Turlough (lake)

Pant y Llyn turlough, South Wales

Turloughs are mostly found on the central lowlands west of the Shannon, in counties Galway, Clare, Mayo, and Roscommon, although a few are also found elsewhere, e.g. in Limerick, Sligo, Longford, and Cork.

Only three turloughs have been identified in Northern Ireland, namely Roosky, Green, and Fardrum Loughs located near Ely Lodge Forest in County Fermanagh.[4] These constitute the most northerly turloughs in Ireland and have been collectively designated a Ramsar site[5] and an Area of Special Scientific Interest.[6] There is one turlough in South Wales, Pant y Llyn.

Most turloughs flood in the autumn, usually sometime in October, and then dry up some time between April and July. However, some turloughs in the Burren 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.

Some turloughs are affected by the oceanic tide: in the summer, Caherglassaun Lough, situated 5 kilometres from Galway Bay, can be seen to flood and empty again twice every 24 hours. Most turloughs flood to a depth of about 2 metres (6.6 ft) but some are much deeper: for example, some of the turloughs near Gort reach about 5 metres (16 ft) deep in midwinter. Turloughs are variable in size: the largest turlough in Ireland, Rahasane turlough, which lies to the west of Craughwell in County Galway, covers about 2.5 square kilometres (1 sq mi).

All of the turloughs are found in limestone 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 great thicknesses of glacial drift deposited during the last glacial period, but in many areas to the west of the Shannon 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 distant 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 increases, 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.

Many of the rivers seen in these areas today are largely artificial, constructed by drainage engineers from the nineteenth century up to the present day, often linking a series of turloughs. For example, much of the River Clare is artificial, and the middle section of its course used to be a huge turlough, the largest in Ireland at 6.5 square kilometres (3 sq mi).

Turloughs usually fill and empty at particular places on the floor of the depression; 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. Some turloughs have a spring at one place and a sinkhole (also known as a "swallet") somewhere else on the floor where water drains away, but many turloughs fill and empty through the same hole. A few 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 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 volumetric flow rate can be quite rapid: water from the turlough may flow underground to a spring at a rate of 100 metres per hour (330 ft/h) 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 mineral deposits on the inside of kettles) 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. 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. In the Burren, the high-water mark is often shown by the shrubby cinquefoil (Dasiphora fruticosa) with its attractive yellow flowers, and meadow rue. Just below the high-water mark, dog violets are abundant and in some turloughs there may be a dense sward of the rare sky-blue turlough violet about one metre further down. Other characteristic plants of turlough sides include marsh orchids and speedwell. About halfway down the sides, and across the bottom of shallow turloughs, silverweed (Argentina anserina) may blanket almost all other plants.

If the turlough has a marshy zone near the swallow hole there may be mint, watercress, 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 fleas and fairy shrimps, some unknown elsewhere in Ireland. These hatch and grow fast, finding safety in the warm fish-free waters. Flatworms and snails are also often abundant; these pass the dry periods in spring mouths or marshy areas.

Rahasane turlough in Galway has been described as the most important turlough in Ireland for birdlife.[7] It is famous for its greater white-fronted geese, whooper swans, wigeon, teal, and many waders in winter.[8] Both Rahasane turlough and Coole Park are Important Bird Areas.

Turloughs provide good summer grazing for cattle, sheep and horses, partly because of the annual deposition of lime-rich silt. However, for many years, farmers have seen the winter flooding as a waste of potential and they have attempted to find some means of draining the turloughs so that they can be used all year round. This has usually been achieved by digging an artificial channel through the turlough, capable of carrying away any water entering the turlough from surface or groundwater – such channels have often been constructed as part of major arterial drainage schemes.

At least a third of the turloughs in Ireland have already been drained and more are being drained each year. This has very serious environmental consequences, because the unique flora and fauna of the turlough cannot survive in the absence of seasonal flooding. Even for farmers, 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.

• Mullygollan Turlough, a SAC
• Karst – Topography from dissolved soluble rocks
• Subterranean river – A river that runs wholly or partly beneath the ground surface

• National Parks and Wildlife Service (c. 1980) Wetlands Discovered. (Duchas, National Parks and Wildlife Service)
• O'Gorman, Fergus (ed) ; Gerrit van Gelderen, Eamon de Buitlear and Richard Mills (ill.) (1979) The Irish Wildlife Book, Irish Wildlife Publications, Dublin. (pages 58–60). ASIN B001F6YLO4, John Coughlan (pub.), revised edition (January 1, 1980).
• Praeger, R. Lloyd (1950) The Natural History of Ireland, William Collins Ltd.; 1st edition, Ireland. ASIN B0000CHMCL
• Webb, D.A. & Scannell, M. (1983) Flora of Connemara and the Burren. Royal Dublin Society, Cambridge University Press. ISBN 978-0521233958.
• F.H. O’Neill & J.R. Martin (2015) Summary of findings from the "Survey of Potential Turloughs 2015", volumes I & II.

Look up Turlough in Wiktionary, the free dictionary.

• Turloughs
• Survey of potential turloughs, 2015 NPWS, as of January 20, 2016.
• The Turlough lake in Westmeath
• Mullygollan Turlough SAC
• Turloughs in general, Carran Turlough by Darrell Watts
• ENFO: Ireland's public environmental and sustainable development information service
• Flora of the turloughs Clare Library