542 
bank-full river has a current sufficient to discharge its content 
in five days, and, allowing for increase of current and discharge 
in these conditions, the maximum flood would require about 
fifty days for its run-off. Accessions of water during the run- 
off would naturally tend to prolong the process to some extent. 
An examination of the hydrograph (PI. VII.) will show that 
the decline from the flood maximum to the ten-foot level (ces- 
sation of overflow) is accomplished in the neighborhood of fifty 
days in some eases, as, for example, in 1898. 
A small part of the flood is thus impounded for the full 
period of the run-off, while in case of the greater part of the 
impounded volume sufficient time elapses before discharge into 
the channel for the breeding of an abundant plankton, so that 
the backwaters become very important factors in determining 
both the quantity and the nature of the plankton in the main 
channel. Contributions from tributary streams form the other 
large factor, but the ratio existing between the contributions 
from the two sources is not easily ascertained. The only data 
available are the areas of the catchment-basins of the tribu- 
taries, the statistics of rainfall, and the estimated volume of 
the impounded water. A rough comparison of these data 
would indicate that during the decline of the flood to the stage 
of initial overflow (bank height), the impounded waters con- 
tribute somewhat more than the tributary streams to the main 
channel. Their contributions decrease as the river falls, and 
the more rapidly below the level of bank height, while those 
from tributary streams come to form an increasing proportion, 
and at low-water stages almost the only natural source, of 
channel waters. 
With a view to setting forth the contrast in the productiy- 
ity of the plankton in all the areas concerned in this problem 
I have drawn up the table following page 342, which gives data 
compiled from Tables III. to [X. showing the average number 
of cubic centimeters of plankton per m.’ of water for each month 
in which collections were made, from June, 1894, through March, 
1899. The silt has been eliminated by estimation, and all cor- 
