Þjórsá, a glacial river, the longest in Iceland, about 230 km, average flow about 400m3/sec. Its northernmost source is on Sprengisandur east of the glacier Hofsjökull and it collects most of its water from the eastern and southern sides of Hofsjökull and from the western side of Vatnajökull through the rivers Kaldakvísl and Tungnaá. Several waterfalls in the river, most of them in the interior. First bridge built in 1895 near Þjótandi.