The calculation is just the difference in elevation between the river's source and the river's confluence or mouth divided by the total length of the river or stream. This gives the average drop in elevation per unit length of river.
- Shaw, Lewis C. Pennsylvania Gazetteer of Streams Part II (Water Resources Bulletin No. 16). Prepared in Cooperation with the United States Department of the Interior Geological Survey (1st ed.). Harrisburg, PA: Commonwealth of Pennsylvania, Department of Environmental Resources (no ISBN).