Colorado River Delta

Overview

The Delta before the early 1900s

Prior to the construction of dozens of large dams on the mainstem and tributaries, the Colorado River carried million tons of sediment annually to its lowest reaches, building a broad alluvial plain from the Salton Sea [link to Salton Sea wiki page] to the Gulf of California. At the southern end of this plain, close to sea level and the Gulf, the Colorado River branched out into multiple distributary channels, creating over 3,000 square miles of freshwater, brackish, and saltwater wetlands: the Colorado River Delta. This shifting mosaic of verdant wetlands was sustained by the river’s inflows, which intermingled with seawater carried upstream by the tides.

Thousands of Cucupá (now also known as Cocopah or Kwapa) people lived on the higher ground of the Delta, farming corn and other crops in seasonally flooded fields and fishing in the channels and the lagoons. European visitors to the Delta from the 1500s onward remarked on the power of the tides and the confounding physiography of multiple channels and soft sediments that hindered upstream travel by boat and by foot. It was not until the early 1900s that visitors, most famously Aldo Leopold, began to document what the Cucupá had long known about the Delta: Its dynamic landscape of wetlands supported numerous and diverse plants and animals. The Delta, Leopold wrote, held “a verdant wall of mesquite and willow…a hundred green lagoons…fleets of cormorants” and many other birds, beavers, and jaguars. The Delta’s waters hosted an equally prolific aquatic fauna, including billions of endemic Colorado delta clams.

The 20th-century drying of the Delta

By the time of Leopold’s exploration in 1922, the ecohydrology of the Delta was already being impacted by water use upstream. Then the closing of Hoover Dam (Lake Mead) in 1935, Morelos Dam (Mexico’s mainstem diversion) in 1950, and Glen Canyon Dam (Lake Powell) in 1963 progressively cut off virtually all of the water and sediment that formerly fed the Delta. By the late 1970s, only a few Cucupá still lived in the Delta, and over 90% of the original wetlands had been replaced by agricultural fields, invasive plant species, or most conspicuously, dewatered and barren sediments. At that time, many observers wrote off the Delta as a “dead” ecosystem, even as a 40,000-acre wetland complex called the Cienega de Santa Clara emerged in the upper Delta, incidentally created by new flows of brackish agricultural drain water from Arizona.

In 1980, Lake Powell finally filled, shortly followed by an unusually wet period in the mid-1980s, a wintertime flood on the lower Colorado in 1993, and then a smaller “pluvial” in the late 1990s. Flows into the Delta surged, even reaching pre-Glen Canyon Dam levels in a few years, temporarily bringing life back to parts of the Delta. The rapid ecological response to these higher flows and the persistence of the Cienega de Santa Clara indicated the potential for long-term restoration of the Delta if even modest inflows could be secured. In 1996, environmental groups on both sides of the border began holding workshops in communities to promote restoration of the Delta. In 2000, the United States and Mexico signed Minute 306, acknowledging for the first time the value of the riparian and estuarine ecology of the Colorado River Limitrophe (the river reach below Morelos Dam) and Delta to both countries.

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Additional resources

State of the Science Report

Chapter # of the State of the Science report, describes...