If you live by or own property that is next to any watercourse, or you have a watercourse that flows through your property, you are a 'riparian landowner'. This means you have 'riparian' rights and responsibilities which have been established in common law for many years.
If the land on the other side of the watercourse is owned by a third party, then it is likely that this person is the ‘joint riparian owner’ for the length of watercourse adjacent to the boundary. Unless the landowners’ deeds show otherwise, it is always assumed that each person owns the area up to the centre line of the watercourse.