Prepared in cooperation with the City of Ithaca, New York, 

and the New York State Department of State

Bathymetric Survey and Estimation of Storage Capacity of 

Lower Sixmile Creek Reservoir, Ithaca, New York

Open-File Report 2016–1157

U.S. Department of the Interior

U.S. Geological Survey
Cover. U.S. Geological Survey hydrographers conducting bathymetric survey of lower Sixmile Creek reservoir, Ithaca, New York. Crest of 

dam is between bedrock outcrops on right side of photograph. (Photograph by James Dye, U.S. Geological Survey.)
Bathymetric Survey and Estimation of 

Storage Capacity of Lower Sixmile Creek 

Reservoir, Ithaca, New York

By John F. Wernly, Henry J. Zajd, Jr., and William F. Coon

Prepared in cooperation with the 

City of Ithaca, New York, and the New York State Department of State

Open-File Report 2016–1157

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Wernly, J.F., Zajd, H.J., Jr., and Coon, W.F., 2016, Bathymetric survey and estimation of storage capacity of 

lower Sixmile Creek reservoir, Ithaca, New York: U.S. Geological Survey Open-File Report 2016–1157, 13 p., 

http://dx.doi.org/10.3133/ofr20161157.

ISSN 2331-1258 (online)
iii

Acknowledgments

This report was prepared with funding provided by the New York State Department of State 

under Title 11 of the Environmental Protection Fund. The authors would like to thank the 

personnel of the City of Ithaca, specifically Michael Thorne, superintendent for the Department 

of Public Works, and Erik Whitney, who assisted the U.S. Geological Survey in gaining access to 

the lower Sixmile Creek reservoir and transporting equipment into the Sixmile Creek gorge. The 

Geographic Information Systems (GIS) Program of the City of Ithaca, specifically Susan Nixson 

and Ruth Aslanis, created the bathymetric surfaces from 1938 and 2015 data that enabled 

calculation of the storage capacity of the reservoir. Scott Gibson, environmental engineer for the 

City of Ithaca, provided historical and other background information on the reservoir and its role 

in the water-supply system for the City of Ithaca.

Jared Marion and Nolan Milliman, both of the U.S. Geological Survey, assisted in the field 

and devised an extraction system for removal of the boat from the Sixmile Creek gorge at the 

end of the project. Thanks are extended to James Dye, of the U.S. Geological Survey, for his 

expertise in operating Global Positioning System (GPS) equipment, processing GPS data, and 

clarifying the presentation of GPS-related information in the report. The authors would also like 

to acknowledge the contributions of U.S. Geological Survey hydrologists Douglas Nagle and 

Benjamin Dietsch, who provided technical reviews of the report.

v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Bathymetric Survey.......................................................................................................................................4

Survey Plan............................................................................................................................................4

Vertical and Horizontal Control...........................................................................................................6

Postprocessing of Bathymetric Data ................................................................................................7

Vertical and Horizontal Control Accuracy ........................................................................................7

Dam and Normal Pool Water-Surface Elevations....................................................................................8

Creation of Bathymetric Surface ................................................................................................................8

Estimation of Surface Area and Storage Capacity ..................................................................................8

Summary........................................................................................................................................................13

References Cited..........................................................................................................................................13

Figures

1. Map showing Sixmile Creek and locations of water-supply reservoirs in Ithaca, 

Tompkins County, New York........................................................................................................2

2. Map showing the lower Sixmile Creek reservoir, including approximate 1938 

reservoir boundary and 1949 landslide location, Ithaca, New York.....................................3

3. Map showing locations of transects for echo-sounder depth measurements, 

target points for manual depth measurements, and shoreline target points from 

the bathymetric survey of the lower Sixmile Creek reservoir, Ithaca, New York ..............5

4. Map showing bathymetric surface of the lower Sixmile Creek reservoir, Ithaca, 

New York, 2015 ..............................................................................................................................9

5. Map showing bathymetric surface of the lower Sixmile Creek reservoir, Ithaca, 

New York, 1938 ............................................................................................................................10

6. Graph showing surface area and storage capacity of the lower Sixmile Creek 

reservoir, Ithaca, New York, 1938 and 2015............................................................................12

Table

1. Surface area and storage capacity of the lower Sixmile Creek reservoir, Ithaca, 

New York, 1903, 1938, and 2015 ................................................................................................11
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to both the National Geodetic Vertical Datum of 

1929 (NGVD 29) and the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
vii

Abbreviations

ADCP acoustic Doppler current profiler

GIS geographic information system

GPS Global Positioning System

OPUS National Geodetic Survey Online Positioning User Service

RMSE root mean square error

RTK real-time kinematic

TIN triangular irregular network

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

Bathymetric Survey and Estimation of Storage Capacity of 

Lower Sixmile Creek Reservoir, Ithaca, New York

By John F. Wernly, Henry J. Zajd, Jr., and William F. Coon

Abstract

During 2015, the U.S. Geological Survey, in cooperation 

with the City of Ithaca, New York, and the New York State 

Department of State, conducted a bathymetric survey of 

the lower Sixmile Creek reservoir in Tompkins County, 

New York. A former water-supply reservoir for the City of 

Ithaca, the reservoir is no longer a functional component 

of Ithaca’s water-supply system, having been replaced by a 

larger reservoir less than a mile upstream in 1911. Excessive 

sedimentation has substantially reduced the reservoir’s waterstorage capacity and made the discharge gate at the base of 

the 30-foot dam, which creates the reservoir, inoperable. 

