The role of bedrock on the longitudinal profile of the Colorado River has intrigued workers for over a century. The river's profile exhibits large-scale (10 to 100 km) variations in geomorphology that are qualitatively associated with changes in rock type. This study provides the first bedrock-strength data to quantitatively test the relation of bedrock-resisting to hydraulic-driving forces in Glen and Grand canyons. The intent of this study is to explore the role, if any, that bedrock has on large-scale geomorphic variations along the profile of the Colorado River. Rock-strength data collected at 84 sites along the river corridor in Glen and Grand canyons include intact-rock strength, fracture spacing , and other characteristics associated with Selby rock-mass strength (RMS). These strength data were statistically related to measurements of channel width, gradient, and calculations of unit stream power. At the canyon scale (100 km), rocks in Grand Canyon have significantly higher intact-rock strength, lower fracture spacing, and higher RMS than those in Glen Canyon. These observations correspond to the fact that Grand Canyon is steeper and narrower, and has greater mean unit stream power. Furthermore, smaller scale, reach-average values of rock strength correlate significantly to width, gradient, and unit stream power, although there are outliers related to local-scale effects such as rapids. The Colorado River runs in a narrower and steeper channel in reaches confined by resistant bedrock ( e.g., Upper Granite Gorge, RM 77-114). In contrast , reaches floored in weaker bedrock (e.g., lower Marble Canyon, river miles 37 to 58) are associated with wider channels and lower gradient. This study confirms previous research linking rock type to the geomorphology of the Colorado River. Results imply that knickzones in the profile are persistent features that reflect a dynamic equilibrium between hydraulic-driving and bedrock-resisting forces, rather than transient waves of incision due to tectonics or drainage integration. They support the hypothesis that bedrock sets the long-term, large-scale template for the Colorado River. Bedrock hypothetically acts as a direct control on the river's width and gradient, particularly when the river is in contact with bedrock. Rock-strength and weathering properties of bedrock within tributary catchments, where debris flows initiate, act as an indirect control through their influence on hillslope-to-river sediment production during episodes, such as today, when the river is not on bedrock.

Colorado River cutthroat trout, Oncorhyncus clarki pleuriticus , occupy less than 12% of their historic range. Restoration and conservation of this species are currently under way across the upper Colorado River basin, but guidance to inform management decisions related to the impacts of climate change on cutthroat is lacking. Shifts in the thermal distribution of freshwater fish have been documented, and will continue to occur as cold water habitat is threatened by warming water temperatures. Coupled air and water temperature data allow for an estimation of potential resistance and resilience to warming, determining the effect that local air has on stream temperature. The United States Forest Service, cooperating with federal agencies, state agencies and private landowners, placed temperature loggers in the water and two air locations at 50 sites. To select a representative subset of sites, six habitat characteristics of each Colorado River cutthroat trout core conservation population were considered. These characteristics include solar input, elevation, watershed area, riparian vegetation, groundwater input, and the 30-year mean maximum July air temperature. Results from coupled temperature loggers indicate that the relationship between air and water temperature in the upper Colorado River basin is neither linear, nor one-to-one. Using Mohseni's (2003) equation, the relationship between air and water temperature was fit to a nonlinear regression curve. Analysis shows that the median rise in daily maximum water temperature is only 0.41°C for a 1.0°C increase in the median daily maximum air temperature. Air temperature exerts the most influence over water temperature; however, these results indicate that there are other characteristics that influence stream temperature. To determine these characteristics, analysis of the six habitat characteristics used for site selection in addition to aspect, slope, and latitude were used to model multiple temperature metrics. The best model, nonlinear water to air temperature relationship, had an R2 between actual and predicted values of 0.71. It also became clear that using multi-metric analysis would provide a much more robust indicator of resistance. This work will allow managers to consider potential climate change resistance or resilience in project prioritization, by understanding potential habitat characteristics to buffer stream warming.

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Thesis (Ph. D.)--University of California, San Diego, 2002. / Vita. Includes bibliographical references (leaves 406-419).

abstract: Water resource systems have provided vital support to transformative growth in the Southwest United States; and for more than a century the Salt River Project (SRP) has served as a model of success among multipurpose federal reclamation projects, currently delivering approximately 40% of water demand in the metropolitan Phoenix area. Drought concerns have sensitized water management to risks posed by natural variability and forthcoming climate change. Full simulations originating in climate modeling have been the conventional approach to impacts assessment. But, once debatable climate projections are applied to hydrologic models challenged to accurately represent the region’s arid hydrology, the range of possible scenarios enlarges as uncertainties propagate through sequential levels of modeling complexity. Numerous issues render future projections frustratingly uncertain, leading many researchers to conclude it will be some decades before hydroclimatic modeling can provide specific and useful information to water management. Alternatively, this research investigation inverts the standard approach to vulnerability assessment and begins with characterization of the threatened system, proceeding backwards to the uncertain climate future. Thorough statistical analysis of historical watershed climate and runoff enabled development of (a) a stochastic simulation methodology for net basin supply (NBS) that renders the entire range of droughts, and (b) hydrologic sensitivities to temperature and precipitation changes. An operations simulation model was developed for assessing the SRP reservoir system’s cumulative response to inflow variability and change. After analysis of the current system’s drought response, a set of climate change forecasts for the balance of this century were developed and translated through hydrologic sensitivities to drive alternative NBS time series assessed by reservoir operations modeling. Statistically significant changes in key metrics were found for climate change forecasts, but the risk of reservoir depletion was found to remain zero. System outcomes fall within ranges to which water management is capable of responding. Actions taken to address natural variability are likely to be the same considered for climate change adaptation. This research approach provides specific risk assessments per unambiguous methods grounded in observational evidence in contrast to the uncertain projections thus far prepared for the region. / Dissertation/Thesis / Doctoral Dissertation Geography 2016

