BillSD,
I installed an AgriDrain system as my spillway when the pond was dug in 2009. I've had two Minnesota winters now and haven't noticed anything amiss. I've actually seen the Agridrain conducting the heat from 8' down up to the surface and melting the snow.
Your experience has me a little worried though. It looks like you had yours installed in open water, rather than buried within your dam. Is that correct?

Z