Physiographic Regions

Figure 1: Relief and Physiographic Regions

Washington, known as the Evergreen State, can be divided into four major physiographic regions (Fig. 1). They are the Pacific Border, the Cascade Mountains, the Rocky Mountains, and the Columbia Intermontane (Shimer 1974). Everything west of the Cascades is the Pacific Border, which can be further subdivided into the Olympic Mountains, the Willapa Hills, and the Puget-Willamette Lowland.

The Olympic Mountains are actually part of the discontinuous Coast Range that runs from Canada to California (Atwood 1940). In the north it is broken from the mountains of Vancouver Island by the Strait of Juan de Fuca. In the south it is broken from the Willapa Hills by the Chehalis Valley. The Olympic Mountains are made of folded, metamorphosed rocks, which have been eroded to form sharp, steep-sided ridges reaching 4,000 to 8,000 feet above sea level (Kimerling, Jackson 1985). The highest peaks, which include Mount Olympus, now harbor perpetual snowbanks and several small glaciers, though during the ice age they experienced severe glaciation.

The Willapa Hills are part of the same range. They are moderately folded marine tuffaceous sandstones and shales with basaltic volcanic rocks and related intrusives. They have been uplifted more than 1,000 to 2,000 feet and eroded by streams to form rounded mountains of moderate relief. The summits and capes are made of resistant igneous rocks. Shore features include sea terraces, sand dunes, many harbors, and drowned valleys. One such drowned valley is the lower course of the Columbia River, the water route through the Coast Range that divides the Willapa Hills in Washington from the rest of the range in Oregon.

The Puget-Willamette Lowland is a trough with a north/south orientation lying between the Coast and the Cascade Mountain Ranges (University of Washington Department of Oceanography 1953). It extends from the Fraser River in British Columbia down through the Cowlitz River Basin in Washington and the Willamette River Basin in Oregon. The Cowlitz and Willamette Lowlands are stream valleys containing alluvial terraces, which have been eroded to low elevations in tilted and folded belts of nonresistant Tertiary rocks. Resistant rocks are responsible for the hills and watergaps (Kimerling, Jackson 1985).

The Puget Lowland was eroded by streams and Pleistocene glaciers, leaving partially drowned valley systems amidst low, rounded plains composed of glacial till and smooth sheets of glaciofluvial gravel deposited during the past ice age, creating numerous channels, sounds, and inlets. The height of these bluffs range from 50 to 500 feet (University of Washinton Department of Oceanography 1953). The average water depth is approximately 200 feet, with depths from 50 to 100 feet close to the shore, tidal flats near river deltas, a narrow beach line, and maximum depths over 900 feet. Ninety-five percent of the water flows through the Admirality inlet. This landscape is very prone to landslides and slumping. They are the principal processes of erosion and most commonly caused by uplift.

The Cascade Mountains are the most prominent relief feature in Washington, and divide the state both physiographically and climatically. They are subdivided into Northern, Middle, and Southern Cascades and extend from Canada to California. Washington shares the Northern Cascades with Canada, and the Middle Cascades with Oregon (Hunt 1967).

The Northern Cascades are a dissected upland underlain by folded, metamorphosed, upper Paleozoic sediments intruded by granites. They have ridges 6,000 to 8,000 feet high. They are home to the composite volcanoes Glacier Peak and Mount Baker, known to the local Lumni, Nooksack, and Skagit tribes as, "Koma Kulshan," which means "white, steep mountain (Scott, DeLorme 1988)." These mountains used to be extensively glaciated, but now consist of many small glaciers.

The Middle Cascades are a broad upwarp composed of underlying layers of early Tertiary tuffs, breccias, lavas, and mudflows (Kimerling, Jackson 1985). These layers are exposed in the Columbia River Gorge and other deep valleys. A thick middle section of Tertiary basalts form the deeply eroded western Cascades, while an upper section of Tertiary and Quaternary andesites and basalts form the less dissected high Cascades lava platform. They are 15 to 25 miles wide, and the crest of the range is 4,000 to 6,000 feet high. They too are home to snowcapped strata volcanoes as well. The famous Mount Rainier is the highest point in Washington. It stand 14,410 feet high and towers more than two miles above its surroundings. There is also Mount Adams and the infamous Mount St. Helens (Easterbrook 1970).

