I'll add. Salination of the river is likely a greater risk. But still small.

As rivers tend to have flow from high land too low. As they enter the ocean. The positive flow (current) prevents the brine area going to far into land.

Because canals tend to only move water with navigation. At each lock there is less, preventing salt water mixing further up where the canal joins the river.

Of course this is to some extent expected and modern canals can limit it. Simply by providing a current from the river going through weirs at each lock.

Exactly as navigational canals used to move water do.