Stream channel adjustment in upland swamps, Barrington Tops, New South Wales, Australia

The study of stable, channelled, upland swamps is relatively new and the processes by which theyattain and maintain stability are of particular interest. Channels can adjust three elements of theirmorphology: cross-section, bedform and planform. This thesis examines unusual extremes in theadjustment of these elements in several swamp channels at Barrington Tops, New South Wales.In these channels, high bank strength is afforded by dense vegetation that has enabled the channelsto achieve unusually low width/depth ratios. The resultant at-a-station hydraulic geometry is suchthat width barely increases with flow stage, depth increases moderately and velocity increasesmarkedly; the channels have particularly high hydraulic efficiencies. Shear stress calculationssuggest that bankfull flows are more than capable of transporting the scant sediment with whichthey are supplied. This economic use of energy means that, to maintain equilibrium, the excess mustbe consumed by the remaining elements; bedform and planform.Bedform magnitude (steepness) is strongly linked to flow resistance and there are large variations inthe scale of bedforms between the channels studied here. In the smaller but more sinuous PolblueCreek, power is moderate and bed features display moderate steepness values. In contrast, the largerbut less sinuous Barrington River and Edwards Creek channels have higher stream powers and theirarmoured bedforms have developed with much greater steepness. The considerable turbulenceassociated with these larger features are unable to destabilise the highly resistant channel banks butare sufficient to generate energy losses that reduce grain shear-stresses and prevent bed erosion.Polblue Creek, Barrington River and Edwards Creek have each developed sinuous planforms, withsome examples of very tight bends. The reduction in the energy slope associated with planformdevelopment and the substantial turbulence that these tight bends generate combine with bedformdevelopment to counter the exceptional hydraulic efficiency of the channel cross-sections. Thedevelopment of extreme bedform and planform morphologies have enabled system stability, despitehigh shear stresses and limited energy expenditure on sediment transport. This research suggeststhat bed armouring, high bank strengths and the development of highly sinuous planforms areintegral to the maintenance of channel equilibrium, as has been demonstrated by channelled-swampstability over the last ~1ka