A Brief Guide to the Parapet View, Sinnott Memorial, Crater Lake National Park by John Campbell Merriam, 1938
The water of Crater Lake is derived from rainfall and snowfall over this crater region, together with snow blown into the depression. The lake is not known to have outlet except by seepage. The conditions of evaporation, seepage, and precipitation are in a state of balance which makes possible this accumulation of water and maintenance of approximately this water level. If the region were at a different altitude, or in a different location, the lake might not have been formed.
It is conceivable that in the course of late stages in its history, and under climatic conditions different from those of the present, the crater may at times have been filled in part with ice.
Existence of Crater Lake was made possible by building of a mountain, in the elevated summit of which there could be formed a wide and deep cavity having no outlet, except by seepage, and no inlet. The conditions required for accumulation of a body of water with the peculiar beauty of this lake are furnished in a crater produced by combination of those tremendous forces found in the power and heat of a volcano.