The Illinois River is 273 miles long from its origin where the Des Plaines and Kankakee flow together to its mouth north of Alton, where it flows into the Mississippi (Illinois Department of Natural Resources). 

The Report of the Rivers and Lakes Commission on the Illinois River, published in 1915, includes the following description: 

"In many respects the Illinois River is one of the most remarkable streams in the United States. Its past importance as an avenue of water commerce, the possibilities of its future in this respect, its fresh water fisheries, its use as the main sewer, so to speak, of the second city in the country, and more recently, the agricultural development on its bottom lands through the construction of levees, all have led to perhaps more thorough studies, with various objects in view than has been received by any other of our rivers."

The introduction to the Integrated Management Plan noted that as of 1995 90% of the state's population lived in the 55 counties that comprise the Illinois River watershed.  Two of Illinois' three UNESCO Ramsar Wetlands of International Importance, The Emiquon Complex and the Sue and Wes Dixon Waterfowl Refuge at Hennepin & Hopper Lakes, are adjacent to the Illinois River.

The reports and other documents on the Illinois River included here address a wide range of issues including settlement, navigation, game wildlife and other biota, water quality, sedimentation, flooding, and land use. Documents focusing on management of the Illinois River and data resources are listed in the right-hand column.