Article: 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 which serves to divide the city into northern and southern segments. The river used to flow into San Diego Bay and its fresh water was the focus of the earliest Spanish explorers.[citation needed] Several reservoirs and Mission Trails Regional Park also lie between and separate developed areas of the city.

Question: What divides the city's northern and southern segments?
San Diego River