The Mississippi at Memphis moves a great deal of water quickly, and the danger is mostly invisible from the bank. Currents run hard beneath a surface that can look placid, the channel shelves away steeply, submerged debris shifts constantly, and barge traffic generates suction and wake that no swimmer of any species contends with. A dog that goes in after a stick is not doing anything a dog would not normally do; it is simply doing it in the wrong river.
So the rule for a Welsh Springer here is a lead near the water's edge, without exception, on the riverfront and on the bluffs above it. This is not a training problem to be solved with a good recall. A reliable recall is worth having and it is worth nothing against a current.
What makes the rule easy to keep is that Memphis provides an excellent alternative. Shelby Farms Park covers several thousand acres on the east side of the city — one of the largest urban parks in the country — with lakes, a large off-leash area and miles of trail. The Wolf River Greenway runs a long connected corridor, and Overton Park adds an old-growth woodland inside the city. A dog rated 4 out of 5 on exercise need can swim, run and be thoroughly worked here without ever going near the main channel.
One consequence to plan for: swimming in warm still water means wet ears, and this breed's long, heavy ear leather closes over the canal and holds moisture. Dry inside the ear after every swim, all year, since Memphis's warm season is long enough that there is no natural break in the habit.