In a league of its own, the Mekong River (pictured above) is the undisputed king of Southeast Asia's river systems. Almost 3,000 miles long, the Mekong starts as a trickling glacial stream 17,000 feet high on Guosongmucha Mountain. From China's mountainous southern Qinghai province, the Mekong flows into Tibet and then returns to southern China before entering the Burma-Thailand-Laos tri-border region. With stretches of churning rapids, the Mekong flows through Laos and along the Thai-Lao border for hundreds of miles. After curving through eastern Cambodia, the Mekong cuts across southern Vietnam and empties through the Mekong Delta into the South China Sea.
The Mekong's northern rapids have historically formed a firm border between peoples, but the river is being partially subdued through blasting and damming. The river is legendary for its enormous fish, including carp and catfish that can grow to hundreds of pounds.
Starting on the Tibetan plateau near the Mekong headwaters, the Salween River plunges south on a 1,700-mile course that takes it across eastern Burma before emptying into the Andaman Sea. One of the world's longest undammed rivers, the Salween forms part of the UNESCO Three Parallel Rivers heritage site in Tibet and southern China.
West of the Mekong, the powerful Irrawaddy (pictured below) is Burma's largest and most important river, flowing over 1,300 miles from north to south before emptying into the Andaman. The sprawling Irrawaddy Delta includes multiple rivers itself and hosts the city of Yangon, Burma's largest, on its eastern end. The Irrawaddy dolphin traverses the delta upstream and downstream and is equally at home in freshwater and saltwater.
At over 200 miles from north to south, the Chao Phraya River (pictured below) flows through the middle of the capital Bangkok before emptying into the Gulf of Thailand. Thailand's most important river system, the Chao Phraya and its arborescent network of tributaries form a watershed encompassing one-third of the country's land area. The river also hosts some of Thailand's legendary floating markets.
The 300-mile Pa Sak River runs north to south, eventually flowing into the Chao Phraya and forming the easternmost extension of the larger river's watershed. The Pa Sak anchors a valley through Thailand's Phetchabun Mountains and flows through some of the country's most remarkable landscape.
The Red River (pictured below) begins in the Yunnan Province of south-central China and flows down into northwestern Vietnam. North of Hanoi, the Lo and Black rivers flow into the Red, and the system's joining with the regional Thai Binh River forms the Red River Delta area, which continues southwards to the Gulf of Tonkin coastline.
Further south, the Lam River enters north-central Vietnam from the northern Laotian mountains and follows a southeasterly course into the Gulf of Tonkin. The Lam is joined by many local tributaries and borders the Hong Linh Mountains.