# The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American Southwest

R E S E A R C H A N D D E V E L O P M E N Ti EPA/600/R-08/134 ARS/233046 November 2008

www.epa.gov



# The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American Southwest

by

Lainie R. Levick, David C. Goodrich, Mariano Hernandez
USDA-ARS Southwest Watershed Research Center Tucson, Arizona

Julia Fonseca

Pima County Office of Conservation Science and Environmental Policy Tucson, Arizona

Darius J. Semmens USGS – Rocky Mountain Geographic Science Center Denver, Colorado

Juliet Stromberg, Melanie Tluczek Arizona State University Tempe, Arizona

Robert A. Leidy, Melissa Scianni USEPA, Office of Water, Region IX San Francisco, California

D. Phillip Guertin University of Arizona

Tucson, Arizona

William G. Kepner USEPA, ORD, NERL Las Vegas, Nevada

The information in this report has been funded wholly by th e United States Environmental Protection Agency under an interagency assistance agreement (DW12922094) to the USDA , Agricultural Research Service, Southwest Watershed Research Center. It has been subjected to both agencies peer and administrative review processes and has been approved for publication. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy. Mention of trade names a nd commercial products does not constitute endorsement or recommendation for use.

U.S. Environmental Protection Agency Office of Research and Development Washington, DC 20460
5765leb08

### Acknowledgements

We gratefully acknowledge the following people fo r their valuable comments and review that significantly improved the quality, value, and accuracy of this document: Dave Bertelsen (University of Arizona Herbarium), Trevor Hare (Sky Island Alliance), Jim Leenhouts (USGS), Kathleen Lohse (University of Arizona), Wa ite Osterkamp (USGS), Sam Rector (Arizona Department of Environmental Quality), Phil Rosen (University of Arizona), Marty Tuegel (U.S. Fish & Wildlife Service), Dale Turner (The Na ture Conservancy), and Wilson Yee (USEPA).

Unless noted otherwise, all phot ographs are © Lainie Levick.

Suggested citation:

Levick, L., J. Fonseca, D. Goodrich, M. Hernand ez, D. Semmens, J. Stromberg, R. Leidy, M. Scianni, D. P. Guertin, M. Tl uczek, and W. Kepner. 2008. Th e Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American
Southwest. U.S. Environmental Protecti on Agency and USDA/ARS Southwest Watershed Research Center, EPA/6 00/R-08/134, ARS/233046, 116 pp.

ii

### Abstract

This report represents a state-of-the-art synt hesis of current knowle dge of the ecology and hydrology of ephemeral (dry washes) and intermitt ent streams in the American Southwest, and may have important bearing on establishing nexu s to traditional naviga ble waters (TNW) and defining connectivity relative to the Clean Water Act. Ephemeral and intermittent streams make up approximately 59% of all streams in the Unite d States (excluding Alas ka), and over 81% in
the arid and semi-arid Sout hwest (Arizona, New Mexico, Nevada, Utah, Colorado and California) according to the U.S. Geological Survey National Hydrography Dataset. They are often the headwaters or major tributaries of perennial streams in the Southwest. This comprehensive review of the pr esent scientific un derstanding of the ecology and hydrology of ephemeral and intermittent streams will help pl ace them in a watershed context, thereby highlighting their importance in maintaining wate r quality, overall waters hed function or health, and provisioning of the essential human and biolog ical requirements of clean water. Ephemeral and intermittent streams provide the same ecol ogical and hydrological functions as perennial streams by moving water, nutrients, and sediment throughout the watershed. When functioning properly, these streams provide landscape hydrol ogic connections; stream energy dissipation during high-water flows to reduce erosion and improve water quality; surface and subsurface water storage and exchange; ground-w ater recharge and discharge; sediment transport, storage, and deposition to aid in floodplain maintenance and development; nutrient storage and cycling; wildlife habitat and migration corridors; support for vegetation commun ities to help stabilize stream banks and provide wildlife services; a nd water supply and water- quality filtering. They provide a wide array of ecological functions including forage, cover, nesting, and movement
corridors for wildlife. Because of the relativel y higher moisture content in arid and semi-arid region streams, vegetation and wildlife abunda nce and diversity in and near them is proportionally higher than in the surrounding uplands . In the rapidly developing southwest, land management decisions must employ a watershed-s cale approach that addresses overall watershed function and water quality. Ephemeral and intermittent stream systems comprise a large portion of southwestern watersheds, and contribute to the hydrological, biogeochemical, and ecological
health of a watershed. Given thei r importance and vast ex tent, it is concluded that an individual ephemeral or intermittent stream segment should not be examined in isolation. Consideration of the cumulative impacts from anthropogenic uses on these streams is critical in watershed-based assessments and land management decisions to maintain overall watershed health and water quality.

iiiiv

### Table of Contents

Acknowledgements

............................................................................................................... ..... ii

Abstract

....................................................................................................................... ............... iii

Table of Contents

.............................................................................................................. ......... v

List of Tables

................................................................................................................. ........... vii

List of Figures

................................................................................................................ .......... viii

1. Introduction

............................................................................................................................ 1

2. Location of Ephemeral and Intermittent Streams

................................................................... 2

3. Definitions

.............................................................................................................................. 6

4. The Watershed Context

....................................................................................................... ... 7

5. Characteristics, Functions, and Ecosystem Significance

....................................................... 9

a. Hydrologic Features

......................................................................................................... 13

i. Variability of ar id and semi-arid region flows and floods

............................................ 15
ii. Types of arid and se mi-arid region floods

.................................................................... 17
iii. Transmissi on losses

..................................................................................................... 18
iv. Ground-water recharge

................................................................................................ 22

v. Landscape and hydrologic connections

........................................................................ 24
vi. Energy dissipation

....................................................................................................... 26
vii. Sediment mobilization, storage, transport, and deposition

........................................ 26

b. Geomorphic Characteristics

............................................................................................. 28

i. Channel-forming processes

........................................................................................... 31
ii. Geomorphic response to land management

.................................................................. 34
iii. Map scale in determining channel network and stream order

..................................... 35

c. Biogeochemical Functions

............................................................................................... 36

i. Cycling of elements and compounds

............................................................................. 36
ii. Detention of imported elements and compounds

......................................................... 37
iii. Particulate detention

.................................................................................................... 3 8
iv. Organic carbon export

................................................................................................. 39

d. Plant Community Support

................................................................................................ 40

i. Physiognomy, density, and species composition.

......................................................... 41
ii. Primary productivity a nd plant water sources

.............................................................. 43
iii. Temporal and spatial patte rns of species diversity

...................................................... 44
iv. Influences of vegetation on ecosystem processes.

...................................................... 45

v. Vegetation and channel morphology

............................................................................ 46
vi. Vegetation and geochemical cycles

............................................................................. 47

e. Faunal Support and Habitat

.............................................................................................. 47

vi. Spatial structure, connectivity, an d corridors in w ildlife habitat

................................... 48
ii. Physical habitat features

............................................................................................... 49
iii. Vegetative habitat features

.......................................................................................... 50
iv. Hydrological habitat features

...................................................................................... 52

v. Faunal abundance and distribution

............................................................................... 52

f. Synthesis of Functions

...................................................................................................... 64

6. Anthropogenic Impacts on Ephemeral and In termittent Streams and Riparian Areas

......... 65

a. Land Development

........................................................................................................... 66

b. Land Uses

......................................................................................................................... 69

c. Water Resources Impacts

................................................................................................. 70

d. Climate Change

.............................................................................................................. .. 71

7. Discussion

............................................................................................................................. 71

a. Clean Water Act Context

.................................................................................................. 71

b. Ecosystem Goods and Services

........................................................................................ 72

c. Management Principles

.................................................................................................... 74

d. Research Recommendations

............................................................................................. 75

8. Conclusions

.......................................................................................................................... 76

9. Literature Cited

..................................................................................................................... 77

vi

### List of Tables

Table 1. List of wildlife species that use ri verine soil exposures in Pima County, Arizona (source: Julia Fonseca, Pima County Office of Conservation Science and Environmental Policy, 2008).....................................................................................................................50

vii

### List of Figures

Figure 1. Illustration of the locations of pr esent day drylands and their categories. (Millennium Ecosystem Assessment, 2005a)

..................................................................... 3

Figure 2. Map showing the San Pedro River Wate rshed’s current and historical perennial reaches (courtesy of The Nature Conservancy, Arizona)

................................................... 4

Figure 3. Map of the Southwestern U.S. s howing the National Hydrography Dataset (NHD) intermittent/ephemeral (red) and perennial (blue) streams

................................................. 5

Figure 4. Diagram of the four interacting zone s of a desert stream ecosystem: the surface stream, hyporheic, parafluvial and riparian zones (from Holmes et al. 1994).

.................. 7

Figure 5. Diagram showing connectivity of landscap e-level processes and attributes important for ecosystem monitoring (from Miller, 2005)

................................................................... 9

Figure 6. Photograph showing ground-wa ter dependent cottonwood trees ( Populus fremontii ) lining an intermittent stream

............................................................................................. 10

Figure 7. Map of the conterminous U.S. showi ng Bailey’s Ecoregions with the dry domain
outlined in red

................................................................................................................ ... 12

Figure 8. Map of the conterminous U.S. with EPA Level II Ecoregions showing most of Nevada, Arizona, Utah, and Ne w Mexico in the North American Deserts classification.

............................................................................................................................... ........... 12

Figure 9. Photographs of an ephemeral stream, sa me location with flow (left), and dry (right), Tucson, Arizona

................................................................................................................ 13

Figure 10. Maps showing average annual pr ecipitation amounts, 1961-1990 (left), and locations of ephemeral/intermittent (red) and perennial (blue) streams (right)

................ 14

Figure 11. Maps showing percent of average an nual precipitation, July and August (left) and percent of average annual precipitation, October through March (right), for comparison, Western Regional Climate Center, h ttp://www.wrcc.dri.edu/precip.html.

...................... 15

Figure 12. Photograph of a flash flood in an ephemeral channel, Southern Arizona

............... 18

Figure 13. Example hydrograph showing typical ra pid rise to peak discharge and long
recession curve of a flash flood

. ....................................................................................... 18

Figure 14. Map of the Walnut Gulch Expe rimental Watershed (WGEW), Tombstone, Arizona, showing major stream network, fl umes and sub-watershed boundaries.

.......... 19

Figure 15. Hydrograph, location map, and photog raph of rainfall-runoff event August 27, 1982, illustrating ephemeral stream channel transmission losses as measured within the WGEW (Goodrich et al., 1997)

........................................................................................ 21

Figure 16. Photographs of Flume 1 at WGEW , dry (left) and with flow (right)

...................... 21

Figure 17. Diagrams of well levels a nd flow depths at the WGEW.

....................................... 23

viiiFigure 18. Photograph of ephemeral and interm ittent stream channels connecting to a perennial reach of Cienega Creek, southeast of Tucson, Arizona. (Photograph: Lainie Levick/Aerial flight courtesy of Lighthawk, www.lighthawk.org)

.................................. 25

Figure 19. Photographs of an ephemeral stream , same location during flow (left), and dry (right), Tucson, Arizona.

.................................................................................................. 26

Figure 20. Photographs from an unusually large flood event in an ephemeral stream that damaged roads and bridges, and flooded n earby homes, Tucson, Arizona, July 31, 2006.

............................................................................................................................... ........... 27

Figure 21. Photograph of sediment-laden floodwat ers in an ephemeral stream, Walnut Gulch, Arizona. (Photograph: USDA-ARS/SWRC)

.................................................................... 28

Figure 22. Photograph showing an ephemeral brai ded stream system, Yuma Wash, southwest Arizona. (Photograph: Susan Howe, Colorado State University)

. ................................... 29

Figure 23. Relation of bankfull channel width to drainage area for different climatic environments (after Wolman and Gerson, 1978).

............................................................ 30

Figure 24. Photograph of a typical wide, flat, an d sandy ephemeral stream channel, Martinez Wash, Arizona. (Photograph: William Kepner, USEPA/ORD)

....................................... 31

Figure 25. Photograph of a small arroyo or incised channel, southern Colorado

. ................... 33

Figure 26. Diagram of digitized stream channels on a subwatershed at WGEW (from Miller et al., 1999a)

. ................................................................................................................... ..... 35

Figure 27. Photograph showing dense corridor of vegetation lining an ephemeral wash, Agua Fria River, north of Phoenix, AZ. (Photograph: William Kepner, USEPA/ORD).

......... 40

Figure 28. Photograph showing riparian vegetati on along a desert stream, western Arizona. 42

Figure 29. Photograph of native palms ( Washingtonia filifera ) in Castle Creek, AZ. (Photograph: William Kepner, USEPA/ORD)

................................................................. 43

Figure 30. Photograph of annual plant species in an ephemeral streambed following spring rains, southern Arizona

..................................................................................................... 45

Figure 31. Photograph of vegeta tion growing in an ephemeral channel bed, Arizona. (Photograph: Lainie Levick/Aerial flight co urtesy of Lighthawk, www.lighthawk.org). 46

Figure 32. Photograph of diverse riparian ve getation, Badger Springs Wash, Arizona.

......... 51

Figure 33. Graph of herpetofauna species by ta xonomic group and by ha bitat type, from BLM surveys in Arizona, 1977-1981 (Jones, 1988)

. ................................................................. 54

Figure 34. Graph of lizard abundance by habita t type at Organ Pipe Cactus National Monument, Arizona, 1989-1990 (from Rosen and Lowe, 1996)

. .................................... 55

Figure 35. Photographs of amphibians that inha bit and breed in ephemeral and intermittent streams (clockwise from top left): Canyon tree frog ( Hyla arenicolor ), lowland leopard frog ( Rana yavapaiensis ), red spotted toad ( Bufo punctatus ), Sonoran desert toad ( Bufo
alvarius , photograph: Shea Burns, USDA-ARS), Egg strand of Sonoran desert toad (photograph: Shea Burns, USDA-ARS)

........................................................................... 56

Figure 36. Photograph of a desert mule deer ( Odocoileus hemionus ), Arizona

....................... 59

ixFigure 37. Illustrations of caddisfly larva (www.sc ientificillustrator.com) (left), and caddisfly adult (www.nps.gov) (right)

. ............................................................................................ 61

Figure 38. Diagram of a conceptual model of th e delivery of invertebrates, sloughed algae and detritus from headwater streams, intermittent and permanent first-order fishless streams to larger fish-bearing streams, illustrating the dependence of downstream reaches on upstream processes (from Cummi ns and Wilzbach, 2005)

. ............................................. 63

Figure 39. Photographs of the Rillito River, Tucs on, Arizona, dry (left) and with flow (right).

............................................................................................................................... ........... 65

Figure 40. Aerial photograph showing ephemeral tributaries to Cienega Creek, a perennial stream, flowing through the small community of Vail, southeast of Tucson, Arizona

.... 67

Figure 41. Photograph of the Santa Cruz Ri ver, Tucson, Arizona, during summer monsoon flows. (Photograph: unknown)

. ........................................................................................ 74

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### 1. Introduction

This report addresses the hydr ological and ecological significance of ephemeral and
intermittent streams in the arid and semi-ari d Southwestern United States (U.S.) for the purpose of illustrating their connection and va lue to perennial stream systems and other “waters of the United States” as protected under the Federal Water Pollution Control Act, otherwise known as the Clean Water Act (CWA). The CWA was establis hed to “restore and maintain the chemical, physical, a nd biological integrity of the Nation’s waters.” Its goal is to prevent pollution of waters of the U.S., and to ensure that our citizens have safe, clean water. Although originally enacted in 1948, the act wa s revised and expanded in 1972, with nearly
annual amendments since then.

In recent years, there have been numerous discussions as to whether ephemeral and
intermittent streams are “waters of the United Stat es” under the Act, and if the act applies to those streams. From 33CFR, Part 328.3, the defini tion of “waters of the United States,” as it applies to the jurisdictional lim its of the authority of the Co rps of Engineers under the CWA, includes (in part):

(1) All waters which are currently used, or were used in the past, or may be susceptible to use in interstate or forei gn commerce, including all waters which are subject to the ebb a nd flow of the tide;

(2) All interstate waters including interstate wetlands;

(3) All other waters such as intrastate lakes, rivers, streams (including intermittent streams), mudflats, sandflats, wetlands , sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds, the use, de gradation or destruction of which could affect interstate or foreign co mmerce including any such waters:

(i) Which are or could be used by interstate or foreign travelers for recreational or other purposes; or (ii) From which fish or she llfish are or could be taken and sold in interstate or foreign commerce; or (iii) Which are used or could be used for industrial purposes by industries in interstate commerce;

(4) All impoundments of waters otherwise defined as waters of the United States under this definition;

(5) Tributaries of waters iden tified in paragraph (s) (1) through (4) of this section (from http://www.usace.army.m il/cw/cecwo/reg/33cfr328.htm).

1This definition specifically includes intermittent streams (paragraph 3), and tributaries of any waters identified in the definition (paragraph 5). From these definitions ephemeral and intermittent streams appear to qualify for pr otection under the CWA; however, there have been some recent court cases that have complicated interpretation of this law. Nadeau and Rains (2007) discussed the Supreme Cour t decisions of June 2006, concerning the determination of jurisdiction under the CWA, and the implication that non-navigable, isolated, intrastate waters need a “significant nexus” to navigable waters to be jurisdictional under the CWA (see also Leibowitz et al., 2008). Although “significant nexus” has not been defined, the goal of the CWA is to “restore and maintain the chemical, physical, and biological integrity of the Nation’s waters” (33 U.S.C. 1251). Nadeau and Rains (2007) therefore, consider a “significant nexus” to ex ist if a headwater stream contributes to the chemical, physical, or biological in tegrity of navigable water.

Ephemeral and intermittent streams are the defining characteristic of many watersheds in dry, arid and semi-arid regions, and serve a critical role in the pr otection and maintenance of water resources, human health, and the environment. This report is a compilation of information that describes the signific ance of ephemeral and intermittent streams to the hydrology, biogeochemistry, flora and fauna of arid and semi-arid region watersheds. This comprehensive review of the pr esent scientific un derstanding of the ecology and hydrology of ephemeral and intermittent streams will help pl ace them in a watershed context, thereby highlighting their importance in maintaining water quality, overall watershed health, and provisioning of the essential hum an and biological requirements of clean water. Individual ephemeral or intermittent stream segments should not be examined in isolation. Given their vast extent and the accumulation of impacts to them over large areas in the rapidly developing southwest, a landscape or watershed-scale appr oach should be employed that considers the
cumulative effects on overall watershed function.

The geographic scope of this repor t is the arid and semi-arid re gions of the conterminous U.S. as defined by the Bailey’s and EPA/Omernik ec oregion classifications (see Figures 7 and 8), but focuses on the states of California, Ariz ona, Nevada, New Mexico, Utah and Colorado. The application of this report is for the EP A Region 9 states of Arizona, California and Nevada. This report does not address management, policy, or regulatory issues.



### 2. Location of Ephemeral and Intermittent Streams

Ephemeral and intermittent streams are found acro ss the Earth’s land surface in arid and semi

arid regions that are commonly referred to as “drylands.” Approximately one-third of the
Earth’s land surfaces are classified as ar id or semi-arid (Whitford, 2002; Millennium Ecosystem Assessment, 2005a), including mo st of the Western U.S. (Figure 1).

These lands are characterized by low and hi ghly variable annual precipitation, where evapotranspiration exceeds precipita tion. It is because of these dry conditions, which result in great contrast between the moist riparian area s and adjacent dry upland communities, that arid and semi-arid region streams are so important. Riparian ecosystems occupy very small portions of the landscape in arid and semi-arid regions, yet they exert substantial influence on

2hydrologic, geomorphic, and ecological proce sses (Shaw and Cooper, 2008), and typically support the great majority of bi odiversity in these regions.

Figure 1. Illustration of the locations of p resent day drylands and their categories. (Millennium Ecosystem Assessment, 2005a)

Some southwestern landscapes confound typical no tions of where water is to be found. The recent CWA discussions generally assume that perennial streams receive water from ephemeral tributaries. But, a person dying of thirst in the Cabeza Prieta National Wildlife Refuge in southwestern Arizona will find surf ace water in the mountai ns, not in the valley floor streams. In the San Pedro Valley of southeastern Arizona, perennial and intermittent stream reaches commonly are found in the tribut aries, as well as along the main stem San Pedro River (Figure 2). In the Mojave Desert of southern Ca lifornia, some mountain streams are physically isolated from downstream hydrologi c systems. Water from these mountain streams takes hundreds or thousands of years to move into and through the regional aquifer and discharge into valley floor streams, springs and wetlands (Izbicki, 2007).

3Figure 2. Map showing the San Pedro Riv er Watershed’s current and historical perennial reaches (courtesy of The Nature Conservancy, Arizona).

4The U.S. EPA, using the National Hydrogra phy Dataset (NHD) (USGS, 2006), has estimated that 59 percent of the streams in the U.S. (e xcluding Alaska) are ephemeral or intermittent (U.S. EPA, 2005). The NHD combines ephemera l and intermittent streams in its mapping, and identifies them as streams, which contain water for only part of the year. The NHD also
identifies start reaches as t hose that have no other streams flowing into them (at the 1:100,000 scale). These reaches can thus be considered headwater or first-order streams (Nadeau and Rains, 2007).

Among the six states being addressed in this report, Arizona has the greatest percentage, 94 percent, of ephemeral and intermittent streams, whereas California has the least, 66 percent. However, it is not just states in the arid Southwest that contain high percentages of nonperennial streams. For example, 86 percent of South Dakota’s streams are ephemeral or intermittent, 81 percent in Kansas, and 84 percent in North Dakota. The percentages of ephemeral/intermittent streams from the NHD for the six Southwestern states that are the subjects of this report are ta bulated in Figure 3, and are illu strated using the NHD stream map (National Hydrography Dataset, http://nhd.usgs.gov/ ).

Arizona 94%
Nevada 89%
New Mexico 88%
Utah 79%
Colorado 68%
California 66%

Figure 3. Map of the Southwestern U.S. showing the National Hydrography Dataset (NHD) intermittent/ephemeral (red ) and perennial (blue) streams.

It should be noted that the NHD may not accurately reflect the to tal extent of ephemeral or intermittent streams; it does not include stream segments less than one mile in length, combines intermittent and ephemeral stream s, and is based on 1:100,000 scale topographic maps. Washes (dry streambeds that contain water only during or afte r a local rainstorm or heavy snowmelt) in the arid Southwest are not c onsistently demarcated. The NHD dataset contains information on naturally occurring a nd constructed bodies of water, paths through which water flows, and related entities (USGS, 2006), and calculates th e percent of streams that are ephemeral or intermittent relative to total stream length using total kilometers of linear streams in watersheds that are totally or partially contained within each state boundary. Watersheds are at the 8-digit Hydrologi c Unit Code (HUC) level (U.S. EPA, 2005).

5

### 3. Definitions

In humid parts of the world, where precip itation exceeds evapotra nspiration, water is plentiful, and rivers will typically flow ceasel essly except in times of exceptional drought or human diversion. In arid and semi-arid regions , flows have a beginning and an ending in time and space, and there are various classification systems for categorizing the permanency of stream flows, or hydrologic continuum.

For this report, we classify streams by the following definitions:

Ephemeral : A stream or portion of a stream whic h flows briefly in direct response to precipitation in the immediate vicinity, and w hose channel is at all times above the ground

water reservoir.
Intermittent : A stream where portions flow conti nuously only at certain times of the year, for example when it receives water from a spring, ground-water source or from a surface source, such as melting snow (i.e. seasonal). At low flow there may be dry segments alternating with flowing segments.
Perennial: A stream or portion of a stream th at flows year-round, is considered a permanent stream, and for which baseflow is maintained by ground-water discharge to the streambed due to the ground-water elevation ad jacent to the stream typically being higher
than the elevation of the streambed.
Headwater : The low order, small stream at the t op of a watershed, when viewed at the 1:100,000 map or image scale; may be perennia l, intermittent, or e phemeral (Nadeau and Rains, 2007).

In addition, for this report we clarify the definition of Riparian area or riparian zone as: the strip of vegetation along an ephemeral, intermittent , or perennial stream, which is of distinct composition and density from the surrounding uplands (see Section 5.d.i Plant physiognomy, density and species compositi on for further discussion).

Many seemingly perennial reaches of a stream are separated by ephemeral or intermittent segments of flow, as a result of differences in geology along the river. Th is variation of flow is common enough in the Southwest that hydrologists use the terms interrupted or spatially
intermittent to describe the spatial segmentation of a river into reaches that are ephemeral, intermittent, or perennial.

The active channel of a desert stream is defined by the hyporheic zone, the zone between the surface stream and alluvial ground water, and the parafluvial zone, the part of the active channel without surface water (Figure 4). A de sert stream ecosystem is composed of four interacting subsystems: the riparian zone, the surface stream, the hyporheic and the parafluvial zones (Holmes et al., 1994). Str eam ecologists are becoming increasingly aware of the importance of what happens below the channel bed (Boulton et al., 1998) and in these interacting zones (Holmes et al. 1994).

6Figure 4. Diagram of the four interacting zone s of a desert stream ecosystem: the surface stream, hyporheic, parafluvial and ripa rian zones (from Holmes et al. 1994).

The hyporheic zone is important to the physical , chemical, and biological integrity of the above-ground portion of the stream. A stream reach that lacks water at all times on the surface may continue to have a thriving hyporheic zone. Water in the hyporheic zone may be discharged into perennial or intermittent reaches of flow downstream. During hyporheic flow, ground water and stream water mix in the beds and banks of ephemeral, intermittent, and perennial streams and sometimes in a larg er region surrounding the stream channel. In these zones, there is substantial biogeochemical cycling of nutrients and trace elements that are essential to aquatic life (Valett et al ., 1994; Boulton et al., 1998; Hibbs, 2008). The parafluvial zone can be extensive in some sy stems, and the potential for surface-subsurface exchange is high; however, less is known about these processes than in the hyporheic zone (Holmes et al., 1994).



### 4. The Watershed Context

Watersheds gradually are becoming regarded as the most appropriate spatial unit for land management, and especially for water-resource management. Managing from a watershed context is more effective than focusing on a spec ific site, such as an individual ephemeral or intermittent stream segment, because actions by humans, wildlife, and nature can have widespread effects, crossing political boundaries and impacti ng downstream water quality and ecosystem health. The accumulation of impact s over large areas in the rapidly developing southwest suggests a landscape or watershed-scale approach th at considers the cumulative
effects on overall watershed function.

Ephemeral and intermittent stream channels are of ten but not always the smallest channels in the watershed, and often represent the headwaters of a stream. Given their large extent, these streams are important sources of sediment, water, nutrients, seeds, and organic matter for

7downstream systems and provide habitat for ma ny species (Gomi et al., 2002) and their inclusion is important in wate rshed-based assessments (Gandolfi and Bischetti, 1997; Miller et al., 1999b).

An understanding of the key ecological and hydrologi cal functions that watersheds perform is required for effective land and water quality management. These watershed functions, outlined by Black (1997), include:

(1) the collection of water from rainfall, s nowmelt, and storage that becomes runoff, (2) the storage of various amounts of water and sediment, (3) the discharge of water as runoff, and the transport of sediment, (4) providing diverse sites and pathways along which chemical r eactions take place,
and (5) providing habitat for flora and fauna.

The two integrative watershed responses to these five f unctions are hydrologic energy attenuation, and the regulated movement or flushing of water through the system which controls the movement of chemicals. Dependi ng on the flow regime, the movement of water
affects the concentration or load of materi als in suspension or solution in the aquatic environment. Black (1997) referred to th is link between hydrology and water quality to demonstrate the importance of considering the entire watershed in the protection of water

resources.

Miller (2005) discussed the conn ectivity of ecosystems in a land scape, and the importance of managing at that scale, noting how the condition of one part of a landscape can affect other portions. Figure 5 illustrates how ecosystem processes, organisms, resources, and
disturbances interact across a landscape. In arid and semi-arid regions, ephemeral and intermittent streams provide much of the eco logical and hydrological connectivity in a landscape. Although lacking perennial flow, they may constitute a large percentage of the stream network in a watershed, and are connected to the larger stream system.

The disturbance or loss of ephemeral and intermittent streams has dramatic physical, biological, and chemical impacts, which are eviden t from the uplands to the riparian areas and
stream courses of the watershed. Barnett et al . (2002) noted that the condition of upland areas is integral to hydrologic function. The amount of precipitation which immediately runs off the land surface, and that which inf iltrates into the soil to either be used for plant growth or to recharge ground water, is dependent on this critical interface. For example, when precipitation falls on the land its fate is affected by the soil and vegetation, which in turn are affected by land uses, both hi storical and current.

8Figure 5. Diagram showing connectivity of la ndscape-level processes and attributes important for ecosystem monitori ng (from Miller, 2005). In box A , landscape units are functionally connected by flows of soil a nd water resources, organisms, disturbances,
and stressors. In box B , degraded conditions in Unit 1 are shown to cause resource enrichment in Unit 2, illustrating the im portance of landscape context. In box C , degraded conditions in Unit 1 are propagated to Unit 3 due to increased size of Unit 1
and decreased size of Unit 2.



### 5. Characteristics, Function s, and Ecosystem Significance

Ephemeral and intermittent streams in arid and semi-arid regions have distinctly different characteristics from perennial streams that ar e in wetter, more humid (mesic to hydric) environments. These complex systems have developed in a climatic regime of wide fluctuations of precipitation, ranging from drought to flood. Anthropogenic uses, such as urbanization, superimposed on that climatic regi me can exacerbate or ameliorate their effects on soils and vegetation, and may affect hydrol ogic and ecological functions throughout the watershed. Stability and resilie ncy to disturbance are important for ecological integrity, but because of the deficiency of water, terrestri al arid and semi-arid region ecosystems do not recover quickly from human-imposed disturba nce, although desert streams recover more quickly than the uplands.

