Deposition and Simulation

of Sediment Transport 

in the Lower Susquehanna River 

Reservoir System

by Robert A. Hainly, Lloyd A. Reed, Herbert N. Flippo, Jr, and Gary J. Barton 

Water-Resources Investigations Report 95-4122

Prepared in cooperation with the

PENNSYLVANIA DEPARTMENT OF ENVIRONMENTAL RESOURCES, 

BUREAU OF SOIL AND WATER CONSERAVATION

Lemoyne, Pennsylvania 

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

For additional information 

write to:

District Chief

U.S. Geological Survey

840 Market Street

Lemoyne, Pennsylvania 17043-1586

Copies of this report may be 

purchased from:

U.S. Geological Survey

Earth Science Information Center

Open-File Reports Section

Box 25286, MS 517

Denver Federal Center

Denver, Colorado 80225
CONTENTS

Page

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

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

Purpose and scope ...................................................................... 1

Description of the study area...............................................................2

Description of the hydroelectric dams and reservoirs ..................................................2

Safe Harbor Dam and Lake Clarke ..........................................................2

Holtwood Dam and Lake Aldred ............................................................5

Conowingo Dam and Conowingo Reservoir ...................................................6

Previous studies ..............................................................................6

Data-collection methods ........................................................................7

Seismic-reflection survey..................................................................7

Bottom material .........................................................................7

Deposition in the Lower Susquehanna River reservoir system...........................................9

Lake Clarke ........................................................................... 10

Sediment distribution .............................................................. 10

Sediment composition ............................................................. 12

Particle size and coal percentage............................................... 12

Nutrients .................................................................. 12

Metals.................................................................... 14

Lake Aldred ...........................................................................17

Sediment distribution .............................................................. 17

Sediment composition ............................................................. 17

Particle size and coal percentage............................................... 17

Nutrients .................................................................. 17

Metals.................................................................... 17

Conowingo Reservoir.................................................................... 19

Sediment distribution .............................................................. 19

Sediment composition ............................................................. 19

Particle size and coal percentage............................................... 19

Nutrients .................................................................. 19

Metals ....................................................................21

Effect of deposition on reservoir storage .....................................................21

Lake Clarke......................................................................21

Lake Aldred......................................................................21

Conowingo Reservoir..............................................................25

Simulation of sediment transport in the reservoir system ..............................................27

Description of the selected model ..........................................................27

Application of model to reservoir system .....................................................28

Development.....................................................................28

Calibration.......................................................................30

Results .........................................................................34

Summary and conclusions......................................................................37

References cited .............................................................................38
ILLUSTRATIONS

Page

Figures 1-8. Maps showing:

1. The Susquehanna River Basin ................................................ 3

2. The location of the three hydroelectric dams on the

Lower Susquehanna River ............................................... 4

3. The thickness of sediment deposition in Lake Clarke.............................. 11

4. The percentage of clay in the sediment deposited in the three reservoirs

on the Lower Susquehanna River......................................... 13

5. Concentration ranges of organic nitrogen in the sediment deposited in the three

reservoirs on the Lower Susquehanna River ................................ 15

6. Concentration ranges of phosphorus in the sediment deposited in the three reservoirs

on the Lower Susquehanna River......................................... 16

7. Concentration ranges of aluminum in the sediment deposited in the three reservoir?

on the Lower Susquehanna River......................................... 18

8. The thickness of sediment deposition in Conowingo Reservoir ...................... 20

9-12. Graphs showing:

9. Water-storage capacity of Lake Clarke, Lower Susquehanna River Basin, from the

time the Safe Harbor Dam was completed in 1931 through 1990................. 22

10. Relations of upstream distance from Chesapeake Bay to average bed elevations 

and cross-sectional areas in Lake Clarke, 

Lower Susquehanna River Basin, 1931 and 1990 ............................ 23

11. Relations of upstream distance from Chesapeake Bay to average bed elevations 

and cross-sectional areas in Lake Aldred, 

Lower Susquehanna River Basin, 1910 and 1990 ............................ 24

12. Relations of upstream distance from Chesapeake Bay to average bed elevations 

and cross-sectional areas in Conowingo Reservoir, 

Lower Susquehanna River Basin, 1928,1959, and 1990....................... 26
TABLES

Page

Table 1. Physical characteristics of the three hydroelectric darns and reservoirs on the

Lower Susquehanna River..................................................... 2

2. Physical characteristics of the Lower Susquehanna River reservoir subareas ................. 5

3. Site identification number and location of water-quality and bottom-material

sampling sites on the Susquehanna River......................................... 8

4. Estimated sediment deposition and composition for subareas of and for

Lake Clarke, Lake Aldred, and Conowlngo Reservoir, 1990 .......................... 10

5. Mean concentrations and deposition of ammonia, organic nitrogen, phosphorus, 

iron, aluminum, and manganese in subareas of and in Lake Clarke, 

Lake Aldred, and Conowingo Reservoir.......................................... 12

6. Initial estimates of fractional distributions of particle loads for selected discharges

of the Susquehanna River at Marietta, Pa. ....................................... 29

7. Initial estimates of fractional distributions of particle loads for selected

discharges of the Conestoga River and Pequea Creek Basins........................ 30

8. Original and revised fractional distributions of particle loads for selected

discharges of the Susquehanna River at Marietta, Pa. .............................. 32

9. Original and revised fractional distributions of particle loads for selected

discharges of the Conestoga River and Pequea Creek Basins........................ 33

10. Summary output from the HEC-6 model showing bed-surface and water-surface 

elevation change, the modeled discharge, and the sediment load passing 

each cross-section, in the Lower Susquehanna River reservoir system, 

December 30-31,1987....................................................... 35

11. Summary output from HEC-6 model calibration run showing 1987 sediment load 

introduced to each reservoir on the Susquehanna River and sediment 

load trap efficiency.......................................................... 36

12. Loads and trap efficiencies for the three-reservoir system on the

Lower Susquehanna River, as computed by the HEC-6 and Conn models,

and by a hand integration method............................................... 36

13. HEC-6 loads and trap efficiencies for the three-reservoir system on the

Lower Susquehanna River, May and June 1972 ................................... 36
CONVERSION FACTORS AND ABBREVIATIONS

Multiply

inch (in.) 

foot (ft) 

mile (mi)

By.

Length

25.4

0.3048

1.609

To obtain

millimeter

meter

kilometer

square foot (ft2) 

square mile (mi2) 

acre

Area

0.09294

2.590

0.4047

square meter 

square kilometer 

hectare

foot per second (ft/s) 

mile per year (mi/year)

Velocity

0.3048

1.609

meter per second 

kilometer per year

cubic foot (ft3) 

acre-foot (acre-ft)

Volume

0.02832

1,233

cubic meter 

cubic meter

cubic foot per second

Flow 

0.02832 cubic meter per second

ton, short

Mass 

0.9072 megagram

degree Fahrenheit (°F)

Temperature 

°C - 5/9 x (°F-32) degree Celsius

pounds per square foot (Ib/ft2)

Pressure 

0.04788 dynes per square rmter

pounds per cubic foot (Ib/ft3)

Density 

16.02 kilograms per cubic meter

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 192P (NVGD of 

1929) a geodetic datum derived from a general adjustment of the first-order level nets o^both the 

United States and Canada, formerly called Sea Level Datum of 1929.

vi
DEPOSITION AND SIMULATION OF SEDIMENT TRANSPORT IN 

THE LOWER SUSQUEHANNA RIVER RESERVOIR SYSTEM

by Robert A. Hainly, Lloyd A. Reed, Herbert N. Flippo, Jr., and Gary J. Barton

ABSTRACT

The Susquehanna River drains 27,510 square miles in New York, Pennsylvania, and Maryland and is 

the largest tributary to the Chesapeake Bay. Three large hydroelectric dams are located on the river, Safe 

Harbor (Lake Clarice) and Holtwood (Lake Aldred) in southern Pennsylvania, and Conowingo 

(Conowingo Reservoir) in northern Maryland. About 259 million tons of sediment have been deposited in 

the three reservoirs. Lake Clarke contains about 90.7 million tons of sediment, Lake Aldred contains about 

13.6 million tons, and Conowingo Reservoir contains about 155 million tons. An estimated 64.8 million 

tons of sand, 19.7 million tons of coal, 112 million tons of silt, and 63.3 million tons of clay are deposited in 

the three reservoirs. Deposition in the reservoirs is variable and ranges from 0 to 30 feet.

Chemical analyses of sediment core samples indicate that the three reservoirs combined contain 

about 814,000 tons of organic nitrogen, 98,900 tons of ammonia as nitrogen, 226,000 tons of phosphorus, 

5,610,000 tons of iron, 2,250,000 tons of aluminum, and about 409,000 tons of manganese.

Historical data indicate that Lake Clarke and Lake Aldred have reached equilibrium, and that they 

no longer store sediment. A comparison of cross-sectional data from Lake Clarke and Lake Aldred with 

data from Conowingo Reservoir indicates that Conowingo Reservoir will reach equilibrium within the 

next 20 to 30 years. As the Conowingo Reservoir fills with sediment and approaches equilibrium, the 

amount of sediment transported to the Chesapeake Bay will increase. The most notable increase will take 

place when very high flows scour the deposited sediment.

Sediment transport through the reservoir system was simulated with the U.S. Army Corps of 

Engineers' HEC-6 computer model. The model was calibrated with monthly sediment loads fo- calendar 

year 1987. Calibration runs with options set for maximum trap efficiency and a "natural" particle-size 

distribution resulted in an overall computed trap efficiency of 34 percent for 1987, much less th=m the 

measured efficiency of 71 percent.

INTRODUCTION

The District of Columbia and the States of Pennsylvania, Maryland, and Virginia have agreed to a 

40-percent reduction in controllable nutrient loads to the Chesapeake Bay by the year 2000. The load of 

nutrients transported to the bay depends, in large part, on the load transported by the Susquehanna River, 

the largest freshwater contributor to the bay. The reservoir system on the Lower Susquehanna R'ver affects 

the loads of sediment and nutrients delivered to Chesapeake Bay, but the magnitude and length of the 

effects are not known.

As part of the Chesapeake Bay Program, the Bureau of Land and Water Conservation of the 

Pennsylvania Department of Environmental Resources and the U.S. Geological Survey (USGS) cooperated 

in a study to evaluate deposition of sediment, nutrients, and selected metals in the three reservoirs on the 

Lower Susquehanna River. The study was conducted during the summer and fall of 1990.

Purpose and Scope

The quantity and chemistry of sediment in the reservoirs formed by the Safe Harbor, Holtwood, and 

Conowingo hydroelectric dams is evaluated in the report. The report presents a comparison of historical 

reservoir-bed elevations with elevations obtained during this study, and an estimate of the rem aining 

sediment storage capacity. The results of calibrating a model to calculate deposition and scour in the 

reservoirs during storms also are presented.
Data from the seismic-reflection profiling, data obtained during the collection of core semples, and 

historical data (Whaley, 1960) were used to map the thickness of bed sediments. The dry density and 

composition data determined from the core sample analyses, and the sediment thickness data were used 

to compute the dry weight and composition of the deposited material in each reservoir.

Description of the Study Area

The Susquehanna River drains 27,510 mi2 in south<entral New York, central Pennsylvania, and a 

small part of Maryland before entering the Chesapeake Bay (fig. 1). The reservoirs in the lower part of the 

Susquehanna drainage were formed by the construction of three hydroelectric dams on the 32-mi reach of 

the river between Conowingo, Md., and Columbia, Pa. (fig. 2). Conowingo Dam is in northern Maryland 

and forms Conowingo Reservoir, which extends into southern Pennsylvania. Holtwood Dam is upstream 

from Conowingo Reservoir and forms Lake Aldred. Safe Harbor Dam is upstream from Lake Aldred and 

forms Lake Clarke.

