Time Scales and Processes of Shoreline Formation in Pluvial Lakes of the Great Basin, Western USA

University of Nevada, Reno

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology.

Noah F. Abramson

Dr. Kenneth D. Adams/Thesis Advisor

May, 2019

THE GRADUATE SCHOOL

We recommend that the thesis prepared under our supervision by

NOAH ABRAMSON

Entitled

Time Scales and Processes of Shoreline Formation in Pluvial Lakes of the Great Basin, Western USA

be accepted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

, Advisor

, Committee Member

, Graduate School Representative

David W. Zeh, Ph.D., Dean, Graduate School

Abstract

This study examines historical beach deposits formed in Walker Lake, Winnemucca

Lake, and Lake Tahoe to characterize the processes that lead to the construction of beach ridges along lacustrine shorelines and compares the volume of individual shoreline features to the lengths of time under which they formed. Maximum fetch distances of < 50 km limit waves generated in these basins. Waves that are capable of significant geomorphic work are generated during events that achieve wind speeds > 10 m/s for 3 or more hours. A 30-year wind record from

the Dead Camel Mountains, NV indicate these wind events occur 5-24 times per year. Modeled maximum total wave swash elevations for the observed significant wind events range from 0.93-

1.93 m across the three lakes studied, which is in general agreement with heights of observed beach ridges. Total incident wave energy delivered to the shorelines of interest for the years 1999,

2007, and 2017 ranged from 0 - 2.5x108 J/m across the three basins, indicating total wave energy

delivered per year can be highly variable. The volumetric analysis of historical beach ridges at

Walker Lake showed a strong correlation between time of formation and increased volume,

therefore, the individual volumes of beach ridges can be used as an indicator to infer relative

durations of lake-level stability. By applying the volume vs. time of formation relationship

developed on the historical shorelines, we estimate lake levels were sustained at 1262 m elevation

for ~4.5 - 5.5 years in Walker Lake during the late Holocene highstand around 3500 cal yr. BP.

When used to estimate durations of time of lake-level stability, volumetric analysis of beach

ridges presents a new technique to further refine pre-historic lake-level curves on an annual to

decadal timescale. However, given large variability in shoreline volumes and rates of

development across basins, it is important to acknowledge differences in sediment supply, wind

conditions, wave energy, shoreline equilibrium, and other parameters relating to beach ridge

formation before applying volumetric relationships from one lake to another.

Acknowledgements

I give heartfelt thanks to all the people that have assisted me with this project. Particularly, Dave Page who helped collect aerial imagery used in this study. Thanks to Ann Millspaugh and Benjamin Serpa for helping me develop computer code to analyze my data. Thanks to Dr. Scott McCoy and Dr. Scott Mensing for providing constructive feedback and helping refine my interpretations made in this study. Most of all thanks to Dr. Ken Adams for giving me the opportunity to work on this project, and whose insight and support allowed me to become a better geomorphologist, critical thinker, and scientist.

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Table of Contents

Abstract ...... i

Acknowledgements ...... ii

Introduction ...... 1

Pluvial Lakes and their Associated Landforms...... 2

Previous Work……………………...…………………………………...... 6

Regional Settings and Hydrology

Walker Lake…………………………………………………………………………………...... 10

Winnemucca Dry Lake…………………………………………………………………………..13

Lake Tahoe………………………………………………………………………………………17

Methods

Wind Regime Characterization and Analysis……………………………………………………21

Wave Hindcasting ……………………………………………………………………………….24

Wave Runup, Wave Setup, and Wind Setup Modeling…………………………………………...27

Wave Refraction……………………………………………………………………………….....29

Wave Energy Modeling…………………………………………………………………………..31

Volumetric Analysis of Beach Ridges……………………………………………………...……..32

Duration of Lake-Level Residency Time for Historical Beach Ridges……………………...... …35

Results

Wind Analysis…………………………………………………………………………………….36

Wave Hindcasting………………………………………………………………………………...39

Wave Runup………………………………………………………………………………………44

Wave Energy……………………………………………………………………………………...49

Volumetric Analysis: Walker Lake……………………………………………………………….53

Volumetric Analysis: Winnemucca Dry Lake……………………………………………...…….58

Volumetric Analysis: Lake Tahoe………………………………………………………………..61

Volumetric Analysis: Late Holocene Highstand at Walker Lake………………………………...63

Discussion

Wind Analysis…………………………………………………………………………………….66

Wave and Runup Modeling……………………………………………………………………….70

Wave Energy……………………………………………………………………………………...71

Evidence for Historical Shoreline Deposits……………………………………………………....72

Shoreline Volume…………………………………………………………………………………74

Applying Volumetric Analysis of Beach Ridges to Paleoshorelines……………………………...77

Conclusions…………………………………………………………...……………………….....81

References………………………………………………………………………………………..85

Tables

Table 1: Volume Analysis of Walker Lake Shorelines…………………………………………...57

Table 2: Volume Analysis of Winnemucca-Dry Lake Shorelines………………………………..60

Table 3: Volume Analysis of Lake Tahoe Shorelines………………...………………………….63

Table 4: Duration Estimations of Late Holocene Highstand Walker Lake………...…………….65

Table 5: Frequency Distribution of 30-Year Significant Wind Events by Direction……...……..69

Figures

Figure 1: Map of Late Pleistocene Extent of Pluvial Lakes in the Great Basin…………………...3

Figure 2: Map of Historical Beach Ridge Complex Walker Lake…………………………………4

Figure 3: Typical Shoreline Features of Pluvial Lakes…………………………………………….6

Figure 4: Map of Walker Lake Hydrography…………………………………………………….11

Figure 5: Historical Lake-Level Fluctuations at Walker Lake…………………………………...13

Figure 6: Tahoe-Pyramid-Winnemucca Lake Basin Hydrography………………………………15

Figure 7: Historical Lake-Level Fluctuations at Winnemucca-Dry-Lake………………………..16

Figure 8: Aerial Photo of Winnemucca-Dry Lake Historical Shorelines………………………...16

Figure 9: Map of Lake Tahoe Hydrography……………………………………………………...19

Figure 10: Map of Baldwin Beach Shoreline Deposits, Lake Tahoe…………………………….20

Figure 11: Historical Lake-Level Fluctuations at Lake Tahoe…………………………………...21

Figure 12: Map of Location of Dead Camel Mountain Weather Station…………………………22

Figure 13: Wave Refraction Mechanics…………………………………………………………..30

Figure 14: Cross-section of Baldwin Beach 1900 m Beach Ridge……………………………….34

Figure 15: Photo of Active Swash Processes, Lake Tahoe……………………………………….36

Figure 16: 30-year Average Windspeed and Annual Significant Wind Event Occurrence………37

Figure 17: Significant Wind Event Wind Rose 1999, 2007, 2017……………………………….38

Figure 18: Results of Modeled Deep-Water Wave Heights...... 40

Figure 19: Results of Modeled Wave Runups……………………………………………………45

Figure 20: Results of Modeled Wave Energy…………………………………………………….50

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Figure 21: Map of Typical Shorelines Analyzed at Walker Lake………………………………..55

Figure 22: Volume vs. Lake Level Residency Time, Walker Lake………………………………56

Figure 23: Volume vs. Shoreline Elevation, Winnemucca-Dry Lake……………………………58

Figure 24: Map of Shorelines Analyzed at Winnemucca-Dry Lake……………………………...59

Figure 25: Volume vs. Lake Level Residency Time, Winnemucca-Dry Lake…………………...60

Figure 26: Volume vs. Lake Level Residency Time, Lake Tahoe……………………………….61

Figure 27: Map of Shorelines Analyzed at Lake Tahoe………………………………………….62

Figure 28: Geomorphic Map of Late Holocene Shorelines, Walker Lake……………………….64

Figure 29: Estimation of Time of Formation for Late Holocene Highstand, Walker Lake………65

Figure 30: Shoreline Evidence for N Directed Winds, Winnemucca-Dry Lake…………………68

Figure 31: Wind Rose of Significant Wind Events for 30-year Wind Record…………………...69

Figure 32: Lake-Level Curve Walker Lake, Late Holocene- Present…………………………….80

Introduction

A key characteristic of closed lake basins is that lake stage is directly related to the relative abundance of inflow, which includes precipitation, runoff, and groundwater inflow vs. outflow, which includes evaporation and groundwater discharge (Mifflin and Wheat, 1979).

These factors make lake-surface elevation in pluvial lakes a sensitive indicator of climatic conditions for a given time period. Past lake levels are often recorded in the form of distinct shoreline deposits, which have been used for over a century to document paleoclimatic fluctuations, paleohydrology, and tectonic deformation in the western U.S. (e.g., Russell, 1885;

Mifflin and Wheat 1979; Adams and Wesnousky, 1998; Adams et al. 1999; Oviatt, 2015).

Shoreline features form as near horizontal, equipotential surfaces associated with a particular lake level. These characteristics allow them to be easily distinguished from other landforms in the field, with topographic data, and in high-resolution imagery. Despite the prominence and marked geomorphic characteristics of shoreline deposits formed in pluvial lakes, little to no research has been done to quantify the rate at which these features form. A variety of time scales are often suggested for shoreline formation from months to years (e.g., Adams and

Wesnousky, 1998), decades (e.g., Thompson and Baedke, 1995), to hundreds or even thousands of years (e.g., Oviatt, 2015), yet much of this evidence is more qualitative than quantitative.

The primary objective of this paper is to first explore in detail the formative processes that lead to beach ridge formation in fetch-limited lacustrine environments. To do this local wind regime, wave energy, and lake levels associated with individual historical beach deposits are characterized at Walker Lake, Winnemucca Lake, and Lake Tahoe in western Nevada and northeastern California. Secondly, volumetric analysis of historical beach ridge deposits in these basins are related to the known durations of time under which they formed. This approach allows us to develop a methodology to relate the volume of material accumulated in a beach ridge deposit to the lake-level residency time at that associated elevation. This relationship is then 2

applied to the late Holocene highstand beach ridge at 1262 m in the Walker Lake basin which was formed ~ 3600 cal yr. BP (Adams and Rhodes, 2019b). This allows us to estimate the duration of time that the 1262 m lake level persisted. Given the abundance and high preservation potential of beach ridge deposits throughout the Great Basin and beyond, this work will have far- reaching impacts. It is our hope that the relationships and methods developed here can be applied to any basin where beach ridge deposits are present. Ultimately, we hope to demonstrate that the morphology of beach ridges can be used as a tool to better refine the lake-level histories in ancient lake basins and the paleoclimatic conditions that created and desiccated the lakes.

Pluvial Lakes and their Associated Landforms

In the Great Basin of the Western United States the extensional tectonic regime has created many internally drained basins that at times, contained large pluvial lakes (Mifflin and

Wheat, 1979) (Figure 1). Pluvial lake basins are commonly surrounded by steep range fronts covered by alluvial fan deposits shed from the uplifted mountain ranges, which provide abundant coarse clastic sediment that can be re-worked by wave energy (Adams and Wesnousky, 1998;

Reheis et al., 2014) (Figure 2.). The sedimentary deposits formed along the margins of pluvial lakes are typically composed of gravel-cobble and occasionally boulder size clasts (Adams, 2003,

2004; Reheis et al., 2014). The coarse nature of these deposits indicates that much of the sediment that composes these features is locally derived and formed purely by locally generated, wave- related processes (Adams 2003; Reheis et al., 2014; Wang et al., 2018). There are a variety of shoreline morphologies that can form along the margins of a pluvial lake (Figure 3.). The conditions that dictate the style and type of shoreline that forms are local slope, shoreline orientation, sediment supply, particle-size distribution, accommodation space, wind strength, fetch length, water depth, and the length of time that lake level remains stable at a particular elevation (Forbes and Syvitski, 1994; Adams and Wesnousky, 1998; Adams, 2003, 2004; Reheis et al., 2014).

Figure 1: Map showing late Pleistocene extent of pluvial lakes in the Great Basin (blue polygons) and the extent of their drainage basins (white). Numbers refer to general locations observed in this study, 1= Lake Tahoe, 2= Walker Lake, 3= Winnemucca Dry Lake.

Figure 2: LiDAR image of Wassuk Rangefront at Walker Lake, NV. Note steep alluviated rangefront and wave modified slope. Series of NE-SW striking lineaments represent historical beach ridge complexes of Walker Lake.

Lacustrine beach features can be broadly categorized as either erosional or constructional

landforms. Common erosional landforms include wave-formed terraces and beach cliffs incised

into bedrock or alluvium (Figure 3). Erosional features typically form on slopes > 6° (Adams and

Wesnousky, 1998) and are defined here as landforms that are created by the removal of sediment

or bedrock by wave processes. For the purposes of this study we have omitted analysis of erosional

shoreline features due to the possible error and large assumptions associated with assuming the

antecedent slope morphology prior to wave modification.

Constructional beach features are defined herein as landforms created by the accumulation of sediment through wave processes. Common constructional features observed along lacustrine beaches includes spits, looped barriers, beach ridges, progradational beach ridge complexes, and pocket barriers (Figure 3). These features often display a convex-up cross- sectional shape whose upper limits are thought to represent the maximum height of wave swash

(Taylor and Stone, 1996; Adams and Wesnousky, 1998; Adams, 2003; Tamura et al., 2018;

Wang et al., 2018). Constructional features commonly form on relatively low angle slopes < 4°

with abundant sediment supply (Adams and Wesnousky, 1998; Reheis et al., 2014). This study is

focused on the processes that create constructional beach ridges and how the volume of material

accreted relates to the length of time over which they form. Constructional beach ridges are an

excellent target for lake-level analysis due to their abundance, distinct geomorphic characteristics,

and high preservation potential.

Figure 3: Typical shoreline features found around margins of pluvial lake basins. From Reheis et al. (2014).

