Ping
Well, not a sinking civ, but even as late as the 1927 Mississippi River floods over thousands of sq miles in the lower river, thousands of flood refugees got to the old Indian mounds across the delta and Mississippi-Louisiana flatlands and waited out the days until the flood waters receded on the mounds.
My opinion? The first Indian mounds were built near the village to serve as a last-minute shelter against sudden catastrophic floods. No mounds? No hills very, very close-by (accessible in minutes by foot)? Then the tribal village died out. Feel the flood waters rise, get to the mound, the people survive.
Then, after centuries of successfully surviving floods, the mounds got higher and higher, more and more expensive to “build a better/bigger/higher/wide mound than the next priest” started up.