Phoenix is in the southwestern United States, in the south-central portion of Arizona, and about halfway between Tucson to the south and Flagstaff to the north. The metropolitan area is known as the "Valley of the Sun", due to its location in the Salt River Valley. It lies at a mean elevation of 1,117 feet (340 m), in the northern reaches of the Sonoran Desert. A photo showing the skyline of Phoenix, looking north. It shows the various buildings of the downtown area, as well as Sunnyslope Mountain in the background Northern skyline, downtown Phoenix, Sunnyslope Mountain clearly visible in background Other than the mountains in and around the city, the topography of Phoenix is generally flat, allowing the city's main streets to run on a precise grid with wide, open-spaced roadways. Scattered, low mountain ranges surround the valley: McDowell Mountains to the northeast, the White Tank Mountains to the west, the Superstition Mountains far to the east, and the Sierra Estrella to the southwest. On the outskirts of Phoenix are large fields of irrigated cropland and several Indian reservations. The Salt River runs westward through the city of Phoenix, and the riverbed is often dry or contains a little water due to large irrigation diversions. The community of Ahwatukee is separated from the rest of the city by South Mountain. According to the United States Census Bureau, the city has a total area of 517.9 square miles (1,341 km2); 516.7 square miles (1,338 km2) of it is land and 1.2 square miles (0.6 km², or 0.2%) of it is water. Even though it is the 6th most populated city, the large area gives it a low density rate of approximately 2,797 people per square mile. In comparison, Philadelphia, the 5th most populous city has a density of over 11,000.