Colorado River System Research Articles

Salt Mineralogy of Las Vegas Wash, Nevada

Las Vegas Wash drains the Las Vegas Basin in Nevada by capturing a series of tributaries and ending in Lake Mead and is being developed into an urban wetland. The soils are part of the Land series and contain high concentrations of pedogenic salts because of local sulfate‐rich parent materials and high evapotranspiration rates. These salts cause damage to property, affect plant communities in the wetlands, and contribute to salinity in the Colorado River System. To gain a better understanding of these salts, two soil profiles were described. Whole soil samples were analyzed for ammonium acetate–extractable and water‐soluble elemental analyses, cation exchange capacity (CEC), pH, electrical conductivity, and particle size. Salt minerals were analyzed using scanning electron microscope (SEM)/energy dispersive spectrometer (EDS) and x‐ray diffraction (XRD). Results of XRD analyses indicate hexahydrite, bloedite, mirabilite, gypsum, thenardite, halite, vivianite, and sepiolite. SEM/EDS analyses found bloedite, eugsterite, halite, hexahydrite, gypsum, thenardite, and possibly kainite. Gypsum occurs at the surface and in all subsurface horizons. The more soluble salts occur at the surface and in two subsurface horizons. This study documents the third occurrence of eugsterite in the USA and is the first study to document salts other than gypsum forming snowball morphology. We interpret the subsurface zones of soluble salts to represent relict water tables where capillary action, combined with subsurface evaporation, has concentrated Na‐Mg sulfates and halite. These relict water tables may represent previous high‐level water tables that were lowered because of increased urbanization resulting in flooding, erosion, and incision of the Las Vegas Wash.

Proximate causes of sexual size dimorphism in Colorado pikeminnow, a long‐lived cyprinid

Evidence for sexual size dimorphism (SSD) and its possible causes were examined in the endangered Colorado pikeminnow Ptychocheilus lucius, a large, piscivorous, cyprinid endemic to the Colorado River system of North America. Individuals representing 18–24% of the upper Colorado River population were captured, measured, sexed and released in 1999 and 2000. Differing male and female total length‐(LT) frequency distributions revealed SSD with females having greater mean and maximum sizes than males. Although both sexes exhibit indeterminate post‐maturity growth, growth trajectories differed. The point of trajectory divergence was not established, but slowed male growth might coincide with the onset of maturation. Differing growth rate was the dominant proximate cause of SSD, accounting for an estimated 61% of the observed difference in mean adult LT. The degree of SSD in adults, however, was also related to two other factors. Evidence suggests males become sexually active at a smaller size and earlier age than females; a 2 year difference, suggested here, accounted for an estimated 12% of the between‐sex difference in mean adult LT. Temporal shifts in gender‐specific survival accounted for an additional 27% of the observed between‐sex difference in mean adult LT. Estimated age distributions indicated a higher number of older females than older males and more younger males than younger females in the population during the period of sampling. Dissimilarity of age distributions was an unexpected result because the male : female population sex ratio was 1 : 1 and estimates of long‐term annual survival for adult males and females were equal (88%). Future assessments of SSD in this population are apt to vary depending on the prior history of short‐term gender‐specific survival. Without recognizing SSD, non‐gender‐specific growth curves overestimate mean age of adult females and underestimate mean age of adult males of given LT. Assuming age 8 years for first reproduction in males and age 10 years for females, the adult male : female ratio was estimated as 1·1 : 1 and mean adult age, or generation time, was estimated as 16·4 years for males and 18·4 years for females.

Experimental infection of the endangered bonytail chub (Gila elegans) with the Asian fish tapeworm (Bothriocephalus acheilognathi): impacts on survival, growth, and condition

Bothriocephalus acheilognathi Yamaguti, 1934, a tapeworm known to be pathogenic to some fish species, has become established in the endangered humpback chub ( Gila cypha Miller, 1964) in Grand Canyon, USA, following the tapeworm’s introduction into the Colorado River system. The potential impact of this tapeworm on humpback chub was studied by exposing the closely related bonytail chub ( Gila elegans Baird and Girard, 1853) to the parasite under a range of conditions that included potential stressors of humpback chub in their natal waters, such as abrupt temperature change and a limited food base. Survival of infected fish under low food rations was considerably lower than that of control fish, and mortality of infected fish began 20 days earlier. Growth of infected fish was significantly reduced, and negative changes in health condition indices were found. No significant negative impacts were revealed from the synergistic effects between temperature shock and infection. Bothriocephalus acheilognathi does present a potential threat to humpback chub in Grand Canyon and should be considered, along with conventional concerns involving altered flow regimes and predation, when management decisions are made concerning conservation of this endangered species.

