The Delta's Remarkably Resilient Flora and Fauna

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

"All this wealth of fowl and fish was not for our delectation alone. Often we came upon a bobcat, flattened to some half-immersed driftwood log, paw poised for mullet. Families of raccoons waded the shallows, munching water beetles. Coyotes watched us from inland knolls, waiting to resume their breakfast of mesquite beans, varied, I suppose, by an occasional crippled shore bird, duck, or quail.  At every shallow burrow were tracks of burro deer. We always examined these deer trails, hoping to find signs of the despot of the Delta, the great jaguar, el tigre." 

-- Aldo Leopold, A Sand County Almanac 

The Colorado Delta of today bears little resemblance to the one Leopold witnessed from his canoe in 1922.  A series of dams, storage reservoirs and diversion canals built in an effort to quench the insatiable human thirst for Colorado River water have drastically diminished both the size and quality of the delta. As precious habitat began to rapidly disappear, the once numerous species declined alongside, and the delta was largely written off as "dead."

Over the last couple of decades however, the delta has shown unmistakable signs of life. Periodic flood-flows that returned to the delta in the early 1980's, as upriver storage reservoirs filled, have begun to restore valuable native riparian corridors and rejuvenate the historically productive Gulf Estuary. Agriculture and municipal wastewater, unfit for human use, is responsible for recreating wetlands that host an impressive array of migratory and resident birds. Several threatened or endangered species currently depend on delta habitat for their survival. 

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PHOTO: The endangered Desert Pupfish (Cyprinodon macularius) lives in warm desert pools, marshes, streams and springs. CREDIT: U.S. Fish and Wildlfife Service

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Although the delta might never again look like its former self, its importance as an oasis in the middle of the desolate Sonoran desert to plants and animals of the sea, sky and land is undeniable. Fortunately, the resilient delta and its remaining native inhabitants have shown the uncanny ability to sustain themselves on very little. If the modest flows from the Colorado continue to reach the delta and are not diverted elsewhere in the future, native species, important in so many ways, will have the chance to endure and prosper.

Flora

By providing food, shelter and breeding grounds for other members of their communities, the various trees, plants and grasses found throughout the region form the structural base of the delta's ecosystems. 

The delta's riverbank forests along the main stem of the Colorado provide essential habitat for numerous avian species. Native gallery forests of cottonwood and willow have begun to reestablish themselves since the return of periodic flooding and provide much better habitat than the invasive salt-cedar vegetation that prospered when the delta was cut off from flood flows. Capable of surviving on flows occurring every three or four years, these native hardwood trees are but one example of the delta's capacity for self-renewal.
 
The wetlands now found in the delta are largely the product of water that drains off of nearby farms. This brackish water is unable to support the hardwood gallery forests that prevailed before upriver diversions cut off the annual flows of freshwater. Salt-tolerant plants like cattails and phragmites dominate the wastewater-fed wetlands. Although not as beneficial as the native vegetation, these plants nevertheless provide crucial habitat for endangered species such as the Yuma Clapper Rail.

Birds

The delta is home to a wide variety of resident and migratory birds. A crucial element in how birds select their habitat is what tree species predominate. Many birds migrating along the Pacific flyway favor the native cottonwoods and willows that grow along the Colorado River in the delta. Because more of these preferred trees are found in the delta than on the Colorado farther upstream in the U.S., delta habitat is essential for the birds of the region and those that travel through semi-annually. Without it, many of them may become threatened or forced to find other, less accommodating habitats. 

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PHOTO: A black-crowned Night Heron (Nycticorax nycticorax) is one of a wide variety of birds found in the delta. CREDIT: George Jameson

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Fish and Marine Mammals

The delta's intertidal zone, where the Colorado River meets the Gulf of California, is home to a multitude of aquatic species. Most no longer flourish as they did when this rich estuary was the fertile breeding ground of the legendary Gulf fishery. The lack of freshwater from the Colorado flowing into the Gulf has dramatically changed both the temperature and salinity in the estuary. As a result, Gulf species dependent on this irreplaceable breeding habitat are in sharp decline. Overfishing has also played a significant part in the waning productivity of the Gulf fishery. The vaquita porpoise, the world's smallest marine mammal, and the totoaba, a steel blue fish growing up to seven feet long and weighing 300 pounds are now virtually extinct. These are but two of the many species harmed by the changed conditions in the delta.

Like other areas of the delta though, the fate of this prolific habitat is not sealed.  With the recent return of freshwater flows to the estuary, productivity is increasing.  Scientists have made a positive correlation between high flood flows reaching the delta and an increase in shrimp productivity. The Mexican government has limited overfishing by establishing the Biosphere Reserve of the Upper Gulf of California and the Colorado River Delta, severely restricting commercial fishing in a core zone. The combination of improved breeding grounds in the delta and protection against overharvesting in the Gulf has the potential to rejuvenate a troubled fishery.

Mammals

Mammals both large and small once roamed the delta, stabilizing a diverse ecosystem and providing meat to the Cucupá Indians. The ubiquitous rabbits, rodents, wild boar, mule deer and big horn sheep that prevailed about 100 years ago have largely been forced out, as the delta no longer provides suitable habitat. The larger mammals in particular struggle to find food and sufficient space in a land that is only a remnant of its former self. Being at or near the top of the complex delta food web and needing vast areas of suitable habitat for survival, these species are often the least able to adapt to the  languishing delta. The health of the delta ecosystems will have to improve markedly if these treasured animals are to make a strong comeback.

View our slideshow Hope for the Colorado River Delta

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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

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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