ENVIRONMENTAL AND HYDROLOGIC SETTINGS OF THE 

LAS VEGAS VALLEY AREA AND THE CARSON AND 

TRUCKEE RIVER BASINS, NEVADA AND CALIFORNIA

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4087

National Water-Quality Assessment Study Unit 

Nevada Basin and Range Study Unit

NATIONAL WATER - QUALITY ASSESSMENT PROGRAM
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

Any use of trade names in this publication is for descriptive purposes 

only and does not imply endorsement by the U.S. Government.

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

333 West Nye Lane, Room 203 Box 25286

Carson City, NV 89706-0866 Denver, CO 80225-0286
FOREWORD

The mission of the U.S. Geological Survey 

(USGS) is to assess the quantity and quality of the 

earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound 

decisions. Assessment of water-quality conditions and 

trends is an important part of this overall mission.

One of the greatest challenges faced by waterresources scientists is acquiring reliable information 

that will guide the use and protection of the Nation's 

water resources. That challenge is being addressed by 

Federal, State, interstate, and local water-resource 

agencies and by many academic institutions. These 

organizations are collecting water-quality data for a 

host of purposes that include: compliance with permits 

and water-supply standards; development of remediation plans for a specific contamination problem; operational decisions on industrial, wastewater, or watersupply facilities; and research on factors that affect 

water quality. An additional need for water-quality 

information is to provide a basis on which regional 

and national-level policy decisions can be based. Wise 

decisions must be based on sound information. As a 

society we need to know whether certain types of 

water-quality problems are isolated or ubiquitous, 

whether there are significant differences in conditions 

among regions, whether the conditions are changing 

over time, and why these conditions change from 

place to place and over time. The information can be 

used to help determine the efficacy of existing waterquality policies and to help analysts determine the 

need for, and likely consequences, of new policies.

To address these needs, the Congress appropriated funds in 1986 for the USGS to begin a pilot 

program in seven project areas to develop and refine 

the National Water-Quality Assessment (NAWQA) 

Program. In 1991, the USGS began full implementation of the program. The NAWQA Program builds 

upon an existing base of water-quality studies of the 

USGS, as well as those of other Federal, State, and 

local agencies. The objectives of the NAWQA 

Program are to:

 Describe current water-quality conditions for a 

large part of the Nation's freshwater streams, 

rivers, and aquifers.

 Describe how water quality is changing over 

time.

 Improve understanding of the primary natural 

and human factors that affect water-quality 

conditions.

This information will help support the development and evaluation of management, regulatory, and 

monitoring decisions by other Federal, State, and local 

agencies to protect, use, and enhance water resources.

The goals of the NAWQA Program are being 

achieved through investigations of 60 of the Nation's 

most important river basins and aquifer systems, which 

are referred to as study units. These study units are 

distributed throughout the Nation and cover a diversity 

of hydrogeologic settings. More than two-thirds of the 

Nation's freshwater use occurs within the 60 study 

units and more than two-thirds of the people 

served by public water-supply systems live within 

their boundaries.

National synthesis of data analysis, based on 

aggregation of comparable information obtained from 

the study units, is a major component of the program. 

This effort focuses on selected water-quality topics 

using nationally consistent information. Comparative 

studies will explain differences and similarities in 

observed water-quality conditions among study areas 

and will identify changes and trends and their causes. 

The first topics addressed by the national synthesis are 

pesticides, nutrients, volatile organic compounds, and 

aquatic biology. Discussions on these and other waterquality topics will be published in periodic summaries 

of the quality of the Nations's ground and surface water 

as the information becomes available.

This report is an element of the comprehensive 

body of information developed as part of the NAWQA 

Program. The program depends heavily on the advice, 

cooperation, and information from many Federal, 

State, interstate, Tribal, and local agencies and the 

public. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch 

Chief Hydrologist

FOREWORD
CONTENTS

Abstract .................................................................................................................................................................^ 1

mtroduction.........................................................................................................^^ 2

National Water-Quality Assessment Program ........................................................................................................... 2

Nevada Basin and Range Study Unit ........................................................................................................................ 3

Report Purpose and Scope ......................................................................................................................................... 3

Environmental Subareas........................................................................................................................................................ 5

Environmental Setting .......................................................................................................................................................... 5

Physiography and Geology ....................................................................................................................................... 5

Las Vegas Valley Area ................................................................................................................................... 5

Physical and Geologic Settings........................................................................................................... 5

Consolidated Rocks............................................................................................................................. 9

Unconsolidated Deposits .................................................................................................................... 9

Carson and Truckee River Basins .................................................................................................................. 12

Physical and Geologic Settings........................................................................................................... 12

Consolidated Rocks ............................................................................................................................ 14

Unconsolidated Deposits .................................................................................................................... 14

Climate, Soils, and Vegetation .................................................................................................................................. 16

Las Vegas Valley Area ................................................................................................................................... 16

Climate................................................................................................................................................ 16

Soils ................:................................................................................................................................... 16

Vegetation ........................................................................................................................................... 19

Carson and Truckee River Basins .................................................................................................................. 19

Climate ............................................................................................................................................... 19

Soils ................................................................................................................................................. 19

Vegetation........................................................................................................................................... 19

Urbanization, Land Use, and Water Use ................................................................................................................... 22

Las Vegas Valley Area ................................................................................................................................... 22

Urbanization ....................................................................................................................................... 22

Land Use ............................................................................................................................................ 22

Water Use ........................................................................................................................................... 24

Carson and Truckee River Basins .................................................................................................................. 24

Urbanization ....................................................................................................................................... 24

Land Use ............................................................................................................................................ 24

Water Use ........................................................................................................................................... 26

Hydrologic Setting...............................................................................................................................................................^ 26

Surface'Water...................................................................................................................,_^ 26

Las Vegas Valley Area ................................................................................................................................... 26

Lakes and Reservoirs ......................................................................................................................... 26

Flood-Control Structures .................................................................................................................... 28

Streamflow Characteristics ................................................................................................................. 28

Stream Water-Quality Characteristics ......................................................................................~........ 29

Carson and Truckee River Basins................................................................................................................... 31

Lakes and Reservoirs.......................................................................................................................... 31

Streamflow Diversions...................................................................................................^.................... 34

Streamflow Characteristics ................................................................................................................. 34

Stream Water-Quality Characteristics................................................................................................. 37

CONTENTS
Ground Water ............................................................................................................................................................ 44

Las Vegas Valley Area ................................................................................................................................... 44

Principal Aquifers .............................................................................................................................. 44

Ground-Water-Quality Characteristics ............................................................................................... 48

Carson and Truckee River Basins................................................................................................................... 50

Principal Aquifers............................................................................................................................... 50

Ground-Water-Quality Characteristics ............................................................................................... 58

Water-Quality Issues ................................................................................................................................................. 65

Las Vegas Valley Area ................................................................................................................................... 65

Carson and Truckee River Basins .................................................................................................................. 65

Summary ............................................................................................................................................................................... 65

References Cited.................................................................................................................................................................... 69

FIGURES

1-11. Maps showing:

1. National Water-Quality Assessment study units, including Nevada Basin and Range study unit................. 3

2. Nevada Basin and Range study unit............................................................................................................... 4

3. Environmental subareas for Las Vegas Valley area........................................................................................ 6

4. Environmental subareas for Carson and Truckee River Basins...................................................................... 7

5. Physiography of Las Vegas Valley area.......................................................................................................... 8

6. Surficial geology of Las Vegas Valley area..................................................................................................... 11

7. Physiography of Carson and Truckee River Basins........................................................................................ 13

8. Surficial geology of Carson and Truckee River Basins.................................................................................. 15

9. Location of precipitation measurement stations in Las Vegas Valley area..................................................... 17

10. Location of precipitation measurement stations in Carson and Truckee River Basins .................................. 20

11. Population density, based on 1990 census, in Las Vegas Valley area............................................................. 23

12. Graph showing water use for Las Vegas Valley, 1990........................................................................................... 24

13. Map showing population density, based on 1990 census, in Carson and Truckee River Basins........................... 25

14. Graph showing water use for Carson and Truckee River Basins, 1990................................................................. 26

15. Map showing streamflow-gaging stations and water-quality monitoring sites in Las Vegas Valley area............. 27

16-19. Graphs showing:

16. Flow duration curve for Las Vegas Wash near Henderson, Nev., water years 1970-88................................. 29

17. Daily mean streamflow for Las Vegas Wash near Henderson, Nev., water years 1970-88............................ 30

18. Boxplots for concentrations of major ions and dissolved solids for water samples from

Las Vegas Wash near Henderson, Nev., water years 1988-92........................................................................ 31

19. Relation of smoothed dissolved-solids concentrations to streamflow percentiles for

Las Vegas Wash near Henderson, Nev., water years 1970-88........................................................................ 31

20. Map showing streamflow-gaging stations and water-quality monitoring sites in Carson and

Truckee River Basins............................................................................................................................................. 32

VI Environmental and Hydro logic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
21-25. Graphs showing:

21. Flow-duration curves for selected streamflow-gaging stations in Carson and Truckee River Basins,

water years 1970-90........................................................................................................................................ 36

22. Daily mean streamflow for selected gaging stations in Carson and Truckee River Basins,

water years 1970-90........................................................................................................................................ 38

23. Boxplots for concentrations of major ions and dissolved solids, and pH for water samples from 

selected stream-water-quality monitoring sites in Truckee and Carson River Basins, 

water years 1961-92........................................................................................................................................ 40

24. Relation of smoothed dissolved-solids concentrations to streamflow percentiles for 

selected stream-water-quality monitoring sites in Carson and Truckee River Basins, 

water years 1970-90........................................................................................................................................ 42

25. Temperature duration curves for selected sites in Carson and Truckee River Basins.................................... 45

26. Map showing general area of principal basin-fill aquifers in Las Vegas Valley area............................................ 46

27. Map showing water-level altitude and selected water-level monitoring wells in Las Vegas Valley area.............. 47

28-30. Graphs showing:

28. Total Colorado River and ground-water use for Las Vegas Valley area, 1970-90.......................................... 49

29. Ground-water levels for selected wells in Las Vegas Valley area, water years 1970-90................................ 51

30. Boxplots for concentrations of major ions and dissolved solids in ground-water

samples from Las Vegas Valley area, water years 1975-87............................................................................ 52

31-33. Maps showing:

31. Location of selected water-quality monitoring wells in Las Vegas Valley area............................................. 53

32. General areas of principal basin-fill aquifers in Truckee and Carson River Basins....................................... 54

33. Water-level altitude and selected water-level monitoring wells in Carson and

Truckee River Basins...................................................................................................................................... 55

34. Graphs showing ground-water levels for selected wells in Carson and Truckee River Basins,

water years 1970-90............................................................................................................................................... 56

35-38. Boxplots for concentrations in ground-water samples:

35. Selected cations, Carson River Basin, water years 1958-89........................................................................... 60

36. Selected anions, Carson River Basin, water years 1958-89............................................................................. 61

37. Selected cations, Truckee River Basin, water years 1972-91......................................................................... 62

38. Selected anions, Truckee River Basin, water years 1972-91.......................................................................... 63

39. Map showing location of selected water-quality monitoring wells in Carson and Truckee River Basins ............ 64

TABLES

1. Divisions of geologic time major geochronologic units..................................................................................... 10

2. Precipitation data for weather stations in Las Vegas Valley area, 1981-91 ........................................................... 18

3. Precipitation data for weather stations in Carson and Truckee River Basins, 1981-91......................................... 21

4. Streamflow characteristics at selected gaging stations in Las Vegas Valley area,

water years 1970-90............................................................................................................................................... 30

5. Streamflow characteristics at selected gaging stations in Carson and Truckee River Basins,

water years 1970-90................................................^ 35

6. Principal aquifers and well information in the Nevada Basin and Range study unit............................................. 50

CONTENTS VII
CONVERSION FACTORS AND ABBREVIATIONS

Multiply

acre

acre-foot per year (acre-ft/yr)

cubic foot per second (ft3/s)

foot (ft)

gallon (gal)

million gallons per day (Mgal/d)

inch (in)

feet per year (ft/yr)

inch per year (in/yr)

mile (mi)

square mile (mi2)

By

0.4047

1233.4

0.002832

0.3048

0.003785

3.78530

25.39999

0.30480

25.399

1.60934

2.58992

To obtain

square hectometer

cubic meter per year

cubic meter per second

meter

cubic meter

million liters per day

millimeter

meters per year

millimeter per year

kilometer

square kilometer

Temperature: Degrees Celsius (°C) can be converted to degrees Fahrenheit (°F) by using the formula °F = [1.8(°C)]+32.

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929, formerly called "Sea-Level 

Datum of 1929"), which is derived from a general adjustment of the first-order leveling networks of the United States and Canada.

Additional abbreviations:

(Ig/L (microgram per liter)

(iS/cm (microsiemens per centimeter at 25°C)

mg/L (milligram per liter)

mg/kg (milligram per kilogram)

VIII Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
Environmental and Hydrologic Settings of the 

Las Vegas Valley Area and the Carson and 

Truckee River Basins, Nevada and California

By Kenneth J. Covay, Juliana M. Banks, Hugh E. Bevans, anc/Sharon A. Watkins

ABSTRACT

In 1991, the U.S. Geological Survey began 

a full-scale National Water-Quality Assessment 

(NAWQA) program to describe the status of 

and trends in the quality of the Nation's water 

resources and to provide a scientific understanding 

of the primary natural and human factors that 

affect water quality. The NAWQA program 

consists of 60 proposed study units representing 

more than two-thirds of the Nation's water use and 

population served by public water supplies. The 

Nevada Basin and Range (NVBR) was selected as 

one of the first 20 study units to be investigated as 

part of the full-scale program.

This report uses available information and 

data to describe the environmental and hydrologic 

settings of the NVBR study unit, which includes 

the Las Vegas Valley area and the Carson and 

Truckee River Basins. It includes descriptions 

of landforms, geology, climate, soils, vegetation, 

land use, water use, surface-water and groundwater hydrology, and water-quality issues for 

the three areas.

The Las Vegas Valley area in southeastern 

Nevada encompasses about 1,640 square miles of 

the Basin and Range Physiographic Province. The 

Carson and Truckee River Basins in west-central 

Nevada and east-central California encompass 

an area of about 7,200 square miles. The basins 

include parts of both the Sierra Nevada and Basin 

and Range Provinces. More than 90 percent of 

Nevada's population resided in the NVBR study 

unit in 1990.

Land use in the Las Vegas Valley area is 

about 79 percent range, 14 percent forest, 5 percent urban, 1 percent open water and wetlands, 

and 1 percent barren. Total water use in 1990 

was about 317,000 acre-feet; 91 percent was used 

for public supplies. Land use in the Carson and 

Truckee River Basins is 50 percent range, 22 percent forest, 13 percent open water and wetlands, 

10 percent barren, 4 percent irrigated agriculture, 

and 1 percent urban. Water use in 1990 was about 

538,000 acre-feet in the Carson River Basin, and 

about 262,000 acre-feet in the Truckee River 

Basin. About 95 percent of the water used in the 

Carson River Basin was for irrigation. About 

59 percent of the water used in the Truckee 

River Basin was for irrigation.

Las Vegas Wash is the major drainage in 

Las Vegas Valley area. Its flow in and downstream 

from Las Vegas is perennial because of urban runoff and, in the lower reaches, tertiary treated sewage effluent, industrial drainage, and discharge of 

saline ground water. Las Vegas Wash transports 

contaminants from these point and nonpoint 

sources to Las Vegas Bay of Lake Mead. Other 

washes in Las Vegas Valley are either intermittent 

or ephemeral.

The Carson and Truckee Rivers are 

perennial and originate in headwater areas in 

the Sierra Nevada. The basins are hydrologically 

closed, with the Carson River terminating in the 

Carson Sink and the Truckee River terminating in 

Pyramid Lake. The Truckee River receives urban 

drainage and tertiary treated sewage effluent from 

the Reno-Sparks urban area. Irrigation in the

ABSTRACT
Carson Desert has resulted in high concentrations 

of dissolved solids, boron, arsenic, selenium, 

molybdenum, lithium, and un-ionized ammonia 

in water, bottom sediments, and biota. Historical 

mining activities have contaminated lower reaches 

of the Carson River, Lahontan Reservoir, and 

Stillwater Marsh with mercury.

Principal unconsolidated basin-fill aquifers 

in Las Vegas Valley are more than 3,000 feet thick. 

Deeper consolidated carbonate-rock aquifers are 

present beneath the valley. The ground-water 

quality deteriorates from headwater areas in the 

Spring Mountains to discharge areas in the southeast. Water quality of the shallow aquifer system 

underlying Las Vegas Valley is affected primarily 

by urban land use. The shallow aquifer system in 

the central and southeastern parts of the valley 

receives secondary recharge from landscape 

irrigation. Downward hydraulic gradients, with 

downward recharge potential, from the shallow 

aquifer to deeper aquifers have resulted from 

withdrawals for public water supplies from deeper 

principal aquifers. This has created a potential for 

contaminated shallow ground water to affect 

deeper potable supplies.

Principal aquifers in the Carson and 

Truckee River Basins are unconsolidated basin-fill 

deposits of interbedded gravel, sand, silt, and clay. 

Aquifers within the basins readily receive, transmit, and store large volumes of water. Ground 

water in headwater recharge areas within the 

Carson and Truckee River Basins generally is a 

calcium bicarbonate type with less than 200 mg/L 

of dissolved solids. Ground water in terminal areas 

contains increased concentrations of sodium, 

chloride, and sulfate; concentrations of dissolved 

solids can exceed 10,000 mg/L. Shallow groundwater recharge in the Carson Desert primarily 

is by irrigation infiltration in the Fallon 

agricultural area.

INTRODUCTION

National Water-Quality Assessment Program

In 1986, Congress appropriated funds for the 

U.S. Geological Survey (USGS) to begin a pilot 

National Water-Quality Assessment (NAWQA) program. The purpose of the pilot NAWQA program was 

to test, develop, and refine methods of assessing the 

quality of the Nation's water resources. The National 

Academy of Sciences (NAS) Water and Technology 

Board reviewed the NAWQA pilot program in 1987 

and reported that implementation of a full-scale program was in the Nation's best interest. The NAS recommended that Congress appropriate the necessary funds 

to proceed with a full-scale NAWQA program in fiscal 

year 1991.

The full-scale NAWQA program will provide 

data and information on the status, trends, and major 

natural and human factors that affect the quality of the 

Nation's water resources. The program will focus on 

large-scale, persistent conditions to provide current 

information to resource managers and policy makers 

at the Federal, State, and local levels. A total of 60 

proposed study units (fig. 1) composed of interrelated 

surface- and ground-water systems will be examined. 

These study units represent more than two-thirds of the 

Nation's water use and population served by public 

water supplies.

The first group of 20 study-unit investigations 

(fig. 1) began in 1991, the second group began in 1994, 

and the third group will begin in 1997. The basic study 

plan allows for 2 years of planning and retrospective 

analysis, followed by 3 years of intensive data collection and 5 years of less intensive activities. Selected 

issues of national concern for the first group of 20 study 

units are pesticides, nutrients, and suspended sediment. 

The status and trends of these water-quality constituents and relations among water quality, human 

activities, and natural factors will be investigated 

by evaluating physical, chemical, and biological 

components of hydrologic systems.

Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
EXPLANATION

NAWQA study units

Began in 1991; Black denotes Nevada 

Basin and Range study unit

Began in 1994 

I I Scheduled to begin in 1997

Figure 1 . National Water-Quality Assessment study units, including Nevada Basin and Range Study unit.

