Changes in Sediment and Nutrient Storage

in Three Reservoirs in the Lower Susquehanna River Basin

and Implications for the Chesapeake Bay

Importance of the Susquehanna

River and Its Reservoir System to

the Chesapeake Bay

The Susquehanna River contributes

nearly 50 percent of the freshwater discharge

to the Chesapeake Bay in a year of normal or

average streamflow. The river also transports

the greatest amount of nutrients (estimates of

nearly 66 percent of the nitrogen and

40 percent of the phosphorus load) from all

nontidal areas in the Chesapeake Bay Basin.

Excessive nutrients in the Bay result in algal

blooms that decrease the amount of light

reaching submerged aquatic vegetation, and

upon decomposition, deplete the oxygen in

the water. In a normal-flow year, the Susquehanna River also contributes about

25 percent of the sediment load from nontidal areas to the Bay. Suspended sediments

also reduce light needed by submerged

aquatic vegetation and can smother livingresource habitat and obstruct fish gills.

A reservoir system consisting of Lake

Clarke, Lake Aldred, and Conowingo

Reservoir is formed by three consecutive

hydroelectric dams on the Lower Susquehanna River (fig. 1). Safe Harbor Dam,

which forms Lake Clarke, was built in 1931.

Holtwood Dam, the smallest of the three,

was built in 1910 to form Lake Aldred. The

largest and most downstream dam,

Conowingo Dam, was built in 1928 and

forms Conowingo Reservoir. Since

construction, the reservoirs have been filling

with sediment and sediment-associated

nutrients. The upper two reservoirs have

reached their capacity to store sediments and

generally no longer trap nutrients and

sediments. Conowingo Reservoir has not

reached storage capacity, however, and is

currently trapping about 70 percent of the

suspended-sediment load, 2 percent of the

total-nitrogen load, and 40 percent of the

total-phosphorus load (fig. 2) that would

otherwise be discharged to the Chesapeake

Bay (Ott and others, 1991).

In 1990, 1993, and 1996, the U.S.

Geological Survey collected information on

the depth to sediment in the reservoirs to

determine the remaining sediment-storage

capacity in the reservoir system and to

estimate when the reservoirs will reach

sediment-storage capacity. In addition,

sediment cores were collected and analyzed

in 1993 and 1996 to determine the nutrient

mass remaining in the Conowingo Reservoir.

The 1996 data collection followed a major

flood in the Susquehanna River Basin.

Figure 1. The location of the three hydroelectric dams and reservoirs on the

Lower Susquehanna River.

39°45′

40°08′

76°30′ 76°15′

Marietta

Wrightsville

Columbia

Washington Boro

Turkey Hill

Urey Duncan Island

Weise Island

Safe Harbor

Islands

Holtwood

Hennery Island

Mt. Johnson Island

Peach Bottom Peach Bottom Generating Station

Wildcat

Troublesome Conowingo

Tunnel

Hills

PENNSYLVANIA

MARYLAND

LAKE CLARKE

LAKE

CONOWINGO

ALDRED

RESERVOIR

Conestoga River

Safe Harbor

Dam

Holtwood Dam

Michael

Run

Broad

Creek Conowingo

Creek

Conowingo

Dam

0 5 10 MILES

0 5 10 KILOMETERS

PENNSYLVANIA

MARYLAND

Study

area

Long Level

Pequea Creek

U.S. Department of the Interior USGS Fact Sheet 003-98

U.S. Geological Survey March 1998
Holtwood Dam

Conowingo Dam

Safe Harbor Dam

3,100,000 tons sediment

75,000 tons nitrogen

4,350 tons phosphorus

2,210,000 tons sediment (70%)

1,500 tons nitrogen (2%)

1,740 tons phosphorus (40%)

