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.‘ on the 18th 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, toan earlier date. The flushing effect of the 
floods of May, 1596, 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 1598 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 detline 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, 15896 (1.30 em.*‘), 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 
