Flows, floods, floodplains and river restoration

Abstract

Returning water to rivers as environmental flows is now a major tool for restoration. Many plans for environmental flows have been modelled and devised but few have been delivered, at least in Australia. Nevertheless, as planned in the latest version of the Murray‐Darling Basin Authority’s (2011) Draft Basin Plan, water recovery for environmental flows is firmly advocated. In environmental flow planning, a key component to restore is periodic (seasonal) floods. Thus, in upland rivers that have been dammed, below-dam floods are seen as vital to reshape channels, to remove unnaturally high levels of sediments accumulated since dams were built and to improve water quality. An example of this is the restoration of the Snowy River below Jindabyne Dam (NSW Office of Water 2010). Environmental flows are released to lift base flow and to introduce variability that mimics pre-dam levels and the seasonal snow-melt flood. The flood is not of the magnitude of the pre-dam flows, but is designed to improve the river by remoulding the channel, removing accumulated sediments and detritus, improving water quality and rebuilding habitats typical of an upland river. In general, restoring floods in constrained upland rivers aims to improve channel conditions. Lowland rivers are usually floodplain rivers, which, in their intact state, regularly (seasonally) flood their floodplains. When the floodplains were flooded, as still occurs in the few remaining unregulated rivers (e.g. Ovens River, Victoria), there is a boom of primary production (bacteria, phytoplankton, macrophytes, tree growth) followed by a pulse of secondary production both of aquatic (zooplankton, macroinvertebrates, fish) and terrestrial fauna (e.g. birds, amphibians, reptiles). The high primary production is fuelled by nutrients coming in with the flood but also by nutrients and organic carbon released from the floodplain (Baldwin & Mitchell 2000). Flooded, intact floodplains are one of the most productive types of ecosystems in the world. The loss of that ecological production arising from river regulation and floodplain alienation in dammed lowland rivers is known, but its size and value are seldom acknowledged in developing outcomes for managed flows. Floods in upland rivers are pulse (sometimes intense) disturbances, whereas in floodplain rivers, floods are regenerative or restorative disturbances, notwithstanding some disruptions of the terrestrial biota (Lake et al. 2006). The ongoing viability of the biota of floodplain ecosystems lies in their capacity to capitalize on floods and to survive the dry periods between floods. To maintain the ecological integrity of floodplains, regular flooding covering the floodplain is essential.