SEDIMENT MOVEMENT FOLLOWING REMOVAL OF SAELTZER DAM, CLEAR CREEK, CALIFORNIA

Clear Creek drains 590 km² of the northern Coast Range and southern Klamath Mountains, flowing into the Sacramento River south of Redding, California. Chinook salmon (Oncorhynchus tshawytscha) spawn in Clear Creek, but their migration to upstream habitats was blocked about 10 km upstream of the Sacramento by Saeltzer Dam, a small diversion structure built in 1912. As part of a larger program to restore Chinook salmon populations, the US Bureau of Reclamation removed Saeltzer Dam in fall 2000 to restore passage to the 16 km-reach above Saeltzer and downstream of Whiskeytown Dam. Prior to dam removal, approximately 19,000 m³ of sediment stored in the reservoir were mechanically removed and placed on a nearby terrace. In 2002, after two years with relatively modest high flows, an assessment of remaining reservoir sediments predicted (based on a sediment transport model) that there would be little sediment transported from the former reservoir site. However, following higher flows in 2003, our partial survey showed erosion and transport of at least 7,600 m³ from the former reservoir deposits. More recent and thorough topographic mapping indicates about 37,000 m³ of sediment was eroded from the reservoir deposit from 2000-2004, and cross section surveys in a formerly heavily-used spawning reach 2 km downstream showed aggradation of up to 0.5 m (Graham Matthews, Weaverville, unpublished data). Facies mapping of the bed downstream of the damsite showed increased extent of gravel bars and finer substrates from 2000–2004. The volume of sediment eroded from the reservoir site approaches the volume of sediment deliberately added to the channel to mitigate for the sediment deficit downstream of Whiskeytown Dam: 45,350 m³ of gravel injected from 1996 to 2004 at five sites over the 26-km reach below the dam.