Effects of Flow Augmentation on Channel Morphology and Riparian Vegetation in the Upper Arkansas River Basin, Colorado

This study examined historic change of riparian plant communities and fluvial geomorphic response of gravel-bedded streams and their floodplains to over 50 years of hydrologic disturbance. Four tributary basins of the Arkansas River were analyzed. Lake Creek, Clear Creek, and Cottonwood Creek are drainages similar in area, physiography, and vegetation composition. However, Lake Creek may receive an instantaneous discharge of approximately 28 m3sec-1 from the Twin Lakes tunnel, over three times the normal flow of the stream during spring runoff. By contrast, Clear Creek and Cottonwood Creek, nonaugmented streams, were used as controls to compare the historic and present condition of natural flowing streams with Lake Creek. Lake Fork was also examined, another augmented stream that carries transmountain water stored in a reservoir to the Upper Arkansas River. Hydrologic data collected from U.S.G.S. gaging stations were used in an attempt to identify relations between specific flow events and corresponding changes in fluvial landforms, riparian vegetation patterns, and channel morphology. Topographic cross sections of the channel and floodplain were surveyed and hydraulic geometry at each field site was examined. Aerial photographs from 1938-39, 1956-57 , and 1988 were incorporated into a Geographic Information System (GIS) to prepare a series of land cover maps of the river corridor at eight field sites on augmented and nonaugmented streams. Results indicate channel morphology and riparian vegetation cover on the active floodplains of Lake Creek and Lake Fork have experienced substantial change from natural physical and biologic conditions as a result of historic and present flow augmentation practices. The increase in water discharge without a natural sediment load in Lake Creek and Lake Fork appears to have caused accelerated rates of channel bank erosion, incision, and bank retreat below transbasin diversion releases. In some downstream reaches characterized by gentler slopes and wider valley bottoms, the width-depth ratio of the channel has increased and sinuosity has decreased. Sites carrying transmountain water were characterized by higher shear stress and specific stream power values and larger bed material compared to control sites. Consequently, the Lake Creek and Lake Fork drainages show temporal and spatial loss of riparian vegetation adjacent to the stream channel in response to historic and present flow augmentation practices. Control watersheds, Clear Creek and Cottonwood Creek, where natural flow regimes exist, did not experience similar magnitudes of change in channel morphology or spatial distribution of riparian vegetation