Lewis Short
Rha, indecl., = Ῥᾶ
* The Wolga, on whose banks grew the radix pontica, Rha ponticum, rhubarb (Rheum Rhaponticum, Linn.), which thence received its name, Amm. 22, 8, 28 (cf. Cels. 5, 23 fin.).
Charlton T. Lewis, Charles Short, A Latin Dictionary