Nedarim 40b נדרים מ׳ ב

in the Euphrates River only during the days of Tishrei alone, as after the summer, when no rain falls, it is clear that the water in the river flows from the source of the river. During the rest of the year, rain and snow are mingled with the river water, and water from rain and snow purify one who immerses in them only when the water is standing.

The Gemara relates: Shmuel’s father fashioned ritual baths for his daughters during the days of Nisan and placed mats in the Euphrates River during the days of Tishrei. He fashioned ritual baths during Nisan due to the concern that the rainwater and snow water that accumulated in the river exceeded the amount of water that originated in the river’s sources. Since immersion in the flowing water would be ineffective, he fashioned ritual baths of standing water elsewhere for them. He placed mats during the days of Tishrei, when they immersed in the river itself, to prevent the mud on the riverbed from acting as an interposition that would invalidate the immersion.

Rabbi Ami said that Rav said: What is the meaning of that which is written: “And you, son of man, make for yourself implements of exile” (Ezekiel 12:3)? That is referring to a lamp, and a bowl,