U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5110

In cooperation with the Pennsylvania Department of Environmental Protection

Bathymetry and Sediment-Storage Capacity Change in Three 

Reservoirs on the Lower Susquehanna River, 1996–2008
Cover.

Photograph showing Conowingo Dam and Reservoir, south of Conowingo, Md. View is looking north. 

Photograph printed with permission of the Excelon Corporation. 
Bathymetry and Sediment-Storage 

Capacity Change in Three Reservoirs

on the Lower Susquehanna River,

1996–2008

By Michael J. Langland

In cooperation with the Pennsylvania Department of Environmental Protection

Scientific Investigations Report 2009–5110

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Langland, M.J., 2009, Bathymetry and sediment-storage capacity change in three reservoirs on the Lower Susquehanna River, 1996–2008: U.S. Geological Survey Scientific Investigations Report 2009-5110, 21 p. 
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................1

Study Area..............................................................................................................................................3

Previous Investigations........................................................................................................................3

Methods and Approach................................................................................................................................3

Office Preparation.................................................................................................................................3

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

Equipment......................................................................................................................................4

Field Methods...............................................................................................................................4

Data Analysis.........................................................................................................................................4

Quality Assurance.................................................................................................................................5

Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008.............................................5

Safe Harbor Dam and Lake Clarke.....................................................................................................8

Holtwood Dam and Lake Aldred.......................................................................................................11

Conowingo Dam and Conowingo Reservoir...................................................................................14

Sediment-Storage Capacity Change in Conowingo Reservoir 1929–2008 .........................................18

Summary........................................................................................................................................................19

Acknowledgments.......................................................................................................................................20

References Cited..........................................................................................................................................20
iv

Figures

1. Map showing location of the three reservoirs in the lower Susquehanna River

Basin ...............................................................................................................................................2

2–4. Graphs showing:

2. An example of a raw graph of the relation between depth and distance

collected and displayed in HYPACKTM in Conowingo reservoir transect

number 17..............................................................................................................................6

3. A generalized example of an edited graph of the relation between depth and

distance displayed in HYPACKTM in Conowingo reservoir transect number 17 .......6

4. Daily mean hydrograph for U.S. Geological Survey streamflow-gaging station

01578310, Conowingo Dam, Maryland, 1996–2008 .........................................................7

5–10. Maps showing:

5. Bathymetric survey transects surveyed in Lake Clarke, lower Susquehanna

River.......................................................................................................................................9

6. Bathymetry showing 10-foot depth ranges in Lake Clarke, lower Susquehanna 

River.....................................................................................................................................10

7. Bathymetric survey transects surveyed in Lake Aldred, lower Susquehanna

River.....................................................................................................................................12

8. Bathymetry showing 10-foot depth ranges in Lake Aldred, lower Susquehanna 

River.....................................................................................................................................13

9. Bathymetric survey transects surveyed in Conowingo Reservoir, lower

Susquehanna River ...........................................................................................................15

10. Bathymetry showing 10-foot depth ranges in Conowingo Reservoir, lower

Susquehanna River ...........................................................................................................16

11. Graph showing change in depth to bottom surface by transect in Conowingo

Reservoir, lower Susquehanna River, 1993 to 2008 ..............................................................17

12. Graph showing changes in vertical cross-sectional area for selected years and 

remaining sediment-storage capacity in Conowingo Reservoir, lower Susquehanna 

River..............................................................................................................................................18

Tables

1. Estimated water volume, net sediment change, and total sediment deposition in

Lake Clarke since 1931, computed using bathymetry data collected in 1996 and

2008 ................................................................................................................................................8

2. Estimated water volume, net sediment change, and total sediment deposition in

Conowingo Reservoir since 1929, computed using bathymetry data collected in

1996 and 2008...............................................................................................................................14

3. Estimated changes in remaining capacity and sediment deposition from the

Conowingo Dam to 11.5 miles upstream in Conowingo Reservoir, lower

Susquehanna River, 1929–2008 ................................................................................................19
v

Conversion Factors, and Datums

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Area

acre 4,047 square meter (m2)

acre 0.4047 square hectometer (hm2) 

acre 0.004047 square kilometer (km2)

Volume

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

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

ton, short (2,000 lb) 0.9072 megagram (Mg) 

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

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).

Abstract

The Susquehanna River transports a substantial amount 

of the sediment and nutrient load to the Chesapeake Bay. 

Upstream of the bay, three large dams and their associated 

reservoirs trap a large amount of the transported sediment and 

associated nutrients. During the fall of 2008, the U.S. Geological Survey in cooperation with the Pennsylvania Department 

of Environmental Protection completed bathymetric surveys of 

three reservoirs on the lower Susquehanna River to provide an 

estimate of the remaining sediment-storage capacity. Previous 

studies indicated the upper two reservoirs were in equilibrium 

with long-term sediment storage; only the most downstream 

reservoir retained capacity to trap sediments. A differential 

global positioning system (DGPS) instrument was used to provide the corresponding coordinate position. Bathymetry data 

were collected using a single beam 210 kHz (kilohertz) echo 

sounder at pre-defined transects that matched previous surveys. Final horizontal (X and Y) and vertical (Z) coordinates 

of the geographic positions and depth to bottom were used to 

create bathymetric maps of the reservoirs.

Results indicated that from 1996 to 2008 about 

14,700,000 tons of sediment were deposited in the three 

reservoirs with the majority (12,000,000 tons) being deposited 

in Conowingo Reservoir. Approximately 20,000 acre-feet or 

30,000,000 tons of remaining storage capacity is available in 

Conowingo Reservoir. At current transport (3,000,000 tons 

per year) and deposition (2,000,000 tons per year) rates and 

with no occurrence of major scour events due to floods, the 

remaining capacity may be filled in 15 to 20 years. Once the 

remaining sediment-storage capacity in the reservoirs is filled, 

sediment and associated phosphorus loads entering the Chesapeake Bay are expected to increase.

Introduction

Sediment, nitrogen, and phosphorus enrichment has 

adversely affected the Chesapeake Bay. Excess nutrients stimulate algal blooms that decay and consume dissolved oxygen, 

causing areas with low concentrations of dissolved oxygen in 

the bay. Algal blooms and sediment block the sunlight needed 

by submerged aquatic vegetation. Excessive sediments can 

remain in suspension also reducing sunlight and can obstruct 

fish gills. In the mid-1980s, the Chesapeake Bay Program 

(CBP), a partnership between the Commonwealths of Pennsylvania and Virginia, the State of Maryland, the District of 

Columbia, the Federal Government, and the Chesapeake Bay 

Commission, began efforts to reduce nutrients and sediments 

in the bay. Improvement in water-quality conditions in the bay 

has been slow; the bay was listed as an “impaired” water body 

under the regulatory statutes related to the Clean Water Act. 

The CBP developed water-quality criteria for the bay (U.S. 

Environmental Protection Agency, 2003) and is implementing 

measures to reduce nutrients and sediments entering the bay in 

an attempt to meet these criteria by 2010.

In a year of normal streamflow, the Susquehanna River 

contributes nearly 50 percent of the freshwater discharge to 

the Chesapeake Bay. It also transports the greatest amount of 

nutrients—estimates of nearly 66 percent of the nitrogen and 

40 percent of the phosphorus load—from non-tidal areas in 

the Chesapeake Bay Basin (Langland and others, 1995). The 

Susquehanna River in a normal-flow year also contributes 

about 25 percent of the sediment load from non-tidal areas to 

the bay (Langland and others, 1995).

