Increasing Central Valley salmon populations in a highly regulated river system is a complex undertaking. Restoring habitats and ecosystem processes, augmenting fish passage, screening diversions, hatchery reform, and improved harvest management are integral components of successful conservation efforts. But restoring a semblance of a natural flow regime — sometimes termed “functional flows” — is often the focus of efforts to improve the status of salmon in Pacific Coast rivers. While it might seem straightforward to reestablish a natural hydrograph in efforts to encourage the ecological and biophysical processes to sustain salmon life stages — well, it’s not. Between obligations related to dam operations (flood control, water storage) and environmental requirements (minimum flows, ramping rates, water temperatures) the operational “decision space” available to managers is often extremely limited. Regulations that constrain the decision space but do not yield hoped for benefits to fisheries can become counter-productive, even harmful to at-risk salmon runs. Central Valley Chinook salmon provide a telling illustration of how well-intended, protective regulations can go wrong.

Studies of juvenile Chinook salmon reported at the most recent Bay-Delta Science Conference affirmed two findings that are highly relevant to Central Valley water management. First, precipitation that leads to elevated river flows, increased turbidity, and improved habitat can boost the productivity and survival of juvenile Chinook salmon.  Second, survival of Chinook salmon through the Delta is not appreciably harmed by water-export operations from the south Delta. As described in a prior blog post, export-related harms can occur — for example by entrainment — but the magnitude and geographic scope of other export-related impacts appears to be much smaller than previously recognized. 

These same studies and methods have informed our understanding of the effects of both river flows and water exports. Tens of thousands of salmon smolts, surgically implanted with acoustic tags, have been released upstream of the Delta. Before acoustic tags were available, estimates of salmon survival relied on releases of coded-wire-tagged (CWT) salmon. Wire-tagged salmon were primarily recovered two to four years later in ocean fisheries or in spawning surveys. Survival estimates from CWT studies were useful but imprecise and, unfortunately, they provided no information about where along the migratory corridor mortality had occurred.  In contrast, acoustic tags yield precise, reach-specific survival estimates. Acoustic tagging studies have now improved our understanding with a relatively high level of certainty that under higher river flow conditions, typically associated with wet winters, juvenile salmon survival improves markedly.  Equally important, acoustic tagging experiments have not shown adverse export effects that were hypothesized. Survival through the interior Delta is poor but remains so regardless of how much water is being diverted from the south Delta (again, see the prior blog post for details). Of course, export volumes above some yet-to-be-identified threshold may produce detectable and biologically significant harms to juvenile salmon.  More acoustic tagging studies during high-export conditions are needed to elucidate this threshold and reliably quantify associated impacts to juvenile salmon survival.   

What’s preventing us from experimenting with spring flows?  

This year’s drought has renewed interest in the idea of releasing waters stored in reservoirs to provide flow pulses for the benefit of out-migrating juvenile salmon. This idea has merit. However, as with high export conditions, experiments are needed to test the magnitude and duration of flows or flow pulses necessary to achieve desired survival of the young fish on their way to the ocean. Among issues to resolve, dry-year flow pulses may not yield the same benefits to juvenile salmon as natural, precipitation-driven flow events that typically occur in the winter.  

Stakeholders concerned about salmon in the Central Valley might assume that a limited water supply prevents spring flow pulses. Not necessarily so. Water supply is a factor, but the biggest obstacles to providing flows for salmon are regulatory. Here is a partial list of regulations that constrain water conveyance when it would be most beneficial to juvenile salmon. 

·        Cross-Delta water transfers can only occur between July and October– when juvenile salmon are not emigrating in Central Valley rivers.

·        The 2009 National Marine Fisheries Services (NMFS) Biological Opinion (BiOp) imposed an Inflow to Export ratio (I:E ratio) that tied spring exports to San Joaquin River inflows. In critically dry water years, the prescribed ratio could not drop below one to one.  In above normal or wet water years, exports were restricted to one-quarter of San Joaquin River inflows. The 2019 NMFS BiOp repealed the I:E ratio because acoustic tagging studies showed it was ineffective, but the California Department of Fish and Wildlife (CDFW) imposed the I:E ratio on State Water Project operations in the 2019 incidental take permit (ITP).

·        In response to the 2019 NMFS BiOp, CDFW also included new triggers in the ITP that can further restrict spring exports based upon small numbers of salmon entrained on a single day — as few as 0.005% of the juvenile production estimate.

·        Delta environmental quality regulations, initially the 1994 Bay Delta Accord, then later the State Water Board’s D-1641, restrict exports to 35% of Delta inflow from February through June.  

