common method of measuring flow through an open
channel is to measure the height or HEAD of the
liquid as it passes over an obstruction (a flume
or weir) in the channel.
The velocity of a river
or stream can also be measured using spot velocity
meters such as turbine or doppler ultrasonics.
There are many types of open channel flow meters
River Flow Meters can be permanent stream flow measurement devices . Also available are Portable Area Velocity Flow Meters which use Submersible Area Velocity Sensors .