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.
It seems clear that the first existing name was Mattaponi, and since the 4 feeder rivers are close together, the syllable names were chosen for the 4 streams, south to north. The Matta (and especially Poni) Rivers look pretty short on the map.
I just read this in the Wikipedia article on the Mattaponi River and it really tickled me. If only all language were so rational!