Wet forest land cools the air. When humid air hits cold air you get rain. We don’t know the extent to which biological systems create their own rain, but we know it’s greater than zero. Also ground water recharging helps restore the water table, which helps keep the flow rate of the river higher for longer.

So yes, keeping water in the system longer doesn’t magically increase the rate you can draw it down… unless it does.

I wonder how beaver activity impacts evaporation rates versus human reservoir-building? Does lots of small ponds (perhaps mostly in wooded areas?) work better than a few massive reservoirs?

Wait, what? You'd need hundreds of thousands of "small ponds" in the woods to approximate a comparison with a human mega reservoir like Lake Mead and associated reservoirs. I don't really understand the comparison.

I think you're underestimating the extent of beaver altered terrain in the west. Estimates are between 60 and 400 million beaver before the fur trade started. We are currently sitting at 6-12 million and there are a number of watersheds where they have to be reintroduced because there are no populations in that area.

I was actually reading the other day about beavers and supposedly as much as 10% of North America was covered in beaver-built reservoirs and ponds. There were a lot of beavers.