If "waste water" in the article is the same as "storm water", then it is no big deal.
It's far from ideal, yes.
The river will flush itself out.
The real positive thing is that it isn't sewage.
Rain gardens are collection points to let the water to percolate back into the ground leaving the sediment in the garden and not reach bodies of water or waste water systems.
http://www.raingardennetwork.com/General Contractors, Civil Contractors, Fed and State DECs usually call them sediment ponds.
Look around newer larger parking lots, you will see some sort of sediment ponds.
Especially newer Walmarts.
They are to keep and treat all of that parking lot run off on their site.
Most of us call them black lagoons.