In 2002, Peter Wilcock and Kenworthy T.A. , following Peter Wilcock (1998), published a sediment bed-load transport formula that works with only two sediments fractions, i.e. sand and gravel fractions. Peter Wilcock and Kenworthy T.A. in their article recognized that a mixed-sized sediment bed-load transport model using only two fractions offers practical advantages in terms of both computational and conceptual modeling by taking into account the nonlinear effects of sand presence in gravel beds on bed-load transport rate of both fractions. In fact, in the two-fraction bed load formula appears a new ingredient with respect to that of Meyer-Peter and Müller that is the proportion formula_93 of fraction formula_103 on the bed surface where the subscript formula_104 represents either the sand (s) or gravel (g) fraction. The proportion formula_93, as a function of sand content formula_106, physically represents the relative influence of the mechanisms controlling sand and gravel transport, associated with the change from a clast-supported to matrix-supported gravel bed. Moreover, since formula_106 spans between 0 and 1, phenomena that vary with formula_106 include the relative size effects producing ‘‘hiding’’ of fine grains and ‘‘exposure’’ of coarse grains.