U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5255

Prepared in cooperation with the International Joint Commission, 

International St. Croix River Watershed Board

A Digital Terrain Model of Bathymetry and Shallow-Zone 

Bottom-Substrate Classification for Spednic Lake 

and Estimates of Lake-Level-Dependent Habitat 

to Support Smallmouth Bass Persistence Modeling
Cover.  U.S. Geological Survey boat on the shore of Spednic Lake, November 2009.
A Digital Terrain Model of Bathymetry and 

Shallow-Zone Bottom-Substrate Classification 

for Spednic Lake and Estimates 

of Lake-Level-Dependent Habitat to 

Support Smallmouth Bass Persistence Modeling

By Robert W. Dudley, Charles W. Schalk, Nicholas W. Stasulis, and Joan G. Trial

Prepared in cooperation with the International Joint Commission,

International St. Croix River Watershed Board

Scientific Investigations Report 2010–5255

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Dudley, R.W., Schalk, C.W., Stasulis, N.W., and Trial, J.G., 2011, A digital terrain model of bathymetry and 

shallow-zone bottom-substrate classification for Spednic Lake and estimates of lake-level-dependent habitat 

to support smallmouth bass persistence modeling: U.S. Geological Survey Scientific Investigations Report 2010–5255, 

18 p.
iii

Acknowledgments

The authors thank Mike Laitta, International Joint Commission, Dan Walters, U.S. Geological Survey,

Laura Flight, U.S. Geological Survey, Rory Saunders, National Oceanic and Atmospheric Administration,

Jon Hortness, U.S. Geological Survey, Lee Sochasky, St. Croix International Waterway Commission,

Lance Wheaton, Maine Guide, Larry Day, proprietor Palfrey Lake Lodge, Peter Cronin, New Brunswick

Department of Natural Resources, Fred Kircheis, Maine Atlantic Salmon Commission – retired, and

Richard Jordan, Maine Department of Inland Fisheries and Wildlife – retired, for their valuable input

and assistance with this project.

v

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Description of Spednic Lake...............................................................................................................3

Hydrography .................................................................................................................................3

Surficial Geology..........................................................................................................................3

Climate...........................................................................................................................................3

Methods of Study...........................................................................................................................................4

Data Collection......................................................................................................................................4

Post-Processing of Echosounder Data.............................................................................................4

Post-Processing of ADCP Data ..........................................................................................................4

GIS Construction of the Digital Terrain Model of Bathymetry.......................................................5

Interpretation of Bottom-Substrate Types........................................................................................5

Bathymetry and Shallow-Zone Bottom-Substrate Classification for Spednic Lake

and Estimates of Lake-Level-Dependent Habitat .....................................................................10

References Cited..........................................................................................................................................17

Figures

1-3. Maps showing:

1. Spednic Lake and St. Croix River Basin...........................................................................2

2. Spednic Lake and executed tracklines............................................................................6

3. Acoustic data points (tracklines) for Spednic Lake.......................................................7

4. Graphs showing, A, linear regression used to normalize intensity as a

function of depth and B, plot of residuals.................................................................................8

5.  Flowchart illustrating the logistic classification process....................................................10

6–8.  Maps showing:

6. Point-classifications of sediment types in Spednic Lake ...........................................11

7.  Bathymetry of Spednic Lake............................................................................................12

8.  Sediment-types of Spednic Lake ....................................................................................13

9.  Time series plot of A, lake level range and B, C, annual historical

quantities of habitat during the spawning period (May 20–June 20)

in Spednic Lake, 1970-2009........................................................................................................16

Tables

1.  Annual historical quantities of habitat in Spednic Lake, 1970-2009...................................15
vi

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

hectare (ha) 2.471 acre

square kilometer (km2

) 247.1 acre

square meter (m2

) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2

) 

square kilometer (km2

) 0.3861 square mile (mi2)

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

meter per second (m/s) 3.281 foot per second (ft/s) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Pressure

kilopascal (kPa) 0.1450 pound per square inch (lb/ft2

) 

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

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
A Digital Terrain Model of Bathymetry and Shallow-Zone 

Bottom-Substrate Classification for Spednic Lake and 

Estimates of Lake-Level-Dependent Habitat to Support 

Smallmouth Bass Persistence Modeling

By Robert W. Dudley1, Charles W. Schalk1, Nicholas W. Stasulis1, and Joan G. Trial2

Abstract

In 2009, the U.S. Geological Survey entered into a cooperative agreement with the International Joint Commission, 

St. Croix River Board to do an analysis of historical smallmouth bass habitat as a function of lake level for Spednic Lake 

in an effort to quantify the effects, if any, of historical lakelevel management and meteorological conditions (from 1970 

to 2009) on smallmouth bass year-class failure. The analysis 

requires estimating habitat availability as a function of lake 

level during spawning periods from 1970 to 2009, which is 

documented in this report. Field work was done from October 19 to 23, and from November 2 to 10, 2009, to acquire 

acoustic bathymetric (depth) data and acoustic data indicating 

the character of the surficial lake-bottom sediments. Historical 

lake-level data during smallmouth bass spawning (May-June) 

were applied to the bathymetric and surficial-sediment type 

data sets to produce annual historic estimates of smallmouthbass-spawning-habitat area. Results show that minimum 

lake level during the spawning period explained most of 

the variability (R2 = 0.89) in available spawning habitat for 

nearshore areas of shallow slope (less than 10 degrees) on the 

basis of linear correlation. The change in lake level during the 

spawning period explained most of the variability (R2 = 0.90) 

in available spawning habitat for areas of steeper slopes (10 to 

40 degrees) on the basis of linear correlation. The next step in 

modeling historic smallmouth bass year-class persistence is to 

combine this analysis of the effects of lake-level management 

on habitat availability with meteorological conditions.

