The Eel River historically supported one of the largest salmon and steelhead runs in California. but populations have dramatically declined. As part of recovery efforts, fish passage assessments were conducted at crossings on tributary streams. An assessment along the now-defunct Northwestern Pacific Railroad, constructed in the early 1900’s through the remote Eel River Canyon, identified a severe barrier at the confluence of Woodman Creek, a 63 km2 drainage. During railroad construction the lower 100 meters of channel was buried with fill material 20 meters deep, and the stream was rerouted into a narrow cut through bedrock, discharging directly into the river, creating a 5 meter tall bedrock waterfall that blocked fish access to the watershed. In the 1990’s the rail was abandoned due to the unstable geology, and no public site access exists.
The project was initiated to remove the fill overlaying the historical channel and permanently plug the bedrock cut. Preliminary investigations involved use of seismic refraction and excavation of test pits to map the buried channel’s depth, and configuration. The subsurface investigations revealed a buried bedrock-boulder channel. The investigations indicated channel daylighting required excavation and disposal of 36,000 m3 of fill.
Given the remoteness of the site, the bedrock plug must remain functional for the indefinite future. It was designed as an 8 meter tall earthen dam using material excavated from the rail embankment. A concrete sill and geomembrane liner in its core is designed to minimize seepage. Large rock placed in layers along the plug’s exposed face is designed to resist scour and erosion from Woodman Creek on one side and the Eel River on the other side.
This presentation will describe site characterization techniques and design approaches employed to daylight the stream and building the plug, and methods used to overcome challenging construction logistics.