In Venezuela, in Monagas State, to be more specific, there is a kind of rivers called "Morichales" which have as a striking characteristic a dark color in their waters. It acquires that color because the moriche palm grows in its surroundings and the roots of this palm transfer tannic acids to the water, causing that dark color similar to tea. But they are still suitable for swimming in them, although their waters are usually colder than other rivers. I assure you it is a good experience to bathe in them ♥☺