

Lake Mead in 2008 (Davis and Moeltner, 2010), and the 

abundance and distribution of this invasive species in the 

lakes is unknown. The density of Asian clams was more 

than 100 individuals/ m2 in the mid-1980s and has declined 

to less than 50 individuals/ m2 in Lake Mead in recent years; 

however, the cause for these declines is not known. 

Management Implications

The establishment of quagga mussel has the potential 

for significant ecosystem-wide effects on the resources 

of Lakes Mead and Mohave. Although quantifiable 

ecosystem impacts from adult quagga mussels have not 

been documented, research is underway on their natural life 

history and on potential impacts to food-web dynamics. One 

predicted outcome of quagga mussel infestations is increased 

water clarity through filtering of the water by the mussels. 

Although increases in water clarity at Lake Mead have 

been documented, this condition is believed to be primarily 

influenced by recent enhancements in phosphorus removals 

from treated wastewaters discharged into Las Vegas Wash.

Ecosystem-wide consequences of quagga mussels might 

include reducing biomass and changing species composition 

of phytoplankton and zooplankton. As a result, nutrients and 

food resources could be shifted from the pelagic community 

to the benthic community, which would benefit bottom 

dwelling invertebrates and fish but potentially harm openwater fish. This transfer of energy to the lake bottom can 

create negative effects associated with increased bottom 

algal growth and a decrease in available dissolved oxygen. 

Although some areas of the lake bottom have been noted 

to be covered with more bottom algae, overall trends and 

impacts are unknown and will require additional monitoring. 

Quagga mussels attached to a boat motor before cleaning. 

Photograph by National Park Service.
154 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Additionally, quagga mussels have the potential to cause an 

increase in concentrations of dissolved inorganic phosphorus 

and nitrogen that may benefit aquatic plants. 

Quagga mussel encrustations have had significant 

impacts on infrastructure including water-supply intakes 

at Hoover Dam and LMNRA, and the docks and marinas 

located on Lakes Mead and Mohave. Significant costs are 

incurred to keep infrastructure and operations free of clogging 

infestations of quagga mussels. In addition, the States of 

Nevada and Arizona, in cooperation with LMNRA, have been 

forced to implement new boating management and education 

programs to prevent the spread of quagga mussels into other 

water bodies. Although the effects of quagga mussels on 

infrastructure are well documented elsewhere, they have not 

been documented to date in large areas of Lakes Mead or 

Mohave.

Non-native crayfish has become an important food 

resource for many fish species within Lakes Mead and 

Mohave (Chapter 5). Although not known to have negative 

consequences in Lakes Mead and Mohave, crayfish have 

created significant negative impacts on nearby desert spring 

and stream resources, and in some cases they have eliminated 

native species. Care is needed to prevent the spread of crayfish 

from Lakes Mead and Mohave into adjacent bodies of water. 

Additionally, New Zealand mudsnails are abundant in certain 

areas of the lake bottom, but have not been noted to generate 

significant ecosystem impacts. Similarly, Asian clams have 

not been found to have a significant impact to lake ecosystems 

or recreational values and their populations have declined 

substantially in recent years.

Data and Information Needs

Agencies with management responsibilities for 

Lakes Mead and Mohave developed interagency response, 

monitoring, and research plans for quagga mussels shortly 

after their detection in Lake Mead. There has been a 

significant volume of coordinated monitoring for both adult 

and juvenile quagga mussels to date, as well as monthly 

monitoring from stations around both lakes for quagga mussel 

veligers. Quagga mussel monitoring should continue at current 

(2012) levels to determine (1) trends, (2) address questions 

on abundance and distribution, (3) establish rates of change to 

the ecosystem and infrastructure, and (4) inventory potential 

recreational impacts, such as piles of dead shells on beaches 

or algal mats resulting from quagga mussel infestation. 

Moreover, additional research is needed on quagga mussel 

ecosystem effects, such as potential changes in food-web 

dynamics, plankton composition, and in dissolved oxygen or 

nutrient cycling.

Detection monitoring for other potential non-native 

nuisance aquatic species, for example giant Salvinia (an 

aquatic fern from Brazil), is needed to assist managers in 

timely and appropriate responses for spread prevention. 