Nevada Basin and Range Study Unit

Water quality in the Nevada Basin and Range 

study unit (fig. 2) is affected primarily by urbanization 

and agriculture. Local issues were defined in NVBR 

liaison committee meetings with representatives from 

agencies and organizations concerned with waterresource management. The committee helped identify 

persistent and widespread water-quality issues affected 

by human activities such as urbanization, agriculture, 

industry, and mining. The principal natural waterquality issues identified were the presence of radionuclides and arsenic in ground water, intrusion or 

discharge of geothermal water, and salinity.

The NVBR study will provide data and interpretation 

needed to address water-quality issues at national, 

regional, and local scales.

Report Purpose and Scope

This report provides an overview of the NVBR 

study unit. It includes a description of environmental 

and hydrologic conditions for the Las Vegas Valley 

area and the Carson and Truckee River Basins. 

Available information and data are used to describe 

physiography, geology, climate, soils, vegetation, 

urbanization, land use, water use, surface-water and 

ground-water hydrology, and water-quality issues.

INTRODUCTION
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Figure 2. Nevada Basin and Range study unit.

Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
ENVIRONMENTAL SUBAREAS

The information in this report was used to 

develop a strategy for further study. The investigation 

strategy is based on subdividing the study area (fig. 2) 

into subareas that are generally homogeneous with 

respect to environmental conditions, both human and 

natural, that influence water quality. This process facilitates the evaluation of significant relations among 

water-quality conditions and selected, important 

human and natural factors that otherwise might 

be obscured.

A conceptual model of basin and range hydrology 

was used to develop a framework of primary subareas. 

The conceptual model designates the Sierra Nevada 

and Spring Mountains, and other high mountains 

and adjacent areas, where precipitation is sufficient, 

as headwater areas that provide water for streamflow 

and ground-water recharge. Headwater areas are 

defined as yielding about 0.2 in. or more of annual runoff, as delineated by Gebert and others (1987). Valleys 

along the eastern slopes of the Sierra Nevada (Carson, 

Eagle, and Washoe Valleys and the Truckee Meadows) 

are included as headwater areas. Basin areas are low 

mountains and valleys that yield less than 0.2 in. of 

annual runoff. Basin-area valleys that receive runoff or 

recharge from adjacent headwater areas can have both 

surface- and ground-water resources. Discharge in the 

terminal areas of the basins is by evapotranspiration 

and streamflow in the Las Vegas Valley and by evapotranspiration in the Carson and Truckee River Basins.

Secondary subareas were delineated by 

subdividing the two primary subareas (headwater 

areas and basin areas) into water-resources categories 

(rivers, lakes, wetlands, and aquifers). Rivers and 

aquifers will be investigated during this cycle of the 

NVBR NAWQA. Aquifer resources were then subdivided into unconsolidated basin-fill deposits, carbonate rocks, and noncarbonate rocks. This cycle of the 

investigation will consider factors that may influence 

water quality of the principal unconsolidated basin-fill 

aquifers that have about 100 ft or more of saturated 

thickness (Vasey and others, 1972; White and others, 

1979; Maurer, 1986) and ground water with less than 

1,000 mg/L of dissolved solids (Thompson and others, 

1984).

The water-resources subareas were then further 

subdivided into selected land uses that either represent 

natural conditions (forest and range) or important 

human effects (urban and suburban land use and agriculture). Headwater areas have forest and range as 

primary land uses; urban and suburban land use and 

agriculture (irrigated pasture and alfalfa) are the principal non-point sources that affect water resources. Basin 

areas are dominated by sparsely vegetated rangeland; 

irrigated agriculture, urban, and suburban areas affect 

water resources. The environmental subareas for the 

Las Vegas Valley area are shown in figure 3; subareas 

for the Carson and Truckee River Basins are shown 

in figure 4.

ENVIRONMENTAL SETTING 

Physiography and Geology

Las Vegas Valley Area

Physical and Geologic Settings

The Las Vegas Valley area (fig. 5) in southeastern

fy

Nevada encompasses about 1,640 mi and includes the 

Las Vegas Valley Hydrographic Area 1 and that part of 

the Black Mountains Hydrographic Area that includes 

Las Vegas Bay. Altitudes range from about 11,900 ft in 

consolidated-rock headwater areas in the Spring 

Mountains, to about 1,600 ft in unconsolidated basinfill deposits of Las Vegas Valley, and to about 1,200 ft 

at Las Vegas Bay of Lake Mead. Las Vegas Valley 

topography is typical of the Basin and Range Physiographic Province with northward-trending mountain 

ranges and valleys. Las Vegas Valley, however, trends 

northwest to southeast. The northern part of the valley 

is within a structurally controlled zone of deformation 

known as the Las Vegas Valley Shear Zone.

formal Hydrographic Areas in Nevada were delineated 

systematically by the U.S. Geological Survey and the Nevada 

Division of Water Resources in the late 1960's for scientific and 

administrative purposes (Rush, 1968; Cardinalli and others. 1968). 

The official Hydrographic Area names, numbers, and geographic 

boundaries continue to be used in U.S. Geological Survey reports 

and Nevada Division of Water Resources administrative activities.

ENVIRONMENTAL SUBAREAS
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Albers Equal-Area Conic projection

Standard parallels 29°301 and 45030-. central meridian -115000'

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

General area underlain by principal 

basin-fill aquifers Saturated thickness is 

100 feet or more (Vasey and others. 1972). 

Ground water has less than 1.000 mg/L 

of dissolved solids (Thompson and 

others. 1984)

Headwater area Annual runoff of 0.02 inch 

or more (Gebert and others. 1987)

Land use Modified from U.S. Geological 

Survey digital data; 1:250,000 scale; 

1973-83

|____I Range Includes barren and other 

[ ] Irrigated agriculture

. . . Study area boundary

Figure 3. Environmental subareas for Las Vegas Valley area.

6 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
120°00'

40°00'

39°00'

119000' 

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10 20 30 40 MILES

10 20 30 40 KILOMETERS

Base from U.S. Geological Survey digital data 1:100.000.1977-65

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -119°00'

EXPLANATION

General area underlain by principal basin-fill

aquifers Saturated thickness is 100 feet or more 

(Vasey and others. 1972; White and others. 1979; 

Maurer, 1986). Ground water has less than 

1,000 mg/L of dissolved solids (Thompson 

and others. 1984)

Headwater area Annual runoff of 0.02 inch or more 

(Gebert and others, 1987)

Land use Modified from U.S. Geological Survey 

digital data; 1:250,000 scale; 1973-83

Range Includes barren and other

Irrigated agriculture

Forest

Urban

 Study area boundary

Figure 4. Environmental subareas for Carson and Truckee River Basins.

ENVIRONMENTAL SETTING
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.»t\v, ".'*R*?IWif!» -Ac\ '* \ /y

\, /

n^- Sunrise Mountain

^T

< Frenchman Mountain

A .. >v*-«4

Base from U.S. Geological Survey digital data 1:100.000.1981-88 

Albers Equal-Area Conic projection. Standard parallels 29°30' and 

45°30'. central meridian -115°00'. Shaded relief from 1:24.000-scale 

Digital Elevation Model; sun illumination from northeast at 30° above 

horizon

10 15 20 MILES

10 15 20 KILOMETERS

EXPLANATION

 .. Study area boundary

Figure 5. Physiography of Las Vegas Valley area.

8 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
Mountains surrounding the valley are elevated 

sharply above the moderately sloping valley floor. 

The mountains are fractured by many faults. Dark gray 

limestone, dolomite, and quartzite form high, rugged, 

steep slopes and less resistant sandstones and shales 

form subdued ridges or valleys (Longwell and 

others, 1965).

Valley lowlands are surrounded by coalescing, 

gently sloping alluvial fans. Large amounts of alluvial 

material are eroded during rainstorms and are continually cut and reworked by gullying. Sediments underlying the alluvium have fluvial, lacustrine, and 

eolian origins.

Drainage from the Las Vegas Valley area is 

southeastward into Lake Mead and the Colorado River 

system. Drainage is primarily ephemeral or intermittent except that the lower reach of Las Vegas Wash 

is perennial because of urban drainage and effluent 

releases. In mountainous areas, channels are narrow 

and steep, but in the valley they are flat and wide. 

Cemented gravels of the late Cenozoic often form cliffs 

along wash channels. Geologic time divisions are 

shown in table 1.

Geologic history of Las Vegas Valley is 

represented by the development of the Basin and 

Range Physiographic Province. Generalized locations 

of consolidated rocks and un-consolidated deposits in 

Las Vegas Valley are shown in figure 6. Repeated periods of volcanic activity, uplift, erosion, and deposition 

took place throughout the Paleozoic and Mesozoic, 

with sedimentation interrupted by erogenic activity. 

Volcanic flows were extruded over broad areas and 

were followed by faulting. Tectonic activity was 

prevalent in the mid to late Tertiary and is evident 

in geologic structures (Bell, 1981).

The Spring Mountains (fig. 5) are the highest features in the valley with an altitude at Mount Charleston 

of about 11,900 ft. Dominant geologic features in these 

mountains are large faults and associated folds.

The rugged Sheep Range is northwest of Las 

Vegas (fig. 5) and has a maximum altitude of about 

9,900 ft. Carbonate rocks are in the northern and southern parts of the range; clastic rocks are near the west 

side. The Sheep Range overlies a thrust fault that is 

exposed in the Las Vegas Range.

The Las Vegas Range (fig. 5) reaches altitudes 

greater than about 7,000 ft with foothills at its base. 

Thrust faults and folds are within the range, but most 

are covered by alluvium. To the east of Las Vegas are 

Frenchman and Sunrise Mountains (fig. 5). Frenchman

Mountain is steep, trends northward, and has an 

altitude of about 2,000 ft. Sunrise Mountain is similar 

to Frenchman Mountain, but is slightly smaller, and 

trends northeast. These mountains are bound on the 

west by numerous faults.

North of Sunrise Mountain is the Las Vegas 

Valley Shear Zone. This zone may be a regional 

ground-water flow path, containing water that discharges from carbonate-rock aquifers (Mifflin, 1968; 

Naff and others, 1974; Hess and Mifflin, 1978; 

Noack, 1988). This zone trends northwestward from 

Sunrise Mountain and corresponds with the deepest 

part of the bedrock within the basin (Plume, 1989).

The River Mountains (fig. 5), southeast of Las 

Vegas, have moderate relief and a rugged surface. 

The River Mountains probably have been faulted to 

their present position from the east (Duebendorfer and 

others, 1990). The northern part of the McCullough 

Range (fig. 5) borders part of Las Vegas Valley on the 

south. This part of the range primarily consists of 

Tertiary volcanic rocks. The consolidated rocks that 

underlie the volcanic rocks are mostly limestone. The 

eastern edge of this range is cut by a steep escarpment.

Consolidated Rocks

Mountains surrounding the valley are composed 

primarily of Precambrian to Tertiary consolidated carbonate rocks, sedimentary rocks, and some igneous and 

metamorphic rocks. Consolidated rocks (fig. 6), except 

for carbonate rocks, usually are a barrier to groundwater flow. Locally, carbonate rocks can transmit large 

quantities of water through solution channels formed 

along faults and joints (Plume, 1985).

The most common types of consolidated rocks 

(fig. 6) are carbonate rocks, siltstone, and sandstone in 

mountain ranges on the west, north, and east sides of 

the valley. The mountains to the south and southeast are 

composed of Tertiary volcanic rocks that overlie Precambrian metamorphic and granitic rocks. Throughout 

the area, there are layers of cemented gravel and lenses 

of caliche.

Unconsolidated Deposits

Beginning in the Miocene, terrestrial and lacustrine sediments accumulated in Las Vegas Valley to 

thicknesses of about 3,000 to 5,000 ft (Longwell and 

others, 1965). Active wash channels, including Las 

Vegas Wash, cut through poorly sorted sediment ranging in size from clay and sand to cobbles and boulders.

Physiography and Geology
Table 1 . Divisions of geologic time major geochronologic units

Subdivisions

Era Period or System

Cenozoic 2 Quaternary2

Tertiary

Mesozoic2 Cretaceous

Jurassic

Triassic

Paleozoic Permian

Carboniferous Pennsylvanian

System

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Precambrian 3

Epoch

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late

Middle

Early

Late

Middle

Early

Late

Early

Late

Middle

Early

Late

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Age estimates 

of boundaries 

in millions of years 1

0.010

1.6(1.6-1.9)

5 (4.9-5.3)

24 (23-26)

38 (34-38)

55 (54-56)

66 (63-66)

96 (95-97)

138(135-141)

205(200-215)

-240

290 (290-305)

-330

360 (360-365)

410(405-415)

435 (435-440)

,

500(495-510)

-570

1 Ranges reflect uncertainties of isotopic and biostratigraphic age assignments. Age boundaries not 

closely bracketed by existing data are shown by -. Decay constants and isotopic ratios employed are cited in 

Steiger and Jager (1977).

2 Modifiers (early, middle, late) when used with these items are informal divisions of the larger unit.

3 Rocks older than 570 million years also called Precambrian, a time term without specific rank.

10 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
36°30'

36°00'

\

115°30' 

I

115°00'

Clark County

rv£ x-4-i-ws.--'

Base from U.S. Geological Survey digital data 1:100.000.1981-88

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -1 15°0ff

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Surflclal geology (data modified from 

Stewart and Carlson, 1978)

[____| Unconsolldated basin-fill deposits

[ I Consolidated carbonate rocks

^^22 Consolidated non-carbonate rocks

 > Study area boundary

Figure 6. Surficial geology of Las Vegas Valley area.

Physiography and Geology 11
Unconsolidated basin fill (fig. 6) is rich in carbonate 

material and includes Miocene clastic and sedimentary 

deposits of siltstone, sandstone, limestone, conglomerates, with some gypsum and lava. Cenozoic deposits 

consist of poorly sorted clay, silt, sand, gravel, and 

conglomerates that overlie older basin fill. Finegrained siltstone and mudstone playa deposits also are 

in lower parts of the valley (Longwell and others, 1965; 

Bell, 1981; Plume, 1989). Basin-fill deposits are laterally and vertically discontinuous because of the 

different depositional environments. Unconsolidated 

sands and gravels are the most productive aquifers in 

the valley.

Carson and Truckee River Basins

Physical and Geologic Settings

In contrast to the primarily carbonate, sandstone, 

and siltstone mountains surrounding Las Vegas Valley, 

the mountains in the Carson and Truckee River Basins 

are primarily granitic and volcanic in origin. Also, 

the Carson and Truckee River Basins are drained by 

perennial streams, in contrast to intermittent streams 

in Las Vegas Valley.

The Carson and Truckee River Basins study area 

in west-central Nevada and east central California

o

(fig. 7) encompass an area of about 7,200 mi . The 

study area includes the Carson River Basin (Carson 

Valley, Eagle Valley, Dayton Valley, Churchill Valley, 

and Carson Desert Hydrographic Areas), the Truckee 

River Basin (Truckee Canyon segment, Lake Tahoe 

Basin, Washoe Valley, Pleasant Valley, Truckee 

Meadows, Sun Valley, Spanish Springs Valley, 

Warm Springs Valley, Tracy Segment, Dodge Flat, 

Pyramid Lake Valley, and Winnemucca Lake Valley 

Hydrographic Areas.) The Fernley Hydrographic Area 

is included in the study area because the Truckee 

Canal, which diverts water from the Truckee River to 

Lahontan Reservoir on the Carson River, flows through 

it. The basins are topographically closed and extend 

from the mountainous Sierra Nevada Province eastward into the Basin and Range Province (fig. 7). A 

closed basin has no outlet and water entering such a 

basin discharges only by evapotranspiration. Altitudes 

range from about 10,900 ft in the consolidated-rock 

headwater areas of the Sierra Nevada, to about 3,900 ft 

in the Unconsolidated basin-fill deposits in the Carson 

Sink and at Pyramid Lake.

In the western part of the Carson and Truckee 

River Basins, the rugged Sierra Nevada are bordered 

on the east by a large northwest-striking fault system 

that extends about 400 mi from south-central to northeastern California (Brown and others, 1986). These 

faults divide granitic rocks on the west from the 

younger volcanic rocks on the east. Vertical displacements along these faults have elevated the granitic 

rocks several thousand feet. Stream channels in these 

mountains have steep gradients in narrow, steep-walled 

canyons. The Sierra Nevada have been glaciated a minimum of three times (Fox, 1982). In glaciated areas, 

stream channels are broad and flat. Glacial features 

include cirques, glacial valleys, moraines, and outwash 

terrace deposits (Fox, 1982).

The Carson River headwaters are located in the 

Carson Range to the west, the Sierra Nevada to the 

south, and the Pine Nut Mountains to the east (fig. 7). 

The East Fork and West Fork (fig. 7) merge in the westcentral part of the Carson Valley to form the main stem 

of the Carson River. Downstream from the confluence, 

the river flows northeastward through Carson Valley 

and east of Carson City. The river then flows between 

the Virginia Range and the Pine Nut Mountains and 

discharges into the Carson Sink, which is bordered by 

the West Humboldt Range and the Stillwater Range.

The Truckee River headwaters are in the 

Sierra Nevada; the river originates from Lake Tahoe 

(Bateman, 1976). From headwater areas, the Truckee 

River flows eastward to enter the Basin and Range 

Physiographic Province near the Truckee Meadows 

and Reno-Sparks urban area (fig. 7). East of the 

Reno-Sparks urban area, the river flows northeastward 

between the Pah Rah and Virginia Ranges. The river 

then flows northward, between the Pah Rah and 

Truckee Ranges, and terminates in Pyramid Lake, 

which is bordered by the Virginia Mountains, the Pah 

Rah Range, and the Lake Range (fig. 7).

There were several episodes of mountain building 

and volcanic activity in the Basin and Range Province 

during the Paleozoic and Mesozoic. Geologic time 

divisions are shown in table 1. Typically, mountain 

ranges within this province were formed during major 

periods of crustal extension in the Cenozoic. In the 

Late Jurassic, the Nevada Orogeny caused extensive 

eastward folding and faulting. Tectonic activity, 

as evidenced by widespread earthquakes, continues 

in the area. The Sierra Nevada are atypical of mountain 

ranges in that their entire length of about 350 mi is 

not breached by a single river (Stanley, 1989).

12 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
TJ

. 

o(O

 o«<

D)a O§5"

to

 n (5'I

w 5' 

ca

O

3a> 

Q.

I

(D

OD 

g

8

12

3 <

1!

^: ^;>i|^v ^- >i^l(l «/,;>.' HgS> 20e ^iv' 9 T;. v^'-^vj «; I. .*> ^>Xr ^Sfc*»»'
During the Tertiary, there was volcanic activity in 

the Carson Sink (Morrison, 1964). During late Tertiary 

and early Pleistocene times, regional and local uplift 

and faulting helped define present-day topography. 

Throughout the region closed basins developed, valley 

sediments were deposited and volcanic activity continued. In the Carson Sink during the Pleistocene, lake 

sediments accumulated in Pleistocene Lake 

Lahonton. The lake covered a large part of the Carson 

and Truckee River Basins and included the present day 

Carson Sink and Pyramid Lake. Climatic changes 

resulted in reduced inflows and high temperatures 

and evaporation rates eventually caused desiccation 

of the lake.

Consolidated Rocks

Consolidated rocks in the Carson and Truckee 

River Basins (fig. 8) primarily are igneous and metamorphic rocks, although some localized consolidated 

sedimentary rocks are present. Composition and thickness of different types vary with location. Consolidated 

rocks usually vary from slightly permeable to completely impermeable, although where fractured can be 

highly permeable.

