This dataset is the estimated altitude of the top of the uppermost fine-grained sediment within the Wood River Valley aquifer system. This map was compiled by describing the estimated altitude of the top of the fine-grained sediment from about 160 well-driller reports for boreholes drilled within the area of the approximate extent of the confined aquifer. A surface was generated using kriging of the point data. This dataset was created in support of the third phase of a continuing U.S. Geological Survey (USGS) study to assess the groundwater resources of the Wood River Valley, south-central Idaho. The first phase was documented in "Water-resource trends and comparisons between partial development and October 2006 hydrologic conditions, Wood River Valley, south-central Idaho" (Skinner and others, 2007), and the second phase was documented in "Ground-water budgets for the Wood River Valley aquifer system, south-central Idaho"(Bartolino, 2009). The third phase is a description of the hydrogeologic framework of the Wood River Valley aquifer system. The Wood River Valley contains most of the population of Blaine County and the cities of Sun Valley, Ketchum, Hailey, and Bellevue. This mountain valley is underlain by the alluvial Wood River Valley aquifer system. The entire population of the area depends on groundwater for domestic supply and rapid population growth since the 1970s has caused concern about the long-term sustainability of the groundwater resource.