To Become Clear, To Become Transparent
intransitive verb, godan verb
Something with lucidity is clear. That's why this word is to become clear or to become transparent.
すむ
You learned this reading when you learned the kanji.
池の水、昨日は濁って緑色だったのに、今はすっかり澄んでいるじゃないか。一体何があったの？
The water in the pond was green yesterday, but it’s totally cleared up now. What happened?