An ecological perspective on floods in Canada

This review presents a summary of the influences of floods on river ecology, both instream and on the adjacent floodplain, mostly in a Canadian context. It emphasizes that ecological impacts and benefits can be highly dependent on flood-generation processes and their magnitude and timing. In Canada, floods can occur under open-water or ice-influenced river conditions. The ecological impacts of floods generated from ice jamming are particularly relevant in Canadian ecosystems due to the potentially higher water levels produced and suspended sediment concentrations that can be detrimental to instream aquatic habitat, but beneficial to floodplains. Large floods provide a major source of physical disturbance. Moderate floods with shorter return periods can be beneficial to aquatic habitats by providing woody debris that contributes to habitat complexity and diversity, by flushing fine sediments and by providing important food sources from terrestrial origins. Floods also influence water-quality variables such as sediment loads, metals and pH, which further influence river ecology. This review points out important links between floods and habitat connectivity and refugia, and the ability of aquatic resources to recover from flood disturbances. The floodplain vegetation also influences bank erosion and channel configuration, as well as many processes contributing to the river ecology, such as allochthonous inputs of carbon, preventing bank erosion, food-web dynamics and other effects. Many species of riparian trees are dependent on floods for reproduction, and floods can limit competitive encroachment of upland vegetation. Floods play a critical role in deltaic environments where high flows provide nutrient supplies and overflows maintain water balances within these environments. Two substantial factors that will likely influence future flood regimes in Canada are climate change and flow regulation. Future research should focus on enhancing our understanding of how floods affect river ecology, including under winter conditions, to better manage important natural resources.