By Nicholas Corline, Emilio Grande, Ate Visser, Jean Moran, Jory Lerback, Tyanna Blaschak, Damon Goodman, Jake Harm, Lauren Tolley-Mann, Dylan O’Ryan, Valerie Muenker, Rollie Nearhood, Amber Lukk, Sarah Howe, and Robert Lusardi

Imagine a giant sponge made of volcanic rock. That’s what scientists have recently discovered in the central Cascades of Oregon, an aquifer that holds fourteen times the volume of water of Shasta Reservoir (Karlstrom et. al. 2025). While the Cascades’ geologically porous volcanic aquifers absorb and store large amounts of snowmelt and rainwater, these aquifers also leak, forming springs from fractures in the volcanic rock. Because volcanic aquifers are relatively shallow the water is not warmed geothermally, and the groundwater remains cold. That makes streams fed from these springs ideal for coldwater fish species, including rainbow trout, steelhead, Coho, and Chinook salmon.  

In a recent study, researchers from UC Davis showed that rainbow trout in a volcanic spring-fed river are 1.6 times longer and 4.5 times heavier in their first year of life than rainbow trout rearing in an adjacent stream fed by precipitation (Lusardi et. al. 2023). Cold spring-fed streams offer fish creature comforts from stable temperatures, stable flows (seasonally and through droughts), and abundant food. Spring-fed streams stay relatively cold in the summer and warm during winter compared to precipitation-driven streams. This is akin to leaving your household thermostat at a comfortable year-round setting. Such habitats provide optimal growing conditions for ectotherms like fish. In addition to stable temperatures, spring-fed streams provide abundant food for trout and salmon throughout the year (Lusardi et al. 2016, 2020). Cold volcanic spring-fed streams may be the Goldilocks habitat for cold-water species where temperature and food conditions are reliably just right. 

Will these habitats persist in a warmer climate? 

How spring-fed systems respond to climate change is critical for both conservation and water management. As climate change increases temperatures and disrupts precipitation patterns, spring-fed streams may act as refuge habitats for numerous native fishes throughout the Pacific Northwest (Lusardi et al. 2016; 2020). For example, a hot, dry year may cause water temperatures in a precipitation-driven stream to increase beyond the thermal limits of native species, such as rainbow trout, or the stream may dry up altogether. However, volcanic spring-fed streams have the potential to maintain steady flows and cool temperatures due to the large cold aquifers that feed them. Although this seems like the probable scenario, it remains unknown how well these aquifers and spring-fed systems can buffer conditions such as prolonged drought or whether all springs will respond uniformly. 

These unknowns have prompted scientists to ask: Where does the water come from? How long does it take water to move through the aquifer? How long can a spring-fed river weather prolonged drought conditions or warming temperatures associated with climate change? 

To understand if we can rely on spring-fed streams to maintain robust native fish populations in the future, researchers at UC Davis, CSU East Bay, Lawrence Livermore National Lab, and California Trout are studying the response of springs and aquifers to climate events and how this will affect rainbow trout growth in Northern California. These researchers are using naturally occurring isotope tracers to characterize the transit time of water, i.e., how long it takes a water molecule to travel along a flow path, from high on Mt. Shasta or Medicine Lake Highlands to spring-fed rivers like the McCloud River and Fall River. The steep topography of the region provides natural variation in stable isotope tracers, such as those found in water and noble gases, which allows researchers to determine the elevation of the water source. Similarly, the group uses radioisotopes such as hydrogen-3 and sulfur-35 to determine the age of spring outflows. Radioisotopes decay at a constant rate, providing a natural ‘clock’ for measuring the time it takes for water to move through volcanic aquifers and emerge from springs. Using these radioisotope tracers, researchers found that water from springs at Mount Shasta City Park is significantly older than that of the McCloud River, having taken a deeper flow path originating high on Mount Shasta. In streams like the Upper Sacramento and Burney Creek, with a mixed hydrology of springs and runoff, radon is used to determine spring-fed groundwater’s contribution to total discharge (e.g. Frei et al. 2021). For instance, tracer data indicates that most groundwater influx at Burney Creek (home of the famous Burney Falls) occurs over a 600 ft-long reach above Burney Falls; a reach over which flows go from near zero to 100 cubic feet per second from volcanic spring flows. 

Using a suite of traces researchers can gain insight into where spring water comes from, how long it takes for it to travel to river headsprings, and the contribution of this water to overall stream discharge. Ultimately, these data will be used to quantify the response of spring-fed streams to climate change. 

How might climate affect fish growth and survival in these spring-fed rivers? 