Previous Work

Abundant research has been conducted regarding the formative processes behind beach

ridge genesis on marine coasts, beginning with Redman (1852) who first introduced the link

between storm waves and beach-ridge development. Subsequent studies (e.g., Gilbert, 1885;

Davies, 1958; Tamura et al., 2018) have emphasized the importance of fair-weather wave swash and non-storm swell in beach-ridge genesis. Considerable debate still exists regarding the modes of sandy beach-ridge construction in marine settings (Taylor and Stone, 1996), however, in pluvial lake basins the relatively low-energy wave regime and coarse nature of the beach deposits indicates that significant geomorphic work must only take place during periods of sustained strong winds that are typically associated with storm events.

Winds blowing across water create waves, the size and period of which depends upon the

velocity, duration of the wind, and the distance of open water (fetch) over which the wind is

blowing (Komar, 1998; Adams, 2003, 2004). Wave heights on lakes are restricted primarily by

relatively small fetch distances and wind durations, and occasionally by the depth of water (CEM,

2006). Given the relatively steep nature of the foreshore, wave heights ~1.5 m or less, and short

surf zones in the basins studied, the depth of water is not considered a limiting factor in wave

growth (Komar, 1998; Adams, 2003, 2004).

Wave ﬁelds on lakes are characterized by locally-generated waves and are therefore much simpler to model than ocean waves where long-period swell must also be considered

(CEM, 2006). Waves reaching the shore of a lake obey predictable and well-established

relationships between wave height, wavelength, and period (Komar, 1998). A relatively simple

method for hindcasting waves is presented in the U.S. Army Corps of Engineers Coastal

Engineering Manual (CEM, 2006). These methods assume that the water body is relatively

simple in shape and wave conditions are either fetch limited or duration limited (CEM, 2006). In fetch-limited conditions, wave height is limited by the available fetch, whereas in duration- limited cases, wave height is limited by the length of time that the winds have blown (CEM,

2006). In pluvial lakes of the Great Basin where fetch distances are typically < 50 km the largest

waves are fetch-limited, with the largest wind events usually achieving fully developed wave

conditions in ~2-5 hours (CEM, 2006). The wave hindcasting techniques we employ in this study allow us to model wave-related shoreline processes based on local wind and beach conditions for the basins of interest. The exact wave hindcasting techniques and models used are further explained in the methods section.

A deep-water wave transforms to a breaking wave through a series of shoaling processes as the waves enter shallower water. During this time, the wave velocity and length progressively decrease as the height increases (Komar, 1998; CEM, 2006). Ultimately, when shoaling waves

enter water that is approximately as deep as they are high they become unstable and break

(Komar, 1998). The shoaling transformation of waves is not as apparent in fetch-limited

lacustrine bodies since locally generated waves with relatively short periods are initially steep in

deep water (Komar, 1998). The wave heights modeled in this study are typically ~1 m in height

and break in depths of 1 meter or less. Due to the narrow surf zones in lake basins, shoaling

processes such as wave refraction are often limited (Adams, 2003, 2004).

When waves break on a beach they produce a swash of landward-moving water whose maximum vertical displacement is related to deep-water wave height, wave period, and local

beach slope (Lorang et al., 1993; Komar, 1998; Kirk et al., 2000; CEM, 2006). This process,

known as wave runup is integral in the formation of beach ridge features and other shoreline

morphodynamic processes. Previous studies (Kirk et al., 2000; Tamura et al., 2018; Wang et al.,

2018) have suggested the height of an individual beach ridge represents the upper limit of swash

height.

When waves break on a beach a temporary mean rise in water level occurs, the

magnitude of which is known as wave setup (Komar, 1998). The magnitude of wave setup is a

function of deep-water wave height, period, and local slope (Komar, 1998; CEM, 2006).

Those studying lake bodies (e.g., Lorang et al., 1993; Komar, 1998; Wang et al., 2018)

have long recognized elevated lake levels at the downwind shores during wind events. This effect

is known as wind setup or seiche and can be approximated as a function of wind speed, fetch

length, average water depth, and the angle of wind to the shore-normal orientation (Wang et al.,

2018). Combined, wave runup, setup, and wind seiche control the magnitude of the super

elevation of lake levels during large wind and wave events, and ultimately the heights of

individual beach ridges. Careful consideration was taken to incorporate these processes into our

beach ridge genesis model.

Previous studies have incorporated wind records, wave hindcasting, wave-energy

dispersion modeling, and wave-runup calculations to help mitigate shoreline erosion and

shoreline retreat issues along lakes. Lorang et al. (1993) used local wind measurements and wave hindcasting techniques to estimate annual wave energy delivered to the shoreline on Flathead

Lake MT, USA. This study concluded excessive erosion and shoreline retreat were caused by

elevated lake levels coinciding with high wind events during winter months. Kirk et al. (2000)

used wave-hindcasting techniques to evaluate total water levels achieved by runup on Lake

Hawea, New Zealand during significant storm events in the 1990’s. The models of wave runup

were then compared to geomorphic features around the lake to determine the erosional and

depositional impacts of these storm events. Both Lorang et al. (1993) and Kirk et al. (2000) used

an integrated approach of observed wind, wave modeling, and geomorphic mapping to assess the

impacts of waves on shoreline processes in fetch-limited lacustrine bodies. However, the focus of

these studies was oriented towards change assessment analysis regarding lake-level regulation in anthropogenically controlled systems.

Researchers have long recognized the utility of analyzing preserved beach sediments to infer past wave, paleowind, and paleoshoreline conditions in lacustrine basins (Russell, 1885;

Orme and Orme, 1991; Adams, 2003, 2004; Wang et al., 2018). Using well-established sediment transport formulae and wave hindcasting techniques Adams (2003, 2004) related particle sizes of gravel beach ridges in the Lahontan basin, NV, USA to infer paleowind strength during the late

Pleistocene. Wang et al. (2018) built upon the methods established by Adams (2003, 2004) to relate beach ridge height to wind conditions in Qinghai Lake in northwestern China, and applied these methods to estimate paleowind conditions in an Eocene lake that formed in the Dongying

Depression of eastern China. While the correlation between beach ridge characteristics such as ridge height and grain size to paleowinds has been well established, a volumetric analysis of these features has yet to be performed. Given the progradational nature of these features (e.g., Tamura

et al., 2018) it is likely that beach ridge growth is a time-dependent process, therefore the volume of an individual ridge can be directly related to the duration of lake-level stability at that associated elevation.

Regional Settings and Hydrology

Walker Lake

Walker Lake lies within a north-south trending topographically closed basin in central western Nevada flanked by the predominately granitic Wassuk Range to the west and the Gillis

Range to the east which is composed of mostly Tertiary volcanic rocks (Stewart, 1978). The structural fabric and large-scale topography of the Walker Lake basin is formed by a combination of active regional extension and right-lateral strike-slip faulting within the Walker Lane, a broad zone of mostly north- northwest trending active extensional and right-lateral strike-slip faults that sits between the Sierra Nevada Mountains and the Basin and Range Province (Stewart, 1988;

Wesnousky, 2005).

In the Late Pleistocene Walker Lake formed the southernmost arm of Lake Lahontan

(Figure 1). It is estimated that Lake Lahontan reached its maximum highstand elevation of ~1330 m ~15,550 cal yr. BP (Adams and Wesnousky, 1998). The post-late Pleistocene history of Walker

Lake is complex, with several large ~ 50 m fluctuations in lake level having occurred since the late Holocene (Benson, 1991; Adams, 2007; Adams and Rhodes, 2019b). These abrupt changes in lake-level have been attributed to climatic fluctuations as well as the occasional diversion of the Walker River into the Carson Sink (Benson and Thompson, 1987; Adams, 2007; Adams and

Rhodes, 2019b). Geomorphic mapping by Adams and Rhodes (2019b) concluded that Walker

Lake reached a maximum elevation of ~1262 m around 3500 cal yr BP.

Figure 4: Map showing the Walker Lake drainage basin boundary (purple line), principle tributaries; the east fork, west fork, and mainstem Walker River, and 1252 m historical highstand of Walker Lake (light blue outline). Green star shows location of historical beach deposits analyzed in this study.

The present-day climatic regime of the Walker Lake basin can be characterized as

Mediterranean, receiving most of its annual precipitation during the winter months as snow in the

Sierra Nevada and other ranges, which melts in the spring causing elevated lake levels to occur

from April-June (Adams, 2007; Lopes and Smith, 2007). The Walker River drains ~10,500 km2 with the West and East forks being the major tributaries that join at the south end of Mason

Valley before flowing into Walker Lake (Adams, 2007; Lopes and Smith, 2007) (Figure 4). A strong rain-shadow effect exists between the western edge of the drainage basin and Walker

Lake, with the highest parts of the watershed along the Sierra Nevada crest receiving 100-170 cm/yr of mean annual precipitation and Walker Lake only receiving ~12 to 20 cm/yr. (Daly et al.,

2008).

The only outflow from Walker Lake occurs as evaporation from the lake surface (Lopes and

Smith, 2007). Allander et al. (2009) determined that the annual lake evaporation rate at Walker

Lake was ~152 cm/yr. Walker Lake reached its historic highstand of 1252 m elevation in 1868

(Adams, 2007). Since the late 19th century, upstream anthropogenic water diversions as well as

regional droughts have caused a dramatic decrease in lake levels (Adams, 2007; Lopes and Smith

2007). Since it’s regression from the 1252 m historic highstand a suite of shoreline features were

formed and subsequently abandoned as lake level continued to drop to its current elevation of

~1194 m at the start of the 2019 water year (Figures 2 and 4). The historical lake-level record in

Walker Lake is relatively robust with sporadic lake-level documentation beginning in 1868

(Russell, 1885) and becoming monthly in 1928 (Figure 5). This lake-level record coupled with

beach deposits that formed during the historical regression allow us to quantify the relationship

between lake-level residency time and beach deposit volume in the Walker Lake basin. For this study, we focus on a suite of well-developed beach ridges at the NW corner of Walker Lake between 1262-1202 m elevation (Figures 2 and 4).

Figure 5: Lake surface elevation of Walker Lake from 1868-2019. Data from Walker Lake USGS gauge near Hawthorne, NV.

Winnemucca-Dry Lake

Winnemucca Dry Lake (WDL) is located in a ~40 km long and narrow (~8 km wide) N-S trending basin located between the Nightingale Mountains to the east and the Lake Range to the west (Figure 6). The Lake Range is composed of primarily Tertiary volcanic rocks (Stewart,

1978) and separates WDL from the Pyramid Lake basin to the west. The Nightingale Range, east of WDL, is composed of Mesozoic metasedimentary rocks and granitics, overlain by Tertiary volcanic rocks (Stewart, 1978). The WDL basin is located near the boundary between the

primarily right-oblique Walker Lane belt to the west (Stewart, 1988; Wesnousky, 2005) and the

Basin and Range Province to the east, which is characterized more by east-west directed

extension (Stewart, 1988).

A low sill at 1178 m, known as Mud Lake Slough (MLS) (Figure 6), connects WDL to the Pyramid Lake basin. At lake levels >1178 m Pyramid Lake and WDL become integrated into

the same lake system. Historically, the primary source of inflow to WDL is the Truckee River via

MLS. The Truckee River, which once fed both Pyramid and Winnemucca lakes (Russell, 1885),

drains ~7050 km2 and includes Lake Tahoe (Figure 6). Most of the flow in the Truckee River is

derived from the highest parts of the basin in the Sierra Nevada mountains where mean annual

precipitation ranges from about 150 to 170 cm/yr. (Adams and Rhodes, 2019a). Precipitation in

the Truckee River basin falls mostly as snow in the winter months that subsequently melts in the

spring. Precipitation at WDL (~1152 m) ranges from about 16 to 20 cm/yr. (Adams and Rhodes,

2019a). Mean annual lake evaporation at Pyramid Lake is reported to average about 125-135

cm/yr. (Houghton et al., 1975; Milne, 1987).

Currently there are no sources of perennial inflow into the WDL basin and no standing body of water exists there, however, in the early historical period a relatively large lake was supported in the basin (Russell, 1885; Hardman and Venstrom, 1941; Harding, 1965; Adams and

Rhodes, 2019a). The recorded lake-level curve for the WDL basin consists of sporadic points separated by several years to decades (Adams and Rhodes, 2019a) (Figure 7). WDL was dry in the 1840s but rose to its historical highstand of about 1175 m in 1882 (Russell, 1885, Adams and

Rhodes, 2019a). The historical highstand of 1175 m was reached again in 1890 after a succession of several wet years (Hardman and Venstrom, 1941; Adams and Rhodes, 2019a). The operation of Derby Dam began in 1906, diverting water from the Truckee River into the adjacent Carson

River drainage basin (Horton, 1997; Adams and Rhodes, 2019a). Water diversion from the

Truckee River caused the level of Pyramid Lake to drop below 1178 m, abandoning MLS, and cutting off the main water supply for WDL (Hardman and Venstrom, 1941; Harding, 1965;

Adams, 2012). Subsequently, it is likely lake levels in the WDL basin dropped at the rate of evaporation of ~125-135 cm/yr. (Houghton et al, 1975; Milne, 1987, Adams and Rhodes, 2019a) until WDL completely desiccated by the early 1940’s (Figure 7). For this study, we focus on a

suite of well-developed beach ridges along the southwestern shore of the former WDL between the elevations of 1175 m and 1160 m (Figure 8).

Figure 6: Map of Truckee River Drainage basin (pink) showing principal lake bodies LT= Lake Tahoe, PYR= Pyramid lake, WDL= Winnemucca-Dry Lake. Truckee River and major tributaries shown in light blue with location of former inflow to WDL, Mud Lake Slough (MLS) (dark blue). WDL lake-surface elevation depicted at 1177 m. Green stars represent beach ridge locations analyzed in this study.

Figure 7: Historical lake-level fluctuations of Pyramid and Winnemucca-Dry Lakes from Adams and Rhodes (2019a).

