tn The construction using הִנֵּה (hinneh) before the participle (here the Hiphil participle מַכֶּה, makkeh) introduces a futur instans use of the participle, expressing imminent future, that he is about to do something.
sn W. C. Kaiser summarizes a view that has been adopted by many scholars, including a good number of conservatives, that the plagues overlap with natural phenomena in Egypt. Accordingly, the “blood” would not be literal blood, but a reddish contamination in the water. If there was an unusually high inundation of the Nile, the water flowed sluggishly through swamps and was joined with the water from the mountains that washed out the reddish soil. If the flood were high, the water would have a deeper red color. In addition to this discoloration, there is said to be a type of algae which produce a stench and a deadly fluctuation of the oxygen level of the river that is fatal to fish (see W. C. Kaiser, Jr., “Exodus,” EBC 2:350; he cites Greta Hort, “The Plagues of Egypt,” ZAW 69 : 84–103; same title, ZAW 70 : 48–59). While most scholars would agree that the water did not actually become blood (any more than the moon will be turned to literal blood [Joel 2:31]), many are not satisfied with this kind of explanation. If the event was a fairly common feature of the Nile, it would not have been any kind of sign to Pharaoh—and it should still be observable. The features that would have to be safeguarded are that it was understood to be done by the staff of God, that it was unexpected and not a mere coincidence, and that the magnitude of the contamination, color, stench, and death, was unparalleled. God does use natural features in miracles, but to be miraculous signs they cannot simply coincide with natural phenomena.