If your place is like mine I'd think you might need to. When I had my old small pond worked up a few years ago (It held no water at the time) It started catching water higher than I'd ever seen it in my lifetime, but the water found it's way out in the sides of the basin around some old sycamore trees lining the pond. Over time, the water fell more rapidly after each rain, until a virtual cavern had eroded in the side of the basin down into the limestone bedrock. The water pops out of the ground some 100 yds below the dam, just like it was running in a pipe, circumventing the entire dam.
Not only do I have the karst caverns below, the problem with some clays formed over limestone bedrock (my clay) is that it tends to form in small natural clumps which don't readily seal without compaction. The best way to compact these clays is with a sheepsfoot roller (I know everyone's getting tired of hearing about sheepsfoot) because it works from the bottom of the lift up kneading and packing the clay clumps. This is stuff I've learned from soils people of course, and I've seen this type of soil structure in my own basin. I don't know what your terrain looks like or what equipment you have available, but what I'd do is this. I'd remove all vegetation and topsoil down to 6 inches. I'd go in with a large tractor and disk the subsoil down to 6-8 inches. I'd go back in with a sheepsfoot roller and pack the ... out of it. Then I'd push the top dirt back over it. Since I have a dozer I'm going to cut all the soil in my basin down a couple of feet and pack it back in in 6" lifts with a, you guessed it, sheepsfoot. Granted, mine's not as big as your's. I dont know if your a do-it-yourselfer but you can get a decent, old track loader for around $16,000, keep it a year or two and sell it. Good luck. I'd like to come check it out sometime, when it's too wet to work on my own!
