What is the ruling if the water has changed due to impurity?

Primary text

If any part of the water has changed due to impurity, all of it is impure, and its ruling follows that of the impurity itself. If only the still water is changed while the flowing water is pure, the flowing water is impure if it is less than two qullahs, but pure if it is two qullahs. If the flowing water is changed and the still water is two qullahs, it is pure; otherwise, it is impure. If only part of the still water is changed, and the unchanging part combined with the connected flowing water totals two qullahs, it is not impure because it is excess water that has not changed, and thus remains pure, similar to the case where the flowing water itself is two qullahs.