Introduction

Spednic Lake is a large, undeveloped lake on the 

United States-Canadian border in the headwaters of the 

St. Croix River Basin (fig. 1); the lake straddles the international border comprising the eastern border of the 

State of Maine and the western border of the province of 

New Brunswick. Spednic Lake is renowned for its smallmouth 

bass and landlocked salmon fishery (New England Forestry 

Foundation, 2010; Maine Department of Inland Fisheries 

and Wildlife, 2010). 

The smallmouth bass (Micropterus dolomieu) population (likely introduced sometime before 1900) precipitously 

declined around the early 1980s. The cause of the population 

decline is disputed and may have been a result of the presence 

of alewife (Alosa psuedoharengus), lake-level management, 

and (or) recreational fishing. In response to the smallmouth 

bass population decline, alewives were denied access to the 

lake, and lake-level and fishery management practices were 

altered. In May 1995, the Maine State Legislature passed a bill 

to prevent alewife migration in the St. Croix River, thereby 

elevating the alewife access issue to international importance. 

The smallmouth bass population has since rebounded and 

is presently (2010) supporting an active bass fishery (Maine 

Department of Inland Fisheries and Wildlife, 2010; International Joint Commission, 2005).

The International Joint Commission (IJC) was formed in 

1912 as a result of the Boundary Waters Treaty Act (effective 

1910). The IJC was invested with the authority to regulate 

flows and water levels for United States-Canadian boundary 

waters, which include the St. Croix River (Flagg, 2007). 

The IJC created the International St. Croix River Board 

(ISCRB) to supervise compliance with IJC Orders of Approval 

that guide the regulation of flows and water levels at hydraulic structures in the St. Croix Basin. In 2007, the Board was 

renamed the International St. Croix River Watershed Board, 

reflecting the watersheds-based approach of the IJC’s 

International Watersheds Initiative. As part of its responsibilities, the ISCRWB keeps the IJC informed about transboundary 

issues relating to the ecological health of the St. Croix River. 

The ISCRWB has identified information gaps and has recommended studies that would develop information to help fishery 

managers in both countries resolve the alewife access issue 

(International Joint Commission, 2005).

1

U.S. Geological Survey

2

Maine Department of Marine Resources, Bureau of Sea Run Fisheries and Habitat
Spednic 

Lake

Grand 

Lake

Big Lake

West 

Grand 

Lake Sysladobsis 

Lake

Junior 

Lake

Grand 

Falls 

Flowage

CANADA

New Brunswick

UNITED STATES

Maine

01AR010 Spednic Lake at St Croix, 

 New Brunswick

01018500 St. Croix River at 

 Vanceboro, Maine

68°

68°

67°15'

67°15'

45°45' 45°45'

45°15'

45°15'

EXPLANATION

Basin boundary

Surface water

International boundary

Gaging station

St Croix

River

St Croix

Basin

CANADA

Quebec

New Brunswick

UNITED STATES

Maine

Bay of Fundy

0 5 10 20 MILES

0 5 10 20 KILOMETERS

Base from U.S. Geological Survey digital files,

1:24,000 scale, Universal Transverse Mercator 

projection, zone 19

2 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

Figure 1. Spednic Lake and St. Croix River Basin.
Introduction 3

In 2009, the U.S. Geological Survey (USGS) entered 

into a cooperative agreement with the IJC to do an analysis 

of historical smallmouth bass spawning habitat as a function of lake level (lake water-surface elevation) for Spednic 

Lake in an effort to quantify the effects, if any, of historical 

lake-level management and meteorological conditions (from 

1970 to 2009) on the survival of smallmouth bass in the first 

summer of life. Study objectives included: deriving lake-level 

smallmouth bass habitat area curves; quantifying available 

smallmouth bass habitat using bathymetry and bottom character data; evaluating meteorological conditions (specifically 

temperature) during smallmouth bass spawning (May 20 to 

June 20), by year, for the historical period 1970 to 2009; and 

predicting the effects of spawning habitat availability and 

thermal conditions on survival of smallmouth bass in the first 

summer of life (May to August) for the same historical period.

Purpose and Scope

The purpose of this report is to document the data and 

methods for deriving smallmouth bass spawning habitat area 

curves and quantifying habitat as a function of lake level. 

Estimating habitat availability as a function of lake level 

requires acoustic bathymetric (depth) data and acoustic data 

indicating the character of the surficial lake-bottom sediments. 

This report describes the collection of acoustic echosounder 

and Doppler current profiler (ADCP) data and documents the 

processes used to check the quality of and analyze these data. 

