Why is the Ganga at Varanasi considered more sacred than elsewhere?

The Ganga at Varanasi flows Uttarvahini — northward, toward the Himalayas — instead of southward like most rivers. The Skanda Purana states all sacred Tirthas of India converge into the Ganga at Kashi on auspicious days. The Linga Purana also describes this Ganga as flowing through the very body of Lord Shiva, charged with his divine grace.