result of sedimentation must be taken into account if sedimentation 

 rates are compared over a period of time. By graphing the rate of 

 fill at each location against its original depth, the effect of changing 

 depths is negated, allowing a direct comparison of sedimentation rates. 



Sedimentation studies made at Lake Meredosia reveal the importance 

 of changing depths when comparing sedimentation rates over a large 

 number of years. A direct comparison of gross average fill per year 

 at Lake Meredosia for the periods 1903-1956 and 1956-1978 was .043 ft. /year and 

 .042 ft. /year respectively. This would seem to indicate that the sedimentation 

 rates are nearly equal. By plotting the rate of sedimentation against 

 the depth (Figure 9) and comparing the location of the regression lines, 

 an increase in the sedimentation rate can be seen. Although the sedi- 

 ment load of the water has increased, unless the effect of changing 

 depths is taken into account, the sedimentation rates cannot be directly 

 compared over a long period of time. 



The streams that flow into the Illinois have a steeper gradient 

 than does the Illinois in its central and lower reaches (Mills, Star- 

 rett, and Bellrose, 1966: 5). As a result of its low gradient and slow 

 current velocity, the Illinois River deposits silt in the bottomland 

 lakes during high water. The effect of drainage has been to reduce 

 the area in which silt can be deposited, thereby increasing the amount 

 of silt in the river and remaining bottomland lakes. 



The construction of the navigation dams on the Illinois and Missis- 

 sippi Rivers slowed the current, compounding this problem. Forbes- and 

 Richardson (1920: xi, xli) reported that the Illinois River's usual 

 rate of flow for ordinary stages varied from lh to lh miles per hour. 

 As a result of higher dams associated with the nine-foot channel and 

 reduced diversion from Lake Michigan, the current velocity is now only 

 about 0.6 miles per hour at ordinary stages. 



Solomon et al. (1975: 67) indicate that on the Mississippi at low 

 and intermediate river flows, pool levels are held above the natural 

 level by the dams, resulting in decreased flow velocity. Carlander 

 (1954: 21) also indicated the current in most of the Mississippi River 

 from St. Paul, Minnesota to Alton, Illinois had been greatly reduced 



144 



