click on image to enlarge
…Both the Ganges and the Yamunā are amartyā (transcendental) rivers, but the Yamunā is still more sanctified for the following reasons.
…And as far as sanctity is concerned, the River Yamunā is more directly connected with Lord Kṛṣṇa than the Ganges. The Lord sanctified the River Yamunā from the beginning of His transcendental pastimes in the world. While His father Vasudeva was crossing the Yamunā with the baby Lord Kṛṣṇa for a safe place at Gokula on the other bank of the river from Mathurā, the Lord fell down in the river, and by the dust of His lotus feet the river at once became sanctified. It is especially mentioned herein that Mahārāja Parīkṣit took shelter of that particular river which is beautifully flowing, carrying the dust of the lotus feet of Lord Kṛṣṇa, mixed with tulasī leaves. Lord Kṛṣṇa’s lotus feet are always besmeared with the tulasī leaves, and thus as soon as His lotus feet contact the water of the Ganges and the Yamunā, the rivers become at once sanctified. The Lord, however, contacted the River Yamunā more than the Ganges. (from purport to SB 1.19.6)
Full verse and purport More
The Hare Krishna Movement 