Figure 8: Young shorelines on the SW side of the WDL basin with the 1175 m historical highstand WDL basin (green). Aerial photo displays sequence of beach ridges below (right) of 1175 m highstand analyzed in this study. These shorelines formed after the sole source of inflow was cut off from the WDL basin in the early 20th century.

Lake Tahoe

Lake Tahoe is a subalpine lake situated at ~1898 m elevation along the California and

Nevada border (Figure 9). Lake Tahoe lies within a fault-bounded graben separating the

easternmost Sierra Nevada from the Carson Range, the westernmost range of the Basin and

Range physiographic province (Stewart, 1988). The Tahoe graben is bounded by active faults on

both the east and west sides of the basin (Kent et al., 2005). Most of the exposed bedrock in the

basin consists of Cretaceous granitic rocks, however, parts of the basin contain occasional

Jurassic metamorphic rocks, Tertiary and Pleistocene volcanic rocks, and a cover of Quaternary

sediments (Saucedo, 2005). The Quaternary history of Lake Tahoe is characterized by three major glacial episodes that include—from oldest to youngest—the Donner Lake, Tahoe, and

Tioga glaciations (Gillespie and Clark, 2011). Numerous glacial deposits around the SW shoreline of Lake Tahoe near Emerald bay have been determined to be associated with the MIS 2

Tioga glaciation (~18 ka) and the MIS 6 Tahoe glaciation (~144 ka) (Rood et al., 2011) Evidence from discrete shoreline deposits at and above 1899 m indicate that lake level, at times, has been much higher than the current natural sill elevation of 1896.88 m. (Adams and Minor, 2002).

Periodic ice dams during the Tahoe and possibly Tioga glaciations downstream from the lake outlet have been proposed to be the cause of these higher lake levels (Birkeland, 1968).

Submerged trees along the southern shore of Tahoe (of middle Holocene age) indicate lake level may have fallen below the natural rim for an extended period during that time (Lindstrom, 1990;

Adams and Minor, 2002). Beach barriers developed near Baldwin Beach and South Lake Tahoe at elevations of 1901-1903 m (Figures 10 and 11) were likely formed in the early 20th century

when lake levels have exceeded 1899 m.

The climate of Lake Tahoe is strongly influenced by the surrounding topography and its

orographic effects on incoming Pacific storms. Elevations range from about 1898.65 m

(maximum lake level) to more than 2750 m along both the Sierra crest to the west of the basin

and the Carson Range bounding the east side of the lake. A strong climatic gradient exists across

the basin where average annual precipitation ranges up to 125 cm on the western side of the basin

but only about 60 cm of precipitation falls along the east shore of the lake (Daly et al. 2008).

Most of the annual precipitation is stored as snowpack during the winter months and is released

during spring snowmelt (Adams and Minor, 2002).

Beginning in 1935 a legally mandated maximum lake elevation of 1898.65 m was set for

Lake Tahoe (Horton, 1997). However, in the early historical period lake levels in Lake Tahoe commonly exceeded 1898.6 m and even reached as high as 1899.29 m in 1907 (Fig. 11). For this study, we focus on two large beach ridge complexes located at Baldwin Beach and Sand Harbor on the southern and northeastern shores of Lake Tahoe, respectively. These prominent features document the historical highstand by the presence of beach ridges with crestal elevations of

~1899-1900 m.

Figure 9: Map of Lake Tahoe drainage basin (red) with locations analyzed in this study (green stars), and sill location.

Figure 10: Satellite and hillshade imagery of Baldwin Beach located on the southern Shore of Lake Tahoe. Historical beach deposits located from 1898.5-1899 m elevation. 1899.6 m barrier displays abundant vegetation indicating greater relative age, likely formed in the early historical period.

Figure 11: Lake Surface elevation of Lake Tahoe 1900- 2019. Data from USGS Lake Tahoe gauge near Tahoe City, Ca.

Methods

Wind Regime Characterization and Analysis

To characterize the local wind regime for the basins studied, long-term hourly wind

records obtained from the Western Regional Climate Center’s Remote Automated Weather

Stations (RAWS) in central western Nevada were used. No proximal long-term wind data are available at Walker Lake, or at Winnemucca Lake. Therefore, a 30-year record of hourly wind measurements taken at the Dead Camel Mountains, Nevada RAWS weather station, ~ 50 km away from each basin (Figure 12), was used as a proxy to represent wind conditions in those basins. While twelve years of wind data is available at Lake Tahoe, existing sites show poor correlation of windspeeds across the lake and failed to report significant wind events reported at

less sheltered wind gauges in the basin during the 2018-2019 winter. This indicates topographic or vegetative wind blocking may exist at the Lake Tahoe weather stations with long term wind data that make them poor indicators of wind conditions on the open lake. Due to these reasons wind data collected at the Dead Camel site was also used to model wave conditions in the Lake

Tahoe Basin. Although using distant wind data to characterize the wind regime in each basin is not ideal, the Dead Camel site provides the only unobstructed long-term wind data for the region.

The longevity of the wind record allows for a better statistical characterization of wind patterns and conditions that lead to wave formation, which is essential for estimating durations of lake- level residency times prior to the instrumental record. While we acknowledge wind conditions in each basin likely varies from the measurements taken at the Dead Camel Mountains RAWS site, care has been taken to choose a site that more or less reflects the climatic and wind conditions for each basin studied as well as the wind conditions on an open lake in the region.

Figure 12: Map showing location of Dead Camel Mountain weather station (red triangle) as well as study locations (green stars).

Typical daily wind speeds and therefore wave conditions do not provide sufficient energy

to transport the coarse clastic material found on many beaches along the margins of pluvial lakes

in the Great Basin (Adams, 2003, 2004). It is typically during the passage of meteorological

depressions when winds become strong enough to create waves that perform significant geomorphic work. These events termed significant wind events after Adams (2003, 2004) require high wind speeds that are sustained for multiple hours for maximum wave growth. To determine the criteria for a significant wind event we used the observed historical beach ridge heights at the locations studied. This value is assumed to represent the average maximum height of wave swash

(sum of wave runup, wave setup, and seiche) during storm events. Using commonly observed beach ridge heights of ~0.5 m as a proxy, wave hindcasting models were used to calculate necessary swash elevations and therefore sustained minimum wind speed, and duration of time to achieve these conditions. Using this method, the minimum wind conditions determined to produce waves of significant geomorphic work have been defined as a windspeed of 10 m/s for three hours or more, similar to the criteria used by Adams (2003, 2004).

After defining the criteria of a significant wind event the wind record from the Dead

Camel RAWS station was analyzed using a python script to determine the frequency of their occurrence on an annual basis. Using the 30-year wind data, the average frequency of significant wind events per year was determined. To calculate annual wave heights and an annual wave energy regime, three wind years were selected based on the number of significant wind events per year. Using the wind data from the minimum, average, and maximum wind years (based on number of significant wind events) the wave regime and wave energy delivered to each shoreline was calculated.

For a given significant wind event, wind direction must also be considered, as large waves only form in the downwind direction. To apply distal wind data to the beaches studied, all wind events that were +/- 45° from the shore normal direction at the beach of interest were

considered. This approach is a deviation from the +/- 27° that is recommended by CERC (1984), however a strong north-south topographic channeling exists in the basins studied, meaning winds are typically channeled north to south, or south to north as they enter from oblique angles (Kirk et al., 2000).

Wave Hindcasting

The following workflow was developed based on wave hindcasting techniques set forth by the U.S. Army Corps of Engineers in the Coastal Engineering Manual (CEM, 2006). These methods were employed in the analysis of waves on for Walker Lake, Winnemucca Lake, and

Lake Tahoe.

CEM (2006) states “Empirical wave prediction requires the basic assumption that universal laws will govern the relationships between dimensionless wave parameters.” A fundamental application of this is the fetch-growth law. Fetch length is the open water distance over which wind can blow for a given water body and is fundamental to controlling the maximum size of waves that can be produced (Komar, 1998). The fetch-growth law states that a constant wind speed and direction over a fixed fetch distance will cause wave heights to reach a stationary fetch-limited state of development (CEM, 2006). For the beaches analyzed in this study, the straight-line maximum fetch distance was measured within 27o of the shore normal orientation in

ArcGIS 10.6.1 following the guidelines of CERC (1984) and CEM (2006). The duration of time at which fetch-limited conditions are achieved ( , ) can be approximated by

𝑥𝑥 𝑢𝑢 𝑡𝑡 . = 77.23 (Equation 1) , . 0 67. (CEM, 2006) 𝑥𝑥 0 34 0 33 𝑡𝑡𝑥𝑥 𝑢𝑢 𝑢𝑢10 𝑔𝑔 Where x = maximum wind fetch in meters, g = acceleration due to gravity, and U10 = windspeed in meters/second taken at 10 m height. Results have the units of seconds. If the wind duration is equal to or longer than this then a fetch-limited situation exists, and wave growth is limited. For

the given observed wind speeds, fetch-limited conditions were calculated and used as a proxy to

define significant wind events.

Wave height and period is a function of wind speed. Wave hindcasting models require

wind speed to be measured and converted to a resultant speed that is 10 m off the ground or water

surface (U10) (CEM, 2006). The RAWS weather station used in this study observes wind speed at

6.1 m above the ground surface. This requires the observed wind speed to be converted to the

resultant windspeed at 10 m off the ground, which can be estimated from

(Equation 2) = 1/7 (CEM, 2006) 10 𝑈𝑈10 𝑈𝑈𝑧𝑧 � 𝑧𝑧 � Where Uz is the observed windspeed and z equals the height of wind measurement. This equation assumes near neutral air-sea temperature differences, which is recommended when exact conditions are unknown (Lorang et al., 1993; CEM, 2006). The U10 conversion is also codified in the U.S. Army Corps of Engineers A.C.E.S software for wave prediction, which was utilized in this study.

The first step in wave hindcasting requires observed windspeed ( ) to be converted to a

10 coefficient of drag ( ) by the following equation 𝑈𝑈

𝐶𝐶𝐷𝐷 (Equation 3) = 0.001 (1.1 + 0.035 ) (CEM, 2006)

𝐶𝐶𝐷𝐷 𝑈𝑈10 Coefficient of drag ( ) is then used to estimate the frictional velocity ( ) for the given windspeed by 𝐶𝐶𝐷𝐷 𝑈𝑈∗

(Equation 4) = (CEM, 2006)

𝑈𝑈∗ �𝐶𝐶𝐷𝐷𝑈𝑈10 Significant wave height ( ) is a statistical analysis of wave heights defined as the

average of the highest 1/3 of waves𝐻𝐻𝑠𝑠 over a stated time interval (Komar, 1998). The use of

significant wave parameters is based on the impression that larger waves are more significant in

wave-related processes such as shoreline morphodynamics (Komar, 1998). It has been shown that

roughly corresponds to a visual estimate of wave heights (Komar, 1998). For this study, we

use𝐻𝐻𝑠𝑠 a series of equations from CEM (2006) to estimate the deep-water significant wave height

( ) as a function of windspeed and fetch length. Estimation of began with the calculation of

∞ ∞ nondimensiona𝐻𝐻 l fetch ( ) by the following equation: 𝐻𝐻

𝑥𝑥�

(Equation 5) = (CEM, 2006) 𝑔𝑔𝑔𝑔 2 𝑥𝑥� 𝑈𝑈∗

Where g is acceleration due to gravity (9.81 m/s2), x = maximum straight-line fetch distance, and

= the friction velocity for a given windspeed (equation 4).

𝑈𝑈∗ Using the non-dimensional fetch length , the nondimensional deep-water wave height can be calculated by 𝑥𝑥�

(Equation 6) = (CEM, 2006) 𝑚𝑚1 �∞ 1 Where is a dimensionless coefficient𝐻𝐻 𝜆𝜆equal𝑥𝑥� to 0.0413 and is a dimensionless exponent

1 1 equal to𝜆𝜆 0.5. Final deep-water wave height ( ) is then calculated𝑚𝑚 by the following equation:

𝐻𝐻∞ (Equation 7) = (CEM, 2006) * 2 𝑈𝑈∗ 𝐻𝐻∞ 𝐻𝐻�∞ 𝑔𝑔 Wave period is the amount of time in which each successive wave crest passes a fixed point and is an important factor in total wave energy delivered to a given shoreline (Komar, 1998). Peak wave period for a given wind event was approximated by first estimating the nondimensional spectral peak period ( ) by the following equation:

𝑇𝑇�𝑝𝑝 (Equation 8) = (CEM, 2006) 𝑚𝑚2 𝑇𝑇�𝑝𝑝 𝜆𝜆2𝑥𝑥� Where is a dimensionless coefficient equal to 0.751 and is a dimensionless exponent equal

2 2 to 1/3. Ulti𝜆𝜆 mately, the peak spectral period ( ) is estimated𝑚𝑚 by the following equation:

𝑇𝑇𝑝𝑝

(Equation 9) = (CEM, 2006) 𝑈𝑈∗ 𝑇𝑇𝑝𝑝 𝑇𝑇�𝑝𝑝 𝑔𝑔 Using fetch measurements for each beach in the study and the series of equations summarized

above we are able to use observed windspeed to estimate deep-water wave heights and wave

period for recorded significant wind events.

Wave Runup, Wave Setup, and Wind Setup Modeling

Wave swash along an active shoreline is a fundamental process that leads to shoreline

deposition and beach ridge formation (Tamura et al., 2018; Wang et al., 2018). The uppermost

limit of beach ridge height represents the maximum height of wave runup, which is also affected

by wave setup, and wind setup (Komar, 1998; Kirk et al., 2000; Tamura et al., 2018; Wang et al.,

2018).

A variety of equations exist to estimate various statistical parameters for wave runup,

however because we are concerned with the maximum elevation of wave swash we estimate the

vertical height of swash that is exceeded by 2% of the runup heights, and the maximum wave

runup height. These parameters can be related to deep-water significant wave height by first

determining the deep-water wavelength ( )

𝐿𝐿𝑜𝑜 (Equation 10) = /(2 ) (CEM, 2006) 2 𝐿𝐿𝑜𝑜 𝑔𝑔 𝑇𝑇 𝜋𝜋 Where T is the wave period in seconds.

