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.

Where does all the water go? Partitioning water transmission losses in a data-sparse, multi-channel and low-gradient dryland river system using modelling and remote sensing

“…Given the strong rainfall seasonality, these arid zone rivers are also characterised by some of the most variable flow regimes in the world, with distinct runoff periods during the November -March wet season, separated by a distinct dry season of generally no flow, (April -October, Knighton and Nanson, 2001). The system is also notable for the water that can be retained year-round within enlarged channel segments, or waterholes, that become disconnected at low flow and which are critical ecological refuge sites (Bunn et al, 2006). River flow in the LEB is unregulated and sustains (albeit in 'boom-and-bust' cycles) high ecological value habitat (Costelloe et al, 2006) for migratory and non-migratory fish (Puckridge et al, 2000) and water-bird (Kingsford et al, 1999) populations.…”

Detecting range shifts among Australian fishes in response to climate change

“…1b), resulting in highly variable patterns of occurrence through time (Unmack 2001;Arthington et al 2005;Bunn et al 2006). In such systems, detection of range shifts becomes difficult because vagrant and previously unsurveyed populations are likely to be encountered relatively often.…”

Climate Change in Oceania – A synthesis of biodiversity impacts and adaptations.

“…Protecting headwaters and identifying and protecting existing thermal refugia will also enhance the ability of cold-water fish and other biota to persist as temperatures rise (Hansen et al 2003). Refugia in arid freshwater ecosystems should be protected from water resource development as these are often the only freshwater sites in the landscape where obligate species can survive (Bunn et al 2006).…”