I believe there where three rivers which they damed hence the three rivers gorge name. So, if it is the dam that's collecting all that trash then that means a whole lot of it was floating out that river to who knows where in the world. There were several villages and numerous homes relocated from the rivers during construction so there must have been even more trash before the dam.
One thing is for sure, WE as a WORLD need to get a handle on this trash thing. This kind of behavior is not sustainable and looks nasty.