Category Archives: Sturgeon

White Sturgeon Recruitment to San Francisco Bay-Estuary in 2024

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In a June 2024 post, I hypothesized factors controlling the white sturgeon population in San Francisco Bay-Estuary.  I concluded the major factor controlling the adult stock size was periodic recruitment of juvenile sturgeon from successful spring spawning and early rearing in the lower Sacramento River.  Successful recruitment only occurs in the wettest years, when there are higher streamflows and cooler water temperatures.

Recruitment of young white sturgeon in significant numbers has only occurred in three years since 2010:  2011, 2017, and 2023.  Recruitment in 2024, an above normal water year, is likely to be poor.  Production of young sturgeon is likely of function of attraction of spawners from San Francisco Bay (to high winter-spring river flows – good in 2024), good spawning conditions (streamflow and water temperatures – good in 2024), and good early rearing and transport conditions in the lower Sacramento River and the north and central Delta (streamflow and water temperatures – poor in 2024). 

The adult spawning stock size may not be as important as spawning conditions, given strong recruitment in wet year 2023 under a very low stock abundance (observed Bay die-off in summer of critical drought year 2022).

There are a number of measures that hold promise to protect and enhance adult stock numbers, recruitment, and survival of white sturgeon.

Spawning Conditions

Flows in the lower Sacramento River (Wilkins gage) should be 8,000 to 10,000 cfs or higher in spring.

Flow in the lower Sacramento River (Wilkins Slough) should be at least 8,000-10,000 cfs.

Early Rearing and Juvenile Transport

Sacramento River inflows to the Delta should be at least 20,000 cfs in spring.

Flow from the lower Sacramento River into the Delta (Freeport) should be at least 20,000 cfs in spring and early summer (April-July).

Delta Conditions

The net flow in the lower Sacramento River channel downstream of the entrance to Georgianna Slough should be at least 10,000 cfs in spring and early summer (April-July).  This will require total Delta diversions, including local agricultural diversions and exports by the State Water Project (SWP) and Central Valley Project (CVP), to be limited to approximately 10,000 cfs. 

Flow in the lower Sacramento River in the Delta below Georgianna Slough should be at least 10,000 cfs in spring and early summer (April-July).

White sturgeon recruitment is best assessed at fish salvage facilities at the water project south Delta export pumps.  Young sturgeon produced in the lower Sacramento River reach the Delta in early summer as shown here in 2023.  Note the high export levels of 20,000 acre-feet per day (approximately 10,000 cfs; in this figure, SWP exports are shown behind CVP exports).

Bay Conditions

Delta outflow to the Bay should be at least 10,000 cfs from spring to early fall (April-October).

Delta outflow to the Bay (DTO) and Rio Vista water temperature May-Oct, 2021-2024.  Recommended Delta Outflow is a minimum 10,000 cfs (purple line).

Minimize Sturgeon Adult Harvest and Pre-Adult Fishing Mortality

Sturgeon sport fishing should be limited to the Bay only, with catch-and-release regulations, with the following further considerations:

  1. Fishing for sturgeon should be closed in the east Bay or north Bay if daily maximum water temperatures are expected to exceed 65ºF (18ºC) in any open water portion gage locations in either Bay portions (possible from late spring through early fall).
  2. CDFW could allow limited harvest through short-term regulations, limited by slot (length range) and number.  (Note that slot harvest in 2024 would allow a small harvest of broodyear 2011 white sturgeon, the most recent abundant broodyear in the adult population).

Under such restrictions, the effects of sport fishing on the sturgeon population would be minimal.

In addition, sturgeon collected at south Delta export salvage facilities should be transported to an appropriate location in the Bay for release (presently, they are released in the Delta).   

Population abundance and recruitment of white sturgeon are mainly a function of annual Central Valley hydrology (river flow), with abundant juvenile production occurring only in the wettest years.  Past harvest has only involved a small percentage of the adult population, while watershed hydrology has orders of magnitude greater effect on recruitment and eventual adult population abundance.