The Reno-Sparks urban area (fig. 7) is in a basin 

caused by faults bordering the area and is surrounded 

by the Sierra Nevada and the Virginia and Pah Rah 

mountain ranges. The consolidated rocks are mostly 

metasedimentary and metavolcanic in origin (Bonham, 

1969). These rocks have low permeability except 

where they are highly fractured.

Exposed consolidated rocks around the Carson 

Valley (fig. 7) are primarily granitic, metavolcanic, and 

metasedimentary. Granite at the surface is a result of 

normal faulting and associated tilting and erosion. 

Consolidated rocks in the Carson Valley dip westward 

(Moore, 1969). Maurer (1986) indicates that depth to 

the consolidated rocks throughout the Carson Valley 

generally exceeds 1,000 ft, with the maximum depth 

exceeding 5,000 ft on the west side of the valley.

Consolidated rocks surrounding Eagle Valley 

(fig. 7) are primarily metamorphosed Mesozoic volcanic and sedimentary rocks, Mesozoic and Cretaceous 

igneous rocks, and Tertiary sandstone and volcanic 

rocks (Moore, 1969; Trexler, 1977). The consolidated 

rocks are nearly impermeable except where fractured 

or weathered, and have not been considered as important sources of water (Szecsody and others, 1983; 

Arteaga, 1986). A recent investigation by the USGS

indicates that these rocks, where fractured or weathered, can be conduits for recharge to the basin-fill 

aquifer (David E. Prudic, U.S. Geological Survey, 

written commun., 1995).

In Dayton and Churchill Valleys, consolidated 

rocks include Jurassic to Quaternary igneous rocks, 

metavolcanic rocks, and sedimentary rocks (Schaefer 

and Whitney, 1992). Consolidated rocks within these 

valleys are not the primary water-bearing units, but, 

if highly fractured, can have high permeability.

In the Fallen area, the depth to consolidated rock 

often exceeds 8,000 ft. These rocks are mainly volcanic, with some metamorphic and sedimentary rocks 

(Willden and Speed, 1974; Hoffman, 1989).

Unconsolidated Deposits

Unconsolidated deposits (fig. 8) are in all valleys 

of the Carson and Truckee River Basins and are the 

primary water-bearing units within the study area. In 

the Reno-Sparks area, the basin fill is derived from the 

surrounding mountain ranges. Quanternary alluvial 

materials consist of clay, silt, sand, and gravel layers. 

These sediments provide most of the ground water for 

the area. Tertiary, semiconsolidated sediments with 

interbedded clay, silt, sand, gravel, and pumice do not 

yield large quantities of water. The basin-fill deposits in 

the Reno-Sparks area arenas much as 4,000 ft thick 

(Cohen and Loeltz, 1964).

Carson Valley (fig. 7) contains Unconsolidated 

Quaternary basin-fill sediments and semiconsolidated 

Tertiary sediments (Maurer, 1986). The Quaternary 

deposits are mostly fine- to coarse-grained Carson 

River alluvium and are the major water-bearing units 

within the Carson Valley (Maurer, 1986). The thickest 

section of the basin-fill deposits is west of the valley 

axis because of downward tilting to the west of the Pine 

Nut Mountains, relative to uplift along the east margin 

of the Carson Range. Much of the eastern part of the 

valley is covered by Tertiary lake and stream deposits 

as much as several hundreds of feet in thickness. Most 

of the Tertiary strata dip moderately westward and are 

covered by Quaternary gravel. These sediments are 

cut by north-trending Quaternary normal faults 

(Moore, 1969).

14 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
120°00' 119°00' 118°00'

40°00' 

Surfictal geology modified from 

Stewarl and Cartson. 1978; 

Jenmngs and others. 1977

Unconsolidated basin-fill deposits 

Consolidated rocks

 Study area boundary

Base from U.S. Geological Survey digital data 1:100.000.1977-85

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -119°00'

Figure 8. Surficial geology of Carson and Truckee River Basins.

Physiography and Geology 15
Eagle Valley (fig. 7) contains Quaternary 

semi-consolidated and unconsolidated alluvium that 

is mostly granitic and metamorphic detritus of clay, 

silt, sand, and coarse materials deposited in alternating 

layers with clay layers that act as confining units 

(Trexler, 1977; Arteaga, 1986).

Basin-fill deposits in the Dayton and Churchill 

Valleys (fig. 7) consist of Cenozoic sedimentary 

deposits (about 500-3,000 ft thick), alluvial fans and 

pediments, lake, river, delta, and flood-plain deposits 

(Schaefer and Whitney, 1992). These depositional 

units consist of clays, silts, sands, and gravel.

Tertiary to Holocene sedimentary deposits are 

in the Fernley area (fig. 7). Lico (1992) indicates these 

deposits, from youngest to oldest, are Holocene eolian 

and alluvial deposits, lacustrine deposits from 

Pleistocene Lake Lahontan, pre-Pleistocene Lake 

Lahontan lacustrine and fluvial sediments, and 

Tertiary volcanic rocks. The combined thickness 

is approximately 1,000 ft.

In the Carson Sink (fig. 8), the unconsolidated 

surficial deposits are fluvial playa and lake, and some 

eolian sediments (Morrison, 1964). The basin floor is 

underlain by interbedded Holocene fluvial and eolian 

deposits, Pleistocene and Holocene lake sediments, 

Cenozoic fan gravel, deltaic deposits, and volcanic 

rocks, and possibly pre-Tertiary sediments. More than 

300 ft of fine-grained lake sediments, consisting of 

deltaic deposits, shoreline sand and gravel, and deep 

lake sediments, were deposited when Pleistocene Lake 

Lahontan covered the area (Lico, 1992). The combined 

thickness of these deposits exceeds 8,000 ft.

Climate, Soils, and Vegetation

Las Vegas Valley Area

Climate

The climate in most of the Las Vegas Valley area 

is arid to semiarid. During 1981-91 at Las Vegas, the 

average monthly summer temperature maximum was 

39°C and the minimum was 21°C; the average monthly 

winter temperature maximum was 15°C and the minimum was 0.6°C (data from National Climatic Center, 

1982-92). Frost-free periods average about 240 days 

in the valley.

Annual precipitation for five selected locations 

(fig. 9) is listed in table 2. Most precipitation is during 

the months of December through March. The average 

annual precipitation of the Las Vegas Valley area is 

about 4 in., although higher altitudes in the mountains 

can receive more than 20 in. (Speck, 1985). Summer 

thunderstorms of short duration and high intensity 

cause local flooding and erosion. Intense rain storms 

are possible in any season and can produce torrents 

of water and debris. Annual precipitation, 1981-91, 

ranged from about 1.3 in. at McCarran Airport (station 

3 in fig. 9) in 1985 to about 24 in. at Red Rock Canyon 

State Park (station 4) in 1983 (data from National 

Climatic Center, 1982-92).

Soils

Soils in Las Vegas Valley are formed from 

weathered basin-fill deposits and consolidated rocks. 

In the southeastern part of the valley, alluvial soil in 

the McCullough Range and River Mountains (fig. 5) 

is developed mainly from weathered, coarse, volcanic 

detritus (Carlsen and others, 1991). Soils within the 

valley are classified as entisols and aridisols. Entisols 

develop on recent alluvium and have weakly defined 

horizons. Aridisols form in desert environments and 

accumulate salts at the surface. Many soils in the valley 

are covered with desert pavement pebbles and 

cobbles that have a dark coating of desert varnish. 

Surficial soils are well drained and composed of fine 

gravelly to fine sandy loam. At various depths below 

the surface throughout most of the valley, a wellcemented soil horizon of sandy, loamy, or clayey 

material (caliche) is present (Speck, 1985).

In the northern part of the basin, soils are formed 

from carbonate-enriched alluvium, siliceous enriched 

silt, and fine-grained sand (Sowers and others, 1988). 

Accumulation of calcium carbonate-rich horizons 

dominate these soils. Laminar calcrete is present 

throughout the valley and is formed when calcium 

carbonate percolates downward into the soil and precipitates in void spaces within the alluvium. Eventually 

calcium carbonate accumulation creates a hydrologic 

barrier. Sowers and others (1988) further suggested 

that the depth to this barrier is controlled by porosity 

and permeability of alluvium.

16 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
36°30'

36°00'

115030'

I

Clark County

\

i"

\

Las Vegas

11 5°00' 

I

I

S f -""^ \ S

Base from U.S. Geological Survey digital data 1:100.000.1981-88

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -1

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

 . . Study area boundary

Precipitation measurement station See table 2

Figure 9. Location of precipitation measurement stations in Las Vegas Valley area.

Climate, Soils, and Vegetation 17
m

3 

3 <t>i&>Q. 

I

3 o

COo*

I

CO 

WO

W

Table 2. Precipitation data for weather stations in Las Vegas Valley area, 1981-91 

[Symbol: --, full-year data not available. Data from National Climatic Center, 1982-92]

Station 

number 

(figure 9)

Station name

Altitude __ (feet 

above

sea 1981 

level)

Total annual precipitation (Inches)

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

Average annual

precipitation,

for indicated

record during

1981-91

(inches)

1

234

Boulder City

Desert National

Wildlife Range

McCarran Airport

Red Rock Canyon

2,530

2,9222,162

3,780

7.64

3.263.149.24

6.56

4.87

3.99

17.87

7.35

6.34

4.86

23.89

13.36

10.38

6.85 -- 4.13 2.58 1.27 -- 5.09 4.66 2.65 -- 7.43 7.21 6.59 9.55 3.01 3.12 2.29 7.96 3.03 1.51 2.11 2.34 5.34 2.82 3.75 7.45 8.34 3.66 3.90 7.72 6.48 4.58 3.76 10.75

State Park 

Sunrise Manor 1,821 3.16 4.83 6.26 2.57 7.81 4.93

&> 

a.o&>3 o

&> 

a

sa&> &>

a.o&>
Vegetation

Precipitation and altitude are major controls that 

influence vegetation zonation in the Las Vegas Valley 

area. Four plant communities are found in Las Vegas 

Valley area. Communities from highest to lowest altitude are (1) fir pine, (2) piny on juniper, (3) blackbrush, 

and (4) creosote. Because precipitation is lacking in 

lower altitudes of the valley, vegetation is sparse and 

some areas are barren. Increased precipitation at 

higher altitudes supports more vegetation.

White fir, ponderosa pine, and some bristlecone 

pine grow above 7,000 ft on the shaded sides of canyons and on the north-facing slopes. Conifer forests 

and aspen grow in high altitudes within the Spring 

Mountains. Above 4,500 ft, wooded areas of pinyon 

pine and juniper are abundant.

Joshua trees and Spanish bayonet grow in central 

parts of the alluvial fans and foothills. In this zone, 

blackbrush is the dominant small shrub and many cacti 

are present. Sagebrush typically is found in this zone.

In lower altitudes, vegetation consists of 

phreatophytes plants that obtain water from or 

below the water table (Maxey and Jameson, 1948). 

Mesquite, salt grass, greasewood, and rabbit brush 

grow throughout the basin in the lowest, most arid 

altitudes. Creosote brush grows in lowland areas and 

on alluvial slopes where the water table is at considerable depth. Along wash channels, sagebrush, tamarisk, 

and creosote are dominant vegetation types, although 

sagebrush and tamarisk do not grow in the same 

community together.

Carson and Truckee River Basins 

Climate

The climate of the Carson and Truckee River 

Basins is classified as humid and subhumid continental 

in headwater areas; these climates are characterized by 

generally cool or mild summers and cold winters. 

Houghton and others (1975) indicate that in more arid 

low-altitude areas the climate is classified as midlatitude steppe, and is characterized by warm to hot 

summers and winter temperatures near or below freezing. In Reno (station 7 in fig. 10 and table 3) during 

1981-91, the average monthly summer temperature 

maximum was 30°C and the minimum was 8.4°C; 

the average monthly winter temperature maximum

was 8.4°C and the minimum was -6.2°C (data from 

National Climatic Center, 1982-92). Other climatic 

characteristics of this area are prevailing westerly 

winds, large daily temperature fluctuations, and infrequent, but severe storms (Garcia and Carman, 1986).

Precipitation in the Carson and Truckee River 

Basins is attenuated by the rainshadow of the Sierra 

Nevada. Average annual precipitation in the Carson 

Desert is less than 5 in., but high altitudes of the Sierra 

Nevada receive more than 30 in. Annual precipitation 

for nine selected locations (fig. 10) are indicated in 

table 2. On the eastern slopes of the Sierra Nevada and 

ranges to the east, annual precipitation increases with 

increasing altitude, but the relation varies seasonally. 

Annual precipitation, 1981-91, ranged from about 2.6 

in. at Fallen (station 2 in fig. 10 and table 3) in 1986 to 

about 52 in. at Mt. Rose (station 6) in 1982 (data from 

National Climatic Center, 1982-92).

Soils

Carson and Truckee River Basin soils are classified primarily as aridisols and ultisols, with some entisols (U.S. Geological Survey, 1970). All soils are dry 

to moist and have gray to brown surface horizons, or 

layers. Aridisols are dry, alkaline mineral soils with 

light-colored surface horizons containing little organic 

material. Layers of calcium carbonate, gypsum or salts 

may accumulate beneath the surface horizon. They 

form mostly in semiarid to arid regions. Ultisols are 

highly weathered, somewhat acidic red to yellow soils 

underlain by clay layers. They normally develop under 

forest vegetation. Entisols are dry mineral soils that 

have formed from alluvial material without developing 

significant layering. All surficial soils are predominately loamy to sandy with intermixed gravel and boulders. They range from being poorly drained in flat 

basin areas to excessively drained on steep slopes 

in mountain areas (Rogers, 1974).

Vegetation

Vegetation in the Carson and Truckee River 

Basins can be divided into several zones that vary from 

barren peaks and alpine meadows in the higher altitudes of mountainous headwater areas to greasewood 

and barren land in the Carson Sink. Alpine meadows in 

higher altitudes consist of grasses and wildflowers. 

Below the alpine meadows, three zones of forest vegetation are in mountainous headwater areas above about

Climate, Soils, and Vegetation 19
120°00' 119000' 118°00'

40°00' -

39°00' -

Carson

National 

WiUlifa 

Refuge stillwater 

/ National 

Wildlife 

Refuge Management _ __/

 Area **"'

EXPLANATION

10 20 30 40 KILOMETERS

 . - Study area boundary

Precipitation measurement station 

See table 3

Base from U.S. Geological Survey digital data 1:100.000.1977-85

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -119°00'

Figure 10. Location of precipitation measurement stations in Carson and Truckee River Basins.

20 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
o 

_5T

09as

10s

Table 3. Precipitation data for weather stations in Carson and Truckee River Basins, 1981-91 

[Leaders, (--), data not available. Data from National, Climatic Center, 1982-92]

Station

number

(figure 10)

1

23

4

5

67

8

9

Station name

Carson CityFallenGlenbrook

Lahontan Dam

Minden

Mount RoseReno-Tahoe International Airport

Virginia CityWadsworth

Altitude 

(feet

above

sea

level)

4,651

3,9666,3554,150

4,700

7,400

4,404

6,340

4,200

Total annual precipitation (inches)

1981

10.95

3.80

20.97

4.22

6.35

38.58

6.68

13.00

4.84

1982

19.21

7.92

37.19

8.73

13.95

52.11

11.10

25.89 -- 1983 21.77 8.45 32.92 10.92 17.90 13.23 25.98 17.36 1984 8.61 3.76 12.86 4.57 5.49 4.28 12.63 4.56 1985 6.28 4.94 -6.10 -«4.99 12.74 6.26 1986 12.88 2.55 --4.19 7.65 --8.94 15.92 4.88 1987 6.81 5.27 14.58 4.23 --7.50 11.06 5.65 1988 4.85 5.83 10.67 3.15 3.87 5.30 8.53 4.54 1989 9.07 4.82 ~5.14 --.-7.34 -6.52 1990 4.00 5.69 10.72 3.81 3.41 ..5.26 ~5.29 1991 ~3.38 «5.37 7.16 6.00 14.97 5.52

Average annual 

precipitation, 

for indicated

record during 

1981-91

(inches)

10.44

5.13

19.99

5.49

8.22

7.33

15.64

6.54
5,500 ft (Bonham, 1969). These forest vegetation 

zones, from highest to lowest altitude, are (1) hemlock, 

white-bark pine, white pine, and red fir; (2) white and 

red fir; and (3) pinyon pine, Jeffrey pine, and white fir. 

At lower altitudes in headwater areas, sagebrush, 

bitterbrush, rabbit brush, and several species of grasses 

are common (Bateman 1976). Pinyon-juniper forest 

grows above altitudes of about 6,000 ft in dryer mountain ranges, including the Pine Nut mountains, Virginia 

Range, and Pah Rah Range.

Carson Valley and Truckee Meadows are in a 

sagebrush zone dominated by big sagebrush. However, 

agriculture (irrigated pasture and alfalfa) and urbanization (particularly in the Truckee Meadows) have 

replaced most of the natural vegetation. Wetland areas 

support plant species such as cattail, spike rush, alkali 

and hardstem bulrush, rushes, and sedges.

In the Carson and Truckee River Basin, downstream from Carson Valley and the Truckee Meadows, 

is a shadscale zone. The shadscale zone is found 

typically in valley bottoms where soil salt concentrations are high and precipitation is low. Dominant plant 

species are four-wing saltbrush, green rabbitbrush, 

green ephedra, winterfat, and a variety of greasewood. 

Near Fallen and Lahontan Reservoir, natural vegetation has been replaced by cultivated farmland and 

irrigated pasture.

Riparian vegetation grows along the lower 

Carson and Truckee Rivers, drains, and canals. Typical 

species are cottonwood, rabbitbrush, willow, thistle, 

rushes, sedges, and salt grass. Salt cedar, an introduced 

species, is common along the lower Carson and 

Truckee Rivers and other waterways.

The wetland areas downstream from Fallen 

support submergent, emergent, and terrestrial 

plant groups. Emergent plant species in the wetlands 

include alkali and hardstem bulrush, and cattail. 

Carson Lake wetland areas (southeast of Lahontan 

Reservoir) contain concentrations of emergent plants. 

Terrestrial species around the marshes include salt 

grass, sedge, and rushes. Away from the marsh areas, 

salt desert shrub species dominate. Barren areas in the 

Carson Sink result from decreased precipitation and 

increasing water and soil salinity.

Urbanization, Land Use, and Water Use

Nevada is rapidly becoming one of the most 

urbanized states in the Nation. The population in 

Nevada for 1990 was about 1,200,000. The Nevada 

Basin and Range study unit has about 1,090,000 

people, more than 90 percent of the State's population 

(Nevada State Demographer, Bureau of Business and 

Economic Research, written commun., 1991).

Las Vegas Valley Area

Urbanization

Most of the population resides in the southeastern 

part of the valley in the cities of Las Vegas and 

Henderson. Population density for the Las Vegas 

Valley area is shown in figure 11. Major reasons for 

population increases include growth of the gaming 

industry, real estate development, recreation, and 

favorable climate. Population for the Las Vegas 

Valley area was about 710,000 in 1990 (Nevada State 

Demographer, Bureau of Business and Economic 

Research, written commun., 1991).

Land Use

Figure 3 shows land-use and land-cover patterns 

in the Las Vegas Valley (U.S. Geological Survey, digital data, 1973-83). Land use in Las Vegas Valley area is 

79 percent range, 14 percent forest, 5 percent urban, 1 

percent open water and wetlands, and 1 percent barren. 

