The geomorphic complexity of streams influences the residence time of carbon by creating storage sites for organic matter. Instream wood increases complexity, particularly where logjams initiate multithread channels, as documented previously for lowland alluvial channels. Analogous processes in mountain streams can substantially increase the residence time of organic matter in streams otherwise characterized by high transport capacity and limited storage. Thresholds in wood mobility that reflect valley and channel geometry and wood dimensions determine the distribution of multithread channels and the mechanics underlying these thresholds. Valleys must have a lower gradient and less lateral confinement to develop multithread channels, but also larger volumes of riparian and instream wood. Hydrostatic forces that mobilize jams increase more slowly in low-gradient, unconfined channels, facilitating jam persistence. Treefall can thus initiate complex behavior that results in a self-enhancing feedback between wood recruitment and channel geometry. Patches of old-growth forest and multithread channels were formerly more widespread. Carbon retention has likely decreased as multithread channels have become less common.