Fishing for answers on what high flows of 2017 do for salmon on Modesto-area rivers

This year has brought the mighty river flows that environmental and fishing groups say are vital to salmon.

A farmer or city water user might disagree: Yes, the fish need high water at times, but not at the 2017 volume. And we should be adding reservoir space to carry over the excess for dry years ahead.

The Stanislaus, Tuolumne and Merced rivers have near-record runoff from their mountain watersheds after five years of drought. It started in fall, when above-average storms provided more water for salmon returning to spawn after a few years in the Pacific Ocean. The skies truly let loose in January and February, forcing reservoir operators to ramp up releases to prepare for the spring snowmelt.

“These flows are helping baby salmon swim safely out to the ocean,” said Patrick Koepele, executive director of the Tuolumne River Trust. “They are flooding floodplains, which helps salmon grow more quickly, and they help other animals and plants that live in and along the river thrive.”

At times during the drought, the Tuolumne dropped below 100 cubic feet per second as measured at Modesto. It topped 16,000 cfs after the Feb. 20 opening of the Don Pedro Reservoir spillway to create space for upstream runoff. The river was at about 4,000 cfs Thursday, far below the flood stage but still much higher than the previous five years.

The Stanislaus River ran low for much of winter because of unused space in New Melones Reservoir, but it is now at about 4,000 cfs near Oakdale. The Merced River at Stevinson has dropped the most, to about 860 cfs Thursday compared with 4,900 a month earlier.

Year-to-year swings in volume shaped the rivers for millennia before the arrival of humans. High flows can leave fresh gravel beds for salmon spawning and distribute downed logs and other elements of diverse habitat.

The currents this year have dislodged most of the water hyacinth, a non-native plant that messes with both fish and boats. And striped bass, an introduced predator of salmon, could be less numerous because it favors slower, warmer water.

Young salmon benefit especially when rivers spread into floodplains, where they can fatten up on insects before the journey to sea. These zones also take pressure off flood-prone towns and farms downstream. The region has far less floodplain than in the time before river diversion, but restoration projects have added back some acreage.

Rushing rivers also can do harm, such as when recently hatched salmon are washed away before than can withstand the journey to the sea.

The Tuolumne is still too high to allow a survey of how much the channel is being reshaped, said Pat Maloney, aquatic biologist for the Turlock Irrigation District. It shares Don Pedro with the Modesto Irrigation District.

State flow plan

The rivers rose this year amid debate over a state proposal to sharply increase their volume from February to June each year. Overall supplies would drop by 14 percent in average years and 38 percent in “critically dry” years, the State Water Resources Control Board projected.

Water suppliers contend that native fish would not need so much if they took nonflow measures such as spawning gravel restoration, predator control and planting of riverside vegetation.

Last week, the TID board endorsed such a plan by San Francisco, another Tuolumne diverter. Flows would be no more than 250 cfs much of the year. They could spike to as high as 7,000 in wet years to promote gravel replenishment, but only for two to four days. The river was that high for several weeks this year.

“We believe that this year will demonstrate that simply sending more water down the river will not improve native fish populations by itself,” Maloney said.

Finding refuge

The three rivers end up in the lower San Joaquin River, where a string of wildlife refuges provides plenty of floodplain. Wednesday, a tour group in the Vernalis area heard about the challenges of massive runoff. It breached a levee along an irrigation district canal in February, putting this site under as much as 10 feet of water.

Willow and cottonwood trees survived, but the flooding damaged valuable brush such as wild rose, coyote bush and elderberry, said Eric Hopson, manager of the San Joaquin River National Wildlife Refuge. He added that it also killed saltcedar, an invasive plant that needs to be removed.

The tour was by a coalition of agencies and groups working to improve flood management while enhancing habitat.

“We would like as much as possible to have natural processes take place,” Hopson said.

This story was originally published June 11, 2017 at 3:47 PM with the headline "Fishing for answers on what high flows of 2017 do for salmon on Modesto-area rivers."