Monthly Archives: May 2024

Warm Water Temperature in lower Sacramento River in May 2024 Migrating Adult and Juvenile Salmon and Sturgeon Are Compromised in Spring of an Above Normal Year Following a Wet Year

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In the third week of May 2024, the water temperatures in the lower Sacramento River recorded at Wilkins Slough increased to 72oF, well above the 68oF water quality standard (Figure 1). These warm water temperatures occurred in a wet spring of an Above Normal water year that is following a Wet water year.

The water temperature spike occurred between prescribed pulse flow releases from Shasta Dam in May (Figure 1).  Three pulse flows were prescribed this spring to promote and assist migration of juvenile salmon into the lower Sacramento River and the Delta.

After the second pulse in early May, the lower river flow was allowed to drop to a drought-level 5000 cfs, causing the high water temperatures.  Shasta Reservoir was virtually full at 4.3 MAF during all of May.

The Central Valley Basin Plan’s water quality objective for the lower Sacramento River is 68oF maximum “during periods when temperature increases will be detrimental to the fishery.” (P. 3-14).  Declining tributary inflows and increased mainstem water diversions contributed to the low flows.  Cooler American River and Feather River inflows to the Sacramento River below Wilkins Slough have kept north Delta water temperatures in the Sacramento River channel at Freeport and Rio Vista cooler at 64-67oF (Figure 2).

Many juvenile salmon emigrate to and through the Delta in spring (Figures 3-5).  High water temperatures in the lower Sacramento River lead to a drop-off in migration and increase in stress and predation, ultimately reducing survival and the numbers of smolts reaching the ocean.  Many spring-run and fall-run salmon smolts stay in the Delta through June and into July (Figures 6 and 7).  A majority of these smolts are wild salmon adapted to emigrate with the late spring snowmelt season; they rear in the Delta prior to entering the ocean.

Adult spring-run and winter-run salmon also migrate upstream through the Bay-Delta to upriver spawning areas in the spring.  They too benefit from the pulse flows, but also suffer stress from the high water temperatures on the journey upstream.  Adult Chinook salmon avoid migration through water whose temperature is at or above 72oF.

Green and white sturgeon also spawn in the spring in the lower Sacramento River from Red Bluff downstream to Verona (river miles 200 to 100).  Optimal water temperatures for spawning and early rearing are 50-65oF.  Water temperatures above 65oF are stressful and lead to poor survival.  Pulse flows stimulate spawning.  Peak water temperatures of 68-72oF in mid-May are considered detrimental to juveniles and adults, as well as lethal to sturgeon eggs and embryos.

The river conditions described above for mid-May 2024 are typical in drought years, but not in wet years.  The pulse flow certainly helps in the salmon and sturgeon migrations.  But lower Sacramento River base flows should not be allowed to fall below the 8,000-10,000 cfs needed to maintain water temperatures at or below 65oF to protect migrating adult and juvenile salmon and sturgeon.

Wet-year recoveries are essential given how poor conditions are in drought years.  The 2020-2022 drought led directly to the complete closure of salmon fisheries in 2023 and 2024 and more stringent requirements for the white sturgeon fishery in 2024.

Good migration conditions must be maintained in Wet years, including sequences of Wet and Above Normal water years, if there is to be any recovery from the multiyear droughts.

Figure 1. Daily-average Sacramento River flow and water temperature at Keswick Dam (RM 300) and Wilkins Slough (RM 120) in spring 2024. Note water temperature at Wilkins Slough (purple line) has exceeded the water quality standard of 68oF for the lower Sacramento River. Afternoon water temperatures on May 15 reached 72oF. Note the three pulse flows conducted by Reclamation (blue line) to support salmon migration helped to lower water temperatures. Note the sharp rise in water temperature in mid-May at Wilkins Slough after cessation of the second prescribed pulse flow, when streamflow dropped below pre-pulse flows.

Figure 2. Daily-average Sacramento River flow at Freeport (blue line) and water temperature a Freeport (green line) and Rio Vista (orange line) in spring 2024.

Figure 3. Rotary screw trap capture rate of juvenile salmon in 2024. Also shown is river flow rate and water temperature and turbidity.

Figure 4. Trawl Catch Index of juvenile salmon near Sacramento in the Sacramento River in Water Year 2024. Also shown is river flow rate and water temperature and turbidity.

Figure 5. Trawl Catch Index of juvenile salmon near Chipps Island in the eastern Suisun Bay in Water Year 2024. Also shown is river flow rate and water temperature and turbidity.

Figure 6. Salvage of juvenile salmon in water year 2023 at south Delta export pumping plants. Also shown are tagged hatchery salmon smolt collections by hatchery release groups and run type, and Delta flow and export rates.