The analyses yielded bathymetric and surficial lake-bottom 

sediment data sets suitable for building a digital terrain model 

(DTM) of the bathymetry and a map of the spatial distribution of surficial-sediment types in Spednic Lake. Historical 

lake-level data during smallmouth bass spawning (May 20 to 

June 20) were used to filter the bathymetric and surficial-sediment type data based on depth, substrate, and slope to produce 

annual estimates of smallmouth bass spawning habitat area.

Description of Spednic Lake

Hydrography

 Spednic Lake has a surface area of about 6,970 hectares 

(ha) and about 170 kilometers (km) of shoreline and is part of 

the St. Croix River Basin (fig. 1; University of Maine, 2010a). 

Outflow from the lake into the St. Croix River is controlled 

by a dam at Vanceboro. The dam was originally a timber crib 

structure built in 1836 that raised the natural lake level 4.6 

meters (m); the dam was replaced in 1965 with a concrete 

structure outfitted with two tainter gates capable of controlling 

lake level over a range of 4.38 m (Flagg, 2007). Lake level 

at the dam has been gaged by the Water Survey of Canada at 

station 01AR010 from January 19, 1967, to the present (2010). 

Mean lake level for the period of record ending December 31, 

2008, was 116.27 m; the minimum recorded lake level was 

113.89 m on February 10, 2002; the highest recorded lake 

level was 117.77 m on May 1, 1973 (Environment Canada, 

2010).

Flow in the St. Croix River at Vanceboro has been gaged 

by USGS streamflow-gaging station 01018500, maintained by 

the United States under agreement with Canada and currently 

operated in cooperation with the IJC from October 22, 1928, 

to the present (2010). Gaging an upstream drainage area of 

1,070 square kilometers (km2), the gage has measured monthly 

mean flows ranging from 13.4 cubic meters per second (m3/s) 

(November) to 27.8 m3/s (May), and mean annual streamflow of 20.6 m3

/s for the period of record ending September 

30, 2009 (U.S. Geological Survey, 2010a). The minimum 

recorded daily mean flow was 0.05 m3/s on October 12, 22, 

and November 4, 1936, when the flow was held back by a 

cofferdam to facilitate dam repairs; the highest recorded daily 

mean flow was 191.6 m3/s on June 3, 1984 (U.S. Geological 

Survey, 2010b). 

Surficial Geology

Spednic Lake lies in a region glaciated until approximately 11,000 to 13,000 years ago when the glacial ice 

retreated in a pattern generally parallel to the present-day 

coastline of Maine and New Brunswick (Randall, 2001; 

Hunter and Smith, 2001). This hydrophysiographic region is 

characterized by moderate to low relief, with commonly small 

or shallow closely spaced valleys, and coarse-stratified drift 

sparsely distributed mostly in eskers (Randall, 2001). As a 

consequence of glacial advance and retreat across this area, 

most surficial geology in the Spednic Lake region is compact 

glacial sediment (till) comprised of a heterogeneous mix of 

sand, silt, clay, stones, and boulders eroded and transported by 

glacial movement (Thompson and Borns, 1985; Marvinney 

and Thompson, 2000). 

Climate

The climate of the Spednic Lake Basin is temperate with 

mild summers and cold winters. The mean annual air temperature at Vanceboro (elevation 128 m) from 1971 to 2000 was 

about 5 degrees Celsius (°C), with mean monthly air temperatures ranging from about -10 o

C in January to about 19 oC 

in July (National Oceanic and Atmospheric Administration, 

2002). Mean annual precipitation during the same 30-year 

period was approximately 1,120 millimeters (mm), which 

was fairly evenly distributed throughout the year (National 

Oceanic and Atmospheric Administration, 2002). Mean 

annual evapotranspiration (loss of water to the atmosphere by 

evaporation from the soil and transpiration from plants) from 

1951 through 1980 was about 430 mm (Randall, 1996). Measured mean annual runoff from 1929 through 2009 was about 

610 mm (U.S. Geological Survey, 2010b).
4 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

Methods of Study

The following sections of the report document the collection and interpretation of data used for bathymetric mapping and classifying surficial lake-bottom sediments, and the 

geographic-information system (GIS) analysis of those data 

to estimate quantities of smallmouth bass habitat as a function 

of lake level. Linear correlations were done using the Pearson 

product-moment correlation coefficient. The limitations of 

these data and their interpretation also are briefly discussed.

Data Collection

Publicly available bathymetric data for Spednic Lake 

were identified and acquired from United States (University of Maine, 2010b) and Canadian sources (Nelda Craig, 

New Brunswick Department of the Environment, Sciences and 

Reporting Branch, written commun., 2009). Data from both 

sources were based on a 1954 survey by the Maine Department of Inland Fisheries and Wildlife. These data were not 

usable for this study because the elevation of the water surface 

at the time of the survey was not recorded, and the horizontal 

positioning of the depth data was estimated, not precisely 

located. However, the data set did provide coarse quality 

assurance for bathymetric data collected for this study.

