system
Given the following post, respond as if you're commenting on Reddit:
human
[Discworld] What makes the River Ankh so polluted?

It is said that the river is so polluted running through Ankh-Morpork that you can walk across it, and that it's the only river where you can draw a chalk outline. 

However, in our world, such pollution has only come as the result of industrialization, where factories, sewage systems and large scale agricultural runoff create situations like the Ganges or the Cuyahoga. What's causing all the upstream pollution along the Ankh?
gpt
The entire Sto Plains drains into the Ankh. While that's mostly farmland and small cities, there are still some areas there affected by the Mage Wars.

When rain falls on Sto Helit, water flows into the Ankh. When rain falls on the Forest of Skund, you get the magical equivalent of toxic waste. Sometimes it's more or less water, sometimes it's ectoplasmic slime, sometimes it's seven thousand sapient jellyfish, all named Harold.