Defining Ecosystem Flow Requirements for the 

Bill Williams River, Arizona 

Edited by Patrick B. Shafroth and Vanessa B. Beauchamp 

Open-File Report 2006–1314 

U.S. Department of the Interior 

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

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2006 

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ii 
Contents 

Chapter 1. Background and Introduction 

By A. Hautzinger, P.B. Shafroth, V.B. Beauchamp, and A. Warner ................................................................1 


Study Area Description ..................................................................................................................................................2 


Flow-Biota Relations.......................................................................................................................................................3 


Flow Categories ...............................................................................................................................................................6 


Flow Timing................................................................................................................................................................6 


Flow Magnitude ........................................................................................................................................................6 


Chapter 2. Hydrology and Fluvial Geomorphology 


By P.K. House, P.B. Shafroth, and V.B. Beauchamp..........................................................................................9 


Regional Hydroclimatology............................................................................................................................................9 


Surfacewater Hydrology................................................................................................................................................9 


Big Sandy and Santa Maria Rivers .....................................................................................................................10 


Bill Williams River...................................................................................................................................................10 


Effects of Alamo Dam Operation on the Bill Williams River Flow Regime ...................................................12 


Representative Crosssections and Stage-Discharge Curves ........................................................................17 


Fluvial Geomorphology .................................................................................................................................................17 


Longitudinal Profile ................................................................................................................................................17 


Predam Condition ...................................................................................................................................................19 


Postdam Geomorphology......................................................................................................................................21 


Conceptual Framework for Assessing Postdam Geomorphic Change on the Bill Williams River ..........23 


Beaver Dams ...........................................................................................................................................................26 


Groundwater Dynamics ...............................................................................................................................................27 


Water Quality .................................................................................................................................................................29 


Chapter 3. Streamflow-Biota Relations: Riparian Vegetation 


 By P.B. Shafroth and V.B. Beauchamp ..............................................................................................................31 


Floodflows.......................................................................................................................................................................31 


Baseflows and Alluvial Groundwater Conditions....................................................................................................32 


Bill Williams River Vegetation Dynamics..................................................................................................................32 


Relationships Between Flow Components and Cottonwood Life History...........................................................34 


Application of the Recruitment Box to the Bill Williams River..............................................................................36 


Seed Germination ...................................................................................................................................................36 


Early Seedling Survival..........................................................................................................................................36 


Later Seedling Survival..........................................................................................................................................36 


Extension of Cottonwood Flow Models to Other Woody Riparian Species .......................................................36 


Flow Connections to Taxa with Warm Season Phenology....................................................................................38 


Flow Connections to Herbaceous Plants..................................................................................................................38 


Chapter 4. Streamflow-Biota Relations: Birds 

 By C.E. van Riper III and C.E. Paradzick .............................................................................................................41 


General Responses of Major Bird Groups to Flood Flows and Low Flows.........................................................41 


Deepwater and Shallowwater Birds...................................................................................................................41 


iii 
Predators and Scavengers ...................................................................................................................................42 


Gamebirds................................................................................................................................................................42 


Forestbirds ...............................................................................................................................................................42 


Specific Examples of Flow-Avian Biota Relationships in the Bill Williams River ..............................................43 


Southwestern Willow Flycatcher ........................................................................................................................43 


Yellow-Billed Cuckoo.............................................................................................................................................45 


Chapter 5. Streamflow-Biota Relations: Fish and Aquatic Macroinvertebrates 

 By D.A. Lytle ............................................................................................................................................................51 


Relationships Between Habitat and Species Composition ...................................................................................51 


Lotic Versus Lentic Habitats.................................................................................................................................51 


Overview of Aquatic Organisms in the Bill Williams River Below Alamo Dam .................................................54 


Aquatic Invertebrates Below Alamo Dam .........................................................................................................54 


Fish Below Alamo Dam..........................................................................................................................................54 


Relationships Between Flow Events and Species Distributions ..........................................................................54 


Resistance Versus Resilience..............................................................................................................................54 


Evolutionary Adaptations to Flood and Drought ...............................................................................................55 


Examples of Flow-Biota Relationships in the Bill Williams River .........................................................................56 


Longfin Dace (Agosia chrysogaster): High Resistance to Floods and Droughts........................................56 


Baetis Mayflies (Ephemeroptera: Baetidae): High Resilience to Floods .....................................................56 


Gomphid Dragonflies (Odonata: Gomphidae): Low Resilience to Floods.....................................................56 


Chapter 6. Streamflow-Biota Relations: Mammals, Reptiles, Amphibians, and Floodplain Invertebrates 

 By D.C. Andersen....................................................................................................................................................59 


Large Herbivores ...........................................................................................................................................................59 


Beavers ...........................................................................................................................................................................60 


Small Mammals .............................................................................................................................................................61 


Bats ..................................................................................................................................................................................62 


Mammalian Predators..................................................................................................................................................62 


Reptiles and Amphibians .............................................................................................................................................62 


Floodplain Invertebrates ..............................................................................................................................................63 


Examples of Flow-Biota Relationships in the Bill Williams River .........................................................................64 


Chapter 7. Ecosystem Functioning 


 By D.C. Andersen....................................................................................................................................................66 


Chapter 8. Summary of Unified Ecosystem Flow Requirements for the Bill Williams River Corridor 


By A. Hautzinger, A. Warner, J. Hickey and V.B. Beauchamp.......................................................................71 


Flow Requirements by Biological Group...................................................................................................................72 


Aquatics Group .......................................................................................................................................................72 


Riparian-Birds Group .............................................................................................................................................74 


Riparian Non-Birds Group ....................................................................................................................................82 


Unification of Biological-Group Flow Requirements ..............................................................................................84 


Unified Baseflows ..................................................................................................................................................84 


Unified Floods..........................................................................................................................................................86 


Discussion of Knowledge Gaps, Priority Monitoring, and Research Needs......................................................89 


Sediment Budget and Transport Dynamics.......................................................................................................89 


iv 
Magnitude and Duration of Flows Required to Achieve the Ecological Flow Requirements...................89 


Connections Between Surface Water, Ground Water, and Related Biotic Communities ........................89 


Role of Floods in the Tropical and Monsoon Seasons ....................................................................................90 


Nutrient Cycling, Decomposition, and the Role of Fire....................................................................................90 


Conclusions ....................................................................................................................................................................90 


References Cited..................................................................................................................................................................91 


Appendix A. Stage-Discharge Relationships Along the Bill Williams River. ..........................................................104 


Appendix B. Birds of the Bill Williams River Corridor .................................................................................................113 


Appendix C. Aquatic Invertebrates of the Bill Williams River Corridor....................................................................122 


Appendix D. Mammals of the Bill Williams River Corridor .........................................................................................126 


Appendix E. Reptiles and Amphibians of the Bill Williams River Corridor...............................................................128 


Appendix F. Terrestrial Invertebrates of the Bill Williams River Corridor ...............................................................130 


v 
Figures

 1. Map of the Bill Williams River Basin, Ariz. ..................................................................................................................4 


2. Canyon reach of the Bill Williams River, Ariz. ............................................................................................................5 


3. Alluvial valley reach of the Bill Williams River, Ariz. .................................................................................................5 


4. Reach map of the Bill Williams River, Ariz ..................................................................................................................6 


5. Annual flood series for the Big Sandy, Santa Maria, and Bill Williams Rivers, Ariz. ........................................11 


6. Average monthly flow for the postdam period on the Bill Williams, Santa Maria, and Big Sandy Rivers, 


Ariz (1969–2003). .....................................................................................................................................................13 


7. Log-Pearson analysis of annual peak flows for the Bill Williams River, Ariz. 	RI = Return interval; Predam 


values are shown with plus symbols, post dam values are shown with open circles. .............................14 


8. Frequency distribution of annual instantaneous peak flows in the high flow categories for the predam (43 


years) and postdam (30 years) periods at Bill Williams River below Alamo Dam, Ariz.............................14 


9. Average monthly flows for the predam (1940–1968) and postdam (1969–2003) periods at Bill Williams River 


below Alamo Dam, Ariz. ........................................................................................................................................16 


10. Distribution of average daily flows for the predam (10471 days) and postdam (12844 days) periods at Bill 


Williams River below Alamo Dam, Ariz. .............................................................................................................16 


11. Longitudinal profile of the Bill Williams River, Ariz., from Alamo Dam to the Highway 95 Bridge. ................18 


12. Lake Havasu Delta and reach upstream in 1953 .....................................................................................................20 


13. Planet Valley in 1953 .....................................................................................................................................................20 


14. Changes in channel width, 1953–1996, Bill Williams River, Ariz...........................................................................21 


15. Lake Havasu Delta and reach upstream in 1964 .....................................................................................................22 


16. Lake Havasu Delta in 1995...........................................................................................................................................22 


17. Changes in transect elevation between 1995 and 1996 .........................................................................................24 


18. Representative predam and postdam storm hydrographs, Bill Williams River, Ariz........................................25 


19. Conceptual model of the erosion potential of different types of flood hydrographs on the Bill Williams 


River, Ariz. ................................................................................................................................................................26 


20. Surfacewater and groundwater flowpaths for the Bill Williams River, Ariz. .....................................................28 


21. Variation in alluvial groundwater levels at three sites along the Bill Williams River.......................................29 


22. Riparian vegeatation growing along the Bill Willims River, Ariz. .........................................................................33 


23. A canyon reach of the Bill Williams River, Ariz., filled with Fremont cottonwood............................................33 


24. A floodplain area along the Bill Williams River, Ariz., dominated by tamarisk ..................................................34 


25. The “Recruitment Box” which defines a zone of floodplain elevation and time in which cottonwood 


seedlings can become established if streamflow conditions are favorable...............................................35 


26. Fremont cottonwood seedlings along the Bill Williams River, Ariz .....................................................................37 


27. Seed release periods of four woody riparian pioneer species at upstream and downstream areas of the 


Bill Williams River, Ariz..........................................................................................................................................37 


28. Herbaceous vegetation along the Bill Williams River, Ariz...................................................................................39 


29. Southwestern willow flycatcher (Empidonax traillii extimus). .............................................................................44 


30. Yellow-billed cuckoo habitat throughout Arizona as predicted through Advanced Very High Resolution 


Radiometer (AVHRR) satellite imagery...............................................................................................................49 


vi 
 31. Advanced Very High Resolution Radiometer (AVHRR) satellite imagery predicting yellow-billed cuckoo 


habitat, overlaid with known yellow-billed cuckoo locations on the Bill Williams NWR .........................50 


32. Beaver dam activity, and lentic habitat created by a beaver dam. .....................................................................52 


33. Typical lotic habitat in a sand-dominated reach, lower Bill Williams River, Ariz. ..........................................53 


34. Gray sanddragon Progomphus borealis (Gomphidae) larvae. Big Sandy River, Ariz.......................................57 


35. A model of the collaboration process used at the Bill Williams Ecosystem Flow Workshop ........................72 


A1. Bill Williams River, Ariz. Transect 1 ........................................................................................................................105 


A2. Bill Williams River, Ariz. Transect 2 ........................................................................................................................106 


A3. Bill Williams River, Ariz. Transect 3 ........................................................................................................................107 


A4. Bill Williams River, Ariz. Transect 4 ........................................................................................................................108 


A5. Bill Williams River, Ariz. Transect 5 ........................................................................................................................109 


A6. Bill Williams River, Ariz. Transect 6 ........................................................................................................................110 


A7. Bill Williams River, Ariz. Transect 7 ........................................................................................................................111 


A8. Bill Williams River, Ariz. Transect 8 ........................................................................................................................112 


vii
Tables

 1a. Floodflows for optimal ecosystem diversity of the Bill Williams River, Ariz. .......................................................7 


 1b. Baseflows for optimal ecosystem diversity of the Bill Williams River, Ariz.........................................................8 


2. Daily streamflow characteristics of the two principal tributaries of the Bill Willimas River, Ariz.................12 


3. Summary statistics of predam and postdam peak flood series for the Bill Williams River, Ariz....................13 


4. 	Comparison of predam and postdam characteristics of daily streamflow on the Bill Williams River near 


Alamo and below Alamo Dam, Ariz.....................................................................................................................15 


5. Summary of flow-biota relationships for riparian vegetation in the Bill Williams River Corridor, Ariz.. .......40 


6. Summary of flow-biota relationships for bird species found along the Bill Williams River, Ariz. ..................47 


7. Yellow-billed cuckoo habitat requirements .............................................................................................................48 


8. Summary of flow-biota relationships for longfin dace, Baetis mayflies and Gomphid dragonflies...............58 


9. 	Summary of flow-biota relationships for for beaver (Castor canadensis), ornate tree lizard 


(Callisaurus ornatus) and bullfrog (Rana catesbeiana) in the Bill Williams River, Ariz.............................65 


10. 	Ecosystem processes and associated functions important in the corridor of the Bill Williams River, 


Ariz. ...........................................................................................................................................................................70 


11. Baseflow requirements established by the Aquatics Group ...............................................................................75 


12. Floodflow requirements established by the the Aquatics Group. .......................................................................76 


13. Bird species life-history requirements by season as established by the Riparian-Birds Group ..................77 


14. Baseflow requirements as established by the Riparian-Birds Group................................................................79 


15. Floodflow requirements established by the Riparian-Birds Group.....................................................................80 


16. Baseflow requirements established by the Riparian Non-Birds Group.............................................................83 


17. Floodflow requirements established by the Riparian Non-Birds Group ............................................................83 


18. Unified baseflow requirements developed at the Bill Williams Ecosystem Flow Workshop . ......................86 


19. Unified floodflow requirements developed at the Bill Williams Ecosystem Flow Workshop. .......................88 


A1. Approximate locations of eight cross sections used to estimate stage-discharge relations along the Bill 


Williams River, Ariz. .............................................................................................................................................104 


B1. Deepwater bird species found within the corridor of the Bill Williams River, Ariz........................................113 


B2. Shallowwater and shore bird species found within the corridor of the Bill Williams River, Ariz. ..............114 


B3. Predator and scavenger bird species found within the corridor of the Bill Williams River, Ariz................116 


B4. Game bird species found within the corridor of the Bill Williams River, Ariz. ................................................117 


B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. ...............................................117 


C1. Aquatic invertebrate species found corridor of the Bill Williams River, Ariz..................................................122 


C2. Fish species found within the corridor of the Bill Williams River, Ariz.............................................................125 


D1. Mammal species found within the corridor of the Bill Williams River, Ariz. ...................................................126 


E1. Reptile and amphibian species found within the corridor of the Bill Williams River, Ariz. ...........................128 


F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. .........................130 


viii 
Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

inch (in.) 2.54 centimeter (cm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Area 

acre 0.4047 hectare (ha) 

square mile (mi2) 2.590 square kilometer (km 2) 

Volume 

cubic foot (ft3) 0.02832 cubic meter (m 3) 

Flow rate 

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

ix 
Chapter 1. Background and Introduction 

By Andrew Hautzinger1, Patrick B. Shafroth2, Vanessa B. Beauchamp2, and Andrew Warner3 

Alteration of natural river flows resulting from the construction and operation of dams 

can result in substantial changes to downstream aquatic and bottomland ecosystems and 

undermine the long-term health of native species and communities (for general reviews, cf. Ward 

and Stanford, 1995; Baron and others, 2002; Nilsson and Svedmark, 2002). Increasingly, land 

and water managers are seeking ways to manage reservoir releases to produce flow regimes that 

simultaneously meet human needs and maintain the health and sustainability of downstream 

Richter, 2003; Richter and others, 2003; Rood and 

others, 2003; Rood and others, 2005; Arthington and 

others, 2006). 

The Nature Conservancy (TNC) has 

developed an approach for defining “environmental 

flows” and is applying this approach to several rivers 

in the United States as a part of a collaboration with 

the U.S. Army Corps of Engineers (USACE) known 

as the Sustainable Rivers Project (Richter and others, 

2006). The Sustainable Rivers Project is designed to 

evaluate and, if necessary, recommend changes to 

dam operations to restore and protect the health of 

rivers and surrounding natural areas while continuing 

to meet human needs for services such as flood 

control and power generation. The definition of 

ecological flow requirements is the first step in a 

methodology, known as ecologically sustainable 

water management, which guides decisionmaking at 

those USACE dam sites across the country that 

participate in the Sustainable Rivers Project (see 

http://nature.org/success/dams.html for more details 

regarding the Sustainable Rivers Project). 

Ecosystem Flows 

water in a river or lake that sustains 

healthy ecosystems and the goods and 

services that humans derive from them. 

the timing, duration, frequency, and rate 

of change of these flow conditions. 

flows”. 

River Ecosystem Flow Workshop and 

ecosystem. 

biota (e.g., Poff and others, 1997; Patten and Stevens, 2001; Hughes and Rood, 2003; Postel and 

An ecosystem flow is the flow of 

Effective quantification of these flows 

includes the ecologically important 

range of flow magnitudes (low flows, 

high flow pulses, and floods), as well as 

Globally, these flows are most 

commonly referred to as “environmental 

In this report, “ecosystem flow 

requirements” are the specific quantified 

flows defined during the Bill Williams 

considered necessary for sustaining the 

health of the Bill Williams River 

This document is part of an effort to define a set of ecosystem flow requirements (see box 

at right) for the Bill Williams River (BWR) downstream of Alamo Dam in western Arizona (fig. 

1), one of the USACE/TNC focus rivers. In addition to USACE and TNC involvement, the 

BWR effort is being undertaken by a multiagency/entity group (the Bill Williams River Corridor 

Steering Committee) that meets on an approximately quarterly basis to coordinate resource 

management and prioritize research needs associated with the BWR. In the context of the BWR 

and Alamo Dam, the focus of both the steering committee and the technical and resource support 

made possible by the USACE–TNC agreement is to use the definition of flow requirements as a 

means to (1) assess current dam operations and their effect on the downstream aquatic and 

1

 U.S. Fish and Wildlife Service, Albuquerque, N. Mex. 2

 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colo. 3

 The Nature Conservancy, Sustainable Waters Program 

1 
riparian ecosystems and (2) identify changes in reservoir management that will restore and 

protect downstream ecosystems. By design, this work to define ecosystem flow requirements 

will also advance the steering committee’s firm commitment to develop and implement an 

adaptive management and monitoring plan. This plan will be used to evaluate ecosystem 

response to current and future management of Alamo Dam. 

The current effort to articulate ecosystem flow requirements for the BWR builds upon 

reoperation analyses that date back to the early 1990s. This earlier work came about through 

recognition by many interested parties that conflicts regarding Alamo Dam operations between 

stakeholders were probable and that any approach to reoperating Alamo Dam had to provide a 

means to resolve these likely conflicts. This effort culminated in the 1994 endorsement of a new 

approach to managing Alamo Dam (as embodied by the Bill Williams River Corridor Technical 

Committee’s (1994) water management plan) and the issuance of a new water control manual by 

the USACE in December of 2003. By the end of the 1990s, it had become clear that 

improvements could be made to existing models of reservoir release routing and that a 

comprehensive adaptive management and monitoring plan to assess dam operations had not been 

developed and implemented. These factors, coupled with advancements made in river science 

over the past decade, have served as motivators to engage in the current approach to generate 

ecosystem flow requirements. 

Much of the content of this document was originally used as background material on 

various aspects of the BWR by participants in a workshop, the Bill Williams River Ecosystem 

Flow Workshop, held in Tempe, Ariz., March 16 to 18, 2005. The purpose of the workshop was 

to define a set of flow requirements for sustaining the long-term ecological health of the BWR 

corridor with the overall goal of maximizing native biodiversity within the BWR flood plain. 

Specific topics covered in this document include surfacewater and groundwater hydrology, 

channel structure and geomorphology, and the ecology of the riverine and riparian systems 

including vegetation, aquatic organisms, small mammals, reptiles, amphibians, avifauna, and 

ecosystem function. The document draws on information that pertains directly to the BWR, as 

well as on relevant publications from other river systems in the Southwest or from around the 

world. The Bill Williams River Ecosystem Flow Workshop was attended by over 50 scientists 

and water and natural resource managers representing over 20 agencies and institutions. 

Workshop participants included experts with a broad array of relevant expertise. The flow 

requirements defined during the workshop built on over 15 years of flow-related work on the 

river system and are designed to support adaptive management of Alamo Dam. The final chapter 

(chapter 8) of this document summarizes the process, discussions, and results of the workshop. 

Study Area Description 

The BWR drains more than 13,000 km2 (5,200 mi2) of rugged, mountainous terrain in 

west-central Arizona. It is the largest tributary of the Colorado River between the Virgin and 

Gila Rivers. The name “Bill Williams River” is applied to the river segment extending from the 

confluence of the Big Sandy and Santa Maria Rivers to the Colorado River confluence at Lake 

Havasu (fig. 1). The watershed of the BWR spans diverse physiography ranging from highelevation forested mountains along the western margin of the central highlands province to lowlying, rugged desert mountains and intervening alluvial valleys in the basin and range province. 

The course of the Big Sandy River approximates the boundary between these two provinces. 

Average annual precipitation in the watershed ranges from approximately 45 cm (17.7 in.) in the 

headwaters to 22.5 cm (8.86 in.) near Alamo Dam (National Climatic Data Center station Alamo 

2

Dam) to 12.1 cm (4.76 in.) near the Colorado River (National Climatic Data Center station 

Parker 6NE). 

The BWR extends about 65 km (40.4 mi), its upstreammost 6.5 km (4.0 mi) now 

consisting of water impounded behind Alamo Dam, a flood-control structure which was 

completed in 1968 and has a reservoir storage capacity of approximately 1,233 m3

 x 106 (43,540 

ft3

 x 106). Downstream of the dam, the BWR flows 58.5 km (36.4 mi) with an average gradient 

of 0.003 (range of 0.001–0.009) to its confluence with the Colorado River (at Lake Havasu) at an 

elevation of 137 m (449 ft) (fig. 1). The BWR passes through canyons (fig. 2) interspersed with 

alluvial valleys, including the 10.6-km-long (6.6 mi-long) Planet Valley, a significant 

hydrological control on flows in the 14.5 km (9.0 mi) of river between the basin and the 

confluence with the Colorado River (fig. 3). Different investigators have divided the BWR into 

reaches, based primarily on differences in valley width, reliability of surface flow, and distance 

downstream of Alamo Dam (House and others, 1999; TetraTech. Inc., 2002; Shafroth and others, 

2004) (fig. 4). No perennial tributaries enter the BWR downstream of Alamo Dam. Channel 

bed and floodplain sediments are dominated by coarse particles (81 percent), primarily sand (67 

percent), and are generally low in electrical conductivity (ca. 1.0 dS/m). Flows of 35.1 m3

/s 

(1,240 ft3

/s) and larger readily transport the poorly consolidated sand. 

Human use is minimal along the BWR corridor. Although extensive alfalfa (Medicago 

sativa) farming and associated groundwater pumping occurred within the Planet Valley Basin 

historically and as recently as the early 1990s, agriculture is currently limited to a single cotton 

(Gossypium spp.) farm along a 2.6-km (1.6-mi) reach of the BWR (within Reid Valley; see fig. 

4). Along the BWR, cattle legally graze only a small area within Planet Valley, though 

trespassing cattle occasionally utilize reaches of the river upstream of Planet Valley. Feral 

burros are present throughout the study area, but their grazing and browsing impacts appear to be 

relatively minor. Four-wheel-drive vehicles commonly drive through a stretch of county road 

(and occasionally on unroaded portions of the flood plain) in the bottomland within the Bill 

Williams River National Wildlife Refuge and also within the flood plain on State of Arizona 

land, private land, and some Bureau of Land Management (BLM) land upstream of Planet 

Valley. 

Flow-Biota Relations 

After a chapter on the BWR’s hydrology and geomorphology (chapter 2), the next five 

chapters (chapters 3–7) of this report contain discussions of relationships between streamflow 

and various physical and biological aspects of the BWR. These chapters were provided to 

workshop participants as background material. Some of these discussions are general, but 

throughout the document we also refer to several categories of streamflow (described below). 

These categories were used during the ecosystem flow workshop to help structure discussions 

related to flow requirements of different parts of the river system. Flows are discussed in terms 

of flow timing or seasonality and flow magnitude (floodflows and baseflows). Discussions of 

flow-biota connections are presented both in terms of broad relationships and also in the context 

of key taxa or guilds about which there was a relatively substantial base of information available. 

3

Figure 1. Map of the Bill Williams River Basin, Ariz. 

4

Figure 2. Canyon reach of the Bill Williams River, Ariz. Photograph by Patrick Shafroth, U.S. 

Geological Survey. 

Figure 3. Alluvial valley reach of the Bill Williams River, Ariz. Photograph by Patrick Shafroth, 

U.S. Geological Survey. 

5

Figure 4. Map of the Bill Williams River, Ariz. Showing river miles upstream from the Highway 

95 bridge crossing and twelve reaches. Reach boundaries were based on a combination of valley 

width and alluvial groundwater dynamics (Shafroth and others, 2004). 

Flow Categories 

Flow Timing 

We have defined four seasons and the precipitation events and flow conditions that may 

occur within each season. These are September 15–November 15: tropical storms; November 

15–April 30: winter-spring frontal storms; May 1–June 30: dry; July 1–September 15: monsoons. 

Flow Magnitude 

We distinguish two flow magnitude groups: floodflows and baseflows. Floodflows 

include flood magnitude, frequency, duration, and rates of flow recession or drawdown 

following the flood. Flood peaks can be flows greater than or equal to 28.3 m3

/s (1,000 ft3/s). 

Flow recessions or drawdowns extend from the flood peak down to baseflow levels, which we 

define here as less than or equal to 2.8 m3

/s (100 ft3/s). Within both the floodflow and baseflow 

groups, we have identified three magnitude classes and estimated various associated physical 

effects (table 1a, b). Direct and indirect effects on particular biota are discussed in more detail 

within each section below. 

6

7


Table 1a. Estimated physical and biological effects of different magnitude floods on the Bill Williams River, Ariz[CPOM = Coarse particulate organic matter; FPOM = fine particulate organic matter] 

Large flood Moderate flood1 Sma

Flow range 30,000 ft3/s or greater 5,000–30,000 ft3/s 100–Channel Channel avulsion. Some channel migration, widening, local No significant changgeomorphology deepening (magnitude and duration geomorphology. 

Channel geometry change and formation dependent). of new channels. Channel widening and deepening. 

Sediment Extensive sediment erosion and deposition, Some bare substrate generated via sediment Turnover of some se

including channel bed sediment, tributary mobilization (erosion and deposition). fans, and channel banks in incised reaches. Complete turnover of instream sediments. 

Beaver Removal of essentially all beaver dams. Removal of most beaver dams. Removal of few bea

some. 

Vegetation Removal of mature trees in some Mechanical damage or removal of smaller Some mechanical da

floodplain locations. woody plants in broad floodplain reaches; riparian vegetation. 

large woody plants damaged or removed in 

Removal of most herbaceous vegetation. narrow reaches. 

Some herbaceous vegetation scoured. 

Flood plain Creation of new off-channel aquatic Refresh and/or rescour existing off-channel Refilling of some (lo

habitats such as pools, destruction or aquatic habitats. channel habitats witfilling of old off-channel habitats. 

Some creation of new off-channel aquatic Some of floodplain 

Wetting of entire flood plain. habitats such as pools, and some destruction 

or filling of old off-channel habitats (magnitude dependent). Most of flood plain wetted. 

Organic matter CPOM and FPOM removed. Some CPOM and FPOM removed. Little CPOM or FPOGroundwater Alluvial groundwater and soil moisture Alluvial groundwater and soil moisture Partial recharge of a

recharge. recharge. soil moisture. 

A wide range of effects is possible within this flow range, representing a gradient between responses associated with large and small fl

1
Table 1b. Estimated effects of different magnitude baseflows on surface and ground water of 

the Bill Williams River, Ariz. 

High baseflow Moderate baseflow Low baseflow 

Flow range 50–100 ft3

/s 10–50 ft3/s 0–10 ft3/s 

Surface flow Surface flow maintained Surface flow maintained in Surface flow absent 

throughout the year in all 

reaches except east end of 

Planet Valley. 

winter months in all reaches 

except east end of Planet 

Valley. 

Surface flow may be absent 

during some or all of the 

spring, summer and fall 

months from several river 

throughout the year from 

several river reaches (e.g., 

upstream end of Rankin 

Valley, from near Swansea to 

west end of Planet Valley, and 

from Kohen Ranch 

downstream on refuge). 

reaches (e.g., upstream end of 

Rankin Valley, from near 

Swansea to west end of Planet 

Valley, and from Kohen 

Ranch downstream on 

refuge). 

During some/all of the spring, 

summer and fall months, 

surface flow also may be 

absent through more of the 

river’s length (e.g., above 

reaches plus more of Rankin 

Valley, from gas pipeline 

crossing down to near 

Swansea, and on refuge from 

Mohave Wash down to Kohen 

Ranch). 

Ground water Alluvial water tables fully Dynamic alluvial water tables, Relatively deep alluvial water 

charged and relatively stable. with summertime declines of tables, including 1–4 m (3–13 

1–3 m (3–10 ft) in intermittent ft) summertime declines in 

flow reaches. intermittent reaches. 

8 
Chapter 2. Hydrology and Fluvial Geomorphology 

By P. Kyle House1, Patrick B. Shafroth2, and Vanessa B. Beauchamp2 

Regional Hydroclimatology 

Streamflow in most large rivers in Arizona corresponds to a distinct regional 

hydroclimatologic pattern with a mixture of regional and local storms. Streamflow in rivers 

draining the central highlands of Arizona is primarily controlled by precipitation from dissipating 

tropical cyclones in the late summer and fall and by regional-scale winter frontal storms in the 

late fall, winter, and early spring (Webb and Betancourt, 1992; Ely and others, 1994; House and 

Hirschboeck, 1997). Isolated summer and fall monsoonal thunderstorms rarely result in 

significant runoff in large river basins, but they can have an important local impact on smaller 

tributaries and can contribute to the system’s overall discontinuous nature of sediment input and 

channel response (e.g., House and Pearthree, 1995; House and Baker, 2001). Interestingly, flow 

regulation on the BWR is such that flashfloods from some tributaries can equal or even exceed 

the peak discharge of maximum flow releases from Alamo Dam (e.g., scenarios described in 

Gatewood and others, 1946; Hansen and Schwartz, 1981). 

The most significant types of flow-generating storms in the BWR Basin and similar 

basins in the Lower Colorado River drainage are demonstrably linked to El Niño or positive El 

Niño Southern Oscillation (ENSO) conditions that vary over decadal and centennial time scales. 

