Why does the upper Mekong River flow through a narrow valley?

The upper Mekong River flows through a narrow valley because it cuts deeply into the rugged landscape of the mountains and plateaus of southwestern China, specifically across the highlands of Tibet and Yunnan. As the river descends from the Plateau of Tibet, its streambed carves through this high-elevation, mountainous terrain, resulting in a long, narrow valley that comprises approximately one-fourth of the river's total drainage area.12