Engineering Activities Influencing Historical Sediment Transport Pathwaysat the Columbia River Mouth, WA/OR

Regional sediment transport dynamics at and adjacent to the MCR were quantified using historical shoreline and bathymetry surveys. Four distinct depositional trends were identified: 1) the modern ebb shoal was deposited approximately 2-mi seaward of the natural shoal in about 90- to 130-ft water depth as a result of jetty construction; 2) the primary area of sedimentation on the ebb shoal is north of the entrance, and it continues to migrate north with time; 3) northward-directed sediment transport from the entrance has resulted in net accretion along the shoreline and on the continental shelf seaward of Long Beach Peninsula; and 4) continual shoreface erosion south of the MCR is the result of sediment diversion by the south jetty to the west and north of the entrance toward the ebb shoal and onto the continental shelf. The long-term sediment budget indicates that approximately 4.4 Mcy of sediment is transported to the continental shelf north of the MCR on an annual basis.