Sediment Transport and Capacity Change in Three 

Reservoirs, Lower Susquehanna River Basin, 

Pennsylvania and Maryland 1900–2012

Open-File Report 2014–1235

U.S. Department of the Interior

U.S. Geological Survey
Cover. Susquehanna River at Conowingo Dam, September 9, 2004. Photo by Wendy McPherson, U.S. Geological Survey.
Sediment Transport and Capacity Change 

in Three Reservoirs, Lower Susquehanna 

River Basin, Pennsylvania and Maryland, 

1900–2012

By Michael J. Langland

Open-File Report 2014–1235

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Langland, M.J., 2015, Sediment transport and capacity change in three reservoirs, Lower Susquehanna River 

Basin, Pennsylvania and Maryland, 1900–2012: U.S. Geological Survey Open-File Report 2014–1235, 18 p., 

http://dx.doi.org/10.3133/ofr20141235.

ISSN 2331-1258 (online)
iii

Contents

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

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

Plan to Reduce Loads to Chesapeake Bay ......................................................................................3

Previous Studies on the Three Reservoirs .......................................................................................3

Purpose and Scope ..............................................................................................................................5

Susquehanna River Sediment Transport ...................................................................................................5

Recurrence Intervals and Total and Scour Sediment Loads..................................................................7

Capacity Change and Total Sediment Deposition ..................................................................................12

Sediment Cores and Spatial Distribution of Sediment in Conowingo Reservoir...............................14

Summary........................................................................................................................................................16

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

Figures

1. Map showing location of the three major reservoirs—Lake Clarke, Lake Aldred, 

and Conowingo Reservoirs—in the Lower Susquehanna River Basin, Pennsylvania 

and Maryland ................................................................................................................................2

2. Idealized schematic of a reservoir and the dynamic of circulation and deposition .........4

3. Graph showing total estimated sediment transported from the Susquehanna River 

into three reservoirs in Pennsylvania and Maryland, 1900–2010 .........................................5

4. Graph showing the annual percent of the total sediment load from the Conestoga 

River and Pequea Creek tributaries, Pennsylvania, in relation to the total sediment 

load transported into the reservoir system, Lower Susquehanna River 

Basin, 1987–2012...........................................................................................................................6

5. Graph showing peak streamflows, by recurrence interval, for the Susquehanna 

River at Marietta, Pennsylvania, and Susquehanna River at Conowingo, Maryland, 

streamgages..................................................................................................................................8

6. Graph showing daily mean streamflow hydrographs for 11 storms with greater than 

400,000 cubic feet per second since 1972 at the Susquehanna River at Conowingo, 

Maryland, streamgage.................................................................................................................9

7. Graph showing number of storm events and mean daily streamflow by season at 

Susquehanna River at Conowingo, Maryland, 1967–2012 .....................................................9

8. Graph showing daily mean streamflow in relation to sediment scour load and 

U.S. Geological Survey scour equation used to predict scour using streamflows 

generally exceeding 400,000 cubic feet per second in the Lower Susquehanna 

River reservoir system...............................................................................................................10

9. Graph showing trend in sediment storage capacity change (percent full) in the 

Conowingo Reservoir; Lower Susquehanna River Basin, Pennsylvania and 

Maryland, since construction, 1929–2012 ..............................................................................13
iv

10. Graph showing trend in sediment storage capacity change (percent full) in Lake 

Clarke, Lower Susquehanna River basin, Pennsylvania, since construction in 1931 

to 2013...........................................................................................................................................13

11. Map showing locations and year for 70 sediment cores collected from Conowingo 

Reservoir, Lower Susquehanna River Basin, Pennsylvania and Maryland......................15

12. Map showing locations of the upper, middle, and lower sections of Conowingo 

Reservoir, Pennsylvania and Maryland ..................................................................................16

Tables

1. Nutrient and sediment load allocation goals for the six states with waterways 

draining into Chesapeake Bay to meet the Total Maximum Daily Loads set by 

the U.S. Environmental Protection Agency ..............................................................................3

2. Average annual sediment loads transported into and out of the Lower 

Susquehanna River reservoir system and estimated trapping efficiency 

for multiple time periods..............................................................................................................6

3. U.S. Geological Survey estimated recurrence intervals, annual exceedance 

probabilities, and estimated peak-streamflow estimates at two Susquehanna 

River streamgages, Lower Susquehanna River Basin, 1968–2012.......................................7

4. Predicted sediment scour loads for storms with an average daily-mean discharge 

at Conowingo, Maryland, greater than 400,000 cubic feet per second.............................11

5. Recurrence intervals for selected streamflows, percent chance of streamflow 

event, predicted scour, total sediment load, and percent scour to total load 

for Conowingo Reservoir, Lower Susquehanna River Basin, Pennsylvania 

and Maryland ..............................................................................................................................11

6. Storage capacity change in Conowingo Reservoir from bathymetric surveys since 

construction.................................................................................................................................12

7. Change in grain-size distribution and deposition for three sediment coring 

studies and projected to 2012 for Conowingo Reservoir, Lower Susquehanna 

River Basin...................................................................................................................................14
v

Conversion Factors

Multiply By To obtain

Volume

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

Flow rate

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Mass

ton per year (ton/yr) 0.9072 metric ton per year

Density

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

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

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

(NAVD 88).

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

Elevation, as used in this report, refers to distance above the vertical datum.
vi
Sediment Transport and Capacity Change in Three 

Reservoirs, Lower Susquehanna River Basin, 

Pennsylvania and Maryland, 1900–2012

By Michael J. Langland 

Abstract 

The U.S. Geological Survey (USGS) has conducted 

numerous sediment transport studies in the Susquehanna River 

and in particular in three reservoirs in the Lower Susquehanna 

River Basin to determine sediment transport rates over the 

past century and to document changes in storage capacity. 

The Susquehanna River is the largest tributary to Chesapeake 

Bay and transports about one-half of the total freshwater input 

and substantial amounts of sediment and nutrients to the bay. 

The transported loads are affected by deposition in reservoirs (Lake Clarke, Lake Aldred, and Conowingo Reservoir) 

behind three hydropower dams. The geometry and texture of 

the deposited sediments in each reservoir upstream from the 

three dams has been a subject of research in recent decades. 

Particle size deposition and sediment scouring processes are 

part of the reservoir dynamics. A Total Maximum Daily Load 

(TMDL) for nitrogen, phosphorus, and sediment was established for Chesapeake Bay to attain water-quality standards. 

Six states and the District of Columbia agreed to reduce loads 

to the bay and to meet load allocation goals for the TMDL. 

