Revisiting fluvial biogeomorphic units across scales

Fluvial biogeomorphic feedbacks happen on multiple spatial and temporal scales, and between them within a nested, hierarchical organization of river systems. Entities such as reach or catchment are often used to place a study in a spatial and temporal context. While hydrogeomorphic units are well defined and operationalized in fluvial geomorphology (e.g., catchment, reach, geomorphic unit etc.), transferring this logic to biogeomorphic units is not straightforward because vegetation and other biota both respond to and actively modify hydrological and geomorphological processes on a wide range of scales potentially not matching with the ones of fluvial geomorphology. Thus, such units inherently reflect the complex nature of riverine process regimes. Considering the two-way interactions of hydro-morphology and vegetation makes the delineation of (fluvial) biogeomorphic units of specific spatial and temporal scales particularly challenging.With our contribution, we provide a synthesis of how hydrogeomorphic and biogeomorphic units have been defined in their spatio-temporal extent across literature from small entities with fast turnover rates to long lasting units like the catchment. We contrast (i) biogeomorphic units defined as standalone entities (e.g., pioneer or mature island), with (ii) biogeomorphic units defined as attributes or states of pre-defined geomorphic units (e.g., bars, islands, banks) within a hierarchical framework. Furthermore, we merge insights into the distribution of the units across scales with the phases of the Fluvial Biogeomorphic Succession model (geomorphic, pioneer, biogeomorphic and ecologic phase). We suggest that such integrated hierarchical perspective can guide research design and monitoring for management purposes.