The ravines and canyons are carved by streams and run off from freshwater springs. Some of these flow year round and others only during the summer rainy season. The most important springs are Pericos, Mal Paso, Apapxtla, Las Ventanas, Cieneguillas, Los Cuervos, and San Miguel. There are two major flows of water—the Río Magdalena and the Río Eslava. The Río Magdalena is the only free flowing river in the Federal District. It begins on the slopes of the Cerro La Palma in the Cuajimalpa borough. Some of the water is captured at a section of the Los Dinamos park to be treated at a plant but the rest flows down to the floor of the valley. The borough has little in the way of structured drainage for wastewater and rainwater. Much of the wastewater flows openly which has contaminated local streams, the Río Magdalena, and the supply of drinking water. The problem is serious enough in the urbanized area to affect the population’s health.