I live near https://en.wikipedia.org/wiki/Oyster_Creek_Nuclear_Generatin.... When the plant was running, it would suck in so much water that one river would run backwards, pulling in brackish water from the bay (near the outlet of another river), and depositing the outflow into a different river, which went back into the bay about a mile away. Both the inlet side and outlet side were rich fishing and crabbing grounds. Now that the plant is shut down, both the inlet and outlet are nearly stagnant, and all the seafood is gone. The outlet was a particularly popular fishing spot, because the warmer water both attracted more fish and greatly extended the fishing season (even through the winter, when you couldn't catch much elsewhere).

My thought is that hot and humid air is holding more moisture as it is, so it’s harder to evaporate more into it.

If they’re drawing from the river to evaporate, and flows are sluggish (because of evaporation upstream or lack of precipitation), they also have to draw less when they need more.