470 



and when they are of short duration and merely flush the chan- 

 nel, as is predominantly the case during the low-water period. 

 Lower levels depress production when they introduce stagna- 

 tion conditions — ^as in the winter under the ice, and when they 

 cut off contributory backwaters or otherwise reduce the run- 

 off of impounded waters of long standing. They increase pro- 

 duction when they lend stability to hydrographic conditions, 

 increase the relative fertilization (sewage) of the stream, and 

 by slackened current afford time for breeding. 



In general terms, production in the backwaters exhibits 

 relations to levels similar to those we have described for chan- 

 nel waters so long as the backwaters retain an intimate con- 

 nection with the channel, that is, generally during high water, 

 and for longest periods when, as in Thompson's Lake, the con- 

 nection with the channel is most intimate. The diversification, 

 as levels fall, of the several regions examined by us, renders 

 generalizations impossible with respect to allof the backwaters, 

 since one or another local factor sooner or later comes in to 

 modify conditions. Moreover, some of the backwaters, as 

 Phelps Lake, are cut off from the channel early and are not 

 affected by changes in river levels, and, in general, the effect 

 of the changes in channel levels, especially the minor ones, is 

 reduced, equalized, or even obliterated, before it reaches the 

 backwaters. 



The season at which the initial stages of the major flood of 

 the year occurs, affects the subsequent production. Thus in 

 1896 and 1897 floods begin early in the year. The result is 

 the carrying away in the run-off of great quantities of organic 

 matter in suspension (Table X.) before they have had time to 

 decay and yield up in solution their nitrogenous and other con- 

 stituents for the support of the plankton. Temperatures are 

 low in these months, decay is not rapid, and the plankton is 

 not produced in large quantities. The result is that the stream 

 is locally impoverished by this early run-off of matters in sus- 

 pension and to some extent in solution. In 1898, on the other 

 hand, the flood does not reach overflow stages till late in Feb- 



