What is the chief river of Southeast Asia, which forms part of the Laos-Thailand border?

The chief river of Southeast Asia that forms part of the Laos-Thailand border is the Mekong River. This river is one of the longest rivers in the world, flowing through multiple countries including China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. Its significance is not only geographical but also cultural and economic, as it supports millions of people who depend on its waters for fishing, agriculture, and transportation.

The Mekong River plays a crucial role in Southeast Asia's ecology and economy. It serves as a vital water source for agriculture, particularly rice farming, which is fundamental to the region's food supply. Additionally, the river is known for its rich biodiversity and the unique ecosystems it supports, further emphasizing its importance to the environment and local communities.

In contrast, the Ganges River is primarily associated with India and is sacred in Hinduism, while the Irrawaddy and Indus rivers are important to Myanmar and Pakistan, respectively, but do not share the same geographical significance in relation to Laos and Thailand.