Ecological–economic assessment of a sediment‐producing stream behind lower granite dam on the lower snake river, USA

Field investigations and simulation modelling studies were conducted on an agriculturally perturbed watershed draining into the regulated Lower Clearwater–Snake Rivers of Idaho and Washington. Ecological–economic analyses were conducted to develop an integrated systems framework to assess the impacts of alternative farming practices on sustainable farming, sediment erosion and stream health on Tom Beall Creek, with ecological implications and consequences to Lower Granite Reservoir on the Snake River. A geographic information system (GIS) was used to spatially delineate field boundaries, soil types, drainage patterns and erosion rates. An AGNPS (agricultural nonpoint source) pollution model was interfaced with the GIS to project water quality impacts of land use in the watershed. The AGNPS model was integrated with a mixed integer linear programming model (MIP) to facilitate modelling cost effectiveness on land as it related to eroded soil entering the receiving streams. Biological health assessment was performed using benthic macroinvertebrates. The biological condition of Tom Beall Creek was rated as slightly and moderately impaired for 1992 and 1993, respectively. A wheat–pea rotation under conventional tillage and cross‐slope farming had the highest weighted average on‐site erosion damage of $20 per ha; the lowest damage was $3.63 per ha using a wheat–barley–fallow rotation with reduced tillage and divided slopes. Simulating a ‘no programme’ policy scenario, the MIP model showed the dramatic impact of the current farm programme with conservation compliance in the region of highly erodible land and high participation rate in the government farm programme. Under current farming practices and > 50 yr spring runoff event, it was estimated that nearly 5000 t of suspended solids would be exported from the Tom Beall watershed. During low flow years, less than 200 t were exported. Sedimentation in Tom Beall Creek is the most debilitating environmental factor impacting the aquatic biota. High erosion rates from all of the anthropogenically impacted streams above Lower Granite Dam tend to shorten the productive storage life of the reservoir, alter the hydroelectric power generating capacity and interfere with the ecological food web and biodiversity. Although the implementation of better conservation practices in anthropogenically disturbed watersheds will not eliminate watershed erosion, it will reduce the frequency and the amounts of sediment dredged from reservoirs in the future, and will reduce on‐site and off‐site costs of erosion and sediment damage.