Mikvaot, Chapter Five, Mishnah Two
1) If it was made to pass over the outside of vessels or over a bench:
a) Rabbi Judah says: behold it remains as it was before.
b) Rabbi Yose says: behold it is like a mikveh, except that one may not immerse anything above the bench.
In this case, the spring was made to pass over movable vessels or over a bench.
Rabbi Judah says that the status of the spring has not changed. This means that the water still counts as “living waters” and even zavim, etc. can immerse in them. Such people can immerse in the spring after it has passed over the vessels or bench, just not in the part of the spring that directly passes over the vessels.
Rabbi Yose says that now that the spring has passed over vessels the status is that of a mikveh. There will need to be forty seahs for the person to immerse but these are not “living waters” so zavim and those who require living waters cannot immerse there. And one cannot immerse directly over the bench or vessels themselves because while the water is there it counts as drawn water.