This arc and point data set contains streamflow-measurement sites and reaches indicating streamflow gain or loss under base-flow conditions along Republican River tributaries in Dundy and Chase Counties, Nebraska during October 6 to 8, 1975 (U.S. Geological Survey, 1977). The streamflow measurements were made to obtain data on ground-water/surface-water interaction. Flow was observed visually to be zero, was measured, or was estimated at 64 sites. 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 plotting locations of streamflow measurements. These points were used to designate stream-reach segments to calculate gain/loss per river mile. Reach segments were created by manually splitting the lines from a 1:250,000 hydrography data set (Soenksen and others, 1999) at every location where the streams were measured. Each stream-reach segment between streamflow-measurement sites was assigned a unique reach number. All other lines in the hydrography data set without reach numbers were omitted. This data set was created to archive the calculated streamflow gains and losses of tributary streams to the Republican River in Dundy and Chase Counties, Nebraska, in October 1975, and make the data available for use with geographic information systems (GIS). If measurement sites are used separately from reaches, the maximum scale of 1:100,000 should not be exceeded. When used in conjunction with the reach segments, the maximum scale should not exceed 1:250,000.