Holocene Lake-Level Trends in the Rocky Mountains, USA

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Holocene lake-level trends in the Rocky Mountains, USA

Bryan Shuman University of Minnesota - Twin Cities, [email protected]

Anna K. Henderson University of Minnesota - Twin Cities

Steven M. Colman University of Minnesota—Duluth

Jeffery R. Stone University of Nebraska-Lincoln, [email protected]

Sherilyn C. Fritz University of Nebraska-Lincoln, [email protected]

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Shuman, Bryan; Henderson, Anna K.; Colman, Steven M.; Stone, Jeffery R.; Fritz, Sherilyn C.; Stevens, Lora R.; Power, Mitchell J.; and Whitlock, Cathy, "Holocene lake-level trends in the Rocky Mountains, USA" (2009). Papers in the Earth and Atmospheric Sciences. 106. https://digitalcommons.unl.edu/geosciencefacpub/106

This Article is brought to you for free and open access by the Earth and Atmospheric Sciences, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in the Earth and Atmospheric Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Bryan Shuman, Anna K. Henderson, Steven M. Colman, Jeffery R. Stone, Sherilyn C. Fritz, Lora R. Stevens, Mitchell J. Power, and Cathy Whitlock

This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/ geosciencefacpub/106 Published in Quaternary Science Reviews 28:19/20 (September 2009), pp. 1861–1879; doi: 10.1016/j.quascirev.2009.03.003 Copyright © 2009 Elsevier Ltd. Used by permission. http://www.elsevier.com/locate/quascirev

Submitted May 3, 2008; revised February 22, 2009; accepted March 9, 2009; published online June 21, 2009.

Holocene lake-level trends in the Rocky Mountains, USA

Bryan Shuman,a, b Anna K. Henderson,b Steven M. Colman,c Jeffery R. Stone,d Sherilyn C. Fritz,d Lora R. Stevens,d Mitchell J. Power,e and Cathy Whitlock e

a Department of Geography, University of Minnesota, Minneapolis, MN 55455, USA b Limnological Research Center, University of Minnesota, Minneapolis, MN 55455, USA c Large Lakes Observatory and Department of Geological Sciences, University of Minnesota—Duluth, Duluth, MN 55812, USA d Department of Geosciences, University of Nebraska–Lincoln, Lincoln, NE 68588-0340, USA e Department of Geography, University of Oregon, Eugene, OR 97403-1251, USA

Corresponding author — B. Shuman; present address: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA; tel 307 766-6442, fax 307 766-2697, email [email protected] Present address for L. R. Stevens — Department of Geological Sciences, California State University–Long Beach, Long Beach, ca. 90840-3902, USA Present address for M. J. Power —Utah Museum of Natural History and Department of Geography, University of Utah, Salt Lake City, UT 84102, USA Present address for C. Whitlock — Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA.

Abstract The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holo- cene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Ho- locene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid- Holocene shorelines in the small lakes (4–110 ha) were as much as ~10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca. 7000– 4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability.

1. Introduction nally-forced changes in the past, even if such changes developed over long-time scales (Overpeck et al., 1991; Webb et al., Studies of drought in the western U.S. during the past few 1993). Both past and future boundary-condition changes have centuries and millennia (e.g., Stahle et al., 2000; Woodhouse, the potential to push climatic conditions beyond the envelope 2003; Cook et al., 2004; MacDonald, 2007; Meko et al., 2007) of natural variability experienced during the past millennium provide insights on patterns of drought generated by “un- (Diffenbaugh et al., 2005; Williams et al., 2007). Here, we ex- forced” climate phenomena such as El Nino and decadal vari- amine such potential by reconstructing Rocky Mountain lake- ability in Pacific climates (e.g., Swetnam and Betancourt, 1998; level trends during the Holocene, including intervals when in- McCabe et al., 2004, 2007; Stone and Fritz, 2006; Stevens and solation and atmospheric composition differed significantly Dean, 2008). Future regional changes will, instead, be forced from today (Berger and Loutre, 1991; Monnin et al., 2001). by rising greenhouse-gas levels (e.g., Stewart et al., 2004; Dif- Climate model experiments have simulated large changes in fenbaugh et al., 2005) and may be analogous to large exter- precipitation and annual moisture balance across the western

1861 1862 Shuman et al. in Quatern ary Science Reviews 28 (2009)

Figure 1. (A) Study sites and their location relative to primary regions of snowpack variability (based on Cayan, 1996). Cayan’s principle components analysis of annual snowpack variation showed three principle components in the Rocky Mountains: an “Idaho pattern” (ID; short dashed border), a “Colorado pattern” (CO; long dashed border), and a “New Mexico pattern” (MN; dotted border), which explain 28%, 8%, and 10% of the western U.S. snowpack variance respectively. Spatial loadings are shown in gray (light, correlation coefficients >0.5; dark, >0.7). Numbered locations refer to sedimentary records of water-level change drawn from the literature: 1, Flathead Lake, MT (Hofmann et al., 2006); 2, Jones Lake, MT (Shapley, 2005); 3, Yellowstone Lake, WY (Otis et al., 1977); 4, Park Pond 2, WY (Lynch, 1998); 5, Forest Pond 2, WY (Lynch, 1998); 6, Park Pond 1, WY (Lynch, 1998); 7, Cottonwood Pass Pond, CO (Fall, 1997); 8, Red Lady Fen, CO (Fall, 1997); 9, Red Well Fen, CO (Fall, 1997); 10, Bear Lake, AZ (Weng and Jackson, 1999); 11, Fracas Lake, AZ (Weng and Jackson, 1999); 12, Walker Lake, AZ (Hevly, 1985); 13, Montazuma Well, AZ (Davis and Shafer, 1992); 14, Potato Lake, AZ (Anderson, 1993). (B) The hypothesis that mid-Holocene aridity in the mid-continent can be explained by increased westerly flow of Pacific-Ocean-derived air masses (e.g., Bartlein et al., 1984; Bardbury and Dean, 1993; Yu et al., 1997) would have the upwind implication of enhanced orographic precipitation during the mid-Holocene.

U.S. during the Holocene in response to orbital change (Bartlein hanced orographic precipitation (Figure 1). et al., 1998; Harrison et al., 2003; Diffenbaugh et al., 2006; Shin Evidence of past lake levels can reveal the range of mois- et al., 2006). Likewise, pollen and macrofossil data from Colo- ture variation over multiple time scales (Benson et al., 1990; rado, Wyoming, and Montana reveal long-term trends in Rocky Harrison and Digerfeldt, 1993; Stine, 1994), but few studies Mountain climates and vegetation (Maher, 1961, 1972; Baker, have documented Holocene lake-level changes in the Rocky 1983; Thompson et al., 1993; Whitlock, 1993; Fall, 1997; Lynch, Mountains (Fritz et al., 2000). A growing number of diatom, 1998; Mock and Brunelle-Daines, 1999); variation in fire ac- geochemical, and geomorphic records from the Rocky Moun- tivity is also tied to climatic changes (Millspaugh et al., 2000; tains indicate both long- and short-term variability in lake lev- Brunelle et al., 2005; Whitlock and Bartlein, 2004; Toney and An- els (Cumming et al., 2002; Langford, 2003; Stone and Fritz, derson, 2006; Power et al., 2006, 2008; Marlon et al., 2006). Ad- 2006; Stevens et al., 2006; Bracht et al., 2008; Stevens and Dean, ditionally, dunes in intermountain basins (Stokes and Gaylord, 2008). Here we synthesize new and existing sedimentological 1993; Langford, 2003; Forman et al., 2006) and immediately east evidence for long-term water-level trends, and compare the of the Rocky Mountains (Forman et al., 2001, 2006; Miao et al., evidence for these trends with the limited sedimentary evi- 2007) were active during the Holocene. However, zonal atmo- dence of shorter-term variations. Three key questions motivate spheric flow has been widely cited to explain dry mid-Holocene this study: conditions in the mid-continent (e.g., Bartlein et al., 1984; Brad- bury and Dean, 1993; Yu et al., 1997), and might be expected to (1) What long-term hydrologic trends are evident in the produce wet conditions in the Rocky Mountains because of en- Rocky Mountains during the Holocene? Holocene lake-level trends in the Rocky Mountains 1863

Figure 2. Study site bathymetry, air photos, core and sub-surface profile locations, and modern climatology. Dashed lines on photos mark bathymetry (note different m intervals among sites); solid lines on photos mark sub-surface profiles; dots indicate core locations. Monthly temperature (T), precipitation (P), and P minus potential evapotranspiration (PET) are shown at left, and are based on the AD 1971–2000 climatologies for the U.S. climate divisions that contain the lakes (NCDC, 1994) and the Thornthwaite (1948) method for calculating PET. Dashed lines represent P; dark gray indicates periods of potential lake and groundwater recharge when PET > P.