U.S. Geological Survey personnel collected bathymetric data 

by using an acoustic Doppler current profiler. Across more 

than half of the approximately 14-acre reservoir, depths were 

manually measured because of interference from aquatic 

vegetation with the acoustic Doppler current profiler. City of 

Ithaca personnel created a bottom-elevation surface from these 

depth data. A second surface was created from depths that 

were manually measured by City of Ithaca personnel during 

1938. Surface areas and storage capacities were computed at 

1-foot increments of elevation for both bathymetric surveys. 

The results indicate that the current storage capacity of 

the reservoir at its normal water-surface elevation is about 

84 acre-feet and that sediment accumulated between 1938 and 

2015 has decreased the reservoir’s capacity by about 68 acrefeet. This sediment load is attributed to annual inputs from 

the watershed above the reservoir, as well as from an episodic 

landslide that filled a large part of the reservoir along its 

northern edge in 1949.

Introduction

Since 1903, water-supply needs for the City of Ithaca 

in Tompkins County, New York, have been wholly or partly 

met by water impounded in one of two reservoirs on Sixmile 

Creek (fig. 1). Several bedrock constrictions along the length 

of the Sixmile Creek gorge made construction of the reservoirs 

technically feasible. The lower reservoir, completed in 1903, 

was formed by a 30-foot (ft) dam and is officially named 

the Sixmile Creek Reservoir (New York State Department 

of Environmental Conservation, 2016), but it is locally 

referred to as the 30-foot-dam reservoir. The upper reservoir, 

completed in 1911, was formed by the 60-foot-high Potters 

Falls Dam, and is officially and locally referred to as the 

Potters Falls Reservoir (Scott Gibson and Matthew Sledjeski, 

City of Ithaca Water and Sewer Division, written commun., 

2015). However, on the U.S. Geological Survey (USGS) 

topographic map for “Ithaca, East” (U.S. Geological Survey, 

2013), this upstream reservoir is identified as Ithaca Reservoir. 

To avoid confusion, herein these reservoirs will be referred to 

as the lower and upper Sixmile Creek reservoirs, respectively.

The lower reservoir covers about 14 acres and was 

relegated to a backup, emergency water supply when the 

upper reservoir was completed in 1911. Until 1959, the lower 

reservoir was used for water supply infrequently and only 

when operational problems or maintenance requirements 

interrupted water supply from the upper reservoir (U.S. 

Army Corps of Engineers, 1981). Presently (2016) the upper 

reservoir is the only operational reservoir of the two; excessive 

sedimentation in the lower reservoir has rendered both the 

valve to the water main in the control tower and the discharge 

gate at the base of the reservoir nonfunctional (Scott Gibson, 

City of Ithaca Water and Sewer Division, written commun., 

2015). Sedimentation of the lower reservoir was a centurylong process of deposition of fine-grained sediment that was 

carried into the reservoir from the Sixmile Creek watershed; 

however, much of this infilling might have occurred prior 

to construction of the upper reservoir in 1911 and from a 

landslide that filled a large part of the reservoir along its 

northern edge, presumably during 1949 (Susan Nixson, City of 

Ithaca Geographic Information System [GIS] Program, written 

commun., 2015; fig. 2).

The USGS provides reliable scientific information to 

describe hydrologic systems and assists in understanding 

how hydrologic and geomorphic conditions can change 

over time. In most instances, this involves documenting 

and analyzing the effects of widely recognized phenomena, 

such as surface-water and groundwater flow and quality. 

On occasion, this involves relatively small-scope studies 

of previously unrecognized phenomena, such as reservoir 

sedimentation and (or) landslides that emerge as natural 

hazards. The USGS works within its strategic science 
2 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York Cayuga Lake Lower Sixmile Creek reservoir ITHACA Upper Sixmile Creek reservoir

Sixmile Creek

76°26' 76°27' 76°28' 76°29' 76°30' 76°31' 76°32' 76°33'

42°28'42°27'42°26'42°25'42°24'

0 2 1 MILES0 2 1 KILOMETERS

Study area Seneca Lake Cayuga Lake

NEWYORK

Location map

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Figure 1. Sixmile Creek and locations of water-supply reservoirs in Ithaca, Tompkins County, New York. The blue shading on the base image is an enhancement to highlight Sixmile Creek and the two reservoirs.
Introduction 

3

0 800 FEET 400

0 200 METERS 100

EXPLANATION

Approximate location  of 1949 landslide

1938 reservoir boundary

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

76°28'10" 76°28'20" 76°28'30"

42°25'30"42°25'20"

Figure 2. The lower Sixmile Creek reservoir, including approximate 1938 reservoir boundary and 1949 landslide location, Ithaca, New York.
4 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York

direction and with its cooperative partners, such as the City of 

Ithaca, to document emerging environmental conditions and 

(or) hazards and to ensure that scientific methods are applied 

effectively to better understand these phenomena and thereby 

inform environmental managers on water resources and 

associated infrastructure.

In 2015, the USGS, in cooperation with the City of Ithaca 

and the New York State Department of State, conducted a 

bathymetric survey of the lower Sixmile Creek reservoir. Data 

from this survey were used to estimate the volume of sediment 

that has accumulated and the current water-storage capacity 

of the reservoir. These results could be of use to City of Ithaca 

personnel faced with a management decision regarding the 

disposition of the 30-ft dam. Options being considered include 

demolition and removal of the dam, renovation of the dam to 

meet current New York State safety standards, and repair of 

the gate at the base of the dam to permit use of the reservoir 

for flood-control purposes.

