The geology of the Bull Run watershed is dominated by rocks formed during its volcanic history, predominantly Columbia River basalts and andesite. Exposed rocks in the watershed include layers of volcanic ash and other rocks formed as a result of volcanic eruptions. The predominantly loamy and well-drained soils in the Bull Run are a combination of the various volcanic soil layers that lie below the organic material on the forest floor.
Elevations in the watershed range from 750 to 4,750 feet above mean sea level. The topography varies from low-relief lava flow surfaces to steep-walled canyons. Most of the westerly lava flows have been deeply eroded by down-cutting streams. Only about 12 percent of the watershed has slope angles greater than 50 percent. Evidence of glaciation in the watershed is limited to elevations above 2,600 feet and is generally in north-facing drainages.
 Sources:U.S. Dept. of Agriculture, Natural Resources Conservation Service Soil Survey, Geographic database forMultnomah County,Oregon; and State Soil Geographic Database forClackamas County,Oregon.