Or it could be that Buell didn't really try to find a ford, choosing to rebuild the bridge, because he was not in all that much of a hurry.
On the other hand, Nelson really wanted to move forward, and convinced Buell to let him.
Nothing about this movement supports that Buell was in a hurry.
Buell's orders to McCook were to move on to Savannah by fords if possible. If what happened after those orders were issued (on the 18th) was that McCook reached the Duck and started rebuilding the bridge then there are two options:
1) McCook did not obey orders to move on by fords if possible, despite Buell explicitly stating this.
2) The river was not fordable.
Meanwhile, Nelson's division (the 4th) reached the Duck around the 20th or 21st. Either Nelson wasn't with the division, or (despite really wanting to move forward) it took him a week or so to decide to go ahead.
There is ample evidence to the effect that the river was in a flood stage around the 20th and the subsequent days, and that (as noted, Nelson explicitly said) the river was falling around the period of the 26th-29th - which is the period when it was forded.
If the river was not fordable as of mid March, then everything everyone did and said hangs together; if the river was fordable but nobody bothered to find one, then we have problems with:
- McCook disobeying orders and not continuing on via fords.
- Ammen being under the impression the river was not fordable.
- The Confederates destroying bridges on a fordable river despite not needing to.
- The rain which took place in mid-March not resulting in the river becoming unfordable.
- Buell explicitly noting that many of the smaller streams were unfordable (but the Duck would have to be fordable despite smaller streams not being).
- Buell being willing to move at 10 miles per day or more when marching, yet "not being in a hurry", but when he did cross the Duck he then put in some of the fastest marching of the war with his rear divisions (which reached Savannah after a march of around 20-25 miles per day).
On top of all that, we have that Ammen saw the Duck "200 yards or more wide" after the river had fallen somewhat (it was 200 yards or more on the 28th, it had been said to be falling on the 27th). The modern Duck at Columbia is not more than about 70-80 yards.