Bathymetric data collection was done from October 19 

to 23, and from November 2 to 10, 2009. Data collection was 

done using a 200 kilohertz (kHz) Navisound 210 echosounder 

and ADCP operating at 1200 kHz, thereby producing two 

concurrent bathymetric data sets for mutual backup, quality 

assurance, and to facilitate interpretation of bottom type. The 

echosounder and ADCP units were used for the bathymetric 

survey to collect a high density of precisely located depth 

data in a relatively short period of time. On a few occasions 

because of logistics or field conditions (extremely shallow 

conditions and hazardous navigation), only the ADCP was 

used. Survey tracklines were planned using HYPACK – a 

hydrographic surveying software package used for data collection and processing of single beam, multibeam, and side-scan 

sonar data. Echosounder data were logged in HYPACK with 

time tags and synchronized with a Hemisphere A100 globalpositioning system (GPS) location referenced to State Plane 

North American Datum of 1983 (NAD 83), ME-1801 Maine 

East US Survey Foot datum; vertical units equal horizontal 

units in US Survey Foot. Approximately 332 km of survey 

tracklines were run (fig. 2). ADCP data were logged in Teledyne RD Instrument’s WinRiver II software with time and 

synchronized to the identical string of GPS data. 

Speed of sound was measured and verified (1,500 meters 

per second (m/s)) using manual tape-downs (a sounding 

weight affixed to a reel-retractable steel tape), and the echosounder and ADCP were configured to these conditions. The 

echosounder was configured on the basis of the depth of the 

transducer in the water and a user-specified value for the 

speed of sound. The ADCP was configured on the basis of the 

depth of the transducer in the water and the ADCP computed 

the speed of sound for each vertical profile on the basis of 

the water temperature and salinity measured at the transducer 

face. The ADCP unit also was configured to maximize data 

collection for interpreting surficial bottom-sediment type to a 

depth of approximately 5.0 m, though depth readings from the 

ADCP were collected to a maximum depth of approximately 

9.8 m. Interpretation of surficial bottom-sediment type was 

limited to 5.0 m to capture the range of depths at which bass 

spawning habitat was most likely to occur over a range of 

lake-level elevations.

Manual tape downs, bottom observations, probing, and 

sampling were done for assembling a data set suitable for 

interpreting surficial lake-bottom sediment types from the 

acoustic data (ground truthing), and quality assuring collected bathymetric data. Fifty-seven observations, probes, or 

samples were done over the course of the survey to document 

various surficial lake-bottom sediment types. Ten manual 

tape downs were collocated within 10 m of echosounder data 

and 11 manual tape downs were collocated within 10 m of 

ADCP data over the course of the survey to confirm that the 

echosounder and ADCP were reading depths accurately. The 

mean difference between 10 tape downs and echosounder data 

was -0.01 m (0.14 m standard deviation). The mean difference 

between 11 tape downs and ADCP data was -0.52 m (0.88 m 

standard deviation).

Depth data were converted to bottom-elevation data on 

the basis of lake level as datum. Daily gaged lake-level data 

were obtained from the Water Survey of Canada (Environment 

Canada, 2010).

Post-Processing of Echosounder Data

Field data were loaded into HYPACK software (version 

9.0 9.1.0.0) for post-processing. A levels-corrections file was 

created using the Manual Tides module. The levels correction file was used to convert depth data to bottom elevations 

on the basis of lake-level time-series data. Echosounder data 

were post-processed on a transect-by-transect basis using the 

Single Beam Editor module. Erroneous readings (obviously 

unrealistic spikes) were censored. Noise was censored from 

the data using the filter function with spike limit set to 0.2 and 

gate step set to 2.0. Following censoring, the bottom-elevation 

data were smoothed using the default 64-sample averaging 

and output from HYPACK for subsequent GIS processing for 

bathymetric mapping (fig. 3).

Post-Processing of ADCP Data

ADCP data were loaded into the Teledyne RD Instruments WinRiver II software for post processing. ADCP data 

were post-processed on a day-by-day basis. For each day, 

latitude, longitude, and average depth of the four beams 
Methods of Study 5

were output from WinRiver II; the depth data were adjusted, 

using a similar levels-correction file as that used for the 

echosounder post-processing, to convert the ADCP depth 

data to bottom elevations on the basis of lake-level time-series 

data. The resulting bottom-elevation data files were imported 

into GIS for bathymetric mapping (fig. 3). A second output 

template was created in the WinRiver II software to output 

all parameters required for interpretation of bottom substrate 

type. The parameters used for determining bottom substrate 

are discussed in the section, “Interpretation of Bottom 

Substrate Type.” 

GIS Construction of the Digital Terrain Model of 

Bathymetry

Echosounder data were the primary bathymetric record; 

ADCP data were secondary. Echosounder data were used for 

91 percent of the transect distance surveyed (302 of 332 km); 

ADCP data were used for the remaining 9 percent (30 of 

332 km) —predominantly in extremely shallow water. The 

median difference between ADCP and echosounder depths 

was +0.08 m (N = 314,272 depth data points within 0.2 m 

along 302 km of common transect lines). Differences in depths 

recorded by the echosounder and the ADCP were functions 

of how the hardware operated. The echosounder measured 

depths using a single transducer configured vertically while 

the ADCP measured depth as an average of four different 

transducers pointed out at 20-degree angles from the vertical; 

thus, depth readings in areas with a highly irregular bottom 

(boulders) could vary greatly at any given point between the 

ADCP and the echosounder. The median difference in depths 

(echosounder depth – ADCP depth) is +0.08 m; because the 

echosounder and ADCP operate using different frequencies, 

the echosounder acoustic signal likely penetrated muddy 

sediments deeper than the ADCP acoustic signal, which may 

explain why the median difference in depths is positive. 

