CHAPTER 2 



ENVIRONMENTAL CONDITIONS 

 THAT HAVE SHAPED THE ESTUARY 



2.1 



GEOLOGIC HISTORY 



The geologic history of Tijuana Estuary 

 and its adjacent coast is poorly studied. Yet it 

 is so different from most of the Nation's 

 estuaries that even the most general 

 descriptions are useful. In his review of sea 

 level and coastal morphology during the late 

 Pleistocene, Bloom (1983a,b) characterized 

 the Pacific Coast as "a total contrast to the 

 trailing continental margin with coastal 

 plains of the Atlantic and Gulf coasts. Active 

 tectonics associated with regional strike-slip 

 faulting characterize the California coast." 



As continental drift shifted North America 

 toward the west, a steep coastline and narrow 

 continental shelf developed (Figure 2.1). 

 Marine terraces were gradually carved along 

 the shores. Then, in the late Cenozoic, 

 tectonic uplift raised alluvial terraces to 

 several hundred meters above modern sea 

 levels; the lowest of these terraces were laid 

 down 125,000 to 80,000 years ago (Ku and 

 Kern I974; Kern 1977; Lajoie et al. 1979). 

 In some places, the youngest terraces have 

 been thrust 40 m above current shorelines. 



Because the shelf has a steep decline, 

 Pleistocene glaciation and receding sea levels 

 did not expose large expanses of coastal land. 

 What is now the Tijuana River presumably 

 cut through these terraces, although the 

 narrow floodplain suggests that flows were not 

 consistently large. The cut that frames 

 Tijuana Estuary is only a few kilometers wide. 



The picture emerges of a sharp, steeply 

 inclined coast with vertically active terraces. 

 Then, in the Holocene, a rising sea began to 



reclaim the exposed margins of the coastal 

 shelf. As Bloom (1983a) goes on to say, "the 

 last 15,000 years of California coastal 

 evolution have been dominated by 

 submergence coincident with deglaciation." 

 The 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, and, 

 without continuous river flow and scouring, 

 their mouths closed between flood seasons. 



The recent geologic factors that have 

 shaped the estuary are thus 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. 

 Longshore drift is generally southward in 

 southern California, with flows interrupted 

 by submarine canyons. However, the precise 

 patterns and seasonal shifts at Tijuana 

 Estuary have not been quantified. What is 

 clear is that catastrophic beach erosion has 

 shifted the shoreline landward in the past few 

 years. How much of that shift is due to recent 

 storms and how much is a general trend due to 

 rising sea level are yet to be determined. 

 Additional discussion of the magnitude of these 

 changes and the impacts on estuarine habitats 

 can be found in Chapter 5. 



The recent history of sea level rise has 

 been summarized by Flick and Cayan (1984) 

 and Cayan and Flick (1985). Data from 1906 

 to the present, taken in San Diego Bay, 

 indicate an average rise of about 20 

 cm/century (Figure 2.2). During recurrent 



