Hydrology of Crater, East and Davis Lakes, Oregon by Kenneth N. Phillips
WATER SUPPLY TO THE LAKE
The water supply of Crater Lake is derived from infalling precipitation, surface runoff, and ground-water inflow. Direct precipitation, chiefly in the form of snowfall, is by far the greatest contributor, because the lake itself covers most of its basin. Snowfall forms drifts on the crater walls and the snow reaches the lake either by avalanching during winter and spring storms or as summer snowmelt and deep percolation. Some precipitation that falls high on the crater slopes may be drained radially outward away from the lake. On the other hand, Williams (1942, p. 129) has suggested the possibility of slight amounts of seepage toward the lake from outside the rim and has observed that:
In general, the drainage is outward from the caldera, following the dip of the lavas and interbedded pyroclastic rocks. At several points, however, particularly where lavas rest on glacial debris, the drainage is reversed and enters the lake. Noteworthy examples may be seen on the cliffs south of Sentinel Point, on Sutton Cliff, and under the topmost flows overlooking Grotto Cove. At these and similar places, copious springs discharge down the caldera walls.
Any small contribution to the water supply of the lake that may result from a reversal of the prevailing outward drainage, as noted by Williams, probably is offset, and may be exceeded, by outward radial seepage from melting snowbanks high up on the walls of the caldera. No allowance is made herein for the possibility of such in-seepage or out-seepage above the level of the lake; rather, the topographic rim is assumed to be the hydrologic divide.