Snake River Plain

Snake River Plain

The Snake River Plain is a broad, crescent-shaped lowland extending roughly 640 km (400 miles) across southern Idaho. It is not a climatic desert in the same sense as the Mojave or Sonoran deserts. Large parts of the plain are semi-arid sagebrush steppe, however, with low precipitation, high summer evaporation, sparse native shrub cover, and extensive dry basalt terrain.

The plain is also geologically unusual because its eastern and western halves developed in different ways. The Eastern Snake River Plain follows the Yellowstone hotspot track, while the Western Snake River Plain is largely a fault-bounded basin that once contained Lake Idaho.

Geography

The Snake River Plain crosses southern Idaho from the Oregon border toward the Yellowstone region. Its curved shape is easily visible on regional maps because it separates mountain ranges north and south of the lowland.

The plain is commonly divided near Twin Falls into two geologic sectors:

• Eastern Snake River Plain: Extends northeast from the Twin Falls area toward the Wyoming border and Yellowstone region.
• Western Snake River Plain: Extends northwest from Twin Falls through the Boise and Treasure Valley region toward the Oregon border.

The frequently cited figure of about 12,000 square miles refers mainly to the eastern geologic plain in USGS studies and should not be used as the area of the entire Snake River Plain. Broader ecological mapping covers substantially more territory.

The plain itself is not part of the classic alternating mountain-and-basin topography of the Basin and Range Province. Basin-and-range faulting occurs around parts of southern Idaho and influenced regional geology, but the Eastern Snake River Plain is a hotspot-related volcanic depression, while the Western Snake River Plain developed primarily through faulting and basin subsidence.

Two Different Geologic Histories

The eastern and western plains meet geographically but preserve different geologic histories.

Eastern Snake River Plain

As the North American Plate moved across the Yellowstone hotspot, large rhyolitic volcanic systems formed progressively from southwest to northeast. After major volcanic episodes, the crust along the hotspot track subsided.

Younger basalt then erupted across much of the surface. These repeated lava flows now cover extensive parts of the Eastern Snake River Plain, often separated by thin layers of windblown sediment or older soils.

The buried rhyolitic volcanic rocks beneath the basalt preserve the older stages of hotspot activity.

Western Snake River Plain

The Western Snake River Plain developed differently. USGS geological work describes it as a fault-bounded depression formed when extension lowered a block of crust between major normal faults.

From roughly 10 million to 2.5 million years ago, much of this basin was occupied by Lake Idaho. Thick layers of lake and river sediment accumulated alongside volcanic deposits.

When drainage eventually shifted northward, the lake disappeared and the modern Snake River drainage developed.

Craters of the Moon and Recent Volcanism

Craters of the Moon National Monument and Preserve lies on the Eastern Snake River Plain along the Great Rift volcanic zone.

The area contains basalt flows, cinder cones, spatter cones, lava tubes, fissures, and other features created by repeated eruptions over approximately the last 15,000 years. The youngest flows formed about 2,100 years ago.

The Craters of the Moon lava field covers roughly 1,600 km² (620 sq mi). Together with the Wapi and Kings Bowl lava fields, it records some of the youngest volcanic activity on the plain.

The Great Rift extends for roughly 85 km (53 miles) and contains eruptive fissures, faults, volcanic vents, and cinder cones.

Climate

Much of the Snake River Plain has a semi-arid continental climate. Mountain ranges surrounding the lowland receive considerably more precipitation, while the interior plain remains comparatively dry.

Parts of the Eastern Snake River Plain receive about 8–14 inches (200–355 mm) of precipitation annually. At Idaho National Laboratory, long-term measurements have been close to 8 inches per year.

Summers are generally warm to hot and dry. Temperatures above 38°C (100°F) occur during heat waves in lower parts of the plain.

Winters are cold, with regular frost and periods below freezing. Snow supplies part of the annual moisture, particularly across the eastern plain and higher sectors.

The combination of limited rainfall, summer dryness, wind, and porous volcanic ground helps produce the open sagebrush-steppe landscape associated with undeveloped parts of the region.

Eastern Snake Plain Aquifer

One of the defining features beneath the Eastern Snake River Plain is the Eastern Snake Plain Aquifer.

Groundwater moves through fractures, lava-flow contacts, cavities, and other openings within stacked basalt flows. These connected volcanic rocks allow water to move much faster than it does through many sedimentary aquifers.

Recharge comes from several sources:

• precipitation and snowmelt;
• water entering from surrounding mountain basins;
• losses from rivers and streams;
• seepage from irrigation canals;
• infiltration from irrigated agricultural land.

