Alexandra Cousteau, granddaughter of legendary marine explorer Jacques Yves Cousteau, discusses her film The Nation's River, which looks at new threats to the Potomac River and what can be done to protect it. The Potomac provides water for 4 million people in Maryland, Virginia, and the District of Columbia. It starts its 382-mile journey to the ocean in the mountains of West Virginia, flows along fields and farmlands, and runs through the Washington, D.C. metropolitan area before it reaches the Chesapeake Bay. It used to be so polluted that is was once called "a national disgrace." Thanks in large part to the Clean Water Act, things have dramatically improved. Now the Potomac is threatened by polluted rainwater -- wastewater overflowing from sewers and agricultural waste. According to a report released by American Rivers on May 15, 2012, the Potomac is one of the the nation's most endangered rivers. Alexandra Cousteau took a 17,000 mile journey across North America to look at the state of rivers. The Potomac was the last stop on her journey.