The Fall-X2 Action – Benefits and Costs

Posted on October 22, 2025 by Tom Cannon

The 2008 Delta Smelt Biological Opinion for Central Valley Project (CVP) and State Water Project (SWP) operations included the Fall X2 Action to protect Delta smelt. The about-to-be implemented “Action 5” that will modify the 2024 biological opinions for the CVP and SWP will eliminate the Fall X2 Action entirely.

The Fall X2 Action required the CVP and SWP to keep brackish water west of the Delta in September and October in wet and above normal water years. The X2 location is defined as the location where salinity is 2 parts per thousand.¹ X2 is located within what is known as the low salinity zone (LSZ, 1-6 parts per thousand).

Delta smelt and longfin smelt concentrate in the LSZ near X2 in summer and fall. Keeping X2 at Chipps Island (KM 71 from the Golden Gate, Figure 1) benefits the populations of Delta smelt and longfin smelt. Less effective options for the Fall X2 Action that require less Delta outflow have included other locations as far east as KM 80.

If freshwater outflow from the Delta is too low (at or below about 6000 cfs), the LSZ is located within the narrow channels of the west Delta. Water in the west Delta tends to be warmer and less productive than water in Suisun Bay. It is also more likely to be pulled south by pumping at the CVP and SWP’s south Delta export facilities.
The higher freshwater inflows needed for the Fall X2 Action also benefit other native fishes in the lower Sacramento River, its tributaries, and the Delta.

Harm comes to Bay-Delta fish in the form of high water temperatures and reduced food concentrations. When water temperatures climb into the range of 70-72°F, native fishes, including smelt, sturgeon, steelhead, and salmon, generally suffer stress-related poor growth and survival.

Warm Delta water blocks or hinders adult salmon in their upstream spawning migrations. (A water temperature of 72°F is the salmon avoidance temperature.) Adult salmon will hold in the Bay because of warm water temperatures, resulting in delayed spawning and the expenditure of the critical energy needed for spawning. Warm rivers cause further delay, diseases, and thiamine deficiency, reducing the ability of salmon to spawn successfully. Higher water temperatures (> 65°F) during spawning migrations also lead to reduced salmon egg and embryo survival. High water temperatures in juvenile rearing habitat and emigration routes lower the growth and survival of juvenile salmon, steelhead, smelt, and sturgeon, and increase predation on all these species.
In the Delta specifically, high water temperatures in the Delta block emigration of juvenile salmon and reduce growth rates and survival of juvenile salmon and smelt. Both salmon and smelt species are highly sensitive to warm and varying water temperatures. Heat transfer and retention during high air temperatures from late-spring to early-fall is greater at lower flows.

In summary, water temperatures above 72°F are highly stressful on native Bay-Delta fishes, often leading to poor growth, low-oxygen stress, more disease, greater susceptibility to predation, and competition from non-native fish, all of which lead to reduced survival. Also, many non-native fish such as largemouth bass thrive in higher water temperatures (72- 80°F).

The Fall X2 Action
The Fall X2 Action focuses on increasing Delta inflow and outflow in the months of September and October when seasonal flows are naturally lower and irrigation demands less, while air temperatures remain high. Late summer and early fall are a key period in the life cycle and overall reproductive success of many Bay-Delta native fishes.

Bay-Delta
The Fall X2 Action is needed to maintain low salinity and water temperature in the optimal range of native Bay-Delta fish. In the 2017-2019 period, with two wet years and one below-normal year, the higher prescribed Delta outflow (DTO) of the Fall X2 Action (Figure 2) in the wet years kept the Bay fresher (Figure 3) and Delta outflows cooler (Figures 4 and 5). In the two above-normal years of 2024 and 2025, the lack of a Fall X2 Action in September 2025 (Figure 6) led to high September water temperatures in the western Delta (Figure 7). The lack of Fall X2 Action in October 2024 (Figure 6) contributed to stressful (>65º F) October water temperatures at Rio Vista bridge in the west Delta (Figure 7). 2020, a below normal water year, provides another example of high September-October water temperatures when the Fall X2 Action was not implemented (Figure 8).

