Your vessel is approaching the International Bridge on the St. Marys River. If the gage on the bridge, read from top to bottom, indicates 124 ft. and the IGLD (1955) is 600 ft., determine the actual vertical clearances between the existing water level and the lowest point of the bridge over the channel.

Correct Answer: Option A — 124 feet

Bridge clearance gages on waterways such as the St. Marys River are physically calibrated to read the actual direct distance from the water surface to the lowest steel of the span. The referenced IGLD datum provides the baseline, but the direct gage reading directly indicates the clearance.