Likewise with the forearc basin... but let's jump ahead to the Cascades. When a subducting plate reaches a certain depth (I seem to remember 150 km- 90 mi- but I'm not certain), hydrated minerals start "sweating out" their water. This water moves into the overlying hot rocks, reducing their melting temperature slightly, and causes them to melt. The magma moves upward, and the portion that reaches the surface erupts as volcanoes. This set of volcanoes parallel to the subduction is called a volcanic arc: most are curved (arc-shaped), but the Cascades are short as arcs go, and its axis doesn't curve much. Hence, forearc ridge and basin: the ridge and basin on the forward side of the arc.
While the Coast Range has passes at about 1000-1200 feet elevation, the Cascade passes are about 5000 to 5500 feet. Typical peaks in the Coast Range are 2000-3000 feet, with the high point 4101 feet at Marys Peak just west of My Favorite Coffee Shop... well, 14 miles as the crow flies. The Cascades peg out at the 10,000 foot range, with Mt. Hood topping out Oregon at 11,249 (Mt. Rainier in Washington is the overall Cascade champ at 14,410).
And in between these two mountain ranges lies the Willamette Valley. While the Coast Range has risen by compression, faulting and folding, and the Cascades by a build-up of lava, the Willamette Valley (aside from fluvial sedimentation), has not been raised up very much. It is referred to as a structural depression- its existence is not due to erosion by the Willamette River. And technically, a valley is created by erosion. While most basins in the west are referred to as valleys (sometimes as holes, like Jackson Hole), many are structural in nature, not erosional.