A system of three reservoirs in the lower Susquehanna 

River (fig. 1) currently traps a substantial part of the phosphorus and suspended-sediment load and to a lesser extent the 

nitrogen load (Ott and others, 1991). The U.S. Geological Survey (USGS) completed bathymetry studies in the reservoirs in 

1990, 1993, and 1996 to document change in depth to bottom 

and to determine remaining sediment-storage capacity.

Purpose and Scope

For each of the three reservoirs in the lower Susquehanna River, this report (1) presents results of the bathymetric 

surveys (depth to bottom) during the fall of 2008, (2) documents the change in water and sediment-storage capacity, 

(3) provides estimates of the remaining sediment-storage 

capacity, and (4) examines long-term capacity changes since 

1929. In addition, the methods and equipment used to collect, 

estimate, and quality assure these data are summarized.

Bathymetry and Sediment-Storage Capacity Change in 

Three Reservoirs on the Lower Susquehanna River,

1996–2008

By Michael J. Langland
2 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Figure 1. Location of the three reservoirs in the lower Susquehanna River Basin.

Marietta

Wrightsville

Columbia

Washington Boro

Turkey Hill

Long Level

Urey Duncan Island

Weise Island

Safe Harbor

Islands

Holtwood

Hennery Island

Mt. Johnson Island

Peach Bottom Peach Bottom

Generating Station

Wildcat Tunnel

Troublesome Conowingo

Hills

PENNSYLVANIA

MARYLAND

Conestoga

River

Pequea Creek

Safe Harbor

Dam

Holtwood

Dam

Muddy Run

Hydroelectric Dam

Michael

Run

Broad

Creek

Conowingo

Creek

Conowingo

Dam

CONOWINGO

RESERVOIR

LA

KE

CLARKE

LA

KE ALDRED

76°30' 76°15'

39°

45'

40°

08'

0 5 10 MILES

0 5 10 KILOMETERS

PENNSYLVANIA

MARYLAND

Study area

Susquehanna 

River Basin
Methods and Approach 3

Study Area

The Susquehanna River Basin extends from south-central 

New York State near Cooperstown, N.Y., through central and 

southern Pennsylvania and northeastern Maryland, draining 

27, 510 mi2, and enters the Chesapeake Bay near Havre de 

Grace, Md. Forests cover about two-thirds of the basin and are 

concentrated in the northern and western parts of the basin. 

Agricultural land use makes up about one-quarter of the basin 

area. Most agricultural land is tilled and is predominantly 

located along the river valleys in southern New York/northern 

Pennsylvania and in southern Pennsylvania. Urban settings 

comprise about one-tenth of the basin, mostly in the southern 

part of the basin. Annual precipitation ranges from an average 

of about 34 in. in southern New York State to over 46 in. in 

areas of central Pennsylvania. The lower Susquehanna River 

Basin is defined as the area downstream from the confluence 

of the West Branch Susquehanna River near Sunbury, Pa., to 

the mouth of the river (Sprague and others, 2000).

Three hydroelectric dams are located in the lower part of 

the Susquehanna River Basin near its terminus with the Chesapeake Bay. Two of the dams (Safe Harbor and Holtwood) are 

in south-central Pennsylvania, and the third dam, Conowingo, 

is in northern Maryland (fig. 1). The most upstream dam is 

Safe Harbor Dam. Constructed in 1931, it forms Lake Clarke 

and is located 32 mi upstream from Chesapeake Bay. Lake 

Clarke extends about 9.5 mi from Safe Harbor, Pa., to Columbia, Pa., and the reservoir width ranges from 800 to 6,600 ft. 

The design water-storage capacity of Lake Clarke was 

150,000 acre-ft. The middle dam, Holtwood Dam, is about 

25 mi upstream from Chesapeake Bay and forms Lake Aldred. 

Holtwood Dam was constructed in 1910 and is the oldest of 

the three dams. Lake Aldred extends upstream from the dam 

about 8 mi to just below Safe Harbor Dam. The impoundment is relatively narrow along its entire reach (reservoir 

width ranges from 1,100 to 3,800 ft) and had an original 

water-storage capacity of 60,000 acre-ft. The largest and 

most-downstream impoundment, Conowingo Reservoir, was 

formed by the construction of Conowingo Dam in 1928. The 

dam is about 10 mi upstream from Chesapeake Bay and was 

originally designed to store 300,000 acre-ft of water. A moredetailed summary of dam and impoundment characteristics is 

provided by Hainly and others (1995).

Previous Investigations

Previous studies have documented changes in the bottomsurface profile in all three reservoirs since 1929. Surveys of 

Lake Clarke by Schuleen and Higgins (1953) reported an 

original water-storage capacity of 144,600 acre-ft in 1931, 

with the volume gradually decreasing to a remaining capacity 

of 78,800 acre-ft in 1950. Six subsequent surveys, completed 

in 1950, 1951, 1959, and 1964 for the Pennsylvania Power and 

Light Company (E.T. Schuleen, Pennsylvania Power and Light 

Company, written commun., 1965), 1990 (Hainly and others, 

1995), and 1993 (Reed and Hoffman, 1997) indicated little 

change in water-storage capacity since 1950. This indicates 

that the reservoir had reached the sediment-storage capacity 

with incoming sediment loads. In 1996, a major ice jam in the 

upper section of Lake Clarke broke and released backed-up 

water. A subsequent survey concluded that this water combined with the already high river discharge to produce a major 

scour event that increased water-storage capacity in all three 

reservoirs (Langland and Hainly, 1997).

Siltation surveys in Lake Aldred in 1939, 1950, and 1961 

were described by Ledvina (1962) and suggested the amount 

of sediment stored in the reservoir decreased over time. Subsequent surveys by the USGS in 1990 (Hainly and others, 1995) 

and 1993 (Reed and Hoffman, 1997) indicated no appreciable 

changes since 1961. Reasons for the reduction in sediment 

storage prior to 1961 included coal dredging and construction 

of the upstream Safe Harbor Dam (Reed and Hoffman, 1997).

Whaley (1960) concluded that the water-storage capacity in Conowingo Reservoir was reduced from the original 

300,000 acre-ft to about 235,000 acre-ft. Subsequent bottomsurface profiles in 1990 and 1993 indicated continuing reductions in water-storage capacity to about 169,000 acre-ft in 

1993 (Reed and Hoffman, 1997). As a result of a major scour 

event in January 1996, a survey completed in 1997 concluded 

that the remaining sediment-storage capacity in Conowingo 

Reservoir actually increased to 170,600 acre-ft (Langland and 

Hainly, 1997). For this current study, the value of 170,600 is 

rounded to 171,000 acre-ft.

Methods and Approach

To accomplish this project, several primary tasks were 

performed. These tasks included office preparation, data 

collection and analysis, and quality assurance. The following sections describe each of the tasks in more detail and the 

approach used in this study.

Office Preparation

Initially, the USGS contacted the hydroelectric dam utility owners to inform them of plans to resurvey the reservoirs. 

