“The chal lenge in managing flows to restore the Delta ecosystem is to do so while also improving the reliability of water supplies … and balancing for other beneficial uses.”
– Delta Stewardship Council Draft Delta Plan
The Challenge of Managing Flows
The Delta has been significantly transformed from its legacy as a 350,000-acre freshwater tidal marsh. At one time, the rivers and tributaries of the Central Valley regularly spilled their banks, triggering an enrich- ment process to the ecosystem that benefitted organisms near and far. Flows would then slow substantially during the summer and fall months. Development of large water projects upstream of the Delta shifted the inflow pattern from winter and spring to summer and fall. “Even without considering the additional effects of through-Delta exports, inflows to the Delta have been tremendously modified from natural conditions,” according to On Developing Prescriptions for Freshwater Flows to Sustain Desirable Fishes in the Sacramento-San Joaquin Delta, a 2010 report by the Center for Watershed Sciences at U.C. Davis. “Changes in San Joaquin River inflow volume and timing to the Delta are especially stark.”
According to the report, the combined effects of water exports and upstream diversions reduced average annual net outflow from the Delta from unimpaired conditions by 33 percent between 1948 and 1968 and by 48 percent from 1986 to 2005. “To be useful, flow prescrip- tions must respond to the changing characteristics of the Delta, includ- ing sea level rise, additional flooded islands, changes in water diversions, and new invasive species,” the U.C. Davis authors wrote. “Adaptive flow prescriptions for desirable fish species are likely to require a more causal, mechanistic, or process basis, so their effectiveness is not outstripped by underlying change and so they can be more easily modified with improvements in scientific under- standing.”
The direction and timing of flow is important because the export pumps of the state and federal water projects frequently cause net reverse flows in
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Old River and Middle River, affecting the Delta’s aquatic ecosystems “both directly and indirectly.” Water flowing toward the pumps in the South Delta increases the probability that “small, weak- swimming fish” such as young smelt or salmon will be pulled into the system and killed, the Delta Stewardship Council’s draft Delta Plan says, adding “depending on the fish species and its size, the closer a fish is to the pumps, the more likely it is to be entrained.” Some fish are caught before being drawn into the pumps and released at a safer spot downstream. While the “salvage” process “decreases the number of some types of fish (includ- ing salmon) entrained at the pumping facilities; many, including Delta smelt, are not able to survive the collection, handling, transport and release,” the draft says.
Beyond the threat of the pumps, fish, including juvenile salmon, are vulnerable because the reverse flow pulls them into sloughs and channels where they suffer from predation. Flow-related stressors such as those caused by pumping can be reduced or mitigated through improved flow management, “better use of current water infrastructure and additional infrastructure improvements,” though “the chal- lenge in managing flows to restore the Delta ecosystem is to do so while also improving the reliability of water supplies … and balancing for other beneficial uses,” the draft Delta Stewardship Council’s Delta Plan says. Johns said the Stewardship
Council’s Delta Plan draft report does not place enough emphasis on the fact that the Delta is a tidal estuary and tides dominate the dispersion of fish. “This is not a river,” he said. “The reverse flow effects mentioned in the report typically are limited to relative- ly small areas on the San Joaquin side of the system when key fish are present and the effects of exports do not at any time reach to the Sacramento system and migrating salmon in that system as some mistakenly believe.