An impoundment does result in a surprising increasing of evaporative loss compared to the free-river run. So a nominally non-consumptive water use such as hydroelectric generation or river cooling of the condenser can involve considerable fresh water loss, usually only important to downstream consumers.
Cooling towers are by definition totally consumptive and are also comparatively expensive so they are mostly used for nuclear plants which make steam at a much lower temperature than coal plants. Thus the small reduction in enthalpy at the top of a cooling tower translates into a couple per cent increase in thermodynamic efficiency and a significant ~5 increase in profit.
Nuclear plants *could* be run at a much higher temperature, but only the French have the guts to do that :)