Which natural feature forms much of the borderbetween Laos and Thailand?

The natural feature that forms most of the border between Laos and Thailand is the Mekong River. Mekong River is by far the biggest river in southeast Asia and by far the most important one in the region. It is also a natural feature that has set the borders between the countries in this part of the world, including these two countries.

Explanation:

it was the first amendment