This arc and point data set contains streamflow measurement sites and reaches indicating streamflow gain or loss under base-flow conditions along the Republican River and tributaries in Nebraska during March 21 to 22, 1989 (Boohar and others, 1990). These measurements were made to obtain data on ground-water/surface-water interaction. Flow was visually observed to be zero, was measured, or was estimated at 136 sites. The measurements were made on the main stem of the Republican River and all flowing tributaries that enter the Republican River above Swanson Reservoir and parts of the Frenchman, Red Willow, and Medicine Creek drainages in the Nebraska part of the Republican River Basin. Tributaries were followed upstream until the first road crossing where zero flow was encountered. For selected streams, points of zero flow upstream of the first zero flow site also were checked. Streamflow gain or loss for each stream reach was calculated by subtracting the streamflow values measured at the upstream end of the reach and values for contributing tributaries from the downstream value. The data obtained reflected base-flow conditions suitable for estimating streamflow gains and losses for stream reaches between sites. This digital data set was created by manually splitting the lines from a 1:250,000 hydrography data set (Soenksen and others, 1999) at every streamflow-measurement site. Each set of stream segments between measurement sites was assigned a unique reach number.