The geomorphological evolution of Corryong Creek, Victoria

Abstract

The present morphology of a river is the outcome of past and present conditions. To understand a fluvial system, the history of a river must therefore be known. Once established, river management activities will be better informed, as a model for channel response is now available. Moreover, using variable-specific studies sometimes creates a bias in attributing geomorphic significance to variables. An integrated approach avoids the underestimation or overestimation of influences because variables are considered collectively. This thesis is driven by this principle. Presented here is an investigation on Corryong Creek, to determine and explain its geomorphological evolution in the period since European settlement. To do so, its history of channel change was first reconstructed, followed by the reconstruction of main geomorphic variables including hydro-climate, catchment land cover, riparian vegetation cover, and river management. Spatiotemporal trends for each parameter were identified, and then compared to decipher interrelationships.

Since European settlement, Corryong Creek has become increasingly unstable, exhibiting an overall pattern of straightening, widening and increased sediment deposition. A change in catchment hydrology is evident, which is due to a combination of severe land clearing and an increase in average rainfall since the mid-20th century. Because climate is a significant influence on flow, periodicities in channel stability are observable through climatic phase shifts between wet and dry years dictated by the El Nino Southern Oscillation and the Indian Ocean dipole. Flood regimes were not observed in the rainfall data. While general patterns in channel stability were observed over longer time periods and larger areas, rates of channel change have not necessarily been consistent across smaller spatial and temporal scales. This revealed the influence of river management and riparian willow cover. Severe channel straightening, bank stabilisation and willow growth have distorted the capacity for channel change to occur. In general, channel straightening has increased channel instability by increasing flow velocities (in addition to directly affecting channel planform), bank stabilisation has stunted erosion rates, and willows enhance channel widening and sediment deposition (but their effect reduces as channels widen). This thesis demonstrates that despite complexity, fluvial systems can still be better understood through an analysis of spatiotemporal trends.