Hydrogeologic setting and preliminary estimates of hydrologic components for Bull Run Lake and the Bull Run Lake drainage basin, Multnomah and Clackamas Counties, Oregon

The hydrogeologic setting was described and preliminary estimates of hydrologic components prepared for the Bull Run Lake and for the Bull Run Lake drainage basin, in the Cascade Range of northwestern Oregon. The 0.73-square-mile lake and the 3.44-square-mile drainage basin lie within the Bull Run Watershed, the principal water supply for the Portland, Oregon, metropolitan area. During periods of high demand or low inflows to the watershed, the City of Portland Bureau of Water Works, releases water from Bull Run Lake to augment the supply. Bull Run Lake is impounded by a natural dam formed by a landslide. Outflow of ground water from the lake through the landslide emerges as springflow at the toe of the landslide and forms the headwaters of the Bull Run River. The approximately 4,300-Mgal (million gallons) discharge of the Bull Run River measured below the springs during the 1993 water year is composed of (1) outflow of ground water from Bull Run Lake through the landslide (approximat