The road to viable Central Valley Chinook salmon populations is paved with cost-effective management actions. Do spring export restrictions fit the bill?

Balancing species protections against human demands on the environment is the central challenge for conservation planners in California. The state’s taxpayers and residents have shown themselves to be supportive of sometimes costly conservation efforts targeting the state’s federal and state listed species and programs targeting the California condor, northern spotted owl, sea otters and desert tortoises. But that support may wane if species conservation pairs a big price tag with limited or indiscernible benefits to the species.  

Students of economics are familiar with the concept of opportunity cost (i.e. benefits lost when choosing one option over another).   A company choosing to invest its available assets in a new manufacturing facility loses the benefits that might accrue from keeping the old facility and instead pursuing a new marketing campaign.  Opportunity cost also applies in conservation of at-risk species.  A recovery strategy that prioritizes out-planting larger riparian buffers may reduce resources available for land acquisition. Regulations that come with substantial costs to resource users may be punitively effective, but if such regulations contribute less to species recovery than other available options, then those regulations represent a misplaced allocation of limited available resources.

Which brings us to the uncomfortable and consequential topic of South Delta water-export regulations. Restrictions on south Delta water exports to urban and agricultural interests in central and southern California are among the most costly and most controversial actions taken for the protection of Central Valley salmon populations. Water export restrictions have been a go-to conservation action because reverse flows in portions of the Delta are thought to be a major factor driving the decline of Central Valley salmon.  The presumed contribution to the low survivorship of out-migrating salmon caused by export operations in the south Delta has guided Delta water policy and management for more than thirty years (for a recent example of media reporting see  However, a close look at the best scientific information that has appeared in recent years indicates curtailing water exports offers few benefits to embattled salmon runs.

What we have learned about juvenile salmon in the Delta?

Intensive juvenile salmon studies began in the early 1970s, intending to but not explicitly testing the hypothesis that south Delta water exports and river inflows were important determinants of salmon survival.  Forty years later, we have an abundance of data suitable for testing hypotheses relating water exports to salmon performance in the Delta.  Unlike the increasingly rare and cryptic delta smelt, juvenile salmon are readily available from hatcheries and amendable to tagging and tracking.   At least 50 million CWT salmon have been released as part of Central Valley and Delta salmon studies.  Acoustic tags (AT) began to be used in 2006 and were widely adopted thereafter with tens of thousands of acoustically tagged fish released as part of Delta salmon studies (see Perry et al. 2018, Buchanan et al. 2018).  Though expensive at $300 each, acoustic tags provide reliable, fine-resolution data on the behavior and fates of tagged juvenile salmon.

CWT studies generally indicate that juvenile salmon survival in rivers is enhanced with increased river flows.  However, because CWT studies are dependent on tag recoveries provided by trawling, ocean fisheries, or spawning surveys, resolution of where, how, or why mortality occurs during Delta passage is largely lacking.  Acoustic studies have affirmed river inflows can benefit juvenile salmon survival, but they also show that benefits particularly accrue in riverine areas — channels with unidirectional flows — or in channels that bridge the transition between river and tidal conditions. In the tidal Delta, flows are naturally bidirectional, that is, directed landward and seaward twice per day as tides ebb and flood.  The tidal Delta remains tidal regardless of river inflows.  As discussed further below, different flow-survival patterns have been observed in the tidal Delta when compared to the non-tidal Delta.  

A relatively large adverse effect of water exports on juvenile salmon might have been expected given tremendous water costs associated with water export restrictions and extraordinary efforts devoted to related fish studies. But study findings for juvenile salmon have not gone as many expected. For juvenile salmon approaching the Delta from the San Joaquin River basin – the fish passing closest to the export facilities — both CWT and AT studies indicate exports are not negatively associated with salmon survival and may be positive as a result of fish salvage operations.  For Sacramento River basin juvenile salmon approaching the Delta from the north, adverse exports effects have been minimal. 

How are these seemingly paradoxical outcomes possible? Three interrelated and to-date underappreciated factors contribute to head-scratching findings — Delta hydrodynamics, juvenile salmon behavior in the Delta, and ecosystem changes.  

How do water-export operations affect Delta hydrodynamics?

South Delta water exports have been the presumptive ground zero for Central Valley water management debates for decades, yet there have been few accessible, quantitative assessments of how exports influence Delta hydrodynamics.  Kimmerer and Nobriga (2008), NMFS (2009) and some others have used particle-tracking models (PTM) to characterize the fate of neutrally buoyant particles in response to South Delta exports. However, investigators now recognize that such particles (without realistic juvenile salmon behaviors) cannot usefully represent how hydrodynamics are likely to affect fish (Anderson et al. 2012; Delaney et al. 2014).  

