Countywide Vertical Datum Conversion Factors in Illinois - This map displays countywide, conversion values to adjust elevations from the National Geodetic Vertical Datum 1929 (NGVD 29) to the North American Vertical Datum 1988 (NAVD 88). Values indicated should be added to current NGVD 29 elevations (not displayed) to obtain the NAVD 88 elevation. Countywide values were calculated using the methodology outlined in the Federal Emergency Management Agency (FEMA) Guidelines and Specifications for Flood Hazard Mapping Partners Appendix B (www.fema.gov/plan/prevent/fhm/dl_cgs.shtm).

Point conversion factors were computed using the CORPSCON utility (http://crunch.tec.army.mil/software/corpscon/corpscon.html).

Major Watersheds of Illinois - This map illustrates general boundaries of major watersheds of Illinois. Shades of blue and green indicate watersheds in the Mississippi River basin. Watersheds with shades of green are located in the Illinois River basin. Shades of yellow and brown indicate watersheds in the Ohio River basin. The Cache River drains to both the Mississippi and Ohio Rivers. Drainage to Lake Michigan is in the Great Lakes basin. Its watershed is shaded pink. A watershed is often considered synonymous with drainage basin, and in this context it is the land area that directly drains to a common stream, river, or lake. Watershed boundaries follow topographic highs. Artificial drainage structures such as canals may convey water across topographic watershed boundaries in some areas shown. Features shown on this map are accurate to the scale at which they were originally developed. Communities are displayed as space allowed. To request a paper copy of the Major Watersheds of Illinois map from The Illinois State Water Survey, please fill out this form. Copies are available free of charge until supplies run out.

Lambert Conformal Conic projection based on standard parallels 30° and 45°

Community Surface Water Supplies Year 2000 - Community water supplies provide nearly 90% of Illinois' citizens with water for residential use. The Illinois Environmental Protection Agency (IEPA) defines a "community water supply" as a public water supply that provides potable water to a minimum of 15 service connections used by year-round residents or regularly serves at least 25 year-round residents. The IEPA issues permits for operation of water supplies serving the public. Data from the Illinois State Water Survey (ISWS) Illinois Water Inventory Program (IWIP) show that 1998 total withdrawals by all public water supply systems exceeded 1,700 million gallons per day (mgd). Community systems withdrawing surface water accounted for 84% of those withdrawals, with Lake Michigan withdrawals exceeding 1,000 mgd. Other surface water sources include interstate rivers (Mississippi and Ohio), intrastate rivers (Fox, Illinois, Kankakee, Kaskaskia, and Little Wabash), and 96 reservoirs.

Due to Homeland Security issues, we can not freely distribute this map through the World Wide Web. For a copy, you will need to send a Letter of Intent on your company’s letterhead, stating why you would like this map. This map is prohibited from being displayed publicly. Please include contact information, as we may call to verify the information. Upon receipt of the letter, it will be forwarded to the Survey Director for approval. You can fax it (217-333-6540) or mail it to:

Illinois State Water Survey
c/o Gloria Marsh
2204 Griffith Drive
Champaign, IL 61820-7495

Illinois River Watershed - The Illinois River watershed (shaded in light blue) is of vital importance to the State of Illinois. This large watershed covers 44 percent of the state and 90 percent of Illinois' population resides within 55 counties wholly or partially included in the watershed. The Illinois River, one of the major tributaries of the Mississippi River, is part of the only inland waterway linking the Great Lakes to the Gulf of Mexico. The Illinois River watershed has a drainage area of 28,906 square miles or sq mi (75,156 square kilometers or sq km), of which approximately 3,058 sq mi (4,920 sq km) are located in Indiana and 1,070 sq mi (1,722 sq km) in Wisconsin. The watershed contains the drainage basins of several of the Illinois' major rivers including the Des Plaines, Kankakee, Fox, Vermilion, Mackinaw, Spoon, Sangamon, and La Moine Rivers.

This map is available online as a 26" x 38" PDF file (6mb).