Study finds minimal increases of stream sediment concentrations after forestry operations

Historical forest harvesting and road construction in the Pacific Northwest sometimes resulted in measurable increases in sediment concentrations in adjacent streams. Significant increases in fine-grained suspended sediment (<2 mm) in streams can adversely affect aquatic organisms.

Current forest management practices have been refined to mitigate and minimize increased sediment from forestry operations. However, managers still wish to ensure that current practices are as effective as possible in reducing sediment transport from roads.

Recently, scientists with Oregon State University, Weyerhaeuser, the U.S. Forest Service, and the Oregon Department of Forestry reported results from a study that examined whether the combined effects of road construction/improvement, forest harvest, and hauling would increase turbidity and suspended sediment concentrations in adjacent streams.

The study addressed two primary questions: (1) Do road crossings in forested areas lead to increased fine-suspended sediment downstream after road construction/improvements and after forest harvest and log hauling? (2) If so, how frequently does sediment transport downstream occur?

The authors report finding “no evidence to suggest that current management practices increased median fine-suspended sediment concentrations in streams above biologically meaningful levels.” Furthermore, turbidity and suspended sediment concentrations below road crossings “appeared to be far less than what was observed in studies under historical forest practices.”

The study, which was part of the Trask Watershed Study, was conducted on the Trask River in the Northern Oregon Coast Range. Funding was provided by the Oregon Forest Industries Council, NCASI, the Oregon Department of Environmental Quality, and Weyerhaeuser Company. Additional indirect support was provided by Oregon State University, the Oregon Department of Forestry, NCASI, the U.S. Forest Service, and Weyerhaeuser.

The abstract follows.

“Transport of fine-grained sediment from unpaved forest roads into streams is a concern due to the potential negative effects of additional suspended sediment on aquatic ecosystems. Here we compared turbidity and suspended sediment concentration (SSC) dynamics in five nonfish bearing coastal Oregon streams above and below road crossings, during three consecutive time periods (“before”, “after road construction/improvement”, and “after forest harvest and hauling”). We hypothesized that the combined effects of road construction/improvement and the hauling following forest harvest would increase turbidity and SSC in these streams. We tested whether the differences between paired samples from above and below road crossing exceeded various biological thresholds, using literature values of biological responses to increases in SSC and turbidity. Overall, we found minimal increases of both turbidity and SSC after road improvement, forest harvest, and hauling. Because flow is often used as a surrogate for turbidity or SSC we examined these relationships using data from locations above road crossings that were unaffected by roads or forest harvest and hauling. In addition, we examined the association between turbidity and SSC for these background locations. We found a positive, but in some cases weak association between flow and turbidity, and between flow and SSC; the relationship between turbidity and SSC was more robust, but also inconsistent among sites over time. In these low order streams, the concentrations and
transport of suspended sediment seems to be highly influenced by the variability of local conditions. Our study provides an expanded understanding of current forest road management practice effects on fine-grained sediment in streams and introduces alternative metrics using multiple thresholds to evaluate potential indicators of biological relevance.”

Reference 

Arismendi, I., J.D. Groom, M. Reiter, S.L. Johnson, L. Dent, M. Meleason, A. Argerich, and A.E. Skaugset. 2017. Suspended sediment and turbidity after road construction/improvement and forest harvest in streams of the Trask River Watershed Study, Oregon. Water Resources Research 53:6763–6783. https://dx.doi.org/10.1002/2016WR020198