When rain falls, the water doesn’t just disappear; it has to go somewhere. Some of it soaks into the ground, replenishing groundwater, while the rest flows downhill over the land’s surface. As water moves, it gathers into small rivulets, which join creeks, which then merge with larger rivers and lakes.

A watershed is the entire area of land that collects and channels water toward a common outlet, such as a stream, river, or lake. Think of watersheds like the veins on a leaf; each small vein drains into a larger one, and they all lead to the stem. Similarly, every drop of water in a watershed follows the natural contours of the land, eventually reaching a larger body of water.

Some helpful illustrations and information about watersheds are available through the National Oceanic and Atmospheric Administration, National Geographic Society, and U.S. Geological Survey. Illinois Extension also has watershed resources as part of the state's Nutrient Loss Reduction Strategy. 

Where can you find watersheds? 

Watersheds are everywhere, including where you live! They direct rainwater and runoff across the land, eventually draining into a river, lake, or even the ocean. Illinois is bordered by two major rivers, the Mississippi and Ohio. Most watersheds drain into the Mississippi River, while much of the remaining watersheds reach the Ohio River by way of the Wabash River. The Illinois State Water Survey map of the Major Watersheds of Illinois illustrates where different watersheds drain. 

A sliver of land in northern Illinois drains into Lake Michigan. Through major engineering, Lake Michigan and the Illinois River have been connected, which allows travel and goods to be shipped between the Great Lakes and the Gulf of Mexico. Later this spring, the Everyday Environment podcast will explore the reversal of the Chicago River!

Why are watersheds important? 

As water flows downhill through a watershed, it serves countless purposes for both nature and people.

- Wildlife and Ecosystems: The first to rely on this water are the animals, trees, and plants that live in and around it. Streams, ponds, and wetlands provide essential habitat, while groundwater replenishes forests and prairies.

- Agriculture: Small ponds and reservoirs help store water for crops and livestock, ensuring food production even during dry spells.

- Communities and Industry: Further downstream, lakes and reservoirs supply drinking water for cities, support food processing, and even cool power plants that generate electricity.

With so many demands, water management becomes a balancing act, especially during droughts when rainfall is scarce. Water supply planning is critical to ensure enough water is available for both people and ecosystems. When water carries pollutants or excess sediment downstream, it must be cleaned before use. Erosion upstream can clog ponds and reservoirs with sediment, reducing storage capacity and making drought impacts even worse.

Planning for the future of Illinois watersheds

By understanding how we use and manage water in a watershed, we can work toward sustainable solutions that keep water clean, available, and beneficial for all. To address these issues, Illinois state agencies created the 2022 Illinois State Water Plan, which determined critical water issues that need attention. Those issues include water quality, climate impacts, how to integrate water management across user needs, long-term funding for projects, water sustainability, Lake Michigan, flood damage mitigation, aquatic and riparian habitat, water use laws and regulations, river navigation, erosion and sedimentation, stream data management, and recreation uses.  Many of these issues are cross-cutting. 

To stay on top of this, the State of Illinois established the Illinois State Water Planning Task Force, which meets quarterly to coordinate and share information on the progress of over 130 recommendations. Agencies already have responsibilities that overlap with some of the critical water issues and steer efforts toward making progress on recommendations. This is an open meeting, and attendees have the opportunity to ask questions or bring up concerns.

Understanding watersheds is essential for protecting our water resources and ensuring they remain clean and available for future generations. As Illinois continues to address water challenges through planning and collaboration, we all have a part to play in protecting our local waters.

This guest article was written by Laura Keefer, Illinois State Hydrologist and Interim Director of the Illinois State Water Survey

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