Flow variability in dryland rivers: boom, bust and the bits in between

Abstract

Australian dryland rivers are acknowledged as being among the most variable and unpredictable in the world in terms of their flow regimes. Although renowned for their spectacular floods over vast and complex floodplains, rivers exist for much of the time as discrete waterholes, which are important refugia for aquatic biota. Recent work has shown that waterholes are filled by surface flows and there is little evidence of groundwater contributions. The permanence of these refugia is largely determined by waterhole morphology and evaporative loss, and some waterholes can clearly persist for 2 years or more without surface flow connection. As a consequence, the spatial distribution of refugia for aquatic biota is determined not only by the physical template but also by the duration of dry spells and the timing of flow events. Flow variability also has a major influence on aquatic production in these systems and ultimately influences food availability for fish and other consumers. During dry spells, aquatic food webs in waterholes are largely supported by algal production in the shallow littoral zone. At the other extreme, during floods, the boom of aquatic production on floodplains provides an immense food resource. However, there are many occasions when in-channel flows (flow pulses) result in periods where neither of these sources of production is likely to be available. Although such flow pulses are essential for the physical persistence and connectivity of waterholes, we propose that they may lead to food limitation and stress for populations of fish and other consumers. Water resource development in dryland rivers often leads to an increase in the frequency and duration of flow pulses, due to reduced floods and elevated base flows. This increase in the 'bits in between' natural boom or bust conditions may help to explain the observed decline in ecosystem health in dryland river systems with significant water resource development.