CIRCULATION AND TEMPERATURE 



61 



We have been able to find no record of the amount of lowering of 

 the waters of Lake Michigan at a given point, by the wind, nor any 

 discussion of the relations of the surface currents to the effects of winds 

 and the vertical circulation. The waves of large lakes rise to consider- 

 able heights, as is familiar to all. They are of much importance in 

 keeping a large amount of gas in solution in the lake waters. 



The current in streams differs from that in lakes in that it is for the 

 most part in one definite direction, while the lake currents often alternate. 

 There are backward flows and eddies at various points in streams, in 

 front of and behind every object encountered in the current (57, p. 124). 

 On the basis of the current, streams are classified as intermittent, swift, 



•w- 



C 



r 



10 



"% 



■ w 





11 



; yfiy ^.^y.-^-i 'Bfmfflf.- 



Fig. 10. — Showing the circulation of the water in a lake of equal temperature. 

 W represents the direction of the wind (after Birge). 



Fig. 11. — The circulation of the waters of a lake of unequal temperature (after 

 Birge). 



moderately swift, sluggish, and stagnant or ponded. The current 

 wdthin the same stream differs at different times, and in different places. 

 As we pass across a stream we find the current swiftest near the surface 

 in the middle, and least swift at the bottom near the sides. 



2. TEMPERATURE 



Temperature has always been regarded as of great importance in 

 the direct control of the distribution of life in water. The tendency of 

 modern investigation is to show that its influence is of great indirect 

 importance, and the belief in its direct importance is correspondingly 

 weakened. 



The temperature in a stream is probably about the same at the 

 various points in any cross-section. The extent to which daily, seasonal, 

 and weather fluctuations in atmospheric temperature affect a lake is 



