Institute for Regional Studies of the Californias

---

Map 13. Topography and Hydrography

Richard Wright

Topography

The relief of the planning area is quite high, with elevations ranging from sea level on the west to more than 3,550 feet (1,082 meters) in Otay Mountain in the far east. The western two-thirds is composed largely of relatively flat marine terraces comprised of conglomerate and other sedimentary rocks that are dissected by steep-sided valleys. In the west, severe erosion has left few remnants of upland areas. To the east, the urban zone of Otay Mesa-Mesa de Otay is located in the large areas of relatively flat upland areas. The eastern one-third is the most rugged section and is characterized by deeply dissected terrain developed on rocks that are largely igneous in nature.

Hydrography

Surface water flow in the planning area is the culmination of drainage from the Tijuana River Watershed, a 1,719 square miles (4,452 square kilometers) basin, two-thirds of which is in Mexico. The Tijuana River is formed by two drainage networks that merge in urban Tijuana, about 10.6 miles (17 kilometers) from the Pacific Ocean. Upstream, on the U.S. side, the network is comprised principally of the Pine Valley Creek, Campo Creek, and the Cottonwood Creek drainages. The last is known as the Río Alamar from the point it enters Mexico to its confluence with the Tijuana River. Two reservoirs in the United States, Barrett and Morena, store water that is conveyed out of the Cottonwood Creek-Río Alamar subbasin into the Otay River watershed to the north where it is a source of water for the City of San Diego. In Mexico, the major tributary to the Tijuana River is the Río de las Palmas that flows into the Abelardo L. Rodríguez Reservoir, a major source of potable water for urban Tijuana along with supplies from the Colorado River. Downstream from Rodríguez Reservoir, water flows through Tijuana partly in a concrete channel to the international boundary. From the boundary, it continues westward through the Tijuana River Valley for a distance of about six miles (nine kilometers) to the Tijuana River Estuary and then into the Pacific Ocean.

Precipitation in the watershed exhibits large seasonal variations with almost all moisture falling during the period November to May, sometimes as intense storms lasting only three or four days. The result is a highly variable stream flow. During the summer and fall seasons, stream flow is almost nonexistent, whereas in the winter and spring it runs at high, sometimes flood, levels. Intense precipitation resulted in severe floods in 1980, 1983, and 1993. These had devastating effects on natural habitat, structures, personal property, and transportation facilities in the Tijuana River Valley. Side canyons of the Tijuana River Valley, such as Goat Canyon-Cañ ón de Los Laureles and Smuggler's Gulch-Cañ ón Matadero are especially subject to flash flooding during heavy rainfall events. It is anticipated that flooding will continue to occur in the study area due to increased urbanization, loss of vegetation on the hillsides, accumulation of sediment and other debris in drainage channels, and an inadequate storm drainage system.

Photo

Colonia Nido de las Águilas on Mesa de Otay adjacent to international border. The unpaved roads on both sides of the border and the housing on the steep slopes encourage erosion. 2000.

Photo

Channelized Tijuana River as it enters San Diego. 2000.