Limitations to Natural Bioremediation of Perchlorate in a Contaminated Site

ABSTRACT Perchlorate (ClO4 −) has been detected in many drinking water supplies in the United States, including the Las Vegas Wash and Lake Mead, Nevada. These locations are highly contaminated and contribute perchlorate to Lake Mead and the Colorado River system. Essential elements for perchlorate bioremediation at these locations were examined, including the presence of perchlorate-reducing bacteria (PRB), sufficient electron donors, occurrence of competing electron acceptors, and ability of PRB to utilize a variety of electron donors. Enumeration of PRB was performed anoxically using most probable number (MPN). Values ranged from ≤20 to 230 PRB/100 ml or ≤20 to ≥ 1.6× 105 PRB/g for Lake Mead water samples and Las Vegas Wash sediments, respectively. 16S rRNA sequences revealed that isolates were γ -proteobacteria, Aeromonas, Dechlorosoma, Rahnella and Shewanella. A screening of potential electron donors using BIOLOGTM demonstrated that all isolates were capable of metabolic versatility. Measurements of total organic carbon (TOC), nitrate and dissolved oxygen (DO) indicated limited presence of electron donor at all sites, whereas the electron acceptors varied throughout the Wash and Lake Mead. The persistence of perchlorate in the sites is attributed to lack of available electron donor and/or the presence of competing electron acceptors. A location has been identified where perchlorate biodegradation could be implemented thereby halting the transport of perchlorate to Lake Mead and the Colorado River.

Neglected Taxonomy of Rare Desert Fishes: Congruent Evidence for Two Species of Leatherside Chub

Conservation biologists rely heavily on taxonomy to set the scope for biological monitoring and recovery planning of rare or threatened species. Yet, taxonomic boundaries are seldom evaluated as falsifiable hypotheses that can be statistically tested. Here, we examine species boundaries in leatherside chub (Teleostei, Cyprinidae), an imperiled desert fish native to the Bonneville Basin and upper Snake River drainages of western North America. Recent molecular data hint that this fish could be composed of two distinct taxa that are geographically separated into northern and southern species. To formally test this hypothesis, we evaluated leatherside chub using several different categories of species concepts, including criteria dependent on phylogenetic, morphological, and ecological data. We found that leatherside chub is composed of two reciprocally monophyletic clades (candidate species) characterized by numerous fixed genetic differences for both mitochondrial and nuclear DNA markers; mtDNA sequence divergence between the two clades approached 8%. The candidate species also showed significant differences in cranial shape, revealed by morphometric analysis. Finally, controlled growth and foraging experiments using representative populations from each clade show that candidate species appear to be locally adapted to the thermal environments where they now occur. Combined, these three lines of evidence support the hypothesis that leatherside chub is composed of two species. Moreover, all lines of evidence place these two species within the genusLepidomeda, a group consisting of three additional species of endangered spinedace fishes, and one extinct species, all native to the Colorado River system. Hence, we elevate the two clades of leatherside chub to distinct species status (Lepidomeda copeiin the north andL. aliciaein the south), and argue that each warrants independent conservation and recovery action.

Fish humps in two Colorado River fishes: a morphological response to cyprinid predation?

Extant fishes endemic to the upper Colorado River of the American southwest include only cyprinids and catostomids. A curious attribute in species of both groups is the presence of a large nuchal hump. Largest cyprinid humps occur in humpback chub, Gila cypha, and largest catostomid humps occur in razorback sucker, Xyrauchen texanus. Several authors have suggested the humps confer a hydrodynamic advantage to life in fast flow, but this premise has not been confirmed with experimental work. To test the role of humps in Colorado River fishes, we subjected whole-body casts of preserved specimens with humps and with humps removed to controlled flows in an experimental tank. These tests confirmed that humps increased drag coefficients for X. texanus and G. cypha with no additional lift component. High energetic costs of locomotion and position-holding with a large hump, and the additional metabolic expense of forming large humps, suggest that the humps are not relict structures. Instead, we argue that these large humps represent convergent evolution prompted by predation from a cyprinid piscivore. Colorado pikeminnow, Ptychocheilus lucius, top piscivore in the Colorado River system, is the only native fish capable of consuming large X. texanus and G. cypha, and it also is sympatric with them. However, lack of jaw teeth and a relatively small jaw gape limit the maximum prey size that P. lucius can consume. Based on gape size, about 55% of X. texanus and 71% of G. cypha could be consumed by even the largest P. lucius. However, vulnerability would increase to 73 and 83% respectively if these species did not have humps. Coevolution tends to favor predator defense mechanisms in prey most vulnerable to such a voracious predator. Development of a large nuchal hump provides a deep body that is difficult or impossible for P. lucius to ingest.

