Feshie
Rivers
The channels of rivers draining the Cairngorms range from confined gorges to dynamic braided systems. Many elements of the valleys in which the rivers are inherited: the valley itself may have been cut or adapted by the flow of ice or meltwater and the stream itself is misfit and the valley may be choked by coarse sediments dumped by glaciers, meltwater and the river itself. The ability of the river to shape its own channel is thus closely constrained and some rock-cut channels have changed little in the last 10 thousand years.
A river flowing on a floodplain draped with alluvium has the capacity to adjust its channel geometry in response to changes in discharge and sediment supply. Adjustment can involve
- Channel slope
- Flow velocity
- Channel roughness
- Channel shape
- Channel pattern
- Sediment transport
In the
Cairngorms, the most dynamic channel patterns are the braided systems of gravel
bed rivers. Rivers like the Feshie and the Dorback are overwhelmed by sediment
derived from thick sequences of fluvioglacial gravel and sand. The rivers shift
the debris around, moving it slowly down valley, but they are incapable of
clearing out the vast accumulation of accumulation and, indeed, erosion of
bluffs at the channel margins bring in more debris. This creates some of the
fastest changing floodplains in the
British Isles,
with switching of channels and movement of gravel bars over timescales of 10-100
years.