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The Klamath Basin is a large north-south-striking basin located in south-central Oregon and northwestern California. It occupies 14,700 km2 in Oregon and lies in the transition zone between the Basin and Range and Cascade Range physiographic provinces. This basin contains the largest wetland complex in the western United States and is winter home to the largest concentration of migratory waterfowl in North America. This area also currently supports a $200 million per year agriculturally based economy that includes over 100,000 head of cattle . Klamath Lake, which is located in the central portion of the basin, is the largest freshwater lake in Oregon. Two river systems flow into the lake: the Wood and the Williamson. The Williamson has the largest discharge and enters the lake from the north after flowing through Klamath Marsh. The Williamson River sub-basin covers 3,781 km2 in the northwestern portion of the Klamath Basin (Figure 1). Illian (1970) delineated this sub-basin from topographic divides that he interpreted as groundwater boundaries.
Short-term variations in climate, issues related to water rights, and increases in consumptive uses of ground water resources in the Klamath Basin have prompted a detailed study of the hydrogeology. The first step in the overall study is to produce large-scale geologic maps to better understand the lithologies and structures that influence the movement of water. Portland State University mapped the Wocus Bay (Conaway and Cummings, in preparation) and Soloman Butte (Lee and Cummings, in preparation) quadrangles. These data, as well as mapping done by the Oregon Department of Geology and Mineral Industries, will be incorporated into a basin-wide hydrologic assessment by the U.S. Geologic Survey Water Resources Division and Oregon Water Resources.
Figure 1: Location of study with the Soloman Butte and Wocus Bay quadrangles, the Williamson River basin, and major water bodies highlighted.
This thesis is the compilation of two studies, a hydrogeologic reconnaissance and an investigation of the basin's paleohydrology. The hydrogeology section, Chapter 2, is a preliminary investigation that can be used to provide direction for future work . The paleohydrology section, Chapter 3, concentrates on the geomorphic impacts of the cataclysmic eruption of Mount Mazama.
In Chapter 2 bedrock and surficial deposits, data on river and spring discharge, isotopic composition of surface and ground water, and water-well logs are examined to provide a preliminary view of the modern hydrology of the Williamson River basin. The objectives of this portion of the study are to:
Chapter 3 investigates the record of water levels since the Pleistocene in the area that is currently occupied by Klamath Marsh. The objectives of this chapter are to:
The Klamath Basin has a semi-arid winter rainfall climate . Precipitation data from 1981-1999 from the USDA SNOWTEL site Chemult Alternate at an elevation of 1450 m (USDA, 1999) indicate over seventy-five percent of the 68.15 cm (s ± 23 cm) of average annual precipitation in the Williamson River basin occurs during the months of October through March. The average snow water equivalent is 33 cm (s ± 16 cm) annually (Figure 2).
Large variations in temperature occur both seasonally and diurnally. The warm temperatures in summer (average high 26° C, temperatures in excess of 37° C are common) coupled with the reduced precipitation lead to high evapotranspiration rates. Orographic influences produce a humid alpine climate along the Cascade Range to the west while the basin remains semi-arid. The average annual precipitation 30 km to the west of Chemult at Diamond Lake (elevation 1620 m) (Figure 1) is 128 cm (s ± 36 cm).
Figure 2: Mean monthly precipitation and snow water equivalent with standard deviations from 1981-1998. Data are from USDA SNOTEL site Chemult Alternate (USDA, 1999).
The Klamath Basin is the northwestern most extent of the Basin and Range province. The fault-block-mountains formed as a result of east-west crustal extension that began in the Miocene. In Oregon east of the Cascade Range and south of the Blue Mountains, this extension progressively decreases to the north as four right-lateral strike slip fault zones cross the province in an approximate west-northwest orientation .
The study area also lies partially within the Cascade Range province. The evolution of the Cascades is separated into five episodes of volcanism by Priest (1990).
This area is tectonically active as indicated by young fault scarps developed in alluvial fans and the occurrence of two moderate (M 5.9 and 6.0) earthquakes in 1993, which occurred approximately 30 km northwest of Klamath Falls in the West Klamath Lake fault zone. It has been estimated that this fault zone is capable of generating earthquakes as large as magnitude 7.25 (Lienau and Lund, 1993).
Sherrod and Pickthorn (1992) mapped the Williamson River basin at 1:250,000 scale. In this reconnaissance mapping the basalt and basaltic andesite flows, and vent deposits of the study area were assigned Pliocene to Pleistocene ages. The dominant surficial unit is pumiceous pyroclastic-fall lapilli and ash that are the product of the climactic eruption of Mount Mazama at 7,627± 150 cal yr B.P. . This cataclysmic eruption produced greater than 20 km3 of tephra (dense-rock equivalent) that was distributed over 1.7 million km2 in North America and had a plume height of 55 km during the Plinian phase of the eruption Gardner et al., 1998).