A standard approach to assessing water-project operation effects on fish (wherever they occur not just in the Delta), is to consult available flow data. That approach works great for rivers (check CDEC!), but the Delta doesn’t function like a river. Although there are dozens of flow-gauging stations in the Delta, the dynamic influence of tides, flow control gates/barriers, meteorology, river inflows, and water diversions makes interpreting those data difficult.  What is required is a way to hold noise-inducing factors constant, while boundary conditions of interest like river inflow and export rates are experimentally modified.  Delta hydrodynamic models allow this control. However, hydrodynamic models are complicated to run and results difficult to view and interpret.  To overcome these obstacles, we have developed a web-based toolfor visualizing hydrodynamic effects resulting from water project operations (see Figure 2 for more information).  

Figure 1.  Footprints of hydrodynamic changes resulting from South Delta water exports.   Yellow dashed line indicates Delta area exhibiting reverse tidally averaged “net” flows, but very small changes in instantaneous velocities.  Orange dashed line indicates extent of Delta area exhibiting moderate changes in instantaneous velocities. Red dashed line indicates extent of Delta area exhibiting large changes in instantaneous velocities.  See Figure 2 for examples of hydrodynamic changes representative for these three zones. 

There is a lot to take in about how the Delta functions — what affects hydrodynamics where, and what doesn’t. For the purposes of understanding potential impacts to juvenile salmonids, two aspects of Delta hydrodynamics are important to consider.  First, the Delta does not respond monolithically to water project operations, but rather is composed of several hydraulically connected regions differently influenced by tides, inflows, and diversions.  Second, the distinction between riverine and tidal areas of the Delta is critical. In the Sacramento River arm of the Delta (the North Delta), unidirectional (riverine) conditions extend from the Delta Cross Channel to upstream of Rio Vista under managed, non-flood inflows (Cavallo et al. 2013). In the San Joaquin River arm of the Delta (the South Delta), riverine conditions range between the Head of Old River to Stockton.  The Delta west of Stockton, north of the SWP and CVP export facilities, and south of Rio Vista is tidal under typical river inflow conditions (including any minimum flows potentially required by the SWRQB or other regulatory entities). In the tidal Delta, flows are naturally bidirectional- directed landward and seaward twice per day as tides ebb and flood.   

When South Delta exports exceed San Joaquin River inflows, hydrodynamic conditions, commonly referred to as reverse flows occur in the tidal Delta (Arthur et al. 1996; Andrews et al. 2016).  Despite the descriptor, flows do not literally change direction under these circumstances. Rather, reverse flows are tidally averaged flow conditions – that is, the flows that would remain if tidal flows did not exist.  Of course, tidal flows doexist, and their magnitude is typically much greater than tidally averaged flows. A calculated metric that removes tidal flows is an abstraction from the hydrodynamic reality experienced by fish.  That said, tidally averaged flows moving away from, rather than toward, San Francisco Bay have been termed reverse flows.  Reverse flows and tidally averaged can certainly affect transport patterns and the residence time of Delta waters (e.g. Glibert et al. 2014).  However, reverse flows have also been hypothesized to disorient migrating fish and to “pull” fish from other areas into the South Delta, where habitat conditions are poor and where risk of entrainment in diversion facilities is greatest (Newman and Brandes 2010; NMFS 2009). Reducing reverse flows is considered a primary benefit of the California Water Fix project (

Figure 2. Delta water velocities with zero total exports and with total exports of 9,000cfs (CVP and SWP combined).  Delta map (right side) depicts the proportion of overlap in velocity-frequency distribution with these contrasting export rates.  Green indicates velocities are very similar (high overlap), while orange indicates large velocity differences (low overlap).   Inset panels (left side) depict velocity-frequency distributions at three Delta channels (blue markers on map) at zero exports and for 9,000cfs total exports. More information on the source of these data and an interactive Shiny application is available at  The Shiny application allows the user to select and view hydrodynamic conditions resulting from a variety of operating conditions and for a variety of hydrodynamic metrics.   

Do juvenile salmon respond to reverse flows caused by exports?