The tectonic process at work in Washington is subduction (Fig.2). Due to oceanic spreading, the oceanic Juan de Fuca Plate is being thrust beneath the continental North America Plate back into the mantle of the earth (Keller 1996). This has provided the source of tectonic stress and material needed to keep the Cascades seismically and volcanically active for the past four million years.

Figure 2: Plate Tectonics

East of the Cascades is the Columbia Intermantane and the Northern Rockies. The Northern Rockies have high mountain ridges and deep valleys eroded from folded and faulted rocks of moderately complex structure (Kimerling, Jackson 1985). Some valleys are 10 to 20 miles wide, favoring transportation and settlement. They drain into the Columbia River and its tributaries.

The Columbia Basin accounts for all of the Columbia Intermontane except the southeast corner, which is the northern most part of the Central, or Blue Mountains, most of which are in Oregon. The Blue Mountains are a complex group of folded and faulted uplifts. They are comprised of various rocks of differing resistance. With plenty of orographic rainfall, these mountains have been well dissected, with alluvium filled fault troughs within them. They reach elevations of 6,000 to 10,000 feet above sea level, and tower 2,000 to 5,000 feet above their surroundings. The higher portions used to be glaciated.

The Columbia Basin is just that, an irregular structural and topographic basin. It is actually saucer shaped. In the center of the concave, where the Columbia and Snake Rivers merge, it is depressed to sea level and the rivers flow on level ground. However, the rim reaches heights of 1,500 to 3,000 feet due to the uplift of the surrounding mountains, and where the rivers cut through this elevated rim they are deeply entrenched (Meinig 1968). The basin is underlain with Late Tertiary and Quaternary basalt flows that are somewhat of a mystery. These basalt flows cover 200,000 square miles and are almost two miles thick (Levin 1988). They could have come from an abandoned rift of an Intraplate "Hot Spot" under the continent (Mitchell 1996). They are underlain with fluvial, lacustrine, eolian, and glacial sediments that form terraces and other subordinate physiographic features.

The Columbia and Snake Rivers subdivide the basin into three distinct lanscapes lying to the southeast, west, and north of their junction (Meinig 1968). The landscape to the southeast is partly low plain of sand and gravel, and partly plateau incised by canyons and steep stream valleys. To the west is a series of partly faulted anticlinal ridges and synclinal valleys with watergaps formed by the Columbia and Yakima Rivers crossing the ridges. To the north are the Channeled Scablands. These are a network of anastomosing channels, cataracts, and waterfalls made by the most catastrophic floods yet recognized anywhere in the world (Easterbrook 1996).

During Pleistocene glaciations, part of the Cordilleran Ice Sheet impounded glacial Lake Missoula in Montana (Easterbrook 1993). However, with each melting back of the ice sheet, the ice dam burst sending immense floods across western Montana, northern Idaho, and eastern Washington. The Channeled Scablands were the drainage ways of those gigantic ice age floods.

Climatic Regions

Figure 3: Climatic and Physiographic Regions

Washington State has four climatic regions as well (Fig. 3). They are the Coast, the Western Lowlands, the Cascades, and the Intermontane (Scott, Vasquez, Newman, Sargent 1989). The Coast has a mild, wet marine climate along the coastal strip and into the river valleys of the Willapa Hills. While winters are mild, they are windy, wet, and cloudy. Summers are cool, foggy, and cloudy also, but receive only 10 percent of the annual precipitation. The average temperature is 50 to 60 F with a summer peak of 70 F in August. It has a long freeze-free season of 240 to 260 days. The Willapa Hills receive precipitation over 200 days a year due to orographic lift on its windward slope for an average total of 80-100 inches per year and snow in the higher elevations. The Olympic Mountains cause east-moving cyclonic storms to dump tons of rain and 100 inches of snow in the winter. In the summer they receive more rain than the Willapa Hills due to greater orographic lift and convective disturbances.

The Western Lowlands have dry, sunny summers and moist, mild winters. However, being on the leeward side of the Coast Range, they have a slightly larger temperature range than the Coast with an average July high/low in Seattle of 76 /56 F (Thoman, Richards, Lantis 1996). Daytime humidity during the summer is less than 50 percent. Occasionally dry, hot winds are funneled through the Columbia Gorge from the east, greatly increasing irrigation requirements, electricity demands, and forest fire hazard. There is a 30 to 60 day window within which dry spells can occur during July and August. Then the rainy season starts in September or October, marking the beginning of a mild, moist, cloudy winter. December and January usually see the most precipitation with 16 to 18 days of rain per month for monthly totals of six to nine inches. These rain totals are relatively small considering the number of rainy days due to the low intensity of the precipitation. This area can get snow in the winter, but it usually never amounts to more than 10 inches per year.