(2) What spatial patterns characterize the long-term trends cause lake levels depend heavily on surplus moisture derived in moisture variation? primarily from winter precipitation (Figure 2), and long-term forcing may have generated broad-scale moisture anomalies (3) How similar were long-term moisture variations in the similar to those at interannual time scales (Mock and Brunelle- Rocky Mountains and mid-continent? Daines, 1999; Harrison et al., 2003; Whitlock and Bartlein, 2004; To address these three questions, we studied the lake-level Shin et al., 2006). Indeed, some of Cayan’s (1996) snowpack history of a lake in western Montana and two lakes in Colo- patterns (i.e., the “Idaho” pattern; Figure 1a) are associated rado, and compiled existing sedimentary evidence for shore- with atmospheric and oceanic conditions like those that have line shifts or desiccation events in an additional 14 lakes or been simulated and inferred for the mid-Holocene, including wetlands (Figures 1a and 2). The small size of our new study a deep Aleutian low (e.g., Bartlein et al., 1998; Anderson et al., lakes (5 ha to 1.1 km2) ensures that past lake-level fluctuations 2005) and a cool north Pacific (Kim et al., 2004). Likewise, Lic- capture local hydrologic conditions (Harrison and Digerfeldt, ciardi (2001) compared the histories of four lakes (Chewau- 1993). Each new study lake was chosen to represent a differ- can, Lahontan, Bonneville, and Owens) in the Great Basin and ent region defined by Cayan (1996) for having broadly-coher- showed important synoptic differences like those that exist at ent snowpack variation at interannual scales (Figure 1a), be- short time scales today. 1864 Shuman et al. in Quatern ary Science Reviews 28 (2009)

We present detailed data for our three study sites, and then 3. Methods review all of the other direct evidence of lake desiccation or shoreline changes in the Rocky Mountains (Figure 1a). We dis- Our approach follows the methods of Digerfeldt (1986) and cuss the long-term patterns in these data, some evidence for Shuman et al. (2001, 2005) and uses transects of sediment cores millennial and shorter changes in climate, and comparisons within each lake, in combination with sub-surface profiles col- among the lake-level data, other paleoenvironmental records, lected by seismic and ground-penetrating radar (GPR) geophys- and climate model simulations. ical techniques, to track shifts in the position of near-shore sediments. The method depends on the assertion that wave energy 2. Study sites prohibits the accumulation of sediment in the shallowest areas of the lake. Littoral sediment types (e.g., sands) thus accumulate We targeted small lakes with small watersheds and mini- closer to the center of the basin when water levels are low than mal stream inputs to obtain records that (a) were minimally when lake levels are high. A rise (fall) in lake level provides one affected by local stream processes (e.g., down cutting of lake of the few mechanisms for raising (lowering) the elevation of the outlets; variable contributions of stream-derived sediments energy-defined limit of fine sediment accumulation (the “sedi- to sediment sequences), and (b) were controlled by ground- ment limit” of Digerfeldt, 1986; Dearing, 1997; or “depositional water levels, which integrate multiple years of precipitation. boundary depth” of Rowan et al., 1992). Even as the lake fills Such lakes have been shown to be climatically sensitive and with lacustrine sediment, fine-grained muds can only accumu- to have regionally-coherent trends (Harrison and Digerfeldt, late where the energy of the environment is low; infilling alone 1993; Shuman and Donnelly, 2006). Two lakes are diamicton- cannot cause shoreward expansion of sediment accumulation. dammed lakes fed by groundwater in unconsolidated sedi- Therefore, near-shore sediment accumulation can be used to in- ments near the forest-steppe boundaries of the Flathead Val- fer both high and low-lake stands; and the presence of young ley of Montana and the North Park Valley of Colorado. The sediments near shore can be taken to confirm recent high wa- third lake is a small glacially-scoured bedrock basin in subal- ter levels (see Davis and Ford, 1982; Shuman et al., 2005 for con- pine forest in the San Juan Mountains, Colorado. trast). Previous studies have also demonstrated the potential to Foy Lake in western Montana (48.17°N, 114.35°W, 1006 m find stratigraphic evidence of century-scale lake-level changes elevation, 110 ha surface area) fills a depression in till that (e.g., Abbott et al., 1997). Sediment descriptions used here fol- formed as glaciers retreated from the Flathead Valley before low Schnurrenberger et al. (2003); all lake sediments are referred 13,100 cal yr BP (Smith, 2004). Proterozic sediments comprise to as lacustrine, but sediments deposited in shallow water above the bedrock near Foy Lake, and contain some carbonate units wave base are described as littoral. (Harrison et al., 1986). The lake is primarily a groundwater To estimate past lake levels, we reconstructed the past ele- outcrop with only minor stream inputs, and has a historically- vations of wave base (i.e., the physical control on the bound- widened outlet. The immediate watershed of the lake covers ary between shallow- and deep-water sediment facies, see e.g., ~300 ha, but groundwater may be derived from an additional Shuman, 2003; Anderson et al., 2005). To do so, we find the ~1500 ha. Stevens et al. (2006) and Stone and Fritz (2006) stud- highest elevation location of fine-grained lacustrine sediments ied the diatom assemblages and geochemical composition of at a particular time. Low-lake stands are identified in sub-sur- a core collected from the center of Foy Lake in 39.9 m of wa- face profiles by the truncation of sedimentary units near shore ter. Power et al. (2006) and Power (2006) described the local and in near-shore cores as intervals of low sedimentation rates fire and vegetation history based on an examination of char- (<10 cm/ka; Webb and Webb, 1988), coarse-grained sediment coal and pollen. Foy Lake lies within Cayan’s (1996) “Idaho” lags, and other littoral sedimentary features (Dearing, 1997). pattern of snowpack variation (Figure 1a). High stands appear evident as basin-wide units of fine-grained Hidden Lake (40.51°N, 106.61°W, 2710 m elevation, 6.5 ha sediment that extend to high elevations in the lake basin. surface area) is located behind ridges of unconsolidated sediments on the eastern edge of the Park Range of northern Colo- 3.1. Sub-surface profiles rado. This surficially-closed basin is fed by groundwater from a 94-ha watershed. The ridges, which dam the lake, contain We collected over ten sub-surface profiles in each study boulders composed of Tertiary volcanics and Cretaceous sand- lake to identify the major stratigraphic features. Represen- stones, which outcrop near the lake (Snyder, 1980). These fea- tative profiles are shown here (Figures 3–5). Evidence of on- tures have been interpreted as block-slide deposits (Snyder, lapping sediments (increasing shoreward extent of overlying 1980), but field inspection indicates that they may be moraines, sediment layers) was inferred to indicate time-transgressive which would date to at least 17,300–16,300 cal yr BP (Benson increases in water level. Strong near-shore reflectors indicate et al., 2005). The lake has a maximum depth of 9 m, and sits deposits of coarse-grained littoral sediments (“paleoshore- within Cayan’s (1996) “Colorado” region (Figure 1a). lines”) that are disconformable with underlying sediment Little Molas Lake (37.74°N, 107.71°W, 3330 m elevation, 5 ha units. Paleoshorelines were differentiated from local slumps, surface area) is located at Molas Pass, in Colorado’s San Juan because they were found around the entire lake. At Foy Lake, Mountains, within the region of Cayan’s (1996) “New Mexico” where water depths exceeded 40 m and where water conduc- pattern (Figure 1a). The small lake regularly overflows and has tivity was high (1120 μS), we used an EdgeTech 424 CHIRP one small inflowing stream, which drains a ~275-ha watershed seismic-reflection system. At Hidden and Little Molas Lakes, including a near-by cirque. The lake lies in a glacially-scoured where water depths were less than 10 m and water conductiv- karst feature of the Mississippian Leadville Limestone, which ity was low (20 and 120 μS respectively), we used a Geophysi- is draped by the red silts of the Mississippian Molas Forma- cal Survey Systems, Inc. (GSSI) SIR-3000 GPR with a 200 mHz tion (Blair et al., 1996). The bedrock forms a submerged ridge antenna floated in a raft on the water surface. that partially divides the lake basin (Figure 2). Tertiary volcanics form the upper elevations of the Little Molas Lake water- 3.2. Collection and analysis of sediment cores shed. The maximum depth of the lake is 5 m. Toney and An- derson (2006) analyzed pollen and sedimentary charcoal in a Sediment cores were collected with a piston corer along deep-water core from the lake. Maher (1961) analyzed fossil transects from the center to the shore of each lake (Figures 2 pollen from Molas Lake located 2.3 km to the east. & 3). At Foy Lake, sediments were collected in 2002 with a 5- Holocene lake-level trends in the Rocky Mountains 1865

Figure 3. Sub-surface profiles of study sites. Core locations (white lines) and their water depths are shown. cm diameter modified Livingstone corer in water depths of ments (Nowaczyk et al., 2002) for all three lakes and gamma- 4.0, 7.9, 15.9, and 20.0 m. At Hidden Lake, sediments were col- ray attenuation bulk density measurements (Zolitschka et al., lected in 2005 using 2–3 m long, 10-cm diameter polycarbon- 2002) for the polycarbonate-lined cores from Hidden and Lit- ate tubes with a piston at three deep locations (north, central, tle Molas Lakes. A coulometer was used to measure the total and southern) and along two transects toward shore. A cen- carbonate and total organic content of the carbonate-rich sed- tral transect of cores collected in 6.6, 6.0, 5.5, 5.0, 4.5, and 3.5 m iments from Foy Lake; loss-on-ignition analyses provided in- of water is considered in this paper. A core of the complete formation on the water, organic, and carbonate content (Dean, sediment sequence from the deepest portion of the lake was 1974) of Hidden Lake cores. Sediments were also visually in- collected in 2007. At Little Molas Lake, we used a modified spected to classify facies based on sediment structure and Livingstone corer with 10-cm diameter polycarbonate barrels grain-size and to identify potential unconformity surfaces. to collect a single transect of cores from 3.0, 2.0, 1.5, 1.0, and 0.69 m of water in 2005. 3.3. Chronology Cores were logged and imaged using a Geotek Multi-Sensor Core Logger and DMZ Core Scanner at the National Lacustrine AMS radiocarbon dates were obtained from selected cores Core Repository Core Lab (LacCore) at the University of Min- to provide a chronology for each basin. Dating targeted key nesota. The logs provided magnetic-susceptibility measure- stratigraphic changes, but the chronologies are limited by the 1866 Shuman et al. in Quatern ary Science Reviews 28 (2009)

availability of datable material. For this reason, we emphasize long-term features of the lake histories here, which appear robust regardless of the specific ages of individual stratigraphic changes. All dates were calibrated using CALIB 5.0 (Reimer et al., 2004), and rounded to the nearest 5 years. AMS radiocarbon ages were obtained on macrofossil material, charcoal, and bulk sediment at Hidden and Little Mo- las lakes (Table 1). Four dates from Foy Lake were obtained on 5 cm3 of core material that was processed to concentrate pollen using a heavy-liquid separation method (Brown et al., 1989). The use of bulk sediment for dating at Hidden Lake is sup- ported by comparable ages and sedimentation rates obtained on a sub-set of macrofossil and charcoal samples.

