Treasure Valley and Owyhee Plateau

The highest density of nesting raptors, thousand-foot-deep canyons, and a tall sand dune
States and Coordinates: Idaho-Nevada-Oregon, 41˚ to 45˚ North, 114˚ to 118˚ West
Overview
There are several distinct areas of the shrub-steppe ecoregion of the Columbia Plateau. For the purposes of this discussion, the ecoregion is subdivided into four sections, based on biological or geographic criteria. The first area, the Upper Snake RiverPlain , was discussed in Part I.
The second area is the Treasure Valley and Owyhee Plateau, the subject of this article. The Treasure Valley towns such as Boise, Nampa, Caldwell, Emmett, Ontario, and Weiser were settled around irrigated cropland, made possible by the numerous federal reservoirs and the water supply provided by the Snake, Boise, Payette, Owyhee, and Malheur Rivers. One unique community is the town of New Plymouth (1), N43˚58’ W116˚49’, which was established in 1895 along the Payette River. This was a planned community developed by the Plymouth Society of Chicago. Purchasers of shares were entitled to 20 acres and a town lot. The two town streets were developed in a horseshoe shape and separated by an 80-foot-wide park. The town plan is still evident today.
Surrounding the valleys are semiarid uplands and foothills which include such areas as the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA)(2), areas around Mountain Home, and the Mount Bennett Hills and Picabo Hills. To the north of the Mount Bennett Hills is the unique Camas Prairie, an area of meadows and wetlands to the south of the Sawtooth Range in the vicinity of Fairfield and along US Route 20.
The area around Mountain Home was originally sagebrush steppe, but many sites are being invaded by cheatgrass. The sagebrush-perennial grassland ecosystem is being lost due to wildfire, heavy grazing, and perhaps other disturbances. In the Morley Nelson Snake River Birds of Prey NCA, shrublands in 1979 covered 51 percent of the area. Within 20 years, about half of the shrublands were lost, mostly due to fire. Once cheatgrass is established, it feeds continuous fires that destroy more shrublands. Natural recovery of shrubland will be slow because of the arid conditions. This affects other wildlife as well. For example, blacktailed jackrabbits are associated with shrublands. Golden eagles prefer to eat jackrabbits, and the decline in prey will cause a reduction in the ability of the area to support golden eagles. Numbers of prairie falcons may also be affected. These birds eat ground squirrels. Up to five percent of the world’s population of Piute ground squirrels is found on the NCA, and the ground squirrels also are affected by broad-scale change to annual grassland (Sullivan 2005).
To the south and west of the Treasure Valley are the Owyhee Uplands, a high lava plateau which drains into the Snake River and covers most of southwestern Idaho and southeastern Oregon. Precipitous canyons of the Bruneau, Owyhee, Malheur, and Salmon Falls Creek cut through the uplands in places. The canyons are riddled with caves in many of the walls. Scattered volcanic buttes and cones rise out of the lava plains. The plateau is covered with semiarid vegetation such as sagebrush and grassland, but the Silver City Range rises above 6,500 feet in elevation and supports forests.
The volcanism that created the Owyhee Uplands created copious quantities of volcanic ash which blanketed the area periodically. Lava blocked creeks and created lakes and marshes. Volcanic ash then settled in the lakes, providing nutrients which promoted diatoms and algal blooms. These fell to the bottom and were deposited as siltstone and shale. Plant leaves, fruit, and flowers were deposited in these lakes and preserved as exquisite fossils due to the fine-grained sediments of the volcanic ash. The Succor Creek fossil flora, found in shale formed from volcanic ash in the Owyhee Uplands in Idaho and Oregon, is the largest Tertiary-period (15 million years ago) plant assemblage known from North America, with 160 species identified. Plants at that time were temperate with warm-temperate evergreens in lowlands. Shortly after this, the climate was in transition to a dryer, cooler one and eventually to the shrub-steppe seen today (Graham 2011).
There is one National Historic Landmark in theTreasure Valley-Owyhee Plateau. The Old US Assay Office, Idaho Historical Society, Boise, Idaho (N43˚37’ W116˚12’) was built in the 1870s to serve gold miners on the Clearwater, Wood, and Salmon Rivers and in the Boise Basin. By 1917, Idaho gold mines had yielded $400 million in gold. The office operated until 1933 and is today home of the State Historic Preservation Officer for Idaho.
There is one National Natural Landmark in theTreasure Valley-Owyhee Plateau. Crater Rings(3), part of the Morley Nelson Snake River Birds of Prey National Conservation Area, Idaho (N43˚9’ W115˚48’) are two adjacent pit craters, volcanic conduits which formed when rising lava came into contact with groundwater and exploded.
For a more complete list of sites in this ecoregion, please visit http://sites.google.com/site/enviroramble