E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Persons dwelling on opposite sides of a river. Forsyth derives these words from the Latin rivalis, a riverman. Cælius says there was no more fruitful source of contention than river-right, both with beasts and men, not only for the benefit of its waters, but also because rivers are natural boundaries. Hence Ariosto compares Orlando and Agrican to two hinds quarrelling for the river-right (xxiii. 83).