Hydrogeologic Framework


The groundwater hydrology of the upper Klamath Basin, including the geology, is discussed in detail by Gannett and others (2007). Much of the discussion in this section is from that report.


Geology


The upper Klamath Basin has been a region of volcanic activity for at least 35 million years (Sherrod and Smith, 2000), resulting in complex assemblages of volcanic vents and lava flows, pyroclastic deposits, and volcanically derived sedimentary deposits (fig. 3). Volcanic and tectonic processes have created many of the present-day landforms in the basin. Glaciation and stream processes have subsequently modified the landscape in many places.


The upper Klamath Basin lies within two major geologic provinces, the Cascade Range and the Basin and Range geologic provinces (Orr and others, 1992). The processes that have operated in these provinces have overlapped and interacted in much of the upper Klamath Basin. The Cascade Range is a north-south trending zone of compositionally diverse volcanic eruptive centers with deposits extending from northern California to southern British Columbia. The Cascade Range is subdivided between an older, highly eroded Western Cascades, and a younger, mostly constructional High Cascades. Prominent among the eruptive centers in the High Cascades of the Klamath Basin are large composite and shield volcanoes such as Mount Mazama (Crater Lake), Mount McLoughlin, and Medicine Lake Volcano. The Cascade Range has been impinged on its eastern side by the adjacent structurally-dominated Basin and Range geologic province. The Basin and Range geologic province is a region of crustal extension characterized by subparallel, fault-bounded, down-dropped basins separated by fault-block ranges. Individual basins and intervening ranges are typically 10–20 mi across. The Basin and Range geologic province encompasses much of the interior of the Western United States, extending from central Oregon southward through Nevada and western Utah, into the southern parts of California, Arizona, and New Mexico. Although the Basin and Range geologic province is primarily structural, faulting has been accompanied by widespread volcanism.


The oldest rocks in the upper Klamath Basin study area are part of the Western Cascades subprovince and consist primarily of early to middle Tertiary lava flows, andesitic mudflows, tuffaceous sedimentary rocks, and vent deposits (fig. 3). The Western Cascade rocks range in age from 20 to 33 million years and are as much as 20,000 ft thick (Hammond, 1983; Vance, 1984). Rocks of the Western Cascades overlie pre-Tertiary rocks of the Klamath Mountains, just west of the study area. Western Cascade rocks have very low permeability because the tuffaceous materials are mostly devitrified (changed to clays and other minerals), and lava flows are weathered and contain abundant secondary minerals. Because of the low permeability, groundwater does not easily move through the Western Cascades rocks, and the unit acts as a barrier to regional groundwater flow. The Western Cascades constitute part of the western boundary of the regional groundwater system. Western Cascade rocks dip toward the east and underlie the High Cascade deposits, defining the lower boundary of the regional flow system throughout that part of the study area.


The High Cascade subprovince ranges in age from late Miocene to late Pleistocene; however, most rocks are Pliocene to Pleistocene in age (Mertzman, 2000). Deposits within the High Cascade subprovince in the study area mostly form constructional features and consist of volcanic vents and lava flows with relatively minor interbedded volcaniclastic and sedimentary deposits. An area of numerous late Miocene to Pliocene cinder cones extends from southwest of Butte Valley to northwest of Mt. Mazama (Crater Lake). Quaternary volcanic deposits are associated with a few volcanic centers concentrated in two general areas in the upper Klamath Basin: from Lake of the Woods north to Crater Lake and from Mt. Shasta (south of the study area) east to Medicine Lake Volcano. The High Cascades rocks are relatively thin in southern Oregon and northern California. High Cascade rocks unconformably overlie Western Cascade rocks and are very permeable, relative to the older rocks.


Deposits in the Basin and Range geologic province in the study area range in age from middle Miocene to Pleistocene. The oldest rocks are middle to late Miocene in age, ranging from 13 to 8 million years. These rocks are exposed just south of the study area in the Pit River Basin and probably underlie the Pliocene age lavas south of Clear Lake Reservoir. The older rocks in the Pit River Basin and bounding the eastern part of the study area are mostly silicic domes, flows, and pyroclastic deposits, which generally have low permeability (California Department of Water Resources, 1963) and typically are faulted and tilted.


Late Miocene to Pliocene volcanic rocks of the Basin and Range geologic province are the major water bearing rocks in the upper Klamath Basin study area. These units consist of volcanic vent deposits and volcanic flow rocks throughout the area east of Upper Klamath Lake and Lower Klamath Lake; the units probably underlie most of the valley- and basin-fill deposits in the study area. Late Miocene to Pliocene rocks also form uplands along the eastern boundary of the study area, and form the plateau that extends from the Langell Valley south to the Pit River. The rocks are predominately basalt and basaltic andesite in composition, but silicic vents and lava flows occur locally, notably in the vicinity of Beatty, Oregon.


Tuff cones and tuff rings are the predominant volcanic vent form in the Sprague River subbasin between Chiloquin and Sprague River, Oregon. Tuff cones and rings form when rising magma comes in contact with water, resulting in explosive fragmentation of the volcanic material and formation of hydrovolcanic deposits. These late Miocene to Pliocene rocks typically exhibit high permeability. Permeability locally may be markedly reduced by secondary mineralization from hydrothermal alteration.


