Riparian Vegetation as a Primary Control on Channel Characteristics in Multi-Thread Rivers

While many previous studies have explored the effects of vegetation on single-thread rivers, systematic studies on multi-thread rivers are scarce. Our approach is to synthesize data and ideas from a field-based study of the Waitaki River in New Zealand with results from laboratory flume experiments and a cellular numerical model for which variables can be controlled. The combination of the results from the three approaches suggests that vegetation affects channel planform mainly through reductions in the total channel width, braiding index, and relative mobility of channels. A major driver is the effect of vegetation on flow dynamics and the apparent cohesion of channel bed and banks. By choking off weaker channels, vegetation corrals the flow into fewer, stronger channels with more uniform higher velocities; by strengthening the banks and exposed bars, vegetation reduces channel migration rates and limits bed-material exchange with islands and bars. At present them is no indication that the maximum shear stress is increased by vegetation, only that the low-stress tail of the total stress distribution is cut off. Thus total flow width appears to be relatively sensitive to changes in vegetation intensity while maximum velocity and width of the region of active sediment transport are less so. In natural channel systems, vegetation reduces the total channel width by occupying fleshly exposed areas of bare sediment. Though this happens naturally, it is often enhanced by changes in the flow regime that may be due, for example, to climate change or damming. Colonization by vegetation is not easily reversible, and therefore typically has long-term effects on the system. We investigate historical channel changes on the Platte River in central Nebraska to separate out the effects of discharge reduction and vegetation expansion on channel width, and find that discharge reduction alone cannot account for the current reduced width on the Platte today.