![](media/image1.wmf){width="1.625in" height="0.5986111111111111in"}

# Digital Bathymetric Model of Mono Lake, California

By Christian G. Raumann[^1], Scott Stine[^2], Alexander Evans^1^, and
Jerry Wilson[^3]

Miscellaneous Field Studies Map MF-2393

2002

Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government.

U.S. Department of the Interior

U.S. Geological Survey

# ABSTRACT

In 1986 and 1987, Pelagos Corporation of San Diego (now Racal Pelagos)
undertook a bathymetric survey of Mono Lake in eastern California for
the Los Angeles Department of Water and Power (DWP). The result of that
survey was a series of maps at various scales and contour intervals.
From these maps, the DWP hoped to predict consequences of the drop in
lake level that resulted from their diversion of streams in the Mono
Basin. No digital models, including shaded-relief and perspective-view
renderings, were made from the data collected during the survey. With
the permission of Pelagos Corporation and DWP, these data are used to
produce a digital model of the floor of Mono Lake. The model was created
using a geographic information system (GIS) to incorporate these data
with new observations and measurements made in the field. This model
should prove to be a valuable tool for enhanced visualization and
analyses of the floor of Mono Lake.

# INTRODUCTION

Mono Lake is a hydrographically closed saline lake located on the east
side of the Sierra Nevada in central California (fig. 1). It is the
remnant of Pleistocene Lake Russell and has fluctuated in level
throughout the Late Quaternary, including historic times. At the June
2000 surface level of 6,384.5 ft (1,946.0 m) above sea level, the lake
has an east-west dimension of 13.3 mi (21.4 km) and a north-south
dimension of 9.3 mi (14.9 km). Numerous geologically young volcanic
features are found in the Mono Basin, which include Paoha and Negit
Islands and many small islets near the center of Mono Lake.

In 1941, the City of Los Angeles began diverting water from Mono Lake's
principle feeder streams for domestic use. After several years of
experimentation, the lake level began to fall. Between 1947 and 1982,
the lake's surface

![](media/image2.jpeg){width="6.013888888888889in"
height="4.096527777777778in"}

**Figure 1.** Map showing location of study area and Mono Lake and
vicinity.

level dropped 44.9 ft (13.7 m). Following years of legal proceedings,
the California State Water Resources Control Board in 1984 ordered Los
Angeles to curtail diversions. Since then the lake has risen 11.2 ft
(3.4 m) to an elevation of 6,384.5 ft (1,946.0 m) above sea level as of
June 2000 (Los Angeles Aqueduct Daily Reports and Mono Lake Committee).

The purpose of this project is to produce a modern digital bathymetric
model of the floor of Mono Lake. This model will be an evaluation tool
that allows for better visualization and analyses to help understand the
characteristics and origin of the bottom features of Mono Lake.

# PREVIOUS WORK

Russell (1889) gave the first description of the floor of Mono Lake and
briefly discussed bottom features in a monograph on the Mono Basin
published by the U.S. Geological Survey (USGS). On the basis of
approximately 100 soundings, he produced a map with 25-ft elevation
contour intervals. He determined that the deepest part of the lake was
along the southern border of Paoha Island at 152 ft (the lake stood at
6,410 ft at the time of Russell's study (Stine, 1987, 1990)). He also
stated that the average depth of the lake was between 60 to 62 ft (18.3
to 18.9 m) (Russell, 1889).

The Earth and Planetary Sciences Division of the U.S. Naval Ordnance
Test Station performed a more detailed and thorough bathymetric study of
Mono Lake in July 1964. The survey was conducted using a
continuous-recording fathometer along approximately 200.1 mi (322 km) of
sounding lines. Approximately 3,000 acoustic soundings were used to
construct a bathymetric chart with 10-ft contour intervals. The
resulting report described the major bathymetric features of Mono Lake
(fig. 1; Scholl and others, 1967).

Scholl and others (1967) summarized their study in four key
interpretations that they offered as provisional hypotheses to aid
future work in the Mono Basin. First, most of the small- and large-scale
bathymetric relief is Holocene and was not present beneath Pleistocene
Lake Russell. Second, bottom relief is highest and most irregular in the
vicinity of Paoha Island. Third, Paoha Island was formed by the
intrusion of volcanic features from beneath the central part of Mono
Lake, displacing lacustrine deposits that previously composed the lake
floor. Fourth, a prominent subbottom reflector lies beneath lake muds
over the central part of Johnson Basin to the south of Paoha Island
(Scholl and others 1967).

