Oregon has a number of major geographic regions, many of which include mountain ranges. The Cascade Mountain Range, which runs north-south, divides Oregon into eastern and western regions. The eastern section of the Cascades is made up of a chain of volcanic peaks. The Cascades contain nine snow-capped volcanic peaks more than 9,000 ft (2,700 m) high, of which the highest is Mt. Hood, at 11,239 ft (3,428 m). A dormant volcano, Mt. Hood last erupted in 1865.
At the state's western edge, the Coast Mountain Range rises from the beaches, bays, and rugged headlands of the Pacific coast. Between the Coast and Cascade ranges are fertile valleys, including the Willamette Valley, Oregon's farming heartland. The Klamath Mountains are higher and more rugged than the Coast Range. Thick forests grow on these mountains.
The area of the state to the east of the Cascade Range is made up of dry plateaus cut by river canyons. There are rolling hills in the north-central part of this region, known as the Deschutes-Umatilla Plateau, which ends in the Blue Mountains in the northeast. The smoothly rolling ground in this region is a good wheat-growing area. The Great Basin in the southeast contains weathered ridges and the remains of large prehistoric lakes. On the northern edge of the Great Basin is the Columbia Plateau. The Great Basin merges with the High Lava Plains, which contains many small volcanoes.
The Blue-Wallowa mountains are located in the northeastern part of the state. Basins and valleys, headquarters for large cattle ranches, are scattered through the Blue Mountains. The Wallowa Mountains are near the Idaho border. South of the Blue Mountains are the High Lava Plains, or High Desert. This area contains cinder cones, buttes, and craters. In southeastern Oregon, the Malheur-Owyhee Upland is a high plateau with many deep canyons in an area called the Rimrock Country.
Oregon is criss-crossed by rivers, but the Columbia, along most of the northern border with Washington, is the biggest and most important. Originating in Canada, it flows more than 1,200 miles (1,900 km) to the Pacific Ocean. The Columbia is the 3rd-largest river in the US. Rivers that feed into the Columbia include the Deschutes, John Day, and Umatilla. The largest of the Columbia's tributaries in Oregon, and longest river entirely within the state, is the Willamette. More than half of Oregon's eastern boundary with Idaho is made up by the Snake River, which flows through Hell's Canyon, one of the deepest canyons in North America.
Oregon has 19 natural lakes with a surface area of more than 3,000 acres (1,200 ha), and many smaller ones. The largest lake in Oregon is Upper Klamath Lake, which covers 58,922 acres (23,845 ha) and is very shallow. The most famous lake in the state is Crater Lake, which formed in the crater created by the violent eruption of Mt. Mazama thousands of years ago and is now a national park. It has a depth of 1,932 ft (589 m)—the deepest lake in the US.
The climate in Oregon can be quite different, depending on which side of the Cascade mountains you are on. This is because the Cascades act as a barrier to the warm, moist winds blowing in from the Pacific Ocean. Because of this, much more rain falls in the western part of Oregon, while the eastern part of the state has very little rain and is colder in winter and warmer in summer. Oregon is famous for its rain, but this varies a great deal throughout the state. The average yearly rainfall in Portland is around 37 in (94 cm); while rainfall elsewhere in the state is from less than 8 in (20 cm) in the drier plateau regions to as much as 200 in (508 cm) on the upper west slopes of the Coast Range. As much as 550 in (1,400 cm) of snow falls each year in the highest parts of the Cascades.
Along the coast and in the west, July temperatures average in the upper 50s F (about 14 °C), and January temperatures in the low 40s F (about 5 °C). In the lowlands of the Willamette, Umpqua, and middle Rogue rivers, July temperatures average about 70 °F (21 °C), and January averages about 40 °F (4 °C). The Cascade Range has short, dry, sunny summers. Here, snow begins to fall in October and large patches of snow do not disappear until July. Oregon's record low temperature was 54°F (–48°C), at Seneca on 10 February 1933; and the all-time high, 119°F (48°C) was recorded at Pendleton on 10 August 1898.
