QA_Vol 24_No 1_July 2007 - Australasian Quaternary Association

Australia and NZ Geomorphology Group Workshop CONTINUEDThe Tuross River drains a catchment of c. 2180 km 2 andis a confined, sand-bedded meandering river (Fergusonand Brierley, 1999) with an intermittent/ephemeraldischarge regime. The steep forested headwaters of theTuross River reach an elevation greater than 1000 m asl,and are underlain by Devonian granites. On the coastalside of the Great Dividing Range escarpment, the catchmentis underlain by Ordovician metasediments. In thelowermost 60 km of channel, slope decreases from 0.0011to 0.0007 m m -1 and bed material fines downstream fromcobbles to sand, with mixed sand and mud in the infillingestuary lake which is impounded by a coastal sand barrier(Rustomji et al., 2006).The field trip started at Tuross Head, which overlooks themouth of the wave-dominated estuary of the Tuross River.At the time of our field trip, the estuary was completelyblocked by a large sand barrier (Figure 2a), a commonfeature of the many wave-dominated estuaries of Australia’ssouth-east coast. Paul Rustomji has estimated thatthere has been sufficient inflow to the Tuross estuary forit to be flushed and the barrier breached between 10 and50 times per decade over the past 100 years. By way ofdemonstrating geomorphology in action, a few days afterour visit to the area, over 150 mm of rain fell over thecatchment. This triggered a spectacular breach of thebarrier (Figure 2c).The field trip then moved upstream to our second stop,overlooking the meandering lower reaches of the TurossRiver. Here, Holocene progradation of the river mouthresulted in floodplain aggradation over estuarinesediments. Alec Costin pointed out the low-lying floodbasin depressions along the bedrock margins and palaeochannelsthat have been submerged during periods ofhigh discharge.Figure 2. Geomorphologists trigger estuary mouth opening?Three days after the ANZGG workshop fieldtrip to the TurossRiver on the New South Wales south coast, 150 mm of rain fellover the weekend, resulting in the opening of the estuary mouthin fairly spectacular fashion. A. Before breach (photo taken byJeff Shellberg). B. After breach (photo taken by James Woodford).C. At the breach (photo taken by James Woodford).As we travelled further upstream, dominant alluvialfeatures became apparent, including a prominent highlevelalluvial (levee/floodplain) surface that is typically 10-14 m above the thalweg, and substantial benches that aretypically 6–8 m above the thalweg and less than 50m wide(Ferguson and Brierley, 1999; Rustomji et al., 2006). Forthose participants used to working on near-flat floodplainsthat are many kilometres wide, the narrow valleysand distinct topography of alluvial features were a luxury,although some participants from New Zealand remarkedon the lack of “real rivers” in this part of the world!At our lunch stop a debate arose over the mechanism bywhich Holocene valley aggradation occurs. Paul Rustomjiproposed a model by which aggradation is driven by rivermouth progradation, with a constant rate of aggradationoccurring at all locations along the lower Turossextending tens of kilometres upstream. This model wasvigorously challenged by Tim Cohen, who proposed thatthe rate of floodplain and bed aggradation must diminishin an upstream direction, and should only extend as far as38 | Quaternary AUSTRALASIA 24 (2)