The Bondenau and the Kielstau flow together in the approximately 5 hectare Lake Treß. From the outflow of the lake then the Treene, which opens at Friedrichstadt in the Eider. The lowland surrounding the lake is grazed all year by sheep and stable cattle, so that the landscape remains open and does not fade. North of the lake is an extensive inland dune area with interspersed ponds and bog pools. Here is the habitat of many rare insects such as dragonflies, wild bees, wasps, grasshoppers, grass snake and common viper. The 10 ha area was already protected in 1937. Also this NSG belongs to the approximately 20 km² large nature conservation project "Upper Treen landscape".
Originally the lake was much bigger once. The inhabitants of the village of Grosssolt once lived on the shores of this lake from fishing, as the lake has a rich fish population, u.a. Salmon, showed up. Influences from sewage and agriculture, which are registered by the Bondenau or Kielstau, have u.a. led to the lake becoming increasingly silted up. However, the increasing deposition of soil erosion sediments in the lake over the past centuries also contributed to the lake losing much of its water surface.