A team of researchers who went on a nine day rafting trip to study river flow in bedrock canyons claims that previous notions on flow and incision in bedrock-rivers need to be reconsidered. Their study, which has implications for lowland river floodplains and deltas, appears today in the journal Nature. It was written by lead author Jeremy Venditti from Simon Fraser University, with co-authors from the University of Ottawa and the University of British Columbia.
Current models of bedrock-rivers assume flow velocity is uniform, without changes in the downstream direction. Our results show this is not the case, says Colin Rennie, professor in the Department of Civil Engineering at the University of Ottawa. Our research has important implications for canyon erosion, because a plunging flow pattern results in greater flow force applied to the bed. The scientists conclude that river flow in bedrock canyons is far more complex than first thought, and the way scientists have linked climate, bedrock incision and the uplift of mountains needs to be rethought.
For the first time, we used oceanographic instruments to examine flow through steep bedrock canyons, says Venditti. These instruments are more commonly used to measure three-dimensional river flow velocity in lowland rivers. With the instruments, researchers observed a complicated flow field, in which high velocity flow plunges down to the bottom of the canyon forming a velocity inversion, and then rises along the canyon walls.
River flow velocity in bedrock canyons also influences the delivery of sediment from mountain-rivers to lowland rivers. Sediment delivery controls water levels and stability of lowland rivers, which has important implications for lowland river management, the impact of flooding on infrastructure, availability of fish habitat and more.
To carry out their expedition, the researchers put their lives into the experienced hands of Fraser River Rafting Expeditions, which took them into 42 bedrock canyons. Equipped with acoustic Doppler current profilers to measure velocity fields, they rafted 486 kilometres of the Fraser River from Quesnel to Chilliwack. Their raft navigated turbulent waters normally only accessed by thrill-seeking river rafters.
While this type of expedition may sound like a thrill-ride, the results yielded provide valuable data. Lowland river floodplains and deltas are the most densely populated places on earth, so understanding what is happening in mountain rivers is important. Our continued development of these areas is significantly affected by what is happening upstream, says Venditti.
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