The stage of a stream is the height of the water surface above an established datum plane. The water surface elevation, referred to some arbitrary or predetermined gauge datum, is known as the gauge height. Gauge height is often used interchangeably with the more general term ‘stage’ although gauge height is more appropriate when used to indicate a reading on a gauge. Stage or gauge height is usually expressed in metres and hundredths or thousandths of a metre depending on the accuracy required.