The Umatilla subbasin consists of two primary areas: the Blue Mountain physiographic province and the valley physiographic province (sometimes referred to as the Umatilla plain). The Umatilla River and its tributaries begin in the Blue Mountain physiographic province, which is characterized by deeply incised upland surfaces and a ramp-like slope call the Blue Mountain slope (U. S. Army Corps of Engineers 1947). The Blue Mountain province consists of flat-topped ridges and steep stair-stepped valley walls formed by thousands of feet of Miocene basalt flows that surrounded and largely engulfed the batholithic cores of the mountains (U. S. Army Corps of Engineers 1947). The structural deformation of the basalt and its subsequent erosion created the varied topography of the subbasin. The Miocene basalt belongs to a regionally widespread series of flows known as the Columbia Basin basalt.
Figure 7. Average monthly temperature for three climate stations in the Umatilla subbasin, 1961-1990 (Oregon Climate Service 1999).
Figure 8. Average monthly precipitation at three climate stations in the Umatilla subbasin, 1961-1990 (Oregon Climate Service 1999).
Figure 9. Average monthly precipitation at the Heppner and Boardman climate stations in the Willow subwatershed and Sixmile Canyon area (respectively) (Oregon Climate Service 1999)
Three major basalt formations occur in the Umatilla subbasin: the Saddle Mountains, Wanapum, and Grande Ronde. Each basalt formation is an aggradation of smaller individual flows sharing similar flow histories and chemistry extruded from a regional volcanic vent system and filling the shallow structural basin of the Columbia Plateau (Gonthier and Bolke 1993). The flow thickness can range from five feet to as much as 150 feet, and collectively is estimated to be hundreds to thousands of feet thick (Newcomb 1965). As the mountains were further uplifted and the horizontal basalt layers warped into a series of folds, streams carved canyons through the basalt layers, creating a highly dissected landscape (Davies-Smith et al. 1988).
As the streams leave the canyons of the Blue Mountain province, they cross a wide expanse of plains and terraces known as the valley physiographic province (Newcomb 1965). The valley province is comprised of tertiary and quaternary loess, alluvium, glacio-fluvial, and lacustrine sediment deposits which mantle the Columbia River basin across much of the lower elevations (Newcomb 1965). During the tertiary period, ancestral streams washed the oldest of the valley sedimentary deposits down from the canyons of the Blue Mountains and deposited them along the mountain front (Gonthier and Bolke 1993). Quaternary deposits of wind-borne silt, or loess, blanket much of the tertiary deposits and basalt flows in the subbasin. The source of these loess deposits was likely flood-deposited material left from the massive Missoula Floods that periodically inundated large areas of the Columbia Plateau from 12,800 to 15,000 years ago (Gonthier and Bolke 1993). The highly productive soils that make the region famous for its agriculture are largely derived from these quaternary and tertiary deposits.
There are about 75 different kinds of soil in the Umatilla subbasin ranging from highly fertile loess to volcanic ash derived from eruptions of Mt. St. Helens 21 years ago, Mt. Mazama 6,000 years ago, and Glacier Peak 11,250 years ago (Johnson and Makinson 1988). At higher elevations, the soils were formed in volcanic ash and residuum; other portions formed in loess, colluvium, and residuum (Johnson and Makinson 1988). The lowest elevation portion of the valley physiographic province around Hermiston consists of soils that formed in aeolian sand, loess alluvium and lacustrine sediment on terraces of the Columbia River (Johnson and Makinson 1988). The portion of the valley physiographic province that lies north of the Umatilla River formed in loess, lacustrine sediment, and alluvium on hills, terraces, and piedmonts (Johnson and Makinson 1988). The Umatilla River bounds the final soil unit found in the valley province on the north, and Birch Creek bounds it on the east. These soils were formed in loess, colluvium, and alluvium on hills.