Allowing a limited fishery could also help in continuing to assess the health of the population.  Sport fishers should be asked to contribute important information on the sturgeon they catch.

Salmon and Sturgeon Bay Summer Season 2024

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Starting with the Strawberry Moon

The Strawberry Moon, sometimes called the planting moon, is the June full moon (e.g., June 21, 2024). June is the end the juvenile salmon and sturgeon emigration season from the Central Valley to the Bay.  The Strawberry Moon June 2024 super tides drained the warm river and Delta waters into the Bay driving the remainder of the brood year 2023 salmon and sturgeon with it toward the Bay.

In a wet year, the young salmon and sturgeon are pushing through the Delta throughout June.  But 2024 was just an average water year, with the seasonal salmon and sturgeon emigration to the Bay ending with lower streamflow and higher water temperatures from the rivers through the Delta and then entering the Bay.  At this point, we cannot yet determine if the seasonal event was successful – we just do not know.  Regardless, we now must look toward the Bay to ensure that fish die-offs from warm water, low DO, and toxic algae blooms do not occur again.

What is important now is maintaining the Bay through the summer by ensuring toxic algae blooms and warm water do not stress the salmon and sturgeon present in the Bay.  With water temperatures in the eastern parts of the Bay already high (22oC/72oF) at the beginning of summer, it seems the Bay is destined for another bad summer of toxic algae blooms and dying adult sturgeon. 

What can be done to help keep the Bay cool this summer?  There are three actions that when taken together and in sequence can help keep the Bay cooler:  Keeping the rivers cooler keeps the Delta cooler.  Keeping the Delta cooler keeps the Bay cooler.  There are also some Bay actions that can sustain cooler water temperatures.

  1. Keeping Lower Rivers Cooler

First, how to keep the lower Sacramento and San Joaquin rivers entering the Delta cooler.  Higher streamflows speed the water along in the hot Valley keeping the rivers from absorbing heat.  We are only talking about 5oF or so, but it is an important five degrees. That is accomplished by maintaining adequate (legally prescribed) streamflows with reservoir releases, especially during summer heat waves that are getting hotter and more frequent with each decade (i.e., climate change). 

Figure 1.  Drought year 2022 spring-summer streamflow and water temperatures at Wilkins Slough (RM 120) upstream of the Delta.  Red line shows the target of 68ºF/20ºC needed to protect fish and keep the Delta cool.

Figure 2.  Wet year 2023 spring-summer streamflow and water temperatures at Wilkins Slough (RM 120) upstream of the Delta.  Red line shows the target of 68ºF/20ºC needed to protect fish and keep the Delta cool.

  • The Sacramento River in the North Delta

Next is the Sacramento River channel in the north Delta’s Freeport to Emmaton reach, with Rio Vista in the center.  This reach tends to emulate the input at Freeport, although it tends to warm when flow falls below 20,000 cfs (the inflow from Freeport, Cache Slough, and the San Joaquin River), as it too tends to warm as it slows down and sloshes back and forth with the tides in the summer sun.  Delta diversions[1] can (and often do) take 12,000-13,000 cfs out of the 20,000 cfs Delta inflow (65% is the prescribed limit).

Input water temperatures at Freeport in June of 2023 and 2024, with streamflow of 15,000 cfs, were generally 70oF or higher (Figure 3).  In 2023, inflows of 20,000-40,000 cfs brought water temperatures in the 62-66oF range with slightly higher water temperatures during heat waves.  When streamflow at Freeport fell below 20,000 cfs in summer 2023, water temperatures reached 70oF or higher, especially during heat waves, when water temperatures spiked 2-4oF.  Maintaining 20,000 cfs inflow at Freeport generally would bring water temperatures below 70oF.[2]

Figure 3.  Wet year 2023 and above normal year 2024 spring-summer streamflow and water temperatures of the Sacramento River at Freeport (RM 90) at entrance to the Delta. 