Climate Change

Key Questions

• What are the potential impacts of climate change on 

water quality related to drinking water and recreation?

• What are the potential impacts of climate change on 

ecosystems, especially to fish or other aquatic-living 

resources, and to birds?

Key Scientific Findings

Observations and studies have shown that many natural 

systems are being affected by regional and global climate 

changes, particularly temperature increases. Because these 

changes will likely affect the hydrological cycle, changes 

in climate may have a large impact on water resources and 

management of those resources. For example, reduced 

precipitation and a series of the hottest years on record from 

2000 to 2004 in the Southwestern United States resulted in 

streamflows that were lower than those during the 1930s 

Dust Bowl or the 1950s drought. Declines in lake levels 

have occurred previously in response to precipitation 

patterns; however, conditions from 2001 to 2010 have 

been unprecedented in their effect on lake levels and lake 

management.

Models of future climate in the Southwest indicate 

that Lake Mead water levels will be affected by declines in 

snowpack within the watershed, increases in evaporation, and 

increases in water use and supply demands. Temperatures 

in the Colorado River Basin are projected to increase from 

5 to 6oF (2 to 3oC) in the 21st century, with slightly larger 

increases in the Upper Colorado River Basin. Lower lake 

levels have serious implications for the availability of water 

for irrigating crops, drinking water, and power generation 

(Chapters 1 and 2). Moreover, projections of precipitation 

in the Southwestern United States show the 30-year average 

in 2070–2099 will be drier than average conditions during 

1950–1979. The most accurate models project that flow in the 

Colorado River Basin will be reduced by 5–20 percent from 

current flows.

Management Implications

With projections of increased temperatures and reduced 

precipitation and flows in the Colorado River Basin, managing 

and preventing water shortages will involve difficult choices 

on how water is used. Reduced Colorado River flows, 

combined with increasing demands for water uses, will result 

in the potential for longer periods of low water levels in Lake 

Mead. Long periods of lower water levels have the potential 

to alter water mixing within the lake, which can in turn 

cause alterations in water temperatures, basic water-quality 
Chapter 7

Management Implications of the Science 155

measures, and food-web dynamics. Lower lake levels also change tributary 

inflows, create more areas of broad shallow deltas, and result in formation 

of new stream channels with the potential to re-suspend sediments. These 

conditions may expose new shoreline areas below the previous high 

water lines, creating expanded or additional 

riparian habitats near tributary inflows, but also 

extensive new areas subject to invasive plants. 

Lowered lake levels may cause various 

changes to habitat and limnology. Razorback 

suckers have historically used very specific 

areas in Las Vegas Bay to spawn; further 

research is needed to understand how lower 

lake levels would affect reproduction of these 

fish. Organic contaminant concentrations also 

have increased over time, perhaps due to lower 

lake levels, but the influence of this trend on 

aquatic organisms is unknown. Additionally, 

increased water temperature corresponding 

to lower lake levels and increasing air 

temperatures will influence the potential for 

algal blooms, which can further reduce oxygen 

levels in the water column.

Projected changes in climate also include 

increased summer rainfall intensities and 

the potential for more frequent flash floods. 

Past flooding events in Las Vegas Wash have 

resulted in elevated nutrient and sediment loads 

as well as temporary spikes in bacteria and 

potential pathogens.

Low water levels from 2001 to 2010 caused the formation of a bathtub ring made of salts precipitated 

out of the water. Photograph by National Park Service.

Working together, high water quality can be maintained for future generations to enjoy. Photograph by National Park Service.
156 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Data and Information Needs

Recent research (Bureau of Reclamation, 2011) on 

climate influences within the Colorado River watershed is 

focused on water availability and supply. No such research, 

however, has been conducted on the potential effects 

of chronically lower water levels in Lake Mead on lake 

configurations, hydrology, and water quality. Information 

is needed on aquatic biota and the ecosystem in general, 

including impacts from ecosystem stressors. Many of these 

effects would be synergistic, enhancing potential impacts from 

the invasive quagga mussel or environmental contaminants. 

