This can be done in a number of methods and much will depend on the type of river and consideration for other users of the river such as boats. Essentially a collection system is placed in the river to funnel the fresh water through a large pipe to carry it to the offshore storage area. A one way valve and low power pump may be sensible to maintain fresh water flow into the system. Simple sensors checking the rate of river flow would be used to ensure no salt water enters the collection system. Thus if the river flow were to drop then the pump would shut off.
Diagram of the water
catchment system
As the diagrams show, in the mouth of the river a catchments funnel is placed, fresh water flows along the connecting pipe to a circular offshore reservoir.
Below is shown a system where the river flow is sufficient to maintain fresh water in.
Below is shown a system where the the underwater pipe is connected into the final weir.
Fresh water flows along the connecting pipe to a circular offshore reservoir.
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