He was also asked—may Allah have mercy on him—about a large quantity of water whose colour changes through standing, or whose colour and taste change but not its smell: does it remain suitable for ritual purification?

He answered:

Praise be to Allah. Water altered by standing or by the place in which it collects remains pure and purifying, by agreement of the scholars.

As for a flowing river, if it is known to have been altered by an impurity, its water is impure. If pure and impure substances capable of altering it mix with it, and there is doubt whether the change was caused by something pure or impure, it is not judged impure on the basis of doubt alone. Generally, these large rivers are not altered by the channels along their banks. However, if it becomes clear that the water has been altered by an impurity, it is impure. If it has been altered by something that is not impure, there are two well-known scholarly views concerning its suitability for ritual purification. And Allah knows best.