The geometry of a reach is determined by the cross sections characteristics of the shape and volume of the channel. Dimensions are identified with respect to a single mark to enable the calculation of the local slopes. Cross sections are identified following the curvilinear abscissa of the channel (x-axis in length).

In general, it is chosen as the origin supply tank dam and orientation as the direction of flow. If the model is meshed or branched we need several long abscissa for the entire model.

The cross sections can be defined in three different ways: abscissa-elevation, width-elevation and parametric. The type of description may vary from one data section to another within a reach.