Thinning forests for fire safety also boosts snowpack by up to 30 percent, Cascades study finds

For years, water managers and forest managers in the western United States have been working toward different goals, sometimes at cross-purposes. Hydrologists want more snowpack - the mountains' frozen water reservoir that feeds rivers through spring and summer. Fire ecologists want thinner forests - less fuel to carry the increasingly severe blazes that have burned through millions of acres in recent decades.

A field experiment in Washington State's Yakima River Basin suggests these goals may be more compatible than the competing interest groups have assumed.

University of Washington researchers, working with The Nature Conservancy, experimentally thinned approximately 150 acres of forest on Cle Elum Ridge and measured what happened to the snowpack before and after. The results, published in Frontiers in Forest and Global Change in March 2026, found snowpack increased by 30 percent on north-facing slopes and 16 percent on south-facing slopes in the thinned areas. The study has particular relevance for a region currently in its third consecutive year of water restrictions.

How trees steal snow

The mechanism is counterintuitive if you think of trees as helpful. Dense forest canopies intercept snow before it reaches the ground, where it is then lost to evaporation or sublimation - converting directly from solid to vapor without ever melting into the soil. In a dense stand, a substantial fraction of snowfall never makes it to the snowpack at all. It hangs in the canopy and disappears.

Thinning reduces this interception. When canopy gaps are large enough - the study identified an optimal range of 4 to 16 meters in diameter - snow falls through to the ground, accumulates, and stays cold enough to persist into the spring melt season. On north-facing slopes, which receive less solar radiation and therefore retain snow longer, the researchers calculated an additional 12.3 acre-feet of water generated per 100 acres of thinned forest. That is meaningful volume in a watershed managing scarcity.

South-facing slopes showed a smaller benefit, 16 percent versus 30 percent, because increased sun exposure after thinning can accelerate snowmelt even as more snow reaches the ground. The net effect was still positive, but managers working on south-facing aspects would need to weigh the tradeoff more carefully.

A measurement problem hiding in plain sight

One of the study's more useful findings was about how forest managers currently measure their work. Standard practice is to count trees per acre - a straightforward metric that is easy to quantify and compare across sites. But counting trunks does not directly measure canopy cover, and canopy cover is what determines whether snow gets intercepted.

Two stands with identical tree density can have dramatically different canopy structures depending on the species, age, and spacing of trees. A forest thinned to a target density of, say, 80 trees per acre might still have a canopy continuous enough to intercept most snowfall if the remaining trees are large and their crowns touch. The same tree count in a different configuration could produce the beneficial gaps the study identified.

The researchers argue that incorporating canopy gap measurements into management planning - not just tree counts - would allow managers to predict snowpack outcomes more reliably. This does not require new technology; aerial imagery and lidar surveys can map canopy structure at landscape scales. It requires adding a variable to the decision framework that has been absent from most operational planning.

Wildfire risk and the case for overlap

The fire ecology case for thinning is well established. Dense forests in the western United States carry more surface fuel, higher crown density, and greater continuity - conditions that allow fires to spread faster, burn hotter, and climb from the ground into the canopy. Thinning reduces all three risk factors. Prescribed fire or mechanical thinning followed by removal of slash is now standard practice in many forest management plans.

What has been less clear is whether the type of thinning that benefits fire resilience - creating gaps, reducing continuity, lowering overall density - is compatible with the type that benefits snowpack. The Cle Elum Ridge experiment suggests the answer is yes, within a range. The optimal gap size for snowpack accumulation, 4 to 16 meters in diameter, is consistent with thinning prescriptions designed to reduce fire risk. Managers do not need to choose between the two objectives; the same treatment can serve both.

The caveat is that "forest thinning" is not a single action. The intensity, spatial pattern, and residual structure after thinning all affect outcomes. A thinning that creates the right gap size in one location might leave too much or too little canopy elsewhere. Site-specific planning, calibrated to slope aspect, elevation, and existing forest structure, would be needed to reliably achieve dual benefits across a landscape.

Water security in a drying region

The Yakima River Basin is among the most water-stressed watersheds in the Pacific Northwest. Agriculture, municipalities, and salmon habitat all compete for water that originates as mountain snowpack. Three consecutive years of restrictions have focused attention on every potential source of additional storage, including the forests themselves.

If thinning treatments applied across a meaningful fraction of the basin's headwater forests could increase snowpack by even a fraction of the percentages observed at Cle Elum Ridge, the volumetric gain in acre-feet of water could be significant at the watershed scale. Translating plot-level findings to landscape-scale projections requires careful modeling and additional validation sites, but the directional signal from this experiment is encouraging for water managers looking for interventions with multiple co-benefits.

The study does not resolve every tension between timber interests, conservation groups, and water managers. But it does make the case that at least some of those tensions are less inherent than assumed. Reducing wildfire risk and protecting water resources, the researchers conclude, do not have to be competing goals.