Many lakes whose existence depends wholly on rainwater runoffs are seasonal. The phenomenon is not particularly mysterious—the lake forms when rainwater accumulates in a depression, and disappears when the water dries off. However, in Ireland, there is a type of disappearing lake that does not evaporate into the air; it drains underground. They are known as turloughs.
Lough Bunny in The Burren, County Clare, Ireland, is a turlough. Every summer it drains into fissures around the lake's northern end. Photo: Tom Fahy/Flickr
Turloughs mostly occur in the central lowlands west of the River Shannon, although some have been found elsewhere. The central requirement for a turlough to form is a porous bed of limestone. Limestone is a sedimentary rock composed of the skeletal fragments of marine organisms such as coral and molluscs. Their main component is calcium carbonate, which is soluble in water. Rainwater is slightly acidic, because it contains dissolved carbon-dioxide, which causes it to slowly eat away at the limestone rocks. Over millions of years, the limestone develops cracks which widens by rainwater seepage, eventually forming large underground caverns and tunnel systems through which water moves in and out. When this happens the limestone is said to have karstified.
About 65 percent of Ireland is made up of limestone. These rocks were deposited from coral reef and other marine life some 400 million years ago, when much of Ireland was beneath a warm tropical sea. The karst landscape formed some 250 millions years ago. By this time, tectonic movement had pushed Ireland well out of water. Subsequent glacialization deposited a thick layer clay and sand, over which bogs and lakes formed.
Lough Aroolagh, County Galway, Connaught, Ireland. Photo: Keith Ewing/Flickr
The east of the River Shannon is still covered by a thick layer of glacial drift, but in many areas to the west of the Shannon the limestone is pure and the drift cover is thin. In these areas, rainfall disappears underground, flows through openings in the rock and then rises at distant springs. In winter, when the underground water table rises, and the underground routes are overburdened with water, rainwater will not drain into the ground but start pooling at the surface to create lakes, or turloughs.
Turloughs are very predictable. They appear and disappear in pretty much the same places year after year, although new turloughs sometimes appear where there weren’t any before. Depending on the amount of rainfall, turloughs can appear almost instantly, taking as little as an hour to form and disappear in an equally short time. Sometimes the water disappears through a hole on the floor of the depression, called swallow holes, which can be seen when the lake is dry.
A dry turlough near Gort, County Clare. Photo: Dr Mary Gillham Archive Project/Flickr
Most Turloughs have an inflow spring at one end, and a swallow hole somewhere else on the floor where water drains away. But some turloughs fill and empty through the same hole. These turloughs are filled by surface rivers and streams flowing into them as well as by water rising from underground. The water sinking in the swallow hole travels underground and can re-emerge kilometers away at yet another spring or turlough. Some turloughs can also be connected to the ocean and are affected by oceanic tides. The Caherglassaun Lough, situated 5 kilometers from Galway Bay, near Gort, can be seen to flood and empty again twice every 24 hours.
Most turloughs flood to a depth of about 2 meters but some are much deeper. Some of the turloughs near Gort reach about 5 meters deep in midwinter.
Although turloughs are seasonal, they support a variety of animal and plant life. Frogs and newts commonly use turloughs to spawn. Some fish species such as the stickleback live in turloughs while it is wet, and retreat into underground cracks in the rock when waters are low. Some troughs have water fleas and fairy shrimps found nowhere else in Ireland. The country’s largest turlough, Rahasane in Galway, at 2.5 square kilometer is one of the most important breeding ground for birds. It is famous for its greater white-fronted geese, whooper swans, wigeon, teal, and many waders in winter.
Ballinacourty turlough, January 2005.
Ballinacourty turlough, May 2005.
During summer, when the turloughs dry up, they become great grazing ground for cattle, sheep and horses, partly because of the annual deposition of lime-rich silt. Many farmers have sought to drain turloughs so that the land can be used throughout the year. At least a third of the turloughs in Ireland have already been drained by digging channels and more are being drained each year. But the benefits are not always that great. Many farmers have discovered that seasonal flooding is, in fact, necessary to keep the land fertile, without which farmers have to use fertilizer. The poorly developed soil is also not able to withstand the presence of animals through the winter.
Related: Loughareema–The Lake That Randomly Vanishes
Researchers consider turloughs as one of “the most threatened and fragile habitats in Europe”. Others complain that the conservation status of turloughs in Ireland is “unfavourable”. In one report, prepared by the Trinity College Dublin, and funded by the National Parks & Wildlife Service, the researchers wrote:
Efforts should be made to improve the conservation status of turloughs. For those already considered to be in favourable conservation status, this should involve on-going monitoring of the biological communities and the pressures, particularly phosphorus in floodwater. For other turloughs, more active conservation is required; some trial restorations could be attempted to improve their ecological status, mainly through grazing management and control of source inputs of nutrients. … Effective conservation of turloughs can be delivered while also ensuring that the livelihoods, land and property of local landowners are maintained.
References:
# Waldren, S. Turlough Hydrology, Ecology and Conservation, https://www.npws.ie/sites/default/files/publications/pdf/Project_Report_consolidated_all.pdf
# Turloughs – Ireland’s unique wetland habitat, https://www.researchgate.net/publication/223829810_Turloughs_-_Ireland's_unique_wetland_habitat
# Wikipedia, https://en.wikipedia.org/wiki/Turlough_(lake)
They call them mud puddles in Mississippi
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