306 



swept away all but a vestige of the rich plankton of the earlier 

 weeks. The amount remaining was so small that its quantita- 

 tive changes were swamped in the errors of the volumetric 

 method and silt estimation. The large amount of silt carried 

 in this and subsequent floods of the winter is due to the fact 

 that bottom-lands and fields covered with a rich vegetation were 

 now submerged for the first time in two years (PI. VII.), and 

 vast quantities of debris from this region and tributary streams 

 now entered channel waters. 



In comparison with other years December in 1895 is, in 

 spite of its fortnight of flood,- the most productive December 

 recorded (see table on p. 292), averaging 1.14 cm^ to .76 in 1896, 

 .56 in 1897, and .99 in 1898. It shares the large development 

 of the preceding month, and with it presents the most marked 

 late autumnal development in channel waters, though falling 

 far below the production of some of the permanent backwaters 

 in this and other years in this season. The unusually stable 

 hydrographic conditions in the river doubtless contribute in 

 large measure to this exceptional development. That low tem- 

 peratures alone do not prevent the development of a large win- 

 ter plankton is apparent from this December development of 

 2.6 cm.^ per m.'* and 11.1 cm.^ per square meter at temperatures 

 but little above 32°. 



The year 1895 as a whole may be summed up as one of mid- 

 winter stagnation followed by excessive spring and early sum- 

 mer development of the channel plankton, of midsummer and 

 equinoctial floods, which check development at that season, of 

 stable autumnal conditions and exceptional production in late 

 autumn, and of catastrophic reduction by flood to a minimum. 

 As a whole the year was one of exceptionally heavy production 

 when expressed in terms of plankton per cubic meter. This is 

 seen in the high average^ — 3.22 cm.^ of all catches, 5.31 cm.^ of 

 monthly averages. When total production is considered it may 

 be that the decreased volume of water at the time of the maxi- 

 mum in the low water of June will at least counterbalance the 

 excess per cubic meter, and that the toted j_)ro(luctio)i will not 



