Karst Lake

Seasonal Lake - Turlough - Ephemeral Lake

Schmiecher See, Schwäbische Alb, Deutschland.

Karst lakes look like any other lake ... on the first glimpse. But often there is something special with those lakes.

There may be no visble 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.

The explanation of all this strange behavour are caves. The lake is fed by springs below the water level and emptied by swallow holes, also below the water level. Often the same cave works as spring and swallow hole, depending on the season.

Another way to interpret this situation, is to say, the lake is a part of the ground water. The surface of the lake is the water table. When the water table in the hills around a depression rises above the ground of the depression, it is filled with water. When the ground water lowers, the lake dries up.

It is pretty difficult to decide, if a lake is true karst lake. The geological examination is very difficult and for some lakes the oppinions differ. Missing rivers in and out of the lake, are visible on a map, and give a first hint. Not all seasonal lakes are karst lakes but this is a second hint. And of course the existence of karst around the lake is necessary.

In English often the word turlough is used for karst lake. This name is Gaelic, and describes the typical karst lakes of Ireland, especially Galway. Tuar loch translates dry lake. It also found its way into the scientific literature (Waltham & Lowe: Dictionary of Karst, p 38).

Turloughs are seasonal lakes that occur on the Carboniferous Limestone in Ireland, where they frequetly occupy glacially excavated depressions. They usually fill via inputs of underground water between autumn and spring, and drain in summer via swallow holes or estavelles.

A relation of the different terms describing the geologic feature is a bit tricky. Seasonal lakes are all lakes which are seasonal, although they my have no connection to karst in any way. A subset of all seasonal lakes are the karst lakes, which are seasonal lakes in an karst environment, seasonal because of karst related processes.

Turloughs are again a subset of karst lakes, as this term is mainly used for Irish karst lakes. 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.

Ephemeral lakes are again a subset of seasonal lakes. But the name only suggests a short period of flooding, sometimes only for a few days in spring. Many karst lakes are ephemeral lakes, but not all ephemeral lakes are karst lakes.