The greater of two tributary streams that came together to form the long river Gelion. The Greater Gelion rose in the foothills of Mount Rerir at the northern end of the Blue Mountains, and flowed southwestward for some hundred miles, marking the eastern border of the lands under Maglor's control. Roughly halfway along its course it was joined by a tributary of its own, flowing out from Lake Helevorn. These combined streams ran on through the lands southward of Maglor's Gap, until they met the Little Gelion that came down from Himring to the northwest. The combined waters of the Great Gelion and Little Gelion thus gave rise to one of the great rivers of Beleriand, with the full river Gelion flowing on for hundreds of miles to meet the Great Sea in the distant south.