Why does stream water in the Ituri Forest often look like strong tea?

The stream water in the Ituri Forest appears the color of strong tea because it is steeped in the tannin-rich leaves that cover the forest floor.1

As rainwater flows through the forest, it leaches these phenolic compounds from the decaying organic matter. Tannins are naturally occurring substances found in the roots, wood, bark, and leaves of many woody plants, and they are responsible for the characteristic color and astringency associated with tea.2