Parah, Chapter 8, Mishnah 10. This portion of the chapter examines which water sources qualify for the mei chatas, the water used with the ashes of the parah adumah. Our Mishnah names specific rivers and disqualifies them, each for its own reason.
The Mishnah opens: "Mei Karmeyon umei Pugah pesulin, mipnei shehein mei bitzim", the waters of the Karmeyon and the Pugah are unfit, because they are marsh waters. Marsh water is water that carries sand and mud along with it. As to where these rivers are, the Rambam and the Rosh understand the Karmeyon and the Pugah to be rivers in Eretz Yisroel, so that our Mishnah is speaking of four rivers altogether: the Yardein, the Yarmuch, the Karmeyon and the Pugah. The Aruch identifies the Karmeyon and the Pugah with the Amanah and the Parpar, the rivers of Damascus.
What is it about silt and grit that disqualifies such water? Two approaches are offered. According to the first, the solid matter floating in the water constitutes a chatzitzah: when the water is gathered up, the mud comes between it and the walls of the container, so the drawing is not valid. According to the second, once swamp water empties into a river its current becomes sluggish and the river no longer moves along on its own; and a stream whose flow has been choked off in this way loses the status of mayim chayim, living spring water, which is precisely what the mei chatas requires.
The Mishnah continues: "Mei HaYardein umei Yarmuch pesulin, mipnei shehein mei ta'arovos", the waters of the Yardein and the Yarmuch are unfit, because they are mixed waters. These rivers are disqualified not for their own quality but because unfit rivers empty into them and mingle with them. (The Yarmuch is a tributary that comes from the Transjordan side and joins the Yardein.)
The Mishnah then defines the category: "Ve'eilu hein mei ta'arovos? Echad kasher ve'echad pasul shenit'arvu", these are mixed waters: one fit river and one unfit river that flowed together. In such a case no water may be drawn from the point where they meet. "Sheneihem kesheirin venit'arvu kesheirin", if both rivers are fit and they flow together, the water remains fit. "Rebbi Yehuda posel", Rebbi Yehuda declares even that unfit.
Rebbi Yehuda's concern is a broader one: if we permit drawing water at the junction of two fit rivers, people will come to draw at the junction of two rivers where one of them is unfit, and they will not distinguish between the cases. The halachah, however, does not follow Rebbi Yehuda; where both rivers are fit, water may be taken from the place where they join.