Summary

As we have learned, the Illinois River basin is large; it covers 44% of the land area of the state and includes within it 46% of the state’s agricultural land, 28% of its forests, and 37% of its surface water and streams. Perhaps even more remarkable is that over 95% of the urban area within Illinois lies within the Illinois River Basin (Terstriep 1987).

This large population and its dependence on the river to meet often conflicting demands are responsible for most of the problems facing the river today. Prior to 1850, humans had little impact on the land; however, in the last half of the twentieth century, a series of events begin that leave the river changed forever. Among these are:

• Dams constructed between 1872 and 1893 at Henry, Copperas Creek, LaGrange, and Kampsville create a 7-foot deep navigational channel from the Mississippi River to La Salle;
• The reversal of the Chicago River in 1900 and the diversion of Lake Michigan water into the Illinois River increases the volume of water - mostly untreated sewage - entering the river;
• Industrialization and the use of the rivers to dispose of wastes from a diverse assortment of businesses - distilleries, slaughterhouses, and tanneries - further pollutes the system;
• The invention of farm equipment that allows the clearing of much of the land for agricultural crops eventually exacerbates sedimentation, a primary type of non-point source pollution; and
• The construction of levees throughout the basin between 1900 and 1920 removes 185,000 acres of land from the floodplain.

During this period when significant events occur throughout the river basin, less obvious events or trends also occur that affect the river: clearing of forests, drainage of wetlands, urban development, expansion of agriculture, levee construction, and an increase in navigation. Each of these produces physical changes that impact the river. The more obvious ones are listed below (Terstriep 1987).

• Clearing forests

  Elimination of habitat
  Reduced infiltration
  Increased runoff
  Increased erosion and sedimentation
   

• Wetland drainage

  Reduced flood storage
  Increased runoff volume
  Increased peak flows
  Elimination of habitat
   

• Urban development

  Reduced infiltration
  Accelerated runoff
  Increased runoff volume and flood peaks
  Elimination of habitat
  Increased pollution
   

• Agricultural expansion

  Reduced infiltration
  Increased runoff
  Elimination of habitat
  Increased chemical runoff and sedimentation
   

• Levee construction

  Reduced flood storage
  Increased flood stages
   

• Navigation dams - short term

  Creation of deep pools
  Increased area of backwater lakes
  Increased low flows
   

• Navigation dams - long term

  Increased sedimentation
  Increased barge traffic
  Sediment resuspension
  Shoreline erosion
  

Do you know why each activity results in these impacts upon the river? Can you think of other activities that are taking place in the Illinois River basin and what affects they might have?