This annual Water-Data Report for Illinois contains current water year (Oct. 1, 2003, to Sept. 30, 2004) and historical data of discharge, stage, water quality and biology of streams; stage of lakes and reservoirs; levels and quality of ground water; and records of precipitation, air temperature, dew point, solar radiation, and wind speed. The current year's (2004) data provided in this report include (1) discharge for 178 surface-water gaging stations and for 10 crest-stage partial-record stations; (2) stage for 27 surface-water gaging stations; (3) stage for 8 reservoirs; (4) water-quality records for 36 surface-water stations; (5) sediment-discharge records for 16 surface-water stations; (6) water-level records for 15 ground-water wells; (7) precipitation records for 49 rain gages; (8) records of air temperature, dew point, solar radiation and wind speed for 1 meteorological station; and (9) biological records for 6 sample sites. Also included are miscellaneous data collected at various sites not in the systematic data-collection network.

Data were collected and compiled as a part of the National Water Information System (NWIS) maintained by the U.S. Geological Survey in cooperation with Federal, State, and local agencies.

This series of Water-Data Reports for Illinois began with the 1961 water year with a report that contained only data relating to the quantities of surface water. For the 1964 water year, a similar report was introduced that contained only data relating to water quality. Beginning with the 1965 water year, the report format was changed to include data on quantities of surface water, quality of surface water and ground water, and ground-water levels. Because of the increase in hydrologic data collected in Illinois, a two-volume format was published for water years 1978 to 1997. Greater requests for hydrologic data in electronic format and the addition of meteorological and biological data to the report have shown a need to provide data in an electronic report. Beginning in 1998, hydrologic data were published on the Web and on compact disc.

Prior to introduction of this series and for 10 water years (1961-70) concurrent with it, water-resources data for Illinois were published in USGS Water-Supply Papers. Data on stream discharge and stage and on lake or reservoir content and stage, through September 1960, were published annually under the title "Surface-Water Supply of the United States, Parts 3A, 4, and 5." For the 1961-70 water years, the data were published in two 5-year reports. Data on chemical quality, temperature, and suspended sediment for the 1941-70 water years were published annually under the title "Quality of Surface Waters of the United States," and water levels for the 1935-74 water years were published under the title "Ground-Water Levels in the United States." The above mentioned Water-Supply Papers may be found in the libraries of major cities of the United States and may be purchased from U.S. Geological Survey, Information Services, Denver Federal Center, Box 25286, Denver, CO 80225-0286.

Publications similar to this report are published annually by the USGS for all States. These USGS reports have an identification number consisting of the two-letter State abbreviation, the last two digits of the water year, and the volume number (when applicable). For example, Volume 1 of Illinois' 1997 report is identified as "U.S. Geological Survey Water-Data Report IL-97-1." For archiving and general distribution, the reports for 1961 to current are identified as Water-Data Reports. These Water-Data Reports are for sale in paper copy or microfiche by the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161.

Additional information for ordering specific reports may be obtained from the USGS Illinois Water Science Center at the address given in the section titled "Access to USGS Water Data" or by telephone at (217) 344-0037.

Water-resources data, including those provided in water-data reports, also are available through the World Wide Web on the Internet. The Universal Resource Locator (URL) to the USGS Illinois Water Science Center's home page is http://il.water.usgs.gov/.