Calhoun County is a county in the U.S. state of Illinois. As of the 2020 census, the population was 4,437, making it Illinois’ third-least populous county. Its county seat and biggest community is Hardin, with a population of 801. Its smallest incorporated community is Hamburg, with a population of 99. Calhoun County is at the tip of the peninsula formed by the courses of the Mississippi and Illinois rivers above their confluence and is almost completely surrounded by water. Calhoun County is sparsely populated; it has just five municipalities, all of them villages.

The Illinois River is a principal tributary of the Mississippi River at approximately 273 miles (439 km) in length. Located in the U.S. state of Illinois, the river has a drainage basin of 28,756.6 square miles (74,479 km2). The Illinois River begins with the confluence of the Des Plaines and Kankakee rivers in the Chicago metropolitan area, and it generally flows to the southwest across Illinois, until it empties into the Mississippi near Grafton, Illinois. Its drainage basin extends into southeastern Wisconsin, northwestern Indiana, and a very small area of southwestern Michigan in addition to central Illinois. Along its shores are several river ports, including Peoria, Illinois. Historic and recreation areas on the river include Starved Rock, and the internationally important wetlands of the Emiquon Complex and Dixon Waterfowl Refuge.