Clear Creek Floodplain Rehabilitation
Winner of Governor’s Environmental and Economic Leadership Award (GEELA) in 2006
The Clear Creek Floodplain Rehabilitation Project is a product of the Clear Creek Technical Team, a group of public agencies and private partners whose common vision is to restore and enhance ecosystem function in the lower 16 miles of Clear Creek below Whiskeytown Dam. Historic impacts to Clear Creek vary by reach. Relevant impacts to the downstream alluvial reaches include: nearly 50 years of flow and sediment impairment by Whiskeytown Dam; channel and floodplain alteration from gold mining, and; a valley-scale sediment deficit induced by gravel extraction. The cascade of geomorphic effects from these impacts includes channel incision to bedrock, reduced floodplain function and severely impaired alluvial function. Biological consequences of these impacts included: floodplain stranding of juvenile salmon; predation by non-native species inhabiting abandoned mining pits and severely limited spawning habitat.
The project is part of a basin-wide integrated approach to ecosystem management which emphasizes the restoration of process. Flow releases, exotic species removal, upstream gravel augmentation and ongoing monitoring contribute to an adaptive, integrated approach. Key geomorphic objectives for the floodplain restoration project include: the restoration of sediment transport continuity, enhanced alluvial function and increased frequency of overbank flows. The dynamism associated with each of these key elements benefits all of the species of concern and provides the physical framework for a healthier, more productive system.
Project implementation has been through the Western Shasta Resource Conservation District with most of the funding from CALFED. The focus is on a three mile reach of Lower Clear Creek within BLM lands. Since 2000, GMA has led geomorphic research and hypothesis development, survey and physical data collection, drafting, design input, construction oversight and geomorphic monitoring. The scale of the project dictated a phased approach. In Phase One, off channel pits were filled with over 200,000 cubic yards of dredger spoils to restore floodplain integrity. Phase Two provided for the planting of floodplains with careful developed patches and matrices of native species.
Phases 3A and 3B focused on the restoration of channel form and relocation of the channel away from exposed claypan. While virtually the entire channel was developed using a “soft” approach, two critical locations were reinforced by rootwad revetments with boulder reinforcements. Each of these locations was determined to be highly vulnerable to channel recapture by the old claypan channel, thus threatening millions of dollars of restoration. Further, since the entire project could not be completed at once, grade control structures were installed to provide interim stability for the new gravel-bedded channel. As of 2016, these structures have performed flawlessly during floods approaching 7,000 cfs. With few exceptions, channel capacity functions very near the design threshold of 3,000 cfs (the two-year event) facilitating more frequent floodplain inundation. The channel is highly dynamic, creating new features while maintaining a mostly single thread channel and sediment transport appears to be nearly balanced at the project level. Designed riffles and other evolved alluvial features support a large increase in spawning salmon and juvenile salmon utilize the velocity and predation refuge provided by the rootwad revetments.