U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5185

Flux of Nitrogen, Phosphorus, and Suspended Sediment from 

the Susquehanna River Basin to the Chesapeake Bay during 

Tropical Storm Lee, September 2011, as an Indicator of the 

Effects of Reservoir Sedimentation on Water Quality
Cover. View of the Conowingo Dam on the Susquehanna River in the aftermath of Tropical Storm Lee. Photo taken at 4:30 p.m., 

September 12, 2011. Discharge at time of the photo was 220,000 cubic feet per second. Peak discharge for the flood was 778,000 

cubic feet per second at 4:00 a.m. on September 9, 2011. Photograph by Wendy McPherson, U.S. Geological Survey.
Flux of Nitrogen, Phosphorus, and 

Suspended Sediment from the 

Susquehanna River Basin to the 

Chesapeake Bay during Tropical 

Storm Lee, September 2011, as an 

Indicator of the Effects of Reservoir 

Sedimentation on Water Quality

By Robert M. Hirsch

Scientific Investigations Report 2012–5185

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Hirsch, R.M., 2012, Flux of nitrogen, phosphorus, and suspended sediment from the Susquehanna River Basin to the 

Chesapeake Bay during Tropical Storm Lee, September 2011, as an indicator of the effects of reservoir sedimentation 

on water quality: U.S. Geological Survey Scientific Investigations Report 2012–5185, 17 p.

ISBN 978-1-4113-3452-6
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

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

Hypothesis Regarding the Changing Role of the Reservoirs.........................................................4

Nitrogen...........................................................................................................................................................5

Total Phosphorus ...........................................................................................................................................8

Suspended Sediment ..................................................................................................................................10

Overview of Tropical Storm Lee Fluxes for the Susquehanna River at Conowingo, 

Maryland..........................................................................................................................................11

Implications for the Chesapeake Bay ......................................................................................................12

Summary and Conclusions.........................................................................................................................13

Acknowledgments.......................................................................................................................................14

References Cited..........................................................................................................................................14

Appendix 1. Daily-mean discharge, average flux, and flow-normalized annual flux 

values from the Weighted Regressions on Time, Discharge, and Season 

(WRTDS) model for total nitrogen in the Susquehanna River at Conowingo, 

Maryland, water years 1978–2011..................................................................................................16

Appendix 2. Daily-mean discharge, average flux, and flow-normalized annual 

flux values from the Weighted Regressions on Time, Discharge, and 

Season (WRTDS) model for total phosphorus in the Susquehanna River 

at Conowingo, Maryland, water years 1978 –2011 ...................................................................16

Appendix 3. Daily-mean discharge, average flux, and flow-normalized annual 

flux values from the Weighted Regressions on Time, Discharge, and 

Season (WRTDS) model for suspended sediment in the Susquehanna 

River at Conowingo, Maryland, water years 1978–2011..........................................................17

Figures

1. National Aeronautics and Space Administration Moderate Resolution 

Imaging Spectroradiometer (MODIS) photograph from the Terra satellite, 

September 13, 2011, showing sediment plume extending to near the 

mouth of the Potomac River, a distance of about 100 miles..................................................2

2a–2b. Maps showing—

2a. Location of the Susquehanna River Basin within the Chesapeake 

Bay Basin and the three dams on the lower Susquehanna River 

(Safe Harbor Dam, Holtwood Dam, and Conowingo Dam)...........................................3

2b. Location of Safe Harbor Dam, Holtwood Dam, and Conowingo Dam 

and their associated reservoirs, Lake Clarke, Lake Aldred, and 

Conowingo Reservoir..........................................................................................................4

3–17. Graphs showing—

3. Sediment deposition in the lower 11.5 miles of Conowingo Reservoir 

from the time of dam construction (1929) to the most recent survey 

(2008), in acre-feet and as a percentage of the estimated capacity 

of 138,000 acre-feet.............................................................................................................4
iv

4. Total-nitrogen concentration as a function of time for the Susquehanna 

River at Conowingo, Maryland ..........................................................................................5

5. Estimated daily flux of total nitrogen for water years 1978–2011, 

Susquehanna River at Conowingo, Maryland ................................................................6

6. Daily discharge values for the Susquehanna River at Conowingo, 

Maryland, showing only discharges that exceeded (A) 200,000 cubic 

feet per second, and (B) 400,000 cubic feet per second...............................................7

7. Observed concentration of total nitrogen as a function of discharge 

for the Susquehanna River at Conowingo, Maryland....................................................7

8. Estimated concentration of total nitrogen as a function of discharge 

for the Susquehanna River at Conowingo, Maryland, based on 

smoothing centered on September 1, 1990, or on September 1, 2005 ........................8

9. Estimated annual flux of total nitrogen by water year for the 

Susquehanna River at Conowingo, Maryland ................................................................8

10. Total-phosphorus concentration as a function of time for the 

Susquehanna River at Conowingo, Maryland ................................................................9

11. Observed concentration of total phosphorus as a function of 

discharge for the Susquehanna River at Conowingo, Maryland.................................9

12. Estimated concentration of total phosphorus as a function of 

discharge for the Susquehanna River at Conowingo, Maryland, 

based on smoothing centered on September 1, 1990, or on 

September 1, 2005................................................................................................................9

13. Estimated annual flux of total phosphorus by water year for the 

Susquehanna River at Conowingo, Maryland ..............................................................10

14. Suspended-sediment concentration as a function of time for the 

Susquehanna River at Conowingo, Maryland ..............................................................11

15. Observed concentration of suspended sediment as a function of 

discharge for the Susquehanna River at Conowingo, Maryland...............................11

16. Estimated concentration of suspended sediment as a function of 

discharge for the Susquehanna River at Conowingo, Maryland, 

based on smoothing centered on September 1, 1990, or on 

September 1, 2005..............................................................................................................11

17. Estimated annual flux of suspended sediment by water year for the 

Susquehanna River at Conowingo, Maryland ..............................................................11

Tables

1. Estimated flux of total nitrogen, total phosphorus, and suspended sediment 

for Tropical Storm Lee and water year 2011; and average annual flux for 

water years 2002–11 and 1978–2011..........................................................................................6

2. Time, streamflow, and flux of total nitrogen, total phosphorus, and 

suspended sediment associated with Tropical Storm Lee as a percentage 

of total values for water year 2011, water years 2002–11, and water years 

1978–2011 .....................................................................................................................................12

3. Predicted change in average flux of total nitrogen, total phosphorus, 

and suspended sediment from the mid 1990s to the time when reservoir 

sediment-storage capacity is reached (Langland and Hainly, 1997), and 

observed change in flow-normalized flux, water years 1996–2011....................................13
v

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047.0 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

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

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

Flow rate

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

gallon per day (gal/d) 3.785 liter per day (L/d)

Mass

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

ton per day (ton/d) 0.9072 metric ton per day

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

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

A water year is the period from October 1st to September 30th. It is designated by the calendar 

year in which it ends.

