Fine sediment sources of coastal watersheds with uplifted marine terraces in Northwest Humboldt County, California

Erosion and sedimentation related to roads in mountainous areas have long been a concern for land managers and those charged with protecting soil, water and streams. Erosion in the Mill and Luffenholtz Creek watersheds in Humboldt County, California, with their extensive clay soils, can lead to high turbidity levels in receiving bodies of water. These eroded clays, once suspended in flowing water, can increase the costs of treating water for domestic water supplies. Furthermore a health hazard can result from the creation of disinfection bi-products, such as chloro-tri-halomethanes. Reasons for studying turbidity in these creeks included the use of these creeks for domestic water supplies for the City of Trinidad and the surrounding area and the presence of endangered species of fish. In addition the receiving waters of Trinidad Bay have been designated an Area of Special Biological Significance (ASBS), as well as a Critical Coastal Area (CCA). Local residents were interested in identifying sediment sources and implementing practices to remedy these problem areas. Detailed road and erosion surveys and monitoring of suspended sediment, discharge, and turbidity levels in Mill Creek (3.11 kilometers²) and Luffenholtz Creek (12.95 kilometers²) were completed to determine the sources of turbidity. Two continuous turbidity and stage monitoring stations in lower Mill and Luffenholtz Creeks were maintained along with six additional grab sample locations. Watershed physiographic and land use characteristics were compared for those basin areas upstream from the eight monitoring sites and a statistical analysis was completed to determine whether significant differences existed between turbidity levels in the watersheds. The Luffenholtz Creek watershed, particularly Grassy and 18 Creeks, and the main stem of Luffenholtz, just downstream of the 21 Rock Quarry, were the sub-watershed areas with the highest turbidities. They were also the sub-watershed areas with the highest density of roads, the most miles of roads adjacent to streams, and the highest concentration of identified erosion prone sites. Statistical analysis showed that Luffenholtz Creek was significantly more turbid than Mill Creek, and the site downstream of the 21 Rock Quarry was significantly more turbid than the site upstream of the 21 Quarry. Total instantaneous loads for a three-storm period (5/1-5/4, 5/4-5/9, and 5/13-5/17/2009) representing three of eight major storms for the year, were approximately 0.016 metric tons per kilometer² for Luffenholtz Creek and 0.0006 metric tons per kilometer² for Mill Creek. Although Mill Creek was the watershed with the highest rate of timber harvesting over the past ten years, the steepest stream profile, and a history of more extensive fires, its turbidities were nearly always lower than Luffenholtz Creek. The most likely explanation for increased turbidities in Luffenholtz was the greater road density and road proximities, as well as a legacy of stored sediments in the bankfull channel area from past harvest activities. The sediment was most likely coming from roads and stored sediments. Cooperative efforts are now underway with the local watershed council, local contractors, and local landowners to resolve these correctable erosion problems in both watersheds.