“…Interpreting the variation of channel geometry (Wolman, 1955;Knighton, 1975;Leopold and Wolman, 1957;Speight, 1965;Ferguson, 1975Ferguson, , 1976, evaluating aquatic habitats (Mathur et al, 1985;Bren, 1993;Johnson, 1994;Currier, 1995;Myers and Swanson, 1997), and studying the dispersion of tracers (Hays et al, 1966;Sabol and Nordin, 1978;Nordin and Troutman, 1980;Bencala et al, 1993;D'Angelo et al, 1993;Runkel and Bencala, 1995) 1252 J. A. MOODY AND B. M. TROUTMAN require knowledge of the marginal probability distribution of widths and depths, while flood (Fread, 1985;Rashid and Chaudhry, 1995;Wiele and Smith, 1996) and sediment routing models (Pickup and Higgins, 1979;Miller, 1994;Dietrich and Whiting, 1989) need both the marginal probability distributions and the proper length scales over which relatively small-scale variability can be suitably averaged to provide useful physical parameters for modelling.…”