466 
the total movement (51.9) of 1895 and add it to that of 1896 
(45.7), the resulting figures (42.1 and 55.5) will more nearly 
characterize the hydrographic conditions in which our plank- 
ton collections were made. After making allowances for the 
AVERAGE CHANNEL PLANKTON AND ACCOMPANYING MOVEMENT IN RIVER LEVELS. 
Total movement in river| Plankton per m.? 
Year levels—in feet —in cm.'* 
TSO4 2d op s'sssc5 wise eyo Datete ouoacietene caelela aleuet= 32.9 2.53 
Tits $e ase AERO ACOH een Aes Oo oto 51.9 (42.1)T 5.9L 
T8Q Oe ered cists tlatrciekate rete aree sia teee aatelebene 45-7(55-5), 1.05 
(OYA S DOOR TOC BRO CMI CeO ACOL 44.8 3.28 
i tolo lot et eae mee Tae ero 67.2 2.03 
*Mean of monthly averages. +After transferring December rise to 1896. 
fact that our plankton averages imperfectly represent the 
actual production, it is still apparent that plankton production 
(per m.*) is inversely proportional, in a varying ratio, to the 
extent of the total movement in river levels. 
In not a few of the years (Pl. VII.) the hydrograph falls 
into two periods dominated respectively by high- and by low- 
water conditions. This is seen typically in 1897, and with less 
contrast in 1894, 1895, and 1898, while in 1896 the recurrent 
floods obliterate almost all traces of such a division. In their 
totality these two hydrographic extremes present strong con- 
trasts which must bear important relations to the plankton 
production. They are, moreover, so involved with other fac- 
tors that no simple analysis of their results seems possible. For 
example, the high-water period is predominantly, as a rule, of 
low temperature, considerable ice, dilution of sewage, decreased 
light, increased access of recent water, increased current, de- 
creased occupancy by, and lessened growth of, aquatic vegeta- 
tion, and of greatly increased area and intimate and free con- 
nection with reservoir backwaters. The low-water period, on the 
other hand, is one of higher temperatures, no ice, concentration 
of sewage, increased light, reduced volume of tributary water,— 
which itself is of less recent origin than at high water,—slack- 
ened current, greater domination by, and growth of, aquatic 
vegetation, and of greatly contracted area and restricted con- 