The USGS has been estimating annual sediment loads at the 

Susquehanna River at Marietta, Pennsylvania (above Lake 

Clarke), and Susquehanna River at Conowingo, Maryland 

(below Conowingo Reservoir), since the mid-1980s to predict 

the mass balance of sediment transport through the reservoir system. Using streamflow and sediment data from the 

Susquehanna River at Harrisburg, Pennsylvania (upstream 

from the reservoirs), from 1900 to 1981, sediment loads were 

greatest in the early to mid-1900s when land disturbance 

activities from coal production and agriculture were at their 

peak. Sediment loads declined in the 1950s with the introduction of agricultural soil conservation practices. Loads were 

dominated by climatic factors in the 1960s (drought) and 

1970s (very wet) and have been declining since the 1980s 

through 2012. The USGS developed a regression equation to 

predict the sediment scour load for daily mean streamflows 

greater than 300,000 cubic feet per second for the Lower 

Susquehanna River reservoirs. A compilation of data from 

various sources produced a range in total sediment transported 

through the reservoir system and allowed for apportioning to 

source (watershed or scour) for various streamflows. In 2011, 

Conowingo Reservoir was estimated to be about 92 percent of 

sediment storage capacity. Since construction of Conowingo 

Dam in 1929 through 2012, approximately 470 million tons 

of sediment was transported down the Susquehanna River into 

the reservoir system, approximately 280 million tons were 

trapped, and approximately 190 million tons were transported 

to Chesapeake Bay. Spatial and estimated total sand deposition 

in Conowingo Reservoir based on historical sediment cores 

indicated continued migration of sand downgradient toward 

the dam and the winnowing of silts and clays near the dam due 

to scour.

Introduction

The Susquehanna River is the largest tributary to Chesapeake Bay and transports about one-half of the total freshwater 

input and substantial amounts of sediment, nitrogen, and phosphorus to the bay (Langland, 2009). The loads transported by 

the Susquehanna River to the bay are substantially affected by 

the deposition of sediment and nutrients behind three hydroelectric dams on the lower Susquehanna River near the mouth 

(Reed and Hoffman, 1996). The three consecutive reservoirs 

(Lake Clarke, Lake Aldred, and Conowingo Reservoir) that 

formed behind the three dams (Safe Harbor, Holtwood, and 

Conowingo, respectively) involve nearly 32 miles of the river 

and have a combined design storage capacity of 510,000 acrefeet (acre-ft) at the normal pool elevations (fig. 1). The normal 

pool elevation is the height in feet above sea level at which a 

section of a river is to be maintained behind a dam. A fourth 

dam (York Haven) is located approximately 44 miles upstream 

from Conowingo Dam. Because of the low water head (28 

feet) and low storage area (7,800 acre-ft), the sediment retention at York Haven is substantially less than that of the dams 

located downstream and is not considered in this project. A 

water-storage facility (Muddy Run, located just below Holtwood Dam) is a pump-storage release facility where water 
2 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

Marietta

Wrightsville

Columbia

Washington Boro

Turkey Hill

Long Level

Urey

Duncan Island

Weise Island

Safe Harbor

Islands

Holtwood

Hennery Island

Mt. Johnson Island

Peach Bottom Peach Bottom

Generating Station

Wildcat Tunnel

Troublesome Conowingo

Hills

PENNSYLVANIA

MARYLAND

Conestoga

River

Pequea Creek

Safe Harbor

Dam

Holtwood

Dam

Muddy Run

Hydroelectric Dam

Michael

Run

Broad

Creek

Conowingo

Creek

Conowingo

Dam

CONOWINGO

RESERVOIR

LA

KE

CLARKE

LA

KE ALDRED

76°30' 76°15'

39°

45'

40°

08'

0 5 10 MILES

0 5 10 KILOMETERS

PENNSYLVANIA

MARYLAND

Study area

Susquehanna 

River Basin

01576000

01576754

01576787

01578310

EXPLANATION

U.S. Geological Survey 

surface-water site 

and identifier

01576787

Base from U.S. Geological Survey 1:24,000-scale digital data

Figure 1. Location of the three major reservoirs—Lake Clarke, Lake Aldred, and Conowingo Reservoirs—

in the Lower Susquehanna River Basin, Pennsylvania and Maryland. 
Introduction 3

from Conowingo Reservoir is pumped approximately 400 feet 

uphill to be released when demand for electricity is high also 

is not considered in this project. 

Safe Harbor Dam, built in 1931 with a dam height of 

80 feet (North American Vertical Datum, 1988, referred to as 

“NAVD 1988”) forms the uppermost reservoir, has a design 

capacity of about 150,000 acre-ft, and is considered to have 

reached the capacity to store sediment in the early 1950s. Holtwood Dam, built in 1910 with a dam height of 60 feet NAVD 

1988, is the smallest of the three dams, has a design capacity 

of about 60,000 acre-ft, and is considered to have reached the 

capacity to store sediment in the mid-1920s. Conowingo Dam, 

built in 1929, is the largest and most downstream dam, has 

a height of 110 feet NAVD 1988, and has a design capacity 

of about 300,000 acre-ft. Conowingo Reservoir has limited 

capacity to store sediment and may be in equilibrium (Hainly 

and others, 1995; Reed and Hoffman, 1996).

Plan to Reduce Loads to Chesapeake Bay 

The District of Columbia, the six states with waterways 

draining into Chesapeake Bay (Maryland, Pennsylvania, 

Virginia, New York, West Virginia, and Delaware), the Chesapeake Bay Commission, and the U.S. Environmental Protection Agency (EPA) have agreed to a plan to reduce nutrient 

loads to Chesapeake Bay in an attempt to restore and protect 

the estuarine environment of the bay. The EPA has established 

a Total Maximum Daily Load (TMDL), which mandates sediment and nutrient (nitrogen and phosphorus) allocation goals 

for each of the six states (table 1) with waterways draining 

into Chesapeake Bay (U.S. Environmental Protection Agency, 

2010). The six states and the District of Columbia have each 

written Watershed Implementation Plans (WIPs) to reduce 

loads to the Bay and to meet load allocation goals for the 

TMDL. Each of the states’ plans can be accessed at http://

www.chesapeakebay.net/about/programs/watershed. Of particular interest are the allocations for New York, Pennsylvania, 

and Maryland, the states that have waters draining into at least 

one the three reservoirs in the Lower Susquehanna River.

Previous Studies on the Three Reservoirs

Previous studies by Ott and others (1991), Hainly and 

others (1995), Reed and Hoffman (1996), Langland and 

Hainly (1997), Langland (2009), and Gomez and Sullivan 

Engineers (2012) have documented important information 

on the Lower Susquehanna River reservoirs, including the 

reservoirs’ bottom-sediment profiles, reduced storage capacity, and trap efficiency. Several studies also have determined 

sediment chemistry (Hainly and others, 1995; Langland and 

Hainly, 1997; and Edwards, 2006) and the effects of large 

storm events on the removal and transport of sediment out 

of the reservoir system and into the upper Chesapeake Bay 

(Langland and Hainly, 1997; Langland, 2009; Gomez and Sullivan Engineers, 2012). 

Langland (2009) provides a historical perspective on reservoir filling rates and projects when sediment storage capacity may be reached in the Conowingo Reservoir. When storage 

capacity is reached, a dynamic-equilibrium condition will exist 

between incoming and outgoing sediment and nutrient loads 

discharged through the reservoir system to Chesapeake Bay. 

In the dynamic-equilibrium condition, constituent loads may 

increase during high streamflow scour events, thereby affecting the WIPs to meet sediment and nutrient allocation TMDL 

goals set by the EPA and the State of Maryland water-quality 

standards for dissolved oxygen, water clarity, and chlorophyll 

A. With respect to TMDLs, increased loads may have a greater 

effect on sediment and phosphorus, which tend to be transported in the particulate (solid) phase, and less of an effect on 

nitrogen, which tends to transported in the dissolved phase. 

Table 1. Nutrient and sediment load allocation goals for the six states with waterways draining into Chesapeake Bay to meet the 

Total Maximum Daily Loads set by the U.S. Environmental Protection Agency (2010).

