Grand Canyon Watersheds Protection Act of 2008 - Withdraws the approximately 1,068,908 acres of the Tusayan Ranger District and federal land managed by the Bureau of Land Management (BLM) in the vicinity of Kanab Creek and in House Rock Valley from: (1) all forms of entry, appropriation, and disposal under the public land laws; (2) location, entry, and patent under the mining laws; and (3) operation of the mineral leasing, geothermal leasing, and mineral materials laws. Subjects such withdrawal to valid, existing rights.