Not a manifestation of, or an avatar of Vishnu, but still holding a rightful place in the Vaisnav-theon, is Ganga Devi. You see Ganga Devi, manifesting on our Earth as the River Ganges, flowed from the feet of Lord Vishnu as Vamana Avatar.
Not only is She revered by Vaisnavs for this reason, but She is another form of the Divine Mother for Whom we celebrate Navratri. I will not delve into how the mythical Ganga became a river on Earth. Because what matters is what she represents. Purity, providence, sustenance. All qualities of Divine Mother-hood, indeed. As she continuously flows, she exhibits the qualities of Shakti energy.
Ganga Devi is inexorably linked to the dwarf Avatar Vamana, who stole the entire universe back from the evil Bali. Vamana was told by Bali he could have as much land as he could cover in three strides – an arrogant pact to make with a dwarf indeed. Unfortunately for Bali he did not know he was dealing with God Himself, and Vamana’s three strides covered the entirety of the universe. As he took his third step which covered not only the universe but expanded his territory into the Heavens, Brahma washed Vishnu’s feet, and hence the River Ganges had its headwaters. Since She washed the feet of Vishnu (and also because She fell from the Heavens and flowed from the hair of Shiva, sitting in his abode in the Himalayas), Ganga Devi is considered a purifier of sins. This is why for Hindus, bathing in the Ganges is such a Holy experience.
Krishna told Arjuna in the Bhagavad-Gita, by way of demonstrating His omnipotence: “Of all flowing rivers, I am the Ganges.”
But all rivers flow into the ocean, as all individuals return again to Brahman. For this reason, I argued once at my Temple’s Gita discussion group that all rivers are equally sacred. All of them are givers of life. All are preservers. All flow from the feet of Vishnu. And as we are all but rivers on our way to the Cosmic Ocean, we all flow from Vishnu.