I suppose even a great river like the Anduin can have fordable points if the
land levels out so that the river broadens and slows and cannot cut as
deeply. And the mention of marshes is consistent with there being a
fairly level region. But it's hard to mount an attack on a fortified
position (as Cair Andros must have been by the time the West got there) across
a ford. I think I like PhantomS's suggestion of capturing enemy boats
better. Particularly if they're anything like the landing craft in the
movie!
sic hobbitur ad astra.
(Entire text of a postcard received by JRRT in 1937, according to Letter #17)