9While hydrologists generally re ject the popular con cept of an “underground river,” the sediment below the channel does convey water. For some streams, in current climatic regimes, there may not be a perennial or inte rmittent reach, but water may always be present below the ground and accessible to a rich assemb lage of plant and animal life. This is illustrated in Figure 6, the San Pedro River, Ar izona, an intermittent stream, bordered by a ground-water-dependent cottonwood forest.

Figure 6. Photograph showing ground-wa ter dependent cottonwood trees ( Populus fremontii ) lining an intermittent stream.

Ephemeral or intermittent stream reaches can be headwater reaches or the main stem. Some watersheds consist of only ephemeral or interm ittent streams. Generally, these systems occur in arid and semi-arid regions, and their locati ons can be described us ing climatic factors, latitude, continental pos ition, and elevation. These featur es combine to form the world’s ecoclimatic zones, which are referred to as an ecosystem region or ecoregion.

The classification of ecoregions provides a me thod of characterizing th e ecological areas of the U.S., and allows for the rich mosaic of e nvironmental conditions to be placed in context with one another, enabling their connections to be better understood. Ecoregion
classifications indicate areas of similar envir onmental characteristics, which can serve as a spatial framework for the research, assessment, management, and monitoring of ecosystems and ecosystem components, and can help us to understand the ecosystem s in which ephemeral
and intermittent streams occur. This is usef ul for proper understanding and management of our environmental resources, including wate r (Commission for Environmental Cooperation, 1997).

Both Bailey’s Ecoregions (Bailey, 1976) and the EPA Ecological Regions of North America (Commission for Environmental Cooperation, 1997) , based on Omernik and others, place the

10Southwestern states mainly in dry, desert, or semi-arid classifications. The essential feature
of these classifications is that annual losses of water through ev aporation at the earth's surface exceed annual water gains from precipitation.

Bailey’s Ecoregion classification is based largely on forest a nd climatic factors. This classification system designates four domains: polar, humid temperate, humid tropical, and dry. The first three are based on humidity and thermal characteristics; however, the fourth, the dry domain, is based solely on moisture, a nd is defined as those locations where annual losses of water through evaporation at the ea rth’s surface exceed annual water gains from precipitation. Five of the six st ates considered in this report lie wholly within the dry domain: Arizona, Nevada, Colorado, Utah, and New Mexic o. The deserts of Southern California are also within the dry domain, whereas the rest of the state is within the humid temperate domain (Figure 7). The dry domain includes the arid de sert and the semi-arid steppe, and represents seven Divisions encompassing a wide divers ity in terrain, vege tation structure and composition, climatic regime, hydrologic regime , and ecosystem function. However, the
dominant characteristics are variable rainfall and high evapotranspiration.

The EPA Ecological Regions Clas sification is based on Omernik, who was one of the first to take a more holistic approach by including physical and biotic char acteristics (Commission for Environmental Cooperation, 1997). This cla ssification defines four levels of ecological regions that represent increasingly detailed local characteristics. Level II, which is most similar to Bailey’s Divisions, is illustrated in Fi gure 8. Most of the S outhwestern states fall
into the warm or cold desert ec oregion, the southern semi-arid hi ghlands, or temperate sierras. These areas are described as having an arid to semi-arid climate, with marked seasonal temperature extremes. This aridity is the resu lt of the rain shadows of the Sierra Nevada, Cascade Mountains and Sierra Madre ranges as they intercept the wet winter air masses brought by the westerly and easterly winds.

Both the Bailey’s and EPA Ecoregion classifications illustrate the extent of arid and semi-arid regions in the Southwestern U.S. and provi de a framework for understanding the unique conditions found in this region where most watersheds are dominated by ephemeral and
intermittent streams.

11Figure 7. Map of the conterminous U.S. sh owing Bailey’s Ecoregions with the dry
domain outlined in red.

Figure 8. Map of the conterminous U.S. with EPA Level II Ecoregions showing most of Nevada, Arizona, Utah, and New Mexico in the North American Deserts classification.

12a. Hydrologic Features

Ephemeral streams are unique in that they lack permanent flow except in response to rainfall
events. Intermittent streams fl ow continuously only in places where it receives water from a ground-water source or from seasonal runoff. Ne vertheless, they perform the same critical hydrologic functions as perennial streams: they move water, sediment, nutrients, and debris through the stream network and provide connect ivity within the watershed. These streams experience extreme and rapid variations in fl ood regime (Figure 9), and as a consequence rarely reach process-form equilibrium where fl ow conditions change too rapidly for bedforms to develop a form matching that flow, so sedime ntary structures can give a misleading picture of the flow that occurred (North, 2005).

Although arid and semi-arid region streams perfor m the same functions as perennial streams, their hydrology and sediment transport characte ristics cannot be reliably predicted by extrapolation from humid regions (Scott, 2006; McMahon, 1979). This is due to a much higher degree of spatial and temporal variab ility in hydrologic proce sses and also in the resulting erosion and sedimentation processes th an in humid regions. Desert environments typically produce more runoff and erosion per un it area than in temperate regions for a given intensity of rainfall due to sparse vegetation cover and poorly developed soils with little organic matter (Thornes, 1994).

Figure 9. Photographs of an ephemeral stream, same location with flow (left), and dry (right), Tucson, Arizona.

The variability of flood magnitudes is much gr eater for ephemeral stream channel flows as compared to that of perennial stream systems. For example, Graf (1988) reported that in a humid region in Pennsylvania, the 50-year re turn flood event is roughly 2.5 times the mean annual flow, whereas the 50-year return flow for the Gila River in Ariz ona is about 280 times the mean annual flow. Although this may also be a function of differences in base-flow, the difference is still significant. Some studies ha ve noted that many of our watersheds in the Western States (up to 90 per cent in Arizona, for example) yi eld less than 12.7 mm of runoff

13per unit area, per year, but the vast extent of these arid and semi-arid watersheds makes their total runoff production significant, and their prop er management important (Renard, 1970). Osterkamp and Friedman (2000) compared runo ff and extreme rainfalls of semi-arid areas
with those of other climatic areas in the c onterminous U.S. They found that the magnitudes of intense precipitation in semi-arid areas are ge nerally less than in humid areas, but peaks of infrequent floods are typically larger, with many of the greatest r ecorded unit flood flows occurring in drainage basins of less than 1,000 km 2

.

Most of the Southwest receives less than 500 mm of rainfall per year, and a correlation can be seen between locations with low average a nnual rainfall amounts and locations with ephemeral or intermittent stream flow. Figure 10 shows maps of the average annual precipitation for the Western U.S. for 1961- 1990, and the locations of perennial and ephemeral/intermittent streams from the NHD da taset for comparison. Because of the low rainfall amounts, most stream reaches in the Southwest are ephemeral or intermittent.

Figure 10. Maps showing average annual preci pitation amounts, 1961-1990 (left), and locations of ephemeral/intermittent (red ) and perennial (blue) streams (right). (Western Regional Climate Center, http://www.wrcc.dri.edu/precip.html , and National Hydrography Dataset (NHD), http://nhd.usgs.gov/ . For NHD see also “Concepts and
Contents” at http://nhd.usgs.gov/chapter1 /chp1_data_users_guide.pdf ).

Rainfall patterns in arid and semi-arid regions in fluence when streamflow is most likely. The Great Basin and Mojave Deserts have wet winters and relatively dry summers with sporadic
thunderstorms. The Chihuahuan De sert receives rainfall primarily during the summer. The
Sonoran Desert receives rainfall in both winter and summer (England and Laudenslayer, 1995). Most streamflow events in a large portion of the Sonoran and Chihuahuan Deserts (southern Arizona and New Mexico) occur during the summer monsoon (July through September) from high-intensity, short-duration ra infall events which typically occur as airmass thunderstorms resulting from convective heati ng of moisture-laden air masses (Gochis et

14Figure 11. Maps showing percent of average annua l precipitation, July and August (left) and percent of average annual precipitation , October through March (right), for comparison, Western Regional Climate Center, http://www.wrcc.dri.edu/precip.html

. al., 2006). This warm-season monsoonal rainfall results from a seasonal reversal of

atm ospheric circulation that transports moisture from the Gulf of Mexico and/or the Gulf of California (Hereford et al., 2002). Longer duration rainfall events with embedded highintensity thunderstorms are often the result of dissipating tropical depressions that are
common in the fall and sometimes in the winter (Webb and Betancourt, 1992; Gochis et al., 2006), while the lower-intensity events are ty pical of cool-season pr ecipitation caused by frontal systems originating in the eastern North Pacific Ocean (Hereford et al., 2003).

Significant streamflow events in ephemeral st ream channels occur infrequently from lowintensity cool-season precipitati on unless there has been regular rainfall for several months and the soil is saturated. Still less frequently (for example, approximately 3 to 5 percent of the annual rainfall in southern Arizona, on aver age), runoff and streamflow occurs from the remnants of hurricanes and tropical depressions which track north from lower latitudes. The
influence of both the summer monsoon and in creases in precipitation from tropical depressions decreases northward.

Most of New Mexico and large portions of Arizona and Colorado receive between 30 to 50 percent of their annual preci pitation during just two months, July and August, when the monsoon thunderstorms occur. Figure 11 shows maps of the percent of average annual precipitation occurring during th e summer season (July and Augus t), and during the 6 months of the cool season (October through March) for a comparison.

i. Variability of arid and se mi-arid region flows and floods

Many aspects of arid and semi-arid region floods are highly distinctive. The low annual precipitation in these regions in evitably means low annual runoff, with interannual variability of runoff increasing as annual totals decrea se (McMahon, 1979; Rodier, 1985). In North American arid lands, the vari ability of mean annual runoff is about double that for the

15continental area as a whole (McMahon, 1979). In addition, given the spatially variable patterns of precipitation and runoff in arid and sem i-arid regions, for any given watershed size there is a large range in annual runoff totals (Reid and Frostick, 1997), and basin response can only be extrapolated to a very limited extent (De Boer, 1992). This implies that watershed area usually cannot be used as a reliable surr ogate measure of runoff in arid and semi-arid regions. Goodrich et al. (1997) found that watershed rainfall-runoff response becomes more non-linear with increasing watershed size due to the increasing importance of ephemeral stream channel transmission losses and partial area storm coverage.

With the exception of perennial, mainly allogenic rivers (those that orig inate and are fed from outside of the area, where precip itation and runoff are sufficient to generate flow), most arid and semi-arid region rivers are characterized by long periods without flow. For example, in the Negev Desert in Israel, Reid et al. (1998) conducted flow duration analysis and found that ephemeral stream channels are hydrologically active only 2 percent of the time, or about seven days per year, and that overbank flow can be expected for only 0.03 percent of the time

– about three hours per year. Because of infreque nt flows, process studies in arid and semi

arid region channels are domina ted by the analysis of flood events (Graf, 1988). In the general fluvial literatu re, a flood is usually defined in re lation to a humid region event (i.e., the near or complete exceedance of bankfull). Nevertheless, several authors (e.g. Leopold and Miller, 1956; Schumm and Lichty, 1963; Hedm an and Osterkamp, 1982; Bourke and Pickup, 1999) have referred to the variable size of floods , as this can be important for processes of sediment transport and channel change.

Floods caused by distinctly different climatic processes commonly have distinctly different magnitude and frequency relations. Many studi es have examined the nature of these differences by separating flood data for a station into two or mo re populations on the basis of the climatic causes of the floods (U.S. Army Corps of Engineers, 1958; Elliott et al., 1982; Jarrett and Costa, 1982; Waylen and Woo, 1982; Hi rschboeck, 1987). Results of these studies for different regions have indicated that floods caused only by snowmelt, by rain on snow, and only by rain, form distinct populations; fl oods caused by rain on snow or only by rain tend to have larger magnitudes than do floods caused only by snowmelt. In parts of the arid Southwest, floods caused by precipitation from front al passages in the winter tend to be larger than floods caused by precipitation from conv ectional storms in the summer. In the Southwest and Northeast, floods caused by precipitation from tropical cy clones tend to have greater magnitudes than do floods caused by precip itation from storms other than tropical cyclones. Floods caused by precipitation from tropical cyclones commonly include the peak flow of record (USGS, 1997).

Variability of flow is a natural continuum in arid and semi-arid regions, and is affected by climatic and ecological conditions. For example, the peak water demands of a dense riparian forest for transpiration in dry regions can deplete a stream ch annel of its flow for several hours during a hot summer day. A stream can run continuously for several years, and then go dry, making it difficult to classify the stream as perennial or ephemeral. Increasingly accurate and precise methods of monito ring and measurement, which ma y now detect these natural phenomena, might change a stream classification, without the river itself changing. Stanley et al. (1997) noted that desert streams are “spa tially dynamic ecosystems th at undergo cycles of expansion, contraction, and fragmentation; th at conventionally hydrologic measurements of

16water velocity or volume passing a fixed point represent only one aspect of hydrologic dynamism...”

ii. Types of arid and semi-arid region floods

As well as varying in size, floods in arid and semi-arid regions vary from entirely channeled, to largely unchanneled (Olsen, 1987). In the American Southwest, for example, partly channeled floods occur during major events when river banks are overtopped and flood waters diffuse across vast, low-grad ient plains (Hedman and Os terkamp, 1982). Graf (1988)
described numerous instances of sheet floods from piedmont settings as examples of
unchanneled floods.

Channeled floods in arid and semi-arid region rivers may occur as flash floods, single-peak events, multiple-peak events and seasonal floods (Graf, 1988). The highly variable stream flow in ephemeral and intermittent systems mo st often occurs as a flash flood, lasting only minutes or hours, or persisting for days or w eeks depending on the climatic regime and the
nature of the watershed contri buting area. Flash floods may o ccur any time of the year in response to a short-duration hi gh-intensity precipitation event, and after the watershed has received enough precipitation to generate runoff (Figure 12).

Most commentaries on arid and semi-arid region river floods refer to th e characteristics of flash flood hydrographs (charts s howing change in flow over time), which are typically produced by convectional prec ipitation in small (< 100 km 2 ) watersheds. Due to high runoff
coefficients and the dominance of Hortonian overland flow in runoff generation, these hydrographs are characterized by steep rising and receding limbs and a short time base (Reid, 1994; Dick et al., 1997). For a si mple individual flow event gene rated by a discrete storm, the rapid rise to peak discharge (almost instanta neous) is followed by the recession portion of the hydrograph. The duration of recession is gene rally much longer than the time required to reach peak flow, and the resulting flood wave sh ape is such that almost the entire hydrograph is the recession curve (Figure 13). The recession curve of an ephemeral stream hydrograph has two properties of interest: (1) flow ceases after some period of time, causing the flow to be of finite duration, and (2) th e shape of the curve can be compared to an exponential decay reference curve (C how et al., 1988).

17Figure 12. Photograph of a flash flood in an ephemeral channel, Southern Arizona. (Photograph: USDA-ARS/SWRC)

Less well documented are the single and multiple-p eak floods generated by tropical storms or frontal systems, or the floods associated with seasonal snowmelt or rainfall. Knighton and Nanson (1997) considered that in moving from single-peak to multiple-peak to seasonal floods there is a corresponding reduction in the steepness of the rising limb of the hydrograph and a broadening of the time base of the floods.

120

100

80

60

40

20

0

Time (min)

Figure 13. Example hydrograph showing typical rapid rise to peak discharge and long
recession curve of a flash flood.

iii. Transmission losses

In a spatial as well as a temporal sense, streamflow in arid an d semi-arid region rivers exhibit unique characteristics. Regardless of the source of water, flows in arid and semi-arid region rivers are generally influent, or subject to downstream volume decreases. These decreasing flow volumes principally are due to transmission losses resulting from infiltration ofDischarge (cms)

0 50 100 150 200 250

18stream flow into the unconsolidated alluvi um forming channel boundaries, losses resulting from overbank flooding, and evaporation of fl oodwaters (Babcock and Cushing, 1942; Keppel and Renard, 1962; Sharp and Sa xton, 1962; Lane, 1983; Goodrich et al. 1997; Cataldo et al., 2004). Transmission losses are also an important source of water for ground-water recharge (see next section).

Downstream volume decreases are sometimes neg ligible along small, alluvial or bedrock channels, but for larger alluvial channels they can be of gr eat importance, with many flows failing to travel the full length of the cha nnel (Keppel and Renar d, 1962; Aldridge, 1970), leaving the lower parts of the watershed dry.

The nature of the rainfall event can affect dow nstream reductions in fl ow. When precipitation is widespread, tributary contributions can in crease downstream flows even while losses are still large. For spatially localized even ts, however, in combination with hydrograph attenuation, and in the absence of appreciable tributary inflow s in the lower parts of the watershed, transmission losses can produce signif icant downstream decreases in total flow volume, flood peak, and flow frequencies (K eppel and Renard, 1962; Lane, 1983; Goodrich et al., 1997; Knighton and Nanson, 1997).

A great deal of research on semi-arid regi on hydrology has been conducted at the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona (Figure 14). The WGEW is one of the most intensively instrumented semi-arid experimental watersheds in the world with nearly 100 years of abiotic and biotic data (Moran et al., 2008). The network of over 125 gauging stations has been continuously collecting preci pitation and runoff data for over 50 years.

Figure 14. Map of the Walnut Gulch Experi mental Watershed (WGEW), Tombstone, Arizona, showing major stream network, flumes and sub-watershed boundaries.

19An exa mple of the magnitude of transmission lo sses within the WGEW is presented in Figure
15. This figure shows the August 27, 1982, storm ev ent that was isolated in Sub-watershed 6 and recorded at Raingage 56 on the upper 95 km 2 of the watershed (alth ough not all of that precipitation produced runoff). The spatial patter n of total storm precipitation depth depicted by the isolines in Figure 15 is interpolated from the WGEW rain gauge network. The temporal distribution of rainfall intensity obser ved at Raingage 56 is illustrated in the upper right portion of the figure. The runoff measured at Flume 6 was 246,000 m 3 , with a peak discharge of 107 m 3 /s. Runoff traversing the 10.86 km of dry streambed from Flume 6 to Flume 1 experienced significant infi ltration losses. As a result, 90,800 m 3 of water was absorbed in the channel alluvium, and to tal peak discharge was reduced by 52 m 3 /s. Photographs of Flume 1 with and wi thout flow are shown in Figure 16.

The magnitudes and rates of transmission losses fo r streamflow or flood events in a given arid and semi-arid region river are often highly variable, as both depend on a complex of interrelated factors, in cluding the characteristics of the st orm (e.g., size, position of the storm track, location in relation to the drainage network), the hydrograph (e.g., flow volume and duration), and the channel (e.g., width of the we tted perimeter, porosity and initial moisture content of the channel bed, st ratigraphy of the channel fill ) (Knighton and Nanson, 1997; Reid and Frostick, 1997; Lekach et al., 1998). Catald o et al. (2004) review ed about three dozen approaches for predicting transmission losses in ephemeral streams in the U.S., and concluded that approaches that combine differential equa tions and regression analyses that consider physical processes and st atistical methods have the most promise.

20Figure 15. Hydrograph, location map, and pho tograph of rainfall -runoff event August 27, 1982, illustrating ephemeral stream channel tr ansmission losses as measured within the WGEW (Goodrich et al., 1997).

Figure 16. Photographs of Flum e 1 at WGEW, dry (left) and with flow (right). (Photographs: USDA/ARS-Southwest Watershed Research Center, Tucson, Arizona)

21iv. Ground-water recharge

Ground-water recharge in arid and semi-arid regi ons has generally been viewed as the sum of several different distinct pathways incl uding mountain-block rech arge, mountain-front recharge, spatially distributed recharge, and ephemeral stre am channel recharge. Recent research has expanded this view to include the mediating role of vegetation (i.e. water use by vegetation), and the grea ter role of ephemeral stream cha nnel recharge in basin floors.

“Mountain-front recharge” refers to the contribution from mount ain precipitation to recharge of aquifers in adjacent basins. It includes recharge from the mountain block system and stream channels, and is considered to be the most significant form of ground-water recharge in arid and semi-arid regions, with ephemeral stream channel recharge providing a significant portion in these climates (Goodrich et al., 2004; Coes and Pool, 2005). Basin floor or spatially distributed recharge in arid and semi-a rid regions plays a lesser role in the overall recharge volume due to high evaporation rates, low rainfall, and high water use by desert vegetation (Coes and Pool, 2005).

Advances such as environmental tracers a nd geographic information systems (GIS) based ground-water models have improved our understandi ng of recharge processes (Phillips et al., 2004; Hogan et al, 2004). However, an accurate representation of gr ound-water recharge is difficult since it cannot be measured directly on a basin scale, in addition to other reasons, including the extremely small recharge rates a nd recharge mechanisms that vary greatly in time and space throughout a watershed. The methods used in humid regions, such as a water balance approach, are not applicable in arid and semi-arid regions because these extremely small amounts of recharge are within the meas urement error, and potential evapotranspiration exceeds precipitation (Hogan et al., 2004; Phillips et al., 2004). Also, channel transmission losses are more significant in most ephemeral and intermittent channels than in humid region perennial streams, as noted in the previous section. Therefore, me thods for calculating recharge are indirect, and subj ect to cumulative measurement e rrors. In addition, the annual variability of precipitation in ar id and semi-arid regions makes it difficult to apply recharge models, which simulate the direct recharge to the aquifer from infiltra tion of precipitation. Goodrich et al. (2004) noted that ephemeral stream channel transmission losses play an important role in ground-water surface-water dynamics in numerous arid and semi-arid regions and are potentially signi ficant sources of recharge at the basin scale. However, identification of the processes dr iving these dynamics is difficult. Specificall y, it is difficult to obtain data on the proportion of transm ission losses that become deep ground-water recharge instead of being lost to near-channel evapotranspiration ( ET) and wetted channel evaporation.

This issue was addressed via coordinated fiel d research and modeling within the WGEW. A variety of methods were used to estimate ephemeral stream channel recharge, including ground water, surface water, chemical, isot opic, tree sap flux, micrometeorological techniques, and changes in microgravity. Chan ges in microgravity reflect the gravitational pull of the Earth and indicate the presence of subsurface density variations such as those produced by voids or cavities. A cavity us ually has a lower density than the surrounding

22materials and may be filled with water or sediment, resulting in a very small reduction in the pull of the Earth’s gravit y (Styles et al., 2006).

Figure 17 illustrates the changes in deep ground- water levels due to multiple runoff events during the 1999 and 2000 monsoon as well as associ ated microgravity changes. During the relatively wet 1999 and 2000 monsoon seasons the channel recharge estimated from these methods differed by a factor of about 2.9. A r ough scaling of these rates to the entire basin
shows that these estimates would constitute r oughly 15 percent at the low end of the range and 40 percent at the high end, respectively, of all water recharged annua lly into the regional aquifer as derived from a calibrated ground-water model estimate (Goodrich et al., 2004). However, in 2001 and 2002 no discernable ephe meral stream channel recharge in the intensely studied reach was detected due to weak monsoon seasons, illustrating that ground

water recharge in ephemeral stream channels ca n be significant in some years and negligible
in others.

Figure 17. Diagrams of well levels and flow depths at the WGEW. Diagram (a) at top
left shows Flume 2 well level ch anges (m) and flow depths (m), diagram (b) at bottom shows Flume 1 well level changes (m), flow depths (m) and gravity measurements, and diagram (c) on upper right shows a cross sect ion of well transect above Flume 1 (from Goodrich et al., 2004).

Other studies have also noted the importance of locally recharged monsoon floodwater derived from ephemeral stream channels for maintaining river flow. Using a suite of geochemical tracers and a two end-member mi xing model, Baillie et al. (2007) found that locally recharged monsoon floodwater s is one of the dominant water sources in the main stem

23of the spatially intermittent San Pedro River, wi th these waters comprising 60 to 85 percent of riparian ground water in losing reaches of the main stem and 10 to 40 percent in gaining reaches. Baseflows in the perennial reaches also contained a si gnificant component of monsoon floodwater: 80 percent at the upstream segment, decr easing to 55 percent after several gaining reaches (Baillie et al., 2007). Various other methods of tracers are described in Cook and Herczeg (1999). Coes and Pool (20 05) looked at recharge in the same basin and found that ephemeral stream channel rechar ge occurs during both summer and winter
streamflow events.

v. Landscape and hydrologic connections

Watersheds and their surrounding ecosystems are li nked by the flow of water. In a watershed context, landscape hydrologic connectivity refe rs to the maintenanc e of natural hydraulic
connections of surface and subsurface flow betw een source, headwater, or contributing areas and downstream/down gradient receiving waters. Nadeau and Rains (2007) defined it as “the hydrologically mediated transfer of mass, momentum, energy, or organisms within or between compartments of the hydrologic cycle. ” In arid land streams, this hydrologic connection occurs episodically during flood pulses, yet still pr ovides a substantial amount of the mass, momentum, energy and organisms deli vered to downstream perennial waters, as well as to ground-water recharge.

Freeman et al. (2007) stated that, “The hydrolog ic connectivity of sma ll headwater streams to navigable waters is clear and unambiguous to ecol ogists. Every important aspect of the river ecosystem, the river geomorphic system, and the river chemical system begins in headwater streams.” Kennedy (1977) discussed the intera ctions of stream-ripa rian-vegetation-energy

nutrients-water production-aqua tic life and terrestrial life, noting that the key to wise management of aquatic ecosystems is wise management of the watershed.

As headwater streams occur upstream from, and may ultimately discharge into higher order perennial streams, they connect landscape pr ocesses through their influence on the supply, transport, and fate of water and solutes in th e watershed (Alexander et al., 2007; Leibowitz et al., 2008).

Shaw and Cooper (2008) noted that biotic patterns within ep hemeral stream networks are controlled directly by interacti ons of hydrologic and geomorphic regimes, and indirectly by watershed and stream-network properties. In their study of riparian vegetation and watershed linkages in ephemeral stream systems, they cl assified channels into three types based on physical properties and plant community type s. Their classificat ion system described functional linkages among watersheds, stream reaches, and riparian plant ecology, indicating a strong landscape connection between proces ses in the upper watershed and the lower watershed. For example, they found that st reamflow and ground water regimes in regional flood plain rivers were driven by climatic patterns from distant portions of the upper watershed and were relatively insensitive to local rainfall.

24Figure 18. Photograph of ephemeral and intermit tent stream channels connecting to a perennial reach of Cienega Creek, southeast of Tucson, Arizona. (Photograph: Lainie Levick/Aerial flight courtesy of Lighthawk, www.lighthawk.org)

Delivery of water to a stream is dependent largely on the timing, duration, and amount of water that falls on the surface and subsequently runs off, which is dependent on soil type, and condition of the watershed and buffer. The importance of hydrologic connectivity in arid environments relates closely to the delivery of water, sediment, nutrients, compounds, etc. to
downstream areas. Small tribut aries generally have land-domin ated hydrographs as opposed to stream-flow dominated, because they main ly drain adjacent land surfaces. Numerous observed runoff events originating in the uplands of ephemeral tributar ies at the WGEW have reached the San Pedro River as evidenced by corresponding hydrograph observations at the USGS Tombstone gaging station just downstrea m of the confluence of Walnut Gulch and the San Pedro River. Instrumenting additional watersheds would add to the understanding of these arid and semi-arid systems.

Although observed runoff events are more meani ngful than simulated results, nevertheless, models are useful in understanding a hydrologi c system. In a hydrologic modeling study of ephemeral tributaries to the San Pedro River, Le vick et al. (2006) dete rmined that simulated flows from the uplands would reach the San Pe dro. Using the AGWA/KINEROS model, they
looked at runoff and sediment yield using three design storms : 2-year 1-hour, 5-year 1-hour, and 10-year 1-hour. They determined that unde r predevelopment conditions, even the 2-year

1-hour design storm event (18.47 mm) was enough to fill the void spaces in channel-bed sediment, overcome transmission losses, and cause a small but measurable flow at the watershed outlet, demonstrating a hydrologic conn ection from the ephemeral tributaries to the San Pedro River, nearly ten miles downstream. The simulations showed that larger storm events yielded more flow, as did post-devel opment simulations where impermeable surfaces
in the watershed increased. For more info rmation on the AGWA/KINEROS model, go to http://www.tucson.ars.ag.gov/agwa.

25Figure 19. Photographs of an ephemeral stream, same location during flow (left), and dry (right), Tucson, Arizona.

vi. Energy dissipation

Energy dissipation refers to the transformati on and/or reduction in the amount of kinetic energy of flowing water, which is a functi on of channel roughness, channel morphology, and buffer and landscape vegetation. Stream energy di ssipation is important for the prevention of channel erosion and scour, and increased se diment loads that can degrade water quality.

Water flowing in stream channe ls is subject to two key for ces: (1) gravity that moves the water downslope and (2) friction between the water and channel boundaries that resists the downslope movement. These two forces determine, to a large degree, th e ability of the water to modify the channel geometry and transpor t debris. In addition, channel roughness, slope, and depth determine the velocity of the flow ing water (Leopold et al., 1964; Wakelin-King and Webb, 2007). Channel slopes in the Southw est are often large so when flows do occur they have high velocities and consequently significant energy and stream power.

Dissipation of energy in channels can occur due to vegetation, curvature (stream sinuosity), obstructions (rocks, debris, dams), and the size, character and configurati on of material in the bed and banks. Flow hydraulics and roughness coefficients in some arid and semi-arid channels are strongly influen ced by vegetation, which frequen tly grows on the normally dry channel beds to exploit moisture contained in subsurface sediment.

vii. Sediment mobilization, storage, transport, and deposition

As noted previously, although ephemeral streams do not flow at all times, they still perform the major functions of a stream: the transportati on of water, nutrients, and sediment. Unlike perennial streams that continuously move sediment through the watershed, sediment movement in non-perennial stream channels gene rally occurs as a pulse in response to runoff generated by the short duration, hi gh intensity thundersto rms that are typical of arid and semi

arid regions. These thunderstorms can result in flash floods and yiel d rapidly rising runoff

26hydrographs. The associated high velocity turbul ent flash flows contain heavy sediment loads and push large amounts of coarse sediment through the system. In addition, sediment is moved from the uplands and hillslopes into the channels from overland flow. Figure 20 shows photographs from an unusually large flood event in Tucson, Arizona, that moved large quantities of rock and debris th rough the channel. The rock an d debris plugged up the bridge, causing the floodwaters to leave the channe l, damaging the roadway and flooding nearby
homes.

