The St. Johanner Badesee (or Weizelsdorfer Badesee) is situated between the two villages Feistritz and Ferlach. Geologically this region is characterized by late- and after- Ice Age deposition. Largely the sediments are covered by terrace gravel, which is swept down to the valley by the brooks coming out of the Karawanken.
Quick water renewal. The ground water substance in general is between 7 and 40 m deep. The renewal of this water is very high, which is a consequence of the dominating of weakly permeable ground- , gravel- and sand deposition (Hansen & Anderle, 1973). The lake was formed by taking gravel out of the ground water region from 1980 to 1991. There is no surface inflow or runoff, the lake is only fed by the ground water.
The catchment area is used agriculturally, with 67 % green-land and 33 % farmland. The steep shore slopes were mostly left to a natural succession. The eastern bank is planted with suitable alder. Planting not suitable larch and spruce on the eastern shore caused high dry- damages on the wood. The steep under water slopes only enable the development of SCHWIMMBLATT and reed companies on a narrow shore belt.
Intense usage for bathing. The lake shows intense usage for bathing activities. On the flat south – west bank there is the public bath of the municipality Feistritz, it was opened in 1989.
Ground water study. By order of the Ministries of Environment and Agriculture they conducted a comprehensive study “dredge lakes and their interrelation with ground water”. During this research project they examined the hydrological characteristics, the chemistry and the biology of the St. Johanner Badesee as well as it surrounding ground water field.
Fish population. Above all the population of sun bass is very high, which probably is the reason for the decrease of the natural growing of carp. Lake trout, rainbow trout and brool trout are stocked for angling, a natural growing of these species is not likely.
Because of the little amount of nutrients in the lake the growth and condition of fish is rather small. The bleak show a strong attack of Riemenwurm (Ligula intestinalis). This parasite can reach 20 cm and more and comes into the abdominal cavities of the fish dragged in by water birds and small crayfish. There they stop the development of sexual organs (parasite castration).
The usage for sport fishing is managed by the Fishing Association “Seeforelle”, which also are responsible for the cultivation of the lake and for the planting on the shore embankments and shore zones.