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Silver Platters in Black Holes
Each spring, under the Vernalis Adaptive Management Plan, Delta export pumps suck less water from the San Joaquin River and Valley farmers relinquish some of their precious reserves in an effort to boost flows for young salmon moving downstream, past Old River and the pumps. But a preliminary study by the U.S. Geological Survey is raising questions about how the increased flows are affecting the river’s geomorphology, and in turn, its fish.
Last May, as researchers eyed their laptop from a boat on the river, they were startled to see lots of big blips popping up on the screen, transmitted from a depth sounder. The shape and position of the blips in the water column—and the fact that fish bladders are good reflectors of acoustic energy—led the research team to suspect that the blips were large fish which can prey on young salmon. The large blips popped up every time the boat moved in the vicinity of the river’s bends, explains researcher Jay Cuetara.
After mapping the river’s bottom, the team discovered that very deep holes had been scoured at many of the river’s bends. While scour holes occur naturally at bends, the San Joaquin’s meanders have been heavily armored with riprap, forcing the river to direct its energy downward rather than outward, which scours the holes deeper than they would be naturally, says Cuetara. At certain flow rates, strong back eddies—or recirculation zones—form in the holes. These deep whirlpools may be providing the perfect hiding spots—and habitat—for predators.
According to Jon Burau, the eddies (or recirculation zones) are characterized by upstream flow on the outside of the bend, upwelling at the river banks, and downwelling where the recirculation zone interfaces with the river. Within the center of the zone, the water moves very slowly, if at all, creating still spots where predators don’t even need to swim for their food, but just hang out and wait, says Burau. "The food comes on a silver platter."
Burau suspects that the strength of the eddies varies with the flows. "We know that these recirculation zones simply don’t exist at very low or high flows," he says. "We suspect each scour hole has a range of flows for which zones exist, and that there is a specific rate that creates the strongest zone."
While the VAMP flows are designed to push salmon smolts out of the Delta and into the Bay, they may, ironically, fall within a range of flows that is actually hindering the salmon by helping their predators, says Burau. "This is an excellent example of an unexpected consequence of a management action that could alter the system over the long term," he says.
The researchers are quick to say they are not 100 percent sure the blips were predators. "But we know there’s a ton of something down there," says Cuetara. Their suspicions were strengthened when they saw people catching bass and catfish at the holes.
Though the goal of the study was really to study the VAMP flows—and to try to quantify how many young fish make it past the culverts and barrier at Old River by
understanding the hydrodynamic processes in the river—the team became fascinated by what they saw at the river’s bends. "We’re sort of raising the flag for the first time on this," says Burau. "Let’s say there is predation in these holes. Is it a big deal, little deal? We simply don’t know."
Between Mossdale and Stockton, there could be a dozen large holes, says Cuetara, with predators lurking in each one. By way of follow-up, researchers are now suggesting further studies to document the bathymetry (shape of the river bottom) of the holes, or to at least study one in detail, and to do some gill-netting to see what fish are down there.
"It’s a fundamentally different way of looking at the problem," says Burau. "The VAMP studies drop a bunch of fish in upstream and then trawl down at Chipps Island, among other things. What’s inbetween is a kind of black hole." Burau suggests a joint study in which his agency studies the holes and state or CALFED biologists look more closely at the fish.
If their worst fears are confirmed—if predators are having a serious impact on young salmon—one eventual solution might be to allow the river to erode its bends again as it once did naturally, so that the deep holes will no longer be scoured out. Setback levees could be used to protect farms and other property, says Burau. One question for geomorphologists is how far back the levees would need to be set. "In a relatively short time would the river bump up against the levees again?" wonders Burau. "Historically, the San Joaquin migrated across the entire San Joaquin Valley. But we don’t really have the flows that move things around like they once did."
Contact: Jon Burau (916) 278-3127 or Jay Cuetara (916) 278-3130 LOV
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