Which option best describes the method used to determine flow in an open channel by measuring depth?

• A. Temperature probe
• B. Dissolved oxygen meter
• C. pH meter
• D. Weir height

Answer: (D)

Measuring depth to determine flow relies on a fixed hydraulic structure that links water stage to discharge. A weir provides a known relationship between head over its crest and the amount of water flowing over it. By recording the depth (the height of water above the crest) and using the weir’s rating curve, you can convert that depth into a discharge value. The other instruments listed (temperature probe, dissolved oxygen meter, pH meter) measure water quality and do not translate depth into flow. So the method described is using the weir height.