Deep water wavelength ( ) and significant wave height ( ) can then be used to

𝑜𝑜 ∞ estimate the surf similarity parameter𝐿𝐿 (ξo) (CEM, 2006), which estimates𝐻𝐻 the breaker wave type as

a function of local slope ( ) and wave conditions.

𝛽𝛽 (Equation 11) = ( / ) / (CEM, 2006) −1 2 ξ𝑜𝑜 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝐻𝐻∞ 𝐿𝐿𝑜𝑜 Maximum runup ( ) can then be calculated by the following equation:

𝑅𝑅𝑚𝑚𝑚𝑚𝑚𝑚 (Equation 12) = 2.32 . (CEM, 2006) 0 70 𝑅𝑅𝑚𝑚𝑚𝑚𝑚𝑚 𝐻𝐻∞ξ𝑜𝑜

Runup exceeded by 2% of the runup crests can be estimated by

(Equation 13) = 1.86 . (CEM, 2006) 0 71 𝑅𝑅2% 𝐻𝐻∞ξ𝑜𝑜 Wave setup is the super elevation of mean water level caused by wave action. To estimate wave setup, incipient breaker height and depth must first be determined. Initially, breaker height index

( b), must be approximated as a function of deep-water wavelength ( ) and deep-water

𝑜𝑜 significant𝛺𝛺 wave height ( ) (Komar and Gaughan, 1976). 𝐿𝐿

∞ 𝐻𝐻 / (Equation 14) ( b) = 0.56( ) (CEM, 2006) 𝐻𝐻∞ −1 5 𝛺𝛺 𝐿𝐿𝑜𝑜 The height at incipient breaking ( ) can be determined from the deep-water wave height by

𝐻𝐻𝑏𝑏 (Equation 15) = b (CEM, 2006)

𝐻𝐻𝑏𝑏 𝛺𝛺 𝐻𝐻∞ The breaker index ( ) is used to describe nondimensional breaker height and is given by the

equation 𝛾𝛾𝑏𝑏

= (Equation 16) (CEM, 2006) 𝐻𝐻𝑏𝑏 2 𝛾𝛾𝑏𝑏 𝑏𝑏 − 𝑎𝑎 𝑔𝑔 𝑇𝑇 The parameters a and b are empirically determined functions of beach slope (CEM, 2006) given

(Equation 17) = 43.8(1 ) (CEM, 2006) −19𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑎𝑎 − 𝑒𝑒 and

. = (Equation 18) ( . ) (CEM, 2006) 1 56 −19 5 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑏𝑏 1+𝑒𝑒 Incipient breaker depth ( ) is determined by

𝑑𝑑𝑏𝑏 (Equation 19) = / (CEM, 2006)

𝑑𝑑𝑏𝑏 𝐻𝐻𝑏𝑏 𝛾𝛾𝑏𝑏

The first step in determining wave setup requires the calculation of wave set down ( ) or the maximum lowering of the water level that occurs near the breaking point (CEM, 2006�𝜂𝜂��𝑏𝑏�).� This is given by

(Equation 20) = 1/16 (CEM, 2006) 2 �𝜂𝜂�𝑏𝑏� − 𝛾𝛾𝑏𝑏 𝑑𝑑𝑏𝑏 Using the steps illustrated above in equations 14-20, the input parameters to solve for wave set up

at the still-water shoreline ( ) have been determined (CEM, 2006). can be determined by

𝜂𝜂�𝑠𝑠 𝜂𝜂�𝑠𝑠 (Equation 21) = + (CEM, 2006) 8 𝑠𝑠 𝑏𝑏 8 𝑏𝑏 𝜂𝜂� �𝜂𝜂�� 1+ 2� ℎ 3𝛾𝛾𝑏𝑏 Wave Refraction

Given the relatively straight coastline, orthogonal wave approach, and parallel

bathymetric contours at the beaches studied in Walker Lake and Lake Tahoe, wave refraction is

likely not a large contributing factor in shoaling processes. For these lakes, can be used to

determine wave runup and setup (CEM, 2006). In cases where wave refraction𝐻𝐻∞ occurs (e.g. WDL

basin) the results from equation 22 (Hi) are substituted for when performing wave runup and

setup modeling. 𝐻𝐻∞

The shorelines studied at Winnemucca Dry Lake are located on the southwestern margin

of the former lake and generally strike ~320-330 degrees (Figures 6 and 8). Given the topography of the basin and the observed wind directions, it is highly unlikely significant wind events occur in the shore normal direction. Rather, wind and therefore waves are likely to propagate along the long axis of the basin (~0o or ~180o). As waves enter shallow water the wave crests tend to become more parallel with the bathymetric contours of the shoreline (Figure 13) (Komar, 1998), a process that is known as wave refraction. On relatively straight coasts with parallel bathymetric contours wave refraction will cause the wave rays to spread out (Figure 13), resulting in reduced wave heights (Komar, 1998). To account for wave refraction at WDL, a shoaling coefficient ( )

𝐾𝐾𝑠𝑠

and a refraction coefficient ( )were estimated. and are estimated as a function of wave height, wavelength, wave group𝐾𝐾𝑟𝑟 velocity, and local𝐾𝐾𝑠𝑠 slope𝐾𝐾.𝑟𝑟 This assumes a relatively straight

shoreline with evenly spaced depth contours that are parallel to the shoreline. The calculation of

the refraction and shoaling coefficients is codified within the A.C.E.S. software utilized in this

study. After ( ) and ( ) were estimated the refracted wave height Hi is given by

𝐾𝐾𝑠𝑠 𝐾𝐾𝑟𝑟 (Equation 22) = (CEM, 2006)

𝐻𝐻𝑖𝑖 𝐻𝐻∞𝐾𝐾𝑠𝑠𝐾𝐾𝑟𝑟

Figue 13: Schematic illustration from Komar (1998) showing the process of wave refraction as deep-water waves approach the shoreline. During this process the wave crest at point B in deeper water moves faster than the wave crest at point A causing the wave crest to swing around and become parallel to shore.

The final factor to be considered in driving the superelevation of lake levels during wind and wave events is the wind setup or seiche effect. The following equation was used to determine the total wind setup ( ):

ℎ𝑠𝑠 = (Equation 23) 2 (Wang et al., 𝐾𝐾𝑈𝑈10 𝐹𝐹 ℎ𝑠𝑠 2𝑔𝑔𝑔𝑔 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 2018)

-6 where K is the coefficient of frictional resistance equal to 3.6 x 10 (Wang et al., 2018); U10 is the

windspeed at 10 m height; F is the fetch distance in meters; d is the average water depth (m); and

is the angle (degrees) between the wind direction and the normal to the beach ridge axis (0-

90𝛽𝛽 o).

Wave Energy Modeling

As waves move across the deep-water area where they are generated they transform as they enter

the shallow water near the shoreline of interest. Several mathematical theories have been

developed to express the transformations of waves as they approach the shore and are thoroughly reviewed in Komar (1998). For the purposes of this study, we have applied the assumptions of linear Airy-wave theory, which assumes that near shore waves behave as solitary masses of water propagating shoreward at some average velocity, dictated by their height and the depth of water

(Komar, 1998; Adams, 2003, 2004). An important derivation of linear Airy-wave theory is the energy flux (P), which is the rate of energy density that is carried along by moving waves

(Lorang et al., 1993; Komar, 1998). The distribution of wave energy at various lake levels is

important in shoreline processes and is therefore an important factor to be considered for the

purposes of this study. Following the methods of Lorang et al. (1993) and Komar (1998) we used

a series of equations that estimate wave energy flux (P). Initially the wave energy density (E) of

the wave is calculated by:

(Equation 22) = 1 2 ∞ (Komar, 1998) 𝐸𝐸 8 𝜌𝜌𝜌𝜌𝐻𝐻

Where is the density of fresh water (1000 kg/m3). The flux of wave energy or wave power (P) in Joules/m𝜌𝜌 is

(Equation 23) = (Komar, 1998)

𝑃𝑃 𝐸𝐸𝐶𝐶𝑔𝑔 Where is the deep-water wave group velocity calculated with:

𝐶𝐶𝑔𝑔

(Equation 24) = 0.5 (Komar, 1998) 𝑔𝑔 𝐶𝐶𝑔𝑔 �2𝜋𝜋� ∗ 𝑇𝑇𝑝𝑝 Where is equal to peak wave period.

𝑇𝑇𝑝𝑝 Using the wind regime of the years specified, the wave energy flux (P) for each

significant wind event was calculated for the periods of interest. From this an average, above

average, and below average wave energy year was determined for each basin. This analysis

attempts to develop a simple representation of the annual distribution of wave energy for the

shorelines of interest, which has been derived from actual wind measurements in the region so

that it would reflect natural fluctuations of wind speed, direction, and wind duration. We

recognize the temporal distribution and storm intensity may vary on a year-to-year basis but our approach to defining an average, above average, and below average wave energy year allows this technique to be applied to beach ridges formed prior to the instrumental record or for beaches where appropriate wind records are not available.

Volumetric Analysis of Beach Ridges

A total of four historical beach sites were selected across the three basins based on the

presence of well-developed shorelines within the historical lake-level ranges. At the Lake Tahoe and Walker Lake sites, the locations represent the near maximum fetch distances available for wave growth in those water bodies. In the WDL basin, the site examined is located on the southwestern shore allowing us to characterize shoreline variability with respect to shoreline orientation.

Existing high-resolution LiDAR data sets were downloaded from opentopography.org and used in the volumetric analysis process at Walker Lake and Lake Tahoe. The Walker Lake and the Tahoe Basin LiDAR datasets used in this study were collected by Airborne LiDAR techniques in 2015 and 2010, respectively. Original point cloud data for the areas of interest were downloaded and then converted to a .lasd dataset in ArcCatalog. The LAS datasets were filtered

for bare earth using the “Create LAS Dataset” tool in ArcGIS 10.6.1 and converted to a Digital

Elevation Model (DEM) using the “LAS Dataset to Raster” tool. The horizontal and vertical accuracy of the DEMs is < 0.5 m which is adequate given the natural variability in the heights of shoreline features can be 1 m or greater (Adams and Wesnousky, 1998).

High-resolution topographic data was collected for the WDL site by standard Structure from Motion techniques as outlined in UNAVCO (2016). The platform used was a DJI quadcopter with a continuous shooting camera that captured 60% overlap between photos. Ten ground control points were set every ~0.5 km2 of flight area and were surveyed with a survey- grade Trimble R10 GPS unit. Photos were processed and georeferenced using Agisoft PhotoScan

Pro software to build a mesh from a high-density point cloud. The LAS data from the WDL site was converted to a DEM using the “LAS Dataset to Raster” tool in ArcGIS 10.6.1. Ultimately, the DEM created for the WDL field site has a vertical and horizontal accuracy of ~15 cm.

Beach ridge features often have a distinct slope inflection point where the accumulated

material meets the far field slope (Figure 14). This point on both the shoreward and landward

extents of the feature is referred to as the “base” of the beach ridge. To determine an individual

beach ridges’ volume, polygons were drawn in ArcGIS intersecting the shoreward and landward

base of each beach feature of interest in planview, then a high-resolution DEM was clipped to the

individual polygons using the Clip tool. Base and crestal elevation of an individual beach ridge

typically showed variability along strike of <0.2 m which was averaged along the entire length of

the feature for an individual beach ridge. Using the Surface Volume tool and the average base

elevation of the beach ridge, the volume above base elevation was calculated. The length of beach

ridge was measured along strike for each feature; the total volume was then divided by total

length to estimate the average volume per unit length of an individual beach ridge (m3/m).

As a validation for the modeled volumetric values, several field estimates of beach ridge

profiles were performed at Walker Lake and Lake Tahoe with a stadia rod and hand level as well

as a survey grade GPS. The height of field estimated cross sectional beach ridge profiles were summed over the width of the features measured and compared to volumetric values calculated in

ArcGIS. Mean variance between field-estimated values and computed values is 0.2 m2 , which is

consistent with the natural variability of shoreline features as well as the horizontal and vertical

accuracy of the DEMs used.

Figure 14: Cross sectional profile of active beach ridge (yellow) and historical highstand beach ridge (red) at Baldwin Beach, Lake Tahoe, California. Note distinct break in slope at shoreward and landward extents of beach ridges. Active beach ridge displays erosional escarpment at still water level.

Duration of Lake-Level Residency Time for Historical Beach Ridges

To determine the duration of time over which the beach ridges observed in this study formed, a relationship was developed to relate historical still water levels to the elevations of abandoned beach ridges. Previous studies (Tamura et al., 2018; Wang et al., 2018) have proposed the height of beach ridges represents the uppermost limit of wave swash (sum of wave setup, wave runup, and wind setup) for a given beach. Observations made at Sand Harbor Beach, Lake

Tahoe during a ~10 m/s wind event sustained for 7 hours in March of 2019 showed fresh pine needles and gravel to have washed to the crestal limit of the active beach ridge (Figure 15). The observations supporting previous assumptions made regarding the relationship between total swash elevation and beach ridge height allow for an important distinction to be made; the still water lake elevation does not correspond directly to the crestal height of the beach ridge. Average beach ridge height observed in the basins studied are typically > 0.5m, indicating typical wave

swash elevations achieve 0.5 m in height. These observations lead us to place the range of still water levels associated with a beach ridge between the base elevation of the beach ridge to 0.5 m below crestal height. This method of relating still water level and beach ridge height is based on observed wave events and beach ridge morphology and therefore most likely reflects the underlying processes that leads to beach ridge genesis.

