I believe they are installing these instead of pressure reduction valves on down hill water flow. So it's basically a beneficial idea in a specific scenario. Not something to be applied generally.
Lengthy discussion on r/futurology which I think answers your question. Or, I'm not sure it does. I thought you were asking if the system was zero-sum because you have to use energy to get the water high up enough to generate energy when it goes down? That is addressed. But if you're actually asking about the decrease in water pressure caused by the turbines, I'm not sure. I will think about it.
Thanks -- answered all of my questions. I like the image of thinking that in an ideal situation everyone's water would be stored 10 feet above their house. Since it can't there's extra energy in the system that can be harvested. Would work not only in hilly places like Portland but in any system with elevation changes. Love it.