Getting the dam higher such that water will run around it, not over it, during flooding would be my priority (feet of free-board above gently sloping emergency spillway in virgin soil). Just dumping more dirt on top is unlikely to work well. Any topsoil on the top of the dam really needs to be removed, and the core of the dam raised in elevation in compacted layers. I know this is not what you want to hear, but if you can swing the cost now, it will likely save you money in the long run. It would also be money well spent to lower your water level and install the drain pipe properly now. On my second pond, I went through 2 contractors before hiring folks to fix problems which included replacing a poorly installed drain pipe and raising the dam height. In my case, I did this before the pond filled and I have a worry-free pond. I had not budgeted for this, but things don't always go as planned, and there are a lot of dirt pushers out there. Finding a pond builder is a lot harder. I built my 3rd pond myself but it has yet to be tested, so I cannot claim success yet.