Figure 20. Photographs from an unusually larg e flood event in an ephemeral stream that damaged roads and bridges, and flooded nearby homes, Tucson, Arizona, July 31, 2006.

Channels in arid and semi-arid regions tend to have deep sediments that are mostly sands and gravels, with widely scattered shrubs that are resistant to violent flood waters. However, the unconsolidated alluvium can easily be mobili zed during flows, unlike the clay bedded or armored channels in more humid regions. Th ese deep sediments cause large transmission
losses in the downstream directi on, resulting in reductions in both flow volume and velocity over the length of the stream, and subsequent deposition of bed load materials and coarser suspended sediments in the downstream segments (Whitford, 2002).

Storm water is often completely absorbed in th e channel network before reaching the outlet. Transmission losses and decreasing discharge in the downstream direction thus promote the stepwise movement, deposition and storage of se diment within ephemeral stream networks (Renard, 1975). The effect is a pulsing style of sediment movement that doesn’t always reach the watershed outlet, but is instead remobilized during the next flow and redistributed within
the watershed’s channel networ k (Leopold et al., 1964; Thornes, 1977; DeBano et al., 1995).

Ephemeral and intermittent channels contain a wi de range of sediment size, with the larger material remaining essentially at rest although a significant portion is available for transport (Renard and Laursen, 1975). These channels are typically transport-limited systems as opposed to detachment limited. The large flows that can move great quantities of sediment are relatively infrequent in arid and semi-ari d regions; however the sediment moved by the smaller more frequent flows can add up to a considerable amount (Nichols, 2006).

As a result of decreased flow rates in the downstream direction, more silts and fines are deposited in the channel, which can be adva ntageous to biotic communities. A study of

27ephemeral rivers in the Namib Desert (Jac obson et al., 2000a) found that “Organic carbon, nitrogen and phosphorous were correlated with silt content, and silt deposition patterns influence patters of moisture availability and plant rooting, creating and maintaining micro

habitats for various organisms.” Jacobson conc luded that “...alluviation patterns associated with the hydrologic regime strongly influen ce the structure, produc tivity, and spatial distribution of biotic communities in ephemeral river ecosystems.”

Because the small, uppermost channels of a dr ainage network are important in determining the amount of sediment transported downstr eam during storm events, their removal will
increase sedimentation rates in downstream channels (Meyer and Wallace, 2000). This increased sediment load can have negative effect s on channel stability, fish, invertebrates, and overall stream productivity. However, when sm all or headwater streams are replaced with paved or lined floodways during land devel opment, sediment production may decrease, causing an increase in downstream erosion as sediment starved waters move through the watershed. Figure 21 is a photograph of sediment-laden floodwaters.

Figure 21. Photograph of sediment-laden floo dwaters in an ephemeral stream, Walnut Gulch, Arizona. (Photograph: USDA-ARS/SWRC)

Sediment deposition can have vary ing effects. For example, sediment deposited during flow events can encourage plant germination (i.e. Cottonwood, Populus fremontii ) by providing seed beds and scarifying seeds, but it also can inhibit the growth of seedlings or some types of vegetation, such as non-native saltcedar ( Tamarix ramosissima ). This can be beneficial in some instances where stream restoration efforts are occurring. However, some aquatic species can be adversely affected by exce ssive sediment, which can interfere with reproduction and feeding.

b. Geomorphic Characteristics

The variability in time and space of fluvial processe s is particularly characteristic of arid and semi-arid area rivers (Tooth, 2000a), yet the role of rivers in shaping desert landscapes has

28generally been underestimated by geomorphologist s. As a result, th ere is inadequate information on geomorphic processes and forms in arid and semi-arid regions. This is important because fluvial processes are a cause of so many problems in desert areas (Reid and Frostick, 1989), and also because the geomorphol ogy and hydraulic-geometry relationships of ephemeral and intermittent streams are very different from humid area perennial streams (Graf, 1988; Reid and Frostick, 1989; Thornes, 1994; Tooth, 2000a; Bu ll and Kirkby, 2002).

Although one of the most universal ly recognized traits of arid and semi-arid ephemeral stream channels is their enormous variability in form, several broad generalizations have been used
to characterize them:

• They are often closely spaced, result ing in a high drainage density.

• They have high width-to-depth ratios.

• They are likely to be braided (Figure 22).

• They have low sinuosity relative to their humid counterparts.

Closely spaced channels and a hi gh drainage density are genera lly due to high erosion rates and limited runoff, which produce high sediment concentrations in arid and semi-arid region channelized flows (Reid and Frostick, 1997; Bull and Kirkby, 2002). In headwater areas, this may lead to gullying and/or badland developmen t until such time as shrinking contributing area, stabilizing vegetation, and/or surface armo ring moderate erosion from rain splash and surface flows. High width-to-depth ratios, brai ded channels and low sinuosity are often the result of high sediment concentrations a nd coarse grain sizes (Bull and Kirkby, 2002).

Figure 22. Photograph showing an ephemera l braided stream system, Yuma Wash, southwest Arizona. (Photograph: Susan Howe, Colorado State University)

29In most arid and semi-arid river cross secti ons, depth increases with discharge somewhat faster than does width (Leopold and Maddock, 1 953). The width of channels increases much more rapidly in the downstream direction (Bre schta and Platts, 1986) than is observed in humid regions, resulting in wide channels in the lower reaches. Wolman and Gerson (1978) compiled data from different arid and semi-arid regions and found that channel widths increased rapidly up to a dr ainage area of about 50 km 2 (Figure 23). As drainage area increased beyond about 50 km 2 , channel widths asymptotically approached a value between 100 and 200 m. It is likely that this stabilizati on of channel width for la rger drainage areas is
due to the fact that transmission losses from flows with such a high wetted perimeter compensate for any addition of tributary wa ter (Reid and Frostick, 1997). In ephemeral stream channels with no significant tributary inputs, transmission losses can result in decreasing channel width and capacity in the downstream direction with some ultimately becoming unchannelized alluvial surfaces termed “floodouts” (Dunkerley, 1992; Tooth, 2000b).

Figure 23. Relation of bankfull channel width to drainage area for different climatic environments (after Wolman and Gerson, 1978).

An oscillating pattern of narrow, incised reac hes and wide, shallow reaches has also been observed in ephemeral stream channels (Sc humm and Hadley, 1957; Bu ll, 1997; Pelletier and DeLong, 2004). The wavelength of these oscillat ions ranges from 15 m to over 10 km (Bull, 1997) and has been successfully modeled as a function of channel sl ope, width, and depth (Pelletier and DeLong, 2004). Alternating er osional and depositional reaches migrate progressively upstream, resulting in repeated episodes of incision a nd aggradation at any given point along the channel. Perturbation of these system s by natural or anthropogenic causes can result in the development of continuo us incised channels, or arroyos, as described
in the next section.

In addition to their pronounced widths, the lowe r reaches of ephemeral streams are noted for having particularly flat bed topography; the be ds of single-thread st reams are often near horizontal and planar (Reid and Frostick, 1997) (Figure 24). Cha nnel bars, where present, are also often flat-topped and rise only 10-20 cm above the thalweg (Leopold et al., 1966; Frostick and Reid, 1977, 1979). Be d flatness and channel width are likely related through

30flow depth; wide, shallow fl ows suppress the secondary curr ent cells that encourage the development of bars (Reid and Frostick, 1997) . Rapidly receding fl ows can destroy or modify bedforms such as ripples, dunes, a nd antidunes that may develop at greater flow depths. Bedforms in streams are created when water currents carry loose grains across the horizontal surface of unconsolidated sediments. The size and shape of bedforms are determined by the flow velocit y, direction, and consistency.

Figure 24. Photograph of a typical wide, fl at, and sandy ephemeral stream channel, Martinez Wash, Arizona. (Photogra ph: William Kepne r, USEPA/ORD)

i. Channel-forming processes

Fluvial processes are significant agents of eros ion and deposition in arid and semi-arid regions and thus, over time, desert ri vers can be active land-forming agents (Frostick and Reid, 1987; Reid and Frostick, 1997). Low rainfall in dese rt regions results in weathering processes dominated by mechanical rather than chemical means. Clay production is thus inhibited and silt-sized fractions are predominant in the soils. The lack of bank-stabi lizing clay in arid and semi-arid region ephemeral stream channels may partially explain why these channels typically have wide, shallow, low sinuosity geometries (Schumm, 1961; Scott, 2006). The sparseness of vegetation along some stream banks in arid areas can also contribute to channel widening tendencies (Miller, 1995; Reid and Frostick, 1997). Furthermore, channel-bed armoring is uncommon in desert streams because of the high supply of all sediment sizes, rapid recession of flash flood hydrographs, a nd extended periods of no flow (Reid and Laronne, 1995).

As event size increases sediment can be move d further downstream, but only the largest, least frequent events are capable of flushing sedi ment completely through the system and opening up (widening or incising) channels that have become progressi vely choked with vegetation and sediment. Indeed, Lekach et al. (1992) obs erved that more than 90 percent of the bed

31load yield in an arid-region watershed origin ated from the mid-catchment channels during larger runoff events. High-magnitude, lo w-frequency floods thus control channel development, and their effects tend to be modifi ed very slowly by smaller events. The result is a tapestry of highly varied, transient channel forms that are a reflection of the recent flood history rather than an equilibrium state. In addition, the variable flows in combination with
sediment characteristics will influence the ch aracter of the flood plain (Nanson and Croke, 1992).

The morphology of ephemeral stream channels, in combination with transmission losses, is the result of cyclical patterns of infill and erosion. For example, the smaller, more frequent flows that transport sediment into the channel ne twork can, over time, result in the infilling of channels and decreasing of channel width (e.g., Burkham, 1 972), often in association with the growth of riparian vegetation (s ee section 5.d). In contrast, pe riods of low (base) flow in perennial steams are characterized by low sedi ment loads and can cause channel narrowing by cutting into deposits left duri ng larger flows when ephemera l tributaries are active (e.g., Friedman et al.,1996). Dunkerley and Brown (1999) determined that smaller flows are disproportionately impacted by transmission losses than bank-full flows because flatbottomed channels result in proportionately larger wetted perimeter for a given flow volume. In addition, a steep increase in transmission losses occurs as flows overtop their banks and spread out onto the flood plain, which furt her limits the potential work that can be accomplished by intermediate floods (Graf, 1983; Lange, 2005). Together these relationships encourage aggradation within th e channel network. Over time, however, the threshold flow required to cause a major erosive event is re duced as fine sediment retards infiltration capacity (e.g., Dunkerley, 2008), channel width narrows and the growth of flood plain vegetation encourages the concen tration of flow within the main channel. Both channel widening (e.g., Burkham, 1972; Friedman a nd Lee, 2002) and incision (e.g., Merritt and Wohl, 2003) have been observed when this threshold has inevitably been reached.

In the late 19th century, ephemeral stream channels throughout the American Southwest began to incise into alluvial valleys, creating deep continuous channels that are collectively referred to as arroyos. This arroyo formation episode was one of several periods of channel
incision that are evidenced in the Holocene st ratigraphic record, a nd separated by extended periods of aggradation (Cooke and Reeves, 1976). Arroyos are defined by Elliot et al. (1999) as large-scale, continuous, and persistent erosional features created when stream channels incise into their alluvial valleys (Figure 25). The term arroyo is usually used to refer to incised ephemeral stream channels in the Ameri can Southwest, but it is important to note that
incised channels have also been formed on in termittent streams and have been observed in many regions throughout the world.

Arroyo development is commonly thought to resu lt from a combination of three factors: anthropogenic disturbance, changing climatic conditions, and/or intrinsic geomorphic conditions. Land-use change associated with overgrazing, farming, and timber harvesting was one of the first explanations for arr oyo development (e.g., Thornwaite et al., 1942; Antevs, 1952; Cooke and Reeves, 1976; Fanning, 1999). Reduced vegetation and infiltration
rates associated with these an thropogenic activities were wide spread in the late 1800s, and likely increased both runoff and erosion.

32Figure 25. Photograph of a small arroyo or incised channel, southern Colorado.

A factor that is mentioned less frequently in the literature on cyclic incision episodes is the reason why it is observed primarily in ephe meral stream channels. Aside from obvious differences in their discharge regime, the fundamental difference between ephemeral and perennial streams is that ephemeral stream channels are characterized by sizeable transmission losses when they flow. Nume rous authors have documented substantial transmission losses in ephemeral streams, frequen tly to such an extent that flows infiltrate completely before reaching the watershed out let (Keppel and Renard, 1962; Aldridge, 1970). Schumm and Hadley (1957) suggested that deposition as a result of seepage-induced discharge reductions in the downstream directio n eventually causes dismemberment of the drainage system by the sealing off of tributary channels. Resulting incr eased valley gradients cause the formation of discontinuous gullies and reintegration of the system by arroyo cutting in the fills. Successive episode s of erosion and deposition are then thus the logical course of events as gradients adjust to differential filling along the profile. This explanation is
consistent with observations of alternative er osion and aggradation following a flood event in Yuma Wash, a tributary to the Colorado River in southwestern Arizona (Merritt and Wohl, 2003).

Combining these observations under conditions characterized by high transmission losses and decreasing downstream stream power, aggradatio n will prevail. These conditions are altered during the highest flows when transmission losse s are less significant, and increased stream gradients from prior deposition allow streams to cut into the valley fill. Subsequent smaller flows will be contained within the enlarged ch annel area and subject to reduced transmission loss and increasing stream power in the downstream direction. As a result they will continue to widen and deepen the channel until such time as the downstream distribution of stream power is again decreasing. Whethe r it is one flood even t or a series of events closely spaced

33in time that is needed to upset aggraded conditi ons has still not been resolved, and is likely to vary depending on a suite of other s ite-specific hydrologic conditions.

ii. Geomorphic response to land management

It is difficult to assess the stability of stream systems that are widely ch aracterized as being in a perpetual state of flux. A single cycle of incision and deposition can last decades to centuries, and numerous cycles would have to be analyzed before it could be decisively concluded that any persistent change was ta king place. Despite this, however, enough is known about the functioning of hydrologic and geomorphic systems to make very broad generalizations about the downstream effects of climatic and/or management changes. For instance, if upland surfaces are armored with impervious pavement due to development then it
is known that there will be increased runoff (particularly from smaller storms), but less
sediment delivered to the channels. Over time it can be expected with some confidence that
increased erosion will occur in channels downstream of the developed area, as numerous studies have shown (e.g., Booth, 1990; Chin and Gregory, 2001; Semmens, 2004).

The management of arid and semi-arid lands drained by ephemeral stream channels has a direct impact on the hydrology and geomorphology of the drainage network. Indeed ephemeral streams may be more sensitive to an thropogenic disturbance than perennial streams (Bull, 1997). Impervious surfaces increase th e frequency and magnitude of flooding. Storm sewers and lined drainages increase the rate at which these waters are delivered to the channel network, and thus further increase peak flows (Center for Watershed Protection, 2003). The primary geomorphic consequence of these hydrol ogic changes is the erosional entrenchment of adjacent channels and associated trans portation of the excavated sediment further downstream. Ultimately, as headwater streams e quilibrate to the new flow regime and their importance as a sediment source declines, channel entrenchment will likely shift further and
further downstream.

The cumulative effect of many entrenching channels is a significant increase in sediment load in downstream waters, which may partly expl ain why many TMDLs in the Southwest are
written for sediment. TMDL stands for “Total Maximum Da ily Load,” and is a written, quantitative plan and analysis to determine th e daily maximum amount of a pollutant that a waterbody can receive and still meet water qualit y standards, and includ es an allocation of that amount to the pollutant's sources ( http://www.epa.gov/owow/tmdl/intro.html ). In EPA Region 9, twenty out of twenty-six TMDLs th at have recently been completed include sediment as one of the pollutants ( http://www.epa.gov/region09/water/tmdl/final.html ).

In addition to changes in channel form a nd sediment yield, the geomorphic response to anthropogenic disturbance can also have significant c onsequences for ripa rian ecosystems and water supplies. As streams become entrench ed, formerly rich biological communities on the flood plain can become hydrologically disconnected from ephemeral streamflow and transform into dry terraces. Additionally, as channels become narrower and unconsolidated alluvial bed material is removed, there is less capacity to absorb passing flows and for vegetation to establish.

34Figure 26. Diagram of digitize d stream channels on a s ubwatershed at WGEW (from Miller et al., 1999a). iii. Map scale in determining ch annel network and stream order

Map scale can influence the identification of a stream channel network, which is often the basis for watershed, basin and regional scale assessments of hydrologi c systems and surface processes. As mentioned earli er, ephemeral stream channels are oftentimes the smallest channels in a watershed, or headwater streams, and make up a significant portion of the total stream network, making them important in wate rshed-based assessments. However, because of map scale, they frequently will not be represented on a map.

In conducting a watershed assessment, map scale will influence the level of detail of the drainage network (Miller et al., 1999a). The mo st common source of drainage network data is from 1:24,000-scale U.S. Geological Survey (USGS) topographic maps (i.e. blue-line streams) and studies have f ound that USGS 1:24,000-scale ma ps may grossly underestimate the number and length of drainage networks (Schneider, 1961; Leopol d et al., 1964; Mark, 1983; Heine et al., 2004).

Heine et al. (2004) reported that USGS 1:24,000-scale maps under-represented drainage networks by 64.6 percent in a study in Kansas . Mark (1983) found in a study in Kentucky on twenty-nine small watersheds that USGS 1:24,000-scale maps under-represented the number of source channels with an averag e of 2.15 channels per watershed.

Miller et al. (1999a) compared drainage density of digitized stream channels using different resolution aerial photographs and USGS maps, and found that drainage density decreased with decreasing scale while the underlying struct ure of the drainage pattern was retained (Figure 26).

In a striking study of im portance to arid and semi-arid environments, Leopold et al. (1964) examined the Arroyo de los Frijoles watershe d near Santa Fe, New Mexico. Based on USGS 1:24,000-scale maps, the Arroyo de los Frijoles watershed had no identified stream channels;

35however, after an examination of the contour patterns, the investig ators found a potential drainage network with 258 first-order channels . A field study of one of the first-order
channels revealed that its watershed actual ly contained 86 first-order ephemeral stream channels. The study by Leopold et al. (1964) illustrated that in arid and semi-arid environments even detailed examination of terrain data (i.e. contours or DEM) may still under-represent the number and length of ephe meral stream channels, making the use of “stream order” problematic.

c. Biogeochemical Functions

The biogeochemical functions of ephemeral and intermittent streams include cycling of elements and compounds, removal of impor ted elements and compounds, particulate detention, and organic matter transport. Thes e functions influence water quality, sediment deposition, nutrient availability, a nd biotic functions. Biogeoche mical features are affected directly and indirectly by land-use and land-cover change. Hydrologic modifications such as direct alteration of flow regime and hydrologic flow paths, and indirect alterations such as increased impervious cover in contributing ar eas of the watershed can cause biogeochemical changes. Elimination of the surface-water ground-water connection or disruption of the
connection between a stream and its watershed by large-scale changes such as urban and suburban development also influence biogeoche mical functions (Grimm et. al., 2004).

The spatial and temporal variability of rainfall in the arid Southwest affects the biogeochemical functions of ephemeral and interm ittent streams. These systems are driven by pulse inputs of water, sediment, organic matte r, and other materials during rain events. Periodic flows in ephemeral or intermitte nt channels have a strong influence on biogeochemistry by providing a connection betw een the channel and other landscape elements (Valett et al., 2005). This episodic connec tion can be very important for transmitting a
substantial amount of material into downstream perennial waters (Nadeau and Rains, 2007) where it is an important component of pe rennial food webs (Jac obson et al., 2000b).

i. Cycling of elements and compounds

Cycling of elements and compounds refers to biot ic and abiotic processes that cycle elements and convert compounds from one form to anot her (Lee et al., 2001, 2004) and is an essential ecosystem function performed by ephemeral and intermittent streams. Biotic processes include net primary production wh erein plants and algae take up nutrients from the soil and water, and detritus turnover from which nutrien ts are released back into the ecosystem by microbial activity (Brinson et al., 1995).

Biogeochemical cycling primarily occurs thr ough chemical transformation in response to redox potentials. These reduction and oxidation reactions are affected by the soil profile, wind inputs, and hydrologic input (Brinson et al., 1995). The recycling of elements and compounds is critical to maintain ing their low concentrations in flowing water. As uptake and removal processes primarily occur at the water-sediment interface, physical characteristics such as channel depth and wa ter velocity will affect nutrient recycling (Peterson et al., 2001). In addition, the amount of soil organic matter, coarse and fine woody

36debris, and litter within the channel and soil profile will la rgely determine the ability of ephemeral and intermittent streams to perform certain biogeochemical functions (Lee et al., 2004). In hyporheic zones, ther e is substantial biogeochemical cycling of nutrients and trace elements, which are essential to aquatic life (Hibbs, 2008). Ho lmes et al. (1994) found that the parafluvial zone represents a significant source of nitr ate to a nitrogen-limited stream ecosystem, which might be expected to c ontribute to ecosystem resilience following
disturbance.

Alteration of channel characteri stics (e.g., channel shape and de pth) and organic matter inputwill affect the ability of streams to cycle materials. Because small streams have high surfacearea to volume ratios, they are of ten able to take up and process nutrients at higher rates than larger perennial streams (Pinay et al., 2002) , and are important for maintaining downstream water quality.

Water limitation in arid environments results in patchy, sparse vascular plant cover. As a result, algal and soil microbial activity is importa nt for nutrient cycling in these environments (Belnap et al., 2005). Some dominant plant species such as mesquite ( Prosopis sp.) living along ephemeral streams have nitr ogen fixing bacteria associated with their roots, which can be an important influence on local nitrogen availability (Virginia et al., 1992).

Biological soil crusts, or cryptobiotic crusts, are a mixture of mosses, algae, microfungi, lichen and cyanobacteria that live on and just be low desert soil surfaces. They are usually found in open, undisturbed areas where vegetation is sparse, for example in upland areas adjacent to ephemeral streams. These organic comp lexes help to stabilize desert soils, hold in soil moisture, fix carbon and nitrogen, and can stimulate plant growth (Belnap, 2003). In some soil types, biological soil crusts can increas e infiltration rates. Biological soil crusts can determine the amount, location, and timing of wate r infiltration into desert soils, which, in turn, determines the type and si ze of microbial response. Nutrie nts resulting from this pulse then create a positive feedback as increases in microbial and plant biomass enhance future resource capture or, alternatively, may be lost to the atmosphere, deeper soils, or downslope patches (Belnap et al., 2005).

ii. Detention of imported elements and compounds

Headwater streams and wetlands are in a unique position to intercept nutrients and contaminants from upland environments before they reach larger perennial streams (Brinson, 1988). As water moves through small, shallow channels and comes in contact with sediment, vegetation, coarse and fine woody debris and so il organic matter, elements and compounds are removed from the water, either by direct uptake or by conversion into inactive forms. Important variables in assessing the capacity of ephemeral and intermittent streams to perform
this function include the amount of vegetative cover and soil or ganic matter (Lee et al., 2004).

Nutrient uptake and removal occurs more rapi dly in the small, uppermost channels in a watershed than in larger, downstream channels (Peterson et al ., 2001). During intermediate storms, small headwater channels may serve as co llection points for organic matter. Material accumulated during drier periods can be released downstream during larg e, infrequent storm

37events (Fisher et al., 2001). In an Arizona watershed, Fisher and Grimm (1985) found that ephemeral streams retained between 50 pe rcent and 90 percent of the nitrogen and phosphorous entering the stream during intermedia te storm events. Several authors have hypothesized that headwater streams contri bute significantly to downstream productivity (Freeman et al., 2007; Cummins a nd Wilzbach, 2005; Wipfli, 2005).

The temporally and spatially variable rainfall in the arid Southwest influences nitrogen processing and retention in small streams. Nitrogen cycling is dependent on soil moisture as some processes (e.g., nitrification) only o ccur in aerobic conditions while others (e.g., denitrification) only occur in anaerobic conditions (Pinay et al., 2002). Therefore, the degree of soil moisture will affect the end produc ts of nitrogen processing, which will affect downstream waters. During high moisture conditi on (such as after a storm event), ephemeral streams may experience elevated rates of denitrification (Fishe r et al., 2001; Rassam et al., 2006). Denitrification converts nitr ogen to gaseous forms that can be lost to the air, thereby completely removing it from the system, which can be important in areas that receive excess
nutrients from the watershed. Westerhoff a nd Anning (2000) found that ephemeral streams had higher total organic carbon (TOC) levels than perennial streams, suggesting that ephemeral streams are important for storing a nd processing organic material between large storm events. Because large am ounts of material are only move d into larger streams during extreme rainfall events, much of the nutrient cy cling may occur in the smaller streams.

iii. Particulate detention

Factors important in assessing the capacity of ephemeral and intermittent streams to retain particulates include floodway cross-sectional area, channe l roughness, and sediment supply (Lee et al., 2004). Because headwater, epheme ral and intermittent streams comprise a large percentage of the total watershed channel distan ce, in combination they may have the capacity to store large amounts of sediment and partic ulates. When studying the seasonal dynamics of physical and chemical variables in perennial, intermittent and ephemeral streams, Dieterich and Anderson (1998) found that ephemeral stre ams were very effective in removing and storing suspended sediment from the water co lumn. This stored particulate matter (e.g., sediment, plant fragments) can be released to downstream ecosystems during large storm

events.

The amount of particulate retention will depe nd on the timing, durati on, and amount of water received, as well as the characteristics of the stream channel and the integrity of the vegetative community around the stream (Brinson et al., 1995; Powell et al., 2007). Powell et al. (2007) found little net change in the removal and depos ition of sediments in an Arizona ephemeral stream over a three-year period. However, individual storm events resulted in scour or fill depending on storm severity suggesting that ep hemeral stream channels may undergo cycles
of infill and erosion.

In the southwestern deserts, large amounts of se diment and other particulates are washed into small streams during storms due to compacted soils with low infiltration rates and sparse vegetation in the upland environment (Fisher and Minckley, 1978). Flow events that do not fully connect small streams to downstream wate rs will result in particulate matter accretion

38and storage in the small stream. Sediment depos ition in small streams can affect retention of organic material, for example by burying leaf l itter (Brinson et al., 1995 ). Buried organic matter is processed and transformed in the str eam sediments, thereby making it available for biological uptake (Richardson et al., 2005).

iv. Organic carbon export

Dissolved and particulate organic matter ma y be exported throughout the watershed through mechanisms such as erosion, flushing, displa cement, and/or leaching (Lee et al., 2004). Organic carbon is the primary source of energy for microbial food webs and its export is critical to the productivity of down-gradie nt receiving waters (Allan, 1995; Wetzel, 2006). Variables involved in determining the capacity of ephemeral and intermittent streams to export carbon include the condition of the hydrologic connection to downstream reaches, shallow subsurface substrate permeability and poro sity, soil organic matter content, shrub and herbaceous canopy coverage, amount and stage of d ecay of course litter, and coarse and fine woody debris (Lee et al., 2004).

Thoms et al. (2005) found that anabranch channels (channels that branch off the main river and rejoin it downstream) in Australia were important sources of organic carbon for main channels despite being connected only during fl ood events. Freeman et al. (2007) noted that headwater streams, which can make up most of the length in a river sy stem, are the primary collectors and processors of terrestrially derived organic matter. For example, one study on the San Pedro River found that approximately 98 percent of nutrients came into the river during the summer monsoon thunderstorms from e phemeral tributaries, and that almost 60 percent of that input occurre d as a flux of particulate matter (Brooks et al., 2007).

Organic material brought into and stored in small headwater streams, can be broken down and transformed into forms more readily available for use by biota in larger perennial streams (Richardson et al., 2005). This organic matter ma y originate from terrestrial sources or from algal growth within the stream channels. In ar id and semi-arid environments, algae growth in the channel may be a more important source of organic carbon than terre strial plants due to the low upland plant cover (Jones et al., 1996; Jacobson et al., 2000b). This was also confirmed by Brooks and Lemon (2007) who conc luded that in the San Pedro River, high concentrations of organic matter, and especially high concentrati ons of nitrogen occurred with
the inflow of monsoon runoff from la teral ephemeral tributaries.

The degree to which material wi ll be transported out of stre ams depends on channel integrity and the condition of the downstream hydrol ogic connection. Organic matter may be transported in multiple ways including leachi ng, displacement, and flushing (Brinson et al. 1995). Surface runoff into headwater streams br ings nutrients that may be stored and transferred to ground-water rese rves (Fisher and Grimm, 1985; Belnap et al., 2005). The ground water containing nutrients may reemerge downstream in perennial waters or springs where they can be an important source of nutrien ts to plants and wildlife (Fisher and Grimm, 1985). Ephemeral and intermittent streams can c ontribute water and nutrients to perennial
streams even in the absence of direct above ground flow.

39d. Plant Community Support

Desert washes are easily recognizable by their de nse corridor of vegeta tion that is in strong contrast to the more sparsely vegetated uplands (Figure 27). Th is corridor contributes to the disproportionately high biological di versity of desert environments relative to their total area (Warren and Anderson, 1985).

