Year

2006

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

This study examines the downstream changes in the character of the Gwydir distributary system which flows across the Gwydir fan-plain, a large (~ 7500 km2) low gradient alluvial surface which forms part of the Darling Riverine Plains of southeastern Australia. The Late Quaternary history of the distributary system is evaluated by investigating the chronology and probable discharges of palaeochannels at or near the surface of the fan-plain. Channels of the contemporary distributary system are characterized by downstream declining discharges, in part a result of the interaction of the modern system with remnants of the preceding palaeochannel systems. The hydraulic geometry of the contemporary channels revealed that these distributaries do not have a uniform response to declining discharge, with differences in heights of off-takes leading to differences in sedimentology, hydrology and channel morphology. Of the four distributaries, the Gwydir River is the bedload transporting trunk stream, hence its hydraulic geometry is fundamentally influenced by the need to maintain bedload conveyance as discharge declines downstream. It maintains a relatively deep channel facilitated in part by adoption of an anabranching habit in its lower reaches, and by relatively continuous flow that keeps its bed free of vegetation. Hence it has a relatively low W/D ratio. Contrary to an expectation for clay dominated bedload-free channels with banks of high material strength, depth in the Mehi, Moomin and Carole declines relatively rapidly downstream with little or no change in width. This results in large increases in W/D ratio downstream. It appears that long intervals of no flow in these three distributaries with their elevated off-takes encourage the growth of dense sturdy vegetation on their beds, directing flow energy to the banks during flood events. Planform analyses show that bend radius of curvature and meander wavelength are strongly correlated with discharge, channel depth and flow velocity, but are poorly correlated with width. Width appears to be responding to conditions other than discharge, probably the presence of within-channel vegetation. The negative correlation between sinuosity and unit stream power, and between sinuosity and apparent bank strength, indicate that an increase in channel width allows or indeed requires a decrease in gradient through meandering. Analyses of the palaeochannels of the Gwydir fan-plain revealed that alluvial quartz grains here are unsuitable for traditional thermoluminescence analysis, as it is invariably heavily contaminated with inclusions of feldspar which causes severe age underestimation. An alternative single-grain optically stimulated luminescence (OSL) method shows that downstream fluvial transport of quartz in contemporary channels increases its luminescence sensitivity due to repeated cycles of burial and exposure. Three potential applications of this finding are; a) sediment tracing, b) river hydrology description and c) palaeochannel connectivity analysis Application of the revised OSL technique reveals that the palaeochannels on the surface of the Gwydir fan-plain date from at least 70 ka. The oldest, the Challicum, dates to Oxygen Isotope Stage 4 (OIS 4) and shows limited surface preservation so no reliable palaeohydrological estimates could be made. The largest preserved palaeochannel system, the Coocalla unit with a palaeodischarge of ~ 2550 m3s-1, dates to mid OIS 3 (~ 45 - 35 ka), a time of enhanced fluvial activity across the Murray-Darling basin and nearby coastal systems. Two more well preserved palaeochannels, the Kookabunna and Kamilaroi, were both active during OIS 2 (~ 20 � 15 ka) with discharges of ~ 1240 m3s-1 and ~ 1310 m3s-1, respectively. A period of enhanced aeolian activity has been identified at ~ 4 ka, followed by the establishment of the contemporary system, with a total combined channel capacity mid-fan of ~ 200 m3s-1. The Gwydir fan-plain reveals a Late Quaternary history of flow regime decline by approximately an order of magnitude. The present system of greatly reduced flow competence responds to a marked downstream reduction in discharge with a set of spatial hydraulic geometry and planform changes that are clearly not simply the inverse of more usual systems where discharge increases downstream.

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