Simplify the following text for a Grade 6 native English-speaking person.
Original: The Columbia River is the largest river in the Pacific Northwest region of North America . The river rises in the Rocky Mountains of British Columbia , Canada , flows northwest and then south into the U.S. state of Washington , then turns west to form most of the border between Washington and the state of Oregon before emptying into the Pacific Ocean .
Simplified: The Columbia River is a river in British Columbia , Canada , and the U.S. states of Washington and Oregon .