The Ecology of Tijuana Estuary, California: An Estuarine Profile

c. Dredging at Tijuana Estuary will generate The expense of growing and transplanting the
spoils that differ in quality, depending on the native perennials requires that plants should be
source of sediments (sand dunes or bluffs) and protected with fencing.
proximity to sewage spills. Sandy spoils should be
used to rebuild the dunes, but spoils of low quality
should be taken off the site. The desirability of
b. Dune rebuilt but not fenced. Segments of
dune will be left unfenced to evaluate the
using fine sediments to replenish the dune needs
further evaluation (Williams, pers. comm.).
effectiveness of fencing.
c. Existing topography left; fencing added The
d. The tidal flushing regime needed to maintain role of fencing in helping to build up the dune will
the natural diversity of habitats is difficult to define, be evaluated by this treatment with the
because there are few records of tidal action within next.
the estuary. The studies of marsh vegetation
suggest that there should be continual but not
necessarily vigorous tidal flushing Cordgrass is
not recovering rapldly from the 1984 drought, in
fact, it continued to decline In 1985 We suspect
that sluggish tidal actlon may be necessary at
times to reduce p~ckleweed vigor in order for
cordgrass to have a competitive advantage
6.2.2 Beach and Dune Erosion
Winter storms that coincide with h~gh sea levels
erode the dunes and beach Summer is the
rebullding phase in an annual cycle of removal and
replenishment However, ~f the replenishing sands
are intercepted in therr transport along shore or
downstream, the beach and dunes show a net loss
(tnman 1985) In addlt~on to the annual cycle.
there is a long-term trend for an Increase in mean
sea level, whtch means that the beach will
gradually move inland
It may be impossible to prevent beach and dune
erosion at Tijuana Estuary, however, slowtng the
losses and restoring dune plant and animal
commun~tles are achtevable management goals
Disturbance of beach vegetation has contributed to
the destabil~ratron of the dunes, and replanting IS
des~rable. However, because initla1 attempts to
reconstruct and replant the dune north of the
estuary mouth have iarled, a new approach is
called for The dune south of the mouth will be
restored 111 late summer 1986, after the period of
least tern nesting Dunes will be rebuilt in two
rows, to create a double barrler to storm erosion
Plans for thrs second phase of dune reconstruction
rncorporate four exper~mental treatments that will
be repl~cated along the dune and evaluated over at
least a 2-year period.
a. Dune rebuilt by bulldozers and fenced with a
sand-trapping web This combination is expected
to be most effective In stabtlizing the dune, and the
largest length of dune will be assigned ta thls
treatment Several areas within th~s treatment will
be planted wrth native vegetation to encourage
natural stabilization of the substrate Revegetation
alone rs probably insufficient to control erosion,
there 1s no native grass and plant cover is patchy
d. Existing topography unaltered. These "control
plots" will serve two purposes, a comparison for
other treatments and a refuge for native organisms.
Patches along the beach where native plants
remain will be included in these plots. Dune
insects are expected to flourish in these refuges
and to expand onto the restored dunes.
As the effects of each treatment become clear,
the management program can be modified to
incorporate the most effective measures.
Measures of dune profiles, vegetative expansion,
and insect colonization will insure that restoration
of the total dune ecosystem is assessed and that
revised plans are based on a broad data base.
6.2.3 Streamflow Modifications
Reservoirs can 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 Unlted States portion
of the Tljuana River watershed, Barrett and Morena
Reservo~rs trap streamflow and presumably modify
the timlng and volume of floodflows Water can be
d~scharged from Morena to Barrett, but the gates of
Barrett Reservorr cannot be opened once water is
impounded, thus, drawdown is not possible In
Mex~co, Rodriguez Reservoir has gates that can be
used to lower water levels, and the prolonged
discharges of 1983 indicate the magnitude of
strearnflow change that can occur as a result of
reservoir drawdown Record ftows for 10 months
(March through December) occurred during periods
of little rainfall and usual low streamflows (Table
29) At such times, salrn~tres are lowered (Chapter
5), tidal regimes are probably muted, and nutr~ent
concentrations increase (Covin 1984)
There are several scenarlos under which
streamflow into Tijuana Estuary might be altered by
wastewater disctlarges. The city of fijuana,
Mexico. IS directly upstream of the estuary, and the
land slopes northward towards the United States.
At present, the largest influxes occur as sewage
spi!ls, when Mexican pipelines, designed to carry
sewage west and south to an ~ntertrdal outfall.