watershed is divided into three distinct sections. The northern third, located within the Angeles National Forest of the San Gabriel Mountains, is steep and mountainous; it receives the most precipitation of any part of the basin – per year – and as a result is the source of nearly all the natural runoff. Elevations reach up to at Mount San Antonio (Mount Baldy), the highest point of the range. During the winter, many elevations above are covered in snow. The middle third, the San Gabriel Valley, and the southern third, the coastal plain of the Los Angeles Basin, are separated by the Puente Hills and Montebello Hills. With the exception of some recreation areas and lands set aside for flood control, the valleys are almost entirely urbanized. Approximately 2 million people live in the watershed, divided between 35 incorporated cities. Rainfall is slightly higher in the San Gabriel Valley than the coastal plain due to its proximity to the mountains. However, the climate as a whole is very arid, with only moderate precipitation in winter and nearly none in summer. The lower watershed essentially consists of alluvial plains that once experienced seasonal flooding from the San Gabriel River, creating vast swamps and wetlands. Today very little of this original environment remains. The San Gabriel is one of the largest natural streams in Southern California, but its discharge varies widely from year to year. Between 1895 and 1957 the mean unimpaired runoff at Azusa was estimated at , with a range from . Historically, the San Gabriel River reached its highest flows in the winter and spring, with runoff dropping significantly after early June before rising again with November or December storms. Today, the flow of the San Gabriel River has been dried up in places by dams, diversions and groundwater recharge operations, and increased in other sections by wastewater run-off. Headwaters East Fork The East Fork, long, is the largest headwater of the San Gabriel River; the U.S. Geological Survey considers it part of the main stem. However, it is colloquially known as the "East Fork" to distinguish it from the West Fork of the San Gabriel. Its furthest tributary, the Prairie Fork, originates at Pine Mountain in the Sheep Mountain Wilderness to the southwest of Wrightwood. Draining a high, remote subalpine valley characterized by extensive meadows, it flows west to join with Vincent Gulch, below which the stream is