Oc·mul·gee River (?k-m?l?g?) A river, about 390 km (240 mi) long, of Georgia rising near Atlanta and flowing southeast to join the Oconee River and form the Altamaha 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.