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Rion (rēôn´), ancient Phasis, river, c.195 mi (310 km) long, rising in the Caucasus near the Mamison Pass, W Georgia, and flowing S and W past Kutaisi into the Black Sea at Poti. Its upper course is used to produce hydroelectric power. In its lower course it passes through the lowland of Mingrelia. The Ossetian Military Road follows the upper Rion valley.