The echosounder and ADCP data were 10-percent subsampled because of the high density of data available. The 

echosounder and ADCP were operated in continuous mode, 

providing (after initial processing) more than 4.2 million discrete depth points along the 332 km surveyed, an average of 

one depth sounding every 8 centimeters (cm). Vertical changes 

over the average 8-cm interval were usually negligible (less 

than 0.03 m), indicating that many of the data points were 

superfluous. Consequently, every tenth data point was retained 

for use in the bathymetric analysis. This subsample provided a 

good balance between analytical accuracy and computational 

speed. This subsample was used to create point data with 

northing, easting, and elevation attributes that were subsequently processed in GIS. Horizontal coordinates were in the 

Universal Transverse Mercator (UTM) projection, NAD 83. 

Vertical elevations were measured from the North American 

Vertical Datum of 1988 (NAVD 88). Surveyed edge points, 

particularly in areas where accurate orthoimagery of the lake 

boundary was unavailable, and locations of manual tape downs, 

bottom observations, probing, and sampling were included in 

the GIS processing. In all, nearly 434,000 points were used to 

create the digital terrain model (DTM).

Several data layers were used to constrain the creation of 

the DTM. Lakeshore boundaries were digitized from 2-m aerial 

photography dated July 25, 2007, and assigned an elevation 

equal to the mean lake level on that day, 116.799 m; this was 

the highest lake level for which good controls were available. 

Contours on the bathymetric surface were drawn at seven intermediate elevations ranging from 100 m to 113.4 m on the basis 

of the echosounder and ADCP data. Other line features were 

added to incorporate shallow water observations in areas where 

the boat could not go.

The maximum historical lake level was 117.77 m, but no 

aerial imagery of the lake at that elevation was available. To 

include the near lakeshore land-surface features in the DTM, 

contours at elevations of 116.8 m, 117.6 m, and 118.4 m were 

drawn from 30-m digital elevation model (DEM) data. However, because of the coarseness of these DEM data, the contours 

did not agree with the digitized lakeshore boundary. Consequently, the contours derived from the DEM were manually 

adjusted to the lakeshore boundary with the intent of preserving the slope, as indicated by the positions of the three contour 

lines, as near as possible. These land-surface contours, with 

the point and line features described above, were used to build 

the DTM of elevation data and a second model of slope data at 

9-m resolution. Bottom substrate data, described below, were 

combined with field observations of the shoreline (everywhere 

being rock-gravel) to derive polygons of bottom-substrate 

types. 

Interpretation of Bottom-Substrate Types

Surficial bottom-sediment types were classified using 

a logistic regression, which used one or more explanatory 

variables to estimate the discretization of the response variable 

into two or more nominal response categories. An independent regression was derived for each possible contrast, thus 

for n response categories, there were n-1 logistic regressions 

derived (Helsel and Hirsch, 2002); for this study, three logistic 

regression equations defined four surficial bottom-sediment 

categories.

The logistic regressions were derived on the basis of 

ADCP and visual sediment category data collected at 38 out of 

57 bottom-sediment ground-truth points in areas of the lake less 

than 5 m deep. About one-third of the bottom-sediment groundtruth points (the remaining 19 points) were reserved to validate 

the logistic regressions. The average of the ADCP data at each 

observation point and 10 readings before and after it along a 

transect (21 total readings averaged together) was calculated 

for each of the 38 ground-truth points. Twenty-one readings 

were used at each ground-truth point to make use of the high 

density of data in the vicinity of each point and to ensure that 

the bottom intensity data were representative of the point. 
New Brunswick

Maine

CANADA

UNITED STATES

SpednicLake

UNITESTATE

Maine 

EXPLANATION

International boundary

Lake boundary

Trackline

0 1 2 4 MILES

0 1 2 4 KILOMETERS

45°42'30" 45°42'30"

45°37'30" 45°37'30"

67°37'30"

67°37'30"

67°30'

67°30'

Base from U.S. Geological Survey digital files,1:24,000 scale, Universal Transverse Mercator projection, zone 19

Figure 2.  Spednic Lake and executed tracklines.
New Brunswick

Maine

CANADA

UNITED STATES

S p e d n i c L a k e

UNITESTATE

Maine 

EXPLANATION

International boundaryLake boundary

Acoustic Doppler Current Profiler or Echosounder elevation, in meters above North American Vertical Datum of 1988

98.38−106.0106.01−109.1109.11−111.5111.51−113.5113.51−116.01

0 1 2 4 MILES0 1 2 4 KILOMETERS

45°42'30" 45°42'30"

45°37'30" 45°37'30"

67°37'30"

67°37'30"

67°30'

67°30'

Base from U.S. Geological Survey digital files,scale 1:24,000, Universal Transverse Mercatorprojection, zone 19

Figure 3. Acoustic data points (tracklines) for Spednic Lake.
RESIDUAL, IN COUNTS

DEPTH, IN METERS

0 1 2 3 4 5

-80

-60

-40

-20

80

40

60

20

0

0 1 2 3 4 5

DEPTH, IN METERS

INTENSITY, IN COUNTS

INTENSITY= -4.49 (DEPTH) + 158.82

Adjusted coefficient of determination, R = 0.09 2

50

100

150

200

250

0

8 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

The four surficial bottom-sediment categories were: 

1. Rocky (R)—The coarsest surficial bottom-sediment 

types observed in the study area: boulders, bedrock, and large 

cobbles.

