Physical Geography

IdahoIdaho is a state in the Rocky Mountain region of the United States.

Idaho occupies part of three major physical regions of the United States: the Rocky Mountains, the Columbia Plateau, and the Great Basin.


The Rocky Mountain section extends through the northern, central, and extreme eastern parts of the state. High rugged land prevails, especially in central Idaho. Among the loftiest ranges are the Bitterroot, Salmon River, Seven Devils, Sawtooth, Lost River, and Lemhi mountains. Numerous peaks exceed elevations of 10,000 feet (3,000 m). The highest, at 12,662 feet (3,859 m), is Borah Peak, in the Lost River Range. Major breaks in the mountainous terrain occur in the hilly Palouse region around Moscow and in the prairie lands northwest of Grangeville.

The Columbia Plateau section of the state consists mainly of the Snake River Plain, in the south, and low plateaus and mountains, in the southwest. Arid to semiarid conditions prevail nearly everywhere. On the Snake River Plain, which arcs across the state, are nearly all of Idaho's principal cities and most of its best farmland. Lava flows and other volcanic formations, sand dunes, and canyons are also prominent features of Idaho's part of the Columbia Plateau.

The Great Basin section, in the southeast, is part of a large arid region of the western United States. Scattered low mountain ranges, trending roughly north-south, are the chief features of Idaho's section of the basin.


Idaho slopes generally downward from east to west, beginning at the Continental Divide, and is drained mainly by the Snake River and its tributaries. Among tributaries with large flows are the Salmon and Clearwater rivers; others include the Boise and Payette. Only the narrow northern part of the state, known as the Panhandle, and the Great Basin section of the state lie outside the Snake River basin. The Pend Oreille River drains most of the Panhandle.

Many of Idaho's rivers flow as white-water torrents through gorges and deep, scenic valleys. Hells Canyon, carved out by the Snake River on the Idaho-Oregon border, is one of the deepest chasms in the world.

Lakes are found throughout much of the state. Large natural lakes of great scenic beauty are mainly in the Panhandle, site of Pend Oreille, Coeur d'Alene, and Priest lakes. Sizable man-made lakes, created primarily to provide water for irrigation and electrical power, include Dworshak, Cascade, and American Falls reservoirs. In the high mountains are numerous glacial lakes known for their crystalline waters. Hot springs and waterfalls are also found in the state.


Idaho has a continental climate that varies greatly with elevation and location within the state. Winters in the mountains are long and bitterly cold. Lowlands and valleys, particularly in the far western part of the state, have milder winter weather, mainly because of lower elevation and greater exposure to moderating winds from the Pacific Ocean.

Average January temperatures in the lowlands and valleys range from 20° F. (-7° C.) on the eastern Snake River Plain to slightly above freezing in the Lewiston area. Average temperatures for July vary from about 65° F. (18° C.) in the Panhandle to about 75° F. (24° C.) in the valleys of the southwest. High daytime temperatures and cool nights are characteristic of the climate during summer.

Precipitation is meager throughout much of the state. Only the high mountains receive 40 inches (1,020 mm) or more each year. The Snake River Plain, the driest part of the state, usually receives only 8 to 12 inches (200 to 300 mm) annually. The greatest amount of precipitation occurs in the winter, the least in the summer. The yearly snowfall varies enormously, from less than 20 inches (510 mm) in some valleys to more than 200 inches (5,100 mm) in the high mountains.

Vegetation and Wildlife
Idaho'sIdaho's state flower is the syringa.

Forests cover about two-fifths of the state. They are found mostly in the mountains of central and northern Idaho. Western white pine, ponderosa pine, Douglas fir, western larch, Engelmann spruce, lodgepole pine, white fir, and western red cedar are among the chief trees. All are conifers. Cottonwoods, growing mostly along stream banks in the drier areas, and quaking aspen are the principal deciduous trees. Sagebrush, bunch grass, and other drought-resistant small plants provide a sparse covering throughout the semiarid to arid areas of the south.

Idaho—especially those parts that the federal government has designated as wilderness or primitive areas—is one of the nation's last great sanctuaries for large animals. Elk, moose, black bears, grizzly bears, mule deer, white-tailed deer, cougars, timber wolves, bighorn sheep, and mountain goats are among the animals found in the state. Songbirds, birds of prey, waterfowl, and other kinds of birds are abundant and varied. Lakes and streams abound with trout and other fish.

Interesting facts about Idaho
The St. Joe River, which empties into Coeur d'Alene Lake, is the world's highest commercially navigable river. The St. Joe is navigable for about 50 miles (80 kilometers) east of the lake. The river flows more than 2,100 feet (630 meters) above sea level.
The longest main street in the United States is located in Island Park. The street runs 33 miles (53 kilometers) through the village, which consists mainly of a long stretch of resorts that became incorporated into one town.
Crystal Ice Cave, near American Falls, has a frozen river, frozen waterfall, and other beautiful formations of ice and stone. The cave is located 160 feet (49 meters) below the lava beds of the Columbia Plateau region. Some of the ice formations in the cave are hundreds of years old. While the temperature aboveground may reach 95 to 97 degrees F. (35 to 36 degrees C), the temperature in the ice cave remains at 32 degrees F. (0 degrees C) throughout the year.
Lava Hot Springs is the home of world-famous hot springs. More than 6 million gallons (23 million liters) of steaming mineral water pour out of the springs each day.
The Big Wood River has been called the upside down river. In one stretch, the river is about 100 feet (30 meters) deep and 4 feet (1.2 meters) wide, while a nearby section of the river is about 100 feet (30 meters) wide and 4 feet (1.2 meters) deep.
Idaho'sIdaho's state tree is the western white pine.