It is early October 2023, and the fall-run salmon are running.  The California Department of Fish and Wildlife expects to open the ladder to the American River Hatchery on or about November 1.  There are already salmon in the lower American River from its mouth all the way to Nimbus Dam.

The run expected this year has a mixed forecast.  There was no ocean or river salmon fishery this year because of poor runs in the Central Valley during the 2020-2022 drought.  The fact that fishing was not allowed will improve the 2023 run size (escapement).  The 2020 run (parents of this year’s run) was pretty good (Figure 1).  But when brood year 2020 was rearing and migrating to the ocean, it was a critical drought year 2021, conditions under which juvenile salmon almost always have poor success in making the journey from rivers to the ocean.

Conditions in 2023 have been a mixed bag for the migration of adult spawners from the Bay to Central Valley rivers.  Summer 2023 started with warm water and low flows.  That changed when the Fall X2 requirement kicked in September (Figure 2).  The September Fall X2 flow pulse helped cool the lower Sacramento River and the Bay-Delta (Figure 3), and provided attraction flows for the adult salmon to move into the Bay and on toward the rivers.

An unforeseen problem has faced the running salmon when they hit the north Delta and mouth of the American River: water temperatures have been stressful for migrating adult salmon (>65^oF, Figure 4) despite high flows (3000 cfs) from the American River.  The water quality standard for the lower American River requires water temperatures to be maintained <65^oF in summer.  But water temperatures rose through September, with continuing increased water temperatures of water released from Folsom Reservoir (Figure 5).  Water year 2023 was a very wet year with above-average reservoir storage.  So why was the American River so warm?

The answer is that the powerhouse intakes at Folsom Dam had limited access to the cold-water pool, and the supply of accessible cold water was running short.  To ensure colder water for the hatchery and river for the November spawn, the Bureau of Reclamation began operating the powerhouse intakes in September to draw warmer water from the surface layers of the reservoir as the reservoir slowly drained.

The water temperature for spawning needs to be below 56^oF.  Getting the temperature down that low by November is a tall order, but it was achieved in the 2020-2022 drought (Figure 6) under much lower reservoir storage levels (the present level is 135% of average for October and is much higher than in 2020-2022).  Reclamation achieved November spawning temperatures in those drier, lower-storage years by releasing substantial cold-water from the lower dam outlets, bypassing the power intakes.   The power bypass measure will likely be needed again in 2023 to meet the November temperature targets in the river.

There is a case to be made that Reclamation should also have used the power bypass in September 2023 to maintain 65^oF at the Watt Avenue bridge.  Reclamation’s reluctance to use the power bypass to maintain release water temperature requirements further highlights the need for Reclamation to complete the planned refinements to the Folsom powerhouse intake system that will allow both continuous power generation and greater access to Folsom’s cold-water pool.

How else could this series of events have been avoided or at least improved?

First, summer flows in the lower Sacramento River should have been higher to help keep water temperatures at or closer to the standard of 68^oF (see Figure 3) at the Wilkins Slough gage, located  on the lower Sacramento River upstream of the mouths of the Feather and American rivers.  Flows at Wilkins Slough for wet year 2023 were well below the average for water years 1994-2003, when salmon did relatively well (Figure 7).  These low flows in 2023 contributed to poor conditions in the lower Sacramento River, the Delta, and the Bay.

Second, the water temperature in the  lower American River at the Watt Avenue Bridge (AWB) should have met the water quality standard of 65^oF until September, and should be kept below 60^oF in October (Figure 8).  After November 1, Reclamation should maintain temperatures below 56^oF for spawning.

Such improved conditions would improve the health of adult salmon spawning in the American River and its hatchery, as well as in other rivers and streams in the Sacramento River watershed.

Figure 1. Escapement (run size) for in-river spawning fall-run salmon in the lower American river 1952-2022.

Figure 2: Flow and water temperature at the Rio Vista Bridge July 15 – September 30, 2023.

Figure 3. Air and water temperature on the Sacramento River at Red Bluff (River Mile – RM – 240), and water temperature at Wilkins Slough (RM 120), Freeport (RM 50), and Rio Vista Bridge (RM 25). Also shown is water temperature standard at Red Bluff and Wilkins Slough.

Figure 4. Water temperature in the upper Sacramento River below Keswick Dam (RM 300), lower American River at Watt Avenue Bridge (AWB), the lower Feather River at Gridley gage (GRL), the lower Sacramento River at Wilkins Slough (WLK, RM 120) upstream of the Feather and American Rivers, and Freeport (FPT, RM 46) downstream of the American River.

Figure 5. Daily average water temperatures in the lower American River 9/10-10/3 2023 below Folsom Dam (AFD), at Fair Oaks gage below Nimbus Dam (AFO), at William Pond gage (AWP), and at Watt Avenue Bridge gage (AWB). The water quality standard in all water year types is 65oF maximum (year-round) at AWB.

Figure 6. September through December daily average water temperatures of Folsom Dam releases 2020-2022. Note the sharp drops in water temperatures in middle October from Power Bypass cold-water releases to provide target spawning temperatures (56^oF).

Figure 7. Average daily flows in the lower Sacramento River at Wilkins Slough, upstream of the mouths of the American and Feather rivers in 2023, compared to the average during the decade of strong escapement 1994-2003.

Figure 8. Water temperatures in the lower American River at Fair Oaks (below Nimbus Dam) June-September 2023 with average from prior eight years that included 5 critical dry years.