Idaho Geology

The Plateau Regions

Parts of the greater volcanic Columbia Plateau of Washington

The Clearwater Plateau is the second region of Idaho formed like the Columbia Plateau

Clearwater Plateau

Area drained by the Clearwater River, including the Palouse Hills

Tall rolling hills and gently sloping prairies

A lot of it is ideal for farming for many reasons

Clarkston-Lewiston

Moscow

Post Falls

Snake River

Clearwater River

WA ID

Potlatch River

Dworshak

Clearwater and tributaries like the Potlatch

Snake River on border

Clearwater joining from the east

The Clearwater joins the Snake at the western edge of Idaho

Lowest point in Idaho – 738 feet

at Lewiston

Originally formed by basalt flows

Granite peaks – Cottonwood Butte and Craig Mt.

Exposed as hills uplifted and basaltic lava eroded away

Much of the region formed by wind and water erosion. Covered by a thick layer of rich topsoil of eroded volcanic

material mixed with clay-like glacial sediment blown in from Puget Sound (loess). This makes up the Palouse Hills.

Wheat farms of the Palouse with rich water-holding soil called loess

Clearwater Climate

• Fairly mild

• Summer temp. averages 89 degrees with cool evenings

• Winter temp. averages 26-31 degrees

• Precipitation 20+ inches (25-55 in higher elevations)

• 4-6 month growing season! (the Banana Belt)

Irrigation not needed because of clayey glacial loess

• The grasslands originally covering much of the region have now been generally replaced by domesticated grasses (wheat, oats, barley) and legumes (alfalfa, peas, lentils)

• Crop rotation and fallow fields help offset the loss of nutrients and water

Palouse

Camas Prairie

• At higher elevations through the plateau and as the plateau in the east rises to meet the Rockies, evergreen forests are found. These are mainly………

Pine forests

Owyhee-Bruneau Plateau

3rd Volcanic region in Idaho

Rolling hills and

mountains; dry and sparsely

populated

Mountains

Jump Creek Hills

Poison Creek Hills

French John Hills

Owyhee Mts.

Rivers

Owyhee River

Bruneau River

Jarbridge River

Salmon Falls Creek

Bruneau Gorge

ColumbiaPlateau

lava flows included Owyhee-Bruneau

Lava layers from the many preceding

fissure flows

Owyhee-Bruneau becoming a plateau through uplifting forces. This is the main difference

between it and the Snake River Plain

2000-5000 feet above the SRP;

some peaks rising to 8000 feet

WA, OR, CA

ID

Why is it so dry?

Rainshadow Effect

Climate

• Extreme and semi-arid

• Precipitation 13-20 inches

• Summer average temp. = 88 degrees

• Winter average = 12 degrees

Southern Idaho Vegetation

O-B Summary• Hostile environment and

climate

• Ranching is main industry

• Low population density (more cows and sheep than people)

• Few towns

• Murphy is the county seat of Owyhee County (pop. 600)