The Book of Settlements explains the denomination of the river: " A man named Thorarinn, the son of Thorkell from Alvidra, the son of Hallbjorn Hordukappi, sailed into the estuaries of Thjorsa and did not remove the carved bull's head from the prow of his ship". The river Thjorsa is the longest and the second most voluminous river of the country (364 m³/sec.). It is 230 km (139 miles) long and succeeds river Olfusa by volume (373 m³/sec.). It is the natural border between the districts Rangarvellir- and Arnessysla with its northernmost tributary called The Spring Fed River.

The source of that river is situated a mere 70 km (42 miles) south of the end of the Eyjafiord bay in the North. The main course of the river is towards southwest, following the main tectonic fissure system of the country. The discharge area covers about 7530 km² and most its volume is run off water from the glaciers Vatnajokull and Hofsjokull. The greatest floods measured, which took place in 1948 and 1949, exceeded 3000 m³/sec. The river sometimes, but rarely, almost disappears for a while in the lower regions of its course, when the winter ice breaks up and creates natural dams.

The river transports a great volume of silt and gravel from the erosion areas of the central highlands, about 4,5 million tons per year. A few waterfalls decorate the river on its way to the ocean. Its largest tributary, which is larger than the Thjorsa River itself at the confluence, is Tungnaa. The glacial rivers of Iceland always were great obstacles for travellers, who had to use the ferries or ford them. All the fords of river Thjorsa are situated above its largest island, Arnes, one of which is in the central highlands on the longest highland route between North and South. There were several ferry points and the nearest farms to them were obliged to serve the travellers against payment.

The first bridge was built in 1895 and rebuilt in 1949. The bridge at the edge of the central highlands was built in connection with the construction of the first hydroelectric power stations on the tributary Tungnaa in 1973. River Thjorsa and its tributaries represent 27% of the hydropower of the country. There are several man made reservoirs in the central highlands to secure an even supply of water for the power stations the whole year round (try the link to The National Energy Authority). Seals have rookeries in the estuaries of the river, where they have been hunted from the beginning of the history of the country. Salmon, trout and char migrate upriver every summer and autumn and head for the spring fed- or the run off tributaries to spawn (see The Angling Guide).
River Thjorsa also decorates the landscape with its waterfalls, such as Gljufurleitarfoss (28 m), Dynkur and furthest north Hvanngiljafoss, Thjofafoss,  Trollkonuhlaup and Urridafoss one of the most voluminous waterfalls in Iceland