The Bronx is one of the five boroughs of New York City, and the only one that is located mostly on the mainland. It is conterminous with Bronx County, which was the last of the 62 counties of New York State to be fully incorporated. The Bronx River divides the borough into two; the hilly West Bronx and the flatter East Bronx. Although it is one of the most densely populated areas in the U.S., The Bronx devotes a quarter of its total area to unpopulated space such as parks and zoos. The borough is also home to one of the five poorest Congressional Districts in the U.S., The 16th, alongside many much more affluent areas. During the 1960s and 1970s The South Bronx in particular experienced a period of decline, but has recently experienced some signs of revival.