Long-term changes to water levels in Thirlmere Lakes – drivers and consequences

Many of the world’s rivers and wetlands have degraded with alteration of flow regimes.Mostly this is due to impacts of structures (i.e. dams and weirs), but increasingly miningand access to groundwater have affected flooding regimes to wetlands. Weinvestigated water levels within Thirlmere Lakes National Park, which are primarilyfilled by rainfall but interact with shallow groundwater. We focused on the three of thefive lakes, with the most historical data, and compared inputs from rainfall and waterlevels over 113 years (1900–2012). This first step required development of threedimensionalmodels of the lakes, using topographic surveys. We then superimposedhistorical aerial photographs and ground photographs, allowing determination of waterlevels (1884–2012, n= 49 photographs/maps). We also investigated trends ingroundwater levels (1999-2011) and created groundwater contour maps for twoperiods, before and after 1982, when longwall coal mining began. Rainfall andcatchment inflow patterns were examined at three nearby water bodies (Avon,Warragamba and Nepean Dams), acting as ‘control sites’. There were no significantlong-term trends in rainfall at the three ‘control sites’, although rainfall increasedsignificantly at Thirlmere Lakes (1900 – 2012). There were similar evaporation patternsat all sites and inflows at the three dams had not decreased. Contrastingly, there was asevere decline in water levels at Thirlmere Lakes. The disassociation of inflows andwater levels (1930 -2011) had steadily increased, especially since the 1980’s and wedetected sudden drops in water levels in 1999, 2000 and 2004 in three lakes. This didnot coincide with the main period of water extraction for steam engines and PictonTuberculosis Village at Thirlmere Lakes (1884-1910). Groundwater levels in theshallow aquifer declined by up to 40m after mining commenced in 1982 and thenumber of groundwater bores had increased exponentially (1940-2011). Although therewere no publicly available data on how much groundwater was extracted each year,the serious decline in water levels coincided with increased groundwater developmentand longwall mining (29 longwall panels (2010)). The changes to water levels were notclimate related and so are perhaps best explained by anthropogenic impacts. Eitherthere was considerable pumping of groundwater or longwall coal mining disrupted thegroundwater aquifers, causing diversion of groundwater resources that drainedThirlmere Lakes. Or, changes could be due to a combination of these twoanthropogenic factors. The implications are significant for the ecological character ofthe Thirlmere Lakes. There are many affected obligate aquatic species or speciesreliant on wet habitats, including five species of waterbirds (Australasian bittern Botaurus poiciloptilus, Australian painted snipe Rostratula australis, great egret Ardeaalba, cattle egret Ardea ibis, and Latham’s or Japanese Snipe Gallinago hardwickii)one fish species (Macquarie perch Macquaria australasica), two frog species (giantburrowing frog Heleioporus australiacus, Littlejohn’s tree frog Litoria littlejohni) and twoplant species (smooth bush pea Pultenaea glabra, Kangaloon sun orchid ThelymitraKangaloon) that are also listed as threatened. There are serious implications forgovernments and their responsibilities for managing the values of Thirlmere LakesNational 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 degradethe National Park, its ecological, cultural and recreational values