Jurisdiction

Total maximum daily load allocations

Nitrogen Phosphorus Sediment

Tons per year Million pounds 

per year1 Tons per year Million pounds per year1 Tons per year Million pounds per year1

Delaware 1,500 3 150 0.3 28,900 58

District of Columbia 1,200 2.3 50 0.1 5,600 11

Maryland 19,600 39.2 1,400 2.7 609,000 1,219

New York 4,400 8.8 300 0.6 146,000 293

Pennsylvania 36,900 74 1,400 2.9 992,000 1,984

Virginia 26,700 53 2,700 5.4 1,289,000 2,579

West Virginia 2,800 5.5 300 0.6 155,000 311

1

Rounded.
4 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

However, in this dynamic-equilibrium condition, loads may 

decrease for a short duration owing to increased deposition as 

a result of, and related to, the length of time since a preceding scour event. Hirsch (2012) concludes that the reservoirs 

are very close to this equilibrium condition and that nutrient 

and sediment concentrations and loads have been increasing 

at the Conowingo Dam (the furthest downstream and closest 

to Chesapeake Bay) for the past 10–15 years. Hirsch (2012) 

implies that increasing concentrations and loads are due to the 

loss of storage capacity and a possible decrease in the scour 

threshold of 400,000 cubic feet per second (ft3/s). Reasons 

for the increase are not certain, but likely involve changes in 

particle-falling velocities owing to increased water velocity, 

transport capacities, and bed shear. 

Dams create a change in hydrological dynamics affecting sediment transport and deposition. With increased depth, 

streamflow velocities are reduced within the reservoir. Owing 

to streamflow deceleration as the water enters the reservoir, 

sediment-transport capacity decreases, and the coarser-size 

fractions of the incoming sediment fall out of the water column and are deposited near the upstream end of the reservoir, 

forming a delta near the entrance to the reservoir (fig. 2). As 

the water and sediment continue to flow into the reservoir, the 

delta continues to extend in the direction of the dam, eventually filling the entire sediment storage volume. The process is 

usually slow, governed by the amount of incoming sediment, 

sediment particle size, and streamflow variability. Generally, 

low streamflow results in deposition, whereas during higher 

streamflow some of the sediment is scoured from the upper 

end of the reservoir and transported downstream with a portion transported out of the reservoir. Large reservoirs receiving 

runoff with substantial sediment from natural and (or) anthropogenic sources typically fill (reach equilibrium) in 50 to 

100 years (Mahmood, 1987). 

Water surface

Fine sediments

(silts and clays)

Density current (silts)

Delta

(sand)

Floating debis

Sluiceway

Relatively clear water

(clay)

Turbid inflow

Dam Reservoir bottom

Figure 2. Idealized schematic of a reservoir and the dynamic of circulation and deposition (From Sloff,1997).
Susquehanna River Sediment Transport 5

Purpose and Scope

For this report, the primary objective is to provide a 

historical perspective on sediment transport and to document 

resultant changes in reservoir capacity within three reservoirs 

in the Lower Susquehanna River Basin (fig. 1). Streamgages 

at Susquehanna River at Marietta, Pennsylvania, located 

above Lake Clarke (01576000), with a drainage area (DA) 

of approximately 25,990 square miles (mi2), and the Susquehanna River at Conowingo, Maryland (01578310), located 

below Conowingo Reservoir, with a DA of approximately 

27,100 mi2, are considered to be representative of the streamflow and sediment input to, and output from, the reservoir 

system. Owing to the lack of sediment information from the 

upper two reservoirs (Lake Clarke and Lake Aldred), the 

streamflow and sediment results are considered the cumulative effect of all three reservoirs. In addition, the streamgage at 

Susquehanna River at Harrisburg, Pennsylvania (01570500), 

was used to estimate streamflow at Marietta prior to 1987.

This report presents decadal changes in sediment transport, recurrence intervals for streamflow at two U.S. Geological Survey (USGS) streamgages, river and scour sediment 

transport values, and an evaluation of streamflow and sediment 

transport in the reservoirs. Additional information presented in 

this report includes locations and dates of all sediment cores 

collected by the USGS historically in the Conowingo Reservoir, with grain-size distribution and total deposition of sand, 

silt, and clay for specific locations for multiple time periods. 

Information provided in this report may be useful to managers 

when considering a range of management options dealing with 

streamflow and sediment dynamics in the Lower Susquehanna 

River reservoir system.

Susquehanna River Sediment Transport

Using current and historical streamflow (1900–1987) and 

sediment data (1962–1981) from the Susquehanna River at 

Harrisburg, Pa. (01570500), approximately 25 miles upstream 

from Marietta, and streamflow and sediment data (1987–2010) 

from the Susquehanna River at Marietta, Pa. (01576000), sediment loads were estimated from 1930 to 2010 (by decade) at 

Marietta, and these values were considered to represent input 

to the reservoirs (fig. 3). Loads historically were estimated 

using the USGS ESTIMATOR model (Cohn and other, 1989) 

and more recently using the Weighted Regression on Time 

Discharge and Season (WRTDS) model (Hirsch, 2010). Loads 

were greater in the early to mid-1900s, averaging approximately 87 million tons per decade (8.7 million tons per year), 

owing to large land disturbance activities, including coal 

extraction and agriculture (Williams and George, 1972). In 

the 1950s, agricultural conservation measures were enacted 

(Wedin, 2002; Westra, 2003), helping to reduce sediment loads 

from 87 million tons in previous decades to approximately 

60 million tons. Sediment loads have generally decreased 

20,000

30,000

40,000

50,000

0

20

40

60

80

100

120

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Decadal mean flow, cubic feet per second

Sediment load, million tons

Decade

EXPLANATION

Load to reservoirs

High estimate

Low estimate

Decadal mean flow

Figure 3. Total estimated sediment transported from the Susquehanna River into three reservoirs in Pennsylvania and 

Maryland, 1900–2010.
6 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

from the 1960s through the 1980s as a result of more land 

reverting to forest from farm abandonment, a decrease in land 

disturbance from coal production, and new best-management 

actions to control sediment (table 2). Loads continued to 

decline to an average of 3.5 million tons per year over the 

last 20 years (1991–2012). If not for the large decreases in 

sediment from the basin, the Conowingo Reservoir may have 

reached sediment storage capacity resulting in increased 

loads to Chesapeake Bay decades ago. The larger decreases 

in the loads into the reservoirs versus the nearly stable loads 

out to Chesapeake Bay indicate a loss of trapping efficiency 

over time (table 2). Trapping efficiencies can exhibit a wide 

variation and are dependent on climatic conditions in a given 

time frame.

Climatic extremes are indicated in figure 3, when during 

the 1960s, streamflow every year was below the normal annual 

mean, and 1970–79 was the wettest decade on record since 

1900. Two storm events that caused major flooding occurred 

during the 1970s in the Chesapeake Bay region (Tropical 

Storm Agnes in 1972 and Tropical Storm Eloise in 1975). 

Tropical Storm Agnes produced the highest recorded streamflows at many locations in the Susquehanna River Basin, 

including Conowingo Dam. Since the 1980s, the decadal 

mean streamflow has increased by approximately 17 percent, 

whereas the decadal sediment loads continued to decrease 

by approximately 9 percent, an indication that management 

practices in the Susquehanna River Basin may be helping to 

control sediment that would otherwise reach the streams.