The Cascades form an orographic barrier that creates a major east/west temperature and moisture divide of Washington (Tables 1a & 1b).The Cascades experience sharp temperature and precipitation gradients, and many microclimates depending on elevation and slope aspect (U.S. Army Corps of Engineers 1975). Winter temperatures are much colder at higher elevations. They receive 70 to more than 100 inches of precipitation during the year. The majority of this falls on the west slopes where the average winter temperature is greater than 30 F. The east slopes get much less precipitation and more sun. Elevations between 5,000 to 7,000 feet have winter snowfall totals between 200 to 600 inches and a ground accumulation of about 25 feet. There are snow fields and glaciers year round on the highest peaks. With a very brief summer, the freeze-free season can be less than 30 days out of the year. There is also a high frequency of summer storms in the northern Cascades, but they bring less than 10 percent of the total annual precipitation. Maximum temperatures are from 70 to 80 F during the day, but drop below freezing at night due to the clear, dry atmosphere at the higher elevetions.

The Intermontane has fewer cloudy days than the west, and smaller amounts of precipitation, usually less than 10 inches a year verses the 100 inches that the west gets due to the Rianshadow Effect of being on the leeward side of the Cascades (Meinig 1968). However, it has larger temperature ranges between winter and summer. The Blue Mountains get a little more precipitaion, about one to two inches per month through the fall, winter, and spring for a total of about 20 inches a year. Spring brings convection thunderstorm. July and August have dry, sunny days and cool nights, which are good for ripening wheat around Palouse and orchard crops in the Yakima and Wenatchee Valleys (Baker 1936). Summer temperatures are between 80 to 90 F during the day, and around 55 F at night. Winter precipitation falls in the form of snow with totals somewhere between 10 to 30 inches. Winter temperatures are between 30 to 40 F during the day, and 20 to 25 F at night. The colder winter temperatures allows the snow to stay on the ground longer than it does on the west side of the Cascades. Sometimes cold, dry air masses flow into the basin from the Rockies. These strong winds flow in response to pressure system movements, and can cause severe wind erosion of the Palouse and Columbia borderlands.

Table 1a: Washington Temperatures-East vs West




Annual Average ( F)



January High ( F)



January Low ( F)



July High ( F)



July Low ( F)



Table 1b: Washington Precipitation-East vs West




Annual Average (inches)



Average Snowfall (inches)



Days w/Precip (days)



Clear Days (days)



Humidity (%)



Vegetation and Soils

The three major vegetation zones in Washington are Forest, Shrub-Steppe, and Alpine (Scott, Vasquez, Newman, Sargent 1989). The Alpine zone accounts for the higher elevations of the Olympics and Cascades, where there are year round snow fields and glaciers. The Shrub-Steppe zone accounts for the Columbia Basin, and the Forest zone accounts for everything elso, the Cascades and Olympics below the Alpine zone, the Willapa Hills, the Lowlands, the Rockies, and the Blues (Fig.4).

Figure 4: Vegetation Zones within Physiographic Regions

Blues&Greens=Forest   Yellows=Shrub-Steppe  White=Alpine

There are eleven different soil regions scattered throughout these vegetation zones, and within these soil regions, various combinations of eight different soil types are found (Fig 5).

Figure 5: Soil and Physiographic Regions

A: Soils of steep and very steep mountainous lands are found in the Olympics, Cascades, Rockies, and Blues, so they are forested with Western Hemlock, Mountain Hemlock, Ponderosa Pine, Grand Fir, and Sub-Alpine Fir, with Alpine Meadows at the higher elevations. The soil types found here are: mollisol, which is a zonal soil with dark, organically-enriched surface horizon, ultisol, which is a zonal soil that has been strongly weathered and leached, and alfisol, which is a zonal soil that is grey to brown in color with clay-enriched horizons and some accumulation of organic matter (Strahler, Strahler 1989).