The climate in the Susquehanna River Basin varies considerably from central New Yorl State to 

northern Maryland. The mean annual temperature ranges from 45°F in central New York to 53°F in 

Maryland. The mean growing season ranges from 120 days in the north to 160 days in the sor*h (U.S. 

Department of Commerce, 1990). Mean annual precipitation in the basin is about 40 in. and is fairly evenly 

distributed throughout the year. The mean annual precipitation is highest in the lower basin and lowest in 

the headwaters.

Woodland covers 63 percent of the Susquehanna River Basin and is concentrated in the northern and 

western parts of the basin. Nineteen percent of the basin is tilled cropland, and most of the tiT«d cropland 

is in the lower basin. Extensive, cultivated areas are also along the river valleys in southern NTW York and 

northern Pennsylvania. Urban land occupies slightly more than 9 percent of the basin. Most cf the urban 

areas are along river valleys in southern New York and central Pennsylvania.

Anthracite coal was mined in several areas of eastern Pennsylvania. Fine coal from processing plants 

in the mining region was a large component of the sediment transported by the Susquehanna River from 

the late 19th century through the early 20th century, and "river coal" was routinely dredged f-om pools in 

the river until 1972. After the hydroelectric dams were constructed on the Lower Susquehanna River, large 

amounts of fine coal were trapped in the reservoirs.

DESCRIPTION OF THE HYDROELECTRIC DAMS AND RESERVOIRS

Safe Harbor Dam and Laks Clarks

Safe Harbor Dam, constructed in 1931, is 32 mi upstream from Chesapeake Bay (fig. 2). Lake Clarke 

extends upstream about 9.5 mi from Safe Harbor, Pa., to Columbia, Pa., and has a design capacity of 

150,000 acre-ft (table 1). Streamflow in excess of plant capacity is regulated by flood gates along the top of 

the dam west of the hydroelectric plant.

Table 1. Physical characteristics of the three hydroelectric dams and reservoirs on the 

Lower Susquehanna River

Dam

Safe Harbor 

Holtwood 

Conowingo

Lake or 

reservoir

Clarke 

Aldred 

Conowingo

Year 

completed

1931 

1910 

1928

Elevation 

(feet above sea level)

Normal 

pool

227 

1 170 

109

Flood 

pool

227 

180 

109

Design

capacity 

(acre-feet)

150,000 

60,000 

300,000

Surface 

area 

(square miles)

9.5 

4.0 

12.8

b'^ximum 

turbine 

d'«charge 

(cubic feet 

per second)

110,000 

27,000 

81,000

1 Includes 4.75-foot flash boards.
79" 78' 77' 76' 75'

42 «

41°

40'

I

PENNSYLVANIA

25

I

50 MILES 

I

I 

25 SO KILOMETERS

EXPLANATION 

~ Drainage basir boundary 

 State boundar'

Hydroelectric dams

Municipality

Figure 1. The Susquehanna River Basin.
76-301 76'15'

Marietta

4<T08'

Long Level

Channel

Subarea

39-451

Washington Boro

Washington Boro 

Flats Subarea

TiirkeyHill 

Lower Lake 

Subarea

( Conestoga 

* River

Safe Harbor 

Dam

Upper LakeAldred

Peach Bottom 

Generating Station

Middle Reservoir 

Subarea

PENNSYLVANIA 

MARYLAND

Weise Island Subarea 

Duncan Island Subarea 

Lower Lake A Idred Subarea

Muddy Run 

Hydroelectric Dam

Hennery Island

Mt. Johnson Island 

Subarea

ML Johnson Island

10 MILES Lower Reservoir Subarea 

Conowingo '

10 KILOMETERS

Conowingo 

Dam

Figure 2. The location of the three hydroelectric dams on the Lower Susquehanna River.

4
For the purposes of this study, Lake Clarke was divided into three subareas the Washington Boro 

flats, the Long Level channel, and the lower lake (fig. 2, table 2). The Washington Boro flats are along the 

eastern half of the lake from Washington Boro to Turkey Hill. The surface area, excluding the thr?e large 

islands, is 2.7 mi2. Lake Clarke contains many small islands with land surfaces just above the normal water 

level and numerous sand and coal bars with surfaces just below the normal water level. Many sand and 

coal bars are exposed during low water and much of the area is too shallow for boating. The Long Level 

channel extends along the west side of the lake from about 3 mi upstream of the community of Long Level 

downstream to Turkey Hill. The surface area of the Long Level channel is 3.8 mi2 . The lower lake extends 

from Turkey Hill to the Safe Harbor Dam and has a surface area of 3.0 mi2.

Table 2. Physical characteristics of the Lower Susquehanna River reservoir subareas

Surface area

Reservoir subarea (square 

miles) (acres)

Channel 

length 

(feet)

Maximum 

width 

(feet)

Minimum 

width 

(feet)

Lake Clarke

Washington Boro Flats

Long Level channel

Lower Lake

Total

2.7

3.8

3.0

9.5

1,720

2,440

1,930

6,090

24,600

26,600

22,400

^9,000

4,680

6,600

5,000

8,840

800

3,720

3,000

3,000

Lake Aldred

Upper Lake Aldred

Weise Island

Duncan Island

Lower Lake Aldred

Total

1.2

1.0

1.3

.5

4.0

780

660

830

290

2360

9,200

13,000

14,000

6,400

42,600

5,080

3,800

2,700

2,800

5,080

4,040

2,000

1,200

1,200

1,200

Conowinpo Reservoir

Below Holtwood Dam

Mt. Johnson Island

Middle Reservoir area

Lower Reservoir area

Total

1.5

3.6

4.7

3.0

12.8

990

2,310

3,020

1,890

8,210

17,200

16,600

23,320

20,440

77360

5,160

6,840

7,000

5300

7,000

1,840

5,120

5300

3,100

1,840

1 The total channel length is the sum of the Long Level channel and Lower Lake subareas 

because the Washington Boro flats and the Long Level channel are side-by-side, and not 

consecutive subareas.

Holtwood Dam and Lake Aldred

The Holtwood Dam, constructed in 1910, is about 25 mi upstream of Chesapeake Bay (fig. 2). The 

reservoir formed by the dam, Lake Aldred, extends upstream for about 8 mi. Lake Aldred covers an area 

of about 4.0 mi2 and has a design capacity of 60,000 acre-ft (table 1). Holtwood Dam was constructed 

without flood gates, and river flow in excess of plant capacity spills over the top of the dam to th? west of 

the powerhouse. A coal-fired power plant was constructed adjacent to the hydroelectric plant in 1925. The 

coal plant had a capacity of 73,000 kilowatts and was designed to burn about 200,000 tons of river coal per 

year. Until 1972, most of this coal was dredged from the reservoirs.
For this report, Lake Aldred was divided into four subareas (fig. 2, table 2). The most upstream 

subarea, Upper Lake Aldred, extends from the Safe Harbor Dam to Weise Island and covers 1.2 mi 2. The 

Weise Island subarea extends from Weise Island to just below the Urey Islands. This subarea of the lake is 

13,000 ft long, and the width ranges from 3,800 ft just below Weise Island to 2,000 ft at the Urpv Islands. 

The surface area, not including Weise Island, is 1.0 mi 2. The Duncan Island subarea extends from just 

below the Urey Islands to the narrows. It is 14,000 ft long, and the width ranges from 2,700 ft above 

Duncan Island to 1,200 ft at the Narrows. The surface area is 1.3 mi2. Lower Lake Aldred extends from the 

Narrows to the dam. The width of this subarea ranges from 1,200 ft at the Narrows to 2,800 ft above the 

dam. The surface area is 0.5 mi2.

Conowingo Dam and Conowingo Reservoir

Conowingo Dam, constructed in 1928 about 10 mi upstream of Chesapeake Bay, is the largest 

hydroelectric dam and creates the largest reservoir on the river. Conowingo Reservoir has a surface area of 

12.8 mi2 and a design capacity of 300,000 acre-ft (table 1). The elevation of the river bed at the dam is about 

11 ft above sea level, and the normal pool elevation is 109 ft. River flow in excess of the plant capacity is 

discharged through flood gates installed on the east side of the dam.

Conowingo Reservoir extends about 12 mi from the dam upstream to Hennery Island (fig. 2). A 

3.2-mi section of the river separates the Holtwood Dam from the headwaters of the Conowingo Reservoir 

at Hennery Island. Water velocities in this river section are high, and sediment does not accumulate. The 

remainder of the reservoir was divided into three subareas the Mt. Johnson Island subarea, the Middle 

Reservoir subarea, and the Lower Reservoir subarea (table 2).

The Mt. Johnson Island subarea extends from Hennery Island to just below the Peach Bottom 

Generating Station and has an average width of about 6,000 ft. The surface area is 3.6 mi2. The Middle 

Reservoir subarea extends from below the Peach Bottom Generating Station to Broad Creek. The width of 

the Middle Reservoir subarea ranges from 7,000 ft just below the Peach Bottom Generating Sftion to 

5,500 ft at Broad Creek, and the surface area is 4.7 mi2. The Lower Reservoir subarea extends from Broad 

Creek to Conowingo Dam. The width of this subarea ranges from 5,500 ft at Broad Creek to 3,100 ft at 

Conowingo Creek and increases to about 4,900 ft at the dam. The surface area of the Lower Reservoir 

subarea is 3.0 mi2.

PREVIOUS STUDIES

Anthracite coal was a major component of the sediment transported by the Susquehanm River from 

the late 19th century through the early 20th century. Mined coal was brought to the land surface, broken, 

and washed. At many of the coal-processing areas, the waste water from the washing operations was 

discharged directly to streams. An estimated 10-15 percent of the coal that was mined and run through the 

breakers was discharged in the wash water. Sisler and others (1928) reported that 260,000 tons of coal were 

dredged from the Susquehanna River in 1913 and that half of that amount had a diameter larger than 

2 mm (millimeters). They also reported that continuing deposition and scour were moving a large coal bar 

down the river at a rate of about 3 mi per year. By 1925, dredge operators were recovering 400,000 tons of 

coal a year from the Susquehanna River.

Schuleen and Higgins (1953) reported the results of siltation surveys in Lake Clarke. They reported 

that Lake Clarke contained 144,600 acre-ft of water in 1931, and the capacity was reduced to 78,800 acre-ft 

in 1950. The implication is that nearly 66,000 acre-ft of water storage (about 45 percent of the design 

capacity) was lost because of sediment deposition during 1931-50. In 20 years, the lake trapped about 

74 million tons of sediment, which is an average deposition rate of 3.7 million tons per year. Surveys 

completed in 1950,1951,1959, and 1964 indicated that the amount of sediment in Lake Clarke remained 

fairly constant at 74 million tons from 1950 to 1964 and that the reservoir had reached an equilibrium 

(E.T. Schuleen, Pennsylvania Power and Light Company, oral commun., 1965).

Schuleen and Higgins (1953) also collected suspended-sediment data from the Susquehenna River at 

Columbia and at Safe Harbor during 1948-53. During the 6-year period, sediment discharge a* Columbia, 

upstream of Lake Clarke, averaged 4.46 million tons per year and the sediment discharge measured at Safe 

Harbor, the outflow of the reservoir, was 3.13 million tons per year.
Levin and Smith (1954) reported on a river-coal dredging operation in Lake Clarke that started in 

1953. The operation was designed to dredge 1 million tons of material a year from Lake Clarke rnd 

transport it on barges to the shore where the sand and coal were separated. The dredged material was 

about half sand and half coal. Dredging continued until the flood caused by Hurricane Agnes in June 1972. 

