Dynamics of wood in stream networks of the western Cascades Range, Oregon

Abstract

We develop and test a conceptual model of wood dynamics in stream networks that considers legacies of forest management practices, floods, and debris flows. We combine an observational study of wood in 25 km of 2nd- through 5th-order streams in a steep, forested watershed of the western Cascade Range of Oregon with whole-network studies of forest cutting, roads, and geomorphic processes over the preceding 50 years. Statistical and simple mass balance analyses show that natural process and forest management effects on wood input, transport processes, and decomposition account for observed patterns of wood in the stream network. Forest practices reduced wood amounts throughout the network; in headwater streams these effects are fixed in stream segments bordered by cuts and roads, but in larger channels they are diffused along the channel by fluvial transport of wood. Landforms and roads limited delivery of wood by debris flows to mainstem channels. Network dynamics studies and watershed management plans should include spatial patterns of debris flow initiation and runout, flood redistribution, and reduction of wood in the network by forest cutting and intentional wood removal from channels on time scales of forest succession and recurrence of major floods.