is carried may not be that significant in the overall calculation.
Some wild estimations
Lets say its 1000m long between locks or whatever, 30m wide, and was built 20m deep to allow a small cargo ship to squeeze through
Laden cargo ship that size per a quick bit of googling, might be 20,000 tonnes.
The water in the channel weighs some 600,000 tonnes, so what it carries is only likely to weigh 3%.