293 



revealing only a single midsummer pulse, culminating in the 

 August maximum in a period of maximum heat and lowest water. 



In the light of collections of later years in this and other 

 localities it seems probable that collections at more frequent 

 intervals v^ould have yielded a curve of greater irregularity, 

 with other fluctuations than the single one apparent in the 

 present record. 



It seems probable from the records of 1896 (PI. X.) and 

 1898 (PI. XII.) that the small average (0.74) in June is due to 

 the fact that the dates of collection fall in a period of decline 

 from an April-May pulse, hastened by the rise in May and per- 

 haps reduced in volume by the relatively small contributions 

 of impounded backwaters resulting from the depression of the 

 spring flood. It may also be that the collection of June 29 ex- 

 hibits the flushing, depleting effect of the rise of the preceding 

 ten days. It v^dll be noted that the collection of June 12 lies 

 about four weeks after the crest of the May rise — a location 

 which is attended in 1896, 1897, and 1898 (PL X.— XII.) by a 

 decline to a minimum after a pulse of plankton development. 



The hydrographic conditions of July in 1894 — decline of 

 flood to low-water levels — are approximately realized with va- 

 rying stages of river and rates of decline in all the other years 

 but 1895 (PI. IX.-XIL). In 1894 they attend a tenfold in- 

 crease in the plankton during this month. The movement of 

 production is in the same direction approximately in July in 

 1896, 1897, and 1898, though its development is less in 1896 and 

 1898, and data are lacking for its progress in 1897. In 1894, 

 and to a varying extent in other years, this rise attends among 

 other factors the restriction of contributions from impounding 

 backwaters and the differentiation of what might be called 

 channel plankton proper. The July production in 1894 aver- 

 ages 5.12 cm.^ per m.^ — the largest, with the exception of that 

 for 1895, of any year, and a fact to be correlated with the un- 

 usually stable conditions then prevalent. 



In August of this year the single collection forms the apex 

 of the season's production, reaching 9.67 cm.'' per m.^ — an 



