Sand bank, Venezuela

The 425-mile-long (690 km) Río Caroni flows northward through the Venezuelan state of Bolívar (commonly called Guayana), descending in a series of falls and meeting vast sandbanks on its way. The Caroni, along with the other waterways that cross Guayana, is rich in alkaloids and tannins from the degradation of dense forest vegetation. They are therefore grouped together under the general category of “black” rivers, as opposed to the “white” rivers that descend from the heights of the Andes carrying mud and silt; the waters of the black rivers are dark but clear, whereas the white rivers are murky. Before ending its course in the Orinoco River, the Caroni powers the hydroelectric Guri Dam (in service since 1986), which is one of the most powerful in the world. However, the major drought in Venezuela between 2001 and 2004 has considerably affected the use of dams. Hydroelectric dams produce 19% of electricity worldwide.