The Role of Large Wood in Promoting Channel-Floodplain Connectivity for River Restoration

Colorado State University, Fort Collins CO

Investigators

Abstract

River restoration has initially focused on channel morphology and subsequently gradually expanded to include reconnection of channels and floodplains. Although stream ecologists have recognized the critical ecological role of floodplains for decades, and hydrological, geomorphic, and biogeochemical research now increasingly emphasizes the importance of floodplains, floodplains do not receive the legal protection of navigable water and have been extensively altered throughout the US. Consequently, one of the issues now at the forefront of river management and restoration is how to effectively re-connect altered channels and floodplains. Reconnection requires multiple changes of differing importance in specific settings. Large wood (LW) reintroduction or retention falls within this scope, but guidelines have not been developed regarding how much LW to reintroduce, in what configuration, and where. This research will address an important gap in knowledge regarding the quantitative impacts of LW jams on channel-floodplain hydrologic connectivity and how these impacts can inform river restoration design. Large wood can be a dominant driver of channel and floodplain complexity. LW can alter the local distribution of hydraulic forces, sediment dynamics, hyporheic exchange flows, channel cross-sectional and planform geometry, and channel-floodplain connectivity. These effects operate in a nonlinear fashion as accumulations of LW in a logjam and in progressively larger and more closely spaced logjams create emergent patterns in these response variables. However, thresholds can not yet be predicted for the magnitude of change in response variables with respect to LW quantity or spatial distribution. This is important in the context of river restoration, which reintroduces LW in channels despite inability to predict hydraulic, geomorphic, or ecological responses. This knowledge gap of quantitatively predicting the physical effects of LW on channel-floodplain hydrologic connectivity will be addressed by by (i) measuring LW and associated connectivity in diverse field settings; (ii) assessing threshold responses in connectivity due to river morphologic and LW characteristics; and (iii) developing guidelines for representing LW characteristics and associated hydraulic responses in numerical models. In this context, the project defines channel-floodplain hydrologic connectivity as being present when flow overtops the morphological channel banks and spreads onto the adjacent floodplain. The project objectives are to: (1) collect data from natural channels on LW jam characteristics and associated influences on channel-floodplain connectivity; (2) use these data to develop guidelines for calibrating LW characteristics and hydraulic responses in numerical hydraulic models; and (3) co-generate knowledge and best practices with river restoration practitioners for long-term monitoring of LW jam benefits. The project will integrate new knowledge of how LW jams influence channel-floodplain interactions with ongoing and future investments in river restoration projects, and planned activities are aimed to advance the sustainability of river restoration design, promote restoration monitoring and citizen-science learning to diverse stakeholders and restoration practitioners, and improve STEM education focused on sustainable and resilient natural systems. This award is co-funded by the Environmental Sustainability program in CBET/ENG and the Hydologic Sciences program in EAR/GEO. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.