PROCESSES AND RATES OF SEDIMENT AND WOOD ACCUMULATION DURING THE INTERVAL BETWEEN DEBRIS FLOWS IN HEADWATER STREAMS OF THE OREGON COAST RANGE, U.S.A

Channels that have been scoured to bedrock by debris flows provide unique opportunities to calculate the rate of sediment and wood accumulation in debris flow prone channels, to understand the temporal succession of channel morphology following disturbance, and to make inferences about processes associated with the input and transport of sediment and wood. Dendrochronology was used to estimate the time since the previous debris flow in unlogged basins in the central Oregon Coast Range. Changes in sediment and wood storage were quantified for 13 streams that ranged from 4 to 145 years post-disturbance. The timing of fires and large storm events influenced the temporal pattern of debris flow occurrence. The volume of wood in the channel was strongly correlated with the time since the previous debris flow, and the accumulation rate was linear. The pattern of sediment accumulation was non-linear and appeared to increase as the storage capacity of the channel increased through time. Wood created a physical obstruction to sediment transport and functioned to store the majority sediment in these steep headwater streams. The dominant processes associated with wood recruitment to these channels were landslides and windthrow in upland forests. In the absence of wood, channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for a longer period of time. With an adequate supply of wood, low-order channels have the potential for storing large volumes of sediment in the interval between debris flows and can function as one of the dominant storage reservoirs for sediment in mountainous terrain.