Ancestral rivers and terraces

Ancient and modern meanders on the Goulburn River, Murray Darling River Basin, Australia (Image: Google Earth)

Another week on the road and more fluvial geomorphology! Irene and I attended a delightful UNE function last week giving us the opportunity to drive to Armidale. We travelled uphill via the New England Highway and back down (an hour quicker) along Thunderbolt’s Way through Gloucester. I was reminded of all the fine work by our fluvial colleagues on stream flow regimes, channel patterns, impacts of land clearing, catastrophic events and terrace formation reinforcing the reflections from the trip the previous week in the Murray Valley.

East coast valleys have been the subject of a range of fluvial studies over past decades inspired in part by the work of George Dury who spent 8 years at Sydney University in the 1960s. George came to Geography at Sydney after a period with the USGS fluvial team and encouraged many students and others to look at channel processes and landforms in these valleys. Rob Warner has collated much of this work in an edited volume entitled Fluvial Geomorphology of Australia (Academic Press, 1988). A highlight for me in this book are chapters on NSW rivers, one by Nanson and Erskine on episodic changes of channels and floodplains, the other by Erskine and Warner on geomorphic effects of alternating flood and drought dominated regimes. These papers document the recent history of changes to channel geometry and sediment loads even as far down the Hunter River as Morpeth. Both Warner and Nanson had separately published papers on the evolution of terraces of coastal rivers such as the Bellinger and the Hawkesbury respectively.

After my trip to the Murray Valley, I re-read the excellent summary paper by Jim Bowler on the evolution of the Riverine Plain (Chapter 3 in Davies and Williams, eds. Landform Evolution, ANU Press, 1978). Although more work has been done since then, Jim’s work encapsulates the principles of both morphodynamics and morphostratigraphy in describing and interpreting changes in channel history in this region, in particular the Murray, Murrumbidgee and Goulburn valleys. He stressed the importance of combing information on channel form with sediment types as revealed at the surface and in the stratigraphy. Following the work of others such as Pels, Butler, Schumm and Langford-Smith, he was able to relate the large meanders of so-called ancestral and prior streams to changes in sediment load, regional climatic influences, including glaciation and periglaciation in the highlands, and local upstream channel diversions reflecting aggradation of channels and levees, as well as local tectonic events. The switch from high bedload to a suspended load regime changed the fluvial landscape from wide meander belts and long meander wavelengths to the narrower belts of today accompanying vegetative stabilisation of hill slopes in the highlands. That such patterns do not occur in the Lachlan Valley was attributed to an absence of bedload sediment yield from catchments of lower altitude and hence less periglacial activity than those to the south.

I have not studied fluvial features since returning to Australia in 1971. However, my PhD program at LSU involved mapping, drilling and dating fluvial features contained within the Quaternary prograded coastal landscape of Horry County, South Carolina. On arrival at Baton Rouge I kept hearing about the Deweyville terrace of river valleys along the Gulf Coast. Little had been published and there was much speculation about the age of a terrace deposits that in places possessed “giant meander scars”. A colleague at LSU, Woody Gagliano, and I decided to compare these features with those I had mapped on the Waccamaw and Little and Great Pee Dee Rivers of South Carolina. I had observed larger than present meanders on a terrace on both the Waccamaw and Great Pee Dee. However, the sandy terrace surface of the Little Pee Dee River contained relict braided channels in contrast with present-day meander pattern. We presented our findings in 1964 at the Annual Meeting of the Association of American Geographers in Toronto and published a short paper in 1967 in the LSU Coastal Studies Bulletin No.1. In this paper an attempt was made to correlate these fluvial events for both regions with what was then known of glacial and sea level history of North America over the last 40000 years.

While our attempt at correlation was tentative, I was convinced that we had a story that could be tested. Similarly the story from east Australian rivers and the Riverina as constructed in the 70s and 80s requires further work especially with the application of new dating and other techniques that were not readily available in those years. There are also prospects for tying the fluvial record with the coastal dune history of the Late Quaternary.

Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2017, posted 26 October 2017, for correspondence about this blog post please email admin@australiancoastalsociety.org

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