Upper Sacramento Basin Source Waters Science Study

Northern California's spring-fed rivers, the McCloud, Upper Sacramento, Rising, and Fall rivers, along with Hat Creek and Burney Creek, all draw their exceptional cold, clean water from ancient volcanic aquifers deep within the Cascade Range. These springs have long supported some of the state's most productive wild trout fisheries and provide critical water security for people downstream, yet for decades almost nothing was known about how vulnerable they actually are to drought and a warming climate. Over three years, CalTrout partnered with UC Davis, Lawrence Livermore National Laboratory, Cal State East Bay, and more partners to change that, using isotopic water sampling and hydrologic data analysis to trace exactly where these springs' water comes from and how long it takes to get there.

The completed study found that water transit times across the basin's springs range from under a year to more than 75 years between when snowmelt first infiltrates the ground and when it re-emerges downstream, revealing that some springs are fed by ancient, deeply buried groundwater while others depend on much younger, far more climate-vulnerable sources. That distinction turned out to matter enormously for fish. Springs fed by older, more resilient groundwater, like those at Thousand Springs and Mount Shasta City Park, proved far more stable through drought than younger-water springs such as the one feeding CalTrout's own Trout Camp in Dunsmuir. The temperature and productivity differences between these systems are dramatic enough to reshape how rainbow trout grow and reproduce: fish in warmer, more productive volcanic springs grow up to 3.5 times larger in their first year than those in colder streams, and spawning timing varies by as much as 74 days from one stream to the next.

The study's findings now give water managers and conservationists a clear, science-based toolset for identifying which source waters need the most urgent protection as climate conditions continue to shift, providing a foundation for policy and management decisions across the Upper Sacramento Basin for years to come.