MOUNTAIN river networks often consist of both bedrock and alluvial channel(1-5), the spatial distribution of which controls several fundamental geomorphological and ecological processes(6,7). The nature of river channels can influence the rates of river incision and landscape evolution(1,2), as well as the stream habitat characteristics affecting species abundance and aquatic ecosystem structure(8-11), Studies of the factors controlling the distribution of bedrock and alluvial channels have hitherto been limited to anthropogenic badlands(12), Here we investigate the distribution of channel types in forested mountain drainage basins, and show that the occurrence of bedrock and alluvial channels can be described by a threshold model relating local sediment transport capacity to sediment supply. In addition, we find that valley-spanning log jams create alluvial channels-hospitable to aquatic life-in what would otherwise be bedrock reaches, The formation of such jams depends critically on the stabilizing presence of logs derived from the largest trees in the riverside forests, suggesting that management strategies that allow harvesting of such trees can have a devastating influence on alluvial habitats in mountain drainage basins.