An interesting fact about the Columbia River is that it is the largest river in the world that does not have a delta. In fact, it is one of the few large rivers at all that does not have a delta.
The reason that it does not have a delta is due to the fact that about 75 miles from its meeting with the Pacific Ocean, it departs from it's westward course when it runs into the Portland Hills. It takes a 90 degree angle and heads north. This sudden extreme turning of the river causes it to slow down and drop its sediment. Thus, all the dirt that would end up forming a delta ends up forming a wide flood plain at the point where the Willamette River, the Columbia River and the Portland Hills all meet. One of the major pieces of this misplaced delta is Sauvie Island.
Also, it is somewhat incorrect to say that the Columbia River starts in Canada. While the north fork does, the Clark Fork of the Columbia starts in Montana.