QUATERNARY GEOLOGY OF THE LOWER ELWHA RIVER VALLEY, CLALLAM COUNTY, WASHINGTON

The Quaternary geology of the lower Elwha River valley was mapped and investigated as part of a groundwater resource evaluation associated with the U.S. Bureau of Reclamation’s planned removal of the Elwha and Glines Canyon Dams from the Elwha River near Port Angeles, Washington. The study consisted of existing data compilation and evaluation, geologic mapping, an electrical resistivity imaging survey, and exploratory borings/wells. The lower Elwha River valley is incised into the coastal plain extending from the north side of the Olympic Mountains to the coast of the Strait of Juan de Fuca, and consists of two sub-basins separated by an approximately 200-foot wide narrows in Tertiary sedimentary and volcanic rocks. The valley is located on the north limb of the Clallam syncline and is crossed by the Lower Elwha fault. This reverse (?) fault juxtaposes 35 degree dipping rocks of the Eocene lower Twin River and Crescent Formations, against Miocene upper Twin River strata. Late-Pleistocene Vashon till and recessional outwash locally cap the Tertiary rocks on the coastal plain. Within the valley, there are two inset fluvial terraces and three alluvial units above the active channel and flood plain. Distinct changes in subsurface alluvial stratigraphy across the Lower Elwha fault suggest Quaternary displacement on the fault. In addition, there is a possible paleo-shoreline in the lower sub-basin subsurface.