The River Umber was a fast flowing river, full of cascades, that formed part of the natural border between Aglarond and Thesk. The river has its headwaters in the high glaciers of Thaymount and flowed west. At the town of Nethentir if flowed down the First Escarpment and into Lake Umber, and from there it flowed through the Dragonjaw Mountains into the Sea of Dlurg.