Þjórsá is a glacier river and has its source on the glacier Hofsjökull. It flows out through narrow gorges in the highlands of Iceland. Further downstream, another river, the Tungnaá, flows into it (see also Háifoss), before it enters the lowlands. There it passes the valley of Þjórsárdalur (Thjorsardalur) where the historical farm of Stöng is located. In the lowlands it flows along the eastern border of the Great Þjórsá Lava.