what can be used to replace dams?rivers lake and oceans do not replace dams
Since you included hydropower in your tags, I am assuming you are asking what source of electrical production can replace hydroelectric dams.
Ocean tides can indeed replace dams in this way. Several countries are experimenting with ways to harness the movement of the ocean's waves and/or tides to produce hydroelectricity. There are numerous ways to do this.
While some tidal power plants do require building a sort of dam on or near the shore line, other types do not. Instead, units like the "Salter Duck" float on the surface like buoys, capturing the rolling motion of the ocean's surface and turning it into electricity.
"Swanturbines" developed by the University of Swansea in the United Kingdom look very similar to wind turbines, but they are designed to sit on the bottom of the sea, and are intended to capture the power of the currents that flow by. This particular technology has the advantage of being out of sight, and also is less likely to be damaged by storms. The links below have some good illustrations that show how the different options look and work.
At this point there are several functional tidal power plants in Korea and the UK, with more slated to be built in those countries and in China and Canada in the future.