The Toffaleti equation was developed for large, sand bed river, and does not perform well for coarse particles (Williams and Julian, 1989). Williams, D. T., and Julien, P. Y. (1989). "Applicability index for sand transport equations." Journal of Hydr. Eng., 115(11), 1578– 1581. But some large domain models include large sand-bed rivers as well as coarser tributaries. Because HEC-RAS requires user to select one transport function, these models have to use the same equation for both. HEC 6 addressed systems where Toffaleti was appropriate for down-gradient rivers that have coarse bed-load up-gradient, but integrating the Toffaleti and the Meyer-Peter Muller equations. This combined function replaces the bed component of Toffaleti with the MPM bed load equation which is more appropriate for coarse material.