4. Site-specific analyses

4.1. Foy Lake, Montana

4.1.1. CHIRP data Seismic profiles from Foy Lake show widespread stratified sediments across deep areas of the lake (Figure 3). The Mazama Ash (MZ) layer, dating to ca. 7600 cal yr BP (Zdanow- icz et al., 1999), appears evident as a dark band in the profiles. Biogenic gas obscures the lowermost sediments in many parts of the lake, but appears constrained below specific layers. Less gas is evident in areas where homogenized cones of sediment Figure 4. Interpretation of key features in sub-surface profiles. Dashed line extend upward from the MZ as though these features formed on profile of Little Molas Lake shows elevation of top of bedrock outcrop during the release of gas trapped below the MZ. in comparison to lower extent of near-shore unconformity. A dark reflector in the seismic profiles (Figure 5) extends outward from shore and truncates underlying features including the MZ and the gas-rich layers, but ends before reaching areas of about 20 m water depth. The feature appears in mul-

Figure 5. Details from the near-shore segments of two seismic-reflection profiles across Foy Lake. Eastern segments are shown in the two columns on the left with a schematic interpretation of the major onlapping and truncated reflectors, and unconformities, to the left. Western segments are shown in the two columns to the right with the schematic interpretation to the right. Arrows labeled “PS” indicate possible paleoshoreline features. Holocene lake-level trends in the Rocky Mountains 1867

Table 1. Radiocarbon samples and their calibrated ages based on CALIB 5.0.2 (Reimer et al., 2004) by core. Lab Sample # Site Core Depth in core Material dated 14C age (BP) ± Calibrated age (BP) Max Median Min UCIAMS 24267 Hidden I 50.5 Decalcified sediment 1005 20 952 931 919 UCIAMS 24270 Hidden I 50.5 Female conifer cone 1160 20 1167 1072 1008 UCIAMS 24268 Hidden I 99.5 Decalcified sediment 2145 20 2292 2136 2072 UCIAMS 24271 Hidden I 99.5 Female conifer cone 1115 20 1055 1014 979 UCIAMS 49046 Hidden I (2007)a Decalcified sediment 7630 20 8424 8410 8401 UCIAMS 24273 Hidden J 73.5 Charcoal (>125 μm) 2815 25 2949 2915 2879 UCIAMS 24269 Hidden C 61.5 Decalcified sediment 2845 25 2993 2953 2891 UCIAMS 24272 Hidden C 73 Conifer needle (0.2 mg C) 3465 25 3823 3747 3691 UCIAMS 26032 Hidden C 89.5 Decalcified sediment 3930 20 4422 4370 4299 NOSAMS OS-61011 Hidden E 10.5 Decalcified sediment 415 25 509 491 477 NOSAMS OS-61012 Hidden E 20.5 Decalcified sediment 920 30 906 849 792 NOSAMS OS-61013 Hidden E 32.5 Decalcified sediment 1270 30 1262 1221 1179 UCIAMS 26029 Hidden E 35.5 Decalcified sediment 2060 15 2043 2026 1993 UCIAMS 26030 Hidden E 49.5 Decalcified sediment 3455 20 3819 3718 3647 UCIAMS 26031 Hidden E 68.5 Decalcified sediment 3835 20 4285 4231 4158 NOSAMS OS-60992 Hidden H 6.5 Decalcified sediment 15 35 242 55 −5 NOSAMS 26044 Little Molas E 203 Macrofossil (0.26 mg C) 9255 35 10509 10435 10302 NOSAMS 26045 Little Molas F 30 Wood 1740 20 1696 1654 1617 UCIAMS 18283 Foy C 127.5 Pollen 2680 30 2838 2781 2753 UCIAMS 18284 Foy C 127.5 Pollen 2685 30 2840 2785 2755 UCIAMS 18286 Foy C 233.5 Pollen 5900 35 6747 6719 6671 UCIAMS 18287 Foy C 242.5 Pollen 6930 35 7791 7755 7701 a Collected from the location I in 2007; all other cores from Hidden Lake were collected in 2005. tiple profiles at similar elevations (see “paleoshoreline” in Fig- tains the Glacier Peak (GP) tephra, which dates to 13,100 cal yr ures 3-5). We interpret the feature as the expression of a low- BP (Sarna-Wojcicki et al., 1983), but the thickness of sediment lake stand, overlain by an onlapping unit (Figure 5) resulting between the GP and MZ tephras (Figure 6b) decreases from from a subsequent water-level rise. core A (151 cm) to core C (74.5 cm). In fact, a distinct unconformity surface in core C at the top of Unit 1 is marked by gastro- 4.1.2. Core data pod shells and a sharp transition from the subtle laminations Six major sediment facies were found in the Foy Lake cores (facies 1, 2) of Unit 1 to the carbonate sands (facies 4) of Unit (Figure 6). The facies represent a gradient from deep to shal- 2 (Figure 7d). The unconformity lies 240 cm below the top of low water with a transition from fine laminations to coarse core C, and is 1030 cm below the modern lake surface (bmls). sand and gastropods (Figure 6a). The deepest water facies In core A, a dark, high-magnetic-susceptibility sand layer (0) includes a series of thin organic-and-carbonate lamina- marks the transition from Unit 1 to Unit 2 at 347–350 cm below tions, which are likely varves similar to those observed in the the surface of the core (1937–1940 cm bmls). deeper, central core studied by Stevens et al. (2006) and Power Likewise, unit 4 is 90 cm thick in core A, but only 15 cm et al. (2006). Correlative sediments in shallower portions of the thick or less in cores C and D (Figure 6b). An unconformity at lake contain thick carbonate-rich laminations (facies 1). Cores the base of Unit 4 is evident 140 cm below the top of core C A and C from 15.9 and 7.9 m of water respectively also contain (930 cm bmls; Figure 7b). This greenish contact between the massive marls (facies 2), fine carbonate sands (facies 3), and marl of Unit 3 and the sands of Unit 4 (Figure 7b) appears vi- coarse-grained carbonate sands (facies 4). Carbonate fragments sually to contain abundant glauconite, which would be consis- form the sand-sized grains of facies 3 and 4 and appear to have tent with a non-accumulation surface inhabited by gastropods been precipitated as crusts around littoral plants (Charophyte and other littoral organisms. In core D, a gastropod lag (Fig- casts). The lowermost sediments in shallow-water core D con- ure 6a, facies 5) overlies the unconformity. The unconformity tain a dense lag of gastropod shells (facies 5). The sandy fa- appears to correlate to the prominent unconformity and paleo- cies were not found in the deep-water core from 39.9 m of wa- shoreline in the CHIRP profiles (Figures 3–5). Only Unit 5 is ter that was the focus of previous work (Power, 2006; Power et well defined in all cores, and coincides with the first rise in the al., 2006; Stevens et al., 2006; Stone and Fritz, 2006). Core B in organic carbon content of the cores above 5% in D and C and 20.0 m of water is similar to A, and is not discussed. above 10% in core A (Figure 8). The cores contain five stratigraphic units, which represent repeated transitions from fine-grained facies (0–2) to coarse- 4.1.3. Chronology grained facies (3–5) and back (Figure 6b). Laminated and mas- The two tephras provide the primary age control at Foy Lake sive marls (facies 1, 2) form Unit 1 at the base of cores A and C. and aid core-to-core correlation. Four radiocarbon ages were ob- Massive carbonate sands comprised of Charophyte casts (fa- tained from material processed for pollen analysis. Two of the cies 3–4) then overlie Unit 1 and form Unit 2. The MZ tephra ages were obtained from the same sample at 129–130 cm in core marks a sharp transition (Figures 6b and 7c) back to massive C (the lowermost lamination). Their ages both calibrate to 2840– marl (facies 2), which forms Unit 3 in both cores. Carbonate 2755 cal yr BP, and fall within a few centuries of the varve count sands (facies 4) then form Unit 4 in cores C and A, but tran- age of 2223 cal yr BP for the lowermost varves in the deep-wa- sition abruptly to the fine laminated sediments (facies 0–1) of ter core from the center of the lake (Stevens et al., 2006). Based Unit 5 in the upper 125–140 cm of all of the cores (Figure 6b). on the near-equal thickness of the similar laminated sediments Several of the stratigraphic units are only well represented at the top of cores C and D, we assume an age of ~2800 cal yr BP in the deepest core A (Figure 6b). Unit 1 in cores A and C con- or younger for the base of the laminations in core D. 1868 Shuman et al. in Quatern ary Science Reviews 28 (2009)

Figure 6. Major sediment facies (A) and core stratigraphies (B) from Foy Lake, Montana. Magnetic-susceptibility data are presented as dashed lines; graphs of visually-identified facies (values equal numbers associated with photos in A) are plotted as solid lines. Calibrated ages of two radiocarbon dates in core C and the Mazama (MZ) and Glacier Peak (GP) tephras are shown.

