I just read this in the Wikipedia article on the Mattaponi River and it really tickled me. If only all language were so rational!
- 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.
Makes sense, thanks for providing the explanation I didn't think of!