311 



cally the run-off of the April rains which checked the fall of 

 the March flood (PL X.). This is also a period of rising tem- 

 perature, a rise of 12° (to 82°) attending the decline of river 

 levels and the rising plankton production. The rising plank- 

 ton pulse is, however, flushed out by the entrance of flood 

 waters in the closing fortnight of the month. The plankton 

 falls at once from 3.56 cm. 3 on the 13th to .86 on the 18th with 

 the first stages of the flood, and the fluctuations during the 

 period of rise are erratic, suggestions of recovery and decline 

 appearing in the data. These vagaries may be due to the dis- 

 tribution of local storms, which contributed largely to this 

 somewhat slow rise in river levels. The general effect of the 

 flood seems to be to depress the production and thus to deflect 

 the apex or node and the mean of the curve of production to 

 the left, that is, to an earlier date. The flushing effect of the 

 floods of May, 1896, is apparently greater than that in 1898, as 

 shown by the plankton production. The flood of 1896 did not 

 exceed bank height. Its diluent action is thus concentrated in 

 channel waters. In 1898 the floods occur in overflow stages 

 and are thus diffused over a large area. 



The chemical conditions show but little relation to plank- 

 ton movement in this month. The maximum production fol- 

 lows immediately upon a rise in nitrates, nitrites, and free am- 

 monia, and coincides with a slight decline in the two first 

 named. The decline in production during the rising flood 

 takes place along with considerable increase in nitrates and 

 nitrites. 



The average production in May, 1896 (1.30 cm. 3 ), is less 

 than that of the following years (see table on p. 292), since it 

 does not contain the vernal maximum, and also because it is 

 reduced by flood action. 



The June pulse is not well differentiated in the volumetric 

 data, and its delimitation here becomes largely a matter of 

 conjecture though it stands out more clearly in the statistic- 

 al results (PL LI.). If we follow the latter the pulse termi- 

 nates, at least so far as the chlorophyll-bearing organisms are 



