MODELING SEDIMENT AND WOOD STORAGE AND DYNAMICS IN SMALL MOUNTAINOUS WATERSHEDS

We examine controls on supply and transport of sediment and wood in a small (approximately two square kilometers) basin in the Oregon Coast Range, typical of streams at the interface between episodic sediment and wood delivery by mass movements and frequent fluvial sediment transport. We hypothesize that wood deposited by mass movements forms dams that lead to persistent sediment storage and inhibit coherent propagation of sediment pulses. Field data show that much sediment is stored behind such dams and in terraces after the dams breach. We developed a drainage basin-scale model driven by stochastic storm and fire sequences that combines empirical, stochastic and physical models of forest growth, tree fall, wood decay, soil production and diffusion, landslide initiation, debris flow runout, and fluvial sediment transport. In a 3000-year simulation of the study area, woody debris flow deposits form dams on the main channel and lead to steps in the channel profile and terraces on the valley floor that persist in place even after nearly all deposited wood has decayed. Simulated sediment output from the network is relatively steady and shows little evidence of episodic input. Our results suggest that abundant wood plays a key role in moderating sediment flux from small basins following debris flow events. Debris flow events coincident with a lack of abundant wood, such as might occur following forest harvest, could lead to more episodic sediment flux to downstream, fish-bearing reaches.