Between August 1986 and February 1987, Pelagos Corporation of San Diego
(now called Racal Pelagos and hereafter referred to as Pelagos)
undertook a bathymetric survey of Mono Lake for the Los Angeles
Department of Water and Power (DWP). According to the report produced be
Pelagos for the DWP, "the purpose of the study was to gain additional
knowledge on the lake's bathymetry (the measurement of water depths),
geology, and bottom features" (Pelagos, 1987). The result of that survey
was a series of maps at various scales and contour intervals. Data from
the Pelagos survey form the basis for the digital model developed here.
The Pelagos survey is discussed in more detail below.

Recent bathymetric surveys of other lakes and coastal areas that have
resulted in high-resolution digital models include a 1998 survey of Lake
Tahoe conducted by the USGS in cooperation with the Ocean Mapping Group
from the University of New Brunswick. This survey used a high-resolution
multibeam mapping system that has replaced the acoustic methods used by
Pelagos at Mono Lake as the standard for precision bathymetric surveys
(Gardner and others, 1998). Nevertheless, the Pelagos data are
sufficient to produce digital datasets and maps similar to those
produced for Lake Tahoe as well as make possible similar bathymetric
analysis.

# THE PELAGOS SURVEY DATA AND MAPS

The survey data were collected along approximately 1,020 km (3345.6 ft)
of sounding lines using two 24-ft boats, each equipped with an echo
sounder, subbottom profiler (boomer-type system), sidescan sonar, and
electronic navigation system. Water depths were measured by survey-grade
echo sounders every 15 m (4.6 ft) along each survey line and were
recorded onto floppy disk along with the position data. The digital data
were transferred to a VAX-11/780 VMS mini-computer for editing and
digitizing by a Science Accessories Model GP8 sonic digitizer. A total
of 29,650 depth points were used to construct the bathymetric part of
the topographic maps (Pelagos, 1987).

## Horizontal and Vertical Accuracy

Horizontal positions were obtained using a Motorola Mini-Ranger III
system, which has a manufacturer's stated accuracy of ± 2 m (6.6 ft) for
a calibrated system. Vessel positions during the survey were based on
triangulation between three or more satellites, which allowed for an
accuracy of 2 m (6.6 ft) or less. Furthermore, the Mini-Ranger antenna
was mounted directly above the echo-sounder transducer to avoid any
position-versus-depth offset error (Pelagos, 1987).

The Raytheon echo sounder used by Pelagos had a vertical accuracy of
0.06 m (0.2 ft), and their digitizing system had an accuracy of 0.08 m
(0.3 ft). Their field calibration steps were designed to meet or exceed
the manufacturer's stated accuracy, bringing accuracy of depth
measurements to ±0.15 m (0.5 ft) (Pelagos, 1987).

Boat motion during the survey, such as boat heave, squat, and roll, was
likely a limiting factor on the depth accuracy of the survey (James
Gardner, written commun., 2001). However, no mention was made concerning
boat motion or its effect in the Pelagos report (Pelagos, 1987).

## Combined Bathymetric and Topographic Maps

The final maps drafted by Pelagos show lakeshore topography combined
with bathymetry covering elevations from 6,230 to 6,430 ft (1898.9 to
1959.9 m). These maps have scales of 1:24,000 and 1:12,000 with 5-ft
contour intervals covering the entire lake, and 1:6,000 with 2-ft
contour intervals for the regions around Paoha and Negit Islands. At the
time of the survey, the surface level of the lake was 6,380.7 ft.
Contours at and below 6,370 ft were mapped using the data acquired from
the bathymetric survey, while contours 6,375 to 6,430 ft were mapped
using photogrammetric techniques based on aerial photography from 1982
collected by Pacific Western Aerial Surveys (Pelagos, 1987).

# DIGITAL MODEL PROCESS AND METHODS

The goal of this project was to produce a digital model of the
bathymetry of Mono Lake by building upon and further processing the
Pelagos data. The digital model was created by combining the Pelagos
data with additional field measurements, along with other digital data
obtained from the USGS, in a geographic information system (GIS).

A digital elevation model (DEM) is a raster-based dataset that uses a
uniform grid system to depict elevations over a specified area of the
Earth's surface. Typically, large-scale DEMs are based on and referred
to by a standard USGS 7.5-minute quadrangle, although DEMs are by no
means limited to this geographic delineation. The primary task of the
project was to construct a DEM of Mono Lake that accurately represents
the lake's bathymetry. This DEM could then be used to model and map the
lake floor.

