The longest river in Southeast Asia, the Mekong runs for 2,600 miles from the Tibetan Plateau down to the South China Sea. Inhabitants of the Mekong’s thousands of islands depend on the river for their food and livelihoods, yet it’s also a promising source of clean energy.

The government of Laos’ recent decision to build a hydroelectric dam on the Mekong has raised concerns with residents and with environmentalists, who worry the dam will interrupt fish migration and damage the river’s ecology.