Abstract Late Holocene dome-building eruptions at Mount Hood during the Timberline and Old Maid eruptive periods resulted in numerous dome-collapse pyroclastic flows and lahars that moved large volumes of volcaniclastic sediment into temporary storage in headwater canyons of the Sandy River. During each eruptive period, accelerated sediment loading to the river through erosion and remobilization of volcanic fragmental debris resulted in very high sediment-transport rates in the Sandy River during rain- and snowmelt-induced floods. Large sediment loads in excess of the river's transport capacity led to channel aggradation, channel widening, and change to a braided channel form in the lowermost reach of the river, between 61 and 87 km downstream from the volcano. The post-eruption sediment load moved as a broad bed-material wave, which in the case of the Old Maid eruption took ~2 decades to crest 83 km downstream. Maximum post-eruption aggradation levels of at least 28 and 23 m were achieved in response to Timberline and Old Maid eruptions. In each case, downstream aggradation cycles were initiated by lahars, but the bulk of the aggradation was achieved by fluvial sediment transport and deposition. When the high rates of sediment supply began to diminish, the river degraded, incising the channel fills and forming progressively lower sets of degradational terraces. A variety of debris-flow, hyperconcentrated-flow, and fluvial (upper and lower flow regime) deposits record the downstream passage of the sediment waves that were initiated by these eruptions. The deposits also presage a hazard that may be faced by communities along the Sandy River when volcanic activity at Mount Hood resumes.

Abstract Landslides and floods of lava and water tremendously affected the Columbia River during its long history of transecting the Cascade Volcanic Arc. This field trip touches on aspects of the resulting geology of the scenic Columbia River Gorge, including the river-blocking Bonneville landslide of ~550 years ago and the great late-Pleistocene Missoula floods. Not only did these events create great landscapes, but they inspired great geologists. Mid-nineteenth century observations of the Columbia River and Pacific Northwest by James Dwight Dana and John Strong Newberry helped germinate the “school of fluvial” erosion later expanded upon by the southwestern United States topographic and geologic surveys. Later work on features related to the Missoula floods framed the career of J Harlen Bretz in one of the great geologic controversies of the twentieth century.

Abstract This field trip guide describes a one-day loop from Portland eastward around Mount Hood and returning through the Columbia River Gorge. The purpose is to visit a SNOTEL (SNOwpack TELemetry) site to observe processes and instrumentation applied in automated snowpack data collection, as well as observe geomorphic features related to snowmelt in the western United States. Annual snow accumulation in the higher elevations in the western United States provides a critical source of water for irrigation, hydroelectric power generation, municipal water supplies, and recreation. Snowmelt, however, also can cause various hydrogeologic hazards, such as floods and debris flows.

ABSTRACT This field guide is for a three-day trip from Portland to Klamath Falls, Oregon, and back, traversing many of the physiographic provinces in Oregon, including the Columbia River, the Willamette Valley, the Western and High Cascade Mountains, the High Lava Plains, and the Basin and Range. Geologic field stops on Day 1 will be made along the drive to Klamath Falls at Salt Creek Falls and Crater Lake, and will include short discussions of Oregon's geological history and a brief introduction to geothermal resources in the state. Day 2 includes an extensive introduction to geothermal resources in Oregon and how these relate to Oregon geology, with tours to examine how the Oregon Institute of Technology, the City of Klamath Falls, and several local businesses in the Klamath Falls area use a local geothermal resource. Day 3 is devoted to the return trip to Portland, and will include geological stops at Newberry Crater, Crooked River Bridge, and Mount Hood, with more discussions of geology, geothermal resources, and geothermal exploration in Oregon.