Reductions in river flows associated with hydrologic changes from diversions may reduce water elevations and impact hydrology of riparian wetlands. The U.S, Army Corps of Engineers and other federal agencies currently do not have appropriate tools to evaluate effects of hydrologic changes on wetlands. We describe use of ttEC-RAS modeling to predict and quantify wetland impacts along 10 kin of the Greybull River, Wyoming, USA following a proposed 28,313 L/see diversion for a new off-channel reservoir. Riparian wetlands were delineated and mapped on aerial photographs and topographic maps. A water-surface-profile model of the river was developed using the HEC-RAS model. Floodplain boundaries with and without the diversion were plotted to the same scale and overlaid on the wetland maps to estimate area of wetlands that would not be flooded under the reduced flow model. The model predicted that total area flooded under the 28,313 L/sec diversion in the reach analyzed would be reduced from 167.2 to 149.7 ha and that approximately 0.47 ha of wetland would be impacted by the diversion. We believe that the procedure we used is a valid method to quantify effects of diversions on riparian wetlands and that this procedure should be further evaluated as a method to assess wetland impacts on similar projects.