by Robin Meadows
If you visit the Delta town of Walnut Grove during winter or spring, look for a surprise in the Sacramento River just before it meets Georgiana Slough. A steady stream of bubbles rises from the river bottom, accompanied by flashes of bright yellow strobe lights and low whooshing sounds. It looks like an art installation, especially at night.
But this barrage of light, noise and bubbles is actually there to protect imperiled baby salmon. Formally called a bioacoustic fish fence and dubbed a bubble barrier, the system was installed in 2023 to keep the little fish from going the wrong way in the Delta.
“It’s working great,” says bubble barrier project manager Shahid Anwar, an engineer at the state Department of Water Resources.
Young salmon, or smolts, migrating down the Sacramento River face a choice at its junction with Georgiana Slough. Staying in the river takes them west toward the ocean, where they grow up and spend most of their lives.
Veering left into Georgiana Slough takes baby salmon off course. They end up getting lost in the maze of slow, twisty waterways of the Delta interior, where predators can snap them up, or pulled south to the pumps that export vast amounts of water from the Delta to farmland and cities.
“This junction is very important,” Anwar says. “Once smolts enter Georgiana Slough, their survival rate is very, very low.”
PROTECTING BABY SALMON
The project dates back to 2009, when the National Marine Fisheries Service required the Department of Water Resources to find a way to keep winter-run and spring-run chinook smolts out of the interior and south Delta. These fish are listed under the federal Endangered Species Act, and protecting them is key to continued water exports from the Delta.
The agency decided to try a bubble barrier, which had been shown to help keep invasive carp out of the Great Lakes. A bubble barrier has major advantages over a physical barrier in that boats and water pass right through it. Unimpeded flows into Georgiana Slough are critical to maintaining the quality of water exported from the Delta. Another benefit is that while the sound is very loud in the water, boaters only feel it as slight vibrations and people right by the river don’t notice it all.
A pilot bubble barrier installed across the mouth of Georgiana Slough in 2011 worked well. Researchers released smolts with acoustic tags in the Sacramento River shortly upstream of Georgiana Slough, and used submerged microphones to monitor which way the baby fish swam at the junction. The pilot system cut the number of young salmon going down the slough in half, keeping 67% of them in the river.
The new bubble barrier installed in 2023 works even better. Monthly releases of tagged fish show that it keeps 82% to 92% of baby salmon on course.
“We’ve upgraded it since 2011―it’s pretty sophisticated,” says Department of Water Resources engineer Roger Padilla, who worked with Anwar on the project. Enhancements include brighter lights, higher frequency sounds, and twice as many bubbles. The system also operates automatically and is monitored remotely.
BUBBLE BARRIER BASICS
So how, exactly, does the bubble barrier work? It’s generated by equipment mounted on a 640-foot frame, which is roughly 10 feet below the water surface and is supported by 17 steel piles driven into the river bed.
Along the length of the frame run four rows of perforated pipe, which are fed by compressed air and produce a bubble curtain that goes up through the water. The curtain’s width expands from about one foot at the bottom to as many as twelve feet at the top. Its insubstantial nature notwithstanding, the curtain of bubbles is so cohesive that it always reaches all the way to the surface.
The frame also supports speakers and high-intensity LED lights aimed at the bubble curtain. “The bubbles capture the sound and light to make a virtual wall,” Anwar says.
The speakers sweep through frequencies known to deter salmon. While the precise frequencies are proprietary to the manufacturer, they lie between 5 hertz, which is too low for people to hear, and 600 hertz, which is mid-range between bass and treble.
“It makes a boom-boom noise that pulsates in sync with the lights—fish can’t tolerate much of the sound,” Anwar says, adding that the light shows smolts where the bubble curtain is. One of his colleagues calls it a disco barrier. It’s no wonder the vast majority of baby salmon, which typically migrate in the top third of the water column, decide to swim a different way.
The bubble barrier is in action from mid-November through April or May, depending on exactly when winter-run and spring-run chinook smolts make their way through the Delta. The system operates seasonally through 2030.
This powerful fish protection comes at a hefty cost. The electricity alone exceeds $30,000 per month. The project has run $25 million so far, and ongoing annual expenses are estimated at $5 to $7 million, depending on how many months it operates in a given year.
SALMON WOES
Georgiana Slough is just one of many dangers to baby salmon migrating down the Sacramento River system, cautions Golden Gate Salmon Association director Scott Artis. Other threats include high water temperatures and low flows.
Smolts need flows to migrate to the ocean, and Artis worries that the bubble barrier’s success could mask the impact of low flows. “I’m all for helping prevent these salmon from getting sucked into the pumps,” he says. “But will this new technology divert the state from the greater issue that salmon need more flows?”
Even salmon that aren’t listed are in trouble in California. This is the second year in a row that the state’s ocean salmon fisheries have been shut down due to low numbers of adults returning from the sea. “Entire communities have historically relied on these fisheries,” says Artis, who grew up fishing with his dad and uncle, and has a background in salmon restoration. “We’re not setting salmon up for maximum success―we need to get our act together as a state.”
That said, Georgiana Slough is on the Golden Gate Salmon Association’s top 10 list of wild fish problems, and Artis is excited to see a fix. While aimed at helping federally-listed salmon, the Delta’s new bubble barrier could also benefit the hatchery-raised chinook that are the mainstay of the commercial fishery, he says. “They need that boost—they’re facing a gauntlet of danger.”
