Why are there not many rivers that empty into the pacific ocean from us?

The western part of the United States contains few major rivers because it is in the rain shadow of the Cascade Range, Sierra Nevada, and other coastal mountain ranges. These mountain ranges force most of the moisture from the Pacific Ocean to fall as precipitation on the western slopes, leaving little for the eastern slopes. As a result, the western part of the United States is arid and semi-arid, and few major rivers flow through this region.