This subbasin summary includes an assessment of the Umatilla River, Willow Creek, Juniper Canyon and Sixmile Canyon watershed areas. It has evolved out of the rolling provincial review process, developed by the Northwest Power Planning Council (NWPPC) in February 2000, in response to recommendations by the Independent Scientific Review Panel (ISRP) and the Columbia Basin Fish and Wildlife Authority (CBFWA). The process of developing a subbasin summary was initiated as part of the provincial review process at a November 28-29, 2000 meeting in Pendleton, Oregon.
The primary intent of this document focuses on the Umatilla River subbasin, but includes discussion of other small adjacent watersheds where information is available. Willow Creek, Six Mile Canyon and Juniper Canyon represent significant wildlife habitat, are largely unstudied, and need to be addressed at greater depth in the future. These systems were included in this document because they have not been addressed in other subbasin summaries as part of this process. This summary is an interim document that provides context for project proposals during the provincial reviews, while a more extensive subbasin plan is developed.
The Umatilla River and Willow Creek subbasins are two of a number of subbasins included within the Columbia Plateau province (Figure 1). For the purpose of brevity, when appropriate, the Umatilla, Willow, Six Mile Canyon and Juniper Canyon will be referred to as the Umatilla/Willow subbasin (Figure 2).
Draining an area of nearly 2,290 square miles (Gonthier and Harris 1977), the Umatilla River originates in the Blue Mountains of northeastern Oregon and flows north and west to enter the Columbia River at river mile (RM) 289. Elevations in the subbasin range from about 5,800 feet near Pole Springs on Thimbleberry Mountain to 260 feet at the mouth of the Umatilla River (Figure 3). The south and east portions of the drainage lie on the steep, timbered slopes of the Blue Mountains within the Umatilla National Forest. The remainder of the drainage consists of moderate slopes and level terrain.
To the west of the Umatilla subbasin is Willow Creek, a 79-mile long river that drains into the Columbia River at RM 253. Willow Creek and its tributaries drain an area of about 880 square miles, ranging in elevation from 269 feet at its confluence with the Columbia River, to 5,583 feet at its headwaters near Bald Mountain in the Umatilla National Forest. The upper Willow Creek drainage has a total annual flow of approximately 30,000 ac-ft; however, by RM 4, total annual flow is reduced to an estimated 23,000 ac-ft due to extensive irrigation withdrawals. The U. S. Army Corps of Engineers (COE) constructed a 160-ft high dam just upstream of Heppner, Oregon in an effort to control flash flood events, which in the past have claimed both lives and property. (http://www.nwp.usace.army.mil/op/D/standard/wc/wc.htm).
Figure 1 Umatilla/Willow subbasin within the Columbia Plateau ecoprovince
Figure 2 Assessment units and major features of the Umatilla/Willow subbasin
Figure 3. Elevation ranges in the Umatilla /Willow subbasin
This dam is located at the confluence of mainstem Willow Creek and the Balm Fork of Willow Creek and creates the 14,000 ac-ft Willow Creek Reservoir. The Juniper Canyon watershed, which lies to the east of the city of Umatilla, encompasses 72 square miles and enters the Columbia River at RM 315. Elevations within the drainage range from 105 feet at the Columbia River, to 617 feet at the headwaters. Flows in Juniper Creek are ephemeral, largely resulting from storm events.
Between the Willow Creek and Umatilla drainages lies a 472 square-mile expanse of semi-arid land. This area has seasonal streams, which seldom drain into a Columbia River tributary. These tributaries include Sixmile Canyon and Sand Hollow. Juniper Canyon enters the Columbia east of the Umatilla subbasin, 16 river miles downstream from the Umatilla/Columbia River confluence. The most significant human usage of the area is the Boardman Bombing Range.
The Umatilla subbasin lies within Umatilla and Morrow Counties, Oregon, with a negligible portion of the headwaters located in Union County. Seventy eight-percent of the Willow subbasin occurs in Morrow County, while 22% occurs in Gilliam County. The Umatilla drainage is a part of the historic homelands of the Walla Walla, Cayuse, and Umatilla Indian Tribes. Approximately 6.4 million acres (Confederated Tribes of the Umatilla Indian Reservation 1996) of their lands in northeast Oregon and southeast Washington were ceded to the federal government under the Treaty of 1855. The Tribes maintain reserved rights for these lands that include harvesting salmon, wildlife, and vegetative resources (U. S. Army Corps of Engineers 1997). The Umatilla Indian Reservation is located within the Umatilla subbasin, including the Confederated Tribes of the Umatilla Indian Reservation government headquarters at Mission, Oregon. Today, the lands of the CTUIR encompass approximately 236 square miles of northeastern Oregon (Confederated Tribes of the Umatilla Indian Reservation, 2000).
The entire Umatilla/Willow subbasin falls within Oregon’s North Central Climatic Zone (Zone 6). The local climate is subject to different large-scale patterns depending on location within the subbasin (Figure 4). The major influence to the regional climate is the Cascade Mountains to the west, which form a barrier against warm moist fronts from the Pacific Ocean (Johnson and Clausnitzer 1992). The Columbia Gorge provides a break in the curtain of the Cascade Mountains and occasionally allows moisture laden marine air to penetrate into the northern Blue Mountains. This induces light to moderate precipitation (depending on elevation), and results in vegetation more common to the west slopes of the Cascades (Johnson and Clausnitzer 1992).
Light to moderate precipitation characterizes the climate in the subbasin, which experiences a wide range in annual temperatures, partially as a function of a highly diverse topography. A climatic gradient from northwest to southeast across coincides with increasing elevation. As a result, warm and dry conditions exist in the northwestern, low elevation portion of the subbasin, while cool and wet conditions prevail in the southeastern highlands of the Blue Mountains. The average growing season also changes with this northwest – southeast gradient, decreasing from northwest to southeast.
Figure 4. Precipitation ranges in the Umatilla /Willow subbasin
The subbasin experiences a continental climatic regime in the summer, with warm days, cool nights, and little precipitation. In the low elevation portions of the subbasin to the northwest, daytime summer temperatures of 100F are not uncommon. Mean annual temperature and precipitation have fluctuated over the last century, with recent years showing an increase in precipitation and a slight decrease in temperature (Figure 5 and Figure 6).
Figure 5. Air temperature in Climate Zone 6 (North Central) of Oregon state (1895-1995) (Oregon Climate Service 1999).
Figure 6. Precipitation in Climate Zone 6 (North Central) of Oregon state (1895-1995) (Oregon Climate Service 1999).
Precipitation across the subbasin falls mainly between late fall and early spring. Precipitation is generally adequate for wheat farming in the higher plateau areas, but inadequate for diversified farming (Bureau of Reclamation 1954). Average annual precipitation ranges between 55 inches in the southernmost portions of the Blue Mountains to less than nine inches near the Columbia River (Figure 4). The average monthly temperature and precipitation for the Hermiston, Pendleton and Pilot Rock stations in the Umatilla subbasin show low precipitation, strong seasonal variation, and slight variation by elevation (Figure 7 and Figure 8). These areas are all at relatively low elevations for the subbasin. Average monthly precipitation in the Willow subbasin and Sixmile Canyon area also vary by elevation (Figure 9), as demonstrated by differences recorded at the Boardman (620’ el.) and Heppner (1890’ el.) stations (Figure 9).