At McAlpine L & D, normal upper pool elevation is 420.0 feet (130.8 meters) MSL, equal to 12.0 feet (3.7 meters) on the upper gage. The vertical clearance at the Clark Memorial Highway bridge is 72.6 feet (22.1 meters) above normal pool. What is the clearance if the gage reads 10.6 feet (3.2 meters)?
Official USCG Answer & Rule Citation
Correct Answer: Option A — 74.0 feet (22.6 meters)
When the river gage drops 1.4 feet from normal pool (12.0 down to 10.6), the water level falls, increasing vertical bridge clearance by the exact same differential (1.4 feet). Adding 1.4 feet to the baseline clearance of 72.6 yields 74.0 feet.
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