Owyhee Ecological Province





Owyhee Province in the southeastern corner of Oregon is the western foothills and associated plains of the Owyhee Mountains, which are in southwestern Idaho. In Oregon, this province covers about 1.4 million acres, all in Malheur County. Owyhee Province extends south into Humboldt County, Nevada and east into Owyhee County, Idaho.



The southern portion of Owyhee Province in Oregon is characterized by very extensive, very rocky uplands generally sloping down to the west from the Oregon–Idaho border. This portion of the province, which lies south of Highway 95 going west from Jordan Valley community, encompasses the entire upper watershed of Owyhee River that lies in Oregon from about 6 miles upriver from the community of Rome. This portion of the Owyhee River watershed is a continuous dendritic pattern of basalt-cliff canyons that dissect the huge basaltic plain. The north portion of the province in Oregon consists of lava fields, a few lake basins, and some mountainous areas lying south and east of the major Owyhee River canyon breaks.

Elevations in Owyhee Province in Oregon generally are between 4,000 and 5,500 feet. Highest elevations include Mahogany Mountains at 6,168 feet elevation, which is in the northern part of the province, and Oregon Hill at 6,445 feet elevation, which is near the Nevada border. (Elevations are from USGS 1:250,000 topographic quads, 1958.)

Some valleys and areas with suitable soil are farmed and irrigated, primarily for livestock feed. Rangeland strongly dominates the province in Oregon, which also has small, isolated dry lake basins.



Soils of Owyhee Province are related to very extensive basaltic uplands associated with the Owyhee Mountains in southwestern Idaho that typify the province in Oregon. Soils on plains are moderately sloping, clayey, very stony or rocky, and shallow to very shallow over basalt bedrock or hardpans. On buttes and mountain slopes, soils are relatively steep, loamy, stony, and moderately deep. North of Highway 95 and west from Jordan Valley community, basalt flows cover most of the surface on about 107,000 acres. Some of the flows are relatively small, but there are four sizable ones of about 10,000, 16,000, 23,000 and 53,000 acres.

Two major bottomlands are in Oregon’s portion of the province. These are in the Cow Creek basin and the Jordan Creek basin. The Cow Creek basin contains about 6,000 acres of very deep silty to clayey soils that are moderately well drained. The Jordan Creek basin contains about 12,000 acres of very deep silty to clayey soils that are moderately well drained and about 1,300 acres of very deep clayey soils that are poorly drained.94



The 1941 Yearbook of Agriculture showed no official weather stations in Owyhee Province in Oregon.28 Johnsgard17 shows one: at Danner, about 15 miles west of Jordan Valley community and north of Highway 95. The station is now abandoned. The 22-year record at Danner shows an average annual precipitation of 10.6 inches of which 53% falls in winter (November through March) and 31% in the herbaceous native-plant growing season (April through June). A precipitation map53 shows much of this province in Oregon receives between 10 and 15 inches annual precipitation. Areas in the western part of the province, contiguous to Humboldt Province, receive about 10 inches precipitation; areas on the east side, where the effects of Idaho’s Owyhee Mountains are apparent, receive about 15 inches annual precipitation.

Average January maximum and minimum temperatures at Danner station are 37.1 and 12.3°F, respectively, and the maximum and minimum temperatures for April through June are 70.8 and 35.8°F, respectively.



In Oregon, vegetation associated with the extensive basaltic uplands of Owyhee Province is a shrub–grassland climax type, i.e., with 10% or more natural canopy cover of shrubs. On the extensive sloping stony plains where soils are very shallow, low sagebrush is the dominant shrub. On buttes and mountain slopes where soils are shallow to moderately deep, Wyoming big sagebrush is the dominant shrub. Bluebunch wheatgrass, Sandberg bluegrass, and squirreltail are prominent grasses.

Native vegetation associated with the well-drained bottoms in Cow and Jordan creek basins is a natural grassland climax type, i.e., with less than 10% canopy cover of shrubs. Originally, these bottomlands likely produced a dense, vigorous stand of basin wildrye (Fig. 35).43 Poorly drained bottomlands likely produced basin wildrye, sedges, rushes, sod bluegrasses, and other such species that tolerate alternating wet and dry periods, which would occur in this arid climate.

Figure 35: A dense, vigorous stand of basin wildrye and Nevada bluegrass on a bottomland in the vicinity of Jordan Valley in Owyhee Province, Oregon. The man standing in the grass is Bud Town, SCS Range Conservationist.

Riparian areas in Owyhee Province in Oregon are primarily along the perennial streams in the upper Owyhee River system. These streams run in very extensive basalt-walled canyons and are relatively inaccessible from contiguous uplands. Based on the USGS 1:250,000 topographic quad map, these canyon-walled perennial streams extend about 120 miles collectively in Oregon’s portion of Owyhee Province. The significance of these riparian areas to wildlife is obvious even though there is a myriad of manmade water holes throughout Owyhee Province in Oregon. Vegetation in these perennial-stream riparian areas includes a variety of species which signify different degrees of wetness.

Scattered, small, clayey semiwet meadows in areas such as Mahogany Mountains are typified by species such as Nevada and Kentucky bluegrasses, meadow sedges, timothy, redtop, meadow barley, iris, cinquefoil, and yampa. These are natural grasslands.

