If water depth or velocity change abruptly over a short distance and the pressure distribution is not hydrostatic, the water surface profile is characterized as Rapidly Varying Flow (RVF). The occurrence of RVF is usually a local phenomenon.  RVF can often be observed near the inlet and outlet of culverts, and wherever hydraulic jumps occur. 

For more information on how FishXing approximates Rapidly Varied Flow please see the following topics: Inlet Contraction Velocity, Hydraulic Jump, Free Surface Outlet, Boundary Conditions, and Outlet Plunge