Water resources data for the 2005 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 75 continuous-record streamflow-gaging stations, 5 partial-record stations, 41 special-study and miscellaneous streamflow sites; (2) elevation and contents records for 13 lakes and reservoirs, and water-quality records for 5 lakes and reservoirs; (3) water-quality records for 24 gaging stations and 10 ungaged streamsites; (4) water-quality records for 73 special-study stations; (5) water-level records for 52 network observation wells; and (6) water-quality analyses of ground water from 42 ground-water wells. Site locations are shown in figures 6-19. 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.