The resilience of logjams to floods

Logjams that span the bankfull channel strongly influence hydraulics and downstream fluxes of diverse materials. Several studies quantify the longitudinal distribution of channel-spanning logjams, but fewer studies examine changes in longitudinal distribution in response to disturbances such as floods. We use 10 years of annual surveys of a population of channel-spanning logjams along mountain streams in the Southern Rocky Mountains. Surveys from 2010 to 2019 bracket substantial interannual variability in the snowmelt peak flow as well as a rainfall flood in 2013. We characterised the number of logjams per unit length of valley (logjam distribution density) within and between reaches designated based on longitudinally consistent channel and valley geometry. Our primary objectives are to evaluate the influences on logjam distribution density of (i) spatial variations in valley and channel geometry and (ii) temporal variations in peak annual flow. We hypothesized that logjam distribution densities are resilient to disturbance at both spatial scales. At the creek scale, logjam distribution density correlates significantly with increasing ratio of floodplain width to channel width and wood piece length to channel width. Wide, low gradient reaches with greater distribution density exhibit greater interannual variation in distribution density. These reaches lost jams during the 2013 flood but returned to pre-flood distribution density values by the end of the study. The pattern of greater logjam distribution density in unconfined reaches relative to confined and partially confined reaches is also consistent over the period of the study. We interpret these results as indicating the resilience of logjam distributions to disturbance. The persistence of greater numbers of logjams in wide, low gradient reaches suggests that river restoration employing engineered logjams and wood reintroduction can focus most effectively on these reaches.

Automated summary

This study investigated the influences on logjam distribution density in mountain streams over a 10-year period. It was found that wide, low gradient reaches with greater distribution density exhibit higher interannual variation and are resilient to disturbances, as they can recover to pre-disturbance levels. The results suggest that river restoration efforts focusing on engineered logjams and wood reintroduction can be most effective in these reaches.