Established in 1908, the Siuslaw is made up of a wide variety of ecosystems, ranging from coastal forests to sand dunes.

The Siuslaw National Forest encompasses more than 630,000 acres (2,500 km2) along the central Oregon Coast between Coos Bay and Tillamook, and in some places extends east from the ocean, beyond the crest of the Oregon Coast Range, almost reaching the Willamette Valley.

[5] The forest contains Marys Peak, the highest point in the Oregon Coast Range at 4,097 feet (1,249 m).

Numerous aquatic habitats are found in the forest: marine shore, rivers and streams—1,200 miles (1,900 km), 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).