The 1936 Forest Type Map of Oregon54 shows no stands of western juniper or other trees in Owyhee Province in Oregon. This indicates that, if junipers were on mountain slopes or buttes in the province at that time, they likely were scattered. However, a map that covers the north portion of Owyhee Province in Oregon, shows about 2,500 acres of shrubs and trees on Mahogany Mountains southeast of Malheur Reservoir.55

The shrubs and trees indicated on this map represent groves of curlleaf mountain-mahogany growing at about 5,700 feet elevation (Fig. 36). This is an interesting ecological phenomenon. Plant species growing here and the deep, black silty soils on north exposures in this isolated mountainous terrain are comparable to other isolated high-elevation mountains such as Hart and Steens mountains in High Desert Province of Oregon.

Figure 36: Groves of curlleaf mountain-mahogany on Mahogany Mountains in the northern portion of Owyhee Province, Oregon

In Mahogany Mountains, some of the oldest mahogany plants are huge. They grow in crevices of basalt bedrock outcrops on ridgetops. One mahogany tree measured 18 inches in diameter at 10 inches above ground level and was 20 feet high with a crown 25 feet in diameter. One old stump, probably cut for firewood by old-time sheepherders, was about 22 inches in diameter at the top of the stump, 18 inches above ground level. Most of the mahogany trees were cut a long time ago, and the remaining stumps show that the original stand of mature trees, 12 to 24 inches in diameter, were spaced about 10 feet apart.32 No western juniper trees were recorded in these studies.

By 1962, it was apparent that this high-elevation, well-watered area had been severely grazed by sheep and cattle over many years.41 It is an ideal summer range in an otherwise arid area and likely attracted the greatest concentration of grazing animals, including deer. The dense stands of younger mahogany and reproduction on north-facing slopes in the area suggested that continued close use over the years resulted in the mahogany’s encroaching, or increasing, on these shrub-covered north exposures, which are snowdrift locations that typically have deep, black silty soils.

In 1962, the snowdrift areas had produced an overstory of mahogany and an interesting array of plant species in the understory.41 These included shrubs such as bitter cherry and chokecherry, mountain snowberry, snowbrush, dwarf rock spirea, low Oregon-grape, big sagebrush (the variety was not noted at that early date but likely was mountain big sagebrush), and mahogany reproduction. Distinctive grass species on these sheltered sites included pinegrass, blue and basin wildrye, big, Canby, and Wheeler bluegrasses, a dryland sedge, Columbia needlegrass, and Idaho fescue. This is an interesting shrub–grassland climax type in an isolated high-altitude location. It also proved to be a haven for rattlesnakes.

The 1962 studies of Mahogany Mountains area41 indicated that the upland slopes are a shrub–grassland climax type with big sagebrush and green and gray rabbitbrushes as the major shrubs. Predominant grasses include bluebunch and beardless wheatgrasses, Idaho fescue, Columbia and Thurber needlegrasses, Sandberg and Canby bluegrasses, oniongrass, and squirreltail. Arrowleaf balsamroot was the most prominent forb.



Management Implications

The southern portion of Owyhee Province in Oregon has natural restrictions on management options: extensive, very stony, shallow-soil series and the many continuous basalt-cliff canyons along major drainages and tributaries of Owyhee River. Large grazing units are essentially mandated here, as in High Desert and Humboldt provinces in Oregon, but without the flexibility of vehicular traffic on roads crisscrossing the country. Recreational activities are similarly restricted.

The BLM Vale Project covered much, if not all, of Oregon’s portion of Owyhee Province. Consequently, many rangeland manipulations and improvements were made. This benefitted many resource values such as wildlife, watershed, recreation, water quality, and range condition while at the same time providing great benefits to livestock ranching. As a result, resource management problems in this portion of Oregon’s Owyhee Province appear to be minimized compared with some other provinces. The ranching element around Jordan Valley seems to be effectively sponsoring perpetually beneficial programs on both private and public lands.


Province Demarcation

Owyhee and Humboldt Demarcation

Beginning at the southwest corner of Owyhee Province in Oregon, which is at the Oregon–Nevada border about 13 miles east of where Highway 95 crosses the border and about 4 miles southwest of Oregon Hill, the line of demarcation between Owyhee and Humboldt provinces lies at about 5,000 feet elevation along the western boundary of the sloping basaltic plateau, which is in Owyhee Province, and the hilly uplands to the west, which are in Humboldt Province. The line continues north along the western edge of the plateau to the vicinity of Battle Creek Ranch and on north at about 4,000 feet elevation to about 4 miles west of Jackie’s Butte, in Owyhee Province. From northwest of Jackie's Butte the line of demaracation between Owyhee and Humboldt Provinces veers eastward and northward at about 4,000 feet elevation to about 7 miles north of Jackie’s Butte where Owyhee, Humboldt, and Snake River provinces adjoin at about 4,000 feet elevation.

Owyhee and Snake River Demarcation

From the juncture of Owyhee, Humboldt, and Snake River provinces, the line of demarcation between Owyhee and Snake River provinces meanders north at about 4,000 feet elevation to cross Owyhee River about 6 miles upriver from Rome community, which is in Snake River Province. From this point, the line runs north at 4,000 feet elevation to the vicinity of Arock community, which is in Snake River Province.

From the vicinity of Arock, the line follows north and northeasterly along the rim of Owyhee River canyon at about 4,000 feet elevation. It follows along this elevation, which is the approximate break between the vast basalt plateau and scattered hills of Owyhee Province and the exposed sedimentary and tuffaceous materials lying below the rimrocks to the north, which typify Snake River Province. The line of demarcation continues northeast along the rimrocks to the north of Mahogany Mountains and east to the vicinity of the abandoned community of Rockville, in Snake River Province. The line continues around the Sucker Creek watershed at 4,000 feet elevation north and then east to cross the Oregon–Idaho border about 15 air miles south of the Snake River.