Outside the urban area, most of the land is federally 

owned.

Land in the valley that is administered by the 

Federal Government includes Toiyabe National Forest 

by the U.S. Forest Service, Lake Mead National Recreation Area by the National Park Service, and Red Rock 

Canyon National Conservation Area by the Bureau of 

Land Management. Much of the open rangeland is 

managed by the Bureau of Land Management. The 

U.S. Departments of Energy and Defense also own 

land within Las Vegas Valley. Privately owned land is 

used for residential, commercial, industrial, and warehousing developments. This activity is centered around 

the major cities.

22 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
36°30'

36°00'

\

115°30' 

I

115000' 

I

Clark County

Base from U.S. Geological Survey digital data 1:100.000.1981-88

Alters Equal-Area Conic projection

Standard parallels 29°30' and 45°30I . central meridian -115°00'

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Study area boundary

100 people or more Data from U.S. Bureau of the Census (1992)

Figure 11 . Population density, based on 1990 census, in Las Vegas Valley area.

Urbanization, Land Use, and Water Use 23
Water Use

Ground water was the main source of water for 

the area until 1971, when extensive importation of 

Colorado River water began. Analysis of 1990 wateruse data for the Las Vegas Valley area indicates that the 

total was about 317,000 acre-ft (James E. Crompton, 

U.S. Geological Survey, written commun., 1992). 

This total included all self-supplied withdrawals and 

water-supply deliveries. Water use in the basin is illustrated in figure 12. About 80 percent of the water was 

from Lake Mead. Public water supplies accounted for 

about 91 percent of the water use. Self-supplied water 

for commercial and domestic purposes was about 4 

percent of the total. Private water supplies for industrial 

and mining purposes were about 3 percent of the total. 

Water use for irrigation and agriculture from private 

water supplies was about 2 percent of the total. Estimated consumptive use of water in Las Vegas Valley 

area was about 109,000 acre-ft in 1990.

Lake Mead water is distributed for residential 

and commercial use throughout most of the valley by 

water systems in Las Vegas, North Las Vegas, and 

Henderson. About 113,000 acre-ft of treated sewage 

effluent was returned to Lake Mead (1990) and about 

1,000 acre-ft was used for irrigation.

Industrial

and mining, Irrigation 

3 percent and agriculture,

Commercial

and domestic,

4 percent

2 percent

317,000 Acre-feet

(Data from James E. Crompton. 

U.S. Geological Survey, written commun., 1992)

Figure 12. Water use for Las Vegas Valley, 1990.

Carson and Truckee River Basins 

Urbanization

The largest urban centers are in the Reno-Sparks 

area, Carson City, and on the south shore of Lake 

Tahoe. Population densities are shown in figure 13. 

Population increases have shifted from a historically 

agrarian society to a more urban society. Most urban 

and suburban development has occurred on land that 

primarily was used for agriculture (Welch and others, 

1989). Rapid urban development began in the 1950's 

and continues into the 1990's. Historically, surface 

water was the major source of public supply and 

ground water was used intermittently. Now, ground 

water is the major source of public supply used to meet 

the increased population demands. Population for the 

Carson and Truckee River Basins was about 380,000 in 

1990 (Nevada State Demographer, Bureau of Business 

and Economic Research, written commun., 1991).

Land Use

Historically, land use in the Carson and Truckee 

River Basins was related to mining, logging, and 

agriculture. Figure 4 shows land-use and land-cover 

patterns in the Carson and Truckee River Basins (U.S. 

Geological Survey, digital data, 1973-83). Land use in 

the basins is about 50 percent range, 22 percent forest, 

13 percent open water and wetlands, 10 percent barren, 

4 percent irrigated agriculture, and 1 percent urban.

Headwater areas of the Carson and Truckee River 

Basins are predominantly Federal forest lands and 

urban development is not extensive. However, urban 

development of privately owned land is extensive 

around the north and south shores of Lake Tahoe.

Agricultural lands within the basins are mostly in 

the Newlands Project in the Fallen area, in the Carson 

Valley south of Carson City, and in the Truckee 

Meadows south of the Reno-Sparks area (see fig. 4). 

Agricultural areas in Carson Valley and the Truckee 

Meadows are declining because of urbanization. Few 

major farming communities are in other parts of the 

basins because of the arid climate. Small areas of 

agricultural land are present in the Carson City area. 

These areas are decreasing because of urbanization.

The Newlands Project in the Carson Desert is the 

largest and most intensive agricultural land-use area in 

the Carson and Truckee River Basins. The project was 

initiated with the passage of the Reclamation Act 

of 1902. Derby Dam and the Truckee Canal were

24 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
120°00'

40°00' -

39°00' -

Falton

National

WildlHe

Refuge sb«waler 

National 

WikJIile 

Refuge

0 10 20 30 40 KILOMETERS

 . . . Study area boundary

100 people or more Data from 

U.S. Bureau of the Census (1992)

Base from U.S. Geological Survey digital data 1:100,000.1977-85

Albers Equal-Area Conic projection

Standard parallels 29030' and 45°3Cr. central meridian -119°00'

Figure 13. Population density, based on 1990 census, in Carson and Truckee River Basins.

Urbanization, Land Use, and Water Use 25
constructed in 1905 to divert Truckee River water for 

irrigation in the Newlands Project. Lahontan Dam was 

completed on the Carson River in 1915 to provide storage for Newlands Project irrigation water from the 

Carson and Truckee Rivers. Alfalfa and pasture are 

the principal irrigated crops in the Newlands Project. 

About 68,000 acres of agricultural land are irrigated in 

the Newlands Project, and about 47,000 acres are irrigated in the Carson Valley (California Department of 

Water Resources, 1991b).

Water Use

Analysis of 1990 water-use data for the Carson 

and Truckee River Basins indicates that combined 

total offstream water use was about 800,000 acre-ft 

(James E. Crompton, U.S. Geological Survey, written 

commun., 1992). More than 85 percent of the water is 

from surface-water sources. The Carson River Basin 

totaled about 538,000 acre-ft; water use in the Truckee 

River Basin totaled about 262,000 acre-ft. These values 

include all self-supplied withdrawals and public-water 

supplies. Water use in the basins is shown in figure 14.

Irrigation and agriculture was the largest use 

about 662,000 acre-ft or 83 percent of the total supply; 

95 percent of the water used in the Carson River Basin 

and 59 percent of the water used in the Truckee River 

Basin. Of the amount, about 92 percent was surface 

water, primarily from the Carson and Truckee Rivers. 

The largest deliveries for public supplies in the Carson 

and Truckee River Basins are to the Reno-Sparks and 

Carson City areas. Public supply used about 14 percent 

of the total. Water for self-supplied commercial and 

domestic needs was less than 1 percent of the total. 

Many rural homes outside the urban and suburban 

areas are supplied by private wells. In recent years, 

the quantity of ground water pumped for irrigation has 

decreased; the quantity pumped for domestic supply 

has increased. This change is a result of conversion of 

agricultural lands to residential developments (Garcia, 

1989). Industry and mining used less than 1 percent of 

the total. During 1990, estimated consumptive water 

use in the Carson and Truckee River Basins was about 

367,000 acre-ft; estimated irrigation conveyance loss 

was about 175,000 acre-ft. About 50,000 acre-ft of 

treated sewage effluent were returned to surface and 

ground-water systems (1990), and about 12,000 acre-ft 

were used for irrigation.

Power generation, 

2 percent

Commercial and domestic, 

less than 1 percent

HYDROLOGIC SETTING 

Surface Water

Industrial and mining, . m , m , .. 

less than 1 percent Las Vegas Valley Area

800,000 Acre-feet

(Data from James E. Crompton, 

U.S. Geological Survey, written commun.. 1992)

Figure 1 4. Water use for Carson and Truckee River 

Basins, 1990.

Lakes and Reservoirs

Two major lakes in the Las Vegas Valley area, 

Lake Mead and Lake Las Vegas, are shown in figure 

15. Smaller real-estate lake impoundments in residential developments are not discussed in this report. No 

lakes of natural origin are in the Las Vegas Valley area.

Lake Mead, part of the Colorado River system, is 

the largest reservoir in the NVBR study unit. The lake 

occupies a 114 mi-long reach of the Colorado River 

along the Nevada-Arizona border (Baker and Paulson, 

1980). Lake Mead fills a broad structural basin that was 

formed by faulting in the late Tertiary time (Duebendorfer and others, 1990). The lake is impounded by

26 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
36°30'

36°00'

\

115°30'

I

115°00'

\

Clark County

s

i,\fiK

 '"U

Base from U.S. Geological Survey digital data 1:100.000.1981-88

Albers Equal-Area Conic projection

Standard parallels 29°301 and 45-301 . central meridian -115°00'

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

 . . . Study area boundary

Streamflow-gaging station Referenced to table 4

Tc Stream water-quality monitoring site 

O

Figure 15. Streamflow-gaging stations and water-quality monitoring sites in Las Vegas Valley area.

HYDROLOGIC SETTING 27
Hoover Dam, which was completed in March 1936, 

by the Bureau of Reclamation. Primary uses of the lake 

include flood control, irrigation, public supply, power 

generation, and recreation. Total capacity for Lake 

Mead is 29,755,000 acre-ft, with a usable content 

of 26,159,000 acre-ft (Garcia and others, 1991).

Las Vegas Bay of Lake Mead receives urban 

drainage, treated sewage effluent, industrial drainage, 

and saline ground-water discharge from Las Vegas 

Wash. The discharge of Las Vegas Wash is usually 

more dense than the water of Las Vegas Bay. When it 

is much denser, the wash discharge flows mostly into 

the hypolimnion (below the thermocline) of the bay 

(Nevada Division of Environmental Protection, 1988). 

However, when the density difference between Las 

Vegas Wash discharge and Las Vegas Bay water is not 

large, as was observed during 1985-86, the wash discharge can introduce contaminants directly into 

the epilimnion of the bay (Nevada Division of 

Environmental Protection, 1988).

A Las Vegas development, Lake Las Vegas, 

was designed as a resort facility and community. The 

project, when completed, will include several resort 

hotels and golf courses built around a reservoir constructed in Las Vegas Wash about 2 mi upstream from 

Las Vegas Bay. The project also will incorporate 

commercial, residential, and recreational properties. 

The reservoir is impounded by an earth-fill dam with a 

spillway structure and is filled with water pumped from 

Lake Mead. It has a capacity of about 10,000 acre-ft 

and a surface area of 0.5 mi2. Two 7-ft-diameter tunnels can convey about 1,800 f^/s of Las Vegas Wash 

water under the reservoir^ to avoid contamination of 

reservoir water with sewage effluent. However, during 

floods, Las Vegas Wash flow could exceed the capacity 

of the intake structure and discharge directly into 

the reservoir.

Flood-Control Structures

The Las Vegas Valley area can experience highintensity, short-duration floods any time of the year. 

Most natural washes in Las Vegas Valley are intermittent or ephemeral, but Las Vegas Wash in and downstream from Las Vegas is perennial because of excess 

urban runoff and treated sewage effluent. Surface runoff from intense thunderstorms often exceeds the 

capacity of storm drains, and flooding occurs. Flash 

flooding creates serious hazards when it overflows 

roadways and developed lands.

A Regional Flood Control District Master Plan 

for Las Vegas and other metropolitan areas in the valley 

was adopted in 1986. Recommended flood-control 

measures in the plan include constructing detention 

basins, lining streams, building bridges, and constructing storm drains. These flood-control structures are in 

differing stages of completion. Detention basins have 

been completed on the upper reaches of Las Vegas, 

Flamingo, and Tropicana Washes. Channel lining of 

certain reaches of Las Vegas, Flamingo, Tropicana, 

Pittman Washes, and Duck Creek, has been completed, 

but substantial erosion is present in unlined reaches of 

these washes. Cement-lined reaches move water downstream quickly, thereby increasing velocity and turbulence (which scours the wash bed and banks) and 

eroding channels in unlined reaches.

Streamflow Characteristics

Las Vegas Wash is the major drainage in Las 

Vegas Valley. Major tributaries include Flamingo 

Wash, Tropicana Wash, Las Vegas Creek, and Duck 

Creek. Streamflow in most of the Las Vegas Valley area 

is ephemeral. Figure 15 shows locations of streamflowgaging stations in the valley; selected streamflow data 

are listed in table 4. Las Vegas Wash near Henderson 

(station 6) is the only site with a long period of streamflow record (1957 to present). The stations are 

influenced by urban runoff, and the lower reach of 

Las Vegas Wash is affected by treated sewage effluent. 

Low-flow and high-flow characteristics were determined only for Las Vegas Wash near Henderson for 

water years 1970-88. The 7-day, 10-year, low flow 

was 23 ft3/s and the 1-day, 25-year, high flow was 

1,140 ft3/s. A flow-duration curve for Las Vegas Wash 

near Henderson is shown in figure 16. The average 

streamflow of 58 ft3/s is equaled or exceeded about 

70 percent of the time. A daily mean streamflow hydrograph for Las Vegas Wash near Henderson (fig. 17) 

indicates an increasing trend in base flow, caused by 

increasing sewage effluent from the Las Vegas area. 

Peaks in the graph are caused by intense rainfall runoff. 

Prior to 1955, Las Vegas Wash was ephemeral except 

in short reaches near springs and close to points of 

sewage release.

28 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
1,000

o 

o

LL1 

CO

f£ 

LL1 

D_

LU 

LU 

li.

O 

CD

ID 

O

100

g

LL

LU 

CC

CO

10

0.1 125 10 20 30 40 50 60 70 80 90 95 98 99

PERCENT OF TIME THAT STREAMFLOW 

WAS EQUALED OR EXCEEDED

99.9

Figure 16. Flow duration curve for Las Vegas Wash near Henderson, Nev., 

water years 1970-88.

Stream Water-Quality Characteristics

Variations in and interrelations among chemical 

constituents and physical characteristics are complex 

in aquatic systems. Water quality can be measured 

in terms of chemical characteristics such as total 

dissolved solids and physical characteristics such as 

temperature. Water-quality characteristics can change 

considerably during short time intervals, and most 

measurements are instantaneous, rather than 

representative of long periods or average conditions.

Major-ion and dissolved-solids concentrations 

for Las Vegas Wash near Henderson (station 6, fig. 15) 

are shown in boxplots in figure 18. These boxplots 

show the median value and percentile distributions of 

ions and dissolved-solids concentrations. A percentile

value is the value that is greater than or equal to the 

values of a specific percent of the data. Thus, if 47 is 

the 75th percentile of a particular set of data, the values 

of 75 percent of the data are less than or equal to 47. 

The median is the 50th percentile. As indicated in 

figure 18, the cations with the greatest median concentrations are sodium and calcium; potassium has the 

lowest. Sulfate has the greatest median concentration 

value for anions; bicarbonate has the lowest. The 

median dissolved-solids concentratioa is about 

1,700 mg/L. Milliequivalent analyses indicate that 

sodium and calcium are codominant cations and sulfate 

is the dominant anion. The median value for pH is 7.5 

at Las Vegas Wash near Henderson.

Surface Water 29
m

ET

Table 4. Streamflow characteristics at selected gaging stations in Las Vegas Valley area, water years 1970-90

ivironmental andHydrologic Settings ol

USGS station 

number 

(figure 15 

map number)

09419656(1) 09419658 (2) 09419673 (3) 094196781 (4)

09419679 (5) 

09419700 (6) 

09419753 (7)

Station name

Las Vegas Creek at Lamb Boulevard near Las Vegas, Nev. Las Vegas Wash near Sahara Avenue near Las Vegas, Nev. Flamingo Wash near Torrey Pines Drive near Las Vegas, Nev. Flamingo Wash at Nellis Boulevard near Las Vegas, Nev.

Las Vegas Wasteway near East Las Vegas, Nev. 

Las Vegas Wash near Henderson, Nev. Las Vegas Wash above Three Kids Wash below Henderson, Nev.

Period of 

record

1988-90 1988-90 1988-90 1988-90

2 1979-90 

1970-88 1988-90

Drainage 

area 

(square miles)

46.3 

1,146 

93.6 

215

1,518 1,519

Maximum 

streamflow 

(cubic feet 

per second)

1,070 1,960 

3,920 

4,100

734 

6,510 

4,050

Minimum 

streamflow 

(cubic feet 

per second)

0.00 0.58 

.00 

4.0

45 

4.8 

114

Average annual 

streamflow 1 

(cubic feet 

per second)

115 

58

1 Average not determined for less than 10 years of record

2 Period of record. May 1979 to September 1983, November 1983 to May 1984, and September 1984 to present.

10,000

n 

ff

1 I I I 1 I 1 I I I I I I 1 I I I T s'i

O O111 u-jWH

-, W ^ wS^r

1,000 - *U^

10-

1 1 1 1 1 1 1 1 1 1

1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988

WATER YEAR

Figure 17. Daily mean streamflow for Las Vegas Wash near Henderson, Nev., water years 1970-88.
10,000

NUMBER OF ANALYSES

48 47 47 46 25 47 47 48

DC

LU

CC 

LU 

D.

U) 1.000

LLI

o

z 

o 

o

100

10

CATIONS

$

a

$

e

G 

^

,?

^

/

(?

§

^ ^ # ^ ^ <? //'

<^

^

EXPLANATION 

. 90th percentile 

-U 75th percentile 

Median

25th percentile 

10th percentile

Figure 18. Boxplots for concentrations of major ions and 

dissolved solids for water samples from Las Vegas Wash 

near Henderson, Nev., water years 1988-92.

A graph relating smoothed dissolved-solids 

concentrations to streamflow percentiles for Las Vegas 

Wash near Henderson is shown in figure 19. The graph 

was developed by plotting dissolved-solids concentrations against streamflow percentiles determined from 

the distribution of long-term daily mean streamflow, 

and then smoothing. Smoothing is an exploratory technique that estimates the center of the data the conditional mean of the dissolved solids concentrations as 

streamflow changes (Helsel and Hirsch, 1992). This 

graph indicates some variation in concentration with

|8g

O 2

3,000

2,500

2.000

J 1,500

1.000

0 10 20 30 40 50 60 70 80 90 100 

DAILY MEAN STREAMFLOW PERCENTILE

Figure 19. Relation of smoothed dissolved-solids 

concentrations to streamflow percentiles for Las Vegas 

Wash near Henderson, Nev., water years 1970-88. Daily 

mean streamflow values were converted to percentiles 

to facilitate comparison of relations among stations with 

different magnitudes of flow; 100th percentile corresponds 

to highest recorded daily mean streamflow and the 50th 

percentile corresponds to median daily mean streamflow.

streamflow, but overall, dissolved-solids concentrations decrease as streamflow increases because of dilution. Dissolved-solids concentrations are influenced by 

sustained sewage-effluent discharge, intense rainfall 

runoff, and urban runoff. No other gaging stations in 

the Las Vegas Valley area have a comparable waterquality record.

Carson and Truckee River Basins 

Lakes and Reservoirs

The larger lakes and reservoirs in the Carson and 

Truckee River Basins are shown in figure 20. Several 

high alpine reservoirs are located in the headwater area 

of the Carson River. They are very small, with storage 

capacities ranging from 31 to 2,948 acre-ft (California 

Department of Water Resources, 1991 b). They are used 

by private parties and ditch companies to augment 

summer flow in the Carson River for downstream 

agricultural purposes in Carson and Dayton Valleys, 

including irrigation of alfalfa and pasture, and 

livestock watering.

