The Porter Brook rises from a series of springs among the sedge grass of Burbage Moor, the highest of which is close to the contour near Brown Edge Farm. Its source is just inside the Peak District National Park, to the north-west of the village of Ringinglow. It passes through Clough Hollow, and under Fulwood Lane, where it leaves the National Park. Nearby, at the junction of Fulwood Lane and Greenhouse Lane is a Rotary Club funded toposcope, Finder Cairn, a rotary orientation table which indicates the direction of local landmarks. Just before it reaches the bridge carrying Woodcliffe over it, the river is joined by Mayfield Brook, which rises at a similar level but a little further to the north. Before the junction, Mayfield Brook supplied power to Fulwood corn mill, the highest mill on the system. Soon the combined flow reaches Forge Dam, now a large lake with an island. In Sheffield, "dam" referred to the body of water, rather than the structure which impounded it. Mills used for grinding were known as wheels, and the tail race was known as a goit.