RECOVERY OF A GRAVEL-BED RIVER FOLLOWING VOLCANIC SEDIMENT LOADING

Form and grade of alluvial channels are in dynamic equilibrium when a number of governing variables are in balance. Rate, magnitude, and grain size of sediment inputs to a river system affect this balance. When sediment influx is greater than transport capacity, increased sediment storage occurs and is manifested as channel aggradation downstream of the sediment source. Volcanic eruptions can contribute large influxes of clastic sediment to fluvial systems and, hence, commonly trigger downstream cycles of channel aggradation and degradation.

Return to former bed elevation is one criterion for defining recovery of a channel following an aggradation-degradation cycle, assuming equilibrium conditions do not change. Flume studies and multi-decade field studies suggest that while channel beds may return to former bed elevations following the input of moderately large sediment inputs, they may stay elevated following very large sediment inputs. This study addresses whether post-cycle elevated bed levels can persist longer than a few decades. Limiting pre- and post-cycle bed elevations have been approximated for the Sandy River downstream of Mount Hood for a late 18th century aggradation-degradation cycle, which was triggered by eruptive activity. During the Old Maid eruptive period (AD 1781 to about 1800), steep narrow canyons at the head of the Sandy River received on the order of 10^8 m^3 of dominantly sand- and gravel-size volcaniclastic sediment, primarily as successive pyroclastic-flow and lahar deposits. In a reach of the Sandy River 65-75 km downstream from the volcano, surveyed elevations of dendrochronologically dated alluvial terrace surfaces demonstrate that this sediment input resulted in at least 23-24 m of aggradation within a decade. In the century following cessation of major sediment inputs, the river degraded about 20 m and has remained essentially at its present bed level, at least 3-4 m above its former bed level, since 1910.

Possible reasons for establishment of a new bed elevation following high sediment loading include: (1) establishment of a new equilibrium channel profile in response to changes in magnitude and frequency of sediment-transporting flows and/or erosional regimes, and (2) lateral shifting of the channel onto more erosion-resistant substrates on the valley floor.