Year

2013

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

This study examined the mid to late Quaternary landscape evolution of the Shoalhaven River. It aimed to fill the spatial and temporal gaps of existing work which focused on Tertiary evolution in the upstream reaches, and development of the modern deltaic plains. It provides an understanding of fluvial responses to Quaternary climate that may contribute to both theoretical understanding of features and regional catchment management. This thesis, therefore, examines how far upstream Quaternary sea level changes are recorded; how rivers in confined valleys adjust to long-term changes in flow regime; and the structure of in-channel benches as a modern sedimentary process.

Key results establish that the sedimentary signature of Holocene sea level rise is preserved tens of kilometres inland and into bedrock-confined reaches. At Wogamia, some 32 km from the current coastline, Holocene estuarine depositional environments extend to 2.2 m above present sea level (AHD).

Slightly upstream at Bundanon, where the channel bed lies at approximately -3 m AHD, some ~35 km from the current coastline, a laterally migrating channel with a floodplain built dominantly by vertical overbank deposits is recorded. The absence of estuarine facies at Bundanon is interpreted to relate either to the barrier provided by flow conditions associated with the bedrock constriction downstream, or their removal due to subsequent river-channel migration. Channel migration has occurred since ~4 ka, which coincides with channel stabilisation downstream at Wogamia and the formation of the deltaic plains farther downstream.

A significant outcome is the contrast between the long-term preservation of terraces in Bulls Reach with modern inset, in-channel benches. Bulls Reach lies ~7 km upstream, where the channel bed lies at 6 m AHD at the downstream end, and at 18 m AHD at the upstream end. The river in this confined reach has adjusted to changes in flow regime during the Quaternary, and demonstrates a long history of sediment preservation in this lower gorge of the Shoalhaven River. Three of the four sites suggest considerable lateral stability of the river within its valley for up to 200 ka.

In addition to these old terraces, the polycyclic depositional history of the alluvium here is reflected by Holocene sediments capping these ancient features, and at lower elevation of longitudinal benches of sand and gravel that have been periodically reworked over much shorter periods. This study finds that deposition and erosion of benches is more complex than accounted for in previous models. Deposition may occur across multiple surfaces in a single event, and scouring occurs during small magnitude as well as extreme events. Furthermore, the vertical boundaries described previously in the literature and the suggestion that benches are formed almost exclusively by vertical accretion is not upheld in these results. The location of the bench front may laterally accrete, and/or be laterally or vertically eroded. The existing premise that events of a specific recurrence interval are responsible for the formation of individual benches of a particular elevation cannot, therefore, be supported.

The implications of these findings are that the preservation of diachronous alluvial terraces in the wider reaches reduces the variability of channel dimensions and resulting channel capacity. The youthful benches provide a dynamic form of channel adjustment in response to reduced flows in recent decades.

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