Hydrology of Crater, East and Davis Lakes, Oregon by Kenneth N. Phillips
WATER SUPPLY TO THE LAKE
SURFACE RUNOFF AND GROUNDWATER INFLOW
Most of the water supply for the lake comes from rain and snow falling directly into the lake. The contribution by runoff and percolation from the steep peripheral area cannot, however, be disregarded, and its amount has been estimated by considering precipitation and runoff records in Rogue River basin, over which the moisture-laden winds must pass to reach the lake. Average annual observed precipitation is about 18 inches at Medford (alt 1,300 ft), 42 inches ‘at Prospect (alt 2,500 ft), and 67 inches at Crater Lake (alt 6,475 ft). The long-term average runoff (including ground-water contributions) of Rogue River below South Fork (alt 1,700 ft) is equal to 36 inches depth on its drainage area annually; at the river station near Prospect (alt 2,600 ft) it is 35 inches, and some large spring flows bypass the station; above Bybee Creek (alt 3,600 ft) it is 41 inches. Obviously, the annual precipitation and runoff increase with altitude. However, because of avalanching from the crater walls, the precipitation effective in producing runoff from the peripheral area is less than the computed annual average of 69 inches on the lake itself.
For this report, the average annual runoff from the peripheral area is estimated at 46 inches depth (13,840 acre-ft), including groundwater inflow.
The total water supply reaching the lake has been calculated from the area of the lake (13,140 acres) and the tributary area (3,610 acres). For 1 inch of observed precipitation at the weather station, about 1,386 acre-feet of water reaches the lake, and the average annual observed precipitation of 67.4 inches (1930-62) produces 93,400 acre-feet of water in the lake.