Downstream of Freeport in the central portion of the Sacramento River north-Delta channel at Rio Vista Bridge, water temperature patterns are similar to Freeport although slightly warmer and more erratic (Figures 4 and 5).  Rio Vista is subject to inputs of warmer water from Cache Slough and the San Joaquin River and thus tends to be slightly warmer than Freeport.  Net river flows at Rio Vista are also lower, as Delta water diversions can markedly reduce the Delta inflows by this location.  Operation of the Delta Cross Channel also complicates the flow splits upstream of Rio Vista.  Cooler Delta air compared to the Sacramento Valley is also a factor.  However, the main factors appear to be water temperatures and inflows from upstream (Freeport).  Keeping Freeport streamflows cooler should keep Rio Vista water cooler, especially during summer heat waves like the three in July 2023.

Figure 4.  Water temperatures at the Rio Vista Bridge in the north Delta 2019-2024.  Red line is recommended 72ºF limit.

Figure 5.  Water temperature (daily average) at Rio Vista Bridge in north Delta from May-September 2023 – a wet year.  Also shown are Rio Vista Bridge (Delta) and Red Bluff (Sacramento Valley) daily average air temperatures.

  • The Bay

Summer Bay water temperatures are a function of inputs from the Delta and ocean, and of local air temperatures and tidal circulation.  At the Benicia Bridge between the east Bay (Suisun Bay) and the north Bay (San Pablo Bay), the influence of freshwater inflows is not unlike that in the rivers and Delta.  In wet years like 2023, high freshwater inflows in June kept salinity and water temperature lower (<20oC/68oF) (Figure 6).  Warm water generally comes in from the Delta, especially during the twice monthly tidal-cycle draining of Delta water into the Bay (Figure 7).  A one-foot stage drop from the 500,000-acre Delta into the Bay is 500,000 acre-ft of warm Delta water that over several days can have a measurable effect on the Bay.  The late June 2024 full super moon is already heating the Bay with warm river and Delta water (Figure 7).  A cooler Delta would make for a cooler Bay.

Figure 6.  Hourly water temperatures and salinity (EC) at Benecia Bridge in west Suisun Bay June 2022-June 2024.  Red boxes denote June periods in each year.

Figure 7.  Hourly water temperature (degrees C) and salinity (EC) in east Suisun Bay in late June 2024 after full moon.  Note warm fresher water from upstream (Delta) on ebb tides.

Conclusions and Recommendations

Poor water conditions in the Sacramento River, Delta, and Bay this summer will have negative effects on salmon, sturgeon, and other native fish populations.  Unless there is action to mitigate these effects, the summer fish die-offs that occurred in summers of 2021-2023 in the Bay are likely to occur again this summer.  The following actions can reduce these negative effects:

  • Maintain the water quality standard of 68oF (daily-average) in the lower Sacramento River at the Wilkins Slough gage.  This will require raising river flow from the planned 4000-5000 cfs level to 6000-8000 cfs level or higher (during heat waves).
  • Maintain an average daily Delta freshwater inflow of 20,000 cfs at the Freeport gage.
  • Increase the freshwater inflow above 20,000 cfs and/or reduce Delta water diversions as necessary during heat waves to maintain a daily-average 68oF at the Freeport gage and maximum hourly 72oF at the Rio Vista gage.
  • Consider operational changes to the False River weir, Delta Cross Channel gates, and Montezuma Slough gates, which may also help reduce localized adverse effects.

[1] South Delta exports, smaller regional diversions, Delta agriculture, etc.

[2] Based on review and analyses of many years of data at Freeport and other locations.

Strawberry Moon* – June 2024

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White Sturgeon Update

River and Bay/Delta water conditions during the Strawberry Moon make or break white sturgeon broodyear strength in the Central Valley.  White sturgeon spawn in April and May in a one-hundred-mile reach of the lower Sacramento River between Red Bluff and Knights Landing.  Sturgeon eggs and newly-hatched larvae need cool water 60-65oF to survive.  Within a few weeks of spawning, juvenile sturgeon actively migrate downstream to the Delta during June as river flows drop and water temperatures increase.  If flows are too low and water temperatures too high, the juvenile sturgeon cannot survive the journey to the Delta.  Water conditions in the lower river and Delta during June determine the survival rates of juvenile sturgeon and whether they even make it to the Delta and eventually the Bay.

