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 (Pl. *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 13.8 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 
