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An accounting procedure is developed which determines a flow regime that is capable of transporting an amount of bedload sediment necessary to ensure channel stability downstream. The method allows for sediment buildup in the channel within geomorphic threshold limits during low flow periods. During periods of high runoff, enough water is bypassed to transport the stored sediment. The procedure utilizes only those flows of sufficient magnitude to maintain channel stability over the long run (25–50+ years). An example is presented which determines the volume of water and frequency of release for channel maintenance purposes downstream from a hypothetical water diversion project. Of some 1,200,000 acre feet generated...
We present herein clear field evidence for the persistence of a coarse surface layer in a gravel-bed river during flows capable of transporting all grain sizes present on the channel bed. Detailed field measurements of channel topography and bed surface grain size were made in a gravel-bed reach of the Colorado River prior to a flood in 2003. Runoff produced during the 2003 snowmelt was far above average, resulting in a sustained period of high flow with a peak discharge of 27 m3/s (170% of normal peak flow); all available grain sizes within the study reach were mobilized in this period of time. During the 2003 peak flow, the river avulsed immediately upstream of the study reach, thereby abandoning approximately...
The hydrology and geomorphology of large rivers in America reflect the pervasive influence of an extensive water control infrastructure including more than 75,000 dams. One hundred thirty-seven of the very large dams, each storing 1.2 km3 (106 acre feet) of water or more, alter the flows of every large river in the country. The hydrologic effects of these very large dams emerge from an analysis of the stream gage records of 72 river reaches organized into 36 pairs. One member of each pair is an unregulated reach above a dam, whereas the other is a regulated reach downstream from the same structure. Comparison of the regulated and unregulated reaches shows that very large dams, on average, reduce annual peak discharges...
Many ephemeral streams in western North America flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos. These inherently unstable streams of semiarid regions are sensitive to short-term climatic changes, and to human impacts, because hillslopes supply abundant sediment to infrequent large streamflow events. Discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation; either aggradation or degradation may become dominant. Attainment of equilibrium conditions is brief. Disequilibrium is promoted by channel entrenchment that causes the fall of local base level, and by deposition of channel fans that causes...
American geomorphologic research related to dams is embedded in a complicated context of science, policy, economics, and culture. Research into the downstream effects of large dams has progressed to the point of theory-building, but generalization and theory-building are from this research because (1) it is highly focused on a few locations, (2) it concerns mostly very large dams rather than a representative sample of sizes, (3) the available record of effects is too short to inform us on long-term changes, (4) the reversibility of changes imposed by dam installation and operation is unknown, and (5) coordinated funding for the needed research is scarce. In the scientific context, present research is embedded in...
A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 360014C yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900?1100)...
Analysis of field data and development and application of a dynamic model indicate that the processes that control the number and distribution of age-0 Colorado pikeminnow in the middle Green River are poorly understood. Colorado pikeminnow are a federally endangered species endemic to the Colorado River basin that utilize backwaters during their larval stage. The present agency-mandated field sampling program for backwater habitats may be inadequate because it takes place at a time when the model predicts that most larval fish have drifted beyond the study area. The model predicts that water releases from Flaming Gorge Dam have a large potential effect on larval drift, because high releases at the time of drift...
Billions of dollars are being spent in the United States to restore rivers to a desired, yet often unknown, reference condition. In lieu of a known reference, practitioners typically assume the paradigm of a connected watercourse. Geological and ecological processes, however, create patchy and discontinuous fluvial systems. One of these processes, dam building by North American beavers (Castor canadensis), generated discontinuities throughout precolonial river systems of northern North America. Under modern conditions, beaver dams create dynamic sequences of ponds and wet meadows among free-flowing segments. One beaver impoundment alone can exceed 1000 meters along the river, flood the valley laterally, and fundamentally...
The relative importance of macro-and micro-scale processes in the biosphere is controversial, with some researchers emphasizing micro-scale population dynamics and others macro-scale landscape dynamics. I hypothesize that there is spatial variation in the linkage between and importance of macro-and micro-environmental influences on vegetation, and explore this hypothesis in a case study of riparian vegetation in the mountains of western Colorado. I estimate percent cover, by species, in 115-0.1 ha samples of riparian vegetation at relatively undisturbed sites. Environmental variables measured at each site include several channel characteristics, valley characteristics, geographical coordinates, and drainage basin...