Geo-referenced aerial photographs of York and Lancaster 

Counties in south-central Pennsylvania showing the reservoirs 

and surrounding areas were acquired and geo-referenced for 

incorporation into navigation-support software HYPACKTM. 

Aerial photographs were not acquired for northern Maryland; 

however, the reservoir boundaries were obtained from digitized maps. HYPACKTM is a navigation software package that 

integrates the reservoir-bottom depths with the Differential 

Geo-Positional Satellite (DGPS) positional data and displays 

the location of the boat on a geo-referenced aerial photo of 

the reservoir to aid in navigation. Using latitude and longitude 

coordinates from previous surveys, the pre-existing transects were plotted within HYPACKTM. These transects were 
4 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

determined by complexities inherent in each reservoir such as 

geometry, alignment of the shore, and features such as islands 

and confluence points of tributaries. In general, the transect 

lines were perpendicular to the shore edge. Spacing between 

transects averaged 2,000 ft. The transect lines were used to 

guide the boat during the survey and improve overall boat navigation. Equipment used in the survey was assembled at the 

office and tested in the reservoirs to verify proper operation.

Data Collection

Various pieces of equipment were used to ensure accurate 

depth and geo-spatial locations of the surveys. A standard field 

method of data collection was established to maintain consistency in the bathymetric surveys among the three reservoirs.

Equipment

A brief list of the field equipment used in this study is 

presented below. Additional information can be found in Hittle 

and Ruby (2008).

• DGPS data-processing software.

• An 18-ft Boston Whaler accommodating a crew of two 

people with some protection from the elements.

• An echo sounder and transducer with an appropriate 

frequency for the depth of the reservoirs. This survey 

used a 210 kHz (kilohertz) ODOMTM transducer and a 

NavisoundTM 210 Reson echo sounder, a depth range 

from 0.6 to 600 m (Reson Inc., 2005), and thermal 

recording paper as backup.

• A TrimbleTM DGPS system and a differential correction 

service from Omni StarTM.

• A backup DGPS unit from RockwellTM.

• A laptop computer with the HYPACKTM software package installed.

• A second monitor for the boat operator to view the predefined transects, boat location, and heading, allowing 

the boat operator to follow the transects and maintain 

safe operation of the boat.

• A laser range finder with a compass module enabling 

determination of azimuth and distance measurements 

from the boat to the shoreline.

Field Methods

The staff required to collect the bathymetry and georeferenced position data consisted of two people: the boat 

operator and the equipment operator. The ODOMTM transducer was mounted on the port (left) side of the boat with the 

DGPS antenna mounted directly above the transducer. Each 

day before starting, an offset corresponding to the depth of 

the transducer in the water was measured and entered into the 

NavisoundTM. The laser range finder was used to make the 

shoreline measurements from as close to the transducer position as possible to correlate to the boat position.

Data collection in each reservoir followed the pre-defined 

transect lines created in HYPACKTM to ensure the data collected would closely correspond to the previously collected 

bathymetry data (Langland and Hainly, 1997). The procedure 

for each of the pre-defined transect lines included (1) the boat 

was held stationary at one end of the transect near the shore 

while the laser range finder measured the distance and azimuth (bearing) reading to the shoreline, (2) the echo sounder 

(depth) and DGPS (position) were started and simultaneously 

recorded within HYPACKTM, (3) the boat proceeded to the 

opposite end of the transect and was stopped near the shore 

where distance and bearing readings were again recorded, and 

(4) the recording was stopped and the boat proceeded to the 

next transect to repeat the process. If any gaps or problems in 

the data were identified after the transect was completed, the 

transect was rerun. The completed transects for each reservoir 

are presented on figures later in the report.

Data Analysis

The methods for the analysis of the bathymetry data and 

subsequent estimation of remaining sediment-storage capacity 

are documented in Langland and Hainly (1997). In summary, 

cross-sectional profile data were collected in the reservoirs on 

at least nine occasions from 1939 to 1996. The most recent 

surveys (1990, 1993, and 1996) and new profile data collected 

in 2008 were used to produce bathymetry (depth to bottom) 

contour maps, one for each reservoir, and provide a new estimate of remaining reservoir capacity to store sediment. The 

majority of the cross sections profiled in 1993 and 1996 were 

re-profiled in 2008 to document elevation change.

Water-level elevations fluctuate due to hydroelectric 

activity. One-half to one hour interval water-level elevations 

were obtained from each of the dam operators, adjusted to the 

normal lake level elevations, and prorated over the time of 

each transect. This insured that the bathymetry was referenced 

to the same pool elevation levels used in previous reports. 

Bottom-elevation contours in each reservoir were developed 

from the 2008 data by plotting the cross-sectional water-depth 

data and connecting lines of equal depth using Geographic 

Information System (GIS) software and manual interpretation. The resultant bathymetry maps have a contour interval of 

10 ft; therefore, sediment deposition and sediment scour less 

than 10 ft are not shown.

 Changes in water and sediment-storage capacity and 

sediment mass were estimated in Lake Clarke, Lake Aldred, 

and Conowingo Reservoir by computing the change in water 

volume. Using the mid-point method, cross-sectional length 

multiplied by width between adjacent cross sections multiplied 
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 5

by the average depth equaled the volume. By using the same 

transects as in previous studies, the cross-sectional area was 

the same. Therefore, changes in average depth from the 1996 

and 2008 surveys indicated the change in water volume. Using 

a normalized density of dry sediment of 67.8 lb/ft3, as suggested by Reed and Hoffman (1997), multiplied by the change 

in volume, a change in sediment mass for a cross section was 

estimated.

The estimated change in sediment mass based on 

bathymetry also was compared to the overall load entering and 

leaving the reservoirs to provide an overall estimate of “error” 

and some verification of the change in sediment mass in the 

reservoirs. The sediment loads entering and leaving the reservoirs were estimated from water-quality and discharge data 

from two streamflow-gaging stations, the Susquehanna River 

at Marietta (USGS 01577600) and the Conestoga River at 

Conestoga (USGS 01576757) using the USGS ESTIMATOR 

software program (Cohn and others, 1989) and comparing the 

estimated loads to the Susquehanna River at Conowingo Dam 

(USGS 01578310) for multiple time periods. This provided 

estimated changes in storage capacity, average riverine sediment transport, reservoir trapping efficiency, and deposition 

rates.

Quality Assurance

Even though a complete analysis of the accuracy of the 

vertical (elevation) and horizontal (geographic position) data 

was beyond the scope of this study, every effort was made to 

minimize error. For example, the equipment and procedures 

used in the collection of the bathymetric data were based 

on methods used in other USGS bathymetric survey studies 

(Wilson and Richards, 2006; Hittle and Ruby, 2008). Several quality-assurance diagonal transects were created and 

surveyed so the depths collected at the same point from two 

different paths could be compared.

The NavisoundTM 210 Reson echo sounder has an 

accuracy of 0.06 ft at 210 kHz (Reson Inc., 2005). A built-in 

bar-check utility allowed for depth verification using a correction for the speed of sound. Following the advice of the U.S. 

Army Corps of Engineers (2002) to adequately calibrate a 

fathometer, a multi-depth bar check of the echo sounder was 

undertaken each day to ensure accurate depth soundings. The 

calibration involved lowering a 2 ft by 4 ft plate to a known 

depth of 6 ft below the transducer. If the calibration depth was 

sensed correctly, the speed-of-sound setting was verified. If 

the calibration depth was sensed incorrectly, the echo sounder 

was calibrated to the correct depth. In this study, the echo 

sounder always reported the correct depths for the bar check. 

