The Illinois River (continued)

As a result of these geological events, the rate of fall of the Illinois River is low: 0.03 m per km (0.17 ft per mile) between Hennepin and Pekin, and 0.02 m per km (0.13 ft per mile) from Pekin to Meredosia. The upper reaches of the river drop 60 feet in 57 miles, whereas, the lower reach (from Hennepin south) falls 25 feet over 215 miles - a drop of 1.4 inches per mile (Cruikshank 1998).

Another result is that the current is typically sluggish; it moves at 2.01 to 4.02 km (1.25-2.50 miles) per hour. Because of this gentle slope below the Big Bend at Hennepin, the valley is aggrading rather than eroding as is typical for most river valleys. Below the Big Bend, the rocky bluffs disappear and the floodplain widens dramatically from two to almost twelve miles. These geologic characteristics contribute to the water quality problems, particularly those related to sedimentation. The Illinois River, according to a quote by Harland Barrows in 1910 (reported by Bellrose 1966), “is wholly unequal to the task of washing forward the sediment delivered by its headwaters and its numerous tributaries. This is the reverse of the normal relation between tributaries and their main streams.”

The lower Illinois River is composed of five sub-basins (Figure 1), including: the Vermilion, Sangamon, LaMoine, Spoon, and Mackinaw rivers.

Figure 1: The Sub-basins of the Lower Illinois River Basin (USGS 1998)

Travelers’ accounts of their travels document the slow, shallow waters characteristic of the Illinois River:

In 1821, Henry Schoolcraft travels up the Illinois River and provides the following description of this gentle, sluggish flow:

...The Illinois presents to the eye a smooth and sluggish current, bordered on each side by an exuberant growth of aquatic plants, in some places, reach nearly across the channel...There perhaps is no stream in America whose current offers so little resistance in the ascent...(Cruikshank 1998).

In 1838, the Secretary of War, Joel R. Poinsett, attests to the slow flow of the river in a statement to the U.S. Senate:

The Illinois River...from Peru to its mouth flows with a current so very gentle and uniform as to cause but a few hours difference in the time occupied by steamboats in ascending or descending its stream, an estimated distance of two hundred and fifty miles...The current of the river is so sluggish that it is believed a velocity could not even be these means (constructing wind dams) be given it sufficient to remove the sand bars entirely... (Cruikshank 1998).