rise in sea level cannot begin to keep up with 

 catastrophic sedimentation, and the net effect 

 is for coastal wetlands to fill in more rapidly 

 that they would naturally. 



6.3.2 Beach and Dune Erosion Problems 



Winter storms that coincide with high sea 

 levels erode the dunes and beach. Summer is 

 the rebuilding phase in an annual cycle of 

 removal and replenishment. However, if the 

 replenishing sands are intercepted in their 

 transport along shore or downstream, the 

 beach and dunes show a net loss (Inman 

 1985). In addition to the annual cycle, there 

 is a long-term trend for an increase in mean 

 sea level, which gradually moves the beach 

 inland. 



Disturbance of beach vegetation has 

 contributed to the destabilization of the dunes, 

 and replanting efforts have been underway in 

 recent years (P. Jorgensen and B. Fink, Dept. 

 of Parks and Recreation project, unpubl.). 

 Further dune stabilization, through extensive 

 revegetation efforts, is called for in the 

 restoration plan. 



6.3.3 Streamflow Modifications 



Reservoirs modify estuarine hydrology by 

 reducing total volume of streamflow, by 

 delaying the start of floodflows, or by 

 prolonging the period of wet-season flows. In 

 the United States portion of the Tijuana River 

 watershed, Barrett and Morena Reservoirs 

 trap streamflow and presumably modify the 

 timing and volume of floodflows. Water can be 

 discharged from Morena to Barrett, but the 

 gates of Barrett Reservoir cannot be opened 

 once water is impounded; thus, drawdown is 

 not possible. In Mexico, Rodriguez Reservoir 

 has gates that can be used to lower water 

 levels, and the prolonged discharges of 1983 

 indicate the magnitude of streamflow change 

 that can occur as a result of reservoir 

 drawdown. Record flows for 10 months 

 (March through December) occurred during 

 periods of little rainfall and low streamflows 

 (Table 6.1). At such times, salinities are 



Table 6.1. Streamflow data (acre-ft) for the 

 Tijuana River at the United States-Mexico 

 border. All 1983 flows were augmented by 

 discharges from Rodriguez Dam; flows for 

 March-December 1983 were maximum for 

 the period of record (from IBWC 1983). 



lowered (Chapter 5), tidal regimes are 

 probably muted, and nutrient concentrations 

 increase (Covin 1984). 



Streamflow into Tijuana Estuary has been 

 altered by wastewater discharges from 

 Tijuana, Mexico, since the land slopes 

 northward towards the United States. The 

 largest influxes occurred as sewage discharges 

 that were continuous from the mid-1980s 

 until early October of 1991. Sewage spills 

 still occur when there is a break in the 

 Mexican pipelines, which are designed to 

 carry sewage west and south to an intertidal 

 outfall. Flows exceeding 7,000 m 3 per day 

 (over 2 MGD) have occurred at various 

 unpredictable intervals. 



A major sewage interception system is 

 now under construction, wherein flows from 

 four canyons will be collected and piped to the 

 San Diego treatment plant (City of San Diego 

 Clean Water Program). A new sewage 

 treatment plant will be built at the U.S.- 

 Mexico Border, with plans for it to begin 



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