The Gadar River rises in the Iranian Zagros Mountains near the point where the borders of Iran, Turkey and Iraq meet. From its source, the river first flows towards the southeast and then changes course due east through the Ushnu-Solduz valley. After leaving the valley, the river turns north and flows into marshes bordering Lake Urmia. The length of the river is approximately , its drainage basin is variously estimated as and and its discharge is per second. The Ushnu-Solduz valley has been occupied since many millennia, as testified by the excavations at sites like Hasanlu Tepe and Hajji Firuz Tepe.