If a stream gauge using a current meter consistently reads lower velocities than expected during a flood event, what specific error source related to the instrument's deployment is most likely causing this discrepancy?

The most likely specific error source related to the instrument's deployment causing a current meter to consistently read lower velocities than expected during a flood event is the accumulation of debris on the current meter's propeller. A current meter, also known as a velocity meter, measures the speed of water flow, often by counting the rotations of a propeller exposed to the current. During a flood event, the increased water levels and heightened velocities often dislodge and transport a significantly greater volume of suspended material and floating or submerged debris, such as leaves, twigs, sediment, and other detritus, within the water column. If this debris collects on or becomes entangled in the current meter's propeller, it physically obstructs or impedes the propeller's free and unimpeded rotation. This physical impedance causes the propeller to spin at a slower rate than the actual velocity of the water flowing past it would otherwise dictate. Consequently, the instrument registers a lower number of rotations per unit of time, which is then translated into an artificially lower and underestimated water velocity reading. This issue is directly related to instrument deployment because proper deployment and operation, especially during dynamic flood conditions, require diligent monitoring for debris accumulation and periodic clearing of the propeller to maintain the accuracy of velocity measurements.