Because the reservoir surveys conducted in 1951,1959, and 1961 indicated the amount of sediment 

deposited in Lake Clarke remained about the same, the dredged material was replaced by incoming 

sediments.

Ledvina (1962) reported results of siltation surveys of Lake Aldred conducted by the Penrsylvania 

Power and Light Company and the Holtwood Steam Electric Station. These surveys indicate thet the 

annual amounts of sediment deposited in Lake Aldred are variable. Ledvina reported that the lake 

contained 19.3 million tons of sediment in 1939,13.3 million tons in 1950, and 9.97 million tons in 1961. 

Reasons for the decline in sediment were not given, but coal was dredged from the reservoir du~ing most 

of the period.

Whaley (1960) reported temperature, dissolved oxygen concentrations, velocity distributions, and 

bottom elevations at six cross-sections in Conowingo Reservoir in 1959. His data indicate that thr? capacity 

of Conowingo Reservoir was reduced from 300,000 acre-ft in 1928 to 235,000 acre-ft in 1959. The reservoir 

contained an estimated 92 million tons of sediment in 1959.

DATA-COLLECTION METHODS

Continuous seismic-reflection profiles were run at about 10 locations in each of the lakes to 

determine the areal extent and the thickness of sediment deposition. Bottom-material sampling points 

were selected to characterize the particle size and chemistry of the material deposited in the reservoirs and 

to confirm the sediment thickness as determined by geophysical techniques. Suspended-sediment samples 

were collected during periods of high flow from 1984 to 1989 at the Susquehanna River at Marie'te, Pa., 

and from the Conestoga River at Conestoga, Pa. The sediment deposition data were used, along with 

suspended-sediment transport data, to calibrate and test a model to compute deposition and resuspension of sediment over a range of flows.

Seismic-Reflection Survey

The continuous seismic-reflection profiling method (Gorin and Haeni, 1988; Wolansky and others, 

1982) is based on signals transmitted from a sound source and reflected at air-water, water-sediment, and 

sediment-rock interfaces. Depths are determined by observing the arrival time of the reflected waves and 

applying a velocity to the wave. The velocity of sound is related to compressibility and specific veight of 

the medium through which it travels. The velocity of sound in water is 4,720 ft/s and the velocit" in 

saturated unconsolidated sediments is about 5,000 ft/s. The velocity of sound in bedrock varies 1 -it is also 

greater than velocity of sound in water. The seismic signal penetrates to a depth of 100 ft in fine-<?rained 

sediments but is limited to 5 ft in coarse sediments because gravel and larger particles severely scatter the 

signals. Resolution is 1 to 2 ft. A fathometer, operating at a signal frequency of 192 kilohertz, was also used 

to provide a record of water depth and morphology of the bottom of each reservoir.

Bottom Material

A total of 54 core samples were collected from the bottom material in the reservoirs from October 

1990 to April 1991. The sample-collection sites in each reservoir are listed in table 3. Samples of tt « bottom 

material were collected to a maximum depth of 7 ft with a 2-in. diameter stainless-steel core sampler 

equipped with a plastic liner. Bed-material samples were analyzed for particle-size distribution, 

percentage of coal, dry density, and concentrations of selected nutrient and metal species. Result' of all 

sample analyses are published in "Water Resources Data for Pennsylvania, Water Year 1991, Volume 2" 

(Durlin and Schaffstall, 1992).
Table 3. Site identification number and location of water-quality and bottom-material sampling site~ on the Lower 

Susquehanna River

[Latitude, in degrees, minutes, seconds north; Longitude in degrees, minutes, seconds west]

Lake Clarke

Site 

number

LC 05.01

LC 05.06

LC 07.02

LC 07.03

LC 09.03

LC 10.01

LC 10.02

LC 10.03

LC 10.04

LC 12.01

LC 12.03

LC 12.05

LC 12.07

LC 14.07

LC 14.09

LC 14.10

LC 15.02

LC 15.06

LC 16.03

Latitude

395738

395757

395624

395635

395538

395457

395518

395529

395542

395756

395822

395904

395931

395701

395620

395550

395550

395627

395546

Longitude

0762908

0762755

0762704

0762653

0762501

0762417

0762405

0762358

0762355

0762728

0762738

0762801

0762819

0762754

0762640

0762517

0762603

0762728

0762433

Lake Aldred

Site 

number

LA 01.02

LA 03.02

LA 03.04

LA 04.02

LA 04.03

LA 06.02

LA 06.03

LA 12.03

LA 12.11

LA 12.14

LA 12.18

LA 13.09

LA 13.11

LA 13.12

LA 15.04

LA 15.06

LA 15.13

LA 15.14

LA 16.03

LA 16.05

LA 17.03

LA 17.04

LA 17.07

LA 17.09

LA 17.11

Latitude

394947

395050

395058

395127

395135

395303

395305

395414

395210

395111

394944

395023

395040

395050

394958

395015

395132

395142

395329

395353

395341

395320

395244

395220

395155

Longitude

0762008

0762101

0762045

0762133

0762125

0762219

0762231

0762241

0762226

0762108

0762023

0762057

0762059

0762105

0762035

0762047

0762158

0762218

0762247

0762254

0762219

0762218

0762254

0762248

0762234

Conowingo Reservoir

Site 

number

CO 01.01

CO 01.03

CO 01.05

CO 02.02

CO 02.03

CO 02.04

CO 03.05

CO 04.03

CO 04.05

CO 05.02

CO 07.03

CO 08.01

CO 08.03

CO 08.05

CO 09.02

CO 09.03

CO 10.03

CO 11. 06

CO 12.01

CO 12.05

CO 12.06

CO 13.02

CO 13.05

Latitude

393939

393955

394010

394039

394025

394017

394104

394208

394212

394254

394453

394608

394544

394524

394704

394655

394738

394530

394339

394148

394126

394107

394007

Longitude

0761109

0761058

0761049

0761150

0761152

0761200

0761255

0761402

0761335

0761407

0761441

0761508

0761523

0761545

0761605

0761622

0761716

0761430

0761407

0761318

0761258

0761223

0761124
All chemical analyses were performed at the USGS National Water Quality Laboratory in Arvada, 

Colo. Bed-material particle-size analyses were performed at the USGS Pennsylvania District Sediment 

Laboratory in Lemoyne to determine the percentage of sand, silt, clay, and coal. Samples of the bottom 

material were sieved to determine the weight of sediment that had a diameter of greater than 0.062,0.125, 

025, 0.50,1.00, and 2.00 mm. The percentages of silt and clay were determined by standard particle-size 

analysis techniques (Guy, 1969).

The percentage of coal by volume of each of the six sieved portions was visually estimated. and then 

used to estimate the percentage of coal and sand by weight in each sample. The following example 

demonstrates the method used to determine the percentages of sand and coal by weight in each sieved 

portion of the sediment samples.

A 0.89 g portion of sediment in the 1.00-2.00 mm class was visually estimated to contain 4^ percent 

coal and 60 percent sand, by volume. Specific gravities of 1.7 for coal and 2.4 for sand were assumed. The 

relative weights of coal and sand were determined by multiplying the volume estimate by the assumed 

specific gravity for each particle type.

0.40 x 1.7 = 0.68 (relative weight of coal in this portion) (1)

0.60 x 2.4 = 1.44 (relative weight of sand in this portion) (2)

0.68 + 1.44 = 2.12 (total relative weight) (3)

To determine the actual weight of coal and sand in the portion, the ratio of the total actual weight of 

the sample and the total relative weight was determined.

0.89 g / 2.12 = 0.42 (ratio of total actual weight to total relative weight) (4)

The ratio was then multiplied by the relative weights to determine the actual weight of ccal and 

sand in the portion.

0.68 x 0.42 = 0.29 g (weight of coal in this portion) (5) 

1.44 x 0.42 = 0.60 g (weight of sand in this portion) (6)

The percentages of coal and sand, by weight, in this 1.00-2.00 mm class portion are 33 percent 

(0.29 g / 0.89 g) and 67 percent (0.60 g / 0.89 g), respectively.

DEPOSITION IN THE LOWER SUSQUEHANNA RIVER RESERVOIR SYSTE"

After the hydroelectric dams were constructed on the Lower Susquehanna River, large amounts of 

coal and other sand-size particles and some of the silt and clay particles transported by the river were 

trapped in the reservoirs and no longer reached Chesapeake Bay. The headwaters of the reserve irs 

generally contain large deposits of sand and coal and the middle and lower parts of the reservoirs contain 

less sand and coal and more silt and clay.

About 10 percent of the nitrogen trapped in the sediments is ammonia and the rest is organic 

nitrogen. Together, nitrite and nitrate accounted for less than 0.1 percent of the total nitrogen in each of the 

impoundments and are not discussed in this report. For this report, concentrations of ammonia and 

organic nitrogen are expressed as nitrogen (N) and concentrations of phosphorus are expressed as 

phosphorus (P).

Data from bed-material samples from each of the three impoundments indicated that concentrations 

of iron were generally 2 to 3 times greater than concentrations of aluminum and 12 to 18 times greater than 

concentrations of manganese.
Lake Clarke

Sediment Distribution

Sediment deposition in Lake Clarke was greatest in the Lower Lake area and least in the Long Level 

channel (fig. 3, table 4). The Washington Boro flats, excluding the three large islands, contained about 

15,600 acre-ft (723 million ft3) of sediment. Sediment samples collected in the Washington Boro flats had a 

dry density of about 71 Ib/ft3, and about 25.7 million tons of sediment were deposited in the fhts (table 4). 

The upstream 3 mi of the bed of the Long Level channel is composed primarily of cobbles ancf boulders 

(material with a diameter greater than 64 mm), and few (if any) deposits of fine-grained sediment are 

present. Sediment deposited in the 520-acre downstream reach of the Long Level channel, from Long 

Level to Turkey Hill, has an average depth of 3.7 ft, and the area contains about 1,920 acre-ft o* sediment 

(table 4)

The amount of sediment deposited in the Lower Lake subarea of Lake Clarke is about 43,600 acre-ft 

(1.9 billion ft3); the density of the sediment is about 65 Ib/ft3, the weight of sediment is calculated to be 

about 62.0 million tons (table 4). The total weight of sediment in Lake Clarke is about 90.7 miFon tons..

Table 4. Estimated sediment deposition and composition for subareas of and for Lake Clarke, Lake Ald'-vi, and 

Conowingo Reservoir, Lower Susquehanna River Basin, 1990

Reservoir or 

Reservoir subarea

Design capacity Sediment deposition

(acre-feet) (acre-feet) (tons)

Sand 

(percent)

Silt 

(percent)

Clay 

(percent)

Coal 

(percent)

Lake Clarke

Washington Boro flats 

Long Level channel 

Lower Lake

Lake Clarke

24,600 

47,600 

77,800

150,000

15,600 

1,920 

43,600

61,120

25,700,000 

3,000,000 

62,000,000

90,700,000

52 

26 

18

28

16 

43 

49

39

10 

30 

28

23

22 

1 

5

10

Lake Aldred

Weise Island

Duncan Island

Lower Lake Aldred

Lake Aldred

4,700

19,500

20,500

60,000

4,130

3,280

1,170

8,580

6,600,000

5,200,000

1,800,000

13,600,000

33

46

31

38

35

21

28

29

15

21

21

18

16

12

20

15

Conowinpo Reservoir

Mt Johnson Island

Middle Reservoir

Lower Reservoir

Conowingo Reservoir

41,000

114,000

145,000

300,000

7,120

41,100

56,700

104,920

11,000,000

63,400,000

80,500,000

155,000,000

45

39

5

22

18

36

58

46

7

18

35

26

30

7

2

6

10
4CT08'

39-351

Wrightsville*

Washington Boro

0

EXPLANATION 

Thickness of sediment 

deposition, in feet.