Research is needed to develop an understanding of biotic and 

water-quality responses to changes resulting from lowered 

lake levels, increased lake water temperatures, environmental 

contaminants in water and sediment, and impacts of invasive 

species. Modeling lake configurations also will assist 

managers in responding to infrastructure and management 

adjustments necessary with differing water-level regimes. 

Chapter 7 References

Bureau of Reclamation, 2011, SECURE Water Act section 

9503(c) —Reclamation climate change and water 2011: 

Bureau of Reclamation report to Congress, 226 p., accessed 

August 22, 2012, at http://www.usbr.gov/climate/.

Davis, A., and Moeltner, K., 2010, Valuing the prevention of 

an infestation—The threat of the New Zealand mud snail 

in northern Nevada. Agricultural and Resource Economics 

Review, v. 39, no. 1, p. 56–74.

National Park Service, 2010, Long-term limnological and 

aquatic resources monitoring and research plan for Lakes 

Mead and Mohave: Boulder City, Nev., Lake Mead 

National Recreation Area and University of Nevada, Las 

Vegas, 94 p, accessed August 23, 2012, at http://www.nps.

gov/lake/naturescience/loader.cfm?csModule=security/

getfile&%3bpageid=431205.

Rosen, M.R., and Van Metre, P.C., 2010, Assessment of 

multiple sources of anthropogenic and natural chemical 

inputs to a morphologically complex basin, Lake Mead, 

USA: Palaeogeography, Palaeoclimatology, Palaeoecology, 

v. 294, no. 1, p. 30–43.

Rutzke, M.A., Gutenmann, W.H., Lisk, D.J., and Mills, E.L., 

2000, Toxic and nutrient element concentrations in soft 

tissues of zebra and quagga mussels from Lakes Erie and 

Ontario: Chemosphere, v. 40, no. 12, p. 1,353–1,356.

Twichell, D.C., Cross, V.A., Hanson, A.D., Buck, B.J., Zybala, 

J.G., and Rudin, M.J., 2005, Seismic generation and 

lithofacies of turbidites in Lake Mead (Arizona and Nevada, 

U.S.A.), an analogue for topographically complex basins: 

Journal of Sedimentary Research, v. 75, no. 1, p. 134–148.

U.S. Environmental Protection Agency, 2010, National lakes 

assessment—Technical appendix–Data analysis approach: 

Washington, D.C., U.S. Environmental Protection Agency, 

Office of Water and Office of Research and Development, 

EPA 841-R-09-001a, 63 p.

Wong, W.H., Gerstenberger, S.L., Miller, J.M., Palmer, C.J., 

and Moore, B., 2011, A standardized design for quagga 

mussel monitoring in Lake Mead, Nevada-Arizona. Aquatic 

Invasions, v. 6, no. 2, p. 205–215. 
Glossary 157

Glossary

Alluvial Fan A fan-shaped pile of sediment 

(clay, silt, sand, gravel, or other particulate 

material) that forms where a rapidly flowing 

mountain stream enters a relatively flat valley. 

As water slows down, it deposits sediment 

(alluvium) that gradually builds a fan.

Alluvium Sediment eroded by water, wind, and 

gravity that are deposited by rivers and streams 

in a valley bottom.

Bathymetric (also bathymetry) The 

measurement of the depth of water relative to 

a fixed point (for example, sea level, a certain 

elevation or datum). 

Benthic Something that is attached to or resting 

on the bottom or living in the bottom sediments 

of a water body.

Bioaccumulate Absorption by an organism 

from all sources (for example, water, food, air, 

etc.) of environmental contaminants, such as 

pesticides, other organic chemicals, or heavy 

metals, at a rate greater than that at which the 

substance is lost from the organism. 

Biomagnify A series of processes in an 

ecosystem by which greater concentrations of 

a particular chemical are reached in organisms 

higher up the food chain or food web, generally 

as one organism higher in the food chain eats 

organisms that are lower in the food chain.

Biological diversity Used to describe species 

richness, ecosystem complexity, and genetic 

variation in a particular area.

Bioturbation The process of extensively 

reworking sediment by worms, crustaceans, or 

other organisms. 

Chlorophyll Green pigments of plants. 

Chlorophyll a and b are the two most common 

green pigments in plants.

