EPISODICITY AND GEOLOGIC TIMESCALES: IMPLICATIONS OF DEBRIS FLOWS FOR LANDSCAPE EVOLUTION

In the face of the episodicity of debris flows and consequent sparseness of event-based data, field measurements emphasizing sedimentology, rather than rheology, reveal the dynamics of deposition and evacuation of debris flows on millennial timescales, and quantitative models based on those dynamics reveal their logical consequences for evolution of landscape morphologies over geologic time. In the Oregon Coast Range, bedrock lowering and uplift are in approximate steady state, and debris flows transport much of the hillslope-derived sediment to the fluvial system. Just as the tips of the valley networks in such landscapes bear the signatures of episodic scouring by debris flows, their deposition forces characteristic adjustments at the transition between debris-flow and fluvial processes, where valley morphologies must accommodate episodic deposition in addition to bedrock uplift. Field data from such valleys (contributing area <10 km²) include classification, properties, and radiocarbon dating of sedimentary facies of valley-bottom deposits, and longitudinal stream and cross-sectional valley-bottom profiles of elevation and substrate. These data indicate that episodic debris-flow deposition and concomitant fluvial impoundment lead to complex and persistent valley-bottom depositional assemblages, from those dominated by mainstem-traversing-debris-flow deposits with residence times <500 yrs in valleys with contributing areas <1.5 km² and gradients >5%, to assemblages dominated by fluvial impoundments behind debris-flow fans at tributary junctions and with residence times >1000 yrs in larger, lower-gradient valleys. Quantitative models based on these findings describe valley bottoms where debris dams and associated fluvial impoundments lead to evolution of longitudinal profile gradients twice those of nominal bedrock gradients, and cross-sectional widths and inner gorge depths that grow to accommodate greater deposition and incision rates, respectively. The evolution of accommodation space and steeper gradients in headwater valleys are two mechanisms for local adjustment of the fluvial system to spatial variations in episodic, but temporally stationary, sediment supply.