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

Flood Rows and Water
Requirements of Delta Vegetation

Water deliveries below Morelos Dam since 1980 have
been extremely variable in frequency and volume.
Furthermore, knowledge of the vegetation is based on
a single snapshot inspection in 1997. Hence, it is not
possible to determine the vegetation response to a
particular flow regime without further study. However,
inspection of the vegetation that exists in the delta now
and the recent flow history allows some inferences about
the water requirements.

Since 1980, there have been two prolonged periods with-
out flow: a four-year period from 1988 to 1993, and a
three-year period from 1994 to 1997. However, the 1997
inspection showed cottonwood and willow trees dat-
ing back to both the 1980's and 1993 flow events. It
appears that germination correlates with flood flows. It
also appears that annual flood events are not necessary
for survival of the native tree species. They are capable
of surviving at least three-to-four-year intervals between
major flow events in the delta floodplain. It is not clear
whether their survival depends on local agricultural
return flows or other sources that may recharge the ri-
parian zone during periods in which water does not flow
from the United States. Apparently, delta riparian veg-
etation can survive a period of several years without
water deliveries from the United States, once flood flows
have allowed the seeds to germinate.

The vegetation analysis (see Delta Wetlands and Rivarian
Vegetation below) was based on a satellite image from
July 1997, plus field inspections. The analysis was
conducted following flood releases in January-April
1997 of approximately 260,000 acre-feet (3.2 x 108 m3) of
water at flow rates from approximately 3500 to 7000 fr
per second (100 to 200 m3 per second) below Morelos
Dam. A February 21, 1997, satellite image, plus
low-level aerial and ground surveys during February
flows, showed that the 1997 winter flood was sufficient
to cause overbank flooding of the Colorado River
channels throughout the floodplain between the levees.
In addition, this flood produced runoff from the flood-
plain into the Gulf of California and Laguna Salada, a
dry depression in the delta. During these flows, ocean
salinity was diluted to less than half the salinity of sea-

water at the northern tip of Montague Island. It appears
that flow rates of 3500 to 7000 ft3 per second (100 to 200
m3 per second) are sufficient to inundate the floodplain.
In addition, annual volume releases totaling 260,000
acre-feet (3.2 x lOS m3) in winter and spring are suffi-
cient to produce a vegetation response in summer. The
extent to which the July 1997 vegetation response was
due to early 1997 water releases, and how much might
have occurred without flooding, is not known. How-
ever, copious emergence of seedlings following the
floods was observed, so it is possible to conclude that
this volume of water was sufficient to support the exist-
ing vegetation and stimulate new growth along the
floodplain.

The entire floodplain between the levees is 150,000 acres
(60,000 ha), of which about 25 percent supported
high-density vegetation in July 1997. Potential evapo-
transpiration by wetland and riparian vegetation in the
floodplain is as high as 8 feet (2.5 m) per year. The
high-density vegetation, which consists mai~Iy of
cottonwood, willow, mesquite, salt cedar and cattail, can
use approximately 304,000 acre-feet (3.75 x 108 m3) of
water per year. This is greater than the flows recorded
during January through April 1997. The 1997 vegeta-
tion response may indicate that the floodplain is
recharged by local aquifers in addition to river flows
from the United States.

Based on these observations, it is apparent that irregu-
lar flows since 1980 have contributed to revegetation of
the floodplain despite three-to-four-year intervals of no
cross-border flows. The flood releases from Lake Mead
by the Bureau of Reclamation31 produce sufficient flows
to inundate the remaining floodplain area in the delta
and produce a vegetation response the following
summer.

Preliminary observations suggest that large, continuous
flows of water in the river are not necessary to support
the remaining delta riparian habitats. Conservation and
restoration goals might be achieved through two assur-
ances: 1) that when surpluses (as defined by the current
capacity for use) arise in the Colorado River system, they
will be delivered as flood flows to the delta; and 2) that
agricultural waste flows will continue to be conveyed
there.

31 The Bureau of Reclamation terms these floods "Stage 1 and Stage 2 space-building releases:'

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