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# Unit 6 Reading: Great Basin Lakes — Nature's Rain Gauges

by Kirsten Menking, Vassar College

In this week's modeling project, we explore how geologists use clues in the landscape to uncover Earth's history of climatic change. We take, as our example, the Owens River chain of lakes in eastern California, which repeatedly overflowed during the Pleistocene, leaving behind evidence of water balance fluctuations in ancient shorelines and basin center deposits. We consider what climatic conditions may have been necessary to create lakes at different levels and run experiments to determine the ability of the lake chain to record climatic oscillations of different cycle lengths.

## Great Basin "Pluvial" Lakes

The Great Basin of the western United States stretches from eastern Oregon southward to southeastern California, eastward into Utah, and underlies nearly all of the state of Nevada (Fig. 1). It is a region formed by stretching of Earth's crust and consists of north-south trending fault-bounded mountain ranges separated by tectonically down-dropped valleys, a landscape once described by geologist G.K. Gilbert (1875) as "an army of caterpillars marching north out of Mexico." Hydrologically, the Great Basin has no connection to any of the large river systems that drain the North American continent. As a result, precipitation that falls within its boundaries has no way to leave except by evaporation.

Climatologically, the region is under the influence of storm systems that originate over the Pacific Ocean and work their way eastward on the prevailing westerly winds. As these storm systems cross the Sierra Nevada range in California, the air is forced to rise, causing it to cool and the water vapor it contains to condense to rain and snow that falls on the mountains (Fig. 2). Wrung of most of its moisture, the air continues eastward and loses still more moisture with each mountain range it crosses. For this reason, the interior of the Great Basin is quite arid. Large lakes fed by spring snowmelt exist in a few places, but most valley bottoms instead contain playas (Fig. 3), vast expanses of clay and/or evaporite minerals seasonally flooded by shallow sheets of water that evaporate within a few days to weeks.

During the Pleistocene, conditions were quite different. Wave-cut shorelines (Fig. 4), beach gravels, and aquatic fossils found high above presently dry valley floors prove that large lakes were the rule rather than the exception at various times in the past. Indeed, the valleys of the Great Basin have been named "nature's rain gauges" (Smith and Bischoff, 1997) since they filled up with water during wet climate intervals and dried up during more arid periods. Lakes that fluctuate in size as climate changes are also called "pluvial" lakes from the Latin word pluvia, meaning rain.

The most extensive of the Pleistocene pluvial lakes was Lake Bonneville, which covered much of western Utah during the last glacial maximum and was 10 times larger than its modern remnant, Great Salt Lake (Fig. 5). Another massive water body, Lake Lahontan, covered the northwestern corner of the state of Nevada. Its remnants include Pyramid and Walker Lakes along with playas known as the Carson Sink, Humboldt Sink, and Winnemucca Lake. Both Bonneville and Lahontan grew to incorporate multiple Great Basin valleys, but many other valleys were occupied by single lakes at the same time.

What caused lakes throughout the Great Basin to expand during the Pleistocene? The answer likely has to do with changes in atmospheric circulation. The polar jet stream is a fast-moving current of air that lies at the confluence of the atmospheric polar and ferrel cells where air is rising upward from Earth's surface (Fig. 6). As the air rises, it cools, and the water vapor it contains condenses to rain drops or snow flakes. As such, the polar jet stream is a locus of storm activity.

The position of the polar jet stream meanders quite a bit on a week-by-week basis, but today it generally enters the North American continent from the Pacific Ocean somewhere between 50 and 60° latitude, or about the location of British Columbia, Canada. Climate modeling results (Kutzbach, 1987) suggest that growth of the ice sheet that covered the northern half of North America at the last glacial maximum may have deflected the polar jet stream southward (Fig. 7).

Thus deflected, the locus of storm activity would also have shifted, bringing higher rainfall rates to the desert West. When combined with cooler temperatures from the glaciation and associated lower evaporation rates, these conditions could have promoted the growth of exceptionally large lakes. We know from evidence in ocean sediment cores that periods of glaciation have occurred repeatedly throughout the Pleistocene. If model results for the last glacial maximum are correct, then it is likely that southward deflection of the jet stream occurred on numerous occasions.

## The Owens River chain of lakes

In eastern California, southward migration of the jet stream during global glaciations led to increased snowfall and expansion of glaciers in the Sierra Nevada range. Glacial melt water flowed down dozens of tributaries into the Owens River, where it fed a chain of lakes separated by bedrock sills (Fig. 8). This lake chain may, on occasion, have included Lake Russell, a greatly expanded version of present day Mono Lake in the Mono Basin just north of the Owens Valley, but generally started with the Owens River, which flows into Owens Lake. When Owens Lake filled to its drainage divide, it overflowed into the China Lake basin, which subsequently overflowed into the Searles Lake basin. During exceptionally wet periods, China and Searles Lakes coalesced, filled to a common sill level, and spilled into Panamint Valley and ultimately into Death Valley (Smith and Street-Perrott, 1983; Phillips, 2008). Surface area for the entire chain of lakes exceeded nine times the modern value at such times.

