The surface of Lake Chelan is a little more than three hundred feet above the bed of the Columbia River, which flows through a deep canon only three miles distant. If we could remove the dam of glacial boulders and gravel at the lower end of the lake, the water would be lowered only three hundred feet. The lake would not be drained, for it is very much deeper. Now here is another puzzle for us: the bottom of the lake is more than one thousand feet below the level of the Columbia. We shall have to go still farther back into the past to get a satisfactory explanation this time. Hundreds of thousands of years ago there was no plateau filling central Washington, and no Columbia River crossing it. The Cascade Range stood where we see it to-day, and the region of the plateau was a broad valley, toward which flowed the streams that had already cut canons upon the eastern side of the range. These streams probably united in a river emptying westward into the Pacific by a course now unknown. The shores of the ocean were farther west than at present, for the land stood higher. The canon of Lake Chelan was made by a river of this period, which through many long years gradually deepened and enlarged its channel. The river worked just as we see rivers working at the present time, for throughout all the history of the earth rivers have not changed their habits. Then came the long period of volcanic eruptions. Our Northwest was flooded by fiery lava, which built up the Columbia plateau and buried under thousands of feet of rock the old river valley into which the canon of Chelan emptied. Then streams of water began to flow over the plateau from the higher mountains above the reach of the lava. These streams formed the Columbia River, which sought the easiest way to the sea, and finally excavated a canon for hundreds of miles. In a portion of its course the river came close to the edge of the Cascade Range. The ancient canon of Lake Chelan had been dammed up by the lava, and a lake occupied a portion of the former bed of the river. The Columbia could not cut its channel deep enough to drain the lake, and there it remained. [Illustration: FIG. 60.--LOOKING DOWN LAKE CHELAN FROM THE UPPER END] Then another change came: the climate grew cold and heavy snows gathered upon the Cascade Range. The snow did not all melt during the summers, but went on increasing from year to year. The masses of snow moved gradually down the mountain slopes, growing more and more icy until they became true glaciers.