A numerical model, HEC-6, was used to help determine the effects of the proposed Passaic River Tunnel Flood Protection Project in northern New Jersey. The modeling effort was one of the largest undertaken using HEC-6. A branching and closed loop network of 406 cross sections that included several locations in which reverse flows occur were some of the complicating factors that were dealt with in this study. The results of the study showed that the tunnel operation will not adversely affect sedimentation patterns in the network and should actually decrease the rate of present channel degradation in some reaches. Aggradation in the vicinity of Two Bridges and the Spur Tunnel Inlet could be a problem, which might require periodic maintenance. (MM).