Lahontan Reservoir (fig. 20) is the only significant storage reservoir in the Carson River Basin. It is 

located about 18 mi west of Fallen on the Carson River

r\

and has a drainage area of about 1,800 mi (Garcia and 

others, 1992). The reservoir is impounded by an earthand gravel-filled dam and has a usable storage capacity 

of about 295,000 acre-ft (California Department of 

Water Resources, 1991b). At the spillway, the surface

Surface Water 31
120°00' 119°00'

40°00' -

39°00' -

FaUon

National

Wildlife

Refuge stflwaler

 National 

Wildlife 

Refuge

EXPLANATION

 - . . Study area boundary

4

Streamflow-gaging station Referenced 

to table 5

Stream water-quality monitoring site

Base from U.S. Geological Survey digital data 1:100.000.1977-85

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -11 9°OV

Figure 20. Streamflow-gaging stations and water-quality monitoring sites in Carson and Truckee River Basins.

32 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
r\

area is about 21 mi (Garcia and others, 1992). Water is 

supplied to this reservoir by the Carson River and the 

Truckee River via the Truckee Canal. The reservoir 

supplies approximately 87,500 acre-ft of water annually for irrigation in the Newlands Project (California 

Department of Water Resources, 1991b). A small 

1.92 megawatt hydropower plant supplies power to the 

immediate vicinity. Most excess water and irrigation 

return flows terminate in the Stillwater Marsh area of 

the Carson Sink. Water from Lahontan Reservoir is a 

calcium sodium bicarbonate type with concentrations 

of dissolved solids generally less than 300 mg/L 

(Cooper and others, 1983; Cooper and others, 1985). 

The pH ranges between 6.5 to 7.5 in the winter and is 

uniform with depth, but can exceed 8.5 at the surface 

during summer. Mercury from historical silver and 

gold milling activities in the Virginia City area has 

accumulated in sediments in the lake and concentrations that exceed the recommended level for consumption of 1 mg/kg (dry weight) have been found in the 

tissue of numerous fish species (Cooper and others, 

1983; Cooper and others, 1985).

Lake Tahoe (fig. 20), the largest lake in both 

basins, fills a steep-sided valley. Lake Tahoe has a 

drainage area of 506 mi and a surface area of about 

192 mi2 (Garcia and others, 1992). Lake Tahoe is the 

origin of the Truckee River; outflow to the river from 

the upper 6.1 ft of the lake is regulated by a 17-gate 

concrete dam (California Department of Water 

Resources, 199la). Its usable storage capacity is about 

744,600 acre-ft, although its total volume is much 

larger (California Department of Water Resources, 

1991 a). Water released from Lake Tahoe is diverted, by 

way of Derby Dam and the Truckee Canal, for irrigation in the Carson Desert Newlands Project. Average 

lake depth is about 900 ft, and the maximum depth 

is about 1,650 ft (California Department of Water 

Resources, 199 la).

Chemical data for water samples from Lake 

Tahoe indicate that calcium is the dominant cation 

in solution and bicarbonate is the dominant anion. 

Concentrations of dissolved solids generally are less 

than 170 mg/L and pH is less than 8. Lake Tahoe is in 

the earliest stages of eutrophication. During the period 

1967-86, the transparency of the lake decreased at a 

rate of about 1.3 ft/yr, although it still (1993) exceeds 

65 ft. This decreasing transparency is directly related to 

increasing primary productivity of lake phytoplankton,

which increased at a rate of about 6 percent per year 

during the same period. The sources of nutrients to 

Lake Tahoe include atmospheric deposition, fertilizer 

applications on golf courses and other developed 

areas, abandoned septic fields, and possible leaking 

sewage lines (California Department of Water 

Resources, 199 la).

Small lakes and reservoirs in headwater areas 

of the Truckee River include Donner, Independence, 

Martis, Prosser, Stampede, and Boca (fig. 20). These 

reservoirs were built for irrigation, public supply, flood 

control, fishery enhancement, hydropower, and recreation (California Department of Water Resources, 

199la). Donner Lake has a storage capacity of about 

9,500 acre-ft; the water is used for public supply in 

Reno and Sparks, and for irrigation in the Newlands 

Project. Independence Lake has a usable storage of 

17,500 acre-ft that is used for public supply in Reno 

and Sparks. Martis Creek Lake provides 20,400 acre-ft 

of temporary storage for flood control. Prosser Creek 

Reservoir impounds up to 29,800 acre-ft for flood 

control; water can be released for irrigation in the 

Newlands Project when traded for Lake Tahoe water, 

allowing more water to remain in Lake Tahoe during 

the summer. Stampede Reservoir can impound up to 

226,500 acre-ft of water; the water is released primarily 

to provide fishery flows for Pyramid Lake; incidental 

uses include recreation, flood control, and power 

generation. Boca Reservoir impounds up to 41,100 

acre-ft of water that is used for Truckee Meadows 

irrigation and public supplies for Reno and Sparks.

Pyramid Lake (fig. 20) is the terminus of the 

Truckee River. The drainage area for the lake is about 

2,730 mi2 (Garcia and others, 1992). The lake is a 

remnant of Pleistocene Lake Lahontan. The maximum 

water volume of the lake during water year 1992 was 

about 21,850,000 acre-ft (Garcia and others, 1992). 

Pyramid Lake is a terminal sink in the topographically 

lowest valley in the Truckee River Basin; water leaves 

the area only by evapotranspiration. Pyramid Lake is 

moderately saline with a dissolved-solids concentration that exceeds 5,000 mg/L. Sodium is the dominant 

cation in solution and chloride is the dominant anion. 

The pH is about 9. The water-surface altitude of 

Pyramid Lake has declined from about 3,865 ft in 1882 

(Pyramid Lake Indian Tribal Council, 1982) to 3,800 ft 

in 1992. Most of this decline is the result of diversions 

by the Truckee Canal, which began operating in 1906, 

of Truckee River water for irrigation in the Carson 

Desert Newlands Project near Fallon. The resulting

Surface Water 33
decline of surface-water inflow to the lake combined 

with evapotranspiration rates approaching 4 ft/yr 

(Galat and others, 1981), have caused the dissolvedsolids concentration to increase from about 3,500 mg/L 

in 1882 (Pyramid Lake Indian Tribal Council, 1982) 

to current levels. Pyramid Lake is the habitat of the 

Lahontan cutthroat trout, a threatened species, and 

the cui-ui lake sucker, an endangered species.

Streamflow Diversions

Diversions of water from the Carson and Truckee 

Rivers have been made since the mid-1800's. Many 

systems have been developed to divert water throughout the basins. Controls and regulations have been 

established for allowable diversions from both the 

Carson and Truckee Rivers. Principal diversion 

systems and areas where the water is used, including 

Carson Valley, Newlands Project, Derby Dam, Truckee 

Canal, and Reno-Sparks areas are shown in figure 20. 

During 1990, approximately 196,200 acre-ft of 

diverted surface water were used for irrigation in the 

Carson Valley, about 258,300 acre-ft were used for irrigation in the Newlands Project; and about 83,300 acreft for irrigation and 48,400 acre-ft for public supplies 

were used in the Reno-Sparks area (James E. Crompton, U.S. Geological Survey, written commun., 1990).

The Alpine Decree, issued in 1980, established 

respective Carson River surface-water rights and reservoir storage rights in high alpine reservoirs for parties 

in California and Nevada (California Department of 

Water Resources, 1991b). The Truckee River Agreement, promulgated in 1935, is the current legal basis 

for operation of the Truckee River, including its tributaries and diversions from its source at Lake Tahoe to 

its terminus at Pyramid Lake. Upstream reservoirs are 

operated under supervision of the Federal Water 

Master, who administers requirements of the Orr 

Ditch Decree to achieve mandated streamflow rates 

(Floriston Rates) at the California-Nevada border. 

The Orr Ditch decree, promulgated in 1944, incorporates the Truckee River Agreement and affirms individual municipal, industrial, and agricultural water rights. 

The Truckee-Carson-Pyramid Lake Water Rights 

Settlement Act, Public Law 101-618, was passed in 

1990. This law provides a foundation for developing 

operating criteria for interstate allocation of water 

for irrigation, public supplies, fish and wildlife, and 

recreational uses, and to meet water-quality standards 

(Bohman and others, 1995).

Streamflow Characteristics

The two largest rivers in the study unit, in terms 

of drainage area and streamflow, are the Carson and 

Truckee Rivers. In contrast to Las Vegas Wash, the 

Carson and Truckee Rivers are natural, perennial 

flowing streams. Statistics for selected streamflow 

characteristics for 11 Carson and Truckee River 

streamflow-gaging stations during water years 1970-90 

(fig. 20) are listed in table 5. These stations were 

selected to represent a range of climate, geology, vegetation, and human effects. Table 5 includes period of 

record; drainage area; maximum observed streamflow; 

minimum observed streamflow; average annual flow; 

7-day, 10-year low flow; and 1-day, 25-year high flow.

Low-flow statistics are used to evaluate the 

adequacy of a stream to assimilate industrial- or 

public-supply wastes. These data are needed to help 

preserve a suitable aquatic environment and to fulfill 

water-supply requirements. A common low-flow 

statistic used is the 7-day, low flow that occurs, on 

average, once every 10 years. This low-flow statistic 

varied considerably among the analyzed stations. The 

7-day, 10-year, low flow values for the Carson River 

stations are slightly higher in the western part of the 

basin than in the eastern part because of diversions and 

drought conditions. The 7-day, 10-year, low flow values for the Truckee River stations vary less than those 

for the Carson River stations, because of streamflow 

regulation by reservoirs in the headwater area.

High-flow statistics are used to evaluate floodflow frequencies. High flows result from spring snowmelt or intense rainfall. The 1-day, high flow values 

that occur, on average, once every 25 years, vary 

throughout both basins. Maximum annual streamflows 

usually result from snowmelt, but may result from 

storm runoff.

Flow-duration curves, showing the percentage of 

time that streamflow rates are equaled or exceeded, are 

presented for selected streamflow-gaging stations in 

Carson (fig. 21 A) and Truckee (fig. TAB) River Basins. 

The shape of a flow-duration curve gives an indication 

of the streamflow conditions. For unregulated streams, 

a steep sloping curve indicates a highly variable 

streamflow, which is caused primarily by overland 

runoff; a flat sloping curve indicates that substantial

34 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
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Figure 21. Flow-duration curves for selected streamflow-gag'ing stations in (A) Carson and (B) Truckee River Basins, water years 1970-90.

5to a.OiOto
contributions come from ground-water or surfacewater storage. For a regulated stream, a steep slope at 

the lower end of the curve indicates a minor groundwater contribution to base flow or streamflow depletion 

by diversions; a flat slope at the lower end indicates a 

substantial ground-water contribution to base flow or 

the effects of low-flow releases from reservoirs.

Slopes of the flow-duration curves for the Carson 

River stations (fig. 21 A) are similar at the upper end, 

and represent the influence of direct runoff from snowmelt. The curves for the Markleeville and Carson City 

stations are flat throughout the middle sections and 

lower ends, indicating some sustained base flow. 

Divergence of the curves at the lower end is due 

primarily to irrigation diversions in Carson Valley 

and downstream.

Slopes of flow-duration curves for the Truckee 

River stations (fig. 2\E) are influenced by regulated 

reservoirs in headwater areas, irrigation diversions and 

return flow, and treated sewage effluent in the lower 

reaches. The break in slope between steep upper sections and flat middle sections of the curves for Farad 

and Vista probably are the result of reservoir storage 

and release. The slope of the lower section of the curve 

(50 to 99 percent of the time) is affected by public supply and irrigation diversions and discharge of treated 

sewage from the Reno-Sparks urban area. The absence 

of reservoir-release effects on the slope of the flowduration curve for Nixon is the result of substantial 

diversions of water to the Truckee Canal by Derby 

Dam for irrigation in Carson Desert; the flatter slope 

for that part of the curve, representing streamflow 

equaled or exceeded more than 70 percent of the time 

at Nixon probably is caused by ground-water inflow 

downstream of Derby Dam.

Generally, streamflow is low in late summer and 

gradually increases through autumn and winter to a 

peak during spring snowmelt; peak discharges usually 

are in May or June. Daily mean streamflow hydrographs are shown for selected long-term gaging 

stations in the Carson (fig. 22A) and Truckee (fig. 22B) 

River Basins. These hydrographs indicate seasonal 

flow patterns and longer-term trends in discharge at 

different locations within the basins. Streamflow 

hydrographs reflect not only the natural factors of 

snowmelt and rainfall, but also the influences of 

irrigation diversions, return flows, and sewage-effluent 

contributions. The available snowpack is a major factor 

influencing streamflow patterns.

Streamflow trends for the East and West Fork 

Carson River stations are similar for the period illustrated (fig. 22A). During water years 1976 and 1977, 

there appears to have been a slight decrease in streamflow probably caused by a regional drought. The 

hydrograph for the Carson River near Carson City 

station indicates the greatest decreases in streamflow 

were during water years 1977,1981, 1987,1988, and 

1989. The decreases in streamflows during the late 

1980's at these stations is attributed to below normal 

snowpacks in the Sierra Nevada. The highest streamflows at Carson City during water years 1970-90 were 

in 1979,1980,1982,1983, and 1986. Streamflow at the 

Fort Churchill station is strongly influenced by irrigation during the summer. The Carson River below 

Lahontan Reservoir near Fallen is highly regulated by 

releases from Lahontan Reservoir for irrigation and the 

flow record does not match those of other stations.

Streamflow hydrographs for Truckee River 

gaging stations (fig. 22E) show the effects of severe 

droughts that caused outflow from Lake Tahoe to cease 

during periods in water years 1978 and 1989. Hydrographs for stations at Farad, Reno, and Vista (fig. 22B) 

are comparable in streamflow trends and patterns. At 

the stations below Derby Dam and near Nixon, the 

larger variations in streamflow are most likely caused 

by diversions of water by Derby Dam to the Truckee 

Canal for irrigation.

Stream Water-Quality Characteristics

Five U.S. Geological Survey stream waterquality monitoring sites in the Carson and Truckee 

River Basins, where water-quality samples were 

collected during water years 1970-90, are shown in 

figure 22. Statistical summaries, in the form of boxplots (fig. 23) for concentrations of selected cations, 

selected anions, pH, and dissolved solids are based 

on data that have been published in numerous annual 

reports by USGS. The boxplots indicate that median 

values for all constituents and pH at the three Carson 

and two Truckee River sites increase in the downstream direction. Calcium, sodium, bicarbonate, 

and sulfate concentrations show the largest increase. 

Dissolved-solids concentrations, for at least 90 percent 

of the data, are less than 500 mg/L for all five stations.

Surface Water 37
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38 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
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Figure 23. Boxplots for concentrations of major ions and dissolved solids, and pH for water samples from selected 

stream-water-quality monitoring sites in Truckee and Carson River Basins, water years 1961-92.

40 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
Graphs of smoothed dissolved-solids concentrations versus streamflow percentiles are shown in 

figures 24A and 24B for sites within the basins. The 

graphs were developed by plotting dissolved-solids 

concentrations against streamflow percentiles determined from the distribution of long-term, daily mean 

streamflow values, and then smoothing. Concentrations present during median streamflow increase from 

less than 100 mg/L for the East Fork Carson River near 

Gardnerville, the West Fork Carson River at Woodfords, and the Truckee River at Farad, to 200 mg/L or 

more for the Carson River at Fort Churchill and the 

Truckee River near Nixon.

Graphs for the Carson River sites (fig. 24A) 

indicate that at the West Fork site dissolved-solids 

concentrations decrease slightly as streamflow 

increases. At the East Fork site, concentrations 

decrease as streamflow increases. The curve for the 

Fort Churchill site indicates that dissolved-solids 

concentrations decrease greatly as streamflow 

increases. The curves for sites at Carson City and 

below Lahontan Reservoir show wide fluctuations, 

most likely attributable to irrigation diversions and 

reservoir releases. Overall, all curves show decreases 

in dissolved-solids concentrations as streamflow 

increases.

Garcia and Carman (1986) investigated nutrient 

and sediment loads in the Carson River during 1980. 

The investigation showed that annual nutrient loads 

contributed by the East and West Forks of the Carson 

River, upstream of agricultural activities, were about 

490 tons of total nitrogen and about 130 tons of total 

phosphorus. At Carson City, downstream from the 

agricultural Carson Valley, the annual loads in the 

Carson River had increased to about 670 tons of total 

nitrogen and about 230 tons of total phosphorus. Nutrient loads remained fairly constant from Carson City 

to Fort Churchill. The annual sediment load upstream 

of agricultural activities was about 200,000 tons, 

increasing to about 210,000 tons at Carson City 

and 230,000 tons at Fort Churchill.

Historical silver and gold ore milling activities 

have resulted in elevated concentrations of mercury 

in bed sediments and tissues of aquatic organisms in 

the Carson River downstream from Carson City. This 

part of the Carson River is being investigated as a U.S. 

Environmental Protection Agency Superfund Site.

Intensive agricultural activities in the Carson 

Desert near Fallon have resulted in contamination of 

water, bottom sediments, and biota by irrigation drainage. High concentrations of dissolved solids, boron, 

arsenic, selenium, molybdenum, lithium, and unionized ammonia were detected in areas affected by 

irrigation drainage (Hoffman and others, 1990).

Graphs for Truckee River sites (fig. 24#) at Tahoe 

City and Farad indicate that dissolved-solids concentrations are fairly consistent over the range of streamflow. Flow at Tahoe City is predominantly outflow 

from Lake Tahoe. Flow at Farad is from Lake Tahoe 

and releases from headwater area reservoirs. The curve 

for Truckee River near Nixon indicates much greater 

dissolved-solids concentrations at lower streamflow. 

As streamflow increases, concentrations of dissolved 

solids decrease, indicating a dilution effect. At the 30th 

percentile of flow, dissolved-solids concentrations for 

Tahoe City and Farad are about 60 and 80 mg/L, 

respectively, whereas at Nixon, the concentration is 

about 380 mg/L. At the 80th percentile of flow, dissolved-solids concentrations for Tahoe City and Farad 

are still about the same, but at Nixon, the concentration 

has decreased to about 140 mg/L.

The U.S. Geological Survey Truckee-Carson 

River Quality Assessment in 1979 and 1980 investigated loading in the Truckee River. A steady-state, onedimensional, water-quality transport model was used to 

assess the sources of loadings and processes controlling water quality (Nowlin, 1987). Principal sources 

of dissolved-solids loads from Reno to Derby Dam 

(fig. 20) were the North Truckee Drain, Steamboat 

Creek, and sewage treatment plant effluent. The RenoSparks Wastewater Treatment Facility was the major 

source of nutrient loadings to the river, followed by 

Steamboat Creek.

Downstream from Derby Dam local nonpoint 

sources of dissolved solids and nutrients are increasingly significant during low to median flows (Nowlin, 

1987) primarily because a substantial part of the flow 

and upstream loads are diverted into the Truckee 

Canal. Local agricultural return flows-and groundwater discharge are principal contributors of dissolved 

solids and nutrients to the river, particularly in the 

vicinity of Wadsworth and Dead Ox Wash (about 10 mi 

downstream from Wadsworth).

Surface Water 41
500

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10 20 30 40 50 60 70 80 

DAILY MEAN STREAMFLOW PERCENTILE

90 100

EXPLANATION

1 East Fork Carson River near Gardnervllle, Nev. 

2-------- West Fork Carson River at Woodfords, i

3 - Carson River near Carson City, Nev.

4 - - - - Carson River near Fort Churchill, Nev.

5 ............. Carson River below Lahontan Reservoir

near Fallen, Nev.

