Grey Eagle, Frand Steele, Jeanette Roberts, and Time and Tide at dock in St. Paul, MN

Human impact on the river has been great over the past 150 years. The population along the Illinois River grew from a 0.5 million in 1850 to over 1.6 million twenty years later and to 8.5 million by 1964, by which time 47 percent of the land in the river basin was under cultivation. The river has given residents and visitors fish, mussels, waterfowl, water, and recreation, but people living along the river have used and misused the river and its resources. This population growth affected the use of the river for transportation and sanitation, the use of its resources for food and materials, and the wear and tear on the river and its environment. Siltation and pollution became major factors in the degradation of river quality.

Siltation
Backwater lakes (called lateral levee lakes) are formed in the spring flooding, widening the Illinois River Valley. However, the building of levees to increase farmland in the twentieth century increased siltation, which slowed the river even more. With silt accumulation in the river and lake bottoms, water capacity is lost. For example, between 1925 and 1951, Lake Chautauqua lost more than 18 percent of its capacity because of increased levels of silt. As the silt built up in four- to eight-foot layers, the river bottom had difficulty growing the aquatic plants that feed fish, shellfish, and fowl. River traffic stirred up this silt, making the water consistently muddy.

Pollution
With the construction of the Chicago Sanitary and Shipping Canal, and increased industrial development in Peoria, pollutants from sewage, farming, and industry poured into the river. In 1922, this amounted to the pollution from a population of six million people. Beginning in the early 1920s high levels of pollutants depleted the water of oxygen needed by aquatic plants and animals. Further loss of aquatic plants was due to turbidity of the water, which blocked out sunlight and offered inferior footing for the plant roots to take hold. The reduction in the numbers of aquatic plants and animals led to lack of food supply for waterfowl and fish, resulting in a break in the food chain. Once-abundant waterfowl and fish started to disappear.