National Geographic : 1898 Sep
BITTER ROOT FOREST RESERVE Salmon rivers. On the eastern slope is the Bitter Root river, one fork of which heads in the southeast corner of the Reserve and flows northward through the fertile valley of the same name. This valley separates the Rocky mountains from the Bitter Root range for a distance of about 100 miles and at present has a good agricultural development. The main valley has a width of about 8 or 10 miles, its floor being comparatively level, composed of lacustrine deposits and very fertile under irrigation. When the drainage of the ancient lake occurred there was left a heavy deposit of gravel and other sediment, through which the Bitter Root river is still cutting, and this process has shifted the flood plain back and forth, the result being that in some portions of the valley well defined terraces have been carved out correspond ing to the older floodplains. The Bitter Root river joins the Missoula near the town of the same name and ultimately finds an outlet in the Columbia river through Clarke's Fork and Lake Pend d'Oreille. The streams con stituting this drainage are remarkably straight and of a very steep gradient. Their tangent-like course is due primarily to glacial agencies, and they have not become modified on account of the extreme hardness of the rocks. They seek the straight and direct course and do not loiter amid the inhospitable granite to carve out for themselves gentle curves. In their haste to reach the valley they leap and jump and are tossed from boulder to boulder, now lashing themselves into fleecy whiteness and now circling in emerald eddies as they plunge into some quiet pool, where they find a moment's rest and gather strength for their ever downward course. The beds are filled with boulders, and the sides of the canyons are precipitous and almost entirely bare of vegetation. These streams in their incessant activity are not only continually deepening their own beds in the attempt to reach baselevel, but are gradually working their way westward and capturing the tributaries of the less active affluents of the Clear water, causing what is termed a migration of the divide. The shifting or migration of a divide is due to the weathering or wast ing away of the crest line, and may result from various causes. It seems probable that the main crest of the Bitter Roots has moved to the westward, owing to the fact that the highest points at present are all east of the crest line. Ward peak is 8 miles to the east and about 800 feet higher than the general elevation of the divide, and St Mary's and El Capitan peaks each attain an elevation considerably higher than the divide.