The precision of the bar checks was affected by wave action 

on the reservoirs. The vertical accuracy of the echo sounder 

was conservatively estimated to be ± 0.5 ft. The echo sounder 

recorded the depth data in both analog and digital formats. A 

thermal paper recorder was used for backup to the digital data 

during office processing.

To ensure accurate coordinate-system data, two DGPS 

units were operated. One DGPS recorded and the other was 

used for quality-assurance purposes. Multiple daily field 

checks verified that the units were both within 5–10 ft horizontally. Because of hydroelectric power generation, the reservoir 

elevations could change substantially during the time when 

the data were collected. Using one-half hour or hourly interval 

water-level-elevation data collected from each hydroelectric 

facility, the bathymetry data were adjusted to the normal lakelevel elevations unique for each reservoir. To help improve the 

accuracy of the distance and bearing measurements from the 

transducer location to the shoreline point, the equipment setup 

on the boat was configured so that the transducer was mounted 

on the port side with the DGPS antenna mounted directly 

above the transducer. The operator of the laser range finder 

would make the shoreline-edge measurements from this position and the boat operator on the starboard (right) side reduced 

the roll of the boat during surveys. In addition, the equipment 

operator took detailed notes (weather, current transect number, starting and ending points, etc.) to aid in field and office 

processing of the data. 

Raw bathymetry data were processed within HYPACKTM 

to filter (1) multiple-return acoustic signals in shallow water, 

(2) erroneous DGPS signals, and (3) redundant areas (backtracking) caused by equipment limitations. Raw data were 

viewed (fig. 2) and edited (fig. 3). The edited X and Y (horizontal position coordinates) and Z (vertical depth coordinate) 

were exported into X, Y, Z data files. On the basis of the speed 

of the boat (4 mi per hour) and number of data points collected 

(10 per second), questionable data (spikes) were generally 

removed if the recorded depth of the spike was less than 

one-half of a second (approximately 5 ft of surface travel). 

The X, Y, Z data files were used for the analysis of remaining 

reservoir water and sediment-capacity change and various GIS 

applications.

Bathymetry and Sediment-Storage 

Capacity Change from 1996 to 2008

This section provides an estimate of change in sediment 

on the basis of the volume change from the 1996 and 2008 

surveys, a discussion of long-term transport to reservoirs 

to help determine the rate of capacity loss (fill-in), and an 

estimate of time remaining until the reservoir system is at 

sediment-storage capacity.

The depth-to-bottom data collected from each reservoir 

were adjusted to the normal lake-level elevations established 

by the hydroelectric power utilities. GIS was used to filter 

data points and produce maps showing contoured 10-ft water 

depths. The water depths shown on the maps for the three 

reservoirs should not be used as a boating guide. The release 

of stored water can cause lake levels to change rapidly and 

sediment bars can form in unexpected locations because of 

changes in local currents from storm to storm. In addition, the 
6 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Figure 2. An example of a raw graph of the relation between depth (vertical axis) and distance (horizontal axis) collected and 

displayed in HYPACKTM in Conowingo Reservoir transect number 17.

Figure 3. A generalized example of an edited graph of the relation between depth (vertical axis) and distance (horizontal axis) 

displayed in HYPACKTM in Conowingo Reservoir transect number 17. Erroneous data shown as spikes in figure 2 were removed. Note 

that the depth scales differ between figures 2 and 3. 

0

20

40

60

80

100

120

Bathymetry Profile (raw depth)

DISTANCE FROM WESTERN BANK, IN FEET

DEPT

H, I

N FEET

0 1,000 2,000 3,000 4,000 5,000 6,000 6,118

0

5

10

15

20

25

30

35

Bathymetry Profile (corrected depth)

DISTANCE FROM WESTERN BANK, IN FEET

DEPT

H, I

N FEE

T

0 1,000 2,000 3,000 4,000 5,000 6,000 6,118
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 7

reservoirs contain many partially or fully submerged obstacles 

such as bedrock ledges, rocks, trees, and stumps that were not 

within the transects, and were therefore not mapped.

A change in the depth to bottom elevation of a reservoir 

reflects a change in the volume of water and, therefore, the 

storage capacity of the reservoir. Changes in bottom surface 

indicate a change in the quantity of sediment caused by scour 

or deposition or represent movement in bottom sediments. 

Although major scour events increase the storage capacity in the short term, the mass of incoming sediments in the 

reservoirs fills that capacity relatively quickly, 2–4 years for 

most scour events (Langland and Hainly, 1997). The extreme 

case was Hurricane Agnes in 1972, when an estimated 

20,000,000 tons of sediment was scoured from the reservoirs 

(Gross and others, 1978) and most likely took about a decade 

to fill back in (Reed and Hoffman, 1997). In this report, 

changes in bottom-surface elevations are discussed and compared in two ways: (1) cross-sectional data collected in 1996 

and 2008 were used to estimate change in bottom elevation, 

water volume, storage capacity, and sediment mass within the 

reservoirs, and (2) net sediment-mass change estimated from 

the cross-sectional data was compared to the sediment loads 

entering and leaving the reservoirs estimated from the monitoring data.

A streamflow threshold of at least 390,000 ft3/s (Lang, 

1982) is usually required to scour sediments in the reservoirs. Color changes in the monitored sediments (from light 

brown to dark brown and black) provide an indication when 

streamflows begin to scour the darker anoxic bottom sediments. Between the 1996 and 2008 surveys, three flood events 

occurred with streamflows capable of scouring sediments in 

the reservoirs (fig. 4). On September 19 and 20, 2004, the 

daily mean streamflows at Conowingo were 495,000 and 

545,000 ft3/s, respectively. On April 4, 2005, the daily mean 

streamflow was 394,000 ft3/s, and on June 29, 2006, the daily 

mean streamflow was 403,000 ft3/s. The highest instantaneous 

streamflow (620,000 ft3/s) occurred on September 19, 2004. 

Using the nonlinear relation between streamflow and bedsediment scour (Langland and Hainly, 1997), an estimated 

Figure 4. Daily mean hydrograph for U.S. Geological Survey streamflow-gaging station 01578310, Conowingo Dam, 

Maryland, 1996–2008. The red line represents the estimated flow at which scour of bottom sediments occurs. 

0

100,000

200,000

300,000

400,000

500,000

600,000

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

WATER YEAR

DAILY MEAN FLOW, IN CUBIC FEET PER SECOND

01573810 Conowingo Dam, Maryland

390,000 cubic feet per second
8 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

2,500,000 tons of sediment was scoured from the reservoirs 

between the 1996 and 2008 surveys.

Safe Harbor Dam and Lake Clarke

Depth to bottom data were collected from 19 cross sections in Lake Clarke (fig. 5) and were adjusted to the normal 

lake elevation of 227.2 ft above sea level. The deepest areas 

of the lake are closest to the dam and range from 30 to 50 ft 

(fig. 6). The old Susquehanna and Tidewater Canal forms one 

of the deeper areas in the reservoir close to the west bank, running from just upstream of the dam to Fishing Creek (transect 

number 16). The upper half of the lake is generally shallow; 

average lake depths are about 10–15 ft on the west side, with 

many islands and sand and coal bars on the east side near 

Washington Boro. When the lake level is low, much of the 

area around Washington Boro is too shallow for boating.

