Police originally said the boy had died, but sent a correction later Tuesday that the boy was taken to St. Joseph's Hospital and life-saving efforts were still under way.
The boy was with his mom, dad and 5-year-old sibling were on flat rock at the top of a 20- to 25-foot waterfall when the boy lost his footing and was swept downstream.
Police say they coordinated with public works to take the water down 75 percent reduce to help improve conditions.
The boy was eventually located in the undertow about 90 minutes later and rushed to St. Joseph's Hospital.
"The water has a tremendous amount of power and the flow can take someone away very quickly," said Bob Vander Yacht with Bellingham Police.
Officers are providing support to the family and say they will have an update on the boy's condition Wednesday. Meanwhile the park has reopened.