The purpose of this report is to document the procedures 

followed during collection of bathymetric data from the 

lower Sixmile Creek reservoir and to present estimates of the 

water-storage capacity and volume of accumulated sediment 

in the reservoir. The report describes the collection of depth 

data along transects and at target points, the processing of 

the depth data, and the assessment of the accuracy of the 

depth measurements. These data are available as a separate 

data release through USGS ScienceBase (Wernly and others, 

2016). The report explains how the reference elevation was 

determined for calculation of the reservoir’s storage capacity. 

Two maps of the bathymetric surface of the reservoir, created 

by City of Ithaca personnel, are presented: one from depth 

data collected in this study and one from data collected in 

a 1938 survey. The report describes the computation of the 

water-storage capacity of the reservoir and the comparison of 

the results with similar data produced from the 1938 survey. 

On the basis of these results, the volume of accumulated 

sediment in the reservoir is estimated.

Bathymetric Survey

Bathymetric data were collected with an acoustic 

Doppler current profiler (ADCP), specifically a SonTek 

RiverSurveyor M9 with a 0.5-megahertz integral verticalbeam echo sounder (SonTek, 2015b). An echo sounder 

measures water depths by measuring the time interval required 

for ultrasonic waves to travel, at a known velocity, from a 

known point (transducer face) to a reflecting surface (reservoir 

bottom) and then return. A minimum depth limitation 

applies when the reflecting surface is too close to the echosounder face (generally less than about 1.5 ft for this survey) 

or if aquatic vegetation interferes with the echo-sounder 

transmission. In these cases, depths were manually sensed 

with a fiberglass stadia rod incremented in tenths of a foot. 

A 6-inch steel circular pad, affixed to the bottom of the rod, 

minimized settling of the rod below the top of the soft bottom.

The velocity of sound in water is dependent upon the 

salinity, temperature, and depth. The salinity and relatively 

shallow depths typically encountered in freshwater lakes are 

not major factors that affect the sound velocity (U.S. Army 

Corps of Engineers, 2002). ADCPs have built-in temperature 

sensors that measure water temperature at the transducer face 

and permit the computation of the sound velocity and the 

subsequent accurate measurement of depth. To confirm the 

accuracy of the ADCP’s sound-velocity calculation, surfacewater temperature was measured independently by means of 

a thermistor. Throughout the survey the ADCP temperature 

sensor varied by no more than 0.2 degrees Celsius from the 

independently measured surface-water temperatures, and 

water temperatures recorded at depths of 5 and 10 ft varied by 

no more than 0.1 degrees Celsius from the measured surface 

temperature. Therefore, no variation in sound velocity with 

depth, as a function of change in temperature, was expected in 

this shallow reservoir.

Before surveying each day, the depth readings of the 

echo sounder were checked by means of a bar check to ensure 

accurate depth sounding. A plate was suspended at known 

depths—5 and 10 ft—below the transducer face. The depth 

indicated by the echo-sounder signal that was reflected off the 

submerged plate was compared with the known depth of the 

plate. As an additional check, manual depth measurements 

were made periodically along the surveyed transects and were 

compared with echo-sounder-measured depths.

All data were referenced to the horizontal datum, North 

American Datum of 1983, and vertical datum, North American 

Vertical Datum of 1988 (NAVD 88). The survey was 

conducted from November 16 to 20, 2015.

Survey Plan

Guidelines for conducting a bathymetric survey as 

described by Wilson and Richards (2006) were followed. 

The first set of planned survey transects was arranged 

perpendicular to the gorge section where the dam is located 

(fig. 3). Given the relatively short length of the reservoir 

(approximately 1,400 ft), the minimum transect interval of 

30 ft was selected. Closer spacing was deemed infeasible 

because of the impracticality of maintaining boat position 

along planned transects that were spaced more closely than 

30 ft (Wilson and Richards, 2006).

Site reconnaissance indicated that over half of the 

reservoir—the upstream part—had shallow depths and that 

its bottom was covered by dense aquatic vegetation, which 

interfered with the use of the ADCP and required manual 

measurement of depths. Therefore, a second set of transects 

was created for measuring manually sensed depths. This set 

was oriented similarly to the 30-ft transects, but the spacing 

was increased to 50 ft. Target points, which served as markers 

for manually measured depths, were created along these 
Bathymetric Survey 

5

0 800 FEET 400

0 200 METERS 100

Original survey transect for echo-sounder measurementsQuality-assurance transect for echo-sounder measurementsOriginal survey target point for manual measurementAdditional target point for manual measurementShoreline target point

EXPLANATION

76°28'10" 76°28'20" 76°28'30"

42°25'30"42°25'20"

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Figure 3. Locations of transects for echo-sounder depth measurements, target points for manual depth measurements, and shoreline target points from the bathymetric survey of the lower Sixmile Creek reservoir, Ithaca, New York.
6 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York

transects at 50-ft intervals (fig. 3). Spacing was increased 

because of labor and time constraints and was justified on 

the basis of the gradual change in depths from point to point 

across the reservoir.

