The bathing lake of Ferlach is situated in Rosental in the municipality of Ferlach. It is embedded in the gravel grounds, which were shifted into the valley by the brooks that have its source in the Karawanken. They chiefly consist of lime gravel and are partly put on mighty 30 to 40 m high conglomerates of the hardened fluvial gravel grounds of the Drau (HANSELY & ANDERLE, 1973). The groundwater level is between 0 and 4 m.
Because of the predominance of porous grounds (gravel- and sand sedimentation) the restoration of the ground water is very big. The bathing lake of Ferlach has developed in the process of the construction of the power station Ferlach- Maria Rain in the year 1976. The lake has no inflow, it is fed by the ground water. A surface runoff is flowing into the little Drau.
The groundwater lake is very popular for bathing and characterized by its flat shore zone. It is surrounded by the alder forests of the former pasture wood of the Drau and in the north it is bordered by the dams of the Drau barrier.
Set in of signal crayfish. The bathing lake of Ferlach houses a population of signal crayfish (Pacifastacus leniusculus). This North –American river crayfish was settled down by a calculated setting- in into this former dredged lake in the 1970s. Since then a dense population of crayfish has developed.