According to Mahabahrata "Impelled by time, a Brahmana named Kalki Vishnuyasha will be born towards the end of Kali Yuga. He will possess great energy, intelligence and prowess. He will be born at a village called Shambhala in a blessed Brahmana family. As soon as thought of, vehicles, weapons, warriors, and arms and armors will all be at his command. He will be the imperial sovereign, ever victorious by the strength of his virtue. He will restore order and peace in this world, overcrowded with creatures and contradictory in its laws. That effulgent and greatly intelligent Brahmana will destroy all things. He will be the destroyer of all and He will be the maker of a new Yuga (Satya-Yuga). That twice-born one surrounded by the Brahmanas, will exterminate all the low and despicable mlecchas wherever they will be found."
And when the Satya-yuga returns the balance or Dharma will be restored, as re-established by Kalki, the planet will begin to heal and rejuvenate itself. Those creatures that were once extinct due to the pollution and imbalance caused by humanity will appear again. The sacred rivers of the Ganga and Yamuna and others also reappear. The planet creates what it needs again, and after years of drought and famine the clouds will supply rain again. Everywhere there will be peace and bountifulness.
The Mahabharata also states when Satya-yuga or Krita-yuga begins again: "O descendant of Bharata, when at the appearance of the Krita Yuga, sin will thus be completely destroyed and virtue will flourish, men will again be engaged in religious rites. Well-planted gardens, sacrificial grounds, large tanks, Vedic schools and colleges, ponds and temples, will (all) reappear everywhere; various sacrifices will also begin to be performed at the appearance of the Krita age. Brahmanas will be honest and good. Being devoted to asceticism, they will be Rishis
(and wise sages)The hermitages occupied by the wicked wretches will once more be the homes of men devoted to Truth. Men in general will begin to honor and practice Truth. All seeds sown on earth will grow. O king of kings, every kind of crop will grow in every season. Men will devotedly practice charity, vows, and religious rites. The Brahmanas, devoted to meditation and sacrifices
(vedc rituals), will be of virtuous soul and cheerful disposition. The kings will virtuously govern the earth. In the Krita Yuga, the Vaishyas will devote themselves to trade, the Brahmans will be devoted to their six duties and the Kshatriyas will be devoted to the display of prowess
(for the protection of the citizens)The Shudras will be devoted to the service of the other three orders. Such will be the Dharma in Krita, Treta, and Dvapara Yugas.
credit Mahabharta translated by Kisari Mohan Ganguly
