In addition to what the other answers said I want to add some details about historical problems of organic waste and rivers, as well as issues here in Indonesia today.
You have basically a number of problems that come with throwing waste into the river. These include downstream:
- Downstream effects on water quality
- current and downstream effects on water flow
- pathogen issues.
On the third I don't have anything to add that hasn't been said before by others. I will say that water tends to be assumed to be contaminated before treatment, but greater turbidity interferes with water treatment and so the effects on water quality generally also make it harder to treat the water and make it safe to drink.
The downstream effects on water quality have been a problem that has long been known and where prevention strategies have long been required by law. For example, linen production requires underwater rotting of flax to separate the fibers and while running river water is supposed to provide the best water for this process, it was known that this would foul the water, leading to fines for anyone who retted linen in a river. The alternative was basically small man-made ponds specifically for this purpose even back into the middle ages.
So water quality is a big issue, and this is particularly the case when you start dumping nitrogen-rich waste into rivers. While this is anecdotal, I found the water in my fish pond cleared up when I added stuff to start soaking up nitrates. I assume aquaculture folks here can probably attest to the role of nitrogen in water clarity/lack of turbidity.
Keep in mind that people downstream may try to effectively sanitize and drink what you throw into the river.
The second big issue is that of waste and river flow. One thing that makes flooding worse in Jakarta is that the rivers and storm drains get clogged with sediment and garbage. Adding waste to the river is never a good thing and sediment resulting, as well as pieces not yet decomposed, can settle to the bottom, ensuring that when water levels rise, they rise further than they have before such dumping.
So the short answer is no, and in fact this is a really bad idea.