The Sur de Puebla is in the southwest and accounts for 26% of the state. Other southern subregions include the Sierras y Valles Guerrerenses, the Sierras Centrales de Oaxaca and the Sierras Orientales. Together, they account for about 15% of the state. The hydrology of Puebla is formed by three major river systems. One is based on the Atoyac River, which originates with the melting runoff of the Halos, Telapón and Papagayo mountains along with those from the Iztaccihuatl volcano and waters from the Zahuapan River, which enters from Tlaxcala. This river receives further water from tributaries such as the Acateno, Atila, Amacuzac, Molinos and Cohetzala. The river has one major dam called Valsequllo or Manuel Avila Camacho. This river eventually flows west to the Pacific Ocean. The next system empties into the Gulf of Mexico and consists of the Pantepec, Cazones, Necaxa, Laxaxalpan, San Pedro/Zun, Zempoala, Apulco, Cedro Viejo, Salteros, Martínez de la Torre and other rivers on the east side of the state. This system has two major dams called the Necaxa and Mazatepec. The third is based on the large number of small lakes fresh water springs as well as some volcanically heated springs. The best known of these include Chignahuapan, Agua Azúl, Amalucan, Cisnaqullas, Garcicrespo, Almoloya and Rancho Colorado. Lakes include Chapulco, San Bernardino, Lagunas Epatlán, Ayutla, Almoloyan, Alchichica, Pahuatlán, Las Minas, Aljojuca and Tecuitlapa. Puebla has many different climates owing to its range of altitudes. It has an average temperature of but this varies greatly locally. There is a rainy season from May until October with an overall precipitation of . The state has eleven different climate zones, but five predominate. The centre and south of the state has a temperate and semi-moist climate, with an average temperature of and of rainfall. The southwest has a warm to hot and semi-moist climate with of precipitation and average temperature.