The Valley of Puebla is an expanse of plains crossed by a number of small rivers, streams and arroyos, with the most significant being the Atoyac River. The Atoyac River has its beginning in the runoff of both the Iztaccíhuatl and Popocatepetl volcancos. Cholula is located in the river's upper basin. This river passes through the San Pedro area and forms part of the border between San Andrés and the city of Puebla. There are also tributaries such as the Ametlapanapa, Zapatero and the Rabanillo and number of fresh water springs and seasonal arroyos.