Water resources data for the 1990 water year for Idaho consist of records of stage, discharge, and water quality of streams; stage, contents, and water quality of lakes and reservoirs; discharge of irrigation diversions; and water levels and water quality of ground water. This report contains discharge records for 202 stream-gaging stations and 41 irrigation diversions; stage only records for 2 stream-gaging stations; stage only for 6 lakes and reservoirs; contents only for 21 lakes and reservoirs; water quality for 50 stream-gaging stations, 97 wells, and 1 lake; daily totals for 1 precipitation gage; and water levels for 473 observation wells. Additional water data were collected at various sites, not involved in the systematic data collection program, and are published as miscellaneous measurements. 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 Idaho, adjacent States, and Canada.