Connectivity of Timber Harvest Units to Stream Channels in the Sierra Nevada Mountains, California.

Increases in sediment delivery to streams degrade water quality and aquatic habitat. Previous studies have documented how roads, fires, and mass movements can deliver large amounts of sediment to streams, but there have been few studies on the extent to which timber harvest units are connected to streams. There also is a lack of validated models for predicting the downslope delivery of sediment due to surface erosion in forested areas. This information is critical for predicting the cumulative effects of timber harvest activities in forested watersheds. The objectives of this study were to: (1) determine the proportion of timber harvest units with rills or sediment plumes leading to streams; (2) assess the magnitude of rill erosion; and (3) collect site data to develop models for predicting sediment delivery to streams. Surface erosional features and sediment delivery from harvest units was assessed by walking the boundary between the riparian protection zone and about 200 harvest units. The harvest units were on four National Forests in the northern Sierra Nevada Mountains of California and ranged in age from 2 to 18 years. Only 15 rills and 4 sediment plumes emanated from harvest units rather than unpaved roads. The rills varied in length from 11 to 220 m as compared to 10 to 22 m for the sediment plumes. Preliminary analysis indicates that the longer rills were associated with older harvest units, steeper hillslope gradients, and higher annual precipitation. These results indicate that timber harvest alone generally does not initiate downslope surface erosion, and public concerns should focus on the associated skid trails and unpaved roads.