Crossing Three Deep Valleys: The Sedimentology, Geomorphology and Palaeo-environments of Thirlmere Lakes, N.S.W

The Thirlmere Lakes are a series of five freshwater lakes that reside in ancient, uplifted river meander situated on the eastern margins of the Greater Blue Mountains World Heritage Area near Picton, NSW. These lakes are important ecologically and recreationally, and are generally viewed as being perennially inundated. In the last few decades, however, there has been an observable drying trend in the lakes. This offers an important question; is the recent water loss unprecedented or is it part of natural variability experienced by the lakes? This honours thesis aimed to assess past hydrological variability within one of the Thirlmere Lakes, Lake Couridjah, in order to put the recent trend of water loss into a longer-term context of how the lakes respond to change. This aim was achieved by broadly investigating the palaeo-environments within Lake Couridjah, through examining the sedimentary characteristics and stratigraphic relationships in and between Lake Couridjah and the neighbouring Lake Baraba. This included examining the age, grain size, organic content, elemental composition and carbon isotopes of the sediments. The results of these analyses were then synchronously compared along with other palaeo-environmental research from south eastern Australia. The results of this study found that within the past ~100 ka, Lake Couridjah has experienced significant hydrological change. However, throughout the Holocene (~12 - 0 ka), there was a marked shift into sustained lacustrine conditions resulting in near constant peat formation in Lake Couridjah. While there is some evidence for lake drying during the Holocene, these events are only minimal and do not show complete drying of the whole lake. Furthermore, this study found that lakes Couridjah and Baraba appear to have experienced different hydrological and sedimentological processes through time, which suggests that despite their close proximity, these lakes can operate as independent water bodies. These findings highlight Thirlmere Lakes are unique within the Sydney Basin, as well as a potentially important palaeo-environmental archive for south eastern Australia