What is Aquagium?

Legal Definition
A canal, ditch or watercourse running through marshy grounds. A mark or gauge placed in or on the banks of a running stream, to indicate the height of the water, was called "aquagaugium." Spelman.
-- Black's Law Dictionary
Legal Definition
I. e. aquae agium. 1. A water course. 2. A toll for water.
-- Bouviers Law Dictionary
Legal Definition
A waterway.
-- Ballentine's Law Dictionary