The amount of power that can be generated from a hydroelectric system depends on the amount of water flowing (usually measured in cubic feet per second) and the head (the distance the water has to fall measured in feet). Greater flows and greater heads translate into greater power potential.
That said, there are a significant number of small dams in the United States that were developed for municipal water or irrigation water storage. Some of these dams can be retrofitted with hydroelectric generating equipment. There are also diversion, irrigation, and water supply ditches and pipes that have the potential to produce power with minimal modification.
Small hydropower systems can be used for mechanical processes such as water pumping or to generate electricity both on- and off-grid. Generating power from small hydro resources can require vertical elevation changes of only 2 feet in areas of high flow. Submersible turbines can be supported by as little as 13 inches of water.
When determining the potential power output of a water source, consider the seasonality of the flow. Also know that separate water rights may need to be acquired to generate electricity and that other permitting issues may apply.