Hells Canyon gouges out the border between Idaho and Oregon. The Cascade Mountains for centuries formed effective transportation barriers between western and eastern Oregon and Washington. Even today, heavy mountain snows close interstate highways between the two sections for days at a time. In Idaho, rugged terrain limits north-south travel to one major highway, US 95. The climate and consequently vegetation also vary widely across the region.
Mountain ranges and distance from the Pacific Ocean control the climate of the Pacific Northwest.
When moisture-laden air flows off the Pacific Ocean onto the shores of Oregon and Washington, it encounters coastal mountain ranges. Forced to climb above these barriers, the wet air cools and loses its ability to hold water. Some of these slopes receive over ten feet of rain in a year. The result on the Olympic Peninsula is the creation of continental America's only temperate rain forest. On the leeward side of the mountains, residents of Portland and Seattle enjoy dry, sunny summers and mild, wet winters.