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Wild Central Valley Salmon: Managers Missing an Opportunity

This winter and early spring of 2020 have been drier than normal in the Central Valley. However, precipitation in January, March, and now April provided opportunities to greatly enhance this year’s brood of fall and spring run salmon success. Water managers missed these opportunities by capturing all the water in reservoirs. What happened to prescribed spring flow pulses for salmon in state and federal plans? Is holding the promised water back the “best science”?1 No.

Shasta, Oroville, and Folsom reservoirs, the largest in Sacramento Valley, have released no flow pulses since January 1 to the Sacramento, Feather, and American rivers (Figure1). These are the rivers with the state’s biggest runs of fall-run and spring-run salmon. These three reservoirs now hold 6.5 million acre-feet (MAF) of stored water, over 95% of average for this date. Also available is 2 MAF of water now stored in Trinity Reservoir, which is at 109% of average for this date. Local rainfall and un-dammed tributaries have provided three significant flow pulses in lower rivers and the Delta, but these pulses have not touched the spawning and rearing grounds just downstream of the major dams.

Figure 1. Streamflow (cfs) in the upper section of the lower Sacramento River below Shasta (KWK – Keswick), the middle section of the lower Sacramento River (WLK – Wilkins), the lower section of the lower Sacramento River (FPT – Freeport), the lower Feather River (GRL – Gridley), and the lower American River (AFO – Fair Oaks).

The lower San Joaquin River watershed had a similar record this year, with minimal contribution to Delta inflow and outflow or to flow pulses from reservoirs (Figure 2).  The watershed’s largest reservoir, New Melones on the Stanislaus River, has 121% of average for the date with 1.9 MAF of water in storage.  New Melones did provide a small release in early February and appears to be ramping up releases in early April.  But the State Water Board has already written a letter to the Bureau of Reclamation calling Reclamation out for failing to maintain required flows in the lower San Joaquin River.

Figure 2. Streamflow (cfs) in the lower San Joaquin River (VER – Vernalis), the lower Stanislaus River (RIP – Ripon), and Delta outflow (DTO).

The river flow and Delta outflow pulses in early April have spurred the annual exodus of juvenile spring and fall run salmon from the Delta (Figure 3).  It is important to get as much of the river juvenile salmon production into the Delta as soon as possible to take advantage of this critically-timed pulse in Delta outflow.  The tailwaters of the large dams holds tens of millions of wild fry and juvenile salmon (30-50 mm in length) that need to get downstream to grow and to emigrate into and through the Delta.  These young salmon need reservoir releases to encourage their emigration and improve their growth and survival.  Most importantly, pulses will piggy-back on the present April pulse in Delta outflow.  This need is most pronounced in the San Joaquin salmon watersheds, where flows have been low and few salmon have reached the Delta (Figure 4).  The need to support the fall-run and spring-run salmon emigration extends at least through April and into May, including over 20 million hatchery salmon smolts released from tributary hatcheries (Figures 5 and 6).

Figure 3. Catch of juvenile salmon at exit from the Delta at Chipps Island In 2020.

Figure 4. Catch of juvenile salmon in the lower San Joaquin River at Mossdale at entrance to the Delta In 2020. Note catch was only one so far in 2020.

Figure 5. Timing of the fall-run salmon exiting the Delta in brood years 2005-2018 (2006-2019).

Figure 6. Timing of the spring-run salmon exiting the Delta 2006-2019.