The two other AMS radiocarbon dates on pollen from 237 accumulation. The unconformity dates to 6175–2780 cal yr BP, to 238 cm and 245 to 246 cm in core C bracket the lower un- if a sedimentation rate of 31.2 cm/ka is applied to the 23-cm conformity between Units 1 and 2. The ages for these two massive marl in core C between MZ and the unconformity. dates (6750–6670 and 7790–7700 cal yr BP; Table 1) are incon- The rate equals that between MZ and the varve-like lamina- sistent with their position relative to the MZ and GP tephras tions in core A, and may be a reasonable assumption for core and were not used in our age model. The upper date was ob- C given the similar spacing of CHIRP reflectors below the un- tained 55 cm below the MZ but was 850–920 yrs younger than conformity at the locations of cores C and A (Figures 3–5). the MZ; the lower date was 12 cm above the GP but was 5310– The MZ and GP ashes bracket the lower unconformity in 5400 yrs younger than the GP. core C; a linear sedimentation rate between tephras would equal 23.0 cm/ka. The unconformity is 14 cm above the 4.1.4. Sedimentation rates GP tephra, and thus its lower age is slightly younger than The chronology indicates hiatuses in sediment accumula- 13,100 cal yr BP. The unconformity would last from 12,600 to tion (Figure 8) consistent with the two apparent unconformi- 9760 cal yr BP if a sedimentation rate of 36.8 cm/ka is applied ties in core C (Figure 7b, d; note that a break in the plot of core to the sediments above and below—based on the assump- C at ca. 2000 yrs BP in Figure 8 is due to a lack of data not an tion that the continuous sedimentation rate from core A be- unconformity). The MZ and the two calibrated radiocarbon tween the MZ and GP ashes applies to core C. Alternatively, ages of ca. 2780 cal yr BP bracket the upper unconformity sur- the unconformity represents a period of limited accumula- face in core C. A linear sedimentation rate between the MZ tion over 1040 yrs based on age difference between the me- and the mean age of these dates would equal 11.1 cm/ka and dian calibrated ages of the bracketing but anomalously young is close to Webb and Webb’s (1988) threshold for nonconstant radiocarbon dates. If so, the age range of the unconformity is Holocene lake-level trends in the Rocky Mountains 1869

Figure 7. Four important facies transitions in cores from Foy Lake. (A) Transition from carbonate sands to fine varve-like laminations is shown from core A. (B) The upper unconformity surface, indicated by white dashed lines, and the transition from carbonate sands to broad laminations are show from core C. (C) The Mazama ash layer and associated transition from sands to massive marl is shown for core A, and is representative of a similar transition in core C. (D) The lower unconformity surface between laminated marl and carbonate sand, and including gastropod shells, is shown from core C. Depth intervals of individual images are listed; “Dr.” indicates drive or core sub-section. Breaks in core surfaces occurred because cores dried before imaging.

12,600–11,560 cal yr BP, which is similar to the age of a low- 2400 cal yr BP (Stone and Fritz, 2006). Likewise, Stevens et al. water-level period from ca. 12,500 to ca. 11,000 cal yr BP in- (2006) estimate that the water level rose ~12 m within the past ferred from diatoms (Stone and Fritz, 2006). 2200 years on the basis of diatom and geochemical evidence. Our data confirm that the total lake-level rise since before ca. 4.1.5. Inferred changes in lake level 2800 cal yr BP was >5.3 m based on the depth of the unconfor- Based on the elevations and ages of massive marls (facies mity in core C, and could have been >16 m based on the depo- 2), thickly laminated marls (facies 1), and varve-like lamina- sition of sands (Unit 4) in core A (Figures 6b & 7a). tions (facies 0), we infer that the primary long-term signal at Two additional fluctuations in wave base—and therefore Foy Lake was a rise in the position of wave base, and thus the water level—appear superimposed on the persistently lower- lake level, after early- and mid-Holocene (Figure 8). Given the than-modern levels prior to ca. 2800 cal yr BP (Figure 8). First, depth of the uppermost unconformity in core C and absence the lowest unconformity in core C, and an associated sand of units 1–4 in core D (Figure 6b), wave base in Foy Lake was layer in core A, mark a period when wave base was likely at at or below 930 cm bmls for an extended period between 7600 least 1030 cm bmls and as much as 1940 cm bmls between the and 2780 cal yr BP, and then rose to <525 cm bmls to allow for deposition of the GP and MZ tephras. The unconformity ages the initiation of deposition in core D (Figure 8). The accumu- suggest that the low stand has a Younger Dryas age (12,900– lation of sandy sediments (Units 2 and 4) during much of the 11,600 cal yr BP), which is consistent with low-water levels at Holocene in cores A and C, and as deep as 1940 cm bmls, ap- ca. 12,500–11,000 cal yr BP inferred from diatoms by Stone and pears consistent with a lack of earlier high stands. The infer- Fritz (2006). Second, the transition from sands to marl asso- ence fits with a period of low or fluctuating lake levels at Foy ciated with the MZ is consistent with a relatively high wave Lake inferred from diatom assemblages between 7000 and base for a few centuries to a millennium after the eruption. 1870 Shuman et al. in Quatern ary Science Reviews 28 (2009)

tors represent the extent of the littoral zone during low stands of the lake. The older and larger of the two features is associated with two unconformities and may, therefore, represent a feature formed during two consecutive low stands.

4.2.2. Core data Cores C, D, and E in water depths of 6.0, 5.5, and 5.0 m respectively penetrate the older and most prominent low-stand reflector in the GPR profiles (Figures 2 & 3). Cores F and H in 4.5 and 3.5 m of water penetrate only the uppermost strata that extend nearest to shore. Core F penetrates the uppermost unconformity, which is not evident at the location of core H (Fig- ure 3). The shallow cores (C–H) decrease in thickness shoreward with 91 cm of organic-rich silts in core C and only 6 cm of organic-rich silts in core H (Figure 10). The cores contain two facies: (1) a dark-banded silt with >20% loss-on-ignition at 550 °C and (2) a light gray inorganic silt with <10% loss-on-ignition at 550 °C. Twenty centime- ters of dark-banded silt (facies 1) form the base of cores E, D, and C. These basal sediments are denser and contain less water than sediments above, except for a prominent gray layer of facies 2 that directly overlies the lowermost sediments (Fig- ure 10). Radiocarbon ages from cores E and C indicate that the basal unit of facies 1 also has the same age (ca. 4400–3700 cal yr BP) in both cores, and is much younger than a calibrated basal radiocarbon date of 8410 (8425–8400) cal yr BP from a core collected in the depocenter of the lake (location I in Figure 2b). Facies 2 is similar to the fine silt- and clay-rich matrix of the till surrounding the lake, and forms a distinct 8–19-cm thick layer separating the lower unit of facies 1 from subse- Figure 8. Lake-level interpretation and summary of core data from Foy Lake. quent units in cores E, D, and C (Figure 10). The facies 2 layer Symbol locations indicate linearly-interpolated ages between age control dates from 3750 to 2950 cal yr BP in core C, and from 3720 to points and depth below the modern lake surface. Symbol size indicates facies number (Figure 5a). Symbol color denotes the total organic content (TOC). 2025 cal yr BP in core E. It has a sharp lower contact in cores E Gray shaded area represents the area inferred to have been in the zone of and D at 563 and 610 cm bmls respectively, and becomes pro- sediment accumulation below the wave base, based on the presence of sedi- gressively thicker toward shore where the bases of cores F and ment in the cores. “H” indicates a hiatus in sedimentation based on the pres- H contain an upward fining sequence of silts, sands, and peb- ence of an unconformity surface and low sedimentation rates. A gap near the bles. The layer is denser than other sediments in the cores (Fig- top of core C represents a gap in coloumetry data. ure 10) and likely correlates with the older of the two strong reflectors in the GPR profiles (Figure 9). In a second transect Stone and Fritz (2006) observe an abrupt change in diatom as- of cores at the southern end of the lake (not shown), the layer semblages following the deposition of MZ tephra and infer overlies and fills a mud crack preserved in one core. high water levels from 7600 to 7100 cal yr BP; Power (2006) ob- Two additional light-colored bands of facies 2 are evident served a sharp change in pollen assemblages and fire regimes amid the sediments of facies 1, which comprise the upper- at the same time. most sediments in all of the cores above the prominent gray Our core stratigraphies and sub-surface profiles contain lit- layer (Figure 10). Peaks in density and decreases in sediment tle evidence for lake-level changes on century time scales in water content mark the first of these bands at 39–42 and 29– the past 2000 years, except for subtle facies changes (massive 31 cm depth in cores D and E, where the band is associated to banded marls) in core D (Figure 6b). Geochemical and di- with a calibrated radiocarbon age of 1220 cal yr BP (Figure 10). atom data indicate, however, that the lake level continued to The second band at 10–20 cm depth in cores F, E, and D, be- change with the two basins of Foy Lake merging only at ca. comes increasingly light colored and dense toward shore, and 800 cal yr BP (Stevens et al., 2006; Stone and Fritz, 2006), and is bracketed by ages of 850 and 490 cal yr BP (Figure 10). These therefore, near the end of the dry Medieval period recorded by bands may correlate with the upper two unconformities in the tree-ring data (Cook et al., 2004). GPR profiles (Figure 9). Neither band is evident amid the 6 cm of facies 1 at the top of core H. 4.2. Hidden Lake, Northern Colorado 4.2.3. Sedimentation rates 4.2.1. GPR data The basal ages from cores C and E of 4395 and 4230 cal yr GPR profiles from Hidden Lake show well-stratified sedi- BP were obtained on material 689–690 cm bmls and 568–569 cm ments extending across the lake basin in water depths of >3 m, bmls respectively, and indicate a lack of sediment accumulation and show that only the uppermost strata extend above water >390 cm below the modern “sediment limit” until ca. 4400 cal yr depths of about 6 m (Figure 3). The profiles also show evidence BP (Figure 11). By contrast, sediment had been accumulating at of three near-shore unconformities, which truncate the under- the location of core I since ca. 8400 cal yr BP (Table 1). lying sediments and form the base of onlapping sequences (Fig- Net sedimentation rates in core C reached 22.6–34.7 cm/ka ure 9). Two strong reflectors (labeled “PS” in Figure 9) are asso- from 4370 to 3750 cal yr BP, and were 29.8–56.0 cm/ka in core E ciated with the unconformities and are evident near shore in from 4230 to 3720 cal yr BP (Figure 11). The rates slowed, how- multiple profiles throughout the basin. We infer that the reflec- ever, after statistically similar ages of 3820–3690 cal yr BP in Holocene lake-level trends in the Rocky Mountains 1871

