Columbia River basalts, 10 to 20 million years old, that underlie much of the Bull Run watershed are exposed near the bottoms of steep canyons along the river and its tributaries.
In the western half of the watershed, the Rhododendron formation, rich in sediments, overlies the basalt, and later volcanic flows of basalt and andesite overlie both older formations.
Areas of thick talus occur in the eastern part of the watershed at elevations higher than 2,500 feet (760 m) above sea level, and north-facing slopes above 2,600 feet (790 m) show evidence of glaciation.
Over many centuries, streams in the watershed have carved canyons through the Rhododendron formation to the level of the basalt.
Since basalt resists erosion, water traveling over it remains relatively free of sediments.
Less than 2 percent of the watershed is at high risk for landslides.

The Bull Run River's three reservoirs—Bull Run Lake, Bull Run Reservoir 1, and Bull Run Reservoir 2—are oligotrophic and do not sustain many life forms.
Bull Run Lake is in a steep-sided cirque blocked at its lower end by a series of lava flows topped by debris from a glacial moraine.
Small streams flow into the lake from ridges above it, and water exits the lake mainly by seeping through porous rock to enter the Bull Run River about 0.5 miles (0.80 km) downstream.
Evidence suggests that over the past several thousand years, although forest fires in the area and volcanic activity on Mount Hood or Mount St. Helens  have caused temporary changes in the lake's limnological condition, it "has always returned to conditions similar to those seen at present."
Turbidity is sometimes a problem in Reservoirs 1 and 2 when unstable soils sandwiched between layers of lava erode into tributaries, especially the North and South forks.
Columbia River basalts, 10 to 20 million years old, that underlie much of the Bull Run watershed are exposed near the bottoms of steep canyons along the river and its tributaries. In the western half of the watershed, the Rhododendron formation, rich in sediments, overlies the basalt, and later volcanic flows of basalt and andesite overlie both older formations. Areas of thick talus occur in the eastern part of the watershed at elevations higher than 2,500 feet (760 m) above sea level, and north-facing slopes above 2,600 feet (790 m) show evidence of glaciation. Over many centuries, streams in the watershed have carved canyons through the Rhododendron formation to the level of the basalt. Since basalt resists erosion, water traveling over it remains relatively free of sediments. Less than 2 percent of the watershed is at high risk for landslides.
The Bull Run River's three reservoirs—Bull Run Lake, Bull Run Reservoir 1, and Bull Run Reservoir 2—are oligotrophic and do not sustain many life forms. Bull Run Lake is in a steep-sided cirque blocked at its lower end by a series of lava flows topped by debris from a glacial moraine. Small streams flow into the lake from ridges above it, and water exits the lake mainly by seeping through porous rock to enter the Bull Run River about 0.5 miles (0.80 km) downstream. Evidence suggests that over the past several thousand years, although forest fires in the area and volcanic activity on Mount Hood or Mount St. Helens  have caused temporary changes in the lake's limnological condition, it "has always returned to conditions similar to those seen at present." Turbidity is sometimes a problem in Reservoirs 1 and 2 when unstable soils sandwiched between layers of lava erode into tributaries, especially the North and South forks.
Bull Run River (Oregon)