Conversion Factors

Flux of Nitrogen, Phosphorus, and Suspended Sediment 

from the Susquehanna River Basin to the Chesapeake Bay 

during Tropical Storm Lee, September 2011, as an Indicator 

of the Effects of Reservoir Sedimentation on Water Quality

By Robert M. Hirsch

Abstract 

Concentrations of nitrogen, phosphorus, and suspended 

sediment are measured at the U.S. Geological Survey 

streamgage at Conowingo Dam at the downstream end of 

the Susquehanna River Basin in Maryland, where the river 

flows into the Chesapeake Bay. During the period September 

7–15, 2011, in the aftermath of Tropical Storm Lee, concentrations of these three constituents were among the highest 

ever measured at this site. These measurements indicate that 

sediment-storage processes behind the three dams on the 

lower Susquehanna River are evolving. In particular, they 

indicate that scouring of sediment (and the nitrogen and 

phosphorus attached to that sediment) may be increasing with 

time. Trends in flow-normalized fluxes at the Susquehanna 

River at Conowingo, Maryland, streamgage during 1996–2011 

indicate a 3.2-percent decrease in total nitrogen, but a 55-percent increase in total phosphorus and a 97-percent increase 

in suspended sediment. These large increases in the flux of 

phosphorus and sediment from the Susquehanna River to the 

Chesapeake Bay have occurred despite reductions in the fluxes 

of these constituents from the Susquehanna River watershed 

upstream from the reservoirs. Although the Tropical Storm 

Lee flood event contributed about 1.8 percent of the total 

streamflow from the Susquehanna River to the Chesapeake 

Bay over the past decade (water years 2002–11), it contributed 

about 5 percent of the nitrogen, 22 percent of the phosphorus, 

and 39 percent of the suspended sediment during the same 

period. These results highlight the importance of brief highflow events in releasing nitrogen, phosphorus, and sediment 

derived from the Susquehanna River watershed and stored in 

the Conowingo Reservoir to the Chesapeake Bay.

Introduction

The water and the nitrogen, phosphorus, and suspended 

sediment—the three constituents for which there are 

watershed-specific goals under the Chesapeake Bay 

Total Maximum Daily Load (TMDL) requirements (U.S. 

Environmental Protection Agency, 2012)—that are derived 

from the Susquehanna River Basin and flow past Conowingo 

Dam are critically important to the ecological condition of 

the Chesapeake Bay. Reductions in nutrients are needed to 

limit algae blooms that die and sink to the bottom of the 

Chesapeake Bay and consume oxygen, resulting in hypoxic 

zones where fish and shellfish cannot survive. In addition, 

suspended sediments and these algae blooms limit the penetration of light that is important to grasses and other aquatic 

life on the bottom of the Chesapeake Bay. Estimates indicate 

that, on average, the Susquehanna River contributed nearly 

47 percent of the freshwater and 41 percent of the nitrogen, 

25 percent of the phosphorus, and 27 percent of the sediment 

load to the Chesapeake Bay during 1991–2000 (Gary Shenk, 

U.S. Environmental Protection Agency, written commun., 

2012). In September 2011, Tropical Storm Lee produced an 

estimated 4 to 7 inches of rainfall throughout much of the 

Susquehanna River Basin, with precipitation in some areas 

exceeding 12 inches. This storm produced very large, although 

not unprecedented, flooding in various parts of the basin. It 

was widely noted in the media that the outflows from the 

Susquehanna River past Conowingo Dam produced a large 

plume of sediment that was visible in satellite photographs for 

several days after the flood peak. The visible plume extended 

at least as far as the mouth of the Potomac River, approximately 100 miles downstream from Conowingo Dam. A 
2 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

satellite image from September 13, 2011 (4 days after the peak 

streamflow at Conowingo Dam), is shown in figure 1. It was 

widely recognized that a volume of water and sediment of this 

magnitude would have transported large amounts of nitrogen 

and phosphorus to the Chesapeake Bay (Blankenship, 2011).

The U.S. Geological Survey (USGS) streamgage on the 

Susquehanna River at Conowingo, Maryland (hereafter 

referred to in this report as “Susquehanna River at 

Conowingo”), has a 44-year streamflow record (1968–2011). 

The maximum daily discharge during the flood of September 

2011, associated with Tropical Storm Lee, was 709,000 cubic 

feet per second (ft3/s) on September 9, 2011; this value was the 

second largest annual maximum daily discharge recorded for 

water years 1968–2011. The largest occurred during the flood 

of June 1972, a result of Hurricane Agnes, with a maximum 

daily-mean discharge of 1,120,000 ft3/s. The third and fourth 

largest events in the period were 662,000 ft3/s in 1975 and 

622,000 in 1996, respectively. Therefore, the Tropical Storm 

Lee flood event, although large, was of a magnitude that can 

be expected to occur about once every 20 years as determined 

from the data recorded at this streamgage. 

An analysis of the transport of sediment and nutrients 

during a flood event can benefit from an examination of 

discharge over a period of several days, rather than the peak 

discharge alone. The 2011 flood, when measured as an average 

of the highest 7 consecutive days of flow, was the third largest 

in the period of record (387,000 ft3/s, compared to 712,000 

ft3/s in 1972 and 391,000 ft3/s in 1993). Seven-day average 

maximum flows for 8 of the 44 years of record exceeded 

300,000 ft3/s. By this measure, the Tropical Storm Lee flood 

falls well within a range that can be expected to occur once 

every 10 to 15 years. 

Purpose and Scope

This report examines the flow and water-quality data 

measured at the Susquehanna River at Conowingo streamgage 

during the Tropical Storm Lee event (September 7–15, 2011) 

in the context of the historical water-quality record at this site 

in order to (1) provide the best possible estimate of the flux 

of sediment, total nitrogen, and total phosphorus from the 

Susquehanna River to the Chesapeake Bay during the event; 

(2) evaluate these quantities in the context of long-term average fluxes from the Susquehanna River; (3) use the information from Tropical Storm Lee along with the prior years of 

record to re-evaluate the trends in fluxes of these materials to 

the bay, and (4) explore a hypothesis about the extent to which 

the system of three reservoirs on the lower Susquehanna River 

have begun a transition from a period of high trapping efficiency to one of approximate equilibrium.

Lower Susquehanna River Reservoirs

The three dams on the lower Susquehanna River (in 

downstream order, Safe Harbor, Holtwood, and Conowingo 

Dams) and their associated reservoirs (Lake Clarke, Lake 

Aldred, and Conowingo Reservoir, respectively) are described 

by Langland (2009) and shown in figure 2. The history of 

sediment storage in the lower Susquehanna River has been the 

topic of a number of studies (Langland, 2009; Langland, 1998; 

Langland and Hainly, 1997), all of which generally conclude 

that the pools behind the dams (in total) are on a long-term 

trajectory to becoming filled. In a description of the history 

of deposition in Lakes Clarke and Aldred, Langland (2009) 

concludes that both are in “long-term equilibrium”—that is, on 

average over several years, no net change has occurred in the 

volume of sediment stored in these two reservoirs. Although 

individual flood events have caused sediment scour, the subsequent low to moderate flows have resulted in deposition, so 

little net change in sediment storage has occurred. Repeated 

bathymetric surveys of Conowingo Reservoir show that the 

portion of the reservoir that is more than about 6 miles above 

Conowingo Dam has been at equilibrium at least since 1959. 