While only a portion of the adult white sturgeon spawn each year, a higher proportion of the adult sturgeon likely make the spawning journey from the Bay in the wetter years.  Sporadic recruitment in wetter years is likely a function of high river flow attraction and good spawning and early rearing/emigration conditions (higher streamflow and lower water temperatures).  Thus, wetter years are the key, and the wettest years like 2023 are best. 

There is a paucity of information on the distribution and abundance of early life stages of white sturgeon in the Sacramento River.  The best indicator of broodyear recruitment is the summer salvage numbers at the Delta pumping plants of the State Water Project (SWP) and Central Valley Project (CVP).  In summer 2023, white sturgeon salvage numbers were consistent and relatively high (Figure 1) compared to other years.  Export rates were also high – at maximum levels because of abundant available water (see Figure 1).  Salvage numbers were also higher in 2023 than previous very wet years (2011 and 2017) that had similar high summer export levels (Figure 2).  

In contrast, no juvenile sturgeon were salvage in summer of drought year 2022 (Figure 3).  Conditions during the spawning and post-spawn periods in spring of drought years are simply too poor for any meaningful juvenile production.  Flows in April-May 2022 were only a quarter of the 2023 flows (Figures 4 and 5).  Water temperatures reached lethal levels (68oF) at the beginning of May 2022 compared to the beginning of June in 2023. 

Basically, May and June water temperatures need to be cool 60-68oF for the juvenile sturgeon to grow and survive (Figure 6).  Optimum conditions are provided in a wet year like 2023 (Figure 7), when water temperatures in June are in the mid 60s in the lower Sacramento River as it passes into the Delta (at Freeport).  In an above normal water year like 2024, flows are too low and water temperatures exceed 70oF in June (Figure 7).  In a drought year like 2022, May water temperatures in the lower Sacramento River during heat waves exceed 70oF and June water temperatures exceed 75oF (Figure 8).  Young sturgeon can not survive such high water temperatures.[1]

These basic relationships bring us to the conclusion that recruitment of young sturgeon into the population only comes in wetter years.  The sturgeon inherently know this, and most choose not to undergo a spawning trek except in wet years. 

Thus, it is important to provide the best conditions possible in wet years to maximize sturgeon recruitment that may only be possible in only two or three years per decade.

Wet Year Prescriptions

  1. In the April-May spawning season in the lower Sacramento River, maintain flows and water temperatures lower than 65oF.  Note this is generally possible in wet years (see Figures 5 and 6), but may require a small amount of reservoir storage releases or a limitation on irrigation deliveries.
  2. In June, when juvenile sturgeon emigrate to the Delta and Bay, maintain flows and water temperatures lower than 68oF, with an added flow pulse to stimulate and support emigration, as well as freshening of the Delta.
  3. Maintain higher flows in the lower Sacramento River in June and July nearer historical levels to support juvenile sturgeon emigration (Figures 9 and 10).  Maintain water temperatures no higher than 72oF in the north, central, and west Delta channels (Figures 11 and 12).
  4. Limit combined south Delta exports from SWP and CVP pumping plants to 6000 cfs in June and 9000 cfs in July, as once prescribed in State Board’s D-1485 water quality objectives.  Limit opening the Delta Cross Channel in June and selectively in July (when juvenile sturgeon migration into the Delta is at a minimum).  Once the migration into the Delta is complete, opening the DCC is advised to promote westward flow toward the Bay from the central and west Delta.
  5. Sturgeon “salvaged” at the south Delta fish facilities (Figure 1) should be transported to an appropriate location in the Bay.

Above Normal Year Prescriptions

In above normal years such as 2024, prescriptions should be a close as possible to wet year prescriptions in the hope that some recruitment is possible for the broodyear.

