Nature Responds to Colorado River Delta Pulse Flow

Progress report for the Colorado River Delta Pulse Flow

The Colorado River has been flowing in its delta for more than three weeks, thanks to a cooperative effort by the United States and Mexico to deliver a “pulse flow” of water.

The pulse flow is meant to mimic – albeit at a small scale – the spring floods that historically inundated the delta and were crucial to the community of plants and animals that thrived there.  The small size of the flow – less than one percent of the river’s annual yield – is nonetheless significant because so little water has reached the Colorado’s delta in recent years due to dams and diversions that allow extensive water use in both countries.

We have been watching the pulse flow make its way downstream, wondering how far it will go as water seeps into the sand, recharging the depleted aquifer that straddles the border.  Although the flow will continue for another month, more than 80 percent of the water has already been released into the river below Morelos Dam.  The initial large flows were meant to create habitat. The small flows occurring now are meant to sustain it. (See “American Nile: Saving the Colorado River.”)

In addition to creating the water for the pulse flow, Mexico and the United States have collaborated to establish a program to monitor the impacts of the pulse flow to tell us what happened:  Where did the water go?  How much groundwater recharge took place?  Did the flow change the river channel itself?  How many new trees were generated, and where?  What was the wildlife response?

Scientists continue to collect these data, and will prepare comprehensive reports in due time.  In the meantime, I can share with you some of what I observed during my two weeks on the river:


The water went where it was supposed to!  The scientists who advised the governments on how to engineer the pulse flow suggested that habitat creation would be promoted with flows that rose out of the river banks and inundated the soils, particularly in locations where year-round groundwater is available to sustain any new seedlings.  On a paddle in the first 10 miles below Morelos Dam, we saw ample evidence that the banks were wet, in perfect condition to start seedlings.


The timing looks good!  The pulse flow occurred at the same time that seeds were dropping from cottonwoods and willows, the native trees critical to high-quality river habitat (and so often absent from rivers in the arid West where we have dramatically changed river flows). Some of the seeds that land on newly wetted soils will germinate (some already have), sinking their new roots down in search of water.  This is how the riverside forests rejuvenate and stay healthy.


The birds loved it!  Paddling down the river, I was astounded to hear a riot of birdcalls in the middle of the day (I thought you had to get out early to hear that).  I’ve asked my ornithologist friends.  They say it’s unlikely that more birds were present (my observation was on day 6 of the pulse flow), but that it does seem the birds are vocalizing more than usual.  Happy birds!

In essence, the pulse flow is a demonstration, a test to see how an engineered delivery of water works in the delta.  But that’s not how it feels.  As Jeff Moag, editor-in-chief of Canoe & Kayak Magazine put it: “They say it’s an experiment, but it feels like a river!”

Changing Planet


Jennifer Pitt is the Colorado River Project Director for Environmental Defense Fund. She works with Colorado River water users throughout the Colorado River basin—including seven states in the United States and two in Mexico—to develop practical programs to restore river habitats and to dedicate water to environmental resources. She has worked as a park ranger and a Congressional aide, and has a Masters degree in Environmental Studies from Yale University.