I read, or perhaps hallucinated, a story once about an early European explorer who wrote of having traveled along a river from Point A to Point B — don’t remember where. Only Point A and Point B turned out, upon further investigation, to be on different river systems, and one cannot be reached from the other without significant portage. So early European explorer was denounced as a fraud for several decades (lifetimes?) — a long time anyway . Then one year both river systems experienced unusually large flooding at same time, and a plain between them became a temporary lake, allowing boats to pass from one river system to the other, and early European explorer’s reputation was rehabilitated.
The story may have been set in South America, and it may have been fictional.