Karst Lake

Seasonal Lake - Turlough - Ephemeral Lake

Karst lakes are lakes in karst areas. Since karst areas have no surface drainage, only underground drainage, the existence of lakes is obviously based on their connection to the karst groundwater.

Schmiecher See,Schwäbische Alb, Deutschland.
Swallow holes at the lake floor, Cerknisko Jezero, Slovenia.
Swallow holes at the lake floor, Cerknisko Jezero, Slovenia.

Karst lakes look like any other lake, on the first glimpse. But often there is something special with those lakes. There may be no visible stream flowing in, or none flowing off, sometimes there is neither. Those lakes tend to grow and shrink, the water level rises in spring, when the snow melts or after heavy rains. The level falls in dry periods, some of those lakes disappear completely. Then they are called seasonal lakes, as they only exist in some seasons.

Blind lake a lake that has no surface drainage. Drainage occurs either through karst, through boulders or pore space, or through evaporation.
Karst lake a lake that is connected to the karst groundwater and has no surface inflows or outflows.
Endorheic lake a lake without any drainage into which a watercourse flows. They are typical of arid areas and become saline over time.
Perennial lakes from Latin perennis=all year round, they carry water all year round. Few karstic lakes are perennial and few perennial lakes are karstic lakes.
Intermittent lakes fall partially dry, i.e. they are intermittent lakes and intermittent dry land. According to the rhythm, the following three types are distinguished:
Seasonal lakes are all lakes that are seasonal, i.e. exist in certain seasons and not in others, thus the seasonal change is essential.
Ephemeral lakes are short-lived lakes or ponds that dry up again after a short flooding phase that usually lasts only a few days.
Disappearing lakes are just the opposite, they are lakes most of the time, but disappear irregularly and rather rarely for a short time.

The karst water level depends on the inflow by precipitation and the outflow at springs on the edge of the karst area. If the throughput, determined by the size of the caves that are conduits, is greater than the inflow, the water flows away immediately. The karst water level rises with the distance from the source, the gradient depends on the flow resistance of the cave system. A water level that rises from the source is formed, which we will call in the following normal level of the karst water. If the rainfall is higher than the flow, the drainage becomes constant and the excess water accumulates above the narrow point and the karst water level rises. When the rainfall ends, the flow rate remains the same until the accumulated amount of water has drained off and the previous equilibrium has been restored.

Karst lakes tend to grow and shrink, the water level rises in spring when the snow melts or after heavy rains. In dry periods the water level sinks and many of these lakes disappear completely. The lake is fed by springs below the water level and emptied by ponors, also below the water level. Often times, depending on the season, the same cave serves as a spring and ponore, which is then referred to as the Estavelle. If the lake is empty, the drainage channels and the ponors can easily be viewed. Often it is even possible to penetrate a little bit into the cave under the ponor.

The lake is the karst water level, the water level in the lake rises and falls with the groundwater level. This is simply a result of the physical law of the communicating vessels. If the lake is well above normal, it will obviously only be filled when the karst water level rises sharply. As soon as it sinks again, it dries out again, one speaks of an episodic lake . If it is only slightly above the normal level of the karst water level, in the range of the seasonal fluctuations, it will fill and empty in most years, except perhaps in particularly dry or particularly wet years. Then one speaks of a seasonal lake , the most common form of karst lake. If the lake is in the normal range, it is filled all year round, only the lake level fluctuates.

In English, the word turlough is often used for karst lake. This name is Gaelic and describes the typical karst lakes of Ireland, especially in Galway. Turlough is actually just the Gaelic translation of karst lake, tuar lough means dry lake. This term also found its way into the scientific literature (Waltham & Lowe: Dictionary of the Karst, p. 38). Although numerous sources state that turloughs only exist in Ireland and Wales, this is obviously nonsense. This is simply because karst lakes are called karst lakes and not turloughs where the inhabitants do not speak Gaelic.

Turloughs are seasonal lakes which are rather common on the Carboniferous Limestone karst areas of central and western Ireland, where they frequently occupy glacially formed depressions. They usually fill via springs in the lake bed between autumn and spring, and drain in summer via swallow holes or estavelles. Another definition of turloughs by Coxon (1987) tells three main criteria for distinguishing turloughs from other seasonal lakes:

  1. Seasonal flooding to a depth in excess of 0.5 m for part of the year and a dry floor (apart from small residual pools) for part of the year.
  2. Recharge via ephemeral springs or estavelles.
  3. Emptying to the groundwater-table via swallets or estavelles, with no surface stream outlet.

In many cases it is quite difficult to decide whether a lake is a true karst lake. The geological survey is very difficult and opinions differ on some lakes. Of course, the presence of karst around and under the lake is required. Furthermore, the strong fluctuation of the lake level is a good indication.

And finally there is a single "disappearing" lake on earth, Lough Funshinagh in Roscommon, Ireland, which disappears in minutes and reappears a few days later, again rather fast. And this happens only rarely, probably 20 or more years between two events. We are convinced it's not the only one, but it's the only one we know of and listed it on showcaves.com.