Debris-fan reworking during low-magnitude floods in the Green River canyons of the eastern Uinta Mountains, Colorado and Utah

The magnitude and frequency of tributary debris flows and the historical range of main- stem river discharges are the main factors that create and modify rapids in the Colorado River system. Monitoring of two recently aggraded debris fans in the Green River canyons of the eastern Uinta Mountains shows that main-stem floods with magnitudes between 40% and 75% of the predam 2 yr flood cause significant reworking of fan deposits. Cutbanks formed at fan margins during both small and large flows, indicating that lateral bank erosion is an important reworking mechanism. Armoring of the debris-fan surface limited the degree of reworking by successive floods, even when subsequent flood magnitudes were similar to those that caused significant reworking. Peak discharges increased the width of the reworked zone, decreased fan constrictions, and lowered the water-surface elevation of the ponded backwater. Contrary to predam geomorphic evidence, monitoring indicated that eroded material from recently aggraded debris fans was deposited in bars adjacent to the downstream parts of both fans. This change in the organization of the fan-eddy complex has the potential to alter the location of recirculating eddies and associated areas of fine-grained sediment deposition and storage.

BENTHIC COMMUNITY STRUCTURE OF THE GREEN AND COLORADO RIVERS THROUGH CANYONLANDS NATIONAL PARK, UTAH, USA

We sampled the aquatic benthos at 6 remote sites on the Colorado and Green rivers through Canyonlands National Park, Utah, USA. This study provides the first published description of benthic standing mass, invertebrate community composition, and primary carbon source for this portion of the Colorado River system. High suspended sediment concentrations prohibited growth of primary producers. The primary carbon source for benthic invertebrates was terrestrial organic matter. The invertebrate community was composed of 49 taxa, mostly mayflies, caddisflies, and diptera, which were dominated by filterer/collector species. A smaller portion of the community was made up of predatory stoneflies and odonates. Standing mass of invertebrates on cobble substrates within a given site was stable over the multiyear sample period (1993 through 1996) and was comparable with other southwestern streams (overall mean = 0.41 g/m2 ash-free dry mass ± 0.01 SE). Invertebrate standing mass at each site was controlled by the availability of primary carbon. Primary carbon availability was controlled by supply to the site and retention within the site. Both aspects might be influenced by anthropogenic alteration of the river basin and discharge patterns upstream of the study site.

USDOI's Allocation of Colorado River Water Must Include PPR Deliveries

The US Court of Appeals for the Ninth Circuit ruled that the US Department of Interior (USDOI) is obligated by federal statute to use water from the Colorado River system to supply holders of present perfected rights (PPRs). PPRs are those rights to the system's water that: existed as of June 25, 1929; were acquired under state law; and, have been put to beneficial use.

Climate extremes and adaptive management on the Colorado River: Lessons from the 1997–1998 ENSO event

The Colorado River system exhibits the characteristics of a heavily over-allocated or ‘closing water system’. In such systems, development of mechanisms to allow resource users to acknowledge interdependence and to engage in negotiations and agreements becomes necessary. Recently, after a decade of deliberations and environmental assessments, the Glen Canyon Dam Adaptive Management Program (GCDAMP) was established to monitor and analyze the effects of dam operations on the Grand Canyon ecosystem and recommend adjustments intended to preserve and enhance downstream physical, cultural and environmental values. The Glen Canyon Dam effectively separates the Colorado into its lower and upper basins. Dam operations and adaptive management decisions are strongly influenced by variations in regional climate. This paper focuses on the management of extreme climatic events within the Glen and Grand Canyon Region of the Colorado River. It illustrates how past events (both societal and physical) condition management flexibility and receptivity to new information. The types of climatic information and their appropriate entry points in the annual cycle of information gathering and decision-making (the ‘hydro-climatic decision calendar’) for dam operations and the adaptive management program are identified. The study then describes how the recently implemented program, lessons from past events, and new climate information on the Colorado River Basin, facilitated responses during the major El Niño Southern Oscillation (ENSO) event of 1997–1998. Recommendations are made for engaging researchers and practitioners in the effective use of climatic information in similar settings where the decision stakes are complex and the system uncertainty is large.

