84-2 Wood Dynamics and Ecogeomorphic Processes in Two Gravel-Bed River Floodplains

Abstract:

Wood in river corridors (channels and their floodplains) modifies flows, sediment transport, and geomorphic form, but most research on wood in fluvial systems has focused on wood within channels, ignoring floodplain environments. Understanding channel-floodplain wood fluxes and floodplain wood dynamics is necessary to predict wood transport and storage, close wood budgets, and support management decisions aimed at improving floodplain habitat and ecosystem services. We present field data on floodplain wood and organic matter loads and characteristics on two dynamic gravel bed river floodplains: the Tagliamento River (drainage area: 2580 km²) in Italy and the Sense River (drainage area: 432 km²) in Switzerland.

We measured large wood (>1m in length and >0.1m in diameter) and coarse particulate organic matter (CPOM; material finer than large wood) in accumulations (i.e., jams; n = 242) across the floodplain and in the active channel. In both floodplains, wood and CPOM loads (m³ ha^-1) on the floodplain close to the active channel and on vegetated islands were similar to or higher than loads in the active channel, highlighting the importance of floodplains for wood retention and storage. Some of the highest floodplain wood and CPOM loads were in preferential floodplain flow paths (e.g., side channels); wood and CPOM loads were lowest on the oldest floodplain surfaces farthest from the main channel. Living vegetation, including relatively small flexible vegetation (e.g., Salix spp., Populus nigra), effectively trapped wood and CPOM.

Floodplain relative surface age estimates from historical aerial imagery coupled with plot-based floodplain wood load measurements point to interesting interactions between geomorphic processes and wood decay rates that likely govern floodplain wood loads. Much of the Tagliamento floodplain area has been stable for multiple decades and wood likely decays relatively quickly, resulting in very low wood loads on older surfaces farther from the main channel. In contrast, much of the Sense floodplain area is comparatively young due to frequent shifting of the main channel location across the valley bottom, and wood pieces may be more resistant to decay due to the dominant tree species present; this likely contributed to observations of decayed wood jams on the Sense. Our results provide insight into floodplain ecogeomorphic interactions, with implications for floodplain carbon storage and management of floodplain habitats.

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