[thedoc (OP)]

Pinchas Goldberg  asked the Naked Scientists:
   
Hi
 
I listen to you every Friday morning here in Johannesburg, and find it very interesting.

How do they measure the amount of water flowing over a water-fall?

Yours
Pinchas Goldberg

What do you think?

[syhprum]

You measure the average depth of the river flowing over the falls also the speed at which it is flowing.
From this you can calculate the volume of water flowing over the falls in a given time

[alancalverd]

Easier to move upstream a few yards, where the flow is probably less turbulent. You need to measure the depth/speed profile at several points because the flowrate varies - there are even back-eddies to consider - but the answer will generally  be more precise than any attempt to measure close to the falls.

[evan_au]

These parameters can be a bit tricky to measure, because:

• Natural rivers don't have a rectangular cross-section
  
• The water does not have a uniform speed, but flows more rapidly the further you are from the riverbed and riverbanks

I have heard of people:

• Working out the average depth of a shallow river by wading across the river with a measuring stick, and recording the depth at regular intervals across the river (sonar would work better on a larger river).
  
• Working out the average speed of the river by dropping a float into the water, and seeing how long it takes to pass two points. If you simultaneously drop a number of floats across the width of the river, it will form a shape like a parabola, downstream.