Draft Imnaha Subbasin Summary November 30, 2001 Prepared for the Northwest Power Planning Council Subbasin Team Leader

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The Imnaha River drainage provides a unique and diverse area for soil development due to its geological setting (Figure 3). Varying rock type, topography, and climatic conditions have a large impact on soil-forming processes over the length of the Imnaha River.

Figure 5. Precipitation patterns in the Imnaha subbasin

Soils are generally derived from the weathering of local bedrock or colluvial rock materials (called residual soils). Thus, granitic soils predominate above Indian Crossing (from weathering of the Wallowa batholith) while basaltic soils predominate below Indian Crossing. Residual soils tend to be deeper on north and east facing slopes (capable of supporting conifer stands) and shallower on south and west facing slopes (capable of supporting mainly grasslands). Forces other than weathering of bedrock, however, have also been active in the subbasin. Wind derived soils (loess) and ash deposits from the eruptions of Glacier Peak (12,000 years ago) and Mount Mazama (6,600 years ago) have added greatly to the productivity of the local soils. Ash deposits are very productive with low compactibility and high permeability and water holding capacity, but, because of their low density, are easily erodible. They are generally found on the plateaus where the densest conifer stands are located.

An accelerated process of sedimentation in the upper portion of the subbasin occurs due to the instability of the barren granite mountain peaks. Primary mechanisms of sediment delivery to aquatic habitats in these areas include debris flows and other processes of mass wasting, which are most often triggered by thunderstorms or rain on snow events (Wallowa Whitman National Forest 1993). Low gradient areas and deep pools in the upper and middle portions of the subbasin effectively filter out much of the suspended sediment load delivered to headwater tributaries and mainstem reaches (Art Kreger, USFS Soils Scientist, personal communication February 8, 2001).

The soils that formed from Imnaha basalt along the central part of the valley have much higher clay and coarse sand content than typically found in similar soils throughout the region (Art Kreger, USFS soils scientist, personal communication February 8, 2001). This makes these soils more resistant to erosion along the riverbanks in the central part of the valley. These soils along with those formed on the river terraces have volcanic ash and wind blown silt (loess) content and are well-developed fertile soils that support modern agriculture. They can be a source of sedimentation into the river during flood stages (Tom Smith, NRCS Soils Scientist, personal communication 2/8/01).


The Imnaha subbasin contains vast expanses of relatively undisturbed land. The uppermost part of the subbasin is above the tree line and contains alpine communities (Rose et al. 1992). Below the tree line, the watershed contains a mixture of subalpine communities that grade into forested and grassland stands at lower elevations. Forested communities are more predominant in upstream and eastern portions of the subbasin, whereas grassland communities are more predominant in downstream and western portions of the subbasin (Figure 6). In areas with more intermediate environmental conditions, such as moisture regime and soil type, a mosaic of grassland and forested stands exists.

Figure 6 Current vegetation cover in the Imnaha subbasin.

Forested communities cover approximately 42% of the subbasin (Table 2). At high elevations, subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta), and Engelmann spruce (Picea engelmannii) dominate forested stands (Wallowa-Whitman National Forest 1998; Wallowa-Whitman National Forest 1995; Mays 1992). These high elevation forest communities are found in the headwater areas at the southern end of the subbasin and along parts of the eastern boundary of the subbasin (Figure 6). Grand fir (Abies grandis), Douglas fir (Pseudotsuga menziesii), and ponderosa pine (Pinus ponderosa) dominate low elevation forest communities (Wallowa-Whitman National Forest 1998; Wallowa-Whitman National Forest 1995; Rose et al. 1992). The low-elevation forest communities belong to the northeastern Oregon mixed conifer vegetation type and are found in the eastern and upper elevation portions of the subbasin (Figure 6).

Grasslands cover approximately 43% of the subbasin (Table 2). Most high elevation grasslands in the subbasin belong to the green fescue/Hood’s sedge (Festuca viridula/Carex hoodii) association (Wallowa-Whitman National Forest 1998; Wallowa-Whitman National Forest 1995). These grassland communities occur in the headwaters region of the subbasin. Grasslands at lower elevations belong to a variety of bunchgrass associations with dominants including bluebunch wheatgrass (Agropyron spicatum), Idaho fescue (Festuca idahoensis), Sandberg’s bluegrass (Poa sandbergii), and Kentucky bluegrass (Poa pratensis) (Wallowa-Whitman National Forest 1998; Reid et al. 1991; Nez Perce Tribe et al. 1990). These grasslands belong to the northeastern Oregon canyon grasslands vegetation type. They are found along the steep canyons of the subbasin and generally throughout the northern and western sections of the subbasin (Figure 6).
Table 2. Vegetation cover types in the Imnaha subbasin

Landscape-Level Vegetation Type

Area km2

Area mi2

% of Subbasin

Northeast Ore. Mixed Conifer Forest




Northeast Ore. Canyon Grassland




Modified Grassland




Forest-Grassland Mosaic




Subalpine Fir-Lodgepole Pine Montane Conifer




Subalpine Grassland




Alpine Fell-Snowfields




Grass-shrub-sapling or Regenerating Young Forest




Ponderosa Pine Forest and Woodland












Some plant communities in the subbasin cover little area but have great significance because of high species diversity, importance to wildlife, or their function in the larger ecosystem. Riparian communities in the subbasin are diverse. Riparian communities along the Imnaha range from mixed conifer stands in upper reaches to low shrub and grass stands in lower reaches (Wallowa-Whitman National Forest 1998). In the Big Sheep Creek watershed alone, 80 plant associations have been found in riparian communities (Wallowa-Whitman National Forest 1995). The Zumwalt Prairie, in the northern portion of the subbasin, is one of the best remaining examples of Palouse bunchgrass prairie in North America (The Nature Conservancy 2000). The Zumwalt prairie is significant because of its large size, 220 square miles, and its high quality (The Nature Conservancy 2000). Managed grazing and little agricultural cultivation have allowed the ecological integrity of the Zumwalt prairie to remain high (The Nature Conservancy 2000). One federally listed threatened species, MacFarlane’s four o’clock (Mirabilis macfarlanei), and one federally proposed threatened species, Spalding’s catchfly (Silene spaldingii), have been documented in the Imnaha subbasin (USDA Forest Service Region 6 1999). Over 50 other rare or sensitive plant species have also been documented in the Wallowa-Whitman National Forest (Appendix A).

Introduced species and fewer low-intensity fires have altered the structure and composition of some parts of the subbasin from historical conditions. Some of the successful invaders of riparian communities are diffuse knapweed (Centaurea diffusa), yellow star thistle (Centaurea solstitialis), and leafy spurge (Euphorba esula) (Mason et al. 1993). Grasslands throughout the subbasin have been and are currently grazed. The rangelands are generally in good condition except where cheatgrass (Bromus tectorum), leafy spurge, and knapweeds (Centaurea spp.) have invaded (Wallowa-Whitman National Forest 1998; Mason et al. 1993). Disturbed areas and roadways in the subbasin host a variety of the introduced species listed above and additional species like Canada thistle (Cirsium arvense) (Wallowa-Whitman National Forest 1995). Less frequent wildfires have resulted in grand fir dominating some sites that historically would have been dominated by Douglas fir and ponderosa pine (Mason et al. 1993). Forested stands on many northern slopes lack seral species such as ponderosa pine and western larch (Larix occidentalis) that used to be favored by historical fire regimes (Wallowa-Whitman National Forest 1998). The present fire regime has led to overstocked stands and higher proportions of late seral stands which face insect and disease problems (Wallowa-Whitman National Forest 1998).

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