For quality assurance, a third set of transects was created 

to provide an independent dataset with which the vertical 

accuracies of the ADCP data could be assessed (fig. 3). These 

transects were oriented 45 degrees to the original transects and 

were spaced 90 ft apart, three times as far apart as the original 

transects. Included in this dataset, but not used for qualityassurance purposes, were manually measured target points 

along transects that were spaced every 50 ft in the part of the 

reservoir where vegetation interfered with the ADCP. These 

additional target points were added to the original dataset 

from this part of the reservoir to increase the data density for 

creation of the bathymetric surface.

A final dataset of target points was collected along 

the shoreline of the reservoir to provide an areal boundary 

for the surveyed data (fig. 3). These shoreline points were 

collected at the end of each 30-ft planned transect. Collection 

was performed by placing the ADCP with its real-time 

kinematic (RTK) Global Positioning System (GPS) on the 

bow of the boat and driving the boat, bow-first, to the edge 

of water. When the bow was at the shoreline edge, horizontal 

coordinates were recorded while a depth of zero was assigned.

HYPACK software (HYPACK, Inc., 2015) was used to 

create the transects and the target points over the length of 

the reservoir and thus enabled the collection of consistently 

spaced water depths. A georeferenced aerial image of the 

reservoir was used within HYPACK, both for office planning 

of transects and for guidance of the survey team in the field. 

The survey team used an onboard differential GPS (separate 

from the ADCP’s RTK GPS) that was coupled to a computer, 

on which HYPACK was running, to navigate the transects in 

real time and keep track of their data-collection progress.

Vertical and Horizontal Control

A vertical control point was established on a bedrock 

ledge about 140 ft upstream from the dam and near the 

edge of water. A survey-grade dual-frequency GPS receiver 

(Trimble, 2016), operated in a static RTK and logging datacollection mode, recorded GPS data once per second during a 

4-hour observation session. These GPS data were submitted 

to the National Geodetic Survey Online Positioning User 

Service (OPUS; National Geodetic Survey, 2015) to obtain 

the positional coordinates of the control point. The elevation 

and coordinates of this point, which are an average of three 

independent single-baseline solutions, were determined to 

have a vertical accuracy of 0.046 ft and a horizontal accuracy 

of 0.026 ft, respectively. These accuracy values are peakto-peak errors as reported in the OPUS solution report and 

represent the difference between the maximum and minimum 

values of the coordinates obtained from the three baseline 

solutions (Rydlund and Densmore, 2012). The elevation 

of the control point was referenced to NAVD 88 (using the 

geoid model GEOID12B). The control point was used as a 

reference point from which the water-surface elevation of 

the reservoir was measured by tape-down with a weighted 

steel tape. Such measurements were made at the beginning 

and end of each survey workday. During postprocessing of 

the data, water depths (adjusted for the transducer depth) 

were subtracted from these water-surface elevations to obtain 

reservoir-bottom elevations.

Horizontal survey positions for the ADCP and 

manually sensed data were acquired through the use of a 

RTK–GPS receiver positioned directly above the ADCP 

where depths were measured. This unit received horizontallocation corrections from an onshore RTK base station, 

which was within line of sight of the survey vessel and 

provided a manufacturer-stated horizontal survey accuracy 

of 3 centimeters (cm) or 0.10 ft relative to the base station. 

The software associated with the RiverSurveyor M9 ADCP, 

RiverSurveyor LIVE (version 3.8; SonTek, 2015a), does not 

allow input of a control-point location for the base station, 

nor does it allow for the logging of observations that could 

be postprocessed in OPUS. RiverSurveyor LIVE is designed 

for measuring streamflow where the absolute georeferenced 

location of the ADCP is not critical to a precision of ±3 cm. 

The system uses the base station to correct the ADCP position 

to ±3 cm relative to itself for the purpose of accurately 

measuring stream width to calculate discharge. This system 

was adapted for our bathymetric use by obtaining a high level 

of accuracy for the ADCP relative to the position of the RTK 

base station, whereas, without the ability to enter a controlpoint location for the RTK base station, the absolute position 

of the ADCP may only have attained submeter accuracy.

Latency is the time difference or lag between the time 

when positioning data are received and the time when the 

computed or processed position reaches the data logging 

module and is time tagged. Latency typically results in a 

negative along-track spatial displacement of the depth value. 

This displacement can be minimized by surveying at slow 

speeds and by synchronizing the echo-sounder and GPS 

clocks. Boat speed was kept at a minimum throughout the 

survey. Boat speed averaged 0.90 feet per second (ft/s) during 

the planned transects and 1.60 ft/s during the quality-assurance 

transects. The RiverSurveyor M9 ADCP minimizes latency 

through the use of the integrated RTK–GPS located above 

the ADCP, which directly connects the incoming GPS data 

stream to the ADCP. Unlike other echo-sounder systems, the 

RiverSurveyor M9 ADCP performs internal calculations with 

the GPS data, synchronized by the system with a 10-hertz 

update rate, before the data are stored in the computer. Latency 

was not quantified, but it was considered small enough, given 

the slow survey speed and integrated GPS, that any potential 

errors in the data were negligible for this survey.
Bathymetric Survey 7

Postprocessing of Bathymetric Data

Reservoir water-surface elevations were measured 

twice daily from the surveyed reference point. More frequent 

measurements were unnecessary given the base-flow 

conditions that existed during the survey period. Water-surface 

elevations varied by 0.02 ft or less during a given day and by 

only 0.07 ft over the course of the survey. Reservoir-bottom 

elevations were computed by subtracting the measured depths, 

which included an adjustment for the transducer depth, from 

the mean water-surface elevation for a given day. The resultant 

data were processed by using ArcGIS (version 10.3.1), 

which contains hydrographic software for data viewing and 

editing, data processing, generation of interpolated data, and 

creation of map products (Esri, 2015). Raw transect data were 

reviewed in RiverSurveyor LIVE, the software associated with 

the RiverSurveyor M9 ADCP (SonTek, 2015a), for erroneous 

measurements that could result from false reflections from 

fish, trees, and other submerged objects. In addition, those data 

that did not meet RTK quality, as sometimes occurred when 

the GPS signal was lost near the steep walls of the gorge and 

the upstream face of the dam, were also identified. These data 

were removed, and the remaining transect data were saved as 

edited transects.