To determine how changes in spring stream hydrology will affect native cold-water fish species, researchers at UC Davis are using the ear bones (otoliths) of rainbow trout to link growth to stream habitat conditions. Otoliths function like biological data loggers that record critical life history information as they accumulate mineral material throughout the full life cycle of fish. The accumulation of minerals is analogous to growth rings in a tree, where the width of the ring is indicative of fish growth (see Lusardi et al. 2023). For young rainbow trout, scientists can determine growth at daily increments. Such high-resolution information can show how physical conditions control growth. Preliminary otolith data suggests that spring-fed streams are both ideal for growth and fish also spawn there earlier than in precipitation driven streams, allowing young fish to grow larger in their first year of life. 

Furthermore, analysis of elemental concentrations within otoliths can track individual fish movements and how this aligns with conditions within spring-fed and/or precipitation-derived streams. With otolith data, researchers can relate variation in growth and growing conditions within streams to future scenarios of discharge and temperature determined through isotope hydrology. This information is vital to organizations like California Trout, which help protect these critical habitats for fish and people.

California is entering an era of hydroclimate whiplash with three consecutive years of drought followed by three unusually wet years. As climate change accelerates, Californians will face more extreme swings (Swain et al. 2025). In this shifting landscape, springs are natural stabilizers, helping buffer effects of climate change and offering resilience and a lifeline for communities and ecosystems. Understanding how these systems respond requires collaboration across disciplines – and this research bridges isotope geochemistry, hydrology, and ecology, underscoring the importance of cross-disciplinary approaches in addressing complex environmental challenges. Partnerships between scientific institutions and NGOs such as California Trout also play a crucial role in translating scientific insights into on-the-ground conservation and shaping future policy. This study will provide a scientific foundation to inform the management of ecosystems and communities that depend on springs and helps ensure their long-term protection. Stay tuned for future updates as this collaborative group refines the connections between climate, discharge, and ecological outcomes in these unique ecosystems. 

About the Authors

Nick Corline is a postdoctoral fellow at UC Davis. Emilio Grande is an Assistant Professor in the Department of Earth and Environmental Science at CSU East Bay. Ate Visser is a staff scientist at the Lawrence Livermore National Laboratory. Jean Moran is a Professor in the Department of Earth and Environmental Science at CSU East Bay. Jory Lerback is a postdoctoral fellow at the Lawrence Livermore National Laboratory. Tyanna Blaschak is a project manager for the Mount Shasta-Klamath region at California Trout. Damon Goodman is the Mount Shasta-Klamath Regional Director at California Trout. Jake Harm is a scientist at the Lawrence Livermore National Laboratory. Lauren Tolley-Mann is a graduate student at CSU East Bay. Dylan O’Ryan is a graduate student at CSU East Bay. Valeria Muenker is a graduate student at CSU East Bay. Rollie Nearhood is an undergraduate student at CSU East Bay. Amber Lukk is a freshwater processes assistant specialist at the Center for Watershed Sciences at UC Davis. Sarah Howe is the Lab Manager for the Lusardi Lab in the Department of Wildlife, Fish, and Conservation Biology at UC Davis. Robert A. Lusardi is an Assistant Professor in the Department of Wildlife, Fish, and Conservation Biology and Associated Director of the Center for Watershed Sciences at UC Davis.

Further Reading

Frei, Sven, and Benjamin Silas Gilfedder. Quantifying residence times of bank filtrate: A novel framework using radon as a natural tracer. Water Research 201 (2021): 117376.

L. Karlstrom, N. Klema, G.E. Grant, C. Finn, P.L. Sullivan, S. Cooley, A. Simpson, B. Fasth, K. Cashman, K. Ferrier, L. Ball, & D. McKay. (2025). State shifts in the deep Critical Zone drive landscape evolution in volcanic terrains, Proc. Natl. Acad. Sci. U.S.A. 122 (3) e2415155122, https://doi.org/10.1073/pnas.2415155122.

Lusardi, Robert A., Randy Dahlgren, Erwin Van Nieuwenhuyse, George Whitman, Carson Jeffres, and Rachel Johnson. (2023). Does Fine-Scale Habitat Diversity Promote Meaningful Phenotypic Diversity Within a Watershed Network? Ecology 104(8): e4107. https://doi.org/10.1002/ecy.4107

Lusardi, Robert A., Hammock, Bruce G.,  Jeffres, Carson A., Dahlgren, Randy A., and Kiernan, Joseph 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. https://doi.org/10.1139/cjfas-2018-0484

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

Peters, E., et al., Tracers Reveal Recharge Elevations, Groundwater Flow Paths and Travel Times on Mount Shasta, California. Water, 2018. 10(2)

Swain, D.L., Prein, A.F., Abatzoglou, J.T. et al. Hydroclimate volatility on a warming Earth. Nat Rev Earth Environ 6, 35–50 (2025). https://doi.org/10.1038/s43017-024-00624-z

Visser, A., et al., Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone. Water Resources Research, 2019. 55(2): p. 1429-1450.