Study finds heat waves speed snowmelt substantially, but rivers respond unevenly

Study finds heat waves speed snowmelt substantially, but rivers respond unevenly Lisa Lock Scientific Editor Andrew Zinin Chief Editor A new study by researchers at Montana State University has found that heat waves substantially accelerate the melting rate of mountain snowpack; however, the resulting water doesn't necessarily produce an immediate increase in streamflow. The findings were published Oct. 1 in the paper "Heatwaves drive rapid snowmelt and disrupt streamflow in the western United States," in the journal Environmental Research Letters. The researchers analyzed data collected from 2001 through 2024 at 823 Snow Telemetry, often called SNOTEL, stations across the western United States. Operated by the U.S. Department of Agriculture's Natural Resources Conservation Service, the network measures snowpack, precipitation, temperature and other environmental conditions. "It's a very valuable data set, especially because it contains decades of observational records, providing statistically meaningful information," said Siwei He, assistant professor of civil engineering in MSU's Norm Asbjornson College of Engineering and co-author of the paper. The researchers examined 24 years of snowpack and temperature observations, along with streamflow records from the U.S. Geological Survey. Previous studies focused primarily on individual heat waves or smaller regions. This analysis used data collected from actual observations to investigate how heat waves affect snowmelt rate and streamflow across the western United States. "Our data is not from a model. Our data comes from real-world observations," said Haonan Guo, a third-year civil engineering doctoral student at MSU and the lead author of the paper. For the purposes of this study, the researchers defined a heat wave as a period of three or more consecutive days with temperatures exceeding the 30-year historical average by more than twice the typical amount of temperature variation. Faster snowmelt, but not always more streamflow Of the 823 SNOTEL stations studied, at least 250 experienced a heat wave during the 24-year study period. At 98% of those affected stations, snow melted faster than the historical norm during heat waves. Across the observations, the likelihood of faster-than-normal snowmelt increased by roughly 80% during heat waves compared with normal conditions. Of 168 headwater watersheds affected by heat waves, only 93—or around 55%—showed increased streamflow following heat waves. Faster melting, in other words, did not reliably produce an immediate surge in river flow. "We thought that was very interesting," He said. "If you melt more snow, it's natural to expect more streamflow." Guo calls that mismatch an "asymmetric response." He pointed out that the fractions of watershed area covered by snow and affected by heat waves play important roles, but he also hypothesized that some water may soak into the soil and release gradually, while some snowpack water may be lost to the atmosphere through evaporation. But those are possible pathways, not conclusions established by this study. The researchers have not yet determined the fate of water that did not appear as a measurable streamflow increase. Disrupted timing and lingering melt Mountain snowpack serves as a natural reservoir for water as snow accumulates during winter and gradually melts in spring and summer, supplying water for communities, agriculture and ecosystems. Heat waves can disrupt that seasonal timing, potentially leaving less snow available during the warmer months, Guo said. "If the snow melts faster than the normal rates, it means we might not have enough snow in the spring and summer to release," Guo said, explaining that earlier melting could complicate irrigation and other water needs. The researchers also found that the increased snowmelt persisted after the unusually warm weather ended. Snowmelt rates remained above normal for three to five days, with especially pronounced lingering effects in winter and spring. Guo likened this to a person recovering from a fever: Even after the fever subsides, its effects might not subside for several days. Similarly, snowpack does not return immediately to its previous melting behavior when the temperatures cool. Tracing where the meltwater goes Guo said the next challenge is to trace snowpack losses through different pathways using additional observations and models. That work could help explain why snowpack and river responses diverge and ultimately improve predictions about when mountain water becomes available downstream. "Mountain snowpack is one of the West's most important natural resources, and this research demonstrates why understanding how it responds to a changing climate is so important," said Alison Harmon, vice president for research and development. "This is exactly the kind of work we want to see at MSU: Rigorous science that advances our understanding of complex systems while addressing questions with real consequences for agriculture, communities and ecosystems across Montana and the West." Publication details Haonan Guo et al., Heatwaves drive rapid snowmelt and disrupt streamflow in the western United States, Environmental Research Letters (2026). DOI: 10.1088/1748-9326/aeac3a Journal information: Environmental Research Letters Provided by Montana State University