Vertical and Horizontal Control Accuracy

The vertical-beam echo sounder used in this study 

has a resolution of 1 millimeter (0.003 ft) and an accuracy 

of 1 percent of the measured depth, according to the 

manufacturer’s specifications (SonTek, 2015b). An assessment 

of the vertical accuracy of the measured depths and resultant 

reservoir-bed elevations was possible by comparing results 

for coincident points measured in the original set of transects 

(30-ft spacing) and the quality-assurance set of transects 

(90-ft spacing). Sixty paired intersection points were found 

for comparison. The horizontal distances between the paired 

points averaged 0.48 ft and ranged from 0.07 to 1.05 ft. The 

absolute differences in elevations ranged from 0 to 0.69 ft; 

the median absolute difference was 0.10 ft. The vertical root 

mean square error (RMSE) and the vertical accuracy of the 

elevations at the 95-percent confidence level were computed 

by using the following equations:

2

1

( )

n

i i

i z

Zorig Zqa

RMSE

n

=

−

=

∑ , (1)

where

RMSEz is the vertical root mean square error,

Zorigi is the vertical coordinate of the ith check point 

in the original dataset,

Zqai is the vertical coordinate of the ith check point 

in the quality-assurance dataset,

i is an integer from 1 to n, and

n is the number of points being checked.

Az = 1.960 × RMSEz, (2)

where

Az is the vertical accuracy calculated at the 

95-percent confidence level.

The RMSE of the ADCP-collected elevation data was 0.25 ft, 

and the vertical accuracy was 0.49 ft at the 95-percent 

confidence level.

Echo-sounder-measured depths were also assessed 

by comparison with manual depth measurements. After 

controlling for erroneous echo-sounder depths affected 

by aquatic vegetation, a total of 56 points throughout the 

reservoir were found for comparison. Measured depths ranged 

from 1.28 to 11.4 ft, with differences between manual and 

echo-sounder measurements ranging from −0.10 to 0.81 ft. 

The median absolute difference between manually measured 

depths and those measured with the echo sounder was 0.14 ft. 

Accuracy of these comparisons was affected by boat drift and 

aquatic vegetation. Boat drift introduced uncertainty in the 

comparisons owing to an inability to measure the exact same 

point with the stadia rod as that measured by the echo sounder. 

The maximum difference in depths was measured in an area 

of rapidly changing elevation and likely reflects this source of 

inaccuracy. Echo-sounder depths which were known to have 

been affected by aquatic vegetation from field notes and a 

visual analysis of the data were removed from the comparison. 

However, it is possible that some depth measurements that 

were affected by aquatic vegetation were retained in the 

comparison. The average depth difference of +0.16 ft (manual 

depth minus the echo-sounder depth) indicates that the echo 

sounder measured less depth, and this result could, at least 

partly, be due to unseen aquatic vegetation interfering with the 

echo-sounder transmissions. Evidence for this potential source 

of error was the presence of aquatic vegetation sometimes 

found clinging to the bottom of the stadia rod when it was 

returned to the boat. A second plausible contributor to this 

apparent bias in echo-sounder depth measurements could be 

the settling of the stadia rod into the soft substrate, despite 

the attachment of a 6-inch pad to the bottom of the rod to 

minimize this problem. The vertical accuracy of depths 

manually measured with the stadia rod was not evaluated 

as part of this study, but is assumed to be comparable to the 

computed accuracy of the echo-sounder measurements.

The horizontal positional accuracy of the RTK–GPS 

receiver in the ADCP is 3 cm (0.01 ft), according to the 

manufacturer (SonTek, 2015b). However, horizontal accuracy 

is affected by the multipath environment (that is, proximity 

to trees, buildings, cliffs, and so on), the number of satellites 

in view, satellite geometry, and ionospheric activity. No 

independent testing was conducted to confirm the stated 

horizontal accuracy of the reservoir survey, but for the 

purposes of this study, the exact location of a given surveyed 
8 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York

point in space was deemed to be less important than the 

relative locations among the surveyed points and the distances 

between adjacent points. Therefore, the manufacturer’s level 

of accuracy was accepted as stated.