abstract: The Colorado River Basin (CRB) is the primary source of water in the southwestern United States. A key step to reduce the uncertainty of future streamflow projections in the CRB is to evaluate the performance of historical simulations of General Circulation Models (GCMs). In this study, this challenge is addressed by evaluating the ability of nineteen GCMs from the Coupled Model Intercomparison Project Phase Five (CMIP5) and four nested Regional Climate Models (RCMs) in reproducing the statistical properties of the hydrologic cycle and temperature in the CRB. To capture the transition from snow-dominated to semiarid regions, analyses are conducted by spatially averaging the climate variables in four nested sub-basins. Most models overestimate the mean annual precipitation (P) and underestimate the mean annual temperature (T) at all locations. While a group of models capture the mean annual runoff at all sub-basins with different strengths of the hydrological cycle, another set of models overestimate the mean annual runoff, due to a weak cycle in the evaporation channel. An abrupt increase in the mean annual T in observed and most of the simulated time series (~0.8 °C) is detected at all locations despite the lack of any statistically significant monotonic trends for both P and T. While all models simulate the seasonality of T quite well, the phasing of the seasonal cycle of P is fairly reproduced in just the upper, snow-dominated sub-basin. Model performances degrade in the larger sub-basins that include semiarid areas, because several GCMs are not able to capture the effect of the North American monsoon. Finally, the relative performances of the climate models in reproducing the climatologies of P and T are quantified to support future impact studies in the basin. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2018

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Introduced species can become invasive and cause catastrophic alterations to the system they invade. Both zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) have caused significant ecosystem alterations wherever they have invaded. These Dreissena species have caused changes in water quality and biodiversity and have disrupted energy pathways which can have cascading effects on other trophic levels. Recently quagga mussels invaded Lake Powell, a reservoir located in the southwestern USA, creating the possibility of a trophic cascade that could alter energy flow in the reservoir and change the trophic niche structure of the fishes in the lake. However, due to Lake Powell’s large size, dynamic nature, and complex hydrological structure, the effects of quagga mussels on fish species is uncertain. To determine impacts of quagga mussels on Lake Powell fishes, we quantified trophic niches of five species of sport fish over three years (2017-2019) using stable isotopes of nitrogen, δ15N, and carbon, δ13C. We test the following hypothesis: quagga mussels will cause a shift in trophic niche in more pelagic fishes such that pelagic fishes decrease in trophic position and shift toward use of more littoral energy. In addition, we compare the trophic niche of these species with a previous study on the trophic niche of fish in Lake Powell prior to full colonization of the lake by quagga mussels (2014-2015). In general, fish in the southern region of the lake exhibited a trend of decreasing δ15N suggesting decreasing trophic position and an enrichment of δ13C indicating a littoral energy shift in some species. Fish in the northern region of the lake exhibited a slight increase in trophic position and a shift towards pelagic energy across the same time period. These shifts support the hypothesis with pelagic fish experiencing a trophic niche shift, in the direction predicted, but only in the southern region of Lake Powell. Additionally, this shift is not exclusive to pelagic fish, but happened in all five species. Sediment laden input from the Colorado River may offset the impact of quagga mussels in the northern region of the lake resulting in observed regional differences.

From the Proceedings of the 1974 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 19-20, 1974, Flagstaff, Arizona / In the lower reaches of the Colorado River, damages from the increase in salinity to U.S. water users are now estimated to be about 53 million dollars per year and will increase to about 124 million dollars per year by the year 2000 if no salinity control measures are taken. Physical, legal, economic, and institutional aspects of the salinity problem and proposed actions to mesh salinity control with a total water management plan for the basin are discussed. A scheme is presented for planning under the Colorado River water quality improvement program. Recent legislative action is also discussed which provides control plans to improve the water quality delivered to Mexico as well as upper basin water users. These efforts now under study will assure the continued, full utility of Colorado River water to U.S. users and Mexico. However, more extensive development of the basin's natural resources puts new emphasis on total resources management through improved water and land use planning to conserve a most precious western resource - water.