The California Department of Water Resources (DWR) and the U.S. Bureau of Reclamation (Reclamation) released a Central Valley Drought Contingency Plan Update on August 31, 2021. Part of the Addendum’s section on Sacramento River Temperature Management describes bypassing Shasta Dam’s hydropower generating facilities in order to cool Shasta Reservoir releases to the lower Sacramento River:
Reclamation performed a cold water power bypass test on August 29 to determine the feasibility of using the bypass to cool Sacramento River temperatures in the late summer and early fall. The results of this test and any potential future partial bypass will be discussed with the fishery agencies through the Sacramento River Temperature Task Group to determine the potential benefits and impacts of taking the action and ultimately whether the group recommends a partial power bypass for consideration.
As summarized in the quote above, Reclamation conducted a test on August 29. The test involved releasing cold water from near the bottom of the dam rather than from the power penstock through the Temperature Control Device (TCD) tower on the inside face of the dam. Water temperatures of releases from the TCD steadily increased through late August as the water level in the reservoir continued to fall (Figure 1). Side gate intakes on the TCD were taking in increasing amounts of warmer water from near the surface each day (Figure 2).
The August 29 morning test involved releasing cold water from the bottom of Shasta Reservoir (with minimum TCD release). As expected, it dramatically reduced water temperatures (about 4º F), at the head of Keswick Reservoir immediately downstream of Shasta Dam, while the test was going on (Figure 3). The test was followed by the normal afternoon peaking-power, warmer releases from the TCD, penstocks, and turbines/powerhouse (Figure 4). It was difficult to determine the proportion of releases from the two sources (the blend) in the morning, but it was obvious that water released during the morning of August 29 was cooler than water released at other times during that day or other days that week. The effect of the August 29 test on the Sacramento River below Keswick Dam was barely noticeable (Figure 5).
Releasing cold water from Shasta Dam’s bottom outlet reduces river water temperature below Shasta Dam compared to release through the TCD. However, the Addendum leaves options for a “potential future partial bypass” of Shasta Dam’s power facilities a complete black box.
The prospective objective of a partial power bypass would be to cool the Sacramento River downstream of Keswick Dam, where the salmon are. So many more factors affect release temperatures from Keswick. The release from Shasta of 1000 acre-feet of water over a six-hour period, in a day when the total Shasta release was close to 14,000 acre-feet, offers little insight into a two or three-month strategy for managing the already-diminished cold-water pool that remains in Shasta Reservoir following a spring and summer of excessive releases by Reclamation.
Setting aside the element of time for a moment, Keswick release temperatures depend on the ratio of the colder water released from Shasta Dam to the warmer power releases from both Shasta Dam and Whiskeytown Dam (through the Spring Creek Powerhouse) to Keswick Reservoir. An immediate measure Reclamation can take to reduce the temperature of the Keswick release is to cease the 1000+ cfs it is releasing to Keswick from Whiskeytown. In the short term, this could bring the Keswick release close or closer to the 55º target maximum for the 10-mile reach downstream of Keswick.
Increasing the proportion of releases from Shasta’s bottom outlet would reduce water temperatures in the short-term. However, how long this benefit would last, and whether there would be a net improvement or the opposite over one month or two months, involves calculus of how much cold water the change in operations would drain from the cold-water pool, and how quickly. In this regard, the description in the August Addendum says nothing at all about “the action” or potential actions that “will be discussed” by the Sacramento River Temperature Task Group.
Reclamation has put itself in a position where deciding which fish to save is inseparably deciding which fish to sacrifice. Those may be the same fish: the fish saved now by decreasing water temperatures to protect eggs in the redds may die later when Reclamation runs out of accessible cold water to keep the alevin from those eggs alive in a month. About this, the breezy report of a test says nothing at all.
Then there is the unknown but potentially even more severe consequence of depleting Shasta storage this year in the face of looming disaster if water year 2022 is dry.
This post is part 2 in a series on DWR and Reclamation’s August Addendum to the 2021 Drought Plan.
Figure 1. Water temperature of Shasta powerhouse releases in August 2021.
Figure 2. Shasta Reservoir water temperature profile and TCD intake operation configuration at end of August 2021.
Figure 3. Water temperature of Shasta Dam releases 8/27-9/4 2021. Note August 29 morning power bypass test.
Figure 4. Flow from Shasta Dam August 27 – September 4, 2021.
Figure 5. Comparison of daily average water temperatures of Shasta and Keswick Dam releases in August 2021. Most of the difference is caused by warm water releases from Whiskeytown Reservoir (Trinity water) via Spring Creek power to Keswick Reservoir. Note the apparent small effect of August 29 power-bypass test on both river and TCD water temperature: water temperature leveled off rather continuing upward trend over previous 12 days.