461 



pede the movements of the Entomostraca, and perhaps also of 

 the Rotifera, and thus to exercise a deleterious effect upon 

 them. The excess of moribund and dead individuals of these 

 groups in flood waters is evidence of the destructive character 

 of the silt-laden current. 



When the existence of a current in a body of water involves 

 the run-off of its contents and their replacement by water of 

 more recent origin, the result tends to lower production, and 

 the proportions of the depressing influence will rise with the 

 rate of the current and the contraction of the volume, or with 

 rapidity of replacement, in the main in proportion as they 

 shorten the time for breeding of the plankton. Illustrations 

 of this tendency appear in contrasting our various localities. 

 In Spoon River, where current is rapid and renewal with re- 

 cent water complete and frequent, production (.256) continues 

 in minimum quantity and rises only when current slackens. 

 In the Illinois River the current is perhaps as a rule less rapid 

 than in Spoon River, replacement because of" connecting back- 

 waters less complete and frequent, and the replacing water has 

 a greater proportion of older water by virtue of the longer wa- 

 ter-course and the greater development of contributory im- 

 pounding backwaters. Production is accordingly greater (2.71) 

 in the larger stream. In Quiver Lake conditions in these par- 

 ticulars are extremely varied. As the lake emerges from the 

 general overflow the extent and rate of the current declines, 

 but the proportion of creek and spring water of recent origin 

 is increased with resulting depression in production. When by 

 reason of backwater from the channel at low levels the current 

 is reversed, the run-off checked, and the flood and tributary 

 waters are impounded, — as in the summer floods of 1896, — pro- 

 duction rises. Current with attendant run-off and renewal of 

 water in this lake constitutes one of the important factors in 

 lowering its production (1.75) below that of the other backwaters. 



In Dogfish Lake the factor of current and renewal by recent 

 water is largely eliminated, and production (3.16) exceeds that 

 in the contiguous waters of Quiver Lake. 



