California WaterBlog

By Francisco J. Bellido-Leiva, Nicholas Corline, and Robert A. Lusardi

About 1,500 dams obstruct, modify, and regulate flow in all but one of California’s major rivers. These dams provide Californians with reliable drinking and irrigation water, flood protection for low-lying communities, and hydropower for our electrical grid. But dams also threaten downstream ecosystems by severely disrupting natural processes with potentially dire consequences for native species. Dams affect ecosystems (both upstream and downstream) in many ways, including eliminating habitat inundated by reservoirs, reducing connectivity for migratory and resident fishes, and altering natural flow, sediment, temperature, and nutrient regimes. While several “deadbeat” dams are now being removed, many will continue to play a critically important role in California’s water infrastructure.

In response, alternative management strategies for dam operations are being developed, to reinstate key ecosystem processes. One approach is the California Environmental Flows Framework (CEFF), which focuses on mimicking key parts of the annual hydrograph, and their historical ranges, to regain important ecosystem processes and create biophysical conditions that benefit native species. Previous posts (here, here, and here) and a case study implementation are included in further readings (Yarnell et al. 2024). 

Like flow management, downstream water temperatures can also be controlled to mimic historical thermal regimes or create conditions favorable for native species in novel environments (Olden and Naiman, 2010). The ability of dam managers to control downstream temperatures relies on reservoir thermal stratification (i.e., warm water near the surface and cold water at deeper depths) and infrastructure, such as selective withdrawal devices (SWD). SWDs allow dam operators to release and mix water of different temperatures from multiple depths. California has several of these structures. The largest SWD is at Shasta Dam, which helps provide suitable water temperatures for endangered winter-run Chinook salmon spawning and egg incubation, downstream of Keswick Reservoir. 

Do SWDs subsidize downstream food-webs?

While taking monthly samples of aquatic invertebrates downstream of Keswick Dam (Redding, CA) in the Sacramento River, we found high abundances of pelagic zooplankton. Typically, we would not expect zooplankton production in this part of the Sacramento River due to high turbulence and water velocity. We speculated this prevalence of zooplankton must originate upstream from lentic habitats (i.e. Shasta Reservoir).  As such, we started to ask a series of questions: What is exported from Shasta Reservoir? Do export concentrations vary through time? And, does the SWD at Shasta Reservoir control downstream exports to the Sacramento River? 

To answer these questions, we developed a pilot study that measured food-web resources downstream of Keswick Dam including zooplankton, chlorophyll-a (a proxy for phytoplankton), and nutrients, such as nitrogen and phosphorus. This monthly sampling (Fig. 2) represented a good cross-section of gate operations and conditions in Shasta Reservoir from fully-mixed to a strongly stratified reservoir. 

Critical nutrients associated with ecosystem productivity were released from Shasta Reservoir throughout the year. Exported nutrient concentrations were comparable to highly productive spring-fed systems in the area that receive nutrient-rich groundwater (Lusardi et al. 2016, 2018, 2020, 2021, 2023). Such high nutrient subsidies may enhance primary and secondary production downstream of Keswick Dam. Increases in nutrient concentration exports correlated strongly with the operation of the SWD’s lower gates at Shasta Dam. Nutrients accumulate deep in reservoirs during stratified conditions in summer and are exported to the Sacramento River during operation of deeper gates in fall. 

Food-web resources were continuously exported downstream, including phytoplankton and zooplankton, with distinct peaks in April and January corresponding to reservoir mixing and vertical transport of nutrients to the photic zone. During this study, we estimated that 95 metric tons of zooplankton carbon was exported from the reservoir during the 12-month study period, mostly during April and January. Strikingly, most of this subsidy was utilized within five river kilometers downstream of Keswick Dam. 

Food-web subsidies from large reservoirs

Shasta Reservoir resembles a multilayered cake, where different layers of food web resources (i.e., nutrients and plankton) could be preferentially exported using gate operation and timing to benefit the Sacramento River. The following diagram shows the dynamics observed from our sampling below the Shasta/Keswick complex, in which using the shallower gates during high productivity periods facilitated food-web exports (e.g., zooplankton), while using lower gates during stratified conditions enabled nutrient export. 

