[temp-dude-87844]

Incredibly, this isn't actually true, though it's tempting to think so from looking at the shapes on the map.

The border actually follows the Lusatian Neisse, and then the Oder river, except in the vicinity of the port city of Szczecin; this is the border set by the Allies at the Potsdam Conference after the end of World War 2.

The mine on the eastern side of the river became Poland, but it was still supplying a power plant on the west side of the river in East Germany [1]. Later, in 1962, Poland built its own power plant [2].

The Soviets and the Polish wanted to push this border as far west as possible, while the western allies wanted to set it was far northeast as possible to more closely approximate the German-Polish ethnic divide at the time. Various compromise options were proposed, following various rivers, but in the end the western allies agreed to the Soviet proposal. Germans in these territories gained by Poland were expelled. (Meanwhile, the Soviets took the eastern half of previous Poland and made it [3] part of the Soviet Union, specifically the constituent republics of Belarus, Ukraine, and Lithuania... it was a messy time.)

[1] https://de.wikipedia.org/wiki/Kraftwerk_Hirschfelde [2] https://en.wikipedia.org/wiki/Tur%C3%B3w_Power_Station [3] https://en.wikipedia.org/wiki/Kresy

[enviclash]

The book "Diplomacy" has a precious chapter about two Oder rivers and Stalin fooling the West by picking the most western Oder for the German-Polish border, where nowadays a lot of people has multiple heritages.