Managing regulated flows and contaminant cycles in floodplain rivers

Many river systems in south-eastern Australia originate in relatively wet upland ranges where they are usually highly regulated by large impoundments. They then flow for the majority of their length through semi-arid landscapes of very low relief (Thoms & Sheldon 2000). It is this low relief that drives the hydrology of these river systems, with continued losses of water through evaporation, evapotranspiration, groundwater recharge and a lack of tributaries along the length of the river (Thoms & Sheldon 2000). These rivers have spatial and temporal flow variations that are more extreme and less predictable than those in more humid regions of the world (Lake 2000, Puckridge et al. 1998). This flow variability underpins many ecosystem processes, and regulates the transport of nutrients, carbon and biota within river channels and onto the floodplain (Robertson et al. 1999). Changes to the natural disturbance regime provided by flow variation combined with altered land use practices have resulted in many rivers now containing highly modified sources and concentrations of natural contaminants such as nutrients, salt, and suspended sediment (Robertson et al. 1999). However, the relationships between flow regime, and the sources, sinks and transport of contaminants are poorly understood, making it a difficult task to manage flow releases to best effect for river rehabilitation initiatives.