Sediment input from two monitored tributaries flowing into the reservoir system (Conestoga River at Conestoga 

[01576754] with a drainage areas [DA] of approximately 

470 mi2 and Pequea Creek at Martic Forge [01576787] with 

a DA of approximately 148 mi2), both with long-term streamflow records (1985–2012), was estimated to account for the 

majority of the sediment load entering from the Susquehanna 

River below Marietta, Pa., and into the three reservoirs. 

Although Conestoga River and Pequea Creek have much 

smaller DAs and streamflows than the Susquehanna River, 

these two tributaries in the Lower Susquehanna River Basin 

have large sediment loads from agricultural and urbanization activities. On an annual basis, the sediment load from 

these two tributaries represent less than 10 percent of the total 

suspended-sediment load entering the reservoirs for 21 out the 

26 years (1987–2012). More importantly, only 1 storm event 

exceeded 10 percent when streamflows exceeded 400,000 cfs. 

(fig. 4). Generally, an inverse relation exists between the 

percentage of the total sediment load from the Conestoga 

River and Pequea Creek and the total load transported into 

the reservoirs, indicating a greater influence from the larger 

Susquehanna River Basin as streamflows increase. 

0

5

10

15

20

25

30

35

0 5,000,000 10,000,000 15,000,000

Tributary contribution to total

sediment load, in percent

Total sediment load into the reservoirs, in tons

EXPLANATION

Year with a daily-mean streamflow 

less than 400,000 cubic feet 

per second

Year with a daily-mean streamflow 

greater than 400,000 cubic feet 

per second

Figure 4. The annual percent of the total sediment load from the 

Conestoga River and Pequea Creek tributaries, Pennsylvania, in 

relation to the total sediment load transported into the reservoir 

system, Lower Susquehanna River Basin, 1987–2012.

Table 2. Average annual sediment loads transported into and out of the Lower 

Susquehanna River reservoir system and estimated trapping efficiency for multiple 

time periods.

Time 

period

Average annual 

sediment load 

to reservoirs 

(million tons/year)

Reservoir 

trapping 

(percent)

Average annual 

sediment load 

trapped 

(million tons/year)

Average annual 

sediment load 

to bay 

(million tons/year)

1928–1940 8.7 70–75 6.3 2.4

1941–1950 8.5 65–70 5.8 2.7

1951–1970 5.1 55–60 3.1 2.0

1971–19901 4.9 50–55 2.6 2.3

1991–20122 3.5 45–50 1.3 2.2

1

Includes Tropical Storms Agnes and Eloise, June 1972 and September 1975.

2

Includes Tropical Storm Lee, September 2011.
Recurrence Intervals and Total and Scour Sediment Loads 7

Recurrence Intervals and Total and 

Scour Sediment Loads

Expected streamflows for many recurrence intervals (RI) 

are presented in table 3. A recurrence interval is a statistical 

estimate of the likelihood that a given streamflow will occur, 

based on historical data. The annual exceedance probability is 

the chance that a given streamflow event will occur in the current year. The relation between RI and streamflow is illustrated 

in figure 5 for the two USGS Susquehanna River streamgages 

representing inflow and outflow from the reservoir system—

the Susquehanna River at Marietta, Pennsylvania (01576000), 

and the Susquehanna River at Conowingo, Maryland 

(01578310), respectively—during 1968–2012. Streamflows 

corresponding to various RIs were computed for this study 

using methods described in Flynn and others (2006). Station 

skew for frequency distribution was used at both stations, and 

historical peak streamflows prior to 1968 were not used in the 

analysis. No low outliers were detected. Useful information 

about short-term streamflow includes the bankfull streamflow 

(RI of about 1.5 years) and the mean peak streamflow for the 

period of record (RI of 2.33 years).

A general coincidence in streamflow between the two 

Susquehanna River sites up until about the 1.5-year RI 

(bankfull discharge) is indicated in figure 5, then an increasing 

divergence occurs in RIs as streamflow increases. This is most 

likely due to differences in drainage area between the two sites 

(6 percent) and streamflow regulation and storage of three 

hydroelectric facilities between the streamgages. 

The USGS has been estimating sediment loads at 

Susquehanna River at Marietta, Pennsylvania, since 1987 and 

at Susquehanna River at Conowingo, Maryland, since 1979. 

The annual loads are used to develop a simple in/out model 

to predict the mass balance of sediment transport through 

the reservoir system. The annual loads are used to calibrate a 

scour-prediction equation and estimate the sediment deposition and remaining capacity in Conowingo Reservoir. 

Since 1972, there have been 11 storms with daily mean 

streamflows greater than 400,000 ft3

/s (5-year RI), the streamflow when an average mass wasting event begins, defined 

as the point at which large areas of the bed begin to move 

(Hainly and others, 1995) scouring the sediment in the reservoirs. Most likely some of the finer silt and sand particles 

begin to move before 400,000 ft3/s. Cohesive sediments such 

as clays and fine silts may begin to move off the reservoir 

bottom at streamflows around 200,000 ft3/s, whereas the 

heavier sand and gravels may not move until streamflows 

are greater than 600,000 ft3/s (Schuleen and Higgins, 1953). 

A recent 2-dimensional model simulation of Conowingo 

Reservoir indicated silt and clay movement at around 

250,000 ft3/s (Steve Scott, U.S. Army Corps of Engineers, 

Table 3. U.S. Geological Survey estimated recurrence intervals, annual exceedance probabilities, and 

estimated peak-streamflow estimates at two Susquehanna River streamgages, Lower Susquehanna River 

Basin, 1968–2012.

[ft3/s, cubic feet per second]

Station 01576000 

Susquehanna River at Marietta, Pennsylvania 

(1968–2012)

Station 01578310 

Susquehanna River at Conowingo, Maryland 

(1968–2012)

Estimated 

recurrence interval 

(years)

Annual 

exceedance 

probability

Estimated peak 

streamflow 

(ft3

/s)

Estimated 

recurrence interval 

(years)

Annual 

exceedance 

probability

Estimated peak 

streamflow 

(ft3/s)

1 0.995 113,000 1 0.995 131,000

1.01 0.99 121,000 1.01 0.99 138,000

1.05 0.95 144,000 1.05 0.95 164,000

1.11 0.9 162,000 1.11 0.9 182,000

1.25 0.8 188,000 1.25 0.8 212,000

1.5 0.667 221,000 1.5 0.6667 248,000

2 0.5 265,000 2 0.5 298,000

2.33 0.4292 287,000 2.33 0.4292 323,000

5 0.2 402,000 5 0.2 436,000

10 0.1 514,000 10 0.1 590,000

25 0.04 685,000 25 0.04 798,000

50 0.02 835,000 50 0.02 984,000

100 0.01 1,010,000 100 0.01 1,200,000

200 0.005 1,210,000 200 0.005 1,500,000

500 0.002 1,510,000 500 0.002 1,860,000
8 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

1 1.01 1.05 1.11 1.25 1.5 2 2.33 5 10 25 50 100 200 500

Recurrence interval, in years

0

10,000

20,000

30,000

40,000

50,000

60,000

Peak streamflow, in cubic meters per second

Susquehanna River at Marietta

Susquehanna River at Conowingo

EXPLANATION

Figure 5. Peak streamflows, by recurrence interval, for the Susquehanna River at Marietta, Pennsylvania, and 

Susquehanna River at Conowingo, Maryland, streamgages.

written commun., 2014). For flows less than 400,000 ft3/s, the 

majority of the scoured silts and sands are re-deposited in the 

reservoir system. Daily mean streamflow and number of storm 

days are plotted for 11 storms for the Susquehanna River at 

Conowingo, Maryland, streamgage (fig. 6). Note the general 

pattern of rapid increase on the rising limb to the peak and a 

more general decrease in streamflow on the falling limb. This 

is a typical high-flow response in many rivers and indicates 

that at higher streamflows the reservoirs do not have the 

capacity to store much water above normal pool elevations; 

these reservoirs are normally referred to as “run-of-the-river” 

reservoirs. The number of days with streamflows greater than 

400,000 ft3/s ranged from 1 to 5; the average was about 3 days. 

