there are a lot of factors to hydroelectric power but the main ones include. How much water moves through your source per min. The rate of fall of that water source. MEaning if you move down stream 50 feet how far down in elivation has that water fallen. If you have a stream that doesnt have much flow then you can build a serries of resivores, much like a dam to increase the rate of fall and also the force of the flow. This would increase the amoung of electrical potential that your stream has.
I'm not an expert in this topic but I hope my answer is usefull.