Figure 9. Details from the western near-shore segments of three GPR profiles of Hidden Lake. Radar data are shown in the top row of panels with a schematic interpretation of the major onlapping and truncated reflectors, and unconformities, below. Arrows labeled “PS” indicate strong (dark) reflectors that are possible paleoshoreline features. core C and 3820–3650 cal yr BP in core E. In core C, the prom- paleoshoreline in the GPR data (Figure 9). inent gray layer of facies 2 accumulated between 3750 and The uppermost band of facies 2 in core E dates to 850– 2950 cal yr BP at a net sedimentation rate of 12.3–16.5 cm/ka, 490 cal yr BP (Figure 10), and may correlate with the upper- which is close to Webb and Webb’s (1988) threshold for con- most unconformity evident in GPR profiles near the top of the stant accumulation. In core E, the net sedimentation rate was section (Figure 9). The sedimentation rate between the brack- below the threshold: 7.7–8.7 cm/ka. The rate in core E remained eting dates is 23.3–35.3 cm/ka, which is higher than Webb and low (3.5–4.1 cm/ka) until an age of 1260–1180 cal yr BP. The Webb’s (1988) threshold. The rate, however, is slower than gray layer, therefore, probably coincides with an unconformity that for the immediately underlying sediments in core E (25.5– that spans from ca. 3700 to 2990 cal yr BP at 673–674 cm bmls in 44.0 cm/ka) and for the sediments of equal age in the cen- core C, and from ca. 3700 to 1200 cal yr BP at 549–536 cm bmls tral core, I (47.1–54.2 cm/ka). The relatively low sedimenta- in core E. The ages therefore indicate a 930–2690 yr period of tion rates, and the high near-shore density of the layer, could limited sedimentation >320 cm below the modern “sediment be consistent with a low-lake stand during the Medieval pe- limit” (Figure 11). The different time ranges of slow sedimen- riod. The 6 cm of organic silts (facies 1) in core H only date to tation in cores E and C match with onlapping above the lowest 55 cal yr BP (Figure 11; Table 1).

Figure 10. Core stratigraphies and images from Hidden Lake in northern Colorado. Thin lines show the core water content, and dark lines show bulk density based on gamma-ray attenuation. Dots indicate the location of AMS radiocarbon samples. Italics denotes dates based on macrofossil material or sedimentary charcoal; median calibrated ages are listed. Gray bars marks the position of the layer of low-organic content that correlates with the lower (more prominent) of the two potential paleoshorelines in the radar profiles (“PS” in Figure 9). 1872 Shuman et al. in Quatern ary Science Reviews 28 (2009)

bly declined again by 1220 cal yr BP based on slow sedimentation rates and a second band of facies 2. Similar features may indicate that the lake experienced at least one additional low stand during Medieval times between ca. 850 and 490 cal yr BP. The sedimentary evidence for a Medi- eval low stand is less pronounced than the low stand after ca. 3700 cal yr BP, but seems consistent with a drop in wave base of >50 cm given the differences in the depths of the unconformity (as recorded by density peaks) in cores F and E (Figure 11). The timing of the low stand is consistent with low-water levels at Mono Lake, California, after ca. 840 cal yr BP (Stine, 1994), and with tree-ring evidence for extensive drought in the western U.S. between ca. 1150 and 650 cal yr BP (Cook et al., 2004).

4.3. Little Molas Lake, Southern Colorado

4.3.1. GPR data Four major sedimentary units appear in the GPR profiles from Little Molas Lake (Figures 3 & 4). The lowermost unit (Unit 4, Figure 4) contains few obvious reflectors and does not extend as close to shore as more recent sediments. Unit 4 also laps onto basement material both near shore and near the top of a mid-lake outcrop (at right in Figure 3). Unit 3 is well stratified with multiple reflective bands. It appears to conformably Figure 11. Lake-level interpretation and summary of core data from Hidden overlie unit 4, except near shore and near the mid-lake base- Lake. Symbol locations indicate linearly-interpolated ages between age con- ment outcrop, where the base of unit 3 onlaps over the top of trol points and depth below the modern lake surface. Symbol size indicates unit 4. Unit 2 has three sub-units: a section of highly reflective percent loss-on-ignition at 550 °C. The arrow labeled “MD” marks the lake strata between two sections without strong reflectors. The re- low-stand associated with a Medieval drought. Gray shaded area represents flectors of unit 2 lap over the top of unit 3. the area inferred to have been in the zone of sediment accumulation below Unit 1 (“Late-Holocene sediments” in Figure 4) extends the wave base, based on the presence of sediment in the cores. Ages of core higher along the lake margin and over the mid-lake outcrop F sediments were inferred by stratigraphic correlation to core E. Dashed than any of the older units. Units 2–4 do not cover the top of the lines at base of core A indicate continued but uncollected sedimentation. mid-lake outcrop, and an unconformity truncates units 2 and 3 near shore. Unit 1 overlies the near-shore unconformity and 4.2.4. Inferred change in lake level the top of the bedrock outcrop. An erosional unconformity does The Hidden Lake data track a progressive rise in lake lev- not, however, appear to truncate units 2 and 3 near the mid-lake els since ca. 4400 cal yr BP, which was interrupted by an abrupt, outcrop. The highest extent of units 2 and 3 just below the top of sub-millennial high stand centered at about 4000 cal yr BP and a the outcrop is approximately the same elevation as the lowest brief Medieval-age low stand after ca. 850 cal yr BP (Figure 11). elevation of the near-shore unconformity (Figure 4). Sediments dating to the 19th and 20th centuries extend higher in the lake basin than any earlier sediment and may indicate 4.3.2. Core data that Hidden Lake is currently higher than any time in the Ho- Cores B, C, and D contain similar and complete sequences locene. The basal ages of cores C, E, and I indicate that the sed- from water depths of 3–1.5 m and appear to include contin- iment limit was also much higher in the past 4400 years than uous accumulation since the formation of Little Molas Lake. during earlier portions of the Holocene. Indeed, the basin may Core E, however, penetrates through the near-shore unconfor- have been desiccated before ca. 8400 cal yr BP based on the basal mity in 1 m of water, and core F in 69 cm of water contains age from core I (Table 1). The depth difference between the base only unit 1 where it extends beyond the older units. of core C and modern sediment deposition at the top of core H Cores E, D, and C contain highly similar magnetic suscep- indicates that wave base, and by association the water surface, tibility and bulk density stratigraphies (Figure 12). The lower- have risen by >340 cm since before 4400 cal yr BP. most sediments in all four cores are light pink silts and clays Based on cores C and E, and the lack of onlapping at the base that are dense and have high-magnetic susceptibility. These of the GPR profiles (Figure 9), water levels rose rapidly at first: sediments likely correspond to unit 4 in the GPR profiles the sediment limit rose >120 cm between 4400 and 4200 cal yr (Figure 4). BP. The rise does not appear to have reached above the eleva- Organic-rich banded silts with multiple fibrous intervals tion of core F (450 cm bmls) given the absence of facies 1 be- comprise the remainder of the four cores. A rise in bulk den- low the gray layer (facies 2) at the base of that core (Figure 10). sity at a depth of 228–224 cm in cores C and D and at a depth The rise marks the beginning of a ~700-yr-long high stand that of 195 cm in core E can be tracked among the cores, as can four lasted until ca. 3700 cal yr BP. The overlapping ages below the small susceptibility peaks (gray lines, Figure 12). Above these gray layer in cores E and C are consistent with a rapid decline features (at 85 cm in core C, 80 cm in core D, and 50 cm in core in water level of >120 cm at the end of the high stand. The trun- E), susceptibility declines conspicuously. The low susceptibil- cation of the lowermost reflectors in the GPR profiles (Figure 9), ity sediments have similar thicknesses of about 50 cm in cores increases in core sediment densities (Figure 10), and declines in D and C but are reduced in thickness to 25 cm in core E (Fig- sedimentation rates (Figure 11) show that wave base fell below ure 12). No unconformity surface is apparent in core E, but the the location of core C by 3700 cal yr BP. After the end of the transition from uniformly thick features among cores to the low stand, wave base rose by >125 cm to allow organic deposi- thin layer in core E (Figure 12) matches with the truncation of tion (a band of facies 1) in core E by 2020 cal yr BP, but proba- unit 2 in the GPR data (Figure 4). Holocene lake-level trends in the Rocky Mountains 1873

Figure 12. (A) Core stratigraphies from Little Molas Lake in southern Colorado. Gray lines connect correlative features in the cores. Black symbols indicate the locations of AMS radiocarbon ages on macrofossil material in cores F and E. (B) Age–Depth relationships for cores F and E. Gray lines indicate inferred depths of wave base before (dashed) and during the accumulation of core F (solid).

