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3.2.3.      Environmental flows and natural variability

Ecologists now better understand how flow regime and natural variability, especially the extremes in form of floods and droughts, can be important determinants for ecosystem structure and resilience. A more holistic understanding of ecosystem health has led to a paradigm shift in ecosystem management that considers whole ecosystems containing diverse species with variable flow preferences, sustained by a dynamic flow regime (N. Poff 2009). The variation in flows can act to rejuvenate and maintain aquatic habitats, and changes to the timing of flows can have some of the most significant impacts on freshwater ecosystems (N. Leroy Poff and Zimmerman 2010). Extreme events can exert a selective pressure on ecological populations, renewing biodiversity and building resilience in the system. A shift in thinking is required that moves management interventions away from hard-engineered control in all situations, to accepting change is inevitable (Folke 2003), and to accept that variability can be beneficial. This variability must however be balanced against the requirements for society to be protected against the most extreme events, something that will not always be possible through more ‘soft’ interventions. Reducing human vulnerability to floods through ‘hard-engineering’ options like dams, dikes or channelization could, for example, lead to a reduction of ecosystem functioning (e.g. flow regulation, loss of floodplain connectivity). More examples can be found in chapter 5.2 on flood risk management.