Completing a 53-day journey through its Delta, the Colorado River reaches the tidal zone of the Sea of Cortez (Gulf of California). Photo credit: Francisco Zamora, Sonoran Institute, with aerial support from LightHawk.

After coursing through its delta for nearly eight weeks, the fresh waters of the Colorado River have touched the high tides of the salty sea.

It is the first time in sixteen years that the Colorado River, which flows 1,450 miles (2,334 kilometers) from its headwaters in the Rocky Mountains of Colorado to the Sea of Cortez (Gulf of California) in northwestern Mexico, will have reached its final, natural destination.

This reunion between river and sea is due to an agreement between Mexico and the United States, known as Minute 319, to advance the restoration of the Colorado Delta by releasing a pulse flow and sustaining base flows in a five-year experiment.

The pulse flow, which began on March 23, is now nearing its end. Scientists had not planned on the river reaching its estuary as part of this grand experiment. But that it has, is a wonderful bonus.

This confluence of the river and the high tides signals that “improving estuarine conditions in this upper part of the estuary is possible if restoration efforts continue in the future,” Francisco Zamora, director of the Colorado River Delta Legacy Program at the Sonoran Institute, wrote to me in an email. Zamora took the photos featured in this post on Thursday, May 15, from a low-flying plane operated by LightHawk.

If rivers are born with a destiny, it is to reach the sea. They carry sediment, nutrients and freshwater from the land to the coastal zones, helping sustain the productivity and abundance of marine environments.

Deltas and estuaries – where rivers and seas connect – are some of the most biologically rich ecosystems on the planet.

Before the big dams and diversions of the 20th century, the Colorado’s nutrient-rich freshwater mixed with the Upper Gulf’s salty tides to create the perfect water chemistry and nursery grounds for Gulf corvina, totoaba, brown and blue shrimp, and other fisheries of great commercial and cultural importance to the region and to the indigenous Cucapá.

But over recent decades, a combination of over-fishing and lack of freshwater in-flow has caused fish populations in the Upper Gulf to plummet.

Since the completion of Glen Canyon Dam in 1963, the Colorado has connected with the sea only a few times – mostly during El Niño weather events that brought unusually large amounts of snow and rain to the Colorado Rockies and the upper watershed. The last time the Colorado reached the sea was in 1998.

The estuary is now part of a protected biosphere reserve and no-fishing zone, an attempt to give fish – as well as the highly endangered vaquita, the world’s smallest porpoise – a chance to revive their numbers.

The pulse flow, which was designed to mimic the Colorado’s natural spring flood, is an experiment of historic political and ecological significance: it is the first time the United States and Mexico have made a conscious decision to give some water back to the river to revive the health and habitats of its delta.

One of the planet’s great desert aquatic ecosystems, the delta once boasted some 2 million acres of lush wetlands teeming with birds and wildlife.

Over the past eight weeks, the pulse flow has brought needed water to active delta restoration sites, where conservation groups have planted hundreds of thousands of cottonwoods, willows and mesquite to begin re-establishing habitats for hundreds of species of birds and wildlife.

Timed to coincide with the germination of these native trees, the pulse is also helping new habitats emerge spontaneously along the river.

On the heels of the pulse flow, the Colorado River Delta Water Trust will provide sustaining base flows made possible by purchasing voluntary leases of water from delta farmers.

[Change the Course – a partnership of the National Geographic Society, Bonneville Environmental Foundation, and Participant Media – has partnered with the Delta Water Trust to help provide these crucial base flows.]

Compared with the natural, pre-dam flow of the river through its delta, the volume of water restored through Minute 319 is small – less than 1 percent of the river’s historic flow. But that flow is being strategically timed and directed to produce the highest ecological benefit possible. Teams of scientists are monitoring the effects on the hydrology, vegetation, birds and other ecological features of the delta, so that future flow releases can be even more effective.

The pulse flow experiment did not specifically plan on the river reaching the sea.

But against the odds, at least a small volume of the Colorado River has fulfilled its destiny – and made it home.

Sandra Postel is director of the Global Water Policy Project, Freshwater Fellow of the National Geographic Society, and author of several books and numerous articles on global water issues. She is co-creator of Change the Course, the national freshwater conservation and restoration campaign being piloted in the Colorado River Basin.