The Bondenau and the Kielstau flow together in the approx. 5 hectare Treßsee. The outflow of the lake then becomes the Treene, which flows into the Eider near Friedrichstadt. The lowlands surrounding the lake are grazed by sheep and robust cattle all year round, so that the landscape remains open and not overgrown with bushes. North of the lake is an extensive inland dune area with interspersed ponds and moor ponds. Here is the habitat of many rare insects such as dragonflies, wild bees, wasp species, grasshoppers, grass snakes and adders. The 10-hectare area was placed under nature protection as early as 1937. This nature reserve is also part of the approx. 20 km² nature conservation project "Upper Treene Landscape".
Originally, the lake was once much larger. The inhabitants of the village of Großsolt once lived on the shores of this lake from fishing, as the lake had a rich fish population. Influences from sewage and agriculture, which are brought in by the Bondenau and Kielstau, have led, among other things, to the lake becoming more and more silted up. But also an increasing deposit of soil erosion sediments in the lake in the past centuries contributed to the fact that the lake has lost much of its water surface.