These storms influence the probability of higher than average regional precipitation and, hence, 

streamflow on most rivers in the Southwest (Hirschboeck, 1985; Andrade and Sellers, 1988; 

Redmond and Koch, 1991; Webb and Betancourt, 1992; Ely, 1997; House and Hirschboeck, 

1997). La Niña (or negative ENSO) conditions have an opposite effect and are more likely to be 

associated with below average precipitation and runoff (Redmond and others, 2002). These 

relations are relatively strong for most rivers in the historical record, and some studies indicate 

that they have had similar influence during the last 3,500–5,000 years on rivers in the Southwest 

(Ely, 1997), including the BWR. 

Surfacewater Hydrology 

At the site of Alamo Dam, the contributing drainage area to the BWR is approximately 

11,200 km2

 (4,330 mi2). The Big Sandy River Basin constitutes 65 percent (ca. 7,280 km2 (2,810 

mi2

)) of this area and contributes the majority of streamflow to the BWR. The Santa Maria 

River Basin constitutes the remaining 35 percent (ca. 3,940 km2

 (1,520mi2)). At its mouth, the 

BWR drains an estimated 13,470 km2

 (5,200 mi2), making it the third largest drainage basin that 

lies completely in Arizona. Despite the smaller watershed area, several of the largest reported 

historical peaks on the BWR are comparable to or even larger than corresponding peaks on the 

Salt (16,140 km2

 (6,232 mi2)) and Verde (16,190 km2 (6,250 mi2)) Rivers. Low flows on the 

BWR were typically lower than those on the Salt and Verde Rivers, both of which receive 

perennial flow from tributaries along the margin of the Colorado Plateau. The unregulated BWR 

flow regime was more variable and lower magnitude overall and showed a slightly greater 

response to storms in the late summer and fall than did the Salt and Verde Rivers. Since 1969, 

1

 Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nev. 

2

 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colo. 

9 
the hydrologic regime of the BWR has been significantly altered by flow regulation through 

Alamo Dam, which is located below the Big Sandy-Santa Maria confluence and is now the de 

facto head of the BWR. 

Stream gaging records in the BWR Basin are limited in spatial and temporal scope, but 

adequate data exist to characterize the predam and postdam flow regimes. Each station is shown 

on the map in fig. 1, and the corresponding annual flood series are shown in fig 5. For this 

report, we evaluated the period of record up to and including water year 2003. 

Big Sandy and Santa Maria Rivers 

The only stream gage on the Big Sandy River (near Wikieup, Ariz., USGS #9424450) is 

located near Signal, Ariz. about 48 km (30 mi) south of Wikieup and approximately 24 km (15 

mi) upstream of the Big Sandy River-Santa Maria River confluence. The Big Sandy River drains 

7,280 km2

 (2,810 mi2) at the gage site. The official period of record is 1966–2003. There have 

been two gaging stations on the Santa Maria River , and the composite period of record is 1939 

to the present. The station “Santa Maria River near Alamo” (USGS #9425500) was moved 

upstream to the station “Santa Maria River near Bagdad” (USGS #9424900) in 1967 to eliminate 

the influence of inundation and backwater effects from Alamo Lake. The resultant decrease in 

the contributing drainage area was 806 km 2 (311 mi2

). 

Peak Flows 

On the Big Sandy River, the flood of record of 1,945.4 m3/s (68,700 ft3/s) was recorded 

on February 9, 1993. Most of the peak flow in this event came from Burro Creek, a major 

tributary that joins the Big Sandy River 7 mi. upstream of the gage. Burro Creek recorded a peak 

discharge of 1,565.9 m3

/s (55,300 ft3/s) 3 hours before the peak on the Big Sandy River (fig. 5a). 

The notable historical discharge estimate of 2,831.7 m3/s (100,000 ft3/s) was reported from near 

the gage site in September 1939 (Gatewood and others, 1946). This event resulted from a series 

of tropical storms that flooded large parts of southern California and western Arizona. On the 

Santa Maria River, the flood of record is 951.5 m3

/s (33,600 ft3/s) (at the gage near Alamo), 

which occurred in August 1951 (fig. 5b). 

Average Daily Flows 

Some descriptive statistics of the daily streamflow records from each BWR tributary site 

are summarized in table 2 (from House and others, 1999). On the Santa Maria River, the change 

in the gaging station location in 1967 affected daily flow values. The station near Alamo Dam 

has a 27-year record (1939–66) and recorded no days of zero flow. Over a comparable period of 

time, the record station near Bagdad, Ariz. (after 1967), recorded 6,474 days of zero flow. Thus, 

for approximately 65 percent of the 9,662 days in the record, there was no streamflow at the 

gage. Average monthly flows in the post-Alamo Dam era are illustrated in figure 6. 

Bill Williams River 

There have been two primary gaging stations on the BWR. The earliest stations were 

“Bill Williams River near Swansea” (1910–12), followed by “Williams River near Swansea” 

(1913–15), “Williams River at Planet” (1928–43), and finally “Bill Williams River at Planet” 

(1943–1946). Examination of historical surfacewater records indicates that each of these 

corresponds to the same station located about 1.6 km (1 mi) downstream from the site of Planet, 

Ariz. 

10

Figure 5. Annual flood series for the Big Sandy, Santa Maria, and Bill Williams Rivers, Ariz. 

11

Table 2. Daily streamflow characteristics of the two principal tributaries of the Bill Williams River, 

Ariz. 

[Discharges are in cubic feet per second (ft3/s)] 

Parameter Big Sandy River 

near Wikieup 

Santa Maria River 

near Alamo 

Santa Maria River 

near Bagdad 

Period of record 1966–2003 1939–1966 1967–2003 

(missing 1986–1988) 

Mean 80 34 56 

Median 6 3 0 

Mode 8 2 0 

Standard deviation 646 325 324 

Coefficient of variation 8.1 9.6 6.1 

Count 13,700 9,648 12,584 

Zero values 0 0 8,774 

Missing values 0 0 1,096 

Minimum 1.3 0.1 0.0 

Maximum 26,100 15,500 8,410 

In 1939, a second gage on the BWR was established near Alamo, Ariz. In April 1968, the 

gage was moved 2.7 km (1.7 mi) downstream to a point below Alamo Dam, which was 

completed in that year. The annual flood series for the BWR at Planet and Alamo are both shown 

in the lower graph on figure 5. In the interval 1929–1939 (except for 1937), the most recently 

compiled data in Pope and others (1998) for the Alamo site are extrapolated from the Planet site 

(discharges slightly lower). Each data set is provided in figure 5c for reference and comparison. 

Postdam annual peaks are values reconstructed from reservoir inflow (United States Army Corps 

of Engineers, 2003). 

Effects of Alamo Dam Operation on the Bill Williams River Flow Regime 

Peak Flows 

Three large historical floods in 1891, 1916, and 1927 are listed in the records for the gage at 

Planet and the gage at Alamo (fig. 5c). According to Patterson and Somers (1966), these values 

are estimates made by using floodmarks in “Striped Canyon,” about 37 km (23 mi) upstream 

from Planet. This site is near the present location of Alamo Dam, and the reported discharges, 

though likely maximum estimates (House and others, 1999), are most appropriately related to the 

record from the gage sites nearer to Alamo. The effect of the dam on annual peak discharges is 

extreme. The largest postdam instantaneous peak discharge is 197.6 m3

/s (6,980 ft3/s), which 

occurred in March of 1993. This value is less than 11 percent of the maximum instantaneous 

peak in the official period of record and less than 5 percent of the largest reported historical peak 

discharge. The postdam maximum discharges are limited by the capacity of the dam outlet 

structures. 

To quantify the effects of the dam on peak flow frequency, we analyzed the predam and 

postdam peak flows from the Alamo site data sets by using FLDFRQ3, a Bayesian statistical 

model developed by the U.S. Bureau of Reclamation (described in O’Connell and others, 2002). 

The results are shown in table 3 and figure 7. Weibull plotting positions for both data sets were 

fitted to the log-Pearson 3 distribution, and the values in the table are from the fitted curve. The 

tremendous difference between the two data sets reflects the magnitude of peak flow reduction 

12

Figure 6. Average monthly flow for the postdam period on the Bill Williams, Santa Maria, and 

Big Sandy Rivers, Ariz (1969–2003). 

Table 3. Summary statistics of predam and postdam peak flood series for the Bill Williams 

River, Ariz. 

[RI = Return interval for the specified flow magnitude] 

R.I. Predam Postdam Change 

1.33 2,920 159 95% 

10 63,945 3,949 94% 

20 87,239 5,224 94% 

50 11,4721 6,580 94% 

100 13,2427 7,364 94% 

200 14,7459 7,970 95% 

500 16,3582 8,556 95% 

13 
Figure 7. Log-Pearson analysis of annual peak flows for the Bill Williams River, Ariz. 


RI = Return interval; Predam values are shown with plus symbols, post dam values are shown 


with open circles. 


Figure 8. Frequency distribution of annual instantaneous peak flows in the high flow categories 

for the predam (43 years) and postdam (30 years) periods at Bill Williams River below Alamo Dam, 

Ariz. 

14 
associated with Alamo Dam. Although the regulated flow regime violates most assumptions of 

the statistical analysis, the comparison serves an important purpose by clearly showing the 

magnitude of the change. The steep predam curve represents a highly variable flood regime in 

which the difference between average flows and flood flows is very large, typical of an aridregion river. The flat postdam curve indicates extremely low variability and small differences 

between average and extreme flood conditions. For example, the maximum postdam peak 

discharge corresponds to a 1- to 2-year flood in the predam era. The frequency distribution of 

annual peak flows in the floodflow categories presented above is illustrated in figure 8. 

Average Daily Flows 

Descriptive statistics of the predam and postdam average daily streamflow values in 

Table 4 (House and others, 1999) summarize striking hydrological changes on the BWR 

associated with flow regulation through Alamo Dam. Because Alamo Reservoir serves the 

primary purpose of storage and flood control, it is not depleted by consumptive use, and thus the 

annual flow volume has not been much affected by the existence of the dam, except for the effect 

of evaporation from the reservoir surface, which can be significant under certain conditions 

(United States Army Corps of Engineers, 2003). Climatic factors have resulted in some predam 

versus postdam flow differences. For example, the mean flow in the postdam era (3.6 m3

/s (128 

ft3

/s)) was considerably higher than that in predam era (2.6 m3/s (93 ft3/s)). Median annual flows 

however, were more similar (predam median = 0.27 m3

/s (9.4 ft3/s), postdam median = 0.42 m3/s 

(15 ft3

/s)), reflecting the strong effect of a few particularly wet years in the late 1970s and early 

1990s on average values. Average monthly flows show some effect of dam operations. First, in 

the postdam era, high flows were less likely to occur in December or January (fig. 9). Second, 

on the BWR, flows from May through October were substantially higher in the postdam era than 

predam (fig. 9). The frequency of average daily flows in several categories, including the three 

baseflow categories used in this report (see “Flow Categories” in chapter 1, above), is illustrated 

in figure 10. Baseflows in the 1.4–2.8 m3

/s (51–100 ft3/s) range were relatively uncommon both 

before and after the construction of Alamo Dam (fig. 10). Flows in the range of 0.31–1.4 m3

/s 

(11–50 ft3

/s) have been more frequent in the postdam era, while flows in the 0.003–0.28 m3/s 

(0.1–10 ft3

/s) range were more common in the predam era (fig. 10). Flow regulation by Alamo 

Dam resulted in some zero flow days, which did not occur on the upper BWR prior to the 

construction of the dam. Most of these zero flow days occurred in the first several years 

following dam construction. 

Table 4. Comparison of predam and postdam characteristics of daily streamflow on the Bill 

Williams River near Alamo and below Alamo Dam, Ariz., (modified from House and others, 1999). 

[Discharges are in cubic feet per second( ft3/s)] 

Parameter Predam Postdam Difference 

Mean 93 128 38% 

Standard deviation 692.3 468.7 -38% 

Median 9.4 15 60% 

Mode 11 11 0% 

Coefficient of variation 7.4 3.7 -50% 

Count 10,471 12,844 2,373 

Zero values 0 699 699 

Minimum 0.05 0 0.05 

Maximum 25,200 6,980 -72% 

15 
Figure 9. Average monthly flows for the predam (1940–1968) and postdam (1969–2003) periods 

at Bill Williams River below Alamo Dam, Ariz. 

Figure 10. Distribution of average daily flows for the predam (10471 days) and postdam (12844 

days) periods at Bill Williams River below Alamo Dam, Ariz. 

16 
Representative Crosssections and Stage-Discharge Curves 

The topography of eight crosssections along the BWR was surveyed in January and 

February 1996. Crosssections were perpendicular to the low flow channel and extended from 

valley wall to valley wall. Four crosssections were located in a segment of the river between 

Alamo Dam and Planet Valley, and four were located downstream of Planet Valley (appendix A, 

table A1). Transects were selected to represent different reaches and hence exhibit variation in 

geomorphologic and hydrologic characteristics. 

We used Manning’s equation to estimate stage-discharge relationships at each of the 

crosssections, based on inputs of slope and surface roughness values (Manning’s n; WEST 

Consultants Inc., 1998). We estimated slope from topographic maps (1:24,000) and Manning’s n 

values from published sources (Arcement and Schneider, 1989). These results should be 

considered rough approximations only, as there are several sources of error associated with our 

estimates (e.g., poor calibration, mobile sand bed channel, coarse estimate of slope from 

topographic maps, two dimensional flow, effects of conditions upstream and downstream of the 

crosssections, etc.). Hydraulic modeling results are presented in appendix A (figs. A1–A8). For 

each crosssection, we have displayed estimates of the river stage (elevation), velocity, and wetted 

perimeter associated with flows ranging from 0 to 1,415.8 m3

/s (0 to 50,000 ft3/s). In addition, a 

schematic of the cross-sectional topography and areas inundated at 14.2, 141.6, and 849.5 m3

/s 

(500, 5,000, and 30,000 ft3

/s) is presented to illustrate the approximate distribution of flow 

during floods at the low end of each of the three floodflow categories. Comparisons of results at 

the eight sites help to illustrate some of the variation in flood effects across a range of 

crosssections. 

Fluvial Geomorphology 

The BWR channel is characterized by a series of relatively narrow bedrock gorges 

separated by wider, alluvial reaches. Photographic and field evidence indicates that the bed of the 

river is filled with alluvium virtually throughout the length of the river. Bedrock is shallow and 

occasionally exposed in the narrow gorge downstream from the dam, and it may be shallow in 

other narrow canyon reaches. Minimal data exist to evaluate bedrock depth in detail, however. 

At low flows, the river follows a braided pattern characterized by relatively low sinuosity 

channels separated by medial braid bars composed of sand and gravel. During high flows (prior 

to the dam) the channel apparently occupied nearly all of the late Holocene flood plain, even in 

alluvial valleys that are nearly 3.2 km (2 mi) wide (i.e., Planet Valley). During low flow periods, 

streamflow in the river is intermittent, with surface flow only typical of narrow canyon reaches 

and subsurface flow in the wider valley reaches. In general, the upper ends of the valleys are 

losing reaches, and the lower ends are gaining reaches where throughflow resurfaces. 

Longitudinal Profile 

The longitudinal profile of the BWR was determined from available 7.5-minute 

topographic maps and digital orthophoto quadrangles (fig. 11). The channel length was 

determined by digitizing the thalweg and calculating cumulative distance from point to point 

beginning at the Highway 95 Bridge at Lake Havasu and ending at the stream gage below Alamo 

Dam. Elevations were simply taken from points where the thalweg crossed contour lines on the 

maps. 

17

Figure 11. Longitudinal profile of the Bill Williams River, Ariz., from Alamo Dam to the Highway 

95 Bridge. 

The patterns evident from the longitudinal profile likely reflect the discontinuous nature 

of the channel morphology and (likely) its predam mode of sediment transport and storage. 

Distinct increases in the channel gradient occur in close proximity to entrances to the alluvial 

valleys, and rapid decreases in gradient occur below or near the downstream end of the valleys. 

In the predam condition, sediment conveyed through the canyons was deposited in broad 

(sometimes valleywide) areas of flow expansion below the head of each valley. This sediment 

deposition pattern is an inevitable consequence of the alternating gorge and valley morphology, 

and large-scale flow expansion in the valleys is clearly evident in predam aerial photography 

(House and others, 1999). Aggradation of sediment increases the channel gradient, and incision 

into alluvial fill decreases channel gradient. At the downstream end of each valley, changes in 

channel gradient from steep to gentle reflect incision into the valley fill. In each flood event, a 

wave of sediment liberated from the valley fill is passed into the next gorge and deposited in the 

next area of flow expansion. Over time, the overall morphology is maintained, but waves or 

slugs of sediment have moved farther down the system. In the postdam era, this process is 

maintained but is occurring at a different spatiotemporal scale and subject to some different 

controls, including net decrease in sediment, attenuation of peak discharge, and sustained low 

and moderate flows. 

18

The sediment trapping above Alamo Dam will have inevitable and likely irreversible 

effects on the process of sediment movement through the system, as the available external 

sediment is progressively reduced at the expense of sediment resources in the valley bottom. The 

upper reaches of the BWR will be more heavily impacted by sediment depletion than will lower 

reaches because sediment flushed from the upper part of the BWR is stored in the middle and 

lower reaches, but the zone separating net storage from net loss is expected to move downstream 

over time. 

The vast amount of sediment storage is taking place in the lower reaches of the river 

(below Planet Valley). Channel roughness in the lower reaches is associated with dense 

vegetation cover is conducive to sedimentation by retarding flow velocities and enhancing 

infiltration. The river terminates with a delta in Lake Havasu, and sedimentation is greatly 

enhanced by the interaction of the river with a standing body of water. Base-level rise in Lake 

Havasu can increase sedimentation, but this effect is likely to be restricted to a short reach 

upstream. The effect of base-level fall in Lake Havasu during periods of BWR input (though 

likely rare) would have much more significant geomorphic consequences by enhancing river 

incision into the zones currently receiving a net input of sediment. 

Tributaries can also be important sources of sediment, particularly in arid-land systems. 

According to paleoflood studies and comparison of historical aerial photographs reported in 

House and Baker (2001), however, the relative sediment input by tributaries in the BWR 

watershed is extremely discontinuous in space and time. The reliability of sediment input by 

tributaries is low and the ability of regulated flows to move the higher caliber portion of this 

sediment is also questionable. Photographic comparisons show that the last flow to obliterate 

tributary fans occurred in the 1960s and was definitely larger than the maximum dam releases in 

the modern regime. Furthermore, the tributaries with the largest drainage areas that contribute 

the most sediment most frequently (e.g., Mohave Wash, Castaneda Wash, and Centennial Wash) 

enter the BWR in the lower reaches beginning near Planet Valley, where sediment depletion is 

currently less of a problem. 

Predam Condition 

The predam geomorphology of the BWR has been characterized through analysis of 

predam aerial photographs (House and others, 1999; Shafroth and others, 2002). House and 

others (1999) used aerial photographs from 1953 to map the predam baseline geomorphology of 

the BWR channel at a 1:24,000 scale. The maps indicate that the predam channel morphology 

was dominated by the effects of relatively high-magnitude floods. The 1953 photographs were 

taken after a relatively large flood, which occurred in August 1951 (1,843.4 m3

/s (65,100 ft3/s) 

and a smaller flood in water year 1952 (1,064.7 m3

/s (37,600 ft3/s)), so the specific effects of 

these events are well preserved on the photographs (fig. 12). The predam BWR channel was 

conspicuous in the broad extent of active braided channels and flanking alluvial surfaces. 

Throughout most of the channel length from the present base of Alamo Dam to the downstream 

end of Planet Valley, active alluvial surfaces cover most of the available valley bottom, except in 

Reid Valley, where agricultural activities were constraining the width of the channel somewhat. 

Remarkably, the entire width of Planet Valley was subject to inundation during large predam 

floods, which is evident in the 1953 photographs and in General Land Office survey maps 

compiled after the large flood in 1916 (fig. 13). 

19

Figure 12. Lake Havasu Delta and reach upstream in 1953, showing the resulting broad, active 

channel after extensive flooding in 1951. 

Figure 13. Planet Valley in 1953, showing the resulting broad, active channel after extensive 

flooding in 1951. 

Another study (Shafroth and others, 2002) evaluated changes in channel width as a 

function of streamflow by using a time series of aerial photographs along eight reaches of the 

BWR, including 2 predam years (1953 and 1964), and 4 postdam years (1976, 1987, 1996, and 

2002). Channel width was found to be significantly related to the maximum flood power and the 

average summer flow during the 5 years preceding a photograph year, as well as to whether a 

river reach had perennial or seasonally intermittent flow. Channels narrowed during intervals 

with relatively low flood magnitudes and/or relatively high summer flows (fig. 14). As 

mentioned above, aerial photographs from 1953 reveal large areas with bare, wide channels, the 

likely result of large magnitude floods in both 1951 and 1952 (fig. 13). Between 1959 and 1964, 

flood magnitudes were smaller, and summer flows were larger. As a result, channels were 

considerably narrower in the 1964 photographs (fig. 15). 

20

Figure 14. Changes in channel width, 1953–1996, Bill Williams River, Ariz. Channel width as 

measured on five sets of aerial photography within eight reaches of the river: four perennial and 

four intermittent. Values are means; error bars are one standard error. (See Shafroth and Patten, 

1998; and Shafroth and others, 2002, for more details.) 

It is common for the morphology of large rivers in arid and semiarid settings to be 

dominated by the effects of floods (Schumm and Lichty, 1963; Burkham, 1972; Baker, 1977; 

Wolman and Gerson, 1978). Rivers that undergo large flood-related morphologic changes 

require time to “recover” (i.e., retain preflood channel conditions that are commensurate with 

average or dominant flow conditions) (Wolman and Miller, 1960; Andrews, 1980). In situations 

where the recovery time is longer than the average recurrence interval of channel-modifying 

floods, the floods become the controlling influence on channel and valley bottom morphology. 

This characteristic describes the morphologic evolution of the BWR channel in the 20th century 

and, presumably, for past millennia. 

Postdam Geomorphology 

Flow regulation and trapping of bedload sediment associated with Alamo Dam has 

resulted in a flow and sediment transport regime that is tremendously different from the predam 

condition. Prolonged low flows and periodic prolonged moderate flows coupled with extreme 

attenuation of flood peaks have completely altered the nature of the river, leading to rapid and 

widespread expansion of riparian vegetation, channel narrowing, and channel incision (Shafroth 

and others 2002). Narrowing in the early 1970s was not very dramatic, likely because of the 

very low volumes of water released during the filling of the reservoir, including many days of 

zero flow. By the late 1980s and 1990s, however, channels had narrowed dramatically after 

many years with relatively high baseflows and a continued lack of large floods (Shafroth and 

others, 2002) (fig. 16). 

21

Figure 15. Lake Havasu Delta and reach upstream in 1964 showing vegetation growth and 

channel narrowing resulting from smaller spring floods and higher summer flows between 1959 

and 1964. 

Figure 16. Lake Havasu Delta in 1995 showing vegetation growth and continued channel 

narrowing resulting from smaller spring floods and higher summer baseflows caused by the 

presence of Alamo Dam. 

22

In addition to the extreme attenuation of flood peaks, the effect of the dam has been, of 

course, to trap sediment. In the case of the BWR, this effect is particularly acute because few 

large tributaries enter the river below the dam. The BWR, because of its short mainstem length 

through an arid region of moderate relief, receives only sporadic, local sediment input from 

tributaries. The progressive depletion of sediment from the upper reaches of the BWR is an 

inevitable consequence of bedload loss to the reservoir; however, large amounts of fluvial 

sediment are stored in the alluvial valleys and in the channel bed in canyon reaches along the 

middle and lower stretches of the river. It is uncertain what the long-term geomorphic 

consequences will be of the progressive depletion of valley bottom sediment. Ultimately, most 

liberated sediment will be deposited at the river’s terminus at Lake Havasu and in the delta 

upstream. For example, a crosssection about 4.8 km (3 mi) upstream of the Highway 95 Bridge 

that was surveyed before and after moderate reservoir releases in April 1995 shows considerable 

deposition following this event (fig. 17). 

The occurrence of predam-scale flood peaks in the postdam valley condition would likely 

lead to substantial geomorphic changes, including locally significant erosion and corresponding 

proximate (downstream) deposition. Because the dam traps the vast majority of sediment 

transported to the BWR corridor, the cumulative effect of large flows would be increasingly 

erosive as the largely unreplenished fluvial sediments are transported through the system. Thus, 

attempts to restore predam flows without augmenting the corresponding sediment load will 

impart significant changes to the morphology of the valley bottom that will likely increase in 

scale over time. It may be more appropriate to consider altering flows in a way that is more 

consistent with the scale of the existing river corridor (i.e., a system with a drainage area that is 

ca. 1,300 km2

 (500 mi2)). In this sense, the river can be thought of as an “underfit stream,” that 

is, one that occupies a valley bottom with morphological and sedimentological characteristics 

that are related to throughflows of water and sediment of a much larger scale than those 

associated with the present condition. In any case, once an acceptable flow regime is established, 

the river will have to go through a transitional period of uncertain duration, involving 

redistribution of valley bottom sediment and channel realignment. 

Conceptual Framework for Assessing Postdam Geomorphic Change on the Bill Williams 

River: Spikes and Bricks 

Predam hydrographs of typical floods on the BWR are characterized by steep, rapidly 

rising limbs and only slightly less steep falling limbs, thus resembling “spikes.” Flood durations 

were relatively short. Winter floods were often characterized by multiple peaks associated with 

relatively prolonged frontal storm conditions, whereas summer and fall floods were characterized 

by single peaks. In contrast, postdam flood hydrographs have abrupt rises followed by 

prolonged, flat crests and then by abrupt drops, thus resembling “bricks.” Examples of 

representative hydrographs are provided in figure 18. 

In a simple sense, the hydrographs can be viewed as measures of cumulative energy 

expenditure in the valley bottom (Costa and O’Connor, 1995). The sharply spiking hydrograph 

exerts tremendous amounts of energy over a very short period of time, whereas the brick-shaped 

flow exerts a nearly constant rate of low or moderate energy expenditure over a long period of 

time. There are various possible morphologic consequences associated with each type of 

hydrograph that relate to distinct differences in associated erosion and deposition (e.g., 

Huckleberry, 1994; Costa and O’Connor, 1995). Resulting effects on floodplain sedimentation, 

channel migration (rates and processes), and sediment transport are likely to be considerably 

different in each scenario. 

23

Figure 17. Changes in cross-section topography between 1995 and 1996 following a high flow 

event in April 1995. Upper panel (a) shows elevation change in reach 11 (river mile 3). Lower panel 

(b) shows elevation change in reach 10 (river mile 5.4). 

24 
Figure 18. Representative predam and postdam storm hydrographs, Bill Williams River, Ariz. 

Data are daily values for the gaging station “Below Alamo Dam.” 

Differences in the effects of spikes versus bricks relate also to the associated depth of 

inundation of the channel and adjacent flood plain. The brick, because of its duration and 

relatively low magnitude, may be more conducive to channel migration and mobilization of 

fluvial sediment (except maybe for the largest particles) because of prolonged stress applied to 

channel banks. The predam spike involved significant overbank flow, broad expansive flow, 

floodplain scour and deposition, and mobilization of the coarsest fractions of bedload (in the case 

of the largest floods). The predam spike may also have had a more significant effect on riparian 

vegetation through bending, snapping, and uprooting of plants over a larger range of sizes 

(Phillips and others, 1998). The brick hydrograph may have a locally similar net effect with 

respect to progressive undercutting of riparian plants during protracted flow at constant rates and 

lateral erosion, but outcomes are difficult to predict with confidence without field investigation 

and experimentation with different flow types. The persistence of much of the riparian vegetation 

following a series of protracted releases in 1993 and 1995 indicates that these types of flows 

have a relatively minor influence on the overall abundance of riparian plants (Shafroth and 

others, 2002). 

25

Figure 19. Conceptual model of the erosion potential of different types of flood hydrographs on 

the Bill Williams River, Ariz. 

A conceptual model of spike and brick hydrographs in relation to erosion thresholds is 

provided in figure 19. There, each of the hydrographs presented in figure 18 is shown in relation 

to a series of postulated erosion thresholds, or levels of flow/energy expenditure that must be 

exceeded in order to produce some type of morphologic change in the valley bottom (figure 

based on Costa and O’Connor, 1995, fig. 10 therein). The three different erosion thresholds (1, 2, 

and 3 on the figure) are shown to indicate that some changes require larger amounts of energy. 

The three levels could represent, for example, alluvial bank erosion, tributary debris fan erosion, 

and destruction of mature riparian trees, respectively (fig. 19). Developing a quantitative basis 

for such a diagram and an understanding of how the thresholds may vary over time in relation to 

sequential changes during floods and between floods could be achieved by evaluating the effects 

of different flow levels and durations on relevant physical parameters derived from 

photointerpretation, mapping, and focused field studies. 

Beaver Dams 

Beavers can have a surprisingly significant impact on stream channel morphology and 

attendant ecological phenomena (for thorough overviews, see Butler, 1995; Gurnell, 1998; Baker 

and Hill, 2003; see also chapter 6, this report). Between 1995 and 2005, a series of beaver dams 

appeared along most of the length of the BWR. The persistence of beaver dams on the BWR 

reflects their overall durability relative to the erosive power of the flows to which they have been 

exposed. Beaver dams can include an intricate, interlocking array of woody and herbaceous 

vegetation, mud, and rocks (Gurnell, 1998). Over time, dams trap flotsam and sediment, 

therefore increasing in bulk and strength. Additional strength may be provided by vegetation 

26

growing in the bulk of the dam and penetrating it with a root network. We know of no studies 

that evaluate the strength of beaver dams and document the flow parameters required to induce 

beaver dam failure. Several studies, however, do document the occurrence of “catastrophic” 

beaver dam failure and describe significant impacts on downstream channel morphology (Butler, 

1989; Kondolf and others, 1991; Butler and Malanson, 2005). Most studies of the 

geomorphologic effect of beaver dams have focused on their role in influencing channel 

morphology. Beaver dams are efficient sediment traps (Butler and Malanson, 1995) and, in 

series, create a stepped profile along affected rivers and streams. Hydrologically, beaver dams 

act as buffers on flow magnitudes and may contribute to sustained low flows and higher local 

water tables (Gurnell, 1998). Westbrook and others (2006) documented significant effects on 

groundwater flow patterns and hydrologic processes downstream of dams. It is also likely that 

the failure of a beaver dam that has been particularly effective at trapping sediment could initiate 

local incision that could migrate some distance upstream; however, the response to the failure 

would vary depending on local conditions (e.g., valley and channel gradient, size of beaver dam 

and related sediment reservoir, etc.). 