The timing of different overflow events has been pieced together using both shoreline and sediment core evidence from all of the lake basins, and it appears that for much of the Pleistocene, Searles Lake was at the end of the lake chain (Phillips, 2008). The reason we know this is that the Searles valley contains thick deposits of fairly soluble minerals that could only have come from the evaporation of vast quantities of lake water. The precipitated minerals include trona (Na3(CO3)(HCO3)-2H2O), which is used in the manufacture of glass, and borax (Na2[B4O5(OH)4]·8H2O), which is used as an ingredient in laundry detergent; they and many others have been mined from the Searles lake bed for decades. Had Searles Lake overflowed to Panamint Valley most of the time, the dissolved ions that precipitated to form these minerals would have been transported farther down the lake chain, and Searles Lake would instead have filled with clastic sediments (silts and clays) delivered from upstream. Likewise, had Owens Lake been the terminus of the chain, we would expect to find these evaporites in the Owens Valley. A sediment core taken from Owens Lake in the early 1990s shows no evidence of such minerals prior to the present day, indicating that Owens Lake must have overflowed throughout most of the Pleistocene (Smith et al., 1997; Phillips, 2008). Based on the collective evidence from all of the lake basins, it appears that Owens River water flowed all the way to Death Valley around 150,000 years ago and possibly again 70,000 years ago, and that either Searles or Panamint Valley served as the terminal lake basin much of the rest of the time. Owens Lake only rarely dropped below its spillway level and probably served as the terminus of the lake chain for only a few thousand years at a time before receiving sufficient runoff to again overflow.

Today, all of the lakes but Owens and Mono are dried up, the desiccation occurring as the jet stream shifted back to the north at the end of the last glaciation. In addition to responding to this natural climatic change, Owens and Mono also fell victim to the thirsty Los Angeles basin, which exports Owens Valley and Mono Basin water 200 km to the south to supply millions of people with drinking water (Fig. 9). At the turn of the 20th century, Owens Lake was 14 m deep and supported a thriving waterfowl population. After Los Angeles Department of Water and Power engineer William Mulholland built the Los Angeles aqueduct that intercepts the flow of Owens River tributaries, the lake disappeared, leaving behind a dry salt flat whose sediments were blown into dust storms for decades (http://www.swhydro.arizona.edu/archive/V3_N4/feature4.pdf, Fig. 10). As Owens Valley residents began to suffer from some of the worst air quality in the nation, a series of court orders issued in the 1990s required the city to return some water to the dried-up lake bed and to implement additional dust mitigation strategies, such as the planting of salt-tolerant vegetation that would protect the surface from the scouring effects of the wind.

Before the court orders were put in place, geologists with the U.S. Geological Survey drove a drilling rig out onto the dried-up lake bed and took a 323-m-deep sediment core that extends nearly 800,000 years back in time (Fig. 11 and 12). A team of geologists spent the next several years analyzing the sediments, looking for clues as to how climate had changed in the past. Using diatoms, ostracodes, mollusks, and fish fossils, they reconstructed parameters such as lake salinity and turbidity. From pollen, they were able to keep track of how vegetation changed in the Owens Valley and adjacent mountain ranges, and from mineralogical changes in the sediment, they were able to identify periods of glaciation. All of the evidence combined revealed that Owens Lake was large and overflowing at times when glaciers expanded in the Sierra Nevada and smaller and confined to the Owens Valley during interglacial periods such as today (Smith et al., 1997). In addition, the evidence showed that variations in climate in eastern California appear to have occurred in concert with global glacial and interglacial cycles.

What caused the global cycles in ice volume to which the Sierra Nevada and Owens River chain of lakes responded? Subtle changes in Earth's orbit around the sun amplified by various feedbacks in the climate system are the likeliest explanations (Fig. 13). Orbital variations include changes in the shape of Earth's orbit (eccentricity), which varies between almost perfectly circular and slightly elliptical on a timescale of ~100,000 years. In addition, the tilt of Earth's spin axis relative to the orbital plane varies between ~22 and 24.5° on a timescale of ~41,000 years. Another cycle, precession of the equinoxes, lasts ~20,000 years and affects the timing of Earth's seasons. During World War I, Serbian physicist Milutin Milankovitch developed a theory of Earth's ice ages that called on these variations to drive changes in the amount of solar radiation striking the high northern latitudes where the great continental ice sheets that covered North America and Eurasia began. Milankovitch hypothesized that times when the high northern latitudes received low amounts of solar radiation during the summer months would foster ice sheet growth, since winter snows would be less likely to melt away than during times of high summer solar radiation receipts. Once the ice started to grow, the ice-albedo feedback would have promoted further ice growth as dark soils and forests were replaced by light-colored ice and snow that would have reflected what small amounts of solar radiation struck the planet at these times. Milankovitch's orbital theory for the ice ages is now generally accepted by the climate science community as evidence from lake sediments, ice cores, and even cave deposits has revealed climatic cycles with periods quite similar to those predicted by the theory.

