Link to USGS home page Link to Oregon Water Resources Department Web site Link to Confederated Tribes of the Umatilla Reservation Web site

OREGON WATER SCIENCE CENTER HYDROLOGIC STUDIES

Umatilla Basin Ground-Water Study

Background

Study Area

The focus of the study is located in northcentral Oregon in an area of rolling hills covered in grasslands and desert vegetation. The study area is 5,800 square miles and extends into Washington State (4,200 square miles are within Oregon, 1,600 square miles are in Washington ).

Streams in Oregon drain the Blue Mountains and flow northwest. Many streams are ephemeral. Perennial streams flow into the Columbia River, which flows westerly about 300 miles to the Pacific Ocean . The major stream is the Umatilla River , which drains the eastern half of the study area. Streams in Washington flow southeasterly into the Columbia River .

Geography and Climate

 The Umatilla Basin is part of the Columbia Plateau province and is bounded on the south by the Blue Mountains, on the north by the Horse Heaven Hills, on the east by a subtle topographic divide between the Umatilla River and Walla Walla River drainage basins, and on the west by a subtle divide between Willow Creek and John Day River drainage basins.

The Umatilla Basin climate is characterized by hot, dry summers and wet winters with moderate to low temperatures. Annual basin precipitation amounts range from 5 inches near the Columbia River to more than 12 inches in the Blue Mountains . Sixty to seventy percent of the precipitation occurs from October through March. An average of about 4 inches of rain falls during the period from April through September.

Hydrogeology

The Umatilla Basin lies within the Columbia Plateau, a broad area underlain by volcanic flood basalts, called the Columbia River Basalt Group. A veneer of sediments overlies the basalt. The sediments range in grain size from windblown clay and silt to water lain sand and gravel.

The Columbia River Basalt Group (CRBG) consists of a series of Miocene age (17-6 million years) basalt flows that covers northern and eastern Oregon , central and eastern Washington , and western Idaho . The basalt issued from fissures and vents in eastern Washington , northeastern Oregon and western Idaho . Some eruptions covered tens of thousands of square miles, sending flows hundreds of miles from their source. The Columbia River Basalt Group forms the rolling hills of the Umatilla Basin , as well as the scablands of eastern Washington , uplands within the northern Willamette Valley , and headlands along the central and northern Oregon Coast .

Over 300 flows have been identified. Individual flows range in thickness from 5 to over 100 feet, and total thickness of the series of flows may be as great as 10,000 ft. The top and bottom of individual flows are vesicular and occasionally brecciated. When the hiatus between flows was sufficiently long, soil developed or sediments were deposited on the surface of a flow. If these sediments were preserved by an overlying basalt flow, a sedimentary interbed may be preserved between flows.

The vesicular and brecciated flow tops and bottoms of individual flow within the Columbia River Basalt Group are permeable and an important source of water supply in the Umatilla Basin. Between these interflow zones, the dense flow interiors are relatively impermeable. Conceptually, then, the CRBG is a series of productive aquifers consisting of interflow zones separated by the low permeability flow interiors. The uppermost part of the CRBG is permeable, unconfined and has a good hydraulic connection with overlying alluvial aquifer and, in some cases, streams. Permeable interflow zones at depth are confined by the flow interiors. Although the interflow zones yield large amounts of water initially, continued withdrawals result in large declines in water levels because of low storage properties and limited recharge of water reaching these productive zones through the low permeability flow interiors.

The alluvial sand and gravel deposits are limited to the lower basin between Boardman and Cold Springs Reservoir and in the flood plain of major streams. Ground water in the alluvial aquifer found at shallow depths is unconfined and has a good hydraulic connection with surface water. The main source of recharge to the alluvial aquifer comes from leaky canals and ditches. Additional recharge comes from applied irrigation water. In local areas, leakage from reservoirs and streams represents a significant component of recharge. Recharge from precipitation is a relatively small proportion of total recharge.

The alluvial and shallowest basalt aquifers are the main sources of domestic water for rural residents in the area. The alluvial aquifer is also a major source of municipal water for the cities of Hermiston, Irrigon, and Boardman and an important source of irrigation water in the area between Boardman and Hermiston. Many large capacity irrigation and municipal wells are more than 500 feet deep and open to large intervals of basalt.



Umatilla Basin Ground-Water Study
Oregon Water Science Center Studies
Oregon Water Science Center Home Page

Contact: Terrence Conlon