Which lake is the main source of the Chicago River?

Lake Michigan is the main source of the Chicago River.

Today, water enters the river through the Chicago River Controlling Works and Chicago Harbor Lock at the lakefront. The main stem then runs west through downtown Chicago to Wolf Point, where it joins the North Branch before continuing south and west.

The river’s direction is a famous engineering story. Historically, it flowed into Lake Michigan, carrying city sewage toward Chicago’s drinking-water supply. In 1900, the newly completed Chicago Sanitary and Ship Canal reversed the main stem and South Branch, sending water toward the Des Plaines and Illinois rivers and ultimately the Mississippi system.

A common mix-up is to treat the Chicago River as naturally flowing into the lake today. Its modern connection to Lake Michigan remains the source, but its controlled flow is largely part of a human-engineered system. The river also links the Great Lakes basin to the Mississippi basin through canals.