A turlough, or turlach, is a type of disappearing lake found mostly in limestone areas of Ireland, west of the River Shannon. The name comes from the Irish “tuar”, meaning dry, with the suffix “lach”, meaning a place. The “lach” suffix is often mistakenly spelled and/or thought to refer to the word “loch”, the Irish, Scottish Gaelic and Scots word for lake. They are found in Irish karst (exposed limestone) areas.
The features are almost unique to Ireland, although there is one example in Great Britain in Llandeilo. They are of great interest to many scientists: geomorphologists are interested in how turloughs were formed, hydrologists try to explain what makes turloughs flood, botanists study the unusual vegetation which covers the turlough floor and zoologists study the animals associated with the turloughs.
While the turlough in Esker is relatively small it has the characteristics of the typical turlough. Most turloughs flood in the autumn, usually some time in October, and then dry up between April and July. However, some turloughs can flood at any time of year in a matter of a few hours after heavy rainfall and they may empty again a few days later. The Esker turlough routinely floods in both of these ways. The greater part of the west of Ireland is a limestone area and all of the turloughs are found in such areas. This is because limestone can be dissolved away by rainwater, which becomes mildly acidic by picking up carbon dioxide as it passes through the atmosphere. The cracks or joints in the rock become widened to such an extent that eventually all of the rain falling on the limestone disappears underground and the water moves through the rock openings ranging from cracks a few millimetres wide to large cave passages. The limestone is then said to be karstified.
To the east of the Shannon, the limestone is often covered by a great thickness of glacial drift deposited during the Ice Age but in many areas to the west of the Shannon, such as ours, where the limestone is pure and the drift cover is thin, there is no proper surface river network. In these areas, rainfall disappears underground, flows through openings in the rock and then rises at springs: large springs can be found to the west of the area, flowing into Lough Corrib and Galway Bay. In winter, when the underground water level ,or water table, rises, and when the underground flow rises, and when the underground flow routes to the springs are not capable of dealing with the amount of water entering them, groundwater may appear temporarily at the surface in the form of a turlough.
Turloughs usually fill and empty at particular places on the floor and sometimes an actual hole or passage is visible but more often a hollow with stones in the bottom is all that can be seen and it may not be easy to recognise when it is dry in midsummer. The Esker turlough had many such holes and hollows but over time some of these were closed, primarily for agricultural and safety reasons, and thus covering the natural structures of the turlough. Some turloughs have a spring at one place and a swallow hole, also known as a ‘swallet’, somewhere else on the floor where water drains away, but many turloughs fill and empty through the same hole. Its drainage nowadays depends on a man made waterway to take the water alongside the R339 and enter the Ballybaun River at Ballybaun and continue on it way to the Abbert River at Clooncurreen Bridge. This can cause flooding on the R339 at Esker and Garbally most likely due to the blocking of its natural swallow holes.
The water sinking in the swallow hole travels underground to re-emerge at a spring, which may be several kilometres away. In most rock types, groundwater flows very slowly (from just a few centimetres to a few metres per day), but in karstified limestone the flow rate can be quite rapid as water from the turlough may flow underground to a spring at a rate of 100 metres per hour or more.
Limestone is made up of the mineral calcium carbonate and as water passes through limestone, it dissolves the calcium carbonate – this is what makes hard water and causes furring on the inside of kettles and copper pipes, as the calcium carbonate comes out of solution when the water is heated. Something rather similar happens in turloughs – water that has picked up a lot of calcium carbonate during its underground travel rises in the turloughs and then some of the calcium carbonate comes out of solution and forms a white deposit. If a turlough has emptied recently, a whitish coating on the vegetation on the turlough floor may be visible. This is regularly visible at Garbally when the water empties out and the surface dries quickly. When water comes to the surface in a turlough, it loses carbon dioxide back to the atmosphere and to plants, which use it for photosynthesis, and this loss causes a deposit of calcium carbonate on the surface.
Sometimes a special whitish deposit with the appearance of sheets of paper is found in the turloughs when they dry up. This “algal paper” is made up of filaments of an alga that grows abundantly in warm weather and is then left to dry out in sheets when the turlough empties.
In drainage ditches in a turlough, or in holes made with a soil auger, one may find a white- or cream-coloured deposit beneath the vegetation cover, or beneath a layer of peat. This is often called “white marl”; again, it is made of calcium carbonate. About half of the turloughs contain marl: it was deposited at a time several thousand years ago when these turloughs were not seasonal lakes but were flooded all year round.
Most turloughs have a springy, short-cropped turf of grasses, sedges and herbs. Other characteristic plants of turlough sides include Marsh orchids and speedwell. About halfway down the sides, and across the bottom of shallow turloughs, silverweed (Potentilla anserina) may blanket almost all other plants.
If the turlough has a marshy zone near the swallow hole there may be mint, water cress, pondweeds, aquatic buttercups and knotgrass living a semiterrestrial existence. But most swallow holes when dry are represented by a jumble of rocks, clothed with blackish and dried aquatic mosses (Cinclidotus, the turlough moss, and Fontinalis, usually found in streams).
Many people think that turloughs have no animal life. However, frogs and newts may spawn there and sticklebacks may survive in the larger turloughs, retreating into underground cracks in the rock when waters are low. Shrimps and water lice do the same and where fish are absent there may be a rich fauna of delicate water fleasand fairy shrimps, some unknown elsewhere in Ireland. These hatch and grow fast, finding safety in the warm fishless waters. Flatworms and snails are also often abundant; these pass the dry periods in spring mouths or marshy areas.
Right Of Ways
Up until the 70’s, local farmers had watering rights here for animals. The right of way still exists but since the building of the water scheme it is little used nowadays. One part of the Turlough, on the Skehana side, usually has a flood or pond all year round. While this pond rises in winter and falls in summer I have never seen it to empty completely. When turloughs retain some water all year, they may be important bird haunts. In the early 60’s the turlough at Garbally was a favourite spot for duck shooting. Turloughs, in general, provide good summer grazing for cattle, sheep and horses, partly because of the annual deposition of lime-rich silt.
At least a third of the turloughs in Ireland have already been drained and more are being drained each year. This has very serious consequences from the point of view of the environmentalist – the unique flora and fauna of the turlough cannot survive in the absence of seasonal flooding. Even for the farmer, the benefits are not always as great as anticipated – the stopping of the annual limy silt deposition means that the soil may become impoverished and fertilisers must be used. Also, the poorly developed and delicate soil may not be able to withstand the presence of animals through the winter.