Hydrologic Services Area

Major River Basins

Several major river basins are within the NWS Chicago hydrologic service area. The large Illinois River Basin (dark gray) includes several smaller river basins, including the Fox (green), the Des Plaines (yellow), the Kankakee (orange), and the Vermilion (red). Headwaters of the Illinois River Basin are in southeast Wisconsin, northeast Illinois, and northwest Indiana. The Chicago River basin (light gray) once drained into Lake Michigan, but due to human alterations, most of its water is diverted into the Illinois River via the Chicago Sanitary and Ship Canal.

The Rock River Basin (blue) headwaters are in southern Wisconsin.

The Calumet River Basin (purple) covers the southern sections of the Chicago metro area. The Calumet River once drained into Lake Michigan, but due to human alterations, some of its water is diverted into the Illinois River via the Calumet-Saganashkee Canal and the Chicago Sanitary and Ship Canal.

The Vermilion River Basin (red) covers a small portion of central Illinois between Chicago and Peoria.

More Information

Learn more about drainage basins

Terrain

A map showing the elevation of the NWS Chicago hydrologic services area

The terrain of area river basins has been heavily affected by recent ice ages and the presence of glaciers. Most of the area, especially toward the southeast, has relatively flat terrain with small areas of hilly terrain at locations that were once edges of the glaciers (called moraines). Some rivers, such as the Calumet or the Chicago, flow in between glacial moraines.

Elevations range from about 580 feet near Lake Michigan to 440 feet near the Illinois River in LaSalle to near 1200 feet in Winnebago County.

Land Cover

The land cover of the area has changed dramatically since before the widespread settlement of the area in the 1800s. The area was once mostly covered with prairie, forest, and wetlands (below, left). Numerous large wetlands were present, including the Grand Kankakee Marsh in northwest Indiana, the Inlet Swamp in Lee & Ogle Counties at the headwaters of the Green River, the Winnebago Swamp along the Green River in Lee and Bureau Counties, and the Vermillion Swamp covering portions of Livingston, Ford, and Iroquois Counties at the headwaters of the Vermilion River. Today, much of the land has been altered by humans to support agricultural and large urbanized areas (below, right). Small areas of forest and wetland remain, mostly near rivers and streams. The significant changes to area land cover including the removal of large marshes impacted the hydrology of area rivers in ways that are still noticed today.

1800s	2011

Population

Population density (below, right) is highest in the Chicago Metropolitan Area near Lake Michigan, and in the Rockford area. Other areas are generally more rural with agricultural land uses.

NOTE: Population data is "ambient population" - the average location of persons throughout the 24-hr day. Data was last updated in 2010.