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.
Fig. 463.—View looking upstream across the opaque waters of the Arve to the clear reflecting surface of the Rhone. To the right across the Arve is seen the cement works for recovering the Arve sediments.
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]).
Fig. 464.—The village of Poschiavo in eastern Switzerland, built upon a strath at the head of Lake Poschiavo.
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.