MAGNITUDE AND TIMING OF SEDIMENTATION RESPONSE IN THE SANDY RIVER RESULTING FROM AN ERUPTION (~ A.D. 1760 – 1810) AT MOUNT HOOD, OREGON

A nonexplosive dome-building eruption at Mount Hood, Oregon, about 200 years ago produced lithic pyroclastic flows that were deposited on a southwest-facing debris fan that currently is snow-covered for much of the year. Lahars, presumably triggered by rapid mixing of snow with hot dome-collapse debris, flowed tens of kilometers beyond the fan and were deposited principally in two canyons at the head of the Sandy River. Fresh lahar deposits and older, underlying volcaniclastic sediment formed a thick, erodible valley fill extending more than 30 km downstream from the volcano. Lahar activity (and probably dome extrusion) was at its peak in the early 1780s.

Fluvial erosion of the canyon-fill volcaniclastics by seasonal high flows generated a sediment pulse of dominantly coarse sand and fine gravel, that moved slowly downstream as a broad sediment wave in a river valley incised all the way to its confluence with the Columbia River. High terraces record vertical river-bed aggradation up to 17 m in the lowermost river reach, 61 to 87 km downstream from Mount Hood. Dendrochronology and historical accounts indicate that aggradation at 68 km began sometime after about 1780 and appears to have reached its peak at 80 – 87 km at about the time of Lewis and Clark's visits in 1805 and 1806. However, degradation was well underway at the upper end of this reach by 1810, when the river bed was already more than 6 m below peak level. Downcutting continued at a nonlinearly decreasing rate for nearly a century, with the river bed having achieved its present level by 1910.

Timing of the Sandy River's response to the imposed high sediment load resembles that of other rivers similarly subjected to extreme sediment inputs, but the magnitude of channel aggradation is unusually large, considering the localized, nonexplosive nature of the eruption and the low to moderate rainfall intensities typical for the region. This prodigious vertical response is likely due to lateral confinement of the channel by the incised valley.