Mountain streams, the often-overlooked sources of many Western U.S. rivers, are facing a significant threat. As climate conditions continue to evolve, these vital water sources may experience substantial reductions in groundwater-fed streamflow, or base flow, by the century’s end.
A recent study published in Earth’s Future led by Northern Arizona University (NAU) researchers reveals a concerning trend. Since 1950, headwater watersheds across the western United States have seen a steady decline in base flow. Projections using 75 years of data from 115 headwater basins across 11 states suggest a potential decrease of 45% to 65% in these flows if current climate trends persist.
Caelum Mroczek, the study’s lead author and a research affiliate in NAU’s School of Earth and Sustainability, noted, “Headwaters are where much of our downstream water supply originates, but they’re often some of the least monitored parts of a watershed.” According to Mroczek, these systems are already experiencing widespread declines in base flow due to warming temperatures, reduced snowpack, and drier conditions.
Headwater streams are crucial, comprising nearly 88% of the western U.S. river network and providing a significant portion of the region’s surface water. As these streams heavily depend on groundwater, any alteration in groundwater recharge can severely impact downstream water availability.
The research team categorized watersheds into four distinct groups, ranging from high-elevation, snow-fed basins to lower-elevation, rain-dependent systems typical in southern Arizona’s arid regions. Despite regional variations, a common pattern emerged: peak base flow is occurring earlier in the year than in the past.
Mroczek explained that historically, melting snow would recharge groundwater gradually, sustaining streams through late spring and summer. With rising temperatures, snow is melting sooner and more precipitation is falling as rain, causing water to arrive earlier and reducing groundwater contributions during late summer when it’s most needed.
“We’re seeing increases in base flow during late winter and early spring, but then substantial decreases in June and July,” Mroczek added. This shift means water is becoming increasingly out of sync with the needs of people and ecosystems.
Key factors influencing future base flow include antecedent moisture—the moisture level of the landscape in preceding months—along with snowpack and temperature. These elements are particularly critical in mountain watersheds where snow acts as a natural water reservoir.
The study’s findings are particularly concerning under high-emissions scenarios, where reliance on fossil fuels persists. In such cases, the most vulnerable headwater systems could suffer the greatest relative losses, potentially losing millions of acre-feet of water.
The implications extend beyond mountain streams. Base flow supports water supplies, fish and wildlife habitats, and streamflow during dry periods. A decline in groundwater contributions could heighten the risk of drought and wildfires throughout the region.
For Mroczek, the study emphasizes the importance of long-term water trends over annual weather variations. “It’s easy to remember a wet year or a dry year,” he said. “But these systems operate over decades. Water may still come out of the tap today, and reservoirs may still be functioning, but what we’re seeing is a gradual reduction in the natural storage systems that support water availability over time.”
Despite their significance, much remains unknown about the West’s smallest streams due to their remote and unmonitored nature, leaving researchers with limited data to assess climate change impacts.
“These are some of the most important parts of the watershed, but we don’t have much data on them,” Mroczek stressed. “Improving monitoring in headwaters will be critical for understanding and managing future water resources.”
The research, supported by the Arizona Board of Regents Technology and Research Initiative Fund program, was co-authored by SES professor Abraham Springer and assistant professor of data science Benjamin Lucas.
Jill Kimball | NAU Communications
(928) 523-2282 | jill.kimball@nau.edu
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