Figure 1. National Aeronautics and Space Administration 

Moderate Resolution Imaging Spectroradiometer (MODIS) 

photograph from the Terra satellite, September 13, 2011, 

showing sediment plume extending to near the mouth of the 

Potomac River, a distance of about 100 miles (modified from 

National Aeronautics and Space Administration, 2012).
Introduction 3

Figure 2a. Location of the Susquehanna River Basin within the Chesapeake Bay Basin and the three dams on the lower Susquehanna 

River (Safe Harbor Dam, Holtwood Dam, and Conowingo Dam).
4 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

Deposition in the lower 11.5 miles of the reservoir from 1929, 

when the dam was constructed, to the most recent bathymetric 

survey in 2008, is shown in figure 3 (Langland, 2009). The 

equilibrium sediment capacity of this part of the reservoir 

is estimated to be 138,000 acre-feet; as of 2008, 118,000 

acre-feet of storage (86 percent of capacity) had been lost 

to sediment deposition. Results of the several surveys of the 

major portion of the reservoir (the lower 11.5 miles) show that 

deposition is asymptotically approaching this estimated capacity (fig. 3). 

According to Langland (2009): 

“Once the sediment-storage capacity is reached, 

sediments will no longer be effectively trapped and 

loads transported out of the reservoir will approach 

the loads transported into the reservoir. However, the 

reservoirs will not be constantly filled to capacity 

with sediments because of short-term changes from 

storms that cause scour and a subsequent reduction 

in exported sediments until the scoured amount is 

refilled. Therefore, the amount of sediment transported out of the reservoirs will not always be in 

equilibrium with the amount of sediment transported 

into the reservoirs.” 

Hypothesis Regarding the Changing Role of the 

Reservoirs

The data analysis in this report is designed to explore the 

hypothesis that, as the reservoir moves toward this equilibrium 

condition, the extent of scour resulting from each high-flow 

event is greater than it would have been in the past, when the 

volume of sediment storage was smaller than it currently is. It 

is assumed in this analysis that the transition from the situation in which the reservoirs trapped a large fraction of the 

sediment that enters them to the equilibrium condition is likely 

to be gradual. The working hypothesis of this analysis is that 

deposition of sediment and its attached nitrogen and phosphorus diminishes gradually, although the gradual decrease may 

be punctuated by episodic scour events. 

Langland and Hainly (1997) described the conditions of 

the mid 1990s and also made a projection of the equilibrium 

condition. In the mid 1990s, the reservoirs were trapping 

about 2 percent of the nitrogen, 40 percent of the phosphorus, 

and 70 percent of the suspended sediment that would have 

entered the Chesapeake Bay from the Susquehanna River if 

the three reservoirs did not exist (Langland and Hainly, 1997). 

If it is assumed that no change occurs in the average annual 

inputs of nitrogen, phosphorus, and sediment to the reservoir 

system, the average annual flux out of each of the reservoirs 

will increase as they fill. The changes projected by Langland 

and Hainly (1997) were a 2-percent increase in total nitrogen, 

a 70-percent increase in total phosphorus, and a 250-percent 

increase in suspended sediment. In 1997, the time required 

to fill the reservoirs (in the absence of additional scour-producing floods) was estimated to be 17 to 20 years (Langland 

and Hainly, 1997). In this report, the data collected at the 

Susquehanna River at Conowingo streamgage are analyzed 

to explore the extent to which a transition to this equilibrium 

state is underway.

Figure 2b. Location of Safe Harbor Dam, Holtwood Dam, 

and Conowingo Dam and their associated reservoirs, Lake 

Clarke, Lake Aldred, and Conowingo Reservoir (modified from 

Langland, 2009, fig. 1).

Figure 3. Sediment deposition in the lower 11.5 miles of 

Conowingo Reservoir from the time of dam construction (1929) to 

the most recent survey (2008), in acre-feet and as a percentage 

of the estimated capacity of 138,000 acre-feet (data from 

Langland, 2009, table 3).
Nitrogen 5

In the past, the discharge at which flows through the 

reservoirs initiate substantial scour of stored sediment and the 

attached nitrogen and phosphorus was thought to be about 

390,000 ft3/s (Lang, 1982). This discharge is commonly 

referred to as the “scour threshold.” The physical mechanism 

that would bring about the gradual changes hypothesized is 

the following: For any given discharge of water through the 

reservoirs, the velocity and water-surface slope are likely 

to increase as the reservoirs fill with sediment as a result of 

the decrease in reservoir cross-sectional area through which 

the water can flow. As velocities increase, rates of settling 

decrease, decreasing sediment deposition and increasing the 

shear stress on the reservoir bed. The increase in shear stress 

would cause scour to occur at a lower discharge than in the 

past, and would increase the extent of scour at discharges that 

exceed the scour threshold. 

This report describes the evolving “behavior” of water 

quality for the Susquehanna River Basin as measured at 

Conowingo Dam but does not discuss the hydrodyamics of the 

specific scour or deposition processes. The behavior can be 

thought of as a characterization of the response of the watershed (including the reservoirs) to the full range of hydrologic 

conditions. The response is quantified by the concentrations 

of nitrogen, phosphorus, or sediment at the Susquehanna at 

Conowingo streamgage. The hydrologic condition is quantified by the streamflow at the streamgage. The statistical methods used in this study are designed to describe this evolving 

behavior by characterizing the changing relation between concentration and discharge over the period of record. This report 

examines total nitrogen, total phosphorus, and suspended 

sediment; however, because nitrogen transport is less related 

to the presence of particulates than is phosphorus transport, 

the hypothesized changes can be expected to be smallest for 

nitrogen, larger for phosphorus, and greatest for suspended 

sediment. 

The report explores changes in the behavior of the 

Susquehanna River to search for evidence that the changes 

in flux predicted by Langland and Hainly (1997) have begun. 

Knowing the status of this transition from the reservoirs being 

a trap for these materials to being at steady state is crucial to 

the ongoing adjustment of actions needed to meet Chesapeake 

Bay restoration goals. 

Nitrogen

The USGS National Water Information System (NWIS) 

database contains data for 988 samples that were analyzed 

for total nitrogen (hereafter in this report, the term “nitrogen” 

without a qualifier represents total nitrogen) from August 3, 

1978, through September 29, 2011. The average concentration in the 981 samples collected before September 2011 was 

1.75 milligrams per liter (mg/L). Five samples were collected during the September 2011 flood event; concentrations 

in these samples ranged from a maximum of 10.4 mg/L on 

September 8, the day before the maximum flow, to a minimum 

of 1.4 mg/L 5 days after the peak flow. The average concentration in these five samples was 3.9 mg/L, more than twice the 

long-term mean. Nitrogen concentrations in all 988 samples 

collected over the period of record are shown in figure 4. 