The 1972 event (Tropical Storm Agnes) was the largest flood 

in the Susquehanna River Basin since 1889, when recording 

of streamflow began at Harrisburg, Pa. The second largest 

recorded flood event, using daily mean streamflow data, in the 

Susquehanna River Basin since 1972 was in 2011 (Tropical 

Storm Lee; fig. 6). Note that more than one event is plotted for 

1984 and 2011.

Streamflow can also be examined on a seasonal basis to 

determine the volume and timing of streamflow events over 

a given time period. To increase the number of streamflow 

events, daily mean streamflows greater than 300,000 ft3/s at 

Susquehanna River at Conowingo were tabulated and are 

shown in figure 7. The highest number of daily mean streamflow events greater than 300,000 ft3

/s occurred during March–

May (spring), whereas the greatest daily mean streamflows 

per storm event occurred during June–August (summer) and 

September–November (fall). The summer value was most 

likely biased high owing to the daily mean streamflow for 3 of 

the 8 events, each more than 1,000,000 ft3/s during Tropical 

Storm Agnes. The higher streamflows tended to occur in the 

spring and fall, coinciding with the spring “freshet,” usually 

a result of snowmelt, and the fall Atlantic Ocean hurricane 

season, respectively. 

The USGS developed a regression equation to predict 

the sediment scour load for daily mean streamflows greater 

than 300,000 ft3

/s for the Lower Susquehanna River reservoirs (fig. 8). The equation is based primarily on streamflow 

and estimated loads from six storm events during 1993–2011 

(table 4), on bathymetry (bed-elevation change) data for the 

reservoirs using the Reed and Hoffman (1996), Langland and 

Hainly (1997), Langland (2009), and Gomez and Sullivan 

Engineers (2012) studies, and on a comparison of estimates of 

sediment inflow and outflow from the reservoirs. Additional 

information for Tropical Storm Agnes (1972) and Tropical 

Storm Eloise (1975) (Gross and others, 1978) was used to 

calibrate the curve. The regression equation was then used 

to predict scour loads for an additional three storms between 

1972 and 1990 and for nine storms prior to 1972, with little to 

no sediment or bathymetric data, for daily mean streamflow 

greater than 400,000 ft3

/s (table 4), on the basis of historical 

trapping efficiency (table 2) and storage. For the 1936 sediment scour estimate, the current regression equation totaled 

approximately 5 million tons; however, owing to the small 
Recurrence Intervals and Total and Scour Sediment Loads 9

1972 (Tropical Storm Agnes)

1975 (Tropical Storm Elois)

1979

1984

1984

1993

1996

2004 (Tropical Storm Ivan)

2006

2011

2011 (Tropical Storm Lee)

EXPLANATION

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Number of days since start of storm

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

Streamflow, in cubic feet per second

Figure 6. Daily mean streamflow hydrographs for 11 storms with greater than 400,000 cubic feet per second since 1972 at the 

Susquehanna River at Conowingo, Maryland, streamgage. (Hydrograph years with no associated storm name represent unnamed 

storms.)

35

30

25

20

15

10

5

0

Number of storm events

Winter

(Dec.–Feb.)

Spring

(Mar.–May)

Summer

(June–Aug.)

Fall

(Sept.–Nov.)

Season

Mean daily streamflow, in cubic feet per second

392,000 356,000 680,000 498,000

Figure 7. Number of storm events and mean daily streamflow by 

season at Susquehanna River at Conowingo, Maryland, 1967–2012. 

A storm event is defined as daily mean discharge greater than 

300,000 cubic feet per second.
10 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

0

2

4

6

8

10

12

14

16

18

300,000

Sediment scour load, in millions of tons

400,000 500,000 600,000 700,000 800,000 900,000 1,000,000 1,100,000 1,200,000

Daily mean streamflow, in cubic feet per second

(y = 0.00003x 2 – 0.0219x + 5.1454)

Scour load prediction

Figure 8. Daily mean streamflow in relation to sediment scour load and U.S. Geological Survey scour equation used to predict 

scour using streamflows generally exceeding 400,000 cubic feet per second in the Lower Susquehanna River reservoir system.

amount of sediment estimated to be stored in Conowingo 

Reservoir at the time, the estimate was reduced to 3.5 million 

tons of scoured sediment. Revisions to the equation sometimes result from updates with data from new flood events. 

The curve and subsequent scour prediction provide 

a useful and quick reference for potential scour from the 

reservoir system to the upper Chesapeake Bay at or soon after 

flooding events when information may be needed quickly to 

ascertain potential environmental effects. Although not exact 

as a scour predicting tool, the equation is updated with data 

from each flood event, resulting in a new, slightly different 

equation. Complications in the predictions include errors in 

the methods used to estimate the daily and monthly loads, 

the amount of sediment entering the reservoir system, and 

the amount of streamflow and time above a certain scour 

threshold, generally 400,000 ft3

/s. In addition, the length of 

time since a previous scour event, which may increase or 

decrease the amount of scoured sediment, and the changing 

scour/deposition dynamics resulting from increased velocities (potential to lower the scour threshold) as Conowingo 

Reservoir nears storage capacity, all contribute to scour 

prediction error. 

Using the data from table 3 and converting the annual 

exceedence probability to percent, changes in bottom surface 

based on the bathymetric studies, the annual sediment load 

estimates from Marietta and Conowingo (above and below 

the reservoirs), plus estimates of scour, were combined to 

produce a range in total sediment transported through the 

reservoir system and an apportioning to source (percent scour 

to total load) for various streamflows (table 5). The ranges 

in scour and estimates of total loads transported out the 

reservoir system allow for differences in season, total volume 

of potential scour streamflow, and errors in the estimates. 

As previously discussed, the streamflow when mass scour 

is estimated to begin is approximately 400,000 ft3

/s. Results 

from a U.S. Army Corps of Engineers two-dimensional model 

and a recent USGS report by Hirsch (2012) indicate that the 

threshold has decreased with time. Because figure 8 indicates 

scour might occur at streamflows below 400,000 ft3

/s, table 5 

shows estimated scour as low as 300,000 ft3/s. The uncertainty 

associated with scour estimates less than 400,000 ft3/s is 

greater than the uncertainty for scour estimates greater than 

400,000 ft3/s. 

The percent scour to total load, based on frequency of 

streamflow events, ranges from 20 percent to 37 percent 

(average 30 percent) for streamflows of 400,000–800,000 ft3/s. 

