Making urban rivers 'swimmable' again

Rivers are the lifeblood of a landscape. And yet humans have altered them in disastrous ways. So, how realistic is it to think urban dwellers everywhere could once again swim in their waterways? Efforts to make the Seine "swimmable" for this year's Olympics draw on lessons from successful restoration projects in other parts of the world, as author and naturalist Sy Montgomery explains to The World's Carolyn Beeler.