It rises as four streams in Spotsylvania County, each of which is given a shorter piece of the Mattaponi's name:
- The Mat River and the Ta River join in Spotsylvania County to form the Matta River;
- The Po River and the Ni River join in Caroline County to form the Poni River;
- The Matta River and the Poni River join in Caroline County to form the Mattaponi River.
From the confluence of its tributaries, the Mattaponi flows generally southeast through Caroline County, where it collects the South River; in its lower reaches it defines the boundary between King William and King and Queen counties. At West Point, it meets the Pamunkey River to form the York River.
Over the past few years, residents of the counties surrounding Mattaponi River have hotly debated a proposed reservoir, with environmental groups such as the Sierra Club, members of the Mattaponi Indian tribe, and residents of King William County opposing Newport News Waterworks in their efforts to build the King William Reservoir.
- U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed April 1, 2011