Where did the dirt come from?In other words it really doesn't matter one way or the other. I think it would have been more realistic if they had placed some when it was first built, but, if they didn't then it is a moot point. I have to ask where all that dirt is coming from. Is it just run off? Curiosity though, why would cleaning it be beneficial in this application? Overall, less earth in the riverbed?
Simple. RoA has a concrete bottom but its banks are mostly "natural" land. In other words, dirt.
Rain plus dirt equals dirt going wherever the rain runoff carries it. In this case, into RoA.
And yes, they do put dye in the water to keep it brown enough that you can't see the track for the Liberty Belle.