The majority of the Bull Run (close to 95 percent) is forested. Old- and medium-growth trees such as Douglas-fir, western hemlock, and Pacific silver fir make up much of the forest. These forests provide a thick canopy cover and a layer of organic material on the ground that nourishes smaller plants such as salal and sword fern. This rich layer also acts as a filter for water that moves throughout the watershed, cleaning it as it flows to the river below.   

Non-forested land in the Bull Run is mostly made up of steep, unvegetated slopes covered with rock (called talus) and water bodies such as streams and reservoirs. Aside from one wetland that is a seasonal meadow where deer sometimes graze, the majority of the watershed provides very limited forage for most large mammals, due to the lack of meadow habitat and the density of the forests.