A Parshall Flume helps a user determine how much water is flowing in an open channel, creek/river/stream, canal, or non-full pipe.  

It does this by accelerating slow, subcritical flow to a faster, critical / supercritical state.  In doing this, a known relationship between how much water is flowing through the flume and the water level at a specific point in the flume.

This in turn allows a user to calculate the flow rate of the flume by taking one depth measurement - greatly simplify the calculation of the flow rate.