The Colorado River System and Neogene Sedimentary Formations along Its Course: Apparent Sr Isotopic Connections

Abstract Water samples from the Colorado River and its tributaries provide a present‐day 87Sr/86Sr profile of the Colorado River system from Moab, Utah, down to the Gulf of California. 87Sr/86Sr values around 0.7097 are prevalent above the Grand Canyon, and values of 0.7103 or higher are observed below the Grand Canyon. 87Sr/86Sr values close to the seawater ratio of 0.70917 are seen near the Colorado River delta. All tributaries except the Green River contribute Sr that is lower in 87Sr/86Sr than the main Colorado. The cause of the increase in 87Sr/86Sr through the Grand Canyon and Lake Mead area has not been found. Samples of carbonate and gypsum from the lower lacustrine member of the mid‐Miocene to Pliocene Bidahochi Formation in northern Arizona yield average 87Sr/86Sr of 0.70972. This agrees quite well with weighted averages of present‐day Colorado system rivers that flow into the area, which can be computed at 0.70972 or 0.70984, depending on assumptions. This correspondence argues that the present...

Nonnative Fish Control and Endangered Fish Recovery: Lessons from the Colorado River

Native freshwater fish populations are declining in North America and more than 100 fishes are federally listed as threatened or endangered. The Colorado River system in the southwestern United States has been especially affected. Most of its native fish populations are in decline, including four “big river” fishes, Colorado pikeminnow (Ptychocheilus lucius), humpback chub (Gila cypha), bonytail (G. elegans), and razorback sucker (Xyrauchen texanus), whose populations have declined so precipitously that they are endangered. Physical habitat alteration caused native fish declines, but nonnative fishes pose a more serious threat to native fishes than previously thought. Nonnative fish control measures needed in the upper Colorado River system, identified in part by a workshop of experts, include: (1) preventing movements of game fishes out of impoundments and curtailing future stockings, (2) reducing numbers of small, nonnative cyprinids in shoreline habitat used as rearing areas by young native fishes, and (3) increasing the harvest of channel catfish (Ictalurus punctatus) and common carp (Cyprinus carpio) in the mainstream. Large-scale implementation of these nonnative fish control measures has proven difficult. Recovery efforts have identified the need for developing nonnative fish control strategies and testing methodologies, but no solutions have emerged. A holistic approach, including ecosystem recovery plans, should be used in systems where more than one species share common problems, such as interactions with nonnative fishes. Nonnative fish interactions should be suspected as a potential cause of declining native fish populations.

Operation of the Colorado River System

In this paper, I will present my personal views about the operation of the Colorado River system and why I believe the operational philosophy of the river is working. I must stress that I do not represent the official views of the federal or state organizations involved in the operation of the river. Rather I am an academic who has been fortunate to work on various efforts to model the operation of the Colorado River for almost 20 years. During that time, I have worked with representatives of both state and federal agencies in the basin. I often teach about the Colorado River system in my graduate classes at Colorado State University and I feel this basin is an excellent case study for understanding complex operational issues of large river basins. It is an example of how federal and state governments have balanced competing pressures and demand and developed a shared operational process. The paper is organized to begin with some history of the operational legislation for the river. The current shared governance operational system is directly related to this legislation, as well as, the underlying political system of the USA. It can only be objectively evaluated within that context. Next, the paper will discuss the operational philosophy of the system and describe how the system is administered. It will discuss who makes the operational decisions, what is decided and when are these decisions made. Then the scientific tools used to support the operational process will be described. It is important to appreciate the value of reliable, scientifically based information for such a complex, contentious basin. Finally, I will summarize the experiences to date, look ahead to future challenges facing the river, and provide my suggestions for the application of shared governance operations to other rivers, such as the Han River Basin. I have included a reference section that provides references to materials available on the Internet, so that more detailed information can easily be retrieved by interested parties.