A streamflow of 800,000 ft3/s has a recurrence interval of 

25 years. As indicated in table 5, streamflows greater than 

800,000 ft3/s generate the greatest amounts of scour and an 

increasingly higher proportion of total sediment load. The 

load from bed scour has an upper limit owing to the maximum 

sediment carrying capacity of the water and increasing bed 

shear as a result of compaction of the bed sediments. This 

upper limit was not determined as part of this study. 
Recurrence Intervals and Total and Scour Sediment Loads 11

Table 4. Predicted sediment scour loads for storms 

with an average daily-mean discharge at Conowingo, 

Maryland, greater than 400,000 cubic feet per second.

[ft3/s, cubic feet per second]

Date

Daily-mean 

discharge1

(ft3

/s)

Sediment scour 

storm load event 

(million tons)

May 1936 740,000 3.5

January 1940 493,000 1.3

January 1943 486,000 1.2

May 1946 528,000 0.9

November 1950 495,000 1.8

April 1960 451,000 1.5

February 1961 466,000 1.6

February 1970 434,000 1.3

March 1964 571,000 2.9

June 1972 1,020,000 2

13.5

September 1975 662,000 2

4.4

March 1979 462,000 1.6

February 1984 470,000 1.7

March 1986 406,000 0.8

April 1993 409,000 1.1

January 1996 622,000 4.0

September 2004 495,000 2.1

June 2006 403,000 0.5

March 2011 403,000 0.5

September 2011 709,000 3.5

Total estimated scour 49.7 1

All flow estimates prior to 1968 are based on a drainagearea ratio with the streamflow gage at Susquehanna River at 

Harrisburg, Pa.

2

Revised estimates from Gross and others, 1978.

Table 5. Recurrence intervals for selected streamflows, percent chance of streamflow event, predicted scour, total sediment load, 

and percent scour to total load for Conowingo Reservoir, Lower Susquehanna River Basin, Pennsylvania and Maryland.

[Total sediment load is scour plus watershed load.]

Streamflow 

(cubic feet per second)

Recurrence interval 

(years)

Percent chance of flow 

event per year

Predicted sediment 

scour range1

(million tons)

Range in predicted 

total sediment load2

(million tons)

Range in percent 

scour to total load

1,000,000 60 1.7 10.5–15.5 27.1–31.0 39–49

900,000 40 2.5 6.6–11 21.8–26.2 30–42

800,000 25 4 4.5 -7.5 17.2–20.2 26–37

700,000 17 5.9 3.5–6 13.1–15.6 27–38

600,000 10 10 1.8–4 7.9–10.1 22–40

500,000 5.7 17.5 1–3 4.9–6.9 20–42

400,000 4.8 21 0.5–1.5 2.4–3.4 21–44

300,000 2.1 52 0–0.5 0.5–1.5 0–33

1

Predicted scour from U.S. Geological Survey scour equation, bathymetry results, and literature estimates.

2

Predicted total load based on transport regression equation, bathymetry results, and literature estimates.
12 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

Capacity Change and Total Sediment 

Deposition

On the basis of previous studies (Whaley, 1960; Hainly 

and others, 1995; Reed and Hoffman, 1996; Langland and 

Hainly, 1997; Langland, 2009; and Gomez and Sullivan Engineers, 2012), capacity and volume change were estimated for 

six time intervals for which bathymetric results were available 

(table 6; fig. 9). From construction in 1929 to the first survey 

in 1959 (30 years), the Conowingo Reservoir lost about half 

of the sediment storage capacity (96 of 198 million tons). 

The capacity to store sediment was reduced by an additional 

31 percent by the next survey 31 years later in 1990 (155 of 

198 million tons), indicating a reduction of incoming sediment, a loss of trapping efficiency, or both. The largest flood 

event during 1959–90 occurred in June 1972 when Tropical 

Storm Agnes removed approximately 13.5 million tons of 

sediment from Conowingo Reservoir (fig. 9). Table 6 indicates 

that in 2011, the Conowingo Reservoir was about 92 percent 

full and that 17 million tons storage capacity remained of an 

estimated equilibrium sediment storage capacity of approximately 198 million tons. 

Figure 9 shows that the rate of filling continues to follow a non-linear pattern since construction in 1929. Tropical 

Storm Agnes in 1972 had the greatest effect with regard to 

sediment removal in the reservoir system over 80 years; the 

reservoir most likely was refilled by the end of the 1970s. The 

rate of filling has slowed as a result of a reduction in incoming sediments from the basin and changes in reservoir scour 

and deposition dynamics. As the reservoir fills with sediment, 

the velocity of water increases owing to diminished volume, 

which could increase the bed shear thus inducing more scour 

and reducing the amount of time for sediments to settle out 

of the water column, thereby decreasing deposition. Approximately 8 percent remains of the original 146,000 acre-feet of 

sediment storage capacity (table 6). As the capacity is reduced, 

sediment concentrations and loads to the upper Chesapeake 

Bay may increase, owing to an increase in velocity through 

the reservoirs. Hirsch (2012) indicates that increases in sediment concentrations and loads are occurring and suggests the 

increases are occurring at streamflows less than 400,000 ft3/s. 

In four previous USGS reports (Hainly and others, 1995; 

Reed and Hoffman, 1996; Langland and Hainly, 1997; and 

Langland, 2009) estimates of time for Conowingo Reservoir 

to reach the “dynamic equilibrium” phase were based upon the 

documented rate and pattern of filling in the most upstream 

reservoir, Lake Clarke. Reed and Hoffman (1996) discuss 

the data in terms of loss of water storage. In this report, the 

rate of change in sediment storage capacity (percent full) is 

based on the change in water-storage capacity (fig. 10). The 

rate of sediment deposition was approximately 20 million 

tons every 5 years starting in 1931 such that Lake Clark has 

been in dynamic equilibrium since approximately 1950. 

Subsequent surveys have confirmed the reservoir no longer 

effectively traps sediment except for short periods of time. In 

Table 6. Storage capacity change in Conowingo Reservoir from bathymetric surveys since construction.

[--, not applicable]

Year

Reservoir 

capacity 

(acre-feet)

Change in 

reservoir capacity 

(acre-feet)

Sediment 

deposition 

(acre-feet)

Total sediment 

deposition 

(tons)

Net gain/loss between 

bathymetric surveys1

(tons)

Percent 

full

1929 280,000 -- 0 0 -- 0

1959 215,000 65,000 65,000 96,000,000 96,000,000 49

1990 175,000 40,000 105,000 155,000,000 60,000,000 78

1993 169,000 6,000 111,000 164,000,000 9,000,000 83

1996 171,000 -2,000 109,000 161,000,000 -3,000,000 81

2008 162,000 9,000 118,000 174,000,000 13,000,000 88

2011 157,000 5,000 123,000 181,000,000 7,000,000 92

Equilibrium 2

146,000 11,000 134,000 198,000,000 17,000,000 100

1

Numbers in black represent deposition; numbers in red represent scour.

2

Note the equilibrium capacity previously has been reported at 142,000 acre-feet. The volume was adjusted after the 2011 bathymetry 

survey when more detailed information near the dam became available. 
Capacity Change and Total Sediment Deposition 13

1996, approximately 2.5 million tons were scoured from Lake 

Clarke and the resulting bathymetry data indicated a slight 

reduction in total sediment deposition and resulting increase 

in capacity (fig. 10). Reasons for different patterns in the rate 

of filling in Lake Clark (linear through 1950, fig. 10) compared to Conowingo Reservoir (non-linear, fig. 9) probably 

are due to the fact that Conowingo Reservoir encompasses a 

longer reach length, a larger surface area, and has about twice 

the capacity. 

