The primary value of this site is its substantial amount of relatively undisturbed low-elevation coniferous forest, the only large area of protected habitat of this type in a contiguous block within the fragmented checkerboard landscape south of Interstate 90. It contains 5,670 hectares of old-growth forest, with trees as old as 850 years. The watershed acts as a de facto reserve, and is most significant from a conservation perspective because of the large amount of mature Western Hemlock forest.
Recent King County Breeding Bird Atlas surveys detected 103 breeding species. The site supports an assemblage of species associated with mature coniferous forest, including Northern Goshawk, Marbled Murrelet, Northern Spotted Owl, Vaux's Swift, Pileated Woodpecker, Red-Breasted Nuthatch, and Brown Creeper. Breeding Peregrine Falcons have also been confirmed. Twenty-five percent of the breeding Common Loons in Washington nest in the watershed. These nesting pairs typically produce the majority of the state's fledgling loons on a yearly basis.
At present, the habitat in the Cedar River Watershed is well protected. The Seattle City Council approved a Habitat Conservation Plan that prohibits logging. Over the 50-year period covered by the plan, the proportion of mature forest in the watershed will increase to 85 percent.