Arghandab is a river in Afghanistan, about 400 kilometers (250 mi) in length. It rises in the Hazarajat country north-west of Ghazni, and flows south-west falls into the Helmand 30 km (19 mi) below Girishk. In its lower course it is much used for irrigation, under the control of the Helmand and Arghandab Valley Authority, and the valley is cultivated and populous; yet the water is said to be somewhat brackish. It is doubtful whether the ancient Arachotus is to be identified with the Arghandab or with its chief confluent the Tarnak, which joins it on the left out 50 km (31 mi) SW of Kandahar. The Tarnak, which flows south of Kandahar, is much shorter (length about 320 km or 200 miles) and less copious. The ruins at Ulan Robat, supposed to represent the city Arachosia, are in its basin; and the lake known as Ab-i-Istada, the most probable representative of Lake Arachotus, is near the head of the Tarnak, though not communicating with it. The Tarnak is dammed for irrigation at intervals, and in the hot season almost dries up.