Concentrations from 2011 stand out as high values, but the 

highest value, 20.1 mg/L, was measured on September 20, 

2004 (during the flood associated with Hurricane Ivan), at 

a discharge nearly equal to the discharge of September 8, 

2011 (the date of the highest nitrogen concentration during 

Tropical Storm Lee). Nitrogen concentrations associated 

with the particulates are very high in these extreme high-flow 

samples. In the four samples with the highest concentrations 

(all from 2004 and 2011), the particulate fraction of the total 

nitrogen ranged from 65 to 98 percent of the total. In contrast, 

the average for all the samples collected during the 2002–11 

time period was 12 percent (Joel Blomquist, U.S. Geological 

Survey, written commun., 2012). The biogeochemical processes by which the dissolved and suspended fractions of 

nitrogen enter the food web of the bay differ in that the 

suspended fraction may become available more slowly than 

the dissolved fraction. The nutrient-reduction goals for the 

bay are stated in terms of total nitrogen, however, and highflow events with their associated large particulate fractions are 

highly relevant to the efforts to achieve the water-quality goals 

for the bay; therefore, the analysis presented here is in terms 

of total nitrogen.

In order to evaluate the history of nitrogen concentrations and flux, a statistical model of nitrogen concentration 

as a function of discharge and season over the 34-year period 

of record was constructed. In this study, the method used to 

construct the model is known as “Weighted Regressions on 

Time, Discharge, and Season” (WRTDS), which is described 

by Hirsch and others (2010). In that publication, WRTDS was 

applied to concentrations of nitrate and total phosphorus at all 

nine of the Chesapeake Bay River Input Monitoring stations 

(a set of nine monitoring locations just above the head of tide 

on major tributaries to the Chesapeake Bay) for the 31-year 

period ending with water year 2008. The statistical smoothing 

methods used are designed for large (more than 200 observations) water-quality datasets more than 20 years long and 

Figure 4. Total-nitrogen concentration as a function of time for 

the Susquehanna River at Conowingo, Maryland. 
6 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

for which complete daily streamflow records are available. 

The method creates a statistical model that can be used to 

determine the expected value of concentration for each day 

in the period of record, and from that value it determines the 

expected value of flux for each day. Estimates of flux for the 

period of record are shown in figure 5 and summarized in 

table 1. 

The record includes no estimated daily flux values greater 

than 4,000 tons per day (tons/d) prior to 2004 (fig. 5). Since 

that time, two events (a total of 5 days) have had estimated 

flux values ranging from about 7,000 to 18,000 tons/d. 

Whether this marked change in the last decade is a result of 

a few days of high streamflow or a result of a change in the 

response of the Susquehanna River at Conowingo Dam to high 

streamflow cannot be determined from figure 5 alone. Daily 

discharge values for the same period are shown in figure 6. 

Although the September 2011 high flow was the highest discharge in the record, no general tendency toward an increase 

in the frequency of extreme high flows (for example, above 

thresholds such as 200,000 or 400,000 ft3/s) is observed. 

The Mann-Kendall test (Mann, 1945; Kendall, 1938) was 

used to evaluate the possible presence of a monotonic trend in 

the annual number of exceedances of a 400,000-ft3/s threshold 

at the Susquehanna at Conowingo. The null hypothesis is that 

the expected number of exceedances per year is unchanged 

over the record. The 400,000-ft3/s threshold was used in this 

test because it is approximately the discharge that has been 

considered to be required to generate substantial scour in the 

reservoir. The value of Kendall’s tau for this dataset is 0.06, 

which is near zero, the expected value of tau (which can 

range from -1 to +1) if there were no trend. The p-value for 

rejecting the null hypothesis was 0.6. These results indicate 

that there has been no statistically significant change in the 

frequency of flood events that exceed 400,000 ft3/s. Results 

of a Mann-Kendall test of exceedances of 400,000 ft3/s in the 

122-year-long streamgage record for the Susquehanna River at 

Harrisburg, Pennsylvania (PA), also indicate no change in the 

frequency of large streamflow events. Therefore, it is unlikely 

that the predominance of high fluxes in the last decade is 

solely an artifact of the streamflow pattern.

Two other hypotheses could explain this substantial 

increase in the number of high-flux days and the magnitude of 

fluxes: an increase in inputs to the reservoirs from upstream, 

and an increase in scouring of nitrogen-rich sediments within 

the system of reservoirs. The first hypothesis appears to be 

unlikely, as recent analyses of the long-term water-quality 

records for the Susquehanna River at Marietta, PA, and the 

Conestoga River at Conestoga, PA, show significant improvements (decreases) in total nitrogen during the 1985–2010 

and 2001–10 periods (Langland and others, 2012). The 

drainage basins of these two monitoring sites cover about 98 

percent of the total drainage area of the Susquehanna River 

at Conowingo streamgage. The absence of similar changes 

at these sites indicates that a hypothesis that the high concentrations at the Susquehanna River at Conowingo result 

from changes occurring upstream from the reservoirs is 

not a reasonable explanation for the changes observed. The 

same hypothesis was considered for the total-phosphorus 

and suspended-sediment results (discussed farther on in this 

report). Langland and others (2012) document improvements 

(decreases) in total-phosphorus concentrations for the periods 

1985–2010 and 2001–10 at both the Susquehanna River at 

Marietta, PA, and the Conestoga River at Conestoga, PA. The 

suspended-sediment concentrations show significant improvements, although the changes for the 2001–10 period at the 

Figure 5. Estimated daily flux of total nitrogen for water years 

1978–2011, Susquehanna River at Conowingo, Maryland.

Table 1. Estimated flux of total nitrogen, total phosphorus, and suspended sediment for Tropical Storm Lee and water 

year 2011; and average annual flux for water years 2002–11 and 1978–2011.

Flux Total nitrogen Total phosphorus Suspended sediment

Tropical Storm Lee, in tons 42,000 10,600 19,000,000

Water year 2011, in tons 135,000 17,400 24,300,000

Highest for any water year prior to 2011, in tons 135,000 8,200 11,700,000

Average annual, water years 2002–11, in tons per year 79,000 4,800 4,800,000

Average annual, water years 1978–2011, in tons per year 71,000 3,300 2,500,000
Nitrogen 7

Susquehanna River at Marietta, PA, were not significant. 

Therefore, the scour hypothesis is the most likely explanation 

for the increased flux of nitrogen (and the other two constituents considered in this report) at high flow. 

