Water resources data for the 2004 water year for Pennsylvania consist of records of discharge and water quality of streams; contents and elevations of lakes and reservoirs; and water levels and water quality of ground-water wells. This report, Volume 1 contains (1) discharge records for 79 continuous-record streamflow-gaging stations, 8 partial record stations, 19 special-study and miscellaneous streamflow sites; (2) elevation and contents records for 13 lakes and reservoirs, and water-quality records for 6 lakes and reservoirs; (3) water-quality records for 38 gaging stations and 12 ungaged streamsites; (4) water-quality records for 82 special-study stations; (5) water-level records for 52 network observation wells; and (6) water-quality analyses of ground water from 16 ground-water wells. Site locations are shown in figures 6-18. Additional water data collected at various sites not involved in the systematic data-collection program are also presented. These data together with the data in Volumes 2 and 3, represent that part of the National Water Data System operated by the U.S. Geological Survey and cooperating State, local, and Federal agencies in Pennsylvania.