Page:Popular Science Monthly Volume 47.djvu/186

176 It has been my good fortune to make explorations in Montana, Wyoming, Idaho, and the Yellowstone Park which have cleared up some of these difficulties. The presence of trout in Yellowstone Lake and the total absence of all fish from the other large lakes of the park was one of the most interesting of these anomalies, and it is to its explanation that this article is devoted.

It is certain that all the streams and lakes of the territory covered by the lava flow were wiped out of existence by the fiery flood, and all terrestrial and aquatic life destroyed. Many long years must have passed before this lava sheet became sufficiently cooled to permit the formation of new streams; but a time finally came when the rains, falling upon the gradually cooling rock, were no longer converted into steam and thrown back into the air, only to condense and fall again, but, being able to remain in liquid form upon the rock, sought lower levels, and thus new streams began to flow. And then the fishes in the connecting streams below, which had not been destroyed by the lava flow, began to invade the desolated region and repeople its waters.

The rhyolite, obsidian, and trachyte were very hard and eroded slowly, but when the streams reached the edge of the lava field they encountered rock which was comparatively soft and which wore away rapidly. The result is that every stream leaving the Yellowstone Park has one or more great waterfalls in its course where it leaves the lava sheet. Notable among these streams are Lewis River, the outlet of Lewis and Shoshone Lakes; Yellowstone River, the outlet of Yellowstone Lake; Gardiner, Gibbon, and Firehole Rivers, and Lava, Lupine, Glen, Crawfish, Tower, and Cascade Creeks, all leaving the lava sheet in beautiful falls, varying from thirty feet to over three hundred feet in vertical descent. The following is a list of the principal waterfalls in the streams in and about the park, each one of which is supposed to form an insurmountable barrier to the ascent of fish: