Discovery said Root, who led the channel since March 2004, is returning to the UK, where she previously headed programming at a BBC network for five years. Root was president and general manager of Discovery Channel as well as Science Channel.
Zaslav predecessor Billy Campbell hired Root.
John Ford--who came back to Discovery in September to lead the Discovery Times and Military Channels after four years at the National Geographic Channel--replaces Root, although he won't lead Science Channel.
Root said that "after much consultation with friends and family, I have decided that this is the best time for me to leave Discovery and return to the United Kingdom to pursue other opportunities in the media industry."
Ford headed programming at NGC since 2003, following 13 years at Discovery where he worked on TLC and HD Theater, among other duties.
Discovery Times Channel will be re-branded as Investigation Discovery.