Meander cut-off process
The photographs referred to are found at www.apamapa.co.uk in the Abhainn Srath na Sealga river study section.
The process by which the meander at Eilean a’ Chip on the Abhainn Srath na Sealga in Wester Ross (photograph 8) became abandoned is different from that given in text-books. There is little or no evidence of a narrowing of the neck of this meander during the river’s change of course. This is likely to be the typical pattern seen in areas where periods of spate regularly occur in a river’s regime.
The process, admittedly with a slight narrowing of the neck of the meander, appears to have been taking place on the River Bran, east of Achnasheen (NH1858 and NH1958) in the early years of 2000 though the change of course is still not complete in 2009.
The process is illustrated in the accompanying diagrams. These show a cross-section along the length of a river along line X-Y at the neck of a meander and shown in plan to the right of figure A. Gradient has been exaggerated for clarity with the river flowing in a general direction to the left.
Figure A
The arrow symbols indicate the direction of flow of the river, away from the viewer on the right and towards the viewer on the left. This shows the river at a time of average discharge.
Figure B
During spate, water has spilled over the river bank and is flowing over the floodplain between the two limbs of the meander (photograph 10). The difference in height of the river between average and spate discharge is shown with a lighter shade of blue.
Figure C
After repeated periods of spate, over a number of years possibly, the upstream river bank (1) has been eroded allowing water to flow across the floodplain even at time of only slightly greater than average discharge. It may be that the heightened level of the water is due to impounding of the flow caused by partial blockage of the main channel, round the meander, as a result of deposition during spate (2 indicates the general are of this though it would be behind or downstream of the cross-section). This was witnessed on Abhainn Srath na Sealga (photograph 9).
Figure D
Flow continues over the floodplain causing the slow erosion of the neck from the down-river side. As this takes place, a waterfall (3 and photograph 11) gradually progresses upstream leaving a narrow channel bounded by banks of floodplain material (4 and photograph 13). This part of the process will become a little quicker as the flow of water kills the floodplain vegetation on the floodplain thereby reducing the binding effect of the roots (photograph 12).
The final stage in this process is the breach of the upstream bank (5) when the waterfall nears it and the river’s course is changed to a more direct route across the meander’s neck (photograph 14).