rivers, mileages are often given on navigation aids, such as buoys, 

 markers, and lights. 



Both rivers have been subject to a variety of influences besides 

 the construction of the locks and dams associated with the 9-foot navi- 

 gation channel. For example, the navigational improvements associated 

 with the 4%-foot channel and the 6-foot channel on the Mississippi 

 River caused some long-term changes in the morphometry and hydrology 

 of the river (U.S. Army Engineer District, St. Louis, 1975: 2-3). Both 

 rivers were subjected to increasing sediment loads as agriculture in- 

 tensified in the drainage basin, and to increasing pollution loads as 

 population centers increased. Virtually the entire Illinois River was 

 affected following the 1900 diversion of sewage effluent from Chicago 

 into the upper reaches of the river. Draining and leveeing of the 

 floodplain has markedly reduced the amount of natural habitat. Insofar 

 as possible, we have tried to sort out the effects attributable to the 

 nine-foot navigation project from the other effects, but many of our 

 conclusions are necessarily of a qualitative, rather than a quantitative 

 nature. Another handicap was that maps and tables showing the acreage 

 of wetland and aquatic habitat existing before and after construction of 

 the navigation dams did not become available until after our report was 

 completed. 