Groundwater Dynamics 

Most investigators agree that the BWR groundwater flow system is a variation on a twolayer aquifer (Vionnet, 1995; Wilson, 2001). The system has two types of reaches: (1)canyon 

reaches in which relatively young and permeable (Quaternary) alluvium overlies fractured 

bedrock and (2)valley reaches in which young and permeable alluvium overlies a set of older, 

somewhat less permeable alluvial deposits that overlie fractured bedrock. The upper unit is the 

Bill Williams alluvial aquifer. It is composed of unconsolidated to loosely consolidated stream 

alluvium and interlayered tributary deposits. The lower unit, the basin fill aquifer, is 

predominantly weakly to moderately consolidated tributary alluvium, but it likely contains some 

mainstem alluvium as well, depending on its age in relation to the history of the river’s 

development. Both of these units overlie a presumptive, highly fractured and faulted, regional 

bedrock aquifer. Previous investigations indicate (assume) that little water is lost to the regional 

aquifer. Canyon reaches along the BWR, thus, are overlaid by relatively thin veneers of the 

alluvial aquifer overlying the regional bedrock aquifer. Wider valley reaches, most importantly 

Planet Valley, are underlaid by a greater thickness of the alluvial aquifer and, likely, the basin fill 

aquifer (fig. 20). 

The most significant groundwater repository along the river occurs in Planet Valley, 

which has the deepest, most highly permeable aquifer downstream from the dam (Harshman and 

Maddock, 1993). Elsewhere, a shallower aquifer persists in sandy alluvium below most of the 

length of the river (except for immediately below the dam where, locally, much alluvial cover 

has been removed or isolated through incision by dam-released flows, and bedrock outcrops in 

the streambed are visible in some sections immediately downstream from Alamo Dam (P.K. 

House, personal observation, 1995–98)). The Planet Valley aquifer serves as a major hydrologic 

buffer on the BWR system and is a critical component of maintaining a reliable baseflow in the 

lower reaches of the BWR (Turner, 1962; Jackson and Summers, 1988; Harshman and Maddock, 

1993; Vionnet, 1995). 

 The buffering effect of Planet Valley on low and moderate flows is to reduce their 

magnitude and attenuate their variability markedly. These effects are a function of the scale of 

the alluvial aquifer, its permeability, and, presumably, underlying bedrock structural control at 

the entrance to the canyon at the west end of the valley (Jackson and Summers, 1988). Thus, 

despite variation in timing and size of low and moderate flows at the head of Planet Valley, the 

27

Figure 20. Surfacewater and groundwater flowpaths for the Bill Williams River, Ariz. 

output at the downstream end typically averages about 0.28–0.31 m3

/s (10–11 ft3/s). During 

large flows, the effects of the groundwater system are overwhelmed by the surface discharge 

(surface discharge greatly exceeds the infiltration capacity of the aquifer). The buffering effect is 

apparent during and following the recession of the flood wave. 

Surface and subsurface flow in the BWR channel contributes to the dynamics of shallow 

alluvial ground water. The alluvial water table levels are influenced by the depth of alluvium 

that overlies the bedrock and the volume of water in the system. On the BWR, in reaches with 

relatively deep alluvium (e.g., Reid Valley, Rankin Valley, and Planet Valley), surface flow 

commonly disappears during much of the year, resurfacing in areas where bedrock is closer to 

the surface and valley widths are narrower. Thus, a given low flow release results in different 

groundwater dynamics in different reaches of the river (fig. 21). Another factor affecting 

groundwater levels is evapotranspiration, particularly during the warmer months of the growing 

season. Finally, alluvial ground water apparently responds to a combination of antecedent 

conditions and current conditions, resulting in different water levels in different years or seasons 

that have the same discharge rate (fig. 21). 

28

Figure 21. Variation in alluvial groundwater levels at three sites along the Bill Williams River 

(BWR Transect 1, BWR Transect 5, BWR Transect 7), during a period of varying low flow releases 

from Alamo Dam, Ariz., from April 1995 to August 1997. 

Water Quality 

Only sporadic data exist pertaining to historical trends in water quality in the BWR. 

Reports available on the Environmental Protection Agency STORET Legacy Web site 

(http://www.epa.gov/storpubl/ legacy/gateway.htm) describe the results of measurements made 

near the dam in 1983. A report from 1996 (Ashby and others, 1996) describes problems with 

excessive concentrations of hydrogen sulfide gas in the discharge from Alamo Lake that resulted 

in degradation of concrete in the outlet works, corrosion of electrical components, and obnoxious 

odors below the outlet. The United Sates Army Corps of Engineers (2003) referred to this 

problem in relation to corrosion of electrical components and in restricting access to the outlet 

works for routine inspections(United Sates Army Corps of Engineers, 2003). The buildup of 

hydrogen sulfide is associated with strongly anaerobic conditions that develop from intense 

thermal stratification and associated isolation of hypolimnetic waters (Ashby and others, 1996). 

Water quality downstream from Alamo Dam is not well documented. The USACE stated 

that the principal concern was the effect of anaerobic conditions in the reservoir and that the 

downstream impact of Alamo Lake’s water quality on the BWR was negligible because of the 

small releases typical of the dam (United Sates Army Corps of Engineers, 2003). Harshman and 

Maddock (1993) summarize the results of water quality measurements below Planet Valley by 

the Arizona Department of Water Resources (dating to 1995). Overall, water quality in the 

reported measurement period 1975-91 was good. Seasonal spikes in overall low levels of nitrates 

and fecal coliform occurred in some years during low flow periods. The late 1970s and early 

1980s data show the most variability (Harshman and Maddock, 1993). Harshman and Maddock 

29

(1993) also reported that the groundwater and surfacewater chemistries were nearly identical in 

during this time. 

There may be two other potential impacts to water quality: (1) irrigation return flows 

from Lincoln Ranch, which might contain higher levels of nutrients and possibly some herbicide 

or pesticide residues, and (2) mine tailings and perhaps other byproducts of mining operations 

that can be transported down washes (e.g., the old Planet Mine and mines in Mineral Wash and 

the Swansea area) and into the BWR. We are not aware, however, of any data quantifying any 

effects of these potential sources. 

30

Chapter 3. Streamflow-Biota Relations: Riparian Vegetation 

By Patrick B. Shafroth1 and Vanessa B. Beauchamp1 

It is widely accepted among riparian ecologists that the composition and dynamics of 

riparian (streamside) vegetation reflect direct and indirect effects of streamflow (e.g., Hughes, 

1997; Friedman and Auble, 2000; Stromberg, 2001). The flow regime is often the driving 

variable in these systems, strongly affecting other aspects of the riverine environment such as 

fluvial processes (e.g., channel widening, meandering) and alluvial groundwater dynamics. 

These factors, overlaid on the geologic and climatic setting, form the physical “stage” on which 

vegetation dynamics play out. 

Aspects of the life history of many riparian plants are associated with different flow 

components (e.g., Mahoney and Rood, 1998; Karrenberg and others, 2002), and much of the 

research on relationships between streamflow and riparian vegetation has been conducted in the 

southwestern United States (Stromberg and others, in press), including studies on the BWR 

(Shafroth and others, 1998, 2000, 2002). Most research to date has focused on cottonwood 

species (Populus spp.; often the dominant floodplain tree), for which relationships between flow 

and the establishment, growth, and survival of vegetation have been well quantified. Literature 

on relationships between flow components and herbaceous species on southwestern systems is 

beginning to emerge (e.g., Bagstad and others, 2005). In this chapter, we summarize some of the 

general connections between flow and riparian vegetation, as well as some details for species 

that have been the subject of more intensive study. 

Floodflows 

Floodflows influence riparian vegetation in various ways. Large floods may have 

sufficient energy to remove or damage woody vegetation from significant portions of the flood 

plain, whereas small floods may only remove or damage vegetation within the highest energy 

flow paths. Floodflows entrain and transport sediment, leading to erosion, deposition (and 

perhaps associated burial of vegetation), and consequent changes to fluvial surfaces. Floods 

often drive dominant fluvial processes (e.g., channel meandering, widening, sediment 

deposition), which in turn determine the nature of substrates upon which riparian vegetation 

becomes established and grows (Scott and others, 1996). Infrequent, large-magnitude flood 

events can result in the establishment of new cohorts of woody vegetation throughout a river 

system (“general replenishment model” sensu Hughes, 1994). Smaller magnitude floods can 

result in more spatially limited establishment of new cohorts (“incremental replenishment 

model” sensu Hughes, 1994). Sediment deposition associated with floods can also elevate flood 

plains, making plants there less susceptible to future flooding and leading to an increased 

importance of autogenic (successional) processes in determining vegetation change on the 

highest surfaces. Changes in elevation above the stream channel or water table can alter water 

availability, perhaps affecting plant growth. 

Floodflows also wet flood plain areas, replenishing soil moisture that riparian plants 

utilize, often depositing fine-textured substrates and promoting decomposition of forest floor 

litter (Ellis and others, 1999). Floods can also play a role in dispersing the seeds of riparian 

 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colo. 

31 

1
species (“hydrochory”; see Merritt and Wohl, 2002). Flood regimes include flows that differ in 

their timing (e.g., winter versus summer or fall), magnitude, frequency, and duration, 

contributing to the high dynamism associated with riparian plant communities on unregulated 

rivers. Over decades, floods of different magnitudes, timing, etc., create a mosaic of sites and 

vegetation patches along rivers (Stromberg, 1998; Lytle and Merritt, 2004). The flow regime 

interacts with the geologic and geomorphic setting so that the same flows may influence 

vegetation differently in different reaches of the same river (Shafroth and others, 1998; Cooper 

and others, 2003; Stromberg and others, in press). 

Baseflows and Alluvial Groundwater Conditions 

Low flows are typically important for maintaining the relatively high water availability 

on which riparian plants depend for growth and survival. Low flows replenish ground water 

through infiltration and percolation. On southwestern U.S. flood plains, differences among 

species in tolerance of low or high soil moisture result in somewhat predictable variation in the 

abundance of dominant species along gradients of water availability (Stromberg and others, 

1996). Saltcedar or tamarisk (Tamarix spp.) is more drought tolerant than is cottonwood or 

willow (Salix spp.) (Busch and Smith, 1995; Horton and others, 2001a, b; Rood and others, 

2003) and thus can dominate river reaches where flows are typically lower and ground water is 

deeper (Stromberg, 1998; Shafroth and others, 2000; Lite and Stromberg, 2005). 

Bill Williams River Vegetation Dynamics 

Riparian vegetation along the BWR is dominated by several woody species common to 

low-elevation southwestern riparian ecosystems, including Fremont cottonwood (Populus 

fremontii) and Goodding’s willow (Salix gooddingii) (figs. 22 and 23), tamarisk (Tamarix 

ramosissima) (fig. 24), seep willow (Baccharis salicifolia), and mesquite (Prosopis spp.). 

Woody vegetation abundance (cover, density, basal area) varies largely as a result of water 

availability, with perennial reaches supporting the most abundant vegetation. Herbaceous 

vegetation tends to be quite sparse, except adjacent to perennial channels where water and light 

availability are high. 

Research on the BWR revealed that periodic floods and summer baseflows are key flow 

components that influence riparian vegetation abundance (Shafroth and others, 2002). Largemagnitude floods can remove vegetation and produce bare sites for new growth, and they likely 

did on a fairly regular basis historically. Riparian vegetation growth and survival can be limited 

by periods of very low flow that do not provide sufficient moisture or replenish alluvial ground 

water sufficiently for the water-demanding riparian plants, especially during typically hot and 

dry parts of the growing season (i.e., May–September; Shafroth and others, 2000). Conversely, 

periods with higher low flows during the growing season, or occasional pulses that replenish soil 

and ground water, can enable relatively high growth and survival of riparian vegetation. 

Vegetation at a particular point in time or space often reflects the sequence of flow events over 

previous years and decades, particularly with respect to the magnitude of floodflows and summer 

flows. 

Vegetation patterns can also be influenced by local geomorphology, which influences the 

effects of flood flows and the availability of ground water. Along the BWR, river reaches differ 

in the distribution and nature of surfaces where vegetation typically grows. For example, canyon 

reaches tend to have narrower flood plains and a less complex arrangement of channels than do 

wider valley reaches, which may have multiple channels and a broad flood plain. Along the 

BWR, there are three significant alluvial basins, in each of these, there are important 

32

Figure 22. Riparian vegetation growing along the Bill Williams River, Ariz.. Fremont cottonwood 

and Goodding’s willow dominate the upper canopy level with tamarisk present in the lower 

canopy. Photograph by Patrick Shafroth, U.S. Geological Survey. 

Figure 23. A canyon reach of the Bill Williams River, Ariz., filled primarily with Fremont 

cottonwood. Photograph by Patrick Shafroth, U.S. Geological Survey. 

33

Figure 24. A floodplain area along the Bill Williams River, Ariz., dominated by shrubby tamarisk, 

with smaller patches of cottonwood and willow trees. Photograph by Patrick Shafroth, U.S. 

Geological Survey. 

differences between the upstream and downstream portions of the basins. Notably, the upstream 

portions of the basins are influent or “losing” reaches, where much of the surface flow infiltrates 

into relatively deep alluvium, resulting in lower alluvial water tables, especially during times of 

low flow and high evapotranspiration. At the downstream end of the basins, bedrock is closer to 

the surface again, and ground water in the basin rises concomitantly, resulting in higher water 

tables and the presence of surface flow during times of the year that surface flow may be absent 

in the upstream reaches. (See chapter 2, this report, for additional discussion.) 

Relationships Between Flow Components and Cottonwood Life History 

A substantial body of research has elucidated relationships between streamflow and the 

germination and establishment of cottonwood species throughout semiarid and arid Western 

North America (for reviews, cf. Braatne and others, 1996; Mahoney and Rood, 1998; Karrenberg 

and others, 2002). Flood timing, elevation of seedling establishment, and flood recession rates or 

availability of soil moisture during the first growing season are all important components of 

cottonwood recruitment and have been synthesized into a “Recruitment Box” model which uses 

aspects of the annual hydrograph to estimate areas of potential woody riparian seedling 

establishment (Mahoney and Rood, 1998) (fig. 25). 

34

Figure 25. The “Recruitment Box” which defines a zone of floodplain elevation and time in 

which cottonwood seedlings can become established if streamflow conditions are favorable 

(modified from Mahoney and Rood, 1998). 

Key aspects of cottonwood germination and establishment requirements are as follows: 

• Seed germination. Successful germination requires the presence of moist mineral substrate 

during the period of cottonwood seed release. 

• Early seedling survival: adequate soil moisture. Continued, sufficient soil moisture is 

needed during the first year to enable young seedlings to survive and to promote root growth. 

This moisture may result from water stored in the soil following the flood event, from 

gradually receding floodwaters, from precipitation, or from a combination of the above. In 

the recruitment box model, Mahoney and Rood (1998) estimate 2.5 cm/d (1 in./d) as a 

generalized maximum rate of water table decline. 

• Later seedling survival: protection from scour, desiccation. Seedlings need to establish at 

an elevation (relative to base level), the low end of which is high enough to be protected from 

high flows that can scour seedlings, and the high end of which is low enough to avoid 

desiccation. Based on a synthesis of previous cottonwood studies, the Recruitment Box 

model includes a successful seedling recruitment zone of 60–150 cm (24–59 in.) above the 

annual low water level (Mahoney and Rood, 1998). 

35

Application of the Recruitment Box to the Bill Williams River 

Seed Germination 

Shafroth and others (1998) examined woody riparian seedling establishment along the 

BWR in the context of the above hydrologic variables following high flow releases from Alamo 

Dam in 1993 and 1995 (fig. 26). One aspect of this study was a germination model, which 

combined water surface levels and seed dispersal phenology to predict which sites supported or 

did not support seedling establishment. This germination model is similar to one component of 

the recruitment box model in that germination locations are determined largely by the 

coincidence of the availability of seed and the availability of moist soil. Seed dispersal 

phenology of Freemont cottonwood and other common, woody, pioneer plants on the BWR is 

presented in figure 27. 

Early Seedling Survival 

Shafroth and others (1998) observed maximum rates of surface water decline at sites 

where seedlings survived for both the 1993 (average 1.2–4.4 cm/d (0.47–1.73 in./d)) and 1995 

cohorts (average 2.8–4.2 cm/d (1.1–1.6 in./d)) on the BWR that were similar to the 2.5 cm/d (1 

in./d) generalized rate of Mahoney and Rood (1998). In experimental studies of cottonwood 

species, growth was reduced, but plants survived declines of up to 10 cm/d (3.9 in./d) (Mahoney 

and Rood, 1991, 1992). Comparisons of drawdown rates across studies can be confounded 

because different reported drawdown values may have been averaged over different periods of 

time, affecting different cottonwood species growing in different soils under different climatic 

conditions (cf. Cooper and others, 1999). Although precipitation during a period of rapid 

drawdown may enhance seedling survival, rainfall is typically very sparse along the BWR during 

the months following seed germination. 

Later Seedling Survival 

Removal of germinants or seedlings by high flow events subsequent to germination or 

establishment can be an important cause of seedling mortality in western riparian ecosystems 

(Stromberg and others, 1991, and 1993; Johnson, 1994; Auble and Scott, 1998; Friedman and 

Auble, 2000). On the BWR, removal of seedlings and saplings established in 1993 by high 

flows late in the same year (ca. 28 m3

/s (990 ft3/s)) and in early 1995 (ca. 188.9 m3/s (6,670 ft3/s)) 

was not a primary cause of mortality at the quadrat scale (Shafroth and others, 1998). The 

apparent inability of flows up to 188.9 m3

/s (6,670 ft3/s) to remove substantial numbers of 

seedlings and saplings illustrates an impact of Alamo Dam, as unregulated flows would have 

been much larger and more destructive to young plants. The absence of high flows from 1995 to 

2004 allowed the continued survival and growth of woody vegetation which became established 

in 1995 at floodplain elevations generally considered too low to allow long-term survival. High 

flows in fall 2004 and winter 2005 removed many of these plants (P.B. Shafroth, personal 

observation). 

Extension of Cottonwood Flow Models to Other Woody Riparian Species 

Most of the work relating aspects of flood hydrographs to the establishment of riparian 

vegetation has focused on cottonwood species; however, a few studies have looked at applying 

these models to other species (Shafroth and others, 1998; Amlin and Rood, 2002; Horton and 

Clark, 2001). It stands to reason that other pioneer plants, which, like cottonwood, typically 

require bare, moist substrates for successful seed germination and seedling establishment, would 

36

Figure 26. Fremont cottonwood seedlings along the Bill Williams River, Ariz. Photograph by 

Patrick Shafroth, U.S. Geological Survey. 

Figure 27. Seed release periods of four woody riparian pioneer species at upstream and 

downstream areas of the Bill Williams River, Ariz. Dispersal at the downstream site (140 m above 

sea level) is represented by a solid line and dispersal at the upstream site (230 m above sea level) 

is represented by open squares. 

37

have similar requirements. The most common other woody pioneer taxa on the BWR are 

Goodding’s willow, tamarisk, and seep willow. Each of these species has a unique period of 

seed dispersal, though they may be overlapping at times (fig. 27). Goodding’s willow disperses 

seed later than does Fremont cottonwood on the BWR and thus tends to germinate either in 

response to later floods or to a later period of the flood recession limb. In the second case, 

Goodding’s willow would generally become established at a lower elevational position within 

the bottomland than would Fremont cottonwood (closer to ground water but more vulnerable to 

scour from future high flows). Seed dispersal of the nonnative tamarisk species (Gaskin and 

Schaal, 2002) begins later than that of Fremont cottonwood on the BWR (Shafroth and others, 

1998—though not all rivers in Western North America—e.g., Cooper and others, 1999) and 

continues throughout the growing season and into fall months. Thus, tamarisk is not nearly as 

dependent as cottonwood or willow on precisely timed floods for establishment. 

Flow Connections to Taxa with Warm Season Phenology 

Less is known about the importance of floodflows that occur during the summer or fall 

months, although flows in these seasons historically made up approximately one-half of the 

largest magnitude floods in a given year on the BWR. Some effects that are understood relate to 

the reproduction of species whose phenology is tied to these later precipitation events. For 

example, the seeds of mesquite trees germinate in response to late summer moisture, whether 

from precipitation or flooding (Stromberg and others, 1991). Floods may help promote mesquite 

seed germination by scarifying seeds and by providing moisture. The growth of mesquite trees is 

also somewhat tied to these summer storms, and floodwaters at this time of year can promote 

more vigorous growth by providing a supplement of soil moisture and maintaining relatively 

high water tables. Mesquite growth and productivity are strongly influenced by the availability 

of ground water, with maximum growth occurring in floodplain settings where water tables are 

typically 5–6 m (16.4–19.7 ft) below the surface (Stromberg and others, 1992, 1993). 

Flow Connections to Herbaceous Plants 

The distribution and abundance of herbaceous riparian species are also linked to 

streamflow. Both the disturbance created by flood flows and the elevated base flows which 

follow flood events are important in regulating the distribution and abundance of herbaceous 

species in the riparian zone (fig. 28). The bare, moist sites that are favorable for pioneer woody 

species establishment are also favorable for a large number of annual or biennial herbaceous taxa 

adapted to these environments (“ruderals” sensu Grime, 1979). Annual species typically have a 

rapid and positive response to flooding because disturbance clears suitable germination sites. 

Many riparian perennial species have the capacity for clonal spread and can also respond 

positively to flooding, even after scouring floods have damaged vegetation. After the flood pulse 

passes, elevated baseflows allow for increased survival and growth of many herbaceous riparian 

species. Generally, the superposition of the effects of various floods over decades results in a 

complex set of microsites, with niches for a variety of plant taxa. 

A recent study of herbaceous vegetation response to flooding and rainfall on the San 

Pedro River Ariz., found that flooding had a positive effect on herbaceous richness and cover 

through a combination of disturbance and water availability effects (Bagstad and others, 2005). 

Annual species had a strong positive response to flooding, presumably because of removal of 

litter and existing understory vegetation and increases in light availability. Perennials responded 

less strongly than did annuals to flood disturbance. Scouring forces locally reduced richness of 

perennial vegetation, but the clonal habit of many of these species allowed for survivorship and 

vegetative spread after burial and fragmentation caused by flood disturbance. Alluvial water 

38

Figure 28. Herbaceous vegetation along the Bill Williams River, Ariz. Photograph by Patrick 

Shafroth, U.S. Geological Survey. 

tables and stream flows remained elevated for several months after the flood, which greatly 

benefited hydric (wet site) annual and perennial species through increased flow permanence and 

increased dispersal (hydrochory) of seeds and vegetative propagules. Mesic (moist site) species 

(both annual and perennial) responded to a combination of flood soil wetting, groundwater 

recharge, and rainfall, while xeric (dry site) species responded primarily to increased rainfall. 

Dewatering of streams through flow regulation, water diversion, or groundwater pumping 

can lower baseflows and increase the frequency and extent of intermittent flows, causing shifts in 

the abundance and composition of herbaceous vegetation. Stromberg and others (1996) found 

that many herbaceous riparian species had a narrow tolerance range for depth to ground water, 

and in particular, the abundance of obligate wetland herbaceous species (such as bulrush (Scirpus 

spp.)) declined sharply at groundwater depths below 0.25 m (0.82 ft). Along the San Pedro 

River, cover and richness of herbaceous vegetation declined continuously across a gradient of 

permanent to intermittent flow. Hydric species were the most negatively affected by increases in 

flow intermittency, while mesic perennials (such as bermuda grass (Cynodon dactylon) increased 

at sites with intermittent flow (Stromberg and others, 2005). 

Summaries of flow-biota relationships for woody plant establishment, plant growth, and 

plant diversity are presented in table 5. 

39

Table 5. Summary of flow-biota relationships for riparian vegetation in the Bill Williams River Corridor, Ariz. 

Process Moderate to large floods 	 Small floods High to moderate baseflows 

Woody plant establishment 

Winter-spring floods and associated drawdowns create conditions suitable for recruitment of woody pioneer vegetation and may result in extensive areas of new establishment. The frequency of flows that result in successful establishment largely determine mix of age classes present along the river. (5–10 years between successful establishment events is common along southwestern rivers.) Monsoon and tropical season floods stimulate reproduction of a suite of species with phenology tied to these seasons, such as mesquite. 

Small-magnitude floods may result in limited areas of new establishment, often adjacent to the main channel. 

High baseflows, particularly during the summer months after a recruitment event, will promote seeding survival and growth. 

Low bestablthe wrootinseedlidesicc

Plant growth	 Floods during all seasons replenish soil 

moisture and alluvial ground water, enhancing growth of riparian plants. Floods during dormant season (ca. November–January) are least effective in this regard. 

Small floods replenish Stable, high baseflows nutrients and fine sediments enable rapid growth of on the flood plain, which riparian vegetation. promotes growth of woody overstory and herbaceous understory species without widespread destruction of existing vegetation. 

Lowewater riparipromodistribbetter surviv

40


Plant diversity Floods during all seasons create a Small floods create a mosaic High baseflows allow for Low b

diversity of sites suitable for a diverse of patches on the flood plain the presence of obligate dynammix of herbaceous and woody plant which differ in time since wetland species and some species. disturbance and support generally higher species exam

vegetation in a range of richness. speciesuccessional stages. 
Chapter 4. Streamflow-Biota Relations: Birds 

By Charles van Riper III1 and Charles E. Paradzick2 

Riparian ecosystems, and particularly the BWR corridor, provide critical resources such 

as water, vegetation, and abundant food for resident and migrating birds in the xeric environment 

of the southwestern United States (Grinnell, 1914; Rosenberg and others, 1991). Riparian 

habitat makes up less than 1 percent of the landscape in the Southwest yet supports more 

migrating and breeding birds than do all other western habitat types combined, with 

approximately half of the breeding species being riparian obligates (Anderson and Ohmart, 1977; 

Johnson and others, 1977; Knopf and others, 1988). Riparian areas also serve as critical stopover 

habitat for neotropical migratory birds, supporting 10 times more birds than do surrounding 

uplands during migration (Stevens and others, 1977). Aspects of the life history and habitat 

relationships of many southwestern birds are tied to both floristic composition and structural 

characteristics of the riparian plant community. Thus, the spatial and temporal distribution and 

abundance of riparian birds along the BWR are largely a function of interrelationships between 

streamflow, fluvial processes, and the riparian plant community (vegetation dynamics are 

discussed in detail in chapter 3, this report; see also Scott and others, 2003). 

General Responses of Major Bird Groups to Flood Flows and Low Flows 

This chapter discusses different bird groups and how they are generally influenced over 

the annual cycle by flow components of the BWR. Over 300 bird species have been recorded 

along the BWR corridor, these species are listed in appendix B. This list is stratified first by 

taxonomic group and then broken down by general bird-habitat affiliation. By placing birds 

within general habitat affiliations, we are better able to summarize relationships between 

streamflow and avian life histories across multiple species. The general habitat affiliations that 

we utilized were (1) deepwater birds, (2) shallowwater birds, (3) predators/scavengers, (4) 

gamebirds, and (5) small forestbirds (see appendix B). Streamflow-biota relationships for bird 

species found along the BWR are summarized in table 6 at the end of this section. 

Deepwater and Shallowwater Birds 

Many of the bird species and much of the avian biomass along the BWR corridor are 

concentrated in the delta region. The two bird groups most greatly influenced in this stretch of 

the river corridor are the deepwater and shallowwater birds. Riverflows work in concert with 

Lake Havasu water levels to influence numbers and distributions of birds within these two 

groups. 

The deepwater birds respond primarily to resources either transported down the BWR or 

deepwater resources influenced by river and delta nutrient upwelling. These birds also respond, 

in a secondary fashion, to deeper water regions along the river behind beaver dams and in deep 

scoured pools. During high flood events, water levels and turbidity increase in the stream and 

especially in the delta. The turbidity particularly influences deepwater birds because underwater 

prey is hidden. For example, the numbers of western grebes (Aechmophorus occidentalis) 

1

U.S. Geological Survey, Southwest Biological Science Center, Sonoran Desert Research Station, Tucson, Ariz. 2

Salt River Project, Environmental Division, Phoenix Ariz. 

41 
decrease during high flow events because fish prey become more difficult to find. This effect of 

high flow events also occurs for mergansers (Mergus spp.) and other diving deepwater birds. 

The shallowwater birds respond to variable flow regimes principally through the addition 

(or deletion) of available foraging habitat provided by exposed sandbars along the BWR. There 

is some use of shallowwater areas and exposed sandbars along the entire river, but the majority 

of shallowwater birds are found in the delta region. Low flows in the BWR and lower lake 

levels would proximately enhance the use of the river and delta region by shallowwater birds 

through the exposure of more sandbar habitat; however, higher flows and flood events that are 

ultimately needed to bring sediment for sandbar building negatively influence shallowwater bird 

numbers on the BWR. These higher flow events remove some of the dense stands of cattail 

(Typha spp.), bring outside nutrients for the shallowwater birds’ prey base, and create shallow 

pools and backwater marshes along the entire river corridor, thus ultimately enhancing 

shallowwater-bird habitat. 

Predators and Scavengers 

The avian group that is most greatly influenced by variable flows of the BWR is the 

predators and scavengers group. These birds respond directly to the large biomass of resources 

along the entire river drainage and especially in the delta. Avian predator numbers increase as 

available prey (birds, herps, mammals) biomass increases, and their numbers decline as prey 

biomass is reduced or disappears from the BWR. Thus, avian prey numbers and biomass along 

the entire riparian corridor of the BWR corridor directly influence avian predator densities. Any 

positive relationships between flow components and bird numbers will enhance numbers of 

predator and scavengers. Large flood events that cause numbers of bird, herp, and mammal 

deaths will enhance resources for scavengers along the entire drainage. Potential prey that is not 

killed but that is negatively influenced by large flood events will be easier for avian predators to 

capture. Prey that is killed during the high flows will increase food resources for scavenger birds 

(e.g., turkey vultures (Cathartes aura)). 

Gamebirds 

Of the remaining two avian groups, gamebirds are probably directly influenced the least 

by flow components. The great majority of upland gamebirds (e.g., quail), utilize the riparian 

corridor directly or indirectly for water and food resources (such as seeds), as well as for escape 

cover. Thus, steady baseflows can provide a stable water resource. Floods that inundate flood 

plains and terraces can enhance herbaceous vegetation productivity and increase associated food 

resources such as seeds or insects. 

Forestbirds 

The last group is forestbirds, which contains the greatest number of species and is 

probably influenced the most by BWR flow. Generally, infrequent, large-magnitude flood 

events can result in vegetation destruction and removal, channel migration, and floodplain 

sediment erosion and deposition. While these processes may remove preferred habitat of some 

avian taxa, the resulting reworked flood plain can create a more complex landscape pattern of 

available habitats, including sites for establishment of new cohorts of woody vegetation, and 

interspersed remnant stands of mature vegetation. Maximizing habitat heterogeneity both 

horizontally (forest patch composition) and vertically (foliage height diversity) across the 

landscape can increase available avian niches. Smaller magnitude floods can result in more 

spatially limited establishment of new cohorts, which tends to be (proximally and ultimately) 

more beneficial for avian communities. Between flood events, perennial baseflows benefit 

42

riparian-associated birds by providing stable sources of water and more suitable aquatic habitat 

for insect reproduction. The forestbirds group contains the greatest number of species of 

concern, including the endangered southwestern willow flycatcher (Empidonax traillii extimus) 

and the yellow-billed cuckoo (Coccyzus americanus occidentalis), both of which are discussed in 

more detail below. 

