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.

Almost 0.9 km 3 (dense-rock equivalent) of rhyodacite tephra, lava domes, and lava flows erupted from numerous vents on the southwest, southeast, and northeast flanks of South Sister volcano during late Holocene time. Eruptions occurred during two brief episodes between 2300 and 2000 14 C yr B.P., separated by a dormant interval of as long as several centuries. The eruptions of each episode were probably fed by dikes, on the basis of the following: the alignment of vents, the chemical uniformity of eruptive products, and stratigraphic evidence that the eruptions of each episode occurred over a short interval of time. Each eruptive episode began with the explosive eruption of air-fall tephra. Small pyroclastic flows and hot pyroclastic surges erupted from a few vents and traveled as far as 3 km. Rapid snowmelt accompanied the early phase of each episode and triggered small lahars. Each episode culminated with the extrusion of lava domes and flows. The distribution of late Quaternary mafic vents around the area of Holocene rhyodacite vents suggests that a magma chamber with a maximum area) extent of 30 km 2 may lie beneath the south flank of South Sister. The chemical uniformity of the eruptive products of each episode is consistent with each having tapped a relatively small homogeneous portion of a compositionally zoned magma chamber of much greater volume than the erupted products. Alternately, if this chemical uniformity reflects the generation and rapid ascent and eruption of a crustal partial melt, then a large magma chamber need not be present.