B: Soils of nearly level to very steep loessial uplands are found on the Columbia Basin and consist of mollisol and aridisol, which is a zonal soil found in arid regions with light-colored horizons and low organic content. The vegetation here Steppe, Bunchgrass, Shrub-Steppe, and Sagebrush.

C: Soils of nearly level to strongly sloping valleys, terraces, plateaus, and till plains are aridisol, mollisol, and inceptisol, which is a zonal soil with dark surface horizon and high organic content. They are also found on the Columbia Basin and support mainly grasslands and shrubs with some Ponderosa Pine and Grand Fir where they reach up into the Cascades and Rockies.

D: Soils of nearly level to moderatly steep channeled scablands are mollisol and inceptisol. They are found in the northern portion of the Columbia Basin, and support mainly grasslands with some shrubs and Ponderosa Pine.

E: Soils of sandy, wind- and water-laid deposits are made up of entisol. This soil type is without any zonal development, and occurs in sandy areas where the parent material is still accumulating. In these regions to the east of the Cascades on the Columbia Basin there are grasslands and shrubs, but on the west coast below the Willapa Hills there is Sitka Spruce.

F: Soils of steep and very steep canyon areas are in the Rockies and the eastern portion of the Columbia Basin. In both locations the soils are mollisol, entisol, aridisol, and inceptisol., but while Ponderosa Pine and Grand Fir grow in the Rockies, Steppe and Bunchgrass are growing on the basin.

G: Soils of gently sloping and steep upland areas, such as in the western portion of the Columbia Basin between the Columbia and Yakima Rivers, are aridisols, mollisols, and inceptisols. Mostly shrubs and sagebrush grow here with Ponderosa Pine and Grand Fir near the base of the Cascades.

H:Soils of gently sloping to steep glacial plains, terraces, and foothills, like those that make up the Puget Lowland, are inceptisol, mollisol, and histosol. Histosol is a highly organic soil found in bogs. This entire area is thick with Douglas Fir.

I: Soils of nearly level to gently sloping alluvial lands make up the floodplains of the Pacific Border, and are inceptisol, mollisol, and histosol. They are forested with Sitka Spruce, Western Hemlock, and Douglas Fir.

J: Soils of the moderately steep to very steep mountain foothills and foot slopes of the Willapa Hills, the Olympic Mountains, and the Cascades are comprised of inceptisol, ultisol, alfisol, and spodosol. Spodosol is a zonal soil that is high in organic matter and lime with clay leached from its surface layer. These soils support Sitka Spruce and Western Hemlock.

K: Finally, soils of nearly level to strongly sloping terraces, foothills, and valleys found along the lower course of the Columbia River are inceptisol and ultisol, and also support Sitka Spruce and Western Hemlock.

Even though a soil region exists in different areas of Washington, they have the same soil types. However, even though the soil types are the same, the vegetation can be different (U.S. Army Corps of Engineers 1975). It depends on natural influences such as elevation or temperatures and precipitation due to the east/westclimatic divide (Fig 6).

Figure 6: Precipitation, Elevations, Soils, and Vegetation


Washington has one of the largest quantities of water in the nation (Microsoft Corporation 1996). However, remembering that Washington is divided by the Cascades into a humid western half and a arid eastern half, it is easy to understand why great differences exist between western and eastern Washington in terms of the availability of water resources (Mead, Brown 1962). Great differences in the demand for water also exist depending on land use and population.

Water withdrawals in western Washington are smaller, and mainly for public use, even though it has three times the population of the east. Surface-water withdrawals are only 584 million gallons per day (Mgal/d) from the Puget Sound drainage basin (U.S. Geological Survey 1990). In eastern Washington, where only 25 percent of the population resides, water withdrawals are five times larger and mainly for irrigation (Mitchell 1996). The two largest surface-water withdrawals are 2,790 and 1,590 Mgal/d in the east from the Upper Columbia and Yakima drainage basins respectively (U.S. Geological Survey 1990). In both cases, most of the water withdrawn is from surface-water sources, and within Washington's water budget (Table 2). However, conflicts among competing uses of surface water have created a greater reliance on ground water. The largest ground-water withdrawal is 597 Mgal/d), that is half of all ground-water withdrawals, and comes from the Columbia River basalt aquifer, in eastern Washington.