Coliform bacteria Total coliform bacteria are 

a particular group of bacteria that are used as 

indicators of possible sewage pollution. This 

group includes coliforms that inhabit the intestine 

of warm-blooded animals and those that inhabit 

soils.

Effluent Outflow from a particular source, such 

as a stream that flows from a lake or liquid waste 

that flows from a factory or sewage-treatment 

plant. 

Endemic Animals that are only found in a 

certain area or region.

Epilimnion The top layer of a lake where the 

sunlight penetrates and provides energy for plants 

and algae to grow. See Thermal Stratification.

Estrogenicity The degree to which a compound 

can stimulate female hormones to be produced in 

an organism.

Euphotic zone Zone to which light penetrates in 

a water body, such that photosynthesis can take 

place and plants can grow. 

Eutrophic The condition of a water body when 

nutrient concentrations are high enough to limit 

oxygen for organisms in a water body.

Eutrophication The process by which a body of 

water acquires a high concentration of nutrients, 

especially phosphates and nitrates, which 

typically promote excessive growths of algae. 

As the algae die and decompose, the amount of 

available oxygen in the water is depleted, in turn 

causing the death of other organisms, such as 

fishes.

Extirpation Complete loss of a species in a 

certain geographic area, although the species still 

exists elsewhere in its range.

Feeding guild A group of species having 

similar ecological resource requirements and 

foraging strategies and therefore having similar 

roles in the community.

Feminization Developmental changes resulting 

in the occurrence of female reproductive tissue in 

genetic males.

Food web A summary of the feeding 

relationships within an ecological community.

Forage fish Any fish eaten by large predatory 

fish, birds, or mammals associated with an 

aquatic ecosystem.
158 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Forb A broad-leaved herb (not a grass), 

especially one growing in a field, prairie, or 

meadow.

Full pool The volume of water at which a 

reservoir is fully utilized for all purposes. 

Geomorphic Pertaining to landforms, or 

contour of the land, and processes that form 

landforms.

Hormone A chemical substance produced 

within the body of an organism; once secreted, 

hormones travel through the bloodstream to 

control and regulate the activity of specific 

tissues and cells in other parts of the body.

Hypolimnion The usually cold, dense 

bottom layer of a stratified lake. See Thermal 

Stratification.

Intersex An organism that possesses a mixture 

of male and female reproductive tissues.

Invasive species An introduced alien species 

that is likely to cause harm to the natural 

ecosystem, the economy, or human health.

Legacy pesticides Persistent, toxic substances 

previously used to repel or destroy pests; though 

no longer in use, these substances remain in the 

environment due to their decades-long half-lives.

Lentic Standing water including ponds, lakes, 

and reservoirs.

Limiting nutrient A chemical necessary 

for plant growth but is available in smaller 

quantities than needed for plants to increase their 

abundance.

Limnology The study of inland waters focusing 

on ecological systems interacting with their 

drainage basins and the atmosphere.

Littoral Pertaining to the shallow area of a 

water body adjacent to the shore.

Macrophyte An aquatic plant that grows in or 

near water that can be observed without the use 

of optical magnification.

Mesotrophic Waters that have a moderate 

amount of dissolved nutrients that are usually 

clear and have beds of submerged aquatic plants.

Micronutrients Chemicals required by living 

organisms throughout life in small quantities to 

control important physiological functions.

Nutrient loading Quantity of nutrients entering 

an ecosystem in a given period of time.

Oligotrophic Waters that are poor in nutrients 

have low primary productivity and are usually 

very clear.

Passive samplers Devices able to acquire 

a sample of environmental chemicals from 

a discrete location without the active media 

transport by pumping or purge techniques.

Pathogen Disease-causing organisms that may 

be present in any body of water. Inadequately 

treated drinking water may contain human 

pathogens, and pathogens may also be of 

concern to recreational waters as well as wildlife. 

Pathogens include various types of bacteria, 

viruses, protozoan parasites, and other organisms.

Perennial plants A plant that lives for more 

than 2 years. 

Photosynthesis Synthesis of chemical 

compounds by organisms with the aid of light. 

