Contemporary morphological change in braided gravel-bed rivers: New developments from field and laboratory studies, with particular reference to the influence of riparian vegetation

Contemporary (event to decadal-scale) morphological changes in two large braided rivers in Canterbury, New Zealand, are described, along with laboratory studies that support the field observations. In the process, some new developments in field and laboratory methods for investigating morphological change in braided rivers are presented, and Paola's (2001) hypothesis that braiding tendency should be influenced by a river's ability to turn over its bed within the characteristic time for riparian vegetation to establish and grow to a mature, scour-resistant state is examined. The lower Waitaki River has been regulated for hydropower since 1935, and since then vegetation has encroached over the riverbed and braiding intensity for a given discharge has reduced. Measurements of vegetation removal by floods indicate that floods are not able to turn over the bed fast enough to contain vegetation encroachment, and the present braiding state is held by virtue of a regular spraying programme. In contrast, on the unregulated and sparsely vegetated Waimakariri River, remotely sensed high-resolution topographic surveys using LiDAR and digital photogrammetry have shown that even sub-annual floods turn over large proportions of the braidplain. The laboratory studies show that a braided river will evolve into a single-thread channel when its bed is invaded by vegetation and floods are too infrequent to contain the vegetation growth. Collectively, the field and laboratory evidence confirms that Paola's (2001) dimensionless time-scale parameter is a reasonable first-order predictor of whether floods or vegetation will achieve ascendancy, driving a river towards either braided or single-thread end-points, respectively.