Major peaks are: the North, Middle, and South Sisters (all above 10,000'), along with Broken Top, the best example in the Pacific Northwest of the effect of glaciation. Collier Glacier is the largest glacier in Oregon. Other geologic features include Rock Mesa, Collier Cone, and Yapoah Crater. Alpine meadows, waterfalls, lava fields, glaciers, and glacial lakes are abundant.
Three Sisters Wilderness area covers 281,190 total acres. There are approximately 260 miles of trail within the wilderness. The Pacific Crest National Scenic Trail traverses the wilderness for 40 miles. Most trails entering the wilderness begin in dense forests of Douglas-fir on the western slopes of the Cascades and ponderosa pine on the eastern slopes.
Forests in this wilderness consist mainly of Douglas-fir, silver fir, sub-alpine fir, mountain hemlock, western hemlock, true fir, lodgepole pine, and ponderosa pine. Alpine meadows are abundant.
The Three Sisters Wilderness became part of the National Wilderness Preservation System with the Wilderness Act of 1964.