I'll call it impossible. Erosion would quickly wear down the edges, if you start out with a ragged edge like this you'll probably end up with a number of canyons until the water flow settles into one that happens to be the deepest. Also, you'll need to have as much water flowing into your lake as flows out of your lake. You can actually work that out mathematically if you really want to. iirc such massive "floods" did happen in earth's past, but they were very temporary and always left a heavily eroded canyon-y terrain behind.