Lake Chapala (, ) is Mexico's largest freshwater lake. It lies in the municipalities of Ocotlán, Chapala, Jocotepec, Poncitlán, and Jamay, in Jalisco, and in Venustiano Carranza and Cojumatlán de Régules, in Michoacán. Geography Geographic features It is located at , southeast of Guadalajara, Jalisco, and is situated on the border between the states of Jalisco and Michoacán, at 1,524 metres (5000 feet) above sea level. Its approximate dimensions are from east to west and averages 12.5 km (7.8 miles) from north to south, and covers an approximate area of . It is a shallow lake, with a mean depth of and a maximum of . It is fed by the Río Lerma, Río Zula, Río Huaracha, and Río Duero rivers, and drained by the Rio Grande de Santiago. The water then would normally flow northwest into the Pacific Ocean. No water has flowed out of the lake in over 30 years due to a fall in the supply of incoming water from the Lerma River. Islands The lake contains three small islands: Isla de los Alacranes (most visible from the town of Chapala), Isla Mezcala (the largest island, featuring an old Spanish fort and an old Mexican prison), and a third very small island next to Isla Mezcala called La Isla Menor. Mountains There are many mountains and sierras that surround Lake Chapala: Cerro Viejo-Chupinaya-Los Sabinos (Sierra San Juan Cosalá), Cerro San Miguel-Chiquihuitillo, Cerro San Bartolo-Los Ocotes, Cerro Gomeño, Cerro de García-Del Picacho-El Gachupín (Sierra del Tigre). Ecology The city of Guadalajara has relied on Lake Chapala as a principal source of water since the 1950s. Shortly after, a few consecutive years of poor rainfall dramatically decreased the water level of the lake. The level rebounded until 1979, when Lake Chapala's water level began rapidly decreasing due to increases in urban water consumption. Erosion due to deforestation along the lake as well as the Lerma River has led to increased sedimentation of the lake, also contributing to loss of lake depth. The shrinking depth has also raised the lake's average temperature, resulting in increased evaporation. Simultaneously, the waters of Lake Chapala are polluted by municipal, industrial and agricultural wastes, coming primarily from the Lerma River. The increased presence of nutrients from the pollution combined with the warmer water has been a boon to an invasive species of water hyacinth. The increase in water pollution has had devastating effects on