Figure 24. Relation of smoothed dissolved-solids concentrations to streamflow percentiles 

for selected stream-water-quality monitoring sites in (A) Carson and (B) Truckee River 

Basins, water years 1970-90. Daily mean streamflow values were converted to percentiles 

to facilitate comparison of relations among stations with different magnitudes of flow; 

100th percentile corresponds to highest recorded daily mean flow and 50th percentile 

corresponds to median daily mean streamflow.

42 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
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EXPLANATION

Truckee River at Tahoe City, Calif. 

Truckee River at Farad, Calif.

3 - - - - Truckee River near Nixon, Nev.

Figure 24. Continued.

Surface Water 43
Water temperature influences physical, chemical, 

and biological processes in water. Elevated stream temperature can create undesirable aquatic conditions such 

as a decrease in dissolved oxygen content, putrefaction 

of sludge, and obnoxious odors. Temperature greatly 

influences the biological "health" of a stream. Higher 

water temperatures increase metabolic and respiration 

rates and influence reproduction, mortality, and growth 

rates of aquatic organisms. The quantity of benthic 

organisms and their distribution also decrease when 

temperature increases beyond tolerable levels. The 

upper temperature tolerance limit for a well-balanced 

benthic community is about 32°C.

Duration curves of water temperature for three 

sites in the Carson and Truckee River Basins are shown 

in figure 25. Observed temperatures do not appear to be 

in the range considered harmful to the biological community. Eighty percent of temperatures were 20°C 

or less.

Ground Water

Las Vegas Valley Area

Principal Aquifers

The general area of principal aquifers (fig. 26) in 

the Las Vegas Valley area is defined for this report as 

where the saturated thickness is 100 ft or more (Vasey 

and others, 1972) and the ground water has less than 

1,000 mg/L of dissolved solids (Thompson and 

Chappell, 1984). These aquifers are within a 550 mi 

area of basin-fill deposits that are thousands of feet 

thick and consist primarily of unconsolidated sediments. Consolidated carbonate-rock aquifers are 

present beneath basin-fill deposits, but are not currently 

used as sources of water supply. Prior to development 

most ground water flowed eastward and southeastward 

through Las Vegas Valley, down the hydraulic gradient 

that is perpendicular to the water-level contours shown 

in figure 27.

The ground-water hydrology of the Las Vegas 

Valley aquifer system is complex because the basin-fill 

aquifer is composed of discontinuous beds of clay, silt, 

sand, gravel, and caliche. The following description of 

the Las Vegas Valley aquifer system is based primarily 

on reports by Maxey and Jameson (1948), Malmberg 

(1965), Harrill (1976), Van Denburgh and others

(1982), Mines and others (1993), and Morgan and 

Dettinger (1996). In the valley, three general aquifer 

zones have been described as (1) shallow aquifers, 

(2) near-surface aquifers, and (3) deep aquifers. All 

aquifers may be either confined or unconfined 

depending on local conditions.

The shallow aquifers have been described as the 

upper 30 ft of saturated sediments (Van Denburgh and 

others, 1982). In northwest Las Vegas, the shallow 

aquifers are perched due to ground-water withdrawals 

that have caused declining water levels in deeper aquifers. The shallow perched aquifers were first described 

by Harrill (1976). The perched aquifers developed 

from secondary recharge due to large amounts of landscape irrigation; secondary recharge during 1979 was 

estimated at 44,000 acre-ft (Morgan and Dettinger, 

1996). Water levels in the central part of Las Vegas are 

shallow and have fluctuated little during 1979-89. 

Shallow ground water discharges by the downgradient 

movement of water into Las Vegas Wash and its 

tributaries (fig. 27). Discharge also is by evapotranspiration, and possibly by downward percolation to 

deeper aquifers.

The near-surface aquifers occur from about 30 

to 200 ft beneath the water table (Van Denburgh and 

others, 1982). Recharge for these aquifers is mostly by 

upward flow from deeper aquifers, with some secondary recharge from sewage, irrigation, and industrial 

wastewater (Harrill, 1976). Discharge from the nearsurface aquifers is mainly through evapotranspiration. 

These aquifers are semiconfined and confined. Boundaries and flow directions for these aquifers are not fully 

understood; however, these aquifers have the potential 

of transmitting lesser quality water to deeper principal 

aquifers, especially in areas where pumping rates 

from the deeper principal aquifers are high (Hines 

and others, 1993).

The principal aquifers are more than about 200 ft 

beneath the water table and are semiconfined and confined. These principal aquifers correspond with the 

"shallow, middle, and deep zones" described by Maxey 

and Jamison (1948), the "principal aquifers" described 

by Harrill (1976), and "developed-zone aquifers" and 

"deep-zone aquifers" described by Morgan and 

Dettinger (1996). The upper and middle zones of these 

aquifers have been affected by pumping because they 

are primary sources for public-supply water. The deep 

zones of these aquifers, below about 1,000 ft, may 

contain large amounts of water, but do not yield 

it readily.

44 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
30

20

10

Carson River near Fort Churchill, Nevada 

Water years 1971-84

i.... 11

C/3

ID 

C/3

LJJ 

O

C/3 

LJJ 

LJJ

rr 

o

LLJ 

Q

LLJ

rr

I

rr

LJJ Q_

2

LU

0.1 1 2 10 20 30 40 50 60 70 80 90 95 98 99

20 30 40 50 60 70 80 90 95 98 99

12 5 10 20 30 40 50 60 70 80 90 95 98 99

PERCENT OF TIME THAT TEMPERATURE WAS EQUALED OR EXCEEDED

99.9

Truckee River at Clark, Nevada 

Water years 1977-90

99.9

Truckee River near Nixon, Nevada 

Water years 1979-90

99.9

Figure 25. Temperature duration curves for selected sites in Carson and Truckee River Basins.

Ground Water 45
36°30'

36°00'

115°30'

I

115°00'

Clark County

\

Base from U.S. Geological Survey digital data 1:100.000.1981-88

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°3ff. central meridian -115000'

10 15 20 MILES

10 15 20 KILOMETERS

EXPLANATION

General area underlain by principal basin-fill 

aquifers Saturated thickness is 100 feet or 

more (Vasey and others. 1972). Ground water 

has less than 1.000 mg/L dissolved solids 

(Thompson and others, 1984)

 . . Study area boundary

Figure 26. General area of principal basin-fill aquifers in Las Vegas Valley area.

46 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
115°30'

36°30'

36°00'

\

115000' 

I

Clark County

,/ \

s / J ~v- \ -/ "#

^

 ^ " 'VV

/ ^x .^ V x

/ / 

/ r1..

{. /' / " ^x^V V

\ y/y <? x^x ' X & 

-^

V ^..«--^" 3 ^s>5^^# / 

^-- *£t£j^/s

Base from U.S. Geological Survey digital data 1:100.000.1981-88

AJbers Equal-Area Conic projection

Standard parallels 29°30' and 45°30'. central meridian -115°00*

20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

 . . Study area boundary

 2.100 Water-level contour Shows altitude, in feet above 

sea level (from Bedinger and others, 1984)

A 1 Water-level monitoring well and Identifier

Figure 27. Water-level altitude and selected water-level monitoring wells in Las Vegas Valley area.

Ground Water 47
Prior to urban development in Las Vegas Valley, 

annual recharge to aquifers was by precipitation in 

headwater areas of mountains to the northwest and 

totaled about 33,000 acre-ft (Morgan and Dettinger, 

1996). Surface water from Lake Mead is injected as 

artificial recharge at an annual rate of about 16,000 

acre-ft (Coache, 1991). Natural discharge was by 

aquifer leakage, springflow, and subsequent evapotranspiration (Harrill, 1976; Bell, 1981; Morgan and 

Dettinger, 1996). Currently, discharge primarily is by 

ground-water withdrawals for public supply, which 

was about 38,400 acre-ft in 1990 (James E. Crompton, 

U.S. Geological Survey, written commun., 1992).

Under natural conditions, ground water flows 

from recharge areas in the northwest toward discharge 

areas in the southeast. Ground-water withdrawals 

caused the water level in northwest Las Vegas to 

decline about 125 ft during 1944-90 (Coache, 1991). 

Large cones of depression have developed in nearsurface and deep aquifers around major pumping 

centers. These cones of depression are deep enough 

to disrupt the natural ground-water flow. As a result, 

horizontal gradients controlling ground-water flow in 

the near-surface and deep aquifers have reversed along 

a linear area that extends approximately north and 

south through the center of the valley. Ground water, 

which under natural conditions generally flowed east 

and southeast toward natural discharge areas, now 

flows toward the pumping centers. Natural hydraulic 

gradients caused ground water to flow upward from the 

near-surface and deep aquifers into shallower aquifers, 

but now gradients are downward in areas of large drawdown (Dettinger, 1987). In addition to water-level 

declines and cessation of springflow, there has been 

local, irreversible land subsidence (Bell, 1981).

The deep aquifers (more than about 1,000 ft 

beneath the land surface) consist of basin-fill sediments 

that have relatively low permeability (Morgan and 

Dettinger, 1996). The deep aquifers yield little water 

to wells; however, they have large storage capacities 

(Malmberg, 1965). Discharge from these aquifers is 

by upward leakage and underflow.

In addition to the basin-fill aquifers, ground water 

also flows in fractures and solution channels within 

Paleozoic carbonate rocks underlying the valley. 

Lyles and others (1986) indicate that ground-water 

flow within these carbonate rocks is part of a regional 

flow system. Regionally, carbonate-rock aquifers 

currently are not heavily pumped and provide water

for numerous springs along the western edge of the 

valley. Spring discharge provides some local irrigation 

water, but if these regional carbonate-rock aquifers are 

developed and pumped heavily for other uses, springs 

may cease to flow (Moosburner and Harrill, 1985).

Figure 28 compares imported Colorado River 

water and ground-water withdrawals in the Las Vegas 

Valley area. Prior to 1971, ground water was the primary source of supply. In order to reduce ground-water 

level declines and subsidence that resulted from heavy 

ground-water pumping, increasing amounts of Colorado River water were imported beginning in 1971. 

While ground-water withdrawals have declined 

slightly since about 1974, importation of Colorado 

River water has more than tripled. Approximately 

268,000 acre-ft of Colorado River water were imported 

into the Las Vegas Valley area in 1990 (Coache, 1991).

Table 6 describes principal aquifer and well characteristics and figure 27 shows ground-water-level 

contours in feet above sea level for the Las Vegas 

Valley area. Water-level depths below land surface for 

five selected wells (fig. 27) in the Las Vegas Valley area 

are plotted in figure 29 for water years 1970-90. All 

wells tap the basin-fill aquifers. These water-level 

graphs indicate the variations of water levels throughout the area and their response to different effects. 

Three of the wells (wells 2, 3, and 5 in fig. 27) show 

a continuing decline in water levels during the period 

of record.

Ground-Water-Quality Characteristics

Ground-water-quality characteristics in Las 

Vegas Valley have been influenced by the pumping 

induced reversal of the pre-development vertical 

hydraulic gradients, which was upward from the principal aquifer to the overlying shallow and near-surface 

aquifer zones. Because the quality of shallow ground 

water commonly is poor, there is now a potential for 

water quality to be degraded at depth. Principal aquifers are most susceptible to degradation from shallow 

ground water in parts of the valley where there are no 

confined layers to impede downward movement and 

mixing or where pumping is heavy. The local release of 

poor-quality water through compaction of fine-grained 

sediment caused by land subsidence occurring in the 

Las Vegas Valley could also affect these deeper aquifers. The potential for interaction between poor-quality 

water in the shallow aquifer and water in deeper 

aquifers has not been fully determined.

48 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
OC 300

<

LU

>

CC 

LU 

Q_

LU 

LLJ

o

<

LL 

O

CO 

Q

200

Colorado River Imports 

Ground-water withdrawals

1972 1974 1976 1978 1980 1982 

CALENDAR YEAR

1984 1986 1988 1990

Figure 28. Total Colorado River and ground-water use for Las Vegas Valley area, 1970-90. Data from Coache 

(1991). See figure 27 for well locations.

In the northern Spring Mountains, the primary 

recharge area, ground-water quality is good. In 1981, 

ground-water quality in this area met established 

Nevada State drinking-water standards for all determined constituents (Plume, 1985). However, septictank effluent has locally affected ground water in the 

Kyle Canyon area of the Spring Mountains where 

urban development is expanding (Plume, 1985).

Dissolved-solids concentrations in ground waters 

in the northwest part of the valley range from 200 to 

400 mg/L; dominant ions are calcium, magnesium, 

and bicarbonate, with only small amounts of sulfate. 

Throughout much of the valley, calcium and magnesium are codominant and bicarbonate concentrations 

range from 100 to 300 mg/L. Along the northern and 

western margins of the valley, no obvious chemical 

distinction is discernible between shallow and deep 

water, and generally water quality is good.

Ground water in the southwestern part of the 

valley is derived primarily from the southern Spring 

Mountains. It is influenced chemically by clastic sandstone, conglomerate, shale, limestone, iron-rich red

beds, and gypsum deposits (Dettinger, 1987; Plume, 

1989). In this part of the valley, ground water contains 

more sulfate and chloride than ground water in the 

northwestern area. The ground water in the southwestern part has slightly more calcium than magnesium, 

and slightly more sulfate than bicarbonate. Chloride is 

only a small fraction of the total anion concentrations 

in the water. Water obtained from intermediate and 

deep wells in the southern part of the valley has 

dissolved-solids concentrations ranging from 

700 to 1,500 mg/L.

Fluoride exceeds drinking-water standards in 

water from some wells in the northeastern part of the 

valley. Fluoride in well water probably originates from 

the mineral fluorite in this part of Las Vegas Valley. 

Fluorite is present in igneous and sedimentary rocks 

(Dettinger, 1987). Nitrate concentrations in the shallow 

ground water are spatially variable and are related to 

the local wastewater disposal, landscape irrigation, and 

fertilizer use in developed areas.

Ground Water 49
Table 6. Principal aquifers and well information in the Nevada Basin and Range study unit

Well information

Aquifer

Basin-fill aquifers: 

Alluvial and lacustrine 

deposits, confined and 

unconfined

Volcanic rock aquifers: 

confined and unconfined

Carbonate rock aquifers: 

limestone and dolomite, 

generally confined

Common range 

May exceed

Common range 

May exceed

Common range 

May exceed

Depth 

(feet)

100-500 

1,200

100-1,200 

1,800

600-2,000 

5,000

Yield 

(gallons per 

minute)

200-1,000 

5,000

20-1,000 

3,000

50-1,000 

3,400

Remarks

Upper 500-1,000 feet most permeable 

and generally contains fresh water. 

Provides almost all water pumped 

by major users in State.

Provides public water to Fallen area.

Aggregate thickness of carbonate 

sections ranging from 10,000 to 

30,000 feet. Aquifer not heavily 

pumped, although it supplies water 

to springs

The concentrations of dissolved solids in shallow 

ground water of Las Vegas Valley are affected by large 

volumes of secondary-recharge water that contribute 

additional dissolved solids. Secondary recharge and 

rising water levels infiltrate unsaturated soil and sediment containing large amounts of evaporites and other 

soluble minerals, particularly in the southeastern part 

of the valley. Dissolution of these evaporite and soluble 

minerals further degrades ground-water quality. 

Evapotranspiration also increases chemical-constituent 

concentrations in discharge areas and, subsequently, 

degrades ground-water quality from about 20 to 50 ft 

below land surface.

Dettinger (1987) indicates that average concentrations in water from wells throughout the valley in the 

shallow aquifers are 310 mg/L for calcium, 310 mg/L 

for chloride, 1,500 mg/L for sulfate, and 3,000 mg/L 

for dissolved solids. In shallow wells in the central and 

southeastern parts of the valley, dissolved-solids concentrations range from 2,000 to more than 7,000 mg/L 

(Dettinger, 1987). Along Las Vegas Wash, in the southeastern part of the valley, chloride concentrations 

exceed State drinking-water standards. Upward flow 

gradients in the southeastern part of the valley and low 

permeability of the near-surface aquifers limit downward transport of dissolved solids from the shallow 

aquifer, unless the gradient is reversed. Water chemistry in deep wells in the southeastern part of the valley 

may reflect interaction with volcanic rock. Water samples from these deep wells have low concentrations of 

bicarbonate and do not contain large amounts of 

sulfate or chloride.

Boxplots of selected water-quality constituents 

are shown for water samples from Las Vegas basinfill aquifers in figure 30. The plots are derived from 

chemical analyses of water samples collected from 

1970 to 1990. Locations of the wells where samples 

were collected are indicated in figure 31.

Almost 50 percent of the analyses exceeded 

the Nevada secondary maximum contaminant level 

(SMCL) of 250 mg/L for sulfate, and about 50 percent 

of the analyses exceeded the SMCL of 500 mg/L for 

dissolved solids (fig. 30). Natural dissolved-solids concentrations greater than 2,000 mg/L in the basin-fill 

aquifers principally result from dissolution of gypsum 

and other evaporite salts (Thomas and Hoffman, 1988). 

Dissolution of gypsum also results in increased sulfate 

concentrations.

Carson and Truckee River Basins 

Principal Aquifers

General areas of principal aquifers in the Carson 

and Truckee River Basins, where saturated thickness 

is 100 ft or more (Vasey and others, 1972; White and 

others, 1979; Maurer, 1986) and concentrations of 

dissolved solids in ground water are less than 1,000 

mg/L (Thompson and others, 1984; fig. 32) are in 

unconsolidated basin-fill deposits 'and are dependable 

sources of water. In the Fallen area, a consolidated 

volcanic-rock (basalt) aquifer also is a principal source 

of public water supplies. Hydraulic characteristics 

differ among the different types of aquifers.

50 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
0

50

100

150

0

LU 50o

< 100DC

=> 150

QZ 2005 °O 50

LUCD 100

LU 150 LL

~- 2°o

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5 50_JEU 1 °°^ 150

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0

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100150

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Well 3

Well 4

Well 5

1 1

1 1 1 1 1 1 1 1 1 1

i___i_____i

1 1

1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

WATER YEAR

Figure 29. Ground-water levels for selected wells in Las Vegas Valley area, water years 1970-90. See figure 27 for well locations.
Principal aquifers and well characteristics are in table 6 

and ground water-level contours are shown in figure 33 

for the Carson and Truckee River Basins. Basin-fill 

aquifers readily receive, transmit, and may store large 

volumes of water (Glancy, 1986). Basin-fill deposits 

consist of layers of discontinuous clay, silt, sand, and 

gravel. Highest water yields are from aquifers composed of sand or gravel and are usually within 500 to 

1,500 ft of land surface, although some can be less than 

500 ft (Moosburner and Harrill, 1984).

NUMBER OF ANALYSES

68 68 68 68 62 68 68 68

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EXPLANATION 

 90th percentile 

JL 75th percentile 

-Median

-25th percentile

-10th percentile

Figure 30. Boxplots for concentrations of major ions 

and dissolved solids in ground-water samples from 

Las Vegas Valley area, water years 1975-87.

Aquifers in headwater areas of the Carson and 

Truckee River Basins are in alluvium along canyon 

bottoms, where ground-water levels are controlled by 

adjacent streams (Welch and others, 1989). In mountainous headwater areas, the presence of some ground 

water depends on the permeability of consolidated 

rock. Permeability and saturation of consolidated rock 

are controlled by rock type and the extent of weathering and fracturing. Water can be found in fractured 

consolidated rock within most of the area, although 

the more known productive aquifers are restricted to 

alluvial fill in valley bottoms.

