Benefits - Renewable energy and is environmentally friendly. Water is abundant in many areas (ex: Hoover Dam, Kurobe Dam in Japan). More easily accessible than geothermal power since geothermal power is obtained from beneath the ground.
Costs - requires building of structures which can take a lot of resources, both time and money. Could be more susceptible to disasters as Dams are usually built on rivers or large bodies of water which would eventually affect nearby towns if a disaster does occur.
Geothermal Power
Benefits - renewable form of energy, environmentally friendly. Can be physically isolated to one area to minimize environmental disasters (most geothermal power plants are located in areas far away from towns or populated areas).
Costs - feasibly, harnessing geothermal power is limited to areas of hot spots on the map (like Hawaii, volcanic activity, etc). Also it can be costly to produce technology to effectively use geothermal energy as a source of power.
{This is not my original answer, I too picked it up from somewhere}
There would be dead animals overloading the Earth. Think about it, without worms and other things that break down dead things they would still be here and just sit there.
According to plate tectonics theory, Earth's outer shell is divided into multiple plates that slowly glide over the mantle. This slowly changes Earth's surface over time by merging, or separating, continents.
