Actually, there was a detailed article about a micro-hydro system in coastal Oregon that relied on a rain-fed creek. The designers included a small pumped storage component to hold water during wetter months for use in the summer. It wasn't perfect, but they managed to get consistent output by carefully monitoring watershed data. Some neighbors tried a simpler setup without storage and basically ended up with a dry turbine by August. The geography definitely matters, but creative engineering can stretch what's possible. You gotta have a plan for when the stream isn't running full.