Dam and Normal Pool Water-Surface 

Elevations

Before discussion of the creation of a bathymetric surface 

from the measured depths and calculation of the storage 

capacity of the reservoir, the issue of the reference elevation 

of the reservoir’s normal pool should be clarified. The normalpool water-surface elevation is defined as the elevation of the 

ponded water under base-flow conditions and is determined by 

the crest of the dam. Substantial discrepancies exist in regards 

to the crest elevation of the 30-ft dam at the lower Sixmile 

Creek reservoir. (Unless otherwise noted, the following 

cited elevations are referenced to the National Geodetic 

Vertical Datum of 1929 [NGVD 29].) The City of Ithaca has 

historically used 583.5 ft for the crest elevation (Scott Gibson, 

City of Ithaca Water and Sewer Division, written commun., 

2015). However, a 1938 survey map of the reservoir indicates 

a crest elevation of 584 ft (Susan Nixson, City of Ithaca GIS 

Program, written commun., 2015), and a recent engineering 

assessment of the 30-ft dam by C.T. Male Associates (2014) 

gives the crest elevation as 585 ft. Two other separate sources 

concur in an elevation of 583 ft. A U.S. Army Corps of 

Engineers (USACE; 1981, p. 3) dam-inspection report cites 

the original as-built crest elevation of 201 ft (local datum) 

that was “approximately equal to USGS mean sea level minus 

382” (or 583 minus 382 = 201 ft). This means that the USACE 

value started with a crest elevation of 583 ft. In addition, 

the USGS “Ithaca, East” topographic map (U.S. Geological 

Survey, 2013) gives 583 ft as the reservoir’s mean watersurface elevation.

As mentioned previously, static GPS was used to obtain 

the elevation of a reference point from which the water-surface 

elevation was determined each day of the survey. The mean 

water-surface elevation under base-flow conditions (that is, the 

normal pool) was 582.5 ft, referenced to the North American 

Vertical Datum of 1988 (NAVD 88), or 583.0 ft, referenced 

to NGVD 29. (The conversion from NGVD 29 to NAVD 88, 

−0.54 ft, was computed by using VERTCON [National 

Geodetic Survey, 2016]). The NGVD 29 value, which agrees 

with the USACE and USGS elevations of 583 ft, was used as 

the reference elevation for the subsequent calculation of the 

reservoir’s storage capacity.

Creation of Bathymetric Surface

The bathymetric data that were collected by the USGS 

were used by personnel of the City of Ithaca GIS Program 

(Susan Nixson, written commun., 2015) to produce a 

triangular irregular network (TIN) of the bottom of the 

reservoir (fig. 4). The TIN was projected in a State plane (New 

York Central) coordinate system. These data are available as a 

separate data release through USGS ScienceBase (Wernly and 

others, 2016).

A similar TIN surface was created from data collected by 

City of Ithaca personnel during a 1938 survey of the reservoir 

(fig. 5). The outline of the reservoir in 1938 is shown in 

figure 2 for comparison with the present-day reservoir extent. 

At that time, manually sensed depths were measured in a fixed 

pattern across the reservoir. The reservoir’s water-surface 

elevation in 1938 was unknown, but the crest elevation of 

the dam, which would have approximated the water-surface 

elevation, was noted as 584 ft. As discussed in the previous 

section, 583 ft has been identified as the crest elevation of the 

dam. Therefore, although initial calculations of the reservoir’s 

bottom elevations used 584 ft as the reference elevation for 

converting depths to elevations, the computed storage data 

presented in the following section are based on a reference 

elevation of 583.0 ft, NGVD 29 (582.5 ft, NAVD 88). This 

adjustment brought the 1938 survey into agreement with the 

elevation of the reservoir’s current normal pool and the water 

surface at the time of the USGS 2015 survey and thereby 

permitted direct comparison of reservoir storage capacities 

computed from the 2015 and 1938 datasets. The maps 

presented in figures 4 and 5 are available as a separate data 

release through USGS ScienceBase (Wernly and others, 2016).

Estimation of Surface Area and Storage 

Capacity

Presumably the maximum depth of the original reservoir 

was the height of the dam, 30 ft. The greatest depth noted 

on the bathymetric survey of 1938 was 20.0 ft, which was 

measured about 15 ft upstream from the dam, near the 

water-supply outflow tower on the eastern gorge wall. This 

difference in depths indicates that by 1938, about 10 ft of 

sediment had accumulated at the point of measurement 

upstream from the dam. The greatest depth measured in 

2015, 16.6 ft, was about 130 ft upstream from the dam and 

also near the eastern gorge wall. These depths cannot be 

directly compared because they are from different points 

in the reservoir; however, shallower depths were recorded 

at about the same point just upstream from the dam during 

both surveys. A depth of 16.3 ft was measured in 1938 about 

the same distance upstream from the dam as the maximumdepth measurement, but near the center of the channel. At a 

similar location in the channel in 2015, the measured depth 

was 13.5 ft. This comparison indicates that 16.5 ft of sediment 

had accumulated at this point since the dam was constructed 

in 1903, but less than 3 ft had accumulated since 1938. These 

data indicate that the rate of sedimentation was much greater 

prior to 1938—about 0.39 foot per year (ft/yr)—than between 

1938 and 2015—0.036 ft/yr. If annual depth measurements 
Estimation of Surface Area and Storage Capacity 

9

0 800 400 FEET0 200 100 METERS

EXPLANATION

Elevation, in feet (NAVD 88)

582 to 584

580 to <582578 to <580576 to <578574 to <576572 to <574570 to <572568 to <570565 to <568

76°28'10" 76°28'20" 76°28'30"

42°25'30"42°25'20"

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Figure 4. Bathymetric surface of the lower Sixmile Creek reservoir, Ithaca, New York, 2015.
10 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York

0 800 400 FEET

0 200 100 METERS

76°28'10" 76°28'20" 76°28'30"

42°25'30"42°25'20"

EXPLANATION

Elevation, in feet (NAVD 88)

582 to 584580 to <582578 to <580576 to <578574 to <576572 to <574570 to <572568 to <570564 to <568

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Bathymetric surface created by GIS Program of the City of Ithaca, New York

Figure 5. Bathymetric surface of the lower Sixmile Creek reservoir, Ithaca, New York, 1938.
Estimation of Surface Area and Storage Capacity 11

had been made, they likely would have shown the greatest rate 

of sedimentation between 1903–11, before the construction of 

the upper Sixmile Creek reservoir, which acts as a sediment 

trap for loads that previously were carried into and deposited 

in the lower Sixmile Creek reservoir. 

