谷 shows water splitting and flowing down between mountains (the forked strokes on top) with 口 below as the valley mouth where the water drains out — the place where water runs between hill and hill, a valley. One reading sees water (水) half-sinking into the ground, only partly visible.

Mandarin: gǔ, 3rd tone. 山谷 (shāngǔ, mountain valley), 峡谷 (xiágǔ, gorge / canyon), 谷歌 (Gǔgē, the Chinese transliteration of "Google"). Note that the grain character 穀 merged into 谷 in simplified Chinese, so modern 谷 carries both "valley" and "grain" — 五谷 (wǔgǔ, the five grains).

Japanese: on-reading コク (koku), kun たに (tani). 谷 (tani, valley), 渓谷 (keikoku, ravine), 谷間 (tanima, valley floor). It is extremely common in Japanese surnames and place names — 谷 (Tani), 長谷川 (Hasegawa), 渋谷 (Shibuya, the Tokyo district).

Korean hanja-sound "gok": 溪谷 (gyegok, valley stream), 峽谷 (hyeopgok, gorge), 幽谷 (yugok, deep valley). 栗谷 (Yulgok), the pen name of the scholar Yi I, uses it too.

Memory aid: water forking down between mountains (八) toward an open mouth (口) — a valley.