It is often used in family and geographical names, where it is generally pronounced KAWA. Examples are 立川 TACHIKAWA, Rising-river and 小川 OGAWA, Small-river.
The Chinese added sandbanks to the river to indicate a bit of land surrounded by water. They drew the kanji in final form as 州. It retains the meaning of sandbank, and has the extended meanings of land surrounded by water and a separate political state. It is pronounced SU when used by itself, and SHŪ when used in compound words. 本州 HONSHŪ, main-state, is the name of the main island of Japan. 九州 KYŪSHŪ, nine-states, is the southern-most of the main islands of Japan. It took its name from the island’s nine ancient provinces.
The Chinese saw that if you squeeze a river you get water. They drew the pictograph for water therefore first as and finally 水. Used as a kanji by itself 水 is pronounced MIZU. Used with other kanji in compound words it is generally pronounced SUI, as in 水 分 SUIBUN, water-portion, meaning moisture. In what looks to be an exception to the pronunciation rules, 大水 big-water, meaning flood, is pronounced ŌMIZU, but in this case 水 water is actually a word by itself modified by 大 big, and is not part of a compound word.