Figure 15: Fresh gravel and debris deposited along the crest of the active beach ridge at Sand Harbor, Lake Tahoe during significant wind event of 10 m/s sustained for ~7 hours. Dog 42 cm height at back for scale

Results

Wind Analysis

Average monthly windspeed was determined from the 30-year wind record at the Dead

Camel Mountain site and is presented in Figure 16a. Based on wave modeling results for the

basins studied, a significant wind event was defined as a windspeed of 10 m/s that was sustained for 3 hours or more. The wind record indicates significant wind events occur on average of ~12 times per year with a standard deviation of 5 (Figure 16b). Over the 30-year record the maximum

number of significant wind events per year is 24 and minimum number of events is 5 (Figure

16b). The results of this wind analysis were used to select three individual years with the

minimum, average, and maximum number of significant wind events, which were then used to

model annual wave conditions and wave energy. The years selected for wave modeling are 2007

(5 significant wind events), 1999 (12 significant wind events), and 2017 (24 significant wind

events). Wind rose analysis for the 1999, 2007, and 2017 wind years indicate the wind direction

of significant wind events for this region is predominately S-SW and very rarely N-NW (Figure

17). The most commonly observed hourly windspeed during significant wind events is 12-13 m/s,

and highest hourly windspeed achieved was 18.33 m/s (Figure 17).

(a.) Average Monthly Windspeed Dead Camel Mountains, NV 5

Avg. Monthly Windspeed (m/s) Windspeed Monthly Avg. 0 3/31/1987 5/9/1991 6/17/1995 7/26/1999 9/3/2003 10/12/2007 11/20/2011 12/29/2015 Average Monthly Windspeed (m/s)

Figure 16: (a). Average monthly windspeed (m/s) 1987-2018 from the Dead Camel Mountain, NV. (b). Annual number of significant wind events by year (1987-2018) from the Dead Camel Mountain, NV.

(a.)

1999 Significant Wind Event Wind Rose (Average Year) N 60% 50% NW NE 40% 15-16 30% m/s 13-14 20% m/s 10% 12-13 W 0% E m/s

SW SE

(b.)

2007 Significant Wind Event Wind Rose (Minimum Year) N 80%

NW 60% NE

40% 13-14 m/s 20% 12-13 m/s W 0% E 11-12 m/s 10-11 m/s

SW SE

(c.)

2017 Significant Wind Event Wind Rose (Maximum Year) N 60% 50% NW NE 40% 18-19 m/s 30% 17-18 m/s 20% 16-17 m/s 10% 15-16 m/s W 0% E 13-14 m/s 12-13 m/s 11-12 m/s 10-11 m/s SW SE

Figure 17 a-c: Wind roses of significant wind events for 1999, 2007, and 2017 from the Dead Camel Mountains, NV. Minimum, Average, and Maximum year refers to number of significant wind events observed for 30-year record

Wave Hindcasting

Using wind data from years 1999, 2007, and 2017 and the wave hindcasting techniques outlined

in CEM (2006), deep-water wave heights were modeled in Walker Lake, Winnemucca Dry Lake, and Lake Tahoe. A maximum fetch distance of 17.9 km, 40.3 km, and 40.8 km was measured for the study areas at Walker Lake, WDL, and Lake Tahoe, respectively. Maximum deep-water wave heights modeled for the Walker Lake, WDL, and Lake Tahoe basins are 1.4 m, 1.3 m, and 2.17 m respectively (Figure 18). Modeled deep-water wave heights in the WDL basin are typically much larger than those in the Walker Lake basin due to a larger fetch length; however, a refraction coefficient was applied to the models for the WDL basin due to the area of interests’ orientation with respect to maximum fetch length (CEM, 2006).

(a.)

Modeled Wave Heights at Walker Lake and Observed Windspeed 1999

1.5 25 1.4 Wave Height (m) Windspeed (m/s) 23 1.3 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13 Wave Height (m) 0.5 (m/s) Windspeed 11 0.4 0.3 9 0.2 7 0.1 0 5 1/1/1999 5/1/1999 Date 9/1/1999 1/1/2000 (b.)

Modeled Wave Heights at Walker Lake and Observed Windspeed 2007 1.5 25 1.4 23 1.3 Wave height (m) Windspeed (m/s) 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13 Wave Height (m) 0.5 (m/s) Windspeed 11 0.4 0.3 9 0.2 7 0.1 0 5 1/1/2007 5/1/2007 9/1/2007 1/1/2008 Date

(c.)

Modeled Wave Heights at Walker Lake and Observed Windspeed 2017 1.5 25 1.4 Wave Height (m) Windspeed (m/s) 23 1.3 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13

0.5 (m/s) Windspeed 11 0.4

ModeledWave Height (m) 0.3 9 0.2 7 0.1 0 5 1/1/2017 5/1/2017 9/1/2017 1/1/2018 Date

(d.)

Modeled Wave Heights at WDL and Observed Windspeed 1999 1.5 25 1.4 Wave Height (m) Windspeed (m/s) 23 1.3 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13 0.5 Wave Height (m) 11 (m/s) Windspeed 0.4 0.3 9 0.2 7 0.1 0 5 1/1/1999 5/1/1999 9/1/1999 1/1/2000 Date

(e.)

Modeled Wave Heights at WDL and Observed Windspeed 2007 1.5 25 1.4 Wave Height (m) Windspeed (m/s) 23 1.3 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13

Wave Height (m) 0.5 11 (m/s) Windspeed 0.4 0.3 9 0.2 7 0.1 0 5 1/1/2007 5/1/2007 9/1/2007 1/1/2008 Date

(f.)

Modeled Wave Heights at WDL and Observed Windspeed 2017 1.5 25 1.4 Wave Height (m) Windspeed (m/s) 23 1.3 1.2 21 1.1 19 1 0.9 17 0.8 15 0.7 0.6 13 Wave Height (m) 0.5 (m/s) Windspeed 11 0.4 0.3 9 0.2 7 0.1 0 5 1/1/2017 5/1/2017 Date 9/1/2017 1/1/2018

(g.)

Modeled Wave Heights at Lake Tahoe and Observed Windspeed 1999 2.5 25 Wave Height (m) Windspeed (m/s) 23 2 21 19 1.5 17 15 1 13 Wave Height (m) 11 (m/s) Windspeed 0.5 9 7 0 5 1/1/1999 5/1/1999 9/1/1999 1/1/2000 Date

(h.)

Modeled Wave Heights at Lake Tahoe and Observed Windspeed 2007 2.5 25

Wave Height (m) Windspeed (m/s) 23 2 21 19 1.5 17 15 1 13 Wave Height (m) 11 (m/s) Windspeed 0.5 9 7 0 5 1/1/2007 0:00 5/1/2007 0:00 9/1/2007 0:00 1/1/2008 0:00

Date

(i.)

Modeled Wave Heights at Lake Tahoe and Observed Windspeed 2017 2.5 25

Wave Height (m) Windpeed (m/s) 23 2 21

1.5 17

1 13 Wave Height (m) 11 (m/s) Windspeed

0.5 9

0 5 1/1/2017 5/1/2017 9/1/2017 1/1/2018 Date Figure 18 a-i: Modeled Wave heights at Walker Lake (a-c), Winnemucca Dry Lake (d-f), and Lake Tahoe (g-i) for the years 1999, 2007, and 2017. Wave hindcasting was performed with observed windspeeds and durations (orange), plotted on the secondary y-axis. Note change in primary y-axis scale for Lake Tahoe (g-i).

Wave Runup

Using the modeled deep-water wave heights and periods, wave runup totals for each significant wind event were calculated for the years 1999, 2007, and 2017. The maximum modeled runups for the beaches studied at Walker Lake, WDL, and Lake Tahoe are 1.4 m, 1.5 m, and 0.94 m, respectively (Figure 19). The average wave runup for significant wind events across the three years of interest in Walker Lake, WDL, and Lake Tahoe are and 0.85 m, 0.86 m, and

0.58 m respectively. Slopes at the beaches studied range from 0.5°-3.5°. The modeled wave runups do not consider wind direction for the significant wind events observed, and therefore do not represent the actual wave runups that occurred during the time periods of interest. The results presented regarding wave runup capture the realm of possible runup heights at the beaches of

interest for the range of observed windspeeds, fetch lengths, and local slope. Wind direction is accounted for and discussed further in the following section.

(a.)

Modleled Wave Runup at Walker Lake 1999

1.8 Wave Runup (m) 1.6

1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/1999 3/12/1999 5/21/1999 7/30/1999 10/8/1999 12/17/1999 Date

(b.)

Modleled Wave Runup at Walker Lake 2007

1.8 Wave Runup (m) 1.6

1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2007 3/12/2007 5/21/2007 7/30/2007 10/8/2007 12/17/2007 Date

(c.)

Modeled Wave Runup at Walker Lake 2017 1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2017 3/12/2017 5/21/2017 7/30/2017 10/8/2017 12/17/2017 Date

(d.)

Modeled Wave Runup at WDL 1999 1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6 Total Runup height (m) height Runup Total 0.4

0.2

0 1/1/1999 3/12/1999 5/21/1999 7/30/1999 10/8/1999 12/17/1999 Date

(e.)

Modleled Wave Runup at WDL 2007

1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2007 3/22/2007 6/10/2007 8/29/2007 11/17/2007 Date

(f.)

Modeled Wave Runup WDL 2017

1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2017 3/12/2017 5/21/2017 7/30/2017 10/8/2017 12/17/2017 Date

(g.)

Modleled Wave Runup Lake Tahoe 1999

1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/1999 3/12/1999 5/21/1999 7/30/1999 10/8/1999 12/17/1999 Date

(h.)

Modleled Wave Runup at Lake Tahoe 2007 1.8

1.6 Wave Runup (m) 1.4

1.2

0.8

0.6 Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2007 3/12/2007 5/21/2007 7/30/2007 10/8/2007 12/17/2007 Date

(i.)

Modeled Wave Runup Lake Tahoe 2017 1.8

1.6

1.4 Wave Runup (m)

1.2

0.8

0.6

Total Runup Height (m) Height Runup Total 0.4

0.2

0 1/1/2017 3/12/2017 5/21/2017 7/30/2017 10/8/2017 12/17/2017 Date Figure 19: Modeled heights of wave runup at Walker Lake (a-c), Winnemucca Dry Lake (d-f), and Lake Tahoe (g-i) for the years 1999, 2007, and 2017. Runup modeling was performed with modeled deep-water wave heights from observed significant wind events and local slope measurements performed in ArcGIS 10.6.

Wave Energy Using the modeled deep-water wave heights from the significant wind events during the

1999, 2007, and 2017 wind years, the total incident wave energy was calculated for the Walker

Lake, WDL, and Lake Tahoe field sites using a series of equations outlined in Komar (1998). The wave energy estimations presented herein account for differences in wind direction and a specific shorelines orientation. It is important to note that the deep-water wave height and period was used for wave energy estimation after Lorang et al. (1993), therefore, these estimates do not account for wave energy lost during shoaling processes.

The total incident wave energy delivered to the NW Walker Lake site for the years 1999,

2007, and 2017 is estimated to be 4.8x107 J/m, 6.2x106 J/m, and 1.2x108 J/m respectively (Figure

20a-c).

Given the WDL sites’ orientation and location along the southern margin of the basin, no

significant wind events occurred within the range of acceptable wind directions as modified from

CERC (1984) that would produce significant incident wave energy for the time periods of interest. Similarly, the Baldwin Beach site at Lake Tahoe is not located in a favorable location to receive wave energy from the recorded significant wind events, and no incident wave energy from significant wind events was considered for the time periods of interest.

The total incident wave energy delivered to the Sand Harbor, Lake Tahoe site for the years 1999, 2007, and 2017 is estimated to be 9.8 x107 J/m, 4.2x107 J/m, and 2.5x108 J/m respectively (Figure 20d-f).

(a.)

Walker Lake NW Shore Wave Energy 1999

4000 1207 Wave Energy (KJ/m) Lake Level (m) 3500 1206.5 3000 1206 2500

2000 1205.5

1500 Lake Level (m)

Wave Energy (KJ/m) 1205 1000 1204.5 500

0 1204 1/1/1999 3/1/1999 5/1/1999 7/1/1999 9/1/1999 11/1/1999 Date

(b.)

Walker Lake NW Shore Wave Energy 2007

4000 1201.5 Wave Energy (KJ/m) Lake Level (m) 3500 1201

3000 1200.5

2500 1200

2000 1199.5

1500 1199 Lake Level (m) Wave Energy (KJ/m) 1000 1198.5

500 1198

0 1197.5 1/1/2007 3/1/2007 5/1/2007 7/1/2007 9/1/2007 11/1/2007 Date (c.)

Walker Lake NW shore Total Wave Energy 2017

4000 1200

1199 3500 Wave Energy (KJ/m) Lake Level (m) 1198 3000 1197 2500 1196

2000 1195

1194

1500 Lake Level (m) Wave Energy (KJ/m) 1193 1000 1192 500 1191

0 1190 1/1/2017 3/1/2017 5/1/2017 7/1/2017 9/1/2017 11/1/2017 Date

(d.)

Sand Harbor Total Wave Energy 1999 9000 Wave Energy (KJ/m) Lake Level (m) 1898.6 8000 1898.5 7000 1898.4 6000 1898.3

5000 1898.2

1898.1

4000 Lake Level (m)

Wave Energy (KJ/m) 1898 3000 1897.9 2000 1897.8

1000 1897.7 1/1/1999 3/1/1999 5/1/1999 7/1/1999 9/1/1999 11/1/1999 1/1/2000 Date

(e.) Sand Harbor Total Wave Energy 2007

9000 1898.4 Wave Energy (KJ/m) Lake Level (m) 8000 1898.2

7000 1898

6000 1897.8

5000 1897.6

4000 1897.4 Lake Level (m) Wave Energy (KJ/m)

3000 1897.2

2000 1897

1000 1896.8 1/1/2007 3/1/2007 5/1/2007 7/1/2007 9/1/2007 11/1/2007 1/1/2008 Date

(f.)

