Sforno on Deuteronomy 21:4/ספורנו על דברים כ״א:ד׳

1אל נחל איתן, where people do not normally walk.

2וערפו, a discreet death, not one drawing attention to itself. The animal does not experience pangs of death, not knowing of its impending fate. Naturally, people knew even less about this impending procedure. This procedure parallels what the murderer had done when he ambushed an unsuspecting victim in a location which was isolated so that the court never found out who the murderer was. Had the court known of the identity of the murderer they would have dealt with him instead of this heifer having to be killed.