469 



that is, levels above the average are accompanied by a fall in 

 production to an amount below the average, and those below, by 

 a rise in production above the average. Thus, in this period 

 higher levels depress production and lower levels tend to in- 

 crease it. Two of the 5 apparent exceptions are in October and 

 November, 1894, when insufficient data are available, and one 

 is in July 1897, when the customary vernal conditions (PL XI.) 

 encroach upon the low-water period. 



The cause of this changed relationship of levels and produc- 

 tion in these months of predominantly low water is to be found 

 in the relation which summer rises in levels bear to the im- 

 pounding function of the backwaters. These summer rises are 

 rarely above bank height. They flush the channel, are not ex- 

 tensively impounded in the backwaters, run off quickly, and 

 accordingly depress production. The months of lower average 

 levels are more stable, and, owing to slackened current, a more 

 abundant plankton breeds, other things being equal, than in 

 the more rapid current in the summer months of higher levels- 



The relations in December between production and levels 

 are again reversed. Indeed, suggestions of this reversal appear 

 in November. In this month minimum temperatures are again 

 reached and higher levels prevail, and production now is higher 

 in the years of high levels and falls below the monthly mean 

 in every year of low levels. The cause of this relation does not 

 seem to lie in hydrographic conditions. It may possibly be in- 

 volved in the changed sewage and bacterial content of the 

 channel that accompany the increased current and the decline 

 in temperatures. The submergence of the summer's growth of 

 vegetation in the margin of the river and its connecting back- 

 waters in years of higher levels may also be a contributory 

 factor. 



We thus find that in channel waters higher levels favor 

 production only when they increase the impounding function 

 and by long duration afford time for production and run-off of 

 the plankton, and when they make available additional sources 

 of nutrition. They depress production when they first appear, 



