of 27 and 40 cm in the same location in late June, 1974 and 1975. The scant 

 data available indicate that turbidity has increased in the Mississippi 

 River, although this increase is not as great as in the Illinois River. 



Several factors have been responsible for the increased sedimentation 

 rate. The dramatic intensification in agrarian practices since the 

 middle 1800 's has greatly increased soil pollution in the Illinois and 

 Mississippi River valleys. Clearing of fencer ows and bottomland timber in 

 addition to emphasis on row crop production have created serious erosion 

 problems. In 1886 Illinois farmers planted nearly 6 million acres of 

 row crops (Aldrich, 1965) , approximately 17 percent of the state. The 

 number of acres increased to over 10 million by the early 1900' s (Aldrich, 

 1965: 12) or 28% of the state planted in row crops. A slight increase to 

 33.3% was calculated from the Illinois Cooperative Crop Reporting Service 

 annual summary in 1945. A jump to 50.6 percent was calculated in 1974 

 and 54.8 in 1976. When using 52 counties in the state which drain 

 completely or partially into the Illinois River, a similar increase is 

 found. In 1945 41.1 percent of these counties were planted in row crops. 

 By 1974 this figure had jumped to 57.8 percent and increased by 3.7 per- 

 cent in just two years to 61.5 percent in 1976. 



We have no data to illustrate this change on the Mississippi drainage 

 system, but Barnickol and Starrett (1951: 274) indicate that soil pollution 

 has long been associated with the Mississippi below the mouth of the Mis- 

 souri, but it was not generally noted in the upper part of the river 

 until after the development of intensive farming in the Middle West. The 

 drainage of bottomland lakes and marshes and the channelization of tribu- 

 taries have increased the amount of sediment entering the rivers. 



Sedimentation rates are influenced by several factors among which 

 are water flow, seasonal variability of the flow, sediment load, charac- 

 ter of the sediment, and geometry of the system. Although all of these 

 factors play a role in sedimentation, there is a high correlation between 

 sedimentation rates and water depth in the backwater lakes of the Illinois 

 River. Deep water areas of these lakes show higher sedimentation 

 rates than those with shallower waters. The constant reduction in depth as a 



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