Boomer,
It sounds like all the wall slope will be under water. Is that correct? If it is, and the clay is stable, I would think it could stay in that shape for a very long time. My guess your biggest issue might be if the water level drops a bit and you get some wave action, that could start eating into the bank.
We're in the process of removing clay to build a road, and the borrow area will eventually be under water. At the waters edge we are going to leave a 20 ft wide band that will slope from 1 ft below water level to 1 ft above water level. Then we will cover this with rock to prevent erosion. Then at the 1 ft depth we are going to drop almost straigh down to about 15 ft, to prevent weed growth.
We've got a 10 year old, 25 acre pond and in the borrow area have a similar straight drop off, and my son has scuba dived along it and said the bank still goes straight down.
Bill
