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

MANAGE EXISTING FLOWS FOR THE
BENEFIT OF DELTA ECOSYSTEMS

As documented in this report, remnant delta riparian
and wetland ecosystems can be sustained with a mod-
est amount of freshwater and continued use of
agricultural wastewater. Current hydrologic conditions
and the level of water development in the basin suggest
that excess water will continue to be available for some
time. Nevertheless, restoring and protecting the delta's
ecological viability requires some assurance that the cur-
rent regime is not changed to decrease flows south of
Morelos Dam. Existing flood flows can be managed to
good advantage if they are released as pulse flows at a
rate sufficient to flush pollutants and encourage seed
germination with overbank flooding. Irrigation return
flows and wastewater are important perennial sources,
and should be preserved. With water quality monitor-
ing and improvements, wastewater flows would be of
even greater benefit to delta ecosystems.

. Protect perennial and pulse flows in the delta. Based
on our preliminary analysis of past flows, we
estimate that remnant riparian habitat could be
supported by a perennial flow of 32,000 acre-feet
(4 x 107 m3), supplemented by a flood flow of 260,000
af (3.2 x 108 m3). The flood flow should be released at
3500-7000 cfs (100-200 m3fs), a rate sufficient to
inundate delta riparian areas within the levees. This
pulse flow should occur on average once every four
years, which is the average periodicity for floods
under the current regime of Colorado River devel-
opment and management. Perennial flows should
maintain a core area of 150,000 acres (60,000 ha) of
riparian and wetland habitat. Periodic pulse flows
should saturate 250,000 acres (100,000 ha), contained
within the existing systems of levees. On an annual-
ized basis, these perennial and pulse flows would
represent less than 1 percent of the base flow of the
Colorado River. (See recommendation below to Change
Institutional AIrangements and Agreements to Support
Delta Ecosystems.)

. Assure release of flood flows to sustain delta
ecosystems during extended periods of drought.
Precipitation in the Colorado River basin in the years
since major upstream dams have filled has produced
flood flows in the delta on average every 4 years.
However, there is evidence in the long-term hydro-

logic record that significant drought periods are likely
to occur, possibly enduring 8 or 10, or even 20 years.
The delta's water needs should be met during
drought periods through a mix of dedicated and
purchased water deliveries, at the volumes and
frequencies detailed above. (See recommendation to
Establish Market Mechanisms and Funding Sources
for Delta Preservation below.)

. Abandon plans to operate the Yuma Desalting Plant.
Under no circumstances should la Cienega de Santa
Clara be dewatered through the diversion of waste-
water from the MODE canal. The Bureau of
Reclamation is considering proposals to allow the
plant to be used to treat agricultural wastewater from
California's Imperial Irrigation District. This plan can
be supported if it does not affect water supplies to la
Cienega de Santa Clara. BOR, SEMARNAP-INE
CNAIBWC

. Develop and implement water quality monitoring
programs. Wetlands supported by wastewater are in
danger of accumulating toxins at levels poisonous to
resident fauna. Water quality monitoring is needed
in the agricultural drains that flow into la Cienega de
Santa Clara, the Rio Hardy fRio Colorado wetlands,
and EI Indio marsh. Monitoring should focus on the
levels of pesticides, nutrients, selenium, arsenic,
boron, and other contaminants discharged by these
drains. CNA SEMARNA~ BOR, WelIton-Mohawk
Irrigation District

CHANGE INSTITUTIONAL ARRANGEMENTS
AND AGREEMENTS TO SUPPORT
DELTA ECOSYSTEMS

The 1997 research documented in this report captured a
snapshot of the delta returning to ecological health, yet
precariously dependent on water that is not dedicated
to its ecosystems. Current upstream use, reservoir man-
agement, agricultural drainage, and hydrologic
conditions have produced sustaining perennial and
pulse flows. But just as natural systems are dynamic, so
are the man-made systems and arrangements that
determine use of Colorado River water. Restoring and
protecting the delta's ecological viability will require
assurance that flows south of Morelos Dam are not
diminished.

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