How exactly do dams contribute to global warming?

The
argument here is that global warming is caused by the presence of methane in the dams.  The
larger the dam, the more methane that is collected in the dam.  The growing amount of rotted
organic material that is present in the dam helps to generate the methane that traps more heat
within it than carbon dioxide.  The belief here is that the bigger the dam, the more methane
is...