Watershed Sediment Reduction

Abstract

Unpaved roads are a significant source of sediments that degrade aquatic habitats. During the 1990s, counties in northwestern California faced road maintenance challenges as they were required to reduce sediment produced by their facilities and operations in order to comply with new regulations implementing provisions of the U.S. Endangered Species Act and Clean Water Act. In 1997, five counties in the region joined forces to address this problem, particularly with respect to anadromous fish. Together they obtained grant funding to study the effect of their activities on salmonid populations, develop mitigations, and undertake improvements. This paper reviews the sediment reduction treatments and implementation strategies employed by Mendocino County at two case study sites. Both are on resource roads 14 to 20 ft wide experiencing typical summer volumes of fewer than 50 vehicles per day and annual rainfall exceeding 40 in. An erosion inventory identified these road sections as being significant sediment contributors within their respective watersheds. The sediment reduction projects were completed between August and October 2002 with the use of low-impact-to-hydrology designs modified to comply with AASHTO guidelines. Where these features could not be accommodated, standard best management practices for gravel roads were employed. Compared with adjacent, untreated sections of road, both project sites performed well during exceptionally wet winters. Maintenance requirements have been significantly reduced, and sediment delivery is estimated to be down 84% at one site. Grant funding made these projects cost-effective for the county, but more economical alternatives are suggested for locally funded projects.