Water falling as rain or snow across Illinois begins a journey determined by the state’s topography. The path water takes is dictated by two major drainage basins, creating a flow pattern that ultimately connects the state to two different oceans. This movement follows the gradients of the landscape until it reaches a major river system.
The Vast Mississippi River Watershed
The vast majority of precipitation (95% of the state) flows into the expansive Mississippi River watershed. The state sits almost entirely within the greater Mississippi basin, meaning the land slopes generally downward toward the west and south. This ensures that most of the state’s surface water is destined for the main river artery.
Water collects in a network of major tributaries before joining the Mississippi River. The Illinois River, a principal tributary, collects drainage from nearly half of the state. It begins near the Chicago metropolitan area and flows southwest until it meets the Mississippi near Grafton, Illinois. Further south, the Kaskaskia River carries water from central Illinois westward, while the Big Muddy River drains the southwestern region.
The Ohio River also plays a significant role, forming the southern and southeastern border of Illinois before its confluence with the Mississippi at Cairo. These waterways funnel precipitation into the larger system.
The Great Lakes Drainage Anomaly
A small exception to this pattern exists in the northeastern corner of the state, primarily in the Chicago area. Naturally, a tiny portion of Illinois once drained east, across a low continental divide, into Lake Michigan and the Great Lakes basin. This natural divide separates the Mississippi watershed from the Great Lakes watershed.
However, this natural flow was altered by a massive civil engineering project completed around the turn of the 20th century. To address sanitation issues, engineers reversed the flow of the Chicago River, redirecting it away from Lake Michigan and toward the Des Plaines River. This change sent the city’s water into the Mississippi River system, preventing pollution of the city’s drinking water source.
This engineering feat means that most water falling in the Chicago area today is diverted to join the dominant southwestern flow. Despite this diversion, a small segment of the far northeastern region still naturally drains into Lake Michigan. This area represents the state’s only direct contribution to the Great Lakes system.
The Ultimate Journey to the Ocean
The final destination for water originating in Illinois depends on which of the two major drainage basins it enters. For the vast majority of the state’s water, the journey continues south along the Mississippi River. This waterway carries the combined flow of its tributaries approximately 2,340 miles to its mouth.
After collecting water from Illinois’ major rivers, the Mississippi flows through several states before emptying into the Gulf of Mexico. This is the oceanic endpoint for nearly all precipitation that falls on the state. The retention time for this water, from the upper reaches of the Illinois River to the Gulf, can be approximately 90 days.
Conversely, water that naturally drains into Lake Michigan follows a different and much longer path to the Atlantic Ocean. This water exits the Great Lakes system through the St. Lawrence River, traversing the North American continent to reach the sea. The Gulf of Mexico receives the overwhelming share of Illinois’ surface water.