A Framework for the Ecogeomorphological Modelling of the Macquarie Marshes, Australia
Abstract:The Macquarie Marshes is a system of permanent and semi-permanent marshes, swamps and lagoons interconnected by braided channels. The Marshes are located in the semi-arid region in north western NSW, Australia, and constitute part of the northern Murray–Darling Basin. The wetland complex serves as nesting place and habitat for many species of water birds, fish, frogs and crustaceans, and portions of the Marshes was listed as internationally important under the Ramsar Convention. Over the last four decades, some of the wetlands have undergone degradation, which has been attributed to flow abstraction and regulation at Burrendong Dam upstream of the marshes. Among the many characteristics that make this wetland system unique is the occurrence of channel breakdown and channel avulsion, which are associated with decline of river flow in the downstream direction typical of dryland streams. Decrease in river flow can lead to sediment deposition, decrease in channel capacity, vegetative invasion of the channel, overbank flows, and ultimately result in channel breakdown and changes in marsh formation. A similar process on established marshes may also lead to channel avulsion and marsh abandonment. All the previous geomorphological evolution processes have an effect on the established ecosystem, which will produce feedbacks on the hydrodynamics of the system and affect the geomorphology in return.
In order to simulate the complex dynamics of the marshes we have developed an ecogeomorphological framework that combines hydrodynamic, vegetation and channel evolution modules. The hydrodynamic simulation provides spatially distributed values of inundation extent, duration, depth and recurrence to drive a vegetation model based on species preference to hydraulic conditions. It also provides velocities and shear stresses to assess geomorphological changes. Regular updates of stream network, floodplain surface elevations and vegetation coverage provide feedbacks to the hydrodynamic model. We perform preliminary tests by running continuous simulation over several years and compare the results to existing hydrological, vegetation and geomorphological data to assess the model capabilities and limitations. We also analyse the effects of the implementation of a number of water management strategies.