Cascade Range - Geography

Geography

The Cascades extend from Lassen Peak (also known as Mount Lassen) in northern California to the confluence of the Nicola and Thompson Rivers in British Columbia. The Fraser River separates the Cascades from the Coast Mountains. The highest volcanoes of the Cascades, known as the High Cascades, dominate their surroundings, often standing twice the height of the nearby mountains. They often have a visual height (height above nearby crestlines) of 1 mi (1.6 km) or more. The highest peaks, such as the 14,411 ft (4,392 m) high Mount Rainier, dominate their surroundings for 50 to 100 mi (80 to 160 km).

The northern part of the range, north of Mount Rainier, is known as the North Cascades in the United States but is formally named the Cascade Mountains north of the Canada – United States border, reaching to the northern extremity of the Cascades at Lytton Mountain. Overall, the North Cascades and southwestern Canadian Cascades are extremely rugged, with many of the lesser peaks steep and glaciated, with valleys quite low relative to its peaks and ridges, resulting in great local relief. The southern part of the Canadian Cascades, particularly the Skagit Range, is geologically and topographically similar to the North Cascades, while the northern and northeastern parts are less glaciated and more plateau-like in character, resembling nearby areas of the Thompson Plateau.

Because of the range's proximity to the Pacific Ocean and the region's prevailing westerly winds, precipitation is substantial, especially on the western slopes, with annual snow accumulations of up to 1,000 in (25,000 mm) in some areas. For example, Mount Baker recorded the largest single-season snowfall on record in the world in the winter of 1998–1999 with 1,140 inches (29,000 mm). Prior to that year, Mount Rainier held the world record for snow accumulation at Paradise in 1978. It is not uncommon for some places in the Cascades to have over 500 in (13,000 mm) of annual snow accumulation, such as at Lake Helen, near Lassen Peak. Most of the High Cascades are therefore white with snow and ice year-round. The western slopes are densely covered with Douglas-fir, Western Hemlock and Red alder, while the drier eastern slopes are mostly Ponderosa Pine, with Western Larch at higher elevations. Annual rainfall drops to 9 in (230 mm) on the eastern foothills due to a rainshadow effect.

Beyond the foothills is an arid plateau that was created 16 million years ago as a coalescing series of layered flood basalt flows. Together, these sequences of fluid volcanic rock form a 200,000 sq mi (520,000 km2) region out of eastern Washington, Oregon, and parts of Northern California and Idaho called the Columbia River Plateau.

The Columbia River Gorge is the only major break in the American part of the Cascades. When the Cascades started to rise 7 million years ago in the Pliocene, the Columbia River drained the relatively low Columbia Plateau. As the range grew, erosion from the Columbia River was able to keep pace, creating the gorge and major pass seen today. The gorge also exposes uplifted and warped layers of basalt from the plateau.