CONVEX VALLEY CROSS-SECTIONS OF MOUNT RAINER RIVERS AND THE ROLE OF FORESTS AND LOGJAMS AS LIMITATIONS TO CHANNEL MIGRATION

Several large rivers draining the slopes of Mount Rainier, Washington exhibit distinctive convex valley cross-sections with the active channel occupying the highest elevations. Relative elevation maps (REMs) of the upper White River valley clearly shows locations where the active river channel is situated well above adjacent forested floodplain. The lateral gradients in these locations well exceed the longitudinal gradients of the river, leading to rapid side channel development through the floodplain. The difference in grade would appear to setup situations in which the active channel would reoccupy lower elevations of the valley floor, dramatically changing its planform. The presence of old forests several centuries old on low lying floodplain and the lack of young forests colonizing abandoned river channels indicates that major changes in channel planform are rare. When new channels start to develop within forest floodplain they tend to recruit large wood as they expand. Standing and fallen tree trunks increase surface roughness of the floodplain, limiting expansion of new channels as well as trapping sediment, thereby raising the floodplain surface and diminishing the gradient differences driving new channel development. The forests and logjams are thus effective mechanisms in controlling channel planform and valley morphology. The low relative elevation of floodplain channels allows them to more effectively intercept the water table and provide important ecological habitat for native aquatic species. Rivers such as the Upper White provide a powerful example of how large trees and wood can have a dominant role in channel dynamics and alluvial valley morphology.