% Less Than 10

3 10 To 20

8 Greater Than 20

Safe Harbor

Safe Harbor 

Dam

SMILES

0 5 KILOMETERS

Figure 3. The thickness of sediment deposition in Lake Clarke.

11
Sediment Composition

Particle size and coal percentages

In Lake Clarke, the percentage of sand ranged from 52 percent in the Washington Boro flats to 

18 percent in Lower Lake (table 4). The average of all samples in Lake Clarke was 28 percent. Silt averaged 

39 percent and clay averaged 23 percent. About 10 percent of the sediment deposits was coal. The highest 

percentage of coal 22 percent was in samples from Washington Boro flats. Elsewhere in Lake Clarke, 

5 percent or less of coal was found. Averaged estimates of the percentage of clay in the sediments 

deposited in all three reservoirs are shown on figure 4.

Nutrients

The concentration of ammonia in sediments deposited in the reservoirs appears to be related to the 

particle size of the bottom material. The concentration of ammonia in the sediments deposite-i in the 

Washington Boro flats subarea of Lake Clarke averaged 175 mg/kg, the lowest concentration-' in the lake 

(table 5). This area also had the smallest percentage of silt and clay of any area in the lake, 26 percent 

(table 4). Sediments deposited in Lower Lake, above Safe Harbor Dam, contained an average ammonia 

concentration of 456 mg/kg. Sediments in this area contained an average of 77 percent silt and clay (table 

4). The total quantity of ammonia in sediment in Lake Clarke was calculated at 34,100 tons, and the 

average concentration was 376 mg/kg (table 5).

Table 5. Mean concentrations and deposition of ammonia, organic nitrogen, phosphorus, iron, aluminum, and 

manganese in subareas of and in Lake Clarke, Lake Aldred, and Conowingo Reservoir, Lower Susquehanna River Basin

[mg/kg, milligram per kilogram]

Ammonia 

Reservoir subarea M N

mg/kg tons

Organic 

nitrogen 

asN

mg/kg tons

Phosphorus 

asP

mg/kg tons

Iron as Fe

mg/kg tons

Aluminum

mg/kg tons

Manganese

mg/kg tons

Lake Clarke

Washington Boro flats 175

Long Level channel 433

Lower Lake 456

Lake Clarke 376

4,500

1,300

28,300

34,100

3,710

2,670

3,330

3,410

95,300

8,000

206,000

309,000

490

1,000

970

835

12,600

3,000

60,100

75,700

15,200

13,000

18,900

17,600

391,000

39,000

1,170,000

1,600,000

4,010

5,400

6,870

6,010

103,000

16,200

426,000

545,000

990

1,400

1,800

1,560

25,400

4,200

112,000

142,000

LakeAkfrod

Weise Island 212

Duncan Island 58

Lower Lake Aldred 277

Lake Aldred 162

1,400

300

500

2,200

2380

2380

4,320

2,740

17,000

12,400

7,780

37,200

710

470

640

610

4,700

2,440

1,150

8,290

19,200

11,700

18,000

16,200

127,000

61,000

32,400

220,000

9,080

4,690

7,280

7,160

59,900

24,400

13,100

97,400

1,060

580

1,200

900

7,010

3,000

2,160

12,200

Conowinqo Reservoir

Mt Johnson Island 173

Middle Reservoir 230

Lower Reservoir 573

Conowingo Reservoir 404

1,900

14,600

46,100

62,600

3,440

2,960

3,010

3,020

37,900

188,000

242,000

468,000

600

750

1,100

920

6,600

47,500

88,400

142,000

28,000

27,000

22,000

24,400

308,000

1,710,000

1,770,000

3,790,000

9,730

10,800

10,200

10,400

107,000

685,000

819,000

1,610,000

1,200

1,400

1,910

1,650

13,200

88,800

153,000

255,000

12
76'IS1

4V08

\» Safe Harbor 

,S«J» Harbor

39*45*

Peach Bottom D 

Generating Station

PENNSYLVANIA 

MARYLAND

EXPLANATION 

Percent clay in 

depositedsediment.

 j LessThanIO 

] 10To20 

\ 20To30 

3 30To40

Hennery Island

10MILES

J

Conowingo

10KILOMETCRS Conowingo OaniN

Rgure 4. The percentage of day in the sediment deposited in the three reservoirs on the Lower 

Susquehanna River.

13
Ranges of concentrations of organic nitrogen in the sediments deposited in all three reservoirs are 

shown in figure 5. Lake Clarke contained 309,000 tons of organic nitrogen, and the average concentration 

was 3,410 mg/kg (table 5). Concentrations of organic nitrogen in the sediments deposited in Lake Clarke 

ranged from 3,710 mg/kg in the Washington Boro flats to 2,670 mg/kg in the Long Level channel subarea.

Similar to ammonia, the concentration of phosphorus in the sediments deposited in thr reservoirs 

was generally greater in areas where the sediment was mostly silt and clay. Concentration ranges of 

phosphorus in the sediments deposited in Lake Clarke and the other two reservoirs are shown in figure 6.

Concentrations of phosphorus in the Washington Boro flats section of Lake Clarke averaged 

490 mg/kg. Phosphorus concentrations were highest in Lower Lake; phosphorus concentrations in this 

section averaged 970 mg/kg. Sediments in Lake Clarke contained 75,700 tons of phosphorus and the 

average concentration was 835 mg/kg.

Metals

Concentrations of each metal were of the same magnitude in all three areas of Lake Clarke, but 

because the Lower Lake area contains substantially more sediment than the other areas, it contained the 

greatest quantity of metals (table 5).

Iron deposition in Lake Clarke totaled 1,600,000 tons, and the average concentration in the 

sediments was 17,600 mg/kg (table 5). Concentrations of iron in the sediment deposited in the Washington 

Boro flats subarea of Lake Clarke averaged 15,200 mg/kg, and the sediment contained 391,000 tons of iron. 

Concentrations of iron in the sediments in the Lower Lake subarea averaged 18,900 mg/kg, and the 

sediment contained 1,170,000 tons of iron.

Concentrations of aluminum in the sediment collected from Lake Clarke averaged 6,010 mg/kg, and 

the total quantity of aluminum was calculated at 545,000 tons (table 5). Aluminum concentrations in the 

sediments deposited in the Washington Boro flats subarea of Lake Clarke averaged 4,010 mg/kg, and the 

subarea contained 103,000 tons of aluminum. Aluminum concentrations in the sediments in the Lower 

Lake subarea above the dam averaged 6,870 mg/kg and the area contained 426,000 tons of a'uminum.

In Lake Clarke, the average concentration of manganese in the sediment deposited in the 

Washington Boro flats, 990 mg/kg, was about half the concentration in the sediment deposited in the 

Lower Lake, 1,800 mg/kg (table 5). Sediment in the Washington Boro flats contained 25,400 tons of 

manganese, the Lower Lake subarea contained 112,000 tons, and total manganese deposition in Lake 

Clarke was 142,000 tons.

14
76-301

40TJ81

39-451

\« Safe Harbor 

,Safe Harbor 

VD«m

Peach Bottom D 

Generating Station

PENNSYLVANIA 

MARYLAND

EXPLANATIpN 

Concentration of 

organic nitrogen, in 

milligrams per kilogram as N

3 2,000 To 3,000 

| 3,000 To 4,000 

i Greater Than 4,000

Hennery Island

10MILES

Cooowingo

10KILOMETERS Conowingo Dcrrt

Figure 5. Concentration ranges of organic nitrogen in the sediment deposited in the three reservoirs 

on the Lower Susquehahna River.

15
76-301 76-151

40-08'

35T451

J\|i Washington Boro

\» Safe Harbor 

.Sate Harbor

Peach Bottom G 

Generating Station

PENNSYLVANIA 

MARYLAND

EXPLANATION 

Concentration of total 

phosphorus, in 

milligrams per kilogram. 

3 400 To 600

~\ 600 To 800

] 800 To 1,000

1 Greater Than 1,000

Hennery Island

Peach Bot toff

Conowingo

10MILES

10KILOMETERS

Conowingo Darr v

Figure 6. Concentration ranges of phosphorus in the sediment deposited in the three reservoirr on the 

Lower Susquehanna River.

16
Lake Aldred

Sediment Distribution

Sediment thickness in Lake Aldred is estimated to be less than 10 ft throughout the lake. The most 

upstream area of Lake Aldred, the area just downstream of Safe Harbor Dam, had little or no sediment 

deposition. About 4,130 acre-ft (180 million ft3) of sediment were deposited in the 660-acre Weise Island 

area; the dry weight of this sediment was about 6.6 million tons (table 4). The 830-acre Duncan Island area 

contained about 3,280 acre-ft (143 million ft3) of sediment and the dry weight was about 5.2 million tons. 

The 290-acre Lower Lake Aldred area contained about 1,170 acre-ft (50.8 million ft3) of sediment and the 

dry weight of the sediment was about 1.8 million tons. Total sediment deposition in Lake Aldred was 

13.6 million tons.

Sediment Composition

Particle size and coal percentage

The percentages of sand, silt, clay, and coal in bottom-material samples collected from eacl of the 

subareas in Lake Aldred are summarized in table 4. The percentages for these samples were less variable 

than those in Lake Clarke. Sand content was highest in the Duncan Island area. The percentage cf sand 

ranged from 31 to 46 and averaged 38 percent. Clay and coal averaged about 18 and 15 percent, 

respectively.

Nutrients

In Lake Aldred, the maximum concentration of ammonia, 470 mg/kg, was in a bed sample collected 

from a channel west of the Urey Islands (fig. 2). The average concentration in the Weise Island sul irea was 

212 mg/kg, and the deposition of ammonia was 1,400 tons (table 5). Concentrations of ammonia in the 

Duncan Island subarea were the lowest in the lake, 58 mg/kg, and the deposition of ammonia was only 

300 tons. In Lower Lake Aldred, concentrations of ammonia increased from the Narrows toward the 

Holtwood Dam. The average concentration of ammonia in the sediment in Lower Lake Aldred was 

277 mg/kg, and 500 tons of ammonia were deposited in the sediment. The average concentration of 

ammonia in all the samples from Lake Aldred was 162 mg/kg, and 2,200 tons of ammonia were deposited 

in the lake.

The greatest average concentration of organic nitrogen in sediments from the three reservoirs was 

from Lower Lake Aldred (fig. 5, table 5). Lake Aldred contained 37,200 tons of organic nitrogen. 

Concentrations averaged 2,580 mg/kg in the Weise Island subarea, 2,380 mg/kg in the Duncan Inland 

subarea, and 4320 mg/kg in the Lower Lake Aldred subarea.

Phosphorus concentrations in the sediments deposited in the Weise Island subarea, the Duncan 

Island subarea, and the Lower Lake Aldred subarea averaged 710,470, and 640 mg/kg, respectively (fig. 6, 

table 5). Total phosphorus deposition in Lake Aldred was 8,290 tons.

Metals

In Lake Aldred, the concentration of iron in the sediments deposited in the Weise Island subarea, in 

the Duncan Island subarea, and in the Lower Lake Aldred subarea averaged 19,200,11,700, and 

18,000 mg/kg, respectively (table 5). The average concentration of iron in the sediments in Lake Aldred 

was 16,200 mg/kg, and the lake contained 220,000 tons of iron.

