Iraqi Hanafi jurists treated the precise point at which impurity entered running water as affected whether the impurity was visible or invisible.
Some Hanafi jurists of Khurasan and Balkh distinguished visible from invisible impurity: where no visible impurity or residual effect exists, they allowed use even at the point of entry.
The latter view was preferred in some Hanafi references. The dispute concerns the legal significance of the contaminant’s presence and identifiable effect at the point of entry.