2. Gravelly (G)—Coarse surficial bottom-sediment types: 

small cobbles and sand and gravel 

3. Muddy (M)—Fine surficial bottom-sediment types: 

mud and silt.

4. Vegetated (V)—This classification was included because 

the acoustic data collected in areas with aquatic vegetation often 

resembled a rocky bottom type because of irregular acoustic 

reflections from the vegetation; it was thus necessary to derive 

specific methods to identify this bottom type to differentiate 

it from rock. Field observations noted vegetated areas most 

commonly coincided with muddy or sandy surficial bottomsediment types. In the final classification of surficial bottomsediment types for mapping, all types classified as vegetated 

(V) were converted to muddy (M).

Because intensity of the reflection of sound from the lake 

bottom was one of the explanatory variables, it was first necessary to normalize all ADCP intensity readings as a function 

of depth, because intensities decreased with depth. Average 

intensity values in the vicinity of the water-sediment interface 

(lake bottom) were normalized to depth using a linear regression (eq. 1, fig. 4) derived from a 5-percent random sampling of 

all bottom intensity readings collected over the entire survey in 

areas less than 5 m deep (n= 12,155 data points). The sample, 

on the basis of computer-generated random-numbers, represented all days of data collection and yielded a manageable data 

set with which to derive the normalization regression equation. 

For the data collected in less than 5 m of water, a linear relation between depth and intensity seemed to yield a satisfactory 

regression equation (adjusted coefficient of determination, 

R2 = 0.09; p-value less than 0.0001; residual standard error: 

15.05) evaluated on the basis of linearity and distribution of 

residuals (Helsel and Hirsch, 1992).

Figure 4. A, linear

regression used to

normalize intensity as a

function of depth and B,

plot of residuals.

A

B
Methods of Study 9

	 	 Ina

Ia = (b–aD) 		 (1)

where

I

na is the normalized average intensity in the 

vicinity of the lake bottom (plus or minus 

0.40 m), in counts;

Ia is the raw ADCP intensity in the vicinity of 

the lake bottom, in counts;

D is the water depth, in meters; and

b, a are derived regression coefficients.

Following normalization of intensity data as a function 

of depth, several explanatory variables for deriving logistic 

regressions were computed from the ADCP record. After 

evaluating several potential variables, the following had the 

greatest explanatory power: depth variance (eqs. 2 and 3), 

average bottom intensity, local bottom variation, and intensity 

ratio (eq. 4). The depth variance was the computed variance 

in depth among the four acoustic transducers of the ADCP; 

a smooth, featureless bottom would have a low variance, and 

a rocky, irregular bottom would have a high variance. Average 

bottom intensity was computed as the average bottom intensity from transducer number one in the vicinity of the lake 

bottom (plus or minus 0.40 m). In this case, a single transducer was used for ease of computation and to avoid losing 

bottom-variation information by averaging four transducers 

together. Transducer number one was always oriented in the 

same location on the ADCP unit throughout the survey. Local 

bottom variation was computed as the difference in depth 

to the lake bottom from transducer number one between the 

point of interest and the depth to lake bottom from five prior 

readings. Thus it was a measure of how quickly the elevation 

of the lake bottom varied along the transect. The intensity ratio 

was the ratio of the depth at which the first intensity reading 

for transducer number one was equal to or greater than 170 

counts was encountered, divided by the total depth at the lake 

bottom. The intensity ratio effectively detected the presence 

of vegetation, and thus, differentiated between vegetated areas 

and rocky areas. 

	 	 (2)

where

Da is the average depth of the four acoustic 

transducers of the ADCP, in meters; 

D1 is the depth from transducer number one, in 

meters;

D2 is the depth from transducer number two, in 

meters;

D3 is the depth from transducer number three, in 

meters; and

D4 is the depth from transducer number four, in 

meters.

(D –Da) +(D –Da) +(D –Da) +(D –Da) 1 2 3 4

3

D v= 2 2 2 2 [ ]

where

Dv is the variance in depths recorded by the 

four acoustic transducers of the ADCP, in 

square meters. 

	 	 	 D D = 0 ratio Db(4)

where

Dratio is the ratio of the depth at which the first 

intensity reading equal to or greater than 

170 counts is encountered, divided by 

the depth at the water-sediment interface 

(lake bottom); 

D0 is the depth at which the first intensity reading 

equal to or greater than 170 counts is 

encountered, in meters; and 

Db is the depth at the lake bottom, in meters. 

The three derived logistic regressions were linear 

equations of the form:

= m(D )+n(I )+p(S)+q(D )+r v z na ratio (5)

where 

z is the logit;

S is a measure of local bottom variation, in 

meters;

 m, n, p, q are derived regression coefficients; 

and

r is the derived regression intercept.

