Games : Hydro Plant not allowing water flow through

In my city, I have a hydro plant that is sitting across a river quite near my city. This dam holds back all of the water and there is a significant drop in water level before and after. When I say significant difference, I mean that on the up-river side the river is lapping at the edges of the river and at the down-river side, the river is totally empty.

However, my problem is that the hydro plant doesn't let any water through, and therefore is not generating any power (0MWh out). I can't for the life of me work out why this is happening, and it's bugging me because I don't want to have to buy another solar plant just yet.

The river in question runs out of a lake, joins with another river and then runs out into the sea. Although I do have water pumps on the upriver side of the dam, they are not consuming anywhere near enough water to totally drain the river.

Are there any ideas as to why I can't get my free river power?

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