A line in long array where they wind betwixt green islands,
They take a serpentine course, their arms flash in the sun—hark to
 the musical clank,
Behold the silvery river, in it the splashing horses loitering stop
 to drink,
Behold the brown-faced men, each group, each person a picture, the
negligent rest on the saddles,
Some emerge on the opposite bank, others are just entering the ford—while,
Scarlet and blue and snowy white,
The guidon flags flutter gayly in the wind.
Like  Kipling,  Whitman brings a sense of journalistic prose to this poem, like a front-line reporter bringing us the latest war news. The description is a clear one – “brown-faced men” – skin dark and rough as would be if one were in the sun for a long time. 
 “Each group, each person a picture” – Here we get a sense of the uniformed soldier, the requirements of their station being one of looking their best at all times while in the ranks. 
 “The Negligent rest on the saddles” – Historically, and for anyone who is an equestrian, it is/was customary to dismount while the horse drinks. 
 The lifting of the weight from the animal’s back lessens the strain on their neck as they lower their heads and dip their mouths. Swallowing, too, is eased allowing the horse to consume greater amounts in a shorter time; something which can be vital for a moving army. Any soldier resting on their saddle would have truly been negligent in their duty and could easily be punished. In the end, Whitman provides us with a great deal of information in but a few poetic words… in many ways, the epitome of good writing.
Cavalry Crossing a Ford