No. 373-] CLASSIFICATION OF LAKES. 27 
slightly. Soon the surface and bottom layers come to have 
substantially the same temperature, and vertical currents extend 
from top to bottom. This is the “ period of spring circulation,” 
or the “ spring overturning.” It lasts several weeks, but varies 
in duration in different years. As the season advances the 
surface water becomes warmer than that at the bottom, and 
finally the difference becomes so great that the wind is no 
longer able to keep up the circulation. Consequently, the 
bottom temperature ceases to rise, the water becomes ‘directly 
stratified,’ and the lake enters upon the period of ‘summer 
stagnation.” During this period, which extends from April to 
November, the bottom temperature remains constant and the 
water below a depth of about twenty-five feet remains stagnant. 
This bottom temperature during the summer varies with differ- 
ent years, depending upon the meteorological conditions at the 
time when the period begins. In the autumn, as the surface 
cools, the water becomes stirred up to greater and greater 
depths, until finally the “ great overturning” takes pla¢e, and 
all the water is in circulation. At this time there is a slight 
increase in the bottom temperature. Then follows the “ period 
of autumnal circulation,” during which the surface and bottom 
layers have substantially the same temperature. In December 
the lake freezes and “ winter stagnation ” begins. 
Thus during the year there are two periods of circulation and 
two periods of stagnation. These physical changes have an 
important effect upon the quality of the water. During the 
stagnation periods much of the suspended matter in the water 
settles to the bottom, where there is already a large accumula- 
tion of organic matter. This decomposes, robbing the lower 
layers of water of all the oxygen present. Decomposition then 
goes on under the influence of the anzrobic bacteria, and the 
water becomes charged with the products of decay. By the 
end of the stagnation period the lower layers have a very high 
color and a bad odor. At the overturning the foul water is 
carried into circulation, and its effect is noticed throughout the 
entire body of water. Nor is this all. The circulating water 
brings up from the bottom certain micro-organisms which have 
been lying dormant, and the products of decay alluded to 
