HYDROLOGIC ALTERATION AND SEDIMENTATION IN THE UPPER HENRY’S FORK WATERSHED

AbstractHydrologic Alteration and Sedimentation in the Upper Henry’s Fork WatershedAmelie Jeanne CharnauxThe Henry’s Fork of the Snake River is venerated by the global recreational community as one of the finest trout fishing streams on the planet.  Furthermore, this remarkable waterway flows within the bounds of one of the most important ecological corridors in the equally world-renowned Greater Yellowstone Ecosystem.  While the recreational and biological features of this corner of Idaho may capture the interest of the broader public, the waterway is equally significant to the livelihoods of local interests, such as the ranching and farming communities.  With the stakes running high for all interest groups, a case study of the Henry’s Fork watershed provides a valuable baseline narrative for understanding decision-making related to water resources not only in Idaho, but also in other Western states.Environmental impacts of the Henry’s Lake Dam and management of the stream below the structure were evaluated by literature review and GIS mapping, with research emphasis placed on the ecological integrity and fisheries of the Upper Henry’s Fork Subbasin.  The literature review focused on natural processes in stream ecosystems and anthropogenic impacts, with the goal of providing information for the development of management policies that minimize the negative impacts of current flow management and land use.Literature sources overwhelmingly agree that the alteration of natural hydrologic regimes is the most serious and continuing threat to the sustainability of river ecosystems.  In recent decades, downstream recreational fishing declined on the upper Henry’s Fork due to increased sedimentation, inciting investigation as to the source.  Three major anthropogenic factors targeting one section of the river, the Henry’s Lake Outlet, set the stage for excess sedimentation.First, the Henry’s Lake Dam was constructed in 1923, impacting the Outlet’s hydrology through changes in the timing, magnitude, and frequency of low and high flows.  Second, an artificial stream channel was constructed in the 1920s to bypass the meandering Outlet in order to increase conveyance capacity of irrigation water from Henry’s Lake to downstream water users.  Third, long-term livestock grazing along this section of river dramatically reduced riparian and upland vegetation, triggering the loss of stream-bank stability and increasing erosion and sedimentation.  These management practices have resulted in significant loss of biodiversity in the stream ecosystem and an increased rate of erosion in the Outlet.The Henry’s Lake Outlet restoration project, led by the Henry’s Fork Foundation, provides the opportunity to predict potential effects of large-scale restoration in the Henry’s Fork watershed.  The project seeks to reduce sediment delivery downstream from the Outlet by rerouting flow from the straightened channel into the historic channel.  The project tests the hypothesis that, by restoring the meandering stream channel, and thereby adding a half-mile to the Outlet, bank erosion and channel instability will decrease, the interaction between stream and riparian habitats will improve, and overall ecosystem health will benefit.In spite of the proactive intentions of the project, it will not change the current management of stream flow.  In order to restore the Henry’s Lake Outlet to a state of dynamic equilibrium in terms of erosion and sediment load, the flows from Henry’s Lake Dam must more closely reflect the natural hydrologic regime.  However, the ability to implement full restoration of the Outlet is complicated by the conflict between the  requirements for ecosystem health and economic and socio-political pressures, a story common to many water systems throughout the West.  Ultimately, it is hoped that this research may be integrated into policy and conservation strategy to mitigate streambank erosion and sedimentation in the Henry’s Fork Subbasin.