INFLUENCE OF BEDROCK GEOLOGY AND FOREST MANAGEMENT ON SEDIMENT YIELD IN FIRST ORDER WATERSHEDS, HUMBOLDT COUNTY, CALIFORNIA

Small, first order streams in the Van Duzen and lower Eel watersheds owned by Pacific Lumber Company were surveyed and small sedimentation basins installed to estimate of sediment yield. A random study design was developed to investigate the effects of bedrock type and forest management on sediment yield. The drainage area for these streams was about 5 to 10 ha.

The erosion processes within the scope of this study were surface erosion from hillslopes and stream channel erosion. Large-scale mass wasting processes were excluded. The three most common sedimentary formations in the study are are the Coastal Belt Franciscan, Yager, and Wildcat. The latter is particularly poorly consolidated.

The key element of the study was the installation of temporary sedimentation basins in 30 randomly chosen channels to quantify sediment yield from watersheds with different management conditions (“treatment sites”, typically clear-cut harvest) and second-growth stands not recently entered (“control sites”). Two classes of treated sites were sampled: sites harvested under stricter regulations of a Habitat Conservation Plan (HCP) and sites harvested prior to implementation of the HCP. Channel surveys were also conducted to characterize conditions and identify existing evidence of channel erosion and surface erosion in areas adjacent to the channel.

Results from winter 2001/02 suggested that there may be differences in sediment yield among geologic substrates, however, the sample data were insufficient to detect a statistically-significant difference in sediment yield as a function of geologic substrate (ANOVA, p > 0.05). Treatment effects were significantly different (ANOVA, p < 0.05); the HCP sites had significantly lower sediment yield. Control sites had sediment yield comparable to the pre-HCP treatment sites.

The apparent treatment effect on sediment yield may be confounded by the study design wherein the HCP treatment sites have experienced only one or two post-harvest winters, while the pre-HCP treatment sites experienced relatively severe winters 1996-1998. The fact that the control sites tended to have higher sediment yield than the HCP treatment sites suggests that legacy effects of management may also be an important factor. HCP harvest regulations probably reduced the likelihood of harvest in portions of the landscape more vulnerable to erosion.