Naturally there are limits. In Sweden it is regulated in Vattendomar that sets highs and lows and a ton of other things. Regulated rivers are precisely that from a legal standpoint.

Have no idea what the order of maximum effect is with relation to water flows. Should not be much lower then total maximum effect though, not much point in installing more effect then the Vattendom allows water flow, though there is probably an effect on each others maximum allowed flow in the case of rivers running together. Total installed hydro capacity in Sweden is about 16 GW in Sweden.

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