Please check out what happened to the Animus River in Colorado.  The first part of the story is that the EPA and its contractors made an error, and a torrent of arsenic, cadmium and lead in millions of gallons of waste water from a closed mine flowed into the river (8/5/15). However, who put all that stuff there in the first place and never cleaned it up?  It was a mining company.  They extracted gold for their customers and ran off with the profits while leaving us with the cleaning bill. These are the "negative externalities" corporations would rather not have on the balance sheet.  And now, our river "bed" is all laced for us...