Conclusion

A 2019 scientific paper on the white sturgeon population in the Central Valley [2] suggested: “that population growth was most influenced by the survival of sexually mature adults, and low levels of exploitation are needed to maintain a stable population.”  This conclusion points to harvest and recent Bay die-offs of adult sturgeon in controlling the population level.

While this may be partially correct in pointing to the fishery and habitat degradation as the largest problems facing the white sturgeon population, my conclusions here point to the ultimate control being (1) lack of juvenile recruitment except in the wettest years and (2) limits on recruitment in some wet years.  High juvenile recruitment in 2023 under a very low adult population level points to recruitment being most important in the long run and best explains the long-term decline in the white sturgeon population.

White sturgeon were recommended for listing under the state endangered species act in June 2024,[3] prompted by low population levels and recent adult die-off’s in San Francisco, San Pablo, and Suisun bays.  It would be unfortunate if improving juvenile recruitment were not included in any actions taken to bring about recovery.

Figure 1.  Salvage of juvenile white sturgeon at south Delta fish facilities with export rates in wet year 2023.  Maximum export rates are approximately 11,400 cfs or 23,000 acre-ft per day.

Figure 2.  Salvage of juvenile white sturgeon at south Delta fish facilities with export rates 2011-2023.  Three wet years noted.

Figure 3.  Salvage of juvenile white sturgeon at south Delta fish facilities with export rates in drought year 2022. 

Figure 4.  Flows and water temperature in the lower Sacramento River below Wilkins Slough (RM 120) April-August of drought year 2022. Water quality objective of 68oF noted as red line.

Figure 5.  Flows and water temperature in the lower Sacramento River below Wilkins Slough (RM 120) April-August of wet year 2023. Water quality objective of 68oF noted as red line.

Figure 6.  Water temperature at Wilkins Slough gage April-June 2013-2024.  Three wet years and above normal 2024 noted. Also noted in red line is the 68ºF water quality objective.

Figure 7.  Comparison of June 2023 and 2024 Freeport gage water temperatures and flows with 2023 Red Bluff air temperature.

Figure 8. Comparison of May-July 2022 Freeport (RM 50) and Wilkins Slough (RM 130) gages Sacramento River water temperatures and flows with 2022 Red Bluff air temperature.

Figure 9.  Comparison of May-June 2016-2024 Freeport (RM 50) Sacramento River streamflows.  Red years are wet years.  Year 2024 is above normal.  Years 2021 and 2022 are critically dry years.   Years 2016, 2018, and 2020 are below normal years.

Figure 10.  Flows and water temperature in the lower Sacramento River at Hamilton City (RM 146) June-October of wet year 2023 and average for years 1995-2004.  Note summer historical flows were higher than in recent wet year.  Flows were historically higher to maintain water quality objective of 56-68ºF in lower 100 miles of river upstream of the Delta.

Figure 11.  Water temperatures at Rio Vista Bridge in north Delta 2019-2024.  Red line denotes recommended summer water quality standard to protect white sturgeon.

Figure 12.  Water temperatures at Emmaton in west Delta 2019-2024.  Red line denotes recommended summer water quality standard to protect white sturgeon.

* The Strawberry Moon (or Planting Moon) is the given name of the June full moon. 

[1] https://calsport.org/fisheriesblog/?p=938

[2] Blackburn et al. (2019).  Available at: https://afspubs.onlinelibrary.wiley.com/doi/abs/10.1002/nafm.10316

[3] https://calmatters.org/environment/2024/06/white-sturgeon-gains-california-protection-species/

Sturgeon Moon #3 – August 30 Blue Moon

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August 2023 has come to an end, following the second “Super Moon” of the month – a Blue Moon. The first full moon of the month was called a “Sturgeon Moon,” originally coined in reference to the ease of catching sturgeon in the Great Lakes during a full moon in summer. The cycle began with the quarter moon and neap tide on July 24 until the full moon on August 1.1

This year’s Super Moons have made a mess of San Francisco Bay, as they did in summer 2022. This year’s August moons have again, regrettably, led to a die-off of sturgeon and other Bay fish.2 The Super Moons’ strong tides caused warm water from the Delta to drain into the Bay, making the usually cool Bay unseasonably warm. (The ocean Blob may have added to some of this summer’s warm Bay water.) The warm water and the associated algal blooms (and their die-offs) have led to unprecedented low dissolved oxygen levels in the Bay, which can kill fish.

