River stability issues in Northern Australia

The Northern Territory Government plans to convert pastoral leases in the Daly River region to agriculture. Prior to this development it is important to understand the sediment dynamics of the system and the potential impacts on the river of water abstraction for irrigation. The Daly River is one of the iconic river systems of northern Australia. There is considerable debate about the effects that potential development of the river basin may have on the river channel. It has been suggested that there has been considerable siltation in the estuary since 1880, and recent observations of the channel indicate that there are instabilities in the channel planform. The question as to whether these instabilities are driven by hydrological or anthropogenic influences is crucial to the decision on future development and how it should be managed. This paper proposes the application of rational regime techniques to alluvial reaches of the Daly River to help determine if the channel dimensions are in equilibrium or are being subjected to a change from a previous equilibrium state. Considerable work has been carried out in idealised laboratory free-form channels to investigate and test rational regime approaches. However, much less has been done to apply these methods to field conditions. There are several reasons for this. For example, the argument about the so-called channel-forming discharge, the scepticism that such systems may yield to a pseudo-deterministic approach, and the fact that to solve the equations which describe the system, an extremal hypothesis has to be applied with little theoretical justification.