Lower Rivers
The Fall X2 Action requires roughly 5000 cfs of extra Delta inflow for two months (assuming needed flow is not provided by foregone river diversions or Delta exports). Releases from Shasta, Oroville, and/or Folsom reservoirs generally provide this inflow. That amounts to roughly 10,000 acre-feet of water per day, or 600,000 acre-feet for the full September-October period. In recent wetter water years, the CVP and SWP have offset some (or all) of this water cost by reducing reservoir releases in the summer and not meeting water quality standards for water temperatures in the lower Sacramento River. Flows in the lower Sacramento River at Wilkins Slough have been near 5000 cfs, when it takes about 10,000 cfs to maintain the 68º F standard.

The Cost
In summary, the amount of fresh water required to keep X2 in Suisun Bay in late summer and early fall of above-normal and wet water years requires approximately 600,000 acre-feet of stored project water or forgone diversions from rivers and exports from the Delta. That amounts to about 5% of total CVP and SWP storage capacity. It is also about 5% of the total annual upstream, in-Delta, and Delta export water use in the Bay-Delta watershed.

The about-to-be implemented “Action 5” that will modify the 2024 biological opinions for the CVP and SWP will eliminate the Fall X2 Action, and this water cost, entirely.

Figure 1. Bay-Delta and key water accounting and water quality locations.

Figure 2. Daily average Delta outflow (cfs) in September-October of water years 2017-2019. Fall X2 Action was implemented in wet year 2017 and 2019.

Figure 3. Salinity (EC) of east Bay near Chipps Island at Mallard Slough 2017-2019.

Figure 4. Water temperature in west Delta at Emmaton in September-October 2017-2019.

Figure 5. Water temperature in west Delta at Rio Vista in September-October 2017-2019.

Figure 6. Daily average Delta outflow (cfs) in September-October of above-normal water years 2024 and 2025. Fall X2 Action was implemented in September of 2024 and October of 2025.

Figure 7. Water temperature in west Delta at Emmaton and Rio Vista in September-October 2024 and 2025.

Figure 8. Daily average Delta outflow (cfs), Freeport river flow, and Emmaton and Rio Vista water temperature in July-October of water year 2020. The Fall X2 Action was not implemented in below-normal water year 2020.

Also measured as about 3000 EC. See https://pubs.usgs.gov/sir/2014/5041/pdf/sir2014-5041.pdf ↩

2025 Sturgeon Moon

Posted on September 12, 2025 by Tom Cannon

The 2025 Sturgeon Moon (August 9 full moon) wreaked havoc on San Francisco Bay as it had in recent years¹. The main effect was warm fresh water from the Delta draining into the Bay (Figure 1). The tidal effect of the super moon dropped water levels sharply about two weeks before the super moon (Figure 2). Warm water built up in the Bay over several days (Figure 3). A plankton bloom appeared in the North Bay soon thereafter (Figure 4). Is anyone checking on the sturgeon in the Bay? Will the coming heat waves over-heat the Bay?

Figure 1. Fresh water drained into the Bay at Martinez for several days in late July, beginning about 14 days prior to the full moon.

Figure 2. Water level of the Bay at Martinez July and early August 2025.

Figure 3. Water temperatures increased at Martinez with the warm inputs from the Delta and Suisun Bay.

Figure 4. Small plankton blooms appeared around the peripheries of San Pablo and Suisun bays and in Montezuma Slough at the end of July with the warm freshwater inputs. Source: https://www.cbr.washington.edu/sacramento/data/query_river_graph.html

https://calsport.org/fisheriesblog/?s=Sturgeon+moon&submit=Search ↩

California Salmon Recovery – Series Introduction

Posted on December 16, 2024 by Tom Cannon

This Introduction is the first in a series of posts that summarize elements of my recommended plan for Central Valley salmon recovery. Central Valley salmon runs over the recent five decades have varied from a total near 100,000 to near a million spawners with an average of 200-300 thousand (Figure 1). About 90% of the salmon runs have been fall-run. Spring-run and winter-run salmon have declined and are listed under the state and federal endangered species acts. All four runs are supported by hatcheries that raise about 30 million juveniles per year for release to rivers, the Bay-Delta, and the coastal ocean. Each of the run types are made up predominantly of hatchery-produced salmon. A few small stocks are comprised mainly of wild salmon, while most runs have a small “wild” or natural-born component. Rebuilding wild salmon stocks while providing for historic levels of commercial and recreational salmon harvest is the dual program mission.

