I thought it could be Dirty water canal
for direct translation, the answer given is correct. Also, ordinarily no canal is designed for dirty water, which I think 'dirty water canal' implies. Also also, just as a side note, 'mfereji' is also a tap
Why is it machafu here and not mchafu?
Because it's a canal of dirty water, not a dirty canal of water.
maji machafu = dirty water (ma class)
mfereji mchafu = dirty canal (m-mi class)
There's a table showing all the forms here
Thanks for a succinct and perfect contribution which answers the question as well as providing us with a nugget of language wisdom. You've got precision and a sense of proportion.
Why can't it be "The ditch with dirty water"?
I understand the word Mfereji to be both canal and pipe. Hence Mfereji wa mani machafu should translate as a dirty water pipe ?