Carbon dioxide is used as a raw material for 

photosynthesis and oxygen is a product.

Phytoplankton Phytoplankton are microscopic 

organisms that live in watery environments, both 

salty and fresh.

Planktonic Related to floating or weakly 

swimming organisms at the mercy of the 

waves and currents. Animals of the group are 

called zooplankton and the plants are called 

phytoplankton.

Productivity A measure of the rate at which 

new organic matter is formed and accumulated 

through photosynthetic and chemosynthetic 

activity of producer organisms; in lake 

management,it is frequently used to express 

levels of primary production of plankton.

Riparian Areas adjacent to rivers and streams 

with a high density, diversity, and productivity 

of plant and animal species relative to nearby 

uplands.

Recruitment The addition of new individuals 

into a fish or wildlife population by reproduction, 

commonly measured as the proportion of young 

in the population just before the breeding season
Glossary 159

Reservoir A pond, lake, or basin, either natural 

or artificial, for the storage, regulation, and 

control of water.

Return flow That part of a diverted body of 

water that is not consumptively used and is 

returned to its original source or another body of 

water.

Specific Conductance A measure of the 

ability of water to conduct an electrical current 

as measured using a 1-cm cell and expressed in 

units of electrical conductance, i.e. Siemens per 

centimeter at 25°C. Specific conductance can be 

used for approximating the total dissolved solids 

content of water by testing its capacity to carry 

an electrical current.

Supersaturation A solution that contains a 

higher than saturation concentration of a solute.

Thermal Stratification Vertical temperature 

stratification that shows the following: The upper 

layer of the lake, known as the epilimnion, in 

which the water temperature is virtually uniform; 

a stratum next below, known as the thermocline, 

in which there is a marked drop in temperature 

per unit of depth; and the lowermost region or 

stratum, known as the hypolimnion, in which the 

temperature from its upper limit to the bottom is 

nearly uniform.

Thermocline The area of marked temperature 

change usually between a warm top layer 

of a lake and the cold bottom part of the 

lake. However, in some lakes the contrast in 

temperatures can be reversed. See Thermal 

Stratification.

Trophic Pertaining to nutrition or a position in a 

food web or food chain.

Turbidity Reduced clarity of water because of 

suspended particles, usually sediments. 

Xenobiotic A chemical found in an organism 

that is not normally produced, or expected to be 

present in the organism, naturally. Often used in 

the context of pollutants.
160 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

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Chapter 1

Appendix A 161

Common name Status

Relative abundance

(observation frequency)

Mead Mohave

Diving birds

Red-throated Loon U r

Pacific Loon U r

Common Loon W i i

Pied-billed Grebe R c c

Horned Grebe M i i

Red-necked Grebe U r

Eared Grebe M a a

Western Grebe M,W,B a c

Clark’s Grebe M,W,B a c

Brown Pelican U r

American White Pelican M a r

Double-crested Cormorant R,M,B c c

Wading birds

Least Bittern U r

Great Blue Heron R,B c c

Great Egret M,W i i

Snowy Egret M,S c i

Reddish Egret U r

Cattle Egret U r

Green Heron S r i

Black-crowned Night-Heron M i i

White-faced Ibis M i i

Waterfowl

Greater White-fronted Goose U r

Snow Goose M,W i r

Ross’s Goose U r

Canada Goose M,W c r

Mute Swan U r

Tundra Swan U r

Wood Duck U r r

Gadwall W a r

American Wigeon M,W a r

[Species names follow the checklist of the American Ornithologists’ Union. General status of each species is listed, along with its relative abundance on each 

lake (blanks spaces indicate non-detection of the species). Status: R, resident; S, summer visitor; M, migrant; W, winter visitor; U, unknown, few observations; 

B, breeding documented or highly suspected. Relative abundance (observation frequency) on each lake: a, abundant: regularly observed, often in large numbers; 

c, common: regularly observed; i, common but infrequently observed: observed often but sporadically and in low numbers; r, rare: few observations; blank space, not 

detected]

Common name Status

Relative abundance

(observation frequency)

