Extreme hydrological events and the ecological restoration of flowing waters

Abstract

Summary Extreme hydrological events (floods, droughts) in flowing waters can alter habitat availability and structure and produce significant shifts in biodiversity, species assemblages and ecosystem processes. Extreme floods and droughts occur over large spatial extents and can greatly alter links between streams and their catchments. The nature and strengths of these links differ between degraded and intact streams and their catchments, strongly influencing the responses to and recovery from extreme events. It is essential that these catchment–stream linkages be identified and considered when planning stream restoration. Despite the scarcity of comprehensive, long‐term monitoring of most stream restoration projects, our understanding of the influence of extreme events on running waters suggests both positive and negative impacts on restoration efforts. Extreme hydrological events can impair stream restoration efforts giving rise to delays or complete failures in achieving restoration objectives. Alternatively, extreme floods may facilitate progress towards restoration goals by engendering key ecological processes. Understanding the impacts and the temporal–spatial scales over which extreme events alter the ecological structure and function of streams (in both natural and damaged systems), as well as their underlying mechanisms, is a challenge for stream restorers that will only progress with the accumulation of long‐term data. Restorers should recognise and plan for hydrological extreme events and their interactions with other disturbances (e.g. fire, urban development). Increasing connectivity of restoration efforts along with their spatial extent can lessen the negative effects of extreme hydrological events on stream restoration. Successful ecological stream restoration requires long time spans and thus with a changing climate the prediction of more frequent and intense extreme events only strengthens the need to incorporate their likelihood into restoration planning and implementation.