The Gwryne Fawr rises at Blaen Grwyne Fawr on the southern slopes of Rhos Dirion in the Black Mountains and follows a southeastward course for several miles, its flow interrupted by the presence of Grwyne Fawr Reservoir, the only waterbody within this range of hills. Some way below the reservoir, Mynydd Du Forest clothes the sides of the valley. Though a public road penetrates the valley as far as the north end of the forest, the valley is very sparsely populated. It is only approaching the hamlet of Partrishow that the valley takes on a farmed appearance. The river now heads west along a valley whose alignment is guided by the presence of a geological line of weakness known as the Neath Disturbance.