Vegetative communities along ephemeral and interm ittent streams provide structural elements of food, cover, nesting and breed ing habitat, and movement/mig ration corridors for wildlife
that are not as available in th e adjacent uplands. Functional services of these communities include moderating soil and air temperatures, stabilizing channel banks and interfluves, seed banking and trapping of silt and fine sediment favor able to the establishment of diverse floral and faunal species, and dissipati ng stream energy which aids in flood control (Howe et al., 2008).

In arid and semi-arid regions, plants have adapted to limited water, high temperatures, and high evaporation rates. These stre sses are only partly alleviated in locations that concentrate water, whether they are perennial, ephemeral, or intermittent stream networks. Such areas are also subject to periodic dist urbance from flood flows. Limited water and flood disturbance thus are key factors that stru cture the vegetation of ephemera l streams (Nilsen et al., 1984; Friedman and Lee, 2002).

Figure 27. Photograph showing dense corridor of vegetation lining an ephemeral wash, Agua Fria River, north of Phoenix, AZ. (Photograph: William Kepner, USEPA/ORD)

The factors affecting riparian vegetation in arid and semi-arid regi ons are not as well understood as in humid regions with perennial ri vers. In turn, the influence of vegetation on ephemeral or intermittent stream systems is not well studied. It is, however, understood that the vegetative structure of desert landscapes refl ects the effects of low rainfall. Regardless of low rainfall, even in the driest of deserts, there are productive patc hes (Whitford, 2002).

40i. Physiognomy, density, and species composition

In ephemeral and intermittent streams, the structure and composition of the vegetation is
related to the size of the stream and patterns of flow, although most of the diversity is comprised of herbaceous species (Bagstad et al., 2005). In their study of vegetation along the San Pedro River, Stromberg et al. (1996) found that depth to ground water and bottomland elevation and inundation frequency exerted the greatest influence on species composition, followed by soil texture, soil moisture holding capac ity, light availability and site elevation. In another study along the San Pe dro River, all annuals showed strong increases in richness and cover in the year following a large fa ll flood, while hydric perennials had a small net increase in richness, indicati ng that both disturbance and increased moisture conditions caused by floods, as well as moisture from seasona l rains, contribute to increased richness and cover of herbaceous plants in the bottomlands of the San Pedro River, a spatially intermittent desert river (Bag stad et al., 2005).

In regions with seasonal precip itation, depth to ground water is particularly important since ground water is closely coupled with stream fl ow to maintain wate r supply to riparian vegetation (Groeneveld and Griepentrog, 1985). As the hydrologic regime shifts from perennial to ephemeral, vegetation composition shifts towards more drought-tolerant species, vegetation cover declines, riparian woodlands give way to riparian shrublands, and canopy height and upper canopy vegetation volume declin e (Leenhouts et al., 20 06; Stromberg et al., 2007).

The composition of riparian vegetation al ong desert streams reflects the vegetation composition of its watershed. The plants growing along large ephemeral or intermittent
stream channels or smaller ones below about 5,000 feet elevation incl ude species that are obligately associated with riparian environmen ts and ones that typify the surrounding desert uplands (Figure 28). The species composition of ephemeral and intermittent streams within
the arid and semi-arid Southwestern U.S. thus varies widely depending on species composition of the watershed and floristic province, as well as with drainage size, climatic regime, latitude, longitude, elevation, aspect, and soil char acteristics.

41Figure 28. Photograph showing riparian vege tation along a desert stream, western
Arizona.

Along small desert washes, vegetation compositi on and structure overlap considerably with those of the surrounding desert uplands (Blo ss and Brotherson, 1979; Warren and Anderson, 1985) and consist primarily of small, xerophytic shrubs and trees. Stem and leaf succulents and perennial grasses often ar e present, and annual grasse s and forbs become seasonally abundant during wet periods. Collectively, th e drought-tolerant vegetation that borders ephemeral streams is referred to as xero riparian vegetation (Johnson et al., 1984).

As water availability increases, the vegetation becomes increasingly distinct from the upland vegetation with respect to physiognomy and species composition. The vegetation becomes taller (Shreve and Wiggins, 1964) and tree canopy cover can increase (Sponseller and Fisher, 2006). Xeroriparian species ar e still present, bu t mesoriparian and hydroriparian species increase in abundance. Ephemeral streams w ith intermediate wate r availability support drought-tolerant shrubs such as wolfberry ( Lycium spp. ) or brickellbush ( Brickellia spp. ) and small-leaved trees such as acacia ( Acacia greggii )

, blue palo verde ( Parkinsonia floridum )

, or velvet mesquite ( Prosopis velutina ) (Hardy et al., 2004). Along intermittent and perennial streams, riparian scrublands in clude seepwillow or batamote ( Baccharis glutinosa ), broom ( Baccharis sarothroides or B. emoryi ), arroweed ( Pluchea sericea ), and tamarisk ( Tamarix
chinensis ) (Brown et al., 1977). Broad-leaved trees with relatively high water needs (e.g., the mesoriparian species Arizona walnut ( Juglans major )

, and the hydrorip arian species Fremont
cottonwood ( Populus fremontii )) are typically sustained on larg e washes by floodwater stored in perched ground-water reservoirs.

The additional water availability in the botto mland and riparian zone of a perennial to
intermittent stream in the Sonoran Desert resu lts in greater plant di versity than the arid upland, as measured at temporal scales th at capture seasonal va riance in resource and disturbance pulses, and at spa tial scales that capture the environmental heterogeneity of bottomlands. Although periodically limited by in tense flood disturbance, diversity remains

42high in bottomlands because of the combinati on of moderate resource levels (ground water, seasonal flood water) and persiste nt effects of flood disturbance (high sp atial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall) (Stromberg, 2006). For example, native palms ( Washingtonia filifera ) occur in only two locations in Arizona, both of which are washes (Figure 29).

Figure 29. Photograph of native palms ( Washingtonia filifera ) in Castle Creek, AZ. (Photograph: William Kepner, USEPA/ORD)

Vegetation structure also shifts as watershed size and flood intens ity increase. On large, dry ephemeral streams with intense flood scour, species composition shif ts towards pioneer species. For example, in Sonoran Desert washes, desert broom ( Baccharis sarothroides ), a pioneer xeroriparian shrub that produces prolific numbers of wind-dispersed seeds, was more
abundant in washes as watershed size increas ed (Warren and Anders on, 1985). Other pioneer species include burrobush ( Ambrosia salsola ), a xeroriparian shrub that is adapted to disturbance through capacity for clon al growth, and desert willow ( Chilopsis linearis )

, a drought-tolerant tree that produces wind-dispersed seeds. Zona tion can occur between fluvial surfaces within an ephemeral-stream bottomlan d, with the pioneer species sometimes being
more abundant in the active channel bed than on the stream banks or flood plain (Bloss and Brotherson, 1979).

ii. Primary productivity and plant water sources

Plant productivity in arid and semi-arid regions is often low for much of the year, punctuated by bursts of activity following ra in and runoff events. For exam ple, Smith et al. (1995) found that burrobush ( Ambrosia salsola ) is typically dormant (not actively transpiring) for most of the year in a desert wash setting.

Patterns of primary productivity and evapotrans piration vary depending on whether the main water source for the vegetation is direct pr ecipitation, channel flow, or stored water

43(Leenhouts et al., 2006; de Soyza et al., 2004). When stored wate r is accessible, productivity and evapotranspiration of plant species can be high for much of the growing season (Atchley et al., 1999). De Soyza et al . (2004) found that plants along an ephemeral stream channel responded more to channel flow than direct precipitation, indicating the importance of maintaining intact channel netw orks throughout a watershed.

Productivity patterns also vary seasonally depending on phenology, morphology, and physiological adaptations of the plant species. Some of the perennial plants that grow along desert washes are evergreen (e.g., creosote ( Larrea tridentata )), and can maintain some level of productivity year-round. Others are summ er drought-deciduous (e.g., desert lavender ( Hyptis emoryi )), or winter cold-deci duous (e.g., desert ironwood ( Olneya tesota )) (Nilsen et al., 1984).

iii. Temporal and spatial patterns of species diversity

Non-perennial streams with active flood regimes contain a high di versity of plant species that varies depending on the location within the wa tershed. The complex longitudinal gradients encompassing changes in flood intensity, climate, and water availability result in a wide range of biological conditions along the stream length (Lite et al., 2005; Shaw and Cooper, 2008).

During seasonal dry periods, plant species divers ity levels along ephemeral stream channels typically are low, with values much lower th an along perennial streams and also often lower than in adjacent uplands (Leitner, 1987). Du ring seasonal wet periods, however, diversity levels along some ephemeral stream channels ca n equal that along perennial stream channels (Stromberg et al., in press).

Species type and composition are affected by flow regime. Stromberg et al. (2006) found that in the San Pedro River there is a sharp decline in the riverine marsh type as perennial flows become intermittent. As flows become more ephemeral, and the stream channel loses vegetation cover and widens, hydromesic pioneer forests (cottonwood-willow ( Populus fremontii-Salix gooddingii )) give way to mesic pioneer shrublands (dominated by saltcedar (Tamarix ramosissima) , an introduced species) as tolerance levels for survivor ship relative to ground-water depth and fluctuation are exceeded.

Species diversity varies with seasonal rain and stream flow patterns, and also varies on longer temporal scales. Following infrequent large wi nter floods, stream flow can be sustained for several months in ephemeral stream reaches of large rivers that drain humid mountains. During this period of sustained runoff, the ephemeral stream washes can support a high density and diversity of wetland (hydroriparian) plant species (S tromberg et al., in press). These “ephemeral wetland” commun ities develop with a recurrenc e interval of perhaps once per decade or more, depending on the flow regime of the particular stream.

Water from rainfall and flood fl ows can trigger a pulse of germ ination of annual and perennial plant species in ephemeral streambeds (Figure 30). Because the dry-season cover of the woody vegetation is low, and cover of bare soil is fairly high, the seasonal resource pulses can

44result in very high diversity levels in comp arison to those of the more densely vegetated perennial streams.

Figure 30. Photograph of annual plant species in an ephemeral streambed following spring rains, southern Arizona.

Ephemeral reaches of spatially intermittent rivers maintain diverse soil seed banks (Stromberg et al., in press). Many of the plant species that establish along ephemeral stream channels during water pulses arise from soil seed banks . Along a spatially in termittent stream in central Arizona, the soil seed banks of ephe meral stream reaches included a mixture of species adapted to xeric, mesic, and hydric c onditions (Stromberg etal., in press). For example, viable seeds of the wetland taxa Juncus (rushes) were found in ephemeral stream reaches. In contrast, wetland species were not found in the soil seed bank of a smaller tributary that was ephemeral over its entire length.

iv. Influences of vegetation on ecosystem processes

Miller (2005: p. 18) noted that “the most important functions in dryland ecosystems are those
that control the retention of water and nutri ent resources because productivity and diversity cannot be sustained in systems that fail to reta in these resources.” Vegetation in ephemeral stream channels plays a key role in resource retention by protecting soils from wind and water erosion, slowing floodwater velocity, and m oderating temperatures. Ephemeral stream vegetation also influences biogeochemical cycl es by providing leaf litter, and food and cover for wildlife. In some cases, vegetation can inte rcept rainfall, preventing it from infiltrating into the soil, and influencing the local water balance and ecosystem processes (Owens et al., 2006; Miller, 2005). Vegetation st ructure and diversity determin e wildlife species diversity and abundance, and if a porti on of habitat on which a species depends is damaged or destroyed, the breeding popul ation of that species could be lost (Anderson and Ohmart, 1977),

45Changes in the abundance or composition of the plant community thus affect an array of ecosystem functions and processes. Plants that have the greatest effects on the structure and functioning of dryland ecosystems are small tr ees, shrubs, sub-shrubs and perennial grasses (Whitford, 2002).

v. Vegetation and channel morphology

Vegetation in arid and semi-arid regions is largel y controlled by the availability of water, with flood disturbance and edaphic conditions furt her shaping plant distribution patterns. Depending on attributes of the pa rticular stream, the highest de nsity of vegetation may occur along the stream bank or within the channel bed (Figure 31). By providing channel and stream bank roughness through standing or downed material, vegetation can influence flow velocities, flow depths, bank and flood plain er osion, and sediment transport and deposition, and can be a major factor contributing both to ch annel stability and to channel instability (e.g., Heede, 1985). Vegetation along the stream bank stabilizes the soil th rough the reinforcing nature of their roots, and prevents er osion (Groeneveld and Griepentrog, 1985).

In ephemeral stream channels, vegetation ma y establish on sand bars, and subsequently initiate the formation of various depositional features such as small current shadows, bars, benches, ridges, or islands (Tooth and Nanson, 2000). Spatially extensive assemblages of any plant species have the potential to alte r geomorphology and geomorphic processes through bioturbation, alteration of nutrien t or fire cycles, and patterns of succession (Lovich, 1996).

Figure 31. Photograph of vegetation growing in an ephemeral channel bed, Arizona. (Photograph: Lainie Levick/Aerial flight courtesy of Lighthawk, www.lighthawk.org)

In humid climates, the spatial distribution of riparian vegetation is related through the flow and associated disturbance regimes to fluvial landforms that create establishment sites or stress the persistence of es tablished vegetation (Hupp and Os terkamp, 1996). “In semi-arid

46and arid areas, bare sites are relatively abundant but water-ava ilability, particularly in the seedling establishment phase, is especially limiting. Thus, in dry climates, patterns of establishment may be strongly influenced by surface flow (floods), whereas ground-water levels my greatly influence persiste nce” (Hupp and Osterkamp, 1996, p. 293).

vi. Vegetation and geochemical cycles

The dominant plant species of many ephemeral streams are leguminous trees that harbor nitrogen-fixing bacterial symbi onts. These trees influence ge ochemical cycles and local pools of nitrogen (Virginia et al., 1992). The levels of nitrogen-fixation are related in part to plant productivity, and vary temporally and spatia lly along gradients of water availability.

The trees and shrubs that grow along ephemeral streams vary in the degree to which they resorb nutrients in senescing leaves. The nitrogen-fixing honey mesquite ( Prosopis glandulosa ) had the greatest resorpti on in one multi-species st udy (Killingbeck and Whitford, 2001).

e. Faunal Support and Habitat

The riparian environments created by ephemeral and intermittent streams in the arid and semi

arid Southwest provide and maintain important habitat for wildlife, and are responsible for much of the biotic diversity, ye t the scientific literature on this topic is limited. The following sections present the current understanding of th e contribution of ephemeral and intermittent streams to the biotic integrity of southwestern watersheds.

Riparian systems are one of the ra rest habitat types in North Amer ica. In the arid Southwest, about 80 percent of all animals use riparian re sources and habitats at some life stage, and more than 50 percent of breeding bird species ne st chiefly in riparian habitats (Krueper 1993).
It has been estimated that over half of all wild life species in Arizona depend on riparian areas (Arizona Riparian Council, 2004 ). Riparian habitat is the area between the stream channel and the upland terrestrial ecosystems. The st rongest contrasts between these areas are found
in arid and semi-arid lands where water is a limiting resource (e.g., Ceballos, 1985).

Because ephemeral and intermittent stream channe ls have a higher moisture content and more abundant vegetation than the surrounding areas, they are very important to wildlife. Frequently, these streams may retain the only available water in the area, with perennial segments or permanent pools interposed wher ever hydrogeological condi tions allow. These isolated perennial waters can support fauna not found in an otherwise ephemeral system.

The microclimates created in and around ephe meral and intermittent streams are utilized extensively by wildlife, and es pecially by less mobile species that cannot avoid the harsh desert environment by moving to more favorable microclimates. As a result, these areas generally support the greatest concentrations of wildlife, providing the primary habitat, predator protection, breeding a nd nesting sites, shade, move ment corridors, migration stopover sites, and food sources.

47The importance of streamside or riparian vegetation communities to wildlife is well recognized (Carothers, 1977), and is heightened in the arid an d semi-arid Southwestern U.S. due to the high ambient temperatures and intense aridity outside of the riparian community.

i. Spatial structure, connectivity, and corridors in wildlife habitat

The spatial structure of wildlife habitat is desc ribed by patch size, number of patches, density and distribution, in addition to the geometric complexity of the patches (Johnson and Lowe, 1985). The term “connectivity” has been used to describe how spatial arrangement and quality of elements in the landscape affect m ovement of organisms among habitat patches. The term “corridor” refers to a connecting feature in the landscape, and “habitat corridor” generally refers to a linear st rip of vegetated land that pr ovides a continuous or near continuous pathway between two larger habita t blocks (Bennett, 1999 ). One of the key functions of intact and functional migration co rridors is to link patches in the landscape.

Ephemeral and intermittent stream channels provide important wildlife movement corridors in arid and semi-arid regions because they contai n continuous chains of vegetation that wildlife can utilize for cover and food. In addition, during the summer monsoon season small floods
create a more-or-less continuous co rridor of water that allows dispersal of herpetofauna such as garter snakes and amphibians, which are ac tive during the summer. Winter rains do not serve the same function, since the cold temperatures preven t much activity in amphibians or reptiles. This dispersal mechanism allows genetic interchange between subpopulations that are isolated for most of the year, and allows recolonization of site s where subpopulations may be lost due to drought or disturbance.

Various authors (e.g. Meyer et al., 2007) have recognized the importance of small stream and headwater habitats, including those of ephemeral and intermittent streams, as vital parts of the biological integrity of U.S. wate rways. The degradation of th ese habitats and loss of their connections to larger streams have negative cons equences not only to the inhabitants of these streams, but also for the dive rsity of downstream a nd riparian ecosystems, and the biological integrity of the entire river network.

Habitat fragmentation caused by human activities can jeopardize the survival of wildlife species by diminishing their ability to access the resources they need, retain genetic diversity, and maintain reproductive capac ity within a population; howev er, conservation biologists have debated these concepts because of lack of detailed information (Hilty et al., 2006). Recent studies are attempting to cl arify this issue. For example, in their study of predator use
of corridors in the northern California wine -growing region, Hilty et al. (2006) found that mammalian predator detection rates were 11 times higher in riparian area s than in vineyard
locations. More research in to this topic is needed.

As previously mentioned, nearly 81 percent of all streams in the six Southwestern states are ephemeral or intermittent (USGS, 2006) and in many watersheds most stream channel reaches are ephemeral or intermittent. From a strictly numerical standpoint, th en, the degradation of ephemeral or intermittent streams diminishes ecosystem functions in most southwestern
watersheds.

48The structural components of wildlife habitat are considered below in terms of the physical, vegetative, and hydrological contributions that ephemeral and intermittent streams provide to the landscape. Habitat needs vary accordi ng to species, but this overview treats larger taxonomic groups more generally.

ii. Physical habitat features

The habitat provided by desert st reams contracts and expands dramatically in size due to the extreme variations in flow, which can range from high-discharge fl oods to periods when surface flow is absent. This spatial variation in habitat or ecosystem size is a fundamental, defining feature of these str eams (Stanley et al., 1997).

Regardless of whether it is pe rennial, a stream affects the substrate it flows upon, creating habitat for various species of wildlife. Some physical featur es of wildlife habitat along ephemeral and intermittent streams include the deposits of river material (sediment and debris), the exposure of rock and subsurface so il layers by erosion, the provision of shade through topographic relief, the creation of microclimatic zone s, and the sequestration of
moisture and nutrients in alluvium.

River bank material provides shelter for numerous species of wildlife in the arid Southwest, including reptiles, amphibians, birds, mammals and invertebrates. Bank shelters are created through the action of water, wi nd, and gravity, independent of whether the river contains water year-round. In fact, dry wash embankme nts are notoriously full of small caves and
crevices critical in the lif e of desert animals such as the desert tortoise ( Gopherus agassizii ) (Van Devender, 2002). These shelters not only provide refuge from predators, but also critical protection from extreme heat and arid ity. Table 1 lists species observed to use riverine soil exposures in the Pima County, Arizona, area, and includes bats, birds, snakes, lizards, mammals, insects and amphibians.

Stream alluvium is often looser than the soils or colluvium of surrounding uplands, which enhances the potential for e xploitation by specialized sand-bu rrowing species of wildlife. Woody debris swept in from the watershed collect s in the flood plain and stream channel as flood wrack (brush piles), creating additional co mplex, high-value shelters. If the stream incision is deep enough, it may create a cool er canyon environment in which heat and
moisture loss are retarded.

49Table 1. List of wildlife species that use riverine soil expo sures in Pima County, Arizona (source: Julia Fonseca, Pima County Office of Conservation Science and Environmental Policy, 2008). Species Soil exposure use Townsend’s Big-eared Bat Roost, maternity roost Western Pipestrelle Year-round roost

Pallid Bat Night roost Myotis species Roost, maternity roost Mexican Long-tongued Bat Roost, maternity roost Lesser Long-nosed Bat Roost Burrowing Owl Roost, nest
Barn Owl Roost, nest
Great-horned Owl Roost, nest

Common Raven Nest Rough-winged Swallow Nest
Cliff Swallow Nest
Black Phoebe Nest Say’s Phoebe Nest
Rock Wren Nest Green Kingfisher Roost Belted Kingfisher Roost, nest

Western Diamondback Rattlesnake Shelter, foraging Mojave Rattlesnake Shelter

Sonoran Desert Toad Shelter

White-throated Woodrat Nest Rock Squirrel Den
Desert Tortoise Shelter, den Desert Spiny Lizard Shelter Clark’s Spiny Lizard Shelter Coyote Den
Kit Fox Den Mud-dauber Wasp Hive Ringtail Cat Den Striped and Hog-nosed Skunks Den Kissing Bugs Foraging

Javelina Day camp Raccoons Foraging, shelter

iii. Vegetative habitat features

The abundance and diversity of riparian vegeta tion, as compared to uplands, is a critical
wildlife habitat feature of arid and semi-arid region streams (Figure 32). Each of the plant

50communities found along these streams offers distinct and notable habitat features to wildlife, summarized in Brown (2004).

Major washes with shallow ground-water zones are typically lined wi th phreatophytic trees including Fremont cottonwood ( Populus fremontii ), Arizona sycamore ( Platanus wrightii ), Arizona ash ( Fraxinus velutina ), distinctive shrubs such as willow ( Salix ssp ), seepwillow ( Baccharis ssp ), burrobrush ( Ambrosia monogyra ), and saltcedar ( Tamarix ramosissima ), or dense grass stands of sacaton ( Sporobolus ssp ). A special case is presented by those southwestern ephemeral streams carrying discha rge of treated municipal sewage or other
effluent.

Figure 32. Photograph of diverse riparian vegetation, Badger Springs Wash, Arizona.

Ephemeral and intermittent streams which lack a shallow ground-water system or effluent discharge nonetheless give rise to a distinctive vegetative habitat from the surrounding uplands, often referred to as xeroriparian habi tat. Tree canopy of ephe meral streams generally includes subtropical legumes such as mesquite ( Prosopis ssp ), catclaw acacia ( Acacia greggii ), ironwood ( Olneya tesota ), and blue palo verde ( Cercidium floridum ). Netleaf hackberry ( Celtis reticulatata ) and Arizona sycamore ( Platanus wrightii ) have been identified as providing exceptional cover for nesting bird s on intermittent streams (Powell and Steidl, 2002), and mesquite has been identified as th e key provisioner of food for many migrating birds (Van Riper and Cole, 2004).

Krausman et al. (1985) found that xeroriparian vegetation provides forage, thermal cover, and travel zones. For example, deer in their Arizona study sites were much more dependent on xeroriparian systems than were deer in west Texas. In arid parts of western Arizona, many birds such as red-tailed hawks ( Buteo jamaicensis ), and Gila woodpeckers ( Melanerpes uropygialis ), can become dependent on ephemeral stream s for nesting sites, as this is where large trees occur due to the increas ed moisture (Johnson and Lowe, 1985).

51iv. Hydrological habitat features

Channel flow is a visually prominent aspect of the hydrological character of a stream, but it is seldom the only hydrological habi tat feature of biological signif icance. A stream may also possess moist banks fed by capillary flow from ground water, which offer sites for turtle or insect reproduction. A hyporheic (s ubsurface) zone of flow with a distinct invertebrate fauna may underlie a dry streambed. Flooding, erosion, or man-made excavations can give rise to in-channel or off-channel pools where amphibian s breed. Springs, which may exist within the
stream channel or in the flood plain, can offer distinct chemical compositions or thermal refugia from the main stream. Stream diversi on and irrigation systems may spread the river’s flow into fields via irriga tion canals lined with cottonwoods and willows, altering habitat conditions in both the aquatic and riparian comm unities. Together, these natural or man-made hydrological features provide a wi de variety of important hab itat conditions for aquatic and terrestrial organisms.

Stream corridors naturally guide the movement of wildlife. Movement is essential to wildlife survival, whether it be the day- to-day movements of individua ls seeking food, shelter, or mates, dispersal of offspring (e.g., seeds, fle dglings) to new areas, gene flow, migration to avoid seasonally unfavorable conditions, r ecolonization of unoccupied habitat after environmental disturbances, or shifting of a species ’ geographic range in response to global climate change (Beier et al., 2006).

v. Faunal abundance and distribution

Fauna using ephemeral or intermittent waters include fish, mammals, amphibians, reptiles, birds, and invertebrates. Most desert species have developed adaptations to the water-limited conditions of arid and semi-arid regions, allowing them to survive under adverse environmental conditions (Ward and Associ ates, 1973; Louw and Seely, 1982; Williams, 2006). The natural flow regime in arid lands is a key factor favori ng native species over exotics that are adapted to lake and pond c onditions (Minckley and Meffe, 1987; Poff et al., 1997). However, the variability of climate a nd flow regime, which influences species’ abundance and diversity, makes evaluation di fficult unless surveys are conducted over a period of years in different community type s (Anderson et al., 1977; Boulton and Lake, 1992).

Habitat studies seldom partition the various microhabitats that water creates for wildlife, and do not often attempt to separate the natural continuum of flow conditions th at exist in a given area into perennial and non-perennial component s, particularly since such conditions may exist along the same stream at the same time. The following sections describe in greater detail wildlife uses and benefits of ephemeral and intermittent streams in arid and semi-arid lands, including the collective c ontributions that water itself makes to faunal composition of
southwestern streams. Since habitat values differ among taxono mic groups, the discussion is grouped taxonomically.

52Reptiles and Amphibians

Reptiles and amphibians are diverse and abundant in arid and semi-arid regions, and have a variety of physiological and behavi oral adaptations that enable them to conserve energy and moisture during harsh conditions of high temper ature or low humidity and survive in dry environments. These adaptations include be havioral heat avoida nce involving going underground, becoming nocturnal or subterran ean, reducing activity levels, developing resistance to dehydration, developing the ability to absorb water through their skin, developing the ability to use temporary waters for breeding, and having rapid larval and egg development. In addition, most arid and semi-a rid region reptiles can withstand high levels of electrolyte levels in their body fluids, and have relatively im pervious skin which reduces water loss (Stebbins, 1995).

Many researchers have noted that the high diversit y of plants and the a ssociated microhabitats in desert riparian systems provides preferred habitat for herpetofauna . Some herpetofauna prefer desert washes to other types of desert habitats, and have the hi ghest number of habitat specific species than other dese rt habitat types (Jakle and Gatz, 1985). However, the data have been limited until recently. For example, at the Symposium on the Management of Amphibians, Reptiles, and Small Mammals in North America (U.S. Department of Agriculture, 1988), many of the pr esenters noted the lack of data on amphibians, reptiles and small mammals, possibly due to the difficulty in surveying them because of their small size and secretive habits, or because they have hi storically not been considered important.

Many, indeed most species of snakes and lizards preferentially utilize xeroriparian habitat because of the dense cover provided by the shrub, vine and groundcover layers of annual and perennial plants. Jones (1988) reported on an extensive survey by the BLM from 1977 through 1981 on Arizona’s herpetofauna. Th is was one of the most comprehensive inventories of herpetological communities ever conducted in North America with 27,885 array-nights in 16 habitat types over a five-year period, on approximately 3,441.296 ha (8.5 million acres) of public land. It was also an impo rtant effort to associate herpetofauna with ecosystems. For the Mixed Riparian Scrub (als o called xeroriparian) ha bitat type, the results indicated a high number of species and species diversity for snakes, amphibians, and lizards relative to the other habitat type s. Figure 33 is a graph of th e number of species (turtles, amphibians, snakes, and lizards) for each habitat type. Mixed Riparian Sc rub (xeroriparian) is represented by the “MR” habitat type.

In their study of the herpetof auna at Organ Pipe Cactus National Monument, Arizona, Rosen and Lowe (1996) also found that xeroriparian habitat was preferred by lizard species and certain snake species and hypothe sized that it was due to high er prey abundance, higher relative humidity, and the presen ce of denser vegetation for cover. During drought peaks, almost every non-riparian-dependent snake specie s used the xeroriparian habitat as refugia, although those that normally used that habitat ty pe had a higher survival rate. They also found that lizard species were most abundant in mesquite woodland or bosque and xeroriparian habitats (see Figure 34). At Or gan Pipe Cactus Nationa l Monument, mesquite

53woodland is restricted to ephemeral and inte rmittent streams, and therefore constitutes a special type of xeroriparian habitat.

Figure 33. Graph of herpetofauna species by taxonomic group and by habitat type, from BLM surveys in Arizona, 1977-1981 (Jones, 1988). “MR” represents Mixed Riparian Scrub (xeroriparian) habitat type.

Rosen (2005), in his review of the herpetofauna of the 126-mile San Pedro River looked at the riparian herpetofauna assemblages in three reac hes of the river from the Mexican border to
the confluence with the Gila River. He looked at histor ic and current records and found a large number of species that occurred in the lowe r (mainly ephemeral) reach of the river that were not found in other reaches, although many species occurred in all reaches.