The changing relation between streamflow and 

total-nitrogen concentration at the Susquehanna River at 

Conowingo is shown in figure 7. Concentrations in samples 

collected at discharges greater than 100,000 ft3/s are separated 

into two groups—those prior to 2000 and those from 2000 to 

the present. For discharges less than about 300,000 ft3/s, the 

two datasets are similar, but at discharges greater than about 

300,000 ft3/s, the post-2000 concentrations are distinctly 

higher than those for the previous 24 years. An analysis of 

covariance (Helsel and Hirsch, 2002, p. 316–318) was performed on the data shown in figure 7. The null hypothesis was 

that for discharges greater than 100,000 ft3/s, the logarithm of 

the concentration followed a single model with the explanatory variables of log(Q) and (log(Q))2

 , where Q is discharge. 

The alternative hypothesis was that the relation between 

concentration and discharge for the period prior to 2000 was 

significantly different from that for the period beginning in 

2000. The null hypothesis was rejected (p < 0.0001)—that is, 

the relation between log total-nitrogen concentration and log 

discharge is significantly different for these two periods. 

The WRTDS smoothing procedure was used to evaluate 

the relation between discharge and concentration around the 

midpoints of the two periods depicted in figure 7 (1978–99 

and 2000–11). The dates assigned to the two curves in the 

graph define the midpoints used in this smoothing procedure. 

Data from a range of as much as 10 years before and after 

those dates were used, but more weight was placed on the data 

collected near those dates and less on those collected several 

years from those dates. Similarly, data from all seasons were 

used but more weight was placed on the time of year specified 

(September 1). This time of year was selected because it is 

in the middle of the tropical storm season, a time when many 

(but not all) of the highest streamflow events have occurred. 

For the pre-2000 period, concentrations vary little across the 

range of discharges shown (5,000–500,000 ft3/s) (fig. 8). For 

2000 and later (which includes the high flows of 2004 and 

2011), concentrations at discharges less than about 100,000 

ft3/s are slightly lower than those for the previous decades. For 

discharges greater than about 175,000 ft3/s, however, concentrations rise steeply. For example, at 500,000 ft3

/s, estimated 

concentrations are three to four times those for the same 

discharge in the previous decades, indicating that the relation 

between discharge and nitrogen concentration has changed. 

Concentrations at the highest discharges have been substantially higher since 2000 than they were prior to 2000 (figs. 

7 and 8). Because relatively few observations are available at 

discharges greater than about 200,000 ft3/s, the curve shown in 

figure 8 is only a rough approximation; nevertheless, it clearly 

shows the pattern of lower concentrations since 2000 at the 

lower discharges (less than about 100,000 ft3/s) and higher 

concentrations since 2000 at higher discharges (especially 

above about 300,000 ft3/s).

The nitrate fraction of the total-nitrogen flux (not 

described in this report) likely has not undergone a similar 

marked increase in flux at high discharges; in fact, for virtually all flows and seasons of the year, nitrate concentrations 

have been decreasing. The observed increase in total nitrogen 

at extreme high discharges in recent years is a result of the 

increase in the particulate fraction of the total nitrogen. This 

increase would be expected if the frequency and intensity of 

scouring of the reservoir sediments has increased and deposition rates have declined as the reservoirs fill with sediment. 

The annual flux of total nitrogen by water year is shown 

in figure 9 together with a curve that represents the “flow-normalized flux” of total nitrogen. Using the flow-normalized flux 

removes the effect of year-to-year variations in streamflow 

but preserves the effect of the normal seasonal and random 

Figure 6. Daily discharge values for the Susquehanna River at 

Conowingo, Maryland, showing only discharges that exceeded 

(A) 200,000 cubic feet per second, and (B) 400,000 cubic feet per 

second.

Figure 7. Observed concentration of total nitrogen as a function 

of discharge for the Susquehanna River at Conowingo, Maryland. 
8 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

components of the streamflow record. The flow-normalization 

method is described by Hirsch and others (2010). 

The flow-normalized flux results indicate that after 

removing the year-to-year variations that result from variations 

in streamflow, the overall trend has been a decline in the flux 

of total nitrogen since the late 1980s. The flow-normalized 

results indicate an average rate of change in total-nitrogen 

transport of about -0.5 tons per year per year (tons/yr/yr), or 

-0.6 percent per year, from 1990 through 2011.

In general, because the dominant form of nitrogen (about 

70 percent of the average annual flux) is nitrate, which is relatively unaffected by scour or deposition processes, the filling 

of the reservoirs appears to have little or no detrimental effect 

on the progress toward decreasing the total-nitrogen flux to the 

bay; however, the filling of the reservoirs does appear to be 

affecting the variability of the annual nitrogen flux to the bay. 

Total Phosphorus

The USGS NWIS database contains data for 951 samples 

that were analyzed for total phosphorus (hereafter in this 

report, the term “phosphorus” without a qualifier represents 

total phosphorus) collected from August 3, 1978, through 

October 4, 2011. The average concentration in the 943 samples 

collected before September 2011 was 0.067 mg/L. In the five 

samples collected over the course of the flood associated with 

Tropical Storm Lee, concentrations ranged from a maximum 

of 2.31 mg/L on September 8, the day before the maximum 

flow day, to a minimum of 0.11 mg/L 5 days after the peak 

flow. The average concentration in these five samples was 

0.88 mg/L, about 13 times the long-term mean up to the time 

of Tropical Storm Lee. Phosphorus concentrations in all 951 

samples collected over the period of record are shown in figure 

10. All of the values in the 34-year record that are higher than 

0.4 mg/L occurred during 2004–11. The first of these was a 

value of 1.17 mg/L, which was measured on September 20, 

2004, when the mean daily discharge was 545,000 ft3/s. The 

second was a value of 0.87 mg/L, measured on March 12, 

2011, when the mean daily discharge was 414,000 ft3/s. The 

other three values greater than 0.4 mg/L were associated with 

Tropical Storm Lee—2.30, 0.73, and 0.88 mg/L on September 

8, 10, and 11, respectively, when discharges were 592,000, 

493,000, and 365,000 ft3/s, respectively. The 365,000-ft3/s discharge on September 11 was the 22nd highest daily discharge 

for a sampled day in the 34-year record, yet it resulted in the 

third highest concentration in the record, indicating that for 

discharge values in the range of a few hundred thousand cubic 

feet per second, phosphorus concentrations associated with a 

given discharge have been considerably higher in recent years 

than in the earlier part of the record. 

For all samples collected from 2000 to 2011, an average 

of 71 percent of the total phosphorus was in the particulate 

phase (compared to 12 percent for total nitrogen). During the 

high-flow events of 2004 and 2011, however, the particulate 

phase constituted 98 to 99 percent of the total-phosphorus concentration (Joel Blomquist, U.S. Geological Survey, written 

commun., 2012). The biogeochemical processes by which the 

dissolved and suspended fractions of phosphorus, like those 

of nitrogen, enter the food web of the bay differ—that is, the 

suspended fraction may become available more slowly than 

the dissolved fraction. The nutrient-reduction goals for the bay 

are stated in terms of total phosphorus, however, and highflow events with their associated large particulate fractions are 

highly relevant to the effort to achieve the water-quality goals 

for the bay. Therefore, the analysis presented here is in terms 

of total phosphorus. 

