The water flows cleanly here, just a few miles from the source of the Hudson River, deep in the Adirondack Mountains. The big puffs of foam that form on the surface of the water aren't evidence of agricultural runoff, but rather the result of rainwater leeching through the forest floor. One early summer morning, as I plied my fishing rod along the foggy river, I caught sight of what looked like just such a foam puff, a bit downstream. I didn't pay it much attention until I realized, almost subconsciously, that it was moving upriver. I stopped casting and turned to watch. Something was swimming slowly, determinedly, in my direction.
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