The slope in the Illinois River is very gentle in the Lower Illinois region. The La Grange and Peoria Locks & Dams are the most downstream locks and dams in the Illinois River, and they regulate the water level in the Lower Illinois WSPR. Under normal streamflow conditions, the water levels downstream and upstream of La Grange only change by about 10 feet. The water level only changes about 10 feet downstream of Peoria Lock and Dam as well. Meanwhile, the water level lifts about 138 feet upstream of Peoria. 

The Illinois River National Wildlife and Fish Refuge Complex is a mosaic of backwater lakes, bottomland forests, floodplain wetlands, and upland forest and includes 4 refuges which are:  

The Illinois River National Wildlife and Fish Refuge Complex is vital for wildlife conservation, providing (1) places for waterfowl and other migratory birds to stop, rest, feed, and nest, (2) biological diversity and habitats for resident wildlife such as channel catfish and bluegill, and (3) habitats for two endangered bird species, peregrine falcon and least tern, and threatened species such as bald eagles.

Macoupin Creek is the largest tributary to the Illinois River in the Lower Illinois WSPR, and it is approximately 100 miles long and joins the Illinois River near Hardin, IL. The drainage area of the creek is 675 square miles. The original Macoupin Creek channel meanders for about 5 miles before joining the Illinois River and has been replaced with a straight channel connecting with the Illinois River. The two channels form the Macoupin Island. The river valley in the lower Macoupin Creek is narrow with a width of less than a mile. The river valley is primarily flat and has a small gradient along the river channel. The last 10 miles of Macoupin Creek are Illinois public waters and thus require minimum flow for any withdrawals in this section of the creek. Macoupin Creek is critical for water supply purposes as it provides drinking water to about 20 municipalities through 5 public water supply systems, Carlinville, Gillespie, Otter Lake Water Commission, Palmyra-Modesto, and Greenfield.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the  Illinois Department of Natural Resources Office of Water Resources.