The volcanic rocks of the Basin and Range geologic province are interbedded with, and locally overlain by, late Miocene to Pliocene sedimentary rocks. The sedimentary rocks consist of tuffaceous sandstone, ashy diatomite, mudstone, siltstone, and some conglomerates. These units are exposed both in down-dropped basins and in up-thrown mountain blocks, indicating that the deposits in part represent an earlier generation of sediment-filled basins that have been subsequently faulted and uplifted. These sedimentary deposits are typically poor water producers, and often serve as confining units for underlying volcanic aquifers.


The youngest stratigraphic unit in the upper Klamath Basin consists of late Pliocene to Holocene sedimentary deposits. Those deposits include alluvium along modern flood plains, basin-fill deposits within active grabens, landslide deposits, and glacial drift and outwash. Very thick accumulations of silt, sand, clay, and diatomite underlie the westernmost basins, such as the Upper Klamath Lake, Lower Klamath Lake, Butte Valley, and Tule Lake subbasins. For example, up to 1,740 ft of basin-fill sediment underlies the town of Tulelake, California. Sediment near the base of the deposit at Tulelake has been assigned an age of 3.3 million years on the basis of radiometric ages of interbedded tephra, paleomagnetic data, and estimates of sedimentation rates (Adam and others, 1990). Gravity data suggest that the sediment-fill thickness may exceed 6,000 ft in the Lower Klamath Lake subbasin and may be in the range of 1,300 to 4,000 ft in the Upper Klamath Lake subbasin (Sammel and Peterson, 1976; Veen, 1981; Northwest Geophysical Associates, Inc., 2002; Braunsten, 2009).


Groundwater Hydrology


The upper Klamath Basin has a substantial regional groundwater system. The late Tertiary and Quaternary volcanic rocks that underlie the region are generally permeable, with transmissivity estimates ranging from 1,000 to 100,000 ft²/d, and compose a system of variously interconnected aquifers. Sedimentary rocks, primarily fine-grained lake sediments and basin-filling deposits, are interbedded with the volcanic rocks. The regional groundwater system is underlain and bounded on the east and west by early Tertiary volcanic and sedimentary rocks that have generally low permeability. Eight regional scale hydrogeologic units are defined in the upper Klamath Basin on the basis of surficial geology and subsurface data (Gannett and others, 2007).


Groundwater flows from recharge areas in the Cascade Range, upland areas in the basin interior, and from eastern margins, toward stream valleys and interior subbasins. Groundwater discharges to streams throughout the basin, and most streams have some component of groundwater discharge (base flow). Some streams, however, are predominantly groundwater fed and have relatively constant flows throughout the year. Large amounts of groundwater discharge to streams in the Wood River subbasin, the lower Williamson River area, and along the margin of the Cascade Range. Much of the inflow to Upper Klamath Lake can be attributed to groundwater discharge to streams and major spring complexes within a dozen or so miles from the lake. This large component of groundwater buffers the lake somewhat from year-to-year variations in annual precipitation, but not from multi-year drought cycles. There are also groundwater discharge areas in the eastern parts of the basin, for example in the upper Williamson and Sprague River subbasins and in the upper Lost River subbasin.


The groundwater system in the upper Klamath Basin responds to external stresses such as climate cycles, pumping, lake-stage variations, and canal operation. This response is manifest as fluctuations in hydraulic head (as represented, for example, by fluctuations in the water-table surface) and variations in groundwater discharge to springs. Basinwide, decadal-scale climate cycles are the largest factor controlling head and discharge fluctuations. Climate-driven water-table fluctuations of more than 12 ft have been observed near the Cascade Range, and decadal-scale fluctuations of 5 ft are common throughout the basin. Groundwater discharge to springs and streams varies throughout the basin by a factor of two or more in response to decadal-scale climate cycles. Climate-driven interannual variations in groundwater discharge total hundreds of cubic feet per second.


The response of the groundwater system to pumping is generally largest in areas of irrigation pumping. Annual drawdown and recovery cycles of 1 to 10 ft are common in pumping areas. Long-term drawdown effects, where the water table has reached or is attempting to reach a new level in equilibrium with the pumping, are apparent in parts of the basin. In general, impacts of pumping on streams and springs are diffuse and difficult to measure. In several instances, however, reductions to spring discharge resulting from nearby pumping are well documented through direct measurement. 


Hydraulic Properties of Hydrogeologic Units


Gannett and others (2007) provide a summary of the hydraulic properties (transmissivity and storativity) of regional hydrogeologic units in the upper Klamath Basin based on 32 aquifer tests and specific capacity tests from 288 wells. Transmissivity is the product of the hydraulic conductivity and aquifer thickness. Hydraulic conductivity is the unit volume of water that will move through a unit area of aquifer under a unit hydraulic gradient per unit time, and has dimensions of length per unit time, such as feet per day. Transmissivity then has dimensions of feet squared per day. Storativity is the unit volume of water an aquifer takes into, or releases from, storage per unit area per unit change in head. The volume of water has the units of length cubed (such as ft³), the area has units of length squared (such as ft²), and the change in head has units of length (such as ft). Because the volume is divided by the other two quantities, storativity is dimensionless.


