THE EFFECTIVENESS OF STREAM CROSSING DECOMMISSIONING AS A SMALL STREAM RESTORATION TECHNIQUE FOR CONFINED CHANNELS, NORTHERN CALIFORNIA

In the restoration of managed wildland watersheds and streams, roads are often targeted for treatment because they represent a significant source of anthropogenic sediment and they can be effectively treated to reduce both episodic and chronic sediment delivery. For over a decade, the California Department of Fish and Game (CDFG) has administered a regional fisheries restoration program to improve and protect salmonid habitat. The focus of this and other efforts has been to restore channel processes in small confined streams and to reduce sediment delivery through road decommissioning.

In 2004, with partial funding from CDFG, we inventoried over 80 miles of decommissioned road between San Francisco and the Oregon border. The objectives of the study were to evaluate the effectiveness of current decommissioning techniques and to recommend improved restoration practices. Over 450 decommissioned stream crossings were inventoried from a variety of road types, land ownerships and geologic terrains. Data collection included pertinent site variables; treatments; geomorphic and hydrologic site data; and post decommissioning erosion volumes. Decommissioning practices at each site were compared to standardized protocols. Tape and clinometer surveys defined excavation geometries and excavated fill volumes.

Stream crossing decommissioning has significantly reduced potential long term sediment delivery. In the short term, freshly excavated crossings experience post implementation erosion and channel adjustments that typically total <5% of the original erodible fill volume. A small amount of erosion is unavoidable and can be attributed to bedrock and soil types, storm events, and vegetative recovery. Most post-treatment erosion was attributable to poor implementation practices. Typical operational mistakes include: poor upstream profile transitions (knickpoints), oversteepened excavation sideslopes, incomplete excavation, uncontrolled emergent groundwater, and poor spoil disposal techniques. Specific improvements in problem recognition, prescription development and implementation practices can reduce foreseeable problems and improve the overall cost-effectiveness of stream crossing decommissioning and channel restoration.