The state is divided into five distinct regions: (1) the Coast Ranges, bordering the Pacific coast; (2) the Cascade Range, a parallel range to the east; (3) the Willamette Valley, a narrow lowland that separates the two ranges in the northwest; (4) the Columbia Plateau in the northeast; and (5) the Great Basin in the southeast.
The Coast Ranges consist of the Oregon Coast Range and the Klamath Mountains. Beginning at the Washington border, the Oregon Coast Range juts southward along the northern two-thirds of the coast. Low peaks with gentle slopes prevail, most being 2,000 to 4,000 feet (600 to 1,200 m) in height. The Klamaths, formed by the erosion of a former upland, are generally higher. Several peaks attain heights of more than 7,000 feet, (2,100 m). Like the Oregon Coast Range, they are fringed by narrow coastal lowlands and steep cliffs. Throughout the region dense forests mantle the slopes.
The Cascade Range, an extension of the Sierra Nevada of California, is the most rugged part of the state. Summits rise 5,000 to 7,000 feet (1,500 to 2,100 m) above sea level; some, especially the volcanic peaks, tower considerably higher Among them are Mount Hood, Mount Jefferson, and Three Sisters Mount Hood rises to 11,239 feet (3,426 m)—the highest point in the state.
The Willamette Valley, a continuation of the Puget Sound Basin of Washington, is a gently rolling lowland between the Oregon Coast Range and the Cascade Range. Not only is this one of Oregon's largest valleys, but it is also the economic heart and the most populous part of the state.
The Columbia Plateau is composed of thick lasers of lava In some areas, such as the Columbia and Snake river valleys, the lax a is covered by deep alluvial deposits, in others, notably the Blue and Wallowa mountains, remnants of the original surface are exposed. Throughout most of the region, the Columbia River's tributaries cut deep valleys Most spectacular of these is Hells Canyon, a gorge carved by the Snake River along the Oregon-Idaho border Except in part of central Oregon, there are few extensive areas of flat land.
The Great Basin consists of a series of basins, in which flat to hilly land prevails. Occasionally the terrain is broken by relatively high ridges, buttes, and mountains, such as the Fremont and Steens mountains. Except on wooded slopes, grasslands prevail Dotting the region are lava beds, salt flats, and desert.Oregon's state tree is the Douglas-fir.
Draining much of northern Oregon is the Columbia River system—the Columbia itself and tributaries including the Willamette, Deschutes, John Day, and Snake They are by far the longest and largest rivers in the state. Rivers of the Coast Ranges that drain directly into the sea are general!) short and swift. The Rogue and the Umpqua are the longest. Short, seasonally flowing streams in the Great Basin end either in brackish lakes or in dry basins.
Freshwater lakes, including Upper Klamath and Crater lakes, lie primarily m the Cascades. Few occur to the west East of the Cascades are reservoirs impounded by dams and brackish natural lakes, both intermittent and permanent.
An unbroken belt of western mountains gives Oregon two climatic zones a mild, wet region west of the Cascade crest, and a more rigorous, drier climate to the east.
In the west, moist, tempering winds from the Pacific Ocean bring mild temperatures and moderate to heavy precipitation, mainly during winter. Along the coast January temperatures average 38° to 45° F. (3° to 7° C.); July temperatures average about 62° F. (17° C.). Precipitation varies from 55 to 130 inches (1,400 to 3,300 mm) a year, the larger amounts occurring on upper mountain slopes Toward the east summers grow warmer, winters cooler, and precipitation more moderate. There is little snowfall except on the higher slopes.
East of the Cascades, precipitation drops sharply, and temperature ranges increase. The mountains receive the most precipitation—35 to 60 inches (890 to 1,520 mm) a year Annual precipitation on the Columbia Plateau is generally less than 20 inches (510 mm); in the Great Basin, less than 10 inches (250 mm). In both the Plateau and Basin, January temperatures average below freezing, and July averages range from 68° to 80° F. (20° to 2 7° C.). Snowfall varies from 15 to 30 inches (380 to 760 mm) in the Columbia Valley and the Great Basin to 50 to 70 inches (1,270 to 1,780 mm) in the mountains.
Oregon's vegetation generally corresponds to its two climatic zones. Dense evergreen forests predominate in western Oregon, where there is abundant rain and a mild climate. The Coast Ranges are forested chiefly with Douglas fir, Sitka spruce, western hemlock, and various cedars. The western slopes of the Cascades have thick forests primarily of Douglas fir, silver fir, and western hemlock; the eastern slopes have sparser forests chiefly of ponderosa pine, with some Douglas fir, western hemlock, larch, and lodgepole and white pine.
Grasslands and shrubs predominate east of the Cascades, where a relatively dry climate prevails. Scattered mountain ranges in this area, however, are forested. The Great Basin has mainly desert scrub vegetation.