Proper lake creation

I am trying to make lakes into my terrain, but I am failing to have some natural shape. If I dig hole into terrain, lake does not look naturally. So far I dint figure out, how to flood small part of terrain, or make hole into terrain according real water level. I can dig hole, but it never has shape according to surrounding terrain. It would be perfect to be able to setup something like “flood terrain on 531 metres in this area”.
Create water device without flow restructuralization produces nice natural lakes, but it is not possible to limit it via mask only on some locality (mask has no effect). Bug? If this worked, it could be solution.