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FISH AND WILDLIFE RESOURCESSubbasin DescriptionGeneral LocationLower Snake River Subbasin, Columbia Plateau Province (Figure 1). Lower Snake River dams and hatcheries (Figure 2).
Figure 2. Lower Snake River dams and hatcheries (Mendel 1995) TributariesSeveral small tributaries with perennial water flow that likely contain fish populations are included in this subbasin. They generally drain an arid landscape and they have similar climate and land use. Some of these streams drain the north side of the Snake River in Whitman County (e.g. Alkali Flat Creek, Penawawa, Almota, Wawawai and Steptoe Canyon creeks) others drain from the south, primarily in Garfield County (Alpowa, Deadman and Meadow creeks). Little is known about most of these streams, but there is a recent effort by several agencies to sample fish populations and habitat conditions in them. Only two tributaries with the most information will be described in this document. Alpowa CreekAlpowa Creek, located in southeastern Washington (Figure 3) begins in the Blue Mountains at an elevation of approximately 4,000 feet above sea level and joins the Snake River at Lower Granite Lake about seven miles west of Clarkston, Washington. Major seasonal and ephemeral tributaries of Alpowa Creek include Page, Pow Wah Kee Gulch, Clayton Gulch, and Stember creeks. 
Figure 3. Location of Alpowa Creek in the Lower Snake River Subbasin. Deadman CreekGarfield County contains several watersheds (e.g., Pataha, Deadman, Meadow Creek, New York Gulch, Dry Gulch creeks) that drain into the Snake River. For this summary, these creeks have been grouped and identified as the Deadman Creek watershed (Figure 4) located in WRIA 35 and included in sub-region 17060106 of the Pacific Northwest as delineated by the Water Resources. Area on the Northside of Snake River in Whitman CountyAn area of 281,000 acres lies north of the Snake River in Whitman County (Figure 5). This area contains watersheds of (Alkali Flat Creek and Penawawa Creek). For this summary, these creeks have been grouped and identified as the “Area north of the Snake River in Whitman County. This area has drainages very similar to Deadman Creek in Garfield County. Small Area North of Tucannon and Pataha Creek in Columbia CountyAn area of 14,000 acres lies north of the Tucannon and Pataha Creek in Columbia County (Figure 6). This area contains mostly pasture some farmland on the very top of the ridges. For this summary, this area has been grouped and identified as the “Area north of Tucannon and Pataha Creek in Columbia County. 
Figure 4. Location of Deadman Creek watershed in the Columbia Plateau Province. 
Figure 5. Location of the area north of the Snake River in Whitman County 
Figure 6. Location of area north of the Tucannon and Pataha Creek in Columbia County. Drainage AreaReservoirsThe four dams on the Lower Snake River impound more than 96% (137 miles) of the Snake River in Washington from Asotin, Washington, to the confluence with the Columbia River at Pasco, Washington. Also impounded is the lower 3.7 miles of the Clearwater River in upper Lower Granite Reservoir. The remaining 6.0 miles of the Snake River below Ice Harbor Dam forms the uppermost reach of McNary Reservoir (Lake Wallula) on the Columbia River. The entire reach lies within a canyon cut through the Columbia plateau. The physical characteristics of each reservoir were summarized in Bennett et al. (1983), and all reservoirs generally share similar morphometry (Table 1). Lower Granite is the longest reservoir, whereas Little Goose has the largest surface area. Mean depth ranges from 48-57 feet; Ice Harbor Reservoir is the shallowest. Three major tributaries enter this section. The Clearwater River joins the Snake River in upper Lower Granite and the Palouse and Tucannon rivers join near the midpoint of Lower Monumental Reservoir. TributariesAlpowa CreekThe entire drainage area of the Alpowa Creek watershed is 82,944 acres (130 square miles). Most of this area is very arid landscape with several seasonal canyons that enter the mainstem Alpowa Creek. Deadman CreekThe total watershed area is 214,560 acres of which 121,000 acres are cropland. The watersheds of Garfield County, excluding Pataha and Deadman creeks contain over 55 miles of perennial streams. Table 1. Physical characteristics of Lower Snake River reservoirs in Washington and Idaho (Bennett et. al. 1983)
