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Title: The floodplain sedimentology of Cooper Creek. Why billabongs (waterholes) in the Channel Country start and stop
Authors: Nanson, GC
Larsen, JR
Cendón, DI
Fagan, SD
Jones, BR
Keywords: Floods
Issue Date: 24-Aug-2009
Publisher: Universidad Nacional de Tucumán, Fundación Miguel Lillo
Citation: Nanson, G. C., Larsen, J. R., Cendón, D. I., Fagan, S. D. & Jones, B. G. (2009). The floodplain sedimentology of Cooper Creek. Why billabongs (waterholes) in the Channel Country start and stop. Presentation to the 9th International Conference on Fluvial Sedimentology, San Miguel de Tucuman, Argentina, 24th-28th August 2009. (pp. 50-51).
Abstract: The Channel Country of western Queensland consists of a low gradient anastomosing channels inset in muddy floodplains in a desert climate. Annual transmission losses are large, averaging 75-80% of total volume along a ~400 km reach of Cooper Creek. Billabongs (waterholes) are common and consist of local channel expansions in the mud-lined anastomosing network, as well as isolated scour-channels on a floodplain that is formed of 2-3m of clay-rich mud. Both the primary anabranching and overbank flood channels carry water during large floods but only the billabongs store water for long periods of drought, making them vital to the ecology and agriculture of the region. Despite penetrating a very extensive underlying Pleistocene-age sand body, they are impermeable much of the time. However, transects away from several billabongs reveal marked increases in salinity and provide compelling evidence that they act as fresh-water ‘entry valves’ through an otherwise impermeable muddy floodplain. The base of each billabong is scoured during floods, enabling large volumes of surface water to be dumped into the 10 m deep saline aquifer. As flows decline the billabongs self-seal with mud such that fresh water can be stored for a year or more. Transmission losses along Cooper Creek were initially believed to be due to evaporation, however, the lack of any solute concentration between gauging stations along the ~400 study reach, suggests that of the ~1.8 km3 of surface flow lost from the ~3.0 km3 entering the upper end of the study reach, most leaks through the base of the billabongs. Such massive losses also account for why the billabongs in the form of large and efficient channels, terminate after just a few kilometres. They reform repeatedly at locations where conditions are suitable, but their numbers decline downstream in accordance with the loss of total flow-volume.
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