the bottomland acres were drained (Mills, Starrett, and Bellrose, 1966: 

 5). The second major alteration was the implementation of the nine-foot 

 channel. By comparing mean water levels before and after the 

 nine-foot channel project at dams 24, 25, and 26, the increase in water 

 levels was found to be approximately 9, 10, and 9 feet respectively. 



As a result of man's activities, the rate of sedimentation and the 

 turbidity have increased since the 1900' s. The apparent main factor 

 responsible for this increase has been the expansion of row crop produc- 

 tion. Construction of the locks and dams and municipal sewage effluents 

 have compounded this problem. These activities have changed the clarity 

 of the river and the nature of the bottomland lakes in the Illinois and 

 Mississippi Rivers. Detailed studies to determine what percent of 

 sediment results from agricultural erosion and what percent from construc- 

 tion of the dams have never been undertaken. 



An increase in the turbidity of the Illinois River can be demonstrated 

 by comparing data collected by Kofoid in 1903 and the Illinois Natural 

 History Survey in 1974 and 1976. Kofoid (1903: 179) used a white porcelain 

 plate to determine that the majority of transparencies in the Havana, Illinois 

 area were between 8 and 20 inches. He stated that during floods, the 

 water was turbid; however, the water cleared following its decline. In 

 1974, the majority of Secchi disk readings taken in the Alton Pool, 

 Illinois River, were between 7 and 9 inches. Bottomland lakes located 

 in the middle Illinois River valley were more turbid; the majority of 

 readings collected by the authors in 1976 were between 4 and 7 inches. 



A three-fold increase in Jackson Turbidimeter Units (JTU's) taken 

 under similar conditions between 1897 and 1964 occurred in the La Grange 

 Pool, Illinois River (Mills, Starrett, and Bellrose, 1966: 7). A compari- 

 son of JTU's taken at Lake Chautauqua in 1955 (Jackson and Starrett, 1959: 

 13) with JTU's obtained in 1977 under analogous conditions indicates 

 an almost two-fold increase. Although these data are from La Grange Pool, 

 the increase in turbidity at Lake Chautauqua is indicative of the general 

 change that occurred in the bottomland lakes of the Illinois River. 



There were few turbidity measurements in the Mississippi River. In 

 July, 1921 Galtsoff (1923: 372) measured a transparency of 61 cm near 

 Montrose, Iowa. The Illinois Natural History Survey recorded transparencies 



142 



