The River and the Rock: River Potholes of Wales By Dewi Roberts
At the last meeting Dewi Roberts gave a very interesting talk on the formation of fluvial potholes. His talk was accompanied by his superb photography taken both above and under the water and which had been taken mainly in welsh rivers.
General View of a pothole by Dewi Roberts
Potholes are an erosional and very visible feature of a fluvial landscape and occur mainly in the upper reaches of a river were the flow of water has sufficient energy to carry the boulders, cobbles , pebbles and sand that are required to cause abrasion of the bedrock. Therefore one of the main factors in pothole formation is hydraulic. They are one of the ways in which rivers erode the landscape but they create superb river scenery.
Underwater image of turbulence moving cobble by Dewi Roberts
Formation begins with a minor depression in the bedrock. The turbulent flow of the river will then develop eddy currents which enter the depression. This will then cause the sediment to enter the depression and begin to abrade the sides and the bottom causing it to enlarge. A type of positive feedback mechanism which causes an increase in turbulence and eddy currents. The pothole will continue to deepen and widen over time unless the erosive effects are halted for some reason. The sediment particles involved in the erosion are known as tools.
Potholes on the Clywedog River by Dewi Roberts
If the pothole continues to erode overtime then eventually neighbouring potholes may coalesce which can lead to the development of an inner channel through which water can flow and produce areas of sculpted rock.
Abergwesyn Common by Dewi Roberts
Other factors that play a role in the formation of potholes will be the type of rock over which the river flows and the presence of joints, veins or dykes. The bedrock topography will have an influence on flow and turbidity and joints can lead to block quarrying which may limit the length of time available for pothole formation.
A thoroughly enjoyable talk especially as we were taken under the water to see the process at work.
Dewi along with Stephen Tooth and Hywel Griffiths have produced a book entitled:
River and the Rock, The - River Potholes of Wales
This should prove a very interesting read and the photography will be superb.
Dewi also has an exhibition at The Aberystwyth Arts centre:
Wed 17 May 2023 to Sun 23 July 2023
“Inspired by the Rheidol, Ystwyth, Dulas and other Welsh rivers, Hydro Psyche explores the benefits of re-connecting with nature. A collaborative project by painter Karen Pearce, photographer and videographer Dewi Roberts and writer and performer Liz Pearce that fuses paintings, film, photography and sound to celebrate the beauty and power of rivers and the life within them.”