Purity: Extent of Impurity in Flowing Water Connected to a Spring

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 28

If an impurity makes contact with flowing water, the amount of flowing water that changes in smell, colour, or taste by means of the impurity is impure. Flowing water that is connected to a spring is pure even if it is less than kurr; and if the water that is on the other side of the stream is equal to kurr or it is connected to the spring by means of water that has not changed, it is pure; otherwise, it is impure.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)