Extreme events in running waters

Summary Extreme climatic events are a natural feature of climate variability and a major organising force in running waters. Climate change is shifting the occurrence of extremes, and understanding the far‐reaching consequences for river and stream ecosystems is a research priority. The aim of this special issue is to unite research on the wide array of contrasting extreme events affecting river and stream ecosystems globally, including heat waves and hot days, fires, droughts, heavy rainfall and floods, tropical cyclones, storm surges and coastal flooding. This issue contains reviews as well as observational and experimental case studies that assess the ecological impacts of extreme events at multiple levels of biological organisation, from individuals to ecosystems and food webs, in river and stream ecosystems from the poles to the tropics. Together, these papers synthesise knowledge of extreme events and their effects, identify research gaps and propose new hypotheses and approaches to guide future research. The contributions reveal that impacts of single events such as catastrophic floods, droughts and heat waves are highly context dependent, ranging from deleterious to beneficial, and are contingent upon event magnitude, extent and timing relative to life cycles of constituent species. Not all extreme events generate extreme ecological impacts, but combinations of events that are cumulative, compound or cross key biological or physical thresholds may have the most adverse ecological consequences. Long‐term monitoring programmes and sensor networks are essential in describing rare and usual events. Other approaches, including experiments, will be needed to gain a stronger mechanistic understanding of extreme events, and to predict and manage future events that may be more frequent and intense.