Call me naïve or call me a taxi, but it seems that a family as focused on the environment as the Trails would certainly know their local river fairly well. I’ve known citizens in New York that have never been inside the Empire State Building, or in London that have not visited the British Museum or Tower of London. Yet they weren’t architects or historians.
On the other hand, perhaps Mark did know about that location and deliberately went there. After all, if you are looking for a bassigator, perhaps it would tend to hide among the true alligators!? But as it is night, unexpected events overcame the hunting party.