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 cases, 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 productiv- 

 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 IX. showing the average number 

 of cubic centimeters of plankton per m. {i 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- 



