522 U. S. BUREAU OF FISHEEIES 



field party of 11 went aboard in June and continued the survey work 

 begun in 1930. Through the cooperation of the United States Engi- 

 neer Corps this party covered the Mississippi River from Lake 

 Keokuk south to Cairo, 111. ; the Ohio River from Cairo, 111., to the 

 mouth of Green River above Evansville, Ind. ; and the Tennessee 

 River from its mouth at Paducah, Ky., to the Hiwassee River, some 

 40 miles above Chattanooga, Tenn. During this survey many tribu- 

 tary streams were examined and an extensive study made of the 

 Lake Wilson (Ala.) region. The boat returned to Keokuk, Iowa, 

 the last of September and the collections were sent to the laboratory 

 at Columbia, Mo., for analyses and review. 



After the quarterboat was tied up for the winter, parties working 

 by auto continued field work in the Lake Keokuk region and north 

 on the Mississippi River to Winona, Minn.; also in northern Ar- 

 kansas and in the Lake Worth region near Fort Worth, Tex. Dur- 

 ing the last of December field parties working south through Mis- 

 souri, Arkansas, Tennessee, Mississippi, Louisiana, and Alabama 

 studied the St. Francis, the White, the Black, the Pearl, the Tom- 

 bigbee, the Warrior, and the Tennessee Rivers and their tributaries. 



POLLUTION STUDIES 



In view of the fact that the 1930 survey on the Mississippi River 

 demonstrates that erosion silt is a factor of major importance in 

 many fisheries problems in the inland river systems, particularly 

 problems in which pollution is involved, the pollution studies made 

 by this unit have centered around erosion silt during the past year. 

 In both the field work and the laboratory studies effort has been 

 made to secure qualitative and quantitative data on the effect of 

 erosion silt alone and as a carrier of other substances. The results 

 of the investigations to date may be summarized as follows : 



Mass of silt. — The amount of silt already deposited on the bottom 

 of the streams studied exceeded even that which the 1930 survey 

 suggested would be found. In certain places the silt deposit ex- 

 ceeded 10 feet, and in one locality on the Tennessee River, 27 feet 

 in thickness. Behind both temporary and permanent obstructions 

 to stream flow the deposition of erosion silt was proceeding rapid.ly 

 in all of the streams studied, soundings regularly showing deposits 

 from 1 to 4 feet in depth. 



Mechanical action of silt. — The field work demonstrated that wher- 

 ever the current of the water is slowed sufficiently not only is the 

 bottom covered with silt deposits but a colloidal suspension of silt 

 particles was to be found just above the top of the semisolid silt, so 

 that in many places there was a progressive gradation from the 

 muddy river water through a thick colloidal suspension to the semi- 

 solid mass of deposited silt. Laboratory tests showed that the top 

 layers of the deposited silt and the bottom layers of the suspension 

 supported very little weight, so that even small animals mired down 

 readily in the silt deposit. From the field work it was noted that 

 6 inches of erosion silt, or frequently less, was sufficient to completely 

 change the bottom fauna and to eliminate most of the sedimentary 

 animals usually found at the bottom of unpolluted streams. 



