Post 02823
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When, toward the close of the glacial period, the ice mantle began to shrink and recede from the lowlands, the lower portions of the rivers were developed, issuing from cavelike openings on the melting margin and growing longer as the ice withdrew; while for many a century the tributaries and upper portions of the trunks remained covered. In the fullness of time these also were set free in the sunshine, to take their places in the newborn landscapes; each tributary with its smaller branches being gradually developed like the main trunks, as the climatic changes went on. At first all of them were muddy with glacial detritus, and they became clear only after the glaciers they drained had receded beyond lake basins in which the sediments were dropped.
This early history is clearly explained by the present rivers of southeastern Alaska. Of those draining glaciers that discharge into arms of the sea, only the rills on the surface of the ice, and upboiling, eddying, turbid currents in the tide water in front of the terminal ice wall, are visible. Where glaciers, in the first stage of decadence, have receded from the shore, short sections of the trunks of the rivers that are to take their places may be seen rushing out from caverns and tunnels in the melting front,—rough, roaring, detritus-laden torrents, foaming and tumbling over outspread terminal moraines to the sea, perhaps without a single bush or flower to brighten their raw, shifting banks. Again, in some of the warmer cañons and valleys from which the trunk glaciers have been melted, the main trunks of the rivers are well developed, and their banks planted with fine forests, while their upper branches, lying high on the snowy mountains, are still buried beneath shrinking residual glaciers; illustrating every state of development, from icy darkness to light, and from muddiness to crystal clearness.
Now that the hard grinding sculpture work of the glacial period is done, the whole bright band of Sierra rivers run clear all the year, except when the snow is melting fast in the warm spring weather, and during extraordinary winter floods and the heavy thunderstorms of summer called cloud-bursts. Even then they are not muddy above the foothill mining region, unless the moraines have been loosened and the vegetation destroyed by sheep; for the rocks of the upper basins are clean, and the most able streams find but little to carry save the spoils of the forests,—trees, branches, flakes of bark, cones, leaves, pollen dust, etc.,—with scales of mica, sand grains, and boulders, which are rolled along the bottom of the steep parts of the main channels. Short sections of a few of the highest tributaries heading in glaciers are of course turbid with finely ground rock mud, but this is dropped in the first lakes they enter.