THE EPHEMERAL EXISTENCE OF LAKES

Lakes as settling basins.—Of all the processes which conspire to blot out the lakes with which our northern landscapes are dotted, the one of greatest importance is in most cases a process of filling by the sediments brought in by tributary streams. The carrying of sediment in suspension depends, as we know, upon the velocity of the current, and as this is checked where it reaches the lake margin, all coarser material is at once deposited to form a delta, while the finer sediments are held longer in suspension and finally settle in thin layers over the entire bottom of the lake. Clay deposits surrounded by coarser sediments are thus characteristic of filled lake basins.

How waters are clarified by their passage through a lake is indicated by a comparison of a river system such as the St. Lawrence, with a river like the Missouri and Mississippi. Not only are the lower stretches of the St. Lawrence in striking contrast with the muddy floods of the Missouri and Mississippi; but the delta, which is so remarkable a feature in the Mississippi, has no counterpart in the northern river.

The most noteworthy examples of settling are, however, furnished by the lakes of Switzerland, for the reason that Swiss rivers are heavily charged with rock flour produced beneath the numerous glaciers at the valley heads, and, further, because these rivers descend with turbulent currents to near the borders of the larger lakes. To look out upon the murky waters of the upper Rhone, where they enter Lake Geneva near Villeneuve, and then to watch the flood of crystal water which issues from the lake and passes under the bridge at Geneva, is an object lesson which no traveling student should miss (Fig. 462). Yet even more instructive is a visit to the Bois de la BÂtie at the junction of this clear stream with the Arve, a half hour’s walk only below Geneva. The waters of the Arve have come on a steep descent directly from the glaciers of the Mont Blanc district, and as they meet the cleared waters of the Rhone, they flow beside them down the common valley without mingling. Dull and opaque, the Arve waters can be discerned for a long distance as a white belt against the left bank of the river, sharply defined against the blue reflecting surface of the Rhone waters (Fig. 463). Upon the banks of the Arve, just above its junction, a cement manufactory has been established to utilize the clays which are here deposited.

Wherever lakes are contained in long and narrow valleys, the greater part of the tributary drainage enters at the upper end, and the delta which there forms extends from bank to bank. As it continues to advance into the lake, the earlier water basin is gradually transformed into a level plain of delta deposit, a feature so common as to be deserving of a special name. The Scottish lochs, which are lakes of this type, are each extended in a longer or shorter delta plain described as a strath, and this local term may well be given a general application (frontispiece). The city of Ithaca, the seat of Cornell University, is built upon a strath at the head of Lake Cayuga, and numberless Scottish and Swiss hamlets have been located upon such fertile plains (Fig. 464).

Drawing off of water by erosion of outlet.—Next in importance to the filling up of lake basins as a factor in their early extinction is the cutting down of their channels of outflow. Whenever the walls of the outlet are cut in rock, this draining process is apt to be slow, for the reason that the outlet stream is of filtered water and so lacks the necessary cutting tools. By far the larger number of lakes are, however, held back by dams of loose drift deposits laid down by the earlier continental glaciers; and so the very clarity of the water promotes the erosion of the outlet by allowing the stream’s full burden of sediment to be lifted and then removed from the channel.

The pulling in of headlands and the cutting off of bays.—The removal of projecting headlands by wave action, though it increases the area of the lake, yet it decreases directly the volume of lake water through formation of the built terrace, and indirectly in far larger measure through the transformation of bays into quiet lagoons within which the extinguishing process of peat growth is set in operation.

Lake extinction by peat growth.—The first condition for the growth of lake vegetation is quiet water. Within small lakes, such as the kettle basins upon moraines, aquatic vegetation develops rapidly, and bogs of peat might almost be included among the most important distinguishing marks of a glaciated country. Within larger lakes it is only after barrier beaches have been thrown across the mouths of the bays to form natural breakwaters for the waves that this process of lake extinction by peat growth can become effective.

Many erroneous notions are still held concerning the prime importance of sphagnum in peat formation, owing to the peculiar local conditions under which the early studies were made. Within the glaciated districts of the United States, the formation of peat involves the successive growths of a number of zones of vegetation and the formation of a floating bog which advances into the lake from the shores, followed in turn by belts of low shrubs, tamaracks, and lastly deciduous trees (Fig. 465).

In most cases the first plants to develop in a quiet lake are the water lilies, though these are sometimes preceded by chara and floating bladderwort. Next behind the water lilies come the sedges, which form a mat of floating bog by their grasslike stems sinking down in the water and being there interwoven with the rhizomes below. This mat of sedge is often so firm that cattle may advance upon it to the water’s edge, but it is separated by a layer of water from the bed of growing peat at the bottom of the lake (Fig. 466). This bed of peat appears to grow upward toward the surface and become joined to the shore end of the floating bog by decaying vegetation which is dropped from the bottom of the mat above.

In order behind the floating bog come the advanced plants of the conifer group, with sphagnum and low shrub here upon a peat base extending to the lake bottom. Behind the belt of shrubs arise the tamaracks and spruces, and lastly, toward the shore, come the deciduous trees and especially poplars, maples, and marginal willows. Upon the margin of the basin there is usually a low trench, or “fosse”, filled with water during wet seasons, as a result, no doubt, of seasonal inwash that does not reach the residual lake toward the center of the basin.

Extinction of lakes in desert regions.—In arid regions there are special causes of lake extinction. Thus the blowing in of sand and dust carried for long distances in the air, a by no means negligible factor even in humid regions, here assumes large importance. The now exposed basins of extinct desert lakes afford the evidence, however, of an even greater factor of extinction, in climatic change. The clouds, which at one time found their way into the drainage basin of a lake, may later through the rise of a mountain barrier be cut off, and so with reduced water supply a period of lake desiccation is begun. When, in this process of drying up, the lake level has fallen below that of the outlet, the saline content of the waters begins to increase, and later a stage is reached, as in Great Salt Lake, when the sodium salts are precipitated. When the lake has become extinct, these deposits remain as a witness to the changed climatic condition.

The rÔle of lakes in the economy of nature.—It is natural, in considering the extinction of lakes, to give some attention to the rÔle which they play in the economy of nature. That lakes filter the water of rivers, and prevent the formation of important delta deposits, has already been noticed. A curious exception to this general rule is furnished by the great delta at the head of Lake St. Clair, just below the outlet of Lake Huron. This anomaly is, however, explained by the peculiar currents of Lake Huron, which are so directed as to sweep the beach sand into the swift current of the outlet, to be deposited in the quiet waters of Lake St. Clair (Fig. 467).

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).

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.

The first lake ice of early winter forms in most cases with air temperatures a few degrees only below the freezing point of the water. When later a severe “cold wave” arrives, the ice cover is contracted and becomes too small for the lake surface. To this contraction it yields and opens cracks up which the water rises, and in the prevailing low temperature this water is quickly frozen and the lake cover again made complete. Skaters are familiar with the opening of these cracks and the loud “roaring” which accompanies it on cold mornings, the sharp skate runners sometimes starting a crack in the strained ice, as does a light scratch upon glass that is in a similar strained condition.