Since construction of Conowingo Dam in 1929 until 

2012, approximately 470 million tons of sediment was 

estimated to be transported by the Susquehanna River into the 

reservoir system, approximately 280 million tons were trapped 

(with Conowingo Reservoir trapping about 62 percent), and 

approximately 190 million tons of sediment was transported 

to Chesapeake Bay, indicating a trapping efficiency over the 

85 years of approximately 60 percent for the reservoir system. 

Using the average estimated scour to total load of 30 percent 

(table 5), approximately 57 million tons was predicted to be 

from scour in the reservoirs. Twenty of the storms for which 

scour is estimated account for 50 million tons or 90 percent of 

the predicted total scour.

1920 1940 1960 1980 2000 2020

Year

Percent full

EXPLANATION

Surveyed data

Estimated value

y = -0.009x 2 + 36.568x – 37073

R 2 = 0.998

0

20

40

60

80

100

Tropical Storm

Agnes

Estimated filling rate

Figure 9. Trend in sediment storage capacity change (percent full) in the Conowingo 

Reservoir; Lower Susquehanna River Basin, Pennsylvania and Maryland, since 

construction, 1929–2012. Values are estimated from a combination of methods and 

assume a gradual reduction in long-term trapping efficiency from 75 to 55 percent.

EXPLANATION

Surveyed data

Estimated data

1920 1940 1960 1980 2000 2020

Year

Percent full

0

20

40

60

80

100

Figure 10. Trend in sediment storage capacity change (percent full) in Lake Clarke, 

Lower Susquehanna River basin, Pennsylvania, since construction in 1931 to 2013. 

(Modified from Reed and Hoffman, 1996, based on change in water-storage capacity 

[1931 through 1990].)
14 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

Sediment Cores and Spatial 

Distribution of Sediment in Conowingo 

Reservoir

A natural consequence of any reservoir is a change in the 

sediment carrying capacity of the inflowing water; velocity is 

reduced, thereby enhancing the deposition of sand. A certain 

amount of alluvial material, primarily sands, is beneficial to 

areas downstream. The heavier sands help support underwater 

grasses, which protect young fish from predators, and transport nutrients essential to life in the upper Chesapeake Bay. To 

aid in the identification of spatial distribution of sediment by 

grain-size class (sand, silt, and clay), the locations of 70 USGS 

cores collected over three periods are presented in figure 11. 

Beginning with the 1990–91 collection (23 locations; Hainly 

and others, 1995), efforts were made to sample as closely to 

previous sampling points as possible so comparisons could 

be made over multiple time intervals. For the 1996 sampling 

(Langland and Hainly, 1997), 29 cores were collected, and for 

the 2000 sampling (Edwards, 2006), 18 cores were collected. 

Particle-size results have been compiled and are available in 

Cerco (2012).

The Conowingo Reservoir was divided into three sections 

(upper, middle, and lower) to examine sediment deposition 

and particle size fractions (fig. 12; Langland, 2009). This partitioning is based on common conveyances, depositional areas, 

and state of equilibrium. In general, sediment storage capacity 

in the upper and middle sections is considered to be in a state 

of dynamic equilibrium; over the long term, the sections are 

neither net scour nor deposit areas. The upper section accounts 

for about 19 percent of the total area of the Conowingo Reservoir, of which about two-thirds is considered to contain very 

little sediment as a result of steep channel slopes, high water 

velocities, and the effects of the Muddy Run hydroelectric 

water-storage facility near the top of the pool (Hainly and 

others, 1995). The middle and lower sections of the reservoir account for approximately 50 and 31 percent of the total 

area, respectively.

Changes in average total sediment deposition and in 

total sand deposition in the Conowingo Reservoir from the 

three sediment coring studies (1990–91, 1996, and 2000) 

are presented in table 7. Projections to the year 2012, based 

on the historical changes, are also included in table 7. The 

average percentage of sand/silt/clay is based predominantly 

on the uppermost 1 foot of the sediment cores, areas most 

prone to bed scour and movement. Results of evaluations of 

Table 7. Change in grain-size distribution and deposition 

for three sediment coring studies and projected to 2012 for 

Conowingo Reservoir, Lower Susquehanna River Basin.

Location

Total sediment 

deposition 

(tons)

Average 

sand/silt/clay 

(percent)

Total sand 

deposition 

(tons)

1990 study

Upper 11,000,000 80/13/7 8,800,000

Middle 64,000,000 39/41/20 24,000,000

Lower 80,500,000 5/60/35 4,000,000

1996 study

Upper 11,200,000 82/12/6 9,200,000

Middle 62,000,000 42/39/19 26,000,000

Lower 89,800,000 8/56/36 7,200,000

2000 study

Upper 11,500,000 83/12/15 9,500,000

Middle 63,000,000 43/40/17 26,000,000

Lower 103,000,000 15/73/12 15,500,000

2012 (projected)

Upper 11,500,000 84/12/14 9,660,000

Middle 64,000,000 45/39/12 27,500,000

Lower 108,000,000 20/70/10 21,600,000

the sediment cores indicate the highest percentages of sands 

are in the upper section of the reservoir. This is an area where 

sands are deposited as a result of the loss of streamflow velocity upon entering the top of the impounded reservoir with a 

general downgradient distribution of sands to fines. Results 

also indicate minor changes in the percentage of sands in 

the upper section. Sand increased in the middle section from 

approximately 39 to 45 percent (1990–2012) as a result of 

continual displacement (scour) of fines and transport of sand 

during high-flow events. The middle section had the greatest 

amount of sand deposition. The lower section is the active area 

for sediment deposition and had the greatest increase in sand, 

from 5 to 20 percent (1990–2012). Silt was the dominant class 

of grain size in the lower section of Conowingo Reservoir and 

the dominant class transported in and out of the reservoirs. 

Clay fractions in the lower section have been reduced from 

approximately 35 percent in 1990 to 12 percent in 2000, indicating this is also an active area for scouring of fines. 
Sediment Cores and Spatial Distribution of Sediment in Conowingo Reservoir 15

C

O

N

O

W

I

N

G

O

R

E

S

E

R

V O I R

XY

XY

XY

XY

XY

XY

XY

XY

39°

47'

76°18'

39°

40'

76°10'

0 1 2 MILES

0 1 2 KILOMETERS

PENNSYLVANIA

MARYLAND

Conowingo Creek

Broad Creek

Fishing Creek

Location of sediment core 

and year of collection

1990

1996

2000

EXPLANATION

CONOWINGO DAM

Base from U.S. Geological Survey 1:24,000-scale digital data,

Universal Transverse Mercator Projection: Zone 18, NAD 1983

Figure 11. Locations and year for 70 sediment cores collected from Conowingo Reservoir, Lower Susquehanna River 

Basin, Pennsylvania and Maryland.
16 Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, PA and MD, 1900–2012

39°

40'

39°

45'

76°15' 76°10'

