 At the western edge of the fell lies Devoke Water, which claims the title of largest tarn in the Lake District.It lies at an altitude of 235 m (770 ft) and is approximately 1 km long (east-west) and 0.4 km wide.The southern border between Birker Fell and Ulpha Fell traces a line between Devoke Water and the valley of Crosby Gill, a large stream that drops down the southern flanks of the fell to the village of Ulpha in the Duddon Valley.