Elias Mutani 
The Wanderer and the Golden Paperknife [PDF ebook] 

The Ruvu River has without any reasonable explanation become poisonous. Virunga villagers now have to travel long distances to fetch usable water. Liam, a Canadian teenager on holiday in Virunga, joins up with Fanaka and Heri, from the village, in trying to solve the mystery of why and who is responsible for poisoning the river.
'The story is full of suspense surrounding Roki hills bordering the poisonous Ruvu River. It is interesting and I learned a lot about Tanzanian culture. This story unfolds very methodically as the author does a superb job of predicting the reader’s responses and bringing things full circle, which makes for a very satisfying read …-’
Summer Edward