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Mahoning (məhōn´Ĭng), river, c.90 mi (140 km) long, rising in NE Ohio, E of Canton. It flows northwest to Alliance, then northeast past Warren, where it turns southeast to flow past Youngstown into NW Pennsylvania and joins the Shenango River to form the Beaver River. The river drains a fertile valley. Berlin Dam (completed 1943), on the upper Mahoning, provides flood control and water supply.