Hydrology of the Benninger fen

The Benninger fen is situated in the dry valley of Memmingen. During the last ice age the river of Iller deposited large alluvial terraces of gravel in a melt water lead in this region.
Because of the highly water-permeable gravel strata there is no autochthonous surface draining off throughout the whole valley (dry valley). The two streams leading through the valley rise outside the valley, without other tributaries in the region. As the gravel has been deposited upon almost water-impermeable molasse strata, it serves as ground water lead to the dry valley which, coming from the South, ends in the region of Memmingen. In the area of the Benninger fen the dry valley of Memmingen narrows (bottleneck), at the same time the terrain is a little bit deeper in this region. For this reason large quantities of ground water come to the surface in the Benninger fen.

Because of the compound of the gravel the ground water is highly calcareous. At the outlet of the springs the solved lime is precipitated by the release of CO2 out of the carbonic acid supersaturated water and by the assimilation activities of cyanelles and mosses in form of tufa deposits. Partly large deposits upon the gravel strata have been formed since the last ice age. Because of a vast number of anthropogenic changements in the region of the groundwater outlet the hydrologic conditions of the Benniner fen deteriorated considerably.

The most important reasons for this are above all:

  • Cultivation of the South and the Southwest and the drainage involved
  • Drainage of agricultural acreage
  • Diversion and piping of surface waters
  • (Active and passive) conduit of ground water through the canalization
  • Taking of drinking water
  • Pollution level and eutrophication

Within the scope of the LIFE project calculated measures should help to raise the discharge from the springs of this region as a whole and by this the seasonal fluctuations should also be reinforced.