Since dogs are allowed on leash, they might be able to get in the water, but perhaps on a long leash. I doubt it's a problem if you stay away from the swimming beach.
The problem with that is the "swimming" beach is like the only water access and it's usually crowded with boaters who stop there to party. I'd recommend taking dogs to granite falls park across the river by the Walmart, better area to access water, much quieter, and there's no park fee Hell's Gate is a glorified boat launch and not much else.
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