890,000 tons sediment

73,500 tons nitrogen

2,610 tons phosphorus

AVERAGE ANNUAL

INPUT TO RESERVOIR SYSTEM

AVERAGE ANNUAL

AMOUNT TRAPPED IN

RESERVOIR SYSTEM

AVERAGE ANNUAL

OUTPUT TO CHESAPEAKE BAY

70%

40%

2%

SEDIMENT

NITROGEN

PHOSPHORUS

Percentage trapped in reservoir system:

When the ice jam broke in Lake Clarke,

most of the backwater was released in

about 2.5 hours, resulting in a surge of water

and ice of approximately 180 Mgal/min

(million gallons per minute). This flood

surge added to an existing river flow of about

200 Mgal/min. An estimated peak flow of

approximately 370 Mgal/min moved through

Safe Harbor Dam. The flood surge continued

through Lake Aldred into Conowingo Reservoir. Operators at Conowingo Dam opened

flood gates to help the surge pass. The peak

flow at Conowingo Dam was about

410 Mgal/min.

Effect of Floods on the Reservoirs

During floods, large amounts of nutrients

and sediment are transported into and out of

the Susquehanna River reservoir system.

These nutrients and sediments, along with

those already trapped in the reservoirs, are

available for deposition, resuspension, and

scour and removal from the reservoirs. Scour

from the reservoirs during floods increases

the remaining sediment-storage capacity.

The three most recent floods—in June 1972,

September 1975, and January 1996—

removed about 36 million tons of sediment

from the reservoirs.

The high flows that occurred shortly after

the ice jam in Lake Clarke broke during the

January 1996 flood had a major effect on

storage capacities and the movement of bottom sediment in all three reservoirs. The

amount of flow that causes scour of bottom

sediments from the reservoirs (about

180 Mgal/min) (Lang, 1982) was exceeded

in all three reservoirs. During the flood, the

amount of sediment transported through the

reservoir system (3.2 million tons) and

scoured from sediment trapped in the reservoir system (11.8 million tons) totaled

15 million tons of sediment delivered to the

Chesapeake Bay. This is about 16 times the

normal annual sediment load to the Bay.

Using the average annual amount of sediment deposited in the reservoirs (2.5 million

tons on the basis of data from 1959 to 1993),

the amount of sediment scoured from the

three reservoirs will probably be replaced by

reservoir trapping in only 5 to 6 years.

Large amounts of sediment were

deposited in Lake Clarke upstream from the

ice jam, and even larger amounts of sediment

were scoured near and downstream from the

ice jam. A net amount of 2.3 million tons of

sediment was estimated to have been scoured

from Lake Clarke between 1993 and 1996,

nearly all of it during the January 1996 flood.

Sediment was deposited and scoured in

many areas of Lake Aldred during the flood.

However, because sediment thickness is less

in Lake Aldred than in the other two reservoirs and the narrow channels preclude much

sediment deposition, the net change in

reservoir storage capacity and sediment mass

most likely was minimal.

Sediment was deposited and scoured

throughout the length of Conowingo

Reservoir (fig. 3). Results from the 1996

bottom-surface survey indicate approximately 2.4 million tons of net sediment were

scoured from the reservoir between the 1993

and 1996 surveys. In addition, about

6.9 million tons of sediment estimated to

have been deposited between the 1993 and

1996 surveys also was scoured and removed

from the reservoir. Therefore, total scour in

The January 1996 Flood

January 1996 witnessed an extremely

unusual and widespread floodextensive

flash flooding approached or exceeded previously recorded flood peaks in many areas of

the Susquehanna River Basin (Thompson,

1996). Warm temperatures and winds, the

consequent melting and runoff from 3-4 ft

(feet) of snow on the ground, and an additional 3 in. (inches) of rain falling in a short

time created a rapid rise in river levels. This

rapid rise broke up the thick ice pack on

many streams and rivers, forming many ice

jams around bridges and natural obstructions

in the rivers. An ice jam formed just downstream from Harrisburg, causing river levels

to rise 8 ft in 1 hour. When that jam broke,

the increased flow and ice from Harrisburg

and areas downstream reached a constriction

in Lake Clarke, causing another ice jam to

form about 2.5 mi (miles) upstream from the

Safe Harbor Dam. Behind the ice jam, the

lake level rose an average of 10 ft (the maximum rise near the ice jam was 17 ft), causing

extensive flooding in the middle and upper

parts of Lake Clarke.

