Question: Can smaller willows do serious damage to a dam if kept in check?

I know that you don't want trees growing on your dam because the roots can penetrate the clay core and eventually create channels for leaks. But my research found that willows have very shallow root systems so I didn't think they would cause nearly the same type of problem as other trees as long as their size is kept in check.

I have a deep pond with a small stand of willows toward one end of the dam and I keep them in check with some clippers so they have created about a 12' tall by 8' wide bush overhanging the water. It looks attractive and is the only shady spot for the fish on the south facing dam of this pond other than the dock toward the other end of the dam. I'd like to keep them but won't do so if most of you think it's a really bad idea.

Thanks for your help.