Mead Mohave

Waterfowl—Continued

Mallard R,W,B c c

Blue-winged Teal M i r

Cinnamon Teal M a i

Northern Shoveler M,W a r

Northern Pintail M,W a r

Green-winged Teal M,W a r

Canvasback M,W i r

Redhead M,W i i

Ring-necked Duck W i i

Greater Scaup U r

Lesser Scaup W c c

Surf Scoter U r r

White-winged Scoter U r

Black Scoter U r

Long-tailed Duck U r r

Bufflehead W i i

Common Goldeneye W i a

Hooded Merganser M r i

Common Merganser M,W a c

Red-breasted Merganser M i i

Ruddy Duck M a c

Aquatically associated raptors

Osprey M,W i i

Bald Eagle M,W c c

Peregrine Falcon R,B i i

Marsh birds

Black Rail U r

Virginia Rail W r

Sora M,B i

Common Moorhen U r r

American Coot M,W,B a a

Appendix A.—List of 96 species compiled from observations made during

monthly aquatic bird surveys on Lakes Mead and Mohave over a 5-year 

period (2004–2009). 
162 A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave

Common name Status

Relative abundance

(observation frequency)

Mead Mohave

Shorebirds

Black-bellied Plover U r

Snowy Plover M,S,B i

Semipalmated Plover M i

Killdeer R,B c c

Black-necked Stilt M c i

American Avocet M a i

Spotted Sandpiper M,W,B i i

Spotted Redshank U r

Greater Yellowlegs M i

Willet M i

Lesser Yellowlegs M r

Whimbrel M r

Long-billed Curlew M r r

Marbled Godwit M r

Sanderling M r

Western Sandpiper M a r

Least Sandpiper M a

White-rumped Sandpiper U r

Baird’s Sandpiper U r

Pectoral Sandpiper U r

Appendix A.—List of 96 species compiled from observations made during

monthly aquatic bird surveys on Lakes Mead and Mohave over a 5-year 

period (2004–2009).—Continued 

[Species names follow the checklist of the American Ornithologists’ Union. General status of each species is listed, along with its relative abundance on each 

lake (blanks spaces indicate non-detection of the species). Status: R, resident; S, summer visitor; M, migrant; W, winter visitor; U, unknown, few observations; 

B, breeding documented or highly suspected. Relative abundance (observation frequency) on each lake: a, abundant: regularly observed, often in large numbers; 

c, common: regularly observed; i, common but infrequently observed: observed often but sporadically and in low numbers; r, rare: few observations; blank space, not 

detected]

Common name Status

Relative abundance

(observation frequency)

Mead Mohave

Shorebirds—Continued

Dunlin M r

Long-billed Dowitcher M i

Wilson’s Snipe M r

Wilson’s Phalarope M i

Red-necked Phalarope M r r

Aerialists

Sabine’s Gull U r

Bonaparte’s Gull M i r

Franklin’s Gull M i r

Ring-billed Gull M a a

California Gull M c c

Herring Gull M i r

Caspian Tern M c

Black Tern U r r

Common Tern M i r

Forster’s Tern M i r

Parasitic Jaeger U r

Belted Kingfisher W i i

Publishing support provided by the U.S. Geological Survey

Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Nevada Water Science Center

U.S. Geological Survey

2730 N. Deer Run Road

Carson City, Nevada 89701

http://nevada.usgs.gov/

95

95

215

95

93

93

93

93

15

15

NEVADA

NEVADA ARIZONA

River

Colorado River

HOOVER DAM

Willow Beach

National Fish 

Hatchery (USFWS)

DAVIS DAM

Las Vegas

Henderson

Boulder City

Lake Mead

Fish Hatchery

(NDOW)

Alfred Merritt

Smith Water 

Treatment 

Facility (SNWA)

Muddy

Logandale

Overton

Searchlight

Laughlin

Gregg Basin

Overton Arm

Callville Bay

Temple Basin Virgin Basin

Las Vegas Bay

Boulder

Basin

Pearce 

Ferry

Boulder Canyon

Black 

Canyon

Virgin 

Canyon

Iceberg

Canyon

Virgin River

Wash

LasVegas

LAKE

MOHAVE

MEAD

LAKE

Nevada

Arizona

Study Area

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Rosen and others— A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave—Circular 1381

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