It's part of Pittsburgh and Denver. It stretches from Alberta to New Orleans. It's the Mississippi watershed--one of the world's great river systems--that drains more than a million square miles of forests, farmland and cities in North America. The Mississippi watershed is home for a great variety of wildlife and a key waterway for shipping. But it also carries a heavy load of pesticides and other pollutants. And for the people who have to draw their drinking water from the river, the high concentrations of toxic chemicals are a cause for concern. As part of our series "The Thirst for Safe Water," Brenda Tremblay traveled down the Mississippi from St. Louis to New Orleans