From 谷 (たに) (tani, “valley, ravine, gorge”) + 川 (かわ) (kawa, “river, stream, brook”). The kawa changes to gawa as an instance of rendaku (連濁).
谷川 (hiragana たにがわ, rōmaji tanigawa)
谷川 (hiragana たにがわ, rōmaji Tanigawa)