Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.).