With population of 1,423,851 San Diego is the eighth-largest city in the United States and second-largest in California. The city is known for its mild year-round climate, natural deep-water harbor, extensive beaches, long association with the United States Navy, and recent emergence as a healthcare and biotechnology development center.
The city is the seat of San Diego County and is the economic center of the region as well as the San Diego–Tijuana metropolitan area. San Diego’s main economic engines are military- and defense-related activities, tourism, international trade, and manufacturing; the city’s airport, San Diego International Airport, is the busiest single-runway airport in the United States.
The presence of the University of California, San Diego (UCSD), with the affiliated UCSD Medical Center, has helped make the area a center of research in biotechnology. The city lies on approximately 200 deep canyons and hills separating its mesas, creating small pockets of natural open space scattered throughout the city and giving it a hilly geography.
Traditionally, San Diegans have built their homes and businesses on the mesas, while leaving the urban canyons relatively wild. Thus, the canyons give parts of the city a segmented feel, creating gaps between otherwise proximate neighborhoods and contributing to a low-density, car-centered environment. The San Diego River runs through the middle of San Diego from east to west, creating a river valley that serves to divide the city into northern and southern segments. During the historic period and presumably earlier as well, the river has shifted its flow back and forth between San Diego Bay and Mission Bay, and its fresh water was the focus of the earliest Spanish explorers. Miguel Costansó, a cartographer, wrote in 1769, “When asked by signs where the watering-place was, the Indians pointed to a grove which could be seen at a considerable distance to the northeast, giving to understand that a river or creek flowed through it, and that they would lead our men to it if they would follow. That river was the San Diego River. Several reservoirs and Mission Trails Regional Park also lie between and separate developed areas of the city.