The Rioni River is the principal river of western Georgia. It flows westward from the Caucasus Mountains to the Black Sea. The city of Kutaisi, once the capital of ancient Colchis, lies on its banks.
Known to the ancient Greeks as the Phasis River, it was first mentioned by Hesiod in his Theogony (l.340); later writers like Apollonius Rhodius (Argonautica 2.12.61), Vergil (Georgics 4.367) and Aelius Aristides (Ad Romam 82) considered it the easternmost limit of the navigatible seas.
The rioni were established for the first time in the 4th century BC by Servius Tullius: they were only four and they were called regiones (the plural of regio).
After the fall of the Roman empire and the decline of Rome as a cultural center, the population decreased and the political division in rioni was lost.
The limits of the rioni became more definitive and official in the 13th century: their number increased to 13 and it remained like this until the 16th century.