"In front of San Fernando de Atabapo, three rivers converge:
the crystalline waters of the Orinoco, from the high mountains;
the sandy waters of the largest tributary of the Orinoco,
the Guaviare, which crosses the Colombian plains; and the fresh,
clean waters of the Atabapo, which appear black due to the tannin
from surrounding vegetation."
Orinoco Online

Orinoco River
Orinoco River Resources
Venezuela

Map thanks to Expedia.com