Water-resources data for the 2001 water year for California consist of records of stage, discharge, and water quality of streams, stage and contents in lakes and reservoirs, and water levels and water quality in wells. Volume 4 contains discharge records for 191 gaging stations, stage and contents for 53 lakes and reservoirs, gage-height records for 1 station, and water quality for 18 stations. Also included are 3 miscellaneous partial-record sites, and 3 parital-record water-quality 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.