Map of Colorado River pulse moving toward Sea of Cortez


Photo credit: Francisco Zamora, Sonoran Institute.

Update May 16: The Colorado River has now reached the delta for the first time since the 1990s. Conservationists will study environmental effects of the pulse for years to come.

Update May 13 from Karl Flessa: "Image taken Monday May 12.  Tidal waters in foreground, Colorado River in background. Connection will likely take place on or slightly before Thursday's high tide."

Update May 1 from Karl Flessa at the University of Arizona: "The river has resumed advancing, likely in response to the removal of an earthen dam downstream of the CILA restoration site and new, scheduled pulse flow deliveries from the Km 27 wasteway.  We don't know how far this mini-pulse will go."

On March 23 at about 8:15 a.m., a 2012 agreement between the U.S. and Mexico that was years in the making finally got real. Morelos Dam at the international border began releasing water in an experiment meant to restore a parched Colorado River Delta. The pulse of water may even allow the river to reach the Gulf of California for the first time since the late 1990s (and even so, reaching the ocean hasn't been a regular occurrence since the 1960s). The agreement, known as Minute 319, amends the 1944 U.S.-Mexico Water Treaty to guide bi-national management of Colorado River water until 2017. Over the course of eight weeks, the current flow, which is mostly through the Morelos dam, will release about 105,000 acre-feet of water (an acre-foot is about 326,000 gallons); the final release is planned for May 18. 

Karl Flessa, co-chief scientist of the Minute 319 Monitoring Program, is coordinating multi-agency efforts to understand hydrologic and biological effects of the pulses. Among other things, they're monitoring how much the water table is rising, how many cottonwood and willow seedlings will spring up near the river as a result, and how many more birds show up next year. As far as whether the experiment will reconnect the river with the Gulf in any definitive way, Flessa isn't very optimistic:

"I wasn’t encouraged when I saw the flow at the last crossing on Saturday (April 13). The river’s mouth is blocked by a low sand bar that causes the river’s flow to pond up before the Gulf. Connecting that ponded-up water with the Gulf may depend as much on the effect of the tide as the level of the water. The upper Gulf has a tidal range of 8 meters or so during new and full moons, so there’s a better chance of connection during those times of the month than at other times. And there’s no one geographic spot that’s easy to get to on the ground, where one can definitively state ‘the river has reached the sea.’ We may need to depend on satellite images and observations from small planes. And finally, to be even more downbeat over what is a really upbeat event, the amount of water that may reach the Gulf will be pretty small, and not likely to have much ecological impact. We’d all love to see it reach the gulf, but it’s not one of the stated objectives (of Minute 319). It will have great symbolic value, that’s for sure.”

This map from the Bureau of Reclamation shows the rate at which the water from the dam has moved down the Colorado, beginning in March. Flessa said Wednesday that the next stretch of river will be fairly difficult to monitor. "It's now in a zone that's really hard to get access to -- no more crossings, lots of shallow water and branching channels." Check back for updates we get on the river's progress. Tay Wiles is the online editor of High Country News. She tweets @taywiles.