Answer: Mainland Southeast Asia is drained by five major river systems, which from west to east are the Irrawaddy, Salween, Chao Phraya, Mekong, and Red rivers. The three largest systems—the Irrawaddy, Salween, and Mekong—have their origins in the Plateau of Tibet. These three rivers are somewhat atypical: their middle and upper drainage basins are not broad catchment areas with many small tributaries feeding larger ones but rather consist of a few streams confined to narrow, closely spaced valleys.
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