You know, it gets cold in Alaska...
Very cold, which would have killed the ants naturally during the first winter. This would have been much more believable in a California or Florida setting. It's also nearly impossible to drown an ant. It takes a day or two; a few minutes of torrential water from a dam would only wash them downstream.
Oh, and wasn't that driftwood found on a bayshore? I've never seen a river flow downhill from the ocean before. If it was a lake, how did the wood drift up the river?