Unlike conventional dams, which halt the flow of rivers and release the water gradually to generate electricity, new ideas in hydro power focus on benefitting from moving water rather than controlling it. The basic idea is simple and works very much like wind power: place turbines in an area where water will turn them and produce energy.
While water doesn't move as quickly as wind does, it's denser and generates more energy per square foot. (The downside to this power is that underwater turbines must be built to withstand more force than wind turbines.) It's more dependable than wind and solar power; tides, ocean currents and waves are predictable and don't diminish with cloud cover.