The Toluca Valley is a valley in central Mexico, just west of the Valley of Mexico (Mexico City), the old name was Matlatzinco. The valley runs north–south for about , surrounded by mountains, the most imposing of which is the Nevado de Toluca Volcano. It is one of the highest valleys in Mexico and for this reason has a relatively cold climate. Since the 1940s, there has been significant environmental degradation in the valley, with the loss of forests, soil erosion, falling water tables and water pollution due to growth in industry and population. In the pre-Hispanic period, it was a buffer region between the Aztec Empire and Purépecha Empire. From the Aztec period until the 19th century, it was part of the region controlled by Mexico City, but today it is the center of the State of Mexico, which has its capital in Toluca, the main city of the valley.
Physical geography and climate
The Toluca Valley is a broad highland valley located immediately west of the Valley of Mexico. It is one of the highest valleys in Mexico with an average elevation of 2,570 meters above sea level ("masl"). The valley is part of the Trans-Mexican Volcanic Belt, with its soils of volcanic origin mostly dating from the Upper Cenozoic. Mountains sides have tertiary volcanic rock, mostly basalt and andesite along with pyroclastic matter and breaches. The valley floor is composed of lacustrine and sedimentary materials. The valley covers an area of about 700 square kilometres, running north south with a length of 35 km. Its width is about 20 km. Its northern border is marked by the Guadalupana Volcano, the Cerro el Aguila and the Sierra Monte Alto. Its southern extension is marked by the Tenango and Zempoala Volcanoes. Its eastern boundary is marked by the Sierra de Las Cruces and La Iglesia mountains and the west is defined by the Nevado de Toluca and the Sierra Morelos. These mountains, along with the elevation of the valley in general cause the area to experience cooler temperatures than the rest of the region.
The most distinctive of the surrounding mountains is the Nevado de Toluca (4,690 m asl) to the southwest. The Nevado is the fourth highest mountain in Mexico at 4,600 m asl, formed by successive eruptions with deposited layers of igneous materials. Its peak has snow most of the year and is surrounded by temperate forests. The mountain has important effects on the climate and hydrology of the Toluca Valley The piedmont is characterized by hills covered by broadleaf forests. On the upper slopes there are conifers. The highest elevations have alpine grasslands, known as Zacatonal. While the area is in the tropical latitudes, its high altitudes along with cold are from the Sierra de Las Cruces and Nevado de Toluca gives the area a semi-cold to cold climate. The average monthly temperature varies between 5°C at altitudes over 3,000 m and 20°C lower in the valley.
The climate is classified as C(w2)(w)bi(g) under the Köppen climate classification which means a sub humid temperate climate with a long summer, winter rain of less than 5%, isothermal with maximum temperatures occurring before the summer solstice. The area has a rainy and dry season with the rainy season extending from mid May to mid October. Most precipitation occurs on the sides of the Nevado de Toluca of up to 1200 mm per year. The valley floor receives an average of 900 mm per year. Almost all precipitation occurs during the rainy season with most coming in June. After this, there is a short dry spell in August before rains return in September.
It is part of the upper course of the Lerma River, which is the main source of surface water.
Political geography
The Toluca Valley is in the southeastern section of the State of Mexico, which accounts for most of the state's territory. This state is the most populous in Mexico with over 14 million people. The state used to center on Mexico City historically, but after this was separated into a Federal District, the political center of the state eventually moved to the valley, with its capital in the city of Toluca. Today, most of the state's population is still on the fringes of the Mexico City metropolitan area, but the second highest concentration of people is in the Toluca Valley. Sixty percent of the valley's population of over 1,350,000 lives in the Toluca metropolitan area. The rest live in the over 400 other communities of the valley. Due to the political shift, the Nevado de Toluca volcano which towers over the valley has become the cultural symbol for the State of Mexico.