Fig. 467.—Map to show anomalous position of the delta in Lake St. Clair, due to the peculiar currents in Lake Huron (after maps by Cole).
As regulators of the flow of rivers, lakes perform an important function. Such disastrous floods as are characteristic of the spring season within the basin of the lower Mississippi could not occur in the lower St. Lawrence, for the reason that the great basins of the lakes serve as distributing reservoirs. The annual floods, upon which the agriculture of Egypt depends, are explained by the flood waters from the high mountains of Abyssinia entering the Nile below the lakes of its upper basin.
In one further respect large inland bodies of water have an important function as regulators. It is the property of water to respond but slowly to the variations in the quantity of heat which reaches the earth’s surface from the sun. A larger quantity of heat must be added to or abstracted from a body of water, in order to change its temperature by one degree, than would be required for a like change in the same bulk of earth or rock. Thus bodies of water by more slowly acquiring the summer’s heat retard the coming spring, and by storing up this energy and carrying it over into the autumn the warm season is prolonged and early frosts prevented. The fruit belts about the lower Great Lakes are thus dependent upon this regulating property of the lake waters. The discomfort of the long spring of raw weather is thus compensated by an unusually salubrious harvest season.
Ice ramparts on lake shores.—Small ridges known as ice ramparts are formed upon lake shores by the action of lake ice, though subject to so many qualifying conditions that the range of their occurrence is somewhat limited. Within districts where a winter ice cover of some thickness is formed, the shores of lakes are apt to present ridges of bowlders parallel to and near the water’s edge, and such lakes have sometimes become known as “wall lakes” ([Fig. 468]).
Fig. 468.—A bowlder wall upon the shore of a small lake in the Adirondacks of New York.
In many cases these small ridges have been formed at the time of the spring “break up” of the ice; for the ice cover, when once loosened, is drifted in great rafts first against one shore, and later, with a change of wind direction, against another. Under the impact of such heavy rafts, the half-submerged bowlders near the shore are forced up the beach until they lie in a ridge or bowlder wall.
At other times such bowlder walls, and far more interesting ridges as well, result from a kind of ice shove independent of the wind, but caused by expansion within the ice itself during a sudden rise of temperature of the surrounding air. Such ice ramparts require for their explanation a consideration of the sequence of events from the time the ice cover closes the lakes.
