Before revision: A reach is a length of a stream or river, usually suggesting a level, uninterrupted stretch.Macfarlane, Robert, "Landmarks", Hamish Hamilton Press, 2015
After revision: A reach is a length of a stream , river, or arm of the sea extending up into the land,straight,They are traditionally defined by the capabilities of sailing boats, as a stretch of a watercourse which, because it is straightish, can be sailed in one reach (that is, without tacking). usually suggesting a level, uninterrupted stretch.Macfarlane, Robert, "Landmarks", Hamish Hamilton Press, 2015