## The Pelagos Dataset

Two 9-track tapes were received on loan from Pelagos. These tapes
contained archived digital files of the survey, but the nature of the
digital data was not clear. To extract the data from these tapes, a
UNIX-based computer at the USGS in Menlo Park, Calif., that supports the
VAX/VMS format was used. After extraction, the files were transferred to
a CD-ROM. Approximately 240 files were analyzed and the 40 files
essential to constructing the DEM were identified. These files were in a
generic data (DAT) format.

When viewed in a spreadsheet application, the data are shown as points
with horizontal and vertical California State Plane coordinates (North
American Datum of 1927) with elevations in feet for each point. The
number of points in each file varies from approximately 33 (for A3.dat)
to 45,020 (for D4.dat) depending on the size of the area and the
complexity of relief covered by the DAT file. Pelagos also supplied
multiple printed maps of grids showing the geographic coverage of the
DAT files. When combined into a single grid system, the data contained
in the 40 DAT files cover the entire lake up to a surface elevation of
6,430 ft (fig. 2).

The points contained in the DAT files, when converted to DBF format and
viewed as point coverages using ESRI's ArcView GIS software, represent
vertices of contour lines digitized by Pelagos at 5-ft elevation contour
intervals (fig. 3). These files show that the original combined
topographic and bathymetric maps produced by Pelagos were digitized and
then the line information was saved as point data. The entire area
covered by the original maps was then divided into the grid that is
shown in figure 2. The average size for each of the grid cells is 10,000
by 10,000 ft (3,048.04 by 3,048.04 m).

![](media/image3.jpeg){width="5.375in" height="3.7631944444444443in"}

> **Figure 2**. Location of digitized contour sections of Mono Lake
> (from Pelagos Corporation,1987) shown in California State Plane
> coordinates. Circled letter and number indicates DAT file designation.

##### ![](media/image4.jpeg){width="5.001388888888889in" height="3.9166666666666665in"}

> **Figure 3.** Location of contour section DAT file C7 and example of
> digitized line as point coverage contained in file.

Because the data supplied by Pelagos are in essence the digitized
hand-drawn contours of the printed maps (although represented by points
rather than lines) and not the actual 29,650 survey points, it was
necessary to approach and process the data in a way that would not
lessen or corrupt the integrity of the original data source (that is,
the survey depth points). The accuracy of the final digital model relies
upon the original cartographic work done by Pelagos and the data
processing performed thereafter.

## Data Processing

The first step was to construct a seamless vector coverage of the entire
lake with lines (or arcs as they are referred to in ESRI's ARC/INFO
software package) representing contour lines at 5-ft intervals.

This process involved several steps. Initially, the points contained in
each DAT file were converted into arcs. Next, all 40 coverages were
combined to produce one coverage with elevation contours for the entire
lake and immediate shoreline. At this point, it was apparent that
significant editing was necessary to clean up the dataset and remove
artifacts, especially along the seams where the arcs in each of the
individual coverages shown in figure 2 join together.

In addition editing the bathymetric contours prior to DEM creation, some
corrections were made to the Pelagos contours in the area containing
numerous islets northeast of Negit Island (fig. 4). The original mapping
in this area was inaccurate because of navigational hazards. Additional
data collected in the field in 1989 were used to rectify the contours in
this area; these changes are documented below.

![](media/image5.jpeg){width="6.497222222222222in"
height="5.2652777777777775in"}

> **Figure 4.** Map showing location of islets northeast of Negit
> Island. Bathymetric contour interval is 10 ft. Lake level 6,390 ft.

The next step was to construct a DEM using the seamless vector coverage
created previously. The standard USGS large-scale DEM has either 10- or
30-m grid spacing, meaning that for each 10 by 10-m, or 30 by 30-m grid
space on the ground there is one elevation assigned to this space. For
this project, a 10-m grid spacing was used. No information was available
regarding the techniques Pelagos used to convert their survey data
points into contours, but after experimenting with different grid
spacings, a 10-m spacing produced a relatively smooth grid without
significant artifacts due to generation from contour lines. The command
TOPOGRID in ARC/INFO, which is an interpolation method specifically
designed for the creation of hydrologically correct DEMs, was used to
produce the bathymetric grid. Additional discussion concerning the
interpolation methods used in TOPOGRID is provided by Hutchinson and
Dowling (1991).

