The Piedra (Spanish: Río Piedra) is a river in the Iberian System area, Spain. It is a tributary of the river Jalón.
The waters have a high concentration of calcium carbonate which is deposited on plant life near the source, giving the river its name Río Piedra, which means 'stone river' in English.
The population of the endangered European freshwater crayfish in the river and its tributaries has practically disappeared owing to the introduction of the North American signal crayfish (Pacifastacus leniusculus).

Quelle: Wikipedia