John Gill Commentary 2 Kings 3:22

"And they rose up early in the morning, and the sun shone upon the water, and the Moabites saw the water over against them as red as blood:" — 2 Kings 3:22 (ASV)

And they rose up early in the morning
To watch the motions of their enemies, and be upon their guard against them:

and the sun shone upon the water ;
with which the valley was filled:

and the Moabites saw the water on the other side as red as blood ;
so it appeared through the rays of the sun reflected on it.