Specific Examples of Flow-Avian Biota Relationships in the Bill Williams River 

Southwestern Willow Flycatcher 

The willow flycatcher (Empidonax traillii) is a riparian obligate songbird that breeds 

throughout much of North America. The southwestern willow flycatcher (SWFL) is one of 

several recognized subspecies (fig. 29). The current SWFL breeding range includes southern 

California (from the Santa Ynez River south), Arizona, New Mexico, extreme southern portions 

of Nevada and Utah, and extreme southwest Colorado. Records of probable historical breeding 

SWFL exist for western Texas, but recent surveys are lacking. Similarly, breeding records for 

Mexico are rare and are restricted to extreme northern Baja California del Norte and Sonora 

(Unitt, 1987; Browning, 1993). 

Unitt (1987) was the first to recognize the precipitous decline in SWFL populations 

throughout its range, and he noted that the greatest reductions were probably in Arizona. Causes 

of declines were primarily due to the loss, fragmentation, and modification of riparian habitats. 

In response to the few fragmented populations remaining, the U.S. Fish and Wildlife Service 

listed the species as endangered in 1995. Critical habitat was designated in 2005 (U.S. Fish and 

Wildlife Service, 2005), after the first rule was set aside because of a court challenge in 2001. A 

recovery plan was completed for the species in 2002; the plan summarizes information 

concerning the bird’s habitat preferences, the role hydrology plays in habitat formation and 

persistence, and the actions managers can take to improve riparian conditions for the bird. One 

recovery management action identified was the need for the evaluation of river regulation on 

recruitment and persistence of pioneer riparian forest and the consideration of flows that could 

increase available SWFL nesting habitat. 

The SWFL breeds in dense riparian habitats along rivers, streams, wetlands, and reservoir 

deltas. The largest known populations occur at Cliff-Gila Valley New Mex., Roosevelt Lake 

Ariz., the lower San Pedro River and nearby Gila River Ariz., the Rio Grande in both Colorado 

and New Mexico, and the San Luis Rey River Calif. Riparian forest on the BWR, including the 

upper end of Alamo Lake, has supported nesting birds since 1994. Alamo Lake Delta contains 

one of the largest populations in Arizona (Paradzick and Woodward, 2003). 

Qualitative descriptions of low elevation (<900 m (<3000 ft)) riparian forest suitable for 

SWFL nesting include dense stands of trees with high canopy cover and with close proximity to 

standing water or saturated soil (Sogge and others, 1997). The size of vegetation patches that 

support SWFL can vary widely but generally is less than 10 ha (24.7 acres) (Paradzick and 

Woodward, 2003). Quantitative analysis of vegetation requirements has better defined these 

attributes. SWFL nesting habitat in Arizona has been studied at three scales: landscape, patch, 

and within-patch. Although all three scales are interrelated, each suggests an important aspect of 

habitat needs. At the large spatial scales, Hatten and Paradzick (2003) found that SWFL nest 

sites in Arizona had greater foliage density (forest) and edge in the 4.5-ha (11.1-acre) 

neighborhood size, and greater amounts of flood plain or flat terrain within the 41-ha (101.3 

acre) neighborhood size compared to random unoccupied sites. These selection patterns were 

similar to those found by Brodhead (2005) along the Rio Grande River. At the patch scale, 

SWFL preferred forest stands with high density (500–1,300 stems/ha (1,235–3,212 stems/acre)) 

43

Figure 29. Southwestern willow flycatcher (Empidonax traillii extimus). Photograph by Alex 

Smith, Arizona Game and Fish Department. 

small (5–15 cm diameter at breast height (dbh) (2–6 in dbh)) tamarisk or willow stems, high 

foliage density in the mid (4–7 m (13–23 ft)) and upper (7–9 m (23–30 ft)) canopy, and high 

canopy cover (>85 percent), and most occupied patches were adjacent to water (Paradzick, 

2005). Within patches, nests were placed closer to water and canopy openings and had higher 

stem densities of young trees compared to unoccupied plots (Allison and others, 2003). 

Together these data provide a picture of essential habitat components for SWFL. Broad 

flood plains allow for a mosaic of dense forest patches interspersed with openings and, possibly 

because of lower frequency of flood scour compared to canyon reaches, for the persistence of 

dense forests over time. Such configurations may be important for adult and juvenile SWFL 

refuge, foraging, and dispersal (Hatten and Paradzick, 2003). The preference for young dense 

forest provides the dense canopy layer and high midstory foliage for nesting. The importance of 

water or saturated soil may not only influence tree growth, vigor, and stand and foliage density 

(Horton and others, 2001a, b), which could increase nest concealment and reproduction, but may 

also provide a cooler within-patch microclimate and increase the local insect food base. 

The U.S. Fish and Wildlife Service (1995) listed tamarisk as a cause of SWFL decline; 

however, regional data on SWFL show that over 50 percent of the territories were located in 

stands dominated or co-dominated by tamarisk (Sogge and others, 2003). Willow was the most 

common native plant associated with occupied habitat, while tamarisk was the most common 

exotic. Other native vegetation taxa can include seepwillow, cottonwood (usually in the 

overstory), or box elder (Acer negundo). While cottonwood sometimes occurs in SWFL patches 

44

and has been used as a nest tree, Paradzick (2005) found that SWFL selected willow or tamariskdominated patches but not cottonwood-dominated patches. SWFL nest success and reproduction 

monitoring (A. Tudor, oral commun.), insect foodbase research (Durst, 2004), and physiology 

data (Owen and others, 2005) all point to tamarisk as viable nesting habitat. 

Flow regimes and associated fluvial processes that produce vegetation that fit the 

descriptions above are likely to benefit SWFL (Graf and others, 2002). The preference for young 

dense forest and floodplain mosaics highlights the importance of reoccurring recruitment events 

to create and sustain habitat over the longterm. Along low-elevation rivers in the Southwestern 

United States, young willow trees of this size (5–15 cm (2–6 in)) are typically 4–10 years old, 

while tamarisk are slightly older (7–20 years old). Thus, every 5–10 years, sufficiently large 

floodflows with a recession limb, the tail-end of which is timed to coincide with willow and 

tamarisk seed dispersal (April–May on the BWR; fig 27), should allow for establishment of new 

cohorts. The timing of the drawdown should position willow and tamarisk at relatively low 

topographic positions, enhancing the creation of edges adjacent to water. 

The floodflows that are required to create sites suitable for establishment of new woody 

riparian vegetation may also destroy SWFL habitat, causing local extinctions. These local 

extinctions and/or migrations to new suitable habitats are processes that may simply be a natural 

part of the metapopulation dynamics, which appear to be characteristic of SWFL (U.S. Fish and 

Wildlife Service, 2002). Similarly, moderate summer flood events that inundate the flood plain 

but do not necessarily destroy SWFL occupied patches could reduce ground predator (e.g., 

snakes, lizards) densities and thus increase nest success, a pattern observed at other willow 

flycatcher breeding sites (Cain and others, 2003). 

The strong tie of the SWFL to hydrological conditions is also suggested by Johnson and 

others (1999), who noted complete SWFL reproductive failure in response to lack of river flow 

in New Mexico. On the BWR, relatively large low flows would be likely to benefit SWFL by 

supporting dense growth of the vegetation, by maintaining high soil moisture, and by providing 

open water habitat for insect production. Conversely, SWFL habitat would be unlikely to occur 

where baseflows are too low to support perennial flow (standing water) and therefore cause a 

lowering of groundwater tables at established sites, limiting the establishment or persistence of 

dense vegetation. 

Yellow-Billed Cuckoo 

The western yellow-billed Cuckoo was historically distributed across a vast geographic 

area of the western United States, east to Texas and in northern Mexico (Hughes, 1999). Birds 

are often found nesting and breeding in riparian habitats composed of gallery cottonwood-willow 

and mixed broadleaf species, as well as in mesquite bosques. Historically, yellow-billed cuckoos 

were considered widespread and locally common in Arizona (Swarth, 1914; Phillips and others, 

1964) and typically nested in mature riparian forests and woodlands along central and southern 

Arizona drainages (Hamilton and Hamilton, 1965). 

Habitat structure appears to be a key factor for retention of the yellow-billed cuckoo in 

the Southwest. study in California demonstrated that yellow-billed cuckoos have significant 

macro-habitat requirements and that size of the habitat patch is extremely important. Using 

patch size, Gaines and Laymon (1984) identified four classes of habitat quality (table 7). Further 

research by Laymon and Halterman (1989) found that configuration of the habitat was important 

for the yellow-billed cuckoo, and they proposed “primary” and “secondary” habitat quality 

classes. In Arizona, the western yellow-billed cuckoo has been associated with broadleaf 

deciduous riparian habitat dominated by cottonwood and willow (Hamilton and Hamilton, 1965; 

Gaines, 1974; Gaines and Laymon, 1984; Halterman, 1991). Habitat destruction, degradation, 

45

and fragmentation have caused a range retraction and population declines of the yellow-billed 

cuckoo throughout Arizona (Laymon and Halterman, 1987). As a result, the western yellowbilled cuckoo is a candidate for listing as an endangered species with the U.S. Fish and Wildlife 

Service (2001). The yellow-billed cuckoo is probably the avian species of most concern along 

the BWR corridor. This bird requires large (80+ ha) patches of mature cottonwood and willow 

trees with an understory of smaller trees and shrubs. A recent analysis of the projected 

distribution of this bird throughout the state shows that riparian habitat is an essential component 

of determining if this sensitive species is able to occupy an area (fig. 30). 

The BWR corridor currently serves as a critical core area that sustains yellow-billed 

cuckoo residency (fig. 31). More importantly, the vegetation structure of the river corridor 

allows this bird to find suitable breeding habitat each year. In other areas of the State where 

smaller riparian patches occur (e.g., Verde Valley), yellow-billed cuckoos do not breed every 

year (M. Johnson oral commun.). 

Components of the vegetation that provide resources for this bird are the microclimates 

created by a mature, closed canopy cottonwood-willow forest with a structured understory. 

Yellow-billed cuckoos also use tall, dense, tamarisk-dominated habitat. This type of habitat 

allows birds to successfully nest at a time of the year (June), when it is too hot for many 

passerine species to successfully raise young. This later nesting is mandated by the abundance of 

prey. The yellow-billed cuckoo is a sit-and-wait predator and, as such, relies on larger food 

items such as cicadas and large moth larvae. These food items emerge and become available 

later in the year than do many other insect species. 

Flow regimes and associated fluvial processes that produce large tracts of closed canopy, 

largely cottonwood-dominated forest are necessary to benefit the yellow-billed cuckoo. Flows 

that enable seedling establishment of cottonwood are periodically necessary (see chapter 3, this 

report), but yellow-billed cuckoo habitat might benefit from slightly longer intervals between 

floods so that trees have time to grow and form a closed canopy. Mortality of trees in the 20- to 

40-year age range appears to be relatively common along the BWR (Shafroth, Andersen, van 

Riper, personal observation). Thus a 20–40 year recruitment flood recurrence interval might 

represent a bare minimum recurrence interval to support yellow-billed cuckoo habitat. On the 

BWR, much of the best cuckoo habitat occurs where the moderate floods released from Alamo 

Dam (ca. 169.9–198.2 m3

/s (6,000–7,000 ft3/s)) create some new establishment sites but do not 

destroy much of the former vegetation. Yellow-billed cuckoo habitat would benefit from 

moderate to large baseflows, which would maintain relatively high alluvial water tables and 

water availability to support dense cottonwood forest and perhaps higher insect abundances. 

46

Table 6. Summary of flow-biota relationships for bird species found along the Bill Williams River, Ariz. 

Biota Resilience Moderate to large floods Small floods High to moderate 

to floods baseflows 

Deepwater and shallowwater 

Moderate Mortality of deepwater birds 

may increase as floods create 

Less of an effect than large floods on deepwater birds. 

May inundate sandbars for long periods, leading 

birds turbidity in the delta, which Shallowwater birds lose to increased mortality and 

hides prey. Shallowwater birds access to foraging habitat on emigration because of temporarily lose access to sandbars but habitat is loss of foraging habitat. foraging habitat on sandbars, but ultimately replenished by floods ultimately build sandbars flooding. and bring in nutrients. 

Predators and High Predator and scavenger Response of predators and Response of predators and 

scavengers populations should increase 

because of increased mortality of 

scavengers to small floods is dependent on the response 

scavengers to moderate floods is dependent on the 

potential food resources. Prey of prey biomass. response of prey biomass will be negatively affected by and number of organisms large floods and will be easier to killed in the flood event. capture. 

Gamebirds High May increase mortality through Would enhance population Would benefit growth and 

the removal of escape cover growth because of increases survivorship because of and/or drowning but ultimately in cover and food increases in cover and will build cover and increase availability without food (seed) availability. food availability through widespread destruction of increased seed production. cover. 

Forestbirds High Loss of trees will have an Loss of understory Provide a constant water 

immediate negative impact on vegetation patches will source and deliver populations but is needed to negatively affect nutrients to vegetation. rejuvenate vegetation that birds populations, but impacts Nutrient availability rely on for cover and food will not be as great as with cascades up the food resources. large floods. May rejuvenate pyramid, increasing prey 

herbaceous vegetation over biomass and creating time. better breeding and stop- 

over habitat. 

47 
Table 6. Summary of flow-biota relationships for bird species found along the Bill Williams River, Ariz. —Contin

Biota Resilience Moderate to large floods Small floods High to moderate 

to floods baseflow 

Willow High Local breeding areas will be Fewer local breeding Promotes vegetation flycatcher eliminated because of vegetation 

removal, but destruction of gallery 

territories will be disturbed from vegetation removal, but 

growth and increases habitat for flycatchers. 

forests will lead to an increase in recruitment of vegetation will Provides open and shrubby undergrowth or dense, eventually provide new standing water habitat that young woody vegetation which is habitat. has a positive impact on the preferred habitat for this prey base. species. 

48


Yellow Moderate Openings in closed canopy forest Birds will be less effected than Maintenance of alluvial billed after flooding will negatively affect by large floods. They benefit water tables will increase Cuckoo population growth. through the creation of some vegetation growth and 

new establishment sites provide more food and without widespread habitat. destruction of existing vegetation. 

Table 7. Yellow-billed cuckoo habitat requirements as identified by Laymon and Halterman (1989). 

Habitat quality Size (hectares) Width (meters) 

Unsuitable <15 100 Marginal 20–40 100–200 Suitable 41–80 >200 Optimal >80 600 
Figure 30. Yellow-billed cuckoo habitat throughout Arizona as predicted through Advanced 

Very High Resolution Radiometer (AVHRR) satellite imagery, clipped to perennial watercourses 

and buffered to 500 m (from Wallace and others, written commun.). 

49

Figure 31. Advanced Very High Resolution Radiometer (AVHRR) satellite imagery predicting 

yellow-billed cuckoo habitat, overlaid with known yellow-billed cuckoo locations on the Bill 

Williams NWR (from Wallace and others, written commun.; Laymon and Halterman, 1989; K. Blair, 

oral commun.). 

50

Chapter 5. Streamflow-Biota Relations: Fish and Aquatic 

Macroinvertebrates 

By David A. Lytle1 

Of all the organisms in the BWR ecosystem, fish and aquatic invertebrates are the most 

immediately affected by flood, drought, and other flow events. For this reason, sensible 

management of Alamo Dam flows is essential for maintaining healthy macroinvertebrate 

populations and reestablishing native fish species. Unfortunately there is no single “optimal” 

flow regime that would favor all native taxa simultaneously. The BWR aquatic fauna appears to 

be a mixture of species that vary greatly in their flow regime requirements. For example, some 

taxa require fast-flowing riffles, while others need standing ponds; some species have fast life 

cycles and recover rapidly from floods, while other species may be slow to recover. This section 

reviews the types of aquatic habitat that have occurred historically on the BWR, catalogs the 

different taxa occupying these habitats, and then discusses the management options for 

maintaining healthy populations of these organisms. 

Relationships Between Habitat and Species Composition 

Lotic Versus Lentic Habitats 

 The presence of flowing (lotic) versus standing (lentic) water can be a major control on 

the species composition of both fish and aquatic macroinvertebrates. In turn, the distribution of 

lotic versus lentic habitats is a direct consequence of flow regime because floods can convert one 

type to the other via removal of beaver ponds (lentic to lotic) and overbank flows (creation of 

side-channel lentic habitats). Because of these physical-biological connections, different flow 

regimes have the potential to favor very different aquatic communities. In general, aquatic 

species are primarily adapted to either a lotic or lentic existence, but some BWR taxa are found 

in both habitats (appendix C). 

Lentic habitat types 

Four major types of lentic habitat occur on the BWR, and each has the potential to harbor 

different aquatic species: 

1.	 Beaver ponds. Ponds associated with beaver dams typically have a permanent hydroperiod, 

although some may dry during extended droughts (fig. 32). Characteristic beaver pond 

communities include exotic species such as largemouth bass (Micropterus salmoides), green 

sunfish (Chaenobryttus cyanellus), carp (Cyprinus carpio), and crayfish, as well as native 

aquatic insects such as odonates, dytiscid beetles, and aquatic Hemiptera. Stable baseflows, 

abundant riparian vegetation, and rarity of high-magnitude floods have apparently favored 

beaver populations on the lower BWR in recent years. 

2.	 Lakes. Both Lake Havasu and Alamo Lake have a permanent hydroperiod, although water 

levels fluctuate depending on rainfall and season. Many exotic fish species have been 

introduced into Alamo Lake intentionally, accidentally, or illegally, probably because of easy 

access and popularity with the public. At least 15 exotic fish species have been found in 

Oregon State University, Department of Zoology, Corvallis, Oreg. 

51 

1 
Alamo Lake (appendix C); these species are potential source populations for invasion


downstream in the BWR. 


3.	 Channel pools. Channel pool habitats may occur seasonally in the active river channel 

because of reduction in baseflow during dry seasons. This transformation from lotic to lentic 

can also alter local community structure. In smaller montane streams typical of the BWR 

headwaters, lotic communities predominate in winter and consist of species that utilize 

flowing water such as baetid mayflies (Baetidae), net-spinning caddisflies (Hydropsychidae), 

and blackflies (Simuliidae) (Bogan and Lytle, written commun.; Bogan, 2005). During dry 

periods in late spring and summer, these stream reaches “cook down” to stream pools 

dominated primarily by lentic taxa such as dragonflies, dytiscid beetles, and a variety of 

hemipteran species. It is possible that the seasonal oscillation from lotic to lentic habitats 

may enhance species diversity through temporal coexistence (Bogan & Lytle, written 

commun.; Bogan, 2005). Some species do well in both lotic and lentic conditions: the native 

speckled dace (Rhinichthys osculus) can tolerate the high temperatures and low oxygen levels 

associated with lentic habitats, but it also requires fast-flowing conditions for spawning 

(John, 1963, 1964). 

4.	 Off-channel habitats. The wide flood plain in many reaches of the BWR has produced 

remnant channels that are only wetted after flood events. At present no data are available 

concerning which species use these habitats in the BWR and whether there are off-channel 

“specialist” species not found in other lentic habitats. Off-channel habitats could be 

biologically important because their seasonal hydroperiod can exclude fish, or at least limit 

fish abundances to low levels. Exclusion or limitations of fish abundances could provide 

habitat for invertebrate species that are adapted to fish-free environments (McPeek, 1989, 

1990) or that require fish-free habitats to complete portions of their life cycle (Smith and 

Larson, 1993). 

Figure 32. Beaver dam activity, and lentic habitat created by a beaver dam. Photograph by 

David Lytle, Oregon State University. 

52

Lotic Habitat Types 

Lotic habitats occur seasonally or when discharges from Alamo Dam are sufficiently high. Three 

general types of lotic habitats are apparent within the BWR drainage: 

1.	 Montane bedrock streams. These small headwater streams may fluctuate from high flows in 

winter to very little moving water during summer. Hydroperiod is permanent in reaches 

where the water table is forced up by bedrock. These habitats occur upstream of Alamo Dam 

and harbor an exceptionally rich diversity of benthic macroinvertebrates (Lytle, 2000). 

Anurans such as the native lowland leopard frog (Rana yavapaiensis) can also be found in 

spring-fed streams such as Peoples Canyon on the Santa Maria River, although chitrid fungus 

remains a threat to these populations (Bradley and others, 2002). Diversity is high partly 

because these small streams have not been invaded by exotic fish or crayfish. As such these 

streams could be important population reservoirs for the main stem BWR, although Alamo 

Lake may present a barrier to downstream dispersal of many taxa. 

2.	 Beaver dam outflows. When baseflows are sufficiently high, flowing water occurs at the 

outflows of beaver dams. In some areas where beaver dams are dense, these outflows can be 

very short, flowing less than a meter into the next beaver pond. The lotic nature of these 

outflows, however, may harbor different taxa than do the pools (simuliid flies, baetid 

mayflies, and nonnative red shiners (Notropis lutrensis)). 

3.	 Channel stream habitats. These are the flowing-water counterparts to the channel pool 

habitats described above (fig. 33). 

Figure 33. Typical lotic habitat in a sand-dominated reach, lower Bill Williams River, river mile 

5.4. Photograph by Patrick Shafroth, U.S. Geological Survey. 

53

Overview of Aquatic Organisms in the Bill Williams River Below Alamo Dam 

Aquatic Invertebrates Below Alamo Dam 

Benthic macroinvertebrates were surveyed qualitatively around 1980 (United States Fish 

and Wildlife Service, 1981) and quantitatively during the 1990s (Vinson, 1994, 1995, 1996, 

1999). Most of these surveys targeted lotic riffle habitats and thus do not characterize the 

macroinvertebrate communities in lentic beaver pond habitats. The BWR aquatic invertebrate 

fauna is typical of low-elevation Arizona rivers that possess unstable (nonbedrock) benthic 

substrates. Mayflies such as Baetis spp. (hereafter interpreted to include the new genus Fallceon) 

can be extremely abundant where suitable flowing-water riffles are present, reaching densities 

above 10,000/m2

 at some sites (Vinson, 1999). Other taxa that reach high abundances locally 

include the microcaddisfly Hydroptila, the baetid mayfly Acentrella, and chironomid fly larvae. 

Fish Below Alamo Dam 

The fish fauna below Alamo Dam is dominated by nonnative species including 

largemouth bass, green sunfish, and red shiner (United States Fish and Wildlife Service, 1992). 

Nonnative mosquitofish (Gambusia affinis) are found throughout the BWR drainage (United 

States Fish and Wildlife Service, 1992) and are abundant in beaver dam ponds below Alamo 

Dam (M. T. Bogan, personal observation, 2004). Historically, the native endangered razorback 

sucker (Zyrauchen texanus) may have used the lower BWR during periods of sustained high 

flows (C. Minckley, oral commun, 2004), but reduced flows on the BWR and reduced razorback 

populations in the main stem Colorado River have prevented their use of the BWR in recent 

decades. The native longfin dace (Agosia chrysogaster) occurred throughout the BWR and its 

tributaries as late as the early 1990s (United States Fish and Wildlife Service, 1992), but 

currently it is rare below Alamo Dam. 

Alamo Lake contains a number of fish populations that could be relevant to restoration 

efforts downstream. Alamo Lake harbors a long list of nonnative species that could become 

established downstream, including bluegill (Lepomis macrochirus) and four species of bullhead 

(Ictalurus melas and I. natalus) and catfish (I. punctatus and Pilodictus olivaris)(see species list 

in appendix C). In tributaries above Alamo Lake, there are populations of five native species: 

longfin dace, speckled dace, roundtail chub, Gila mountain sucker (Pantosteus clarki), and desert 

sucker (Catostomus insignis). Thus, populations of both native and nonnative fish could 

potentially move below Alamo Dam and become established if flow conditions below the dam 

were to become suitable. 

Relationships Between Flow Events and Species Distributions 

Resistance Versus Resilience 

Aquatic species vary in both their resistance and resilience to flood and drought events. 

Resistance refers to an individual’s ability to directly withstand or avoid an event and includes 

such traits as survival of desiccation via diapause (some Plecoptera species) and escape from 

floods by using behavioral mechanisms (some native fish species, aquatic Hemiptera). Resilience 

is a population-level phenomenon and refers to a species’ ability to rebound numerically 

following a disturbance. A typical resilience trait is a fast life history (rapid growth rate, short 

time to maturation, high fecundity) that results in a high population-level rate of increase 

following flood mortality. Some mayfly species, such as Baetis, show this life-history syndrome 

(Gray, 1981). 

54

The ability of aquatic organisms to disperse and recolonize also determines how 

populations will be affected by flood and drought events (Gray and Fisher, 1981). All aquatic 

insects in the BWR drainage are capable of leaving the aquatic habitat either by flight or by 

crawling. Dispersal typically occurs during the adult stage, which can last from several months 

(many aquatic beetles and true bugs) to only a few days (most mayfly species), although 

juveniles of some species are capable of leaving the water temporarily (the giant water bug 

(Abedus herberti), Lytle, 1999; and many semiaquatic Hemiptera such as water striders). Thus 

the seasonal timing of floods and droughts relative to the seasonal phenology of the dispersalcapable stage may be an important control on aquatic insect populations. Species also differ 

widely in their dispersal abilities. For example, the dragonfly Anax junius is capable of crosscontinental migration while the giant water bug Abedus herberti is only capable of crawling short 

distances (Lytle and Smith, 2004). Because of these differing dispersal capabilities, proximity to 

source populations (in headwater reaches above Alamo Lake, or possibly in off-channel habitats) 

might determine recolonization rates. 

Evolutionary Adaptations to Flood and Drought 

All of the aquatic insects known from the BWR and its tributaries are native species (see 

appendix C), and at least some of them are known to possess adaptations for surviving flood and 

drought. Flood-adapted species include the giant water bug Abedus herberti, which uses rainfall 

as a cue to temporarily move to protected riparian areas during floods (Lytle, 1999), and the 

caddisfly Phylloicus aeneus, which has an adult life stage that is synchronized to avoid summer 

monsoon floods (Lytle, 2002). 

By contrast with the invertebrates, a large part of the BWR fish fauna is nonnative, and 

some of these species have been shown to be disproportionately vulnerable to floods as 

compared to native species. For example, nonnative fish can be killed or displaced by severe 

floods, at least in smaller canyon streams (Meffe, 1984; Minckley and Meffe, 1987; Dudley and 

Matter, 1999). Many native fish species in the Sonoran Desert, including all five species found in 

the BWR watershed, exhibit adaptations to regular flood events and do not suffer population 

declines even after extremely large floods (Eby and others, 2003). Because of the resilience, of 

native fish in this drainage, floods have been proposed as a management tool for favoring native 

over nonnative fish species (Moyle and Light, 1996; Marchetti and others, 2004). It is not clear 

how this type of management strategy could be implemented on the main stem of the BWR 

because large flows may become dissipated over the relatively wide canyon bottom. 

Species will be affected by different components of the flood regime depending on what 

‘mode of adaptation” (Lytle and Poff, 2004) they possess. Lytle and Poff (2004) describe three 

such modes, and provide a comprehensive list of flow regime adaptations for aquatic animals 

and plants (see table 1 in Lytle and Poff, 2004). 

1.	 Life-history adaptations. Life-history adaptations typically involve the synchronization of 

events such as reproduction and growth in relation to the occurrence of flow regime events. 

For organisms with life-history adaptations, the seasonal timing of flood and drought events 

is critical. Examples of organisms with life history adaptations include caddisflies, which 

mature into the aerial adult stage in order to avoid these same floods. 

2.	 Behavioral adaptations. Behavioral adaptations enable animals to respond directly to 

individual flood or drought events, often by reacting to a correlated environmental cue. 

Native fish such as the Sonoran topminnow (Poeciliopsis occidentalis) respond to the rising 

limb of the hydrograph and use this as a cue to orient into the current to avoid displacement 

(Meffe, 1984). Thus, the rate of change from baseflow to flood conditions is an important 

55

factor. The giant water bug Abedus herberti uses rainfall as a cue that a flood is imminent 

(Lytle, 1999). While this species (and possibly other Hemiptera) is adept at surviving floods 

by this means, this strategy may fail during controlled release floods that occur on days 

without rain. 

3.	 Morphological adaptations. Morphological adaptations include traits such as a streamlined 

body profile for living in fast water (e.g., heptageniid mayflies), large gill surface for 

respiration in low-oxygen conditions (e.g., Callibaetis), and so on. Organisms with certain 

suites of morphological traits will be favored under specific flow conditions (lotic versus 

lentic). 

Examples of Flow-Biota Relationships in the Bill Williams River 

The aquatic species of the BWR differ widely in terms of resistance, resilience, and 

evolutionary adaptation to flow events. While detailed information is not available for most taxa, 

it is possible to use general ecological and life-history information to make informed predictions 

about how flows might affect populations on the BWR. The following section focuses on three 

currently or historically important aquatic taxa. Relationships between life-history and 

streamflow for these species are summarized in table 8 at the end of this section. 

Longfin Dace (Agosia chrysogaster): High Resistance to Floods and Droughts 

Like other native desert fishes, the longfin dace is facing population declines and 

extirpation because of competition with nonnative fishes and changes in river hydrology. In 

Arizona, it is native to the BWR and Gila River drainages, and has been introduced to the Virgin 

River (Minckley, 1973). Because the longfin dace is adept at coping with the extreme floods and 

droughts typical of desert rivers, appropriate flow management could help reestablish 

populations below Alamo Dam. In contrast to many of nonnative fishes, the longfin dace has 

reasonably high resistance to both flood and drought conditions, and it likely possesses 

behavioral adaptations for surviving flood and drought. 

Baetis Mayflies (Ephemeroptera: Baetidae): High Resilience to Floods 

Baetis mayflies possess some of the fastest life cycles of all aquatic insects: in Arizona 

desert streams, Baetis (=Fallceon) quilleri can develop from egg to reproductive adult in less 

than 10 days, which potentially allows 35 generations per year (Gray, 1981). Baetis mayflies are 

also renowned for their ability to drift downstream and recolonize new habitats after a flood 

(Allan, 1995; Lytle, 2000). For these reasons, they have exceptionally high resilience to flood 

events. The BWR harbors at least four genera of mayflies in the family Baetidae, including 

Baetis (inclusive of Fallceon), Pseudocloeon, Acentrella, and Callibaetis. All but Callibaetis, a 

pond dweller, are found in fast-flowing, riffly habitats. Baetis feed by scraping attached algae 

and diatoms (periphyton) from the surfaces of rocks. Baetis mayflies can reach phenomenal 

densities in streams—up to tens of thousands of larvae per square meter (Vinson, 1999). For this 

reason they are likely an important food resource for fish, other invertebrates, and terrestrial 

animals such as birds. 

