The Water Resources Division of the U.S. Geological Survey (USGS), in cooperation with Federal, State, and other local governmental agencies, obtains a large amount of data, collected each water year, pertaining to the water resources of Illinois. These data, accumulated during many years, constitute a valuable data base for developing an improved understanding of the water resources of the State. To make these data readily available to interested parties outside the USGS, the data are published annually in this report series entitled "Water Resources Data - Illinois."

Water-resources data for Illinois for the 1998 water year (October 1, 1997 - September 30, 1998) consist of records of stage, discharge, and water quality of streams; stage of lakes and reservoirs; water level and water quality of ground-water wells; precipitation and biology. This report contains (1) discharge for 166 surface-water gaging stations and for 11 crest-stage partial-record stations; (2) stage for 13 surface-water gaging stations; (3) stage for 2 reservoirs; (4) water-quality records for 12 surface-water gaging stations; (5) sediment-discharge records for 9 surface-water gaging stations; (6) water-level records for 1 observation well; (7) water-quality records for 19 wells; (8) precipitation records for 31 rain gages; and (9) biological records for 6 sample sites. Additional water data were collected at various sites not involved in the systematic data-collection program and are published as miscellaneous water-quality analyses.

This series of annual 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 present, in one volume, 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 have shown a need to provide the report on compact disc (CD). Beginning in 1998, hydrologic data were published on CD with limited paper copies available from the Illinois District office at the address given in the CD insert.

Prior to introduction of this series and for several 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, Branch of Information Services, Box 25286, 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 1971-74 water years also 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 District Office at the address given in the CD insert 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 Illinois District's home page is: