 Those few watercourses (mainly in the Thames catchment) which branch off a major channel and then rejoin it or another watercourse further downstream are known as distributaries or anabranches and are labelled (d).The list is (or at least will be when completed) essentially a list of the main rivers of England (as defined by the Environment Agency) and which includes those named watercourses for which the Environment Agency has a flood defence function.Difficulties arise otherwise in determining what should and what should not be included.