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

Salinity also may have a profound influence on a key-
stone species of the northern gulf. Shrimp populations
are believed to increase with flood flows from the delta.
Researchers have demonstrated a positive correlation
between freshwater discharges to the Gulf of Mexico
and shrimp catches (Galindo-Beet et al.). Shrimp in the
northern Gulf take a year to reach harvest size, and since
1981 the size of the northern Gulf shrimp catch has cor-
responded with freshwater flows of the previous year
(Glenn et aI., 1998). Research indicates that 25 percent
of the variability of the shrimp catch is attributable to
river flows (Galindo-Beet et al.). Before dam building,
Colorado River floods may have affected salinity across
a wide area of the upper Gulf. Recent flood events have
affected salinity at the mout,h of the river near Montague
and Pelicano islands.39 In January 1998, freshwater flows
of 7135 cfs (202 m3/ s) were found to have diluted ocean
water at the northern end of Montague Island to a
salinity of 20 parts per thousand (ppt). Further research
is needed to quantify the relationship between
freshwater flows and Gulf near-shore marine species.

RESTORATION OBJ ECTIVES

The flows of 1997 helped restore vegetation to the delta,
and the research team considered this a major change
in the habitat value and health of the delta. It demon-
strates the resilience of delta ecosystems and offers hope
for the future. It also is worth noting that even in its
diminished state, the delta has richer and more diverse
ecosystems than the Colorado River between the Grand
Canyon and Morelos Dam, a stretch of river five times
greater in length and with a perennial flow of water
(Balogh, 1996).40

The 1997 fieldwork suggests that modest annual flows
(below Morelos Dam) of 32,000 acre-feet (4 x 107 m3)
could maintain and even improve the cottonwood-wil-
low habitat in Zones 1 to 3. Further, pulse flows on a
par with January to April 1997 releases, which are likely
to occur on average every four years under the present
Colorado River management regime, could sustain an
area that includes Zones 4 and 5 as well. The 1997 win-
ter flood totaled 260,000 acre-feet (3.2 x 108 m3), which
was discharged at 3500-7000 cfs (100-200 m3/ s), a rate
sufficient to inundate the delta's floodplain. This flow
regime (that is, 260,000 af every four years) represents
less than 1 percent of the Colorado's average annual

flow. Due to the resilience of the delta's native riparian
vegetation, the most important ecosystem functions can
likely be supported by only a fraction of the historic
flows, much of which could be derived from resource-
ful use of agricultural wastewater. However, due to
sustained flood flows during 1998 and 1999, it is not yet
possible to quantify with certainty the required volume
and frequency of these floods. In addition, freshwater
flow needs of delta fisheries and Gulf near-shore ma-
rine species have not been quantified, and should be
considered as the delta's water needs are determined.

The importance of the timing of flood releases is not yet
known. Although the Colorado's predevelopment flow
regime would flood the delta after spring snowmelt
(April-July) and again during monsoons (August), the
1997 flood occurred in late winter Ganuary-April). These
floods were successful in stimulating vegetation, and
one factor may be the mild climate in the delta.

Water quality problems in some riparian wetlands will
require mitigation, if only to protect humans who come
into contact with the water or eat the local wildlife and
fish. Continued monitoring of water quality in Zones 4,
5, and 7 should be designed to identify the need for pe-
riodic flushing flows or the procurement of alternative
water sources. Specific recommendations for improv-
ing the management of water supplies and water quality
are provided in the final chapter.

Understanding its ecological needs is an important com-
ponent of preserving the delta, but good science alone
will not suffice. The greatest threat to the health of the
delta may be the absence of any formal provisions
between Mexico and the United States recognizing the
ecological values in the delta and providing water to
support them as part of the overall apportionment of
Colorado water. Resource and environmental manage-
ment agencies in the United States tend toward the
position that their responsibility for ecosystem protec-
tion ends at the international border. The challenge is
not only one of water management, but also one of
mustering the political will and cooperation to manage
the delta as a transnational resource. The following
chapter discusses the numerous institutions that must
be considered during the process of policy reform, as
well as several related issues that may provide strategic
opportunities to improve conditions in the delta.

39 During flood events, the salinity of the Colorado River, upstream of the confluence with the Rio Hardy, was low (average of 0.61 ppt) compared to salinity
in the Colorado River at Imperial Dam in the U.S. (average 0.865 ppt) and at Morelos Dam (average 1.01 ppt).
40 The stretch above Morelos Dam contains 82,500 acres (33,400 ha) of vegetation, compared to 150,000 acres (60,000 ha) in the delta.

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