EFFECTS OF ANTHROPOGENIC HYDROLOGIC ALTERATION ON FLUVIAL AND RIPARIAN GEOMORPHOLOGY IN THE HENRY'S FORK SNAKE RIVER WATERSHED, IDAHO

Hydrologic regime is the primary physical driver of geomorphic processes in stream channels and riparian areas. Hydrologic regimes in river systems throughout the intermountain west have been altered by irrigation and hydroelectric power generation. Such alteration can therefore have substantial impacts on geomorphic and hence ecological function in riparian areas, which generally support the greatest density of biomass and species diversity in arid and semi-arid landscapes. We present a method for quantifying hydrologic alteration due to storage reservoirs, irrigation diversion, and hydroelectric generation over an entire watershed and assessing its potential effects on riparian ecology. Our method consisted of reconstructing “unregulated” flows at each of nine gauging stations distributed throughout the watershed and then comparing regulated and unregulated flows using a variety of statistical methods, including the Indicators of Hydrologic Alteration. Division of the watershed's large rivers into reaches and calculation of mean floodplain width in each reach allowed for identification of areas where alteration of peak flow frequency and magnitude could affect floodplain and riparian processes. Results show that alteration is greatest immediately below the two major dams, Island Park Reservoir and Henrys Lake, due to storage and delivery, and immediately below a large hydroelectric power plant diversion. Other areas of high alteration were lower in the watershed below points of high irrigation diversion. Because many of the watershed's streams are deeply incised into recent volcanic bedrock formations and/or are dominated by ground water, few alluvial stream reaches occur in the watershed, and hydrologic alteration therefore has little effect on channel and riparian morphology in most reaches. In the two lower watershed reaches where alluvial floodplains are present, hydrologic alteration has affected peak flow characteristics very little. However, management of Henrys Lake has altered timing, magnitude and frequency of peak flow events in a headwater alluvial reach, greatly affecting riparian processes there. Our work is currently being used as the scientific basis for collaborative water resources management in the Henry's Fork watershed.