Geologically, this ecoregion is very complex containing bedrock of sedimentary, igneous, and metamorphic origin. Historic and current glaciation has sculpted the mountainous landscape filling many of the intermountain valleys with glaciofluvial deposits and moraine. Climate in the ecoregion is heavily modified by elevation resulting in modifying influences from such things as rain shadows and thermal inversions. Cooler, more boreal character is present in the north, moderating maritime influences in the west, and drier, continental conditions in the east. Majority of the precipitation falls as snow and occurs in fall, winter, and spring.
Summers are generally dry.
Natural disturbance is predominantly fire, periodic flooding, insects, and disease.