Concentrations of aluminum in the sediment deposited in Lake Aldred are shown in figure 7. The 

highest average concentration of aluminum in the sediment (9,080 mg/kg) was in the Weise Islard 

subarea. The subarea contained 59,900 tons of aluminum. Total aluminum deposition in Lake Aldred was 

97,400 tons, and the average concentration was 7,160 mg/kg.

Average concentrations of manganese ranged from 580 mg/kg in sediments deposited in the 

Duncan Island subarea to 1,200 mg/kg in the sediments above the Holtwood Dam. Lake Aldred contained 

12,200 tons of manganese, and the average concentration was 900 mg/kg.

17
76'3<r 76-151

4CT081

35T451

Marietta

\« Safe Harbor 

..Safe Harbor 

\D>m

Peach Bottom D 

GeneratingStation

EXPLANATION 

Concentration of 

aluminum, in 

milligrams per kilogram

3 4,000 To 7,000

% 7,000 To 10,000

1 Greater Than 10,000

Hennery Island

Peach Bottom

PENNSYLVANIA 

MARYLAND

Conowingo

10 MILES

I

10KILOMETERS Conowingo Dam

Rgure 7. Concentration ranges of aluminum in the sediment deposited in the three reservoirs on thr Lower 

Susquehanna River.

18
Conowingo Reservoir

Sediment Distribution

The Conowingo Reservoir contains more sediment than the other two reservoirs combined. 

Sediment deposition in the three reservoirs totaled 259 million tons, of which 155 million tons was in 

Conowingo Reservoir (table 4). Sediment thickness was least in the Mt. Johnson Island subarea and 

ranged from zero in the upper reaches near Hennery Island to 5 ft in the middle and lower area'' of the 

reservoir (fig. 8). Sediment deposition in the Mt. Johnson Island subarea was about 7,120 acre-ft 

(310 million ft3), the average depth was 3.1 ft, and the dry weight of the deposited sediment wa-' about 

11 million tons. Sediment deposition in the 3,020-acre Middle Reservoir area was 41,100 acre-ft (1.79 billion 

ft3) and had an average thickness of 13.6 ft. The dry weight of the sediment was about 63.4 miUron tons. 

Sediment was thickest in the Lower Reservoir area, which had an average thickness of about 30 ft. The 

volume of the sediment in this area was about 56,700 acre-ft (2.47 billion ft3). The dry weight of the 

sediment was 80.5 million tons.

Sediment Composition

Particle size and coal percentage

Steep gradients of sand and clay composition were measured in the Conowingo Reservor. About 

45 percent of the sediment deposited in the Mt. Johnson Island subarea was sand; in the Lower Reservoir 

area, immediately above Conowingo Dam, the sediment was only about 5 percent sand (table 4). About 

7 percent of the sediment was clay at the upper end, and 35 percent was clay above the dam. Coal ranged 

from 2 to 30 percent throughout the reservoir.

Nutrients

Concentrations of ammonia in samples collected from the Conowingo Reservoir ranged from 

13 mg/kg in a sample collected in the Mt. Johnson Island subarea to 730 mg/kg in a sample coll -rted near 

the Conowingo Dam. The average concentration of ammonia in the sediment deposited in the Mt. Johnson 

Island subarea was 173 mg/kg, and the area contained 1,900 tons of ammonia (table 5). Sediment in the 

Mt. Johnson area averaged 25 percent silt and clay. The average concentration of ammonia in the 

sediments deposited in the Middle Reservoir area was 230 mg/kg, and the subarea contained 14,600 tons 

of ammonia. The average concentration in the Lower Reservoir subarea, just above Conowingo Dam, was 

573 mg/kg, and the area contained 46,100 tons of ammonia. Silt and clay made up 93 percent of the 

sediment in the Lower Reservoir area above Conowingo Dam. The total ammonia deposition ir the 

sediments in Conowingo Reservoir was 62,600 tons (table 5). About 63 percent of the total ammonia 

deposition in the three reservoirs (98,900 tons) was stored in Conowingo Reservoir. The average 

concentration in the three reservoirs was 381 mg/kg.

Concentrations of organic nitrogen in the sediments deposited in Conowingo Reservoir a-e shown 

in figure 5. The average concentrations of the three subareas had little variation (table 5). Average 

concentrations were 3,440 mg/kg in the Mt. Johnson Island subarea, and the subarea contained 37,900 tons 

of organic nitrogen. Concentrations in the Middle Reservoir area averaged 2,960 mg/kg, and the subarea 

contained 188,000 tons of organic nitrogen. The Lower Reservoir area contained the most organr nitrogen, 

242,000 tons, and the average concentration was 3,010 mg/kg. The three reservoirs contained 814,000 tons 

of organic nitrogen, and the average concentration in the sediments was 3,140 mg/kg.

Sediments deposited in the Mt. Johnson Island subarea had an average concentration of phosphorus 

of 600 mg/kg (table 5) and the sediment content was 25 percent silt and clay (table 4). Phosphorus 

concentrations in the Middle Reservoir area averaged 750 mg/kg, and concentrations in the Lo^ei 

Reservoir area averaged 1,100 mg/kg. Concentrations of phosphorus measured in the sediments 

deposited in Conowingo Reservoir are shown in figure 6. The sediments contained 142,000 tons of 

phosphorus and the average concentration of phosphorus was 920 mg/kg. About 226,000 tons of 

phosphorus was deposited in the three reservoirs and the average concentration was

19
76-201 16'lff

39-Sff

39'45'

39-401

0

Holtwood 

Dam Muddy Run 

Holtwood / Hydroelectric Dam

Hennery Island

EXPLANATION 

Thickness of sediment 

deposition, in feet.

3 0 To 10

H 10 To 20

H Greater Than 20

4 MILES

0 4 KILOMETERS

Mt. Johnson Island

Peach Bottom

Conowingo 

Dam

PENNSYLVANIA 

MARYI.AND

Conowingo

Figure 8. The thickness of sediment deposition in Conowingo Reservoir.

20
Metals

Concentrations of iron in sediment deposited in Conowingo Reservoir were higher than those in 

sediment deposited in the other two reservoirs (table 5). The average concentration of iron in the Mt. 

Johnson Island subarea was 28,000 mg/kg, the average concentration in the Middle Reservoir area was 

27,000 mg/kg, and the average concentration in the Lower Reservoir area, just above the Comwingo 

Dam, was 22,000 mg/kg. The largest concentration, 37,000 mg/kg, was measured in sedimen* deposited 

north of Mt. Johnson Island. The average concentration of iron in Conowingo Reservoir was 

24,400 mg/kg, and the reservoir contained 3.79 million tons of iron. The three reservoirs contained 

5.61 million tons of iron, and the average concentration was 21,600 mg/kg.

Concentration ranges of aluminum in the sediment deposited in the three reservoirs are shown in 

figure 7. Average concentrations of aluminum in sediment deposited in the three subareas of Conowingo 

Reservoir ranged from 9,730 mg/kg in the subarea around Mt. Johnson Island to 10,800 mg/kg in the 

Middle Reservoir area (table 5). Concentrations of aluminum in sediments deposited in the Lower 

Reservoir subarea of the lake averaged 10,200 mg/kg. The reservoir contained 1,610,000 tons of 

aluminum; 107,000 tons in the subarea around Mt. Johnson Island, 685,000 tons in the Middle Reservoir 

area, and 819,000 tons in the Lower Reservoir area. Total aluminum deposition in the three res«rvoirs was 

2,250,000 tons, and the average concentration was 8,700 mg/kg.

Average concentrations of manganese in the sediments deposited in the Conowingo Rerervoir 

ranged from 1,200 mg/kg in the Mt. Johnson Island subarea to 1,910 mg/kg in the area above Conowingo 

Dam (table 5). Manganese deposition was 13,200 tons in the Mt. Johnson Island subarea, 88,800 tons in the 

Middle Reservoir area, and 153,000 tons in the Lower Reservoir area. The load of manganese deposited in 

Conowingo Reservoir was 255,000 tons, and the average concentration of manganese in the sediments was 

1,650 mg/kg. Total manganese deposition in the three reservoirs was 409,000 tons, and the average 

concentration was 1,580 mg/kg.

Effect of Deposition on Reservoir Storage

The reservoirs formed by the dams act as a sediment trap, reducing the load of sediment nutrients, 

and metals that would otherwise be transported to Chesapeake Bay. As the reservoirs fill with sediment, 

the amount of sediment deposited decreases, and the amount transported to the bay increases Transport 

to the bay can also be increased during periods of very high flow, when previously deposited materials 

can be scoured from the reservoirs and transported to Chesapeake Bay.

Lake Clarke

Lake Clarke was surveyed in 1931,1939, several times from 1940 to 1964, and in 1990. The reservoir 

water-storage capacity calculated from the survey data is shown in figure 9 . The capacity decreased at an 

average rate of about 3,400 acre-ft per year for the first 19 years. Since 1950, the capacity of the reservoir 

has been almost constant.

The average cross-section bed elevations of Lake Clarke in 1931 and in 1990 and the crofs-sectional 

area in the lake for the same years are shown in figure 10. The cross-sectional area of Lake Clarke ranges 

from about 75,000 to 110,000 ft2. From 1954 to 1972, about 1.0 million tons of sand and coal per year were 

dredged from the reservoir. The surveys indicate that the dredged material was replaced by incoming 

sediments.

Lake Aldred

Average cross-section bed elevations and areas of Lake Aldred have changed little since the 

construction of Holtwood Dam in 1910 (fig. 11). This indicates that the reservoir reached equilivrium soon 

after construction, with respect to sediment deposition.

Over the long term (1910-90), sediment deposition in Lake Aldred decreased the cross-settional area 

only slightly (fig. 11). Holtwood Dam has no flood gates and river flow in excess of plant capacity is spilled 

over the breast of the dam. At river discharges where reservoir sediments are expected to scour 

(400,000 frVs), the increase from the normal pool elevation in Lake Aldred is about 15 ft.

21
1 3

«<o

CAPACITY OF LAKE CLARKE, IN THOUSANDS OF ACRE-FEET

oi pi -*j o» <p o -* 

o o o o o o o 

2° 3 

o «o n> "

S 2!

o> ce

55 T

§s 

tJ

CD CD 

a S

- Q. 

CDC?

So

IS

SS

8* 

8«

CO

I

I

0> 

0>

5 "

-< _» m m

> *

3

CD

CD 

CD

I

DAM COMPLETED

START OF DREDGING

1972 FLOOD - DREDGING STOPPED

i , i i , i , . . . i . . i , i . . . . i . , . . i . , , , i .i. . . i . , . , i . , . . i . . . . i
fc

LU

co

LU

1

<

I

<

240

230

220

210

200

190

180

170

160

WATER SURFACE 

\

1931

32 33 34 35 36 37 38 39 40 41 42

LLJ 

LLJ 

LL 

LLJ 

CC

o

co 50,000

cr

I

LU

co

LU

Si 100,000

o

35

150,000

LU

co

co 

co O 

cr 

a

200,000

1931

32 33 34 35 36 37 38 39 40 

DISTANCE UPSTREAM OF CHESAPEAKE BAY, IN MILES

41 42

Figure 10. Relations of upstream distance from Chesapeake Bay to average bed elevations and 

cross-sectional areas in Lake Clarke, Lower Susquehanna River Basin, 1931 and 1990.

23
BJ

z

tu§

sco

Ul

IU

El

190

180

170

160

150

140

130

120

110

100

90

24

FLOOD POOL

NORMALPOOL

25 26 27 28 29 30 31 32

25 26 27 26 29 30 31 32 

DISTANCE UPSTREAM OF CHESAPEAKE BAY, IN MILES

Rgure It. Relations of upstream distance from Chesapeake Bay to average bed elevations and 

cross-sectional areas in Lake Aldred, Lower Susquehanna River Basin, 1910 and 1990.