A rise in susceptibility marks the uppermost sediments in the location of core F and the sediments became more organic cores E, D, and C (Figure 12), and probably corresponds to and fine grained, and thus less dense and magnetic, over time. unit 1 in the GPR profiles (Figure 4). Notably, the thickness of The near-shore extent of unit 1 in the GPR profiles beyond the the uppermost sediments (25–35 cm) is again similar in cores uppermost extent of older units (Figure 4) matches with this E, D, and C (Figure 12). interpretation. The uppermost rise in susceptibility in cores Core F is short (25 cm long) and contains dark silts that are E, C, and D may indicate a correlative increase in streamflow denser and have higher susceptibility than the sediments of into the lake and an increase in the deposition of volcanically- the other cores (Figure 12). The bulk density declines through- derived minerals in the past 1650 years. The evidence for in- out the core, but susceptibility increases to a peak at 20 cm and creased runoff and high water levels during this period are then declines until a second rise begins in the upper 10 cm. consistent with a trend toward mesic vegetation recorded in fossil pollen data (Toney and Anderson, 2006). 4.3.3. Chronology The extent of the sediment units on the mid-lake outcrop A basal AMS date from core F on a wood fragment from (Figure 3) is inconsistent with previous periods of when wave- 25 cm calibrates to 1695–1620 cal yr BP. A second AMS date on base elevations equaled or exceeded those of the past 1650 yrs. macrofossil material at 221 cm near the base of the organic silts GPR units 2 and 3 did not accumulate on top of the outcrop in core E calibrates to 10,510–10,300 cal yr BP (Table 1; Fig- and show no evidence of having been eroded from there; these ure 12). Based on these two dates, the average net sedimen- units lap onto the outcrop with their uppermost reflectors ex- tation rates for cores F and E are 15.1 and 21.1 cm/ka respec- tending to just below the top of the basement material. There- tively. Therefore, the top ~35 cm of core E, which contain the fore, wave base was probably at or below the top of the out- uppermost susceptibility rise, accumulated during the time crop before ca. 1650 cal yr BP. represented by core F. The unconformity near shore below Unit 1 (Figure 4) may have resulted from sediment infilling near shore or a period 4.3.4. Inferred change in lake level of extremely low-water levels before ca. 1650 cal yr BP, but The sediments in core F represent a significant rise in lake the later would have also produced an unconformity over the level. Wave base likely rose by >30 cm before ca. 1650 cal yr mid-lake outcrop (which is not evident). However, the onlap- BP because the base of core F is ~30 cm shallower than the cor- ping at the base of units 2 and 3 (Figures 3 & 4) may indicate relative sediments in core E (Figure 12b). As the sediment limit earlier declines in wave-base elevation. The limited shoreward shifted higher than before, sediment began to accumulate at extent of unit 4 (Figure 4) shows that wave base was proba- 1874 Shuman et al. in Quatern ary Science Reviews 28 (2009) bly several meters lower than today before ca. 10,500 cal yr 11,500 and 3000 and at ca. 6000 cal yr BP respectively (Hevly, BP. The core and GPR data show no evidence for water-level 1985; Anderson, 1993). Similarly, the core from Montezuma’s changes after ca. 1650 cal yr BP, despite evidence for dry Medi- Well, Arizona, contains intervals of gravel and shallow-wa- eval conditions at other near-by lakes (Davis, 1994). The small ter macrofossils that date from >8000 to 1500 cal yr BP and in- amplitude of the changes at Little Molas Lake likely resulted terrupt otherwise fine-grained organic sediment accumulation from the ability of the lake to overflow; the observed change in (Davis and Shafer, 1992). wave base may indicate changes from seasonal or intermittent overflow to constant overflow.

4.4. Other direct evidence of Holocene lake-level change in the Rocky Mountains

Foy, Hidden, and Little Molas lakes show stratigraphic evidence for high water levels in the past few millennia. Sedi- ment cores and sub-surface profiles from other lakes throughout the Rocky Mountain region also contain similar evidence (Figure 13). The data appear consistent among lakes both within Cayan’s (1996) regions (Figure 1), indicating that our study sites may be regionally representative, and across the Rocky Mountains as a whole. Drier-than-modern conditions appear evident at all sites between ca. 7000 and 4500 cal yr BP (Figure 13). Near Foy Lake, Hofmann et al. (2006) used seismic profiles to infer a late-Holocene (<3000 cal yr BP) high stand of Flat- head Lake. Like Foy Lake, low levels during the mid-Holocene began after the deposition of the MZ ash. In central Montana, geochemical analysis of carbonates at Jones Lake shows a transition from aragonite-dominated sediments to calcite-dominated sediments beginning at ca. 3100 cal yr BP and likely indicates a late-Holocene rise in lake level (Shapley, 2005). The last occurrence of aragonite at Jones Lake follows a multi-step decline that ended by ca. 1400 cal yr BP, and thus parallels the sedimentary evidence for lake-level rise at Foy by 2800 cal yr BP and the diatom evidence for the highest levels at Foy Lake after ca. 1250 cal yr BP (Stevens et al., 2006). In Wyoming, but also within Cayan’s (1996) “Idaho” pattern of snow variability (Figure 1), three kettle lakes in the Wind River Range show evidence of low-water levels during the mid-Holocene. Two contain sedimentary hiatuses at 8000– 3100 and 7500–5700 cal yr BP respectively, and the third lake did not begin to accumulate sediment until after 5700 cal yr BP (Lynch, 1998). Seismic-reflection profiles and paleobiotic data from Yellowstone Lake, 100 km northwest of the Wind River kettle lakes, may also indicate low-water levels during the mid-Holocene (Otis et al., 1977; Ariztegui et al., 2000; John- son et al., 2003; Theriot et al., 2006; Morgan et al., 2007). The profiles show onlapping of recent sediments and submerged shoreline features (Johnson et al., 2003; Morgan et al., 2007), which cores confirm date after ca. 7000 cal yr BP (Ariztegui et al., 2000; Pierce et al., 2002). Pierce et al. (2002) obtained a radiocarbon age of 3835 cal yr BP near the top of a submerged beach sand 5 m below the modern lake surface. Unfortunately, the volcanic and tectonic history of the Yellowstone Caldera Figure 13. Summary of regional lake-level data over the past 13,000 years. complicates the climatic interpretation of these data. Gray lines show the inferred changes in wave-base elevation (WBE), and by as- In central Colorado within the region of snow variabil- sociation, lake level at Foy, Hidden, and Little Molas Lakes; dashed lines indicate when lower-than-modern levels are inferred but the specifics of any additional ity that includes Hidden Lake, Cottonwood Pass Lake con- fluctuations have not been constrained. Black dots indicate the ages of radio- tains peat layers that date to ca. 6200–5000 cal yr BP suggest- carbon dates at the base of late-Holocene high-stand sediments. Numbered ing shallow or wetland conditions. Near-by wetlands also bars indicate the timing of low-lake stands at additional study sites, which are accumulated sediment only in the past 5400 and 2900 years labeled as in Figure 1a: 1, Flathead Lake, MT (Hofmann et al., 2006); 2, Jones as though they had been dry until the later half of the Holo- Lake, MT (Shapley, 2005); 4, Park Pond 2, WY (Lynch, 1998); 5, Forest Pond cene (Fall, 1997). In northern Arizona, and within the region 2, WY (Lynch, 1998); 6, Park Pond 1, WY (Lynch, 1998); 7, Cottonwood Pass of snow variability that includes Little Molas Lake, Weng and Pond, CO (Fall, 1997); 8, Red Lady Fen, CO (Fall, 1997); 9, Red Well Fen, CO Jackson (1999) report hiatuses attributable to low-lake levels (Fall, 1997); 10, Bear Lake, AZ (Weng and Jackson, 1999); 11, Fracas Lake, AZ (Weng and Jackson, 1999); 13, Montazuma Well, AZ (Davis and Shafer, 1992); at Bear Lake from 7400 and 4230 cal yr BP and at Fracas Lake 14, Potato Lake, AZ (Anderson, 1993). The timing of low stands at Yellowstone from 8500 and 2000 cal yr BP. Cores from Potato and Walker Lake, WY (Otis et al., 1977; Figure 1a site 3), and Walker Lake, AZ (Hevly, 1985; Lakes in Arizona also contain evidence of desiccation between Figure 1a site 12) is not shown because it is poorly constrained. Holocene lake-level trends in the Rocky Mountains 1875

