MAPPING THE RESPONSE OF RIPARIAN VEGETATION TO POSSIBLE FLOW REDUCTIONS IN THE SNAKE RIVER, IDAHO

This study was initiated to determine the general effects of potential flow reductions in the middle Snake River (Swan Falls Dam downstream to the Idaho-Oregon border) on its riparian vegetation. Considerable w ater from the river is currently used to irrigate the adjacent Snake R iver Plain, and increased demand for water in the future is likely. Th e problem was subdivided into several research components including: f ield investigation of the existing riparian vegetation and river envir onment, hydrological modeling to calculate the effects of one flow sce nario on hydrological regime, and integration of vegetation and hydrol ogical modeling results with a Geographic Information System (GIS) to map the riverbed, island, and bank conditions under the scenario flow, Field work was conducted in summer 1990. Riparian vegetation along 40 U.S. Geological Survey cross-sections was sampled at approximately 1. 25 mile intervals within the 50 mile long study area. Cross-section an d flow data were provided by the U.S. Geological Survey, GIS mapping o f land/water cover using ARC/INFO was based on 1987 aerial photographs . Riverbed contour maps were produced by linking cross-section data, t opographic contouring software (ANUDEM),and GIS, The maps were used to spatially display shallow areas in the channel likely to become veget ated under reduced flow conditions. The scenario would reduce flow by approximately 20% (160 MAF) and lower the river an average of 0.5 ft. The scenario flow could cause a drop in the elevation of the riparian zone comparable to the drop in mean river level and expansion of the l ower riparian zone into shallow areas of the channel, The GIS maps sho wed that the shallow areas of the channel more likely to become vegeta ted under the scenario flow are located in wide reaches near islands. Some possible ecological consequences of the scenario flow include a g reater area of riparian habitat, reduced flow velocity and sedimentati on in shallow channels leading to channel deactivation, increased isla nd visitation and nest predation by predatory mammals due to loss of a water barrier between some islands and banks, and larger populations of alien plant species in the new riparian vegetation.