It would tie into the existing clay in the dam at a point just below or right at full pool, so there would be a seal. The seep only has the pressure of three feet of water at the bottom. It doesn't take much clay to hold back a few inches of water. Going all the way to the top of the dam would be unworkable as the trench would be too deep to work in. I would be happy if we could get it to hold at one foot below full pool. It looks OK at that stage.