Year

2004

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Abstract

The Minnamurra River estuary, located on the southeastern coast of Australia, provides an opportunity to examine the sedimentary records of the stages of infill of a barrier estuary through to maturity. By describing and interpreting the diverse geomorphological and stratigraphic units of Minnamurra River estuary and comparing the long-term Holocene morphodynamic evolution with the short-term engineering time scale changes, the nature of Holocene estuary evolution in a bedrock confined valley in a microtidal environment is investigated, providing an evolutionary context for contemporary estuary changes. Management concerns for the Minnamurra River estuary are also investigated, in particular the study includes examination of estuary sediment geochemistry and patterns of recent sedimentation and erosion. The Minnamurra River estuary is characterised by three main depositional environments: the marine depositional environment including the marine flood-tide delta and sand barrier, the estuarine depositional environment incorporating the central mud basin, and the fluvial-dominated environment including the fluvial delta, riverine channel and the alluvial plain. During the final stages of the postglacial marine transgression, the Minnamurra River estuary became a sediment sink that has been largely infilled by marine and fluvial sediments. Examination and quantification of past and present patterns and rates of sedimentation has been assessed through a detailed stratigraphy constructed from 42 auger drill holes, supplemented by full core drill holes, shorter PVC cores from recent depositional environments and surface sediment sampling. A detailed chronology of the infilling of the barrier estuary was also established using radiocarbon dating and 210Pb analysis. 210Pb was used to examine sediment deposition at four sites in the Minnamurra River estuary. Cores were collected from two cut-off embayments, the edge of the main channel and from a tributary off the main channel, Rocklow Creek. Average sedimentation rates ranged from 0.3 - 0.8 cm/year and provided 52 - 148 years of sedimentation history. These results provide a means of comparing infilling rates throughout the Holocene evolution of a mature barrier estuary. In particular, they have afforded the opportunity to quantify the amount of sediment that bypasses the estuary, which is now in a mature form. The reduced accommodation space of a river-dominated estuary means that the amount of sediment which used to be deposited in the extensive central mud basin that acted as a sediment sink, must now be deposited elsewhere. In the Minnamurra River estuary a small percentage of the sediment is deposited in the remaining cut-off embayments or in overbank deposition during time of high flow. The remaining sediment, calculated to be approximately 90% of the total sediment supplied to the Minnamurra River estuary, bypasses the system and is deposited in the nearshore. Recent trace metal levels recorded in the surface sediment samples and short cores show there was little change in the levels of trace metals measured in the sediment. Most trace metal results were below the ISQG-Low trigger value, however there were a few exceptions. Arsenic, copper and nickel levels of several sediment surface samples were between the ISQG-Low and ISQG-High ANZECC (2000) sediment quality guideline values. Only copper levels exceeded the ISQG-High ANZECC sediment quality guidelines in a few sediment samples. Although geomorphological evolution of wave-dominated estuaries is relatively well known, the understanding of wave-dominated estuaries that have reached a river-dominated stage is less well known and as a result, has been investigated in this thesis. This research has described and interpreted the Holocene transformation of the estuarine ecosystem of the Minnamurra River estuary from a wave dominated system 8500 years BP to the present river-dominated system with its restricted estuarine habitats and geomorphology. A five-stage evolutionary model has been devised for the Minnamurra River estuary depicting the changes of infill that occurred during the Holocene. During the first stage of Holocene evolution, 8500 - 6500 years BP, the Minnamurra River valley was inundated by rising sealevel. Initial marine deposition occurred semi-enclosing the valley, forming a central mud basin. The rate of infill increased substantially in stage two (6500 - 4500 years BP) when barrier progradation further enclosed the central mud basin. During stage three (4500 - 2500 years BP) extensive infilling led to continued barrier progradation and further infill of the central mud basin by fluvial deposits, which also may have resulted in a number of cut-off embayments forming. In the fourth stage of estuary infill, the central mud basin was largely filled and characterised by an extensive swamp. During this stage the estuary progressed towards becoming river-dominated with a confined channel, extensive floodplain and an increase in intertidal environments. The fluvial channel form meandered through the floodplain and the sand barrier sequence. The last 100 years encompasses the fifth stage of the evolutionary model for the Minnamurra River estuary in which the estuary was impacted by anthropogenic influences. During this stage the swampy floodplain was drained and the meandering channel in the fluvial and estuarine depositional environments was straightened. Accommodation space during the final stage of the model is significantly reduced with only a few cut-off embayments remaining. The results provided a chronology of the deposition of transgressive deposits, barrier progradation, central mud basin development and subsequent fluvial expansion. They also provided the opportunity to calculate volume of Holocene estuary infill. Approximately 98,000,000 m3 of sediment was deposited during the Holocene. The framework of geomorphology and management adopted in this study can be applied to other estuaries. The future success of estuary management requires a combined effort to integrate management in a comprehensive sense by linking geological, biological and chemical aspects with social and economic factors, framed within the context of the longer-term estuary evolution and recent estuary changes. The evolution of the Minnamurra River estuary has undergone five geomorphologically distinct phases associated with rising and later falling sea-level, estuary confinement from barrier development, fluvial delta progradation and anthropogenic influence by draining the floodplain and channel straightening. Sedimentation rates differed in these five geomorphologically distinct phases. The model of estuary evolution proposed in this thesis endeavours to synthesise divergent hypotheses in the literature with three key features. These three key features are firstly hydrodynamics that combines the three processes (waves, tides and river); secondly, geomorphology, which is shaped by the three processes; and thirdly percentage infill that is controlled by the balance between accommodation space and sediment supply.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.