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 le 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, 
