The Mamoré is a large river in Bolivia, which unites with the Beni to form the Madeira, one of the largest tributaries of the Amazon. It rises on the northern slope of the Sierra de Cochabamba, east of the city of Cochabamba, and is known as the Chimoré down to its junction with the Chapare. Its larger tributaries are the Chapare, Secure, Apere and Yacuma from the west, and the Ichilo, Guapay, Ivari and Guapore from the east . Taking into account its length only, the Guapay should be considered the upper part of the Mamore; but it is shallow and obstructed, and carries a much smaller volume of water. The Guapore, or Itenez, also rivals the Mamore in length and volume, having its source in the Serra dos Parecis, Mato Grosso, Brazil, a few miles from streams flowing north-ward to the Tapajos and Amazon, and southward to the Paraguay and Paraná rivers. The Mamore is interrupted by rapids a few miles above its junction with the Beni, but a railway 300 km long has been undertaken from below the rapids of the Madeira. Above the rapids the river is navigable to Chimore, at the foot of the sierra, and most of its tributaries are navigable for long distances. Franz Keller (see References) gives the outflow of the Mamore at mean water level, and not including the Guapore, as 2,530 cub. in. per second, and the area of its drainage basin, also not including the Guapore, as 9,382 m².