The Role of Sediment Supply on Fish Habitat Dynamics of a Morphologically Active River Widening

ABSTRACT Restoration measures, such as river widening, aim to reactivate key morphodynamic processes, which are critical drivers of fluvial habitat dynamics. While some evidence supports the important role of sediment supply on river widening's morphology, its link to fish habitat availability and dynamics remains unclear. To quantify this link, we used one‐sided river widening morphologies formed under varying sediment supplies (100%, 80%, 60%, and 20% of the channel's transport capacity), upscaled from laboratory. The experiments started from a channelized alternating bar morphology that was subjected to a steady 1.5‐year flood until equilibrium, followed by a 30‐year flood event. We delineated the habitat for juvenile and adult brown trout ( Salmo trutta ) and evaluated their spatial dynamics across discharges, using 2‐D hydrodynamic modeling and suitability curves. Our results showed that the near‐equilibrium sediment supply morphologies (i.e., 100% and 80%) increased habitat at average to high flow conditions, with the 30‐year flood further increasing low‐flow habitats. In contrast, the widening morphologies formed with the reduced‐sediment supply of 60% and 20% did not show an improvement in habitat quantities following both formative events. We found that the correspondence of the spatial habitat dynamics to morphodynamic processes depends on habitat location: juvenile habitats, typically occupying channel margins, corresponded to shoreline length dynamics, whereas adult habitats, typically found within the channels, corresponded to elevation change dynamics, both influenced by sediment supply. Our insights expand the understanding of sediment‐habitat dynamics and can further assist holistic, resilient widening restoration strategies across multiple scales.