Bathymetric measurements, or depths, in the digital model are
represented in feet above sea level and not feet below the surface of
the lake, which is reasonable because Mono Lake is a hydrographically
closed lake characterized by fluctuating surface elevations. Lake bottom
measurements represented as elevations will remain relatively unchanged
as the lake surface rises or falls, unlike water depths (feet below the
surface), which will vary for any point on the lake bottom depending on
the surface elevation of the lake.

## Lake Level Selection

The 6,390-ft contour was chosen as the lake level for the upper
elevation limit of the DEM because this is the closest contour line to
the target lake level elevation of 6,391 ft mandated by the California
State Water Resources Control Board. Once this level is reached, DWP can
then begin diverting water from streams feeding Mono Lake in substantial
amounts. It is important to note, however, that the geomorphic processes
that accompany a rising lake make it difficult to accurately predict
where the 6,391-ft contour will be once the lake reaches this level.

## Dataset Accuracy

Generating a grid from digitized hand-drawn contours for which
documentation of vertical or horizontal accuracy regarding the contours
is unavailable greatly complicates any evaluation of grid accuracy,
making it difficult (if not impossible) to determine the accuracy for
this digital bathymetric model of Mono Lake. Interpolation of the
contours using various grid spacings only reflects the detail and
accuracy of the original contouring, which is a very subjective process
with accuracy being a reflection of the cartographer's interpretation of
the original data points.

# CORRECTIONS TO BATHYMETRY

On the basis of fieldwork conducted in 1989 consisting of spot depth
measurements made with a leadline from a 17-ft canoe and on analyses of
aerial photos from 1930, '40, '56, '64, '72, '75, and '82, the following
corrections were made to the bathymetry around the informally named
islets northeast of Negit Island (table 1; fig. 4) (Stine, 1991). These
corrections were then incorporated into the vector contour dataset prior
to creation of the bathymetric grid.

# MAPS

## Sheet 1

This map incorporates shaded relief with full-spectrum color gradient
for the bathymetry along with a grayscale shaded-relief image of the
surrounding topography derived from USGS 30-m DEMs. The 20-ft
bathymetric contour lines are from the vector coverage used to generate
the bathymetric grid. Hydrographic and transportation vector data are
from 1:24,000- and 1:100,000-scale USGS Digital Line Graphs.

## Sheet 2

This map shows the viewing location and angles for 10 perspective views
located at various points around Mono Lake. The perspective views show
selected bathymetric and topographic features around Mono Lake at
varying amounts of vertical exaggeration.

