The acoustic Doppler current profiler is a velocity area method using a boat, launch or towed platform to traverse the river to make a single measurement of discharge normally for plotting on the stage-discharge relation. The ADCP collects measurements of velocity depth and position as it passes across the measuring section. The velocity is measured by the Doppler principle and the area is measured by tracking the bed to provide river depth and boat position. The method has therefore similarities to the moving boat method (Chapter 2) where a current meter is used instead of the acoustic Doppler profiler.