In GS, before you get access to renewable power options (Hydroelectric, Geothermal, Solar, Wind), occasionally some cities have some innate renewable power sources. Early on, it happened with two cities that had originally settled on resources (one on Quicksilver, one on Horses) but throughout the game I have seen this local, renewable power value fluctuate from 2 power, some have 4 power, some have 6 power. None of them have hydroelectric, geothermal, or any improvements that provide power. What are the rules regarding this local renewable power and what determines their value? Thanks!