Hypothesis of Historical Effects From Selenium on Endangered Fish in the Colorado River Basin

Anthropogenic selenium contamination of aquatic ecosystems was first associated with cooling reservoirs of coal-fired power plants in the late 1970s, and later with drainage water from agricultural irrigation activities in the 1980s. In the 1990s, selenium contamination has been raised as a concern in the recovery of currently endangered fish in the Colorado River system. Widespread contamination from seleniferous drain waters from agriculture has been documented in the upper and lower Colorado River basins. Historically, irrigation started in the upper Colorado River basin in the late 1880s. In the 1930s, selenium concentrations in various drains, tributaries, and major rivers in the upper and lower Colorado River basins were in the 100 s and 1000 s of µg/L. Native fish inhabiting large rivers such as the Colorado pikeminnow and razorback sucker were abundant before 1890, but became rare after 1910 to 1920, before the influence of mainstem reservoirs in the upper and lower Colorado River. A hypothesis is presented that selenium contamination of the tributaries and major rivers of the Colorado River basin in the 1890 to 1910 period caused the decline of the endangered fish and continues to inhibit their recovery.

Cryopreservation of Sperm of the Endangered Razorback Sucker

Abstract The endangered razorback sucker Xyrauchen texanus is endemic to the Colorado River system in western North America and is threatened with extinction because of limited recruitment. To assist in management and recovery efforts, we developed methods for the cryopreservation of sperm, evaluated the influence of various factors on motility of thawed sperm, and examined the effect on fertilization of cooling rate and the addition of caffeine. Sperm samples cryopreserved with 10% methanol (MeOH) had significantly higher postthaw motility than did samples preserved with 5% or 20% MeOH or with 5% or 10% dimethyl sulfoxide, N, N-dimethylacetamide, glycerol, propylene glycol, or ethylene glycol. Sperm samples cryopreserved in 0.5-mL and 2.5-mL straws had significantly higher postthaw motility than did samples cryopreserved in 0.25-mL straws. Exposure to 10% MeOH for up to 30 min did not significantly influence sperm motility before freezing or after thawing. Cooling rate (−21°C/min or −91°C/min) did not si...

IMPACTS OF A SEVERE SUSTAINED DROUGHT ON COLORADO RWER WATER RESOURCES1

ABSTRACT: The impacts of a severe sustained drought on Colorado River system water resources were investigated by simulating the physical and institutional constraints within the Colorado River Basin and testing the response of the system to different hydrologic scenarios. Simulations using Hydrosphere's Colorado River Model compared a 38‐year severe sustained drought derived from 500 years of reconstructed streamflows for the Colorado River basin with a 38‐year streamflow trace extracted from the recent historic record. The impacts of the severe drought on streamflows, water allocation, storage, hydropower generation, and salinity were assessed. Estimated deliveries to consumptive uses in the Upper Basin states of Colorado, Utah, Wyoming, New Mexico, and northern Arizona were heavily affected by the severe drought, while the Lower Basin states of California, Nevada, and Arizona suffered only slight shortages. Upper Basin reservoirs and streamflows were also more heavily affected than those in the Lower Basin by the severe drought. System‐wide, total hydropower generation was 84 percent less in the drought scenario than in the historical stream‐flow scenario. Annual, flow‐weighted salinity below Lake Mead exceeded 1200 ppm for six years during the deepest portion of the severe drought. The salinity levels in the historical hydrology scenario never exceeded 1100 ppm.

The source hydrology of severe sustained drought in the southwestern United States

This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the southwestern United States. The focus is on the Colorado River basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River as well as the four major rivers in northern California. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River are defined on the basis of annual flow at Lees Ferry. Possible spatial manifestations of the Colorado River drought scenarios for input into a Colorado River system simulation model are developed by disaggregating the Lees Ferry flow to monthly flows at 29 source locations required by the model. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models applied to both the Colorado River basin and the combined flow in four major California rivers. The risk of severe sustained drought occurring concurrently in the Colorado River basin and California is also assessed.

