Elwha Delta Morphology and Grain-size Distribution

Abstract

New sediment is being released from the Elwha River dam reservoirs into the Strait of Juan de Fuca for the first time in 100 years, but it is unknown where this sediment will settle and how it will affect the geological environment. In this study, we examined the overall profile of the Elwha River delta and how it is influenced by the new sedimentation. Beach surveys, fathometer transects, and grain-size analyses were combined to evaluate the entire delta front. Grain-size and delta slopes were not found to be correlated in this study. The foreshore of the delta had the steepest slope and was comprised of sandy substrate. The low-tide terrace consisted of large cobbles and had a near horizontal slope. Offshore, the substrate varied widely but no relationship was found between substrate composition and slope. Profiles of the subaqueous delta revealed much longer topsets to the east of the river mouth, suggesting eastward littoral transport. Furthermore, three submarine ancient spits to the east of the delta demonstrate this historical transport pattern. One reason the substrate morphology of the Elwha delta does not look predicted is because the Elwha has been starved of sediment for the past century. Data from this study established a platform for further studies and hints at a future relationship that could be discovered.