Locations of five wells completed in basin-fill 

deposits and one well near Fallen (well number 5), 

in a basalt aquifer are shown in figure 33. Water-level 

hydrographs for these six wells are shown in figure 34 

and indicate areal and temporal variations of waterlevel depth and varying responses to hydrologic 

conditions such as withdrawals, recharge, and 

periods of drought.

Major water-bearing units in Carson Valley 

(fig. 32) are unconsolidated Quaternary basin fill 

and semiconsolidated Tertiary sediments. Quaternary 

deposits are fine- to coarse-grained alluvium deposited 

by the ancestral Carson River and are about 3,000 to 

4,000 ft thick (U.S. Geological Survey, 1988). Tertiary 

sediments crop out in long ridges on the east side of 

the valley and are mostly fine-grained lake and fluvial 

deposits (U.S. Geological Survey, 1988). Total thicknesses are greater than 1,000 ft and the deposits probably underlie most of the valley. Ground water in Carson 

Valley is under confined and unconfined conditions, 

although there is no single confining layer that extends 

across the entire valley. Depth to water in unconfined 

aquifers ranges from about 5 to 100 ft (Maurer, 1986). 

Artesian flow is present in coarse Carson River and 

alluvial fan deposits that are confined under finegrained sediment. Wells in confined aquifers usually 

flow; confined heads range from 5 to 20 ft above land 

surface (Maurer, 1986). More than 1,000 acre-ft of 

ground water was discharged from approximately 100 

flowing wells in 1982 (U.S. Geological Survey, 1988).

The unconfined water table in the Carson Valley 

seasonally rises about 3 to 5 ft during spring runoff and 

in areas where flood irrigation methods are used. Water 

levels decline near the end of the summer because of 

decreased streamflow and high rates of evapotranspiration, which is estimated to be about 80,000 acre-ft/yr 

(U.S. Geological Survey, 1988). In the eastern part of

52 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
36°30'

36°00'

\

115°30'

I

115°00'

I

Base from U.S. Geological Survey digital data 1:100.000.1981-68

Alters Equal-Area Conic projection

Standard parallels 29°30t and 45°30', central meridian -115°00'

10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

 . . Study area boundary

 Water-quality monitoring wells

Figure 31. Location of selected water-quality monitoring wells in Las Vegas Valley area.

Ground Water 53
120°00' 119°00' 118°00'

40°00' 

39000- _

General areas underlain by principal basin-fill 

aquifers Saturated thickness is 100 feet or 

more (Vasey and others. 1972; White and others, 

1979; Maurer, 1986) and ground water has less 

than 1.000 mg/L of dissolved solids 

(Thompson and others, 1984)

 . Study area boundary

Base from U.S. Geological Survey digital data 1:100.000,1977-85

Albers Equal-Area Conic projection

Standard parallels 29°30 and 45°30'. central meridian -119°001

Figure 32. General areas of principal basin-fill aquifers in the Truckee and Carson River Basins.

54 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
120°00' 119°00' 118°00'

40°00' -

39°00' -

Carson

National Sink

Wildlife

Refuge StiHwaler 

National 

Wildlife 0} 

Refuge

 Study area boundary '

 1.554 Water-level contour Shows altitude, in feet 

above sea level (data from Bedinger and 

others. 1984; California State Water Resources 

Control Board. 1979)

Water-level monitoring well and Identifier

Base from U.S. Geological Survey digital data 1:100.000.1977-B5

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°301 . central meridian -1

Figure 33. Water-level altitude and selected water-level monitoring wells in Carson and Truckee River Basins.

Ground Water 55
Environ

i

DJ O.

x: 

o.

3 o

(O

(O (0

o(0

(D (O DJ (0

2L3T x:DJa oDJ

iver Basins, Nevada anCa

0

20

40

60

Well 1

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20

40

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Well 2

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. * * 

. . i i i i . . . . i * i "i i i i i . i i

Oi 

2040

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0

20

40

en

WellS _ i i i i i i i <.»«**i*.|. . r---i "i i * i i i i i i * i * i *

i i i i i i i i i i i i i i i i i i i i

Well 4

, " , , . , , * , ",', "v, ,', , , , , ,-,.,

LLI oLTLLI 20

:> 40

60

0

204060

WellS

Well 6

1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

.WATER YEAR

Figure 34. Ground-water levels for selected wells in Carson and Truckee River Basins, water years 1970-90. See figure 33 for well locations.
Carson Valley, ground water flows toward the center of 

the valley and then northward (Maurer, 1986). Depths 

to water in most areas of the valley are shallow, but 

depths increase beneath alluvial fans at the base of 

the Sierra Nevada and at the base of the Pine Nut 

Mountains to the east.

Eagle Valley (fig. 32) is underlain by unconsolidated and semiconsolidated deposits ranging from less 

than 100 ft to more than 800 ft in thickness. The unconsolidated deposits of sand and coarse sediments are 

derived from nearby granitic and metamorphic rocks, 

and form a generally unconfined aquifer that yields 

large quantities of water. Semiconsolidated alluvial 

deposits are heterogeneous mixtures of sand, silt, and 

clay, with discontinuous clay layers that serve as confining beds; finer grained sediments yield less water.

The Eagle Valley ground-water basin was 

described by Arteaga (1986) as a two-layer aquifer 

system. The upper layer, 0 to 50 ft below land surface, 

is unconfined. The lower layer, deeper than 50 ft, has 

semi- or completely confined conditions. However, 

Szecsody and others (1983) indicate the ground-water 

system in Eagle Valley is unconfined.

Ground-water flow directions in Eagle Valley are 

complicated by consolidated-rock barriers, but flow 

generally is eastward toward the Carson River (Welch 

and others, 1989). The valley ground-water basin is 

divided structurally into west and east sections by an 

uplifted section of consolidated rock; also by several 

north-northeast trending faults. These structural 

features influence flow directions and movement of 

ground water. Maximum depths to bedrock for the 

western and eastern sections are about 1,200 and 

2,000 ft, respectively.

Total ground-water withdrawals in Eagle Valley 

were estimated to be 9,000 acre-ft/yr by Arteaga and 

Durbin (1978); withdrawals that large might lower the 

potentiometric surface and increase depth to water. 

Comparisons of depth-to-ground-water maps from 

1966 to 1980 indicate a net valley-wide decline of 

10 to 20 ft (Szecsody and others, 1983).

Principal aquifers in Dayton and Churchill 

Valleys (fig. 32) have been described by Schaefer and 

Whitney (1992) as basin-fill deposits with thicknesses 

to about 3,000 ft. Ground-water flow in Dayton and 

Churchill Valleys is eastward, along the same general 

course as the Carson River. Water levels have declined 

since irrigation and public-supply withdrawals have

increased (Schaefer and Whitney, 1992). Depth-towater ranges from about 200 ft below land surface near 

mountain fronts, to near surface in areas close to the 

Carson River. Agriculture along the Carson River 

primarily uses surface water rather than ground water.

Basin-fill aquifers in the Fallen area (fig. 32), 

described by Glancy (1986), are in unconsolidated sedimentary deposits originating as fluvial, pluvial, and 

eolian detritus. These aquifers are grouped as (1) a 

shallow alluvial aquifer from near land surface to about 

50 ft below land surface, (2) an intermediate-depth 

alluvial aquifer extending from about 50 ft to depths 

of about 500 ft at Fallen, (3) a basalt aquifer that is 

exposed at the surface near Fallen but normally ranges 

in depth from about 60 to 1,000 ft below land surface, 

and (4) a deep-alluvial aquifer that underlies the intermediate-alluvial and basalt aquifers, at depths of 500 to 

1,000 ft (Glancy, 1986).

Alluvial aquifers consist primarily of sand and 

gravel deposits within the basin fill. These aquifers 

contain water-bearing deposits that are hydraulically 

separate, during short periods of time; but during long 

periods of time are hydraulically dependent upon each 

other (Glancy, 1986). The basalt aquifer is surrounded 

by basin fill and transmissivity is highly variable. 

Hydraulically, the basalt and basin-fill aquifers are not 

separated; basalt aquifer transmissive zones are in partial contact with the highly transmissive zones of the 

basin-fill aquifers. Because of the partial contact, major 

stresses on either alluvial or basalt aquifers will cause 

them to react hydraulically with each other (Glancy, 

1986). The basalt aquifer near Fallen is the most 

heavily pumped volcanic rock aquifer in the state, 

although it is not as heavily pumped as the alluvial 

aquifers (Moosburner and Harrill, 1984).

Ground-water flow in the shallow alluvial aquifer 

in the Carson Desert (less than about 50 ft deep) is 

generally eastward and discharges into the Stillwater 

Wildlife Management Area, the Carson Sink, and 

Carson Lake. Flow directions and magnitudes differ 

within the aquifer and are influenced by drains, canals, 

and irrigation practices (Lico, 1992).

A principal aquifer in the southern part of Lake 

Tahoe Basin is unconfined basin-fill composed of 

glacial outwash material that exceeds 600 ft in thickness (Loeb, 1987). Ground water flows toward the lake 

with a general gradient of about 15 ft/mi and discharges 

into the lake at a rate of 1,400 acre-ft/yr (Loeb, 1987).

Ground Water 57
In the Reno-Sparks area (fig. 32), much of the 

basin fill consists of alternating fluvial deposits of clay, 

silt, sand, and gravel (Sierra Pacific Power Company, 

1986). General flow directions of ground water are 

from southwest to northeast toward the Truckee River. 

Fine-grained alluvium stores large amounts of ground 

water because of porosity and thickness, but yields 

small amounts of water. Alluvium of medium- to 

coarse-grained sand or gravel stores and transmits 

the largest volumes of ground water.

Ground-Water-Quality Characteristics

Concentrations of dissolved solids increase from 

less than 200 mg/L in most headwater areas to greater 

than 10,000 mg/L in terminal discharge areas of the 

Carson Sink. The increase in dissolved-solids concentrations (primarily sodium, chloride, and sulfate) is 

caused by increasingly longer contact with aquifer 

deposits, evapotranspiration, geothermal influences, 

the presence of evaporite deposits in downstream valleys, and the effects of human activities (agricultural 

and urban drainage). The following is a summary of 

reports on ground-water quality within the Carson and 

Truckee River Basins.

The NAWQA pilot study of the Carson River 

Basin evaluated available ground-water-quality data 

through 1987. In a report by Welch and others (1989), 

chemical constituents with 30 or more samples in each 

of the Carson River Basin hydrographic areas were 

evaluated with respect to Nevada State maximum 

contaminant levels (MCL's) and secondary maximum 

contaminant levels (SMCL's). In headwater areas, 

dissolved-solids concentrations were less than 

200 mg/L; fluoride exceeded the MCL in 2 of 10 samples. In Carson Valley, dissolved-solids concentrations 

generally were less than 500 mg/L; fluoride exceeded 

the MCL In 10 of 302 samples, arsenic in 4 of 276 samples, and nitrate in 3 of 265 samples. In Eagle Valley, 

dissolved-solids concentrations generally were less 

than 200 mg/L; fluoride exceeded the MCL in 3 of 

114 samples and arsenic exceeded the MCL in 3 of 

99 samples. In Dayton Valley, dissolved-solids concentrations ranged from 200 to 2,000 mg/L; fluoride 

exceeded the MCL in 15 of 158 samples, arsenic in 4 of 

113 samples, and dissolved solids exceeded the SMCL 

in 58 of 184 samples, primarily because of sulfate, 

which exceeded the SMCL in 53 of 194 samples. 

In Churchill Valley, dissolved-solids concentrations 

generally ranged from 200 to 1,000 mg/L; arsenic

exceeded the MCL in 6 of 55 samples and dissolved 

solids exceeded the SMCL in 7 of 70 samples, because 

of sulfate, which exceeded the SMCL in 6 of 72 samples. In Carson Desert, dissolved-solids concentrations 

ranged from near 1,000 to more than 10,000 mg/L; 

fluoride exceeded the MCL in 10 of 186 samples; 

arsenic in 107 of 190 samples, and dissolved solids 

exceeded the SMCL in 72 of 190 sites, because 

of chloride, which exceeded the SMCL in 69 of 

209 samples.

In Carson Valley, calcium bicarbonate, sodium 

bicarbonate, and calcium sodium or sodium calcium 

bicarbonate are the most common water types, 

although other chemical types of water are present 

(Garcia, 1989). Ground-water quality differs from west 

to east, with lower dissolved-solids concentrations on 

the west side of the valley (Maurer, 1986). The effects 

of rapid population growth and subsequent development of ground-water resources in Carson Valley are 

concerns for State and county officials (Garcia, 1989). 

Most ground water in Carson Valley is acceptable for 

public and agricultural uses. Further development of 

residential areas using septic systems may result in 

more frequent occurrences of high nitrate concentrations in ground water. Treated sewage from urban areas 

and effluent from the Lake Tahoe Basin are used to 

irrigate golf courses and agricultural land at several 

locations within the valley.

Ground water in Carson Valley near a regional 

landfill and near the airport has been affected by 

organic compounds. However, limited valley-wide 

monitoring indicates that contamination by organic 

compounds is not a widespread problem (U.S. 

Geological Survey, 1988).

Worts and Malmberg (1966) investigated water 

from aquifers within Eagle Valley to identify potential 

water-quality concerns. Samples from 22 wells were 

analyzed for major ions and were considered to be 

good water quality. In nonthermal water, calcium and 

sodium are the dominant cations and carbonate is the 

dominant anion in the valley. Thermal waters in Eagle 

Valley are sodium sulfate or calcium sulfate types. 

Both sodium sulfate and calcium sulfate enriched 

waters are widespread throughout the Carson and 

Truckee River Basins. Chemical similarities of these 

water types suggest that deposits of gypsum and anhydrite may be the source of calcium and sulfate 

(Szecsody and others, 1983). Some ground water 

within Eagle Valley has chemical compositions that 

indicate mixing of thermal and nonthermal water.

58 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
Water-quality concerns for Eagle Valley include 

locally high concentrations of iron, manganese, sulfide, 

fluoride, nitrate, and arsenic. Other water-quality concerns include gasoline leaking from underground storage tanks and industrial spills of synthetic organic 

compounds (U.S. Geological Survey, 1988). High 

nitrate concentrations in ground water of areas where 

septic systems have been used for the disposal of 

domestic sewage has become a major concern in the 

early 1990's.

Boxplots for concentrations of selected ions in 

ground water in the Carson River Basin are shown in 

figures 35-38. Boxplots are arranged by constituent 

with respect to hydrographic area, and allow for analysis of distribution in and comparison of concentrations 

in the hydrographic areas. Locations of the wells from 

which these boxplots are derived are shown on 

figure 39.

In the Carson River Basin (fig. 35), median 

concentrations of calcium, magnesium, sodium, and 

potassium are less than 100 mg/L in ground water; 

except the median concentration of sodium is about 

700 mg/L in the Carson Desert. Median concentrations 

of bicarbonate are less than 200 mg/L (fig. 36), except 

in the Carson Desert, where the median concentration 

approaches 400 mg/L. Median sulfate concentrations 

increase from less than 30 mg/L in Carson Valley to 

more than 100 mg/L in the Carson Desert (fig. 36). 

Median chloride concentrations increase from less than 

10 mg/L in Carson Valley to more than 500 mg/L in the 

Carson Desert (fig. 36).

Van Denburgh and others (1973) determined that 

ground water in headwater recharge areas within the 

Truckee River Basin is a calcium bicarbonate type, 

except where there are increased dissolved-solids 

concentrations. Water with high dissolved-solids 

concentrations is commonly a sodium bicarbonate 

type. As ground water flows away from recharge areas, 

dissolved-solids concentrations increase and water 

becomes a sodium chloride type. In some geothermal 

areas, sulfate is the dominant anion. Ground water in 

most of the Truckee River Basin, except in the Tracy 

area, is suitable for agricultural purposes. Groundwater quality is poor north of Reno and near Fernley, 

Wadsworth, and Nixon.

Ground-water quality in the basin-fill aquifers 

beneath the southern part of the Lake Tahoe Basin was 

described by Loeb (1987). Loeb reported that concentrations of nitrate as nitrogen ranged from 0.006 mg/L 

to 2.55 mg/L and were higher in water from shallow

wells and in downgradient wells closer to the lake. 

Measurements of specific conductance ranged from 

about 70 to 400 p,S/cm (microsiemens per centimeter). 

The higher specific conductance values were associated with higher chloride concentrations measured in 

wells near the junction of major highways. Chloride 

sources could be from road salt applied to remove snow 

and ice from the highways.

Ground-water samples collected from shallow 

test wells in unconsolidated deposits along the eastern 

shore of Lake Tahoe by Thodal (1992) had specific 

conductance values ranging from 131 to 1,560 [iS/cm. 

Dissolved-solids concentrations ranged from 82 to 994 

mg/L. High specific conductance and dissolved-solids 

concentrations also were associated with high chloride 

concentrations.

Ground-water quality in the southern part of 

the Reno-Sparks area ranges from poor to excellent 

(William F. Guyton Assoc., 1986). In the Reno-Sparks 

area, arsenic, manganese and iron concentrations can 

approach or exceed SMCL's or MCL's. The poorest 

quality water is primarily to the east near the Virginia 

Range. In the southern part of the Reno-Sparks area, 

dissolved-solids concentrations range from 669 to 

2,670 mg/L. Water of good quality, with low dissolvedsolids concentrations and moderate to high total hardness, is in the north-central and western part of the area. 

Analytical results indicate that dissolved-solids concentrations in the western part of the Reno-Sparks area 

range from 168 to 262 mg/L and total hardness ranges 

from 83 to 208 mg/L. Arsenic concentrations in ground 

water in much of the central Reno-Sparks area 

approach the MCL of 50 p.g/L for public supply (Van 

Denburgh and others, 1973; William F. Guyton Assoc., 

1986).

In the Truckee River Basin (figs. 37 and 39), 

median concentrations of calcium, magnesium, 

sodium, and potassium are less than 30 mg/L, except 

in the Tracy area, where median concentrations of 

calcium, magnesium, and sodium exceed 30 mg/L and 

in the Fernley area where the median concentrations of 

sodium exceed 200 mg/L. Median bicarbonate concentrations increase from less than 90 mg/L in the Lake 

Tahoe Basin, to about 300 mg/L in the Tracy area. 

Median sulfate concentrations increase from less than 

2 mg/L in the Lake Tahoe Basin to more than 100 mg/L 

in the Tracy area.

Ground Water 59
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CARSON VALLEY EAGLE VALLEY DAYTON VALLEY CHURCHILL VALLEY CARSON DESERT

CHEMICAL CONSTITUENTS

Figure 36. Boxplots for concentrations of selected anions in ground-water samples from Carson River Basin, water years 1958-89.
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120°00' 119°00' 118000'

40°00' 

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0 10 20 30 40 KILOMETERS

 Study area boundary

 Water-quality monitoring wells

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Figure 39. Location of selected water-quality monitoring wells in Carson and Truckee River Basins.

64 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
Water-Quality Issues

Las Vegas Valley Area

Las Vegas Wash conveys treated effluent from 

sewage-treatment plants, industrial drainage, urban 

runoff, storm runoff, and sediment to Las Vegas Bay of 

Lake Mead. In Las Vegas Bay, phytoplankton growth, 

pathogenic bacteria, and viruses also are potential 

problems (William Burke, National Park Service, 

oral commun., 1993). The presence of these organisms 

creates a nuisance and health hazard for recreational 

users of Lake Mead.

