437 



51.9 ft. It is not so much the extent of movement in levels as it 

 is distribution which produces this depression in production. 

 Repetition of floods at relatively brief intervals is the cause of 

 low production in channel waters in 1896. 



Not only is the mean production in the river below normal, 

 but all of the monthly averages are likewise from 97 to 35 per 

 cent, below their averages except those of April and December, 

 which are 23 and 7 per cent, above. Hydrographic conditions in 

 these two months of higher production are such as to favor in- 

 crease in plankton, since in both cases there is a period of 6-8 

 weeks of slowly declining levels with little or no interruption 

 in which a more abundant plankton becomes established. 



The relation of production in the channel and the back- 

 waters in 1896 is also very different from that in 1895. While 

 in 1895, owing to low levels in general and to the prolongation 

 of rising levels, the backwaters were contributing but a slight 

 and interrupted run-off to the channel, and production, as shown 

 in monthly averages, was in the case of backwaters examined 

 predominantly lower than in the channel, we find in 1896, owing 

 to higher levels, that there is more impounding, and, owing to 

 the slow declines, a larger continuance of it and more run-off 

 to the channel. There are, for example, 157 days of falling 

 levels above 6 ft. distributed through 10 months, while impound- 

 ing and run-off continues for 90 days more at lower levels and 

 in decreased volume. Not only are backwaters thus contribut- 

 ing to the channel for a much longer period in 1896, but their 

 plankton content is predominantly higher than that in the 

 channel. An examination of the relative production (see table 

 on p. 436) reveals but 3 out of the 58 monthly averages of pro- 

 duction in backwaters, excluding Spoon River, which are less 

 than coincident production in the channel. These are for Dogfish 

 and Quiver lakes in July, and for the latter in September — both 

 months of lowest water, and consequent predominance of creek 

 and spring water in Quiver and of vegetation in both lakes. 

 This relatively greater production in the backwaters is not due 

 to increased absolute production as compared with 1895 except 



