Well, that's depressing: a new report from Environment America ranks the Big Sioux River as having the 13th largest amount of toxic industrial pollution discharges in 2010. The sole discharger into the Big Sioux measured in this report is Smithfield Foods, John Morrell's processing plant in Sioux Falls, which dumped 2.95 million pounds of yuck into the public waters past the Falls.
In all of the surrounding states, only Nebraska has one facility causing more harm to waterways... and that's the Tyson Fresh Meats processing plant pouring 4.62 million pounds of pollution into the Missouri River. I'm assuming this source is the Dakota City beef facility, for whose toxic discharges Tyson paid the feds a two-million-dollar penalty in 2009.
Surprising to me is the fact that even with all that Bakken-frackin' oil activity, North Dakota saw much less toxic discharge into its waterways than did South Dakota, under a million pounds. Maybe South Dakota state government is putting us more at risk of polluted waterways with its pressure on local governments to approve giant hog farms than with its push to drill for oil.