Hah! I did the same thing about two years ago. Went to Walden on a lunch hour [with a fishing rod] and saw all manner of trout porpoising and leaping around that cove. They were so freshly stocked that they weren't interested in feeding, just in swimming around in something that wasn't concrete. There was even a dead trout on the ramp that had been squished by the stocking truck.
So, uh, where are these alders that you like?