How is impurity handled in moving water according to the view of Al-Qadi and his companions?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
Every segment (jar'iyyah) of running water is evaluated based on its own condition. If impurity is flowing with the water, the segment ahead of it is pure because the impurity has not reached it, and the segment behind it is also pure because the impurity has not reached it. The specific segment containing the impurity is judged pure if it amounts to two qullahs, unless it becomes changed by the impurity; if it is less than two qullahs, it becomes impure. If the impurity is stationary at the side, bottom, or in a depression of the river, then every segment passing over it is impure if it is less than two qullahs, but pure if it reaches two qullahs, unless it changes due to the impurity.
Supporting text
The segment (jar'iyyah) encompassing the impurity and the immediately surrounding water (ahead and behind) that would typically be affected by its spread is considered the basis for judgment. If the impurity is spread out, each part of the extended impurity is judged by the volume considered for a small impurity in stagnant water, and the entire width of the river aligned with the impurity is not treated as a single segment. This is done to prevent a small impurity from contaminating a large volume of water or to prevent a large impurity from failing to contaminate a large volume, as the area aligned with a large impurity is extensive, while the area aligned with a small impurity is limited. The ruling equates the treatment of extensive impurity with that of small impurity, arguing that the Lawgiver made an equivalent ruling for stagnant water (the primary basis), necessitating a similar ruling for running water (the derived case).