168 
of minor rises, which flush the river at short intervals during 
August and September, and a rise to bank height in November 
—fluctuations which favor the reservoir action of the perma- 
nent backwaters, and at the same time introduce much silt and 
interrupt and diversify the plankton cycle. Of all the years of 
our operations at Havana this was the one of highest average 
level—8.02 feet (8.11 at Copperas Creek)—and greatest move- 
ment (66.2 feet). The dilution of the sewage, the increased 
current and silt, and the flushings incident to such hydro- 
graphical conditions tend under most circumstances to de- 
crease the relative amount of the plankton, though doubtless 
they also tend to increase the total production of the stream. 
In brief, the year was a typical one of high water with much 
delayed run-off and interrupted low-water period. 
In 1899 (Pl. XIII.) we find another year conforming very 
closely to the normal hydrograph in its main outlines. We are 
concerned only with the first three months, at the close of 
which occurs the maximum (14 feet) of the spring flood. The 
greater part of the rise occurs in a brief period at the close of 
February, and the declining waters or more stable conditions 
at other times reduce considerably the flushing and silt attend- 
ing most winter floods, such, for example, as that of the pre- 
ceding year. The decline in February also afforded a good op- 
portunity for the reservoir action of the permanent backwaters 
under midwinter conditions. Our collections of 1899 thus cover 
a period of winter flood of more than usual stability. 
The wide range of hydrographical conditions during the 
six years of our plankton work at Havana have afforded a 
unique and, up to this time, unexampled opportunity to follow 
the effect of flood and drouth, of changing season, and of yearly 
fluctuations upon the life in the waters of a stream, and to give 
to the conclusions here reached the confirmation which repeti- 
tion alone can bring. 
TEMPERATURES. 
The fluctuation in the temperature of the river water con- 
stitutes for the plankton one of the most marked evidences of 