Gomphid Dragonflies (Odonata: Gomphidae): Low Resilience to Floods 

Adult gomphid dragonflies are known as clubtails from the distinct enlargement at the tip 

of the abdomen. During the summer months, the green and yellow marked adults can be seen 

patrolling stream reaches and ponds in search of flying insect prey. The larvae of some are 

known as sanddragons because they occupy sandy river substrates (fig. 34). Gomphids are also 

56

predatory as larvae, feeding on other insects and even small fish. At least three gomphid genera 

are known from the BWR, Progomphus, Erpetogomphus, and Ophiogomphus. Gomphid larvae 

are classified as lotic and are found in slower-moving reaches of rivers and streams, although 

they sometimes occur in ponds (Merritt and Cummins, 1996). Unlike many other aquatic insect 

larvae, gomphids are thought to require at least 2 years to reach maturity. Because of the longer 

time to maturity for this group, populations may be slow to respond to major hydrologic events 

(low resilience), especially if the event occurs during a time of year when no adults are present 

aerially. It is possible, however, that their long-lived adult stage might provide a fitness 

advantage during large flood or drought events (sensu Lytle, 2002). 

Figure 34. Gray sanddragon Progomphus borealis (Gomphidae) larvae. Big Sandy River, Ariz. 

Photograph by L. McMullen, Oregon State University. 

57

Table 8. Summary of flow-biota relationships for longfin dace, Baetis mayflies and Gomphid dragonflies. 

Biota Resilience to 

floods 

Moderate to large floods Small floods High to moderate 

baseflows 

Longfin High resistance May cause short-term mortality, Unlikely to have a Provide more suitable dace to floods and but could trigger spawning that significant effect. habitat than low 

droughts. ultimately increases population baseflows. 

sizes. May also remove nonnative fish, which could release longfin dace from competitive pressure. 

Baetis High resilience Floods in the fall and winter are Floods during the summer Could favor mayfly mayflies to floods. likely to reduce mayfly 

populations because fewer aerial 

monsoon season may reduce mayflies, but 

populations, especially following large floods 

adults are present to recolonize populations can rebound that create new lotic during these cold months. This from aerial adults. Small habitat by breaching reduction would be offset if floods may enhance beaver dams. recolonization could occur via drift recolonization by cleaning from upstream refugia, but because riffles of fine sediments of the barrier of Alamo Dam, and enhancing algal recolonization is unlikely.. growth. 

Gomphid Low resilience to Will likely reduce Gomphid May reduce Gomphid May not be ideal, as dragonflies floods. populations. Resilience will also 

be low if a flood occurs when few 

populations, especially if floods are strong enough 

Gomphid larvae are visual predators that 

adults are present to recolonize to scour stream channels. typically occupy slower (likely during the cold winter river sections. Other months). If the return interval of desert stream predators large floods is sufficiently long (>3 such as belostomatid years), large floods may have long- hemipterans appear to term positive effects by removing do best when flows are beaver dam ponds and creating high in the winter (high lotic habitat for recolonization. productivity of algaefeeding prey species) 

and then low in late spring and summer (prey become spatially concentrated). 

58 
Chapter 6. Streamflow-Biota Relations: Mammals, Reptiles, 

Amphibians, and Floodplain Invertebrates 

By Douglas C. Andersen1 

The animals that use the flood plain and associated lotic and lentic aquatic habitats of the 

BWR can be classified as either obligately or facultatively riparian (e.g., see Andersen and 

others, 2000). Facultatively riparian species may use the riparian zone extensively, but 

individuals can also survive and reproduce using only upland habitats. Thus, the maintenance of 

their populations is not strictly dependent upon the presence of the habitats associated with a 

riverine ecosystem. Obligately riparian species are those whose life cycle requires resources or 

habitats provided only by the riparian (here, riverine riparian) environment; however, even 

obligately riparian species may be found outside the riparian zone under particular 

circumstances, such as while dispersing. 

The riverine ecosystem created by flows in the BWR contains a large number of aquatic, 

semiaquatic, and terrestrial animals that range in size from microarthropods associated with 

decomposing detritus to large mammals, both herbivores and predators, that use the riverine 

corridor as a source of food, shelter, and water. These organisms form a complex trophic web 

that ties together not only the various components of the riverine ecosystem (e.g., the stream, 

parafluvial zone, and vegetated flood plain) but also the riverine ecosystem to the surrounding 

desert. Even though our understanding of the mechanisms involved in determining the structure 

and functioning of this riverine ecosystem is rudimentary, work in other ecosystems suggests that 

a small number of species or groups of species will have a disproportionately large influence 

(Hooper and others, 2005). This section discusses some mammals known or considered likely to 

be in that group. We also discuss how the nature of the flow regime will influence populations 

of these and other riparian animals, including reptiles, amphibians, and insects. Mammals 

(appendix D), reptiles and amphibians (appendix E), and floodplain invertebrates (appendix F) 

encountered within the BWR corridor are listed in the appendices at the end of this document. 

Large Herbivores 

It is well established that long-term grazing and browsing by large herbivores can result 

in major changes in vegetation (Hobbs, 1996; Augustine and McNaughton, 1998). Overgrazing 

by large herbivores alters the structure and may reduce the abundance of preferred food plants, 

allows populations of nonpreferred plants to increase, and may promote invasion by undesirable 

exotic species. Historically, grazing by native ungulates using the BWR corridor was probably 

limited by predators or by hunting pressure from Native Americans or settlers who kept ungulate 

populations at relatively low levels. Use of the riparian zone by livestock, however, did degrade 

the vegetation. Adverse effects on vegetation and soil are a common consequence of livestock 

use of riparian areas in the arid Western United States (Belsky and others, 1999). The four 

Bureau of Land Management grazing allotments on public lands along the river below Alamo 

Dam are now being managed to facilitate recovery of the riparian vegetation to something 

approximating its pristine (i.e. presettlement) state. Although there has been no licensed 

livestock grazing since 1988, overgrazing may still be occurring in areas where the numbers of 

feral burros are excessively high. 

 U.S. Geological Survey, Denver Field Station, Denver, Colo. 

59 

1
The natural flow regime probably had little effect on large herbivores. Flash floods likely 

occasionally injure or kill individual large mammals caught in them, but even some flash floods 

may actually feature a rising limb sufficiently slow to allow floodwaters to be avoided by simply 

moving into the upland. 

A very small number of elk (Cervus elaphus) may currently use the flood plain below 

Alamo Dam. If present, these individuals are a spillover from a larger group of animals using 

the Big Sandy and Santa Maria River drainages above the dam (Clint Adams, Arizona Game and 

Fish Department Biologist, oral. commun.). The elk rely on the shade, forage, and other 

resources provided by the floodplain environment to a greater extent than do the more substantial 

numbers of desert mule deer (Odocoileus hemionus) that can be found thoughout the corridor. 

Along with seeking water, the deer use the flood plain to obtain green food when desert forage 

declines in palatability or availability. Desert bighorn sheep (Ovis Canadensis) enter the corridor 

to access water, but they likely avoid dense vegetation and preferentially feed and rest in open 

desert environments. Mammalian predators are discussed below. 

Beavers 

The influence of beaver dams and their associated ponds on fluvial geomorphic processes 

were discussed in chapter 2 of this report. In this chapter, we discuss aspects of their population 

biology and some of their links to biological and biogeochemical processes taking place in the 

BWR ecosystem. Aerial photographs taken in May 2002 show much of the BWR low flow 

channel occupied by beaver ponds (Shafroth and others, 2004). It is likely that these rodents take 

up residence and build dams along all reaches bordered by cottonwood or willow or where other 

vegetation (e.g., cattail) provides suitable forage. There has been no research examining the 

effects of a flood pulse on beavers in the desert Southwest. Small floods (0.014–140 m3

/s (500 to 

5,000 ft3

/s)) can damage beaver dams, but assuming that food caching is unnecessary and that 

desert beavers typically build bank dens rather than lodges, this damage would not necessarily 

translate into strong detrimental effects on survival. Casual observations following a flood in 

this range on the BWR in autumn 2004 showed that many ponds retained water (i.e., the 

environment upstream of the damaged dam remained lentic rather than lotic) and that beavers 

were still present. We hypothesize that beavers are neither displaced nor forced to disperse 

following small floods but that they may be made more vulnerable to predators while repairing 

the dam and restoring pond water level to its pre-flood value. 

We speculate that a moderate flood (140–850 m3

/s (5,000–30,000 ft3/s)), however, can 

remove most beaver dams and, where current velocity is high, sweep individual beaver 

downstream. Lower magnitude, but protracted, flows can potentially impact a beaver dam via 

persistent shallow overflow, undercutting, or lateral channel migration. Dam removal will lead 

to the complete draining of most ponds and exposure of most bank dens, forcing animals to seek 

or construct new dens. The vulnerability of beavers to predators will be dramatically increased 

until dams are rebuilt and the security associated with the pond and its dens restored; however, it 

has been noted that beavers can quickly recover from the effects of flood damage. 

(Leidholtbruner and others, 1992). 

A large flood (>850 m3

/s (> 30,000 ft3/s)) not only destroys beaver dams and sweeps 

individual beaver downstream, but also can lead to stream avulsion and thereby eliminate flow in 

the channel the dam formerly blocked. We hypothesize that these floods directly kill individual 

animals (e.g., via drowning or physical injury) and that increases in vulnerability to coyotes and 

other predators (Collen and Gibson, 2001) and social strife among displaced animals (Payne 

1984), perhaps combined with reductions in food resources (Fryxell, 2001), leads to a dramatic 

rise in the mortality rates among beavers that survive the flood. Thus, floods of this magnitude 

60

set the stage for a complete reshuffling of the beaver community and the spatial and temporal 

pattern of pond generation. 

Beaver ponds increase the area of surface water (and the linear extent of shoreline) and 

thereby increase the hydrologic connectivity between stream and flood plain. The ponds also 

raise the local water table, which can enhance vertical connectivity within the alluvium. The 

slower flow velocities in ponds can facilitate sediment accumulation, dense algal growth, organic 

matter buildup, and nutrient transformations. For example, Harper (2001) found higher organic 

matter in sediments of beaver ponds than in adjacent stream habitat in southern Nevada. 

Assuming relatively high carbon availability, the anoxic conditions in pond sediments may 

facilitate denitrification and result in the ponds serving as a sink for nitrogen. Anoxia and 

subsequent denitrification could be important from a water quality standpoint if nitrogen is being 

added to the BWR as a consequence of upstream human activities, such as agricultural 

operations. 

Beavers cut vegetation to obtain both food and dam-construction materials. The effects 

of this cutting on vegetation dynamics within the flood plain are unclear. It appears that beavers 

on the BWR do not create large caches of food material, presumably because they experience 

only short and mild winters. Thus, the well-documented accounts of beavers “eating out” groves 

of preferred tree species in boreal forests and other northern locations may not be applicable to 

warm desert environments. Although beavers certainly cut both cottonwood and willow, which 

likely affects local stand structure, the most important influence of beavers on vegetation along 

the BWR may be through their pond-building activity and the resulting saturated soils and 

relatively stable water tables necessary to maintain cattail and other marsh species (Stromberg 

and others, 1996). Beavers cut tamarisk and incorporate the stems into dams, but the extent to 

which tamarisk is used for food is unknown. Hensley and Fox (1948) include it in a list of food 

plants but provide no evidence of actual consumption. Brazell and Workman (1977) failed to 

find evidence of tamarisk use in a specific effort to do so in a field study. 

Small Mammals 

Small mammals can also influence vegetation structure through selective herbivory 

(Howe and Lane, 2004). A somewhat less “aquatic” obligate-riparian mammal using the BWR 

corridor is the Arizona cotton rat (Sigmodon arizonae), a generalist herbivore that resides in 

mesic, dense herbaceous habitat (e.g., mixed grass and cattail) such as that in the delta area and 

perhaps some upstream areas featuring a perennially high water table. This species could 

potentially influence the nature of herbaceous vegetation in the areas it inhabits. The presence of 

the Arizona cotton rat along the river, however, is based on only two sightings; no individuals 

have been captured. 

Small mammals are more vulnerable to harm from flood pulses than are medium- or 

large-sized mammals. Even low flood pulses could potentially result in destruction of Arizona 

cotton rat nests and the downstream displacement of individuals. If the rising limb of the flood 

pulse provides sufficient time, most small mammals simply move toward higher ground, which 

may strand individuals on shrinking islands or force those that are able to climb up into trees or 

other vegetation (Andersen and others, 2000). A rapid inundation will drown small mammals 

caught in places from which they cannot escape (e.g., below ground), and even moderate 

currents will overcome species that swim poorly. Several of the small mammals using the BWR 

flood plain can escape floodwaters by climbing (e.g., deer mice (Peromyscus maniculatus) and 

cactus mice (P. eremicus)). If they manage to survive the flood, at least some small mammals 

swept downstream can employ strong homing instincts to return to the area from which they 

were carried. An example of the variable effects among species is the finding that several of the 

61

cactus mice captured on the BWR flood plain prior to an autumn 2004 flood pulse (probably 

<141.6 m3

/s (<5,000 ft3/s)) were recaptured in the same locations after the flood, whereas all 

white-throated woodrats (Neotoma albigula) present prior to the flood had disappeared (Kathleen 

Blair, oral commun.). 

Bats 

At least 13 bat species use the BWR corridor (Brown, 1996). Although most of the bats 

roost in abandoned mines in adjacent upland, both hoary bats (Lasiurus cinereus) and yellow 

bats (L. xanthinus) were documented as using floodplain trees as diurnal roosts (Brown, 1996). 

All 13 species are insectivorous (at least in part) and primarily use the river and riparian area for 

foraging and as a source of drinking water. Thus, the flow regime will affect these species both 

directly and indirectly through the extent and configuration of surface water and effects on insect 

production. Large numbers of bats would obviously have the potential to consume large 

numbers of insect prey. Nevertheless, the roles of these species as determinants of riverine 

ecosystem structure are unknown. 

Mammalian Predators 

Numerous mammalian predators in addition to bats rely on the floodplain for food 

(Hoffmeister 1986), but few species other than perhaps the raccoon are likely to restrict their 

movements to within the riparian corridor. The largest predator present is the mountain lion 

(Felis concolor), a wide-ranging species whose prey would include deer (Odocoileus hemionus), 

peccaries (javelina; (Tayassu tajacu)), young burros (Equus asinus), and beavers. Bobcats (Felis 

rufus) are undoubtedly also present. These cats prey extensively on cottontails (Sylvilagus 

audubonii) and jackrabbits (Lepus californicus) when they are available, but also consume mice, 

lizards, and other small prey. Other carnivores include the coyote (Canis latrans), gray fox 

(Urocyon cinereoargenteus) and kit fox (Vulpes macrotis). Coyotes might be able to kill an 

adult beaver found away from water, but most prey would be smaller and less formidable (Gese 

and others, 1996). The foxes probably venture into the riparian zone primarily for water or when 

foraging for rodents and reptiles in adjacent upland becomes unprofitable. All the larger 

carnivores would prey on young beaver (kits) made vulnerable by a flood or other circumstance. 

Striped skunks (Mephitis mephitis) and spotted skunks (Spilogale gracilis) would also use the 

riparian zone, the former probably more extensively than the latter. Like the raccoon (Procyon 

lotor), the skunks are opportunistic omnivores, including invertebrates as well as small 

vertebrates in their diet. Both the striped skunk and the raccoon are seldom found far from 

water. 

Engulfment in rapidly deepening floodwaters would likely displace and perhaps result in 

injury or even drowning of raccoons or skunks, but the larger, more mobile carnivores could 

likely avoid floods by moving into the upland. Large floods might actually indirectly benefit 

most carnivores, at least in the short term, by making prey more vulnerable, either through the 

prey’s displacement or injury, or through a reduction in vegetation that serves as concealment 

cover. 

Reptiles and Amphibians 

Consequences of flood pulses to terrestrial lizards are probably similar to those of small 

rodents and shrews, with effects differing strongly among those that are arboreal (e.g., ornate tree 

lizard (Callisaurus ornatus)) and those strictly terrestrial. Most lizards and egg-laying snakes 

reproduce during the warm months (April to October), and flooding during this season could 

smother or drown developing embryos and hatchlings. Adult turtles may be able to tolerate all 

62

but high flood pulses because of their strong swimming abilities (spiny soft-shelled turtle 

(Trionyx spiniferous)), climbing and clinging abilities (mud turtles), and ability to stay 

submerged for extended periods of time (Chris Holdren, oral commun.). Turtles and other 

aquatic animals may take refuge in underwater caves (e.g., beaver dens) to avoid currents during 

flood events. Many turtles also have well-developed homing abilities, which would facilitate 

their returning to a pond from which they had been displaced by a moderate flood. Lizards that 

hibernate (e.g., desert iguana (Dipsosaurus dorsalis) and the banded gecko (Coleonyx 

variegatus)) during all or part of the cool season (about November through February) may be 

especially vulnerable to floods from winter frontal storms. It is unclear if innate behaviors 

(habitat selection) associated with searching for hibernacula lead to these species moving from 

floodplain sites to uplands at the end of the warm season. Winter surveys within the Bill 

Williams National Wildlife Refuge indicated that only side-blotched lizards (Uta stansburiana) 

were active (on warm days), suggesting that all other species, including the ornate tree lizard, 

hibernate (Kathleen Blair, oral commun.). 

Consequences of floods to amphibians differ from those of small mammals and reptiles, 

and these consequences probably vary strongly with organism sizes, which is related to 

swimming ability. The strong swimming ability of many species means that effects of small 

floods may be minor and that only moderate and large floods can increase mortality and disrupt 

communities through downstream displacement. The introduced and undesirable (Mueller and 

others, 2006) American bullfrog (Rana catesbeiana) may be least affected by floods because of 

its large size. American bullfrog tadpoles, however, are likely more vulnerable to displacement 

by floods. Based on studies elsewhere in the Southwest (summarized in Clarkson and deVos, 

1986), the tadpole stage lasts for 3–6 months for eggs laid early in the April to early July 

breeding season. Tadpoles from eggs laid late in the breeding season overwinter in that form and 

transform in March or April of the following year. Platz and others (1990) speculated that 

another exotic amphibian present in the lower Colorado River, the Rio Grande leopard frog 

(Rana berlandieri), would eventually find its way into the BWR drainage, but unknown factors 

appear to be hindering invasion (Rorabaugh and others, 2002). The native lowland leopard frog 

may have historically occupied parts of the BWR corridor, but it is now absent. This species 

breeds from March to May and, to a lesser extent, in September and October (Sartorius and 

Rosen, 2000). Tadpoles from the spring eggs metamorphose in early summer. This timing 

minimizes exposure of eggs and tadpoles to flash floods generated by monsoon rains. 

Amphibians may in general face greater hazards as floodwaters recede and the need to find 

shelter in a location with permanent water becomes paramount. Whether flood events and the 

presence of displaced terrestrial and aquatic vertebrates attract predators is unknown. Some 

species may be vulnerable to burial by winter floods. For example, the Colorado River toad 

(Bufo alvarius) may estivate to avoid seasonal drought, spending the dry period under ground 

until “awakened” by rains. 

Floodplain Invertebrates 

Very little is known about how the flow regime influences nonaquatic floodplain 

invertebrates or how these animals may influence floodplain structure and functioning. Apache 

cicadas (Diceroprocta apache), which feed on the roots of floodplain trees and probably other 

vegetation as larvae, emerge from the soil during summer of most years and climb into the 

vegetation immediately prior to adult eclosion. The adults then mate, and females lay their eggs 

inside thin stems and branches, a process that can be especially damaging to sapling cottonwoods 

and willows. Although cicada abundance varies from year to year, these insects are a major 

source of food for some birds and probably small mammals and reptiles as well. Andersen 

63

(1994) postulated that high densities of these insects belowground could influence soil moisture 

properties and therefore herbaceous vegetation. Flooding alone does not appear to markedly 

affect cicada numbers, but the deposition and erosion processes that accompany high flood 

pulses might dramatically influence emergence patterns by burying larvae under material that 

prevents their emergence or exposing larvae to desiccating air and aboveground predators. 

The western viceroy butterfly (Limenitis archippus obsolete) is an obligate riparian 

species in that adults lay eggs on and the larval caterpillars feed on Goodding’s willow and 

possibly Fremont cottonwood (Nelson, 2003). Adults feed on nectar produced by a variety of 

riparian plant species, including tamarisk and seepwillow, and are hypothesized to rely on 

surface water accessible from vegetation-free surfaces for both moisture and mineral intake 

(Nelson, 2003). The flow regime may thus influence the distribution and abundance of this 

butterfly both indirectly, by influencing the plant community that supplies the larval host plants 

and nectar sources, and directly, through the generation of bare alluvial surfaces adjacent to 

surface water. 

Examples of Flow-Biota Relationships in the Bill Williams River 

Mirroring the differences among the aquatic invertebrate and fish species discussed 

above, other animals found in the BWR also differ widely in terms of resistance and resilience to 

flow events. And again, detailed information is typically lacking for most taxa, but ecological 

and life-history information allow us to make informed predictions about how flows might affect 

populations on the BWR. Table 9 focuses on three important riparian-aquatic taxa. 

64

Table 9. Summary of flow-biota relationships for beaver (Castor canadensis), ornate tree lizard (Callisaurus 

bullfrog (Rana catesbeiana) in the Bill Williams River, Ariz. 

Biota Resilience Moderate to large floods Small floods High to moderate 

to floods baseflows 

Beaver 	 Moderate 

resistance to floods, low resistance to drought. 

Floods displace individuals and increase mortality. Ponds would be modified or destroyed, triggering a new period of dam-building and pond creation. Increases in pond water levels and river stage allow access to vegetation otherwise only accessible by traveling away from the safety of the pond. Increased growth of trees and other food plants would deliver indirect benefits. 

Small floods Would provide are unlikely to more suitable have a habitat than low significant baseflows. effect. 

Woreacbuiredpopsecassobasred(anfolleve

Ornate Moderate tree resistance to lizard floods, high 

resistance to drought. 

Could cause mortality via drowning or displacement. Displacement would be particularly injurious to the territorial males. Loss of forest cover associated with a large flood event would constitute a loss of habitat. Population turnover is probably naturally high and populations could rebound quickly. 

Would promote plant community development and increase available habitat, but could also destroy eggs and decrease recruitment. 

Would promote canopy development and increases in prey levels. 

Woonlwoodevprodry woudirepopveg

65


Bullfrog	 Low 

resistance to floods, low resistance to drought. 

Fall and winter floods can flush adults and tadpoles into downstream areas where many would not survive. Large floods spring and early summer floods would damage or destroy egg masses. Negative impacts would be offset if recolonization occurs via drift from upstream refugia or from movement of the mobile juveniles and adults across nonaquatic habitats. Flows during the late summer monsoon season may reduce total numbers (if the flood occurs after cessation of breeding) but enhance conditions for the surviving individuals. 

Would enhance growth and survivorship in all age classes by improving conditions for the lower trophic levels. 

Would promote population growth, especially following moderate floods that created new lentic habitat without destroying existing beavers ponds or emergent marsh habitat. 

Wopoplentthe 
Chapter 7. Ecosystem Functioning 

By Douglas C. Andersen1 

Ecosystem functioning refers to the linked biotic and abiotic processes that transform 

matter and energy and move them into, out of, and through the ecosystem. The movement of 

water and materials through the BWR corridor together with the other processes that support the 

organisms living in the corridor (table 10) are examples of the ecosystem’s functioning. Clearly, 

the river’s flow regime is a major factor affecting that functioning through its direct effects on 

individual organisms and populations as detailed earlier and through its effects on abiotic 

processes like erosion and sediment transport and deposition. 

A number of conceptual models have been developed to explain how the structure of a 

riverine ecosystem is coupled to its functioning. Thorp and others (2006) briefly reviewed these 

models and attempted a synthesis in which they postulate that all riverine ecosystems consist of 

arrays of physical patches that can be delineated by their geomorphological and hydrological 

characteristics. From an ecological perspective, Thorp and others (2006) considered these 

patches to be “functional process zones” (FPZ). Variation in the physical nature of the 

catchment, water quality, and local flow regime determine how these patches (and thus FPZs) are 

distributed in a river network. The same hydrogeomorphic patch type (and thus FPZ) can appear 

repeatedly along a stream, as well as in locations that can be independent of headwater-to-mouth 

longitudinal gradients. Further, the edges of these patches may be sharp or indistinct, and the 

difference in ecological characteristics (e.g., characteristic productivity, organic matter 

dynamics, nutrient dynamics, and community composition) between two patches may be small 

or large. The model of Thorp and others (2006) would seem to be a reasonable way to 

conceptually organize the physical and ecological variability of the BWR ecosystem. Channel 

substrate (bedrock canyon or alluvial valley), flow intermittency (perennial or ephemeral), and 

water velocity at baseflow (affected, for example, by the presence of beaver ponds) are examples 

of attributes differentiating patches. Differences in flow character during floods could further 

distinguish FPZ’s. 

Stream ecologists have suggested that periodic episodes of high mortality or 

“disturbance” generated by unusually high and low flows are the dominant organizing factor in 

most streams, including desert streams (Resh and others, 1988; Grimm and Fisher, 1989). 

Indeed, the metrics used to describe disturbance regimes—magnitude, areal extent, frequency, 

and predictability—are nearly identical to those used to describe flow regimes: the magnitude, 

duration, timing, frequency, predictability, and rate of change in discharge events. Grimm and 

Fisher (1989) found that flash floods, a characteristic of most naturally functioning desert 

streams, greatly reduced both primary producers (algae) and macroinvertebrates in a Sonoran 

Desert stream, Sycamore Creek, but that the system quickly returned to its preflood condition. 

They attributed this high resilience to warm temperatures, stable low discharge, and high light, 

which allowed high rates of instream primary and secondary production. They also found that 

the abundance of algae and other primary producers in the stream, as measured by chlorophyll a 

concentration, could be predicted from the magnitude of the last flood, the time since its 

occurrence, and current discharge (Fisher and Grimm, 1988). Macroinvertebrate abundance, 

 U.S. Geological Survey, Denver Field Station, Denver, Colo. 

66 

1
however, was less tractable, presumably because of nutrient (nitrogen) limitation that affected 

food quality and biotic interactions during drying episodes (Boulton and others, 1992a). Flow 

regime will also influence the subsurface organisms in the stream (hyporheos) (Boulton and 

others, 1992b; Clinton and others, 1996) as well as macroinvertebrates (Cartron and others, 

2003), vegetation (Andersen, 2005; Sponseller and Fisher, 2006), and other biota in the adjacent 

flood plain that are sources of allochthonous inputs to the stream. 

Ecosystem structure and functioning are linked. The kinds, abundances, and dispersion 

patterns of organisms present determine in part the nature and timing of processes that occur, and 

ecosystem functioning in turn influences ecosystem structure. Holmes and others (1996) found 

that microbially mediated denitrification in a Sonoran Desert stream occurred primarily at the 

upstream ends of subsurface (hyporheic) flow paths, where streamwater had just entered the 

subsurface zone and contained relatively high amounts of dissolved organic matter. The location 

and extent of downwelling and upwelling zones along a stream channel are determined in part by 

bed material texture, channel geometry, and hydraulic head, and at least the latter two can be 

influenced locally by the presence of a beaver dam (Westbrook and others, 2006). All organisms 

in an ecosystem contribute to its carbon dynamics and energetics, which can be characterized by 

such measures as the ratio of gross primary production to ecosystem respiration or the difference 

between the two (net ecosystem production or ecosystem metabolism). The daily change in 

dissolved oxygen concentration provides a means to estimate a stream’s overall metabolism and 

to indicate whether the stream is a net source or sink of carbon. A stream’s position on the 

source-sink gradient is determined by the interplay of water quality, stream environment, 

primary and secondary producers, microbes, and other components of ecosystem structure. In a 

comparison of stream metabolism among North American biomes (Mulholland and others, 

2001), only a desert stream (Sycamore Creek) showed positive net ecosystem production. A 

stream’s position on the source-sink gradient is not in itself important from a management 

standpoint, but because whole-stream metabolism integrates so many components, change in this 

characteristic following a shift in flow regime or other ecosystem alteration is considered useful 

as a benchmark against which to gage whether overall stream quality has changed (Young and 

others, 2004). It is important to note, however, that there are no studies of ecosystem energetics 

dealing with both a desert stream and its flood plain in alluvial settings like those along the 

BWR, where most primary and secondary production may take place away from the stream. 

Another measure of function that integrates many of the abiotic and biotic components of 

a riverine ecosystem is the pattern and pace of litter breakdown. Many stream and floodplain 

invertebrates are detritivores, and their abundances and dispersion patterns within the stream 

corridor will have a role in ecosystem nutrient dynamics. The complicated linkages among 

vegetation, climate, and surface and soil decomposition processes that regulate nutrient cycling 

in deserts (Whitford, 1992) are made even more complex in desert riverine ecosystems by the 

addition of groundwater and surfacewater dynamics. Litter breakdown rates will vary through 

time as a result of seasonal and interannual variation in both river discharge and local weather. 

Nutrient accumulation, transformation, and spiraling will occur within the stream (lotic) 

environments as a result of organic matter originating within the stream (autochthonous input) 

and elsewhere, for example, the riparian zone (allochthonous input). The latter,—which is due 

largely to floodplain tree leaf litter, woody debris, and dissolved and particulate organic matter in 

sheet flow from adjacent upland—is ecologically insignificant in some desert streams (Schade 

and Fisher, 1997) but may be important in the BWR ecosystem because of the extensive 

floodplain and gallery forest along some reaches. 

The type, quantity, and fate of litter accumulating on the flood plain may be key factors 

determining the structure and functioning of a desert riverine ecosystem like that of the BWR. 

67

Decomposition on a dry flood plain may be as slow as that in adjacent desert upland, and perhaps 

slower if desert macroinvertebrate detritivores like termites are precluded by floods or high water 

tables (Andersen and Nelson, 2006). Thus, litter can accumulate on dry flood plains for 

extended periods of time, sequestering carbon and nutrients until a flood moistens, submerges, 

redistributes, or perhaps buries the material (Molles and others, 1995, 1998). In a study 

involving artificially inundating a semiarid floodplain forest site where a dike had precluded 

flooding for about 50 years, Valett and others (2005) reported differences in dissolved organic 

carbon, dissolved oxygen, inorganic nutrient content, and other floodwater parameters at the 

artificially flooded site and at a nearby forest site naturally inundated almost every year. They 

attributed these differences to the greater amount of forest floor detritus at the experimental site. 

