The Influence of Context in the Reach‐Scale Distribution of Logjams

ABSTRACT

We analysed the longitudinal distribution of logjams along 17 subalpine stream reaches in the Southern Rocky Mountains of Colorado, USA. Valley length in individual study reaches ranged from 210 to 700 m. Study reaches included old‐growth and younger forest and single‐thread and multithread channel planforms. We hypothesized that multithread reaches contain more abundant logjams per 100 m of channel length and 100 m of valley length. We examined three scenarios of large wood recruitment dominated by individual treefall, downstream transport and mass wood introduction, respectively. We calculated the logjam distribution density, or number of logjams per length of stream, and statistically evaluated the spatial distribution of logjams to determine whether they exhibit complete spatial randomness of location. Our hypotheses were not supported: we found no significant differences in logjam distribution densities by channel length or valley length (1.4 jams/100 m single‐thread, 1.2 jams/100 m multithread) or valley length (1.6 jams/100 m single‐thread, 3 jams/100 m multithread). Total wood load per 100 m of valley length was significantly greater in multithread channels (6.1 m ³ vs. 3.1 m ³ for single‐thread channels). Only two multithread reaches deviate from complete spatial randomness, although most reaches exhibit clustering of logjams at some length scale. Much of the scatter within logjam distribution densities for multithread reaches can be explained by site‐specific features such as mass recruitment during blowdowns or removal of logjams during a large flood shortly before our field study. Each of the three scenarios of wood recruitment operates to some degree in the study reaches. The relative importance of each recruitment mechanism, interacting with valley‐floor geometry within and upstream from each reach, provides insight into observed logjam distributions.