Tijuana River National Estuarine Research Reserve, California
The Tijuana River National Estuarine Research Reserve is one a resever within NOAA's National Estuarine Research Reserve System, a network of estuarine habitats protected and managed for the purposes of long-term research, education, and coastal stewardship. It is located in a highly urbanized environment. The reserve encompasses beach, dune, mud flat, salt marsh, riparian, coastal sage and upland habitats surrounded by the growing cities of Tijuana, Imperial Beach and San Diego.
Three quarters of the reserve’s watershed is in Mexico and the management, education and research issues involve an international perspective. Critical issues confronted by the reserve include habitat restoration, endangered species management, management of the wastewater from Mexico, sediment management, and the integration of recreation and habitat conservation and restoration.
The reserve is home to eight threatened and endangered species, including the Light-footed clapper rail, California least tern, Least Bell’s vireo, salt marsh bird’s beak, cordgrass, white and brown pelicans, and numerous shorebirds. The reserve environment is a saline marsh habitat for most of the year with the Tijuana River being an intermittent stream system in a Mediterranean environment.
Flora was an important factor in the decision to include the Tijuana River Reserve in the reserve system. In addition to having regionally significant species, the Tijuana estuary provides examples of most vegetation communities found in other southern California wetlands. Cordgrass (Spartina foliosa) forms robust stands along tidal channels in the northern reaches of the estuary. Above the Spartina-dominated community are found several succulents, including pickleweed and saltwort. At higher elevations these succulents grade into a cover of shoregrass. At the highest elevations, pickleweed becomes codominant with shoregrass. The reserve's marshes also are home to the endangered salt marsh bird's beak. This once abundant plant has been pushed to the brink of extinction by the pressures of marsh destruction in California.
The reserve boasts more than 370 species of birds, of which about 320 are migratory. Birds at the reserve include four federally-listed endangered birds, in the light-footed clapper rail, the California least tern, the least Bell's vireo and the California brown pelican. Peregrine falcons, bald eagles and golden eagles are all occasional visitors as well. The Tijuana River estuary is located along the Pacific Flyway and is used for staging and wintering by a variety of waterfowl and shorebirds. Wintering waterfowl include pintail, cinnamon teal, American widgeon, surf scoter and ruddy duck.
Shorebirds account for a large portion of the migratory population. While 20 species occur regularly along the sand flats and mud flats of the estuary, four species, the willet, dowitcher, western sandpiper and marbled godwit, account for a large part of the bird population throughout the year. Abundance and species composition fluctuate seasonally and among habitats with the intertidal sand- and mud flats supporting both the largest numbers of individuals and species.
The estuary supports a small mammal population typical of fields and lowland habitats. Rodents, including mice, the California ground squirrel and rabbits are the most common. At least 29 species of fish reside in the small tidal creeks and channels of the estuary. Species in their juvenile stages that are found in the reserve's creeks and streams have included longjaw, mudsucker, northern anchovy and several species of gobies. Adult fish residing here include topsmelt, California killifish, staghorn sculpin and longjaw mudsucker.
Crabs are perhaps the most conspicuous invertebrates in Southern California coastal marshes and the Tijuana River Estuary. Rove beetles burrow in the mud- and sand flats. A large population of coastal tiger beetles also lives in these areas. The largest population of the wandering skipper in the United States resides in the Tijuana estuary. Several species of mosquito can be found in the reserve as well. The globose dune beetle and the Belkin's dune fly, both considered threatened, are also found on the reserve site.
The Tijuana River Reserve has six endangered species of birds, in the light-footed clapper rail, California least tern, Belding's savannah sparrow, least Bell's vireo, California brown pelican and occasionally the peregrine falcon. One endangered plant, the salt marsh bird's beak, is also found at the slough. The snowy plover and one of the reserve's shrimp species are both listed as "threatened" species.
Streamflow in the San Diego region is the most variable in the United States and differences between wet and dry years at the reserve are greater than in any other part of the country. On average, the Tijuana River has its peak flow in March. The Tijuana River can experience many months with no flow. Year-to-year flows are highly variable, as are monthly flows. With such a streamflow history, Tijuana Estuary may be the nation's most variable estuary.
The lower Tijuana River valley and estuary proper is primarily underlain by undifferentiated alluvium and slope wash comprised of poorly consolidated stream deposits of clay, silt, sand and cobble-sized particles that were deposited in the late Pleistocene. These deposits reach a thickness of approximately 40 [[meter (unit}|meters]] in some areas throughout the estuary. It is probable that the last of the major alluvial depositional sequences took place during the late Pleistocene and Holocene time, when sea level was receding and subsequently rising again to its present position, at which time alluvial deposition was greatly reduced.
As continental drift shifted North America toward the west, a steep coastline and narrow continental shelf developed. Marine terraces were gradually carved along the shores. Then, in the late Cenozoic, tectonic uplift raised alluvial terraces to several hundred feet above modern sea levels. What is now Tijuana River presumably cut through these terraces, although the narrow floodplain suggests that flows were not consistently large.
Then, in the Holocene, a rising sea began to reclaim the exposed margins of the coastal shelf. Rivers were drowned and lagoons formed as longshore drift created sandy barriers along the coast. With flooding, most of the coastal embayments filled with sediment. Without continuous river flow and scouring, their mouths closed between flood seasons.
Recent geologic factors that have shaped the estuary are the competing forces of rising sea level, which promotes inland migration of the estuary, and tectonic uplift, which reverses that trend. The location of the shore and the configuration of the mouth are additional variables that influence the size and condition of the estuary.
The lower Tijuana River Valley has a limited number of historical and archaeological sites. Based on local records, there are about 16 documented archeological sites located in the nearby area and along the coastal shore. An additional 10 prehistoric sites have also been reported in a reconnaissance survey of the Spooners Mesa. Surveys have revealed three archaeological sites within the reserve.
An early Spanish explorer observed a native village located in the valley in 1769 but the exact location of the village was not recorded. Smuggler's Gulch is believed to be the site of a camp made by Father Junipero Serra in the 1700s. There are a number of recorded paleontological localities in the reserve associated with two fossil-containing formations—the San Diego Formation and unnamed Pleistocene terrace deposits. The sites are of special significance due to their superb preservation.
Because the Tijuana River Reserve is located directly on the United States / Mexico border, our fellowship program offers opportunities for research in a unique environmental and cultural setting. Current research priorities include invasive species ecology and management, salt marsh restoration and sustainable development of urbanized watersheds.
Partners and Supporters
California State Parks and the U.S. Fish and Wildlife Service are the main land-owning agencies within the reserve. The remaining list of organizations contribute to the reserve program through representation on the reserve’s Management Authority. San Diego University and the Southwest Interpretive Association have a two-year membership. As the Tijuana River Reserve’s lead federal partner, NOAA holds a permanent, non-voting seat on the Management Authority.
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