Does hydroelectricity prevent pollution

It depends on what type of water related potential energy you are tapping into to generate the power. There are essentially four types:

* Tidal power where you use the tides to fill abasindururing high tide and drain the water through a turbine during ebb and low tdes. * Wave power where you use the oscillation of the waves to activate pumps or turbine to store water at an elevation or generate power directly * Hydropower generated by water diverted from a natural reservoir where it originally fell over a cliff and now powers your turbines * Essentially the same as the above but the water is contained in a reservoir you construct by damming a river or stream then use the head to power your generators. All of these systems pollute and degrade the environment in some small measure by the construction of their infrastructure (roads, power lines, control buildings etc)

Only the creation of reservoirs has an ongoing emission, that is methane from the anaerobic decomposition of vegetation at the bottom of the reservoir.