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 Classifications of flowing sediment-water mixtures have, in the past, been based primarily on relative, qualitative differences in the style and rate of movement as well as on morphology and sedimentology of deposits. A more quantitative and physically relevant classification is presented here, based on thresholds in rheologic behavior. The classification is constructed on a two-dimensional matrix in which flows are located according to deformation rate (mean velocity) and sediment concentration, with composition of the mixture constant. Three major rheologic boundaries are crossed as sediment concentration increases from 0 (clear water) to 100 percent (dry sediment): (1) the acquisition of a yield strength—the transition from liquid “normal streamflow” to plastic “hyperconcentrated streamflow”; (2) an abrupt increase in yield strength coinciding with the onset of liquefaction behavior—the transition to “slurry flow”; and (3) the loss of the ability to liquefy—the transition of “granular flow.” These three rheologic boundaries shift according to particle-size distribution and composition of the mixture. Processes controlling flow behavior depend on deformation rate (velocity). Rate-independent frictional and viscous forces dominate at lower velocities and in finer grained mixtures; rate-dependent inertial forces dominate at higher velocities and in coarser grained mixtures. As velocity increases, grain-support mechanisms change from low-energy varieties (buoyancy, cohesion, structural support) to progressively higher energy mechanisms (turbulence, dispersive stress, fluidization). Existing nomenclatures of geologic flow phenomena can fit within this rheologic classification. The morphology and sedimentology of flow deposits commonly can be used to deduce rheologic behavior, but caution needs to be exercised in inferring processes from deposits.