Kos”iu, a river in the Komi ASSR, a left tributary of the Usa River (Pechora Basin). Length, 259 km; basin area, 14,800 sq km. It has its source in the Polar Urals and flows through marshy lowlands. The Kos’iu branches out in its lower reaches. It is fed by rain and snow. The average flow 45 km from the mouth is 120 cu m per sec. The river freezes over in late October or early November and thaws in May. It is navigable in its lower reaches.