Final answer:
The Oder River forms the German-Polish border and, together with the Neisse River, creates the Oder-Neisse Line, which was established post-World War II.
Step-by-step explanation:
The river that forms the German-Polish border as part of the namesake line with Neisse is the Oder River. The Oder-Neisse Line was established after World War II, creating the border between Germany and Poland.
This boundary was confirmed at the Potsdam Conference and has been a significant part of European geography and political history. The Oder River begins in the Czech Republic and flows through Poland and along the border of Germany before emptying into the Baltic Sea.