DESTRUCTION or LAKES. Lakes are normally of brief duration, from the standpoint of logical time. Consequently lakes are mostly of recent origin, and are especially abundant in re gions where some recent accident has happened to drainage; as. for example, where glaciers have been, or where lava flows have recently overspread areas of country. Since lakes net as catchment basins for sediment, they are soon tilled by the contributions brought by the incom ing, streams, by rain-wash, by winds, and by waves. Much of the finer sediment settles in the lake at a distance from the shore; but most of the coarser material accumulates near the shore, and especially in the river deltas. These grow out into the lake, forming., flats at the head of the lakes and protruding deltas on the mar gin. In some cases. where the sediment supply is abundant, as in the Swiss lakes, which receive glacier-fed streams, the growth of deltas from opposite sides of the lake has cut a single lake in two parts. This is clearly illustrated in the ease of lakes Thun and Brienz, which are divided by the delta deposit on which Interlaken is situ ated.
As lakes are shallowed by sediment deposit, organic contributions help finally to fill them. Various forms of vegetation, including lilies, reeds, rushes. cane, and sphagnum, are Very effective in this last stage of lake destruction. When finally filled the lake becomes a swampy plain; the rivers then build the plain up into dry land in their establishment of a slope or grade across the plain.
Lakes are not always destroyed solely by H The outlet stream is always eroding at the barrier, though this work is usually slow, because the lake water has filtered out the sedi ment, so that the outlet is robbed of its cutting tools. Thus Niagara. in flowing from Lake Erie, has cut away but little more of the barrier than the loose soil, and consequently flows practically on the surface of the plain. Where the outlet passes through unconsolidated material it may rapidly lower the lake-level. If a condition like
that at Niagara could exist near a lake, the eating back of the falls would in time reach the lake and rapidly drain it. This will not happen in the case of Niagara, because the layer which causes the cataract dips toward the south. and the fall will disappear before Lake Erie is reached.
A lake may also be destroyed by a change of climate. Thus a series of lakes existed in the great basin of Western United States during the Glacial period. when the climate was cooler and damper. (See LAKE: BONNEVILLE: LAKE LAIION TAN. ) During the Glacial period large lakes also existed along the margin of the ice wherever the glacier formed a dam across north-flowing stream valleys. An enormous lake of this origin ex isted in the valley of the Red River of the North, to which the name Lake Agassiz (q.v.) has been given. Similar lakes appeared in the basin of the Great Lakes, while the Saint. Law rence Valley was ice-filled. The sho•e-lines of these glacial lakes are plainly seen along the mar gin of the Great Lakes. and record a very com plex history, with various outflows which were made available as the iee-front melted farther• and farther back. The deposits made in these extinct lakes form much of the soil of the land along the southern margin of the Great Lakes. Such methods of lake destruction are abnormal; the natural and usual method is filling, corn bined with a partial removal of the barrier over which the outlet flows.
litnuoca.tenr. Geddie, Lake Region of Cen tral Africa ( London, SS1 ) ; Gilbert, "Topo graphic Features of Lake Shores," in United States Geological Surrey Fifth Annual Report (Washington. 1S85); Geikie, Manual of Geology (London, 1893) ; Russell, Lakes of North Ameri ca 1894 ) ; Whipple, Classification of Lakes According to Temperature (Boston, 1898) ; Tarr, Physical Geography (New York, 1897) ; Davis, Physical Geography (Boston, 1900).