Tom Cannon December, 2025

In a 9/12/25 post, I warned of poor summer conditions in the Bay for sturgeon.  This came on the heels of a poor population status assessment by CDFW.

From the Department of Fish and Wildlife: “Recent results from white sturgeon monitoring surveys by the California Department of Fish and Wildlife (CDFW) suggest the white sturgeon (Acipenser transmontanus) population has continued to decline. CDFW fisheries biologists now estimate there are approximately 6,500 white sturgeon between 40-60 inches long in California — down sharply from the previous estimate of approximately 30,000 fish in that size range, based on the 2016-2021 survey average.” https://mavensnotebook.com/2025/07/10/cdfw-scientific-surveys-show-continued-decline-in-white-sturgeon-population/

Not only was the recent adult sturgeon population survey estimate down, but the products of sturgeon reproduction in 2025 were nearly non-existent, a pattern inconsistent with an above-normal water year.   During the wet year 2023, white sturgeon reproduction in the Bay-Delta population was up sharply, as shown by numbers salvaged at the south Delta pumping plant fish salvage facilities (Figure 1).  In contrast, sturgeon salvage numbers were very low in summer of above-normal water year 2024.  In above-normal water year 2025, no sturgeon were collected in the south Delta salvage surveys.

A big reason for the unsuccessful sturgeon reproduction in water years 2024 and 2025 was poor conditions in the spring spawning and early rearing reach of the middle Sacramento River (Figure 2).  Water temperatures were above optimal (>65^oF) and at times stressful (>68^oF) or even lethal (>72^oF) in 2024 and 2025.  Few juvenile sturgeon survive to reach the Delta under these habitat conditions.  This was one of the factors that led the State Water Board and USEPA to set 68^oF as the water quality standard for the Sacramento River two decades ago. This standard is also a condition of the State Water Board water right permits for the state and federal water projects.

The water temperature standard could be met if river flows are maintained in the 8000-10,000 cfs range at the Wilkins Slough gage (WLK) located upstream of the mouth of the Feather River (river mile 120). (Note the water temperature benefit of higher flows in the May and June flow pulses in 2025.)

The residual adult sturgeon population within their Bay summer habitat also experienced unfavorable elevated temperature conditions (>20ºC, 68ºF; Figure 3).

For further detailed discussion of the status of sturgeon in the Central Valley see:  https://calsport.org/fisheriesblog/?cat=20 .

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 (22^oC/72^oF) 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 5^oF 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 70^oF or higher (Figure 3).  In 2023, inflows of 20,000-40,000 cfs brought water temperatures in the 62-66^oF range with slightly higher water temperatures during heat waves.  When streamflow at Freeport fell below 20,000 cfs in summer 2023, water temperatures reached 70^oF or higher, especially during heat waves, when water temperatures spiked 2-4^oF.  Maintaining 20,000 cfs inflow at Freeport generally would bring water temperatures below 70^oF.[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 (<20^oC/68^oF) (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 68^oF (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 68^oF at the Freeport gage and maximum hourly 72^oF 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.

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[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.

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 60-65^oF 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 (68^oF) at the beginning of May 2022 compared to the beginning of June in 2023. 

Basically, May and June water temperatures need to be cool 60-68^oF 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 70^oF in June (Figure 7).  In a drought year like 2022, May water temperatures in the lower Sacramento River during heat waves exceed 70^oF and June water temperatures exceed 75^oF (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 65^oF.  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 68^oF, 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 72^oF 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 68^oF 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 68^oF 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.

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* 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/