The so-called "outlet channel" of the Great Stechlin Lake was created during the construction of the Rheinsberg nuclear power plant. It ensured that the cooling water returned to the lake.
The water in the Stechlin Lake is not only very clean, but also very cold due to its great depth. This was used to cool the GDR's first nuclear power plant. "The cooling water was taken from the Nehmitz Lake and fed into the Great Stechlin Lake through the nuclear power plant's outlet channel. Both lakes are connected by the Polzow Canal, so that a cycle existed."
The nuclear power plant was in operation between 1966 and 1990 and has been dismantled since then.
"According to an earlier concept, the main dismantling and decommissioning work on the nuclear power plant was to be completed by 2014, with the building decontamination taking until 2018. The subsequent fifty-year cooldown period for the building should therefore have ended in 2069; after that, the main buildings could have been demolished. However, this plan had to be abandoned due to safety concerns raised by the responsible supervisory authority in the state of Brandenburg.
The decontamination of the buildings is turning out to be much more complex and time-consuming than long expected. The operator EWN now assumes that the dismantling will take until 2035 or longer. At the moment (as of June 2020) there is no approved concept that would allow the buildings to be demolished.”
Source: Wikipedia “Rheinsberg Nuclear Power Plant”