Total-phosphorus concentration and associated discharge 

data for the periods before and after 2000 are shown in figure 

11. At a discharge greater than about 300,000 ft3/s, the concentrations for a given discharge are substantially higher in the 

later period than in the earlier one. An analysis of covariance 

was performed on these data. The null hypothesis was that 

for discharges greater than 100,000 ft3/s, the logarithm of the 

Figure 8. Estimated concentration of total nitrogen as a 

function of discharge for the Susquehanna River at Conowingo, 

Maryland, based on smoothing centered on September 1, 1990, 

or on September 1, 2005.

Figure 9. Estimated annual flux of total nitrogen by water year 

for the Susquehanna River at Conowingo, Maryland (data are 

listed in appendix 1).
Total Phosphorus 9

concentration followed a single model with the explanatory 

variables of log(Q) and (log(Q))2 , where Q is discharge. The 

alternative hypothesis was that the relation between concentration and discharge for the period prior to 2000 was significantly different from that for the period beginning in 2000. 

The null hypothesis was rejected (p < 0.0001)—that is, the 

relation between log total-phosphorus concentration and log 

discharge is different for these two periods. 

The WRTDS model presents this same information 

through smooth curves that describe the relation between concentration and discharge in the different time periods 

(fig. 12). In the past decade (2001–11), for discharge values 

less than about 75,000 ft3/s, concentrations generally were 

the same as or slightly less than they were in the earlier two 

decades. For discharges greater than about 150,000 ft3/s, however, concentrations for any given discharge were much higher 

during the past decade than they were previously.

The hypothesized shift in the behavior of the system 

such that the discharge threshold for scour within the reservoirs has decreased in recent years and (or) deposition in the 

reservoirs has decreased is strongly supported by the data in 

figures 11 and 12. The overall history of total-phosphorus flux 

from the Susquehanna River to the Chesapeake Bay (fig. 13, 

table 1) illustrates the high degree of year-to-year variability 

in phosphorus flux. This variability appears to be increasing as 

a result of the large increase in phosphorus concentrations at 

extreme high discharges from prior decades to the past decade. 

Although the annual flux of total phosphorus appears 

to have declined from 2004 to 2009, the decline is simply a 

result of the decrease in annual discharge over those years. 

The daily-mean discharge for those 6 years was 66,600, 

50,900, 46,600, 40,400, 40,800, and 31,200 ft3/s, respectively. 

The large increase in 2011 was an outcome of the random 

occurrence of a year of extreme high discharge (66,000 ft3/s), 

although 2011 was not the highest discharge year in the 

record. The flow-normalized record (fig. 13) is more indicative of the long-term change in the behavior of the system; 

it shows a period of decreased flow-normalized flux through 

about 1993, and an increase since that time from about 2,500 

to about 3,900 tons/yr. The rate of change over that period has 

been steady at about +3.3 percent per year. The estimate of 

the change in flow-normalized flux of total phosphorus for the 

period 2000–08, reported by Hirsch and others (2010), was 

+1.9 percent per year, with a change in flux over the period of 

390 tons/yr; figure 13 shows a change of 740 tons/yr for the 

same period. There are two reasons for this difference: The 

first is that the high concentration of 1.17 mg/L in 2004 was 

considered to be suspect because it was about three times the 

maximum value of any observed concentration in the previous 

26 years. Therefore, it was not used in the analysis by Hirsch 

and others (2010). The observation of four additional concentrations above 0.4 mg/L at high discharges in 2011 lent credibility to the 2004 value, which consequently was included in 

the current analysis. Second, the availability of an additional 3 

years of data (for water years 2009, 2010, and 2011) provides 

stronger evidence of the upward trend. Therefore, the current 

analysis confirms the upward trend reported previously and 

increases the estimate of the rate of change during that period.

Figure 10. Total-phosphorus concentration as a function of time 

for the Susquehanna River at Conowingo, Maryland.

Figure 11. Observed concentration of total phosphorus as a 

function of discharge for the Susquehanna River at Conowingo, 

Maryland.

Figure 12. Estimated concentration of total phosphorus as a 

function of discharge for the Susquehanna River at Conowingo, 

Maryland, based on smoothing centered on September 1, 1990, 

or on September 1, 2005.
10 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

The steady increase in the flux of total phosphorus 

reflects the fact that the concentrations associated with the 

highest discharge values have been rising sharply over time. 

Given the hypothesis that this rise is caused by the filling of 

the reservoir, resulting in a decrease in deposition at moderate 

flows and a decrease in the threshold of flow required to cause 

scour of the reservoir sediments, there is no reason to expect 

a reversal of this trend in the absence of changes that would 

cause the sediment that is vulnerable to scour to either be 

removed or be made less vulnerable to scour. 

Suspended Sediment

The USGS NWIS database contains data for 985 samples 

that were analyzed for suspended sediment collected from 

January 29, 1978, through September 12, 2011. The average 

concentration in the 981 samples collected before September 

2011 was 45 mg/L. Four samples were collected over the 

course of the September 2011 flood event; concentrations 

in these samples ranged from a maximum of 2,980 mg/L on 

September 8, the day before the maximum flow, to a minimum of 332 mg/L 3 days after the peak flow. The average 

concentration in these five samples was 1,300 mg/L, more 

than seven times the long-term mean for the period prior to 

Tropical Storm Lee. Suspended-sediment concentrations 

in all 985 samples collected over the period of record are 

shown in figure 14. Those from 2011 stand out as high values, 

but the highest concentration, 3,685 mg/L, was measured 

on September 20, 2004, at a discharge nearly equal to the 

discharge on September 8, 2011. Also among the highest 

values were 937 mg/L on March 12, 2011, and four values 

ranging from 533 to 1,200 mg/L during 2 days of flooding, 

January 21–22, 1996. 

Suspended-sediment concentrations as a function of discharge are shown in figure 15. At discharges greater than about 

300,000 ft3/s, the concentrations for a given discharge are substantially higher in the period before 2000 than in the period 

beginning in 2000. This pattern is similar to that observed for 

total phosphorus (fig. 11) because most of the total phosphorus 

is in the particulate phase when suspended-sediment concentrations are high. An analysis of covariance was performed on 

the data. The null hypothesis was that for discharges greater 

than 100,000 ft3/s, the logarithm of the concentration followed 

a single model with the explanatory variables of log(Q) and 

(log(Q))2 , where Q is discharge. The alternative hypothesis 

was that the relation between suspended-sediment concentration and discharge was significantly different prior to 2000 

than during the period beginning in 2000. The null hypothesis 

was rejected (p < 0.0001)—that is, the relation between log 

suspended-sediment concentration and log discharge is different between these two periods.

The WRTDS representation of the estimated value of 

concentration as a function of discharge (fig. 16) uses a 

smoothed estimate centered around September 1, both for 

the period centered around 1990 and for the period centered 

around 2005. At discharges greater than about 175,000 ft3/s, 

the concentrations during the more recent period were much 

higher than those measured at similar discharges before 2000. 

