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Floodwaters Create Crucial Habitat In Yolo Bypass

Photo / Carson Jeffres, UC Davis

Photo / Carson Jeffres, UC Davis

If you’ve ever driven west on Interstate 80 or north on I-5 out of Sacramento, then you’ve driven over the Yolo Bypass.

The bypass diverts Sacramento River water around Sacramento to flood 59,000 acres of land. The water eventually rejoins the Sacramento River near Rio Vista.

It’s been years since floodwaters this high have inundated the bypass, and scientists are eager to see the changes.

Yolo Bypass map - p

UC Davis ecologists lower a boat off a levee near Woodland, about three miles north of I-5. In dry times, they’d be in a rice field. Carson Jeffres, an aquatic ecologist, steers the boat over a body of water that looks more like Lake Michigan.  We come to a stop and anchor the boat to a treetop poking out of the floodwater.

“We are in about 10 feet of water right now, and as we go farther downstream here, it will actually get a little bit deeper,” says Jeffres.

The water moves fast, covering an area three miles wide and 40 miles long.

“There’s more water going through the Yolo Bypass right now then there is in the Sacramento River by Sacramento,” says Jeffres.

Yolo Bypass Scientists

Carson Jeffres (L) and Eric Holmes (R) are testing the floodwaters of the Yolo Bypass to see how much food is available for fish.  Amy Quinton / Capital Public Radio

As soon as water hits this floodplain, the changes are immediate. Plankton and tiny bugs at the bottom of the food chain proliferate. It’s especially beneficial for young migrating salmon that are born farther upriver and wash down during the floods. Winter and spring run Chinook salmon are endangered, so finding places where food is plentiful may be the only way to revive their populations.

Yolo Bypass sample

Eric Holmes, a research ecologist with UC Davis, holds up a zooplankton sample.  Amy Quinton / Capital Public Radio.


“This is providing this big bug buffet for juvenile salmon as they’re going on their way out to the ocean,” says Jeffres. “This is the dinner they expect every year that when we put levees up and lost this floodplain function, we stopped seeing that.”

Jeffres and Eric Holmes, a research ecologist with UC Davis, are trying to get a better idea of just how much food is being produced under flood conditions. Holmes grabs a zooplankton net.

yolo bypass net

Eric Holmes with UC Davis throws net designed to catch zooplankton. Photo / Carson Jeffres, UC Davis


“It has very small mesh designed to catch very small little critters,” says Holmes.

He places the sample in a clear plastic bag and holds it up to the light. He points to a tiny organism near the edge of the bag.

“That is like a Snickers bar to a fish,” says Holmes.

“We get really excited,” says Jeffres. “These things are about a 16thto a 32nd of an inch long. So they’re really small, but if you have enough of them it actually makes a big meal for a fish.”

Brian Schreier, an environmental scientist with DWR, stands on top of a rotary screw trap. It looks like a giant fan sitting in the water, with an eight-foot wide cone that funnels fish into a cage near the front.

Yolo Bypass Fish Trap

Brian Schreier, an environmental scientist with the California Department of Water Resources stands in the live well of the rotary screw trap.  Andrew Nixon / Capital Public Radio


“As you can hear it’s churning pretty good now because we’ve got really fast water speed and fast current,” says Schreier.

Scientists have sampled fish here for 20 years, but they haven’t been able to observe fish under these flooding conditions in 10 years. Schreier reaches into the well where the fish are trapped and pulls out his catch.

He finds Sacramento pike minnow and baby salmon, smaller than guppies, with full stomachs and lots of baby fat. There are dozens.

YOLO Bypass Fish Trap wide

Rotary screw trap sits in water about 20 miles south of West Sacramento. The California Department of Water Resources has been sampling fish here since 1997.  Andrew Nixon / Capital Public Radio.


“Given that we usually catch maybe a dozen fish a day in that trap, catching maybe 50 an hour is a pretty substantial improvement,” says Schreier.

“When you extrapolate that out to the entire bypass, the fact that we’re getting more than just a handful of salmon means that there are millions of salmon out utilizing the bypass right now.”

Fish aren’t the only beneficiaries when the Yolo Bypass floods. Birds, raptors, and river otters do too. Schreier says right now scientists have a golden opportunity to see how the ecosystem is supposed to function, before humans changed this landscape. 


WATCH:  Environmental scientists at DWR catch and sample migrating fish in the Yolo Bypass:


WATCH: Get a bird’s eye view of the Yolo Bypass from drone video captured by Eric Holmes, UC Davis:

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