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 (PI. 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 iA 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 



