of the Mississippi River near Quincy, Illinois (Mississippi River mile 

 325.0). Light penetration was measured by using a Jackson turbidimeter 

 and averaged 38 cm for the year (Dorris e_t _al . , 1963: 84). Dorris et al. 

 (1963: 84) also found a relationship between stream discharge and light 

 penetration. High stream discharge rates were accompanied by low light 

 penetration and low photosynthesis. High turbidity appeared to be caused 

 by silt loading, rather than by plankton, since photosynthetic oxygen 

 production almost always decreased at the time of high turbidity 

 (Dorris et al. , 1963: 84). 



Recent turbidity data for Pools 24, 25, and 26 were found in Dunham 

 (1971: Table 10) and Colbert et al. (1975: Table 6). Dunham (1971: Table 

 10) measured tailwater turbidities below Locks and Dams 12-26 in 1971. He 

 found that water clarity (Secchi disk) during low water decreased down- 

 stream from a high of 20 cm below Lock and Dam 12 to a low of 10 cm be- 

 low Lock and Dam 26 (Dunham, 1971: Table 10). Colbert et al . (1975: 32) 

 measured turbidity photometrically with a Hach Model DREL laboratory 

 kit, using a Hach absorptometric method. Turbidity values for Pools 24, 

 25, and 26 in 1974 were directly related to current velocity (Colbert et al, 

 1975: 36), and mean values at the surface were 257.4 units during high 

 water in July and 46.2 units during low water in September (Colbert et al. , 

 1975: Table 6). Settleable solids, measured volumetrically with a 

 1-liter Imhoff cone using standard methods, followed the same pattern 

 (Colbert et al. , 1975: 22, Table 6). 



Regarding the quantity of erosion silt in the Mississippi River, 

 Ellis (1931b: 5) wrote: 



The most outstanding factor producing changes in river 

 conditions at present (1930) throughout the Mississippi system 

 was found to be erosion silt. As a result of deforestation, 

 current methods of tilling the land, and various improve- 

 ments incident to commercial progress as road building, 

 the amount of erosion silt which is being received by the 

 various streams of the Mississippi system has been progressively 

 greater during the past 10 years until it now presents perhaps 

 the most acute fisheries problem in our inland rivers. 



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