The Chipko Movement, started by Sunderlal Bahuguna in 1973, focused on the conservation of forests. This movement originated in the state of Uttarakhand in India and aimed to protect trees from being cut down and to promote sustainable development practices.

Option 1, conservation of river water, is not the central theme of the Chipko Movement. While the movement did advocate for the conservation of water resources, its primary focus was on the preservation of forests and the livelihoods that depended on them.

Option 2, protection of birds, is unrelated to the Chipko Movement. Although the movement did contribute to overall biodiversity conservation by protecting forests, its specific goal was not centered around the protection of birds.

Option 3, protection of cheetahs, is also unrelated to the Chipko Movement. The movement primarily addressed the issue of deforestation and its impacts on local communities rather than focusing on wildlife conservation.

Therefore, the correct answer is option 4, conservation of forests, as it accurately reflects the central theme of the Chipko Movement.