Influence of large woody debris on channel morphology and dynamics in steep, boulder-rich mountain streams, western Cascades, Oregon

Abstract

This study used 20-year records of stream channel change and wood to test hypotheses about the long-term influence of large woody debris (LWD) on channel morphology, channel stability, and sediment dynamics in a steep, boulder-rich mountain stream. We compared two nearly adjacent reaches of third-order Mack Creek over the period 1978–1997 after virtually all wood was removed from the channel of the lower reach in 1964. We assessed the long-term legacy of wood removal using repeated cross-section surveys, streamflow data, LWD inventory data, and detailed mapping and longitudinal profile surveys. At each of 11 cross sections in the upper reach and 19 in the lower reach, we calculated areas of scour and fill in response to the two largest floods in the record. We used quasi-likelihood logistic regression models to test the proportion of each reach that experienced change between consecutive surveys over the entire record (1978–1997) as a function of flood return periods. The longitudinal profile of the site without LWD was more variable than the reach with LWD at the finest scale (∼1 m) due to a greater frequency of boulder steps, but the reach with LWD was more variable at the channel unit scale. LWD-created steps 1 to 2.5 m high in the wood-rich reach accounted for nearly 30% of the total channel fall and created low-gradient upstream channel segments one to three channel widths long. As a result, both reaches have the same average slope (about 9%), but nearly three times as much of the channel in the wood-rich reach had a slope of ≤5% as in the reach without wood (20.4% of total channel length vs. 7.5% of channel length). The reach with abundant LWD was less responsive to moderate streamflow events (return period <∼5 years), but it responded similarly to peak flows with a return period of about 10 to 25 years. Although the average magnitude of cross-section changes was the same during the largest flood in the record (25-year return period), the reach without LWD experienced scour and coarsening of the bed surface, whereas the reach with LWD experienced aggradation upstream of LWD features. Mack Creek may be representative of many steep mountain streams in which channel structure is strongly influenced by nonfluvial processes: a legacy of large boulders from glacial or mass movement processes and a legacy of dead wood from ecological processes. Sediment-limited mountain streams with large boulders, when deprived of LWD, appear to exhibit less morphological variation at the channel unit scale, to store less sediment, and to release it more readily than those with LWD.