Currents 73 



with them when they die, and thus removed from cir- 

 culation. 



Increasing breadth of surface means increasing 

 exposure to winds with better aeration, especially 

 where waves break in foam and spray, and with the 

 development of superficial currents. Currents in lakes 

 are not controlled by wind alone, but are influenced as 

 well by contours of basins, by outflow, and by the 

 centrifugal ptdl due to the rotation of the earth on its 

 axis. In Lake Superior a current parallels the shore, 

 moving in a direction opposite to that of the hands of a 

 clock. Only in the largest lakes are tides perceptible, 

 but there are other fluctuations of level that are due to 

 inequalities of barometric pressure over the surface. 

 These are called seiches. 



Broad lakes are well defined, for they build their 

 own barrier reefs across every low spot in the shores, 

 and round out their outlines. It is only shores that are 

 not swept by heavy waves that merge insensibly into 

 marshes. In winter in our latitude the margins of the 

 larger lakes become icebound, and the shoreline is 

 temporarily shifted into deeper water (compare simimer 

 and winter conditions at the head of Cayuga lake as 

 shown in our frontispiece). 



Increasing breadth has little effect on the life of the 

 open water, and none, directly, on the inhabitants of 

 the depths; but it profoundly affects the life of the 

 shoals and the margins, where the waves beat, and the 

 loose sands scour and the ice floes grind. Such a beach 

 as that shown on page 61 is bare of vegetation only 

 because it is storm swept. The higher plants cannot 

 withstand the pounding of the waves and the grinding 

 of the ice on such a shore. 



The shallower a lake is the better its waters are 

 exposed to light and air, and, other things being equal, 

 the richer its production of organic life. 



