Answer:In the name of Allah, Most Compassionate, Most Merciful,
Assalam’aleykum, I pray this finds you in the best of states.
Jazakum Allah khayr for your question!
In the Hanafi school flowing water will not become filthy except if traces (smell, color, taste) of the filth become manifest in this flowing water. [Nur Al-Idah] This is because the filth will not stay where it has fallen when the water is flowing and thus, when there are no traces of this filth in the flowing water, we can be sure that the filth is not here anymore. [Binaya]
As for a small quantity of still water (ie a body of water the surface of which is less than 5 meters square) even a small quantity of filth will make this water filthy, even if the traces of the filth are not manifest. [Nur Al-Idah]
This is because the Messenger of Allah (Allah bless him and give him peace) forbade us to put our hands in a container of water upon waking up before washing our hands three times first. [Bukhari] If there is such a command for a situation where the presence of filth is only possible, we can deduce that if the presence of filth was certain then the water affected by it will be filthy for sure. [Inaya]
And Allah knows best.
I hope that the above answers your question. If something is unclear please do not hesitate to ask again.
May Allah Most High grant you the best of this world and the next. Please keep us in your du’as!
[Ustadh] Sufyan Qufi