As with both solar power and wind power, the renewable energy produced by hydrologic power causes no pollution by itself, but building the equipment to harvest the energy does. More than the sun or wind, it can directed and controlled for regular use, but human regulation of water flow through damming and rerouting may also be damaging to aquatic ecosystems. Hydrologic (otherwise known as hydroelectric) power may also be limited by drought conditions.
The most basic principles of how water creates energy can be explained using an empty half-gallon milk carton, string, masking tape and a jug of water. Punch a single hole in the center of the top of the carton and hang it from a tree outside. Punch more small holes in each bottom corners of the carton and cover with the masking tape. Now fill the carton with water and experiment by removing and replacing the tape from various corners. The force of the moving water will propel the carton in different directions dependent on how much water is released and where.
Ask students to consider the pros and cons of each renewable energy source. Which do they think is the best alternative to fossil fuels, based on the benefits of each? What other types of energy are out there? What are their benefits and drawbacks?