Major tributaries include the Santiam, the Yamhill, the Molalla, the Tualatin, and the Clackamas.
The Willamette arises in three separate forks, the North and Middle Forks which emerge on the western side of the Cascade range between Three Sisters south to Diamond Peak, and the Coast Fork, whose headwaters are in southern Lane County.
At Oregon City, the Willamette Falls mark the terminus of the navigable section of the river. Further north, the river divides Portland into east and west sections. North of Portland, the river splits into two channels around Sauvie Island before flowing into the Columbia.
There are public locks to allow boat traffic around the falls, primarily used for recreational boating.
Although riverboats navigated the upstream part of the Willamette into the first decades of the 20th century, currently there is no commercial traffic on the river above the falls.