Chad,
Welcome to the Forum. In the last two years, I've renovated two ponds myself very similar to yours. Maybe I can give you some pointers.
First, you need a plan. Decide on dimensions and sight it all in. At the same time, you need to drain the remaining water; either pump it out, knock a hole in the dam, or dig out the spillway. On one pond I dug out the old spillway, and on the other knocked a hole in the dam. Do what makes the most sense for you in that location. Clear out all trees and brush from the dam. Follow your plan, including attaching the new segment to the old segment. In doing this be sure to core out for the new dam and also rough up the soil on the existing dam especially where the new meets the old.
Since the rainy season is fast approaching (less than 24 hours from now I'm looking at possibly 25 inches), plan your work accordingly. Leave an open path for the water to drain throughout construction.
Perhaps the biggest problem you will face relates to the muck that is in your old pond. Muck is the accumulation of sediment and plant and other decaying materials that forms a substance that is almost impossible to work with. You can either track-hoe it out (very expensive and you have to find a place to put the muck), dozer it out (again somewhat expensive and still have to find a place for the muck), raise the dam and leave in place, or do what I did...and as far as I know no one else has done this...but it works spectacularly...turn the muck from a liability into an asset by pushing it with a ring of clay into an island that will eventually be submerged. This island then becomes deep water structure which attracts fish provided spawing beds and hang-outs. By combining this with raising the dam, you can really haev a uniques pond bottom that is very "fishey". Like I said, I've done it twice and it works spectacularly.
If you are doing the work yourself, a small dozer and a back-hoe front end loader will do the entire job. I have both and now have two beautiful ponds, where before had shallow mud puddles and cattle watering holes.
If you need any help, send me an e-mail and I'll do what I can remotely from East Texas. You may want to only do your planning work now and wait on construction for next spring, depending on your rain patterns. For me, construction season ends tomorrow with the first rain bands from Rita.
