Plans to restore full tidal flushing to the 

 southern arm of Tijuana Estuary began with 

 the hydrologic analysis. Williams and 

 Swanson (1987) used the 1857 map as a 

 model of what the estuary might need to 

 become a fully tidal estuary. At this time, 

 there was a 305-m-wide (1,000-foot) 

 mouth and an estimated 1.8 million cubic 

 meters (1,500 acre-ft) tidal prism (volume 

 between MHHW and MLLW). About 352 ha 

 (870 ac) of intertidal wetlands were 

 estimated to be present. Tidal sloughs 

 extended 914 m (3,000 ft) inland toward the 

 east, 1,524 m (5,000 ft) north, and 610 m 

 (2,000 ft) south from the ocean inlet. A 

 large area of open water was present in the 

 southwesternmost part of the estuary. 



Between 1852 and the present, there were 

 many changes in geomorphology, hydrology, 

 and hydrodynamics (Chapter 2, Figure 2.15). 

 From the historic maps and aerial photos, it 

 was estimated that 80% of the historic tidal 

 prism had been eliminated during repeated 

 sedimentation events. Sediments flowed in 

 from the watershed and across the beach, 

 depending on the type of storm event. With 

 sea storms, the dunes were both flattened and 

 pushed inland. Major beach retreat (90-120 

 m; 300-400 ft) was documented for the 

 134-year period between 1852 and 1986. 



Williams and Swanson (1987) 

 summarized the major problems that would 

 continue to plague the estuary if these 

 physical processes were not controlled: 1) 

 The estuary would continue to lose its tidal 

 prism, through sedimentation down Tijuana 

 River, down Goat Canyon, and across the beach 

 during storm washovers. 2) Wetlands would 

 continue to be converted to upland habitat. 3) 

 Freshwater inflows could shift large areas of 

 saline wetland to brackish wetland. 4), Wet- 

 land area would continue to decline as a 

 consequence of accelerated sea level rise. 5) 

 The encroachment of development in the river 

 corridor would degrade the riparian habitat 

 and increase flood hazards. 



The hydrologists then suggested an 

 extensive dredging program (Figure 6.3) to 

 be focused on the southern arm of the estuary. 

 In the northern arm, a major new channel was 

 proposed to connect the tidal ponds to the 



Tijuana River channel-a measure that would 

 maintain tidal access to Oneonta Slough after 

 the migrating dune pinches off its current 

 opening to Tijuana River and the ocean inlet. 



Resource mapping and environmental 

 impact assessment followed, with the 

 preparation of an Environmental Impact 

 Review and Environmental Impact Statement 

 by Entrix, et al. (1991). A biological 

 analysis was undertaken by PERL to determine 

 what resources would be affected by the 

 proposed dredging. Using recent air photos 

 and extensive ground truthing, M. Busnardo 

 and J. Tiszler mapped the vegetation in detail. 

 The fauna of the south arm were studied for 

 the first time, with studies of terrestrial 

 arthropods (Appendix K3 in Entrix et al. 

 1991, ), mammals (Appendix K5, ibid.), 

 herpetofauna (Appendix K4, ibid.) and 

 channel benthos (Appendix K2, ibid.). Bird 

 use was evaluated in a comparative study of 

 south and north arms (Appendix K1, ibid.; cf. 

 Chapter 3). 



From these field studies, PERL developed a 

 "constraints map" (Figure 6.4) that detailed 

 all areas considered sensitive, either for the 

 type of community present or the occurrence 

 of rare and valued species. For example, the 

 transition to upland that occurs south of the 

 tidal ponds was considered too valuable to be 

 dredged. Salt flats and high marsh habitat 

 near Border Field overlook were shown to 

 have high densities of Belding's Savannah 

 sparrows and hence were considered too 

 sensitive for conversion. 



Modifications to the 1987 plan were 

 suggested to incorporate new information and 

 ideas. A channel was proposed to be cut near 

 the Visitor Center to improve tidal circulation 

 while also enhancing opportunities for nature 

 interpretation and reducing the need to bring 

 visitors into endangered species habitats. The 

 hydrologists were then asked to redesign the 

 dredging program to avoid sensitive areas. 

 Florsheim et al. (1991) redesigned the 

 hydrology plan and responded to recom- 

 mendations for an adaptive management 

 approach, with two phases (a Model Project to 

 precede full-scale restoration) and a modular 

 approach (subunits that could be constructed 

 as funding became available). 



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