ASSESSMENT OF NEEDS AND APPROACHES FOR EVALUATING GROUND WATER AND SURFACE WATER INTERACTIONS FOR HYDROLOGIC UNITS IN THE SNAKE RIVER BASIN

The Snake River Basin, located in Idaho, Wyoming, Oregon, Nevada, and Utah, is experiencing increasing demands on a finite amount of water. The complexity of water allocation is magnified by the size of the basin and the varied types of interests competing for water. Irrigation interests, municipalities, flood control operators, tribal needs, industrial uses, domestic supply, recreation and species habitat requirements are all exerting demands on the water system. To enhance decision making abilities as related to water allocation, the United States Bureau of Reclamation initiated the Snake River Resources Review (SR3). The SR3 project involves all Snake River drainage areas upstream from Brownlee Dam, located on the Idaho-Oregon border. In support of the SR3 study, this report addresses the ground-water surface water interactions between the tributary basins and Snake River. The entire study area was subdivided into geographic areas utilizing the hydrologic unit boundaries (HUCs) established by the U. S. Geological Survey (USGS). Data were accumulated to identify areas where ground water use may be sufficient to materially impact surface water flow. The data sets include: (1) ground water-rights (location, amount, priority date), (2) water table fluctuations, (3) agricultural land use, and (4) recorded stream flow measurements. Current rules for conjunctive management of the surface water and ground water systems were also reviewed. Aquifers in much of the Snake River basin in Idaho have been modeled by previous efforts. In this study, individual HUCs were compared to one another utilizing the criteria suggested by the Idaho Department of Water Resources (IDWR) for prioritizing areas where additional or more detailed ground water modeling should be performed. From this basin wide approach, 6 areas iii received a High Priority ranking: Teton, Raft, Big Lost, Big Wood, Portneuf, and Salmon Falls. Areas receiving a Medium Priority include Middle-Snake Succor, Bruneau, C. J. Strike Reservoir, Camas, and Blackfoot. Water table fluctuations observed in wells, measured by the USGS, indicate that not all wells in a given region show an equal response to stresses on the water system. Basin-wide changes in the water table seem to correlate with climatic periods of drought or non-drought. Some of the suggestions for further studies in the Snake River basin area include: 1. Creation of a regional ground water model that would tie together the entire Eastern Snake River Plain and its associated tributary basins; 2. An incorporation of ‘time-lags’ within the rules of conjunctive management used by the State of Idaho to address the degree and timing of impacts that justify subsequent regulation of ground water rights based on impacts to surface water flow; 3. Incorporate detailed review of historical surface water use and diversions with all ground water modeling efforts; and 4. Investigate the use of analytical element models for a basin-wide modeling approach, and their possible use as a preliminary modeling effort.