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 Andesitic magmas (herein defined as having c. 55–65 wt% SiO 2 ) are abundant in convergent margin volcanoes. Although andesites and other intermediate magmas can plausibly be produced by a number of mechanisms, a lack of evidence for andesitic liquids relative to the abundance of andesite volcanic rocks, together with widespread evidence for hybridization and recharge, suggests that many erupted andesites form by mixing between relatively mafic magmas that ultimately derive from the mantle wedge, and more felsic magmas or mush zones derived from differentiation and crustal melting. A model of recharge filtering accounts for the high abundance of andesitic volcanic rocks and reflects the simple idea that the processes that create hybridized andesitic magmas in the shallow crust can also initiate volcanic activity, resulting in preferential eruption of andesitic magmas relative to the parental magmas that mix to produce them. This occurs via mafic recharge – intrusion of a mafic magma into a silicic magma or mush within the shallow crust. The overall abundance and variability from volcano to volcano of andesitic volcanism at convergent margins also suggests important roles for hydrous mafic magma in promoting mixing and eruption during mafic recharge, and the crust for modulating the compositions of volcanic outputs.