Where flooding is rare or absent, this buildup of floodplain litter can also contribute to an 

increased incidence and severity of riparian fires. Along many regulated rivers in the American 

Southwest, fire now rivals or exceeds flooding in importance as a disturbance agent (Busch, 

1995), and the ecosystem-level effects of fire differ greatly from that of floods. For example, 

Ellis (2001) found that a fire carried by large amounts of dry litter killed the aboveground parts 

of most floodplain trees, including Fremont cottonwood and Goodding’s willow, at two sites 

along the middle Rio Grande. Many trees of both species appeared to have survived the fire, as 

indicated by the rapid appearance of root or shoot sprouts, but long-term survival was poor in 

areas where the fire was most severe. Two years after the fire, the only cottonwoods with viable 

shoots were located in an area where flooding prior to the fire had helped to reduce the fuel load. 

The duration and magnitude of a flood event will largely determine the degree to which it 

accelerates litter breakdown. A short period of immersion from hours to less than a few weeks in 

quiet water may result in little or no effect (Andersen and Nelson, 2003), whereas a longer period 

of immersion will increase decomposition rates (Molles and others, 1995; Ellis and others, 

1999). Flood magnitude (peak discharge) will determine how much of the flood plain is 

inundated and will affect stream power. Strong currents and entrained sediment and other 

materials can physically degrade litter and transport dissolved and particulate matter to distant 

downstream sites or completely out of the catchment. Flooding, presumably through effects on 

decomposition processes and nutrient transport, also appears to influence leaf chemistry, 

particularly nitrogen:phosphorus ratios (Tibbets and Molles, 2005), and thus litter quality. Low 

litter quality (high carbon:nutrient ratios) can restrict growth and reproduction of detritivores by 

limiting their ability to acquire nitrogen, phosphorus, and other essential nutrients. 

Monitoring floodplain litter breakdown may provide an index of ecosystem quality 

complementary to that of whole-stream metabolism. Molles and others (1998) hypothesize that 

floodplain forests along naturally functioning desert rivers have a characteristic long-term forest 

floor respiration rate—a measure of litter decomposition and soil microbial respiration rate— 

determined by the prevailing flooding pattern. They argue that this rate is artificially low in 

areas where flooding has been reduced or eliminated by regulation and that restoration of 

flooding will lead to a temporary rise to unusually high levels, as decomposer populations and 

detrital-based food webs expand in response to the renewed availability of moisture to support 

litter processing. Over time, the accumulation of litter is reduced, and the system returns to a 

new equilibrium at which the rate of litter generation is again matched by decomposition. The 

restoration of floodplain detritus processing appears to have been successfully initiated in an 

experiment involving managed flooding along the Rio Grande (Ellis and others, 1999). 

The breakdown of litter, along with importation via sediment and flood water, is also the 

means by which nutrients are made available to floodplain plants. The temporal and spatial 

patterns of nutrient spiraling—from uptake by a live plant through litter decomposition, to uptake 

and temporary immobilization in a microbe or other organism, to release in ground or surface 

68

water and transport along flow paths—are only now being explored in desert riverine ecosystems 

(Fisher and others, 1998a; Schade and others, 2001; Tibbets and Molles, 2005; Lewis and others, 

2006). It is clear, however, that the flow regime plays a major part in many of the component 

processes and thus in the rate at which nutrients pass through the system (Fisher and others, 

1998b). Heffernan and Sponseller (2004), for example, have shown that the increase in 

inorganic nitrogen concentration in riparian groundwater observed during flash floods likely 

results from both remobilization of soil nitrogen and inputs from floodwater. In contrast, they 

found no evidence for remobilization of phosphorus, a finding that is consistent with the idea of 

floodwater being the primary source of phosphorus in groundwater during flood events. 

The riparian or floodplain component of riverine ecosystems can provide a number of 

functions that have been labeled “ecosystem services” because of their positive contribution to 

human welfare. Within the BWR corridor, these ecosystem services include the following: 

• transport and storage of water and sediment during high discharge periods 

• release of water following high discharge periods (helping to moderate flows) 

• accumulation and sequestering of waterborne chemicals in sediment and plants 

• nutrient retention (via nutrient spiraling) 

• elimination of potential pollutants (e.g., denitrification of agriculturally derived nitrogen) 

• creation of soil macropores through root processes and animal activity (enhancing soil 

infiltration capacity) 

• streambank stabilization by plant roots (reducing erosion) 

• moderation of stream temperatures (by providing shade and through hyporheic mixing and 

heat exchange) 

• delivery of litter (coarse particulate organic matter) to the stream (and to downstream Lake 

Havasu) 

• maintenance of biodiversity by provision of habitats and resources 

The maintenance or restoration of ecological services provided by the stream and its 

flood plain is one of the goals driving development of flow requirements; however, the natural 

patterns and processes that sustain these services in riverine ecosystems are often not well 

understood or may be impossible to restore. For example, natural functioning as measured by 

sediment retention and transport processes may be difficult to mimic with reservoir releases. 

Stanford (1994), discussing flow recommendations for rivers in the Upper Colorado River Basin, 

noted that the tradeoff between very high (natural) flood peaks and lower peaks of longer 

duration has not been examined in detail. Because releases from reservoirs are typically low in 

sediment compared to natural flood flows, high flows intended to flush fine sediment 

downstream and restore the functional role of cobble reaches may eliminate other functions by 

degrading the channel, lowering the water table, and disconnecting the stream from backwaters 

and wetlands. Thus, in cases involving regulated rivers, new combinations of processes may be 

the best alternative to achieve the desired mix of ecosystem services. A summary of ecosystem 

processes and associated functions important in the BWR corridor is presented in table 10. 

69

Table 10. Ecosystem processes and associated functions important in the corridor of the Bill 

Williams River, Ariz. 

[Examples of process attributes that are influenced by flow regime are italicized] 

Process Role in ecosystem functioning 

Biological processes 

Primary production Contributes to carbon storage; sequesters nutrients; 

magnitude creates physical structure; promotes biodiversity 

 spatial and temporal variability 

Secondary production Modifies physical structure; sequesters nutrients; 

form and magnitude promotes biodiversity 

 spatial and temporal variability 

Consumption and respiration 

 form and magnitude

 spatial and temporal variability 

Succession and other types of community 

change 

susceptibility to invasion

 resilience and resistance to disturbance 

Biogeochemical processes 

Nutrient uptake, immobilization, and release 

 decomposition 

 adsorption in sediments 

Controls nutrient cycling; determines food web 

complexity; regulates energy flow and carbon 

balance 

Generates spatial and temporal heterogeneity; can 

promote biodiversity 

Regulates nutrient spiraling and primary 

productivity; removes pollutants; influences 

biodiversity 

Carbon and nutrient flux across upland­ Regulates nutrient spiraling, promotes upland 

floodplain and floodplain-river ecotones biodiversity; links riverine and upland ecosystems 

river-floodplain exchange rate

Fluvial and hydrological processes 

Sediment mobilization (erosion), transport, and Controls long-term downstream movement of 

deposition materials; generates spatial and temporal 

heterogeneity in landforms (i.e., patch generation 

and destruction) and aquatic habitats; regulates 

nutrient spiraling and primary productivity; links 

river and flood plain 

Coarse and fine particulate organic matter Controls long-term downstream movement of 

mobilization, transport, and deposition carbon and nutrients; generates spatial and temporal 

heterogeneity in landform substrate quality and 

aquatic habitats (i.e., role in large woody debris 

dynamics); regulates nutrient spiraling and primary 

productivity; links river and flood plain 

Inundation, water infiltration, storage, and Contributes to discharge (flood) control, flow 

release maintenance, and groundwater recharge; 

belowground water mixing and movement links 

upland, floodplain, and river 

Movement of dissolved materials Regulates nutrient spiraling and primary 

productivity; links river and floodplain 

70 
Chapter 8. Summary of Unified Ecosystem Flow Requirements for the 

Bill Williams River Corridor 

By Andrew Hautzinger1, Andrew Warner2, John Hickey3, and Vanessa B. Beauchamp4 

During the Bill Williams River Ecosystem Flow Workshop, participants were split into 

three groups according to their specialties, and each group was tasked with drafting flow 

requirements. The three groups were “Aquatics,” which included coverage of fishes, aquatic 

macroinvertebrates and amphibians (hereafter Aquatics Group); “Riparian Vegetation and Birds” 

(hereafter Riparian-Birds Group); and “Riparian Vegetation and Terrestrial Fauna Other than 

Birds” (hereafter Riparian Non-Birds Group). Both of the riparian groups considered vegetation 

because of its central role in supporting fauna. 

At a minimum, the flow requirements defined by each group addressed baseflows and 

floods and defined the magnitude, timing, duration, and frequency of these events, as well as 

rates of change between different flow conditions. Some groups provided additional resolution, 

characterizing low, moderate, and high baseflows and small, moderate, and large floods. After 

flow requirements were developed, each of the three groups presented theirs in a plenary session. 

The participants were then divided into two new working groups tasked with unifying the 

requirements for baseflows and floods, respectively. These “Unified Baseflow” and “Unified 

Flood” groups were formed by way of a remix of the scientists from the three previous biota 

based groups. The process used to define a unified set of flow requirements for the BWR 

corridor is illustrated in figure 35. 

The unified flow requirements defined for the BWR corridor below Alamo Dam are 

presented in figure 36. Each of the building blocks portrayed in this figure represents an 

expected ecological outcome associated with different flow conditions. The flow requirements 

in the figure consist of baseflows and floodflows, both of which are further delineated as low, 

moderate, or high (for baseflows) and small, moderate, or large (for floods) as denoted in the 

key. In addition to the magnitude of flow, the requirements define the necessary timing and 

duration of the events, and rates of change between event types. The frequency of specific event 

types—for example, once every 5 years—is also included and captures the importance of 

interannual variability in flow conditions. 

The recommended river flows denoted in figure 36 would be generated by water releases 

from Alamo Dam and are targets for release from the reservoir recognizing flow attenuation 

downstream. It should be noted that the moderate and large flood flows suggested in figure 36 

could not be attained unless structural modifications were made to Alamo Dam, which has a 

current maximum outlet capacity of roughly 198.2 m3

/s (7,000 ft3/s). It should also be noted that 

these flow requirements—and especially the flood components—are presented with the 

assumption that commensurate changes would be made to the sediment regime below Alamo 

Dam. Tasks needed to identify and implement appropriate sediment management options 

1

 U.S. Fish and Wildlife Service, Albuquerque, N. Mex. 2

 The Nature Conservancy, Sustainable Waters Program 

3

 U.S. Army Corps of Engineers, Hydrologic Engineering Center 4

 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colo. 

71 
i

Flows defined by: 

•Purpose 

•Magnitude 

•Frequency 

l

l

Flow Components: 

•Flood Flows 

•Baseflows 

Requirements 

•Duration 

•Rate of Change 

•Contingencies 

Full Group 

Aquat cs 

RiparianBirds 

Riparian 

Nonbirds 

Floods 

Baseflows 

Floods 

Baseflows 

Floods 

Baseflows 

Unified F oods 

Unified Basef ows 

Unified Flow 

•Timing •Uncertainties 

Figure 35. A model of the collaboration process used at the Bill Williams Ecosystem Flow 

Workshop held in Tempe, Ariz., March 16–18, 2005. 

associated with larger flow events are discussed below in both the Priority Monitoring and 

Research and Conclusions sections. 

Flow Requirements by Biological Group 

The following sections describe flow requirements developed by the workshop’s three 

biological groups (Aquatics, Riparian-Birds, and Riparian Non-Birds). Each section describes 

the approach followed by a group to develop flow requirements and presents the flow 

requirements and any supporting information used to justify the flow requirements. As the text 

shows, each group’s approach was unique, and each group’s section bears distinctive marks in 

both format and content. The three groups also discussed knowledge gaps and uncertainties. 

Aquatics Group 

Approach 

The Aquatics Group accepted the workshop’s general objective of maximizing native 

biodiversity on the BWR below Alamo Dam and agreed that natural flow patterns may be 

important for maximizing some native biodiversity (e.g., fishes, aquatic invertebrates, and a few 

reptiles and amphibians). Currently, there are no established populations of native fish on the 

BWR below Alamo Dam, but there are good prospects for reestablishing them in this reach: flow 

needs were specified in part to support the possible reestablishment of these native species. The 

group secondarily considered the natural flow patterns for any year as percentages of 

exceedances of average daily, predam flows, which confirmed a consistent seasonal pattern. 

Moreover, it was agreed that high flow “bricks” (the characteristic shape of postdam high flows) 

would have significant negative effects on aquatic life and that flow “spikes” (the natural shape 

of predam high flows) and variability are important (see chapter 2, this report). 

72

73 

Baseflows 

Floods 

20 ft3/s (minimum baseflow); up to 2 months; rare; gradual rates of change 

• fragment aquatic habitat to favor native species 

20-50 ft3/s (common baseflow) 

• maintain aquatic habitat 

• maintain established riparian vegetation 

Season Monsoon Tropical Winter-spring 

>30,000 ft3/s; <2 days; 1:25 yr; rapid rise and fall; best after Oct 1 • establish cottonwood and willow • Remove nonnative fish • Remove beaver dams • Create off-channel habitat • Distribute woody vegetation 

10,000-30,000 ft3/

2-day peak; 

recede: 

 <2.5 cm/day  to low flow (2-

weeks) 

1:5 to 1:10 yr 

avoid floods (2 yr

• Recruitment of 

cottonwood and w

• Minimize tamaris• Scour channel • Remove beaver • Flush nonnative • Clear litter (butte• Elevated groundw

200-400 ft3/s; 2-4 weeks; 

1:1 yr; constant flows 

• Native fish spawning 

4,500-5,000 ft3/s; 

7-10 days; 1:3 yr; 

quick peak, recede: 

 7-10 days  <2.5 cm/day 

• Herbaceous growth • Clear beaver dams • General cleansing • Refresh: - riffle habitat - off-channel pools 100-500 ft3/s; short (hours); 

rapid rise/fall; 1:2 yr 

• Herbaceous 

growth 

1,000-2,000 ft3/s; 

short (hours); 

rapid rise/fall; 1:5 yr 

• Herbaceous 

growth 

• Litter 

decomposition 

• Avoids destroying − SW toad larvae − ground snake 

eggs 

Key 

Small/Low 

Moderate 

Large/High 

>30,000 ft3/s; <2 days; 

1:25 yr; rapid rise/fall; best after Oct 1 

• Preparation for cottonwood 

and willow establishment 

• Remove nonnative fish • Remove beaver dams • Create off-channel habitat • Distribute woody 

vegetation. 

JULY AUG. SEPT. OCT. NOV. DEC. JAN. FEB. MAR. AP

Figure 36. Unified flow requirements for the Bill Williams River, Ariz. Building blocks are used to link ecology to 
3

Flow Requirements and Justifications 

Baseflow Requirements 

To maximize aquatic biodiversity and discourage increases in exotic species richness and 

abundance, the Aquatics Group recommended that baseflows during the tropical season (late 

fall) and winter-spring should be elevated, and dry season (summer) baseflows should be 

depressed for a kind of “cook down,” which is consistent with the predam hydrograph (table 11). 

In dry years, dry season baseflows should be 0.15 m3

/s (5 ft3/s), an amount which was thought to 

fragment stream habitat into isolated pools and allow side channel pools to dry, to the 

disadvantage of nonnative fish and to the benefit of aquatic invertebrates. For the monsoon and 

tropical season in dry years, baseflows should be elevated to 0.57 m3

/s (20 ft3/s) to improve 

native fish habitat and provide cooler temperatures and better water chemistry. Winter-spring 

baseflows should be further elevated to 1.4 m3

/s (50 ft3/s) for greater fish habitat and favorable 

riffle conditions for some aquatic invertebrates. For wet years, this whole seasonal pattern 

should be maintained but with increased magnitudes, as follows: dry 0.28 m3

/s (10 ft3/s); 

monsoon-tropical 1.4 m3/s (50 ft3/s); and winter-spring 2.3 m3/s (80 ft3/s). As a rare event, if 

native fish are absent, baseflows should be dropped to 0 m3

/s (0 ft3/s) for up to 2 months to 

eliminate nonnative fish. 

Floodflow Requirements 

A principal function of flood flows is to maximize habitat diversity by occasionally 

removing beaver dams and scouring off-channel habitats. Another consideration was to 

discourage nonnative fish and bullfrogs (bullfrogs are not established on the BWR but are a 

likely invader). Smaller, more frequent floods would support native fish spawning. Return 

intervals rather than conditions in wet and dry years were specified for floods. For the dry 

season, no floods are recommended because flooding would interfere with leopard frog and 

lentic invertebrate reproduction. No large floods were considered necessary for the monsoon 

season, consistent with the predam hydrograph. For the tropical season, one to two spiked floods 

between 141.6 and 849.5 m3

/s (5,000 and 30,000 ft3/s) were recommended, with a return interval 

of 5 years, mainly to remove beaver dams and to create some off-channel habitats (an objective 

that could also be achieved with spring floods). For early winter-spring (mid-November to midFebruary), a moderate, single spike flood between 28.3 and 70.8 m3

/s (1,000 and 2,500 ft3/s) was 

recommended with a return interval of 1–2 years to refresh the riffle habitat after the beaver 

dams have been removed and to refill side and off-channel pools. For late winter-spring (late 

February–early April), yearly “bricked” (relatively constant) small floods between 5.7 and 11.3 

m /s (200 and 400 ft3

/s) are recommended to enhance native fish spawning. A large flood 

exceeding 1,415.9 m3

/s (50,000 ft3/s) during either the tropical or winter-spring season with a 

return interval of 10 years is recommended to flush out nonnative fish and to create off-channel 

habitats (table 12). 

Riparian-Birds Group 

Approach 

The Riparian-Birds group started by considering the groupings of bird guilds that were 

identified in the chapter 4 of this report and modified them by splitting the shallowwater birds 

into shore and marsh birds and identifying forest birds as passerines. For each of the bird guilds 

the group identified aspects of life-history as they related to seasonal rainfall patterns including 

74

aspects about when breeding, nesting, rearing/fledgling, and migration occurred, as applicable. 

When possible, the group also identified how different birds use the different habitats associated 

with the BWR corridor, with an emphasis on the habitat requirements of the southwestern willow 

flycatcher and western yellow-billed cuckoo (table 13). Using this information, the group then 

linked flow requirements to the bird and vegetation life-history requirements. To create one 

prescription for the Riparian-Birds Group, the group then resolved any conflicts in the proposed 

prescriptions. 

Table 11. Baseflow requirements established by the Aquatics Group at the Bill Williams River 

Ecosystem Flow Workshop. 

Extremely low Low Moderate High 

baseflows baseflows baseflows baseflows 

Name “Cook down” Maintenance Winter-spring Winter-spring maintenance 

(low end) maintenance 

Purpose Hyper-dry Dry conditions to Facilitate fish habitat Facilitate fish habitat and 

conditions to fragment habitat in and favorable riffle favorable riffle conditions 

fragment habitat favor of natives. conditions for for invertebrates. 

in favor of invertebrates. 

natives. 

Timing Variable. Variable. Variable. Variable. 

Magnitude 

Dry Year 

Wet Year 

5 ft3

/s 

10 ft3

/s 

0–10 ft3

/s 

0–10 ft3

/s 

10–50 ft3

/s 

n/a (wet year) 

n/a (dry year) 

50–80 ft3

/s 

Frequency Rare Common Common Less Common 

Duration <2 months Months Months Months 

Rate of change Gradual Gradual Gradual Gradual 

Contingency Natives present? Wet or dry year? Wet or dry year? Wet or dry year? 

75

Table 12. Floodflow requirements established by the Aquatics Group at the Bill Williams River Ecosyst

Small floods Moderate floods Large flood

Name Habitat 

establishment 

Native fish spawning Tropical season 

cleansing 

Tropical season nonnative fish flush 

Winnon

Purpose Refresh riffle habitat 

and refill side and 

Enhancement of native fish spawning, favor 

Removal of beaver dams and creation of 

Flush out nonnative fishes and creation of 

Blofish

off-channel pools. lotic invertebrate taxa. off-channel habitat. off-channel habitat. off-

Timing Mid-November to 

Mid-February 

Late February to early April 

September 15 to November 14 

September 15 to November 14 

NovApr

Magnitude 1,000–2,500 ft3/s 200–400 ft3/s 5,000–30,000 ft3/s >50,000 ft3/s > 5Frequency 1:1–2 years 1:1 years 1:5 years 1:10 years 1:1Duration 3–4 days 2–4-weeks Two separate peaks, One peak, spiked. 1 p

both sharp, spiked events. 

Rate of change None cited. Elevated flows should None cited. None cited. Non

be held relatively constant for the full duration. 

Contingencies None cited. None cited. None cited. OK to occur in the 

Winter-spring Season with a combined 

OK Troa co

frequency of 1:10 freqyears. yea

Uncertainties None cited. None cited. Magnitude needed to 

redistribute woody vegetation. Spatial extent of vegetation removal. 

Actions other than flow manneeded to control/eradicate nare important connections btemperature and fish strandi

Relationship with other reset factors (drought, fire, and infestation). 

76 
Table 13. Bird species life-history requirements by season as established by the Riparian-Birds 

Group at the Bill Williams River Ecosystem Flow Workshop. 

Bird group or Season 

species 

 Monsoon Tropical Winter-Spring Dry 

Deep Water Overwinter, migrate, 

breed. 

Shore Fall migration Spring migration occurs 

commences Sept. 1; Feb. 1– May 1; peaks 

forage on exposed about Apr. 1; forage on 

sandbars. exposed sandbars; 

sandpipers overwinter. 

Marsh Foraging; flow Forage. Forage; breeding occurs Fledging occurs midfluctuations OK; black late March– April; limit May; forage. 

rails and clapper rails floods during breeding 

may have different season. 

needs. 

Predators Overwintering. 

Game Birds Forage (seeds and 

herbaceous plants); for 

juvenile survival, need 

standing water near 

upland edges. 

Forage. Forage. Breed end of May, early 

June; forage. 

Passerines Migrants depart. Residents forage and 

overwinter. 

Residents forage and 

overwinter; migrants 

arrive/breed Feb. 

through Mar.. 

Same as winter through 

June 1; rearing through 

June 30. 

Southwestern Fledging and depart. Arrive about May 1; 

willow breed, nest, rear 

Flycatcher nestlings; require 

saturated soils, standing 

water, and early 

successional 

cottonwood-willow 

Western Rear nestlings, Arrive about June 1 or 

yellow-billed fledging, depart; later; breed, nest. 

cuckoo require mature 

cottonwood with 

understory (patch 

size >15 ha) and soil 

moisture. 

77

Flow Requirements and Justifications 

Baseflow Requirements 

Baseflows were designed to account for year-to-year climatic variability. Transitions in 

required flows generally occurred in accordance with the seasons as defined by rainfall patterns, 

with some exceptions (September 15–November15: tropical storms; November 15–April 30: 

winter-spring frontal storms; May1–June 30: dry; July 1–Sept. 15: monsoons; see Chapter 1, this 

report). It is important that these transitions are made over the course of 2–3 days to prevent 

abrupt changes in flows being experienced by the biota. Particularly for lower flows, the effect of 

the aquifer underlying Planet Valley and its hydrologic status creates significant uncertainties 

regarding the ecological effects of baseflows within and downstream of Planet Valley. 

Baseflow requirements include elevated baseflows of 0.57–1.4 m3

/s (20–50 ft3/s) in the 

monsoon and dry seasons to mediate vegetation stress during the hottest parts of the year and 

therefore, provide microclimate humidity for nesting birds and increase aquatic insect forage. 

Lower baseflows of 0.28–0.71 m3

/s (10–25 ft3/s) were recommended during tropical and early 

winter-spring seasons (mid-September to late December), when water stress is expected to be 

less of an issue, and increased baseflows were recommended later in the winter-spring season 

(0.28–1.1 m3

/s (10–40 ft3/s)) to support new vegetation growth (table 14). 

Floodflow Requirements 

Flood flows were recognized as being important for production of herbaceous growth and 

therefore, creation of associated insect habitat for forage and creation of the necessary conditions 

for recruitment of woody riparian vegetation. Small, short-duration floods were prescribed for 

the monsoon season with frequent (2-year return interval) floods ranging from 2.8 to 14.2 m3

/s 

(100 to 500 ft3

/s) to stimulate herbaceous plant growth and provide standing water for insect 

production. 

Infrequent (6-year return interval) monsoon season floods ranging from 14.2 to 28.3 m3/s 

(500 to 1,000 ft3

/s) were also prescribed, with the recommendation that these floods be 

synchronized with a rainfall event to boost downstream flows (table 15). Moderate (5– to 10– 

year return interval, 383.2–849.5 m3

/s (10,000–30,000 ft3/s)) and large (100-year return interval, 

>849.5 m3

/s (>30,000 ft3/s)) flows in the tropical or winter-spring seasons were recommended to 

stimulate recruitment of cottonwoods and willows (table 15). One important caveat of these 

floods is that they have an initially steep rate of change but the receding limb of the flood must 

be managed to provide a stage change of no more than 2.5 cm/day (1 in./day) to the desired base 

flow and then held constant to provide suitable root growth conditions. When cottonwood and 

willow recruitment is successful, subsequent flooding should be avoided for 2 years to allow 

good vegetation establishment. 

78

Table 14. Baseflow requirements as established by the Riparian-Birds Group at the Bill Williams 

Ecosystem Flow Workshop. 

Time period 

(Season) 

Minimum 

Low flo

(ft3

/s) 

w 

Maximum 

Low flo

(ft3

/s) 

w 

Ecological purpose 

(baseflows) 

July 1–Sept. 15 20 50 Elevated baseflows are needed to mediate vegetation 

(monsoon season). water stress during the hot, unpredictable monsoon 

season, augment microclimate humidity for nesting 

birds, and increase/maintain aquatic insect forage base. 

Sept. 16–Dec. 31 10 25 Reduced baseflows are adequate to meet vegetation 

(tropical season and maintenance needs during a generally low water-stress 

winter-spring season in period. 

part). 

Jan. 1–Apr. 30 

(remainder of winterspring season). 

May 1–June 30 

(dry season). 

10 

20 

40 

50 

Increased baseflows during favorable climatic periods 

are needed to support new vegetation growth. 

Elevated baseflows are needed to mediate vegetation 

water stress during the hot, dry season, augment 

microclimate humidity for nesting birds, and 

increase/maintain aquatic insect forage base. 

79

Table 15. Floodflow requirements established by the Riparian-Birds Group at the Bill Williams River Ecosystem 

Small floods Moderate floods 

Purpose Monsoon forage Monsoon forage Winter forage. Cottonwood-willow Cottonwood-willow 

(small magnitude). (intermediate Stimulate herbaceous recruitment primer. recruitment. 

Stimulate herbaceous magnitude). plant growth for Scour (remove) the Stimulate recruitme

plant growth for Stimulate forage, provide bird vegetation, reworking of cottonwood and 

forage and provide herbaceous plant cover, support the flood plain, willow (may result 

standing water for growth for forage flowering and redistributing limited recruitment 

insect production. and provide seeding, boost insect sediment, and tamarisk). 

standing water for productivity, provide establishing bare soil 

insect production. maintenance flow for sites. 

cottonwood and willow, and saturate soils. 

Timing Mid-July to midAugust. Mid-July to mid- August. February 1 to February 28. Tropical season (9/16 to 11/15), preferably Mid-March to end 

April. 

after October 1 to limit tamarisk recruitment. 

Magnitude 100–500 ft3/s 500–1,000 ft3/s 500–5,000 ft3/s 10,000–30,000 ft3/s 10,000–30,000 ft3/s Frequency 1:2 years 1:6 years 1:3 years 1:15 to 30 years 1:5 to 10 years Duration Short, on the order of 

hours. Drop rapidly. Can have multiple small spikes. 

Short, on the order of hours. One peak. Drop rapidly. 

7 days sustained small flood. 

One peak, spiked. Up and down within 1–2 days. 

2-day duration peak with a multiday tail that blends into the desired baseflow. 

80 
Table 15. Floodflow requirements established by the Riparian-Birds Group. —Continued 

Small floods Moderate floods 

None specified, None Taper lower Steep. A specific rate Lower portion of the Steep. T

Rate of but event is specified, but receding limb at of change is not receding limb should the recechange sharp and brief 

to limit tamarisk 

event is sharp and brief to 

2.5 cm/day. important. More 

important that the 

not exceed a stage change of more than 

Feb. thrnot exce

recruitment. limit tamarisk event simply occur 2.5 cm/day. Maintain more th

recruitment. quickly (in mimicry this rate of change Maintai

of natural down to desired base down to hydrograph). or low flow condition flow con

and hold for 2 to 4 to 4 weeweeks to encourage root grodeep root growth. 

Contingencies None cited. Time with El 

Niño or 

None cited. Try to couple this 

with a cottonwoodAvoid floods for 2 years after this event. In a cott

recruitm

prevailing wet willow recruitment Start small with floods fhydrologic flood in the following respect to flood event. Scycle. Try to spring. magnitude to gage small latime peak with effects on vegetation gage the a rainfall event to boost downstream 

and geomorphic change. 

and chageomor

flows. 

Uncertainties Spatial extent of 

surface flows, and as a consequence the ecological benefit, needs to be determined for these flows. 

Spatial extent of surface flows, and as a consequence the ecological benefit, needs to be determined for these flows. 

Even at the upper range, it is uncertain whether the flows will be high enough to benefit mesquite bosque community. 

Magnitude needed to remove vegetation and rework the flood plain is unknown. 

Transition point from peak to start of 2.5 cm/day recession during the receding limb may not be one set value depending on channel/floodplain condition. The time to hold at a low base 

Magnituremove unknowspatial eremoval amount quantifiwith othprocesse

flow after recession fire, and to encourage root unknowgrowth is uncertain. uncertai

moderat

81 
Riparian Non-Birds Group 

Approach 

The Riparian Non-Birds Group identified a suite of riparian obligate mammals, reptiles, 

amphibians, and invertebrates native to the BWR for which the life-history was fairly well known. This 

list included the viceroy butterfly, beaver, ground snake (Sonora semiannulata) , desert shrew 

(Notiosorex crawfordi), southwestern toad (Bufo microscaphus), cotton rat and hoary bat (Lasiurus 

cinereus). For each species, critical times in their life-history such as dormancy, and reproductive 

periods, as well as foraging needs, which may be dependent on or could be impacted by the flow regime 

were identified. A majority of these species were directly or indirectly dependent on cottonwoodwillow habitat or mesquite for habitat or forage. For example, the desert shrew and ground snake forage 

on invertebrates found in cottonwood-willow litter, the hoary bat roosts in cottonwood and willow trees 

and viceroy butterfly caterpillars feed on willow trees. The cotton rat is an exception in that it forages in 

grassy herbaceous areas rather than in the cottonwood understory. Given the importance of these types 

of forest and open floodplain habitats, flow requirements were defined to produce these habitats, 

particularly cottonwood-willow forests and mesquite bosques. These flow requirements were then 

adjusted or added to as necessary to accommodate the mammals and herptofauna discussed, with an 

emphasis on avoiding stressful conditions (large floods or low baseflows) during vulnerable life stages. 

