Frequency and characteristics of sediment delivery pathways from forest harvest units to streams

Abstract

Timber harvest is typically the largest area of anthropogenic disturbance in forested watersheds, and harvested areas may generate from one to five times more erosion than undisturbed areas (Motha et al., 2003). When sediment from harvested areas reaches stream channels it can degrade water quality and aquatic habitat. Streamside management zones (SMZs) are often prescribed to minimize sediment delivery, but there is little information about sediment delivery through these zones. Hence the objectives of this study were to: (1) determine the frequency of sediment delivery pathways (“features”) from timber harvest units; (2) measure the physical characteristics and connectivity of these features; and (3) develop models to predict the length and connectivity of features from harvest units to streams. Nearly 200 harvest units with streamside management zones were assessed on four National Forests in the Sierra Nevada and Cascade Mountains of California. Only nineteen features were found below harvest units ranging in age from 2 to 18 years. Feature lengths ranged from 10 m to 220 m, and the length was significantly related to mean annual precipitation, cosine of the aspect, elevation, and hillslope gradient ( R 2 = 64%, p = 0.004). Six of the nineteen features were connected to streams and five of the six connected features originated from skid trails. The results indicate that timber harvest alone rarely initiated large amounts of runoff and surface erosion, particularly when newer harvest practices were utilized. Sediment delivery from timber harvest may be further reduced by locating skid trails away from streams, maintaining high surface roughness downslope of water bars, and promptly decommissioning skid trails following harvest.