In Las Vegas Valley, water quality of the shallow 

aquifers is affected by urban land use. Rapid population growth and associated water use in the Las Vegas 

area have contributed to shallow perched aquifers. 

The shallow perched aquifer water is of poor quality 

(nonpotable) and has the hydraulic potential to percolate downward and possibly contaminate deeper principal aquifers that provide water for public supply 

(Brothers and Katzer, 1988). The shallow aquifers 

presently discharge in the southeastern part of the 

valley into Las Vegas Wash.

Carson and Truckee River Basins

Water quality is adversely affected by human 

activities in parts of the Carson and Truckee River 

Basins. Urbanization introduces sewage, industrial 

wastes, fertilizers, pesticides, and sediment to the environment. Agriculture introduces irrigation drainage 

with elevated levels of dissolved solids, trace elements, 

and agricultural chemicals. Mining practices have 

introduced acid mine drainage, metals, and sediment 

to water resources (Bevans and Kilroy, 1991).

In the Carson River Basin, there are major concerns about contamination of shallow ground water by 

nitrate from septic systems in some urban and suburban 

areas. There also are concerns about mercury contamination in the Carson River caused by historical oremilling activities. There is contamination of wetlands 

in the Carson Desert and Carson Sink by irrigation 

drainage that contains high concentrations of dissolved 

solids, boron, arsenic, selenium, molybdenum, lithium, 

and un-ionized ammonia. Documented effects of irrigation drainage and mercury on biota in the Fernley 

and Stillwater Wildlife Management Areas include 

(1) water in TJ and Hunter Drains, which discharge to 

the Stillwater Wildlife Management Area, was acutely

toxic to freshwater daphnids and flathead minnows, 

and to saltwater myriad shrimp, sheephead minnows, 

and striped bass, (2) samples of juvenile migratory 

birds from several sites had levels of boron, mercury, 

and selenium that could affect survival, (3) nest success 

was poor, and (4) concentrations of mercury and 

selenium in waterfowl exceeded criteria for human 

consumption (Hallock and Hallock, 1993).

In the Truckee River Basin, the principal waterquality issues are the introduction of nutrients and 

sediment into Lake Tahoe by human activities and the 

discharge of treated sewage effluent into the Truckee 

River downstream from the Reno-Sparks urban area. 

In the Truckee River downstream from the RenoSparks area, principal water-quality issues include 

instream concentrations of dissolved oxygen and 

nutrients, with respect to management of the threatened Lahontan cutthroat trout and the endangered 

Cui-ui sucker, and nutrient loads transported to 

Lahontan Reservoir by way of the Truckee Canal 

and to Pyramid Lake (Nowlin, 1987). Septic fields in 

suburban areas and application of treated sewage effluent for irrigation is increasing in the Truckee River 

Basin and is a potential source of ground-water and 

surface-water contamination.

In 1990, about 1,000 acre-ft of treated effluent 

were used for irrigation in Las Vegas Valley, 7,000 

acre-ft were used in the Carson River Basin, and 

5,000 acre-ft were used in the Truckee River Basin 

(James E. Crompton, U.S. Geological Survey, 

written commun., 1992).

SUMMARY

In 1991, Congress appropriated funds for the 

U.S. Geological Survey to begin a National WaterQuality Assessment (NAWQA) program. The 

NAWQA program will provide data and information 

on the status, trends, and major factors that affect the 

quality of the Nation's water resources. The program 

will focus on large scale, persistent conditions, and 

provide information to resource managers and policy 

makers at the Federal, State, and local levels. Surfaceand ground-water systems will be investigated in 

60 proposed river-basin study units. The first group of 

20 study-unit investigations began in 1991, the second

SUMMARY 65
group began in 1994, and the third group is scheduled 

to begin in 1997. The Nevada Basin and Range 

(NVBR) study-unit investigation, which includes the 

Las Vegas Valley area and the Carson and Truckee 

River Basins, is part of the first study-unit group.

This report provides an overview of the environmental and hydrologic settings in the NVBR study unit. 

Available data and information were used to describe 

physiography, geology, climate, soils, vegetation, 

urbanization, land use, water use, surface- and 

ground-water hydrology, and water-quality issues.

The Las Vegas Valley area in southeastern 

Nevada encompasses about 1,640 mi2. Altitudes range 

from about 11,900 ft in headwater areas of the Spring 

Mountains, to about 1,600 ft in Las Vegas Valley, and 

to about 1,200 ft at Las Vegas Bay of Lake Mead. The 

area is typical of basin and range topography with the 

northwest-trending valley bounded by mountain 

ranges, including the Spring Mountains on the west and 

the Sheep and Las Vegas Ranges on the north. Consolidated rocks in the mountains primarily are carbonate 

rocks, siltstone, and sandstone. Unconsolidated basinfill deposits in Las Vegas Valley are more than 

3,000 ft thick.

The Carson and Truckee River Basins in westcentral Nevada and east-central California are closed f\

basins that encompass an area of about 7,200 mi . 

Altitudes range from about 10,900 ft in headwater 

areas of the Sierra Nevada to about 3,900 ft in the 

Carson Sink and at Pyramid Lake. The area primarily 

is composed of north-trending basins bounded by 

mountain ranges. Major basins include Carson Valley, 

Carson Desert and Sink, Lake Tahoe, Truckee 

Meadows, and Pyramid Lake. Major mountain ranges 

include the Sierra Nevada, Carson Range, Stillwater 

Range, Virginia Range, Pah Rah Range, Lake Range, 

Truckee Range, Virginia Mountains, and Pine Nut 

Mountains. Consolidated rocks in the mountains are 

primarily granitic and volcanic. Unconsolidated 

deposits in major basins are 4,000 to 8,000 ft thick. 

Average annual precipitation in the Las Vegas 

Valley area ranges from more than 20 in. in the higher 

altitudes of the mountainous headwater areas to about 

4 in. on the valley floor. Soils in the valley have formed 

from weathered alluvial deposits and consolidated 

rocks. Entisols have developed on alluvium and are 

characterized by weakly defined horizons. Aridisols 

have developed in desert areas and are characterized 

by accumulated salts in the surface horizon. Laminar 

calcrete, formed by the precipitation of calcium carbonate that has percolated downward into the soil, is 

found throughout the valley and can be a hydrologic 

barrier. Vegetation is primarily controlled by precipitation and communities include fir pine in areas of 

shaded canyon sides and north-facing slopes above an 

altitude of 7,000 ft, pinyon juniper in areas above 4,500 

ft, blackbrush on foothills and alluvial fans, and creosote in lowlands. Phreatophytes grow in lowland areas 

where the depth to the water table is shallow. There are 

some barren areas at low altitudes.

Headwater areas in the Sierra Nevada of the 

Carson and Truckee River Basins receive an annual 

average precipitation greater than 30 in.; Carson Desert 

receives less than 5 in. Soils are primarily aridisols and 

ultisols, with some entisols. Aridisols have developed 

in arid and semiarid areas; ultisols have developed in 

forested areas and are characterized as highly weathered acidic soils underlain by clay layers; and entisols 

have developed in alluvial areas. In headwater areas of 

the Sierra Nevada, vegetation includes alpine meadows 

with barren peaks at the highest altitudes, fir pine 

forests above 5,500 ft, and lower altitude communities 

of sagebrush, bitterbrush, rabbit brush, and grasses. 

Headwater riparian areas support willows, sedges, 

aspen, and cottonwood. Dryer mountain ranges 

including the Pine Nut Mountains, the Virginia Range, 

and the Pah Rah Range support pinyon-juniper 

forests at higher altitudes. Greasewood is the dominant 

natural vegetation in the Carson Desert. Riparian and 

wetland areas in the lower Carson and Truckee River 

Basins support a variety of vegetation including cottonwoods, willows, grasses, rushes, sedges, bulrushes, and 

cattails. Barren land is present in some parts of the 

Carson Sink.

Nevada is rapidly becoming one of the most 

urban States in the Nation. The 1990 population for 

Nevada was about 1,200,000. About 1,090,000 people, 

more than 90 percent of Nevada's 1990 population, 

resided in the NVBR study unit. In 1990, the Las Vegas 

Valley area had an estimated population of about 

710,000. Las Vegas is the largest urban area. Land 

use was 79 percent range, 14 percent forest, 5 percent 

urban, 1 percent water and wetland, and 1 percent 

barren. In 1990, approximately 317,000 acre-ft of 

water were used in Las Vegas Valley. Most of the water 

is from Lake Mead (80 percent). Public water supplies 

accounted for 91 percent of the water used, self66 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
supplied commercial and domestic uses were 4 percent, self-supplied industrial and mining uses were 

3 percent, and irrigation and agricultural uses were 

2 percent.

The estimated 1990 population for the Carson 

and Truckee River Basins was 380,000. The RenoSparks urban area had the greatest concentration of 

people. Land use was 50 percent range, 22 percent 

forest, 13 percent water and wetlands, 10 percent 

barren, 4 percent irrigated agriculture, and 1 percent 

urban. Of the approximately 800,000 acre-ft of water 

used in 1990, more that 85 percent was from surfacewater sources. Irrigation used 83 percent of the water, 

public supplies used 14 percent, self-supplied commercial and domestic uses were less than 1 percent. Selfsupplied industrial and mining uses also were less 

than 1 percent.

Streamflow in the Las Vegas Valley area is 

ephemeral. Flow in Las Vegas Wash downstream from 

Las Vegas is perennial because of urban runoff from 

landscape irrigation and treated sewage effluent. 

Streamflow for Las Vegas Wash near Henderson has 

increased steadily during 1970-88 because of increasing discharge of treated sewage and urban runoff. The 

average Streamflow for this station during 1970-88 was 

58 ft3/s; the 7-day, 10-year, low flow was 23 ft3/s; and 

the 1-day, 25-year, high flow was 1,140 ft3/s. The 

median concentration of dissolved solids for Las Vegas 

Wash near Henderson was about 1,700 mg/L. The principal ions in solution are sodium, calcium, and sulfate. 

Concentrations of dissolved solids decrease as streamflow increases, as a result of dilution by overland runoff 

from storms. Lake Mead on the Colorado River is 

the principal surface-water resource in the Las Vegas 

Valley area. It has a total capacity of 29,755,000 acreft and is used for flood control, irrigation, public 

supply, power generation, and recreation. Las Vegas 

Bay of Lake Mead receives urban runoff and treated 

sewage effluent from Las Vegas Wash.

The Carson and Truckee Rivers are perennial and 

originate in headwater areas in the Sierra Nevada; they 

flow to their respective termini in the Carson Sink and 

Pyramid Lake. Snowmelt is the principal source of 

flow in these streams, which receive no perennial tributary flow from areas outside the Sierra Nevada. The 

Carson River is unregulated in the Sierra Nevada, but 

is affected by irrigation diversions and return flows in 

the Carson Valley and is highly regulated by Lahontan 

Reservoir in its lower reaches. Combined average flow 

of the East and West Forks of Carson River upstream

from Carson Valley during 1970-90 was about 471

o

ft /s; respective 7-day, 10-year low flows were 25 and 

8.9 ft3/s; and respective 1-day, 25-year high flows were

o

5,810 and 1,610 ft /s. The average Streamflow downstream from the Carson Valley for the Carson River 

near Carson City was 413 ft3/s; the 7-day, 10-year low

*j

flow was 3.2 ft /s; and the 1 -day, 25-year high flow was 

10,700 ft3/s. The highly regulated Carson River below 

Lahontan Reservoir (which also stores diverted water 

from the Truckee River for irrigation) had an average 

flow of 532 ft3/s; a 7-day, 10-year low flow of 1.6 ft3/s; 

and a 1-day, 25-year high flow of 2,450 ft3/s.

The Truckee River originates as outflow from 

Lake Tahoe and is highly regulated by Lake Tahoe and 

other major impoundments in the Sierra Nevada. Average flow (1970-90) for the Truckee River at Farad, 

downstream from all major regulating impoundments,

o

was 851 ft /s; the 7-day, 10-year low flow was

117 ft3/s; and the 1-day, 25-year high flow was 7,990

o

ft /s. Downstream from urban and agricultural activities in the Truckee Meadows, average flow of the 

Truckee River at Vista was 883 ft3/s; 7-day, 10-year 

low flow was 121 ft3/s; and 1-day, 25-year high flow 

was 12,100 ft /s. Downstream from Derby Dam, where 

Truckee River water is diverted to Lahontan Reservoir

*j

for irrigation use, the average flow was 562 ft /s; the 

7-day, 10-year low flow was 4.5 ft3/s; and the 1-day, 

25-year high flow was 12,000 ft3/s.

Water-quality conditions in the Carson and 

Truckee Rivers are similar. In headwater areas, represented by the East and West Forks of Carson River and 

the Truckee River at Farad, the concentrations of dissolved solids are less than 100 mg/L during median 

Streamflow. Downstream from agricultural and urban 

influences, concentrations of dissolved solids are 

more than 200 mg/L during median Streamflow near 

Fort Churchill and Nixon. Calcium, sodium, bicarbonate, and sulfate concentrations increase appreciably in 

the downstream direction. Water temperatures for the 

Carson River near Fort Churchill and the Truckee 

River at Clark and near Nixon are not in the range 

considered to be stressful to aquatic organisms. Treated 

sewage effluent is not directly discharged into the 

Carson River, but effluent from the Reno-Sparks area is 

discharged into the Truckee River. The Carson River 

downstream from Carson City is being investigated as

SUMMARY 67
a U.S. Environmental Protection Agency Superfund 

Site because of mercury contamination from historical 

silver- and gold-ore milling activities. Irrigation in 

the Carson Desert has resulted in high concentrations 

of dissolved solids, boron, arsenic, selenium, molybdenum, lithium, and un-ionized ammonia.

Reservoirs and lakes have a prominent role in 

both the Carson and Truckee River systems. Lahontan 

Reservoir impounds the Carson River and diverted 

Truckee River water for irrigation use in the Carson 

Desert. The reservoir has a surface area of about 21 mi2 

and has a storage capacity of about 295,000 acre-ft. 

Calcium, sodium, and bicarbonate are the principal 

ions in solution and concentrations of dissolved solids 

generally are less than 300 mg/L. Mercury from 

upstream historical ore milling activities has accumulated in bottom sediments and fish tissues. Lake Tahoe, 

the source of the Truckee River, is a natural body of 

water with a surface area of about 192 mi and an average depth of about 900 ft. The upper 6.1 ft of Lake 

Tahoe is regulated by a dam in the outlet to supply irrigation water to the Newlands Project for irrigation in 

the Carson Basin. Calcium and bicarbonate are the 

principal ions in solution and the concentration of 

dissolved solids generally is less than 170 mg/L. 

Lake Tahoe is in the early stages of eutrophication and 

controlling nutrient contributions to the lake is an 

important concern. During 1967-86, the transparency 

of Lake Tahoe decreased at about 1.3 ft/yr, although it 

still exceeds 65 ft. Pyramid Lake is a natural body of 

water that is the terminus of the Truckee River. Sodium 

and chloride are the principal ions in solution and the 

concentration of dissolved solids exceeds 5,000 mg/L. 

The water surface of Pyramid Lake has declined from 

an altitude of 3,865 ft in 1882 to 3,800 ft in water year 

1992; primarily due to Truckee Canal diversions. 

Pyramid Lake is the habitat for the threatened 

Lahontan'cutthroat trout and the endangered 

cui-ui lake sucker.

Principal aquifers in the NVBR study unit 

consist of unconsolidated basin-fill deposits. In the Las 

Vegas Valley area these aquifers receive recharge from 

headwater areas primarily in the Spring Mountains 

about 33,000 acre-ft/yr. Ground water flows from 

recharge areas in the northwest part of the valley to 

discharge areas in the southeast. Prior to development, 

ground water was discharged primarily by leakage, 

springs, and evapotranspiration. However, most 

ground water currently is discharged by withdrawals 

for public water supplies. A water-level decline

(caused by withdrawals) of about 125 ft was measured 

in northwest Las Vegas during 1944-90. This decline 

caused compaction of clay layers and resulted in land 

subsidence. A shallow perched aquifer has developed 

under the Las Vegas urban area as a result of secondary 

recharge by landscape irrigation, which was estimated 

to be 43,000 acre-ft/yr in 1979. The perched aquifer 

discharges into Las Vegas Wash and subsequently to 

Las Vegas Bay of Lake Mead. Also, there is potential 

for contaminated water in shallow perched aquifers to 

percolate downward to deeper aquifers.

Dissolved-solids concentrations in ground water 

of principal aquifers in the northern part of Las Vegas 

Valley range from 200 to 400 mg/L; calcium, magnesium, and bicarbonate are the dominant ions. In the 

southern part of the valley, dissolved-solids concentrations range from 700 to 1,500 mg/L; concentrations of 

sulfate and chloride are higher than in the northwestern 

part. In the shallow aquifer, leaching and rising water 

levels caused by secondary recharge have resulted in 

dissolution of evaporite minerals. Subsequent evapotranspiration has further concentrated the shallow 

water resulting in average concentrations of 3,000 

mg/L for dissolved solids, 1,500 mg/L for sulfate, 310 

mg/L for calcium, and 310 mg/L for chloride. Results 

of available water-quality analyses indicate that concentrations of sulfate and dissolved solids exceed 

SMCL's (250 and 500 mg/L, respectively) in about 

one-half of the samples. >

Principal basin-fill aquifers are present in most 

major valleys in the Carson and Truckee River Basins. 

In the Carson River Basin, principal aquifers are in 

Carson, Eagle, Dayton, and Churchill Valleys, and 

Carson Desert. In the Truckee River Basin, principal 

aquifers are in the Lake Tahoe Basin, Washoe Valley, 

Truckee Meadows, Spanish Springs Valley, and Warm 

Springs Valley. In basin-fill aquifers in or adjacent to 

mountainous headwater areas, ground water flows 

toward major streams. Ground-water recharge in the 

Carson Desert is provided by the major delivery 

ditches and streams; ground water flows away from the 

recharge sources and is discharged in the Carson Sink 

by evapotranspiration. Ground water generally is 

shallow, ranging from less than 5 ft in some areas 

on valley floors to more than 100 ft on alluvial fans. 

Ground-water gradients are upward in central parts of

68 Environmental and Hydrologic Settings of Las Vegas Valley and Carson and Truckee River Basins, Nevada and California
valleys and deeper wells often are artesian. Concentrations of dissolved solids range from less than 200 mg/L 

in most headwater-area aquifers, where calcium and 

bicarbonate are dominant ions, to more than 10,000 

mg/L in parts of the Carson Desert, where sodium 

and chloride are the dominant ions.

The most important and pervasive water-quality 

issues in the NVBR study unit result from effects of 

human activities. Urban, agricultural, and mining 

activities have caused or have the potential to cause 

water-quality contamination in the study unit. Landscape irrigation in the Las Vegas urban area has 

resulted in the development of a shallow perched 

aquifers with high concentrations of dissolved solids. 

Water from these shallow aquifers has the potential to 

contaminate deeper aquifers. Effluent from sewagetreatment plants from the Las Vegas and Reno-Sparks 

urban areas has the potential to degrade water quality 

in Las Vegas Bay of Lake Mead and the lower Truckee 

River, respectively. Septic systems in urban and 

suburban areas can contaminate shallow ground-water 

systems. The ground application of treated sewage 

effluent for irrigation in urban and agricultural areas 

is becoming a widespread practice and has the potential to degrade water quality. Irrigation drainage has 

affected wetlands in the Carson Desert and Carson 

Sink. Historical milling of gold and silver ore has 

contaminated the Carson River system downstream 

from Carson City with mercury.

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