Aquifer tests from 26 wells show that the transmissivity of the Tertiary volcanics (predominantly basaltic lavas of Miocene to Pliocene age) varies widely, from 2,700 to 610,000 ft²/d, with most ranging from 24,000 to 270,000 ft²/d. The median transmissivity is about 90,000 ft²/d. Storativity values from aquifer tests in the Tertiary volcanics reported by Gannett and others (2007) range from 0.00001 to 0.15. The 0.15 figure is anomalous and likely due to a partially penetrating observation well and leakage from a confining unit. Most storativity values in the Tertiary volcanics range from 0.00025 to 0.001, and the median is about 0.0005.


Although the number of aquifer tests in Tertiary sediments (or mixtures of the sediments and lavas) is small (6), they provided information on the hydraulic characteristics of the coarse-grained facies of Tertiary sediments. Transmissivity values range from 13,000 to 350,000 ft²/d, with most in the 25,000 to 75,000 ft²/d range. The median value is 54,000 ft²/d. Storativity values range from 0.0005 to 0.015 with most ranging from about 0.0002 to 0.003.


In the early 1980s, the USGS conducted an aquifer test of the geothermal aquifer in Klamath Falls in collaboration with the Lawrence Berkeley Laboratory and the City of Klamath Falls (Benson and others, 1984a, b). The test consisted of four phases: a 1-week pre-test phase during which background water levels were monitored; a 21-day pumping phase during which a geothermal well was pumped at about 720 gal/min and the water discharged to an irrigation canal; a 30-day injection phase during which pumping continued (at about 660–695 gal/min) and the water injected into a second well; and a 1-week recovery phase. Benson and others (1984a) analyzed the data from the test and calculated a permeability‑thickness value (analogous to a transmissivity) of about 1.4 × 10⁶ millidarcy-feet. This converts to a transmissivity of about 3,800 ft²/d. Analysis of the test indicated a storativity of about 0.002. 


Gannett and others (2007) also summarized transmissivity estimates from specific-capacity tests on 288 water well reports for wells producing from Quaternary sediment, Tertiary sediment, and Tertiary volcanic rock. Wells producing from Quaternary sedimentary deposits and Tertiary sedimentary deposits have similar transmissivity distributions, with the former having slightly larger values. The median transmissivity for both units determined from specific‑capacity tests is about 200 ft²/d. The frequency distribution of transmissivities for the Tertiary volcanic deposits is distinct from the other units, with values generally larger by more than an order of magnitude. The median transmissivity of Tertiary volcanic deposits is about 6,300 ft²/d. 


The median transmissivity for late Tertiary volcanic deposits determined from specific-capacity tests (6,300 ft²/d) is lower than that calculated from aquifer tests (about 90,000 ft²/d). This is not unexpected for the following reasons. First, transmissivity values determined from single-well tests can be biased downward by excess drawdown in the pumped well due to well inefficiency (see Driscoll, 1986, p. 244). Aquifer tests with observation wells are not affected by this phenomenon. Second, the large number of specific-capacity tests (173) represents a more or less random sampling of wells (and varying characteristics) in the unit. Aquifer tests, in contrast, are not random but tend to be conducted most commonly on high yielding wells for specific purposes. Regardless, transmissivity values calculated from both aquifer tests and specific-capacity tests are useful for understanding the hydraulic characteristics of hydrogeologic units and the differences between units.


Estimates of the hydraulic properties of the Quaternary volcanic rocks in the Cascade Range are based largely on heat and mass transport models because there are so few wells to provide direct measurements. In simulating groundwater flow and heat transport in the Cascade Range, Ingebritsen and others (1992) estimated the permeability of rocks younger than 2.3 million years to be about 10^–13 ft², which is equivalent to a hydraulic conductivity of about 0.018 ft/d assuming a water temperature of 41°F. The permeability of rocks with ages between 4 and 8 million years was estimated to be 5.4 × 10^–15 ft², which is equivalent to a hydraulic conductivity of about 9.1 × 10^–4 ft/d. Higher near-surface permeability, on the order of 0.018 to 1.8 ft/d, was required in their simulation to match groundwater recharge estimates. Higher near‑surface permeabilities are also suggested by well-test data. A specific‑capacity test of a well near Mount Bachelor yielded a hydraulic-conductivity estimate of 9 ft/d.


Mathematical modeling of groundwater discharge to spring-fed streams in the Cascade Range by Manga (1996, 1997) yielded permeability values for near-surface rocks less than about 2.0 million years old of about 10^–10 ft², which equates to a hydraulic conductivity of about 18 ft/d, assuming a water temperature of 41 °F. This estimate is an order of magnitude larger than the upper value of Ingebritsen and others (1992) for near-surface rocks, where most groundwater flow occurs. The permeability estimates of Manga (1996, 1997) and Ingebritsen and others (1992) are considered to be a reasonable range of values for the younger, near-surface strata in the Cascade Range.


First posted May 5, 2012