I didn't do any research on this back then because it's glaringly obvious that there's absolutely no scientific or logical reason why large hydroelectric systems should not be considered "renewable" energy. Therefore the only reason left was politics, and I left it at that.

I dug up a reference that confirms this:

Los Angeles DWP currently generates 50 percent of its electricity from coal power plants located in Nevada, Arizona and Utah making it one of the dirtiest publicly owned utilities in the country. Approximately 10-12 percent of the city's electricity comes from hydroelectric power plants, with Hoover Dam, located along the Colorado River in Arizona, making up the bulk of this supply.

Environmental groups were opposed to including Hoover Dam in the RPS arguing that it would have reduced the city's investment in new renewable energy by up to 300 MW while setting a bad precedent nationally. Without Hoover, Los Angeles will have to develop more than 1,000 MW of new renewable energy capacity over the next 13 years to meet its 20 percent by 2017 goal.