The length L of vegetated first order watersheds developed by fluvial erosion is related to the width W by L = W²/RU, where R is 3 for converging random walks of step size U, whose observed mean is 3 meters for normal dendritic drainage and 40 meters for basins over 300 meters long that drain directly into third or higher order streams. Bare soil watersheds show the relation L = 0.4 W.