394 



its position is such that the backwaters from the bottom-lands 

 up-stream sweep through it and out to the river through the 

 "cut road," being deflected by the alluvial deposits of Spoon 

 River (PI. II.). Its relation to the river is a peculiar one in 

 that its outlet, or slough, lies at its up-stream end. At stages 

 above six feet the current enters through this slough, and the 

 run-oft' takes place at the lower end through the cut road. 

 Below this level all the run-off must take place through the 

 slough. The direction of the movement in the run-off of the 

 lake is thus reversed as river levels pass this stage. There are 

 no tributary waters of consequence which enter the lake, though 

 a small rill and a few springs enter along the western margin. 

 The main supply is drawn directly from channel waters through 

 the slough, and when levels are stationary there is no in- 

 terchange in either direction. The current sets in or out, at 

 stages below 6 ft., according as the river rises or falls. The re- 

 sult of this condition is that during the higher levels back- 

 waters of overflow and the river water entering by the slough 

 are impounded and drawn off slowly at the lower end of the 

 lake. At stages below 6 ft. a run-off occurs only in the falling 

 stages and in relatively smaller volume through the narrow 

 and tortuous slough. The impounding function is accordingly 

 more highly developed at lower levels, while at lowest levels all 

 interchange ceases. 



This close and intimate dependence of this lake upon the 

 river for its water supply in so far destroys the unity and inde- 

 pendence of the lake as a separate unit of environment, and 

 tends to eliminate the differences in plankton production be- 

 tween it and channel waters. This tendency is counterbal- 

 anced to a considerable degree by the large size of the lake and 

 consequent increase in the time occupied in transit during over- 

 flow, and by the impounding, at lower levels, of entering river 

 water at the upper end of the lake, where it deposits its silt and 

 soon permits the development of the lake plankton in its area. 

 Here, as elsewhere, local factors are most potent at lower levels. 



The effect of the greater size of this lake is thus to equal- 



