Major River Basins

Rock River Basin

The Rock River Basin (below) covers portions of south central Wisconsin and north and northwest Illinois. Any precipitation that occurs in the basin (minus evaporation and transpiration) will eventually make it to the Rock River before reaching the the outlet near the Quad Cities. The headwaters in south central Wisconsin reach as far as Madison and the far western suburbs of Milwaukee. Tributaries of the Rock River include the Pecatonica River, Sugar River, Yahara River, Kishwaukee River, and Green River.

The western portions of the basin are relatively more hilly than the northern and eastern portions. The northern portion of the basin, especially the sections including the Rock River and Yahara River, contains numerous lakes and small dams. Precipitation that occurs in the general area between Madison and Milwaukee usually takes longer to move toward the outlet, often causing lower, but longer, crests. Precipitation that occurs to the west of Madison and Rockford usually moves downstream more quickly, often causing higher, but quicker, crests. The Rock River Basin once contained the Inlet Swamp (at the headwaters of the Green River, a tributary of the Rock) and the Winnebago Swamp (between the Green River and Rock River). These wetlands were drained in the late 1800s.

Numerous small dams have been built along the Rock River and some tributaries. These dams generally have little-to-no storage capacity, which means they have little ability to regulate the flow of water for flood control. Gates at small, run-of-the-river dams such as these simply pass the flow that reaches them.

Fox River Basin

The Fox River Basin (below) covers a small portion of southeast Wisconsin and a narrow corrdior across north and northeast Illinois. Any precipitation that occurs in the basin (minus evaporation and transpiration) will eventually make it to the Fox River before reaching the outlet near Ottawa. The headwaters in southeast Wisconsin reach the western suburbs of Milwaukee. The Fox River has a fairly narrow basin with few major tributaries other than numerous small creeks.

Although the northern portion of the basin is relatively more hilly than areas to the south, the river slope is very shallow. The river flows through numerous lakes in Wisconsin and also the Fox Chain of Lakes in Illinois near the state line. The southern portion of the basin is relatively flatter but has a more pronounced floodplain and steeper river slopes. Precipitation that occurs near and north of the Illinois-Wisconsin state line usually takes longer to move downstream toward the basin outlet due to the lakes and shallower slope. Heavy rainfall occurring in the middle or lower portion of the basin may cause the river to crest quickly; often this occurs prior to the upstream flow reaching the location.

Numerous small dams have been built along the Fox River, although a few have since been removed. These dams generally have little-to-no storage capacity, which means they have little ability to regular the flow of water for flood control. Gates at small, run-of-the-river dams such as these simply pass the flow that reaches them. Fox Chain of Lakes is one of the large lakes along the river but is only able to impact flood crests by a few inches.

Des Plaines River Basin

The Des Plaines River Basin (below) covers a very small portion of far southeast Wisconsin and north and a narrow corrdior across northeast Illinois. Any precipitation that occurs in the basin (minus evaporation and transpiration) will eventually make it to the Des Plaines River before reaching the the outlet near Channahon. The headwaters near the Illinois-Wisconsin state line reach just west of Kenosha. Tributaries of the Des Plaines River include the DuPage River (including west and east branches), Salt Creek, and artificially-diverted water from the Chicago Sanitary & Ship Canal and the Cal-Sag Channel. Just downstream of Channahon, the Des Plaines River meets the Kankakee River to form the Illinois River.

The basin has generally flat terrain throughout. Due to a combination of urbanization and shallow river slopes, heavy rainfall occurring in the middle or lower portion of the basin may cause the river to crest quickly, often prior to the upstream flow reaching the downstream location.

Numerous small dams have been built along the Des Plaines River, but several have been removed in recent years.

Kankakee River Basin

The Kankakee River Basin (below) covers portions of north central Indiana, northwest Indiana, and east central Illinois. Any precipitation that occurs in the basin (minus evaporation and transpiration) will eventually make it to the Kankakee River before reaching the the outlet near Wilmington. The headwaters in north central Indiana reach as far as the South Bend area. Tributaries of the Kankakee River include the Iroquois River, Sugar Creek, and the Yellow River. Just downstream of Wilmington, the Kankakee River meets the Des Plaines River to form the Illinois River. The southern portion of the Kankakee River Basin is covered by the Iroquois River Basin.

The basin has generally flat terrain throughout. Due to the flat terrain, the Kankakee River valley in northwest Indiana and a small portion of east central Illinois was once the site of the Grand Kankakee Marsh, one of the largest freshwater marshes in the United States. The Grand Kankakee Marsh was drained in the late 1800s. Due to shallow river slopes, heavy rainfall occurring in the middle or lower portion of the basin may cause the river to crest quickly, often prior to the upstream flow reaching the downstream location.