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A Colorado River reunion with sea?

To revive ecosystem, U.S., Mexico look past ‘pulse flows’

On Sept. 27, representatives from the U.S. and Mexico approved an update to the 1944 treaty that governs how the two nations manage the Colorado River. The new pact builds upon a 2012 agreement that expires this year.

For many people, the 2014 “pulse flow,” a large release of water from Morelos Dam, on the U.S.-Mexico border, was the defining feature of the 2012 agreement. The agreement also addressed drought, reservoir storage and environmental restoration in the Colorado River Delta. The 2014 release reunited the Colorado River with the Gulf of California for the first time since the late 1990s; it was both a scientific and symbolic success as communities along the Colorado River saw its dry channel once again fill with water.

But the pulse flow also showed that a single release of water may not be the most efficient way to revitalize the Delta. So while the new agreement, called Minute 323, includes environmental water releases, it doesn’t specifically call for another pulse flow.

Human demand for the Delta’s defining feature – water – has devastated its ecosystem, leaving much of it desiccated. Historically, “a verdant wall of mesquite and willow separated the channel from the thorny desert beyond,” Aldo Leopold wrote in “The Green Lagoons,” describing a 1922 canoe trip in the Delta.

“Fleets of cormorants drove their black prows in quest of skittering mullets; avocets, willets, and yellowlegs dozed one-legged on the bars; mallards, widgeons, and teal sprang skyward in alarm.”

Invasive plants like tamarisk have moved in, and farms have replaced most of the wilderness Leopold explored. Today, only small and disconnected pockets of wetland and riverside habitat remain. Environmental groups hope to protect and further restore those places by keeping water in the Delta.

The 2014 pulse flow was meant to mimic one of the Basin’s historic spring floods. The deluges that swamped the Colorado Basin before Hoover and Glen Canyon dams were built were big enough to erode banks and deposit sediment. That cleared the way for new cottonwood and willow seedlings to take root and left the ground wet enough for them to flourish.

The limited amount of water set aside for the 2014 pulse flow meant that it was many times smaller than historic spring floods. It didn’t create much of the bare ground that native plants need. They thrived only in areas that were cleared before the water arrived.

“Water is a scarce and very high-priced commodity to have it be doing stuff that you could do mechanically,” says Carlos de la Parra, an environmental policy expert at El Colegio de la Frontera Norte in Tijuana, Mexico.

Changes to the hydrology of the Basin also meant that more than 90 percent of the water released during the pulse flow seeped into underlying aquifers. Where groundwater levels were already low, the extra water did little to support continued plant growth.

“(Those water table levels) are so deep that they’re beyond the reach of the roots,” says Patrick Shafroth, a plant ecologist at the U.S. Geological Survey.

The timing and size of releases under Minute 323 is still to be determined. The agreement includes 210,000 acre-feet of water over nine years for environmental projects. The 2012 agreement included about 105,000 acre-feet for the pulse flow and more than 50,000 additional acre-feet for smaller releases.

Irrigating specific restoration sites with targeted releases is one possibility for the future. That would deliver water to reaches where it can nurture plants and keep it away from areas with deep aquifers – but it wouldn’t have the symbolic heft of a large release that wets the entire length of the Colorado.

Will there be a repeat pulse flow in the next few years? “I wouldn’t write that off yet,” says Karen Schlatter, who manages ecological monitoring and restoration programs in the Colorado River Delta for the nonprofit Sonoran Institute.

“Connecting the river to the sea is a big achievement.” The 2014 release went beyond ecological gains: the flow of long-awaited water highlighted the benefits of bringing Basin communities together as their residents swam, worked on restoration projects and enjoyed the river.

This article was first published on hcn.org.

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