Figure 2.11. Cordgrass mortality during 

 nontidal conditions in 1984 (photo by J. 

 Zedler). 



2 . 4 STREAMFLOW HISTORY 



Streamflow in the San Diego region is the 

 most variable in the United States, and 

 differences between wet and dry years are 

 greater than in any other part of the country 

 (Pryde 1976). Even small variations in 

 streamflow affect the coastal wetlands, 

 because floodplains are narrow, and there is 

 little volume of estuarine water present to 

 ameliorate the impacts of increased or 

 decreased flows. 



The Tijuana River drains a watershed of 

 448,323 ha, three-fourths of which is in 

 Mexico (Figure 1.1). Of that watershed, 78% 

 is behind dams (IBWC 1983). The effects of 

 reservoirs on streamflow are undetermined, 

 because all three dams were installed before 

 streams were gaged. Morena Dam dates to 

 1912, Barrett to 1922, and Rodriguez to 

 1936. Their respective capacities are 62 

 million m 3 (50,210 acre-ft), 55 million m 3 

 (44,760 acre-ft), and 138 million m 3 

 (111,880 acre-ft). The largest and closest to 

 Tijuana Estuary is Rodriguez Dam, just 

 upstream of the city of Tijuana. It is likely 

 that dry-season flows are lower and that flood 

 flows are delayed by the presence of these 

 reservoirs. 



Flows of the Tijuana River have been 

 measured since 1937 at the Nestor gage, 



which includes 99.6% of the watershed 

 (Figure 1.1). On the average, the Tijuana 

 River has its peak flow in March (Figure 

 2.13). However, as with rainfall data, 

 averages tell little of the relationship between 

 streamflow and estuarine dynamics. Year-to- 

 year flows are highly variable (coefficient of 

 variation = 325%), as are monthly flows 

 (c.v. = 690%). With such a streamflow 

 history, Tijuana Estuary may be the Nation's 

 most variable estuary. It is marine- 

 dominated on a seasonal basis (sometimes for 

 several years in succession); it is 

 occasionally fresh during catastrophic 

 flooding, such as occurred in 1980. 



2.4.1 Major Flood Events 



Monthly flow volumes at Nestor (Figure 

 2.14) show that major floods occurred in 

 1978 and 1980 after a 35-year period with 

 little flow. Log-transformations of the 

 monthly streamflows enhance the years of low 

 flow and de-emphasize high-flow years. 

 Seasonal patterns become much clearer with 

 such transformation, and the winter-flow, 

 summer-drought pattern becomes obvious. 

 Note, however, that several entire years had 

 zero flow at the Nestor gage. During these 

 times, the estuary is presumed to have been 

 filled entirely with seawater. Even in years 

 with seasonal streamflow, the predominant 

 influence has been marine. 



In January and again in February of 

 1980, floods that exceeded all previous flow 

 records simultaneously eroded and filled 

 different parts of the estuary. The Jan. -Dec. 

 total flow of 725.4 MCM (million cubic 

 meters) was 18.6x the average yearly 

 volume. The flooding shifted the course of the 

 Tijuana River from its Mid-Valley channel 

 (Figure 2.8) across several parcels of 

 agricultural land, carving a diagonal channel 

 toward the helicopter landing field, where it 

 connected with the central estuarine channel. 

 A much broader coastal embayment was 

 created at the river mouth. Major 



sedimentation occurred in the southern half of 

 the estuary. Monument Road was buried by 

 half a meter of silt that flowed down Goat 

 Canyon. 



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