Schuleen and Higgins (1953) reported Lake Clarke has 

been in long-term equilibrium with sediment storage (they 

called it “sediment saturation”) except for an area from the 

dam to a distance upstream of approximately 7,000 ft by 1950. 

Subsequent surveys in 1951, 1959, and 1964 revealed this 

area was filled, and the total amount of deposited sediment 

remained constant (E. Schuleen, Pennsylvania Power and 

Light Company, written commun., 1965). Short-term changes 

have occurred since 1965 because of scour events. The 

scoured sediment was quickly replaced, returning the reservoir 

to equilibrium. Because of the lack of high scour-producing 

streamflows, net changes in bottom-elevation data from the 

1996 to 2008 survey were minimal (table 1). An estimated net 

change of 1,700,000 tons of sediment was deposited in Lake 

Clarke between 1996 and 2008. The 1996 study estimated 

2,300,000 tons of sediment were scoured and removed from 

Lake Clarke, so a net deposition was expected. The area of 

greatest amount of scour from the 1996 survey (cross-sections 

24–29) was largely filled. Total deposition of sediment in 

Lake Clarke was estimated at 92,400,000 tons in 2008, and 

the 1990 and 1993 studies estimated 90,700,000 tons. The 

difference (1,700,000 tons or 1.8 percent) was within the error 

introduced by rounding and averaging cross-sectional areas 

and depths, further indicating sediment transport in Lake 

Clarke has been in long-term equilibrium. The total amount 

of sediment upstream from cross-section 18 is estimated at 

18,700,000 tons, and the total amount of sediment from crosssection 18 to the dam is estimated at 73,700,000 tons (table 1).

Table 1. Estimated water volume, net sediment change, and total sediment deposition in Lake Clarke since 1931, computed using 

bathymetry data collected in 1996 and 2008. Cross-section locations are shown in figure 5. 

Lake Clarke

Cross-section 

number

(2008)

Distance

upstream

from dam

(feet)

Water volume (acre feet) Sediment (tons)

1996

Volume

2008

Volume

Volume

change

Deposition (+)

or scour (-)

Total deposition

since 1920

29 685 5,407 4,929 478 692,258 7,132,000

28 2,330 3,989 3,830 160 230,912 5,542,000

27 3,510 3,960 3,691 269 389,693 5,341,000

26 5,360 4,062 4,044 18 25,778 5,852,000

25 7,400 4,349 4,317 33 47,319 6,246,000

24 10,000 5,067 5,026 41 59,128 7,273,000

23 12,860 4,528 4,545 -17 -24,914 6,577,000

22 15,000 4,435 4,456 -21 -29,849 6,447,000

21 17,540 4,956 4,935 21 30,645 7,140,000

20 19,780 4,372 4,392 -20 -28,372 6,355,000

19 21,930 4,030 3,991 39 56,073 5,776,000

18 24,000 2,740 2,713 28 40,054 3,925,000

4-17 to 47,520 12,801 12,660 141 203,547 18,723,000

Total 64,698 63,529 1,170 1,692,300 92,400,000
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 9

Figure 5. Bathymetric survey transects surveyed in Lake Clarke, lower Susquehanna River.

xc-13

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CROSS SECTION AND NUMBER

xc-10 Surveyed in 1996

xc-20 Surveyed in 1996 and 2008

Lancaster County

Safe

Harbor

Dam 0 1 2 MILES

0 1 2 KILOMETERS

76°30' 76°27' 76°24'

40°

39°

57'

39°

54'

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection, Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset

EXPLANATION

COUNTY BOUNDARY

York County
10 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Figure 6. Bathymetry showing 10-foot depth ranges in Lake Clarke, lower Susquehanna River.

York County

Lancaster County

Safe

Harbor

Dam 0 1 2 MILES

0 1 2 KILOMETERS

76°30' 76°27' 76°24'

40°

39°

57'

39°

54'

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection, Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset

EXPLANATION

DEPTH, IN FEET

0 to less than 10

10 to less than 20

20 to less than 30

30 to less than 40

40 to less than 50

50 or greater

COUNTY BOUNDARY
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 11

Holtwood Dam and Lake Aldred

Depth to bottom data were collected in 2008 from 13 

cross sections in Lake Aldred (fig. 7) and adjusted to the 

normal lake elevation of 169.75 ft above sea level. In general, 

depths are shallowest in the upper part of the lake near Weis 

Island where depths range from 10 to 30 ft (fig. 8). Depths 

increased with distance downstream through the narrow gorge 

and decreased again near Holtwood Dam. Unlike Safe Harbor 

Dam, Holtwood Dam contains no flood gates, thus sediment 

can accumulate near the dam rather than exit from the bottom 

near the flood gates. The deepest areas are in spoon-shaped 

depressions, which are called “the deeps,” that are located in 

the middle and lower parts of the lake (fig. 8). When Holtwood Dam was built in 1910, a coffer dam exposed six of 

these “deeps.” Five transects (12, 19, 20, 21, and 22) reveal 

three of these deeps. The depth to the bottom of these “deeps” 

ranges from 80 to 130 ft. Although it is unknown how these 

deeps actually formed, they are all in the narrowest section of 

the reservoir, leading to the common conjecture that the deeps 

formed during the last glacial retreat 10,000 years ago when 

they were “drilled” into the bedrock by whirlpool-type action.

Previously, Reed and Hoffman (1997) estimated 

13,600,000 tons of sediment are stored in Lake Aldred, considerably less than in the other two reservoirs. The amount 

of deposited sediment has remained consistent since 1961 

(Ledvina, 1962), and subsequent surveys in 1990 and 1993 

confirmed the lake is in equilibrium in terms of deposited sediment. The 1996 survey indicated areas of scour and deposition 

but the overall change in deposited sediment in Lake Aldred 

was minimal (Langland and Hainly, 1997). The current study 

estimated the change in stored sediment since 1996 as approximately 1,000,000 tons. As previously mentioned, scourproducing flood events between the 1996 and 2008 studies 

were minimal, thus changes in water and sediment capacities 

also were minimal. With minimal change since 1996, sediment 

thickness remains less than 10 ft throughout most of the lake.
12 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Figure 7. Bathymetric survey transects surveyed in Lake Aldred, lower Susquehanna River.

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Weis 

Island

Blair Island

Duncan Island

Holtwood Dam

Safe Harbor 

Dam

York County

Lancaster County

0 1 2 MILES

0 1 2 KILOMETERS

76°24' 76°21'

39°

54'

39°

51'

EXPLANATION

COUNTY BOUNDARY

CROSS SECTION AND NUMBER

xc-8 Surveyed in 1996

xc-13 Surveyed in 1996 and 2008

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection, Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 13

Figure 8. Bathymetry showing 10-foot depth ranges in Lake Aldred, lower Susquehanna River.

