I'm not an expert, but I'd say it's a long-nose gar. They're able to survive in stagnant water because they have some sort of bladder in their lungs that allows them to gulp and store air from the surface. As to why it's in the woods...I'd assume, as the pool dried up, some animal grabbed it up for a meal.

Sorry didn't see the spot was 200 yards from the river. Yeah, then the water came over the banks, but dried up like the above post says.