The river rises, as the Trefechan Brook, high on the Ruabon Moors in the area known as Newtown Mountain, before flowing through a rather steep valley to two reservoirs above Penycae. At this point it is an important feeder for Dee Valley Water, formerly Wrexham Water. At Penycae, it is joined by the Nant y Crogfyn stream, which also flows down from the slopes of Ruabon Mountain. The Eitha's course then runs south-eastwards to Ruabon, where it was formerly used to power several watermills, passing through the centre of the village. From Ruabon the Eitha passes southwards through a deep, wooded valley within the Wynnstay Estate to join the River Dee near Chirk.