Holtwood Dam

Safe Harbor 

Dam

York County

Lancaster County

0 1 2 MILES

0 1 2 KILOMETERS

76°24' 76°21'

39°

54'

39°

51'

COUNTY BOUNDARY

EXPLANATION

DEPTH, IN FEET

0 to less than 10

10 to less than 20

20 to less than 30

30 to less than 40

40 to less than 50

50 to less than 60

60 to less than 70

70 to less than 80

80 to less than 90

90 to less than 100

100 or greater

Weis 

Island

Blair Island

Duncan Island

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection, Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset
14 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Conowingo Dam and Conowingo Reservoir

Depth to bottom data were collected in 2008 from 

26 cross sections in the Conowingo Reservoir (fig. 9) and 

adjusted to the normal lake elevation of 108.5 ft above sea 

level. This bathymetry study confirmed the deepest areas of 

the reservoir are still near the dam (fig. 10). Along the spillway 

gates, which are located along the dam from the east bank to 

about two-thirds of the way across the river, depths averaged 

about 50 ft. Reservoir depths averaged about 60 ft near the turbine gates, which are located along the remaining third of the 

dam. The spillway and turbine gates are well below the normal 

water surface. The average depths were considerably less 

than in 1996 and are discussed later in this section. Additional 

deeper areas were across from the Peach Bottom Power Plant, 

an area just south of the Pennsylvania-Maryland state line and 

below the confluence with Broad Creek. These deeper areas 

probably resulted from natural hydraulic scouring caused by 

the stream reservoir shape. The shallowest areas were in the 

upper one-third of the reservoir where depths averaged about 

15 ft (fig. 10).

The reservoir was divided into three subareas to estimate 

changes in cross-sectional volumes—the lower part (crosssections 17 to 26), the middle part (cross-sections 6 to 16), and 

the upper part (cross-sections 1 to 5) (fig. 9, table 2). Crosssections 1 to 5 are not presented in table 2 because they were 

not surveyed in 1996; however, they are discussed later in 

this section. Results from the survey indicated approximately 

12,000,000 tons of sediment were deposited in Conowingo 

Reservoir between 1996 and 2008 (table 2). All the net deposition occurred in the lower part of the reservoir. Total sediment deposition in the lower part of the reservoir increased 

by 13,800,000 tons (table 2) to about 103,000,000 tons from 

1996 to 2008. Sediment depths increased in every transect in 

the lower part of the reservoir (fig. 11); the majority increased 

by 8–10 ft. The largest change in water volume (1,983 acre-ft) 

was at cross-section 26, nearest the dam (table 2).

Table 2. Estimated water volume, net sediment change, and total sediment deposition in Conowingo Reservoir since 1929, computed 

using bathymetry data collected in 1996 and 2008.

Conowingo

Cross-section

number

(2008)

Distance

upstream

from dam

Water volume (acre-feet) Sediment (tons)

1996 Volume 2008 Volume Volume

change

Deposition (+) or 

scour (-)

Total deposition

since 1929

26 1,700 15,205 13,222 1,983 2,680,300

25 3,150 10,437 10,052 385 586,900

24 5,530 11,278 9,807 1,471 2,130,120

23 7,950 7,171 5,976 1,195 1,781,480

22 9,880 6,761 5,201 1,560 2,186,230

21 12,275 5,901 4,650 1,251 1,857,460

20 14,050 5,863 5,183 679 776,430

19 16,650 7,736 6,947 789 914,470

18 19,300 6,724 6,086 638 839,350

17 21,700 7,742 7,706 36 52,400

Subtotal 13,800,000 103,000,000

16 24,400 6,723 6,254 469 452,780

15 26,850 6,306 6,159 147 212,360

14 29,450 6,710 6,812 -102 -147,590

13 33,150 6,509 6,851 -343 -496,050

12 35,800 7,616 8,178 -562 -740,100

11 37,500 4,876 4,708 168 243,480

10 39,990 3,195 3,375 -180 -260,460

9 42,150 4,797 5,117 -320 -463,060

8 44,250 4,927 5,389 -462 -557,830

7 47,010 6,472 6,877 -405 -585,760

6 49,800 6,422 6,097 325 470,860

Subtotal -1,800,000 59,500,000

Total 149,371 140,417 8,954 112,000,000 2162,500,000 1

 Numbers may not sum to total due to rounding

2

 Does not include the total sediment deposited in the upper section (numbers 1 to 5) estimated to be 11,000,000 tons.
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 15

Figure 9. Bathymetric survey transects surveyed in Conowingo Reservoir, lower Susquehanna River.

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39°

39'

39°

42'

39°

45'

39°

48'

76°18' 76°15' 76°12'

PENNSYLVANIA

MARYLAND

York County

Lancaster County

Harford County

Cecil County

Conowingo

Dam

Mt. Johnson Island

Peach Bottom

Power Plant

0 1 2 MILES

0 1 2 KILOMETERS

Michael Run

Broad Creek

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection: Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset

EXPLANATION

CROSS SECTION AND NUMBER

SUBAREA BOUNDARY

xc-1 Surveyed in 1996 and 2008

COUNTY BOUNDARY

STATE BOUNDARY

U P P E R

M

I

D

D

L

E

L O W E R

U P P E R

M

I

D

D

L

E

L O W E R
16 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Figure 10. Bathymetry showing 10-foot depth ranges in Conowingo Reservoir, lower Susquehanna River.

EXPLANATION

!

DEPTH, IN FEET

0 to less than 10

10 to less than 20

20 to less than 30

30 to less than 40

40 to less than 50

50 to less than 60

60 or greater

39°

39'

39°

42'

39°

45'

39°

48'

76°18' 76°15' 76°12'

PENNSYLVANIA

MARYLAND

York County

Lancaster County

Harford County

Cecil County

Mt. Johnson Island

Peach Bottom

Power Plant

0 1 2 MILES

0 1 2 KILOMETERS

Michael Run

Broad Creek

State and County Boundaries: USGS 1:100,000-scale data,

Universal Transverse Mercator Projection: Zone 18, NAD 1983

Hydrography data: USGS, 1:24,000-scale National Hydrography Dataset

COUNTY BOUNDARY

STATE BOUNDARY

Conowingo

Dam
Bathymetry and Sediment-Storage Capacity Change from 1996 to 2008 17

Previously, Hainly and others (1995) estimated the 

area affected by turbulence to be within 1.25 mi upstream 

of the dam (transect number 24). This turbulent area is due 

to bottom-release mechanisms that allow water to enter the 

turbines about 98 ft below the normal lake-surface elevation 

of 108.5 ft. Turbulence from these mechanisms caused sediment deposition to be less near the turbine gates and greater 

with increased distance away from the gates. In addition, flood 

gates are positioned about 40 ft below the normal lake surface, 

resulting in turbulence from the flood gates affecting sediment 

deposition in the same manner as the turbine gates, although 

the effect on sediment deposition is less (fig. 9).

From 1996 to 2008, the middle part of the reservoir, 

cross-sections 6 to 16, indicated a net sediment decrease 

of about 1,800,000 tons (table 2) to about 59,500,000 tons. 

Reed and Hoffman (1997) reported the area between 

cross-sections 6 and 16 had been “near” sediment-storage 

capacity since about 1959 and some minor capacity changes 

(about 5 percent of total volume) may have occurred within 

cross-sections 6 and 16, probably as a result of small scour 

events. As previously mentioned, scouring events were 

minimal from 1996 to 2008, with several streamflows at or 

just exceeding the scour threshold. It is possible these “less 

than 1 day” scour threshold streamflows did not have enough 

energy to scour and keep sediment in suspension long enough 

for it to be removed from the reservoir, but only transported 

sediment downgradient in the reservoir.

