The Ecology of Tijuana Estuary, California: An Estuarine Profile

flushing until mid-December 1984. Eight months of
closure had devastating effects on the estuary.
While the reduction of tidal flushing is in itself a
catastrophic event for a marine-dominated system,
the situation was made worse by its coincidence
with a year of near-zero rainfall. Channels became
hypersaline (60 ppt in fall 1984; R. Rudnicki,
SDSU, pers. comm.), shallow creek bottoms
desiccated and turned to brick, and marsh soils
became so dry and hypersaline (over 100 ppt in
September 1984) that large areas of low-marsh
vegetation died out (Figure 14). Bare patches
within the salt marsh were obvious from the air in
August 1984 (Figure 13).
Tidal flushing was re~nstated in December 1984
after dredging of the main estuary channel from the
end of Sea Coast Drlve south to the mouth, and
tidal flushing has cont~nued to the present Sand
that washed eastward was bulldozed back to
recreate dunes. wh~ch are now part of a dune
revegetation program Addit~onal dredg~ng In the
south arm of the estuary IS scheduled for 1986,
and slmllar dune rebutldrng act~v~t~es are planned
shallow embayment became the beach line, and
only a narrow channel remained. Control of vehicle
traffic along the urban periphery of the estuary had
allowed some vegetation regrowth.
Disturbances that occurred south of the estuary
mouth in the 1960's left scars that were still
apparent in 1985. Near the Border Field State
Park overlook, areas used to corral horses and an
exercise track used to rehabilitate race horses
have been slow to recover. Twenty years later, the
highly compacted soils support only limited
vegetation. Military installations, shown as
irregular circles in the 1953 aerial photo (Figure
10) were filled with sands washed from the beach
and dune during the 1983 storm. Standing water
accumulated east of the road to the overlook,
allowing establishment of brackish marsh
vegetation in that area. Reduced soil salinities and
nutrient-rich sewage spills stimulated the invasion
of weedy wetland species, such as brass buttons
(Cotula coronopifolia).
2.4 STREAMFLOW HISTORY
In comparison w~th earl~er estuary cond~t~ons,
several changes had occurred by 1985 (Figure 15)
Natlve dune vegetation was almost entlrely gone
both north and south of the mouth Along the
channels, salt marsh vegetat~on that died back in
1984 had begun to recover in 1985, although
species cnrnpos~tion had changed (Chapter 5)
Okes that once separated the sewage lagoons had
been breached and wtdened (Chapter 6) The
islands In the mairl channel were obl~terated by the
sand washover of 1983 The mouth was artlfrcially
cut, and south of the new mouth, it is clear that the
beach l~ne had moved Inland What was once a
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). 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 191 2, Barrett to 1922, and
Rodr~guez to 1936 The~respective capacltles are
62 mill~on m3 (50,210 acre-ft), 55 mllllon m3
(44.760 acre-ft), and 138 million m3 (111,880
acre-ft) The largest and closest to Tijuana Estuary
is Rodr~guez Dam, just upstream of the clty of
Tljuana It IS ltkely that dry-season flows are lower
and that flood flows are delayed by the presence of
these resefvorrs
Flows of the Trjuana Rlver have been measured
slnce 1937 at the Neslor gage, whtch includes
99 6% of the watershed (F~gure 11 On the average
the Tljuana River has ~ts peak flow in March
(Figure 16) However. as w~th ratnfall data,
averases tell little of the relationshl~ between
streamflow and estuar~ne dynamics Year-to-year
Figure 34. Cordgrass mortality during nontidai flows are h~ghly var~able (coefficient of variatron =
conditions in 1984 (photo by J. fedler).
325%)- as are monthly flows (c v = 690%) With