At its fullest length, the river runs from the lake Aursund near Røros in Trøndelag and runs into the Oslofjord at Fredrikstad. Major tributaries include the Vorma River, which drains Lake Mjøsa, joining the Glomma River at Årnes in Nes. The Lågen drains into Lake Mjøsa, collecting drainage from the large Gudbrandsdalen and significantly increasing the Glomma's flow.[3]