A proverb meaning that people or things that are quiet or unassuming often have deep character or ability (equivalent to 'still waters run deep').
彼は普段はあまり話さないが、仕事は確実だ。まさに深い川は静かに流れるという人だ。
He doesn't talk much, but his work is reliable — truly a case of still waters run deep.
若い頃は目立たなかったが、年を経て力を発揮した。深い川は静かに流れるということわざを思い出す。
He wasn't conspicuous when young but showed his strength with age. It reminds me of the proverb 'still waters run deep'.
教師は外見で判断してはいけない。深い川は静かに流れるように、本当に優れた人は落ち着いていることが多い。
You shouldn't judge teachers by appearances. Like still waters run deep, truly excellent people are often calm.