Black tea: why are the natural pools of the Roncador River black?

Dark, almost black water is most often found in rivers flowing through tropical rainforests. Most often, "black" reservoirs can be found in the Amazon basin and in South America. What explains this color of river waters?

It's all about a powerful vegetable filter in the area of water runoff. Nearby forests supply the soil in the river basin area with fallen leaves, branches, bark, and seeds.

Accumulating for years (and even centuries) and rotting, they secrete tannins and humic acids, creating a soil with a special composition. As a result, the water, passing through a natural vegetable filter, becomes soft, acquires increased acidity and a dark, passable color for strongly brewed tea.