Baxter (1988) noted that in th e Mojave Desert, washes are important habitat for the desert tortoise ( Gopherus agassizii ) although their burrows tended to be in the uplands. Because desert washes contain a highl y diverse plant community, they were probably important foraging locations. McArthur a nd Sanderson (1992) studied plant associations in arroyos and uplands in southeastern Utah in relation to use by desert tortoise. Th ey found that the arroyos with more shrubs and a rougher topography were good den sites and provided more succulent forage than uplands. In Arizona Upland Sonoran desertscrub, th e desert tortoise is absent from the valleys, and occurs only along major upper and middle bajada washes and on rock slopes (Van Devender, 2002).

54Figure 34. Graph of lizard abundance by habita t type at Organ Pipe Cactus National Monument, Arizona, 1989-1990 (f rom Rosen and Lowe, 1996). MAXPEAK is maximum peak value observed for all runs of transects within a habitat type at a site. SEP is the Sensitive Ecosystems Program.

Lowe (1985) discussed the obligate riparian turt le, snake, and amphibian species in riparian ecosystems in southern Arizona and adjacen t Sonora, Mexico, and their local population extinctions. Many reptiles and amphibians depend on permanent springs, seeps, and ephemeral streams for their survival. A lthough these species are widely distributed throughout the region, their narro w ecological distributions an d low densities make them extremely vulnerable to habitat degradation. Im pacts to water quality and quantity, such as acid rain, ground-water pumping, and po llution, are the main threats.

Amphibians are not physiologically well adapted to dry desert conditions; however, they have developed several behavioral ad aptations that allow them to survive there, including the ability to avoid the heat and dryness by burrowing underground for extended periods. Species that do not require permanent water may emerge from underground only after rainfall events (e.g., Couch’s spadefoot ( Scaphiopus couchi) ). Some amphibians are also very tolerant of dehydration and can survive a wate r loss equivalent to about 40 percent of their body weight. They handle dehydration by decreasing the rate of urinary water loss and increasing the rate of water absorption through the sk in. For example, some amphi bians can extract water from moist soil (Mayhew, 1995).

The vast majority of amphibians spend at least part of their lif e cycle in water, but frequently only for breeding. At the Rincon Mountain Un it of Saguaro National Park near Tucson, Arizona, lowland leopard frogs ( Rana yavapaiensis ) depend on bedrock pools in ephemeral streams that retain water year round for breed ing habitat (Parker, 2006). Other amphibian

55species, such as the red spotted toad ( Bufo punctatus ), are found only in arid ecosystems (Mayhew, 1995). The canyon tree frog ( Hyla arenicolor ) is found along temporary, intermittent, and permanent streams, springs, and tinajas in rocky desert canyons in much of the Southwest, and uses the temporary pool s during summer rains for breeding (ArizonaSonora Desert Museum, 2007; Arizona Game a nd Fish Department, 2002). Photographs of some of these species are shown in Figure 35.

In a study of all known occurrences of the California red-legged frog ( Rana aurora draytonii ) in the Central Valley of California (n=143) , approximately 64 percent were found in intermittent streams as opposed to perennial streams (Hayes and Jennings, 1988). Six amphibian taxa were captured on ephemeral streams in three southwestern sites by URS Corporation, an environmental consulting fi rm (2006), including salamanders, frogs, and toads. Tadpoles were commonly observe d in longer-lived ephemeral pools.

Figure 35. Photographs of amphibians that inhabit and breed in ephemeral and intermittent streams (clockwise from top left): Canyon tree frog ( Hyla arenicolor ), lowland leopard frog ( Rana yavapaiensis ), red spotted toad ( Bufo punctatus ), Sonoran desert toad ( Bufo alvarius , photograph: Shea Burns, USDA-ARS), Egg strand of Sonoran desert toad (photograph: Shea Burns, USDA-ARS).

Rosen and Lowe (1996) in their study of he rpetofauna at Organ Pipe Cactus National Monument noted that anurans (toads and frogs) are closely tied to permanent or temporary

56surface water that lasts long enough to allow their eggs to hatch and produce tadpoles. They found that anurans bred successfully in tem porary pools in major washes and ephemeral springs. As little as 7.5 days may be required for the Couch’s spadefoot ( Scaphiopus couchi ) to go from egg to toadlet, with a longer period required for the true toads (genus Bufo ).

Upland desertscrub offers essentially no breed ing habitat to the di verse array of summerbreeding toads and frogs characteristic of s outhwestern deserts and grasslands. The key
natural environments for thei r breeding are the riparian fl ats along major valley washes,
where scour holes in the silts and cl ays hold floodwaters long enough for tadpole development.

Birds

Birds, more than any other animal group, are hi ghly dependent upon riparian and xeroriparian vegetation in arid and semi-arid lands even though they have the ability to migrate seasonally to find favorable climates. This is thought to be due to the vegetative structure, diversity and productivity of riparian areas as compared to surrounding uplands (Johnson et al., 1977; Ohmart and Anderson, 1982; Johnson and Hai ght, 1985; England and Laudenslayer, 1995; Kirkpatrick et al., 2007). In the Lower Colorado River Valley subdivision of the Sonoran Desert, dry washes occupy less th an 5 percent of the area, but support 90 percent of its bird life (Dimmitt, 2000).

Some birds are particularly adapted to the hot , dry conditions found in deserts: they excrete waste in the form of a semi-solid, requiring one-tenth the water used by mammals; some have a nasal salt gland to excrete excess salts; th ey have higher body temperatures than most mammals and can tolerate a wide range of body temperatures; and they can store body heat during the day to be released in the cooler evening hours (England and Laudenslayer, 1995).

Kirkpatrick et al. (2007) looked at seventeen sites in southern Arizona (5 sites that had perennial flowing surface water, 9 sites that ha d intermittent surface water, and 3 sites that had ephemeral surface water) for avian abundance and species richness along riparian areas as compared to uplands. He found that avian sp ecies richness and abundance were substantially higher than in the surrounding uplands, even for th e dry, ephemeral sites. This was attributed to the riparian vegetation. No associa tion was evident betwee n species richness and
abundance in association with surface water at the community level; however, there was a positive association with the volume of velvet mesquite, which provides food sources (high densities of insects and other arthropods).

A study by Stevens et al. (1977) on seven paired sites (riparian and adjacent upland) in Arizona found that the importance of riparian ha bitat to migrant passeri nes is substantial. They found that the parameters influencing the use of riparian habita ts included: specific habitat preferences of the bird (stop-over habita t selection); floral com ponents (niche diversity and vegetation species composition) ; location of habitat (island s ituations and accessibility); and quality of the adjacent habitat (including the amount of grazing and other forms of impact).

57Higher bird species richness was found in dry wa sh systems according to a study of bird use of desert habitats by the California BLM on si xty-six study plots. Th ey found approximately
1.5 times as many breeding species (Kubik a nd Remsen, 1977; Tomoff, 1977; Daniels and Boyd, 1979a, 1979b) and about twice as many wintering species (Daniels, 1979a, 1979b; Henderson, 1979; Remsen et al., 1976; Tomo ff, 1979a, 1979b, 1979c) in the dry washes. This demonstrated that these systems supported a greater diversity of species than did the more common desert scrub possessing overstory vegetation structure.

Skagen et al. (1998) compared mi grating birds use of riparian co rridors versus isolated oases in the San Pedro River and found that “Small, isol ated oases [riverine vegetation isolated from similar vegetation patches] hosted more avian speci es than the corridor si tes, and the relative abundance of most migrating birds did not differ between sites relative to size-connectivity.” They concluded that the protec tion of both the small patches and the more extensive riparian corridors that link these patches is imperative, given the overall habitat limitation in western landscapes. They noted that these areas are cri tical in providing migr ation stopover areas, and therefore affect the breeding succes s of northern bird populations.

Ohmart and Zisner (1993) conducted an extensiv e literature review of riparian habitat in Arizona, which included perennial, intermittent, and ephemeral streams. They found that nearly every species of bird in Arizona was found in riparian habita ts either for breeding, foraging, migration or wintering. This include d fifty-seven ducks, geese, and waterfowl; twelve hawks, falcons, and eagles; forty-two shorebirds (breeding habitat); one quail; and seventy-eight songbirds and other birds.

Few studies attempt to separate the effects of water from vegetation on species diversity. In their survey of southwestern streams, Kirkpa trick et al. (2007) found a relationship between the abundance of four bird species (Black Phoebe ( Sayornis nigricans ), Wilson’s warbler ( Wilsonia pusilla ), common yellowthroat ( Geothlypis trichas ), and song sparrow ( Melospiza
melodia )), and presence and extent of surface water. They were unable to determine whether the association with surface water might be caused by other factors, such as a higher arthropod biomass along “wet” streams (which in cluded intermittent and perennial sites). The majority of bird richness or abundance was not explained by the presen ce of water, but was positively correlated with mesquite volume.

A study conducted by the Point Reyes Bird Ob servatory (PRBO, 2007) during conditions of extreme drought identified 120 bird species usin g dry washes in west ern Arizona, including twenty-five breeding species. Skagen et al. (2005) looked at the ge ography of spring bird migration through riparian habitats in the Sout hwest and found that all riparian habitat types were used to some degree.

Mammals

A wide variety of mammals inhabits the arid an d semi-arid Southwest. Most have adapted to
the harsh conditions and lack of water in one or more of the following ways: heat evasion (daily or seasonal estivation, diurnal or noctu rnal behavior, or seas onal migration), water conservation, water storage, dehydration tolerance, heat tolerance, and heat dissipation (open

58mouthed gaping, or long appendages such as long ears). Many mammals burrow underground during the hottest part of the day to avoi d the heat and increase water conservation.

Nearly all mammals must be able to find fr ee water, making them dependent upon riparian areas to some degree where they can util ize temporary and permanent pools found in ephemeral or intermittent streams. However, some small mammals in desert environments, such as the heteromyid rodents, utilize ripa rian areas but never drink free water, having evolved to meet their water requirements through the metabolism of carbohydrates, an efficient renal system that concentrates urine, lack of sweat glands, and nocturnal habits (Kepner, 1978; Frank, 1988).

Mammals utilize dry washes in many ways. Krausman et al. (1985) determined that xeroriparian washes and their associated vege tation were an important component of desert mule deer ( Odocoileus hemionus ) habitat in Arizona (Figure 36). They noted that the greater plant densities and diversity in washes allowed d eer to find the forage an d cover they require. They also found that in central Arizona deer used washes 42 percent of the time in winter, increasing to 83 percent in summer. In the arid and hot King Valley, Arizona, desert mule deer used washes 99 percent of the time. Deer in these areas used washes for forage, cover, travel lanes, and birth sites.

Figure 36. Photograph of a desert mule deer ( Odocoileus hemionus ), Arizona.

Bellantoni and Krausman (1991), and Ragotzkie and Bailey (1991) found that female mule deer especially used xeroripari an or dry wash habitat for fora ging during early summer. They suggested that in the Southwest, xeroriparian areas provide ther mal cover, forage, and travel corridors for mule deer, and that these areas are most important duri ng the hot, dry period of early summer. In addition, the increased st resses on female mule deer during pregnancy
could increase this habitat selection. Othe r large ungulates, such as Desert bighorn sheep ( Ovis Canadensis ) utilize scattered isolated pools in desert washes (Jones, 1986).

59Various authors have noted that collared peccaries ( Tayassu tajacu ) in Arizona used dry washes during certain times of the year for lo afing and resting (Bigle r, 1974; Bellantoni and Krausman, 1993), or as bedding sites and corr idors (Ticer et al., 1998). Bellantoni and Krausman (1991) found that both mule deer ( Odocoileus hemionus ) and peccaries used dry washes for bedding.

When evaluating potential jaguar ( Panthera onca ) habitat, Hatten et al. (2003) found that riparian areas and major wash complexes, mountain ranges, and associated canyons are potentially suitable geographic features. Jaguars occupy a wide range of altitudes as long as food, water, and cover are available. Perennial and intermittent water sources within 20 km were considered important to dispersing jaguars in an arid environment. They noted that any sources of water, even ephemeral ones, may be important because they are usually associated
with well-defined channels that serve as travel co rridors, and contain riparian vegetation, a cooler microclimate, and higher prey abundan ce. Beier (1995) looke d at juvenile cougar dispersal from their maternal home range in three corridors, includi ng a desert arroyo, in southern California. The study found that the co ugar used all three co rridors even though few drainages had perennial water; however, seeps and springs we re distributed throughout the

area.

A variety of other small mammals utilize dry wash habitats and xeroriparian areas, including the Mesquite mouse ( Peromyscus merriami ) (Kingsley, 2006) and a wi de variety of other rodents (Jorgensen et al., 1995; Kepner, 1978). Duncan (1990) found that spotted ground squirrels ( Spermophilus spilosoma ) often use dry, sandy washes for their burrows. Jorgensen et al. (1995) studied an arroyo in New Mexico to determine rodent use of the wash area (sandy bottom), terraces, and shoulders (stream banks). They found that the terraces and shoulders were used much more than the wash, and that most animals traveled parallel to the arroyo as opposed to perpendicular to it. Th is behavior may be due to the high vegetation density along arroyos that offers predator protection.

In Ohmart and Zisner’s (1993) extensive literatu re review of wildlife usage of riparian areas in Arizona, they compiled a list of fifty-five mammals that use riparian areas in any way for breeding, foraging, cover or migration. They noted the importance of riparian areas to birds, bats and large mammals such as elk and deer fo r migration corridors, a nd that continuity of these areas was important for population expansion and genetic diversity in small vertebrates. They found that only a few mammals in Arizona, su ch as the river otter, beaver, muskrat, or water shrew, were truly tied to aquatic habita ts. These species were unlikely to be found on non-perennial streams, with the exception of effluent-dependent ephemeral streams. For instance, beaver have reocc upied part of the Salt and G ila River systems near the 91 st Avenue waste water treatment plant in Phoenix, Arizona.

Invertebrates

The abundant invertebrates associated with ephe meral, intermittent, and headwater tributaries are important contributions to the biological integrity of river netw orks. Invertebrates constitute a major portion of th e faunal diversity of the earth, and the emergence of aquatic invertebrates from streams is a significant part of the food chain. For instance, Fisher (1991)

60Figure 37. Illustrations of caddisfly larva ( www.scientificillustrator.com ) (left), and caddisfly adult ( www.nps.gov ) (right). reported that flycatchers used a large portion of the insect biomass emerging from Sycamore Creek, Arizona, an intermittent stream.

Ephemeral streams contain rich assemblages of both invertebrates and macroinvertebrates. Kingsley (1998) conducted an exte nsive survey of the inverteb rates at Organ Pipe Cactus National Monument, Arizona, and found a very high species richness in the wash habitats in the Ajo Mountains and Aguajita Wash. He noted that this was to be expected due to the diversity of microhabitats. This study descri bed each of the nearly 1,000 taxa surveyed.

Many invertebrates require a hydrologic connecti on for their spatial disp ersal, even if the connection is ephemeral or intermittent (Nad eau and Rains, 2007). Whiles and Goldowitz (2005) looked at macroinvertebrates in wetland s across a hydrologic gradient from ephemeral to perennial. Although this st udy was conducted in Nebraska, the results were interesting in that they showed the highest taxon richness and dive rsity at the intermittent sites. In northern California, Del Rosario and Resh (2000) compar ed invertebrates in th e hyporheic zones of intermittent and perennial streams, and found th at intermittent streams had lower densities, similar richness, but higher species diversity than perennial streams.

Intermittent streams in the Southwest provi de food sources for the high numbers of
macroinvertebrates found there. Disturbances caused by intermittent flows may actually facilitate high food quality and consequently high levels of insect production in warmtemperate desert streams (F isher and Gray, 1983; Jackson and Fisher, 1986; Grimm and Fisher, 1989; Huryn and Wallace, 2000). For example, in Arizona, macroinvertebrate biomass in Sycamore Creek tends to decline fo llowing extended periods without disturbance (e.g., more than 60 to 80 days) because of reduced food quality resulting from cyclical coprophagy (the consumption of feces) (Grimm and Fisher, 1989; Huryn and Wallace, 2000).

Many invertebrates require standing water for part of their life cycle. For example, the caddisfly ( Limnophilus sp. ) requires water only for the egg, larv a, and pupae stages of its life cycle; the adult is terrestrial (Erman and Nagano, 1992) (Figure 37). Other species live in sediment, either in encysted form, or within the hyporheic zone. Graham (2002) studied temporary pools in watercourses in Wupatki Na tional Monument, Arizona. He found 22 taxa of aquatic macroinvertebrates a nd two taxa of amphibians. Ward and Associates (1973) noted that the life cycles of these species are tr iggered by specific temp erature and/or water conditions, and they may remain dormant or aes tivate during unfavorable or stress periods. Invertebrate species using ephemeral streams are generally good disp ersers, either being swept in from the upland, moving in from the ai r, or colonizing from hydrologically separate perennial sources, including backwaters, pool s, and off-channel ponds (URS Corporation, 2006).

61Some crustaceans (Phylum Arthropoda, Class Crus tacea, e.g., tadpole and fairy shrimp) are able to survive in temporary waters in ephemera l stream channels. Fairy shrimp can complete its life cycle, going from egg to egg, in se ven days during summer, and two weeks during winter. As cysts, these creatures are able to dry with the mud and rehydrate later when water returns, hatching 24 to 36 hours after hydration (Carpelan, 1995).

URS Corporation (2006) sampled several ephe meral streams in Arizona, Colorado, and New Mexico during July and August of 2006. Most mi croinvertebrates were terrestrial, but eightysix aquatic taxa, including copepods, ostracods, and cladocerans were detected. Seventyseven aquatic macroinvertebrate taxa occurred in streams with know n or likely upstream sources of colonizers, and thirty -five taxa occurred in those w ithout. Macroinvertebrate taxa had a high degree of dissimilarity, either betw een study watersheds or within them (URS Corporation, 2006).

However, in general it is difficult to unde rstand the dynamics of these communities in intermittent or ephemeral streams due to the irregular nature of the hydrologic regime and their high sensitivity to climatic fluctuations. Boulton and Lake (1992) suggested studying a number of sites over a period spanning several co mplete cycles of flow to assess adequately these complex interactions. Adams (2000) f ound that he was able to develop baseline biological conditions for water-quality assessment prior to urban development by sampling macroinvertebrates in ephemeral st reams during periods of flow.

A typical conceptual model of the movement of invertebrates through a stream network is exemplified by Cummins and Wilzbach (2005), w ith headwater intermittent stream reaches responsible for delivery of i nvertebrates, sloughed algae, and detritus to the downstream perennial stream reaches (Figure 38). Although their research looked at whether fish were present as a criterion for inclus ion in management plans, they noted that downstream reaches are always highly dependent on upstream processe s, and that successful stream management
should include the entire watershed.

62Figure 38. Diagram of a conceptual model of the delivery of invertebrates, sloughed algae and detritus from headwater streams, intermittent and p ermanent first-order fishless streams to larger fish-bearing streams, illustrating the dependence of downstream reaches on upstream processe s (from Cummins and Wilzbach, 2005).

Fish

Native and non-native fish are abundant in pere nnial streams in the S outhwest deserts. For example, seventy-five native fi sh species have been recorded in Arizona and New Mexico, many of them listed as endangered, although some have been lost due to habitat loss (Hubbard, 1977). Surprisingly, many species of fish, both native and non-native, can be found in isolated perennial pool s in otherwise ephemeral or intermittent streams. For example, four fish taxa were collected during a one year study on ephemeral streams in southern Arizona by URS Corporation (2006) , including two native species and two non

native species.

Native desert fish are adapted to the harsh a nd variable conditions of the desert. Pupfish ( Cyprinodon sp. ) can withstand the high temperatures, alka linity, and salinity of small desert pools (Pister, 1995). Lema (2008) looked at en vironmental factors influencing phenotypic development of the seven species of pupfish inha biting the fresh water pools, saline marshes, and small streams in the Death Valley system.

Although pupfish require permanent water, a few of the hardiest desert fish species can survive in areas that periodically go nearly dry, such as in intermittent streams. Longfin dace

63( Agosia chrysogaster ), for example, survive relatively high water temperatures and low water quality and quantity, and have been found aliv e in moist algal mats where there was not enough water to swim (Hulen, 2007; Rinne and Mi nckley, 1991). Longfin dace have the most widespread distribution of any native fish in the Southwest, and can disperse rapidly once flow returns (Rinne and Minckle y, 1991). The Gila topminnow ( Poeciliopsis occidentalis
occidentalis ) also withstands low flows, high temperatures and poor water quality of intermittent desert streams (Arizona Game and Fish Department, 2001).

Although ephemeral streams only temporarily support fish, they indirectly support fish populations by helping to deliver required nutr ients and other materials to the perennial segments. Cummins and Wilzbach (2005) noted that ephemeral and intermittent streams are important suppliers of invertebrates and detr itus to permanently flowing, receiving streams that support juvenile salmonids. They also acknowledged the connection between headwater streams and downstream perennial waters by noting that it is critical to maintain riparian cover in the headwater systems to prevent incr eased temperatures of the downstream delivery
of water that would interfere with juvenile salmonids.

Intermittent streams are important to some fi sh species. Erman and Hawthorne (1976) found that trout production in California was dependent on intermittent streams. Over a four-year period, 39-47 percent of rainbow trout recruits in Sagehen Creek, California, came from an intermittent tributary that flowed only four months each year. Loggins et al. (1996) found that five native migratory cyprinid fishes were spawning in intermittent tributaries of the Sacramento River: Sacramento pikeminnow ( Ptychocheilus grandis ), hardhead ( Mylopharodon conocephalus ), hitch ( Lavinia exilicauda ), speckled dace ( Rhinichthys
osculus ) and Sacramento sucker ( Catostomus occidentalis ).

Populations of native desert fish es are rapidly dwindling due to destruction of aquatic habitats from urbanization, channelization, land-use change, over grazing, ground-water pumping, dams, water diversions, and pollu tion (Rinne and Minckley, 1991).

f. Synthesis of Functions

Ephemeral and intermittent streams and tributarie s provide a wide range of functions that are critical to the health and stability of arid and semi-arid watersheds and ecosystems in the American Southwest. Most importantly, they provide hydrologic connec tivity within a basin, linking ephemeral, intermittent, and perennial stream segments, thereby facilitating the movement of water, sediment, nutrients, de bris, fish, wildlife, and plant propagules throughout the watershed. They provide wildlife habitat and connectivity to perennial reaches by providing a relatively more vegetated and moister environment th an do the surrounding uplands. The processes that occur during ephemeral and intermittent stream flow include dissipation of energy as part of natural fluvial adjustment, and the movement of sediment and
debris.

Ephemeral and intermittent streams are responsible for a large portion of basin ground-water recharge in arid and semi-ari d regions through channel infiltra tion and transmission losses. These stream systems contribute to the biogeoc hemical functions of the watershed by storing,

64cycling, transforming, and transporting elements and compounds. Ephemeral and intermittent streams support a wide diversity of plant species, and serve as seed banks for these species. Because vegetation is more dense than in surrounding uplands, ephemeral and intermittent streams provide habitat, migration pathways, stop-over places, breeding locations, nesting sites, food, cover, water, and resting areas for ma mmals, birds, invertebrates, fish, reptiles and amphibians. In arid and semi-arid regions, the variability of the hydrol ogical regime is the key determinant of both plant community struct ure in time and space and the types of plants and wildlife present.

Figure 39. Photographs of the Rillito River, Tucson, Arizona, dry (left) and with flow (right).



### 6. Anthropogenic Impacts on Ephem eral and Intermittent streams and Riparian Areas

Anthropogenic uses and activities on th e landscape can have significant impacts

– both good
and bad

– on water quality and the h ealth of a watershed. Human-related disturbances are numerous and include livestock grazing, la nd clearing, mining, timber harvesting, groundwater withdrawal, stream flow diversion, channelization, urbanization, ag riculture, roads and road construction, off-road vehicle use, camping, hiking, and vegetation conversion. Biological stressors include habitat loss, alteratio n, effluent discharge, and degradation from decline in water quality, and changes in ch annel and flow charact eristics (Pima County, 2000).

The CWA has regulated many of these uses, but recent changes to the act have weakened or eliminated that enforcement. While many la nd owners voluntarily employ best management practices for water quality protecti on, not all do. However, in ar id and semi-arid areas, where water is limited and systems do not recover quickl y, it is especially important to employ best management practices for water qualit y protection whenever possible.

As noted earlier, ephemeral and intermittent streams and the adjacent riparian areas perform many of the same functions in a watershed as pere nnial streams. Especia lly in arid regions of the country, riparian areas support the vast majo rity of wildlife species , are the predominant

65sites of woody vegetation incl uding trees, and surround what are often the only available
surface water sources. These features have made riparian areas attractive for human development, leading to their alteration on a scale similar to that of wetlands degradation nationally (National Research Coun cil, 2002). This is especially true in arid and semi-arid regions because riparian areas are typically greener and cooler than other places. However, riparian areas in arid and semi-arid regions are more sensitive to development impacts than in
wetter areas because of their lim ited geographical extent, drier hydrologic characteristics, and fragile nature (e.g., erodible soils).

In general, human-induced changes to natural hydrological regimes in desert streams reduce temporal and spatial heterogeneity of plant hab itats, resulting in the loss of biodiversity and homogenization of plant community compositi on and structure. Given the ecological importance of plant communities in desert ri vers (e.g., for channel bank stabilization and wildlife habitat), there may be significant secondary impacts as well. There is some evidence to suggest that restoration of natural hydrological regimes in e phemeral streams may be partly
sufficient to reverse such deleterious cha nges in plant communities (see for example, Stromberg, 2001).

In the past, riparian habitats represented about 1 percent of the landscape in the West, and it
has been estimated that within the past one hundred years, 95 pe rcent of this ha bitat has been destroyed due to a wide variety of land use pr actices such as river channelization, unmanaged livestock utilization, agricu ltural clearing, water impoundment s and urbanization (Krueper, 1995). Given the vast extent of ephemeral a nd intermittent streams and the accumulation of impacts to them over large areas in the ra pidly developing southw est, a landscape or watershed-scale approach should be employed that considers the cumulative effects on overall watershed function. This section presents so me of the types of human caused impacts on ephemeral and intermittent streams and their associated riparian areas.

a. Land Development

The ecological and hydrological value of ephe meral and intermittent streams has been under appreciated, especially with respect to land conversion and development. Land development includes urban, suburban and exurban developm ent, but is referred to here as urban development.

The Southwest is one of the fastest growing regions of the U.S., having an increase in population of approximately 1,500 percent over the last ninety years. In contrast, the population of the country as a whole has grow n by just 225 percent. Arizona and Nevada have grown the most with population increases of 2,880 percent and 2,840 percent, respectively. Most of the growth in Nevada has been in Las Vegas, with Clark County having a 90-year growth rate of 22,480 percent, growing from 3,284 people in 1900 to 741,459 people in 1990. Maricopa County (Phoenix), Ariz ona, had a one hundred year growth rate of 10,275 percent, with most of that growth occurring between 1 960 and 1990 (Chourre and Wright, 1997).

66Assuming that the significant trend in population growth in the Southwestern U.S. over the last ninety years continues, it is necessary to develop plans to manage and protect streams and riparian areas that consider cumulative im pacts across a watershed. Water and natural resources need to be managed to accommodate future growth, and economies need to be examined to ensure a healthy environment (Chourre and Wright, 1997).

Urban development has the potential to change significantly the hydrologic characteristics of a watershed by covering uplands with impervi ous surfaces, and removal, channelization or armoring of small or headwater streams. Disrupt ion of the natural str eam network interferes with or destroys natural flow patterns a nd sediment-transport f unctions, resulting in downstream flooding and changes to the clarity a nd chemistry of the downstream flows. This can damage wildlife habitat and downstream water supplies for humans (National Wildlife Federation, 2007). Many land-pres ervation efforts have focuse d on upland areas, allowing the lowland bottomlands to continue being deve loped and degraded, although these areas support a rich biota (Rosen et al. 2005). In othe r areas, the bottomlands are protected from degradation, but not the uplands . Figure 40 shows a network of ephemeral streams that flows through a small community southeast of Tucs on, Arizona, to Cienega Creek, a protected perennial stream.

Figure 40. Aerial photograph showing ephem eral tributaries to Cienega Creek, a perennial stream, flowing through the small community of Vail, southeast of Tucson, Arizona. (Photograph: Lainie Levick/Aer ial flight courtesy of Lighthawk, www.lighthawk.org)

The influence of impermeable surfaces asso ciated with urbaniza tion increases as the percentage of impermeable surface increases. Various studies have shown that semi-arid stream systems become irreparably impaire d once the impervious surfaces within the watershed exceed about 10 percent, and experi ence dramatic morphological changes once that

67percentage exceeds about 20 percent (Coleman et al., 2005; Miltner et al., 2003; Schueler, 1994).

As the amount of impervious surface increases, runoff increases and infiltration decreases, starting a chain of even ts that includes flooding, erosion, st ream channel alteration, increases in man-made pollutants, and ecological damage. Floods will become more severe and more frequent, and peak flows will be many times greater than in natural basins. The greater volume and intensity of flooding will cause incr eased erosion and sedimentation downstream.
To facilitate the increased flow and sediment lo ad, streams in urbanized areas tend to become deeper and straighter over time. The resulting bank erosion destroys valuable streamside or riparian habitat and tree cover, leading to higher temperatures, sedi mentation, and disruption of habitat. Ground-water recharge will also be reduced as rainfall runoff leaves the watershed more rapidly than before (University of Connecticut, 1994).

Storm sewers and lined drainages increase the rate at which water is delivered to the channel network, and thus further increases peak flow s and erosion. Sedimentation is increased during construction and road building for new urban areas. Improperly constructed and maintained roads, especially dirt roads, can cau se alterations to hillslope drainage, and alter baseflow and precipitation-runoff relationships, resulting in erosion and sedimentation into the streams (USDA, 2002). The primary geomorphic consequence of these hydrologic changes is the erosional entrenchment of adjace nt channels and associated transportation of the excavated sediment downstream, causing a si gnificant increase in sediment load. Urban areas require storm water management plans bo th during and after c onstruction to control polluted runoff.

