Actually, I side with Sam67858 on this one. I'll even take it a step further and say “the source of our water” is even better English. Although I find it difficult to explain the difference, “water source” sounds weird here. “Water source” sounds like the word you use instead of river when you're deliberately trying to be vague about what that source is. “This place has everything we need: a water source, a level surface, discretion.” If somebody passes by the river and says that this river be could a better substitute for your water source, you could counter with “But this river is our water source” and it would make sense then. But otherwise, if the river is the topic of the sentence and not the focus, “This river is the source of our water.” is the only thing that makes sense.