Salmon require a concentrated flow of water which falls over an obstacle into a pool scoured by the falling water in order to jump; the depth of the pool needs to be around 1.25 times the height to be jumped. They use the undercurrent in the pool, created by the falling water, to make a “running jump” into the cascading water. It is the noise of falling water that attracts Salmon to the places where they jump; the most noise is made by the most water falling and that, of course, is where the waterfall is lowest.

Beaver dams, being created with logs and sticks, leak* and, except in spates have little or no flow over them which Salmon and trout can use to jump.  Natural waterfalls have deep pools under them from which fish can jump; Beaver dams rarely have such features because they are not permanent and water goes through them, dissipating energy, rather than over them to create a drop with erosive power. Waterfalls have centuries / millennia to create deep water below them – Beaver dams do not.

* This is why legislation in Scotland requires dams and weirs to be watertight, so that water flow goes over their tops where it can be used by jumping fish and not through them, where it cannot.

Whilst this paper has been prepared by The Tweed Foundation on the basis of information that it believes is accurate, any party seeking to implement or otherwise act on any part or parts of this paper is recommended to obtain specialist advice. The Tweed Foundation does not accept responsibility under any circumstances for the actions or omissions of other parties occasioned by their reading of this paper.