Over the last few weeks, several scientific studies have confirmed that the methane emissions from hydropower dams and reservoirs can big very big, and in some cases hydropower can emit as much CO2e (Carbon Dioxide Equivalents) as coal powerplants. Studies have confirmed this in tropical areas of the planet over the last two decades, but recent studies have also estimated similar emissions in dry and termperate climates in the U.S.
On Sept 16, 2016, a scientific study in PLOS: One estimated CO2e at Hoover Dam to be approximately the same as coal-fired powerplants. See graph below:
Because Navajo coal powerplant is 4 times larger, it produces about 4 times more climate change emissions than Hoover Dam, although the CO2e is estimated to be similar at both plants.