Clay is a must we have seen many ponds with a partial lining of bentonite on the banks fail bring the clay up and entrain at least 24" into existing banks and should work fine for you.
Many times ground water will work it's way in from top grade or incline water pressure and damage seal and erode
Good luck
Scott


Scott Trava
Catskill Pond
http://catskillpond.com
[email protected]
Returning Catskill Waters To A Simpler Time
EST. 1923