24
Conowingo Reservoir

The capacity of Conowingo Reservoir was 300,000 acre-ft when the dam was completed in 1928. 

Bottom-elevation data reported by Whaley (1960) indicated that the capacity of the reservoir wa' about 

235,000 acre-ft in 1959. The 1990 survey indicated that the capacity of Conowingo Reservoir was about 

196,000 acre-ft.

Although the reservoir has filled considerably since 1959 (fig. 12), bed elevations in the upper third 

of the reservoir were lower in 1990 than in 1959, indicating that the area has been scoured. The r?ason for 

the lowering of bed elevations between 1959 and 1990 may have been the installation of a pump-storage 

generating station in the headwaters of the reservoir in 1968. Station operations increased the maximum 

daily instantaneous water discharge in the headwaters of the reservoir from about 27,000 frVs to about 

57,000 ft3/s.

Cross-sectional areas between river miles 15 and 22 range from about 70,000 to 125,000 ft2, similar to 

the cross-sectional areas in Lake Clarke and Lake Aldred. It appears that each of the reservoirs in the 

system reaches equilibrium with respect to sediment when cross-sectional areas are in the range of 70,000 

to 125,000 ft2, or an average area of 100,000 ft2. The turbulence caused by the bottom-release mechanism at 

Conowingo Dam will probably not allow as much deposition in the lower subarea of the reservoir as has 

taken place in upstream subareas and reservoirs. For the purposes of this analysis, a cross-sectional area 

for equilibrium just above Conowingo Dam was estimated to be 200,000 ft2. It is assumed that th° effect of 

releases from the dam would diminish with distance upstream from the dam and that the average 

equilibrium cross-sectional area would eventually approach 100,000 ft2. On the basis of 1990 cro?ssectional data, this was estimated to take place at a point about 1.25 mi upstream of the dam (river mile 

11).

On the cross-sectional area graph in figure 12, an additional dashed line is shown. The line is drawn 

from the dam at a cross-sectional area of 200,000 ft2 to the estimated reservoir-system equilibrium crosssectional area of 100,000 ft2 at a point 1.25 mi upstream of the dam. The reservoir can store an additional 

34,000 acre-ft of sediment before the downstream section reaches equilibrium with incoming sediments at 

the level shown by the dashed line in figure 12.

Suspended-sediment data, collected from the Susquehanna River at Harrisburg and Conovingo and 

at four major tributaries from 1985 through 1989, were used to calculate sediment deposition in the 

reservoirs for the 5-year period. Over the 5-year period, a total of 12.6 million tons of sediment vas 

transported to the reservoirs, and the total sediment discharge from Conowingo Reservoir was 

3.5 million tons. This indicates that an average of 1.8 million tons of sediment were trapped eacl year, 

primarily in the Conowingo Reservoir. Assuming the dry density of the sediment is 65 Ib/ft3, the capacity 

of the reservoir was decreasing at an average rate of 1,270 acre-ft per year. Water discharge was relatively 

low during 2 years, 1985 and 1988, and for those years, the average loss of capacity was 770 acre-ft per 

year. Deposition averaged 1,600 acre-ft per year during the remaining 3 years when water discharge 

averaged 5 percent below normal. Assuming annual sediment deposition of 1,700 acre-ft, no sco"r from 

very large storms, and deposition only in the Conowingo Reservoir, about 20 years would be required to 

accumulate 34,100 acre-ft in the Conowingo Reservoir.

Even though the exact cross-sectional areas for sediment equilibrium in Conowingo Reservoir are 

not known, figure 12 shows that the reservoir is nearing capacity and that it will be full in the next 20 or 

30 years. Once equilibrium is reached, the incoming loads of sediment and nutrients to the reservoirs will 

pass through the reservoirs and enter Chesapeake Bay. Loads discharged during periods of high flow will 

increase, which is routine in the late winter and early spring, and additional loads may be discharged 

because of scour during extreme flow periods.

25
tD 

til

w

LU

co

LU

g

LU

OJ

120

110

100

90

80

70

60

50

40

30

20

10

WATER SURFACE

1990

\

9 10 11 12 13 14 15 16 17 18 19 20 21 22

a. 

O

LU

co

UJtr

co 

co

8 tr 

o

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

1990 1959

1928

9 10 11 12 13 14 15 16 17 18 19 20 21 22 

DISTANCE UPSTREAM OF CHESAPEAKE BAY, IN MILES

Figure 12. Relations of upstream distance from Chesapeake Bay to average

bed elevations and cross-sectional areas in Conowingo Reservoir, Lower Susquehanna

River Basin, 1928,1959, and 1990.

26
SIMULATION OF SEDIMENT TRANSPORT IN THE RESERVOIR SYSTEM

Description of the Selected Model

A realistic computation of the transport of sediment through large, shallow reservoirs, such as on 

the Lower Susquehanna River, requires a numerical model that can simulate both the hydraulic 

characteristics of the stream and the deposition and scour of different sizes of sediment particles 

Summaries of the basic equations, functional capabilities, limitations, and available documentation for 12 

of the most sophisticated stream-sedimentation models commonly used in the United States (Fen, 1988) 

were reviewed. The U.S. Army Corps of Engineers' HEC-6 computer model was selected as the most 

suitable for this study.

The HEC-6 model is designed for one-dimensional simulation of sediment transport under 

changing conditions of boundary geometry and roughness. Water discharge was assumed to be relatively 

constant between reservoir sections. Features that were paramount in the selection were the abi."ity to 

simulate long-term trends of deposition and scour; scour routines that accommodate the full range of 

grain sizes observed in the inflow; and computation of sediment transport by grain-size fractions wherein 

the algorithms accommodate hydraulic sorting and bed armoring.

Limitations of the HEC-6 model include the inability to simulate density currents, bed forms, or 

lateral gradations in deposition or scour. The coding of inflows, which is composed of a series o* shortduration discharge values that approximate the inflow hydrograph, is cumbersome. Sediment-discharge 

data for outflows must be extracted from the output file and post-processed with auxiliary softvare to 

summarize them as daily-value sequences. Sediment-transport capacity is assumed to be in equ;Ubrium 

with flow hydraulics for each inflow time step, which is a condition that seldom exists in reservo4rs during 

high-flow periods.

The developers of the HEC-6 model recognized that deficiencies in available engineering knowledge 

limited their ability to write routines for exact simulation of the mechanisms of armoring, hydraulic 

sorting, and re-entrainment. Of particular significance was the lack of knowledge on the mechanisms of 

clay transport for concentrations greater than 300 mg/L. Details on the theory, equations, and assumptions 

incorporated in the HEC-6 model are provided in the "User's Manual" (U.S. Army Corps of Engineers, 

1991).

The input files to the HEC-6 model are alphanumerically coded records grouped according to data 

content, geometry, sediment, hydrology, and special commands. Geometric data are in the format of the 

HEC-2 step-backwater program (U.S. Army Corps of Engineers, 1984). Water-surface profiles ar? 

computed with the standard step method of that program. For each cross section, special record^1 define 

bed thickness, limits of the movable and fixed parts of the bed, and the limits of dredging, as an option.

Data on the sediment content of inflows consist of particle-size fractions of mean daily loads 

associated with as many as nine discharge values, which are selected to define the full range of the 

sediment-transport curve. Bedload fractions, if significant, are included with these suspended-sediment 

data. Particle-size fractions of bed deposits in the reservoirs are coded for each of the cross sections.

User-specified variables for sediment-transport computations include a choice of 10 sedirrenttransport functions for sand or, alternatively, user-determined transport coefficients; choices for fallvelocity and bed-shear computation methods; specific gravities for clay, silt, and sand; shear-stress 

thresholds for both deposition and erosion of clay and silt; shear-stress thresholds for mass erosion of clay 

and silt; mass-erosion rate; unit weights of unconsolidated and consolidated bed deposits; compaction 

coefficients; and a grain-shape factor for sand.

In the hydrologic-data records, the outflow discharge at the downstream end of the reservoir and as 

many as nine local inflow and outflow discharges for tributaries and diversions in the modeled reach are 

coded as a single record. Another record indicates the durations, in days, for each of these discharges. 

Temperatures of the inflows are a required input. Sets of discharge, duration, and temperature records are 

sequenced to hydrographically describe the simulation period.

27
Application of Model to Reservoir System

Development

The HEC-6 model for the Clarke-Aldred-Conowingo Reservoir system was prepared fr>m the 

previously described seismic data on water depths and sediment thickness and from the par "dele-size 

fractions of accumulated and inflow sediments. Representative cross-sectional data were developed at 

selected intervals from the centerline of Conowingo Dam to the streamflow gage on the Susquehanna 

River at Marietta, Pa.

Hydraulic calibration of the model generally followed the procedures in the HEC-6 model 

calibration and application document (U.S. Army Corps of Engineers, 1981). Manning's roughness 

coefficients were chosen on the basis of field observations and values previously used in modeling similar 

channels. Starting water-surface elevations at Conowingo Dam were determined with a ratify curve, 

which was developed from the dam-tender's forebay water-surface elevations and corresponding 

discharges at the USGS streamflow gage at the tailrace. Because the HEC-6 model will accept only one 

rating curve for a stream subarea, previous measurements of forebay elevations and discharges for the 

Holtwood and Safe Harbor Dams were used to develop simple dam-geometry models that approximated 

the discharges through and over both hydroelectric dams. The developed hydraulic model c'osely 

replicated the high-water profile of the 1972 flood, as documented by Miller (1974), in the reach from 

Conowingo Dam to Marietta, Pa.

Simulation of sediment transport with the HEC-6 model was calibrated by attempting to reproduce 

monthly and annual inflows and outflows of sediment loads for calendar year 1987. These lo^ds were 

computed with version 90.10 of a program by Cohn and others (1989). The sediment-transpon relations 

for calculating inflow loads were developed from 125 measurements of suspended-sediment 

concentrations and corresponding instantaneous water discharges that were made from 1987-89 at the 

streamflow gage on the Susquehanna River at Marietta, Pa. (USGS number 01576000), and from 410 

similar measurements made during that same period at the streamflow gage on the Conestoga River at 

Conestoga, Pa. (USGS number 01576754). To include the drainage area and loads of the adjacent Pequea 

Creek Basin, which has similar sediment yields, loads computed for the Conestoga River at Conestoga 

were multiplied by 1.34, a factor that is based on the size of the Pequea Creek Basin. The effec^ve drainage 

area represented by the computed sediment inflows of the Susquehanna River, Conestoga Ri-'er, and 

Pequea Creek is about 26,620 mi2, or 98.2 percent of the drainage area at Conowingo Dam.

For consistency, the Cohn program was used to compute monthly and annual loads of sediment 

discharged from the reservoir system. A data set that contained 215 pairs of sediment-concerfration and 

discharge measurements made at the streamflow gage on the Susquehanna River at Conowingo during 

1987-89 was used to calculate the loads. For 1987, the sediment load calculated by the Cohn r«odel for the 

Susquehanna River at Conowingo was 565,000 tons. This load compares closely with the load of 

539,000 tons calculated by directly integrating sediment concentrations and water discharge.

Initial estimates of the mean fractions of 13 standard particle sizes, from clay to medium gravel, 

associated with various flows at Marietta and discharges from the Conestoga River and Peqvea Creek, 

were developed from a manually prepared sediment-transport curve and available particle-size analyses 

of suspended sediment.

