Tolkien puts fords practically at any place he needs them, but in most cases in
rivers that we MIGHT imagine have periods of low flow allowing at least
seasonal fording. Judging from his map, some of his fords are a real stretch to
accept. But the lower Anduin!
Anduin in its lower reaches is a continental River, akin to the Mississippi,
the Nile, the Yangtze, or the Rhine. The idea of fords at Osgiliath is
laughable. Fordable streams are by definition a foot deep or two at most, over
a hard bed. Anduin would need to be several miles wide at Osgiliath to be
fordable; an absurdity.
Nevertheless Tolkien is quite definite in his determination, so we must accept
it.
I would guess he needed fords there because of the unliklihood of bridges being
maintained on a border between two hostile military powers, yet he wanted the
City to be a natural place of commerce and the most credible place where an
attack could cross the river.
No fords are ever mentioned at Cair Andros, where roughly the same conditions
apply as at Osgiliath, with the added complication that the current is if
anything swifter and deeper where an island obstructs the flow.
Why is Cair Andros a natural crossing point, military base and fortification,
as he insists it is? We might say, taking his note about the marshy land
further north, that the river banks opposite and on Cair Andros are flattened
and dry, giving excellent wagon and horse transportation right down to the
water's edge. The current may be swifter, but the distance is halved.
Communication across is easier. Possibly bows are effective across the river at
this point, although I doubt it.
Other Fords mentioned, with greater or less credibility, are: Anduin in the
upper vales at the Carrock; Bruinen (THE Ford of Frodo's flight); Isen (clearly
described in TTT); Snowbourn at Edoras and at Dunharrow; Swanfleet at Tharbad;
Ciril, and Gilrain, in southern Gondor; and of course Sarn Ford in the
Southfarthing.
Tolkien is sparing with his bridges, as is proper in a world that is in decline
as far as imperial engineering goes. Osgiliath had many bridges like most
European capitals (again less likely with Anduin than with the Thames or
Seine!), but all seem to be broken, if repairable. Aside from Morgul Vale, no
other specific bridges are mentioned in the South country -- although their
survival in abandoned Ithilien suggest that Gondor has others in its various
uplands.
There was a great bridge at Tharbad, but it is ruined. Saruman employs his
bridge over the Isen gorge to great tactical effect. Hoarwell and Baranduin
each have bridges, survivors from the realm of Arnor. Rivendell has a defensive
bridge along the same lines as the one at Khazad-dum: narrow and uncurbed.
There are other bridges mentioned but they are not great, simply crossing small
streams.
He used often to say there was only one TORn; that it was like a great river: its springs were at every doorstep, and every path was its tributary. “It’s a dangerous business, Frodo, starting a new topic,” he used to say. “You post it on the board, and if you don’t keep your feet, there is no knowing where you might be swept off to. Do you realize that this is the very site that's read by Ph. D's and 70% women, by non-book viewers and purists, gamers, newbies and lurkers, and that if you let it, it might take you to furious arguments, fluffy hugs, virtual cocktail hours or even further and to worse places?”