Water-quality impacts from urbanization include nonpoint source pollution, considered to be the single largest water quality threat in th e U.S. Pollutants include pathogens, nutrients, toxic contaminants, sediment, and debris. Sediment is of particular concern because many other pollutants tend to adhere to eroded soil particles. These changes to a stream system’s form and function result in degraded system s no longer capable of providing good drainage, healthy habitat, or natural pollutant pr ocessing (University of Connecticut, 1994).

Stream channelization is often applied in ur banizing areas to protec t private property and control stream bank erosion. However, channe lization straightens and steepens the stream, resulting in increased flow velo city and sediment movement. It also reduces moisture content along the stream banks by reducing out of bank flows which disrupts water, sediment, organic matter and nutrient enrichment of the flood pl ain (National Research Council, 2002). In addition, removal of vegetation as part of the ch annelization process degrades wildlife habitat.

Many authors have noted that habitat fragmentation is one of the consequences of urbanization (University of Connecticut, 1999; Aurambout, 2003; Hilty et al., 2006). New developments can alter large areas of land, re moving natural drainage systems and wildlife habitat, and replacing them with houses and roads. Altering, bisecting, or channelizing streams can effectively eliminate the main bi ological functions of the stream channel by disrupting vegetation communities and hydrologic function. Habitat fragmentation reduces wildlife diversity and abundance, and may cause sensitiv e species to disappear (University of

68Connecticut, 1999). A study by the California BLM that inventoried sixt y-six study plots for bird use of desert habitats noted the heavy recreational uses of washes, and the related
disturbance to wildlife and habitat de gradation (England and Laudenslayer, 1995).

b. Land Uses

Besides urbanization, agriculture (livestock and crops) and mining are other major land uses in the desert southwest. Livestock grazing is one of the more common uses of rural land in the Southwest, and has historica lly been a large part of the ec onomy. It occurs primarily on state or federal land, but also on private land. In many areas livestock are provided with watering sources, but frequently they must de pend on the streams for water. Livestock management plans attempt to avoid overuse of an area, but because water is scarce in arid environments, cattle and wildlife tend to linge r near water sources. When they are not properly managed, and remain too long in a ripari an area, cattle can tr ample stream banks, eat the riparian vegetation to the ground, contamin ate the water with wastes, and compact the soil. In addition, livestock gr azing can introduce exotic plan t species from hay or feed brought in from outside the area. Non-native plants may out-c ompete native species, causing disruption of natural ecosystem functions. (Pima County, 2000).

It has been estimated that by the late 1800’s over one and one half million cattle were in Arizona, with another two million in New Me xico. Around that time the Southwest was experiencing its typical clima tic pattern of drought and unpredic table rainfall patterns. The resulting desiccation of the uplands drove cattl e to the riparian areas, which were heavily damaged as a result. When the rains return ed to the denuded landscape, erosive processes took over and down cutting began, forming d eep arroyos and lowering the ground-water reservoir. Marshes and ripari an vegetation disappeared (Rinne and Minckley, 1991; Krueper, 1995).

A study by Siekert et al. (1985) on grazing impacts on ephemeral streams in Wyoming found that seasonal grazing had an impact on cha nnel morphology. Specifically, spring grazing had no effect, but summer and fall grazing resulted in increased channel cross sections. These impacts in and along a stream channel can cause reduced stream bank stability, decreased ground-water recharge, water qua lity degradation, increased erosion and sedimentation, removal of vegetation, increased flood risk due to reduced vegeta tion cover, and dispersal of exotic plants. Many sources have stressed that the cumulative impacts of unmanaged livestock in southwestern riparian ecosystems for the past several hundred years has probably been the single most important factor in ri parian ecosystem degradation (e.g., Krueper, 1995; Wagner, 1978; Ohmart, 1995).

Reducing damage to riparian vegetation from ove r-grazing by livestock is important in arid and semi-arid regions. Riparian vegetation helps stabilize stream banks, reduce water temperatures and evaporation through shading, and provides food and habitat for wildlife. Although in the past it was thought that remova l of stream bank vegetation would increase stream flow, recent studies have shown that in some places open water has higher annual water losses from evaporation than riparian trees from evapotranspiration (Leenhouts et al., 2006).

69Mining is another activity that historically ha s had a large place in the economy and land use
in the American Southwest. Some of the largest copper and gold mines in the world are found here, and some cover many thousands of hectares. However, mining can cause major impacts on riparian areas both adjacent and downstream by altering the local hydrology. Mining not only dewaters the area, it re moves vegetation and soil and changes the topography, severely impacting the watershed. Instream and flood plain gravel mining can
cause alteration to the channel dimensions a nd increase sediment yield. Mining can also decrease water quality by leaching heavy meta ls and toxic chemicals into the surface and ground water (Ecosystem Restorat ion Web site, accessed Sept. 12, 2008, http://ecorestoration.monta na.edu/mineland/guide/problem/impacts/default.htm )

Agriculture has had a long history in the southwestern deserts, and areas such as the Central Valley in California provide much of the country’s food supply. However, most crops must be irrigated due to the low a nnual rainfall. Impacts to local hydrology from agricultural
activities include:

o Increased salinity caused by clearing of native vegetation which raises the ground

water reservoir;

o Reduced flows from ground-water pumping or stream diversions for irrigation;

o Increased nutrients and turbidity from the use of fertilizers that run off into the streams across the land surface or through th e soil, causing excessive algae growth;

o Fish, aquatic invertebrate and bird kills from pesticides that run off into the streams or leach into the ground water. (from South Australia Environmenta l Protection Authority Web site, http://www.epa.sa.gov.au/water_impact.html , accessed August 7, 2008)

c. Water Resources Impacts

The Southwest has experienced rapid growth over the past several decades, straining the already limited water resources. Lack of surf ace water flows has placed increased reliance on ground water for domestic, industrial, and agri cultural uses. Ground- water pumping creates a chain reaction of events that impact the local and regional ecology.

Ground-water pumping lowers the ground-water re servoir killing near-channel vegetation whose roots no longer reach the aquifer, or desi ccates the subsurface to the point that flow frequently is reduced (Stromberg et al., 1996). As riparian vegetation dies, stream banks
become unstable due to the loss of the reinforcin g nature of the plant roots, resulting in bank erosion (Groeneveld and Griepe ntrog, 1985). Lowering of th e ground-water reservoir helps the invasion of exotic and drought tolerant species. Alteratio n and degradation of riparian vegetation can adversely affect wildlife species and human uses of those areas.

Dams and retention or detention basins are freque ntly used in the Southw est to store water or as flood-control devices. They disrupt natural surface flow and sediment transport, interfere with natural geomorphic processe s, alter water temperatures, a nd fragment the natural stream systems both upstream and downstream of the stru cture. Upstream locations may experience

70flooding, whereas downstream locations may be de watered and become sediment starved. As a result, both vegetation and wildlife communities are altered.

Ephemeral and intermittent streams in the Southwest are often the recipients of effluent from waste water treatment plants. Depending on the le vel of treatment, effluent can have various effects on the stream ecosystem. For example, Walker et al. (2005) co mpared four effluent dominated waters in Arizona and found that aqua tic macroinvertebrates were most affected by the levels of nitrogenous speci es, especially un-ioni zed ammonia, and mean diel dissolved oxygen. Specifically, diversity decreased with increasing levels of nitrogen.

As human population in the dese rt Southwest continues to grow, water resources will become even more stressed. Ground-water pumping has long been an important source of water in these areas; however, ground-wa ter depletion, ground surface subsidence and impacts to riparian areas are becoming more common. Further research and understanding of the surface-water ground-water interact ions is needed for better management of these resources (Phillips et al., 2004; Newman et al., 2006).

d. Climate Change

Most climate models predict severe changes for the southwest U.S., including increased warming and drying, intensificati on of droughts, and increased va riability of precipitation. These changes will result in less runoff, reduced snow packs, changes in streamflow patterns, longer and hotter growing seasons, shifts in vegetation growth patterns, and changes in wildfire regimes (CIRMOUNT Committee, 2006; Betancourt, 2007). The Intergovernmental Panel on Climate Change (IPCC) model simula tions suggest that the hydrologic cycle will become more vigorous, with the pos sibility of greater droughts a nd/or floods in some regions and reduced occurrences of these phenomena in other areas (Thompson, 1997). Most of these impacts are already occurring to varying degree s, but acceleration over the next century will make management of land and water resources more complex.



### 7. Discussion

a. Clean Water Act Context

The goal of the CWA is to “restore and ma intain the chemical, physical, and biological integrity of the Nation’s waters,” and to prevent pollution of those waters. Historically, desert washes have been considered to be jurisd ictional under the CWA (for example, 408 F.3d 1113 Save Our Sonoran, Inc. v. Flowers). However, as a result of the Supreme Court decision in
the consolidated cases Rapanos v. United States and Carabell v. United States (“Rapanos,” 2004), the definition of the Nation’s waters or wa ters of the United Stat es jurisdictional under the CWA has required additional clarification, speci fically with respect to tributaries that are “not relatively permanent” (i.e. ephemeral or intermittent streams). Recent guidance from the
U.S. EPA and Army Corps of Engineers (U .S. Environmental Protection Agency, 2007) requires that a “significant ne xus” exist between a non-relative ly permanent tributary and a traditional navigable water of the United States for the tributary to be jurisdictional under the

71CWA. This significant nexus evaluation must c onsider flow characteri stics and functions of the tributary to determine if it has a significan t effect on the chemical, physical, and biological integrity of downstream traditional navigable waters. We believe that the information presented in th is report shows that ephemeral and intermittent
streams in the arid and semi -arid Southwestern U.S. are ecologically and hydrologically connected to downstream waters, and have a significant effect on the chemical, physical, and biological integrity of those waters.

Connectivity in non-perennial stre ams, however, can be difficu lt to demonstrate owing to a lack of data. Stream gages and water quality mo nitoring sites tend to be in perennial reaches, for example. Most ecological studies on wildlife or vegetation have been conducted in wetter environments or in the uplands. Nevertheless, hydrologic models sugge st that in arid and semi-arid region watersheds flow in small tribut aries does reach the pe rennial stream courses (see for example Levick et al., 2006). Other studies have shown that non-perennial streams contribute nutrients, seed sources, or spawni ng areas necessary for biological and aquatic health in downstream perennial waters (see for example Erman and Hawthorne, 1976; Howe et al., 2008).

b. Ecosystem Goods and Services

Many sources recognize that the ecosystem good s and services provided by natural systems are critical to the healthy functioning of the natural environment. They also recognize the significant contribution of these goods and services to hum an welfare and well-being, both directly and indirectly, and ther efore the contribution to the overall social and economic value
of the natural environment.

Ephemeral and intermittent streams are integr al parts of a watershed, and their condition affects the health of the entire ecosystem. Healthy ecosystems perform a diverse array of functions that provide goods and services to society. In this context, “goods” refers to materials that can be sold (e.g., drinking water, tourism, or timber), whereas “services” provide value but cannot be sold (biodiversity, wildlif e habitat, or nutrient cycling) (Whiting, 2000; Wilson et al., 2004)

The Millennium Ecosystem Assessment (2005b) reviewed the consequences of ecosystem change on human well-being. Their multi-year study, published in 2005, presents a state-of

the-art scientific appraisal of the condition and trends in the world’s ecosystems and the services they provide, as well as the scientif ic basis for action to conserve and use them wisely. The report defines well-being as including “the basic material needs for a good life, the experience of freedom, health, personal security, and good social relations, which together, provide the conditions for physical, soci al, psychological, and spir itual fulfillment.”

From the Millennium Ecosystem Assessment (2005b):

Human well-being is supported by ecosystem services, which refers to the benefits received by people from an ecosy stem. These may include:

72• provisioning services such as food, water, timber, fi ber, and genetic resources;

• regulating services such as the regulation of clim ate, floods, disease, and water quality;

• cultural services such as recreational, aesthe tic, and spiritual benefits; and

• supporting services such as soil formation, pollination, and nutrient cycling.

Ecosystems also have value for human we ll-being through the cultu ral services they provide, through, for example, totemic sp ecies, sacred groves, trees, scenic landscapes, geological formations, or rive rs and lakes. These attributes and functions of ecosystems influence the aesthetic, recreational, educational, cultural, and spiritual aspects of human experience. Many changes to these ecosystems, through processes of disruption, contamina tion, depletion, and extinction, therefore have negative impacts on cultura l life and human experience.

In most cultures and regions, people gene rally prefer the ae sthetics of natural environments over built-up or urban ones. Fo r example, real estate values tend to be higher near protected open space, and reflect the willingness of people to pay for this amenity (Wilson et. al., 2004). The benefits provided by nature have inspired art, music and clothing throughout human hist ory. Development and degradation of
natural areas have reduced these benefits. The use of natural areas for recreation and tourism is growing as populations increase. Nature travel conti nues to increase, as does nature tourism, or eco-tourism.

The previous sections of this report demonstrate that ephemeral and intermittent streams provide all of these ecosystem services, and th erefore support overall hum an well being in the
arid and semi-arid Southwest. One of the st rongest points made in the Millennium Ecosystem Assessment was the powerful impact of eco system degradation on people with lower incomes, especially in developing countries. Lower income people depend more on natural areas for well-being than more affluent people because they are less able to replace the ecosystem services with purchased goods.

Although the Millennium Ecosystem Assessment report (2005b) did not address specific ecosystems, such as ephemeral and intermittent stream systems, it did make a strong case for the value of local ecosystems to the cultural di versity and cultural id entity of a society. People have historically identifie d with their environment for th eir sense of culture and value systems. This is especially true in arid a nd semi-arid areas where water is a major concern that receives significant attention and focus. Areas where water concentrates are cherished and valued, whether or not permanent water is present. Desert cultures have traditionally based their lives around water, ra iny seasons and the times when the rivers flowed. This is still true in the desert today: whenever it rain s enough to cause the ephemeral streams to flow, people flock to the rivers to watch the water (Figure 41).

73Figure 41. Photograph of the Santa Cruz River, Tucson, Arizona, during summer monsoon flows. (Photograph: unknown)

c. Management Principles

Ecologically responsible land management attemp ts to meet economic and social objectives while maintaining environmental health. It requ ires a landscape or wa tershed-scale approach that considers cumulative impacts to ensure that all physical, bi ological, and chemical components function together. For example, both the upland areas of a watershed and the riparian or stream course areas must be in a healthy functioning condition for the entire watershed to be healthy and to supply clean wa ter for human and ecosystem use, and in many cases, food and fiber production.

Landscape or watershed health can be described as a measure of the balance of anthropogenic uses and ecological function or integrity (Jones et al., 2002). Ecological integrity is the condition in which the productivity of resources and ecological values , including diversity,
are resilient to disturbance and maintained fo r the long term (Reynolds, 1995). It involves maintaining biodiversity, biologi cal productivity, and ecosystem pr ocesses. Important aspects of ecosystem integrity include energy flow through the food web, water and nutrient cycles, disturbance/recovery cycles, biot ic diversity, evolutionary pro cesses, and human influences. These characteristics and processes function at various rates and across multiple scales. Maintaining ecosystems requires maintaining th ese processes; it is not sufficient to only preserve the individual pieces (Ins titute for River Ecosystems, 1997).

The management of arid and semi-arid lands has a direct impact on the hydrology and geomorphology of the drainage network, in add ition to wildlife habita t. Bull (1977) noted that ephemeral streams are much more sensitiv e to climate or anthropogenic disturbance than are perennial streams. Nadeau and Rains ( 2007) discussed a management approach that considers how hydrological and ec ological systems function at va rious temporal and spatial

74scales. They described a watershed management a pproach that uses an in tegrated set of tools (federal, state, tribal, local) and progra ms (voluntary and regu latory), includes all stakeholders, and applies an ite rative planning or adaptive ma nagement process to address strategically priority water resource goals.

In arid and semi-arid regions, the functions of ephemeral and intermittent streams must be recognized and appreciated to protect and manage them properly; they must not be relegated to second-class status as compared to wetter systems elsewhere in the U.S. Ephemeral and
intermittent streams should not be considered in isolation from the entire watershed. Given their vast extent and the accumulation of imp acts to them over large areas in the rapidly developing southwest, a landscap e or watershed-scale approa ch should be employed that
considers the cumulative effects on overall wa tershed function. Ecosystem protection would be meaningless and ineffective if these supporti ng waterways were significantly degraded.

d. Research Recommendations

Many of the underlying physical, ec ological, and biological proce sses and linkages in arid and semi-arid region systems are not well underst ood or documented. This general lack of information specific to ephemeral and intermittent streams in the arid and semi-arid Southwest leaves a wide range of research opportunities. For exampl e, linking knowledge of past hydrological and channel ch anges to present-day changes in arid and semi-arid region streams should be a key resear ch priority (Tooth, 2000a).

Nadeau and Rains (2007) noted the need for l ong term, large-scale monitoring and research that are integrated across spatial and temporal scales to help provi de the theoretical and empirical foundations necessary to identify problems and problem sources. For example,
more research is needed to determine and cl assify the suites of flora and faunal species dependent on ephemeral and intermittent streams, and their preferred habitat types. Instrumenting more arid and semi-arid region wa tersheds with precipita tion and stream flow gages would greatly advance the understanding of these systems.

The protection of intact sites and the establis hment of “representative” sites would provide reference areas for future studies and comparisons with impacted areas. Predictive models are needed to understand the consequences of a lternative management actions on hydrological, ecological, economic and social systems.

Regarding wildlife, a better understanding of the sp ecies that inhabit these systems will aid in our understanding of the significan ce of arid and semi-arid region streams to wildlife. For example, baseline inventories are needed to determine species composition, abundance, and distribution as compared to adjacent upla nd or perennial areas. Specific ecological information is needed for critical target spec ies, such as habitat requirements and current population numbers a nd distribution.

Much still needs to be le arned about the ecological a nd hydrological interactions on ephemeral and intermittent streams due to variability and the often highly episodic occurrence of extreme events in these systems. There ar e unique challenges for work on these desert

75rivers. Sometimes the environments are inhospi table, but arguably th e greatest challenge is trying to use short-term projects to unders tand arid and semi-arid region streams whose variable behavior sometimes demands years of da ta more than are needed on a mesic river. As noted, ephemeral and intermittent streams constitute the vast majority of drainage ways in the Southwest and they play an integral role in overal l watershed function. Future research is needed for both the long-term monitoring of these systems over a range of conditions, and on developing modeling tools that can be applied to large te mporal and spatial scales.



### 8. Conclusions

When functioning properly, arid and semi-ari d region streams provide many of the same services as perennial streams that affect water quality and ecosystem health. These services include landscape hydrologic connections; su rface and subsurface water storage and exchange; ground-water recharge and discharge; se diment transport, storage, and deposition; flood plain development; nutrient cycling; wildli fe habitat including m ovement and migration corridors; support for vegetation communities that help stabilize stream banks and provide wildlife services; water supply and water qu ality filtering or cleansing; and stream energy dissipation associated with high- water flows that reduces eros ion and improves water quality (USFWS, 1993; BLM, 1998). In addition, ripari an areas associated with ephemeral and intermittent streams help mitigate and control water pollution by removing pollutants and sediment from surface runoff (Sonoran Institute, 2007). Thus, these streams play a significant role in the physical, biological, and chemical integrity of an ecosystem and must be afforded the same importance as other wetter systems in the U.S. in land management decisions.

Effective management of water resources in arid and semi-arid e nvironments requires awareness of the interdepende ncies of hydrologic, biogeochemi cal and ecological processes,
and collaboration between ecol ogists and hydrologists. Stream channel characteristics are based on upland watershed and channel conditions , and physical characte ristics such as the hydrology of the system-driven biological values . Non-perennial streams with active flood regimes contain a high diversity of plant specie s that varies depending on the location within the watershed. The complex longitudinal grad ients along arid and semi-arid region streams encompassing changes in flood intensity, climate, and water availability, result in a wide range of biological conditions along its length. Therefore, to protect water quality and riparian habitat, a watershed-ba sed approach to land management must be taken, involving all stakeholders and applying best management practices. Newman et al. (2006) suggest establishing a monitoring network in water- limited environments to facilitate this collaboration, from the experimental desi gn phase, through interpretation and modeling.

In the rapidly developing southwest, land ma nagement decisions must employ a watershedscale approach that addresses overall watershed function and water quality. As shown in this report, ephemeral and intermittent stream system s comprise a large portion of southwestern watersheds, and contribute to the hydrological , biogeochemical, and eco logical health of a watershed. Given their importance and vast ex tent, consideration of the cumulative impacts from anthropogenic uses on these streams is cr itical in watershed-based assessments and land management decisions to maintain overal l watershed health a nd water quality.

76

### 9. Literature Cited

Adams, J.W. 2000. Macroinvertebrate Monitoring of Redmond and Blakely Ridge Ephemeral Streams. The Xerces Society, Portland, OR 97215.

Aldridge, B.N. 1970. Floods of November 1965 to January 1966 in the Gila River Basin, Arizona and New Mexico, and adjacent ba sins in Arizona; US Geological Survey Water-Supply Paper 1850-C, 176 p.

Alexander, R.B., E.W. Boyer, R.A. Smith, G.E. Schwarz, and R.B. Moore. 2007. The Role
of Headwater Streams in Do wnstream Water Quality. Journal of the American Water
Resources Association . 43(1): 41-59.

Allan, J.D. 1995. Stream Ecology: Structure an d Function of Running Waters . Chapman & Hall. London. 388 p.

Anderson, B.W., J. Drake, R.D. Ohmart. 1977. Population Fluctuations in Nocturnal Rodents in the Lower Colorado River Valley. In: Importance, Preservation and Management of Riparian Habitat: A Symposium, July 9, 1977. U.S. Forest Service General Technical Report RM-43, p. 183-192.

Anderson, B.W., and R.D. Ohmart. 1977. Vegeta tion Structure and Bird Use in the Lower Colorado River Valley. In: Importance, Preservation a nd Management of Riparian Habitat: A Symposium, July 9, 1977. U.S. Forest Service General Technical Report RM-43, p. 23-34.

Antevs, E. 1952. Arroyo cutting and filling. Journal of Geology, v. 670, p. 375-378.

Arizona Game and Fish Department. 2001. Poeciliopsis occidentalis occidentalis

. Unpublished abstract compiled and edited by the Heritage Data Management System, Arizona Game and Fish Department, Phoenix, AZ. 5 p.

Arizona Game and Fish Department. 2002. Hyla arenicolor . Unpublished abstract compiled and edited by the Heritage Data Management System, Arizona Game and Fish Department, Phoenix, AZ.

Arizona Riparian Council (ARC). 2004. Fact Sheet #2: Riparian Functions and Values. Tempe, Arizona.

Arizona-Sonora Desert Museum . 2007. Canyon Tree Frog. Re trieved from Web site Nov. 30, 2007. http://www.desertmuseum.o rg/books/nhsd_canyon_treefrog.php

Atchley, M.C., A.G. de Soyza, W.G. Whitford. 1999. Arroyo wate r storage and soil nutrients and their effects on gas-exchange of shrub sp ecies in the northern Chihuahuan Desert. Journal of Arid Environments 43 (1): 21-33.

77Aurambout, J.P. 2003. A spatial model to estimate habitat fragmentation and its consequences of long-term survival of an imal populations. Department of Natural
Resources and Environmental Sciences Univ ersity of Illinois at Urbana Champaign.

Babcock, H.M. and E.M. Cushing. 1942. Rechar ge to groundwater from floods in a typical desert wash, Pinal County, Arizona. Transa ction of the American Geophysical Union 23, p. 49-55.

Bagstad, K.J., J.C. Stromberg and S.J. Lite. 2005. Response of Herbaceous Riparian Plants to Rain and Flooding on the San Pedro River, Arizona, USA. Wetlands , Vol. 25, No. 1, March 2005, p. 210–223.

Bailey, R.G. 1976. “Ecoregions of the United States” map, Aug. 17, 2001, unnumbered publication. Intermountain Region, USDA Forest Service, Ogden, Utah. http://www.fs.fed.us/land/ecosysmgmt/ecoreg1_home.html

Baillie, M.N., J.F. Hogan, B. Ekwurzel, A.K. Wahi, and C.J. Eastoe. 2007. Quantifying water sources to a semiarid riparian ecosystem, San Pedro River, Arizona, J. Geophysical Res., 112, GO3S02, doi:10.1029/2006JG000263.

Barnett, L.O., R.H. Hawkins, and D.P Guertin. 2002. Reconnaissance Watershed and Hydrologic Analysis on the Upper Agua Fria Watershed. School of Renewable Natural Resources, University of Arizona, Tucson, Arizona.

Baxter, R.J. 1988. Spatial Dist ribution of Desert Tortoises ( Gopherus agassizii ) at Twentynine Palms, California: Implications for Relocations. In: Management of Amphibians, Reptiles, and Small Mammals in North America. Proceedings of the Symposium, July 19-21, 1988. U.S. Forest Se rvice General Technical Report RM-166.

p. 180-189.

Beier, P. 1995. Dispersal of juven ile cougars in fragmented habitat. Journal of Wildlife Management . 59(2):228-237.

Beier, P., D. Majka, and T. Bayless. 2006. Arizona Missing Linkage s: Rincon-Santa RitaWhetstone Linkage Design. Re port to Arizona Game and Fi sh Department. School of Forestry, Northern Arizona University.

Bellantoni, E.S., and Krausman, P.R. 1991. Ha bitat use by desert mule deer and collared peccary in an urban environment. Tu cson, Arizona: University of Arizona. Cooperative National Parks Resource s Study Unit. Technical Report no. 42.

Bellantoni, E.S., and P.R. Krausman. 1993. Ha bitat use by collared pe ccaries in an urban environment. Southwestern Naturalist, 38(4):345-351.

78Belnap, J. 2003. The world at your feet: dese rt biological soil crusts . Frontiers in Ecology and the Environment: 2003: 1(5): 181–189. Ecological Society of America, publishers.

Belnap, J., J.R. Welter, N.B. Grimm, N. Ba rger, and J.A. Ludwig. 2005. Linkages between microbial and hydrologic processes in arid and semiarid watersheds. Ecology , 86(2):
298-307.

Bennett, A.F. 1999. Linkages in the Landscape: The Role of Corridors and Connectivity in Wildlife Conservation. IUCN, Gland, Switzerland and Cambridge, UK. 254 p.

Betancourt, J. 2007. Climate Variability and change in the Southwest. SERDP Workshop: Southwest Region Threatened, Endanger ed, and At-Risk Species Workshop: Managing with Highly Variable Environments Tucson, Arizona, Oct. 22, 2007.

Bigler, W.J. 1974. Seasonal Movements and Ac tivity Patterns of the Collared Peccary. Journal of Mammalogy , Vol. 55, no. 4, p. 851-855.

Black, P.E. 1997. Watershed Functions. Journal of the Amer ican Water Resources
Association , 33(1):1-11.

Bloss, D.A. and J.D. Brotherson. 1979. Vegeta tion response to a moisture gradient on an ephemeral stream in central Arizona. Great Basin Naturalist 39:161-176.

Booth, D.B. 1990. Stream-channel incision following drainage-basin urbanization. Water
Resources Bulletin , v. 26, n. 3, p. 407-417.

Boulton, A.J. and P.S. Lake. 1992. The ecology of two intermittent streams in Victoria, Australia. II. Comparisons of faunal compos ition between habitats, rivers and years. Freshwater Biology , v. 27, p. 99-121.

Boulton, A.J., S. Findlay, P. Marmonier, E.H. Stanley, and H.M. Valett. 1998. The Functional Significance of the Hyporheic Zone in Streams and Rivers. Annu. Rev. Ecol. Syst. 1998. 29:59–81.

Bourke, M.C., and G. Pickup. 1999. Fluvial form variability in arid cen tral Australia. In:
A.J. Miller, and A. Gupta (Eds.), Varieties of Fluvial Form, Wiley, Chichester, p. 249

- 271.

Breschta, R.L. and W.S. Pla tts. 1986. Morphological Fe atures of Small Streams: Significance and Function. Water Resources Bulletin , Vol. 22, No. 3, p. 369 – 379.

Brinson, M.M, F.R. Hauer, L.C. Lee, W.L. Nutter, R.D. Rheinhardt, R.D. Smith, and D. Whigham. 1995. A guidebook for applica tion of hydrogeomorphic assessments to riverine wetlands. U.S. Army Corps of Engineers: Wetlands Research Program Technical Report WRP-DE-11. p. 42-68.

79Brinson, M.M. 1988. Strategies of assessing the cumulative effects of wetland alteration on water quality. Environmental Management , 12(5):655-662.

Brooks, P.D., P.A. Haas, and A.K. Huth. 2007. Seasonal variability in the concentration and flux of organic matter and inorganic nitrog en in a semiarid catchment, San Pedro River, Arizona. Journal of Geophysical Research 112, G03S04,
doi:10.1029/2006JG000275.

Brooks, P.D. and M.M. Lemon. 2007. Spatial variability in dissolved organic matter and inorganic nitrogen concentra tions in a semiarid stream , San Pedro River, Arizona, J. Geophysical Res ., 112, G03S05, doi:10.1029/2006JG000262.

Brown, D.E., C.H. Lowe, and J.F. Hausler. 1977. Southwestern Riparian Communities: Their Biotic Importance and Management in Arizona. In: Importance, Preservation and Management of Ripari an Habitat: A Symposium, July 9, 1977. U.S. Forest Service General Technical Report RM-43, p. 201-211.

Brown, D.E. ed. 2004. Biotic Communities: S outhwestern United States and Northwestern Mexico. University of Utah Press, Salt Lake City.

Bull, W.B. 1997. Discontinuous Ephemeral Streams. Geomorphology, 19 (1997), p. 227

- 276.

