The initial post indicates that the "total" energy of a river with a drop of 90 metres is the same as a lake which maintains the same 90m height throughout (but otherwise has the same dimensions as the river).
What this implies is that the additional Potential Energy that the water in the lake has (by virtue of always being raised by 90m) is exactly the same as the additional Kinetic Energy that the River has by virtue of the water flowing down from a height of 90m.
This is sensible, the Potential Energy of the lake is converted into Kinetic Energy of the river, but the Total Energy remains the same.
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