>however people forget to mention that gravity is a zero-sum game. every area that is being rendered unusable upstream results in creation of new area downstream.
That's not right. Once the lake is created, nearly the same amount of water will be flowing downstream.
With a dam the water flow is more controlled, so there is less flooding downstream. It's not a simple zero-sum game like the grandparent post said though.
Not necessarily true bc that assumes constant rain. Depending in the inconsistency of rainfall, the water flow downstream will be lumpier due to the resevoir requiring a certain level of water.
The other way round: the reservoir catches excess rainwater and feeds it downstream through dams and power plants, often aiming to catch water to the reservoir and keep constant flow, at least not exceeding the generation capacity (trying to avoid bypass discharge). The reservoir is where the "lumps" are caught and level of water can change considerably.
The absence of lumps, i.e. floods downstream, is of course not always good for nature of the water systems, as in many places the river system ecology is based on having some floods. And specific arrangements need to be made for fish to pass dams and power plants.
That's not right. Once the lake is created, nearly the same amount of water will be flowing downstream.