Following an introductory post, this is the sixth post in a series on the lessons learned by the National Marine Fisheries Service (NMFS) from the 2013-2015 drought that devastated Sacramento River salmon populations. This post addresses Lesson #4.
NMFS’s lesson #4 states that the summer water temperatures in the 10-mile reach of the Sacramento River downstream of Keswick dam, the most heavily used reach for spawning by winter-run salmon, are not “correlated with flow.” The lesson is important in that if generally true, high summer flow releases are not important in managing summer water temperatures for the salmon spawning and egg-embryo incubation that takes place close to Keswick Dam. The magnitude of flow releases from the dam appears to have minimal effect on how much summer water temperatures increase in the upper 20 miles of river. Rather, water temperature in this river reach is more a function of distance from the dam and the temperature of the water when released from Keswick.
Keswick is a small re-regulating reservoir that takes in water as it is released from Shasta Dam’s peaking power plant and from the Spring Creek Powerhouse, which generates power with water imported to the Sacramento from the Trinity River. The water from the powerhouses is mixed in Keswick Reservoir as it enters the reservoir at times of day when power is most valuable. “Re-regulation” means that Reclamation holds releases from Keswick to the Sacramento River downstream relatively constant over the course of the day. The water temperature may be cold enough for salmon in the 20 miles of river immediately downstream of Keswick Dam, but water in the river warms quickly, depending on flow, as it moves further downstream through the remaining 200 miles of the lower Sacramento River.
1) Effect of Flow on the Water Temperature of Shasta/Keswick Releases
Water temperature immediately below Keswick in drier years 2014 and 2020 was most certainly related to flow (Figure 1), but not in the sense one might expect. In 2014, high flow magnitudes (releases) caused a loss of access to the cold-water pool in Shasta Reservoir. In 2020, high early summer releases led to reduced late summer releases to conserve the cold-water pool. In both cases, spring-summer water deliveries to downstream water contractors were excessive, leading to limited access to Shasta’s cold-water pool by fall, resulting in high salmon egg-embryo mortality.
2) Effect of Flow Magnitudes into Keswick Reservoir on Temperature Increases within the Reservoir
There is some evidence that the temperature of water increases as it moves through and mixes within Keswick reservoir. Keswick releases are often slightly warmer than upstream Shasta Dam releases into Keswick Reservoir (Figure 2), although the relationship is complicated by releases from Spring Creek Powerhouse of warmer water imported from the Trinity River system. Again, as described above in point 1, the water temperature in Keswick Reservoir and water released from it in the late summer and fall of 2014 was primarily the result of gradual decline in the availability of cold water from the bottom of Shasta Reservoir.
3) Effect of Flow Magnitudes on Water Temperature in Sacramento River immediately below Keswick Dam
Higher flow magnitudes immediately downstream of Keswick Dam do not always translate into lower water temperatures, because the temperature of the source water in Shasta is more important (Figure 3). Temperature increases a short distance from Keswick Dam are small (Figure 4). Even in summer of wet-year 2019, water temperature increases over the 10-mile spawning reach were similar to those in 2020 (Figur e 5). In 2019, flow magnitude had relatively little influence on water temperature even as far downstream as Balls Ferry.
It is worth noting that when dam release temperatures are at or above the upper limit of safe survival (53oF) as they were in summer 2020, then any temperature increase in the 10-mile spawning reach becomes a critical issue for the survival of the eggs and embryos of winter-run salmon. However, the solution is not to increase flow to reduce warming within the 10-mile spawning reach, because that depletes the Shasta cold-water pool and trades a short-term benefit for the long-term impact of reducing the size and accessibility of the Shasta cold-water pool. Rather, the solution is to keep releases low enough over the course of the spring and summer to allow those releases to maintain temperatures lower than the 53ºF threshold.
4) Effect of Flow Magnitudes on Water Temperature in Lower Reaches of the Sacramento River
Summer water temperature in the lower reaches of the Sacramento River is heavily influenced by magnitude of flow (Figures 6 and 7). Lower flows promote higher water temperatures. Downstream of Red Bluff, water temperature depends on air temperature, the magnitude of flow, and how fast the river flows. Lower flow magnitudes reduce the speed with which water moves downstream. Smaller thermal mass of water at low flows, combined with the slower rate with which water moves downstream, causes water to pick up more thermal energy and get warmer quicker. In the lower reaches of the Sacramento River, water no longer depends on the temperature of the water released from Shasta and Keswick reservoirs.
In summary, water temperature in late spring through early fall in the Sacramento River immediately downstream of Keswick Dam is primarily determined by the water temperature of the water released from the dam, not the magnitude of flow. In contrast, flow is the primary management tool available to reduce water temperature in the lower 200 miles of the Sacramento River from late spring through early fall.