*G* Guest...

L.H. the Humber River was fordable in Roman times, though that fact would likely surprise anyone looking at it now.

Having said that, both the Thames at London, & the Seine at Paris, & lots of other major cities I would guess also were, at some point in their history, as its often the case that large settlements spring up at the lowest fordable point of a waterway.

(while 'Ull is still considered something of a major city in U.K. - at least to those that know of it - its somewhat downriver of what was ever any fordable point).