Bull, L.J., and M.J. Kirkby. 2002. Dryla nd river characteristics and concepts. In: Dryland Rivers: Hydrology and Geomorphology of Semi-A rid Channels. L.J. Bull and M.J. Kirkby, eds., John Wiley & Sons Ltd., Chichester, p. 3-15.

BLM (Bureau of Land Management). 1998. Riparian Area Management: Process for Assessing Proper Functioning Condition. Technical Reference 1737-9.

Burkham, D.E. 1972. Channel changes of th e Gila River in Safford Valley, Arizona, 1846

- 1970. US Geological Survey Professional Paper 655G.

Carothers, S.W. 1977. Importance, Preservation, and Management of Riparian Habitats: An
Overview. In: Importance, Preservation and Mana gement of Riparian Habitat: A Symposium, July 9, 1977. U.S. Forest Se rvice General Technical Report RM-43, p. 2

- 4.

Carpelan, L.H. 1995. Invertebra tes of the California Desert. In: The California Desert: An
Introduction to Natural Resources and Man’ s Impact. Vol. II. Latting, J. and Rowlands, P.G., eds. Riverside, Califo rnia: June Latting Books. 2 vols. p. 275-283.

Cataldo, J., C. Behr, F. Montalto, and R.J. Pi erce. 2004. A Summary of Published Reports of Transmission Losses in Ephemeral Streams in the U.S. A Report to the National Center for Housing and the Environment.

80Ceballos, G. 1985. The Importance of Riparian Habitats for the Conservation of Endangered
Mammals in Mexico. In: Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. First North American Riparian Conference, April 16-18, 1985.
U.S.D.A. Forest Service Genera l Technical Report RM-120, p. 96-100.

Center for Watershed Protection. 2003. Impact s of Impervious Cover on Aquatic Systems. Watershed Protection Research Monograph No. 1. www.cwp.org , www.stormwatercenter.net

Chin, A., and K.J. Gregory. 2001. Urbaniza tion and adjustment of ephemeral stream
channels. Annals of the Association of American Geographers, v. 91, n. 4, p. 595

- 608.

Chourre, W. and S. Wright. 1997. Population Growth of the Southwest United States, 1900

- 1990. USGS Web Conference: Impacts of Climate Change and Land Use on the Southwestern United States, July 7 - 25, 1997. Accessed from Web site: http://geochange.er .usgs.gov/sw/changes/anthropogenic/population/ , May 9, 2008.

Chow, V.T., D.R. Maidment, and L.W. Mays. 1988. Applied Hydrology . New York; McGraw Hill. 572 p.

CIRMOUNT Committee. 2006. Mapping New Te rrain: Climate Change and America’s West. Report of the Consortium for Integrated Climate Research in Western Mountains (CIRMOUNT), Misc. Pub., PSW-MISC-77, Albany, CA, Pacific Southwest Research Station, Forest Servic e, U.S. Department of Agriculture, 29 p.

Coes, A.L. and D.R. Pool. 2005. Ephemeral-Stream Channel and Basin-Floor Infiltration and Recharge in the Sierra Vista Subwatershed of the Upper San Pedro Basin, Southeastern Arizona. U.S. Geologi cal Survey Open File Report 2005-1023.

Coleman, D., C. MacRae, E.D. Stein. 2005. Effect of Increases in Peak Flows and Imperviousness on the Morphology of Southern California Streams. Southern California Coastal Water Research Project, Westminster, CA. Technical Report #450.

Commission for Environmental Cooperation (C EC). 1997. Ecological Regions of North America: Toward a Common Perspec tive. Montreal (Quebec), Canada. http://www.epa.gov/wed/pages/ ecoregions/na_eco.htm#CEC1997.

Cook, P.G. and A.L. Herczeg (eds.). 1999. E nvironmental Tracers in Subsurface Hydrology. Kluwer Academic, Dordrecht, 529 p.

Cooke, R.U. and R.W. Reeves. 1976. Arroyos and Environmental Change in the American
Southwest . Clarendon Press, Oxford.

Cummins, K.W. and M.A. Wilzbach. 2005. The in adequacy of the fish bearing criterion for stream management. Aquatic Sciences 67(4):486 491.

81Daniels, B.E. 1979a. Thirty-f irst winter bird-population study. No. 81. Paloverde desert
wash. Amer. Birds 22(1):42.

Daniels, B.E. 1979b. Thirty-first winter bi rd-population study. No. 82. Ironwood-smoketree
desert wash. Amer. Birds 22(1):42.

Daniels, B.E. and J. Boyd. 1979a. Forty-s econd breeding bird census. No. 142. Ironwood

smoketree desert wash. Amer. Birds 33(1):92-93.

Daniels, B.E. and J. Boyd. 1979b. Forty-sec ond breeding bird census. No. 150. Paloverde
desert wash. Amer. Birds 33(1):94.

De Boer, D.H. 1992. Constraints on Spatial Tr ansference of Rainfall-Runoff Relationships in Semiarid Basins Drained by Ephemeral Streams. Hydrological Sciences Journal HSJODN, Vol. 37, No. 5, p. 491-504, October 1992.

de Soyza, A.G., K.T. Killingbeck, and W.G. Whitford. 2004. Plant water relations and photosynthesis during and af ter drought in a Chihuahuan desert arroyo. Journal of
Arid Environments 59 (2004), p. 27–39.

DeBano, L.F., P.F. Ffolliott, and K.N. Brooks . 1995. Flow of water and sediments through Southwestern riparian systems. In: Desired future conditions for Southwestern riparian ecosystems: Bringing interests and concerns together. D.W. Shaw, and D.M. Finch, tech cords. Sept. 18-22, 1995; Al buquerque, NM. General Technical Report RM-GTR-272. Fort Collins, CO: U.S. Depa rtment of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, 359 p.

Del Rosario, R.B. and V.H. Resh. 2000. Invert ebrates in Intermittent and Perennial Streams: Is the Hyporheic Zone a Refuge from Drying? Journal of the North American Benthological Society , Vol. 19, No. 4. p. 680-696.

Dick, G.S., R.S. Anderson and D.E. Sampson. 1997. Controls on flash flood magnitude and hydrograph shape, Upper Blue Hills badlands, Utah. Geology 25, p. 45-48.

Dieterich, M., and N.H. Anderson. 1998. Dynami cs of abiotic parame ters, solute removal and sediment retention in summer-dry headwater streams of western Oregon. Hydrobiologia 379:1–15.

Dimmitt, M.A. 2000. Biomes & Communities of the Sonoran Desert Region. In: A Natural History of the Sonoran Desert. S.J. Phill ips and P.W. Comus, eds. Arizona-Sonora
Desert Museum Press and Univ ersity of California Press.

Duncan, D. 1990. Small mammal inventory of Chiricahua National Monument, Cochise County, Arizona. Technical Report No. 30, Cooperative National Park Studies Unit, University of Arizona, National Park Service.

82Dunkerley, D.L. 1992. Channel geometry, bed material, and inferred flow conditions in ephemeral stream systems, Barrier Range, Western N.S.W., Australia. Hydrological
Processes , v. 6, p. 417-433.

Dunkerly, D.L. 2008. Bank permeability in an Australian ephemeral dry-land stream: variation with stage resulting from m ud deposition and sediment clogging. Earth Surface Processes and Landforms , 33:226-243.

Dunkerley, D. and K. Brown. 1999. Flow be havior, suspended sediment transport and transmission losses in a small (sub-bank-full) flow event in an Australian desert
stream; Hydrological Processes , v. 13, n. 11, p. 1577-1588.

Elliot, J.G., R.D. Jarrett, and J.L. Ebling. 1982. Annual snowmelt and rainfall peak-flow data on selected foothills region streams, South Pl atte River, Arkansas River, and Colorado River basins, Colorado: U.S. Geological Survey Open-File Report 82-426, 88 p.

Elliott, J.G., A.C. Gellis, and S.B. Aby. 1999. E volution of arroyos: incised channels of the
southwestern United States. In: Incised River Channels, S.E. Darby and A. Simon, eds., John Wiley and Sons, New York, p. 153-185.

England A.S. and W.F. Laudenslayer, Jr. 1995. Birds of the California Desert. In: The
California Desert: An Introduction to Natural Resources and Man’s Impact. Vol. II. Latting, J. and Rowlands, P.G. eds. Riversid e, California: June Latting Books. 2 vols.

Erman, D.C. and V.M. Hawthorne. 1976. The quantitative importance of an intermittent stream in the spawning of rainbow trout . Transactions of the American Fisheries Society 105:675-681.

Erman, N.A. and C.D. Nagano. 1992. A review of the California caddisflies (Trichoptera) listed as candidate species on the 1989 Fede ral “Endangered and threatened wildlife and plants-animal notice of review. California Fish and Game 78:45-56.

Fanning, P.C. 1999. Recent landscape history in arid Western New South Wales, Australia: a model for regional change. Geomorphology, 29:191-209.

Fisher, S.G. and W.L. Minckley. 1978. Chemical characteristics of a desert stream in flash
flood. Journal of Arid Environments , 1:25-33.

Fisher, S.G. and Gray, L.J. 1983. Secondary production and organic matter processing by
collector macroinvertebrate s in a desert stream. Ecology 64:1217–24.

Fisher, S.G. and N.B. Grimm. 1985. Hydrol ogic and material budgets for a small Sonoran Desert watershed during three consecutive cloudburst floods. Journal of Arid
Environments , 9:105-118.

83Fisher, S.G. 1991. Presidential address: Em erging global issues in freshwater ecology. Bull.
N. Am. Benthol. Soc . 8:235–45.

Fisher, S.G., J. Welter, J. Schade, and J. Henery. 2001. Landscape challenges to ecosystem thinking: Creative flood and drought in the American Southwest. Scientia Marina

, 65(suppl. 2): 181-192.

Frank, C.L. 1988. Diet Selection by a Hetero myid Rodent: Role of Net Metabolic Water
Production. Ecology , Vol. 69, No. 6, (Dec., 1988), p. 1943-1951.

Freeman, M.C., C.M. Pringle, and C.R. J ackson. 2007. Hydrologic Connectivity and the
Contribution of Steam Headwaters to Ec ological Integrity at Regional Scales. Journal of the American Water Resources Association , 43(1):5-14.

Friedman, J.M. and V.J. Lee. 2002. Extreme floods, channel change and riparian forests along ephemeral streams. Ecological Applications 72:409-425.

Friedman, J.M., W.R. Osterkamp, and W.M. Lewi s, Jr. 1996. The role of vegetation and
bed-level fluctuations in the process of channel narrowing. Geomorphology , 14:341

- 351.

Frostick, L.E., and I. Reid. 1977. The origin of horizontal laminae in ephemeral stream channel-fill; Sedimentology , v. 24, p. 1-9.

Frostick, L.E., and I. Reid. 1979. Drainage-net control of sedimentary parameters in sandbed ephemeral streams. In: A.F. Pitty, (ed.), Geographi cal Approaches to Fluvial Processes. Geoabstracts, Norwich, p. 173-201.

Gandolfi, C. and G.B. Bischetti. 1997. Infl uence of the drainage network identification method on geomorphological properties and hydrological response. Hydrological
Processes 11(4): 353-375.

Gochis, D.J., L. Brito-Castillo, and W.J. S huttleworth. 2006. Hydroclimatology of the North American Monsoon region in Northwest Mexico. Journal of Hydrology 316:53-70.

Gomi, T., R.C. Sidle, and J.S. Richardson. 2002. Understanding Processes and Downstream Linkages of Headwater Systems. Bioscience ; Vol. 52, no. 10, p. 905-916.

Goodrich, D.C., L.J. Lane, R.M. Shillito, S.N. Mi ller, K.H. Syed, and D.A. Woolhiser. 1997. Linearity of basin response as a functi on of scale in a semiarid watershed. Water
Resour. Research , 33(12):2951-2965.

Goodrich, D.C., D.G. Williams, C.L. Unkrich, J.F. Hogan, R.L. Scott, K.R. Hultine, D. Pool,
A.L. Coes, and S. Miller. 2004. Comparison of methods to estimate ephemeral channel recharge, Walnut Gulch, San Pedro River Basin, Arizona. In: Groundwater Recharge in a Desert Environment: The Southw estern United States. eds. J.F. Hogan,

84F.M. Phillips, and B.R. Scanlon, Water Sc ience and Applications Series, Vol. 9, American Geophysical Uni on, Washington, D.C., p. 77-99.

Graf, W.L. 1983. Flood-related cha nge in an arid region river. Earth Surface Processes and Landforms , v. 8, p. 125-139.

Graf, W.L. 1988. Fluvial Processes in Dryland Rivers . Springer-Verlag, Berlin.

Graham, T.B. 2002. Survey of aquatic macroinvertebrates and amphibians at Wupatki National Monument, Arizona, USA: An evaluation of selected factors affecting species richness in ephemeral pools. Hydrobiologia 486:215-224.

Grimm, N.B. and S.G. Fisher. 1989. Stability of periphyton and m acroinvertebrates to disturbance by flash floods in a desert stream. J. N. Am. Benthol. Soc . 8:293–307.

Grimm, N.B., J.R. Arrowsmith, C. Eisinger, J. Heffernan, D.B. Lewis, A. MacLeod, L. Prashad, W.J. Roach, T. Rychener, and R.W. Scheibley. 2004. Effects of urbanization on nutrient biogeochemi stry of arid land streams. In: R. DeFries, G.P. Asner, and R. Houghton, eds., Ecosystem inte ractions with land use change. American Geophysical Union, p. 129-146.

Groeneveld, D.P., and T.E. Griepentrog. 1985. Interdependence of Groundwater, Riparian Vegetation and Streambank Stability: A Case Study. In: Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. First North American Riparian Conference, April 16-18, 1985. U.S.D.A. Fo rest Service General Technical Report RM-120, p. 44-48.

Hardy, P.C., D.J. Griffin, K.A. Juenzi, and M. L. Morrison. 2004. Occu rrence and habitat use of passage neotropical migran ts in the Sonoran Desert. Western North American
Naturalist 64:59-71.

Hatten, J.R., L. Averill-Murray and B. Van Pe lt. 2003. Characterizing and mapping potential jaguar habitat in Arizona. AGFD, N ongame and Endangered Wildlife Program Technical Report 203, Phoenix, AZ. 32 p.

Hayes, M.P. and M.R. Jennings. 1988. Habita t Correlates of Distribu tion of the California Red-Legged Frog ( Rana aurora draytonii ) and the Foothill Yellow-Legged Frog ( Rana boylii ): Implications for Management. In: Management of Amphibians, Reptiles, and Small Mammals in North America. Proceedings of the Symposium, July 19-21, 1988. U.S. Forest Service General Technical Report RM-166. p. 144-158.

Hedman, E.R. and W.R. Osterk amp. 1982. Streamflow charact eristics related to channel geometry of stream in western United St ates. U.S. Geological Survey Water Supply Paper 2193.

85Heede, B.H. 1985. Interactions Between St reamside Vegetation and Stream Dynamics. In: Riparian Ecosystems and Their Manageme nt: Reconciling Conflicting Uses. First North American Riparian Conference, April 16-18, 1985. U.S.D.A. Forest Service General Technical Report RM-120, p. 54-58.

Heine, R.A., C.L. Lant, and R.R. Sengupt a. 2004. Development and comparison of approaches for automated mapping of str eam channel networks. Annals of the
Association of American Geographers 94(3):477-490.

Henderson, R.P. 1979. thirty-first winter bird population study. No. 39. Paloverde
woodland. Amer. Birds 33(1):29.

Hereford, R., R.H. Webb, and S. Graham. 2002. Precipitation histor y of the Colorado Plateau region, 1900-2000. U.S. Geologi cal Survey Fact Sheet 119-02, 4 p.

Hereford, R., R.H. Webb, and C.I. Longpre. 2003 . Precipitation history of the Mojave Desert region, 1893-2001. U.S. Geological Survey Fact Sheet 117-03, 4 p.

Hibbs, B.J. 2008. Forward: Ground Water in Arid Zones. Ground Water, Vol. 46. No. 3.

Hilty, J.A., W.Z. Lidicker, Jr., and A.M. Mere nlender. 2006. CorridorEcology: The Science and Practice of Linking Landscapes for Biodi versity Conservation. Island Press, Wash. D.C., 323 p.

Hirschboeck, K.K. 1987. Hydroclimatically defined mixed distributions in partial duration flood series, In: Singh, V.P., ed., Hydrologic freque ncy modeling: Boston, Mass., D. Reidel, p. 192-205.

Hogan, J.F., F.M. Philips, and B.R. Scanlon (e ds.). 2004. Groundwater Recharge in a Desert
Environment: The Southwestern United St ates. Water Science and Applications Series, Vol. 9, American Geophys ical Union, Washington, DC. 294 p.

Holmes, R.M., S.G. Fisher and N.B. Grimm. 19 94. Parafluvial nitrogen dynamics in a desert stream ecosystem. J. North American Benthological Society , 13(4):468-478.

Howe, S., L.R. Levick, A. Hautzinger. 2008. Hydrology and Ecology of Southwestern Intermittent Streams, Dry Washes, and Adjacent Riparian Zones. In: Proceedings from the Southwest Region Threatened, Endangered, and At-Risk Species Workshop, 22-25 October 2007, Tucson, Arizona. Prepared for the Legacy Resource Management Program, Strategic Environmental Res earch and Development Program, and Environmental Security Tec hnology Certification Program by HydroGeoLogic, Inc., Reston, Virginia.

Hubbard, J.P. 1977. Importance of Riparian Ecosystems: Biotic Considerations. In: Importance, Preservation and Management of Riparian Habitat: A Symposium, July 9,
1977. U.S. Forest Service Gene ral Technical Report RM-43, p.14-18.

86Hulen, T. 2007. Cave Creek: Refuge for Dese rt Fish. Retrieved fr om Web site 11/28/07. http://www.dflt.org/awareness/desert-fish.htm

Hupp, C.R., and W.R. Osterkamp, 1996. Ripari an vegetation and fluvial geomorphic processes. Geomorphology 14:277-295.

Huryn, A.D. and J.B. Wallace. 2000. Life Hi story and Production of Stream Insects. Annu.
Rev. Entomol . 45:83–110.

Institute for River Ecosystems. 1997. Smith River Fisheries and Ecosystem Report. http://www.geocities.com/RainForest/3771/index.html Accessed Aug. 1, 2008.

Izbicki, J.A. 2007. Physical and temporal is olation of mountain he adwater streams in the western Mojave Desert, southern California. Journal of the American Water
Resources Association , Vol. 43, No. 1.

Jackson, J.K. and S.G. Fisher. 1986. Sec ondary production, emergence, and export of aquatic insects of a Sonoran Desert stream. Ecology, 67:629–38.

Jacobson, P.J., K.M. Jacobson, P.L. Angermei er and D.S. Cherr y. 2000a. Hydrologic influences on soil properties along ephe meral rivers in the Namib Desert. Journal of
Arid Environments , 45:21-34.

Jacobson, P.J., K.M. Jacobson, P.L. Angermeier, and D.S. Cherry. 2000b. Variation in material transport and water chemistry along a large ephemeral river in Namib Desert. Freshwater Biology , 44(3):481-491.

Jakle, M.D. and T.A. Gatz. 1985. Herpetofauna l Use of Four Habitats in the Middle Gila River Drainage, Arizona. In: Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. First North American Riparian Conference, April 16

- 18, 1985. U.S.D.A. Forest Service Gene ral Technical Report RM-120, p. 355-358.

Jarrett, R.D., and J.E. Costa. 1982. Multidisci plinary approach to the flood hydrology of foothills streams in Colorado, In: Johnson, A.J., and Clark, R.A., eds., International Symposium on Hydrometeorology, Denver, Colo., 1982, Proceedings: American Water Resources Association, p. 565-569.

Johnson, R.R., L.T. Haight, and J.M. Simps on. 1977. Endangered species vs. endangered habitats: A concept. In: R.R. Johnson and D.A. Jones, eds. Importance, preservation, and management of riparian habitat: A sy mposium. General Technical Report RM

166. U.S. Forest Service, Fort Collins, CO. p. 68-79.

Johnson, R.R., S.W. Carothers, J.M. Simpson. 1984. A riparian classification system. pp 375-382, In: R.E. Warner and K.M. Hendrix, ed s., California Riparian Systems: Ecology, Conservation, and Productive Manageme nt. University of California Press, Berkeley.

87Johnson, R.R. and L.T. Haight. 1985. Avian Us e of Xeroriparian Ecosystems in the North
American Warm Deserts. In: Riparian Ecosystems and Their Management: Reconciling Conflicting Uses. First North American Riparian Conference, April 16

- 18, 1985. U.S.D.A. Forest Service Gene ral Technical Report RM-120, p. 156-160.

Johnson, R.R. and C.H. Lowe. 1985. On the development of riparian ecology. In: Riparian Ecosystems and their management: First North American Riparian Conference. Rocky Mountain Forest and Range Experi ment Station, General Technical Report
RM-120.

Jones, C., R.M. Palmer, S. Motkaluk, and M. Walter. 2002. Watershed Health Monitoring, Emerging Technologies. CRC Press LLC, Florida.

Jones, Jr., J.B., S.G. Fisher, and N.B. Grimm. 1996. A long-term perspective of dissolved organic carbon transport in Sy camore Creek, Arizona, USA. Hydrobiologia , 317:183

- 188.

Jones, K.B. 1986. Deserts. In: Inventory and Monitoring of Wildlife Habitat. A.Y. Cooperrider, R.J. Boyd, and H.R. Stuart, ed s. U.S. Dept. Inter., Bureau of Land Management. Service Center, Denver, CO. p. 123-147.

Jones, K.B. 1988. Distribution and Habitat As sociations of Herpetofauna in Arizona: Comparisons by Habitat Type. In: Management of Amphibians, Reptiles, and Small Mammals in North America. Proceedings of the Symposium, July 19-21, 1988. U.S.
Forest Service General T echnical Report RM-166. p. 109-128.

Jorgensen, E.E., S. Demarais and S. Neff. 1995. Rodent Use of Mi crohabitat Patches in Desert Arroyos. American Midland Naturalist , 134(1):193-199.

Kennedy, C.E. 1977. Wildlife Conflicts in Riparian Management: Water. In: Importance, Preservation and Management of Riparian Habitat: A Symposium, July 9, 1977. U.S.
Forest Service General T echnical Report RM-43, p. 52-58.

Kepner, W.G. 1978. Small mammals of the Black Canyon and Skull Valley Planning Units, Maricopa and Yavapai Counties, Arizona. U.S. Bureau of Land Management,
Phoenix District Office. Technical Note 350.

Keppel, R.V. and K.G. Renard. 1962. Transmission losses in ephemeral stream beds. Journal of the Hy draulics Division , ASCE, v. 8, n. HY3, p. 59-68.

Killingbeck, K.T. and W.G. Whitford. 2001. Nu trient resorption in shrubs growing by design, and by default in Chihuahuan Desert Arroyos. Oecologia 128:351-359.

Kingsley, K.J. 1998. Invertebrates of Organ Pipe Cactus National Monument, Arizona.
U.S.G.S. Cooperative Studies Unit, University of Arizona, and National Park Service. Technical Report No. 60, 193 p.

88Kingsley, K.J. 2006. Evaluation of Mesquite M ouse (Peromyscus Merriami) Status in Pima County, Arizona. SWCA Environmental Consul tants, Tucson, AZ. Prepared for Pima County Regional Flood Contro l District, Tucson, AZ.

Kirkpatrick, C., C.J. Conway and D. LaRo che. 2007. Quantifying impacts of groundwater
withdrawal on avian communities in desert riparian woodlands of the southwestern
U.S. USGS Arizona Cooperative Fish & Wildlife Research Unit, Department of Defense, Legacy Resource Mana gement Program, Project 06-290.

Knighton, A.D. and G.C. Nanson. 1997. Distinct iveness, diversity and uniqueness in arid zone river systems. In : Thomas, D. S. G. (ed.), Arid Zone Geomorphology: processes, form and change in drylands, 2nd edition Wiley, Chichester, p. 185-203.

Krausman, P.R., K.R. Rautenstrauch and B.D. Leopold. 1985. Xeroriparian Systems Used by Desert Mule Deer in Texas and Arizona. In: Riparian Ecosystems and Their Management: Reconciling Conflicting Uses . First North American Riparian Conference, April 16-18, 1985. U.S.D.A. Fo rest Service General Technical Report RM-120, p. 144-149.

Krueper, D.J. 1995. Effects of Livestock Management on Southwestern riparian ecosystems.
In : Desired future conditions for Southweste rn riparian ecosystems: Bringing interests and concerns together. D.W. Shaw, and D. M. Finch, tech cords. Sept. 18-22, 1995; Albuquerque, NM. General Technical Report RM-GTR-272. Fort Collins, CO: U.S. Department of Agriculture, Forest Serv ice, Rocky Mountain Forest and Range Experiment Station, 359 p.

Krueper, D.J. 1993. Conservation priorities in naturally fragmented and human-altered riparian habitats of the arid West. USDA Forest Service. General Technical Report
RM-43. Available online: www.birds.co rnell.edu/pifcapemay/krueper.htm.

Kubik, M.R., Jr., and J.V. Remsen, Jr. 1977. Fortieth breeding bird census. No. 125.
Catclaw-rabbitbrush desert wash. Amer. Birds . 31(1):75-76.

Lane, L.J. 1983. Transmission Losses. In: National Engineering Handbook. IV. Hydrology. Washington, D.C., USDA, Soil Conservation Service, 21 p.

Lange, J. 2005. Dynamics of transmission losses in a large arid stream channel. Journal of Hydrology, 306:112-126.

Lee, L.C., P.L. Fiedler, S.R. Stewart, R.R. Cu rry, D.J. Partridge, J.A. Mason, E.M. Inlander,
R.B. Almy, D.L. Aston, and M.E. Spencer. 2001. Draft Guidebook for Reference
Based assessment of the Functions of Rive rine Waters/Wetlands Ecosystems in the South Coast Region of Santa Barbara C ounty, California. Santa Barbara County Water Agency, Santa Barbara, CA.

89Lee, L.C., P.L. Fiedler, S.R. Stewart, D.J. Partridge, J.A. Mason, E.M. Inlander, and M.C. Rains. 2004. Draft Operational Guidebook for Assessment of the Functions of
Riverine Waters/Wetlands in the Santa Ma rgarita Watershed, Riverside & San Diego Counties, California. San Diego Regiona l Water Quality Control Board, Technical Publication. San Diego, CA.

Leenhouts, J.M., J.C. Stromberg, R.L. Scott, eds. 2006. Hydrologic requirements of and consumptive ground-water use by riparian vegetation along the San Pedro River, Arizona. U.S. Geological Survey Scient ific Investigations Report 2005-5163, 154 p.

Leibowitz, S.G., P.J. Wigington, Jr., M.C. Rain s, and D.M. Downing. 2008. Non-navigable streams and adjacent wetlands: addressi ng science needs following the Supreme
Court’s Rapanos decision. Frontiers in Ec ology and the Environment 2008; 6,
doi:10.1890/070068. Downloaded from Frontiers e-View.

Leitner, L.A. 1987. Plant communities of a large arroyo at Punto Cirio, Sonora. Southwestern Naturalist, 32:21-28.

Lekach, J., A.P. Shick, and A. Schlesinger. 1992. Bedload yield and in-channel provenance in a flash-flood fluvial system. In: P. Billi, R.D. Hey, C.R. Thorne, and P. Tacconi (Eds.), Dynamics of Gravel-bed Ri vers, John Wiley & sons Ltd, p. 537-554.

Lekach, J., R. Amit, T. Grodeck, and A.P. Sc hick. 1998. Fluvio-pedogenic processes in an ephemeral stream channel, Nahal Yael, Southern Negev, Israel. Geomorphology 23,

p. 353-369.

Lema, S.C. 2008. The Phenotypic Plasticity of Death Valley's Pupfish. American Scientist, January-February 2008. Accessed Sept. 11, 2008 from http://www.americanscientist.org/issues/f eature/2008/1/the-phenotypic-plasticity-of

death-valleys-pupfish/1

Leopold, L.B. and M. Maddock Jr. 1953. The Hydr aulic Geometry of Stream Channels and Some Physiographic Implications. U.S.G.S. Professional Paper 252.

Leopold, L.B. and J.P. Miller. 1956. Epheme ral Streams – Hydraulic Factor and their Relation to the Drainage Net. Geologi cal Survey Professional Paper 282-A.

Leopold, L.B., M.G. Wolman and J.P. Miller. 1964. Fluvial Processes in Geomorphology

. W.H. Freeman and Company, San Francisco, 504 p.

Leopold, L.B., W.W. Emmet, and R.M. Myrick. 1966. Channel and hillslope processes in a semi-arid area, New Mexico. US Geol ogical Survey Professional Paper 352-G.

Levick, L.R., D.J. Semmens, D.C. Goodrich, W. Kepner, J. Brush, R.A. Leidy, E. Goldmann. 2006. Simulated Changes in Runoff and Sedi ment in Developing Areas Near Benson, Arizona (in press).

90Lite, S.J., K.J. Bagstad, and J.C. Stromber g. 2005. Riparian plant species richness along lateral and longitudinal gradients of wate r stress and flood disturbance, San Pedro River, Arizona, USA. Journal of Arid Environments 63 (2005) 785–813.

Loggins, R.E., P. Maslin, D. Efseaff, K. Nelson, W. McKinney and T. Moore. 1996. Cyprinoid Usage of Intermittent Streams in the Sacramento River Basin. http://www.ecst.csuchico.edu/~loggins/cyprin.html

Louw, G. and M. Seely. 1982. Ecology of desert organisms . Longman Group Limited, New York. 194 p.