Evaluation of an Introduction of Colorado Squawfish in a Main Stem Impoundment on the White River, Colorado

We evaluated endangered Colorado squawfish (Ptychocheilus lucius) as a potential sportfish in Kenney Reservoir from 1988 to 1990 as part of a mitigation program for the construction of Taylor Draw Dam which impounded Kenney Reservoir on the White River, Colorado. A total of 96,597 fingerling squawfish were stocked into Kenney Reservoir from 1988 to 1990. These fish quickly dispersed throughout the reservoir, and downstream into the White River. A small number moved upstream in 1990. A large proportion, associated with high discharge following stocking, exited over the spillway, and continued to move downstream. Catch-per-unit-effort declined drastically in both the river and the reservoir in 1989 and 1990. The rapid disappearance of squawfish throughout the study area was attributed to flow regime, movement patterns, mortality, sampling ineffectiveness or a combination of these. Telemetry observations of six adult Colorado squawfish revealed no difference in behavior attributable to wild or hatchery origin. Stocking squawfish in Kenney Reservoir did not appear to be an effective mitigation procedure for habitat loss due to impoundment. The Colorado squawfish Ptychocheilus lucius is endemic to the Colorado River Basin. Once abundant, its decline has necessitated full protection as an endangered species (Federal Register, Vol. 39[3]:1175, 1974). The historical range of this large cyprinid extended from Northern Sonora, Mexico to Green River, Wyoming, including much of the mainstem Colorado River and most of its larger tributaries (Colorado River Fishes Recovery Team, 1988). It is now restricted to a fraction of its former range, including the main stem Colorado River from Lake Powell to Palisade; the Green River below Brown's Park; the Yampa River below Craig; the White River below Rio Blanco Reservoir; and in portions of the San Juan River in Colorado, Utah and New Mexico (Colorado River Fishes Recovery Team, 1988; Tyus, 1991). Main stem impoundments on the Colorado River system have largely contributed to the decline of Colorado squawfish populations by blocking migration routes and altering flow and temperature regimes (Tyus, 1991). The White River, previously free-flowing, was impounded by Taylor Draw Dam in 1985, 16 river-kilometers east of Rangely, Colorado, creating Kenney Reservoir. The dam effectively reduced the suitable habitat available to squawfish in the White River by 80 km by preventing access to areas upstream from the dam. According to unpublished findings of the Colorado Division of Wildlife, this represents about 16% of the documented range in Colorado. There are no records of Colorado squawfish reproducing in the White River and no suitable spawning conditions have been identified (Carlson et al., 1979; Haynes et al., 1984). In light of this and because of construction mitigation agreements, Colorado Division of Wildlife (CDOW) and the United States Fish and Wildlife Service began a Colorado squawfish stocking program in 1988, calling for 50,000 fingerling-age I Colorado squawfish to be stocked in each of the three years, 1988-1990. After the first stocking in 1988, surHWESTERN RALIST 38(4):362-369 EMBER Trammell et al.-Colorado squawfish veys by CDOW of the reservoir and river indicated the stocked Colorado squawfish had dispersed throughout the reservoir and moved downstream. The study reported here was begun in 1989 to further evaluate dispersal of the 1988 stock and to assess the results of subsequent stockings of Colorado squawfish in Kenney Reservoir. Our primary objectives were to evaluate Colorado squawfish ecology and behavior as they related to attempts to artificially establish these fish in the reservoir as a sport fish, and to evaluate the potential of a sport fishery based on stocked

Predation on Fish Larvae by Adult Red Shiner, Yampa and Green Rivers, Colorado

Minckley (1991) hypothesized that introduced nonnative fishes, enhanced by river alterations, are primarily responsible for the demise of native fish species in the Colorado River system. Negative interactions with nonnative competitors and predators may impact all life stages of virtually all native fishes of the system. Predation by nonnative fishes on young Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) is hypothesized as one reason for the decline of these two endangered species endemic to larger rivers of the Colorado River Basin (Kaeding and Osmundson, 1988; Marsh

Managing Lower Colorado River

The lower Colorado River system, which is managed by the Bureau of Reclamation (BuRec), comprises three major dams (Hoover, Davis, and Parker). There is a significant hydroelectric capacity in the southwestern system, but to make optimum use of the resource, it must be integrated with available thermal and nuclear sources. The major competing demands on the system are water supply, flood control, and energy production. BuRec and the Western Area Power Administration have undertaken an effort to explore alternative management strategies for the three dams. A model that incorporates a hierarchy of objectives as well as physical, institutional, and political constraints imposed on the system has been developed. It may be used to provide guidance to support the managers of the system in real time as they make hourly operating decisions over the next week. The model may also be used to explore alternative operating strategies for the lower Colorado. The process of formulating, developing, and implementing the model has affected the evolution of improved operations for the lower Colorado.