Bentley provides thousands of engineers with HEC software, documentation, and support for a variety of modeling tasks, from floodplain hydrology to river analysis to GIS integration. These important programs are all included in the HEC-Pack:

Graphical HEC-1 for flood hydrology calculations
HEC-HMS for hydrologic modeling
HEC-RAS for river analysis
HEC-GIS (aka HEC-GeoRAS) for data sharing between HEC-RAS and ArcInfo

HEC-RAS is one of the solutions from HEC-Pack that models and maps the hydrologic and hydraulic characteristics of floodplains and rivers.