34 LAKES OF NORTH AMERICA. 



of the principal currents. When the hirger axis of a lake coincides with 

 the direction of the prevailing winds, a surface current is established 

 through its center, as in the case of lakes Erie and Ontario, with return 

 currents and eddies along the shore and about islands. When lakes lie 

 athwart the prevailing winds the main currents combine with the return 

 currents and form minor swirls, as is shown on the chart in the case 

 of lakes Michigan and Huron. In Lake Superior there is a general 

 circulation which follows the main shore lines, but its course has not been 

 fully determined. It has been found that the currents of the Laurentian 

 lakes have in general a speed of from 4 to 12 miles a day, but in certain 

 observed instances, this is increased to 2^ to 4 miles an hour or from 36 

 to 96 miles a day. 



The currents in the central part of a lake produce slight if any 

 changes on the topography of its basin, but when they follow the shore 

 important results may follow. When the wind blows obliquely to the 

 shore, strong currents are frequently produced which follow the general 

 trend of the coast, but cut across bays and inlets. These currents, with 

 the assistance of waves, sweep along sand and gravel, and produce impor- 

 tant changes on the bottom, particularl}' when the water is shallow. The 

 r61e played by Avaves and currents in modifying topography is considered 

 with some detail in the next succeeding chapter. 



Strong winds blowing in a nearly uniform direction for several days 

 cause the waters of lakes to move with them, and to rise on the shores 

 against which they are driven, so as frequently to produce disastrous 

 inundations. A gale blowing from the north over Lake Michigan has 

 been observed to cause a rise of seven feet at Chicag-o. In Novem- 

 ber, 1892, a storm from the west caused the waters of Lake Erie 

 near Toledo, to fall between eight and nine feet below the normal fair 

 weather level. At the same time, unusually high water was experienced 

 at the east end of the lake. The differences in the level of the waters of 

 Lake Erie, at Buffalo, between a high-water stage produced by an east- 

 ward blowing gale, and a low-water stage accompanying a westward or 

 off-shore gale, has been observed to amount to 15^ feet. An eastward 

 movement of the waters of Lake Superior has been known to accom- 

 pany a gale from the west and to produce an unusual rise in the water 

 of St. Mary's river. 



The height to which the waters reach on lake shores, owing to strong 

 winds, establishes the upper limit of wave-action, and leads to the forma- 

 tion of storm beaches at an elevation of several feet above the normal 



