Hetch Hetchy. Google it. The reservoir was built even though Yosemite was already a National Park -- requiring an act of Congress -- and ultimately playing an integral role in Stephen Mather (recognize that name?) pushing for the establishment of the National Park Service to manage the parks and prevent such desecration from happening elsewhere. Today, there is a strong and active campaign to remove the dam and return the valley to its natural state. The lake is 300 feet deep.