With respect to major rivers the mekong is to vietnam as that irrawady is to

The Mekong River is to Vietnam as the Irrawaddy River is to Burma. Burma (also known as Myanmar) is a country in Southeast Asia. The Irrawadday River flows in a relative north to south manner through the nation and empties into Andaman Sea, an offshoot of the Indian Ocean.

Answer they are produced by collision of tectonic plates;