Groundwater generally moves southwest across the eastern plain. Large volumes eventually discharge from springs and seeps along the Snake River near the Thousand Springs area.

Sagebrush Steppe

Before large-scale agricultural conversion, much of the dry plain supported sagebrush steppe.

Big sagebrush forms the shrub layer across many native sites. Bunchgrasses such as bluebunch wheatgrass and Indian ricegrass occupy the spaces between shrubs, along with rabbitbrush, saltbush, greasewood, and seasonal forbs.

These communities provide habitat for animals including:

• greater sage-grouse;
• pygmy rabbit;
• pronghorn;
• mule deer;
• black-tailed jackrabbit;
• burrowing owl;
• ferruginous hawk.

Native shrubland has been altered in many areas by farming, roads, grazing, invasive annual grasses, and repeated wildfire.

Cheatgrass and Fire

Cheatgrass is one of the most widespread introduced plants on disturbed portions of the Snake River Plain.

The annual grass dries early in summer and can create continuous fine fuel between shrubs. Frequent burning can then remove mature sagebrush, while cheatgrass often recolonizes burned ground more quickly than native shrubs.

Repeated fire can therefore shift some sites from sagebrush and perennial bunchgrass toward annual-grass-dominated vegetation.

The pattern is especially relevant in the western and south-central plain, where dry summers and extensive rangeland allow fires to spread across large areas.

Irrigated Agriculture

Modern land use on the Snake River Plain depends heavily on irrigation.

The Snake River, reservoirs, canals, and groundwater transformed large sections of semi-arid land into agricultural areas. Potatoes, wheat, barley, sugar beets, hay, corn, and other crops are grown across irrigated valleys.

Older USGS basin assessments recorded millions of acres of irrigated land in the upper Snake River system. Water is supplied through a mixture of surface-water diversions and groundwater pumping.

Agriculture also altered aquifer recharge. Leakage from canals and excess irrigation water historically added substantial amounts of water to the Eastern Snake Plain Aquifer. Later changes in irrigation efficiency reduced some of that incidental recharge while groundwater pumping remained an important source of agricultural water.

Groundwater Management

Groundwater use on the Eastern Snake Plain has become a major water-management issue.

The Idaho Department of Water Resources reports that aquifer levels declined from the high levels reached around the mid-twentieth century. Because the aquifer and Snake River are hydraulically connected, groundwater pumping can also affect spring discharge and river flows.

Idaho designated the Eastern Snake Plain Aquifer Ground Water Management Area in 2016. Surface-water users and groundwater users have also been involved in long-running administrative proceedings over how pumping affects senior surface-water rights.

Current management includes groundwater monitoring, recharge projects, pumping reductions in some circumstances, and agreements intended to address the balance between withdrawals and aquifer storage.

Snake River Canyon and Birds of Prey

The western plain contains the Morley Nelson Snake River Birds of Prey National Conservation Area, which protects approximately 485,000 acres along 81 miles of the Snake River in southwestern Idaho.

The canyon provides cliff ledges and cavities used by nesting raptors. Bureau of Land Management surveys report that roughly 800 pairs of hawks, eagles, falcons, and owls use the area during the breeding season.

The surrounding shrub-steppe supplies prey such as ground squirrels and other small mammals. Maintaining the upland hunting habitat is therefore closely connected with the breeding success of canyon-nesting raptors.

Lake Idaho and Hagerman Fossils

Sediments left by ancient Lake Idaho are exposed in parts of the Western Snake River Plain.

At Hagerman Fossil Beds National Monument, these deposits preserve fossils of animals that lived in the region before the modern landscape developed. Fossils include the Hagerman horse, along with remains of beavers, otters, peccaries, mastodons, birds, fish, and other animals.

The fossil-bearing sediments record a period when the western plain contained lakes, wetlands, rivers, and surrounding terrestrial habitats rather than today’s semi-arid agricultural valleys.

Snake River Plain Measurements

The Snake River Plain is a semi-arid volcanic and sedimentary lowland rather than a conventional climatic desert. Its eastern half records passage of the Yellowstone hotspot beneath North America, while the western half developed as a fault-bounded basin later occupied by Lake Idaho.

Basalt flows now form much of the eastern surface and contain the permeable Eastern Snake Plain Aquifer. Above ground, remaining sagebrush steppe occurs beside lava fields, irrigated farms, towns, and transportation corridors. Groundwater management, altered fire regimes, invasive grasses, and loss of native shrubland are measurable land and water issues across the modern plain.