Date of Award
Master of Environmental Management (MEM)
Environmental Science and Management
Sediment control -- Washington -- Toutle River Watershed, Soil conservation, Soil-binding plants, Engineered log jams (Hydraulic engineering)
I sought to evaluate the vegetative response to the installation of the 14 engineered log jams (ELJs) on the North Fork Toutle River (NFTR) Sediment Plain. The NFTR sediment plain is constantly being reworked due to channel bank erosion caused by a combination of processes including flow erosion and gravitational mass failure. Vegetation has the ability to protect the bank from erosion as well as providing other stabilizing effects. The ELJ structures were designed in part to protect localized areas of the sediment plain and allow vegetated islands to develop. The purpose of these vegetated islands is to trap sand sized sediment that would otherwise pass over the spillway at a Sediment Retention Structure (SRS), and serve as a seed bank and possible point for continued vegetation establishment across the rest of the sediment plain. I utilized vegetation transects to collect raw vegetation data and then analysed the data to characterize the plant communities directly downstream from each ELJ. It was found that the ELJs are having moderate success creating protected vegetated islands. The NFTR sediment plain is dominated by pioneer species with Alnus rubra as the most abundant species. A total of 42 species were identified to the species level (20 native species, 22 non-native species). Plant assemblages remain broadly similar across the sediment plain, although wetland indicators species are absent in the northern third of the Study Area. The Study Area was found to have 29.01% vegetation cover, compared to 8.8% vegetation cover in the Control Area. The Study Area was also found to have higher plant species richness and diversity than the Control Area. The vegetation behind the ELJs is able to trap sediment but releases that sediment if the vegetation interacts with the larger branches of the NFTR. It is recommended that the ELJs receive regular maintenance including re-racking of the structures.
Ashley, Todd, "Plant Response to 14 Engineered Log Jams on the North Fork Toutle River, WA Sediment Plain" (2014). Master of Environmental Management Project Reports. 45.