Lovich, J.E. 1996. A Brief Review of the Imp acts of Tamarisk, or Saltcedar on Biodiversity
in the New World. In: Proceedings: Saltcedar Manageme nt and Riparian Restoration Workshop. Las Vegas, Nevada, September 17 - 18, 1996

Lowe, C.H. 1985. Amphibians and Reptile s in Southwest Riparian Ecosystems. In: Riparian Ecosystems and their Management: Rec onciling conflicting uses. First North American Riparian Conference, April 16- 18, 1985. U.S. Forest Service General Technical Report RM-120.

Mark, D.M. 1983. Relations between field- surveyed channel networks and map-based geomorphometric measures, Inez, Kentucky. Annals of the Association of American Geographers 73(3): 358-372.

Mayhew, W.W. 1995. Amphibians of the California Desert. In: The California Desert: An
Introduction to Natural Resources and Man’ s Impact. Vol. II. Latting, J. and Rowlands, P.G. eds. Riverside, Calif ornia: June Latting Books. 2 vols.

McArthur, E.D. and S.C. Sa nderson. 1992. A Comparison betw een xeroriparian and upland vegetation of Beaver Dam Slope, Uta h, as Desert Tortoise habitat. In: Proceedings – Symposium on Ecology and Management of Riparian Shrub Communities. U.S. Dept. of Agriculture, General Technical Report INT-289, p. 25-31.

McMahon, T. A. 1979. Hydrological characteristic s of arid zones. Hydrology of areas of low precipitation, Proceedings of the Canbe rra Symposium. I AHS-AISH Publication No. 128, p. 105-123.

Merritt, D.M., and E.E. Wohl. 2003. Downstream hydraulic geometry and channel adjustment during a flood along an ephemeral arid-region drainage. Geomorphology

,
v. 52, p. 165-180.

Meyer, J.L. and J.B. Wallace. 2000. Lost linkages and lotic ecol ogy: rediscovering small streams. Proceedings of the 41 st Symposium of the Britis h Ecological Society, p. 295

- 317.

91Meyer, J.L., D.L. Strayer, J.B. Wallace, S.L. Eggert, G.S. Helfman, and N.E. Leonard. 2007.
The contribution of headwater streams to biodiversity in river networks. Journal of
the American Water Resources Association , 43(1):86-103.

Millennium Ecosystem Assessment, 2005a . Ecosystems and Human Well-being: Desertification Synthesis. World Resources Institute, Washington, DC, http://www.millenniumassessment.org/ documents/document.355.aspx.pdf)

Millennium Ecosystem Assessment, 2005b. Ecosys tems and Human Well-being: Synthesis. World Resources Institute, Washington, DC, http://www.millenniumassessment.org/documents/document.356.aspx.pdf).

Miller, M.E. 2005. The Structure and Functi oning of Dryland Ecosystems – Conceptual Models to Inform Long-Term Ecological Monitoring. U.S. Geological Survey Scientific Investigatio ns Report 2005-5197, 73 p.

Miller, Scott. 1995. An Analysis of Channe l Morphology at Walnut Gulch - Linking Field Research with GIS Applications. M. S. Thesis, University of Arizona.

Miller, S.N., D.P. Guertin, and L.R. Levick. 1999a. Influences of map scale on drainage network representation. Hydrology and Water Resources in Arizona and the Southwest, Office of Arid Land Studies, Univ. of Ariz., 29:7-15.

Miller, S.N., D.P. Guertin, K.H. Syed and D.C. Goodrich. 1999b. Using high resolution synthetic aperture radar for terrain mappi ng: Influences on hydrologic and geomorphic investigations. In: D.S. Olsen and L.P. Potyondy (Editors). 1999. Wildland Hydrology. American Water Associati on, Herndon, Virginia, TPS-99-3. p. 219-228.

Miltner, R.J., D. White, and C. Yoder. 2003. Th e biotic integrity of streams in urban and suburbanizing landscapes. Landscape and Urban Planning Vol. 69, p. 87-100.

Minckley, W.L. and G.K. Meffe . 1987. Differential selection by flooding in stream fish
communities of the arid Amer ican Southwest. Pages 93-104, In: W.J. Matthews and
D.C. Heins, eds. Community and Evolu tionary Ecology of North American Stream Fishes. Norman (OK): Univer sity of Oklahoma Press.

Moran, M.S., W.E. Emmerich, D.C. Goodrich, P. Heilman, C. Holifield Collins, T.O. Keefer,
M.A. Nearing, M.H. Nichols, K.G. Renard, R.L. Scott, J.R. Smith, J.J. Stone, C.L. Unkrich, J.K. Wong. 2008. Preface to specia l section on Fifty Years of Research and Data Collection: U.S. Department of Agriculture Walnut Gulch Experimental
Watershed. Water Resources Research , Vol. 44, W05S01,
doi:10.1029/2007WR006083.

Nadeau, T.L. and M.C. Rains. 2007. Hydrol ogical connectivity betw een headwater streams and downstream waters: How science can inform policy. Journal of the American
Water Resources Association , 43(1):118-133.

92Nanson, G.C. and J.C. Croke. 1992. A genetic classification of floodplains. Geomorphology

, 4 (1992) 459-486.

National Research Council. 2002. Riparian Areas : Functions and Strategies for Management. National Academy of Science, Committ ee on Riparian Zone Functioning and Strategies for Management, Water Science and Technology Board, Board on Environmental Studies and Toxicology, Divi sion on Earth and Life Studies, National Academy Press, Washington, D.C.

National Wildlife Federation (NWF). 2007. Streams. Online fact sheet, http://www.nwf.org/w ildlife/pdfs/Streams.pdf , accessed May 22, 2007.

Newman, B.D., E.R. Vivoni, and A.R. Groffman. 2006. Surf ace water-groundwater interactions in semiarid drainage s of the American southwest. Hydrologic Processes : v 20, p. 3371-3394.

Nichols, M.H. 2006. Stream Processes in Riparian Areas, Chapter 4 In: Understanding Arizona’s Riparian Areas. Univers ity of Arizona Cooperative Extension.

Nilsen, E.T., M.R. Sharifi, and P.W. Runde l. 1984. Comparative Water Relations of Phreatophytes in the Sonoran Desert of California. Ecology 65:767-778.

North, C. 2005. Drylands Rivers Researc h, Web site (last updated Feb. 16, 2005). University of Aberdeen, Scotland. http://www.abdn.ac.uk/~gmi196/DrylandRivers/

Ohmart, R.D., and B.W. Anderson. 1982. North American desert riparian ecosystems. In:
G.L. Bender, ed., Reference handbook on the deserts of North America, Greenwood Press, Westport, CT. p. 433-479.

Ohmart, R.D., and C.D. Zisner. 1993. Functions and Values of Riparian Habitat to Wildlife in Arizona, A Literature Review. Center for Environmental Studies, Arizona State University, Tempe, AZ. Submitted to Ariz ona Game and Fish Department, Contract
Number G300-25B.

Ohmart, R.D. 1995. Historical and present imp acts of livestock grazi ng on fish and wildlife resources in western ripa rian habitats. Pp 245-279, In: P.R. Krausman (ed.). Rangeland wildlife. The Society for Range Management, Denver, CO. 440 p.

Olsen, H. 1987. Ancient ephemeral stream de posits: a local terminal fan model from the
Bunter Sandstone Formation. In: L.E. Frostick, and I. Reid (eds.), Desert Sediments.
Blackwell Scientific Public ations, Oxford, p. 69-86.

Owens, M.K., R.K. Lyons and C.L. Alejandro. 2006. Rainfall partitioni ng within semiarid juniper communities: effects of event size and canopy cover. Hydrol. Process . 20.

93Parker, J.T.C. 2006. Post-Wildfire Sedimentation in Saguaro National Park, Rincon Mountain District, and Effects on Lowland Leopard Frog Habitat. U.S. Geological Survey Scientific Investigations Report 2006-5235, 35 pp.

Pelletier, J.D., and DeLong, S. 2004. Oscillati ons in arid alluvial-channel geometry. Geology, 32:8, p. 713-716.

Peterson, B.J., W.M. Wollheim, P. J. Mulholland, J.R. Webster, J.L. Meyer, J.L Tank, E. Marti, W.B. Bowden, H.M. Valett, A.E. Hershey, W.H McDowell, W.K. Dodds, S.K. Hamilton, S. Gregory, and D.D. Morrall. 2001. Control of nitrogen export from watersheds by headwater streams. Science , 292:86-90.

Phillips, F.M., J.F. Hogan and B.R. Scanlon. 2004. Introduction and Overview. In: Groundwater Recharge in a Desert Environm ent: The Southwestern United States. Hogan, J.F., F.M. Phillips, and B.R. Scanlon (eds.). Water Science and Applications Series, Vol. 9, American Geophys ical Union, Washington, DC. 294 p.

Pima County. 2000. Biological Stress Assessment, An Overview Discussion of Issues and Concerns. Sonoran Desert Conservation Plan Web site, Reports. http://www.co.pima.az.us/cmo/sdcp/reports/d9/008BIO.PDF

Pinay, G., J.C. Clement, and R.J. Naima n. 2002. Basic principles and ecological consequences of changing water regimes on nitrogen cycling fluvial systems. Environmental Management , 30(4): 481-491.

Pister, E. P. 1995. Fishes of the Ca lifornia Desert Conservation Area. In: The California
Desert: An Introduction to Natural Resources and Man’s Impact. Vol. II. Latting, J. and Rowlands, P.G. eds. Riverside, California: June Latting Books. 2 vols.

Poff, N.L, J.D. Allan, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D. Richter, .R.E. Sparks, and
J.C. Stromberg. 1997. The Natural Flow Re gime: A paradigm for river conservation
and restoration. Bioscience , Vol. 47, No. 11.

PRBO (Point Reyes Bird Observatory). 2007. Final Report: Potential Off-road Vehicle impacts on bird populations of Sonoran Dese rt Xeroriparian Habitats of Western Arizona. Point Reyes Bird Obse rvatory Conservation Science.

Powell, B.F., and R.J. Steidl. 2002. Habitat selection by riparian songbirds breeding in
southern Arizona. Journal of Wildlife Management 66(4):1096-1102.

Powell, D.M., R. Brazier, A. Parsons, J. Wa inwright, and M. Nic hols. 2007. Sediment transfer and storage in dr yland headwater streams. Geomorphology , 88:152-166.

Ragotzkie, K.E. and J.A. Bailey. 1991. De sert mule deer use of grazed and ungrazed
habitats. Journal of Range Management , 44(5), p. 487-490.

94Rassam, D.W., C.S. Fellow, R. De Hayr, H. Hunter, and P. Bloesch. 2006. The hydrology of riparian buffer zones; two case studies in an ephemeral and a perennial stream. Journal of Hydrology , 325(1-4):308-324.

Reid, I. and L.E. Frostick. 1989. Channel form, flows and sediments in deserts. Chapter 6
In: Arid Zone Geomorphology, D.S.G. Thomas (ed.), Belhaven Press, London. p.
117-135.

Reid, I. 1994. River landforms and sediment: evidence of climatic change. In: A.D. Abrahams and A.J. Parsons (Eds.), Ge omorphology of Desert Environments. Chapman & Hall, London, p. 571 592.

Reid, I. and J.B. Laronne. 1995. Bed load sedi ment transport in an ephemeral stream and a comparison with seasonal and perennial counterparts. Water Resources Research , v. 31, n. 3, p. 773-782.

Reid, I. and L.E. Frostick. 1997. Channel form, flows and sediments in deserts. In : Arid Zone Geomorphology: Process, Form, and Cha nge in Drylands, D.S.G. Thomas (ed.), Wiley, Chichester, p. 205-229.

Reid, I., J.B. Laronne, and D.M. Powell. 1998. Flash-flood and bedload dynamics of desert gravel-bed streams. Hydrological Processes , (12):543-557.

Remsen, J.V., Jr., K.H. Berry and E. Wessman. 1976. Twenty-ninth winter bird population study. No. 51. Joshua tree woodland II. Amer. Birds 30(6):1060-1061.

Renard, K.G. 1970. The hydrology of semi-ari d rangeland watersheds. U.S. Dept. of Agriculture, Agricultural Res earch Service. Pub. #ARS 41-162.

Renard, K.G. and E.M. Laursen. 1975. Dyna mic Behavior Model of Ephemeral Stream. Proceedings of the American Society of Civil Engineers, Vol. 101, No. HY5, Journal of the Hydraulics Division , May 1975.

Reynolds, Keith M. 1995. Definition of forest ecosystem health. Protection of Forest Health and Productivity Program. http://www.fsl.orst .edu/home/usfs/fhealth/helthdef.htm. Corvallis, Oregon: Pacific Northwest Forest and Range Experiment al Station, Oregon State University.

Richardson, J.S., R.E. Bilby and C.A. Bonda r. 2005. Organic matter dynamics in small
streams of the Pacific Northwest. Journal of th e Association 41(4): 921-934.

Rinne, J.N. and W.L. Minckley. 1991. Native Fishes of Arid Lands; A Dwindling Resource of the Desert Southwest. Gen. Tech. Rep. RM-206. Ft. Collins, CO. U.S.D.A. Forest Service, Rocky Mountain Forest an d Range Experiment Station. 45 p.

95Rodier, J.A. 1985. Aspects of arid zone hydrology. In: Facets of Hydrology, Volume ii, J.C. Rodda. Wiley, Chichester, p. 205-247.

Rosen, P.C. and Lowe, C.H. 1996. Ecology of the Amphibians and Reptiles at Organ Pipe Cactus National Monument, Arizona. T ech. Report #53. Cooperative Park Studies Unit, The University of Arizona, Tucson.

Rosen, P.C. 2005. Lowland Riparian Herpetofa unas: The San Pedro River in Southeastern
Arizona. In: Connecting Mountain Islands and Desert Seas: Biodiversity and management of the Madrean Archipelago II, May 11-15, 2004, Tucson, Arizona. USDA Forest Service Proceedings RMRS-P-36.

Rosen, P.C., W.R. Radke, and D.J. Caldwell. 2005. Herpetofauna of Lowland Bottomlands of Southeastern Arizona: A Comparison of Sites. In: Connecting Mountain Islands and Desert Seas: Biodiversity and manage ment of the Madrean Archipelago II, May 11-15, 2004, Tucson, Arizona. USDA Forest Service Proceedings RMRS-P-36.

Schneider, W.J. 1961. A note on the accuracy of drainage densities computed from topographic maps. Journal of Geophysical Research 66:3617-18.

Schueler, T. 1994. The importance of Imperv iousness. Watershed Protection Techniques 1 (3):100-111.

Schumm, S.A. and R. F. Hadl ey. 1957. Arroyos and the Semiarid Cycle of Erosion. American Journal of Science , Vol. 255, p. 161-174.

Schumm, S.A. 1961. Effect of sediment characteristics on erosion and deposition in ephemeral stream channels. US Geol ogical Survey, Professional Paper 598.

Schumm, S. A. and R.W. Lich ty. 1963. Channel widening and floodplain construction along
Cimarron River in southwestern Kansas . United States Geological Survey Professional Paper 352-D, p. 71-88

Scott, S.H. 2006. Predicting sediment trans port dynamics in ephemeral channels: A review of literature. Army Engineer Waterways Experiment Station, Vicksburg, MS, Coastal Hydraulics Lab, Report Number : ERDC/CHL-CHETN-VII-6, 9 p.

Semmens, D.J. 2004. Geomorphic modeling and routing improvements for GIS-based watershed assessment in arid regions. Doct oral Dissertation, Un iversity of Arizona, 218 p.

Sharp, A. L. and K.E. Saxton. 1962. Transmissi on losses in a mature stream valley. Journal of the Hydraulic Division, Proceedings of the American Society of Civil Engineers, 88, p. 121-142.

96Shaw, J.R. and D.J. Cooper. 2008. Linkages am ong watersheds, stream reaches, and riparian vegetation in dryland ephemeral stream networks. Journal of Hydrology (2008) 350,

p. 68 – 82.

Shreve, F. and I.L. Wiggins. 1964. Vegetation and flora of the Sonoran Desert. Vol. 1. Stanford Univ. Press, Stanford, Calif.

Siekert, R.R., Q.D. Skinner, M.A. Smit h, J.L Dodd, and J.D. Rodgers. 1985. Channel Response of an Ephemeral Stream in Wyoming to Selected Grazing Treatments. In: Riparian Ecosystems and Their Manageme nt: Reconciling Conflicting Uses. First North American Riparian Conference, April 16-18, 1985. U.S.D.A. Forest Service General Technical Report RM-120, p. 276-278.

Skagen, S.K., C.P. Melcher, W.H. Howe, a nd F.L. Knopf. 1998. Comparative Use of Riparian Corridors and Oa ses by Migrating Birds in Southeastern Arizona. Conservation Biology 12:896-909

Skagen, S.K., J.F. Kelly, C. Van Riper, R.L. Hu tto, D.M. Finch, D.J. Krueper, C.P. Melcher. 2005. Geography of Spring Landbird Migra tion through Riparian Habitats in
Southwestern North America. The Condor 107:212–227.

Smith, S.D., C.A. Herr, K.L. Leary, and J.M. Pi orkowski. 1995. Soil-plant water relations in a Mojave Desert mixed shrub community - a comparison of three geomorphic
surfaces. Journal of Arid Environments 29 (3):339-351.

Sonoran Institute. 2007. Sustainable Water Ma nagement: Guidelines for Meeting the Needs of People and Nature in the Arid West. http://sonoran.org/index. php?option=com_content&tas k=view&id=174&Itemid=204

Sponseller, R.A. and S.G. Fisher. 2006. Drai nage size, stream intermittency, and ecosystem function in a Sonoran Desert landscape. Ecosystems 9:344-356.

Stanley, E.H., S.G. Fisher and N.B. Grimm. 1997. Ecosystem Expansion and Contraction in
Streams. Bioscience , Vol. 47, No. 7.

Stebbins, R.C. 1995. Desert Reptiles. In: The California Desert: An Introduction to Natural Resources and Man’s Impact. Vol. II. Latti ng, J. and Rowlands, P.G., eds. Riverside, California: June Latting Books. 2 vols.

Stevens, L.E., B.T. Brown, J.M. Simpson and R.R. Johnson. 1977. In: Importance, Preservation and Management of Riparian Habitat: A Symposium, July 9, 1977. U.S.
Forest Service General T echnical Report RM-43, p. 156-164. Stromberg J.C., A.F. Hazelton, M.S. White, J. M. White and R.A. Fischer. Ephemeral wetlands on ephemeral reaches: temporal pa tterns of vegetation development along a spatially intermittent river. Wetlands (in press).

97Stromberg, J.C., R. Tiller and B. Richter. 1996. Effects of Groundwater Decline on Riparian Vegetation of Semiarid Regions : The San Pedro, Arizona. Ecological Applications

, Vol. 6, No. 1. (Feb., 1996), p. 113-131.

Stromberg, J.C. 2001. Restoration of ripari an vegetation in the south-western USA: importance of flow regimes and fluvial dynamism. Journal of Arid Environments

, 49:17-34.

Stromberg, J.C. 2006. Seasonal reversals of upland-riparian diversity gradients in the
Sonoran Desert. Diversity and Distributions, School of Life Sciences, Arizona State University, Box 874501, Tempe, AZ 85287-4501, USA.

Stromberg, J.C., S.J. Lite, T.J. Rychener, L.R. Levick, M.D. Dixon and J.M. Watts. 2006. Status of the Riparian Ecosystem in the U pper San Pedro River, Arizona: Application
of an Assessment Model. Environmental Monitoring and Assessment (2006)
115:145–173.

Stromberg, J.C., V.B. Beauchamp, M.D. Dixon, S.J. Lite and C. Paradzick. 2007. Importance of low-flow and high-flow char acteristics to restoration of riparian vegetation along rivers in arid south-western United States. Freshwater Biology (2007) 52, p. 651–679.

Styles, P., S. Toon, E. Thomas and M. Skittral l. 2006. Microgravity as a tool for the detection, characterization and predicti on of geohazard posed by abandoned mining cavities. First Break, Vol. 24. (http://www.firstbrea k.org/files/Technart3_m icro.pdf?PHPSESSID=53627986abd04a 1f703c54896d1159d9).

Thompson, R.S. 1997. Global Climate Cha nge: The 1995 Report by Intergovernmental Panel on Climate Change. USGS Web Conf erence: Impacts of Climate Change and Land Use on the Southwestern United States, July 7 - 25, 1997. Accessed from Web
site: http://geochange.er.usgs.gov/sw /changes/anthropogenic/population/ , May 9,
2008.

Thoms, M.C., M. Southwell and H.M. McGi nness. 2005. Floodplain-river ecosystems: fragmentation and water re sources development. Geomorphology 71(1-2):126-138.

Thornes, J.B. 1977. Channel Changes in Ephemeral Streams: Observations, Problems, and
Models. In: River Channel Changes. K.J. Gr egory, ed. John Wiley & Sons.

Thornes, J.B. 1994. Catchment and channel hydrology. In : A.D. Abrahams and A.J. Parsons (Eds.) Geomorphology of Desert Environm ents. Chapman and Hall, London, p. 257

- 287.

98Thornwaite, C.W., C.F.S. Sharpe, and E.F. Do sch. 1942. Climate and accelerated erosion in the arid and semiarid Southwest, with speci al reference to the Polacca Wash drainage basin, Arizona. U.S. Department of Agriculture Technical Bulletin 808.

Ticer, L.L., R.A. Okenfels, and J.C. Devos, Jr. 1998. Habitat use and activity patterns of urban-dwelling javelina. Urban Ecosystems , 2:141-151.

Tomoff, C.S. 1977. The spring avifauna of the Colorado Desert of southeastern California.
U.S. Dep’t. Interior, Bureau of Land Ma nagement, Calif. Desert Plan Program, Riverside, Calif. Rept. Contr. No. CA-060-CT7-987.

Tomoff, C.S. 1979a. Thirty-fir st winter bird population study. No. 58. Blue paloverde

ironwood-smoketree desert ri parian woodland I. Amer. Birds 33(1):35-36.

Tomoff, C.S. 1979b. Thirty-fir st winter bird population study. No. 59. Blue paloverde

ironwood-mixed shrub and succulent desert wash. Amer. Bird s 33(1):41.

Tomoff, C.S. 1979c. Thirty-fir st winter bird population study. No. 77. Blue paloverde

ironwood-smoketree desert ri parian woodland II. Amer. Bird s 33(1):36.

Tooth, S. 2000a. Process, form and change in dryland rivers: a review of recent research.
Earth-Science Reviews , 51 (2000), p. 67–107.

Tooth, S. 2000b. Downstream changes in dryland river channels: the Nort hern Plains of arid
central Australia. Geomorphology, 34:33-54.

Tooth, S. and G.C. Nanson. 2000. The role of vegetation in the formation of anabranching channels in an ephemeral river, Northe rn plains, arid central Australia. Hydrological
Processes , 14, p. 3099-3117.

URS Corporation. 2006. Aquatic Communities of Ephemeral Stream Ecosystems. Arid West Water Quality Research Project. Prepared for Pima County Wastewater Management Department, with funding by EPA Region IX, Assistance Agreement X

97952101.

U.S. Army Corps of Engineers. 1958. Fr equency of New England floods: Sacramento District, Research Note 1, 4 p.

U.S. Department of Agriculture (USDA), Forest Service. 1988. Mana gement of Amphibians, Reptiles, and Small Mammals in North America. Proceedings of the Symposium, July 19-21, 1988. U.S. Forest Service General Technical Report RM-166.

U.S. Department of Agriculture (USDA), Forest Service. 2002. Management and Techniques for Riparian Restoration, Roads Field Guide, Vol. 1. General Technical Report RMRS-GTR-1-2, Vol. 1. Rocky Mountai n Research Station, Fort Collins, CO.

99U.S. Environmental Protection Agency (USEPA) and U.S. Army Corps of Engineers. 2007.
Guidance on Clean Water Act Jurisdicti on Following the U.S. Supreme Court’s
Decision in Rapanos v. United Stat es & Carabell v. United States. http://www.epa.gov/owow/wetla nds/pdf/RapanosGuidance6507.pdf

U.S. Environmental Protection Agency (EPA). 2005. Letter from Benjamin H. Grumbles, Assistant Administrator, EPA, to Ms. Jea nne Christie, Executive Director, Association of State Wetland Managers, dated Jan. 9, 2005.

U.S. Fish & Wildlife Services (USFWS). 1993. Riparian Issue Paper: Lack of Federal
Section 404 Clean Water Act Protection of Ri parian Areas in the Arid and Semi-Arid Southwest. Arizona Ecological Services Office, U.S. Fish & Wildlife Service.

U.S. Geological Survey (USGS). 1997. Some perspectives on Comate and Floods in the
Southwestern U.S. USGS Web Conferen ce: Impacts of Climate Change and Land Use on the Southwestern United States, July 7 - 25, 1997. Accessed from Web site: http://geochange.er .usgs.gov/sw/changes/anthropogenic/population/ , May 9, 2008.

U.S. Geological Survey (USGS). 2006. National Hydrography Dataset Web site, http://nhd.usgs.gov/index.html

.

University of Connecticut. 1994. Impacts of Development on Waterways, NEMO Program Fact Sheet #3. University of Conne cticut, Cooperative Extension System. http://nemo.uconn.edu/tools/publications /fact_sheets/nem o_fact_sheet_3_s.pdf

University of Connecticut. 1999. Carving up the Landscape, NEMO Program Fact Sheet #10. University of Connecticut, Cooperative Extension System. http://nemo.uconn.edu/tools/publications /fact_sheets/nem o_fact_sheet_10_s.pdf

Valett, H.M., S.G. fisher, N.B. Grimm and P. Camill. 1994. Vertical hydrologic exchange and ecological stability of a desert stream ecosystem. Ecology , Vol. 75, No. 2, p. 548

- 560.

Valett, H.M., M.A. Baker, J.A. Morrice, C.S. Crawford, M.C. Molles, Jr., C.N. Dahn, D.L. Moyer, J.R. Thibault, and L.M. Ellis. 2005. Biogeochemical and metabolic responses to the flood pulse in a semiarid floodplain. Ecology , 86(1), p. 220-234.

Van Devender, T.R. 2002. The Sonoran Dese rt Tortoise. Natural History, Biology, and Conservation. University of Arizona Press, Tucson.

Van Riper, III, C. and K.L. Cole. 2004. Th e Colorado Plateau: Cultural, Biological, and Physical Research, University of Arizona Press, Tucson, 279 p.

Virginia, R.A., W.M. Jarrell, W.G. Whitford, and D.W. Freckman. 1992. Soil biota and soil properties associated with the surf ace rooting zone of mesquite ( Prosopis glandulosa ) in historical and recently desertified habitats. Biology and Fertility of Soils 14:90-98.

100Wagner, F.H. 1978. Livestock grazing a nd the livestock industry. Pp. 121-145, In: H.P. Brokaw (ed.) Wildlife and America. Counc il on Environmental Quality, U.S. Gov. Print. Office. 532 p.

Wakelin-King, G.A. and J.A. Webb. 2007. Thres hold-dominated fluvial styles in an aridzone mud-aggregate river: The uplands of Fowlers Creek, Australia. Geomorphology 85 (2007) p. 114–127.

Walker, D., C. Goforth and S. Rector. 2005. An exploration of Nu trient and Community Variables in Effluent Dependent Stream s in Arizona. EPA Grant Number X-828014

- 01-01, awaiting publication.

Ward, J.S. and Associates. 1973. Environm ental Protection Study, Pantano Wash, South Tucson and Canada del Oro Areas, Tucson, Ar izona. For the Pima Association of Governments. 119 p.

Warren, P.L. and L.S. Anderson. 1985. Gradie nt analysis of a Sonoran Desert wash. In: Riparian Ecosystems and Their Manageme nt: Reconciling Conflicting Uses. First North American Riparian Conference, April 16-18, 1985. U.S.D.A. Forest Service General Technical Report RM-120, p. 150-155.

Waylen, P. and M. Woo. 1982. Prediction of annual floods generated by mixed processes:
Water Resources Research , v. 18, no. 4, p. 1283-1286.

Webb, R.H. and J.L. Betancourt. 1992. Climatic variability and flood frequency of the Santa Cruz River, Pima County, Arizona. U.S. Geological Survey Water-Supply Paper, 2379, 40 p.

Westerhoff, P. and D. Anning. 2000. Concentrati ons and characteristics of organic carbon in
surface water in Arizona: influence of urbanization. Journal of Hydrology , 236(3

- 4):202-222.

Wetzel, R.G. 2006. Wetland Ecosystem Processes, pp. 285-312, In: D. P. Batzer and R. R. Sharitz, eds. Ecology of Freshwater and Estuarine Wetlands . University of California Press, Berkeley.

Whiles, M.R. and B.S. Goldowitz. 2005. Macroi nvertebrate communities in central Platte River wetlands: patterns across a hydrologic gradient. Wetlands , 25(2), p. 462–472.

Whitford, W.G. 2002. Ecology of Desert Systems . Academic Press, San Diego, CA. 343 p.

Whiting, L.R. 2000. The Ecological Paradigm. Retrieved from the Ohio State University
Web site: http://ohioline.osu.edu/paradigm/index.html

Williams, D.D. 2006. The Biology of Temporary Waters . Oxford University Press, UK. 337

p.

101Wilson, M.A., R. Costanza, and A. Troy. 2004. Th e EcoValue Project. Retrieved from the University of Vermont EcoValue Web site: http://ecovalue.uvm.edu

Wipfli, M.S. 2005. Trophic Linkages Between Headwater Forests and downstream Fish Habitats: Implications for Fo rest and fish Management. Landscape and Urban Planning , 72:205-213.

Wolman, M.G. and R. Gerson. 1978. Relative scal es of time and effectiveness of climate in watershed geomorphology. Earth Surface Processes , v. 3, p. 189-208.

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