167 



but this was less disastrous to the plankton than usual since it 

 was in the main due to the spring flood and not to minor 

 changes when the stream was within its banks. This freedom 

 from minor interruptions during the low-water period is some- 

 what unusual, and resulted in a concentration of sewage ap- 

 proaching stagnation and in a marked increase in the fall plank- 

 ton. The overflow period, in which the reservoir action of the 

 bottom-lands as a whole was operative, prevailed during the 

 first five months; the change to low water, during which the 

 reservoir action of the more permanent and diversified waters 

 was in force, took place very rapidly; while the low-water 

 stages, during which it is a minimum, were both pronounced 

 and prolonged. These circumstances combine to emphasize 

 in this year both the unity and the autonomy of the river. In 

 brief, 1897 was a year of normally located but pronounced high 

 and low water, of marked freedom from interruptions, and of 

 unusually favorable conditions for the unity and autonomy of 

 the plankton of the river and for the full development of its 

 normal seasonal cycle. 



In 1898 (PI. XII.) we find another year whose hydrograph 

 approaches the normal in its main features. There is a well- 

 defined period of high water followed by one of much inter- 

 rupted low stage. The spring flood is normally located, con- 

 tinues (above 8 feet) for 164 days, and culminates at 18 feet 

 on April second. The extension of the flood period for 24 days 

 beyond the normal is due largely to the ''June" rise of unusual 

 proportions, which culminated in the last of May at 18.8 feet, 

 and covered a period of five or six weeks. The impounding 

 action of the bottom-lands as a whole is thus shifted forward 

 into the late spring and early summer, while the concentration 

 of the overflow into the channel occurs in the early part of 

 May and again in June, and the conditions of rainfall, season, 

 and overflow combine to favor the production of a relatively 

 large amount of plankton at these times. The decline is rapid 

 in July to low-water stage, which continues but three weeks, 

 the lowest record being 2.5 feet. This is followed by a series 