Although not surveyed in 1996, the upper part of the 

reservoir was surveyed in 2008. Total sediment deposition in 

the upper part of the reservoir was estimated in 1993 to be 

11,000,000 tons. At that time, this area contained less than 

15 percent of the total sediment in the reservoir. The sediment 

thickness was minimal, consisting of mostly sand. Since 1959, 

nearly all the sediment deposition has been below Michael 

Run (cross-section 14, fig. 9). In 2008, change in sediment 

deposition in the upper area of the reservoir (cross-sections 1 

to 5) was less than 1 percent, confirming conclusions of previous studies that this area was in equilibrium with respect to 

sediment-storage capacity.

The predicted change in sediment deposition in the 

reservoir system using bathymetry data was compared to the 

difference in monitored sediment load estimates entering and 

leaving the reservoirs. From 1997 to 2007, the USGS estimated 31,800,000 tons of sediment entered the reservoirs and 

15,100,000 tons of sediment exited the system (Blomquist and 

others, written commun., 2008). This includes an estimated 

2,500,000 tons of scoured sediments. Estimates were made of 

reservoir sediment transport on the basis of 2008 streamflow 

and long-term sediment-transport rates. For 2008, an additional 2,100,000 tons was estimated to have entered the reservoirs and 1,400,000 tons exited (Blomquist and others, U.S. 

Geological Survey, written commun., 2008). The difference of 

16,900,000 tons from 1997 to 2008 was considered the total 

deposition in the reservoirs. This compares reasonably well to 

14,700,000 tons estimated to have deposited in the reservoirs 

using bathymetry mapping and volume calculations.

Figure 11. Change in depth to bottom surface by transect in Conowingo Reservoir, lower Susquehanna River, 1993 to 

2008. Transect location shown on figure 9. 

26 24 22 20 18 16 14 12 10 8 6

0

10

20

30

40

50

60

TRANSECT NUMBER

DEPTH, IN FEET

2008

1996

1993

YEAR OF SURVEY
18 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

Sediment-Storage Capacity Change in 

Conowingo Reservoir 1929–2008

Conowingo Reservoir extends about 15.2 mi from the 

dam upstream to near the base of the Holtwood Dam. From 

Hennery Island (about 11.5 mi upstream from the Conowingo 

Dam and above transect 1), accumulation of sediment was 

minimal because of the high water velocities released from the 

Holtwood hydroelectric plant, the effects of a pump-storage 

generation station between Holtwood Dam and Hennery 

Island, and the natural narrow reservoir channel. Therefore, 

the change in capacity was computed from Hennery Island 

downstream to the Conowingo Dam. The storage capacity 

between Hennery Island and Holtwood Dam was estimated at 

20,000 acre-ft.

Changes in cross-sectional area since 1929 and resultant 

changes in storage capacity are shown for the years 1959, 

1990, 1993, 1996, and 2008 (fig. 12 and table 3). Each successive bathymetric survey indicated a decrease in reservoir 

capacity. The exception was 1996 when reservoir capacity 

increased due to scour from an ice-jam flood event. Reed and 

Hoffman (1997) computed a “sediment-storage capacity line” 

using hydrologic conveyances. The difference between the 

lines for the dated surveys and the sediment-storage capacity represents the remaining storage capacity in the reservoir 

(fig. 12).

In 1929, the original storage capacity from Hennery 

Island to Conowingo Dam was approximately 280,000 acre-ft 

(table 3). By 1959, the capacity was reduced to 215,000 acre-ft 

(Whaley, 1960), an average decrease of about 2,170 acre-ft 

per year. By 1993, the remaining storage capacity was further 

reduced by 46,000 acre-ft to 169,000 acre-ft, an average 

decrease of about 1,800 acre-ft per year, which included the 

20,000,000 and 2,400,000 tons of sediment scoured in 1972 

and 1975, respectively (Gross and others, 1978). From 1996 

to 2008, storage capacity in the Conowingo Reservoir was further reduced by approximately 9,000 acre-ft to 162,000 acre-ft, 

which equates to an additional 12,000,000 tons of sediment 

deposited based on volume change (table 3).

Current reservoir capacity from the dam to Hennery 

Island is 162,000 acre-ft–indicating a sediment deposition 

volume of about 118,000 acre-ft. This sediment volume is 

equivalent to approximately 174,000,000 tons of deposited sediment (table 3). About 20,000 acre-ft or 30,000,000 

tons of sediment can be deposited before reaching the 

Figure 12. Changes in vertical cross-sectional area for selected years and remaining sediment-storage capacity in Conowingo 

Reservoir, lower Susquehanna River.

0 5 10 15 20 25 30 35 40 45 50 55 60

DISTANCE UPSTREAM FROM DAM, IN FEET x 10,000

50

100

150

200

250

300

350

400

VERTICAL CROSS-SECTIO

N

AL AREA, IN CUBIC FEET x 100,000

1929

1959

1990

1993

1996

2008

Sediment-storage capacity

YEAR OF SURVEY
Summary 19

sediment-storage capacity (table 3). This is a reduction of 

9,000 acre-ft (12,000,000 tons of sediment) and about 30 

percent of the remaining capacity since the 1996 survey. Once 

the sediment-storage capacity is reached, sediments will no 

longer be effectively trapped and loads transported out of the 

reservoir will approach the loads transported into the reservoir. However, the reservoirs will not be constantly filled to 

capacity with sediments because of short-term changes from 

storms that cause scour and a subsequent reduction in exported 

sediments until the scoured amount is refilled. Therefore, the 

amount of sediment transported out of the reservoirs will not 

always be in equilibrium with the amount of sediment transported into the reservoirs.

Estimating the time remaining until Conowingo Reservoir reaches the sediment-storage capacity is difficult because 

of possible changes in sediment-deposition rates, changes in 

the amount of sediment transported into the reservoir, and the 

effects of large scour events. Annual deposition rates in the 

Conowingo Reservoir vary depending on the length of time 

and period examined (table 3). Data from 1929 to 1958 indicated the highest deposition rates at 3,100,000 tons per year 

and data from 1959 to 1993 indicated deposition decreased to 

about 2,500,000 tons annually. This decrease was most likely 

related to forest regrowth and more recent implementation 

of sediment-erosion and runoff-control practices and may 

also indicate a decreasing trend in reservoir trap efficiency. 

More recently, Ott and others (1991) reported an estimated 

1,800,000 tons of sediment deposited per year from 1985 to 

1989 in the Conowingo Reservoir. During this time, no scour 

events occurred and 2 of the 5 years had an annual mean 

streamflow well below normal. From 1996 to 2008, the current 

study estimated 1,500,000 tons of annual sediment deposition. Similar to the Ott and others (1991) sediment-deposition 

estimate, minimal scouring events occurred during the interval 

studied, and streamflow was well below normal in 3 of 4 consecutive years (1999–2002).

