326 



is thus washed out, and the apex and mean of the pulse are 

 shifted to the left. Flood conditions thus check the winter 

 production and reduce the monthly average to .27 cm. 3 per m. 3 

 This occurs in the presence of an increasing abundance of 

 nitrates (PI. XLV.). This monthly average is much higher 

 than that of 1896 (.02) under somewhat similar conditions. 

 The levels and amount of movement are about the same in 

 both years (cf. PL X. and XII.), but antecedent conditions dif- 

 fer. In 1896 there is six weeks of flood before February, but 

 in 1898 the flood begins but a fortnight before. The cumula- 

 tive effect of its flushing action is thus less developed in 1898, 

 and production is greater under similar immediate conditions, 

 [t is, however, but one third as great as that of February, 1899, 

 a month of falling levels. 



The March pulse has a duration of 28 (42) days, — from the 

 1st to the 29th (or Apr. 12), — with a maximum amplitude of .77 

 cm. 3 per m. 3 on the 22d. Its mean falls on the 22d (or 26th), 

 43 (or 47) days after that of the preceding pulse, Levels de- 

 cline in the first week of March, but thereafter rise rapidly and 

 continuously to a maximum of 18 ft. on Apr. 2 — a point not 

 surpassed during our operations. The total movement in March 

 is 7.5 ft., and the result is complete overflow following the in- 

 troduction of vast amounts of flood water of recent origin. As 

 a result of this, chlorine and free ammonia reach a flood mini- 

 mum, and the nitrates decline, the excess since previous heavy 

 rains having been leached out from the water-shed. Other 

 forms of nitrogen decline or remain stable, and the oxygen 

 consumed falls as silt declines. Owing to the great expanse of 

 the stream, tributary waters enter more into impounding back- 

 waters and drop their burden of silt there, so that channel 

 waters are now much freer from it than in the earlier stages 

 of the rise. Temperatures begin the vernal ascent, passing 

 from the winter minimum to 50°. In keeping with this and in 

 the presence of rising flood and rapid current the plankton pro- 

 duction begins its vernal increase, attaining an average of only 

 .33 cm. 1 ' per m. 3 — about the same level of production as that in 