5. Discussion centennial record is hard to evaluate because of the small number of radiocarbon ages at Foy and Little Molas Lakes, some 5.1. What long-term hydrologic trends are evident during the millennial-scale patterns can be inferred. For example, multi- Holocene? ple dates from Hidden Lake reveal that the lake was high at 4400–3800 cal yr BP and that a rapid decline of >120 cm in less 5.1.1. Orbital-scale trends than a century followed (Figure 11). The high stand correlates We find evidence that mid-Holocene lake levels were lower with evidence for a period of high water levels between 4500 than today at a wide range of settings (Figure 13). Differences and 3500 cal yr BP inferred from diatom assemblages at Foy among our records indicate that moisture trends were spa- Lake (Stone and Fritz, 2006). Likewise, Miao et al. (2007) sum- tially patterned and modulated by other local factors (i.e., hy- marize the dated evidence of loess deposition in Nebraska at drology) and short-term variability, but important long-term sites ~450–750 km east of Hidden Lake, and include dates of trends exist throughout the region. The trends are consistent 4810–4450 and 5060–4480 cal yr BP on paleosol formation dur- with the effects of orbital forcing, especially given that mid-lat- ing a period of minimal dune activity (Miao et al., 2007). These itude lake-level responses to the seasonal progression of orbital ages are slightly older than the ages of 4420–4300 and 4285– forcing can result in large water-level rises in the last few mil- 4160 cal yr BP, which mark the beginning of the high water lennia (Shuman and Donnelly, 2006). In particular, the regional levels at Hidden Lake, but the subsequent period of maximum rise in water levels by ca. 2000 cal yr BP is consistent with the loess deposition (above the paleosols) in Nebraska dates from persistence of >20 W m−2 insolation anomalies in early autumn 3890 to 2430 cal yr BP when Hidden Lake was low (Figures 10 until after 3000 yrs BP (Berger, 1978). Summer insolation anom- & 11). Therefore, Hidden Lake may have been high when pa- alies declined steadily over the course of the Holocene, but Sep- leosols developed in Nebraska, and low during the periods of tember and October insolation anomalies increased until 6000– maximum aeolian activity. Other episodes of dune and loess 4000 cal yr BP. The autumn insolation anomalies are directly activity date to 9400–6600 and 770–570 cal yr BP (Miao et al., relevant to the lake-level record because they affect a portion of 2007) when Hidden Lake was also low (Figure 11). the year when the balance of precipitation and evapotranspira- Some dune data contrast with the Hidden Lake record, tion shifts from negative to positive today (Figure 2). The insola- however. A period of enhanced aridity has been inferred from tion anomalies were also important for generating atmospheric dune activity at 4300–4100 cal yr BP in the Ferris Dunes of cen- dynamics that probably limited precipitation rates during much tral Wyoming (Stokes and Gaylord, 1993). The Ferris Dunes of the Holocene (e.g., Harrison et al., 2003; Diffenbaugh et al., are located 180 km north of Hidden Lake, and contain ~10 m 2006) and for affecting temperatures in a manner that may have of aeolian deposition associated with ages of 4340–4040 cal yr reduced recharge from spring snowmelt (e.g., Bartlein et al., BP, and interbedded interdune deposits that date from 8290 to 1998; Stewart et al., 2004); see further discussion below. 5650 cal yr BP (Stokes and Gaylord, 1993; Forman et al., 2001). The sedimentary signatures of low-lake stands before ca. Based on these ages, little dune activity occurred in central 2000 yrs BP (Figures 5, 8, & 13) are also more pronounced than Wyoming when Hidden Lake was low, and extensive dune stratigraphic features in our study lakes associated with re- activity took place when the lake was high. The regional con- cent decadal-to-centennial droughts (e.g., Stine, 1994; Cook trast between two moisture indicators (lake levels and dunes) et al., 1999, 2004; MacDonald, 2007; Meko et al., 2007). Lake may depend, in part, on differences in seasonal moisture in- levels rose >6 m from 6000 yrs BP to today at Foy Lake (Fig- fluences on lakes and dunes. Dune activity depends on vege- ure 7) and >3 m at Hidden Lake (Figure 9), but likely fluctu- tation cover controlled by summer moisture, as well as wind ated much less during the last millennium. At Hidden Lake, (Forman et al., 1995, 2001), but lakes may rely heavily on win- potential stratigraphic evidence for a recent low-stand dates to ter precipitation (Figure 2). the Medieval period, but the associated lake lowering would have been substantially less than occurred before 4400 cal yr 5.1.3. Comparison with other proxies BP and between 3700 and 1200 cal yr BP. At Little Molas and Like the dune data, fossil pollen and plant macrofossil re- Foy Lakes, water levels may have continued to rise until af- cords confirm that lake-level trends do not fully represent all ter ca. 1600 and 800 cal yr BP respectively, but we do not find aspects of Holocene hydroclimatology. Specifically, the lake- stratigraphic evidence for pronounced fluctuations in water level data from the southern Rocky Mountains differ from levels at these sites during the last millennium. other evidence from Colorado and Arizona that indicates wet- The blunt character of sedimentary records provides a “filter-than-modern conditions in the early to mid-Holocene as a ter” that is useful for gauging the relative severity of different result of an insolation-enhanced southwestern monsoon (Be- climatic events. Recent, short-term droughts were not strong tancourt et al., 1990; Davis and Shafer, 1992; Thompson et al., enough to generate an obvious sedimentary record, whereas 1993; Fall, 1997; Mock and Brunelle-Daines, 1999; Harrison et changes forced by boundary-condition changes over the course al., 2003). In Colorado, forests expanded downslope into now of the Holocene passed the sedimentary “filter” of climatic sig- dry valleys in Colorado in response to wet conditions before nals and produced obvious stratigraphic evidence. Therefore, ca. 6000 yrs BP (Fall, 1997; Markgraf and Scott, 1981; Mayer et although “megadroughts” are part of the natural variability of al., 2005). Mayer et al. (2005) found evidence of forest parkland the late-Holocene (e.g., Cook et al., 2004), changes in the large- during the early Holocene where sagebrush steppe grows to- scale controls on the climate system in the past (i.e., insolation) day in Middle Park, Colorado, ~45 km from Hidden Lake, but and, by analogy, in the future (i.e., greenhouse gases) have the Hidden Lake was low at the time based on the lack of near- potential to drive regional water levels well beyond the enve- shore sediments older than ca. 4400 cal yr BP (Figures 10 & 11). lope of variability that existed during the past millennium. Little Molas Lakes and other lakes in the southwest U.S. (Fig- ure 13) were also lower than today during the same period. 5.1.2. Millennial-to-centennial variations The coincidence between forested valleys, inactive Wyoming Some moisture changes that took place over millennial-to- dunes (see Section 5.1.2), and low lakes may be explained by a centennial time scales were also large enough to pass through high-relative importance of winter (non-growing season) mois- the sedimentary “filter” and produce direct stratigraphic evi- ture for the lakes (e.g., Prentice et al., 1992). Forest extent and dence. Although the regional patterning of the millennial-to- dune activity, by contrast, may have depended in large part 1876 Shuman et al. in Quatern ary Science Reviews 28 (2009) upon spring and summer precipitation (Forman et al., 1995, tial for large boundary-condition changes to override the in- 2001; Weltzin and McPherson, 2000). Isotope data from southern fluences of the internal modes of climate variability that were Colorado near Little Molas Lake indicate that summer precipi- important during the past millennium (see e.g., Diffenbaugh et tation probably decreased relative to winter precipitation over al., 2006). However, the spatial patterns of response to ENSO the course of the Holocene (Friedman et al., 1988). Likewise, cli- and other modes of Pacific SST variability may vary through mate models simulate lower snowpack than today in the early- time (Shinker and Bartlein, 2009), and the influences of these and mid-Holocene (Bartlein et al., 1998), and the absence of gla- patterns may not generate consistently strong ‘spatial finger- cial activity in Colorado between ca. 8000 and 1000 cal yr BP prints’ at both short- and long-time scales. Indeed, some his- (Benson et al., 2007) indicates long-term periods of low snow ac- toric droughts, such as in AD 1856–1865 and AD 1890–1896, cumulation. Therefore, in addition to the direct effects of inso- have been attributed to the effects of cool La Nina-like condi- lation anomalies on evaporation rates (discussed above in Sec- tions like those inferred for the mid-Holocene (e.g., Overpeck tion 5.1.1), the long-term lake-level trends in Colorado may be and Webb, 2000), and have also not shown a north–south con- related to an increase in winter precipitation, as well as changes trast in the Rocky Mountains and adjoining regions (Herwei- in the volume, rate, and seasonal timing of snowmelt. jer et al., 2006). If so, the lack of a match between the Holocene In other portions of the Rocky Mountains, and even locally lake trends and recent patterns of variability (Figure 1) may within Colorado, vegetation trends track moisture-level changes not preclude a role for the drivers of recent variability (e.g. that are similar to those inferred from the lake-level histories. ENSO) over the course of the Holocene. High-elevation forests near Little Molas Lake became more mesic in the past few millennia when the lake level was high 5.2.2. Comparison with climate model simulations (Toney and Anderson, 2006). Vegetation trends are also consis- Widespread aridity in the Rocky Mountains at 6000 yrs BP tent with our lake-level reconstruction at Foy Lake because local is inconsistent with large wet anomalies over large areas of the forests became closed and mesophytic tree taxa, such as fir Ab( - Rocky Mountains in coupled ocean–atmosphere GCM simula- ies) and spruce (Picea), increased in abundance after 2200 cal yr tions of this period (Harrison et al., 2003; Shin et al., 2006). The BP (Power et al., 2006). A trend back toward more open vegeta- specific inclusion of La Nina-like tropical Pacific SST inone tion near Foy Lake in the past 700 years could indicate local hu- GCM generates enhanced aridity in the central U.S., including man activity, low annual moisture balance, or a shift in