The logit, z, was input to the logistic function (eq. 6) and 

yielded a probability, f(z) (between 0 and 1), that the ADCP 

survey point should be classified into one nominal response 

category, else it should be classified as another category 

(fig. 5). The probability cutoff used between classifications 

was 0.50.

	 	 f(z) =

(1+e )-z

1 (6)

The logistic regression equations were applied to all of 

the ADCP data to differentiate every set of 21 ADCP survey 

points into nominal response categories in the same fashion 

they were derived for the ground-truth points; thus every serial 

group of 21 values averaged together received a nominal classification of either R, G, M, or V (fig. 5). Following this initial 

classification, a smoothing function was applied to help reduce 

(D +D +D +D ) 1 2 3 4

4

D a =

(3)
10 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

Logistic regression

equation

G R

R

G or R

R or V

G V

G or M

M

Start

p<0.5 p<0.5

p ≥ 0.5 p<0.5

p ≥ 0.5 p ≥ 0.5

Figure 5.  Flowchart illustrating the logistic classification process. [R, rocky; G, gravelly; M, muddy;

V, vegetation; ≥, greater than or equal to; <, less than; ]

possible misidentification errors and produce final dominantsubordinate classification determinations (fig. 6). The smoothing function grouped ADCP points classified above into 

groups of five and returned a single dominant-subordinate 

classification point for each group. For example, if a group of 

five points had four or five ‘R’ classifications, it was classified 

as ‘R,’ but if it had three ‘R’ classifications and two ‘G’ classifications, it was classified ‘Rg’ —dominant rock, subordinate 

gravel. In the case of ties, coarser bottom types dominated 

over finer bottom types. For example, if a group of five points 

had two ‘R’classifications, two ‘G’ classifications, and one ‘M’ 

classification, it would be classified as ‘Rg.’

Logistic regression classification of surficial bottomsediment types compared favorably with the observed bottom 

types. Of the 19 observations reserved to validate the accuracy 

of the classification scheme, 74 percent were correctly classified, 5 percent were incorrectly classified, and 21 percent were 

inconclusive because the observed bottom type could not be 

definitively classified. 

Bathymetry and Shallow-Zone 

Bottom-Substrate Classification 

for Spednic Lake and Estimates of 

Lake-Level-Dependent Habitat 

The digital terrain model derived for Spednic Lake 

provided the data used to estimate smallmouth bass spawning 

habitat on the basis of depth (fig. 7), and sediment type (fig. 8), 

for two slope classifications. Smallmouth bass spawning habitat depends on lake level during the spawning period (May 20 

to June 20) and was defined using the following criteria: surficial bottom-sediment dominated by cobble, boulder or gravel 

(bottom-type classifications Rg, G, Gr, and Gm), and depths 

from 1 to 3 m (Clark and others, 1998; Clark and others, 2008; 

Saunders and others, 2002; Rejwan and others, 1999). Habitat 

area was then determined for two bottom slope classifications 

(less than 10 degrees and 10 to 40 degrees). 
New Brunswick

Maine

CANADA

UNITED STATES

Spednic Lake

UNITESTATE

Maine 

EXPLANATION

International boundaryLake boundary

Sediment type classification

Gravel, some mud or rock

Mud, some gravel or rock

Rock, some mud or gravel

0 1 2 4 MILES0 1 2 4 KILOMETERS

45°42'30" 45°42'30"

45°37'30" 45°37'30"

67°37'30"

67°37'30"

67°30'

67°30'

Base from U.S. Geological Survey digital files,scale 1:24,000, Universal Transverse Mercatorprojection, zone 19

Figure 6.  Map of Spednic Lake showing the point-classifications of sediment types
45°42'30" 45°42'30" 45°37'30" 45°37'30" 

67°37'30" 67°37'30" 67°30' 67°30' New Brunswick Maine CANADA UNITED STATES Spednic Lake LAKE BOTTOM ELEVATION, in meters above North American Vertical Datum of 1988 High : 118.4 Low : 98.428 

UNITED STATES Maine 

EXPLANATION International boundary 

0 1 2 4 MILES0 1 2 4 KILOMETERS

Base from U.S. Geological Survey digital files,scale 1:24,000, Universal Transverse Mercatorprojection, zone 19

Figure 7.  Bathymetric map of Spednic Lake.
New Brunswick

Maine

CANADA

UNITED STATES

SpednicLake

UNITESTATE

Maine

EXPLANATION

Sediment type and slope

Rock, slope < 10 degrees

Rock, slope ≥ 10 degrees

Mud, slope < 10 degrees

Mud, slope ≥ 10 degrees

Gravel, slope < 10 degrees

Gravel, slope ≥ 10 degrees

International boundary

Lake boundary

0 1 2 4 MILES0 1 2 4 KILOMETERS

45°42'30" 45°42'30"

45°37'30" 45°37'30"

67°37'30"

67°37'30"

67°30'

67°30'

Base from U.S. Geological Survey digital files,scale 1:24,000, Universal Transverse Mercatorprojection, zone 19

Figure 8.  Sediment-type map of Spednic Lake.
14 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

The bathymetric data and criteria were used to identify areas of suitable habitat during the bass nesting season 

(May 20 – June 20) on an annual basis from 1970 to 2009. 

