Bill, I agree that conditioning is key. Also with swales it seems to be important that the distance of the approach is such that the golfer might not be totally confident in carrying (and holding) the green. Such would have been the case with Merion's 18th certainly.
By your description of how the swale plays at your course, it sounds like it works well for you. Swales create sort of a dead zone where the golfer can't be wishy-washy. He must either carry them completely (and hole the ball stops) or he must play well back and run through them. Balls landing in the swale don't usually make it out, by design.
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It is interesting to consider Merion's swale on 18 in the context of Phil's last two rounds . . .
On Saturday he laid so far back that his shot choices probably came fairly close to replicating that of golfers 100 years ago. He chose to try and carry the ball all the way to the green, and while the shot looked brilliant, he couldn't stop the ball on the surface, and he ended up taking a bogey. What if he had played a running shot landing short of or on the downslope of the swale? Almost unimaginable, I know, but it might have given him a better chance of actually stopping it on the green.
On Sunday he had a long tough shot on the rough, and he tried to run it up, but he landed in the bottom of the swale, and his ball didn't go anywhere. Again, what if he had tried to hit it short of the swale or on the downslope? Would he have had a better chance of running the ball onto the green?