PENNSYLVANIA

MARYLAND

YORK COUNTY

LANCASTER COUNTY

HARFORD COUNTY

CECIL COUNTY

Conowingo

Dam

0 21 MILES

0 21 KILOMETERS

Michael Run

Broad Creek

Base from U.S. Geological Survey 1:24,000-scale digital data,

Universal Transverse Mercator Projection: Zone 18, NAD 1983

UPPER

SECTION

MIDDLE

SECTION

LOWER

SECTION

UPPER

SECTION

MIDDLE

SECTION

LOWER

SECTION

Figure 12. Locations of the upper, middle, and lower sections 

of Conowingo Reservoir, Pennsylvania and Maryland. (From 

Langland, 2009)

Summary

The Susquehanna River is the largest tributary to Chesapeake Bay and transports about one-half of the total freshwater 

input and substantial amounts of sediment, nitrogen, and phosphorus to the bay The loads transported by the Susquehanna 

River to the bay are substantially affected by the deposition of 

sediment and nutrients behind three hydroelectric dams on the 

Lower Susquehanna River near its mouth. The three consecutive reservoirs (Lake Clarke, Lake Aldred, and Conowingo 

Reservoir) that formed behind the three dams (Safe Harbor, Holtwood, and Conowingo) involve nearly 32 miles of 

the river and have a combined design storage capacity of 

510,000 acre-feet (acre-ft) at normal pool elevations. The 

District of Columbia, the six states with water draining into 

Chesapeake Bay (Maryland, Pennsylvania, Virginia, New 

York, West Virginia, and Delaware), the Chesapeake Bay 

Commission, and the U.S. Environmental Protection Agency 

(EPA) have agreed to a plan to reduce nutrient loads to Chesapeake Bay in an attempt to restore and protect the estuarine 

environment of the bay. The EPA has established a Total 

Maximum Daily Load (TMDL), which mandates sediment 

and nutrient (nitrogen and phosphorus) allocation goals. The 

six states and the District of Columbia have written Watershed 

Implementation Plans to reduce loads to the bay and to meet 

load allocation goals for the TMDL.

The U.S. Geological Survey (USGS) has conducted 

numerous sediment transport studies in the Susquehanna River 

and in particular in three reservoirs in the Lower Susquehanna River Basin. Results from these studies were used 

to determine sediment transport rates over the past century 

(1900–2012) and to document changes in sediment and 

water storage capacity in the three reservoirs. When storage 

capacity is reached, a dynamic-equilibrium condition exists 

between incoming and outgoing sediment and nutrient loads 

discharged through the reservoir system to Chesapeake Bay. 

In the dynamic-equilibrium condition, constituent loads may 

increase because of short-term high-flow scour events, thereby 

potentially contributing to non-attainment of the sediment and 

nutrient allocation TMDL goals set by EPA and the State of 

Maryland water-quality standards for dissolved oxygen, waterclarity, and chlorophyll A. However, also in this dynamic 

equilibrium condition, loads may decrease for a short duration 

owing to increased deposition as a result of, and related to, the 

length of time since a preceding scour event.

The USGS has been estimating annual sediment loads 

at Susquehanna River at Marietta, Pennsylvania, and Susquehanna River at Conowingo, Maryland, locations since the 

mid-1980s to predict the mass balance of sediment transport 

through the reservoir system. Using streamflow and sediment 

data from the Susquehanna River at Harrisburg, Pa., prior to 

the mid-1980s, a decadal total of sediment loads was generated from 1900 to 1910. Loads were greatest in the early to 

mid-1900s when land disturbance activities from coal production and agriculture were at the peak. Sediment loads indicate 

a major decline (approximately 87 to 60 million tons) in the 

1950s with the introduction of agricultural soil conservation 

practices. Loads were dominated by climatic factors in the 

1960s (drought) and 1970s (very wet) and have been declining 

from the 1980s until 2012. Sediment input from two monitored tributaries flowing below Susquehanna River at Marietta 

into the reservoir system (Conestoga River at Conestoga and 

Pequea Creek at Martic Forge) were estimated to account for 

most of the sediment load entering the reservoirs from the 

Susquehanna River and tributaries. In general, sediment loads 

from Conestoga and Pequea contributed about 5–10 percent 

of the total riverine load entering the reservoirs at the scour 

threshold streamflow of 400,000 cubic feet per second (ft3

/s).

The number of days with streamflow greater than 

400,000 ft3/s ranged from 1 to 5; the average was about 3 days. 

The 1972 event (Tropical Storm Agnes) was the largest flood 

in the Susquehanna River Basin since 1889, when recording 

of streamflow began at Harrisburg, Pa. The second largest 
References Cited 17

recorded flood event using daily mean streamflow data in the 

Susquehanna River Basin since 1972 was in 2011 (Tropical 

Storm Lee). An examination of daily duration streamflow 

events indicated the highest number of daily mean streamflow 

events greater than 300,000 ft3/s occurred in spring (March–

May), whereas the greatest daily mean streamflows per storm 

event occurred in fall (September–November). 

Since 1972, there have been 11 storms with daily mean 

streamflows greater than 400,000 ft3/s (5-year recurrence 

interval), which is the streamflow when an average mass 

wasting event (when large areas of the bed begin to move) 

begins scouring the sediment in the reservoirs. The USGS 

developed a regression equation that is based on streamflow 

and sediment-load data from 11 storms to predict the sediment scour load for daily mean streamflows greater than 

300,000 ft3/s for the Lower Susquehanna River reservoirs. A 

compilation of data from various sources produced a range in 

total sediment transported through the reservoir system and 

allowed an apportioning to source (total watershed or scour) 

for various streamflows. The percent scour to total watershed 

load, based on frequency of streamflow events, ranges from 

20 to 44 percent (average 30 percent) for streamflows of 

400,000–800,000 ft3/s. In general, for streamflows greater than 

400,000 ft3/s, the average incoming sediment load from the 

Susquehanna River Basin contributes approximately 70 percent of the load transported to the upper Chesapeake Bay.

As of 2011, approximately 8 percent remained of the 

original 146,000 acre-feet of sediment storage capacity. Since 

construction of Conowingo Dam in 1929 through 2012, 

approximately 470 million tons of sediment was transported 

down the Susquehanna River into the reservoir system, 

approximately 280 million tons were trapped, and approximately 190 million tons were transported to Chesapeake Bay. 

Using the estimated scour to total load percentage of 30 percent, approximately 57 million tons of the 190 million tons 

was estimated to be from scour in the reservoirs. Combining 

findings from this analysis and Bob Hirsch’s (2012) report, 

increasing sediment concentrations and loads are due to the 

loss of storage capacity and a possible decrease in the scour 

threshold of 400,000 ft3/s. Reasons for the increase are not 

certain but likely involve changes in particle-falling velocities 

owing to increased water velocity, transport capacities, and 

bed shear. 

A total of 70 cores were collected over three time periods 

(1990–91, 1996, and 2000) to help describe location and 

distribution differences of grain size. The occurrence of sand 

had become more widespread and moved downgradient in 

Conowingo Reservoir. At the same time, finer sediment particles (silts and clays) were being displaced, resulting in lesser 

amounts of fines in the bottom sediments near the dam due 

to scour.

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For additional information:

Director

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

http://pa.water.usgs.gov/

Document prepared by the West Trenton Publishing Service Center
Langland—Sediment Transport and Capacity Change in Three Reservoirs, Lower Susquehanna River Basin, Pennsylvania and Maryland 1900–2012—Open-File Report 2014–1235

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