THE ROLE OF RIPARIAN FORESTS IN RIVER AVULSION AND FLOODPLAIN DISEQUILIBRIUM IN AGGRADING PRO-GLACIAL BRAIDED RIVERS: A NEWLY RECOGNIZED MODEL IN FLUVIAL GEOMORPHOLOGY

Due to climate change and rapidly receding glaciers, the proglacial, braided rivers that radially drain Mount Rainier, Washington are aggrading up to10 times the historic rate (up to 1.3 meters per decade). There are segments of these rivers that exhibit a unique convex valley cross-section associated with aggradation in which the main stem river channel is perched 3 to 6 meters above the adjacent forested floodplain. Lateral gradients across the floodplains can be more than twice that of the river channel. These conditions send water and sediment over the floodplain resulting in either deposition or channel formation. Flows can quickly erode a new or existing side channel that can quickly expand to capture even more of the river’s flow. But the ancient trees on the floodplains indicate little change in the river’s position. We found no instances where the main stem White River channel had moved out of its pre-existing channel into the floodplain despite catastrophic side channel expansions. The channel-floodplain disequilibrium does lead to forest mortality where trees are buried or where they fall into expanding side channels. Floodplains along the Upper White River do have areas of tree mortality, but standing and fallen trees effectively dissipate enough of the river’s energy to prevent it from moving into the floodplain. In these areas sedimentation elevates the floodplain surface to create a more stable surface on which a new forest community is established. This unique situation in which low lying floodplain forests play a fundamental role on river morphology by holding their ground and precluding major channel avulsions appears to have persisted for thousands of years. But observations from Tahoma Creek reveal that increases in river bed aggradation can overwhelm floodplain forests. Aggradation in Tahoma Creek was compounded by dozens of recent debris flows that raised the creek bed over 6 m and triggered an avulsion that sent the creek across its valley and through an area once occupied by an old growth forest. Increasing river bed aggradation rates downstream of receding glaciers on Mount Rainier are triggering channel avulsions and floodplain forest mortality that could alter the long-term patterns of alluvial landform and forest development that had persisted since the last glacial maximum.