What river forms the border between china and the korean peninsula

Amnok / Yalu

Explanation:

The 493 miles (795 km) Amnok/Yalu river forms most the border between China and North Korea, on the Western side of the border.  The rest, to the East, is formed by a shorter river named Tumen,

The name Yalu is used by the Chinese people, while the people from North Korea use Amnok or Amrok. It's a very shallow river, the deepest point is about 8 feet (2.5 m), but it has been used as natural border for centuries.