Alternative silvicultural treatments such as thinning can restore the productivity and diversity of forested watersheds and reduce wildfire hazards, but the hydrologic effects of these treatments are not well defined. We evaluated the effect of even thinning (SE) and group-retention thinning (SG), both with ~ 60 % basal area removal, on snow accumulation in lodgepole pine stands at the Tenderfoot Creek experimental forest, west-central Montana. In 2003 and 2004, the snow water equivalent (SWE) close to the seasonal peak was measured at >250 locations in the SE and SG treatments, and a control. In both years, the mean SWE in the SE treatment was significantly higher than in the control and the SG treatment (P<0.0001). In contrast, the mean SWE in the SG treatment was not significantly different from the control. Spatial variability of SWE was up to 3 times higher in the SG treatment than in the SE treatment or the control. The increased snow accumulation in SE treatments is attributed to reduced interception. In the SG treatment, losses due to wind scour and evaporation offset gains due to reduced interception. These results demonstrate that thinning can have substantially different effects on snow accumulation depending on the spatial arrangement of the treatments.