u 1 ' 



^)J^ 



468 



recent origin of the flood waters of these months, from the di- 

 lution of sewage and increased rapidity of run-off, and from the 

 reduction in time of impounding under these hydrographic con- 

 ditions. 



March-May is a period of rising temperatures, maximum 

 flood levels, and increasing plankton production. The data, in 

 so far as they go, indicate that high levels tend to increase pro- 

 duction and low levels to decrease it, in some instances at 

 least. These months witness the maximum and the initial de- 

 cline of the spring flood, as a rule, and the greatest volume and 

 principal run-off of flood waters whose impounding has been 

 more or less prolonged. The 'proportion of impounded water is 

 greater in higher than in lower levels, and we find, accordingly, 

 production increased in the former and decreased in the latter, 

 as a general rule. It is noticeable that levels in May slightly 

 exceeding bank height, as in 1897 and 1898, yield much greater 

 production (5.62 and 11.30) than levels not attended by overflow, 

 as in 1896, when production falls to 1.30 at a level of 6.58 ft. 



The data of the June period are somewhat aberrant, in part 

 as a result of insufficient data in some years, as 1895, and in 

 part because of the relatively great irregularity of hydrographic 

 conditions in this month in different years. The high produc- 

 tion (30.42 cm. 3 ) in low levels (1.88 ft.) in 1895 attends sewage 

 concentration. The data for the remaining years conform in 

 the main to the conclusions concerning production in the three 

 prior months, namely, that high levels favor and low levels 

 depress production, and for the same reasons above cited. A 

 comparison of production in 1896 and 1898 in June yields con- 

 firmatory evidence on this point. 



With July begins the low-water period proper, which con- 

 tinues during the remainder of the year. Levels do not rise, in 

 the means of the monthly averages, above 5 ft. in this period. 

 The relation which existed between production and high water 

 in March-June is reversed in the period of July-November. An 

 inspection of the table shows that in 20 of the 25 months in- 

 cluded in the table in this period this reversed relation obtains; 



