Just how good are we at managing water?

[FishingLunatic]

Can you explain how they do that? I don't know if you get what I mean but in normal rivers for example, if its slow moving then the water level is low, if the rivers moving fast, its high. How can a canal be full iif not much water is being let out? Also, isn't the majority of river speed because of change in elevation? Canals are flat and the same elevation minus some rocks/cement and stuff that gets thrown in them.
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