What is the smallest hydroelectric system?

i was thinking about taking a water pump and trying to light an LED using a small stepper motor or something. what is the smallest application of this. today i bought a small dc motor and got it to light an LED by putting the spindle to a spinning wheel (bike tire) but there is no way i could get enough power from a water pump to turn that little motor fast enouph to light an led. so maybe i need a bigger dc motor and turbine? anyway...basically what is the smallest application possible to light the smallest energy using light. thank you!