Naval interdiction of river crossing

[USSLockwood]

I assume that gunboats will interfere with
an attempted river crossing but I can't find
any reference in the manual. Am I correct
and how effective are gunboats at blocking
a river crossing?

[Gunnulf]

They should be 100% effective i think. I.e. if an enemy gunboat is on a stretch of navigatable river it will block troops trying to cross. You will need to use your own gunboats to clear them out. Only a single gunboat per section is required.
However some sections of river can freeze in winter I think but can't remember offhand exactly what that critia is and where.

[moni kerr]

You need 4 elements of combat vessels to block a river crossing. They cannot be on passive.

[Jim D Burns]

ORIGINAL: moni kerr

You need 4 elements of combat vessels to block a river crossing. They cannot be on passive.

Actually it varies. Hover your mouse over a river region and it tells you how many combat elements you need. Some rivers need up to 12 elements.

Jim

[Ace1_slith]

Jim, moni kerr is right. You need 4 non-passive elements for 90% chance of blocking crossing. That is the maximium blockade percentage, so there is always a chance someone will get through.
The tooltip you were refering to describes how many elements you would need to blockade a port in that river region.