The forest contains Marys Peak, the highest point in the Oregon Coast Range at . Numerous aquatic habitats are found in the forest: marine shore, rivers and streams—, including the Alsea, Nestucca, Siuslaw, and Umpqua rivers—and 30 lakes. The terrestrial environment can be regarded as two major vegetation zones, one near the coast dominated by Sitka spruce (Picea sitchensis), and the other dominated by western hemlock (Tsuga heterophylla) and Douglas fir (Pseudotsuga menziesii). Western hemlock often grows in the shade under Douglas fir. Other major tree species in the forest are western red cedar (Thuja plicata), red alder (Alnus rubra), and bigleaf maple (Acer macrophyllum). A 1993 Forest Service study estimated that the extent of old growth in the forest was . The Cummins Creek Wilderness and the Rock Creek Wilderness preserve some of this old growth.