Studies from 1959 to 2008 give an average reservoir 

sediment-deposition rate of 2,000,000 tons per year. Using this 

rate, a reservoir trapping efficiency of 55 percent (Langland 

and others, 1997), and assuming there will be no scour 

from large storms, the Conowingo Reservoir could reach 

the remaining sediment-storage capacity of 20,000 acre-ft 

(approximately 30,000,000 tons of sediment) in about 

15–20 years. However, additional time could be added if the 

rate at which sediment is transported into the reservoir is 

reduced. For example, if sediment-transport rates are reduced 

20 percent, an additional 5 to 10 years would be needed to 

reach sediment-storage capacity. Additional time also could 

be added if the sediment trap efficiency is reduced. As the 

reservoir fills, cross-sectional areas will decrease, velocities 

will increase, and sediment trapping efficiency could decrease. 

It is expected that a reduction in trapping efficiency or loss of 

sediment storage will result in increased sediment and nutrient 

loads to the upper part of the Chesapeake Bay.

Summary

Bathymetric surveys of Lake Clarke, Lake Aldred, 

and Conowingo Reservoir in the lower Susquehanna River 

Basin were completed in the fall of 2008 in cooperation with 

the Pennsylvania Department of Environmental Protection. 

Bathymetry data were collected to provide an updated analysis 

of reservoir capacities and sediment deposition. Bathymetric 

data were collected using HYPACKTM software integrating 

a 210 kHz echo sounder, single beam transducer, and DGPS 

unit. Within HYPACKTM, transects of the depth data were 

acquired along pre-defined intervals for each reservoir and 

displayed on a geo-referenced aerial photo. Post-processing 

of the raw data sets eliminated erroneous spikes verified from 

analog data recorded on thermal paper. Diagonal transects 

were used to quality assure depth readings.

Bathymetry data were collected at many of the same transects previously surveyed in 1990, 1993, and 1996. A change 

in the depth to bottom elevations of a reservoir reflects a 

change in the volume of water and storage capacity of the reservoir. Changes in bottom surface usually indicated a change 

Table 3. Estimated changes in remaining capacity and sediment deposition from the Conowingo Dam to 11.5 miles upstream in 

Conowingo Reservoir, lower Susquehanna River, 1929–2008.

Year

Reservoir

capacity

(acre-feet)

Sediment

deposition

(acre-feet)

Sediment

deposition

(tons)

1929 280,000 0 0

1959 215,000 65,000 96,000,000

1990 175,000 105,000 155,000,000

1993 169,000 111,000 164,000,000

1996 171,000 109,000 162,000,000

2008 162,000 118,000 174,000,000

Water and sediment-storage capacity 142,000 138,000 204,000,000
20 Bathymetry and Sediment-Storage Capacity Change in Three Reservoirs on the Lower Susquehanna River, 1996–2008

in the quantity of sediment caused by scour or deposition, or 

represented movement in bottom sediments. The streamflow 

threshold to scour sediments is estimated at 390,000 ft3/s. 

Between the 1996 and 2008 surveys, three flood events 

exceeded that streamflow and scoured an estimated 2,500,000 

tons of sediment out of the reservoirs.

Depth to bottom data were collected from 19 cross 

sections in Lake Clarke. The deepest areas of the lake were 

closest to the dam and ranged from 30 to 50 ft. The upper half 

of the lake was generally shallow; average lake depths were 

about 10–15 ft on the west side, with many islands and sand 

and coal bars on the east side. Previous studies indicated Lake 

Clarke was in long-term equilibrium with sediment storage. 

An increase of 1,700,000 tons of sediment deposition was estimated in Lake Clarke between 1996 and 2008; total deposition 

in Lake Clarke was 92,400,000 tons of sediment. The 1996 

study estimated 2,300,000 tons of sediment was scoured and 

removed from Lake Clarke, so a net deposition was expected. 

The areas of greatest scour in the 1996 survey (cross-sections 

24–29) were largely filled.

Depth to bottom data were collected in 2008 from 13 

cross sections in Lake Aldred. Generally, depths were shallowest in the upper part of the lake and increased with distance 

downstream through the narrow gorge and decreased again 

near Holtwood Dam. The deepest areas were in spoon-shaped 

depressions called “the deeps” located in the middle and 

lower parts of the lake. The location of six deeps were known 

and depths were greater than 80 ft with one deep reaching a 

depth of about 130 ft, which extends below sea level. A total 

of 13,600,000 tons of sediment was estimated to be contained 

in Lake Aldred, considerably less than is contained in the 

other two reservoirs. The amount of deposited sediment has 

remained consistent since 1961; subsequent surveys in 1990, 

1993, and 1996 confirmed the lake was at equilibrium with 

minor changes in deposited sediment. The 2008 study estimated the change in sediment deposition as approximately 

1,000,000 tons, thus changes in water and sediment capacities 

were minimal.

Depth to bottom data were collected from 26 cross sections in the Conowingo Reservoir. The deepest areas of the 

reservoir were near the dam along the spillway gates (50 ft) 

and near the turbine gates (60 ft). The depths were substantially less than in 1996. The shallowest areas were in the upper 

one-third of the reservoir where reservoir depths averaged 

about 15 ft. Results indicated approximately 12,000,000 tons 

of sediment were deposited in Conowingo Reservoir between 

1996 and 2008; all the deposition was in the lower part of the 

reservoir. From 1996 to 2008, total sediment deposition in the 

lower part of the reservoir increased by 13,900,000 tons to 

about 103,000,000 tons. Depths decreased in every transect in 

the lower part of the reservoir by 8–10 ft. The middle part of 

the reservoir, cross-sections 6 to 16, indicated a net sediment 

decrease of about 1,800,000 tons from 1996 to 2008 to about 

59,500,000 tons. This suggested some minor capacity changes 

(about 5 percent of total volume) were occurring within 

cross-sections 6 and 16. The upper part of reservoir contained 

11,000,000 tons of sediment and remained unchanged from 

1996 to 2008.

The change in sediment deposition in the reservoir 

system using bathymetry data was compared to the difference in monitored sediment load estimates entering and 

leaving the reservoirs. The difference in monitored loads was 

about 16,900,000 tons and considered the total deposition 

in the reservoir system. This compared reasonably well to 

14,700,000 tons deposited in the reservoirs estimated using 

bathymetry data and volume calculations.

In 1929, the original capacity from Hennery Island to 

Conowingo Dam was approximately 280,000 acre-ft. This 

was reduced to 215,000 acre-ft by 1959, and further reduced 

to 169,000 acre-ft by 1993. Current reservoir capacity 

from the dam to Hennery Island is 162,000 acre-ft–indicating a deposited sediment volume of about 118,000 acre-ft 

(174,000,000 tons). About 20,000 acre-ft or 30,000,000 tons 

of sediment remain to fill before reaching the sedimentstorage capacity. At the current long-term rate of filling 

(2,000,000 tons per year) and with no occurrence of scour 

events, this capacity could be filled in 15–20 years.

Acknowledgments

The cooperation of Mr. Chris Porse and Mr. Ted 

Rainer (Holtwood and Safe Harbor Hydroelectric facilities, 

respectively), PPL Generation Co., and Donna Fabrizio at 

Conowingo Hydroelectric facility, Exelon Corporation is 

gratefully acknowledged. Additional thanks also to Brian Hattenbach, John Fulton, and Clinton Hittle of the USGS Pennsylvania Water Science Center for their assistance in the project.

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Langland—Bathymetry and Sediment-Storage Capacity Change on the Lower Susquehanna River, 1996–2008—Scientific Investigations Report 2009–5110