The Ecology of Tijuana Estuary, California: An Estuarine Profile

CHAPTER 2
MAJOR FACTORS THAT HAVE SHAPED THE ESTUARY
2.1 GEOLOGIC HISTORY embayments filled with sediment, and, without
continilous river flow and scouring, their mouths
The geologic history of Tijuana Estuary and its closed between flood seasons,
adiacent coast is poorly studied. Yet it is so
different from most of the Nation's estuaries that
The recent geologic factors that have shaped
even the most general descriptions are useful. In
the estuary are thus the competing forces of rising
his review of sea level and coastal morphology
sea level, which promotes Inland migration of the
during the late Pleistocene, Bloom (1983a,b)
estuary, and tectonic upl~ft, whrch reverses that
characterized the Pacific Coast as "a total contrast
trend The location of the shore and the
to the trailing corltinental margin with coastal
conftguratlon ot the mouth are addrt~onal variables
plains of the Atlantic and Gulf coasts. Active
that influence the srze and cond~tion of the estuary
tectonics associated with regional strike-slip
Longshore drift is generally southward In southern
faulting characterize the California coast."
California, w~th flows interrupted by submarine
canyons However, the precise patterns and
As continental drift shifted North America seasonal sh~fts at Tijuana Estuary have not been
toward the west, a steep coastline and narrow quaritified What is clear IS that catastrophic beach
contrnental shelf developed (Figure 4) Marine
terraces were gradually carved along the shores
Then In the late Cenozo~c, tectonic upl~ft raised
ailuvlal terraces to several hundred meters above
modern sea levels, the lowest of these terraces
were Iatd down 125,000 to 80,000 years ago (Ku
and Kern 1974, 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 Tljuana Rlver presumably cut through
these terraces, although the narrow fioodplaln
suggests that flows were not consrstently large
The cut that frames Tijuana Estuary IS only a few
kilometers wide
The plcture emerges of a sharp. steeply inclined
coast with vertically acttve terraces Then, In the
Holocene, a rising sea began to reclarm the
exposed margins of the coastal shelf As Bloorn
(I%$%) gftes on :o say. "the !ast 15.05?0 years of
Californ~a coastal evolution have been dtjmrnated
by submergence coincident with deglaciation' The
rivers were drowned and lagoons formed as
longshore drift created sandy barrrers along the
coast W~th flooding most of the coastal
eroslon has shifted the sllorel~ne 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 rrsir-ig sea level are yet to be determined
The recent htstory of sea level rlse has been
sumrnarized by Flick and Cayan (1984) and Cayan
and Flick (1985) Data from 1906 to the present.
taken in San Dfego Bay, indicate an average rise of
about 20 cm/century (Figure 5) Dur~ng recurrent
El Nino events, the rate of sea level rise has been
even higher The hlghest sea level on record
occurred in January 1983, when predicted sea
levels were exceeded by 26 cm due to the
coincidence of El Nino, high spring tides, and a
major sea storm Still, the water levels might have
been even higher if these three events had also
coincided with the hlghest pornt In the 19-year tidal
cycle (Cayan and Ftick 1985) The future locatlon
and configuration of the shorelrne will depend on
both the chron~c and catastrophic increases In sea
level While the former will shlft estuarine habitats
~nland the latter will lap away at the marlne
margins
The 1904 map of Tijuana Estuary (Figure 6)
prov~des the earliest indication of hiStOrlC
cond~tions Three features that are clearly mapped
are not evident in later photographs F~rst, there IS