[e_engineer]

In many places around the world submerged beaches and ancient tree stumps have been found that provide an indication of where the sea level once was. I would try to find a submerged beach, then focus my efforts on the river valleys upstream from the submerged beach.

[RightWhale]

ancient tree stumps

Then there is dendrochronology and you're home free if it's inside the date range they have catalogued. What is it now, 4000 years or more?