Clear as mud is still pretty good compared to clear as the darkest night
1. If you want to make the lake's bank equal all the way around, then a spill way is a good idea. Since there's a few feet of elevation different, spillway is not needed. The lowest part of the lake will be your spillway. Just make sure to dig a bit deeper on the higher elevation end to create a nice equalized depth between the lowest elevation depth to the higher end. For the lowest end, if you don't want your bank to erode away heavily during a high rain event, best to line the bottom of the earthen spillway with a coconut mat (used in construction all the time), lined with a layer of compact earth, and then lined with a layer of 2" to 3" rocks as energy dissipation system to prevent slope erosion.
If you want to find out unique creek-like energy dissipation design, look at the local Department of Transportation and Natural Resource Department for creek embankment restoration for creek/spring restoration and beautification processes, utilizing energy dissipation methods. Rain garden is also used for slope erosion control for downstream pond owners as well when there's overspills occurring annually.
2. If you can drill down even deeper, and see where the clay will end, this will provide you the info on what kind of plants is good to replant in the lake. Plants that have deep boring roots, if you have have the large clay strata, will allow an escape for water to the lose soil below the existing clay layer later. So, it's all how much you are willing to drill and test how deep your clay layer really is.
Well, hope that mud is becoming a bit clearer..lemonade clear maybe?
