From its source in Colombia, the Río Negro flows about 1,400 miles to become one of the largest tributaries of the Amazon River at the Meeting of Waters in Manaus, Brazil. It is also the largest blackwater river in the world, its water stained by decomposing organic matter that seeps in from swamps. These slow-moving, dark waters beautifully reflect the colors of the sky, including the rainbows which form over the Amazon basin, as seen here. The river also plays a crucial role in the ecosystem, as the home of many plants and animals, including the critically endangered Brazilian barefaced tamarin, the giant river otter, and nearly 700 species of fish.