Dry Valley

A dry valley is a valley formed by water erosion, but without a permanent river, because surface water sinks into the permeable bedrock.

photography — When the Hungerbrunnen is dry, the Hungerbrunnen Valley is a dry valley. Schwäbische Alb, Deutschland.

Dry valleys, river valleys that do not have a river, are often mentioned as a characteristic of a karst area. They develop as river valleys at a time when the area still drains above ground. This can be due to a correspondingly high receiving watercourse as well as a lack of karstification and thus low permeability of the subsoil.

If the receiving watercourse, the draining river valley, deepens quickly and a side valley cannot keep up due to insufficient water volume, the groundwater level shifts lower and the valley dries out. Of course, this also requires a previous initial karstification in the area of the groundwater, which has already widened fissures and thus allows the water to penetrate the rock body. The water seeps away and forms a karst groundwater surface below the dry valley, which is an area of increased cave formation.

The process of falling dry is slow, and so various typical karst phenomena occur during the transition period. For example, a stream may flow through the upper part of the valley, which then seeps into swallow holes. Intermittent reactivation of the river is common. The karst water level has already been lowered, and the valley is dry, but during snow melt and after heavy rains, the karst water level rises, springs begin to flow again and the valley once again has a stream. When this phase is over and the karst water level remains permanently below the valley level, the valley is a dry valley.

However, real dry valleys can also be reactivated in special cases. During the ice age, the ground was frozen many metres deep. The summer was too short for the ground to thaw completely, so there was still a layer of ice under the thawed ground. This effect is called permafrost. The frozen ground is watertight because ice seals the cracks. While the karst body was sealed in this way, drainage had to take place above ground in a karst area! The accumulation of meltwater and rainwater on the soil results in solifluction (soil flow), whereby the soil is so heavily enriched with water that the mud flows down the slope. This process is triggered by solar radiation and the associated warming, so that it mainly occurs on south-facing slopes. The result is typical asymmetrical valley profiles that cannot be explained otherwise in a karst region.