According to the U.S. Census Bureau, Champaign county has a total area of 2,584 km² (998 mi²). 2,582 km² (997 mi²) of it is land and 2 km² (1 mi²) of it (0.07%) is water.

Because Champaign County landscape is situated on a large and very flat plateau, it had virtually no natural drainage, so that much of the County consisted of wetlands until drainage ditches were built and swamp lands were reclaimed, beginning in the 1870s. This was an example of an upland marsh, which resulted in a high incidence of malaria before the late nineteenth century.

The topography of Champaign County was formed by the Wisconsin Glacier about 10,000 years before the present. Lobes of ice from what is now Lake Michigan crossed the county, creating a deep pile of glacial soil in Champaign County, up to 300 feet thick, topped by numerous moraines forming small, flat watersheds with no outlets. The water supply of Champaign County is a prevalent factor in the county. Champaign County is situated on the divide between the Ohio and Mississippi Rivers. Rivers flow out of Champaign County to the east, west, and south. The Kaskaskia River has its origin to the northwest of Champaign, draining the western side of that City. The Kaskaskia flows toward the southwest, joining the Mississippi south of St. Louis, Missouri. The Embarras River, on the other hand, drains the south-central portion of the Champaign-Urbana Metropolitan Area, originating in southeastern Champaign and flowing through the experimental fields on the southern part of the campus of the University of Illinois. The Embarras is tributary to the Wabash River and Ohio River systems. The northeast corner of Champaign, the central portion of the University campus, and the northern part of Urbana are drained by the Boneyard Creek, which flows into the Saline Ditch, a tributary of the Vermilion and Wabash Rivers.

Source: A Standard History of Champaign County, Illinois, by J. R. Stewart, published by The Lewis Publishing Company, Chicago And New York, 1918.