Topography/GeomorphologySeveral mountain ranges, with intervening valleys and plains, lie within the Snake River Basin, a semiarid expanse formed by successive flows of basaltic lava. The Snake River flows across a major physiographic region of the Pacific Northwest known as the Snake River Plateau and along the southern portion of the Columbia Plateau. The Snake River Plateau extends from southwestern Oregon across southern Idaho and includes parts of Nevada and Utah. The Columbia Plateau extends south from the upper curve of the Columbia River to the Blue Mountains, west to the Cascades, and east above the Snake River, just east of the Washington-Idaho state line. These two regions are comprised mainly of lava flows covered with soil. In areas where the Snake River has cut canyons, the dark basalt rock is a primary surface feature. Many of the soils of the Snake River Plateau are light and highly erodible with low rainfall limiting the ability of vegetative cover to reestablish once removed. This results in heavy sediment loads in the river, especially during the spring runoff season. The Miocene and Pliocene basalt flows that covered the region and diverted the Columbia River northward and westward to its present location are largely responsible for the topography of the Columbia Basin. Each basalt formation accumulated from individual flows ranging in thickness from 10-300 feet. Known as the Columbia River Basalt, the lava flows overlie the Precambrian Belt-Purcell Supergroup. The current topography of the region results from a combination of erosion and underlying structural deformation of the basalt. The Alpowa Creek and its tributaries have cut canyons into the Columbia River Basalt since the Miocene epoch, resulting in terraces comprised of weathered stream gravel. Since Alpowa Creek currently has insufficient discharge to transport this quantity of gravel, the outcrops indicate past episodes of higher velocity and flow (Foley 1976), suggesting periods of aggradation and mass wasting dominated the geologic formation of Alpowa Creek. Topography of the Deadman watershed is primarily long slopes intersected by steep canyons. Landforms are mainly flat to moderately sloping. Slopes are complex, being irregular, concave and convex in shape. Elevations range from 650 feet above sea level at the confluence of Deadman Creek with the Snake River to 2,800. ClimateThe Pacific Ocean, Cascade Mountains, and prevailing westerly winds largely influence the climate of this subbasin. The Cascades intercept the maritime air masses as they move eastward, creating a rain shadow effect that reaches as far as the Blue Mountains. These moisture patterns combined with differences produce warm and semiarid conditions along the reservoir system to cool and relatively wet at the tributary headwaters. Alpowa CreekApproximately 70-85% of the precipitation in the Alpowa Creek drainage falls from November through April. The watershed experiences precipitation mostly as rain. Although precipitation records for the Alpowa Creek area are absent, data exists for the nearby cities of Anatone and Pomeroy. In Anatone, precipitation varies from 0.71–2.12 inches in August and May, respectively, with the winter months and May showing the greatest variability in average monthly precipitation. The average monthly precipitation recorded at Pomeroy ranged from 0.59-2.16 inches in July and January, respectively, with the greatest variability in average precipitation occurring in the winter and spring. Amounts of precipitation in the Alpowa watershed varies from 14-18 inches depending on topography. Deadman CreekWithin the Deadman Creek watershed, average annual precipitation ranges from 11 inches on cropland in the western portion of the watershed to 25 inches near Mayview at the head of Casey Creek. Most of the precipitation occurs between September and June. Temperatures range from -22 degrees oF to 109 degrees oF. The frost-free growing season within the watershed averages 110-140 days. SoilsThroughout the subbasin, the mountain and plateau soils are dominated by wind-blown silt (loess) deposits. Volcanic ash from the eruption of Mt. Mazama can be found at higher elevations around mountain summits and north-facing canyon slopes. Plateau tops and shoulder slopes are characterized by silt loams moderately to well drained and highly erosive. Numerous soil series (e.g., Larkin, Tolo, Gwin, Walla Walla, Asotin, Chard, Athena, and Palouse) can be found in the Lower Snake River Subbasin. |
