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 em., 
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 8, 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. 
