The Columbia river has entrenched itself in a canyon-like valley around the northern and Western side of the lava plains; Snake river has cut a deeper canyon farther south-east where the plains are higher and has disclosed the many lava sheets which build up the plains, occasionally revealing a buried mountain in which the superposed river has cut an even narrower canyon.
The Snake river (which receives all the drainage of Idaho except small amounts taken by the Spokane, the Pend Oreille and the Kootenai in the N., all emptying directly into the Columbia, and by some minor streams of the S.E.
There are also deep canyons which have been cut by the rivers in their present courses, especially by the Snake river and its tributaries.
Under the Carey Act the Twin Falls project, deriving water from the Snake river near Twin Falls, and irrigating more than 200,000 acres, was completed in 1903-1905.
Trend, forms the water-parting between the streams tributary to the Humboldt river in Nevada and those that flow into the Snake river through Idaho and Oregon and thence to the Pacific Ocean.
The most prominent features of this part of the state are the arid Snake river plains and three mountain-like elevations - Big, Middle and East Buttes - that rise from their midst.