After the river system schematic is completed, the next step for the modeler is to enter the cross section data. Cross section data represent the geometric boundary of the stream. An accurate representation of the stream channel and the overbank area (floodplain) are absolutely necessary to create an accurate hydraulics model. Cross sections are located at relatively short intervals along the stream to characterize the flow carrying capacity of the stream channel and its adjacent floodplain. Cross sections are required at representative locations throughout the stream and at locations where changes occur in discharge, slope, shape, roughness, at locations where levees begin and end, at hydraulic structures (bridges, culverts, inline weirs/spillways, and lateral weirs/spillways), and closely spaced around stream junctions. Cross sections can be entered by hand in the Geometric editor, or they can be extracted from terrain data within a GIS (i.e. ArcGIS using the HEC-GeoRAS add in).