The spot where the Virgin River curves around the Temple of Sinawava used to be the end of the line for some of Zion National Park’s native fish.
A dam was built here in 1957 to house a water line. The concrete structure was only a couple of feet high, but that was more than enough to trap fish downstream and cut them off from around 80 miles of river.
“Our native fish are not very good jumpers,” park scientist Robyn Henderek wryly noted.
Now, fish can swim those miles as they please. After several years of planning, the park removed the dam this spring.
Fish need to be able to travel freely throughout a river’s path for a variety of reasons, Henderek said, and they require different types of habitat at different life stages. Baby fish prefer calm backwater sanctuaries. Adults have to swim upstream to lay eggs.
Between cities and farms diverting water and climate change intensifying drought, native fish across the Colorado River Basin face a lot of challenges. Henderek hopes removing this dam can give Zion’s fish a better chance at survival.
“I've been working on this project, quite literally, since I arrived here in Zion National Park,” said Henderek, who came to the park in 2018. “So, to see this like actually happen is just so fulfilling.”
It may just be the first step, too.
The park is working on design plans to remove two more dams on the Virgin River and install a fish ramp at a third over the next five years.
The river is warming as climate change — driven by fossil fuel emissions — heats temperatures across Utah. So, trapping fish downstream prevents them from seeking refuge in cooler flows at higher elevations.
“Our native fish in Zion National Park and the Virgin River system are cold water species that begin to exhibit behavioral changes and sometimes mortality when water temperatures increase,” Henderek said, which southern Utah is expected to see more of in the future.
Before construction crews put the first jackhammer to the dam, park scientists partnered with the U.S. Fish and Wildlife Service to research which actions would most help its four native fish species — desert sucker, flannelmouth sucker, speckled dace and Virgin spinedace. USFWS also provided most of the funding for the project through its National Fish Passage Program.
The research team fastened beacons to each species and monitored their movements with antennas buried in the river bottom for a full year. Hendrick described them as “kind of like microchips, like you would put in a dog.” That allowed the team to determine the swim speed and jumping capabilities of each animal.
Then they designed the dam’s replacement with the slowest, worst jumping species in mind. In this case, it was the flannelmouth sucker. None of the fish from that species were able to swim past the Temple of Sinawava dam, even during high-water floods. She will survey the fish for another year to confirm the results of the dam’s removal.
The effect of returning native species to upstream habitats extends beyond just the fish. Their return could have positive impacts on everything from aquatic invertebrates to streambed erosion.
“There's really no way to quantify the transcending benefits of achieving free fish movement on all the other species in our ecosystem because of how complex our ecosystem is,” Henderek said.
The project also allowed the park to update the aging infrastructure inside the dam. It’s a pipeline that carries water from a natural spring to the bathrooms and water bottle filling stations at a trailhead near The Narrows, a popular hike for Zion’s millions of annual visitors. The pipeline is now buried beneath the river bed.
This design leaves a slight downslope in the river path, Henderek said, around a 2% slope over 250 feet rather than a 2.5-foot drop over three feet. Still, her team wanted to do what they could to help the fish make it up that gradual incline. So, they added a series of boulders in the river that offer pockets of refuge where fish can take breaks as they swim upstream.
Park crews also returned the riverbank to its historical role as a floodplain. Volunteers and staff planted willow, cottonwood and other native plants propagated from elsewhere in the park to help stabilize the soil.
“When there's flash floods that come through, they can keep this area from just all sliding back into the river,” volunteer Janel Bassett said as she motioned toward a tree sapling.
It’s all part of Zion’s long-term plan to restore the Virgin River corridor.
The closer the river can come to resembling how it flowed decades ago, the better it may be equipped to handle what the future may hold.
“Many other river systems across the West have been dammed, diverted and otherwise dewatered as a result of providing space for humans,” Henderek said. “Zion National Park preserves a section of that wild river that we need to continually be asking ourselves, ‘How can we better protect and enhance?’”
