180 



conditions, to 165 cm. at low water under the ice. The turbidity 

 here is almost entirely due to silt, that at flood being largely 

 composed of earth and clay, giving a black or yellow tinge to 

 the water. The amount of comminuted vegetable debris found 

 in the waters is considerable. 



In Thompson's Lake (Table VIII.) the turbidity is not so 

 frequently marked by the extremes seen in the other bodies of 

 water examined, the range being from 115 cm. in the declining 

 waters of May, 1896, to 6 cm. in invading floods, and again in 

 the late autumn of 1897, when high winds roiled the shallow 

 waters. As a general rule the turbidity of this lake is somewhat 

 less than that of the river, but as great or greater than that 

 of other backwaters. This turbidity is often due in part 

 to the heavy planktons occurring here, and also to the floccu- 

 lent debris loosened from vegetation and stirred up from the 

 mobile bottom by fish and the waves. Very little silt enters 

 the lake except at times of inundation, especially with back- 

 water from Spoon River. Owing to its origin the silt in this 

 locality is usually of finer, more flocculent material than that 

 found elsewhere. 



In Quiver Lake (Table V.) the extremes are much more 

 marked than in any other locality, ranging from 3.5 cm., 

 in flood water from the river, to 260 cm. on June 5, 1896, in clear 

 impounded water. In winter, under the ice, the bottom was vis- 

 ible on December 3, 1896, in 260 cm. of water. A great deal of 

 variation in turbidity occurs in this lake. In years of low water, 

 as 1894 and 1895, when vegetation is abundant, the turbidity is 

 very slight, the bottom being visible much of the time. In the 

 three succeeding years the lake was free from vegetation, and 

 the turbidity was considerably increased as a result largely of 

 the increase in the plankton. The sources of the silt in this 

 body of water are varied ; occasional freshets in Quiver Creek, 

 which enters the upper end of the lake (PI. II.), invading floods 

 from the river, and debris from vegetation and the bottom put 

 in suspension by the wind, current, or movements of fish, — all 

 contribute their share to the pollution of the otherwise clear 

 water of this lake. 



