 The valleys of Etla and Ocotlán are traversed from north-west to south by the Atoyac River which provides water for a small strip of land bordering the river, when it periodically floods.To provide water for crops elsewhere in the valley away from the river, the Zapotecs used canal irrigation.By using water from small streams, the Zapotecs were able to bring water to Monte Albán, situated 400 meters above the valley floor.