HYDRAULIC RESPONSE OF A MOUNTAIN CHANNEL TO LARGE-SCALE DEBRIS FLOW DISTURBANCE

Squire Creek is a tributary to the North Fork Stillaguamish River and is located in the northern Cascades of Washington near the town of Darrington. A rockfall occurred on a ridge overlooking Squire Creek during the morning of February 25, 2002 and triggered a large debris flow that deposited an estimated 0.1 – 1.0 million cubic meters of debris on the valley floor and temporarily blocked Squire Creek. The Squire Creek debris flow provides a rare opportunity to examine the hydraulic response of a hillslope-coupled mountain channel to a large-scale debris flow disturbance. Squire Creek drains a watershed area of 25.7 square kilometers and has a bankfull width of approximately 18 meters at the study site. Results of channel surveys conducted 16 – 18 months post disturbance show a well developed step pool morphology. Step locations typically coincide with the largest clasts, and step spacing is negatively correlated with channel gradient. Ongoing field studies are examining relations between flow resistance and changes in both step height and spacing such that constant channel width and mean velocity are maintained along profile during this early stage of channel recovery. It is hypothesized that fluvial bed surface organization (i.e., bed-surface armoring, step height and spacing, and flow resistance) is more responsive in the short term to large-scale mass wasting events in mountain channels than are changes in gradient. These short-term adjustments appear to exert first-order control on the morphology and ecology of low-order mountain channels that receive bed material from mass wasting events.