RECENT GROUNDWATER RESOURCE INVESTIGATIONS IN THE BIG LOST RIVER BASIN, IDAHO, AND RAFT RIVER BASIN, IDAHO-UTAH

The eastern Snake River Plain aquifer (ESPA) and its tributary basins provide important groundwater resources for communities and irrigation supply for agriculture in southern Idaho. Updated hydrogeologic characterizations of the Big Lost River Basin and Raft River Basin, both tributary to the ESPA, are intended to address water resources challenges. These basins have experienced persistent declining groundwater-level trends, decreased streamflow, and concerns about long-term groundwater availability. However, variable climatic inputs, geological factors, and basin operations highlight similarities and differences with implications for overall resources.

The Big Lost River Basin is located north of the ESPA in parts of Butte and Custer counties in Idaho. The Big Lost River and Mackay Reservoir are used extensively for irrigation via a complex canal network, supplemented with connected groundwater resources. The Raft River Basin is south of the ESPA in parts of Cassia, Oneida, and Power counties in Idaho and Box Elder County, Utah, at the intersection of metamorphic core complex, basin and range, and Snake River Plain geological provinces. Few available surface-water resources exist in the Raft River Basin, with greater reliance on groundwater supply.

The Idaho Geological Survey, a non-regulatory state agency, prepared groundwater budgets for the Big Lost River and Raft River basins and a hydrogeologic framework and three-dimensional geological model for the Raft River Basin as part of broader multi-agency investigations. Project goals were to improve estimates of groundwater underflow exiting the basins, support numerical modeling efforts, and provide datasets and interpretation to assist stakeholders and managers.

Annual groundwater budgets were informed by the respective hydrogeologic frameworks and incorporated available datasets such as land-use coverage, climatic datasets, streamflow and surface-water diversion records, groundwater level and pumpage datasets, lithologic and well construction logs, and irrigation practices. Perceived data gaps were regarded as uncertainty in the budget residuals, or the difference between budget inflow (recharge) and outflow (discharge) terms, as the basis for suggestions for future work.