Abstract:
Esmond Creek is a tributary to the Siuslaw River located in the Oregon Coast Range. It is 18 km in length and drains a watershed area is 48.9 km². Average channel gradient of the study reach is 0.9%. In 1988 a landslide
occurred in the Esmond Creek watershed involving approximately 250,000 cubic yards of material, of which a small portion was delivered to Esmond Creek. In 1984 and 1991 8% and 17.5%, respectively, of the study reach was influenced by beaver ponds. The effects of the landslide, log jams and beaver dams on channel morphology were assessed by comparing width and depth data collected in 1984 (pre-slide) to width and depth data
collected in 1991 (post-slide). In addition, sediment samples were analyzed to investigate landslide, beaver-pond and log-jam influences on particle size distributions. Field observations in 1991 indicate the longitudinal influence of observed sediment deposition from the landslide extended approximately 2.5 km downstream from the slide input. These field observations concurred with analysis of 1990 aerial photograph. Statistical analysis revealed no changes in channel width and depth associated
with the landslide. However, reaches in which major changes in channel width and depth had occurred were associated with the occurrence of beaver dams and ponds. In general, the presence of beaver ponds tends to increase
channel width and depth. Particle size analysis of sediment from floodplain,
point-bar and channel-bed locations revealed localized decreases in the geometric mean diameter of particles due to the interaction between sediment input from the landslide and instream structures (e.g., beaver dams and
log jams). The channel bed, typically a zone of sediment transportation, was altered by these structures to function as a zone of fine sediment deposition upstream from the
structures.
