An intermittent lake is a natural hydrological phenomenon consisting of an enclosed depression or flat plain (most commonly situated within a karst polje) that fills with water from surface and subterranean tributaries during periods of prolonged precipitation or snowmelt, and partially or completely dries up during dry spells. Unlike permanent lakes, it lacks a continuous surface outflow, draining predominantly via vertical infiltration into the karstified underground.

Hydrological Mechanism

The functioning of an intermittent lake depends directly on fluctuations in the karst water table and the conduit discharge capacity of the subterranean drainage network:

  • Filling (Inundation Phase): During prolonged rainfall or rapid snowmelt, underground karst conduits are unable to evacuate the influx of water fast enough. The karst water table rises hydrostatically, forcing water to surge to the surface through karst springs, vauclusian resurgences, and estavelles (reversible openings that act as ascending springs during flood conditions and as swallow holes during recession). The polje floor is thus transformed into an extensive lake.

  • Drainage (Drying Phase): As water inflow recedes and the regional karst groundwater table drops, water begins to drain vertically into the subsurface through sinkholes, swallow holes (ponors), caves, and bedrock fractures. The lake basin gradually empties, returning the floor to dry meadows, agricultural fields, or pastures.

Examples and Specific Forms

  • Lake Cerknica (Cerkniško jezero): The most famous and comprehensively researched intermittent lake in the world, covering up to 30 km² at maximum inundation, widely recognized as the historical cradle for studying this phenomenon.

  • Pivka Intermittent Lakes (Pivška presihajoča jezera): A network of 17 smaller seasonal lakes in the Upper Pivka Basin (such as Palško Lake and Petelinjsko Lake), which fill exclusively upon a pronounced hydrostatic rise of the karst water table through estavelles and bedrock fissures.

  • Floodplain Lakes with Intermittent Character: Certain flood-prone lowlands (such as the Ljubljana Marsh / Ljubljansko barje) do not exhibit typical vertical karst drainage, but retain standing water due to impermeable subsoil and constrained surface outflows. During extreme hydrological events, they temporarily assume the character of a vast shallow lake, albeit with a longer filling and drainage frequency.