Once a Mighty Delta: History

Posted: 01-Mar-2003; Updated: 20-Jun-2003

For eons, before irrigation projects began diverting large amounts of water from the Colorado River for agriculture in the early 20th century, a great delta existed at its mouth. The volume of water reaching the Colorado River Delta varied widely throughout the year, flooding the banks during spring snowmelt and flowing more modestly in the fall and winter, forming a dynamic ecosystem. Undammed, the river carried nourishing silt from high in the Rocky Mountains and throughout the Southwest as it flowed toward the Gulf of California, creating an expansive alluvial plain of dense wetlands and salt flats amid the arid Sonoran desert. As many as four hundred plant species and prolific wildlife flourished in this rich desert estuary. For 2,000 years the indigenous Cucupá made their living by fishing, hunting and gathering the delta's bounty of plants and animals. 

In the 16th century, Spanish sea captains sailed up the delta, believing that the Baja peninsula was an island and that the delta would lead to the Pacific. Two hundred years later the U.S. military ran supply boats from the Gulf of California to forts as far north as Yuma, Arizona. The river once carried so much freshwater downstream that when it collided with the Gulf of California's formidable tides that inundate the lowest reaches of the delta, an enormous wall of water would form. One such tidal bore capsized a passenger steamship in 1922, killing 86 of the 125 passengers and stranding the rest.

The 20th century brought development in the Colorado River watershed that changed the delta dramatically. The U.S. federal government dammed the river under policies designed to attract people to the desert West, and water was siphoned off to grow crops, shrinking the volume of water flowing down to the Gulf. Later, cities in the southern Rocky Mountains and throughout the Southwest began to divert water for domestic and industrial uses. 

A complex system of rights developed to allocate water for consumption. At present, more than 10 major reservoirs on the river are kept filled near capacity to maximize electricity production and to ensure that water obligations can be met during drought years. The Colorado River is one of the most controlled, and most controversial, in the world.

Today, the delta covers less than one tenth of its original expanse, consisting of about 150,000 acres south of Yuma, Arizona and Mexicali, and north of the Gulf of California. Much of what comprised the delta's upper reaches has been converted to irrigated farmland, while the formerly vegetated lower reaches of the delta are now barren mud or salt flats. Wildlife habitat has been severely reduced.

Despite the vast uses and storage of water upstream, the Colorado River Delta is not really the dead, water-starved ecosystem that it is often thought to be. Since the last reservoirs on the river filled in 1981, floods caused by heavy rains and snowfall have once again brought water into the delta. A big flood in 1983 was first considered an aberrant event, but repeated flooding through 1993 inundated the delta. No Colorado River water is officially allocated to support delta wetlands, but occasional, inadvertent floods in the last several decades have supported them nonetheless.

The return of modest floods to the Colorado River Delta has resulted in dramatic gains in ecosystem health. The once "dead" ecosystems have demonstrated a remarkable resilience. These important native habitats support the resident and migratory birds that depend on this rare green in the midst of the Sonoran desert. Principle habitats in the delta include:

Riparian forests: Some 2000 acres of native cottonwood and willow trees have re-established along the banks of the river in the upper delta.  These forests are important to neotropical migrating bird species such as the Southwest willow flycatcher, listed as endangered in the U.S. Native riparian forests are exceedingly rare in the southwestern U.S. (less than 250 acres remain on the lower Colorado River) as altered river flows have allowed the non-native salt-cedar, a shrubby tree of little habitat value, to flourish in their place.

Brackish wetlands: Marshy wetlands flourish where agricultural drain water from the U.S. and Mexico flows into uncultivated lands in the delta. These habitats resemble the former delta's watery habitats but differ in that the vegetation must thrive on  brackish water. Consequently, cattails and reeds flourish where native vegetation once grew. Nevertheless these wetlands support vast numbers of birds. 

The Ciénega de Santa Clara:  This large, open water wetland is a unique habitat in the delta, kept alive by water that drains from farms in the U.S.  The Ciénega is home to an impressive diversity of birds, including pelicans, ducks of all sorts, and the endangered Yuma clapper rail.  Its waters sustain one of the largest remaining populations of desert pupfish.

Intertidal Wetlands: In the delta's southernmost reaches, salt flats are regularly inundated by tides from the Gulf of California. The endemic Palmer's salt grass thrives here.



View our slideshow Hope for the Colorado River Delta

LEARN MORE

The Colorado River Delta
Once a Mighty Delta: History
The Delta's Remarkably Resilient Flora and Fauna
A Decades-Long Tug-of War over Water
The People of the Delta: Yesterday and Today
Restoring the Delta

Another Step Forward for Restoring the Colorado River Delta (10/22/2002 article)

Mapping Conservation Strategies in the Colorado River Delta: A State-of-Knowledge Workshop (October 15-17, 2002)
A Delta Once More (1999 report from Environmental Defense)

University of Arizona web site