Thermally-driven upvalley wind in the upper East River Valley in the Colorado Rocky Mountains often unexpectedly stops in mid-morning and reverses back to downvalley wind. We use a comprehensive observational data set for a nearly two-year long period to analyze the wind system and boundary layer evolution in this high-altitude valley and determine the reason for this early wind reversal. Days with short upvalley wind predominantly occur during the warm season when the valley floor is free of snow and the convective boundary layer grows well above the height of the surrounding ridges. Upvalley wind persists throughout the day only on a few days during the warm season. We link differences in valley wind evolution to wind direction at upper levels at and above ridge height and propose forced channeling mechanisms to describe coupling between valley and upper-level wind when the convective boundary layer grows above ridge height. The frequency distribution of upper-level wind direction is such that channeling in the downvalley direction is favored, which explains the predominance of days with short upvalley wind. The deep convective boundary layer is supported by the presence of a deep weakly stably stratified residual layer with high aerosol content, which is regularly present over the mountain range during the warm season. On days when the convective boundary layer does not grow above ridge height, for example when the valley floor is covered by snow, thermally-driven upvalley wind is able to persist throughout the day independent of upper-level wind direction.