Sand Harbor Total Wave Energy 2017 9000 1899 Wave Energy (KJ/m) Lake Level (m) 8000 1898.5 7000 1898 6000

5000 1897.5

4000 Lake Level (m) 1897 Wave Energy (KJ/m) 3000 1896.5 2000

1000 1896 1/1/2017 3/1/2017 5/1/2017 7/1/2017 9/1/2017 11/1/2017 1/1/2018 Date

Figure 20: Annual wave energy model for Walker Lake (a-c) and Lake Tahoe (d-f) for the years 1999, 2007, and 2017. Lake elevation for the corresponding period is plotted on the secondary y- axis. Note different scales on primary y-axis for Walker Lake and Lake Tahoe.

Volumetric Analysis: Walker Lake The NW shore of Walker Lake contains the most prominent and abundant historical beach ridge complex of all the basins analyzed in this study. The beach ridges found at the NW

Walker Lake site are commonly composed of sand-gravel sized grains with abundant pebble- cobble sized clasts 3-5 cm in diameter. Clasts are primarily granitic with occasional mixed volcanics. The crests of individual ridges are often mantled with coarse pebble-cobble sized grains. Beach ridges are occasionally cut by small ~1-10 m wide ephemeral channels (Figure 21) providing cross sectional views of internal stratigraphy, which display fine ~1-5 cm thick, continuous, horizontal – shallow lakeward dipping alignment of dark minerals interpreted to be bedding features.

Twenty-six individual beach ridges were analyzed within the elevation range of 1244-

1202 m. Based on the historical lake-level curve, these features are estimated to have formed

between the years ~1920- 2000 (Figure 5 and Table 1). Volumes estimated in this suite of beach

ridges ranges from 0.3 – 6 m3/m with an average beach ridge volume of ~2.5 m3/m (Figure 22

and Table 1). The time scales under which these features formed ranges from ~120 - 1000 days

(Figure 22). Height of the beach ridges analyzed at the NW Walker Lake site ranges from 0.4 -

1.6 m and averages 0.78 m (Table 1).

Figure 21: Map of well-defined suite of historical beach ridges on the NW margins of Walker Lake. Figure shows examples of type of features selected for volumetric analysis (yellow). Note as slope steepens to the south of the mapped shorelines, constructional features become erosional in morphology. Historical fan deposits (Qfw) shed off the Wassuk Range may provide sediment replenishment to active beach ridges.

Volume vs. Lake Level Residency Time NW Walker lake 6

/m) 4 3 y = 0.9578ln(x) - 2.5328 3 R² = 0.7678

2 Volume (m 1 Volume (m3/m) Log. (Volume (m3/m)) 0 0 200 400 600 800 1000 1200 1400 1600 Time of Lake Level Residency (Days)

Figure 22: Results of volume of individual beach ridges vs. time of lake-level residency at their corresponding elevations along the NW margins of Walker Lake.

Table 1: Results of analysis of volumetric analysis of 29 beach ridge features at NW Walker Lake. Shoreline Volume Time of Lake-Level Height (m) Field Elevation(m) (m3/m) Residence (days) Estimated Volumes (m3/m) 1232 1.93 124 0.9 1230 1.61 150 0.7 1229 1.69 65 0.5 2.15,1.54 1228 1.55 98 0.6 1.86, 1.05 1228 4.39 262 1 1227 0.55 57 0.4 0.85,0.67 1226 2.78 120 0.6 1224 5.91 1007 1.4 1224 2.83 188 0.7 1224 0.84 36 0.4 1221 1.48 62 0.5 1220 2.36 176 0.9 1217 3.62 297 1.1 1217 3.01 422 0.7 1214 4.12 356 1 1214 2.31 390 1.6 1211 3.00 200 0.6 1211 3.52 1033 1.1 1209 2.03 100 0.6 1209 1.58 69 0.6 1208 1.66 55 0.6 1207 1.81 253 1 1206.8 0.35 32 0.5 1204 2.90 176 0.7 1202 4.56 924 0.7 1202 2.29 212 0.9 1202 1.26 135 0.4 ++ Elevations reported more than once were measured in different locations.

Volumetric Analysis: Winnemucca-Dry Lake

The SW shore of WDL contains relatively small and sparse beach ridge features within the historical lake-level elevation range (Figures 23 and 24). These beach ridges are commonly composed of abundant pebble-cobble sized clasts 3-10 cm in diameter. Clasts are primarily basaltic with occasional andesites and other mixed volcanics. Individual beach ridges at the WDL site are highly regular in their vertical spacing, each separated by ~1 m elevation.

There are ~ 5 distinct historical shoreline elevations that contain beach ridges sufficient for volumetric analysis (Figures 23, 24, and Table 2). Volumetric analysis was performed at 2-3 three different locations at elevations 1175 m, 1170 m, 1169 m, 1168 m, and 1167 m to characterize variability along strike. Volumes ranged from 0.82 – 2.28 m3/m with an average

beach ridge volume of 1.4 m3/m (Table 2). Beach ridge heights ranged from 0.3 - 0.5 m and

average 0.4 m.

Shoreline Volumes of Historical Beach Deposits Winnemucca-Dry Lake 2.4 /m)

3 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 Volume per unit length (m length unit per Volume 0.2 Volume 0 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 Elevation (m)

Figure 23: Results of volumetric analysis of individual beach ridges at their corresponding elevations along the SW margins of Winnemucca-Dry Lake.

Figure 24: Map showing historical beach ridges analyzed (pink) and their corresponding elevations, as well as the 1175 m contour representing the historical highstand at Winnemucca dry Lake (turquoise).

The volumetric relationship developed on the historical shorelines at Walker Lake

(Figure 22) was first applied to estimate the durations of time over which shorelines in the WDL

basin between 1175 and 1167 m formed (Table 2). Estimations of time of formation based on

observed volumes of the WDL beach ridges indicates a range of times of ~33-141 days for a

single beach ridge with an average of 69 days (Figure 25, Table 2).

Table 2: Results of volumetric analysis and modeled time of formation (orange) for historical WDL shorelines based on shoreline volume curve developed at Walker Lake. Location Shoreline Volume per Time of lake- height (m) Elevation(m) unit level length(m3/m) residence(days) S1 1175 0.82 33 0.3 N1 1175 1.18 48 0.5 N1 1173 2.18 137 0.5 S1 1170 1.40 61 0.4 C1 1170 2.21 141 0.4 S1 1169 1.68 81 0.5 C1 1169 1.73 86 0.4

Volume vs. Lake Level Residency Time NW Walker Lake and 6 WDL

4 y = 0.9578ln(x) - 2.5328 /m) 3 R² = 0.7678 3

2 Volume (m

1 Walker Lake WDL 0 0 200 400 600 800 1000 1200 1400 1600 Time of Lake Level Residency (Days) Figure 25: Modeled times of formation for WDL beach ridges (orange) based on observed beach ridge volumes at the WDL site and the volume vs. time relationship developed on historical beach ridges at Walker Lake (blue).

Volumetric Analysis: Lake Tahoe

Constructional beach ridges are relatively rare along the margins of Lake Tahoe, however

a large ~40 m wide beach ridge at Sand Harbor and a series of beach ridges at Baldwin Beach

contain the largest and longest-formed features analyzed in this study (Figures 26 and 27). The

far-field slope at Sand Harbor and Baldwin beach is ~ 0.5º or less, and both beaches are exposed

to the near maximum fetch distances possible at Lake Tahoe, however they are located on

opposite sides of the lake. The beach ridges found at both Sand Harbor and Baldwin Beach are

composed of a quartz-rich, coarse sand with occasional feldspar and volcanic lithic fragments.

Occasional well-rounded, pebble sized clasts ~0.5-1 cm in diameter are present at these beaches however cobble-sized material is very rare to absent. Some rounded cobbles were found along the

1900 m beach ridge crest near the mouth of Taylor Creek at Baldwin Beach.

Estimated volumes of individual beach ridges from Baldwin Beach ranges from 3.06 –

28.7 m3/m, and beach ridge height ranged from 0.4-2.4 m (Table 3). The cumulative time that the

individual beach ridges at Baldwin Beach and Sand Harbor were active is estimated to range from

3.4-25 years (Table 3).

Volume vs. Lake Level Residency Time Lake Tahoe 45 Volume

/m) 40 3 35 30 25 20 15 10 5

V olume Per Unit Length (m 0 0 5 10 15 20 25 30 Years

Figure 26: Results of volume of individual beach ridges vs time of lake-level residency at their corresponding elevations at Sand Harbor and Baldwin Beach, Lake Tahoe.

Figure 27: Map showing historical beach ridges analyzed at Sand Harbor (A) and Baldwin Beach (B), Lake Tahoe.

Table 3: Results of volumetric analysis of 4 beach ridge features at Lake Tahoe. Location Shoreline Volume per Unit Time of Height (m) Elevation(m) Length(m3/m) Lake-level Residence (days) Baldwin 1900 28.79 1247 1.5 Beach Baldwin 1898.8 3.43 2693 0.4 Beach Baldwin 1898.5 3.07 9261 0.8 Beach Sand 1900 38.81 1247 2.4 Harbor

Volumetric Analysis: Late Holocene Highstand Walker Lake

Beach ridges representing the late Holocene highstand at Walker Lake are located at about 1262 m elevation (Adams and Rhodes, 2019b) (Figure 28). IRSL dates indicate the 1262 m highstand occurred in the Walker Lake basin ~3620 + 300 cal yr BP (Adams and Rhodes, 2019b).

For this study the volume of the 1262 m beach ridge was measured at three different locations

(Figure 28 and Table 4). Large variability was observed in beach ridge height and volume along

strike of the 1262 m highstand and ranged from 9.67-33.44 m3/m (Figure 28, Table 4).

Using a modified equation from the volume vs. time of lake-level residency developed on the historical shorelines at Walker Lake (Figure 22) the time of lake-level residency was estimated for the late Holocene highstand at Walker Lake (Figure 29 and Table 4), from this we estimate lake-level was sustained at 1262 m for ~4.5-30 years during the late Holocene highstand.

Figure 28: Geomorphic map of beach ridges (blue polygons) at NW Walker Lake. Features between 1253-1262 m were dated to the late Holocene by Adams and Rhodes (2019b), and the late Holocene highstand reached 1262 m ~3620 + 300 cal yr BP.

Lake Level Residency Time vs. Volume Walker Lake

2500

2000

1500

1000

y = 61.221x1.4753 500 R² = 0.6135

Time of Lake level Resdidency (days) 0 0 5 10 15 20 25 30 Volume (m3/m) Volume (m3/m) Holocene Highstand (1262 m)

Figure 29: Estimates of times of formation for WDL beach ridges based on volume vs. time relationship developed on historical beach ridges at Walker Lake. Note: power law function used to estimate times of lake-level residency for late Holocene highstand (see discussion).

Table 4: Results of time of formation estimates for late Holocene highstand shorelines at Walker Lake based on shoreline volume curve developed from historical beach ridges, location numbers shown in Figure 28. Shoreline Shoreline Volume per unit Time of lake-level Height (m) Location Elevation(m) length(m3/m) residence(days)

Holocene 1262 33.44 10856 1.1 Highstand 1

Holocene 1262 9.67 1741 0.8 Highstand 2

Holocene 1262 10.73 2028 1 Highstand 3

Discussion

Wind Analysis

Regional and local wind conditions are perhaps the most important meteorological

consideration for predicting or characterizing shoreline processes in any lacustrine basin (CEM,

2006). Given the < 50 km fetch distances and coarse nature of the available sediment at Walker

Lake, WDL, and Lake Tahoe it is reasonable to assume that significant geomorphic work along

these shorelines only occurs during exceptionally strong and relatively long-lasting wind events.

Analysis of the 30-year wind record at the Dead Camel Mountain site shows that average wind speeds show little annual variability (Figure 17a), however, the number of significant wind events

per year may differ greatly (Figure 17b). The average monthly wind speeds observed at the Dead

Camel Mountain site are largely the result of downslope wind energy from the Sierra Nevada

Mountains (Jewell and Nicoll, 2011), while large magnitude wind events with the strongest wind

speeds typically accompany the passing of meteorological depressions and landfall of storm

events (Neiman, 2010). Large annual variability in storm magnitudes and frequency across the

western United States (McCabe, 2009) can explain high annual variability in the occurrence of

significant wind events. This phenomenon has large implications for the rates of development for

shoreline features in pluvial lake basins as the rate significant wind event occurrence can vary by

an order of magnitude from year to year (Figure 17b).

Wind direction is an important wind parameter related to wave growth and shoreline formation in the basins studied. In the western Great Basin and Sierra Nevada mountains, the bulk of storms associated with significant wind events arrive from the southwest quadrant

(Neiman, 2010), which favors the formation of winds from the S-SW direction. The three years selected for wind analysis and wave modeling indicate a strong persistence of S-SW, occasionally

W, and rarely NW wind directions (Figure 17). Waves form and propagate in the downwind

direction on lake bodies, leaving the upwind shore relatively calm and wave free (Komar, 1998;

CEM, 2006), therefore, it should be expected that the most well developed and prominent

historical shorelines should be located at the north margins of the lakes studied. Both the WDL

and Baldwin Beach sites are located in the typically upwind portion of their respective lake basins yet display prominent beach ridges ~1-2 m in height. These features indicate the occurrence of significant wind events out of the NW (Figure 30). A significant wind event wind rose constructed for the entire 30-year wind record at the Dead Camel mountain site indicates significant wind events occur out of the NW - NE 1 - 4 % of the time for the entire record (Figure

31, Table 5). The low frequency of significant wind events out of a northerly direction today coupled with the presence of very well-developed beach ridges along the southern margins of

WDL and Lake Tahoe suggests the formation of prominent beach ridges can occur from only a few events in a very rapid amount of time (days-weeks). This evidence supports the conclusions reached by Adams and Wesnousky (1998) who noted the very rapid formation of beach ridges in the spring of 1997 on Pyramid Lake, NV.

