flushing, with three treatments planned for 

 initial experimentation: full tidal flushing, 

 tide waters excluded to maintain relatively 

 dry saline soils, and tide waters impounded to 

 create waterlogged soils. Initially, there will 

 be 8 replicates for each of the three tidal flow 

 regimes. After a year of study, freshwater 

 inflows will be added to half the mesocosms in 

 each tidal treatment for a fully-crossed two- 

 factor experiment. 



Funding has been requested from the U.S. 

 Fish and Wildlife Service and from NOAA's 

 Sanctuaries and Reserves Division. Results 

 are expected to guide pickleweed marsh 

 restoration projects at Tijuana Estuary and 

 other restoration programs in southern 

 California. 



6.3 MANAGEMENT! NEEDS 



Tijuana Estuary is an urban estuary 

 subject to the chronic and acute impacts of an 

 enormous human population. The City of 

 Imperial Beach surrounds the northern arm 

 of the estuary. Agricultural lands and a large 

 system of levees for flood control have 

 modified the Tijuana River floodplain. Just 

 upstream, the city of Tijuana, Mexico, 

 includes over 2 million inhabitants, most of 

 whom do not even have access to a sewer 

 system. The watershed above Tijuana is 

 scheduled for rapid and extensive 

 development. At the same time, the 

 downstream estuary is supposed to serve as a 

 reserve for research and education, a refuge 

 for endangered species, and a state park. 



Management problems are numerous, but 

 four stand out as having the greatest impact on 

 estuarine ecology: sedimentation of channels, 

 erosion of the beach and dunes, inputs of 

 sewage, and modification of streamflow. Each 

 problem has multiple causes. Increased 

 sedimentation follows disturbance of soil- 

 stabilizing vegetation both within the 

 watershed and on the beach. The beach and 

 dunes erode when storms and high sea levels 

 coincide. Sewage spills are almost entirely 

 traceable to breaks in Mexican sewage lines, 

 although local leaks are not unknown. 

 Streamflow augmentation is associated with 



reservoir and wastewater discharges; their 

 effect is greatest during summer, when the 

 river might otherwise be dry. 



These many disturbances are interrelated: 

 dune sands contribute sediment to channels, 

 and sewage spills alter streamflows. The 

 causes of these problems must often be dealt 

 with separately; for example, dunes can be 

 stabilized with fencing or vegetation to resist 

 storm damage, but nothing can control sea 

 levels. Mitigation of impacts, on the other 

 hand, requires a comprehensive approach, 

 because altering habitat in one area can 

 potentially affect the entire ecosystem. For an 

 estuary that is managed primarily for its 

 native wetland communities and endangered 

 species, passive management (leaving nature 

 alone) might seem preferable to active 

 manipulation of environmental conditions. 

 But disturbances have had significant impacts 

 on the estuary, and the issue is not whether, 

 but how much, intervention is required to 

 maintain native species. 



6.3.1 Sedimentation Problems 



Throughout southern California, estuaries 

 and lagoons have been filling in rapidly, as 

 hillsides within their watersheds are 

 disturbed and developed. Vegetation that might 

 slow erosion is sparse in Mexico, where 

 grazing is more common and fires more 

 frequent in the landscape (Minnich 1983). 

 The extremely erodible soils move 

 downstream with winter rains, and 

 catastrophic sedimentation occurs at the coast. 

 Mugu Lagoon lost 40% of its low-tide volume 

 with cumulative sedimentation during the 

 floods of 1978 and 1980 (Onuf 1987). 

 Sedimentation is a natural process, but the 

 rates have accelerated, and the system's 

 ability to respond by changing its 

 configuration has been constrained by 

 peripheral developments. 



Other natural events that counter the 

 impacts of sedimentation may also be 

 augmented by human activities. For example, 

 the greenhouse effect may be accelerating sea- 

 level rise and somewhat compensating for 

 sediment accretion. However, the average 



1 26 