Figure 2. Average annual concentrations of suspended sediment, nitrogen, and

phosphorus input into the reservoir system and output to the Chesapeake Bay.
Conowingo Reservoir was

approximately 9.1 million tons of sediment.

Slightly more than 80 percent of the total

sediment scoured was from the lower third

of the reservoir.

How Much Nitrogen and

Phosphorus are Stored in

Conowingo Reservoir?

Nutrient samples were collected from the

lower third of the reservoir, where the sediment thickness is greatest. On the basis of

data from Langland and Hainly (1997) and

data collected from previous studies (Hainly

and others, 1995; Reed and Hoffman, 1997),

about 670,000 tons of total nitrogen and

130,000 tons of total phosphorus remain in

the lower third of the reservoir. These

amounts are equivalent to about 9 times the

annual total-nitrogen load and about

50 times the annual total-phosphorus load transported to the Chesapeake Bay by the Susquehanna River during a year of normal streamflow.

How Much Longer Will the Conowingo

Reservoir Trap Sediment and

Nutrients?

Although the Conowingo Reservoir is not

filled with sediment, little space remains. Changes

in the cross-sectional areas of space available for

sediment and nutrient storage from 1928 to 1996

and the approximate level of maximum sedimentstorage capacity are shown in figure 4. From the

upper end of the reservoir to about 28,000 ft

upstream from the dam, the reservoir has virtually

no sediment-storage capacity left, and the capacity from 28,000 ft downstream to the dam has

been greatly reduced between 1928 and 1996. As

a result of scour during the January 1996 flood,

sediment-storage capacity in the Conowingo

Reservoir increased by approximately 1,600 acreft (acre-feet), which is equivalent to 2.4 million

tons of sediment. About 29,000 acre-ft remain to

be filled, or 43 million tons of sediment can be

deposited before reaching the sediment-storage

capacity of the reservoir (shaded area on figure 4).

Estimating the time remaining until the reservoir reaches sediment-storage capacity is difficult

Figure 4. Changes in vertical cross-sectional area for selected years and crosssectional area at sediment-storage capacity in the Conowingo Reservoir.

(From Langland and Hainly, 1997.)

0 10,000 20,000 30,000 40,000 50,000 60,000

DISTANCE UPSTREAM FROM CONOWINGO DAM, IN FEET

50

400

100

150

200

250

300

350

VERTICAL CROSS-SECTIONAL AREA, IN SQUARE FEET (× 1,000)

1928

1959

1990

1993

1996

APPROXIMATE LEVEL OF

REMAINING STORAGE CAPACITY

Figure 3. Estimated areas of scour and deposition in

Conowingo Reservoir, 1993-96. (From Langland and Hainly, 1997.)

0

0

2 MILES

2 KILOMETERS

1

1

76°14′

SEDIMENT SCOUR

SEDIMENT DEPOSITION

Peach Bottom

Michael Run

Conowingo

Broad Creek

76°18′ 76°10′

39°40′

Mt. Johnson

Hennery

LOWER

MIDDLE

UPPER

Dam

Power Plant

Island

Island

CROSS-SECTION

EXPLANATION

 MAXIMUM SEDIMENT-STORAGE

CAPACITY

IN 1996

LOCATION

39°43′

39°47′
because the amount of sediment transported

and deposited in the reservoirs depends on such

factors as land use and management practices

in areas draining to the reservoirs and the

amount of rainfall in the upper Susquehanna

River Basin. Occurrences of large storms that

cause scour are unpredictable. Assuming a constant average storage rate of about 2.5 million

tons each year (Ott and others, 1991) and no

scour caused by floods, the Conowingo Reservoir could be at full sediment-storage capacity

in 17 to 20 years.