Originating at nearly 6,000 feet in elevation, the Umatilla River headwaters flow out of the Blue Mountains through narrow, well-defined canyons. After leaving the mountains, the North and South Fork join to form the mainstem, a 90 mile reach of river which flows through a series of broad valleys that drain low rolling lands (U. S. Army Corps of Engineers 1997; Oregon Department of Environmental Quality 2000). The mainstem Umatilla River has eight main tributaries: the North and South Forks of the Umatilla River and Meacham Creek in the upper basin; Wildhorse, Tutuilla, McKay and Birch Creeks in the mid basin; and Butter Creek in the lower basin (Table 1).
Intermittent flows with spring peaks characterize flows in Juniper Canyon. The lower reaches of Willow Creek are also intermittent, while the upper portion maintains several perennial streams. Isolated storm events may cause locally high flows for short periods during the summer and early fall (Oregon Department of Water Resources 1988). The primary tributaries of Willow Creek are Eightmile Creek and Rhea Creek, while the primary tributaries in Juniper Canyon include the North and South Forks of Juniper Canyon.
All the primary tributaries of the Umatilla River drain the Blue Mountains and enter the Umatilla River from the south. Wildhorse Creek drains the divide between the Umatilla River and the Walla Walla River to the north. The North and South Forks of the Umatilla River and Meacham Creek account for approximately 14% of the Umatilla River subbasin drainage area, yet supply 40-50% of the average flow to the Umatilla River (Umatilla National Forest 2000). Average annual discharges are 223 cfs for the Umatilla and 193 cfs for Meacham Creek. Peak annual discharges for the Umatilla, at the city of Umatilla, average 6,321 cfs (Appendix A). Water runoff peaks in April, while the lowest flows generally occur in September (Umatilla National Forest 2000). The average monthly discharge of the Umatilla River (measured at RM 2.1) varies from 23 cubic feet per second (cfs) in July to1095 cfs in April (low flow at the mouth occurs in July rather than September because of upstream removals for irrigation), a difference that reflects the seasonal variation in precipitation.
Table 1. Mainstem length and drainage areas of streams within the Umatilla subbasin.
Area (sq. miles)
Distance from the mouth of the Umatilla River (miles)
North Fork Umatilla
South Fork Umatilla
Tributary to Columbia R.
Tributary to Columbia R.
In the plateau area, many intermittent streams are tributaries to the Umatilla River. Deep, incised channels characterize most of these creeks, with most only carrying water during periods of snowmelt or sustained rainfall. Little runoff from lands in the lower Umatilla subbasin occurs because of low precipitation, flat surface relief, and sandy soils (Bureau of Reclamation 1954). The Umatilla River below McKay Creek shows a decrease in the mean monthly instream flow in the downstream direction from Yoakum (RM 37.7) to the city of Umatilla (RM 2.1) (Figure 10). This decrease in flow is evident during both the summer and winter months, when surface water is diverted for storage and groundwater is recharged. The differential in water between the two stations is greatest in April and May when over 400 cfs of surface flow is lost in the 35-mile reach. Despite the loss between the two stations, flows have been improved by an inter-basin transfer of water from the Columbia River through the target flow period of September - June. Many of the larger tributaries lose surface flow during the summer through parts of their lengths. Sections of Birch, McKay, Butter, and Meacham Creeks are all subsurface during low flow periods (Oregon Department of Environmental Quality 1998). These losses are manifested in the mainstem Umatilla River flow at various tributary confluence points (Figure 11).
Figure 10. Mean monthly discharge for stream gages at Yoakum (RM 37.7) and the city of Umatilla (RM 2.1) and instream flow recommendation (Confederated Tribes of the Umatilla Indian Reservation 1999).
Figure 11. Discharge in the lower Umatilla River by river mile, July 14-17, August 11-14, September 8-11 (Kreag and Threlkeld 1991).
