178 



The wind conditions on the river and the lakes adjacent to 

 it— which are p^enerally elongated in the direction of the main 

 stream (PL II.) — are somewhat peculiar. Owing, it may be, to 

 the configuration of the river valley, or perhaps still more to 

 the bordering forest of the contiguous bottom-lands, the pre- 

 vailing direction of the wind is either up or down the river or 

 lake, especially during the summer season. The effect of an 

 up-stream wind is greatly to increase the disturbance of the 

 surface when wind and current are thus opposed. These winds, 

 when prolonged and violent, decidedly affect the levels of the 

 different parts of the lakes, and, for example, in Thompson's 

 Lake (PI. II.) determine at low-water levels whether the lake 

 shall discharge its waters into the river or itself receive an 

 access of river water. Owing to the mobile condition of the 

 abundant bottom deposits, at low stages the winds also add 

 very much to the silt in suspension in the water, and thus hin- 

 der the penetration of light. 



The effect of varying sky conditions lies primarily in their re- 

 lation to the temperature of the water, but is due in a less degree 

 to the influence of light upon the multiplication of chlorophyll- 

 bearing organisms — the primal food supply of the plankton — 

 and upon the movements of these and other plankton organisms. 

 The abundant silt in suspension in waters of the river and most 

 of the adjacent lakes doubtless hinders the penetration of the 

 sunlight, but modifies to a much slighter extent its effect upon 

 temperatures. Wind and sky conditions combine to favor or 

 prevent the appearance of the "water-bloom." This is a char- 

 acteristic green scum which coats the surface of the river, and 

 occasionally of the lakes, on still, warm days in midsummer. 

 On cloudy or windy days the minute organisms {Euglena, Chlam- 

 ydomonus, etc. ) which form the bloom do not rise to the sur- 

 face. The conditions of wind and sky are thus important fac- 

 tors in the economy of limnetic life and, by reason of their rel- 

 atively greater effectiveness in the river and its adjacent 

 waters as contrasted with the typical lake, add to the elements 

 of instability in the fluviatile environment. 