In contrast, the concentrations at discharges less than 100,000 

ft3/s during the more recent period have been slightly lower 

than those measured at similar discharges before 2000. These 

results indicate that some of the actions taken to improve 

water quality in the Susquehanna River Basin may be having a 

beneficial effect at low to moderate flows.

The flux of suspended sediment from the Susquehanna 

River to the Chesapeake Bay (fig. 17, table 1) follows a 

pattern that is similar to that for total phosphorus (fig. 13), 

except that the highest fluxes (in 2004 and 2011) are even 

more extreme. For example, the flux for 2011 is estimated 

to be 60 times the 2009 flux, even though the discharge in 

2011 was only two times the 2009 discharge. In contrast, the 

total-phosphorus flux for 2011 was about 12 times the flux for 

2009 because, although both total phosphorus and suspended 

sediment are nonlinearly related to streamflow, the relation for 

suspended sediment is much more nonlinear than the relation 

for total phosphorus. The flow-normalized flux of suspended 

sediment decreased through about 1993, and since that time 

it has increased from about 1.5 million to about 3.1 million 

tons/yr, slightly more than doubling the annual flow-normalized flux over that 18-year period. The rate of increase has 

been steady at about 85,000 tons/yr/yr, or an average rate of 

change of about 3.7 percent per year over the period 2000–11. 

Additionally, the highly nonlinear nature of the relation 

between suspended-sediment flux and streamflow results in 

greatly increased year-to-year variability. 

Figure 13. Estimated annual flux of total phosphorus by water 

year for the Susquehanna River at Conowingo, Maryland (data 

are listed in appendix 2).
Overview of Tropical Storm Lee Fluxes for the Susquehanna River at Conowingo, Maryland 11

Overview of Tropical Storm Lee 

Fluxes for the Susquehanna River at 

Conowingo, Maryland

The contribution of the flood resulting from Tropical 

Storm Lee on the Susquehanna River constituted a considerable fraction of the flux of total nitrogen, total phosphorus, 

and suspended sediment to the Chesapeake Bay in water year 

2011. The relative contribution of this event was greatest for 

suspended sediment and least for total nitrogen; this observation also is true for the relation between the Tropical Storm 

Lee flux and the total flux during the last decade (2002–11) or 

the entire 34-year period (table 2), because the long-term total 

fluxes of total phosphorus and particularly suspended sediment 

are largely determined by the fluxes that occur during brief 

high-flow events. 

Concentrations and fluxes of all three constituents at high 

streamflows have increased over the past approximately 10 

years (2002–11). Because the period 1997 through 2003 was 

virtually devoid of high-flow events (there were no days with 

average flows greater than 400,000 ft3/s during this 7-year 

period), the timing of this change cannot be identified with 

any precision; however, the change began to become apparent between the high flows of January 1996 and those associated with Hurricane Ivan in 2004—although analysis results 

are available for only one water-quality sample collected in 

this extreme high-discharge range. The concentration data 

from the extreme high flow in March 2011 followed by those 

for September 2011 strongly support this interpretation. The 

Figure 14. Suspended-sediment concentration as a function of 

time for the Susquehanna River at Conowingo, Maryland.

Figure 17. Estimated annual flux of suspended sediment by 

water year for the Susquehanna River at Conowingo, Maryland 

(data are listed in appendix 3).

Figure 15. Observed concentration of suspended sediment as a 

function of discharge for the Susquehanna River at Conowingo, 

Maryland. 

Figure 16. Estimated concentration of suspended sediment as a 

function of discharge for the Susquehanna River at Conowingo, 

Maryland, based on smoothing centered on September 1, 1990, or 

on September 1, 2005 (note that, in contrast to figures 8 and 12, the 

vertical scale is a logarithmic scale in order to show the separation 

between the two curves at the lower discharges).
12 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

discharge at which the increase occurs also is impossible to 

identify with precision, though it lies in the range of about 

175,000 to 300,000 ft3/s. Furthermore, the relative roles of the 

two processes that likely are occurring—decreased deposition 

and increased scour—cannot be determined from this analysis. 

Implications for the Chesapeake Bay

Future changes in flux are tied to the spatially and temporally complex processes of scour and deposition and the many 

changes in the watershed that are driven by economic, demographic, and agricultural conditions, as well as management 

actions designed to move the watershed toward the waterquality goals associated with the Chesapeake Bay restoration. 

The hypothesized changes in scour and deposition processes 

that appear to be underway can be expected to be a major 

impediment to progress toward meeting those goals, especially 

for phosphorus and suspended sediment. The net result of 

changes upstream in the watershed and within the reservoirs 

has been a trend of generally increasing fluxes of phosphorus 

and suspended sediment of slightly more than 3 percent per 

year since 2000, and a decrease of about 0.3 percent per year 

for nitrogen. The results of this analysis are consistent with 

the hypothesis that the fluxes from the Susquehanna River to 

the Chesapeake Bay are already on a trajectory toward a new 

steady-state condition of average annual fluxes such as those 

predicted by Langland and Hainly (1997), with large increases 

in the flux of phosphorus and suspended sediment (table 3). 

(The year 1996 was used as a base year for this estimate of 

change because it is approximately the time of Langland and 

Hainly’s estimates, and also because 1996 is approximately 

when the flow-normalized flux values for phosphorus and 

suspended sediment began to increase.) 

The filling of the reservoirs is not a simple process in 

which filling occurs up to a specific time after which the reservoirs, once full, remain at steady state. Rather, filling is an 

asymptotic process in which average rates of deposition gradually decrease and average rates of scour gradually increase 

until they become equal (on average). The approach to fullreservoir status also is highly stochastic, with an alternation 

of short scour events balanced by longer depositional periods. 

The analyses in this report indicate that the final stages of this 

asymptotic filling process are already (2012) well underway, 

and not a projection of a future occurrence. 

Additional monitoring and ongoing evaluation of 

the changes that are currently occurring in the reservoirs 

will be crucial to the evolution of plans to meet the loadreduction goals for the Susquehanna River watershed and the 

Chesapeake Bay. The results presented here highlight the need 

to consider the management of the sediment in these reservoirs as a part of future strategies for reducing loads of total 

phosphorus and suspended sediment to the bay. Some of these 

issues are currently under consideration as part of the Lower 

Susquehanna River Watershed Assessment (see http://mddnr.

chesapeakebay.net/LSRWA/index.cfm). Accurate predictions of 

the future evolution of these changes in the system will depend 

on (1) continuing to collect water-quality and flow data (especially during storm events) at Conowingo Dam and at sites 

upstream from the reservoirs; (2) supplementing analyses of 

these standard samples with in situ measurement of characteristics such as turbidity to improve estimates of inputs to and 

outputs from the reservoirs; (3) conducting a detailed analysis 

of the mass balance of sediment and nutrients for the reservoir reach, using data from multiple monitoring sites; and (4) 

simulating the spatially and temporally variable deposition and 

scour processes within the reservoir system through the use 

of physically based representations together with empirically 

based parameter-estimation techniques. 

