WSP12647 ‎(2)‎

• File Number: 8282.400
• Description: Colorado River Operations and Accounting - Deliveries to Mexico
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 6/1/1999
• Author: Environmental Defense Fund
• Title: A Delta Once More - Restoring Riparian and Wetland Habitat in the Colorado River Delta - Environmental Defense Fund - 06-01-99
• Water Supply Pro - Doc Type: Report/Study

OJJ3:.4

In summary, the delta's wetlands and riparian zone
currently cover 150,000 acres (60,000 ha). A quarter of
this is high biomass, woody riparian vegetation, and
16,000 acres (6500 ha) is emergent marshland dominated
by cattail and other hydrophytes. Most of the remain-
ing area is more sparsely vegetated by scrub associations
such as tamarisk, arrowweed, and halophyte shrubs.
Approximately 2200 acres (900 ha) of salt flats in the
tidal portion of the river are covered by Palmer's salt
grass, a grass endemic to the northern Gulf of
California. The woody riparian vegetation is dominated
by the native cottonwoods and willow in the northern
part of the riparian zone, but by the more salt-tolerant
tamarisk, a nonnative, in the southern part of the delta
as the river approaches the tidal zone. The riparian zone
of the Colorado River in Mexico has larger areas of
native riparian forest and marsh habitats than the
upstream stretch in the United States. Based on projec-
tions from other studies on the lower Colorado River,
delta riparian vegetation can potentially support 120,000
migratory and resident summer birds. Marsh habitat
supports several endangered avian and fish species.
These observations point to the high ecological value of
the Colorado River delta.

THREATS TO DELTA HABITATS

The principal threats to delta habitats are insufficient
and unreliable water supplies and their relatively poor
water quality. Satellite and aerial photography indicates
that most of the present delta vegetation has been re-
stored and maintained by flood releases over the last 20
years (Glenn et aI., 1996). A 1972 series of aerial photo-
graphs showed that most of the riparian zone was bare
soil and sparse mesquite trees. A satellite image taken
in May 1992 after four years without flow in the river
showed high-density vegetation in Zones 1 to 5 confined
to the edges of the river channels. Increased salinity due
to tidal flows had allowed tamarisk to invade.

By contrast, after flood releases in 1997 and early 1998,
high-intensity riparian vegetation (R1) occupied
approximately 30 percent of the floodplain, with evi-
dence of widespread seed germination of native trees

ENVlRONllriENTAL DEFENSE FUND

as well as tamarisk. Data for 1997 and 1998 also show
that peak flows of3500-7000 cfs (100-200 m3 Is) are suf-
ficient to inundate nearly the entire floodplain between
the levee system below Morelos Dam, and to dilute sig-
nificantly the salinity of ocean water in the tidal zone.
In the absence of such flooding, the marine influence
extends 34 miles (56 km) upriver from the mouth (Payne
et aI., 1992). Thus, it appears that the reestablishment of
native forest species in Zones 1 to 3 has been a direct
consequence of the return of overbank flooding below
Morelos Dam since the filling of Lake Powell.

Several potential changes could effect the precarious
health of the delta. Reductions in water supplies avail-
able to the delta could occur under several scenarios.
Additional storage and diversion projects in the basin
(e.g., the Animas-La Plata in Colorado) are still under
consideration. Were such projects built, they could vir-
tually eliminate the kind of flows that have been found
to support the regeneration of native trees in the delta.
Similarly, a proposal by California to change the criteria
used by the Bureau of Reclamation to define periods of
surplus and shortage would trigger greater releases for
consumptive use and increase the frequency of periods
without flood releases. Finally, if the Yuma desalting
plant comes on line, it will have devastating conse-
quences for la Cienega de Santa Clara, which will receive
the plant's concentrated brine waste (Glenn et aI., 1992).
[See Box4.]

Water quality threats to the delta include high concen-
trations of selenium in the Rio Hardy IRio Colorado
wetlands and la Cienega de Santa Clara,37 and high
salinity. Selenium is present throughout the lower
Colorado River watershed as a naturally occurring trace
element that is harmless in low concentrations but can
be toxic to fish and wildlife at higher concentrations as
it accumulates in the food chain (Presser, 1994; Presser
et aI., 1994; Ohlendorf, 1986). It often is found in soils in
arid and semiarid climates, particularly in ancient sea-
beds. Selenium is dissolved by irrigation water, and then
evaporation can increase its concentration in both wa-
ter and sediments. In the lower Colorado, selenium
levels are highest in oxbow lakes and backwaters

37 Water sampling and analysis was carried out at 19 sites along the Rio Hardy/Rio Colorado wetlands in July and August 1998, and at 10 sites in la
Cienega de Santa Clara in 1996 and 1997. The sampling and analysis followed EPA and Arizona Department of Environmental Quality procedures. The
results for the Rio Hardy/Rio Colorado are summarized in "Information Database and Local Outreach Program for the Restoration of the Hardy River Wet-
lands, Lower Colorado River Delta, Baja California and Sonora Mexico" (Valdes-Casillas et aI., 1998a). The results of research on water quality in the Cienega
are summarized in "Bioaccumulation of Selenium in the Cil~nega de Santa Clara. Colorado River Delta. Sonora, Mexico:' (Garcia-Hernandez, 1998).

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