Most Peruvian rivers originate in the peaks of the Andes and drain into one of three basins. Those that drain toward the Pacific Ocean are steep and short, flowing only intermittently. Tributaries of the Amazon River have a much larger flow, and are longer and less steep once they exit the "sierra". Rivers that drain into Lake Titicaca are generally short and have a large flow. Peru's longest rivers are the Ucayali, the Marañón, the Putumayo, the Yavarí, the Huallaga, the Urubamba, the Mantaro, and the Amazon.