A mixed-size sediment transport model using only two fractions, sand and gravel, offers practical advantages in terms of the required input and conceptual advantages by readily incorporating the nonlinear effects of sand content on transport rate. A two-fraction model is developed from flume and field transport observations. Sand and gravel transport rates are collapsed to a single transport function using a scaling parameter representing incipient motion for each fraction. The incipient motion function is consistent with established values in the limit of pure sand and gravel beds and shows a sharp change with increasing sand content over the transition from a clast- to matrix-supported bed. A two-fraction model captures an important and previously unmodeled nonlinear effect of sand content on gravel transport rate and suggests important implications for a number of fluvial processes, including the response of channels to sediment supply and the mechanism for abrupt sorting in gravel/sand transitions.