Idaho is extremely mountainous. Its northern two-thirds consists of a mountain massif broken only by valleys carved by rivers and streams, and by two prairies: the Big Camas Prairie around Grangeville and the Palouse Country around Moscow. The Snake River Plain extends E-W across Idaho from Yellowstone National Park to the Boise area, curving around the southern end of the mountain mass. A verdant high-mountain area encroaches into the southeastern corner; the rest of Idaho's southern edge consists mostly of low, dry mountains. Among the most important ranges are the Bitterroot (forming the border with Montana), Clearwater (the largest range), Salmon River, Sawtooth, Lost River, and Lemhi mountains. More than 40 peaks rise above 10,000 ft (3,000 m), of which the highest is Mt. Borah, at 12,662 ft (3,862 m), in the Lost River Range. Idaho's lowest point is 710 ft (217 m) near Lewiston, where the Snake River leaves the Idaho border and enters Washington.

The largest lakes are Pend Oreille (180 sq mi/466 sq km), Coeur d'Alene, and Priest in the panhandle, and Bear on the Utah border. The Snake River—one of the longest in the US, extending 1,038 mi (1,671 km) across Wyoming, Idaho, and Washington—dominates the southern part of the state. The Salmon River—the "River of No Return," a salmon-spawning stream that flows through wilderness of extraordinary beauty—separates northern from southern Idaho. The Clearwater, Kootenai, Bear, Boise, and Payette are other major rivers. There are ice caves near Shoshone and American Falls, and a large scenic cave near Montpelier. Near Arco is an expanse of lava, craters, and caves called the Craters of the Moon, another scenic attraction. At Hell's Canyon in the northernmost part of Adams County, the Snake River cuts the deepest gorge in North America, 7,913 ft (2,412 m) deep.