I found this good tutorial on exporting the river cross-section and their reach to HEC-RAS using Terrain Module. There are many methods and procedures to draw the geometric data for HEC-RAS, but I like how this web page explains the concept of geometric data in the HEC-RAS model, such as River, Reach, and Cross-Section.
Some a good article from that website:
River networks define the connectivity of the river system. A river system is a collection of reaches, all oriented downstream. A reach is defined in HEC-RAS as starting or ending at junctions – locations where two or more streams join together or split apart. Thus, a river may be composed of one or more reaches. In Terrain, each reach a terrain feature.
Channel cross-section data is used in HEC-RAS to characterize the flow capacity of the river and floodplain. Cross-section data includes station-elevation coordinates, main channel bank stations, downstream and reach lengths.
Station-elevation data represent the ground surface at designated locations in a river reach. Cross-sections are taken perpendicular to flow direction both in the main channel and in the overbank areas. Bank stations separate the main channel area from the adjacent floodplain (termed the left and right overbank areas).
Reach lengths are used to define the distance between cross-sections and are used for energy loss calculations in HEC-RAS. Reach lengths are considered for the left overbank, main channel, and right overbank areas and indicate the flow path between cross-sections.
The Terrain Module will automatically calculate reach lengths and bank positions from left, right, and main channel features.