The sediment-transport curve for Marietta was developed from a selected set of sedirrentconcentration data collected at the Marietta gage (107 of 125 calculations) during 1987-89, and 15 load 

calculations made for the flood of June 1972 at the gage on the Susquehanna River at Harrisburg, Pa., 

about 25 mi upstream of Marietta. The curve was given a positive bias of 4 percent 2 percent to allow for 

480 mi2 of ungaged area and 2 percent, as an estimate, for unmeasured bedload.

28
Because no particle-size determinations of suspended sediment were available at the Mar etta gage, 

9 particle-size analyses samples collected from the Susquehanna River at Harrisburg, Pa., in 1980,1981, 

and 1989 were used to estimate transport curves for 10 size fractions (1 clay, 4 silt, and 5 sand sizes) at 

Marietta. Throughout the observed range of water discharge, clay loads were reduced by 50 percent to 

convert the loads, as determined by standard laboratory analysis, to "natural" loads. Loads of very fine silt 

were increased by the same amounts that clay loads were reduced. This conversion was based on 

comparison of a mechanically and chemically dispersed particle-size distribution with a mechanically 

dispersed particle-size distribution collected from Bixler Run, a stream located near the center cf the 

Lower Susquehanna River Basin, during a wintertime flood in 1965. Laboratory particle-size analysis 

results in a misrepresentation of the actual sizes of particles suspended in the streamflow caused by the 

physical and chemical breakdown of colloids during the analyses (Guy, 1969). Better model results would 

probably be obtained if in situ (undispersed) particle-size data were available. The initial HEC-6 input of 

fractional distributions of particle sizes in the total sediment load at Marietta and from ungaged areas, 

representing the contribution from 26,470 mi2, are listed in table 6 for selected discharges.

Table 6. Initial estimates of fractional distributions of particle loads for selected discharges of the 

Susquehanna River at Marietta, Pa.

Discharge, in cubic feet per second

Particle size

Natural clay

Very fine silt

Fine silt

Medium silt

Coarse silt

Very fine sand

Fine sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

1,000

034

.45

.08

.06

.02

.04

.01

0

0

0

0

0

0

10,000

0.28

.40

.10

.08

.04

.04

.03

.01

.01

.01

0

0

0

35,000

0.26

.38

.105

.085

.055

.045

.035

.01

.01

.01

.005

0

0

50.000

0.25

.36

.11

.09

.06

.05

.04

.01

.01

.01

.01

0

0

100,000

0.23

33

.12

.10

.07

.05

.04

.02

.01

.01

.01

.01

0

200,000

0.22

.30

.12

.11

.07

.055

.045

.02

.015

.015

.015

.01

.005

500,000

0.19

.27

.13

.12

.075

.055

.045

.03

.02

.02

.02

.015

.01

1,000,000

0.18

.25

.14

.13

.08

.05

.04

.035

.025

.02

.02

.02

.01

29
The sediment-transport curve for the Conestoga River and Pequea Creek Basins was developed 

from the concentrations of 108 suspended-sediment samples collected during 1985-89 at the Conestoga 

River gage. The transport curve was partitioned into particle-size curves on the basis of 11 prrticle-size 

analyses of suspended sediment collected at the gage from 1987 through 1989. As with the Susquehanna 

River at Marietta curves, half of the "laboratory" clay loads were shifted to the very-fine-silt fraction in 

approximating the "natural" size distributions of loads. The initial input of fractional particle-size 

distributions for the Conestoga River and Pequea Creek, representing 630 mi2, are listed in table 7.

Table 7. Initial estimates of fractional distributions of particle loads for selected discharges of the 

Conestoga River and Pequea Creek Basins

Discharge, in cubic feet per second

Particle size

Natural clay

Very fine silt

Fine silt

Medium silt

Coarse silt

Very fine sand

Fine sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

80

0.40

.56

.03

.005

.005

0

0

0

0

0

0

0

0

100

0.36

.54

.06

.02

.01

.01

0

0

0

0

0

0

0

1,000

0.30

.47

.11

.06

.03

.015

.01

.005

0

0

0

0

0

2,000

0.29

.45

.12

.065

.037

.018

.013

.007

0

0

0

0

0

5,000

0.27

.40

.14

.086

.053

.023

.015

.01

.003

0

0

0

0

10,000

0.25

.38

.15

.095

.060

.025

.020

.01

.01

0

0

0

0

30,000

0.22

.34

.165

.105

.070

.037

.033

.017

.013

0

0

0

0

90000

020

.30

.17

.12

.080

.048

.042

.020

.015

.005

0

0

0

Bed composition throughout the three reservoirs was determined from particle-size anilyses of bed 

materials. The clay and very-fine-silt fractions were adjusted to "natural" size fractions in the same 

manner as were the suspended-sediment loads. Fractional particle-size distributions at the selected cross 

sections were determined from an interpolation of particle-size fraction profiles along each rfservoir. The 

bed sections below each dam and at cross sections in the swift-water parts of the study reach were 

assumed, for coding purposes, to have a thin layer (usually 0.01 ft) of silt.

Calibration

The HEC-6 model was calibrated on the basis of observed inflows for calendar year 1987. The goal 

was to approximate the trap efficiency of the reservoir system calculated by the difference between the 

reference inflow and outflow loads determined with the Cohn model. No load observations rre available 

for evaluating how well the transport and deposition of sediment was simulated within individual 

reservoirs or between reservoirs. Also, no particle-size data are available to evaluate the simulation of clay, 

silt, and sand loads discharged at Conowingo Dam.

Step-wise hydrographic data were used to partition inflow to the reservoirs for calendrr year 1987. 

Thirty-four time steps were used to code the 1987 hydrograph. Water-discharge values for th? Conestoga 

River-Pequea Creek contributions were determined from the equation

QC = log (Q5765 + 0.5) / 1.05, (7)

where Qc is total discharge of Conestoga River and Pequea Creek Basins, and

Q5765 is mean discharge at the Conestoga River at Lancaster, Pa., a long-term streamflow-gaging 

station (USGS number 01576500).

Data from the long-term streamflow-gaging station at Lancaster were used instead of multip'ying 

discharge for Conestoga River at Conestoga by 1.34 to include discharge from Pequea Creek. The greater 

length of water-discharge record at the Lancaster streamflow-gaging station should increase the accuracy 

of die estimate of water discharge for the Conestoga and Pequea Creek Basins.

30
Water-temperature data were derived from annual trend curves for stream temperatures rt the 

Harrisburg and Lancaster gages (Flippo, 1975). Temperatures were specified at a sufficient number of 

intervals to describe the seasonal trend of water temperatures.

Sediment properties and transport parameters were selected in accordance with guidelines 

provided by the HEC-6 User's Manual (U.S. Army Corps of Engineers, 1991). Initially, the specific 

gravities for clay and silt were those determined by analysis of bed deposits. However, a final va'ue of 2.65 

was selected because of the high transport rate initially calculated for these fractions. A value of 2.1 for 

sand was determined by proportional weighting of the specific gravities of the coal and sand fractions. 

Depositional shear-stress thresholds of 0.022 and 0.024 Ib/ft2 were estimated for the surficial and deeper 

bed layers, respectively. Provision was made for 140 iterations of the Exner equation. Even with this many 

iterations, it was sometimes necessary to code short time steps for low to moderate inflows in order to 

avoid math errors in the computations.

Model computations for the calibration year of 1987 resulted in low trap efficiencies, even though 

the computational options that gave the highest trap efficiencies were selected. For the year, the initial 

HEC-6 model run resulted in a trap efficiency of 33.8 percent, as compared to the reference efficiency of 

71.0 percent from the Conn model. Differences between the HEC-6 model results and the reference trap 

efficiencies for both high- and low-flow periods were similar.

Inspection of the HEC-6 model simulation transport summaries indicated that no sand ard only 

small amounts of coarse silt passed through the reservoirs during 1987. This result was reasonab1 ^ because 

the peak water discharge during the year was 236,000 f^/s. Model simulations indicated that much of the 

finer silt and virtually all the clay would pass through the reservoir system. The resulting sedirrent load 

calculated as leaving the reservoir system in 1987 was significantly more than the load simulated in the 

Cohn model, as well as the load computed by integrating the concentration and flow data. The option 

selected to obtain realistic trap efficiencies was a further shifting of the particle-size distribution-' listed on 

table 6 so that less clay and more silt and sand would enter the system. Consequently, transport curves for 

the various fractions of the inflow sediment were adjusted to provide for coarser-grained loads. Total 

tonnage/discharge relations were not changed. The adjustment to the initial curves was made I *' 

successive trial steps until the computed trap efficiency for the reservoir system was within one standard 

error of the Cohn efficiency value. The original and revised fractional distributions of particle loads for the 

Marietta inflow and the Conestoga River-Pequea Creek inflows are given, respectively, in tablef 8 and 9.

31
Table 8, Original and revised fractional distributions of particle toads for selected discharges of the Susquehanna River at Marietta, P

[O, original value; R, revised value]

Discharge, in cubic feet per second

Particle size

Natural day

Wry fine silt

Fine silt

Medium silt

Coarse silt

Wry fine sand

Fine sand

Medium sand

Coarse sand

Wry coarse sand

Wry fine gravel

Fine gravel

Medium gravel

1,000

0

034

.45

.08

.06

.02

.04

.01

0

0

0

0

0

0

R

0200

.180

.190

.197

.182

.049

.002

0

0

0

0

0

0

10,000

0

028

.40

.10

.08

.04

.04

.03

.01

.01

.01

0

0

0

R

0.166

.156

.165

.174

.160

.065

.057

.036

.017

.004

0

0

0

35,000

0

026

.38

.105

.085

.055

.045

.035

.01

.01

.01

.005

0

0

R

0.148

.143

.151

.156

.136

.076

.082

.062

.035

.011

0

0

0

50,000

0

025

.36

.11

.09

.06

.05

.04

.01

.01

.01

.01

0

0

R

0.142

.138

.147

.150

.134

.079

.088

.068

.040

.014

0

0

0

100,000

0

0.23

.33

.12

.10

.07

.05

.04

.02

.01

.01

.01

.01

0

R

0.132

.129

.137

.140

.124

.086

.097

.082

.052

.020

.001

0

0

200,000

0

022

30

.12

.11

.07

.055

.045

.02

.015

.015

.015

.01

.005

R

0.120

.118

.126

.130

.117

.094

.105

.093

.064

.029

.003

.001

0

50

0

0.19

.27

.13

.12

.075

.055

.045

.03

.02

.02

.02

.015

.01
Table 9. Original and revised fractional distributions ofpartide loads for selected discharges of the Conestoga River and Pequea Cre

Basins

[O, original value; R, revised value]

Discharge, in cubic feet per second

Particle size

Natural clay

Very fine silt

Fine silt

Medium silt

Coarse silt

Very fine sand

Fine sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

80

O

0.40

.56

.03

.005

.005

0

0

0

0

0

0

0

0

R

0.320

.200

.190

.180

.110

0

0

0

0

0

0

0

0

100

O

0.36

.54

.06

.02

.01

.01

0

0

0

0

0

0

0

R

0310

.195

.185

.175

.115

.020

0

0

0

0

0

0

0

1,000

O

0.30

.47

.11

.06

.03

.015

.01

.005

0

0

0

0

0

R

0.240

.150

.143

.136

.130

.098

.075

.028

0

0

0

0

0

2,000

O

0.29

.45

.12

.065

.037

.018

.013

.007

0

0

0

0

0

R

0.220

.140

.135

.131

.134

.110

.091

.037

.002

0

0

0

0

5,000

O

0.27

.40

.14

.086

.053

.023

.015

.01

.003

0

0

0

0

R

0.200

.131

.128

.126

.134

.123

.106

.049

.003

0

0

0

0

10,000

O

0.25

.38

.15

.095

.060

.025

.020

.01

.01

0

0

0

0

R

0.190

.125

.122

.122

.134

.133

.117

.053

.004

0

0

0

0

O

0.22

.34

.16

.10

.07

.03

.03

.01

.0

0

0

0

0
Results

A summary of the calibration results is listed in tables 10 and 11. An example of a detailed output 

summary, the cumulative bed change, as well as the water-surface elevation, thalweg elevation, the 

modeled discharge, and sediment loads, by size fraction for each cross section, for the December 30-31 

time step is listed in table 10. The sediment load listed is the amount passing each section, in torts per day. 

