[piasa]

It could be dust from bare red soil if there has been drought an dust storms upstream. Or it could be mud from red clay if there's been flooding. Any areas out there with soil the color of Georgia clay?

Or it could be something leached out from layers of ground that had long been dry before being inundated by the construction the dams.

[Zhang Fei]

I'm no expert, but a quick Google search yielded this:

Much of the soil is acidic red clay, but irrigation and heavy fertilizer use - both organic, and chemical - have earned Chinese farmers high yields. The highest grain yields in the country come from the south, for example the Sichuan basin and the lower Chang Jiang (Yangtze River) valley. Non-staple crops include cotton and tea; potatoes and wheat are grown in the hilly areas.