## Hydrologic balance of a lake

While ancient shorelines and basin center deposits of Great Basin pluvial lakes provide evidence that climate must have been much wetter during glaciations, it would be nice to be more quantitative and to determine just how much wetter conditions must have been to create lakes of various sizes. To answer this question, it is helpful to create a model that keeps track of the various inflows to and outflows from each lake and that incorporates information on each basin's topographic characteristics so that we can determine when a particular lake has reached the overflow point. We can then experiment with various combinations of runoff and evaporation to see which conditions could have led to the lake levels we know existed at various times in the past.

In constructing our model of the Owens River chain of lakes, we use the following equation to keep track of the water balance within each individual lake (Fig. 14):

 (dV)/(dt) = alphaPA_B - EA_L + G_i - G_o + S_i - S_o

where  (dV)/(dt)  is the change in lake volume with time, P is the amount of precipitation falling on the drainage basin that feeds the lake, A_B is the area of the lake basin, alpha is a runoff coefficient describing the percentage of the precipitation that makes its way to the lake without being intercepted by plants or soaking into the soil to enter the groundwater system, E is the rate of evaporation of lake water, A_L is the lake surface area, G_i is the amount of groundwater discharging into the lake, G_o is the amount of water leaving the lake to enter the groundwater system, S_i is the amount of water spilling into the lake from the next lake higher in the chain, and S_o is the amount of water spilling out of the lake basin to the next lake lower down the chain (Mifflin and Wheat, 1979; Phillips and others, 1992).

The low permeability of the the lake sediments in each basin in the chain prevents much groundwater exchange with the lake, so we can ignore G_i and G_o. For a steady state lake that is neither gaining nor losing water by spillover,  (dV)/(dt) , S_i, and S_o all equal zero, so the equation above becomes:

 0 = alphaPA_B - EA_L  or

 alphaPA_B = EA_L .

Let us think about what this means. Since the area of the basin is a fixed parameter, any increase in the amount of runoff will create a larger lake given the same evaporation rate. Likewise, a decline in evaporation rate would result in a larger lake if runoff were to remain constant.

Basin geometry also affects how lakes respond to climate. Consider two hypothetical cone-shaped lake basins that contain the same volume of water (Fig. 15). The basin with the more shallowly dipping sides exhibits a much larger surface area for that volume of water than does the basin with the steeply dipping sides. At the same time, the depth of water in the shallowly dipping cone is much lower than that in the steeply dipping cone when measured from the point of the cone to the water surface level. Changes in basin geometry can occur as a result of tectonics (e.g. stretching of Earth's crust could deepen a lake basin) or deposition (e.g. filling of a basin with sediment changes its capacity to store water). We will assume, for our Owens Lake chain model, that the lakes have remained constant in geometry over time, but will also explore how changing this assumption affects the rate at which these lakes respond to climatic variations.

## References

Gilbert, G.K., 1875, Report on the geology of portions of Nevada, Utah, California, and Arizona examined in the years 1871-1872: U.S. Geographical and Geological Surveys West of the 100th Meridian, v. 3, part 1.

Kutzbach, J.E., "Model simulations of the climatic patterns during the deglaciation of North America," in Ruddiman, W.F., and Wright, H.E., Jr., eds., North America and adjacent oceans during the last deglaciation: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-3, p. 425–446.

Mifflin, M.D., and Wheat, M.M., 1979, "Pluvial lakes and estimated pluvial climates of Nevada," Nevada Bureau of Mines and Geology Bulletin, n. 94, 57 p.

Phillips, F.M., Campbell, A.R., Kruger, C., Johnson, P., Roberts, R., and Keyes, E., 1992, "A reconstruction of the water balance in western United States lake basins to climatic change," New Mexico Water Resources Research Institute Technical Completion Report, n. 269, v. 1.

Phillips, F.M., 2008, "Geological and hydrological history of the paleo-Owens River drainage since the late Miocene," in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic Drainage History of the Southwestern Great Basin and Lower Colorado River Region: Geologic and Biotic Perspectives: Geological Society of America Special Paper 439, p. 115–150.

Reheis, M.C., Adams, K.D., Oviatt, C.G., and Bacon, S.N., 2014, "Pluvial lakes in the Great Basin of the western United States — a view from the outcrop," Quaternary Science Reviews, v. 97, p. 33–57.

Smith, G.I., and Bischoff, J.L., 1997, "Core OL-92 from Owens Lake: Project rationale, geologic setting, drilling procedures, and summary," in Smith, G.I., and Bischoff, J.L., eds., An 800,000-Year Paleoclimatic Record from Core OL-92, Owens Lake, Southeast California: Boulder, Colorado, Geological Society of America Special Paper 317, p. 1–8.

Smith, G.I., Bischoff, J.L., and Bradbury, J.P., 1997, "Synthesis of the paleoclimatic record from Owens Lake core OL-92," in Smith, G.I., and Bischoff, J.L., eds., An 800,000-Year Paleoclimatic Record from Core OL-92, Owens Lake, Southeast California: Boulder, Colorado, Geological Society of America Special Paper 317, p. 143–160.

Smith, G.I., and Street-Perrot, F.A., 1983, "Pluvial Lakes in the Western United States," in Wright, H.E., Jr., ed., Late Quaternary Environments of the United States: Minneapolis, University of Minnesota Press, p. 190-211.

These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.