These policies have made it nearly impossible, and for that matter very risky, for water managers to attempt spring conveyance to benefit salmon. When not limited by a stricter operating rule — I:E ratio or OMR flow restrictions — D-1641 alone promises to take a 35% bite out of additional river flows that might be conveyed between February and June. With a guaranteed loss of 35%, would it be surprising if operators eschewed spring reservoir releases in favor of nearly unencumbered conveyance that is available between July and September?  

In fact, that is exactly what has happened — spring season Sacramento River flows and Delta water diversions have both declined since export regulations were imposed (see Figure 1, panels A and B), while summer flows and exports have increased (Figure 1, panels C and D). Restricting spring exports was undoubtedly a well-intended attempt to protect juvenile salmon, but it now appears we have traded what would be a substantial benefit to salmon (higher flows provided by conveyance of water from reservoir storage through rivers and to the Delta) for something that appears to have much less value to at-risk salmon runs (reduced fish entrainment). Regulations have not increased spring river flows and have not reduced the volume of water exported from the Delta. Instead, regulations have shifted stored-water conveyance to summer, likely benefitting largemouth bass at the expense of Chinook salmon.  

Figure 1 — Patterns of spring (March-May) and summer (June-August) Sacramento River flows into the Delta and total south Delta exports (CVP+SWP). Acre-feet (y-axis) and years (x-axis). Fitted lines show that both exports and Sacramento River inflows to the Delta have declined in spring months (Panel A and Panel B). In contrast, both exports and Sacramento River inflows have increased during summer months (Panel C and Panel D). Data from DWR’s Dayflow. 

With different regulatory incentives, the quantities of water potentially available for water conveyance in the spring are not at all trivial. In three recent drought years — 2013, 2014, and 2015 — a total of more than one million acre-feet was released from reservoir storage for Cross-Delta transfer during summer and early fall (DWR and SWRCB 2015).  One million acre-feet is roughly the equivalent of 5,000 cfs for 100 days. Had regulations allowed or, better yet, encouraged those one million acre-feet to be transferred in the spring (rather than in summer and fall), juvenile salmon could have benefitted from flow pulses of substantial magnitude and duration in all three drought years.  Spring flow pulses of this kind, particularly if strategically planned to coincide with favorable, cool-weather conditions and hatchery releases, could likely yield dramatic benefits to salmon while also maintaining or improving water-supply reliability.

Of course, augmenting river flows to benefit salmon is not a new idea. The California State Water Resources Control Board (SWRCB) has recommended flow increases in San Joaquin basin tributaries and is considering similar actions in the Sacramento basin. CDFW has sought flow increases via Voluntary Agreements, and NMFS has included flow-pulse concepts in recent Biological Opinions affecting Central Valley rivers.  Unfortunately, the desire for increased spring flows has not been matched by an interest in revisiting regulatory constraints to spring flows or in considering whether new scientific information supports export regulatory policies developed 30 years ago.  Instead, calls for increased river flows have almost universally been accompanied by a requirement that augmented flows not be recovered in the Delta. Projects proposed under the California Water Commission’s Water Storage Investment Program (WSIP) have been similarly constrained, limiting opportunities to achieve ecosystem benefits the program requires for public funding. The unsurprising result of this policy is that little water is available to provide spring flow pulses, particularly in dry water years when fish need them most. The SWRCB presumably has the authority to mandate higher river flows, while also forbidding commensurate diversions from the Delta.  However, if emerging scientific information identifies a means to convey water and provide a substantial net benefit to salmon at the same time, can we afford to dismiss that opportunity?


The difficulty of making progress in the Delta has been attributed to the “wicked” complexity of the Delta ecosystems, its ecological communities, and the imperiled species that reside there or pass through its troubled waters (Luoma et al. 2015). Indeed, conservation planning for salmon that migrate through the Delta is not simple, but its challenges are only complicated and compounded by inflexible regulations and inattention by regulators to emerging scientific information. If we are sincere about meeting “co-equal goals” and about the effective application of adaptive resource management, we need to do better.  Experimental spring-flow pulses to benefit salmon while providing water conveyance may be one of our best opportunities to show that we can.




Department of Water Resources (DWR) and State Water Resources Control Board (SWRCB). 2015.  Background and recent history of water transfers in California.  July 2015. Report prepared for the Delta Stewardship Council.  


Luoma, S, et al. 2015.  Challenges facing the Sacramento-San Joaquin Delta: Complex, chaotic, or simply cantankerous?  San Francisco Estuary and Watershed Science: 13(3). 

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