وَأَمَّا الْمَاءُ الْجَارِي إِذَا وَقَعَتْ فِيهِ نَجَاسَةٌ جَازَ الْوُضُوءُ مِنْهُ إِذَا لَمْ يُرَ لَهَا أَثَرٌ؛ لِأَنَّهَا لَا تَسْتَقِرُّ مَعَ جَرَيَانِ الْمَاءِ.
Meaning: As for running water, if impurity falls into it, wudu from it is permissible when no effect of that impurity appears, for impurity does not remain settled while the water flows.
Flowing water is distinguished by movement and the identifiable effect of impurity. Where no relevant effect remains, the ruling differs from stagnant water.
If impurity leaves an observable effect at the point of use, the stated permission does not extend to that affected water.