Peak flows in Willow Creek near Arlington, Oregon occur in January, while higher upstream near Heppner, Oregon they occur between March and April. Peak annual discharges for Willow Creek, near Arlington, average 4,575 cfs (Appendix B). Monthly discharge in the Willow Creek subbasin varies by gauging station. At the lowest elevation (station #14036000) peak runoff occurs in January, whereas higher up in the drainage, near Heppner, peak runoff occurs between March and April (Figure 12). Base flows typically occur during the months of July – September, during which time channels may run intermittent for prolonged periods (Oregon Water Resources Department 1988). Hydrologic data for Juniper Canyon is limited.
Figure 12. Mean monthly flows for Willow Creek at three gauging stations: Willow Creek above Willow Lake, Willow Creek at Hepner, and Willow Creek near Arlington, OR.
Most flooding events in the Umatilla/Willow subbasin result from rain-on-snow events. This usually occurs when snow accumulates between 1,500-3,500 feet elevation in the Blue Mountains and then is rapidly melted by rain and warm winds (Washington Department of Natural Resources 1998). Sixty-two percent of the Umatilla subbasin falls within the 1,500-3,500 foot range in what is termed the transient snow zone, an area that substantially contributes to the flood regime in the subbasin (Figure 13).
The most damaging floods occur as winter flooding events, commonly from December through February. A second common mechanism for flooding is rain-on-frozen soil events, which generally affect the lowland agricultural areas. These events often lead to high surface erosion in agricultural lands. A less common flooding mechanism is heavy summer thunderstorms.
Figure 13. Transient snow zone elevation band in the Umatilla subbasin
Significant flooding has occurred 26 times since 1865. The U. S. Army Corps of Engineers (1955) identified the storm of May 26-30, 1906 as a “standard project general storm,” meaning it produced a flood exceeded only on rare occasion. The 1906 flood was chosen as a benchmark because of its occurrence during a period of higher temperatures, which resulted in a greater percentage of the precipitation falling as rain and having a greater contribution to snowmelt runoff (U. S. Army Corps of Engineers 1955). Table 2 shows the inches of rainfall for each primary tributary for 120 hours during the storm. In the Willow Creek Subbasin, significant peak flows have been recorded in 1965, 1974 and 1979 (Appendix B). One of the most devastating floods in the history of the United States occurred as a flash flood in the Willow Creek subbasin on June 14, 1903 and resulted in the drowning of 247 people in the Heppner area (http://www.nwp.usace.army.mil/op/D/standard/wc/wc.htm).
Table 2. Umatilla standard project general storm (U. S. Army Corps. of Engineers 1955).
Drainage Area (mi2)
Storm Rainfall (in.)
North/ South Forks of the Umatilla
The hydrology of the Umatilla River is heavily influenced through irrigation, and by releases of water from McKay Reservoir. Water is released from McKay Reservoir at RM 51 during peak irrigation periods. These releases contribute flows to reaches that were historically completely withdrawn by diversions downstream. During irrigation season, the primary source of inflows is from irrigation return flows and drains, with the larger tributaries contributing little to the Umatilla River. Irrigation diversions and drains dominate the hydrology of the river as the diversions remove water added for irrigation from McKay Reservoir. Streamflow drops considerably and temperatures rise with the reduction in flow at the diversion points. Where irrigation drains enter the river, stream flows show a modest increase and temperatures often show a slight decline. The impact of McKay Reservoir on the Umatilla River downstream is to lower mean monthly instream flows during the winter and increase them during the summer when stored water is used for irrigation (Figure 14). The reservoir has reduced mean monthly discharge in the Umatilla River during winter months. Mean annual flow differs between the two periods as well, with an average of 8,528 cfs between 1906-1926 and 7,987 between 1928-1984. The change in mean annual flow without a change in annual peaks reflects a change in distribution of the flow levels. Extensive channel alterations have occurred upstream of the gage, but it appears that they have had little impact on the peak flow at Yoakum.
Figure 14. The impact of McKay reservoir releases on the Umatilla River at Yoakum (RM 37.7) (U. S. Geological Survey 1999).