Robbie Pennington: A big pool, maybe about 900m long. Deep along the north (Scottish) bank, progressively shallower towards the southern (English).
Easily accessed at NT 807 393 on the A398 between Birgham and Kelso, where there is some car parking and steps + jetty.
The pool makes a good start for a swim downstream to Wark, on the English side, though it's better to start at the top of the pool by Carham Hall. (Carham Hall Nursing Home was good enough to allow parking when we swam from there to Wark).