What Will Happen When the

Reservoirs Fill?

Once the reservoirs reach sediment-storage

capacity, the sediment and nutrients being

transported to the Chesapeake Bay by the

Susquehanna River will likely equal those

being transported into the reservoir system. If

all other conditions remain constant, when the

reservoirs fill, there will be a 250-percent

average annual increase in the suspendedsediment load, about a 2-percent average

annual increase in the total-nitrogen load, and a

70-percent average annual increase in the totalphosphorus load moving into Chesapeake Bay

from the Susquehanna River Basin (Langland

and Hainly, 1997). Also, after capacity has

been reached, an even greater increase in the

annual loads of sediment and nutrients

transported to the Chesapeake Bay will occur

during major scour-producing floods. It is

important to note that the reservoir system will

be in a state of flux with sediments because of

short-term changes from storms that cause

scour. Therefore, the amount of sediment

transported out of the reservoir system will not

always be in equilibrium with the amount of

sediment transported into the system.

The Chesapeake Bay Program has set a goal

of reducing controllable nutrient inputs to the

Bay by 40 percent by the year 2000. This goal

for the reduction of nutrient inputs will be more

difficult to meet and maintain once the reservoir system exceeds its trapping ability and,

more importantly, sediment and nutrient loads

delivered to the Chesapeake Bay increase.

References Cited

Hainly, R.A., Reed, L.A., Flippo, H.N., and

Barton, G.J., 1995, Deposition and

simulation of sediment transport in the

Lower Susquehanna River Reservoir

System: U.S. Geological Survey WaterResources Investigations Report 95-4122,

39 p.

Lang, D.J., 1982, Water quality of the three

major tributaries to the Chesapeake Bay,

The Susquehanna, Potomac, and James

Rivers, January 1979 - April 1981:

U.S. Geological Survey Water-Resources

Investigations 82-32, 64 p.

Langland, M.J., and Hainly, R.A., 1997,

Changes in bottom surface-elevations in

three reservoirs on the Lower Susquehanna River, Pennsylvania and Maryland,

following the January 1996 flood—

Implications for nutrient and sediment

loads to Chesapeake Bay: U.S.

Geological Survey Water-Resources

Investigations Report 97-4138, 34 p.,

3 pls.

Ott, A.N., Takita, C.S., Edwards, R.E., and

Bollinger, S.W., 1991, Loads and yields of

nutrients and suspended sediment

transported in the Susquehanna River

Basin, 1985-89: Susquehanna River

Basin Commission Report, Publication

no. 136, 253 p.

Reed, L.A., and Hoffman, S.A., 1997,

Sediment deposition in Lake Clarke, Lake

Aldred, and Conowingo Reservoir,

Pennsylvania and Maryland, 1910-93:

U.S. Geological Survey Water-Resources

Investigations Report 96-4048, 14 p.,

3 pls.

Thompson, R.E., 1996, Statewide floods in

Pennsylvania, January 1996: U.S.

Geological Survey Fact Sheet 103-96, 2 p.

Michael J. Langland—1998

For further detailed information about

the USGS study described here,

the reader is referred to the report

by Langland and Hainly (1997).

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on the Chesapeake Bay

can be found by accessing the

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Region Home Page at

http://wwwpah2o.er.usgs.gov/chesbay/

Additional information about other

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can be found by accessing the

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Network Home Page at

http://www.chesapeake.org/

For information on USGS programs

and activities in Pennsylvania,

please visit our web site at

http://wwwpah2o.er.usgs.gov/

or contact:

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USGS, WRD

840 Market Street

Lemoyne, PA 17043-1586

(717) 730-6900

Fax: (717) 730-6997

Email: dc_pa@usgs.gov

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