Simplify the following text for a Grade 6 native English-speaking person.
Original: The Alaska Range is a relatively narrow , 650-km-long ( 400 mi ) mountain range in the southcentral region of the U.S. state of Alaska , from Lake Clark at its southwest end to the White River in Canada 's Yukon Territory in the southeast .
Simplified: The Alaska Range is a series of mountains in Alaska , in the United States . The mountain range stretches about 400 m from northwest to southeast .