Fine sediment sources in coastal watersheds with uplifted marine terraces in northwest Humboldt County, California
This article is part of a larger document. View the larger document here.Abstract
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, increasing the costs of treating water for domestic water supplies. Detailed road and erosion surveys and monitoring of suspended sediment, discharge, and turbidity levels in Mill Creek (3.11 km2) and Luffenholtz Creek (12.95 km2) were completed to determine the sources of turbidity. 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. Although Mill Creek was the watershed with the highest rate of timber harvesting during the past 10 years, the steepest stream profile, and a history of more extensive fires, its turbidities were nearly always lower than Luffenholtz Creek.