Mo·non·ga·he·la River (m?-n?ng?g?-h??l?, -h??-) A river rising in northern West Virginia and flowing about 200 km (125 mi) generally north into southwest Pennsylvania, where it joins the Allegheny River at Pittsburgh to form the Ohio River.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.