I think impounded waters equates to normal pool level. Which would be 915. Flood pool and winter pool levels are different. But they are not going to change the regulation to what ever that days pool level is.
Im pretty sure you would get a warning. But if you're caught there again, you have no justification. Rule of thumb was lakes always extended to the first riffle. But that's not always the case either. And of course how would one know where the lake level mark would be, without it being marked.
But as for the James, it doesn't matter what the water level is or how far up you can run a boat. There is a sign on the Y-Bridge clearly marking the beginning and ending of the lake. And this may be why they put up a sign. And I think there is another one on the White river, or some type of marker. On the Kings, the first riffle or two, are in Missouri or right at the state line, depending on what the level is.