Considering the described application, and my experience on the ranch in northern B.C; I am thinking that you're looking for more of an indicator of water flow. To that, I would mechanically devise a simple "deer-scarer" of the typical Japanese bamboo sort. Though adapt it to simply be a flag that is held up constantly by the water flow, and it simply falls when the water stops.
Your prime mover is such a simple setup to begin with, that there is very little to work on. Unless you want to install power supply and circuitry for broadcast lighting, or even radio transmission, you're best off having a "visual indicator". Instead of a "water wheel" that would be constantly turning, think of a "water lever" that is held up by water hitting the lower/shorter end. When the water stops, the higher/longer end falls, due to gravity.
Now if you want that to hit a switch, which turns on a light, than is detected by a sensor, which processes a circuit, that sends a signal, that gets relayed through a transmitter, that gets bounced back by a satellite, that gets received by a dish, that gets uploaded to the intwebz, which then gets filtered by the NSA, before being wifi'd to your location, that is then sent to your BlueTooth, then by all means go for it.