The California Legislature prohibited the construction of new dams, diversions, and reservoirs on 47 miles of Mill Creek in Tehama County in 1995. The dam prohibition occurred in response to a state study of Mill Creek, which found the river to be eligible and suitable for state protection. Due to opposition by local landowners (including Sierra Pacific Industries), the Legislature chose to simply prohibit dams on the river instead of granting it full state wild and scenic protection. New dams are prohibited on Mill Creek from its source in Lassen Volcanic National Park to where the stream flows out of the northern Sierra foothills into the Sacramento Valley. The river flows through mixed public and private lands, including Lassen Volcanic National Park and Lassen National Forest managed by the National Park Service and Forest Service. In a separate study, the Forest Service found Mill Creek to be both eligible and suitable for federal designation.
The state study found Mill Creek to possess extraordinary scenic, recreation, fish, geological, cultural, and scientific values. From its geothermal sources on the slopes of Mt. Lassen, Mill Creek flows through dense conifer forests, meadows, and rugged lava rim rock canyons into the Sierra foothills. The creek flows through the Wild Cattle Mountain and Mill Creek roadless areas recommended by the Forest Service for wilderness protection, as well as the existing Ishi Wilderness further downstream. These areas offer pristine wilderness-oriented recreation and experts-only whitewater boating. The creek supports some of the last remaining wild runs of threatened spring Chinook salmon and winter steelhead in the Sacramento Valley. Mill Creek’s biotic integrity is very high due to its undammed and roadless nature and the presence of native anadromous fish. The entire area is rich in Native American cultural values and is the setting for Theodora Kroeber’s biography about the “last” Yahi Indian, “Ishi In Two Worlds.”