Mikvaos 5:6

If a wave of 40 seah got detached from the sea and fell on a person or on utensils, they are purified. Any place containing 40 seah is fit for immersing oneself and objects. One may immerse in trenches, in ditches, and even in a donkey track whose water mingles with water in a valley. Beis Shammai say that one may immerse utensils in a torrent (i.e., rainwater rushing down a mountainside) but Beis Hillel say that one may not do so. They agree that one may dam the torrent with utensils and immerse in it but the vessels used as a dam are not immersed through this.

Mikvaos 6:1

Any water that mingles with a mikvah is like a mikvah. One may immerse in the holes and clefts of a cavern as they are. One may not immerse in a cavern’s ditch unless it connects to the mikvah through an opening the width of a water skin’s spout. Rabbi Yehuda says that this the case when the wall between the ditch and the mikvah stands on its own; if it doesn’t stand on its own, then one may immerse in it “as is.”