The simplest explanation of a cauld is akin to that of a weir on a river, a point where the river is semi-dammed by a man made barrier of concrete or constructed stonework to hold back water above the cauld to form a pool from which water can be diverted to power a water driven mill such as that which is still at Mertoun, although now derelict and the mill race filled in at the head (the rest of the mill race can be seen extending down past the poplars and under Mertoun bridge where it rejoins the river).
A by-product of this is to create a holding pool for migratory fish that is of benefit to the landowner as it increases the number of salmon taken on his waters by holding them below the cauld on low water and resting above. The exertions of a fish travelling up a cauld tire them out and they rest in the pool above it before continuing up the river. Usually the centre of a cauld has a gap where the water rages through to form a hole below the cauld, the other fish holding area. The sides of a cauld are at shallower angles than the centre to allow migratory fish to pass up the river system when water levels allow.
A cauld such as that on the Ettrick can be fitted with a fish counter to count the numbers passing up it.They are also a favourite spot for poachers with a gaff but are well monitored by the Tweed Bailiffs and Ghillies. Watching fish pass over a cauld is a good pasttime in the off season, it just gets you itching to fish if you are a salmon and seatrout fisherperson!
Caulds on a beat are in general frowned upon by landowners above them as they can restrict the number of fish migrating upwards in the system. This issue has been brought up time and time again in the house of Lords
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