A camel’s gait and pace can be conspicuous in the motion and bearing of its neck. The poet’s reference to the camels’ necks therefore selects a vivid sign of swift progress rather than merely naming an anatomical part.
That visible cue anchors the sense of caravan movement, which is then extended imaginatively to the valley floors. The expression is more precise than a generic statement of speed.
The necks enrich the image; the principal innovation remains the attribution of “flowing” to the ground rather than to the animals.