Table 2: Water Budget in (Mgal/d)




    4,700 Consuptive Use
Precipitation 129,000 45,000 Evapotransporation
Surface Water Inflow 141,000 219,000 Surface Water Outflow
  270,000 268,700  

Agricultural, industrial, mining, domestic, and commercial purposes, as well as thermoelectric power generation are the principal offstream uses of water in Washington, while navigation, waste dilution, recreation, fish and wildlife propagation, as well as hydroelectric power generation are the principal instream uses (Fig. 7). All of these compete for and are capable of affecting the quality of the water in Washington (Hunt 1974). Although water quality is not a widespread problem in Washington, there is evidence of saltwater intrusion, lake eutrophication, and contamination of ground water locally.

Figure 7: Water Usage

It is the responsibility of the Washington State Department of Ecology (WDOE) to manage the State’s water resources. A water-right permit from the WDOE is required before any person of entity can begin using public surface- or ground-water resources for beneficial use, with the exception of domestic well producing less than 5,000 gallons per day.

With regards to surface-water resources, they have developed minimum instream-flow requirements that are in effect for 14 river basins and along the main stem of the Columbia River for the protection of fish, wildlife, and other instream resources.

With regards to ground-water resources, they have taken measures to protect shallow aquifers from excessive drawdown and water-quality degradation. They have developed policies related to saltwater intrusion in aquifers in island and coastal areas. They have established ground-water management areas, which provide for efficient management of water resources to meet future needs, while recognizing existing water rights, and ground-water subareas where depth zones are designated and withdrawals are regulated to maintain a safe sustaining yield of ground water.

Selected References

Atwood, Wallace W. The Physiographic Provinces of North America. New York: Ginn and Company, 1940.

Baker, O.E. Atlas of American Agriculture. Washington, D.C.: U.S. Government Printing Office, 1936.

Easterbrook, Don J. and Rahm, David A. Landforms of Washington. Washington: Union Printing Company, 1970.

Easterbrook, Donald J. Surface Processes and Landforms. New York: MacMillan Publishing Company, 1993.

Easterbrook, Donald J. Western Washington University, Bellingham, Washington. Lecture, 9 October 1996.

Thoman, Richard S.; Richards, Kent David; and Lantis, David W. Encarta Encyclopedia, s.v. "Washington Geography," Washington: Microsoft Corporation, 1996.

Hunt, Charles B. Natural Regions of the United States and Canada. California: W.H. Freeman and Company, 1974.

Hunt, Charles Butler. Physiography of the United States. California: W.H.Freeman and Company, 1967.

Keller, Edward A. Environmental Geology. 7/e. New Jersy: Prentice-Hall, Inc., 1996.

Kimerling, A.J. and Jackson, P.L. Atlas of the Pacific Northwest. 7/e. Oregon: Oegon State University Press, 1985.

Levin, Harold L. The Earth Through Time. 3/e. New York: Saunders College Publishing, 1988.

Mead, W.R. and Brown, E.H. The United States and Canada. New York: Hutchinson Educational, LTD., 1962.

Meinig, D.W. The Great Columbia Plain. Washington: University of Washington Press, 1968.

Microsoft Corporation. Encarta World Atlas. Washington: Microsoft Corporation, 1996.

Mitchell, Robert. Western Washington University, Bellingham, Washington. Lecture, 1 November 1996.

Mitchell, Robert. Western Washington University, Bellingham, Washington. Lecture, 13 November 1996.

Scott, J.W.; Vasquez, Colin R.; Newman, John G.; and Sargent, Bruce C. Washington Centennial Atlas. Washington: Western Washington University Center for Pacific Northwest Studies, 1989.

Scott, James W. and DeLorme, Roland L. Historical Atlas of Washington. Oklahoma: University of Oklahoma Press, 1988.

Shimer, John A. Field Guide To Landforms in the United States. New York: The MacMillan Company, 1974.

Strahler, Arthur N. and Alan H. Elements of Physical Geography. 4/e. New York: John Wiley and Sons, Inc., 1989.

University of Washington Department of Oceanography. Puget Sound and Approaches: A Literature Survey. v.I. Washington: University of Washington Press, 1953.

U.S. Army Corps of Engineers. Washington Environmental Atlas. Washington, D.C.: U.S. Government Printing Office, 1975.

U.S. Geological Survey. National Water Summary 1987-Hydrologic Events and Water Supply and Use: Washington. Washington, D.C.: U.S. Government Printing Office, 1990.