The presence of large shorelines along the southern margins of WDL and Lake Tahoe may also be due to the use of distal wind data used in this study. Lorang et al. (1993) and Kirk et al. (2000) acknowledge the topographic channeling of winds in confined lake basins, a process that likely occurs in both WDL and Lake Tahoe. Due to problems with existing weather station sites and a dearth of long-term wind data for the region, the exact local wind regimes for the basins studied is not available or is not a good indicator of wind conditions on the open lake. It is highly possible that the more common westerly significant wind events are topographically channeled at WDL and Lake Tahoe to produce significant waves travelling in the north-south direction within those basins.

Figure 30: Map of historical beach deposits at WDL site, asymmetrical arcuate beach ridge feature at 1170 m elevation indicates barrier grew from north-south direction (red arrow) indicating wind direction was out of the N-NW (black arrows).

Table 5: 30-year record of significant wind events from Dead Camel Mountains, Nevada characterized by relative frequency of direction and windspeed.

N NE E SE S SW W NW 10-11 m/s 0% 1% 1% 1% 5% 11% 8% 1% 11-12 m/s 1% 1% 1% 3% 11% 21% 13% 2% 12-13 m/s 1% 2% 2% 4% 16% 33% 17% 3% 13-14 m/s 1% 3% 2% 4% 18% 37% 18% 3% 14-15 m/s 1% 3% 2% 4% 19% 40% 19% 3% 15-16 m/s 1% 3% 2% 4% 20% 43% 19% 3% 16-17 m/s 1% 3% 3% 4% 21% 45% 19% 3% 17-18 m/s 1% 3% 3% 4% 21% 45% 19% 3% 18-19 m/s 1% 3% 3% 4% 21% 45% 19% 3% >19 m/s 0% 0% 0% 0% 0% 0% 0% 0%

30-Year Significant Wind Event Wind Rose N >19 m/s 50% 18-19 m/s NW 40% NE 30% 17-18 m/s 20% 16-17 m/s 10% 15-16 m/s W 0% E 14-15 m/s 12-13 m/s 11-12 m/s SW SE 10-11 m/s S

Figure 31: Wind rose of significant wind events 1987-2018 from the Dead Camel Mountains, NV.

Wave and Runup Modeling

Waves effectively transfer the wind energy from the atmosphere to the shoreline and drive formative shoreline processes such as sediment accumulation and shoreline erosion (Komar,

1998; Adams, 2003,2004). The modeled deep-water wave heights across the three basins of interest range from 0.4-2.1 m for the periods of interest (Figure 18). Overall, Lake Tahoe had the largest deep-water wave heights of the basins studied, and Walker Lake the smallest. This is due in large part to available fetch in each individual basin. However, WDL would have similar modeled deep-water heights to Lake Tahoe if wave refraction was not taken into account.

Previous studies have suggested beach ridge height is controlled by the maximum height of wave swash (Komar, 1998; Kirk et al., 2000; Tamura et al., 2018; Wang et al., 2018). Modeled

maximum total swash elevations for the observed significant wind events are 1.3 m, 1.56 m, and

0.93 m for the Walker Lake, WDL, and Lake Tahoe basins, respectively. Far field slope of the

beach has a large influence on the maximum elevation achieved by wave swash. For example,

despite having the smallest modeled deep-water wave heights the NW Walker Lake site, which

has a far-field slope of 3.6o, displays a larger maximum runup value than the Baldwin Beach site at Lake Tahoe which has the largest modeled deep-water wave heights but a slope of only ~0.5o.

At Walker Lake, the maximum beach ridge height observed is 1.6 m, which is only 30 cm above the maximum modeled swash elevation (Figure 19, Table 1). At Lake Tahoe, the

Baldwin beach ridges at 1898.8 m and 1898. 5 m display maximum heights of 0.4 m and 0.8 m, which is in general agreement with the annual maximum runup values (Figure 19). The heights of historical highstand beach ridges at Baldwin Beach and Sand Harbor are 1.5 and 2.4 m (Table 3) which is significantly taller than the maximum modeled runup value for that basin. It is likely that the historical highstand features are compound beach ridges that were built over a range of lake levels. Therefore, the crests of these features were likely formed during the historical highstand of

1899.29 m (Figure 11), which would correspond to a total swash elevation of 0.81 and 1.71 m at

Baldwin Beach and Sand Harbor, respectively. These heights are much more consistent with modeled runup values. In the WDL basin, the maximum-modeled swash height is 2.4 m, which is significantly larger than the observed maximum beach ridge height of 0.5 m. This is suspected to be due to an underestimation of wave height reduction during wave refraction and wave shoaling processes in the wave models. Alternatively, given the rare nature of significant wind events (as defined for this study) from a northerly direction, it is possible that more common, lower windspeed events from the north played a significant role in forming these features. These lower windspeed events would have smaller runup values than those modeled in this study.

While the heights of individual beach ridges are in good agreement (overall) with modeled swash values, the modeled swash elevations are typically smaller than the observed beach ridge heights. Given that it is highly unlikely the coarse sediment that composes these features is transported by other mechanisms (Komar, 1998; Tamura et al., 2018), it is probable that the assumptions made in the wave models may be the cause of these discrepancies. For example, the use of distal wind data for runup modeling may underestimate local wind conditions in the individual basins. It is also possible that wind conditions were different for the exact periods in which the shorelines were active than the three wind years used in this study.

Wave Energy

Wave energy distributed on the shoreline is an important parameter to both the construction of beach features as well as shoreline erosion (Lorang et al., 1993; Komar, 1998;

Kirk et al., 2000). The wave energy calculated for Walker Lake, WDL, and Lake Tahoe for the years 1999, 2007, and 2017 considers modeled wave heights and periods as well as wind direction. Total incident wave energy delivered to a shoreline can be highly variable for any given year. The Sand Harbor site on Lake Tahoe is estimated to have received a total 2.5x108 J/m in

wave energy over the course of the 2017 calendar year while only receiving 4.2x107 J/m during

the 2007 year (Figure 20). Variability in total annual incident wave energy can fluctuate by an

order of magnitude based on the number of significant wind events for a given year (Figure 16

and 20), which ranges from 5-22 events per year for the 30-year record. Additionally, wind

direction and shoreline orientation may play a very significant role in the amount of wave energy

delivered to a given shoreline. For the 1999, 2007, and 2017 years, no significant wind events

occurred from the NW direction within a 45o radius of the WDL and Lake Tahoe long axes

(Figure 17). Therefore, no annual incident wave energy was calculated at the Baldwin Beach or

WDL sites for the three years of interest. By comparison, Sand Harbor received a total of

2.92x108 J/m of wave energy during the 1999, 2007, and 2017 calendar years, cumulatively.

While the lack of observed NW winds may be a shortcoming of the distal wind data discussed

previously, this illustrates the large influence that wind direction, shoreline orientation, and

shoreline location may have in the rates of shoreline development.

Evidence for Historical Shoreline Deposits

In the Walker Lake, Lake Tahoe, and WDL basins large-scale lake-level fluctuations have occurred in response to climatic changes since the late Pleistocene (Russell, 1885;

Birkeland, 1964; Adams and Rhodes, 2019 a,b). Therefore, a large assumption made in this study is that the shorelines analyzed formed during the historical period with little to no inheritance of morphology from previous lake cycles. While deep-water marl capping the intermediate shorelines in Lake Bonneville indicate that beach ridge features have the potential to survive large

50-100 m transgressions (Jewell, 2007) several lines of evidence suggest the features analyzed are historical.

In the Walker Lake basin several ~ 2 m wide stream channel cuts expose the internal stratigraphy of some of the shorelines within the historical range. In cross-sectional view these features display faint horizontal, even, parallel, and continuous alignment of dark minerals

interpreted to be bedding features. Tamura et al. (2018) demonstrated on marine coasts that beach ridges formed over multiple surge events spanning several months-decades would commonly display erosive sedimentary features such as hummocky-cross bedding. Therefore, the intact horizontal bedding features at Walker Lake are interpreted to indicate the beach ridges were likely formed during the most recent lake level cycle.

In the Lake Tahoe basin, the natural sill elevation at the Truckee River was surveyed by

Adams and Minor (2002) at 1896.8 m elevation. Until anthropogenic damming of the outlet at

Lake Tahoe in the late 19th century, lake elevation had probably not appreciably exceeded its natural sill elevation since the Late Pleistocene (Birkeland, 1968). Several beach ridge features at

1898.5 -1901 m elevation can be observed near the mouth of Taylor Creek near Baldwin Beach recording lake elevations higher than the natural sill (Figure 10). These features correspond to periods within or slightly above the historical lake-level record at Lake Tahoe (Figure 11). These observations indicate that the beach ridges analyzed at Lake Tahoe for this study were most likely formed during the historical period.

After the construction of Derby Dam in 1906, inflow to the WDL basin via Mud Lake-

Slough (MLS) was drastically affected (Harding and Venstrom, 1946), and by ~1913 the abandonment of MLS had cut off the sole source of inflow to the basin (Adams and Rhodes,

2019a). It can be reasonably assumed that lake-level fell at the rate of evaporation beginning

~1913. By assuming evaporation rates of ~125-135 cm/ yr taken in neighboring Pyramid Lake

(Houghton et al., 1975; Milne, 1987), and an average mean annual precipitation on the WDL surface of 20 cm/yr. (Daly et al., 2008) mean annual evaporation rate would have caused WDL to fall by ~105 - 115 cm/yr. The uniform spacing and ~1 m vertical separation of beach ridges from

1170 - 1167 m suggest these features formed as lake level dropped at the annual rate of evaporation (Figure 24), which would strongly suggest they formed during the historical lake-

level regression. Additionally, small trenches dug into the WDL shorelines between 1175- 1167

m elevation did not reveal any soil development, indicating a relatively young age.

Shoreline Volume

Using the historical lake-level curve for Walker Lake (Figure 5), and volumetric analysis

of a suite of shorelines along the NW margins of Walker Lake (Figure 2), a time vs. beach ridge

volume relationship was developed (Figure 22 and Table 1). This relationship shows a strong

correlation between shoreline volume and the estimated time of formation for this sequence of

beach ridges. The curve developed shows up to nearly 5 m3/m of material can be accumulated in a single beach ridge within a year. Interestingly, the rate of development appears to stabilize within ~3 years, suggesting some sort of equilibrium of shoreline development may be achieved in that time. The strong relationship between beach ridge volume and lake-level stability at the corresponding elevation may be due to the progradational nature of beach ridge genesis. In general, beach ridges develop by wave swash and longshore sediment transport on prograded shorelines (Anthony, 1995; Tamura et al., 2018). On marine coasts where sea-level is relatively stable, individual ridges are subsequently abandoned as they are isolated from the active beach due to beach progradation, causing km-wide plains of individual beach ridges to develop within a narrow elevation range (~1-2 m) (Tamura et al., 2018). In pluvial lake basins, suites of closely spaced beach ridges often form within a wide elevation range, recording the rise and fall of lake

levels. However, progradational barrier complexes do form in pluvial lakes in places with

abundant sediment supply and relatively stable lake levels (Adams and Wesnousky, 1998).

Therefore, given the progradational nature of their genesis on marine and lacustrine coasts, it is

fitting that individual beach ridge volume shows a positive relationship to time of formation on

pluvial lakes.

At Lake Tahoe, the observed beach ridges do not obey the same volume vs. time of formation relationship as those observed at Walker Lake (Figure 26). The most voluminous

shorelines observed in this study are located at Baldwin Beach and Sand Harbor, Lake Tahoe

(Figure 27). The shorelines near ~1900 m elevation at Baldwin Beach and Sand Harbor have

volumes of ~28 m3/m and ~38 m3/m, respectively (Table 3). Both of these shorelines formed in an estimated 3.4 years when lake level exceeded the legal maximum limit during discrete times in in the early 20th century (Figure 11). The beach barriers located at 1898.5 and 1898.7 m elevation

at Baldwin Beach (Figure 10) are considerably smaller than the historical highstand barriers,

despite lake levels having been within the elevation ranges of these features for ~25 years (1898.5

m barrier) and ~7 years (1898.7 m barrier). Lake level often exceeds the crests of the 1898.5 and

1898.7 m barriers and it is suspected that during this time much of the accreted sediment that

composes these features is winnowed out and transported by littoral processes or reworked and

deposited further upslope. Compared to beach ridges at Walker lake and WDL, the Lake Tahoe

beach ridges are relatively fine grained and predominately composed of medium-coarse sand

which is conducive to being transported by longshore currents and other forms of littoral transport

(Komar, 1998; Kirk et al., 2000; Wang et al., 2018). It is likely much of the sediment emplaced

when the 1898.5 m and 1898.7 m barriers are active is removed during subsequent period of

relatively high lake level (>1898.6 m). Therefore, it is likely these features formed during the last

lake-level cycle to fall within the elevation ranges of these features before the imagery was

collected in 2010. This demonstrates possible error in volumetric analysis that may be caused by

fine grained beach deposits or many lake-level fluctuations within a small elevation range.

The sediment budget for shorelines in pluvial lakes and other lacustrine bodies is relatively simple (Adams and Wesnousky, 1998; Kirk et al., 2000; Adams, 2003, 2004; Reheis et al., 2014). Sediment contributions at the Walker Lake and WDL sites is predominately from local sediment sources such as alluvial fans and intermittent washes (Figures 22 and 24). Additionally,

material eroded from nearby erosional shorelines and beach cliffs may be transported by littoral

processes and deposited where slopes are < 6° (Adams and Wesnousky, 1998). These sources are

thought to be important for the sediment budgets of the beach ridges at the NW Walker Lake field

site (Figure 21).

