Jeff, It's hard telling about your cottonwoods. I noticed a groove of cottonwood saplings at my pond just the other day, but they are growing at the inlet of the pond about 5 to 15 feet from the bank. It's not like the pond would be leaking there, just damp. I also have a huge cottonwood growing at the base of my dam along with a willow of similar size. They about 24 inches in diameter. They are real close to the drain pipe. The dam does not leak, but the water table is very close to the surface there.

I'll cut my saplings down as soon as it cools off this year just to keep an opening for wind to come through. As far as the large trees at the base of the dam...they have been there since the pond was originally dug (50's or 60's) and have not caused any trouble. No roots were discovers when the pond was renovated in 2016. They will stay for beautification reasons as I would bet the roots are going down to the water table and not up to the pond through the dam.