ABSTRACT The field trip guide describes nine stops that examine the mechanisms and timing of some of the abundant and often gigantic landslides that occur along the Winter Ridge–Slide Mountain escarpment in south-central Oregon. Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding escarpment. Subsidence is accommodated on the 58-km-long Winter Rim fault system, a normal fault which is capable of producing M w ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1 g on the bedrock footwall. Gigantic rock slides cubic kilometers in volume scallop the southwestern portion of the escarpment, and their deposits run out as rock avalanches several kilometers onto the basin floor. Limit-equilibrium slope stability analyses support observations that these gigantic bedrock landslides initiate within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces one to two kilometers in length; are insensitive to groundwater fluctuations; and, are stable under static conditions. Strong ground motions appear requisite to trigger landsliding and are necessary to replicate the long, shallow failure surfaces. Landslide, colluvial, and lacustrine deposits on the hanging wall have undergone widespread post-emplacement deformation, which may involve large-scale seismogenic lateral spreading and flow sliding controlled by the saturated, fine-grained basin fill.

Abstract Over the past four decades, ongoing deformation of an 18-m-thick peat deposit within the flat-lying Mercer Slough has resulted in damaging deflections, and near-collapse in three cases, of pile-supported Interstate 90 bridges and a major water line on the east side of the slough. The peat is partially underlain by a dense sand unit, which includes a highly pressurized aquifer that produces artesian flow 1–2.5 m above the ground surface. Inclinometers on the east side of the slough show the peat flowing toward the structures and then apparently directed west along the interstate centerline. Large displacements recorded in several inclinometers near the center of the slough suggest a length of deforming peat that approaches 600 m, which is likely initiating retrogressively. Potential causal mechanisms include poor engineering characteristics of the peat, presence of high hydrostatic pressure transmitted within and beneath the peat, seasonal water-level variations of Lake Washington and induced hydraulic gradients within the peat, dredging of the Mercer Slough channel, puncturing of the underlying aquifer by numerous pile foundations, and fill placement along the eastern margin of the slough. The peat is flowing around the pile/shaft foundations; however, excessive lateral loads are still being applied to the foundations in a poorly understood and unpredictable manner. The most severe deflections have occurred in the outermost structures where the peat is primarily flowing transverse to the structures.

Abstract The deeply incised central Columbia River valley of Washington State and its tributaries expose mid to late Tertiary basalt flows and clastic sedimentary rocks, pre-Tertiary crystalline bedrock outcrops where the river flows along the eastern slope of the Cascade Mountains between Wenatchee and Chelan. River incision has primarily been driven by the uplift of the Cascades, deposition of the voluminous Columbia River basalts, and the formation of the Yakima fold belt. Glaciation during the Pleistocene, the terminus of which reached Chelan and the northern Waterville Plateau, infused large quantities of sediment into the valley. Concurrently, catastrophic glacial outburst floods, unprecedented in size, repeatedly swept down the river from the north and over the Quincy Basin in the south. Trip stops include some of the early engineering works, principally the dams, where much of the regional stratigraphy was developed and challenging engineering solutions were required for difficult geologic conditions. Stops also exemplify the pervasive large-scale landsliding, common where basalts overlie weak sedimentary rocks. Due to the steep topography, transportation corridors and other developments are widely threatened by rockfall and debris flow hazards. Seismicity is also a regional hazard; the largest historic earthquake in eastern Washington, moment magnitude 6.5-7.0, was sited near Chelan.