Environmental Description:  This association occurs on valley bottoms and toeslopes of valley sides in a glaciated mountainous region. The valleys are flanked by rugged mountains of primarily granitic origins and have been modified by glacial activity. Valley floors are generally wide, with gentle gradients in the lower reaches, while middle reaches are somewhat narrower or U-shaped. Glacial and alluvial deposits comprise the valley floors. The climate of these montane valleys is cool and semi-arid. Summers are warm, but winters are very cold, and morning frost in low-lying pockets is common throughout the warm season. Average annual precipitation is 25 cm (10 inches), much of which falls during November to April in the form of snow. Valley bottoms are typically snow-covered from November through late April or May.

The association is found from 2070 to 2240 m (6790-7350 feet) elevation, in lower and middle reaches of tributaries. It is associated with valley bottoms greater than 100 m wide. Slopes vary from 1-7%. Gradients from 1-3% represent sites in the center of flat bottoms, while larger gradients reflect the tendency of this type to be perched on valley edges, convex brows of terraces, and toeslopes. The wetter examples are large open bogs, with small, dispersed rivulet channels running through. Drier examples occur on a low-gradient, widely meandering stream with shallow pools and a bed of fine angular gravels. The soils are typically composed of organic materials, in some cases over saturated mineral layers, with large rocks protruding as shrub-covered hummocks. The wettest sites are organic mats floating on water or muck.