Open-channel flow can be classified and described in various ways based on the change in flow depth with respect to time and space. The fundamental types of flow dealt with in open-channel hydraulics are:
* Time as the criterion
** Steady flow
*** The depth of flow does not change over time, or if it can be assumed to be constant during the time interval under consideration.
** Unsteady flow
*** The depth of flow does change with time.
* Space as the criterion
** Uniform flow
*** The depth of flow is the same at every section of the channel. Uniform flow can be steady or unsteady, depending on whether or not the depth changes with time, (although unsteady uniform flow is rare).
** Varied flow
*** The depth of flow changes along the length of the channel. Varied flow technically may be either steady or unsteady. Varied flow can be further classified as either rapidly or gradually-varied:
**** Rapidly-varied flow
***** The depth changes abruptly over a comparatively short distance.  Rapidly varied flow is known as a local phenomenon. Examples are the hydraulic jump and the hydraulic drop.
**** Gradually-varied flow
***** The depth changes over a long distance.
** Continuous flow
*** The discharge is constant throughout the reach of the channel under consideration. This is often the case with a steady flow. This flow is considered continuous and therefore can be described using the continuity equation for continuous steady flow.
** Spatially-varied flow
*** The discharge of a steady flow is non-uniform along a channel. This happens when water enters and/or leaves the channel along the course of flow. An example of flow entering a channel would be a road side gutter. An example of flow leaving a channel would be an irrigation channel. This flow can be described using the continuity equation for continuous unsteady flow requires the consideration of the time effect and includes a time element as a variable.