Why didn't Morgulduin pollute the lower Anduin?

[The Mouth of Sauron]

As they left Henneth Annun Frodo and Sam were warned by Faramir not to drink any water emanating from the Morgul Vale. Yet the Morgulduin flowed from there into the Anduin at Osgiliath. So was the lower Anduin polluted?

[Inziladun]

Hm. I would still go with what I said here, with the "pollution" being less along the lines of Sharkey's men in the Shire, and more like the spiritual decay emanating from Minas Morgul.

[William Cloud Hicklin]

Fortunately, the element Morgulium has a half-life of only about 20 minutes.

[Zigûr]

Water was one of the substances of Arda with the lowest concentration of "Morgoth-element", was it not? Perhaps the spiritual corruption of the Morgulduin (I always saw it as a "magical" evil rather than, say, the physical pollution from the New Mill) was too dilute by the time it reached the Anduin proper.

[Mithalwen]

I suppose normally one would wish to take drinking water from as near the source as possible an d given the choice would go for the tributary rather than the main river that has been sewer and laundry as well as tap for every community and their beasts along its great length. However here I think it would aswell as being dilute would also have the benign influenceof Ulmo and his vassals to counteract the malign one of Sauron. Ulmo was the Valar most emgaged with Middle Earth and even in Mordor Frodo and Sam managed to find water to sustain then even if it was not pleasant. Maybeva small example of divine influence?

[Tuor in Gondolin]

Perhaps the power of Ulmo was able to restrain the Morgulduin influence on the Great River. In Unfinished Tales Ulmo says to Tuor noting his (Ulmo's) ebbing
power in streams near Thangorodrim:

...Doom is strong, and the shadow of the Enemy lengthens; and I am diminished, until in Middle-earth I am become now no more than a secret whisper. The waters that run westward wither, and their springs are poisoned, and my power withdraws from the land

But, of course, his power is based in ocean deep waters and so would naturally extend up the broad Anduin estuary. Sort of like Sauron's noxious gases doing battle with the west winds and swirling around the borders of Mordor during the War of the Ring around Minas Tirith.