This U.S. Geological Survey (USGS) data release contains daily-mean streamflow and estimated-daily base flow for 225 streamgages in the Chesapeake Bay watershed ranging from 1913 to 2016 (beginning and end dates may vary). There is a table containing hydrograph-separation results by six methods for 225 sites (Hydrograph_separation_results_for_225_streams_in_the_Chesapeake_Bay_watershed) and a summary table with hydrograph-separation results for each site and method (Hydrograph_separation_summary_for_225_streams_in_the_Chesapeake_Bay_watershed). Quantitative estimates of base flow are necessary to address questions of the vulnerability and response of aquatic ecosystems to natural and human-induced change in environmental conditions. Base flow is generally not measured directly, but is estimated from observations of streamflow and (or) stream water chemistry. Base flow was estimated using PART (Rutledge, 1998), HYSEP (Fixed, Local minimum, and Slide; Sloto and Crouse, 1996), BFI (Wahl and Wahl, 1988 and Wahl and Wahl, 1995), and a Recursive Digital Filter (Eckhardt, 2005 and Collishonn and Fan, 2013) in selected watersheds throughout the Chesapeake Bay watershed. The references to the above citations are in the Supplemental Information section of this metadata record. These data support the following publication: Raffensperger, J.P., Baker, A.C., Blomquist, J.D., and Hopple, J.A., 2017, Optimal hydrograph separation using a recursive digital filter constrained by chemical mass balance, with application to selected Chesapeake Bay watersheds: U.S. Geological Survey Scientific Investigations Report 2017-5034, 51 p., https://doi.org/10.3133/sir20175034.