Dry cooling tower exists but they are more expensive. Natural cooling with the evaporation eventually runs into the problem due to built-up of salts which forces you to replace non-evaporated cooling water with fresh river water and lets you dump the heated water into the river (which is limiting factor due to environmental concerns). The less water is flowing and the higher the temperature of the river are the sooner this point is reached

My hunch is evaporative cooling uses not very much water. You could also fill a small dam when the water flow is higher. All means higher cost, of course.