How is a canal different from a river?

The channel is artificially dug, while the river is formed by natural forces.
There are many differences. In this case, the canal can be both sluice and sluiceless (if the water levels are different). Sometimes the composition of the water can be different and therefore sluice canals are built for the passage of the ship from salt water to fresh water in order to prevent an environmental disaster.
If the river itself flows into salt water, then no action is taken – the ships go without obstacles.