Schädler, S. and Kingsford, R.T. (2016). Long-term changes to water levels in Thirlmere Lakes – drivers and consequences. Centre for Ecosystem Science, UNSW, Australia.

Many of the world’s rivers and wetlands have degraded with alteration of flow regimes when their water supply is modified. Mostly this is due to impacts of structures (i.e. dams and weirs), but increasingly mining and access to groundwater have affected flooding regimes of wetlands. We investigated water levels within Thirlmere Lakes, which are primarily filled by rainfall but interact with shallow groundwater. 

We focused on the three of the five lakes with the most historical data and compared inputs from rainfall and water levels over 115 years (1900–2015). This first step required development of three-dimensional models of the lakes using topographic surveys. We then superimposed historical aerial photographs and ground photographs (1884–2012, n= 49 photographs/maps), allowing determination of water levels. We also investigated trends in groundwater levels (1999-2011), creating groundwater contour maps for two periods, before and after 1982, when longwall coal mining began. Rainfall and catchment inflow patterns were also examined at three nearby water bodies (Avon, Warragamba and Nepean Dams), acting as ‘control sites’. There were no significant long-term trends in rainfall at the three ‘control sites’, although rainfall increased significantly at Thirlmere Lakes (1900 – 2015). There were similar evaporation patterns at all sites and inflows at the three dams had not decreased. Contrastingly, there was a severe decline in water levels at Thirlmere Lakes. The disassociation of inflows and water levels (1930 -2015) had steadily increased, especially since the 1980s. In particular, we detected sudden drops in water levels in 1984, 1997 and 1998 in the three lakes. This did not coincide with the main period of water extraction for steam engines and Picton Tuberculosis Village at Thirlmere Lakes (1884-1910).

Groundwater levels in the shallow aquifer declined by up to 40m after mining commenced in 1982, also coinciding with an exponential increase in the number of groundwater bores (1940-2011). Although there were no publicly available data on how much groundwater was extracted each year, the serious decline in water levels coincided with increased groundwater development and longwall mining (29 longwall panels (2010)).

These serious declines in water levels in the three Thirlmere Lakes were not climate related and are best explained by anthropogenic impacts. There are two potential explanations for the draining of Thirlmere Lakes, which may have acted synergistically: either there was considerable pumping of groundwater or longwall coal mining disrupted the groundwater aquifers, causing diversion of groundwater resources.

The implications are significant for the ecological character of the Thirlmere Lakes. There are many affected obligate aquatic species or species reliant on wet habitats, including five species of waterbirds (Australasian bittern Botaurus poiciloptilus, Australian painted snipe Rostratula australis, great egret Ardea alba, cattle egret Ardea ibis, and Latham’s or Japanese Snipe Gallinago hardwickii) one fish species (Macquarie perch Macquaria australasica), two frog species (giant burrowing frog Heleioporus australiacus, Littlejohn’s tree frog Litoria littlejohni) and two plant species (smooth bush pea Pultenaea glabra, Kangaloon sun orchid Thelymitra Kangaloon) that are listed as threatened. There are serious implications for governments and their responsibilities for managing the values of Thirlmere Lakes National Park along with the Blue Mountains World Heritage Area of which it is a part. These major changes in flooding regimes to Thirlmere Lakes will continue to degrade the Thirlmere Lakes National Park and its ecological, cultural and recreational values. Most importantly, identifying the relative importance longwall mining and groundwater pumping on this deterioration should be a priority, requiring detailed analyses of changes to groundwater pumping volumes and identifying flow paths for water supplies to Thirlmere Lakes.