For the 1938 and 2015 surveys, the ArcMap surfacevolume tool (Esri, 2015) was used to compute the surface area 

and storage capacity of the reservoir in 1-foot increments at 

and below 583 ft, NAVD 88 (583.5 ft, NGVD 29) (table 1; 

fig. 6). A comparison of these results, including those at 

the normal pool elevation of 582.5 ft, NAVD 88 (583 ft, 

NGVD 29), indicates that at a normal pool water-surface 

elevation (1) the surface area decreased from 19.6 acres in 

1938 to 13.9 acres in 2015 and (2) the capacity of the reservoir 

decreased from 152 acre-feet (acre-ft) in 1938 to 83.7 acre-ft 

in 2015. Based on these results, the volume of sediment that 

has accumulated in the reservoir between 1938 and 2015 is 

about 68 acre-ft. This sediment load is attributed to annual 

inputs from the watershed above the reservoir, as well as from 

a landslide that filled a large part of the reservoir along its 

northern edge in 1949 (see fig. 2).

No data could be found from which the surface area 

and total storage capacity of the reservoir at the time it was 

constructed in 1903 could be computed. However, the U.S. 

Army Corps of Engineers (1981) dam-inspection report cites 

a 1925 report from the City of Ithaca Engineer to the New 

York State Engineer and Surveyor, wherein a total capacity of 

287 acre-ft and a surface area of 20 acres at the crest elevation 

are given. For lack of any other information or confirmation 

of when these values were computed, they are presumed to 

represent the capacity and surface area of the reservoir at the 

time of its completion in 1903. Based on these data, the total 

volume of sediment that has accumulated in the reservoir since 

1903 is about 200 acre-ft.

Table 1. Surface area and storage capacity of the lower Sixmile Creek reservoir, Ithaca, New York, 1903, 1938, and 2015.

[NAVD 88, North American Vertical Datum of 1988; NGVD 29, National Geodetic Vertical Datum of 1929; --, no data]

Reference elevation,

in feet

Surface area, 

in acres

Storage capacity, 

in acre-feet

NAVD 88 NGVD 29 1903 1938 2015 1903 1938 2015

583 583.5 -- 19.7 13.9 -- 162 90.7

1

582.5 1583.0 220 19.6 13.9 2287 152 83.7

582 582.5 -- 19.0 13.6 -- 143 76.8

581 581.5 -- 17.8 13.0 -- 124 63.6

580 580.5 -- 16.8 12.0 -- 107 51.1

579 579.5 -- 15.7 10.5 -- 90.9 39.8

578 578.5 -- 14.4 8.69 -- 75.9 30.2

577 577.5 -- 13.1 7.25 -- 62.1 22.3

576 576.5 -- 11.8 6.15 -- 49.7 15.6

575 575.5 -- 10.5 5.14 -- 38.5 9.94

574 574.5 -- 9.10 3.13 -- 28.7 5.71

573 573.5 -- 7.33 1.67 -- 20.5 3.36

572 572.5 -- 5.91 1.08 -- 13.9 2.01

571 571.5 -- 4.70 0.70 -- 8.63 1.13

570 570.5 -- 3.37 0.36 -- 4.55 0.61

569 569.5 -- 1.90 0.23 -- 1.93 0.31

568 568.5 -- 0.88 0.14 -- 0.57 0.13

567 567.5 -- 0.16 0.06 -- 0.12 0.03

566 566.5 -- 0.04 0.00 -- 0.02 0.00

565 565.5 -- 0.01 -- -- 0.01 --

564 564.5 -- 0.00 -- -- 0.00 --

1

Normal pool water-surface elevation; that is, the elevation of the ponded water under base-flow conditions.

2

From U.S. Army Corps of Engineers (1981).
12 Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York

20

0

406080100120140160180

50

10152025

564 566 568 570 572 574 576 578 580 582 584

e feet r ca ni , yti capa Storage c

ser ca ni , aer a ecafr uS

Elevation, in feet, referenced to the 

North A

merican Vertical Datum of 1988

Surface area, 1938

Surface area, 2015

Storage capacity, 1938Storage capacity, 2015

EXPLANATION

Figure 6. Surface area and storage capacity of the lower Sixmile Creek reservoir, Ithaca, New York, 1938 and 2015.
References Cited 13

Summary

A bathymetric survey of the lower Sixmile Creek 

reservoir in Tompkins County, New York, was conducted by 

the U.S. Geological Survey, in cooperation with the City of 

Ithaca, New York, and the New York State Department of 

State, during November 2015. The usable storage capacity 

of the reservoir was estimated from collected data. This 

estimate could help city managers with decisions regarding 

the disposition of the 30-foot (ft) dam that creates the 

reservoir and the possible use of the reservoir as a floodcontrol structure.

