MOUNTAIN RIVER METAMORPHOSIS

Stanley Schumm defined river metamorphosis as “the almost complete transformation of river morphology” driven by natural or human-induced changes. Schumm’s conceptualization was primarily based on low-gradient alluvial rivers in the U.S. Great Plains and the Riverine Plains of southeastern Australia. Metamorphosis can also occur in relatively low gradient, laterally unconfined segments of mountain rivers, which can alternate through time between a single- or multi-thread planform. Steeper, laterally confined segments of these rivers always have a single-thread planform. A threshold defined by valley geometry separates river segments that are consistently single-thread from those that alternate through time between single- and multi-thread. A stable, persistent multi-thread planform occurs where one of two biotic drivers is present: channel-spanning logjams associated with the large and abundant instream wood recruited from old-growth forests, or numerous small dams associated with active beaver colonies. Research in the headwater mountain rivers of the Colorado Front Range indicates that many multi-thread river segments have metamorphosed to single-thread planform as a result of loss of old-growth forest or beaver. These metamorphosed single-thread river segments take on characteristics that make it difficult for the planform to revert to multi-thread, supporting the idea of alternative stable states in these rivers, and creating challenges for river rehabilitation. The loss of geomorphic complexity when multi-thread rivers metamorphose to a single-thread planform results in loss of retention of fine sediment and organic matter: the metamorphosed rivers become ‘leaky’ with respect to fine sediment and nutrients.