a standalone program for analysis and modeling of river hydrology, flow profiles, floodplain mapping, and sediment transport
HEC-RAS, the Hydrologic Engineering Center's River Analysis System, is a free, publicly available program developed by the US Army Corps of Engineers.
HEC-RAS is used to run hydraulic calculations on river systems and structures. The program can calculate one and two-dimensional flow simulations under multiple flow profiles and structural scenarios. It can be used to project downstream effects of structural modifications, generate floodplain maps, and much more.
The program also contains built-in mapping tools, and can export simulation results in common GIS formats for use in other programs. There also exists a related ArcMap plugin, HEC-GeoRAS, to facilitate the transfer of information between programs.
More information, documentation, demos, &c, can be found at the Hydrologic Engineering Center website.