Doctor of Philosophy
School of Geosciences
Fagan, Simon D., Channel and floodplain characteristics of Cooper Creek, Central Australia, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1966
This thesis is an investigation of the formation and distribution of the different channel patterns that co-exist on the broad, low-gradient floodplain surface of the Cooper Creek, an ephemeral, arid-zone river in south west Queensland, Australia. Although previous research described some aspects of the channel as being inherited from wetter Pleistocene periods, all the alluvial surface features were found to be consistent with a contemporary origin broadly in equilibrium with present environmental conditions.
The anastomosing channel system is comprised of interconnected narrow and deep channels with resistant, cohesive boundaries. Levees are formed whose size is related to the size of the proximal channel, and their textural variations are more complex than the usual simple distal fining trends. Levee-like landforms are also formed by the deposition of sediment by flow converging on the channel from the floodplain surface. These forms were termed 'eevels' here to indicate that they form in the opposite way to levees, and there are textural differences between these forms and levees. The planform of anastomosing anabranches was analysed by both the direct measurement of channel planform and the fractal method and is consistent with that of freely meandering channels. Results suggest that the disruption of the channel boundary materials by wetting and drying cycles enables and determines bank erosion and channel migration, which seems to be greater in smaller channels due to greater fracturing of the boundaries of these channels. Smaller channels can be much more sinuous than larger channels and frequently form cutoffs.
Elements of the extensive floodplain channel-system formed here interact with and influence the evolution of the larger, inset anastomosing channels. The angles of bifurcation junctions between anastomosing channels tend to be much larger than confluence angles, and a comparison with the angular geometry of junctions of braid-form or floodway channels and anastomosing channels suggests that the latter develop by the enlargement of floodways in a gradual avulsion process. Like the floodplain-surface channels here, the anastomosing channels form in response to high flows and their formation has the effect of increasing the efficiency of overbank flow transport by the floodplain. Reticulate and braid-form patterns cover more than 80% of the floodplain surface, and their distribution and occurrence at both the small and large scale is shown to be determined by overbank flow patterns. Flow patterns also influence the morphology of reticulate networks which have both transitive and space-filling aspects. Braidform channels co-occur with anastomosing channels where the depths, velocities and inundation frequencies of overbank flow are relatively high. Reticulate patterns occur where overbank flow is weaker and very infrequently inundated areas are unchanneled. The variation in floodplain-surface pattern occurrence is probably due to the mechanical effect of overbank flow power on the formation of prominent gilgai with which the reticulate pattern co-occurs.