461 
pede the movements of the Hntomostraca, 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 ina 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 Ilnois 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. 