Flow Requirements and Justifications 

Baseflow Requirements 

As with the Riparian-Birds Group, this group also recognized the importance of baseflows for 

maintaining water levels for plant growth and flood flows for stimulating recruitment of cottonwoods 

and willows. Wet-year low flows between 0.28 and 1.4 m3

/s (10 and 50 ft3/s) and dry-year low flows 

between 0.28 and 2.8 m3

/s (10 and 100 ft3/s) were recommended with the contingency that these flows 

should vary within this range to stimulate deep root growth which would allow vegetation to survive 

drought conditions (table 16). 

Floodflow Requirements 

Floodflow requirements included wet-year winter-spring floods greater than 198.2 m3/s (7,000 

ft3

/s) 1 out of every 10 years and dry-year winter-spring floods of 56.6 m3/s (2,000 ft3/s) every 2–3 years 

to create conditions necessary for cottonwood and willow reestablishment and to flush out beaver dams 

(table 17). The Riparian Non-Birds Group also recognized the importance of slow flood recession (<2.5 

cm/day) and avoidance of other floods for 2 years after a recruitment event to promote successful tree 

establishment. Summer monsoon pulse floods (28.3–56.6 m3

/s (1,000–2,000 ft3/s) once every 2–3 years) 

were also recommended to stimulate herbaceous plant growth and litter decomposition (table 17). 

82

Table 16. Baseflow requirements established by the Riparian Non-Birds Group at the Bill Williams 

River Ecosystem Flow Workshop. 

Wet year—Low Flow Dry Year—Low Flow 

Purpose Maintain water levels for plant growth. Maintain water levels for plant growth. 

Timing all year all year 

Magnitude 10– 50 ft3

/s 10–100 ft3/s 

Frequency Continuous. Continuous. 

Duration Continuous. Continuous. 

Rate of change Flows should vary between high and low Flows should vary between high and low 

magnitude. magnitude. 

Contingency No scouring floods of over 500 ft3

/s after 

first year post-event and 500–1,000 ft3

/s 

Raise baseflow overall during the driest part of 

year to reduce stress (10–100 ft3/s). 

floods permissible in the second year 

Table 17. Floodflow requirements established by the Riparian Non-Birds Group at the Bill Williams 

River Ecosystem Flow Workshop. 

Wet-year flood Dry-year—winter flood Dry-year—summer flood 

Purpose Scouring of channel 

and flood plain for 

Scouring of channel and for 

reestablishment of cottonwood 

To help herbaceous plant growth 

and litter decomposition. Timing 

reestablishment of and willow. Timing in accord is to avoid destruction of ground 

cottonwood and with seeding of these trees, but snake eggs and southwestern toad 

willow. Timing in before salt cedar seeding and to larvae. 

accord with seeding avoid dormancy and breeding of 

of these trees, but animals earlier in the year. 

before Tamarix To flush out beaver dams, to 

maximum seeding elevate water table for 

herbaceous plants, to clear out 

litter for viceroy butterflies, and 

to wash out exotic species. 

Timing Late Feb–Apr. 1 Late Feb–Apr. 1 July 15–Sept. 15 

Magnitude >7,000 ft3

/s 2,000 ft3/s 1,000–2,000 ft3/s 

Frequency 

Duration 

1:10 

2–7 days 

Once every 2–3 years 

2–3 days (at Lake Havasu) 

Once every 2–3 years 

Hours 

Rate of change Quick start up and 

gradual recession— 

Quick start up and gradual 

recession—about 2.5 cm/day 

Quick start up and shut down. 

about 2.5 cm/day (change in water level). 

(change in water 

level). 

Contingency No further scouring 

floods afterwards for 

2 years—maintain 

Raise baseflow overall during 

the driest part of year to reduce 

stress (10–100 ft3

/s). 

Flooding will likely depend upon 

seasonal weather conditions. 

high baseflows. 

83 
Unification of Biological-Group Flow Requirements 

Unified Baseflows 

Process 

A constant theme in the unified baseflows discussions dealt with the recognition that the BWR 

exhibited great longitudinal variability, such that at a given baseflow release out of Alamo Dam, many 

different flows would likely be expressed over the project area. While this spatial variation certainly 

complicated the analysis, it was also considered a factor likely to provide a range of appropriate flows 

for a wide suite of the biota via the anticipated constant baseflow releases from the dam. 

One point of difference between the groups related to varying degrees of interest in the seasonal 

timing of flow changes. The Aquatics Group was interested in an increase in the winter baseflows, 

while the riparian groups saw a buffering benefit via higher baseflows during the heat of the summer. 

Similarly, the Aquatics Group’s typical flow requirements were more static than were those of the 

riparian groups, which recommended more frequent intraseason and interseason variations in baseflows 

(e.g., to facilitate root development). The Aquatics Group indicated that there were fairly high levels of 

uncertainty associated with the recommended flow magnitudes, but this group had an appreciably 

higher level of confidence with regard to the shape of a recommended hydrograph. 

All three biotic groups identified the need to have different baseflows based on antecedent 

conditions (e.g., in the event of floods that recruit new growth, higher baseflows would be desirable for 

a period of time afterwards to support the newly established seedlings). The Aquatics Group especially 

focused on different baseflow regimes being a function of either being in a dry year or a wet year. 

The most pronounced difference between the three groups’ requirements was that the Aquatics 

Group emphasized the need for a period of extremely low baseflows or no flows, which they referred to 

as a “cook down” event that would tend to favor native over nonnative aquatic species. It was pointed 

out that while many native species have mechanisms to survive periods of extreme drought, the issue of 

recolonization by natives is of concern because of the current lack of refugia. Neither of the riparian 

groups articulated this “cook down” element as a beneficial flow component. 

Results 

Extremely low Baseflows (~0–5 ft 3/s) 

As noted above, extremely low baseflows were only advocated by the Aquatics Group and were 

not explicitly carried forward into the unified baseflows. The need for a rare “cook down” event was 

tied to the desire to fragment stream habitat into isolated pools to the anticipated advantage of the native 

species. The two riparian groups were concerned with the risks to the riparian vegetation associated 

with severely depressed water levels. 

The unification process forged a compromise in which there would be no releases below 0.57 3

m /s (20 ft3

/s), somewhat based on the longitudinal variation (described above), which would likely 

create “cook down” conditions in limited areas along the river during periods of lower baseflow 

releases. It was agreed by all that it is crucial that appropriate monitoring be conducted to better 

understand how the lowest flows manifest throughout the project area. 

Low Baseflows (0–10 ft 3/s) 

Again, the Unified Baseflow Group recommended that there be no baseflows below the 0.57 3

m /s (20 ft3

/s) level. Hence, the recommendation for extremely low baseflows (0–0.28 m3/s (0–10 ft3/s)) 

was essentially the same as the Aquatic Group’s “cook down” recommendation described above. 

84

Moderate Baseflows (10 to 50 ft 3

/s) 

All three groups saw important benefits associated with moderate baseflows, especially in 

acknowledgment that these moderate baseflows will probably be the system’s most common flow 

condition. Both riparian groups advocated flows in this category to range from 0.57 to 1.4 m3

/s (20 to 

50 ft3

/s), with a major advantage being groundwater recharge and an associated higher water table for 

benefit to vegetation. The Riparian Non-Birds Group also saw a possible benefit in decomposition of 

plant materials (processes that require some degree of moisture), although there exists uncertainty as to 

what level of flows would be needed to support this function. The Aquatics Group saw benefit in 

keeping flow levels consistently elevated from September15–April 30. The group also suggested 

making different baseflow prescriptions for wet or dry flow years. 

While the groups expressed appreciation that water availability will often be a strongly limiting 

factor, all three groups saw distinct advantages for baseflows to be typically higher rather than lower. 

Aquatically, higher baseflows would improve water quality conditions (e.g., lower temperatures, higher 

dissolved oxygen) and would thus tend to reduce the biological stressors during the heat of the summer; 

however, longer term high baseflows were not advocated by the Aquatics Group to be included in the 

reconciled baseflow requirement. With respect to vegetation, higher baseflows would typically translate 

into higher water table elevations, which would be advantageous for riparian plants. All three groups 

also saw advantages from occasional baseflows on the lower end of this range. As described above, the 

Aquatic Group advocated the “cook down” concept. Both riparian groups indicated an interest in 

occasional short-duration lowering of water levels to facilitate more extensive root development. Short 

duration drops in water table level were especially of interest in those periods of time following 

establishment events. 

High Baseflows (50–100 ft 3/s) 

There was little expressed interest in baseflows in the 1.4–3 m3/s (50–100 ft3/s) range. Group 

discussion focused on how this type of flow was not a common part of the natural hydrograph, except 

typically on the declining limb of a flood, and therefore flow requirements for these types of flows 

would more logically be left to the Unified Flood Flows Group. It is also likely that the goals associated 

with the higher end of the moderate baseflows (up to 1.4 m3

/s (50 ft3/s)) also would apply to this flow 

category. With the uncertainties associated with how a given low flow will manifest in a given reach, 

the group consensus was to not explicitly call for baseflows in the 1.4–2.8 m3

/s (50–100 ft3/s) range. 

Unified Baseflows 

Most of the baseflows recommended by the three biotic groups had significant overlap with each 

other and were easily unified (table 18). The Unified Baseflow Group’s discussions centered primarily 

on issues related to uncertainties on how baseflows would be expressed across the project area. This 

inability to route or model baseflows through the project area, especially through the Planet Ranch 

reach, was a constant issue during the Unified Baseflow Group’s discussion. It was the group’s 

unanimous opinion that resources needed to be prioritized to address flow-routing questions. 

It was viewed as a critical objective that the BWR system be sufficiently understood such that 

physical goals (tied to ecological objectives) be rendered in terms of quantifiable flow levels. Further, 

there emerged a philosophical consensus that these goals and objectives not be hardwired but that they 

rather have built-in ranges and variations such that they more closely mimic natural dynamics. This 

mimicry of the natural dynamics was thought to be especially critical in transition zones of the 

hydrograph, as typified by the declining limb of a flood hydrograph. 

85

Table 18. Unified baseflow requirements developed at the Bill Williams Ecosystem Flow Workshop . 

Extremely low baseflows (0–5 ft3

/s) Moderate baseflows (10 ft3/s–50 ft3/s) 

Name “Cook down” Maintenance 

Purpose Hyper-dry conditions to fragment Aquatic and riparian habitat maintenance 

habitat in favor of natives. and (contingency) stimulate root 

development with younger plants. 

Timing n/a ~ 1year long 

Magnitude 20 ft3

/s 10– 50 ft3/s 

Frequency No flows<20 ft3

/s Common 

Duration No flows<20 ft3

/s Weeks/months 

Rate of change n/a Gradual 

Contingency Monitor and evaluate ??? 

Unified Floods 

Floodflow requirements, developed independently by the Aquatics, Riparian-Birds, and Riparian 

Non-Birds Groups, were merged into one set of requirements referred to herein as Unified Floods (table 

19). Floods were defined by name, purpose, timing, magnitude, frequency, duration, rates of change, 

contingencies, and uncertainties. 

Process 

The Unified Floods Group began by comparing floods recommended by each of the three 

groups. In general, the independent requirements were more similar than dissimilar. The 

recommendations of the Riparian groups corresponded especially well for their winter-spring moderate 

floods designed to regenerate cottonwood and willow riparian areas and their monsoon small floods to 

stimulate growth of herbaceous plants. The biggest difference was for the frequency and magnitude of 

large floods. The Aquatics Group recommended both the largest and most frequent large flood event, 

calling for floods of greater than 1,415.9 m3

/s (50,000 ft3/s) with a return period of 1:10 years. The 

Riparian-Birds Group recommended smaller and less frequent events calling for floods of greater than 

849.5 m3

/s (30,000 ft3/s) with a return period of 1:100 years. The Riparian Non-Birds group did not 

request any floods near those magnitudes. 

The unification process involved pulling different recommended floods into single events 

without sacrificing any purposes of the original flood events. When the Unified Floods Group was 

concerned that an original purpose might not be accomplished by the flood event after merging, this 

concern was discussed. If the discussion did not resolve the concerns, the flood in question was treated 

as a separate flood event in the Unified Floods requirements. 

Results 

Small Floods (100–5,000 ft 3

/s) 

Both Riparian groups recommended periodic small floods in the monsoon season to support 

herbaceous plant growth, which is an important source of forage for insects, birds, and animals. The 

Aquatics Group also advocated a flood designed to facilitate fish-spawning conditions. Other groups 

recommended small floods to reorganize channel and sandbar sediments and stimulate herbaceous 

growth, which provides winter foraging habitat for birds, and to remove beaver dams and cleanse the 

system of exotic fish and aquatic invertebrates. The magnitude was set at the upper end of the range, 

while the timing was set according to the requirements of the Riparian-Birds Group to encourage an 

advantageous creation of seasonal forage for passerines and upland game birds (table 19). 

86

Moderate Floods (5,000–30,000 ft 3

/s) 

The Riparian Non-Birds group had two flood events that were merged into this unified flood. 

Some purposes of their cottonwood and willow recruitment event (scouring, removal of beaver dams, 

litter, and exotics) were also partially taken care of by the winter forage and cleansing flood discussed 

above (table 19). An important contingency for this flood is incorporation of the annual native fish 

spawn. To accommodate the spawn, when the recession limb is between 5.7 and 11.3 m3

/s (200 and 400 

ft3

/s), flows should be held relatively steady for 2–4 weeks before continuing to gradually recede. 

Large Floods (Greater than 30,000 ft 3/s) 

The Aquatics Group had two flood events that were merged into this unified flood event. 

Purposes of their tropical season cleansing event (removal of beaver dams and creation of off-channel 

habitat) were also partially taken care of by the unified cottonwood and willow recruitment flood 

discussed above. 

The combined frequency (1:10 years) recommended by the Aquatics Group for blowing out 

nonnative fishes in the tropical and winter-spring seasons was decreased during unification (1:25 years), 

but this reduction in frequency was offset by decoupling their blowout floods in the tropical and winterspring seasons. In other words, Unified Floods Group called for two independent high flood events, 

whose purposes include blowing out nonnative fishes, each with a frequency of 1:25 years. 

The increase in magnitude during unification was acceptable to the Riparian-Birds Group, 

though it was reinforced that the value of this flood (in terms of setting the stage for renewal of riparian 

habitat) would be maximized if coupled with a cottonwood-willow recruitment flood in the following 

spring. 

The second large flood recommendations came from the Aquatics and Riparian-Birds Groups, 

and aligned quite well with each other. The increase in frequency from the Riparian-Birds Group’s 

requirements (1:100 to 1:25 years) was acceptable, but representatives from that group stressed that the 

BWR is a critical resource for migratory birds and that these large floods should start near the bottom of 

the specified flow ranges to allow scientists to gage the effects on bird habitat before progressing to 

higher flows. 

Unified Floods 

Most floods recommended by the biotic groups had significant overlap with each other and were 

easily unified. Debate within the Unified Floods Group focused mainly on (1) the degree of flexibility 

in timing floods designed to cleanse the system, which was agreed to be quite high; (2) the frequency of 

large floods, which essentially meant striking a balance between protecting the current ecological value 

of the BWR by cautiously exploring high flows and the need to reestablish those high flows as a 

powerful disturbance mechanism that encourages more dynamic and natural behavior of the ecosystem; 

and (3) the philosophical relationship between the cottonwood-willow recruitment flood and the higher 

and less frequent floods designed to rework the riparian areas and blow out the nonnative fishes, which 

was described by the group as an analogy of the recruitment floods being used to manage crop rotation 

for the cottonwood and willow while the higher floods were used to clear the fields. 

The single most important caveat for the Unified Floods is that these requirements assume that 

changes to the flood regime will be implemented with commensurate changes to the sediment budget of 

the BWR below Alamo Dam. This coupling of the flood regime and sediment regime is of critical 

importance. If any of the moderate or large floods (peak flows greater than or equal to 283.2 m3

/s 

(10,000 ft3

/s)) were implemented without an accompanying sediment adjustment, the effects 

downstream are expected to be quite negative. 

87

Table 19. Unified floodflow requirements developed at the Bill Williams Ecosystem Flow Workshop. 

Small floods Moderate floods 

Name Purpose 

Monsoon forage (low) Stimulate herbaceous growth for forage. 

Monsoon forage (high) Herbaceous plant growth and litter decomposition. 

Native fish spawning Enhancement of native fish spawning. 

Winter forage and cleansing Herbaceous growth, beaver dam removal, general cleansing. 

Cottonwood and willow recruitment 

Stimulate mixed recruitment of cottonwood and willow (with a little tamarisk). 

Nonflusripa(tro

Nonblomandistwooveg

Timing Magnitude 

Mid-July to mid-August. 100–500 ft3/s 

Mid-July to midAugust. 

1,000–2,000 ft3/s 

Late February to early April. 

200–400 ft3/s 

February 1 to February 28. 

4,500–5,000 ft3/s 

March 15 to April 30. 

10,000–30,000 ft3/s 

Sep

NovpreOctlimrecr

>30

Frequency 1:2 years 1:5 years Annually 1:3 years 1:5–10 years 1:2Duration Very short, Very short, rapid 2–4 weeks. Peak quickly then 2-day spike with a multiday One 

rapid drop (hours). One peak. recede for 7–10 tail that blends into spik(hours). Can days. recession. dowhave multiple dayshort spikes. 

Rate of None, but Quick start up and Elevated flows Taper lower Lower receding limb should Stechange event is sharp 

and brief to 

shut off. should be held 

relatively 

receding limb at 2.5 cm/day. 

not fall more than 2.5 cm/day. Recede down to a 

keybe q

limit tamarisk constant for low baseflow and hold at recruitment. the full the low base for 2–4 weeks 

duration. to encourage deep root 

growth. 

Contingency None cited. Look for link to 

regional climactic conditions. 

None cited. None cited. Avoid floods for 2 years 

after this event. Start small, but be aggressive in terms of increasing magnitude of peak within specified range. Accommodate annual native fish spawning flood as part of recession. 

Try this cottwilrecrin tspri

88 
Discussion of Knowledge Gaps, Priority Monitoring, and Research Needs 

Each of the biotic groups noted uncertainties and knowledge gaps identified while 

formulating their flow requirements. Several knowledge gaps were echoed by multiple 

biota-based groups and by the subsequent flow-based groups. The following sections 

summarize knowledge gaps from all of the workshop groups; these gaps presented 

roughly in order of priority. 

Sediment Budget and Transport Dynamics 

The sediment budget of the BWR has been very much altered by the presence of 

Alamo Dam, which is effectively a sink for all coarse sediment mobilized upstream of the 

lake. Below the dam, unregulated tributaries continue to be productive sediment 

sources, but the ability of flows in the BWR below Alamo Dam to redistribute this 

sediment has been greatly reduced by decreases in large flood flows. There are 

unresolved questions about (1) the sustainability and evolution of the current channel and 

riparian forest since Alamo Dam was constructed, (2) potential negative effects resulting 

from implementation of the ecological flow requirements—the flood flows in 

particular—without commensurate adjustments to the sediment budget, and (3) gradation 

of sediment deposited in floodplain areas and whether these materials are conducive to 

plant growth. The workshop participants articulated the need to develop sediment 

budgets and sediment transport models to evaluate this critical system component. 

Magnitude and Duration of Flows Required to Achieve the Purposes of the 

Ecological Flow Requirements, Especially for the Prescribed Floods 

There was a general uncertainty about whether the different components of the 

ecological flow requirements would achieve their designed purposes, especially with 

respect to the flood flows. This uncertainty was mentioned by all the groups in ways that 

implied (1) that the groups anticipate thresholds of flood magnitude that will need to be 

met or exceeded to accomplish specific functions, (2) that the flood flow requirements 

formulated in this workshop reflected the biotic groups’ best estimates of these 

thresholds, and (3) that there is a need to better define these thresholds through 

experimentation and adaptive management. Specific uncertainties listed by the groups 

related to this topic included flow magnitudes needed to remove beaver dams, mobilize 

sediment, remove vegetation, stimulate decomposition, rework floodplain areas, and 

depress the nonnative fish population. It was agreed that further streamflow and 

groundwater level monitoring efforts, done in tandem with acquisition of biological data, 

were key to further develop the linking of flows to ecological responses. 

Connections Between Surface Water, Ground Water, and Related Biotic 

Communities 

The BWR below Alamo Dam courses through a series of narrow canyons and 

wider alluvial valleys. The valleys tend to have a moderating effect on stream flows 

where surface flows recharge underlying aquifers in times of high flow, and those 

aquifers later augment surface flows in times of low flow. There was a general 

uncertainty regarding connections between stream flow and the underlying aquifers, 

especially with regard to Planet Valley. This uncertainty was spatial and temporal in 

nature (how do surfacewater releases from Alamo Dam translate downstream to Lake 

89

Havasu, what are the groundwater responses, and how do these relationships vary 

seasonally?) and was a concern for baseflows and, to a lesser extent, small and moderate 

floods. These questions were related back to rooting dynamics and the sustainability of 

riparian vegetation as a function of groundwater levels. A need to better understand the 

impacts of (and to develop contingencies for changes in) groundwater pumping in the 

basin was also noted. 

Role of Floods in the Tropical and Monsoon Seasons 

Tropical and monsoon floods occur less frequently and tend to be of shorter 

duration and lower magnitude than those in the winter-spring season, but they can still be 

powerful flood events. Also, because tropical and monsoon seasons follow the dry 

season (during which any flows above minimal baseflows are quite rare) flow 

fluctuations in those seasons may be important hydrologic stimuli from an ecological 

perspective. There was a general uncertainty regarding the ecological roles of high flows 

in the tropical and monsoon seasons, including whether tropical floods were important 

precursors for successful recruitment of cottonwoods and willows during winter-spring 

floods, effects of monsoon floods on tamarisk, and whether there would be ecological 

consequences if tropical floods were not prescribed. 

Nutrient Cycling, Decomposition, and the Role of Fire 

There was a general uncertainty regarding connections between flow, 

decomposition, nutrient cycling, and fire. Knowledge gaps noted by the groups created 

an interrelated sequence of uncertainties beginning with fundamental questions about the 

drivers of decomposition and how nutrients cycle within the system. These questions led 

to recognition of uncertainties about the balance of litter production and decomposition 

and the potential increase in fire frequency in situations where production outpaces 

decomposition. This discussion of a potential increase in fire frequency, led to the 

question of the role of fire as a disturbance mechanism for riparian forests. 

Conclusions 

The results from this workshop will be used to further the commitment of the U.S. 

Army Corps of Engineers and the other members of the Bill Williams River Corridor 

Steering Committee to make science-based management decisions with regard to the 

operation of Alamo Dam and the affected environment. This redoubled commitment on 

the part of member agencies builds upon over 15 years of flow-related work on the BWR 

system. The history on the BWR provides a body of work that has consistently 

acknowledged the core need to meaningfully apply the concept of adaptive management. 

The implementation of an effective adaptive management scheme is of high 

priority to the Bill Williams River Corridor Steering Committee, and it is clear to the 

committee that with complicated systems like that of the BWR, the ability to adaptively 

manage is proportional to the commitment to monitoring and experimentation. The 

workshop’s defined flow requirements will be evaluated to identify which components 

can be immediately implemented and which will need further study. The knowledge and 

research gaps that were identified will be used to develop a strategic monitoring plan and 

priority research agenda, and opportunities will be identified to better link reservoir 

operations with scientific research. 

90

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103

Appendix A. Stage-Discharge Relationships Along the Bill 

Williams River. 

Table A1. Approximate locations of eight cross sections used to estimate stage-discharge 

relations along the Bill Williams River, Ariz. Cross sections are in downstream to 

upstream order. River miles are miles upstream of Highway 95 Bridge (see fig. 4). 

Cross section number Location description Reach (fig. 4) Analysis 

1 River mile 3 Reach 11 fig. A.1 

2 River mile 5.4 Reach 10 fig. A.2 

3 River mile 6.3 Reach 10 fig. A.3 

4 River mile 8.8 Reach 10 fig. A.4 

5 River mile 24.5 Reach 7 fig. A.5 

6 River mile 19.3 Reach 5 fig. A.6 

7 River mile 28.6 Reach 3 fig. A.7 

8 River mile 30.9 Reach 1 fig. A.8 

104

Figure A1. Bill Williams River, Ariz., Transect 1 with stages marked for discharges of 500, 

5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

105 
Figure A2. Bill Williams River, Ariz., Transect 2 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

106 
Figure A3. Bill Williams River, Ariz., Transect 3 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

107 
Figure A4. Bill Williams River, Ariz., Transect 4 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

108 
Figure A5. Bill Williams River, Ariz., Transect 5 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

109 
Figure A6. Bill Williams River, Ariz., Transect 6 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

110 
Figure A7. Bill Williams River, Ariz., Transect 7 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

111 
Figure A8. Bill Williams River, Ariz., Transect 8 with stages marked for discharges of 

500, 5000 and 30,000 ft3

/s. Lower graphs show (a) stage-discharge, (b) wetted perimeterdischarge, (c) velocity-discharge, and (d) cross-sectional area-discharge relationships. 

112 
Appendix B. Birds of the Bill Williams River Corridor 

Table B1. Deepwater bird species found within the corridor of the Bill Williams River, Ariz. 

Group/common name Scientific name Nativity Group/common name Scientific name Loons Duck, geese, and swans Red-throated loon Gavia stellata native Tundra swan Cygnus columbianus Pacific loon Gavia pacifica native Greater white-fronted goose Anser albifrons Common loon Gavia immer native Snow goose Chen caerulescens Yellow-billed loon Gavia adamsii native Ross’s goose Chen rossii Grebes Canada goose Branta canadensis Least grebe Tachybaptus dominicus native Canvasback Aythya valisineria Pied-billed grebe Podilymbus podiceps native Redhead Aythya americana Horned grebe Podiceps auritus native Ring-necked duck Aythya collaris Red-neck grebe Podiceps grisegena native Greater scaup Aythya marila Eared grebe Podiceps nigricollis native Lesser scaup Aythya affinis Western grebe Aechmophorus occidentalis native Surf scoter Melanitta perspicillaClark’s grebe Aechmophorus clarkii native White-winged scoter Melanitta fusca Storm-petrels Long-tailed duck Clangula hyemalis Black storm-petrel Oceanodroma melania native Bufflehead Bucephala albeola Least storm-petrel Oceanodroma microsoma native Common goldeneye Bucephala clangula Boobies Barrow’s goldeneye Bucephala islandica Blue-footed booby Sula nebouxii native Hooded gerganser Lophodytes cucullatuBrown booby Sula leucogaster native Common gerganser Mergus merganser Pelicans Red-breasted merganser Mergus serrator American white pelican Pelecanus erythrorhynchos native Ruddy duck Oxyura jamaicensis Brown pelican Pelecanus occidentalis native Cormorants Frigatebirds Neotropical cormorant Phalacrocorax brasilMagnificent frigatebird Fregata magnificens native Double-crested cormorant Phalacrocorax auritu

113 
Table B2. Shallowwater and shore bird species found within the corridor of the Bill Williams River, Ariz. 

Group/common name Scientific name Nativity Group/common name Scientific name Sandpipers, phalaropes, and allies Stilts and Avocets Greater yellowlegs Tringa melanoleuca native Black-necked stilt Himantopus mexicaLesser yellowlegs Tringa flavipes native American avocet Recurvirostra amerSolitary sandpiper Tringa solitaria native Gulls and Terns Willet Catoptrophorus semipalmatus native Parasitic jaeger Stercorarius parasSpotted sandpiper Actitis macularius native Franklin’s gull Larus pipixcan Whimbrel Numenius phaeopus native Bonaparte’s gull Larus Philadelphia Long-billed curlew Numenius americanus native Heermann’s gull Larus heermanni Marbled godwit Limosa fedoa native Ring-Billed gull Larus delawarensis Western sandpiper Calidris mauri native California gull Larus californicus Least sandpiper Calidris minutilla native Herring gull Larus argentatus Baird’s sandpiper Calidris bairdii native Thayer’s gull Larus thayeri Dunlin Calidris alpina native Western gull Larus occidentalis Short-billed dowitcher Limnodromus griseus native Sabine’s gull Xema sabini Long-billed dowitcher Limnodromus scolopaceus native Caspian tern Sterna caspia Wilson’s snipe Gallinago delicata native Common tern Sterna hirundo Wilson's phalarope Phalaropus tricolor native Arctic tern Sterna paradisaea Storks Forster’s tern Sterna forsteri Wood stork Mycteria americana native Black tern Chlidonias niger Ibises Cranes White-faced ibis Plegadis chihi native Sandhill crane Grus canadensis 

114

Table B2. Shallowwater and shore bird species found within the corridor of the Bill Williams River, Ariz. —Conti

115


Group/common name Scientific name Nativity Group/common name Scientific name 

Duck, geese, and swans Herons, bitterns, and allies Wood duck Aix sponsa native American bittern Botaurus lentiginosus 

Gadwall Anas strepera native Least bittern Ixobrychus exilis 

Eurasian wigeon Anas penelope native Great blue heron Ardea herodias 

American wigeon Anas americana native Great egret Ardea alba 

Mallard Anas platyrhynchos native Snowy egret Egretta thula 

Blue-winged teal Anas discors native Little blue heron Egretta caerulea 

Cinnamon teal Anas cyanoptera native Cattle egret Bubulcus ibis 

Northern shoveler Anas clypeata native Green heron Butorides virescens 

Northern pintail Anas acuta native Back-crowned night-heron Nycticorax nycticorax Green-winged teal Anas crecca native 

Lapwings and plovers Black-bellied plover American golden-plover Snowy plover Semipalmated plover Killdeer 

Pluvialis squatarola native 
Pluvialis dominica native 
Charadrius alexandrinus native 
Charadrius semipalmatus native 
Charadrius vociferus native 

Table B3. Predator and scavenger bird species found within the corridor of the Bill Williams River, Ariz. 

