The delimination of international boundaries in rivers and lakes are open to some unique questions. The natural, physical qualities of those geographical phenomena, which, as a part of state territory, in contrast to the definition of territory, is not immovable but movable and is even naturally a fluid element, gives it its unique characteristics. There are geographical, cartographical, even mathematical problems which the legal world had to deal with while dealing with boundaries in rivers and lakes. The two main problems are the exact location of the line and the changeable character of every river and lake, which could change the location of the line. Part of the Iraki-Iranian border runs along the Shatt al Arab river and its history reveals problems concerning the delimination of a boundary in rivers.