Most Delta salmon studies (and the regulatory prescriptions drawn from them) have been predicated upon a conceptual ecological model in which tidally averaged flows exert a strong influence on the fate of juvenile salmonids.  For example, Brandes and McClain (2001) identified Delta channels with reverse flows — thought to be problematic for migrating juvenile salmonids — in the introduction of their paper. More recently, the National Research Council of the National Academies (NRC 2012) concluded “losses [of juvenile salmonids] are substantive and are at least in part attributable to pump operations that alter current patterns into and through the channel complex, drawing smolts into the interior waterways and toward the pumps.” 

Despite the management focus on reverse flows, it’s increasingly apparent juvenile salmon (and perhaps other strong-swimming Delta fishes) may be unaware of and relatively unaffected by tidally averaged flows.  Evidence for this comes from acoustic tagging studies (Perry et al. 2018; Buchanan et al. 2018), from improved understanding of salmon orientation (here), and from expert panels studying the issue (SST 2017; Monismith et al. 2014).  These expert reviews have identified changes in channel velocities or changes in flow direction, not tidally averaged flows, as the hydrodynamic factors likely to influence juvenile salmon.

Do these findings indicate exports don’t alter hydrodynamics in ways that cause juvenile salmon to be entrained at the state and federal export facilities?  No, exports clearly affect juvenile salmon and can cause entrainment.   The revelation we need to acknowledge and incorporate into our management strategy is — the locations and the consequences of export effects on juvenile salmon are quite different than we had previously understood. 

Alhough thresholds of for export-caused impacts on out-migrating salmon require further investigation, available information indicates that export effects begin in the Delta where instantaneous velocities, not tidally averaged flows, substantially diverge from natural conditions (Figure 1, orange dashed line).  The greatest risk of entrainment of and directional confusion in salmon occurs in portions of the Delta where exports can dramatically alter instantaneous velocities and can literally cause flows to be directed toward the export facilities for a period greater than a normal tidal cycle (Figure 1, red dashed line).

What is driving patterns of salmon survival in the South Delta?

Poor survival for juvenile salmon approaching the Delta from the San Joaquin River is not a new phenomenon. Tagging studies indicate survival has been persistently low since at least 2001 (Figure 3).  The factors driving this decline are uncertain, but may be similar to causative factors observed in the Delta’s Pelagic Organism Decline (POD), including habitat degradation and the proliferation of invasive species in the Delta, including non-native predatory fishes (Baxter et al. 2018). 

Figure 3. Estimated survival of coded wire tagged (CWT) and acoustic-telemetry (AT) study releases into the San Joaquin River portion of the Delta.  See Buchanan et al. (2018) for additional information.

Water project operations have likely played a roll as a well, though not necessarily as expected. Cloern and Jassby (2012) report that since 1956 water exports have increased in nearly all months of the year. Less recognized is the fact that total April-May exports — the peak season for juvenile salmon emigration– have been substantially reduced since 1990 (Figure 4). However, as described here, and as Peter Moyle recently observed, acoustic tagging studies demonstrate entrainment and then salvage at the CVP export facilities provides for better survival to Delta exit for juvenile salmon than does natural migration through the gauntlet of poor habitat, even when tidally averaged flows are not reversed (Buchanan et al. 2018; SJRG 2011; SJRG 2013).  

Figure 4. CVP and SWP combined exports in April and May from 1956 through 2017 (DayFlow data).  While exports have increased in nearly all months since 1956 (Cloern and Jassby 2012), April-May exports have declined substantially since 1990. 

What does it all mean?

On one of the best-studied topics of the Delta, multiple lines of evidence indicate that spring-export restrictions are not delivering benefits to juvenile salmon that were reasonably expected.  Salmon habitat in the south Delta has degraded to the point that being entrained to Central Valley Project export facility and transported to the western Delta better assures survival during out-migration than does natural passage, even under favorable hydrodynamic conditions.  

Operations of water project facilities have caused considerable damage to Central Valley Chinook salmon populations and to certain other Delta fishes, but these facilities will continue to function, even with Cal Water Fix.  The challenge for fisheries biologists is to make wise choices about mitigating diversion impacts in ways that most effectively contribute to salmon survival and species recovery.  Actions that are costly to water users may be punitively satisfying, but they represent poor bargains if benefits to salmon and their habitats — opportunity costs — are not being realized. 

It’s beyond the scope of a simple blog post to specify alternative export management regime.  It’s a complex subject; impacts on environmental factors like water quality and species other than Chinook salmon need to be considered. Minimally, findings summarized here suggest spring export restrictions for the ostensible benefit of juvenile salmon should be reconsidered.  Preferentially pumping from the CVP export facilities at a rate that maximizes successful fish salvage appears particularly promising. 


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