Proof that the earth is made of squishy stuff?

Over the last couple weeks I have been haunted by a problem. If the earth were totally rigid the lake shoreline would be at a higher elevation at the center than it is at the edges because water has mass and gravity which would make it bulge up in the middle. I did some rough calculations and it seems the amount it would bulge is the same amount as what I am measuring.
The flat earthers are going to have a field day with this. 🤦
It makes sense though: if you had a sheet of flexible floating material the gravity field over the sheet would be the same before and after adding a weight because the weight would displace its own mass of the fluid that the sheet is floating on.
Both scenarios: "the earth is rigid" and "the earth is squishy" would leave the same elevation difference in the ancient shoreline.
So how would I tell which is correct?
Well for the "earth is squishy" theory the rebound would take some time, perhaps hundreds of years, because the earths mantle is not a runny liquid.
For the "earth is rigid" theory however the bulge would go away as soon as the water did.
Unfortunately even hundreds of years is an extremely short period of time in geological terms.
Something happening that fast just isn't going to leave a long lasting mark unless...
After the boneville flood the shore of the remaining lake would be devoid of vegetation and would erode very quickly. The eroded material would end up flowing to the lake where it would stop at the shore, like a whole bunch of little river deltas. This would have continued for many years until plants colonized the land and started holding back the soil. But importantly that would happen much faster than the rebound. So as the rebound happened the erosion rate would be fast and then slow way down. So I should see a bump in the terrain just above the lower shoreline near the center of the lake but not at the edges. And guess what! That is exactly what I see!
If the elevation difference was due to gravity the bump would not exist.
I'll look at it a bit closer and gather more evidence in the coming week to confirm but I'm very excited to have potentially solved the problem.
What do you all think?