The U.S. Geological Survey collected bathymetric data 

with an acoustic Doppler current profiler. In areas where 

aquatic vegetation interfered with the acoustic Doppler 

current profiler signal, depths were manually measured. The 

maximum depth of the reservoir, 16.6 ft, was measured about 

130 ft upstream from the dam. Depths closer to the dam were 

about 13.5 ft, far shallower than the original 30-ft depth.

City of Ithaca personnel created a bottom-elevation 

surface from the collected depth data. A second surface was 

created from depths that were manually measured during 

1938. These maps and the bathymetric data from which they 

were created are available as a separate data release through 

USGS ScienceBase. Surface areas and storage capacities were 

computed at 1-ft increments of elevation for each bathymetric 

survey. An elevation of 583 ft, National Geodetic Vertical 

Datum of 1929 (or 582.5 ft, North American Vertical Datum 

of 1988), which corresponds to the elevation of the crest of 

the dam and that of the reservoir’s normal pool or watersurface elevation under base-flow conditions, was used as 

a reference elevation for these computations. The results 

indicate that the current storage capacity of the reservoir at its 

normal water-surface elevation is about 84 acre-feet and that 

sediment accumulated between 1938 and 2015, has decreased 

the reservoir’s capacity by about 68 acre-feet. This sediment 

load is attributed to annual inputs from the watershed above 

the reservoir, as well as from an episodic landslide that filled 

a large part of the reservoir along its northern edge in 1949. 

Historical data from circa 1925 suggests that the original 

storage capacity of the reservoir was 287 acre-feet and that 

about 200 acre-feet of sediment has accumulated in the 

reservoir since its creation in 1903.

References Cited

C.T. Male Associates, 2014, Engineering assessment for 

Six Mile Creek dam: C.T. Male Associates, 27 p. plus 

appendices.

Esri, 2015, ArcGIS: Esri Web site, accessed December 1, 

2015, at http://www.esri.com/software/arcgis/.

HYPACK, Inc., 2015, HYPACK hydrographic survey 

software, version 15.0: HYPACK, Inc., Web site, accessed 

November 2015 at http://www.hypack.com.

National Geodetic Survey, 2015, OPUS Online Positioning 

User Service: National Geodetic Survey database, accessed 

November 2015 at http://www.ngs.noaa.gov/OPUS/.

National Geodetic Survey, 2016, VERTCON—North 

American vertical datum conversion: National Geodetic 

Survey Web site, accessed January 2016 at http://www.ngs.

noaa.gov/TOOLS/Vertcon/vertcon.html.

New York State Department of Environmental Conservation 

(NYS–DEC), 2016, New York State inventory of dams: 

NYS–DEC, Division of Water, Bureau of Flood Protection 

and Dam Safety [dataset available at NYS–DEC Natural 

Resources and Environmental Protection Maps Web 

page], accessed May 2, 2016, at http://www.dec.ny.gov/

pubs/103459.html.

Rydlund, P.H., Jr., and Densmore, B.K., 2012, Methods of 

practice and guidelines for using survey-grade global 

navigation satellite systems (GNSS) to establish vertical 

datum in the United States Geological Survey: U.S. 

Geological Survey Techniques and Methods, book 11, 

chap. D1, 102 p. with appendixes.

SonTek, 2015a, RiverSurveyor LIVE (RSL): SonTek Web 

page, accessed June 20, 2016, at http://www.sontek.com/

softwaredetail.php?RiverSurveyor-LIVE-RSL-34.

SonTek, 2015b, RiverSurveyor® S5 and M9: SonTek Web 

page, accessed June 20, 2016, at http://www.sontek.com/

productsdetail.php?RiverSurveyor-S5-M9-14.

Trimble, 2016, Trimble R8 GNSS System: Trimble Web 

page, accessed March 14, 2016, at http://www.trimble.com/

Survey/trimbler8gnss.aspx.

U.S. Army Corps of Engineers, 1981, Phase I inspection 

report, National Dam Safety Program, Sixmile Creek Dam: 

U.S. Army Corps of Engineers, New York District, 94 p.

U.S. Army Corps of Engineers, 2002, Engineering and 

design—Hydrographic surveying manual: U.S. Army Corps 

of Engineers EM 1110–2–1003, 510 p.

U.S. Geological Survey, 2013, Ithaca East, NY: U.S. 

Geological Survey US Topo 7.5-minute map, 1 sheet, 

scale 1:24,000.

Wernly, J.F., Zajd, H.J., Jr., and Coon, W.F., 2016, Geospatial 

dataset of bathymetric survey of lower Sixmile Creek 

reservoir, Ithaca, New York, 2015: U.S. Geological Survey 

data release, accessed September 2016 at http://dx.doi.

org/10.5066/F7G15Z0S.

Wilson, G.L., and Richards, J.M., 2006, Procedural 

documentation and accuracy assessment of bathymetric 

maps and area/capacity tables for small reservoirs: U.S. 

Geological Survey Scientific Investigations Report 2006–

5208, 24 p.

For additional information write to:

Director, New York Water Science Center

U.S. Geological Survey

30 Brown Road

Ithaca, NY 14850

Information requests:

(518) 285-5602

or visit our Web site at:

http://ny.water.usgs.gov

Publishing support by: 

The Pembroke Publishing Service Center
Wernly and others—Bathymetric Survey and Estimation of Storage Capacity of Lower Sixmile Creek Reservoir, Ithaca, New York—Open-File Report 2016–1157

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20161157