Very wide. It can be up to 7 miles wide.
The correct name for the 'River Mersey' is 'The River of Mercy' given at the time of slavery when the so called cargo was deemed as sick or the prices needed to be increased due to an over supply....
the widest recorded river is 30 miles wide!!!
That all depends on the river: It's size, what river it is, and the location of it.