The Rioni or Rion River (Georgian: რიონიRioni, Greek: ΦᾶσιςPhasis) is the main river of western Georgia. It originates in the Caucasus Mountains, in the region of Racha and flows west to the Black Sea, entering it north of the city of Poti (near ancient Phasis). The city of Kutaisi, once the ancient city of Colchis, lies on its banks. It drains the western Transcaucasus into the Black Sea while its sister, the Kura River, drains the eastern Transcaucasus into the Caspian Sea.
The term "pheasant" and the scientific name Phasianus colchicus are derived from "Phasis" and "Colchis", as this was said to be the region from which the common pheasant was introduced to Europe in ancient times (the ring-necked pheasants seen in the present day were later introduced from East Asia; see Common pheasant for details). It is said that "the failure of Kolkhis to emerge as a strong kingdom or to be maintained as a province of Rome has been blamed on the pestilential climate of the Phasis Valley, a situation remarked upon by travelers down to modern times, when the swamps were finally drained.".
The Rioni is the longest river wholly within the borders of Georgia. The river is 327 kilometres (203 mi) long, and its drainage basin covers about 13,400 square kilometres (5,200 sq mi). It starts on the southern slopes of the Caucasus Mountains at 2,960 metres (9,710 ft) above sea level.