The greater part of the northern border and a small section of the border with Pakistan are marked by rivers; the remaining boundary lines are political rather than natural. The northern frontier extends approximately southwestward, from the Pamir Mountains in the northeast to a region of hills and deserts in the west, at the border with Iran. The border with Iran runs generally southward from the Hari River across swamp and desert regions before reaching the northwestern tip of Pakistan. Its southern section crosses the Helmand River.