While sediment supply was not directly measured or quantified in each of the locations

studied it is important to acknowledge the role it may play in determining the rates of shoreline

development as well as the overall volumes of shorelines at a particular location. The 1900 m

beach ridges at Baldwin Beach and Sand Harbor are significantly larger than any other historical

beach ridges observed in this study. At Baldwin Beach, Taylor Creek enters Lake Tahoe

immediately to the southeast of the prominent beach ridge features (Figure 10, 27b). It can be

seen in LiDAR imagery that the suite of beach ridge features at Baldwin Beach are notably wider

near the mouth of Taylor Creek (Figures 10 and 27b). Given that the 1900 m barriers at Baldwin

Beach and Sand Harbor were only active during discrete times when lake levels were particularly

high, it can be inferred that these periods were particularly wet and high runoff from local creeks

may have contributed more sediment than typically available to these features. Additionally,

during these times the high lake levels exposed much of the shoreline that had not seen wave

energy since the Late Pleistocene (Birkeland, 1968) therefore, high erosion along the lake

coupled with longshore sediment transport likely contributed to the particularly rapid and

voluminous development of these features.

The historical highstand features at Lake Tahoe illustrate potential issues that may arise from applying a volumetric analysis of beach ridges from one basin to determine lake-level residency times in another. For example, it would be inappropriate and erroneous to apply the volume vs. time relationship developed at Walker Lake on shorelines in Lake Tahoe given the drastic differences in sediment supply and sedimentological characteristics of the beach ridges.

The shorelines present at the WDL site provide a unique opportunity to examine the

variability of shoreline volumes along strike as well as the inter-annual variability in shoreline

development. As previously discussed, it is likely that the suite of shorelines between 1175 m and

1167 m each formed in around 1 years’ time as WDL fell at the rate of evaporation (Adams and

Rhodes, 2019a). Shoreline volumes across the elevations measured display a minimum value of

0.821 m3/m and a maximum value of 2.21 m3/m (Figure 25). The differences in volume of these shorelines may be a result of variability in the number of significant wind events that occur in a favorable direction for a given shoreline per year. Additionally, variance in sediment supply may occur on an annual basis, for example intermittent washes may provide pulses of hillslope sediment to the active shorelines during periods of particularly intense rainfall, however, the occurrence of these events can be highly variable from year to year (Coe et al., 1997).

To examine variability along strike, the volumes of individual shorelines at equal elevations were measured at 3 different locations at the WDL study area (Figure 24). From 5 individual shorelines measured in at least two different locations maximum variance along strike is 0.812 m3/m with an average of 0.325 m3/m. The central field area typically displays the largest

shoreline volume where several small intermittent channel networks are visible (Figure 24), which likely provided additional sediment to the shorelines when they were active. Overall, shoreline volume variability along strike was relatively low in the WDL basin, however, this may differ where local sediment supply is more variable such as at the Baldwin Beach site in Lake Tahoe.

Applying Volumetric Analysis of Beach Ridges to Paleoshorelines

The volumetric relationship developed on the historical shorelines at Walker Lake was first applied to estimate the durations of time over which shorelines in the WDL basin between

1175 and 1167 m formed. Estimations of time of formation based on observed volumes of the

WDL beach ridges indicates a range of times of ~33-141 days for a single beach ridge with an

average of 69 days (Figure 25, Table 2). Given beach ridge heights of 0.3-0.5 m and an

evaporation rate of ~1 m/yr, this would suggest lake level would have been within the active

range of the observed beach ridges for ~36 - 109 days for any given year. These results strongly

suggest that volumetric analysis of beach ridges can be used to reasonably estimate the durations

of time over which individual shorelines formed. Although the curve used to estimate the time of formation was developed in a different lake basin, both the WDL site and Walker Lake site have comparable modeled wave heights, and similar simple sediment budgets consisting principally of reworked alluvial fan material with some contributions from local intermittent streams and washes. Given these similarities it is reasonable to apply the volumetric relationship developed in

Walker Lake to the WDL site.

Large variability was observed in beach ridge height and volume along strike of the 1262 m late Holocene highstand beach ridge at Walker Lake, particularly at the easternmost segment of

the feature (Figure 28, Table 4). Nearby fluvial terraces (Figure 28) suggest the mouth of the

paleo Walker River was likely located nearby to the east of the beach ridges, which would have

provided additional sediment to the eastern extent of the mapped beach ridge.

Initially, applying the original best fit equation developed on the historical shorelines at

Walker Lake (Figure 22) yielded estimates of time for the duration of the Holocene highstand of

~937-2823 years. These estimates disagree with IRSL ages placed on a 1241 m shoreline ~3000

Cal yr BP by Adams and Rhodes (2019b) (Figure 32). Fetch distances are considerably larger for

Walker Lake at 1262 m elevation (~43 km) than they are where the historical shorelines are

measured (~17-18 km), this would result in significantly larger waves affecting the shoreline

during the late Holocene. Deep-water wave heights estimated for the 1262 m beach ridge at the

location studied are estimated to reach 1.68-1.86 m at fully developed wave conditions for a 12 -

13 m/s wind event. Additionally, due to its proximity to the mouth of the Walker River it is

suspected that the location studied had a much larger sediment budget than the historical site.

Therefore, the ~3 year shoreline equilibrium observed in the historical beach ridges at Walker

Lake (Figure 22) likely does not apply to the Holocene highstand beach ridge selected for this study. Because of the suspected larger sediment supply and larger modeled wave heights for the late Holocene highstand, a power law equation was fit to the data collected at the historical beach ridges (Figure 29), this equation does not result in a relatively quick equilibrium state for rates of shoreline development and is believed to better represent the rates of development that would have occurred at the Holocene highstand beach ridge. Using this relationship and the observed volumes, the estimates for duration of a sustained 1262 m lake level are 4.77-29.74 years (Table

4). It is likely the maximum estimate of 29.74 years may be skewed by the anomalously high volume at that location. Using the volumes of the two westernmost beach ridges at the study site we estimate a 1262 m late Holocene lake level was sustained for ~4.5-5.5 years (Table 4). It is important to acknowledge that it is possible an equilibrium state of beach ridge development could have been achieved during the formation of the 1262 m beach ridge and lake levels could have been sustained for longer periods of time than our current estimates suggest. Therefore, the

4.5-5.5 year estimate made in this study may only represent a minimum duration estimation of the late Holocene highstand at Walker Lake. In light of this, further work is needed to recognize the morphological characteristics of beach ridge equilibrium to reduce uncertainty in inferring durations of lake-level stability from shoreline volumes.

Figure 32: Late Holocene to present lake-level curve for Walker Lake from Adams and Rhodes (2019b).

During the early and middle Holocene, the Walker River is thought to have been diverted

into the Carson Sink leading to very low lake levels in Walker Lake (Bradbury et al., 1989;

Benson et al., 1991; Adams and Rhodes, 2019b). By ~5500 cal yr BP the Walker River was likely

rerouted back into the Walker Lake basin (Benson et al., 1991; Adams and Rhodes, 2019b) and

by ~4000 cal yr BP a relatively rapid lake-level transgression (Figure 32) in Walker Lake coincides with the late Holocene Neopluvial period (Allison, 1982). Based on various radiocarbon and optically stimulated luminescence (OSL) ages, Adams and Rhodes (2019b) show a period of relatively high lake levels in Walker Lake (> 1255 m) from ~ 4000 - 2800 cal yr BP,

with at least two minor lake - level drops to about 1240 – 1245 m (Figure 32). These absolute

dates place a maximum duration of the late Holocene highstand of ~250 years. Therefore, the 4.5-

5.5 year duration estimates based on beach ridge morphologies are within the IRSL age

constraints placed by Adams and Rhodes (2019b).

From the water balance modeling of Hatchett et al. (2015) and Adams and Rhodes

(2019b) we can infer that lake stage in Walker Lake likely stabilize at ~1245 - 1255 m under modern average precipitation with no water diversion. Additionally, the water balance modeling

by Hatchett et al. (2015) suggests that using the precipitation and temperature averages from the

particularly wet 1971-2000 time period, a lake-level transgression from 1250 m to a steady state

lake level of ~1260 m could be achieved in ~50-100 years. A sustained 1262 m lave level in

Walker Lake would require slightly above average precipitation (under modern climatic

conditions), therefore, using beach ridge volumes it is estimated the 1262 m lake-level (and above

average precipitation) was sustained for ~4.5-5.5 years’ time during the late Holocene highstand.

It is important to acknowledge similar high lake levels in Walker Lake could be achieved

by decreasing temperatures and evaporation rates (Hatchett et al, 2015). Additionally, watershed-

scale increases in runoff contributing areas (Hatchet et al., 2015) may also play a significant role

in the efficiency and volume of water moved into Walker Lake during sustained wet periods.

Regardless, the combination of existing lake-level curves, water balance modeling, and beach

ridge analysis indicates that the sequence of events leading up to the late Holocene highstand was

likely the result of a relatively long-lasting (~102 years) climatic event. Additionally, it is reasonable to assume that the late Holocene highstand at Walker Lake could have been achieved through a series of climatic regimes that have occurred within the historical period, given enough time.

Conclusions

This study first examines the processes that lead to the formation of beach ridges along the margins of pluvial lakes in order to better characterize regional controls on their genesis.

Waves are the primary transport mechanism of sediment for constructing beach ridges along lake shorelines (Adams, 2003, 2004). Given the relatively coarse nature of available sediment and fetch distances < 50 km in the basins studied, wave energy sufficient for beach ridge development likely only occurs during significant wind events where windspeeds are > 10 m/s for three hours

or more. Analysis of a 30-year hourly wind record from the Dead Camel Mountains, NV shows the occurrence of significant wind events averages 12 times per year, with a minimum and

maximum of 5 and 22 times per year, respectively. Significant wind events are commonly

associated with the passage of meteorological depressions that typically produce winds from the

W-SW direction, yet despite this, prominent shorelines have formed in the historical period on the

southern shore of WDL and Lake Tahoe. The NE-N facing shorelines indicate prominent beach

ridge development can occur very rapidly as N directed significant wind events only comprise 1 -

4% of significant wind events over the 30-year wind record. We acknowledge the wind data used may not represent the exact wind conditions for each basin, or the criteria used to parse significant wind events may favor winds from the W-SW.

Using wind data from the 1999, 2007, and 2017 calendar years we were able to model annual wave conditions and wave energy on Walker Lake, WDL, and Lake Tahoe for the minimum, average, and maximum wind years for the entire 30-year wind record. Deep water wave heights are largely controlled by fetch distance, windspeed, and wind duration. Maximum deep-water wave heights in Walker Lake, WDL, and Lake Tahoe are estimated to reach 1.4 m,

1.3 m, and 2.2 m, respectively. Overall, the highest modeled deep-water wave heights correspond to the largest fetch distances, however in the WDL basin significant wave refraction is suspected to reduce deep water wave heights by as much as ~1m during shoaling processes.

Modeled maximum total swash elevations for the observed significant wind events are

1.93 m, 1.56 m, and 0.93 m for Walker Lake, WDL, and Lake Tahoe, respectively. The modeled wave swash values generally come within ~0.3 m of maximum beach ridge heights observed in each basin but overall fail to achieve maximum beach ridge heights, indicating the models used may slightly underestimate deep water wave heights or swash elevations for each basin.

Wave energy delivered to the shorelines of interest was calculated from deep water wave heights over the years 2007, 1999, and 2017 which correspond to the minimum, average, and maximum wind years. Because no significant wind events occurred from a northerly direction within 45o of the WDL or Lake Tahoe long axes no wave energy was modeled for the three years

of interest at the WDL site or Baldwin Beach, Lake Tahoe. In Walker Lake and Sand Harbor,

Lake Tahoe total annual incident wave energy delivered to a given shoreline ranges from 6.2x106

J/m (Walker Lake 2007) up to 2.5x108 J/m (Sand Harbor 2017). These data illustrates that total

wave energy at a given shoreline can be highly variable due to variations in wind direction and

number of significant wind events per year, which has large implications for rates of shoreline

development both spatially and temporally.

Using volumetric analysis of beach ridges that fall within the historical lake levels of

these basins, and the historical lake-level records, the volume of beach ridges vs. time of

development was examined for each basin. In Lake Tahoe, particularly voluminous shorelines at

historical highstand levels at ~1900 m elevation developed in ~3-4 years’ time. The rapid rate of

development can be attributed to an abundant sediment supply and robust wave energy in that

basin. In the WDL basin it is suspected that the historical shorelines each formed within 1 years’

time as the lake fell at the rate of evaporation (Adams and Rhodes, 2019a). Despite similar time

scales of development beach ridge volume varied by up to 1.13 m3/m in the WDL basin, which is likely due to annual flux in wave energy or differences in sediment supply. The NW shore of

Walker Lake contains a prominent suite of historical beach ridges that show a strong positive trend between observed volume and times of formation. Additionally, the Walker Lake data suggests that after about 3 years of activity, rates of beach ridge development rapidly decreases and may reach an equilibrium state.

The Late Holocene highstand reached a maximum elevation of 1262 m in the Walker

Lake basin (Adams and Rhodes, 2019b) and formed a well-developed beach ridge that averages

10 m3/m in volume. By applying the volume vs. time of formation relationship developed on the

historical shorelines in Walker Lake to the Late Holocene highstand feature, we estimate lake

levels were sustained at 1262 m for ~4.5-5.5 years, however, we recognize that it is possible that

lake-level could have been sustained for longer periods of time than the volume would indicate if

an equilibrium state of development had been achieved on that shoreline.

The volumetric analysis of historical beach ridges at Walker Lake showed a strong correlation between time of formation and increased volume, which indicates at minimum, the individual volumes of beach ridges can be used as an indicator to infer relative durations of lake- level stability. Furthermore, when used to estimate durations of time of lake-level stability, this presents a new technique to further refine pre-historic lake-level curves on an annual-decadal scale. However, given large variability in shoreline volumes along strike and across basins it is important to acknowledge differences in sediment supply, wind conditions, wave energy, shoreline equilibrium, and other parameters relating to beach ridge formation before applying volumetric relationships from one lake to another.

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