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

Today, the Colorado River irrigates more than 3.7 mil-
lion acres (1.5 million ha) of farmland in the
southwestern United States and Mexico, and supplies
water to nearly 30 million people. It is one of the most
highly regulated and diverted rivers in North America:
virtually every drop is accounted for in the allocation of
water among 9 states (7 in the United States and 2 in
Mexico) and 27 native tribes that have rights to use it
(Pontius, 1997). While irrigated agriculture tops the list
of Colorado River water uses in the United States and
Mexico, the second largest consumption of water is
evaporation from reservoirs.s Diversions out of the
Colorado basin, such as water piped to Los Angeles,
are the third largest draw, and are followed by munici-
pal and industrial uses. Federal hydroelectric plants
along the Colorado have a total generating capacity of
about 4425 megawatts (MacDonnell and Driver, 1996).

THE DELTA TODAY-
A CONTEMPORARY GEOGRAPHY

Where the Colorado River meets the United States-
Mexico border, the Morelos Dam stands as the last major
structure in the river's mainstem. [See Figure 2] For about
eight miles, the border follows the river, after which the
river is fully in Mexico. Water managers refer to two
reference points along the reach of the Colorado that
forms the border: the Northern International Boundary
(NIB) at Morelos Dam, and the Southern International
Boundary (SIB) where the river leaves the United States.

The Colorado River delta is surrounded today by an area
known as the Mexicali and San Luis Rio Colorado
agricultural valleys. At present, the delta encompasses
approximately 150,000 acres (60,000 ha), in a basin
surrounded by nearly 500,000 acres (200,000 ha) of
irrigated agriculture land. Two rivers form its core: the
Colorado, and also the Rio Hardy, a tributary to the
northwest. In addition, water from the Wellton-Mohawk
Canal empties into the eastern delta.

These freshwater sources support delta vegetation,
found in riparian areas and wetlands. Although
constrained by levees, the delta is broadly bound by the
Cucapa Mountains to the west, and the Sonoran Mesa
to the east. Two islands, Montague and Pelicano, mark

where the delta meets the Gulf of California to the south.
Another significant feature is the Laguna Salada, a dry
depression west of the delta into which Colorado River
waters drain when flows are high.

Although the Colorado basin drains 244,000 square miles
(632,000 km2), including 2000 square miles (5200 km2)
in northern Mexico, most of its water does not reach the
delta. During the twentieth century, river flows into the
delta have been reduced nearly 75 percent, from an
annual average between 1896 to 1921 of 16.7 million acre-
feet (maf) (20.7 x 109 m3) (Fradkin, 1981), to an annual
average between 1984 and 1999 of 4.2 maf (5.2 x 109 m3)
(Glenn et aI., 1999). This reduction in water has resulted
in major changes to the delta: less silt, fewer nutrients,
higher salinity, and higher concentrations of pollutants.6
Erosion - rather than accretion - is now the dominant
physical process in the delta (Thompson, 1968), a highly
unusual condition for a river delta. Like other river
deltas at risk, such as the Nile's, the Colorado's delta
has actually begun to decrease in size (Stanley and
Warne, 1993).

The loss of freshwater flows to the delta over the past
century has reduced delta wetlands to about 5 percent
of their original extent, and nonnative species have
compromised the ecological health of much of what
remains. Stress on ecosystems also has allowed
invasive plants to choke out native species along
Colorado River riparian areas. Native forests of
cottonwood and willow have yielded to sand and
mudflats dominated by the nonnative tamarisk (also
known as salt cedar), arrowweed, and iodinebush, a
transformation that has decreased the habitat value of
the riparian forest (Briggs and Cornelius, 1997).

The Colorado River delta and its estuary ecosystems
may bear the worst effects of the river's heavy upstream
use and development, and development in the delta
further compromises its health. Much of the upper delta
has been converted to irrigated farmland, and levees and
channels have changed the physical delta significantly.
In the lower delta, where salinity makes agriculture
impossible, the effects of upstream water diversion and
development are clearly seen.

5 Allocations made under the laws and compacts that make up the Law of the River (see Chapter 2) do not account for 1.5 maf in annual evaporative losses
from mainstem reservoirs (Pontius, 1997).
6 The natural ecology of most of the world's large river systems has been disrupted by dams, flow diversions, channelization of the riverbed, and alteration
of riparian zones by agricultural activities which in turn reduce flows, sill accretion, and nutrient loads to their deltas.

4