While the degree of harm has not been as bad as last year’s summer blooms that were aggravated by the 2022 drought, this year’s algal blooms have also harmed fish despite generally beneficial wet-year conditions. Last summer, there was die-off of nearly a thousand adult white sturgeon in the greater San Francisco Bay due to algal blooms. More dead white sturgeon adults also showed up on Bay beaches again this summer.3

The Bay turned warmer under this summer’s Super Moons than under those last summer (Figure 1). Blooms are still happening, as indicated by high turbidities and chlorophyll levels in portions of the Bay (Figures 1-3). The draining of warm Delta water to the Bay just before the 2023 Super Moons (Figures 4 and 5) warmed the Bay (Figure 6). Low dissolved oxygen continues to plague the Bay (Figure 6). The most recent bloom is depicted in Figures 7 and 8.

The summer 2023 Super Moons and their algal blooms make a complicated story, with the effects of various factors implicated in the blooms, and their role in fish die-offs in the Bay, yet to be fully determined. My concern centers on how warm the Delta becomes in summer before it drains into the Bay during the lunar tidal cycles (Figure 9). The lower rivers and Delta received too little flow from major Central Valley reservoirs for a wet year with full reservoirs. This is an increasing trend that deserves a lot more attention to ensure protection of the Bay’s fish and other public trust values. Otherwise, the trend will simply be chocked up to climate change.

Graph of North Bay (San Pablo Bay at Richmond Bridge) water temperature and turbidity in summer 2023.

Figure 1. North Bay (San Pablo Bay at Richmond Bridge) water temperature and turbidity in summer 2023. Note the presence of the three blooms indicated by high turbidity levels (>100 FNUs)

Graph of Chlorophyll levels at Martinez CA gage between East and North San Francisco Bay in summer 2023.

Figure 2. Chlorophyll levels at Martinez CA gage between East and North San Francisco Bay in summer 2023. Note three periods (green circles) of blooms located at this site.

Graph of Chlorophyll levels at Grizzly Bay gage (in northwest East Bay) in summer 2023.

Figure 3. Chlorophyll levels at Grizzly Bay gage (in northwest East Bay) in summer 2023.

Graph of Salinity (EC) at eastern Suisun Bay gage (in east Bay) in summer 2023.

Figure 4. Salinity (EC) at eastern Suisun Bay gage (in east Bay) in summer 2023. Red arrows indicate periods of draining prior to and between full moons.

Graph of Daily average (tidally filtered) discharge at Pittsburg gage in Suisun Bay summer 2023.

Figure 5. Daily average (tidally filtered) discharge at Pittsburg gage in Suisun Bay summer 2023. Red circles indicate drainage rates to Suisun Bay prior to two Super Moons (August 1 and 30).

Graph of Hourly dissolved oxygen levels in Grizzly Bay in summer 2023.

Figure 6. Hourly dissolved oxygen levels in Grizzly Bay in summer 2023. Note inverse relationship with chlorophyll levels in Figure 4. The low dissolved oxygen levels (<5 mg/l) began with the first bloom (August 1) and continued through August. Also note the Bay water quality standard is a minimum 6 mg/l dissolved oxygen level for fish health.

Satellite imagery of chlorophyll levels in San Francisco Bay on 8/29/2023.

Figure 7. Satellite imagery of chlorophyll levels in San Francisco Bay on 8/29/2023.

Graph of water temperature, chlorophyll, and salinity in Suisun Bay in summer 2023.

Figure 8. Water temperature, chlorophyll, and salinity in Suisun Bay in summer 2023. Note algal bloom in late August that began after the late-August draining of the Delta into the Bay when water temperatures reached 24oC (75oF).

