Appendix A – Study Area Description Appendix A Detailed Study Area Descriptions This Appendix provides more detailed descriptions of the geography and hydrology of the five transboundary regions that make up the study area. These summaries draw heavily on previous work on the U.S.-Mexico border project conducted by Parsons Engineering Science, Inc. (U.S. EPA, 2000), as well as USGS factsheets for three of the basins (Central Desert/Closed Basins: Papoulias et al., 1997; Rio Grande Basin: Blackstun et al., 1996; and Lower Rio Grande Basin: Buckler et al. 1997). A.1 Pacific/Salton Sea Transboundary Basins The Pacific/Salton Sea Basins contain watersheds that drain either to the Pacific Ocean or to inland seas. The basins drain an area of 14,000 square miles (36,000 km2). These basins have a very dry, semiarid climate with few fresh water resources. Flow is primarily from east to west, with stream flows originating from precipitation in the mountains flowing toward the Pacific Ocean. The flow in these streams is controlled through a series of hydraulic structures, including reservoirs. The Tijuana River is one of the main streams in the basin and one of the City of Tijuana’s major natural resources. The river flows northwest through the city of Tijuana before crossing into California near San Ysidro and then flowing into the Pacific Ocean. Figure A-1 shows the Pacific/Salton Sea Basins and their most important characteristics. Figure A-1. Pacific/Salton Sea Basins. A-1 Appendix A – Study Area Description A.1.1 Geography of the Pacific/Salton Sea Basins The San Diego, Cottonwood-Tijuana, and Salton Sea watersheds are the most important watersheds within the Pacific/Salton Sea Basins. They also cover part of the North-East Baja California basin in Mexico. The San Diego watershed encompasses San Diego County, parts of southwestern Riverside County, and southwestern Orange County. It comprises three distinct areas: the coastal plain, the central mountain valley area, and the eastern mountain valley area. The coastal plain ranges from sea level to about 1,200 feet (370 m) above sea level and extends for 10 miles (16 km) inland from the coast. The central mountain valley area is characterized by ridges and basins, which extend from the coastal plain northeast to the Elsinore fault zone. The basins range in elevation from 500 to 5,000 feet (150 to 1,500 m) above sea level, with the exception of the El Cajon area, where the mountain elevation reaches only 1,500 feet (1,500 m). To the northeast of the Elsinore fault zone, in the area known as the eastern mountain valley area, the valleys range from 1,000 to 3,500 feet (300 to 1,100 m) above sea level, while the surrounding peaks reach elevations of 4,000 to 7,500 feet (1,200 to 2,300 m). South of San Diego, the Tijuana watershed is separated from the San Diego watershed by the San Isidro and San Miguel Mountain range at the southern end of the San Diego County, and by the Sierra Juarez in Mexico. The highest mountain elevation is Sierra Juarez at 6,500 feet (2,000 m) above sea level. Land use varies considerably and ranges from urbanized areas to wilderness such as the Cleveland National Forest. Major cities include San Diego in California and Tijuana, Tecate, Rosarito, and Ensenada in Baja California. Smaller cities and towns include Descanso in California; various suburbs of San Diego; and Valle Bonito, San Luis, and La Joya in Baja California. There are also numerous Indian lands on the U.S. side of the border, including the Campo, La Posta, Manzanita, Cuyapaipe, Barona Ranch, Capitan Grande, Cahuilla, Santa Rosa, Pechanga, Mission, Pala, Rincon, La Jolla, San Pasqual, Los Coyotes, Santa Ysabel, and Mesa Grande Reservations. The northern part of the basin encompasses the Camp Pendleton Marine Corps base, and further down along the coast are many scenic beach areas, on both the Mexican and U.S. sides of the border. The Tijuana watershed serves as habitat for coastal shrubs and a chaparral ecosystem that extends from Baja California into California near the Pacific Ocean. This chaparral ecosystem gives way to pine forests and coastal vegetation along valleys and intermittent streams. Among the more serious threats to this ecosystem are erosion and slope instability. Increased sedimentation from urbanization and unregulated road development has negatively affected the flora and fauna and has also significantly affected other resources in the area. In addition to erosion and sedimentation concerns, estuarine and wetland areas have been reduced significantly in this basin, to the point where only 20 to 40 percent of the original wetland area remains intact. The watershed also contains several environmentally sensitive areas, such as the Tijuana River Estuary, which straddles the U.S.