Effects of flow augmentation on stream channel morphology and riparian vegetation: Upper Arkansas River basin, Colorado

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We characterized changes in stream morphology and riparian vegetation cover at eight field sites in tributary basins of the upper Arkansas River, Colorado, USA. Three of these sites have experienced flow augmentation, while five others are responding to natural flow conditions. Data analyzed for the eight field sites included hydrologic data from U.S. Geological Survey gaging stations, extensive field surveys of riparian vegetation and channel and floodplain morphology, and land-cover maps compiled from historic aerial photographs of field sites. Hydrologic analyses indicated that augmented peak annual floods in the Lake Creek basin are more than double those of Clear Creek or Cottonwood Creek, similar tributaries where flow augmentation does not occur. Likewise, augmented peak floods on Lake Fork are roughly double those of a similar tributary basin with a natural flow regime. Flow duration curves from these basins also indicated that flows of a given magnitude are sustained over longer periods of time in the Lake Creek and Lake Fork basins than in comparable basins lacking flow augmentation. Historic changes in the channel pattern at augmented sites indicated a shift from highly sinuous meandering channels to less sinuous or braided channels. Data from field sites also indicated that along augmented reaches, bankfull channel width and widthdepth ratios are greater than in comparable sections that lack flow augmentation. Similarly, substrate size on depositional bars at augmented sites is substantially coarser than on streams without flow augmentation. Median particle size for the three sites along augmented reaches ranges from 38 to 56 mm. By contrast, sites where natural flows occur have median particle sizes between 15 and 26 mm. Historic aerial photograph analysis indicated that over the last 56 years, loss of riparian vegetation cover at sites where flow augmentation occurs was greater than that at sites where natural flows occur. Bottomland areas immediately adjacent to the active channel have experienced up to a 10% decrease in riparian vegetation cover at sites affected by augmented flows. These losses primarily occur along channel margins, with commensurate increases in exposed depositional bars and active channel features. By contrast, along reaches where natural flows occur, riparian vegetation cover generally shows a net increase or decrease of less than 2% of the bottomland area. Substantial change in channel and floodplain morphology suggests that augmented streams in the upper Arkansas River basin are continuing to adjust their shape and channel dimensions to achieve a new equilibrium. The complex response of mountain streams to flow augmentation over a relatively short geologic time period makes the prediction of future long-term conditions difficult. Restoration planners working in this or similar watersheds must recognize the historic and potential future impacts of flow augmentation when evaluating sites for restoration efforts.