The total number of adult salmon produced in the populations includes escapement to the spawning rivers described above as well as adult salmon harvested in the ocean, Bay-Delta, and rivers, prior to the spawning runs. These estimates are of the human harvest (Figure 2) and do not include harvest by marine animals or illegal fishing. Human harvest rates over the past four decades have been 50-70% on average except in years when the fishery is restricted (2008-2010 and 2023 and 2024) (Figure 2). Government agencies often restrict harvest when the Sacramento Fall Run Index falls to near a target minimum of 122,000, a condition commonly referred to as “overfished.”

The goal of my recovery plan is to return total adult salmon escapement to near a million fish, with a further harvest of 500,000 adult fish. The harvest would focus on hatchery fish, while habitat improvements would focus on rebuilding wild, genetically pure populations and their critical habitat.

The primary tools of the proposed recovery program include hatcheries, wild salmon sanctuaries, improvements in water project management and infrastructure, habitat enhancements, and changes to fishery management. Hatchery programs would include “production” hatcheries to produce marked smolts for release for harvest and maintaining spawning populations in rivers that cannot sustain wild populations. “Conservation” hatcheries would support the protection, maintenance, and development of “wild,” “genetically-pure” populations.

The actions I propose are not radical or new: they have been included in many plans and program recommendations for decades. Many have been successfully implemented on a small scale in the Central Valley or in other large salmon watersheds of the Pacific Northwest and Alaska.

Figure 1. Total Central Valley Chinook salmon escapement by run type 1975-2023. (Source: Grandtab)

Figure 2. Sacramento River watershed salmon population index 1983-2023. Source: NOAA Fisheries and Pacific Fisheries Management Council. Note there was no harvest allowed in 2024, as in 2023.

Increasing River, Delta, and Bay Summer Freshwater Flows Proves Viable Action

Posted on July 29, 2024 by Tom Cannon

In Above Normal water year 2024, an increase in summer freshwater flows released from reservoirs to the Bay-Delta estuary has proven a viable action to reduce threats to Central Valley fishes. Water management actions in early July heat waves alleviated extreme water temperatures that threaten the native fishes in rivers and the Bay-Delta. These actions can be described as adaptive management experiments to test their potential performance for the update of the State Board’s Bay-Delta Plan.

Actions

Increasing lower Sacramento River flow from 5000 cfs to near 8000 cfs has helped lower Wilkins Slough gage (WLK) water temperatures from 70-72^oF to the water quality standard of 68^oF (Figure 1), despite record-high air temperatures (Figure 2).
Increasing Delta inflow at Freeport (FPT) from 14,000 cfs to 20,000-22,000 cfs (a combination of increased Wilkins Slough, Feather River, and American River flows) has lowered Freeport water temperatures in the north Delta from 72^oF to 70^oF (Figure 1).
A rise in Delta outflow from 8,000 cfs to 12,000 cfs has helped reduce Rio Vista (RVB) water temperatures at the Delta’s exit to the Bay from 75-76^oF during the early July heat wave to 72^oF after the heat wave (Figure 2).

Benefits

The actions may not seem that dramatic, but they are very important to the river, Delta, and Bay environments and to the salmon, smelt, steelhead, sturgeon, and other native Central Valley fishes that depend on these habitats. Water temperatures in the 72-75^oF range are highly stressful or lethal to these native fishes. Such temperatures favor non-native predatory and competing fishes. Water temperatures of 68-72^oF are at the upper favorable limits for the native fish and are necessary to maintain viable growth, survival, and reproduction. These temperatures also help ensure that dissolved oxygen is adequate and that algae blooms do not reach excessive levels. In other words, they promote a healthier ecosystem. Water temperatures near or above 75^oF, which occurred in the Delta of drought years 2021 and 2022 (Figure 3) under extreme low flows (Figure 4), are deadly to native Delta fishes.

Conclusion

Although water managers in 2024 probably did not have these bold actions in mind to save fish (flows were increased to allow maximum summer water diversions from the Delta), their fortuitous implementation clearly highlights early summer flow measures that should be included in the update of the Bay-Delta Plan. Two additional actions I would recommend are higher flows in the San Joaquin River to provide some minimal benefit to the San Joaquin’s native fish community, and reductions in water exports.

