Yes, you are right. But, the run-of-the river electricity projects are more feasible with small mountain streams with high downstream velocity, and the water should run almost throughout the year. China has many such localized devices, but may not be suitable for a flat country like BD. It may work in Nepal, Bhutan and MM, though.
We are talking here about commercial production and not something that can move only a rice or lentil mill. Even that may not be possible. Can you cite one possible run-of-the water in BD that can produce a commercial-scale 100 MW of power in the planes of BD? I recall, our Hasina once asked people to produce power from Haor and Baor. I wonder, how it is possible? She thought since the name is hydro-electricty and Haors have water, therefore, power can be generated there, although there is no Head there. Water is still.