The geology of this basin is currently being mapped at a larger scale to be incorporated in a detailed hydrologic model. The Soloman Butte (Lee and Cummings, 1999), and Wocus Bay (Conaway and Cummings 1999) quadrangles have been mapped at a scale of 1:24000, and mapping of the Wildhorse Ridge quadrangle was conducted in 1999. The Wocus Bay and Soloman Butte quadrangles are investigated in this study because of proximity to Klamath Marsh, the course of the Williamson River, and the abundance of north-northwest-striking faults (Figure 1).
This mapping has furthered the understanding of the influences of structure on the geologic evolution of the basin. Volcanic centers are aligned along the north- and northwest-striking structural grain and are flanked by small sedimentary basins. The lithologies encountered are predominately basalt, basaltic andesite and andesite with interbedded sediment and pyroclastic deposits.
The stratigraphy and structure of the Wocus Bay quadrangle is described in Conaway and Cummings (in preparation). The following has been summarized from this source. The stratigraphy of the Wocus Bay quadrangle has been divided into four groups. Group 1 units are exposed predominately in the eastern half of the quadrangle and extend into adjacent quadrangles to the east (Buckhorn Springs), to the north (Military Crossing), and northeast (Wildhorse Ridge) (Figure 3). Deposition of these units occurred about 4 Ma. The predominate unit is a lithic-bearing, poorly welded rhyolitic tuff that is overlain by a sparsely phyric trachy andesite dated at 4.09± 0.05 Ma, and a basaltic andesite dated at 3.89± 0.08. Also included in this stage are the trachyte, trachy andesite, and trachy dacite exposed at Little Wocus Butte, Wocus Butte, and the lower western flanks of Wocus Butte. The silica contents of these rocks are from 61 to 67 weight percent, the highest for bedrock units in the quadrangle.
Group 2 units are concentrated in the southeastern portion of the quadrangle and were deposited around 2 Ma. These basaltic andesite erupted from Soloman Butte to the southwest and from a local vent.
Group 3 is composed of high alumina olivine theoliite in the northwest portion of the Wocus Bay quadrangle and in the adjacent quadrangle to the north (Military Crossing). This unit has a characteristic diktytaxitic texture and is dated at 1.62± 0.47 Ma. The final group (4) includes pyroclastic-fall and pyroclastic-flow deposits from eruptions of Mount Mazama during the middle Holocene. These deposits overlie a well-developed paleosol and average approximately 3.5 m in thickness across the quadrangle.
Figure 3: Location of quadrangles, water bodies, and major water courses in the Williamson River basin.
North-northwest striking, high-angle normal faults have dissected this quadrangle forming horsts, grabens, and tilted fault blocks. Deformation is thought to have occurred in two stages. Stage 1 deformation is recorded by ramping structures. Ramping structures are not found in units younger than those of group 1. Stage 2 deformation is characterized by dip-slip displacement that ranges from 15 m to 130 m and is thought to have occurred during the Pleistocene. Structure has influenced the extent of units and placement of volcanic centers in the quadrangle.
The source for at least some of the andesite flows of Group 2 appears to be Soloman Butte, a 300 m stratovolcano located in the northeastern corner of the Soloman Butte quadrangle. This correlation suggests that the Solomon Butte center is approximately two million years old, the age determination for one of the Group 2 flows in the Wocus Bay quadrangle. Lee and Cummings (in preparation) indicate that the Solomon Butte volcano overlies tholeiitic basalt flows that overlie basalt hydrovolcanic deposits and interfingered with fluvial/lacustrine siltstone and mudstone. The basalt flows and hydrovolcanic deposits line the canyon of the Williamson River west and southwest of Solomon Butte. West of the canyon of the Williamson River, a columnar jointed basaltic andesite flow caps the Chemult Plateau and overlies basalt hydrovolcanic deposits across an erosional unconformity. Pleistocene olivine-bearing andesite flows and cinder cones lie in the northwestern corner of the quadrangle and flows from one of these centers forms the bedrock barrier on the Williamson River at Kirk Sill. As in the Wocus Bay quadrangle, the Solomon Butte quadrangle is covered by a blanket of pyroclastic-fall deposits from Mount Mazama. Pyroclastic-flow deposits cover the basaltic andesite flow of the Chemult Plateau and the upper reaches of the Williamson River in the Solomon Butte quadrangle (Lee and Cummings, in preparation). The modification to these deposits upstream from the Williamson River canyon, within the canyon, and near the mouth of the canyon are discussed in a later section of this study.
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