DIALA, the largest tributary of the Tigris rises in the Per sian highlands near Merivan and, for a short distance south of Haleboja, forms the Perso-Iraq frontier. It joins the Tigris just south of Baghdad. After leaving the Jebel Hamrin the river runs over shingle deposits and is extensively used for irrigation. The principal canals are the Beled Ruz and the Khorassan on the left bank and the Khalis and its branches on the right. Baquba lies at the end of the Khorassan canal, 52 miles from the mouth. This region is extremely fruitful and the river is navigable as far as this point from December to April. The metre gauge railway operat ing from Baghdad to Kirkuk runs along the valley of the Diala and offers a further outlet for the development of this well watered region.