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How do Flows Potentially Restore Salmon Habitat?

February 2014

How do flow releases potentially restore salmon habitat? That’s one of the questions Delta Science Fellow Erin Bray sought to discover in her 2011-2013 research—a study that will help inform one of the recommendations outlined in Chapter 4 of the Delta Plan; to create more natural functional flows.

The Delta Stewardship Council’s Delta Science Fellows Program is intended to bring junior scientists together with Delta agency scientists and senior research mentors to work collaboratively on data analysis and research projects relevant to Delta policy and management.

In 2009, plans were underway to release water from Friant Dam on an interim, experimental basis, as part of a court-approved settlement to restore a self-sustaining Chinook salmon fishery to the San Joaquin River. Bray’s project investigated the effects of those releases, as well as other factors, on physical properties such as water temperatures and the flow of water through the pores between gravel (intragravel) that create viable habitat for Chinook salmon.

Through field experiments and modeling, Bray addressed two spatial scales: the scale of a gravel bar (where subsurface water flow is important for spawning salmon) and the whole-river longitudinal scale (whose hydraulic and thermal conditions are important for migrating juvenile and adult salmon).

What she found was somewhat unexpected. “Contrary to what one might think, the intragravel flow through gravel bars is greater during low flows than during high flows, i.e. the connectivity between surface water and groundwater is greater,” Bray said. “The rate at which water exchange occurs between the river and the groundwater aquifer beneath the river—which is important for salmon eggs—can vary by orders of magnitude over just one gravel bar.”

High flows released from a dam can mobilize sand and gravel. Where the sand and gravel settle out on the riverbed also controls how and how rapidly heat is transported through the gravel bars. This is important because incubating salmon eggs are highly sensitive to warm temperatures. Having more flow in the channel to exchange with groundwater in the bed of the stream through gravel bars can be good when the surface water is cold and there are salmon eggs in the gravel, but not so good when the surface water is warm.

At the whole river scale, the thermal regime is important for migrating juvenile and adult salmon. These are controlled largely by how much incoming solar radiation is absorbed by the water column and the riverbed (controlled mainly by water depth) and the volume of water in a reach to dilute the heat, she said.

Along the length of the middle San Joaquin, solar radiation varies only slightly, but flow depth and reach volume varies irregularly due to flow releases, irrigation schedules, and diversions, contributing to increasing water temperatures with distance downstream from Friant Dam, Bray said.

“During times or locations where channel losses are greatest, water temperatures are sensitive to absorbed shortwave radiation and water depth and are not buffered alone by flow release temperatures. This may limit adequate migratory salmon habitat over a given length of stream,” she concluded.

Bray, who is a postdoctoral student at UC Santa Barbara, was also selected as a 2013 Fellow. In her current research, she is studying gravel bars in rivers to identify what physical features facilitate groundwater exchanges that create good Chinook salmon spawning habitat.

Click here for more information on the DSP Fellows program.

Coequal goals

The Delta Stewardship Council was created in legislation to achieve the state mandated coequal goals for the Delta. "'Coequal goals' means the two goals of providing a more reliable water supply for California and protecting, restoring, and enhancing the Delta ecosystem. The coequal goals shall be achieved in a manner that protects and enhances the unique cultural, recreational, natural resource, and agricultural values of the Delta as an evolving place." (CA Water Code §85054)