The morphology and formation of floodplain-surface channels, Cooper Creek, Australia

Floodplain-surface channels are prominent features of the floodplain of Cooper Creek, a low-energy, anastomosing river in semi-arid southwest Queensland. Three distinct floodplain-surface environments and corresponding pedological variations are identified. Braided patterns characterized by large-scale braid bars separating wide, shallow channels occupy 44% of floodplain area. Gilgai, undulations of the soil surface characteristic of many vertisol soils, are subdued or absent from the braided areas. The reticulate pattern occupies 39% of the floodplain surface and is characterized by densely developed networks of small channels with angular planforms. The prominently developed gilgai in these areas play a significant role in the formation of the reticulate pattern. High, rarely inundated areas of the floodplain are unchannelled and occupy the remaining 17% of the floodplain surface. Analysis of the distribution of the patterns over a 370-km-long reach of the river reveals that sensitive feedbacks between fluvial and pedogenic processes determine pattern expression. The energy of inundating flows is controlled by floodplain width, transmission losses and subtle floodplain topography variations. The patterns are distributed along an energy continuum. Braided patterns occur in higher energy areas in which fluvial erosion prevents gilgai expression. Reticulate patterns are confined to intermediate energy areas in which inundation frequencies are sufficient to provide wetting and drying frequencies necessary for gilgai formation and flow energy sufficiently low to allow gilgai expression. Unchannelled areas are neither inundated frequently enough to cause gilgai formation, nor with sufficient energy to cause channel formation.