Afghanistan shares its northern boundary with the Oxus River, known as the Amu Darya in modern geographical literature, which constitutes its northern boundary with the Central Asian republics, especially Tajikistan, Uzbekistan, and Turkmenistan. Historically, this river was the boundary between Greater Iran and Turan, the ancient Central Asian region. Today, it is a wide natural boundary that stretches for several hundred miles along the northern edge of Afghanistan.