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Are you sure it was a Copperhead and not a Watersnake?
They have a superficial resemblance to one another, but the watersnake is basically harmless.
It seems to me that watersnakes are like a thousand times more common than copperheads. I see watersnakes nearly every time I fish certain lakes, but I have only seen a Copperhead in New Jersey once in my life.
The first clue is whether it takes to water. Watersnakes, as their name suggests, are found around and in water.
Here are some pictures to show the differences:
Northern Watersnake:
COPPERHEAD:
