The highest elevations in the east and are formed by volcanic basalt. This rock varies in color between black and red. The San Juan/Tequisquiapan River crosses the municipality in the south on which are the Centenario Dam, near the town of Tequisquiapan and the Paso de Tablas Dam near the border with Hidalgo. The latter has an inactive hydroelectric plant. There are subsurface currents of water which feed a number of freshwater springs on either side of the San Juan River. Near the river a number of fruit and nut trees are grown such as walnuts, peaches, pomegranate, avocado, apricots, limes, guavas and fig. There is also vineyards which have not yet reached peak potential. The main dam for the municipality is the El Centenario Dam, located just south of the town of Tequisquiapan. Although its reservoir is important for water storage, it has had serious problems with pollution from industrial waste. The first major disaster occurred in 1999, when over 2,000 wild ducks died from the contaminated water. A more recent spill, traced to a textile plant in San Juan del Río killed more than 6,000 fish in 2005. This incident also caused health concerns for those living near the water. The area around the dam is subject to flooding during the rainy season, which at times has affected the hotels and restaurants which are nearby. The flooding and contamination problems have had a negative effect on tourism in general. The climate is temperate with two well-defined seasons each year. Most rain falls between June and October, with the dry season extending from November to April. Annual precipitation is 511.8mm. Summers are hot and winters are moderately cold leading to an average annual temperature of 17.5C. The coldest months are October to February, and freezes are not uncommon, occurring an average of 18 days per year. Dominant winds flow from the northeast to the southwest. Natural vegetation is suited for temperate and relatively dry climates. In the higher elevations, there are forests of low and medium height. In the hotter, dryer lowlands vegetation such as pirul (Schinus molle), mesquite, palo bobo (Tessaria integrifolia), various types of cactus and arid land scrub. Population growth since the Conquest, and especially in the 20th century has had a negative impact on wildlife in the municipality and other parts of Querétaro. A number of species have already disappeared, such as the jaguarondi and