Figure 7. Salvage of juvenile salmon in water year 2024 at south Delta export pumping plants. Also shown are tagged hatchery salmon smolt collections by hatchery release groups and run type, and Delta flow and export rates. Wild fry and smolt groups are noted; they can be segregated given the general lack of tagged hatchery smolts for the size group and time period.

Sacramento River Spring Pulse Flows – 2024

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The US Bureau of Reclamation (Reclamation), in cooperation with other state and federal agencies, conducted two short-term flow release experiments from Shasta Reservoir in the spring of 2024 to help juvenile salmon reach the ocean. 2024 is a relatively wet year, and it follows a very wet year in 2023 that left a lot of water in storage in Shasta and other Reclamation reservoirs.

The peak emigration to the ocean of wild and hatchery spring-run and fall-run salmon smolts usually occurs in the April-May time period, but may extend into June, especially in wetter years. Reclamation and its partners are developing pulse flow prescriptions for a flow management plan as part of the Action for the Long Term Operation of the Central Valley Project and State Water Project. “Spring pulse flows are designed to improve survival rates of outmigrating spring-run Chinook salmon smolts through the Sacramento River.” 1

The major sources of spring-run salmon smolts include Clear, Battle, Deer, Mill, and Butte creeks, and the Feather River Fish Hatchery. These smolts must pass through the lower Sacramento River, Delta, and Bay. The Feather River Fish Hatchery released its smolts into the lower Feather River in mid-March.

Pulse flows from Shasta Dam pass through Keswick Reservoir, a small regulating reservoir immediately downstream of Shasta Reservoir, and then into the Sacramento River. Pulse flows from Keswick Dam at river mile 300 of the Sacramento River also help move juvenile fall-run salmon downstream. Most Sacramento River fall-run salmon spawn and rear in the 20 miles of the river immediately downstream of Keswick Dam. Pulse flows help move juveniles from this reach downstream to reaches where the flow from tributary rivers and streams augments the flow in the mainstem Sacramento.

The goal of the pulse flows was to release extra flow (3000-6000 cfs) from Shasta reservoir to maintain 11,250 cfs below Keswick Dam for several days (see first pulse in Figure 1). With tributary inflow, river flow near Red Bluff 50 miles downstream was about 15,000 cfs during the first pulse. By the time the first pulse reached Wilkins Slough, nearly 200 miles downstream, the overall flow reached 16,000 cfs (Figure 2), after 3000-5000 cfs of agricultural diversions and multiple tributary inflows. At the end of the pulse flow period, Keswick Dam releases were dropped 6000 cfs (see Figure 1). By May 5, Sacramento River flow at Wilkins Slough had dropped about 8000 cfs. Declining tributary inflows and increased diversions also contributed to the lower Wilkins Slough flows following the pulse flow.

Without adequate flow, juvenile salmon have poor survival during their downstream journey. Low flows increase juvenile salmon’s vulnerability to predation. Low flows also contribute to stressful warm water temperatures. On May 1, 2024, water temperature in the river at Sacramento was below 65oF (18oC), with peak catch of juvenile salmon in the trawl survey (Figure 3). In contrast, in critically dry year 2022, May 1 water temperatures exceeded 65oF, with less than half the 2024 river flow (Figure 4). By late May 2022, water temperatures exceeded 70oF.

In conclusion, the releases of pulse flows from Shasta and Keswick reservoirs had very little signature in the lower Sacramento River due to intervening flows and diversions. The pulse flows were most beneficial in the upper river, where they made up a third to half the river flow. In a wet year like 2024, the benefit is somewhat limited by already high natural flows, although any additional pulse flows later this spring could be of greater benefit with expected declining natural flows and higher air temperatures. In contrast, pulse flows in dry years would provide much greater net potential benefit because of poor natural flows and limited reservoir storage.

The net benefits of pulse flows from Shasta Reservoir must also be considered, of course, in combination with Reclamation’s management of agricultural water deliveries and water stored in Shasta Reservoir for summer temperature management.

Figure 1. Streamflow in the upper Sacramento River below Keswick Dam (RM 300) April 19 through May 19, 2024. Note three-day flow pulses beginning April 23 and May 7.

Figure 2. Streamflow in the middle Sacramento River below Wilkins Slough (RM 120) April 19 through May 19, 2024. Note signatures of the three-day flow pulses show up on April 26 and May 10.

Figure 3. Juvenile salmon trawl catches and water conditions near Sacramento in winter-spring 2024. Note flow pulse in late April.

Figure 4. Juvenile salmon trawl catches and water conditions near Sacramento in winter-spring 2022. There were no flow pulses from Keswick Dam until irrigation releases commenced in May. Catch of juvenile salmon ceased once water temperatures reached or exceeded 70oF in late May.