Parah, Chapter 6, Mishnah 5. This chapter deals with the ways in which the water intended for the mei chatas can be spoiled, and our Mishnah takes up a case in which the water is never properly gathered at all.
"Hamafneh es hamayim lesoch hagas o lesoch hagevim, pesulim lezavim, v'limtzora'im, u'lekadesh bahen mei chatas, mipnei shelo nitmalu b'chli." A person turned the flow of a spring aside, channeling it into a winepress or into pits and cisterns. That water is posul for the purification of a zav, and for a metzora, and for sanctifying the water of the chatas, since it was not filled into a vessel.
The reason is straightforward once we recall the requirement. Water for these purposes must be gathered in a keli, a vessel. Here the man dug a channel from the spring and led the water off into a winepress or into cisterns, interrupting its natural course, so the water came to rest in something that is not a vessel at all. A winepress hewn in the ground and pits dug in the earth are simply not kelim. Therefore the water is of no use for a zav, for a metzora, or for the sanctification of the mei chatas.
It makes no difference that the water genuinely came from a spring and reached those places directly from it. Spring water is precisely what these purifications call for, but living water alone is not enough: it has to be drawn into a vessel. Had the winepress and the cisterns carried the status of kelim, the water in them would indeed have been fit, since it arrived from the spring. But they are not vessels, and so the water they hold can never be sanctified as mei chatas, because sanctification requires a vessel.