Did the Nile turn to blood?

Commentators are divided on whether the plague of blood has reference to literal blood. Stuart points out that the same Hebrew word is a synonym for the color red.

Duane Garrett has a 5-point argument that it isn't actually hemoglobin. For instance, he points out that the Egyptians used sand as a filtration device to make the river water drinkable. But that would be futile if it was hemoglobin. I agree with most of his arguments. But here's one I find more dubious:

Had the whole river turned to literal blood, it would have been a catastrophe of unimaginable proportions. The Nile in Egypt is almost 600 miles long. If it had all become literal blood under the Egyptian sun, the whole river would have become a thick, decaying sludge of biological waste. No potable water would  have been available for the entire population for months or even years. It is difficult to calculate how long it would have taken waters from the sources of the Nile far to the south in Ethiopia to wash away the tens of millions of gallons of blood as well as the coagulated and decomposing remains of that blood. D. Garrett, A Commentary on Exodus (Kregel 2014), 284-5. 

Although I agree with Stuart and Garrett that the miracle probably didn't mean God changed the water into hemoglobin, I don't think that's a good objection:

i) Does the account require the Nile, throughout the length of Egypt, to be affected? Contextually, the description is centered on a stretch of the Nile near the palace and thereabouts. Pharaoh and his entourage are the primary audience for this plague. To be sure, 7:20-21 describes the plague in comprehensive terms, but that's hyperbolic since most of Egypt is desert. 

ii) Even if it was more extensive, why assume that the plague is supernaturally produced but naturally resolved? If God supernaturally changes the water to hemoglobin (or whatever), the cessation of the plague might just as well or better involve God supernaturally changing it back to water. Miraculous contamination followed by miraculous restoration.

iii) Even if we grant for argument's sake that it wasn't supernaturally restored, the Nile is a dynamic system, not a self-enclosed lake. Not only is it flushed into the ocean from upstream, but I assume that in the Delta region the Nile is to some degree a tidal river, subject to coastal intrusion. So the "blood" would be diluted or replaced from both ends–provided that the affected area was fairly confined (i).