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Location of Study Area

The Klamath River originates at Upper Klamath Lake in Oregon and flows in a southwesterly direction through the Cascade Range and the Klamath Mountains and discharges to the Pacific Ocean. The study described herein is limited to the upper Klamath Basin, which is considered to be the part of the basin upstream from Iron Gate Dam (figure 1).

study area map

The upper Klamath Basin encompasses approximately 8,000 square miles and is located in south-central Oregon and northeastern California. The Oregon part of the basin (more than 5,600 square miles) lies primarily in Klamath County with smaller parts in Jackson and Lake Counties. The California part of the basin (more than 2,300 square miles) lies in Modoc and Siskiyou Counties.

Geography and Climate

The upper Klamath Basin spans parts of the Sierra-Cascade Mountain province to the west and the Basin and Range province to the east. Down faulted valleys and fault block mountains of the Basin and Range province terminate against the Cascade Mountains (Illian, 1970). In the upland areas of the basin to the north, the Wood and Williamson Rivers originate from the eastern flank of Mount Mazama (Crater Lake). To the east, the Sprague and Lost Rivers flow westward from more arid parts of the basin. The California portion of the basin to the south is characterized by closed lake basins that are more typical of the Basin and Range Province.

The upper Klamath Basin climate is characterized by hot, dry summers and wet winters with moderate to low temperatures. Annual basin precipitation amounts range from 15 inches at valley floors to more than 70 inches in the Cascade 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 (Oregon State Climatologist; Oregon State Water Resources Board, 1971; Illian, 1970).


The upper Klamath Basin lies in a transitional zone between the Cascade Mountains and the Basin and Range Province and as a result the geology is complex. Geologic mapping exists for the entire study area at 1:250,000 scale. The geology of the basin is one of the important factors controlling the occurrence and movement of ground water.

The geology of the upper Klamath Basin consists primarily of volcanic deposits with lowland fluviolacustrine deposits. Materials described by Newcomb (1958), Illian (1970), Leonard and Harris (1974), and Sherrod and Pickthorn (1992) include: consolidated volcanic rocks consisting largely of lava; unconsolidated to semi-consolidated volcanic ejecta deposited around eruptive centers; and lowland fluviolacustrine deposits consisting of diatomite, water-lain volcanic sediments, tephra, and lava.

Newcomb (1958), Illian (1970), and Leonard and Harris (1974) described the basin's primary hydrogeologic units as: 1) a highly permeable lower (older) basalt unit which serves as the principal aquifer in the area; 2) the Yonna Formation (a medial zone of stratified lacustrine deposits consisting of tuff, agglomerate, shale, diatomite, sandstone, and volcanic ash with some volcanic intrusives or interbeds of thin lava flows) which primarily confines ground water; and 3) upper, younger units (lava flows forming cap rock in places, eruptive deposits, and alluvium) which occur above the water table or yield small quantities of perched water.

More recent work indicates that the geology and water-bearing characteristics may be more complex than previously described. Sherrod and Pickthorn (1992) described and mapped a more complex stratigraphic sequence. They noted that local interfingering occurs between fluviolacustrine strata and volcanic rocks, and that the entire sequence becomes younger to the west. These stratigraphic relationships are in some ways similar to relationships seen in other areas on the eastern flank of the Cascade Range. Sherrod and Pickthorn (1992) abandoned the Yonna Formation as a valid stratigraphic unit.

The upper Klamath Basin is in part a composite graben formed by north to northwest trending normal faults. Vertical displacements are generally less than 330 feet but locally exceed 1,000 feet. In addition to faults, several anticlines exist within the Yonna Valley (Leonard and Harris, 1974; Sherrod and Pickthorn, 1992). Basin-and-range-style faulting has divided the Klamath Basin into a series of small subbasins. Leonard and Harris (1974) indicate that geologic structures generally impact ground-water flow locally rather than having basin-wide impacts. They note that ground water moves freely across fault zones in most areas.

Illian (1970) and Leonard and Harris (1974) indicated that regional, intermediate, and local ground-water flow occurs within the upper Klamath Basin. Ground-water flow between subbasins was speculated to occur although supporting data were limited. These earlier workers identified uplands as the primary ground-water recharge areas for all the flow systems because of greater precipitation and permeability. They note that discharge occurs locally at mountain slope springs and nearby lowlands and regionally at the lowest basin elevations, (such as Lower Klamath Lake) via upward seepage and springs.

A geothermal system within the basin is indicated by the occurrence of hot springs and hundreds of warm water wells in the vicinity of the City of Klamath Falls and areas to the south near Olene Gap and the Klamath Hills. Sammel (1976) developed a conceptual model of the geothermal system in which meteoric waters in a deep regional flow system circulate to depths of up to 10,000 feet by way of interconnected fracture zones. These waters are heated to 130 C before they move upward into the shallow ground-water system along basin and range faults. Most of the thermal discharge does not reach the surface, but moves outward from the fault conduits into permeable zones in basalts where it mixes with cooler, shallow ground water. The relation of the thermal ground-water system to the shallow non-thermal system is not well understood.

Flowing artesian wells in the vicinity of Upper Klamath Lake (Snyder and Morace, 1997) and a large number of springs indicate that strong upward components of ground-water flow occur in many parts of the basin. This ground-water discharge plays an important role in providing discharge to Upper Klamath Lake and baseflow to streams in the basin.


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