-Mexico border. The estuary is approximately 2,000 acres (800 hectares) of salt water marsh with several stretches of open water. The estuary is generally open A-2 Appendix A – Study Area Description to the ocean, and its water quality generally is the same as that of the shoreline open ocean waters. However, during periods of excess runoff, a variety of wastes originating upstream in the Tijuana River in Mexico can be carried into the estuary. Tidal flushing is considered to be crucial to the estuary’s health, and thus a program to control erosion, manage sediment, and strategically dredge parts of the estuary has been initiated. In the North East Baja California basin, the major surface water is Laguna Salada. The Sierra Juarez range discharges surface runoff to the Laguna Salada. As the range slopes towards the Sea of Cortez, the mountains give way to sand dunes and wetlands. Of these dune areas, one of the most important is Constitution National Park, located south of Laguna Salada, which has been designated as a protected area by the Mexican government. The Salton Sea watershed stretches north from the northeast section of Baja California in Mexico into the southeast portion of California in the United States. The watershed has a gross contributing drainage area of 7,500 square miles (19,000 km2), most of which is in the United States. The western boundaries of the watershed are contiguous with the western boundaries of the Imperial Valley and the eastern side of the Anza Borrego area in California. To the north, the basin is bounded by the Salton Sea along California Route 10 from the San Bernardino National Forest through the Joshua Tree National Monument and to the Colorado River, which forms the eastern side of the boundary. The southern boundary of the watershed is formed as the Imperial Valley lowlands drop to the Sea of Cortez. The watershed’s central feature is the flat, fertile Imperial Valley. The Imperial Valley consists primarily of farming communities, although there are several larger cities in the basin, including the border city of Mexicali in Baja California, a thriving manufacturing center. The main communities in the watershed on the U.S. side of the border are Calexico, El Centro, and Brawley, which are all located along California Route 86 east of the New River. Other communities within the basin area on the California side of the border include Blythe, Indio, Palo Verde, Salton Sea, Seeley, and Westmorland. A.1.2 Hydrology of the Pacific/Salton Sea Basins The mountain ranges running along the coasts of California and Baja California divide the precipitation falling there: precipitation that falls on the western slopes flows toward the Pacific Ocean, and precipitation that falls on the eastern slopes flows east into the Imperial Valley and the lands below Mexicali and on into the Sea of Cortez. In California, a series of stream systems originating in the highlands flow west to the Pacific Ocean. These streams include the Aliso, San Juan, San Mateo, San Onofre, San Marcos, and Escondido Creeks, and the Santa Margarita, San Luis Rey, San Dieguito, San Diego, Sweetwater, Otay, and Tijuana Rivers. Most of these streams and creeks consist of both perennial and ephemeral segments, primarily because of man-made controls and impoundments throughout the watersheds. This has created a series of reservoirs and lakes, which include Vail, O’Neill, Henshaw, Hodges, and Sutherland Lakes, and the Lower Otay, Sweetwater, and San Dieguito Reservoirs. Further south, major streams in Mexico include the Santo Tomas Pino, Las Palmas, Las Cabaza, Agua Caliente, and El Baron. A-3 Appendix A – Study Area Description Little rain falls within the basin, which is semi-arid. On the U.S. side of the border, 70 to 90 percent of the region’s water has to be imported from northern California and the Colorado River. The basin is served by the Metropolitan Water District of Southern California (MWD), which serves more than 16 million people in the California coastal plain. The MWD manages the importation and distribution of water from the Colorado River and the California State Water Project. Small amounts of water are also available from the regional impoundments described above. Despite the fact that most of the rivers flowing through this basin are not used for human water consumption, they are very important as natural systems that can carry pollutant loads and polluted runoff downstream. Of particular concern in this respect is the Tijuana River. The Tijuana River originates east of the city of Tijuana, Mexico, then flows west through the center of Tijuana, where it is heavily channelized. In Tijuana, the river is joined by the Alamar, another highly channelized watercourse. As the river flows west through Tijuana, it also bends north and flows near San Ysidro, California. The Tijuana River drains approximately 1,286 square miles (3,300 km2), approximately two-thirds in Baja California and one-third in California.