Thirlmere Lakes; A degraded environment or an environment sensitive to natural hydroclimate variability?

The Thirlmere Lakes are a series of five freshwater lakes trapped in an ancient uplifted river meander, located on the eastern margin of the Greater Blue Mountains World Heritage Area, near Thirlmere, NSW. The lakes are ecologically, scientifically and recreationally important and are considered a permanently inundated water lacustrine system. The last few decades have resulted in observable drying trend visible throughout many of the individual lake systems. This provides an important question: is the recent water loss in the lakes of Thirlmere Lakes National Park (TLNP) a result if a degraded environment in the TLNP, or are the lakes just sensitive to natural hydroclimate variability. This research report will aim to address this question by assessing the past hydrological variability of Lake Baraba in the TLNP, in order to understand the palaeo-environment of Lake Baraba and put into perspective the recent trend in water loss in the TLNP. Understanding the palaeo-environments of Lake Baraba was achieved by broadly investigating the sedimentary and stratigraphic characteristics of Lake Baraba, examining the grainsize, age, organic content and the elemental composition of the sediments. The results of these analyses were synchronously compared to one-another and compared to the context of other palaeo-environmental publications from south-east Australian palaeo-climate and lake level research throughout the late-Quaternary, particularly the Holocene. The results found that within the last ~35 ka, Lake Baraba has undergone significant hydrological and sedimentological change, however throughout the Holocene (~12-0 ka), there was a marked shift to stable lacustrine conditions in Lake Baraba, resulting in a constant peat formation record since ~ 9.3 ka years cal. BP. While there is some evidence of drying phases on the lake margin during the mid-Holocene, these events are only minimal and are not recorded in the centre of Lake Baraba. Additionally, the research found apart from the evidence of pre-Holocene peat development (~35 ka cal. years BP) and their close proximity, Lake Baraba and Couridjah highlight different hydrological and sedimentological conditions inferring that they currently operate as independent water bodies, however they may become connected via surface water flow during periods of high rainfall. Therefore, these finding suggest that the Thirlmere Lakes are significantly unique within the Sydney Basin, presenting itself as a possible important palaeo-environmental archive throughout the late-Quaternary, in particular the Holocene in the context of south-east Australia.