**Table 1.** Corrections made to bathymetry of Mono Lake northeast of
Negit Island

+---------------------+------------------------------------------------+
| **Islet name\* or   | **Correction(s) to bathymetry^+^**             |
| area**              |                                                |
+---------------------+------------------------------------------------+
| Geographic          | Raised summit contour from 6,370 to 6,380 ft.  |
+---------------------+------------------------------------------------+
| Steamboat           | Raised summit contour from 6,410 to 6,420 ft.  |
+---------------------+------------------------------------------------+
| Little Norway       | Raised summit contour from 6,400 to 6,420 ft.  |
+---------------------+------------------------------------------------+
| Krakatoa            | Raised summit contour from 6,410 to 6,420 ft.  |
+---------------------+------------------------------------------------+
| Hat                 | Raised summit contour from 6,370 to 6,380 ft.  |
+---------------------+------------------------------------------------+
| Tie                 | Raised summit contour from 6,375 to 6,390 ft.  |
+---------------------+------------------------------------------------+
| Spot                | Raised summit contour from 6,375 to 6,385 ft.  |
+---------------------+------------------------------------------------+
| Twain               | Raised summit contour from 6,410 to 6,420 ft.  |
+---------------------+------------------------------------------------+
| Midget              | Raised summit contour from 6,370 to 6,380 ft.  |
+---------------------+------------------------------------------------+
| Siren               | Raised summit contour from 6,370 to 6,380 ft.  |
+---------------------+------------------------------------------------+
| LaPaz               | Raised summit contour from 6,370 to 6,380 ft.  |
+---------------------+------------------------------------------------+
| Java                | Raised summit contour from 6,400 to 6,415 ft.  |
+---------------------+------------------------------------------------+
| Little Tahiti       | Raised the contours of the four principle      |
|                     | summits of Little Tahiti from (variously)      |
|                     | 6,380-6,415 ft to 6,420 ft.                    |
+---------------------+------------------------------------------------+
| Castle Rocks        | Added contours (to a summit contour of 6,380   |
|                     | ft) to reflect the presence of Castle Rocks,   |
|                     | an islet absent from the Pelagos bathymetry.   |
+---------------------+------------------------------------------------+
| Area north of       | Presence of the gentle rise from 6,360 ft to   |
| Geographic          | 6,370 ft that was mapped by Pelagos            |
|                     | immediately to the north of Geographic cannot  |
|                     | be confirmed; therefore, this rise was         |
|                     | removed.                                       |
+---------------------+------------------------------------------------+
| Area northwest of   | Removed large submerged shelf northwest of     |
| Little Tahiti       | islet and modified lake bottom in this area as |
|                     | follows:                                       |
|                     |                                                |
|                     | 1\. Enclosed contours in far western part to   |
|                     | reflect presence of Saddle,                    |
|                     |                                                |
|                     | the summit contour of which was raised from    |
|                     | 6,370 to 6,380 ft.                             |
|                     |                                                |
|                     | 2\. Enclosed the contours immediately          |
|                     | northeast of Saddle to reflect the             |
|                     |                                                |
|                     | presence of Comma, the summit contour of which |
|                     | was raised from                                |
|                     |                                                |
|                     | 6,375 to 6,380 ft.                             |
|                     |                                                |
|                     | 3\. Enclosed the contours immediately          |
|                     | northeast of Saddle to reflect the             |
|                     |                                                |
|                     | presence of Comma, the summit contour of which |
|                     | was raised from                                |
|                     |                                                |
|                     | 6,375 to 6,380 ft.                             |
|                     |                                                |
|                     | 4\. Drew contours (to a summit contour of      |
|                     | 6,380 ft) to reflect the                       |
|                     |                                                |
|                     | presence of Muir, which was absent from the    |
|                     | Pelagos bathymetry.                            |
|                     |                                                |
|                     | 5\. The bathymetry that actually exists in the |
|                     | area to the north of Little                    |
|                     |                                                |
|                     | Tahiti, where Pelagos erroneously drew the     |
|                     | broad, submerged shelf,                        |
|                     |                                                |
|                     | remains unknown.                               |
+---------------------+------------------------------------------------+

\*Names of islets are informal (Stine, 1991).

^+^1 ft = 0.3048 m.

# ACKNOWLEDGMENTS

We would like to thank Peter Dartnell, John Vogel, Sean Stone, and
Robert Lugo for their support, contributions, and assistance with this
project.

# REFERENCES CITED

Gardner, J.V., and Dartnell, P., 1999, Bathymetry and selected
perspective views of Lake Tahoe, California and Nevada: U.S. Geological
Survey Water Resources Investigation Report 99-4043.

Gardner, J.V., Mayer, L.A., and Hughes-Clarke, J., 1998, The bathymetry
of Lake Tahoe, California-Nevada: U.S. Geological Survey Open-File
Report 98-509.

Hutchinson, M.F. and Dowling, T.I., 1991, A continental hydrological
assessment of a new grid-based digital elevation model of Australia:
Hydrological Processes, v. 5, p. 45-58.

Los Angeles Aqueduct Daily Reports and Mono Lake Committee, Monthly Mono
Lake Levels 1979-2000, accessed June 2000, at URL
http://www.monolake.org/library/lakelevel/monthly.htm

Pelagos Corporation, 1987, A bathymetric and geologic survey of Mono
Lake, California: San Diego, Calif., project report.

Russell, I.C., 1889, Quaternary history of the Mono Valley, California:
U.S. Geological Survey Annual Report 8, pt. 1, p. 261-394.

Scholl, D.W., Von Huene, R., St. Amand, P., and Ridlon, J.B., 1967, Age
and origin of topography beneath Mono Lake, a remnant Pleistocene lake,
California: Geological Society of America Bulletin, v. 78, p. 583-600.

Stine, S., 1987, Mono Lake: the past 4,000 Years: University of
California, Berkeley, Ph.D. dissertation, 615 p.

Stine, S., 1990, Late Holocene fluctuations of Mono Lake, eastern
California: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p.
333-381.

Stine, S., 1991, Geomorphic, geographic, and hydrographic basis for
resolving the Mono Lake controversy: Environmental Geology and Water
Science, v. 17, n. 2, pp. 67-83.

[^1]: U.S. Geological Survey, Menlo Park, California

[^2]: California State University Hayward, Dept. of Geography and
    Environmental Studies

[^3]: Racal Pelagos Corporation, San Diego, California