the sea- in the southern Rocky Mountains, but the sign of the regional sonal timing of precipitation (Power et al., 2006). The later two moisture anomaly was reversed when realistic greenhouse-gas possibilities are not recorded in the sedimentary record of lake- forcing (i.e., lower than preindustrial mid-Holocene levels of level change (including diatom and geochemical data; Stevens et CO2 and CH4) was included in a fully forced and coupled ex- al., 2006). More work is needed to fully understand the seasonal periment using the same model (Shin et al., 2006). Harrison et climatic patterns and other factors that explain both spatially- al. (2003) provide a conceptual model for producing aridity in variable fire and vegetation trends (e.g., Thompson and Ander- the northern Rocky Mountains when ocean–atmosphere feed- son, 2000; Whitlock and Bartlein, 2004; Brunelle et al., 2005) and backs amplify a monsoon over the southern Rocky Mountains, the lake-level trends documented here. but their model simulations do not produce the hypothesized pattern and instead generate wet conditions across most of the 5.2. What spatial patterns characterize the long-term trends in mois- central U.S. in contrast to datasets there. ture variation? The most accurate match between data and model results comes from regional-scale model simulations with more accu- 5.2.1. Comparison with patterns of interannual-to-multidecadal variability rate topography than in the GCMs (Diffenbaugh et al., 2006). Many short-term moisture patterns, including snowpack In the regional model results, seasonal insolation forcing at variability (Figure 1), emphasize a north–south contrast in the 6000 years BP drives a robust pattern of enhanced aridity (i.e., Rocky Mountain region (e.g., Trenberth et al., 1988; Cayan, 1996; not significantly changed by other forcing such as SST varia- Mock, 1996; Dettinger et al., 1998; McCabe et al., 2004). Like- tion) in most high-elevation areas of the Rocky Mountains wise, comparison of lake-level histories in the Great Basin has (Diffenbaugh et al., 2006). The regional model’s success in sim- shown variable synoptic responses to climate forcing (Licciardi, ulating the moisture-balance patterns inferred from the data 2001). The rise in lake levels at long-time scales documented underscores the importance of fine-scale processes in creat- here, however, is evident throughout the Rocky Mountains; all ing the patterns of response, and indicates that the long-term of the fourteen lakes discussed were lower than today ca. 7000– lake-level trends are probably driven by a hierarchy of local 4500 cal yrs BP (Figure 13). To the west, throughout the Great to global responses to orbital change. For example, local re- Basin, both small and large lake systems also record evidence sponses probably include effects on surface energy-budgets of aridity before ca. 3000 yrs BP (e.g., Quade et al., 1998; Brough- and soil moisture, which mediate the regional-scale (e.g., topo- ton et al., 2000; Benson et al., 2002; Briggs et al., 2005). Therefore, graphic) modification of synoptic-scale climatic patterns. lakes throughout much of the interior of the western U.S. do not show the spatial variability that exists at short time scales, but 5.3. How similar were long-term moisture variations in the Rocky rather widespread aridity during the mid-Holocene. Mountains and mid-continent? This inference is significant because some datasets - indi cate meaningful changes in Pacific SST patterns during the Intense aridity prevailed in the Great Plains and Great Holocene that are similar to those associated with the El Nino Lakes region during the mid-Holocene (Webb et al., 1983; Southern Oscillation (ENSO) on annual scales (Gagan et al., Baker et al., 1992; Forman et al., 1995, 2001; Laird et al., 1996; 1998; Overpeck and Webb, 2000; Koutavas et al., 2002). Like- Grimm, 2002), and may be expected to coincide with an am- wise, the mechanisms that shift North American precipita- plified east–west moisture gradient with wet conditions in the tion patterns on interannual time scales, such as atmospheric Rocky Mountains. A widely-stated hypothesis links dry mid- circulation changes associated with shifting Pacific SST -gra continent conditions during the mid-Holocene to the west- dients have probably also applied over the time scale of the ward flow of air masses of Pacific origin (e.g., Bartlein et al., Holocene (e.g., Harrison et al., 2003; Shin et al., 2006). Aridity 1984; Bradbury and Dean, 1993; Yu et al., 1997; Lewis et al., in the Rocky Mountains, thus, could underscore the poten- 2008). According to the hypothesis, strong zonal atmospheric Holocene lake-level trends in the Rocky Mountains 1877 flow carried the air masses into the mid-continent, where re- References gional aridity was generated by the loss of moisture from the Abbott et al., 1997 ► M. B. Abbott, M. W. Binford, M. Brenner and K. R. Kelts, A 3500 air masses as they travelled over the mountains in the western 14C yr high-resolution record of water-level changes in Lake Titicaca, Bolivia/Peru, U.S. (Figure 1b). Our data do not support this conclusion at or- Quaternary Research 47 (1997), 169. bital time scales because we do not find evidence of enhanced Anderson et al., 2005 ► L. Anderson, M. B. Abbott, B. P. Finney and S. J. Burns, Re- orographic precipitation in the Rocky Mountains. gional atmospheric circulation change in the North Pacific during the Holocene inferred from lacustrine carbonate oxygen isotopes, Yukon Territory, Canada, Quater- Both the mid-continent and the Rocky Mountains were drier nary Research 64 (1) (2005), 21–35. than today during the mid-Holocene. Therefore, our data, like Anderson, 1993 ► R. S. Anderson, A 35,000 year vegetation and climate history from historic analyses and modeling efforts (e.g., Diffenbaugh et al., Potato Lake, Mogollon Rim, Arizona, Quaternary Research 40 (3) (1993), 351–359. 2006; Shinker et al., 2006), are inconsistent with the “zonal-flow” Ariztegui et al., 2000 ► D. Ariztegui, F. Ansellmetti, G. O. Seltzer, K. D’Agostino and K. Kelts, Identifying paleoenvironmental change across South and North America using hypothesis of mid-continent aridity. Instead, aridity in both the high-resolution seismic stratigraphy in lakes. In: V. Markgraf, Editor, Interhemispheric Rocky Mountains and mid-continent is probably the direct re- Climate Linkages, Academic Press, New York (2000), 227–240. sult of seasonal insolation anomalies (i.e., increased evapo- Baker 1983 ► Baker, R. G., 1983. Holocene vegetational history of the western United transpiration), and a hierarchy of insolation-induced effects, in- States. In: Wright Jr. H. E. (Ed. ), Late-Quaternary Environments of the United States. Uni- cluding (a) broad synoptic changes that enhanced atmospheric versity of Minnesota Press, Minneapolis, MN, 109-127. Baker et al., 1992 ► R. G. Baker, L. J. Maher Jr., C. A. Chumbley and K. L. V. Zant, Pat- subsidence and, thus, prohibited precipitation over the mid- terns of Holocene environmental change in the midwestern United States, Quater- continent (Harrison et al., 2003; Diffenbaugh et al., 2005), (b) a nary Research 37 (1992), 379–389. reduction of cool-season moisture advection resulting, in part, Bartlein et al., 1998 ► P. J. Bartlein, K. H. Anderson, P. M. Anderson, M. E. Edwards, C. J. Mock, R. S. Thompson, R. S. Webb, T. Webb III and C. Whitlock, Paleoclimate simu- from low winter insolation, (c) reduced soil moisture (Forman lations for North America over the past 21,000 years: features of the simulated cli- et al., 2001), and (d) surface–atmosphere feedbacks like those mate and comparisons with paleoenvironmental data, Quaternary Science Reviews 17 linked to snowpack volume and extent in the southwest U.S to- (1998), 549–586. day (Zhu et al., 2005; McCabe and Clark, 2006). Bartlein et al., 1984 ► P. J. Bartlein, T. Webb III and E. C. Fleri, Holocene climate change in the northern Midwest: pollen-derived estimates, Quaternary Research 22 (1984), 361–374. 6. Conclusions Benson et al., 2002 ► L. Benson, M. Kaskgarian, R. Rye, S. Lund, F. Paillet, J. Smoot, C. Kester, S. Mensing, D. Meko and S. Lindstrom, Holocene multidecadal and multi- Well-documented mid-Holocene aridity in the central U.S. centennial droughts affecting Northern California and Nevada, Quaternary Science Reviews 21 (2002), 659–682. extended into the headwater regions of the Rocky Mountains, Benson et al., 2005 ► L. Benson, R. Madole, G. Landis, and J. Gosse, New data for Late and demonstrates a high sensitivity of key water supplies to Pleistocene Pinedale alpine glaciation from southwestern Colorado, Quaternary Sci- global changes. The stratigraphies of Foy, Hidden, and Little ence Reviews 24 (1–2) (2005), 49–65. Molas Lakes show consistent evidence of high lake levels dur- Benson et al., 2007 ► L. Benson, R. Madole, P. Kubik and R. McDonald, Surface-expo- sure ages of Front Range moraines that may have formed during the Younger Dryas, ing the past two millennia, and low-lake levels throughout the 8. 2 cal ka, and Little Ice Age events, Quaternary Science Reviews 26 (11–12) (2007), majority of the Holocene. Aside from these long-term trends, 1638–1649. however, the records differ significantly with unique millen- Benson et al., 1990 ► L. V. Benson, D. R. Currey, R. I. Dorn, K. R. Lajoie, C. G. Oviatt, S. nial-scale fluctuations documented by stratigraphic features at W. Robinson, G. I. Smith and S. Stine, Chronology of expansion and contraction of four Great Basin Lake systems during the past 35,000 years, Palaeogeography, Palaeo- Foy and Hidden Lakes. The Foy Lake stratigraphy indicates a climatology, Palaeoecology 78 (3–4) (1990), 241–286. low stand that may date to the Younger Dryas chronozone (spe- Berger, 1978 ► A. 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The trends indicate ety of America, Denver (1993) 215–238. that changes in the broad-scale controls on climate can signifi- Briggs et al., 2005 ► R. W. Briggs, S. G. Wesnousky and K. D. Adams, Late Pleistocene cantly alter the availability of water in the Rocky Mountain re- and late Holocene lake highstands in the Pyramid Lake subbasin of Lake Lahontan, gion and associated river basins. Large shifts in boundary-con- Nevada, USA, Quaternary Research 64 (2) (2005), 257–263. Broughton et al., 2000 ► J. M. Broughton, D. B. Madsen and J. Quade, Fish remains dition changes in the future (i.e., greenhouse-gas concentrations) from Homestead Cave and lake levels of the past 13,000 years in the Bonneville Ba- could, therefore, also produce dramatic hydrologic impacts. sin, Quaternary Research 53 (3) (2000), 392–401. Brown et al., 1989 ► T. A. Brown, D. E. Nelson, R. W. Mathewes, J. S. Vogel and J. R. 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