ABSTRACT Development along the coasts of Oregon and Washington is threatened by a variety of natural hazards, including coastal erosion, landslides, earthquakes, and tsunamis. Property losses have increased significantly in recent years due to past land-use and management practices and an intensification of the physical processes that drive coastal change. This field trip will visit a number of sites that document or illustrate the processes that shape Pacific Northwest coastal geomorphology and create hazards, including potentially catastrophic tsunamis generated by the Cascadia subduction zone. New research documenting ocean processes (including the role of changing wave climates, storm surges, El Nino's, and sea-level rise), tsunamis, and the effects of coastal subduction caused by great earthquakes will be covered. Also examined is the human response, which includes constructing coastal engineering structures, the establishment of coastal “erosion” hazard zones, and various mitigation efforts that are being implemented to prepare for future tsunamis. The field trip concludes on the southern Washington coast at Cape Disappointment State Park adjacent to the Columbia River, where construction of the Columbia River jetties, river flow regulation, and dredging and disposal activities have affected the sediment budget of the Columbia River littoral cell, resulting in changing sediment conditions and management practices for this cell.

Ground penetrating radar (GPR) records of groundwater surface (GWS) reflections have been analyzed from 40 across-barrier profiles, totaling 50 km in combined length, taken from barrier spits and beach plains of the Columbia River littoral system. The barriers and beach plains host shallow fresh-water aquifers in the prograded beach deposits and abandoned foredune ridges, totaling 10–30 m in thickness. Study results demonstrate that GWS reflections could be traced continuously at subsurface depths of 1–15 m with the GPR 100 MHz and 50 MHz antennae using 400 V and 1000 V transmitters. Boreholes (62 in number) and lake water levels (24 in number) provide ground-truthing of the across-barrier GWS trends interpreted from the GPR profiles. The GWS rises in elevation (4–8 m above base level) under high, broad foredune-ridges and drops under interdune ridge valleys (1–3 m above base level). Continuous profiles of GWS demonstrate that lakes, ponds, and bogs of the barriers and beach plains are “windows” into the shallow coastal aquifer. The GPR records demonstrate that the GWS slopes either to seaward (0.003–0.04 gradient) or to landward (0.001–0.05 gradient) from divides under the largest, shore-parallel dune ridges in the barriers. The GWS gradients indicate that subsurface contaminant transport from the developed dune ridges will be intercepted by intervening lakes and ponds in the interdune-ridge valleys. The GPR records also establish the effect of drainage ditches in lowering GWS elevations (1–2 m) in sensitive wetlands located 100s of meters in distance from the constructed ditches.