I have made major changes in MILHY2 to improve the predictive capability of the downstream conveyance estimation, specifically under out-of- band conditions. Therefore, turbulent exchanges of momentum between segments of flow in a cross-section and multiple-routing reaches have been incorporated. However, the overall structure of the MILHY3 code remains unchanged since MILHY2. All new computations occur within existing subroutines, some of which have been extensively rewritten. Keywords: Flooding; Computerized simulation; Computer programs. (KT)