Figure 1. DTO = Delta Outflow to the Bay, FPT = Sacramento River Freeport gage, WLK = gage below Wilkins Slough on lower Sacramento River upstream of the Delta, RVB = Rio Vista Bridge in Sacramento River channel northwest Delta, near entrance to eastern Bay.

Figure 2. Average daily air temperatures at Red Bluff (KRDD) in Sacramento Valley, Modesto (KMOD) in San Joaquin Valley, and Rio Vista Bridge (RVB) in west Delta May-July 2024. Note Delta air temperatures are generally 5-15^oF lower in the Delta than the valleys. Also note the record or near-record air temperatures in early July.

Figure 3. Average daily water temperatures in the north Delta channel of the Sacramento River at Freeport May-July 2021-2024 and average of years for decade 2001-2010.

Figure 4. Average daily (tidally filtered) streamflow in the north Delta channel of the Sacramento River at Freeport, May-July 2021-2024 and average of years for decade 2001-2010.

Bay Warms in July 2024

Posted on July 16, 2024 by Tom Cannon

This is a follow-up to a July 8 post on summer 2024 aquatic habitat conditions in the Bay-Delta Estuary. In this post, I focus on mid-July conditions after yet another summer heat wave. I am concerned that conditions are building for yet another sturgeon die-off this summer. Sturgeon mortality is caused by excessively warm water and algae blooms that eventually lead to rapid algae die-off and associated extremely low dissolved oxygen levels (<5 mg/l) throughout the Bay. Such conditions became acute in summer of drought year 2022 and led to the deaths of thousands of adult sturgeon and other Bay fishes.

Conditions in the Bay are already worse this year at the beginning of summer than in 2022 or 2023 (Figure 1). Water temperatures and chlorophyll concentrations are higher, with dissolved oxygen concentrations falling.

Figure 1. Water temperature (C), dissolved oxygen (mg/l), and chlorophyll concentration (micrograms per liter) in Grizzly Bay 2022-2024.

Of greatest concern is the already warm water temperatures in parts of the Bay-Delta despite a relatively high Delta inflow with cooler than normal water temperatures (Figure 2). Water temperatures have exceeded 75^oF in the west Delta and east Bay low-salinity zone of the estuary (the prime summer habitat of endangered Delta smelt). Water temperatures of 75-77^oF (24-25^oC) are lethal to Delta smelt.¹

In the decades of the 80’s and 90’s, Delta smelt were still relatively abundant although suffering severely in drought years. Water temperatures rarely exceeded 70^oF in Suisun Bay (Figure 3). The reason for the difference is likely a combination of higher air temperatures, lower Delta outflows, and higher Delta water diversions in recent decades. In some years lower, warmer Delta inflows aggravate the problem, but not this year (2024) when inflows were kept high to sustain high Delta water diversions.

One area of warming of the lower Sacramento River channel that deserves special attention is the north Delta below the Delta Cross Channel and Georgianna Slough bifurcation. Much of the water destined for south Delta exports is diverted from the Sacramento River through these channels. In the reach below the diversion channels the river temperature increased several degrees (73 to 76^oF) in early July. One explanation is that of the 21,000 cfs Delta inflow at Freeport only 5000 cfs remained below the entrance to Georgianna Slough (Figure 4). The missing flow passed into Georgianna Slough, the Delta Cross Channel, and Miners/Steamboat Sloughs, all reaches where the flow warmed to even a greater degree. At Rio Vista and the west Delta, where the water exits the Delta for the Bay, water temperatures reached 76^oF as total outflow increased to 10,000 cfs from the 5000 cfs in the lower Sacramento River channel and 5000 cfs returning warmer water from the Cache Slough complex and San Joaquin River channel. Waters in the wide, large, open channel at Rio Vista also slowed, with a stronger influence of the tides, especially during the recent heat waves (Figure 5).

I contend that the high rate of Delta water diversion at Georgianna Slough and the Delta Cross Channel contributed to the warming by forcing cooler water from the Sacramento River Delta inflow into the central Delta where it warmed or was diverted.

I recommend closing the DCC and reducing Delta water diversions in July and August to reduce Bay-Delta water temperatures and minimize the potential for excessive algae blooms, low dissolved oxygen, and sturgeon die-offs in the Bay this summer. I further recommend that Delta inflows be sufficient to maintain water quality standards for water temperatures in the lower Sacramento River above the Delta.