The most important filter was to determine the areas covered with 

1 to 3 m of water for the entire spawning season. For every 

year, 1- and 3-m depths from the minimum and maximum lake 

levels were used to create inundation areas. The areas that met 

the 1- and 3-m criteria for the maximum lake level were intersected with the areas that met the 1- and 3-m criteria for the 

minimum lake level; from these inundated areas, the sum of 

area with suitable bottom substrate were classified by slope to 

estimate the location and amount of annually available habitat 

suitable for bass spawning (table 1, fig. 9).

Minimum lake level during the spawning period 

explained most of the variability (R2 = 0.89) in available 

spawning habitat for the less-than-10-degree slope category on the basis of linear correlation (fig. 9). Habitat in 

the less-than-10-degree slope category composes most of 

the near-shore areas of the lake (fig. 8). The change in lake 

level during the spawning period explained most of the 

variability (R2 = 0.90) in available spawning habitat for the 

10- to 40-degree slope category on the basis of linear correlation (fig. 9). The assumption is that the availability of bass 

spawning habitat has the potential to limit the number of nest 

sites and thus the production of young bass. Poor survival of 

smallmouth bass in their first growing season has been linked 

to changes in water level (Ploskey and others, 1996; Neves, 

1975; Clark and others, 1998) and temperature (Shuter and 

others, 1980, 1985; Finlay and others, 2001; Goff, 1985; 

MacLean and others, 1981). Thus, the next step in modeling 

historic smallmouth bass reproductive success is to combine this analysis of the effects of lake-level management on 

habitat availability with meteorological conditions (from 1970 

to 2009). 
Bathymetry and Shallow-Zone Bottom-Substrate Classification for Spednic Lake and Estimates of Lake-Level-Dependent Habitat 15

Table 1.  Annual historical quantities of habitat in Spednic Lake, 1970-2009.

Year

Lake level during spawning period, in meters Habitat area during spawning period, in hectares

Maximum Minimum Bottom slope 

less than 10 degrees

Bottom slope 

from 10 to 40 degrees

1970 117.55 117.05 883 12.7

1971 117.32 117.00 868 14.0

1972 117.56 117.38 1,170 15.3

1973 117.54 117.21 1,010 14.1

1974 117.56 117.45 1,200 15.8

1975 117.46 117.19 1,010 14.6

1976 117.58 117.40 1,170 15.3

1977 117.57 117.17 967 13.5

1978 117.43 116.49 373 8.8

1979 117.55 117.30 1,110 14.8

1980 117.04 116.75 663 14.0

1981 117.21 116.65 525 11.8

1982 117.31 116.82 677 12.6

1983 117.50 117.31 1,130 15.3

1984 117.56 117.22 1,010 14.1

1985 116.29 116.12 464 13.6

1986 117.23 116.95 829 14.3

1987 116.92 116.83 765 15.5

1988 117.14 116.85 746 14.1

1989 117.59 117.44 1,180 15.5

1990 117.53 117.21 1,010 14.2

1991 117.32 117.24 1,090 16.1

1992 117.40 117.20 1,040 15.2

1993 117.44 117.28 1,110 15.5

1994 117.35 117.16 1,100 15.1

1995 117.29 117.11 992 15.2

1996 117.48 117.36 1,180 15.8

1997 117.37 117.12 979 14.7

1998 117.32 117.11 939 15.0

1999 117.11 116.86 766 14.5

2000 117.48 117.32 1,150 15.4

2001 116.76 116.66 626 15.2

2002 117.10 116.96 873 15.4

2003 117.45 117.26 1,070 15.2

2004 116.42 116.03 387 12.0

2005 117.40 117.26 1,090 15.6

2006 117.56 116.82 616 10.6

2007 117.42 117.25 1,072 15.4

2008 117.32 117.10 979 14.9

2009 117.30 117.14 1,010 15.4
16 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

Figure 9.  Time series plot of A, lake level range and B, C, annual historical quantities of habitat during the spawning period

(May 20–June 20) in Spednic Lake, 1970–2009.

EPOLS MOTT OB, T

ATI BAH GNI N

W

A SP

LESS

THA

N 10 DEGREES, IN HECTARES

YEAR

0

200

400

600

800

1,000

1,200

1,400

SPA

W

NING HABITA

T, BOTTOM SLOPE

10–40 DEGREES, IN HECTARES

YEAR

1970 1975 1980 1985 1990 1995 2000 2005 2010

0

2

4

6

8

10

12

14

16

18

LA

KE LEVEL, IN

METERS

YEAR

115.8

116.0

116.2

116.4

116.6

116.8

117.0

117.2

117.4

117.6

117.8

1970 1975 1980 1985 1990 1995 2000 2005 2010

1970 1975 1980 1985 1990 1995 2000 2005 2010

A

B

C
References Cited 17

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18 Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat

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Prepared by the Pembroke, Lafayette, and Denver Publishing Service Centers.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Maine Water Science Center

196 Whitten Road

Augusta, ME 04330

rmlent@usgs.gov

or visit out Web site at:

http://me.water.usgs.gov
Dudley, Schalk, Stasulis, and Trial—Digital Bathymetry and Substrate Classification for Spednic Lake and Estimates of Smallmouth Bass Habitat—Scientific Investigations Report 2010–5255