River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs)

Discharge is calculated as follows:

cross sectional area of a river or stream   x mean velocity of water

  1. Describe three relationships shown on the diagram above e.g. as discharge increases, velocity increases


The wetted perimeter of a river refers to that part of the channel that is in contact with water. It represents the friction that slows down the river velocity.

The longer the wetted perimeter, the more friction between channel and water.



The hydraulic radius is a measure of efficiency in a river channel. It is calculated by comparing the wetted perimeter with the cross-sectional area of the channel.

Hydraulic radius = cross-sectional area / wetted perimeter

The higher the hydraulic radius, the more efficient the river.



  1. Make simple sketches of the diagram above
  2. Read 107 - 109 (Core Book): Discharge, stream flow and channel shape.
  3. Make detailed notes under the different headings.