Video of hippo defecation with fake farting sound effects, you can find other vids of hippo defecation too.
They do something I’ve never seen another quadruped do which is they wag their tail violently while defecating which flings feces everywhere. Compare to a horse, elephant, etc which just let them drop.
Everything else living in a river defecates in it, too. Human body wastes cause disease because we form such dense populations. If a hunter-gatherer human population was shitting in a river it wouldn’t be a big deal to the people downstream because it would be a tiny, tiny fraction of the volume of water. By contrast, there are 500 million people living in the Ganges river basin.