Water resources data in this volume for the 1981 water year consists of records of stage discharge, and water quality of streams and wells; stage, contents, and water quality in lakes and reservoirs; and water levels in wells. This report contains discharge records for 162 gaging stations; stage and contents for 12 lakes and reservoirs; water quality for 63 stations and 27 wells; water levels for 78 observation wells. Also included are 1 low-flow partial-record station and 32 water quality partial-record stations. These data represent that part of the National Water Data System operated by the U.S. Geological Survey and cooperating State and Federal agencies in California.