For 10 points - name this river, the longest contained in Massachusetts, which at the end of its course separates Cambridge from Boston.
A famous marathon starts and ends in the same places as this river, but the river takes 80 miles for the trip.  The EPA aims to have the notoriously polluted water in the most populated areas swimmable by 2005.  The river rises at Hopkinton, Middlesex County, and meanders to the Atlantic Ocean, becoming navigable at Watertown.  Swampy land reclaimed at its mouth now forms an area called the Back Bay.  For 10 points - name this river, the longest contained in Massachusetts, which at the end of its course separates Cambridge from Boston.
A famous marathon starts and ends in the same places as this river, but the river takes 80 miles for the trip.  
Charles River