Logjams in a mountain stream network: Patterns, biogeomorphic associations, and anthropogenic impacts

Abstract

Logjams, in‐channel accumulations of downed wood, play important roles governing biophysical processes in river ecosystems. In this study conducted in the mountainous inland Pacific Northwest, we leveraged a basin‐scale inventory to carry out a comprehensive analysis of logjam frequency, volume, and their associations with a suite of biophysical landscape characteristics and anthropogenic influences. We complemented this analysis with one that relies on the concept of process domains. We found that logjam frequency increased with drainage area up to ~10–30 km2 and declined thereafter, while the mean jam volume steadily increased downstream up to the area of >300 km2. Combined, these variables yielded a nonlinear downstream pattern of large wood storage in jams, peaking at drainage areas of ~10–100 km2. Logjam characteristics were associated with multiple predictors, used as proxies for biogeomorphic processes relevant for large wood supply and redistribution: bank erosion, landslides, avulsions, wood availability in riparian forest, and wood transport. Additionally, anthropogenic disturbances were inferred to strongly influence logjam‐forming processes in the study basin. A riprapped forest road disrupted a process chain responsible for large wood supply, wherein erosion undercuts slope toe, ultimately triggering landslides. Timber harvest influenced large wood supply by reducing forest biomass. Overall, these findings contribute to improved understanding of logjams: they provide insight into the complex suite of interacting natural and anthropogenic factors which, by regulating large wood inputs and redistribution, generate a basin‐scale pattern in jam frequency and volume. We believe that our study will be useful in informing river management and restoration practices.