Where do I start...
The Ganges River is filthy and polluted but this is not an issue isolated to India. Many waterways and ecosystems in Asia (and really all around the world) are being decimated because of how humans have used water in recent history.
Over-logging, unsustainable fishing, damming, toxic factory runoff, introduction of fertilizers in farming, shrimping operations, etc.
Here's an example of how over-logging destroys ecosystems. Forests hold soil in place with their roots. When entire forests are removed, there is nothing to hold this soil back. This is a major cause of landslides when it rains. Also, without trees in place, the loose soil runs off into river systems causing major flooding due to the silt buildup. Couple this with the irresponsible damming of rivers and you have on your hands thousands of human lives lost due to man toying with nature. Forests are also the most biodiverse parts of the world so when they are destroyed, countless numbers of endemic species are also being destroyed.
These water issues are going to lead to massive death tolls in the future. Humans are NOTHING without clean water. If there was one thing that could cause a mass war to break out, it would be this. Issues between China and Southeast Asia over China's damming of the major river's that feed the Southeast Asian peninsula. This issue is only going to get worse in the future with China's massive South-to-North Water Divergence project. Issues between Pakistan and India over India's damming of the rivers leading to Pakistan's breadbasket. Pakistan and India both hold nuclear weapons.
I went off on a tangent but the Ganges is a great starting point in learning about how humans are really destroying this world in a very real way. This isn't some abstract concept like "global warming" but a real issue with very real consequences that we have already begun to feel.