A summary of the fractional sediment loads in acre-feet passed between the reservoirs during t're 

calibration run is listed in table 11. Also included are the trap efficiencies of each reservoir for ti ? total 

sediment load. The input at river mile 32.110 reflects the tributary inflow of the Conestoga Rive* and 

Pequea Creek.

These data indicate that about 33 percent of the clay, 61 percent of the silt, and 100 percen* of the 

sand in the inflow sediment loads during 1987 a typical year were trapped by the reservoirs Thirtyfive percent of the inflow sediment as computed in the HEC-6 model simulation, or 563,000 torn, passed 

through Conowingo Dam.

Summaries of annual loads and trap efficiencies obtained after the revision to the model input 

parameters for the calibration year of 1987 and the verification years of 1988 and 1989 are giver for the 

HEC-6 model in table 12. Loads computed in the Conn model and by a hand-integration technique also 

are presented in table 12. Mean inflows for each of the 117 time steps used to input the 3 years of 

hydrographic data into the model agreed within 10 percent with the corresponding mean water discharges 

at the streamflow-gaging station on the Susquehanna River at Conowingo, Md. (USGS number 01578310). 

Annual inflow and outflow water discharges agreed within 3 percent.

High-flow sediment transport was not simulated in the HEC-6 model as well as the data of table 12 

indicates. The results of simulated sediment transport for May and June 1972 are summarized in table 13. 

The peak flood of record occurred during this period. The simulation indicated that clay and si1* were 

scoured from the reservoirs in amounts equal to about half of the load measured during the 2-nonth 

period. Additionally, the simulation indicated that 86 percent of the sand in the inflow was trapped. These 

results are inconsistent with data collected from the Susquehanna River at Harrisburg, Pa., and at 

Conowingo, Md., during the flood of June 1972. From June 21 through June 30,1972, the meas\ired 

sediment load transported by the Susquehanna River at Harrisburg was 7.52 million tons, and the load 

transported by the Susquehanna River at Conowingo was 34.8 million tons. Sediment scoured from the 

three reservoirs may have been more than 23 million tons instead of the 2 million tons of deposition 

simulated in the model.

34
Table 10. Summary output from the HEC-6 model showing bed-surface and water-surface elevation change, the 

simulated discharge, and the sediment load passing each cross section in the Lower Susquehanna River 

reservoirs/stem, December30-31, 1987

_ . . Water- Section Bed-surface 

number elevafon change e|evation 

(feet) (feet)

44.900

44.400

43.580

43.000

41.230

39.240

37.810

37.000

35.750

33.810

32.840

32.382

3Z381

32.380

32.300

3Z110

31.010

29.530

28.670

27.330

26.440

25.970

24.960

24.691

24.690

24.650

24.270

23.170

22.500

21.230

20.010

18.400

17.260

16.000

14.800

13.840

12.520

11.530

10.320

9.743

9.742

9.740

-0.01

-.01

-.01

-.05

-.08

.43

.08

.07

.05

.03

.02

-.01

.06

-.01

.00

-.04

.00

.17

.04

.04

.04

.01

-.02

.00

-.01

.00

.00

.00

-.06

.01

.05

.04

.03

.04

.04

.03

.03

.02

.01

.01

.00

.01

238.29

237.95

234.12

226.93

226.03

226.02

226.02

226.01

226.01

226.01

226.00

226.00

226.00

192.80

169.18

168.41

168.30

168.29

168.28

168.27

168.27

168.26

168.26

167.50

159.44

109.72

108.14

107.41

107.21

107.14

107.13

107.12

107.11

107.11

107.11

107.11

107.11

107.11

107.11

107.11

107.11

1Q7.11

Thalweg 

elevation 

(feet)

229.49

228.99

230.49

220.95

217.92

208.43

206.08

193.07

193.05

194.03

193.02

192.99

175.06

174.99

155.00

157.%

152.00

137.17

139.04

146.04

126.04

93.01

124.98

150.00

149.99

104.00

70.00

75.00

81.94

93.01

89.05

84.04

81.53

65.04

65.04

74.03

54.03

56.52

42.01

42.01

42.00

90.01

Water 

discharge 

(cubic feet per 

second)

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,000

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

34,550

Fractional sediment load 

(tons per day)

Clay

359

359

359

359

359

319

294

278

260

246

238

237

237

237

237

244

241

232

226

220

217

214

214

214

214

214

214

214

214

208

190

173

160

148

138

128

121

116

113

112

112

112

Silt

1,421

1,421

1,421

1,421

1,421

795

627

552

480

435

412

410

410

410

410

426

418

394

378

362

354

347

347

347

347

347

347

347

347

335

291

254

228

205

187

170

159

150

145

144

144

144

F*nd

660

667

667

675

161

0

0

0

0

0

0

0

0

0

0

33

23

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

35
Table 11. Summary output from HEC-6 model calibration run showing 1987 sediment toad introduced to ea^h reservoir 

on the Susquehanna River and sediment load trap efficiency

[--, not applicable]

Reservoir

Clarke

Aldred

Conowingo

Entry /Exit 

River Mile

44.900

32.381

32.381

32.110

24.691

24.691

9.74

In

Clay

Out

(acre-feet)

325.15

1 (325.15)

28230

58.52

(341.02)

327.99

(327.99)

-

28Z50

-

-

327.99

-

257.55

Trap 

efficiency 

(percent)

13

-

-

4

-

21

In

Silt

Out

(acre-feet)

705.68

(705.68)

423.25

82.62

(505.87)

463.39

(463.39)

423.25

-

-

463.39

-

305.33

Trap 

efficiency 

(percent)

40

-

-

8

-

34

In

Swd

Cut

(acre-feetl

382.78

(382.78)

.58

39.62

(40.20)

14.27

(14.27)

-

0.58

-

-

14.27

-

.00

Trap 

efficiency 

(percent)

100

-

-

65

-

100

1 Numbers in parentheses are the total measured inputs to the reservoir.

Table 12. Loads and trap efficiencies for the three-reservoir system on the Lower Susquehanna River, a-s computed 

in the HEC-6 and Cohn models, and by a hand-integration method

[-, not applicable]

HEC-6 model

Year

1987

1988

1989

Flow 

type

Inflow

Outflow

Inflow

Outflow

Inflow

Outflow

Load, in thousands of tons

Clay

384

258

253

165

516

400

Silt

788

305

930

358

1,982

901

Sand

368

0

557

0

1,308

0

Total

1,594

563

1,740

523

3,806

1,301

Trap 

efficiency 

(percent)

-

64.7

-

69.9

-

65.8

Cohn

Total load 

(thousands 

of tons)

1,945

565

1,850

428

3,730

990

model

Trap 

efficiency 

(percent)

-

71.0

-

76.9

-

73.4

Integration method

Total load 

(thousands 

of tons)

539

450

917

Trap 

efficiency 

(percent)

-

72.3

-

75.7

-

75.4

Table 13. HEC-6 loads and trap efficiencies for the three-reservoir system 

on the Lower Susquehanna River, May and June 1972

Load (thousands of tons)

Clay Silt Sand Total

Inflow

Marietta

Conestoga-Pequea 

Total inflow

1,122

211

1,333

4,791

637

5,428

5,560

484

6,044

11,473

1,332

12,805

Outflow

Conowingo Dam 2,003 7,903 860 10,766

TVap efficiency (percent): -50 -46 86 16

36
SUMMARY AND CONCLUSIONS

The Susquehanna River drains 27,510 mi2 in New York, Pennsylvania, and Maryland and is the 

largest tributary to the Chesapeake Bay. Three large hydroelectric dams span the Susquehanra River. Safe 

Harbor (Lake Clarke) and Holtwood (Lake Aldred) are in southern Pennsylvania, and Conovingo 

(Conowingo Reservoir) is in northern Maryland, about 10 mi upstream of the Chesapeake Bay. The 

reservoirs behind the dams have trapped large quantities of sediment, nitrogen, and phosphorus.

In the fall of 1990, sediment deposition in the three reservoirs amounted to about 259 trillion tons; 

Conowingo Reservoir, about 155 million tons; Lake Clarke, about 90.7 million tons; and Lake Aldred, 

about 13.6 million tons. The sediment from all three reservoirs was composed of 64.8 million tons of sand, 

19.7 million tons of coal, 112 million tons of silt, and 63.3 million tons of clay. About 33 percer*: of the 

sediment in the three reservoirs was sand and coal. The percentage of sand and coal ranged from 

75 percent in the upper part of Conowingo Reservoir to 7 percent in the lower part of Conowingo 

Reservoir. Sediment in the lower part of Conowingo Reservoir averaged 58 percent silt, 35 percent clay, 

2 percent coal, and 5 percent sand. The sediment in the reservoirs contained about 814,000 tons of organic 

nitrogen, 98,900 tons of ammonia as nitrogen, 226,000 tons of phosphorus, 5,610,000 tons of iron, 

2,250,000 tons of aluminum, and about 409,000 tons of manganese. Deposition in the reservoirs was 

variable and ranged from areas of little or no deposition to depths of about 30 ft.

Lake Aldred and Lake Clarke reached equilibrium with incoming river sediment by 1910 and 1950, 

respectively, and are no longer storing sediment. The original capacity (in 1928) of the reservoir formed by 

Conowingo Dam was about 300,000 acre-ft. By 1959, deposition of sediment reduced the capacity to 

235,000 acre-ft, and by 1990, the capacity was only 196,000 acre-ft. A comparison of the cross-sectional data 

from Lake Aldred and Lake Clarke with those of Conowingo Reservoir indicates that the Conowingo 

Reservoir will probably reach equilibrium in the next 20 or 30 years. As the reservoirs fill, the percentage of 

sediment, nitrogen, phosphorus, and metals transported by the Susquehanna River that is deposited in the 

reservoirs will decrease, and the percentage that reaches Chesapeake Bay will increase. Histoncal inflow 

and outflow data indicate that the reservoirs scour when the flow of the Susquehanna River at 

Conowingo, Md., exceeds 400,000 ftVs.

The U.S. Army Corps of Engineers' HEC-6 sediment-transport model was used to simulate 

sediment-transport dynamics. Although the model selected was determined to be the most suitable model 

available to this study, simulated trap efficiencies were lower than measured values. During tf ^ year when 

the model was calibrated, the measured trap efficiency of the reservoir system was about 76 percent. The 

maximum efficiency that could be reproduced by the model using 'natural' particle-size distribution was 

about 34 percent. Measured trap efficiencies were reproduced only after the particle-size distribution of 

the inflow was shifted to a more coarse grained sediment.

Additional channel geometry and sediment-transport data may slightly improve the results of this 

model. The major limitation of the model appears to be the algorithms used to transport the various 

particle-size fractions. A review and redefinition of these algorithms are required to significantly improve 

the results of this model and the understanding of the sediment-transport dynamics of the Lower 

Susquehanna River reservoir system.

37
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39