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The big disadvantage of free-standing turbines is that they are less efficient in transforming the mechanical energy of water into electrical energy than turbines in dams are.
Then there is the matter this article so conveniently overlooks the ability for the lake to store the kinetic energy of the water----to even out the power production.
Most stream/river flows are very seasonal---
Here in the Upper Thames watershed---a series of "flood control" dams were built---when their true purpose was to provide a steady stream flow to flush the municipal sewage treatment plants discharge away---particularly London.
Undershot wheel which depend upon flow/velocity were not popular hence the wide adoption of impounds, mill-races and large diameter overshot millwheels.
The mill pond stored the overnight flow for the next days operation.
These in stream turbines will be as unreliable a power source as wind turbines---if that.
They perhaps have an application in tidal estuaries.