Going with the flow: Nature responds to the Colorado River pulse flow

Jennifer Pitt

This post first appeared on National Geographic’s Water Currents blog.

For almost a month now, the Colorado River has been flowing in its delta – a region that has been deprived of regular river flows for half a century – all thanks to a cooperative effort by the United States and Mexico to deliver a “pulse flow” of water. The pulse flow was designed 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 made to allow extensive use of the river’s water in both countries.

We have been watching the pulse flow make its way downstream (see a map of the progress), 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% 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.

In addition to acquiring the water for the pulse flow, the United States and Mexico have collaborated to establish a program to monitor the impacts of the pulse flow: 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? As the scientists collect the empirical data to answer these questions and more, I wanted to share with you some of what I observed during my two weeks on the river:

The water went where it was supposed to

Scientists 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). A portion 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, as I thought you had to get out early to hear that. I’ve asked my ornithologist friends, and they say it’s unlikely that more birds were present, but agreed that it does seem like 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!”