The Natural Habitat of Piranhas

Piranhas only exist in nature in South America. And has never been introduced in any other place neither by accident nor deliberate.
The natural ranges of the Piranhas are (se map above) the Orinoco, the Guyana, the Amazon, the Rio Sao Francisco, the Rio Paraguay and the Rio Parana.

The majority of Piranhas live in the so called whitewater rivers. A whitewater river carries loamy water with a great deal of fine particles in suspension thus giving it a milky yellow-gray appearance.
Some Piranhas live in the poor blackwater rivers and some in clearwater rivers. But in these cases they live near where two rivers meet and white- or blackwater mix with whitewater.
In general Piranhas are amazingly tolerant to different water conditions. They live in rivers with a temperature of 24 - 30 degrees C (75 - 86 F).

Piranhas in the Ecosystem

Why are there only Piranhas in the South America? The most likely reason is a phenomenon called the Pororoca. This happening occurs only in the South American rivers due to their relief conditions. 3500 km from the mouth, the Amazon flows at an altitude of just 150 m. Compare this with the Congo that lies at an altitude of 400 m. The result is that the tide rises into the rivers, several hundred km. The masses of freshwater offer an increasing resistance to the saltwater until the compressed mass 'explode'. Thunderously the waters rise to a 6 to 8 meters high giant wave, that rolls towards the ocean. This is the Prororca!
The Prororca kills a large number of animals and leaves the waters filled with cadavers. Now the Piranhas come into the picture as the only fast-acting underwater health squad available. Without the Piranhas the amount of contagious diseases would be tremendous.
Of course Piranhas attack living creatures as well. Specially when hungry, for example if they have been trapped in a drying lake. Like hyenas, Piranhas sometimes hunt on their own and like them they not only feed on dead animals, but attack everything that is sick, weak or unfit to live long.