Now 'Baccy, you know this is not the traditional understanding. The Torah has no "source." It was written in its entirety by G-d before the universe(s) was/were created ("974 generations" according to the midrash) and was in its entirety dictated to Moses letter-for-letter. Even the ultimate rationalist, RaMBa"M, required the latter belief as absolutely fundamental to Judaism. As for the "limited flood" argument, that's just another nineteenth century liberal Xian theory that, so far as I know, has absolutely no precedent in Jewish tradition.
You should really read Rabbi Shlomo Rotenberg's Toledot `Am `Olam, or at least consult the Seder `Olam.
Sometimes I worry about you.
I don’t disagree with anything you said.
You would do well not to confuse casual internet parlance with deep (and more precise) theological discussion.
The “source” for the non-Torah versions would, of course, be eye-witness accounts passed down trhough generations of the same event dictated to Moshe by HaShem. The two routes are not inconsistent.