Graph of San Francisco Bay water temperatures from 2007-2023.

Figure 9. San Francisco Bay water temperatures from 2007-2023. Note 2023 reached 70oF (see Figure 2) a recent record reached not even reached in critical drought years 2014, 2015, and 2022.

  1. Spring tides always happen when the Moon is at the full or new phase, which is when the Sun, Moon and Earth are in alignment. Neap tides occur around the first and last quarter phase of the Moon, when the Moon’s orbit around Earth brings it perpendicular to the Sun. ↩
  2. https://calsport.org/fisheriesblog/?p=4398 ↩
  3. https://www.sfchronicle.com/climate/article/fishkill-18279379.php ↩

Sturgeon Moon 2 – August 30

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In an August 9 post, I described the effects on San Francisco Bay and its sturgeon from the August 1 Sturgeon Moon.  I hypothesized that the draining of warm water from the Delta into the Bay over several days of the strong neap tide during the Sturgeon Moon caused an algae bloom and unseasonably warm water in the Bay (Figure 1) that was killing sturgeon, as it had in summer 2022.  I blamed the warm water on low river flows and high water diversions in the Central Valley that caused the Delta to reach 75oF and the Bay to subsequently reach an unprecedented 72-73oF.  The warm water, abundant sunshine, and generally high nutrients caused the bloom and the low dissolved oxygen levels that resulted in fish dying.

These events are about to reoccur with the August 30 Super Moon.  Once again, warm water will drain from the Delta on several days of strong neap tides the week before the Super Moon (probably around August 24).   The bloom should appear about August 28, about two days before the full moon.

The key question is how warm the Delta will be when it drains into the Bay.  This depends on air temperatures, river flows, and Delta outflow (the product of reservoir releases, Delta inflow, and water diversions).  With an expected general heat wave August 14-17, there is reason to be concerned that water draining from the Delta could be warm once again.

There have been several mitigating factors since the August 1 Super Moon.  Reservoir releases have increased slightly over the past month (Figure 2).  The strength of the spring and neap tides has decreased slightly following the August 1 full moon (Figure 2).  Higher Delta inflows (Freeport) have reduced Delta water temperatures slightly (Figure 3).

To minimize the strength of the potential bloom, warming, and fish die-off in the Bay, it is essential to keep Delta water temperatures down before the August 25-26 neap tide.  Several interdependent actions come to mind: (1) Increase lower Sacramento River flows over the next 10 days by several thousand cfs to get Wilkins Slough water temperatures down to about 68oF.  (2) Ensure that the extra Wilkins flow reaches the Delta at Freeport to keep Delta inflow up several thousand cfs.  (3) Increase Delta outflow during the August 22-24 spring tide by reducing south Delta exports, to minimize the build-up of warm water in the Delta prior to when the Delta drains to the Bay on the August 25-26 neap tide.

These actions will hopefully minimize the damage caused by Central Valley water management to the Bay ecosystem and specifically to the white sturgeon population during the next Super Moon cycle.

Satellite image

Figure 1. San Francisco Bay algae bloom on July 30, 2023. Source

Graphs showing Delta inflow (Freeport) and Delta outflow to Bay in week since the August 1 Super Moon.

Figure 2. Delta inflow (Freeport) and Delta outflow to Bay in week since the August 1 Super Moon. Note the spring tide has gotten slightly stronger and Delta inflows have increased (due to increased Folsom and Oroville reservoir releases)

Graph showing Delta inflow (tidally filtered and hourly) from the Sacramento River and water temperature at Freeport July-August 2023.

Figure 3. Delta inflow (tidally filtered and hourly) from the Sacramento River and water temperature at Freeport July-August 2023.

Graph showing Lower Sacramento River streamflow and water temperature at Wilkins Slough gage July-August 2023. Water temperature remains high (>20C, 68 F) under low streamflow.

Figure 4. Lower Sacramento River streamflow and water temperature at Wilkins Slough gage July-August 2023. Water temperature remains high (>20oC, 68oF) under low streamflow.