442 



months represented also exhibit a plankton content above the 

 average. The causes of this large production are to be found 

 mainly in the prolonged low water, slackened current, and sew- 

 age contamination of the last half of the year. No large ver- 

 nal pulse appears in the records of the river or its backwaters. 

 It was either intercalated between collections, and thus escaped 

 detection, or the early winter flood, as in the previous year, by 

 its washing away sources of nutrition prior to the season 

 and temperature of greatest plankton development, tended to 

 depress production below normal at this season. It was to be 

 expected, however, that a large plankton development took 

 place in the stable conditions attending the three months of 

 declining levels which followed (PI. XL) the crest of the spring 

 flood. If such a development took place it would tend to raise 

 still higher the level of production established by our records 

 for this year. 



The relation which the backwaters bear to channel produc- 

 tion in 1897 is correlated with the hydrographic conditions. In 

 January-June, a period of continued high water, the plankton 

 content in the backwaters exceeds that in the channel in 22 of 

 the 30 monthly averages (see table on p. 441), or, omitting Spoon 

 River, which does not properly belong in the category of back- 

 waters, in 21 out of 25. 



This was a period of extensive and long-continued impound- 

 ing and of high levels, and, in the last three months (PI. XL), 

 of rapid decline and therefore of speedy run-off and rapid cur- 

 rent in channel waters, factors which favor the breeding of the 

 plankton in the reservoir regions and cut down the time for its 

 development in the channel, in which barren tributary waters 

 of recent origin and plankton-rich backwaters impounded in the 

 more or less current-free areas for a greater or less length of 

 time, depending upon the direction and rate of change in river 

 levels, are mingled in varying proportions. 



In the low-water period, July-December, stability of hydro- 

 graphic conditions continues throughout, while the extreme 

 low levels maintained for so long a time make the channel wa- 