Group/common name Scientific name Nativity Group/common name Scientific name New world vultures Caracaras and falcons Turkey vulture Cathartes aura native Crested caracara Caracara cheriway California condor Gymnogyps californianus native American kestrel Falco sparverius Hawks, kites, eagles, and allies Merlin Falco columbarius Osprey Pandion haliaetus native Peregrine falcon Falco peregrinus White-tailed kite Elanus leucurus native Barn owls Mississippi kite Ictinia mississippiensis native Barn owl Tyto alba Bald eagle Haliaeetus leucocephalus native Typical owls Northern harrier Circus cyaneus native Flammulated owl Otus flammeolus Sharp-shinned hawk Accipiter striatus native Western screech-owl Megascops kennicottii Cooper’s hawk Accipiter cooperii native Great horned owl Bubo virginianus Northern goshawk Accipiter gentilis native Elf owl Micrathene whitneyi Common black-hawk Buteogallus anthracinus native Burrowing owl Athene cunicularia Harris-hawk (Historic) Parabuteo unicinctus native Long-Eared owl Asio otus Red-shouldered hawk Buteo lineatus native Short-Eared owl Asio flammeus Broad-winged hawk Buteo platypterus native Northern saw-whet owl Aegolius acadicus Swainson’s hawk Buteo swainsoni native Zone-tailed hawk Buteo albonotatus native Red-tailed hawk Buteo jamaicensis native Ferrugious hawk Buteo regalis native Golden eagle Aquila chrysaetos native 

116

117


Table B4. Game bird species found within the corridor of the Bill Williams River, Ariz. 

Group/common name Scientific name Nativity Group/common name Scientific name NTurkeys Pigeons and doves Wild turkey Meleagris gallopavo native Rock dove Columba livia nNew world quail Band-tailed pigeon Patagioenas fasciata nGambel’s quail Callipepla gambelii native White-winged dove Zenaida asiatica nRails, gallinules, and coots Mourning dove Zenaida macroura nBlack rail Laterallus jamaicensis native Inca dove Columbina inca nClapper rail Rallus longirostris native Common ground-dove Columbina passerina nSora Porzana carolina native Common moorhen Gallinula chloropus native American coot Fulica americana native 

Table B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. 

Group/common name Scientific name Nativity Group/common name Scientific name Cuckoos and Roadrunners Hummingbirds Yellow-billed cuckoo Coccyzus americanus native Broad-billed hummingbird Cynanthus latirostris Greater roadrunner Geococcyx californianus native Black-chinned hummingbird Archilochus alexandrGoatsuckers Anna’s hummingbird Calypte anna Lesser nighthawk Chordeiles acutipennis native Costa’s hummingbird Calypte costae Common nighthawk Chordeiles minor native Calliope hummingbird Stellula calliope Common poorwill Phalaenoptilus nuttallii native Broad-tailed hummingbird Selasphorus platycercSwifts Rufous hummingbird Selasphorus rufus Black swift Cypseloides niger native Kinglets Vaux’s swift Chaetura vauxi native Golden-crowned kinglet Regulus satrapa White-throated swift Aeronautes saxatalis native Ruby-crowned kinglet Regulus calendula 
Table B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. —Continued 

Group/common name Scientific name Nativity Group/common name Scientific name Gnatcatchers Tyrant flycatchers Blue-gray gnatcatcher Polioptila caerulea native Olive-sided flycatcher Contopus cooperi Black-tailed gnatcatcher Polioptila melanura native Greater pewee Contopus pertinax Woodpecker and allies Western wood-pewee Contopus sordidulus Lewis’ woodpecker Melanerpes lewis native Willow fycatcher Empidonax traillii Acorn woodpecker Melanerpes formicivorus native Hammond’s flycatcher Empidonax hammondii Gila woodpecker Melanerpes uropygialis native Gray flycatcher Empidonax wrightii Williamson’s sapsucker Sphyrapicus thyroideus native Dusky flycatcher Empidonax oberholseri Yellow-bellied sapsucker Sphyrapicus varius native Pacific-slope flycatcher Empidonax difficilis Red-naped sapsucker Sphyrapicus nuchalis native Black phoebe Sayornis nigricans Ladder-backed woodpecker Picoides scalaris native Eastern phoebe Sayornis phoebe Northern flicker Colaptes auratus native Say’s phoebe Sayornis saya Gilded flicker Colaptes chrysoides native Vermilion flycatcher Pyrocephalus rubinus Creepers Ash-throated flycatcher Myiarchus cinerascens Brown creeper Certhia americana native Brown-crested flycatcher Myiarchus tyrannulus Wrens Tropical kingbird Tyrannus melancholicuCactus wren Campylorhynchus brunneicapillus native Cassin’s kingbird Tyrannus vociferans Rock wren Salpinctes obsoletus native Thick-billed kingbird Tyrannus crassirostris Canyon wren Catherpes mexicanus native Western kingbird Tyrannus verticalis Bewick’s wren Thryomanes bewickii native Crows and jays House wren Troglodytes aedon native Steller’s jay Cyanocitta stelleri Winter wren Troglodytes troglodytes native Western scrub-jay Aphelocoma californica Marsh wren Cistothorus palustris native Pinyon jay Gymnorhinus cyanocepKingfishers American crow Corvus brachyrhynchos Belted kingfisher Ceryle alcyon native Common raven Corvus corax 

118 
Table B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. —Continued 

Group/common name Scientific name Nativity Group/common name Scientific name Shrikes Vireos Loggerhead shrike Lanius ludovicianus native Bell’s vireo Vireo bellii Swallows Yellow-throated vireo Vireo flavifrons Purple martin Progne subis native Plumbeous vireo Vireo plumbeus Tree swallow Tachycineta bicolor native Cassin’s vireo Vireo cassinii Violet-green swallow Tachycineta thalassina native Hutton’s vireo Vireo huttoni Bank swallow Riparia riparia native Warbling vireo Vireo gilvus Cliff swallow Petrochelidon pyrrhonota native Red-eyed vireo Vireo olivaceus Barn swallow Hirundo rustica native Mockingbirds and Thrashers Chickadees and titmice Gray catbird Dumetella caroliMountain chickadee Poecile gambeli native Northern mockingbird Mimus polyglottoBridled titmouse Baeolophus wollweberi native Sage thrasher Oreoscoptes monVerdin Brown thrasher Toxostoma rufum Verdin Auriparus flaviceps native Bendire’s thrasher Toxostoma bendiBushtits Curve-billed thrasher Toxostoma curviBushtit Psaltriparus minimus native Crissal thrasher Toxostoma crissaNuthatches LeConte’s thrasher Toxostoma leconRed-breasted nuthatch Sitta canadensis native Thrushes White-breasted nuthatch Sitta carolinensis native Western bluebird Sialia mexicana Waxwings Mountain bluebird Sialia currucoideCedar waxwing Bombycilla cedrorum native Townsend’s solitaire Myadestes townsTanagers Swainson’s thrush Catharus ustulatHepatic tanager Piranga flava native Hermit thrush Catharus guttatuSummer tanager Piranga rubra native American robin Turdus migratorWestern tanager Piranga ludoviciana native Varied thrush Ixoreus naevius 

119

Table B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. —Continued 

Group/common Name Scientific name Nativity Group/common name Scientific name Sparrows Wood-warblers Green-tailed towhee Pipilo chlorurus native Blue-singed warbler Vermivora pinus Spotted towhee Pipilo maculatus native Golden-winged warbler Vermivora chrysoptera Canyon towhee Pipilo fuscus native Orange-crowned warbler Vermivora celata Rufous crowned sparrow Aimophila ruficeps native Nashville warbler Vermivora ruficapilla American tree tparrow Spizella arborea native Virginia's warbler Vermivora virginiae Chipping sparrow Spizella passerina native Lucy’s warbler Vermivora luciae Brewer’s sparrow Spizella breweri native Northern parula Parula americana Black-chinned sparrow Spizella atrogularis native Yellow warbler Dendroica petechia Vesper sparrow Pooecetes gramineus native Chestnut-sided warbler Dendroica pennsylvanica Lark sparrow Chondestes grammacus native Magnolia warbler Dendroica magnolia Sage sparrow Amphispiza belli native Black-throated blue warbler Dendroica caerulescens Savannah sparrow Passerculus sandwichensis native Yellow-rumped warbler Dendroica coronata Grasshopper sparrow Ammodramus savannarum native Black-throated gray warbler Dendroica nigrescens Le Conte’s sparrow Ammodramus leconteii native Black-throated green warbler Dendroica virens Fox sparrow Passerella iliaca native Townsend’s warbler Dendroica townsendi Song sparrow Melospiza melodia native Bay-breasted warbler Dendroica castanea Swamp sparrow Melospiza georgiana native Black-and-white warbler Mniotilta varia White-throated sparrow Zonotrichia albicollis native American redstart Setophaga ruticilla Golden-crowned sparrow Zonotrichia atricapilla native Prothonotary warbler Protonotaria citrea Dark-eyed junco Junco hyemalis native Worm-eating warbler Helmitheros vermivorum McCown’s longspur Calcarius mccownii native Ovenbird Seiurus aurocapilla Old world sparrows Northern waterthrush Seiurus noveboracensis House sparrow Passer domesticus exotic Louisiana waterthrush Seiurus motacilla 

 MacGillivray’s warbler Oporornis tolmiei 

120

Table B5. Forest bird species found within the corridor of the Bill Williams River, Ariz. —Continued 

Group/common name Scientific name Nativity Group/common Name Scientific name Wood-warblers (cont.) Finches and allies Common yellowthroat Geothlypis trichas native Purple finch Carpodacus purpureus Hooded warbler Wilsonia citrina native Cassin’s finch Carpodacus cassinii Wilson’s warbler Wilsonia pusilla native House finch Carpodacus mexicanus Painted redstart Myioborus pictus native Red crossbill Loxia curvirostra Yellow-breasted chat Icteria virens native Pine siskin Carduelis pinus Blackbirds Lesser goldfinch Carduelis psaltria Bobolink Dolichonyx oryzivorus native Lawrence’s goldfinch Carduelis lawrencei Red-winged blackbird Agelaius phoeniceus native American goldfinch Carduelis tristis Eastern meadowlark Sturnella magna native Evening grosbeak Coccothraustes vespertinuWestern meadowlark Sturnella neglecta native Cardinals and allies Yellow-headed blackbird Xanthocephalus xanthocephalus native Northern cardinal Cardinalis cardinalis Rusty blackbird Euphagus carolinus native Rose-breasted grosbeak Pheucticus ludovicianus Brewer’s blackbird Euphagus cyanocephalus native Black-headed grosbeak Pheucticus melanocephalCommon grackle Quiscalus quiscula exotic Blue grosbeak Passerina caerulea Great-tailed grackle Quiscalus mexicanus native Lasuli bunting Passerina amoena Bronzed cowbird Molothrus aeneus native Indigo bunting Passerina cyanea Brown-headed cowbird Molothrus ater native Varied bunting Passerina versicolor Hooded oriole Icterus cucullatus native Painted bunting Passerina ciris Bullock’s oriole Icterus bullockii native Starlings Scott’s oriole Icterus parisorum native European btarling Sturnus vulgaris Larks Silky-flycatchers Horned lark Eremophila alpestris native Phainopepla Phainopepla nitens 

121

Appendix C. Aquatic Invertebrates of the Bill Williams River Corridor 

Table C1. Aquatic invertebrate species found corridor of the Bill Williams River, Ariz. 

122


Species/group common name 

Species/group scientific name Nativity Habitat Species/group common name Species/group scientific name Na

AQUATIC INSECTS Dragonflies and damselflies Gomphidae Mayflies Order Ephemeroptera —continued Progomphus na

Baetidae Erpetogomphus compositus naBaetis native lotic Ophiogomphus naCallibaetis pictus native lentic Libellulidae Acentrella native lotic Perithemis naPseudocloen native lotic Sympetrum illotrum naFallceon native lotic Somatochlora naParacloeodes native lotic Stoneflies Order Plecoptera Leptohyphidae Nemouridae Trichorithodes fallax native lotic Malenka naEphemerellidae Water bugs Order Hemiptera Drunella native lotic water boatmen Corixidae Heptageniidae Graptocorixa serrulata naRhithrogena native lotic water striders Gerridae 

Dragonflies and Order Odonata Gerris nadamselflies Coenagrionidae broad-shouldered water Veliidae 

Argia vivida native lentic/lotic Rhagovelia naEnallagma praevarum native lentic Microvelia naZoniagrion, native Naucoridae Calopterygidae Ambrysus naHetaerina native lentic giant waterbugs Belostomatidae Cordulegastridae Abedus herberti naCordulegaster native lentic/lotic Belostoma naAeschnidae toad bugs Gelastocoridae Anax junius native lentic Gelastocoris oculatus na
Table C1. Aquatic invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name 

Species/group scientific name Nativity Habitat Species/group common name Species/group scientific name N

Aquatic moths Order Lepidoptera predaceous diving beetles Dytiscidae 

Pyralidae Agabus naPetrophila native Rhantus na

Caddisflies Order Trichoptera Deronectes nasaddlecase makers Glossosomatidae Stictotarsus corpulentus na

Culoptila native lotic Oreodytes naProtoptila native lotic Liodessus naHelicopsychidae Laccophilus decipens na

snail case makers Helicopsyche native lotic Copelatus nanetspinning caddis Hydropsychidae water scavenger beetles Hydrophilidae 

Cheumatopsyche native lotic Tropisternus obscurus naSmicridea native lotic Berosus na

microcaddis Hydroptilidae Enochrus na

Hydroptila native lotic Hydrochus naMetrichia native lotic Helochares naLeucotrichia native lotic Laccobius naOchrotrichia native lotic riffle beetles Elmidae 

long-horned caddis Leptoceridae Zaitzevia na

Nectopsyche dorsalis native lotic Microcylloepus naOecetis native lotic Optioservus na

Beetles Order Coleoptera Dryopidae 

Haliplidae Posthelichus productus naPeltodytes simplex native lentic Helichus striatus na

Hydroscaphidae Hydroscapha natans na

water penny beetles Psephenidae n

123

Table C1. Aquatic invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name 

Species/group scientific name Nativity Habitat Species/group common name Species/group scientific name Nativity 

Flies Order Diptera NON-INSECT AQUATIC ARTHROPODS biting midges Ceratopogonidae Scuds, sideswimmers Order Amphipoda 

Probezzia native lotic Hyalellidae 

soldier flies Stratiomyiidae Hyalella azteca native 

Euparyphus native lotic Gammaridae Caloparyphus native lotic Gammarus native Nemotelus native lotic Talitridae native 

midges Chironomidae native lentic/lotic Sow bugs Order Isopoda dance flies Empididae Onisciidae native 

Hemerodromia native Sphaeromidae 

black flies Simuliidae Exosphaeroma native 

Simulium native Order Decapoda 

crane flies Tipulidae Palaemonidae 

Hexatoma native grass shrimp Palaemontes exotic Holorusia native crayfish Astacidae Antocha native Cambarus exotic Erioptera native Procambarus clarki exotic 

common flies Muscidae native dixid midges Dixidae 

Dixella native 

124

1 

125


Table C2. Fish species found within the corridor of the Bill Williams River, Ariz. 

Species/group Species/group Nativity Distrib1 . Habitat Species/group Species/group Nativity Dicommon name scientific name common name scientific name 

Cyprinidae Centrarchidae 

longfin dace Agosia chrysogaster native LAUH lentic largemouth bass Micropterus salmoides exotic LAgoldfish Carassius auratus exotic A lentic smallmouth bass Micropterus dolomieui exotic U carp Cyprinus carpio exotic L lentic green sunfish Chaenobryttus exotic AU roundtail chub Gila robusta native U lotic cyanellus bonytail chub Gila elegans native L? bluegill Lepomis macrochirus exotic A red shiner Notropis lutrensis exotic LAU lotic redear sunfish Lepomis microlophus exotic A fathead minnow Pimephales promelas exotic L shellcracker L. microlophus x C. exotic A speckled dace Rhinichthys osculus native UH lotic cyanellus 

Catostomidae Cichlidae 

desert sucker Catostomus insignis native U lotic Mozambique Tilapia mossambica exotic A Gila mountain Pantosteus clarki native U lotic mouthbrooder sucker Zilli’s tilapia Tilapia zillii exotic U razorback sucker Zyrauchen texanus native L? Clupeidae 

Ictaluridae threadfin shad Dorosoma petenense exotic A 

black bullhead Ictalurus melas exotic AU lentic/lotic yellow bullhead Ictalurus natalus exotic AU lentic/lotic channel catfish Ictalurus punctatus exotic A lentic/lotic flathead catfish Pilodictus olivaris exotic A lentic/lotic 

Poeciliidae 

mosquitofish Gambusia affinis exotic LAUH lentic/lotic 

Distribution: L – lower Bill Williams River below Alamo Dam, A – Alamo Lake only, U – upstream of Alamo Lake, and H – headwa
Appendix D. Mammals of the Bill Williams River Corridor 

Table D1. Mammal species found within the corridor of the Bill Williams River, Ariz. Species/group common name Species/group scientific name Nativity Species/group common name Species/group scienti

Order Chiroptera Order Rodentia Phyllostomatidae Sciuridae 

California leaf-nosed bat Macrotus californicus native Round-tailed ground squirrel Spermophilus tereticau

Vespertilionidae Cliff chipmunk Eutamias dorsalis 

Pallid bat Antrozous pallidus native Castoridae Townsend’s big-eared bat Corynorhinus townsendii native Beaver Castor canadensis Big brown bat Eptesicus fuscus native Cricetidae Hoary bat Lasiurus cinereus native Cactus mouse Peromyscus eremicus Western yellow bat Lasiurus xanthinus native Deer mouse Peromyscus maniculatWestern red bat Lasiurus blossevellii native White-throated woodrat Neotoma albigula California myotis Myotis californicus native Muskrat Ondatra zibethicus Cave myotis Myotis velifer native Arizona cotton rat Sigmodon arizonae Yuma myotis Myotis yumanensis native Western harvest mouse Reithrodontomys megaFringed myotis Myotis thysanodes native Heteromyidae Arizona myotis Myotis occultus native Desert pocket mouse Chaetodipus penicillatWestern pipistrelle bat Pipistrellus hesperus native Bailey’s pocket mouse Perognathus baileyi 

Molossidae Merriam’s kangaroo rat Dipodomys merriami 

Western mastiff bat Eumops perotis native Desert kangaroo rat Dipodomys deserti Pocketed free-tailed bat Nyctinmops fermorosaccus native Pacific pocket mouse Perognathus longimemMexican free-tailed bat Tadarida brasiliensis native Geomyidae 

Order Lagomorpha Botta’s pocket gopher Thomomys bottae Leporidae Erithoziontidae 

Desert cottontail Sylvilagus audubonii native North American porcupine Erethizon dorsatum Black-tailed jackrabbit Lepus californicus native 

126 
Table D1. Mammal species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group scienti

Order Carnivora Order Artiodactyla Canidae Tayassuidae 

Coyote Canis latrans native Collared peccary Tayassu tajacu Gray fox Urocyon cinereoargenteus native Suidae San Joaquin kit fox Vulpes macrotis native Wild boar Sus scrofa  Procyonidae Cervidae Northern raccoon Procyon lotor native Mule deer Odocoileus hemionus Ring tailed cat Bassariscus astutus native Elk Cervus elaphus  Mustelidae Antilocapridae Western spotted skunk Spilogale gracilis native Pronghorn Antilocapra americana Striped skunk Mephitis mephitis native Bovidae American badger Taxidea taxus native Bighorn sheep Ovis canadensis  Felidae Order Perissodactyla Mountain lion Felis concolor native Equidae Bobcat Felis rufus native Wild burro Equus asinus 

Order Insectivora Soricidae 

Crawford's desert shrew Notiosorex crawfordi native 

127

128


Appendix E. Reptiles and Amphibians of the Bill Williams River Corridor Table E1. Reptile and amphibian species found within the corridor of the Bill Williams River, Ariz. 

Species/group common Species/group scientific name Nativity Species/group common name Species/group 

name 

REPTILES Class Reptilia Poisonous lizards HelodermatidaTurtles and tortoises Order Testudines Gila monster Heloderma suspSoftshell turtles Trionychidae Blind snakes LeptotyphlopidSpiny softshell Trionyx spiniferus (Apalone spinifera) exotic Western blind snake Leptotyphlops hBox and water turtles Emydidae Typical snakes Colubridae SliderA Chrysemys scripta exotic Coachwhip Masticophis flaPainted turtleA Chrysemys picta exotic California kingsnake Lampropeltis gMusk and mud turtles Kinosternidae Western patchnose snake Salvadora hexaSonoran mud turtle B Kinosternon sonoriense native Checkered gartersnake Thamnophis flaAlligators, crocodiles and Order Crocodylia Gopher snake Pituophis melancaimans Alligatoridae Glossy snake Arizona eleganAmerican alligatorC Alligator mississippiensis exotic Spotted leafnosed snake Phyllorhynus dLizards and snakes Order Squamata Lyre snake Trimorphodon Banded geckos Eublepharidae Spotted night snake Hypsoglena torWestern banded gecko Coleonyx variegatus native Pit vipers Viperidae Iguanid lizards Iguanidae Western diamondback Crotalus atrox Desert iguana Dipsosaurus dorsalis native Mojave rattlesnake Crotalus scutul

Phrynosomatidae Sidewinder rattlesnakeA Crotalus cerast

Side blotched lizard Uta stansburiana native Long-tailed brush lizard Urosaurus graciosus native Zebra-tailed lizard Callisaurus draconoides native Ornate tree lizard Urosaurus (?Callisaurus) ornatus native Mojave fringe-toed lizardA Uma scoparia native Whiptail lizards Teiidae Western whiptail Aspidoscelis tigris native 

A The species is undocumented but probably present, based on its known range and documented presence nearby. B The species is possibly present, based on its historic distribution (currently uncommon native species) or known nearby presence (intr

The American alligator was introduced and is likely now extirpated. C 
Table E1. Reptile and amphibian species found within the corridor of the Bill Williams River, 

Ariz —Continued. 

Species/group common Species/group scientific Nativity 

name name 

AMPHIBIANS Class Amphibia 

Frogs and toads Order Anura 

True toads Bufonidae 

Woodhouse’s toad Bufo woodshousei native 

Red spotted toad Bufo punctatus native 

Southwestern toad Bufo microscaphus native 

Great plains toad Bufo cognatus native 

Colorado river toadA Bufo alvarius native 

Spadefoot toads Pelobatidae 

Couch’s spadefoot toad Scaphiopus couchi native 

True frogs Ranidae 

Lowland leopard frog Rana yavapaiensis native 

American bullfrogB Rana catesbeiana exotic 

Rio Grand leopard frog B Rana berlandieri exotic 

Treefrogs Hylidae 

Pacific treefrogA Hyla regilla native 

Salamanders Order Caudata 

Mole salamanders Ambystomatidae 

Tiger salamanderB Ambystoma tigrinum exotic A

 The species is undocumented but probably present, based on its known range and documented presence nearby. 

B

 The species is possibly present, based on its historic distribution (currently uncommon native species) or known 

nearby presence (introduced exotic species). 

129

Appendix F. Terrestrial Invertebrates of the Bill Williams River Corridor 

Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. Species/group common name Species/group scientific name Nativity Species/group common name Species/group scientific n

Class Arachnida Order Uropygi Order Araneae Thelyphonidae Theridiiae Vinegarone Mastigopoproctus giganteu

Western widow Latrodectus mactans native Order Amblypygi Miturgidid spiders Miturgidae Tailless whipscorpion Tarantulidae 

Syspira sp. native Paraphrynus mexicanus 

Wolf spiders Lycosidae Order Solpugida 

Pardosa sp. native Eremobatidae Varicosa gosiuta native Windscorpion Eremobates gracilidens 

Spitting spiders Scytodidae Eremothera sculpturata 

Scytodes sp. native Class Insecta (Hextapoda

Scorpions Order Scorpionidae Odred Dermatera 

Iuridae Labiduridae 

Giant hairy desert scorpion Hadrurus arizonensis native Riparian earwig Labidura riparia 

Buthidae Order Orthoptera 

Centeroides scorpions Centeroides exilicauda native Acrididae 

Vaejovidae Grasshopper Schistocerca nitens 

Vajovid scorpions Vaejovis confusus native Tridactylidae 

Order Pseudoscorpionidae Pygmy mole cricket Neotridactylus apicalis 

Daddy longlegs Phalangiidae Gryllidae 

OPILIONES Cricket Gryllus sp. Gagrellidae Order Mantodea 

Harvestman Trachyrhinus marmoratus native Mantidae 

Agile ground mantis Litaneutra minor 

130 
Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group s

Order Blattaria Metallic wood-boring beetles Buprestidae Blattellidae Acmaeodera co

Cockroach Blatella vaga Anthaxia (Mela

Polyphagidae Tiger beetles Cicindelidae 

Cockroach Argenivaga bolliana Megacephala caWebspinners Order Embiidina Scarab beetles Scarabaeidae 

Order Hemiptera Anomala flavillCoreidae Anomala noctur

Leaf-footed bug Leptoglossus clypealis native Cyclocephala h

Reduviidae Cyclocephala loZelus renardii native Diplotaxis anxi

Assassin bug Reduvius personatus native Diplotaxis knau

Pentatomidae June beetle Polyphylla cavif

Predatory stink bug Podisus acutissimus native Stenosphenus d

Order Homoptera Dryopidae Tibicinidae Long-toed water beetle Posthelichus pr

Apache cicada Diceroprocta apache native Click beetles Elateridae 

Order Neuroptera Bostrichidae 

Antlions Myremeleontidae Apple twig borer Amphicerus bic

Order Coleoptera Chrysomelidid

Ground beetles Carabidae Leaf beetle Plagiodera ariz

Pinacodera sp. native 

131

Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group sc

Coccinellidae Weevils Curculionidae 

Ladybird beetle Chilocorus cacti native Ophyrastes sp. Nine-spotted ladybug beetle Coccinella septempunctata native Order Diptera Convergent ladybug beetle Hippodamia convergens native Mosquitos Culicidae Mealybug destroyer Cryptolaemus montrouzieri exotic Culoseta inornata 

Tenebrionidae Culex quinquefasc

 Darkling beetle Diapersis sp. native Chaoborinae 

Pechalium subvittatum native Chaoborus sp. Eleodes armata native Horse flies Tabanidae Eleodes sp. native Tabanus sp. Zopherus sp. native Silvius sp. Centrioptera sp. native Mydas flies Mydidae Cryptoglossa verrucosa native Opomydas limbat

Long-horned beetles Cerambycidae Pseudonomoneura Cottonwood twig borer Oberea quadricallosa native Mydas ventralis Giant root borer Prionus sp. native Robber flies Asilidae 

Meloidae Eccritosia zamon 

Arizona blister beetle Lytta auriculata native Efferia spp. 

Zonitis punctipennis native March flies Bibionidae 

Leaf Beetles Chrysomelidae Bibiodes halterali

Plagiodera sp. native Plagiodera arizonae native Cryptocephalus binotatus native 

132

Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group s

Order Lepidoptera Riodinidae Danaidae Fatal metalmark Calephelis nem

Monarch Danaus plexippus native Wright’s metalmark Calephelis wrigQueen Danaus gilippus native Palmer’s metalmark Apodemia palm

Nymphalidae Mormon metalmark Apodemia morm

Painted lady Vanessa (Cynthia) cardui native Lycaenidae Mourning cloak Nymphalis antiopa native Marine blue Leptotes marina Viceroy Limenitis (Basilarchia) archippus Pygmy blue Brephidium exiCommon buckeye Jumonia coenia native Cerannus blue Hemiargus ceraRed admiral Vanessa atalanta native Reakirt’s blue Hemiargus isolRed spotted purple Limenitis arthemis native Great purple hairstreak Atlides halesus Variegated fritillary Euptoieta claudia native Gray hairstreak Strymon melinu

Papilionidae Leda hairstreak Ministrymon led

Black swallowtail Papilio polyxenes native Hesperiidae Zelicaon (Anise) swallowtail Papilio zelicaon native Common checkered skipper Pyrgus commun

Pieridae Small checkered skipper Pyrugus scriptu

Checkered white Pontia protodice native MacNiell’s sootywing skipper Hesperopsis graOrange (Alfalfa) sulphur Colias eurytheme native Funeral duskywing Erynnis funeralCloudless sulphur Phoebis sennae native Northern white-skipper Heliopetes ericeSleepy orange (Nicippe yellow) Eureme nicippe native Orange skipperling Copaedes auranDainty sulphur Nathalis iole native Fiery skipper Hylephila phyle

Libytheidae Euphala skipper Lerodea eufala 

Snout butterfly Libytheana carinenta native 

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Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group sc

Noctuidae Pteromalid wasp Pteromalidae 

Black witch Ascalapha odorata native Ichneumonidae 

Cossidae Ichneumonid wasps Compsocryptus ap

Carpenter moth Prionoxystus robiniae Melanichneumon 

Sphingidae Netalia sp. 

Big popular sphinx Pachysphinx modesta native Sphecid wasps Sphecidae White-lined sphinx Hyles lineata native Thread-waisted wasp Ammophila aberti Tobacco hornworm moth Manduca sexta native Ammophila sp. Ants and wasps Order Hymenoptera Bembecinus quinq

Formicidae Sand wasp Bembix melanasp

Harvester ant Pogonomyrmex californicus native Chalybion califorNative Fire ant Solenopsis aurea native Liris argentata 

Apidae Liris muspa 

European honey bee Apis mellifera exotic Liris beata 

Anthophoridae Microbembex aur

Large carpenter bee Xylocopa varipuncta native Microbombex nig

Anthophora sp. native Moniaecera sp. Diadasia sp. native Philanthus gibbosBethylidae Prionx foxi 

Bethylid Epyris myrmecophilus native Black and yellow mud dauber Sceliphron caeme

Chrysididae Trypoxylon sp. 

Cuckoo wasp Parnopes concinnus native 

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Table F1. Terrestrial invertebrate species found within the corridor of the Bill Williams River, Ariz. —Continued 

Species/group common name Species/group scientific name Nativity Species/group common name Species/group sc

Halictidae Pompilidae 

Green metalic bee Dialictus sp. native Spider wasp Ageniella joannae 

Agapostemon sp. native Agenioideus birkmTiphiidae Anoplius fulgidus 

Five-banded tiphiid wasp Myzinum quinquecinctum native Aporinellus fascia

Myzinum spp. native Aporinellus taeniaColocistis brevis native Apornellus mediaColocistis castanea native Pepsis pallidolimbParatiphia sp. native Pepsis thisbe Tiphia sp. native Pompilus parvuluMutillidae Scoliidae 

Thistledown velvet ant Dasymutilla gloriosa native Scarab hunter wasp Campsomeris toltRed velvet ant Dasymutilla magnifica native Megachilidae Velvet ant Dasymutilla satanas native Mason bee Osmia sp. 

Chyphotes spp. native Formicidae Photopsis sp. native Rough harvester ant Pogonomyrmex ru

Vespidae 

 Paper wasp Polistes sp. 

Pterocheilus pimo

135