Oregon's contrasting physical features are characterized by great forested mountain slopes and treeless basins, rushing rivers and barren playas, lush valleys and extensive wastelands. The major determinant for these unusual climatic differences is the Cascade Range, a rugged mountain chain running north to south c.100 mi (160 km) inland. As the eastward-moving air masses, warmed by the Alaska Current and heavy with moisture from the Pacific Ocean, rise and meet the cooler mountain temperatures, rain is precipitated over the western third of Oregon. Dry air and continental climate prevail over the eastern two thirds of the state.
The Pacific shoreline (c.300 mi/480 km) is bordered by narrow coastal plains of sandy beaches, luxuriant pastures, and occasional jutting promontories. About 25 mi (40 km) inland, the rugged Coast Range rises to heights of 4,000 ft (1,220 m) to serve as the western wall of the Willamette Valley. In the valley, where the navigable Willamette flows north through miles of rolling farmlands into the Columbia River, lie the agricultural, commercial, and industrial centers of the state. Portland, the largest city, whose metropolitan area contains nearly half the state's population, straddles the Willamette near its junction with the Columbia. Salem, the capital, and Eugene, the second largest city, lie southward in the valley, which is sealed off in the south by the low range of the Calapooya Mts.
The snowcapped volcanic peaks of the Cascades are E of the Willamette, with beautiful Mt. Hood rising to the state's highest elevation (11,235 ft/3,424 m). Mighty stands of timber, many protected as national forests, cover the slopes. Eastward the Cascades level out into high plateaus drained in the north by the Deschutes and the John Day rivers. To the south a variegated pattern of marshland and mountain merges in the east into the semiarid Basin and Range Region. Little vegetation grows here, and the absence of potable water makes habitation difficult.
North of this area rise the pine-covered Blue and Wallowa mts., which in some places extend to the Snake River to form precipitous gorges. Other parts of the region where the Snake cuts through the plateau are more level and have been made productive through irrigation. Oregon's irrigation projects include the Deschutes, the Umatilla, and the Vale; the Klamath, shared with California; and the Boise and the Owyhee, shared with Idaho.