Geomorphology of a dryland fluvial system: the Lower Balonne River, southern Queensland

Abstract

The floodplain of the Lower Balonne River is in the upper reaches of the Murray–Darling Basin. The region has been extensively developed for agriculture, in particular irrigated cotton, and is highly productive. Multidisciplinary investigations to inform land management generated extensive sets of remotely sensed data including Landsat TM, airborne gamma-ray radiometrics, aerial photography, ASTER imagery and digital elevation models. These datasets provided the basis for regolith and geomorphic mapping. The wealth of data has allowed characterisation of the Lower Balonne River system, which is typical of many of the dryland rivers of southern Queensland. The geomorphic map of the Lower Balonne floodplain has eight major units based on landform and geomorphic processes. Bedrock consists of the slightly deformed and extensively weathered marine Cretaceous Griman Creek Formation. Coincident with erosion and weathering, Paleogene quartz gravels were deposited and are now extensively cemented and preserved as remnants forming zones of inverted relief. Much of the present landscape consists of a series of juxtaposed depositional units that have infilled an incised valley system. The different depositional units show the paleo-Balonne River migrating to the west. This is interpreted to be a result of tectonic depression and tilting to the west, causing avulsion and anastomosing of the paleochannels. The modern Balonne River system consists of a number of easily recognised segments. In the north, the modern channel is incised as a single channel. To the south the channel opens out onto an anastomosing plain with branching and reconnecting small-scale channels. Source-bordering dunes, currently inactive, have also formed along the western and eastern sides of the modern river and are prominent in large dunes in the south along the present Moonie River. Their absence in older landscape elements points to increasing aridity over time in the river system.