Question

why is the river clean near the source?

Answer 1

Water pollution that occurs as a result of a single, identifiable source, such as: Waste water from a drain. Sewage from toilets directly drained into rivers.
Some parts of the river, closest to the source in the Himalayan Mountains, are clean and reliably provide drinking water.
I think the most important work should be done firstly is to prevent all pollution sources along the the river body.