Could the SWD help manage downstream food webs? 

Although the primary objectives of the SWD are to maintain water exports, hydroelectric power generation, and cold-water releases for spawning winter-run Chinook salmon, Shasta Dam operations and internal reservoir conditions also control nutrient and food resource exports to the Sacramento River. Given that most habitats downstream of dams are highly altered, export subsidies could be another tool to manage these habitats for native species, particularly under a changing climate. Recent research points to a coupling of temperature and food availability in providing habitat heterogeneity and ecosystem productivity in stream environments (Lusardi et al. 2020; Armstrong et al. 2022). Is it possible to better operate SWDs to manage reservoir productivity to support downstream ecosystems? Our future research aims to quantify the importance of these subsidies to the downstream food web dynamics and understand how operation constraints might be balanced to enhance productivity in the Sacramento River.

Dr. Francisco J. Bellido-Leiva is a Postdoctoral Scholar with the Center for Watershed Sciences at UC Davis.

Nicholas J. Corline is a Ph.D. candidate in the Department of Forest Resources and Environmental Conservation at Virginia Tech.

Dr. Robert A. Lusardi is an Assistant Professor in the Department of Wildlife, Fish, and Conservation Biology.

Further Reading

Armstrong, J. B., Fullerton, A. H., Jordan, C. E., Ebersole, J. L., Bellmore, J. R., Arismendi, I., Penaluna, B.E. and Reeves, G. H. (2021). The importance of warm habitat to the growth regime of cold-water fishes. Nature Climate Change, 11(4), 354-361.

The California Environmental Flows Framework –https://ceff.ucdavis.edu/

Corline, N. J., Bellido-Leiva, F., Alarcon, A., Dahlgren, R., Van Nieuwenhuyse, E. E., Beakes, M., and Lusardi, R. A. (2023). Reservoir-derived subsidies provide a potential management opportunity for novel river ecosystems. Journal of Environmental Management, 345, 118852.

Lusardi, R. A., Bogan, M. T., Moyle, P. B., and Dahlgren, R. A. (2016). Environment shapes invertebrate assemblage structure differences between volcanic spring-fed and runoff rivers in northern California. Freshwater Science, 35(3), 1010-1022.

Lusardi, R. A., Dahlgren, R., Van Nieuwenhuyse, E., Whitman, G., Jeffres, C., and Johnson, R. (2023). Does fine‐scale habitat diversity promote meaningful phenotypic diversity within a watershed network?. Ecology, e4107.

Lusardi, R. A., Hammock, B. G., Jeffres, C. A., Dahlgren, R. A., and Kiernan, J. D. (2020). Oversummer growth and survival of juvenile coho salmon (Oncorhynchus kisutch) across a natural gradient of stream water temperature and prey availability: an in situ enclosure experiment. Canadian Journal of Fisheries and Aquatic Sciences, 77(2), 413-424.

Lusardi, R. A., Jeffres, C. A., and Moyle, P. B. (2018). Stream macrophytes increase invertebrate production and fish habitat utilization in a California stream. River Research and Applications, 34(8), 1003-1012.

Lusardi, R. A., Nichols, A. L., Willis, A. D., Jeffres, C. A., Kiers, A. H., Van Nieuwenhuyse, E. E., and Dahlgren, R. A. (2021). Not all rivers are created equal: The importance of spring-fed rivers under a changing climate. Water, 13(12), 1652.

Olden, J. D., and Naiman, R. J. (2010). Incorporating thermal regimes into environmental flows assessments: modifying dam operations to restore freshwater ecosystem integrity. Freshwater Biology, 55(1), 86-107.

Yarnell, S., Murdoch, L., Bellido-Leiva, F., Peek, R., and Lund J. (2024). Flow management through a resilience lens: Allocation of an environmental water budget using the Functional Flows Adaptive Implementation Model. In M. Thoms & I. Fuller (Eds.), Resilience and Riverine Landscapes (pp. 470-488). Elsevier.