624 



U. S. BUREAU OF FISHERIES 



and the bacterial counts from water at considerable distances from 

 these sources of pollution were extremely high. 



(c) The enormous quantities of erosion silt entering the Missis- 

 sippi have produced a condition of supersaturation so that any slow- 

 ing of the water either by man-made or by natural obstructions re- 

 sults in a rapid deposition of large quantities of silt. This silt first 

 of all is smothering out all sorts of bottom life by simply burying 

 the bottom animals under a layer of soft ooze, varying from a few 

 inches to 7 or 8 feet in thickness, in which they are unable to live. 

 Besides this factor, and it in itself is serious, this silt carries down 

 with it quantities of undestroyed organic material put into the river 

 by the sewage of the larger cities. This organic matter is buried in 

 a thick colloidal suspension and the decomposition of the organic 

 fraction proceeds more slowly than if the waste were in the moving 



Figure 9. — U. S. Engineers' Quarterboat No. 3-'iS, used as a floating laboratory in a 

 limnological study of the upper Mississippi River 



stream. As a result the oxygen demand of water rises rapidly and 

 the oxygen saturation falls in those portions of the river where the 

 water is slowed down and the silt deposited. Large volumes of 

 methane, and various sulphurous gases were found rising from some 

 of these silt deposits, and the fauna reduced to bloodworms and 

 other forms indicative of low oxygen conditions. 



The silt situation was found very acute, as practically all the river 

 bottom is receiving a deposit of this silt-sewage mixture except the 

 swifter portions of the channel proper. Control of the silt and sew- 

 age situations is badly needed regardless of the construction of dams 

 or levees, as these merely make more immediately acute conditions 

 which will doom the desirable fauna of the river unless checked soon. 

 Large areas of dead mussels which had been killed by the silt-sew- 

 age layer were found at many stations and the changes in fish fauna 

 from the bass-crappie group to the carp-sucker group were very 

 evident in the localities where these conditions of pollution were 

 very severe. 



The replacement of the valuable fresh-water mussels, such as the 

 yellow sand shell, the pigtoe, the niggerhead, and the mucket, by 



