Why can oceangoing ships travel as far inland as Portland and Vancouver?

Oceangoing ships are able to travel inland as far as Portland, Oregon, and Vancouver, Washington, primarily because these cities are located within the reach of tidal influence, which extends 140 miles (225 km) up the Columbia River.

To ensure the river remains navigable for deep-draft vessels, the channel is maintained through regular dredging. While these cities serve as the upper limit for direct oceangoing navigation, the river system has been further opened to inland barge traffic as far as Lewiston, Idaho, through the construction of a series of locks integrated into the river's dam system.12