Understanding the changes that are taking place in 

these reservoirs is crucial to effective implementation of 

the Chesapeake Bay Total Maximum Daily Load (TMDL) 

requirement. Success in reducing fluxes of nitrogen, phosphorus, and sediment from the parts of the Susquehanna River 

Basin upstream from the reservoirs may be counterbalanced 

by changes in deposition and scour in the reservoirs, with 

a potential resulting net increase in the average fluxes of 

nitrogen, phosphorus, and sediment to the bay. These increases 

Table 2. Time, streamflow, and flux of total nitrogen, total phosphorus, and suspended sediment associated with 

Tropical Storm Lee as a percentage of total values for water year 2011, water years 2002–11, and water years 

1978–2011.

Storm total Water year 2011 Water years 2002–11 Water years 1978–2011

Time 2 0.2 0.06

Streamflow 12 1.8 0.6

Flux

Total nitrogen 31 5 1.8

Total phosphorus 61 22 9

Suspended sediment 78 39 22
Summary and Conclusions 13

would be even larger if upstream reductions were not successful. Strategies for reducing contaminant inputs to the bay 

must be evaluated and adjusted as new information regarding 

recent and future changes in the watershed and the reservoirs 

becomes available, and as engineering solutions that could be 

undertaken in the reservoirs are considered . 

The results of this study highlight the importance of 

ongoing monitoring of river inputs to the bay and of frequent 

reanalysis of the data that are being collected. The phenomenon described in this report was revealed primarily through 

analysis of a relatively small number of samples collected during high-flow conditions. Determining and interpreting future 

changes in water quality during high-flow conditions will be 

crucial to the development of effective water-quality management plans for the Susquehanna River and the Chesapeake 

Bay.

Summary and Conclusions

The three dams at the downstream end of the 

Susquehanna River are important in mitigating the downstream transport of nitrogen, phosphorus, and suspended 

sediment from the Susquehanna River watershed to the 

Chesapeake Bay. The reservoirs are known to be more than 80 

percent filled with sediment. A consequence of that filling is 

that they are no longer a major sink for the nitrogen, phosphorus, and sediment coming from the watershed, but rather are 

approaching steady state, with an approximately equal balance 

between the fluxes of these materials that enter the reservoir 

and those that leave the reservoir and enter the Chesapeake 

Bay. Water-quality measurements made during the large flood 

in the aftermath of Tropical Storm Lee (September 2011), 

as well as high flows that occurred in March 2011 and after 

Hurricane Ivan (September 2004), indicate that this decrease 

in the effectiveness of the watershed/reservoir system at 

trapping nitrogen, phosphorus, and sediment is well underway. Results of analyses of samples for total nitrogen, total 

phosphorus, and suspended sediment all demonstrate that the 

relation between concentration and discharge has changed 

substantially over the past 10 to 15 years (from some time 

between 1996 and 2004 to 2012). The concentrations of total 

nitrogen, total phosphorus, and suspended sediment measured 

in samples collected at discharges greater than about 100,000 

or 200,000 cubic feet per second currently (2012) are substantially higher than they were 10 to 15 years ago. 

Trends in the flow-normalized flux of total nitrogen, total 

phosphorus, and suspended sediment were estimated by using 

all of the water-quality data collected at the Susquehanna 

River at Conowingo, Maryland, streamgage since 1978. The 

results indicate that, over the period 1996–2011, the flow-normalized flux of total nitrogen, total phosphorus, and suspended 

sediment has decreased 3.2 percent, increased 55 percent, and 

increased 97 percent, respectively. These changes represent 

the combined effects of changes in the fluxes of these constituents from the watershed upstream from the reservoirs and of 

changes in deposition and scour within the reservoirs. It has 

long been predicted that, given no change in upstream inputs 

to the reservoirs, the filling of the reservoirs would cause 

only modest changes in nitrogen flux (because sediment has a 

limited role in nitrogen transport), large increases in phosphorus fluxes, and even larger increases in suspended-sediment 

fluxes. The evidence presented in this report indicates that the 

predicted changes are not just a theoretical issue for future 

consideration, but are already underway. These changes in the 

reservoirs are already overwhelming the progress being made 

to reduce constituent loads from the Susquehanna River watershed. Therefore, efforts to reduce nutrient and sediment inputs 

to the Chesapeake Bay will need to include consideration of 

changes in the trapping of sediment entering, and scouring of 

sediment in, the reservoirs along with the management actions 

implemented upstream in the watershed. Continued analysis of 

water-quality and discharge data that help to improve understanding of the future trajectory of these changes (with and 

without engineered modifications of the reservoirs) will be 

crucial to planning for the achievement of restoration goals for 

the Chesapeake Bay.

Table 3. Predicted change in average flux of total nitrogen, total phosphorus, and suspended sediment from the mid 

1990s to the time when reservoir sediment-storage capacity is reached (Langland and Hainly, 1997), and observed 

change in flow-normalized flux, water years 1996–2011.

Constituent

Predicted change in average flux 

(Langland and Hainly, 1997)

(percent)

Observed change in flow-normalized flux, 

water years 1996–2011

(percent)

Total nitrogen +2 -3.2

Total phosphorus +70 +55

Suspended sediment +250 +97
14 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

Acknowledgments

This study was conducted as part of the U.S. Geological 

Survey (USGS) Hydrologic Research and Development 

Program and the USGS Ecosystem Program. The author 

thanks Mike Langland and Joel Blomquist for providing 

valuable knowledge about the history and status of the 

Susquehanna River reservoirs. The report was significantly 

improved as a result of the reviews provided by Mike 

Langland and Terry Schertz of the USGS and Gary Shenk of 

the U.S. Environmental Protection Agency, Chesapeake Bay 

Program Office. The illustrations were prepared by Timothy 

Auer and the report was edited by Dale Simmons, both of 

the USGS. Scott Phillips, USGS Chesapeake Bay Program 

coordinator, provided excellent advice and support to facilitate 

expedited publication. This report was highly dependent on 

data derived from the long-term monitoring carried out as a 

partnership of the USGS with the State of Maryland and the 

Chesapeake Bay Ecosystem Program. Most important, this 

work could not have been accomplished without the efforts 

of USGS hydrologic technicians and hydrologists who have 

collected nearly 1,000 water-quality samples over 34 years 

and have operated the streamgage at the Susquehanna River at 

Conowingo, Maryland. 

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2012, at http://pubs.er.usgs.gov/publication/sir20125093. 

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

bottom-surface elevations in three reservoirs in 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., available online at http://pubs.er.usgs.gov/

publication/wri974138.

Mann, H.B., 1945, Nonparametric tests against trend: 

Econometrica, v. 13, no. 3, p. 245–259 (also available 

online at http://www.jstor.org/stable/1907187).

National Aeronautics and Space Administration, 2011, MODIS 

Web, accessed June 4, 2012, at http://modis.gsfc.nasa.gov/

gallery/individual.php?db_date=2011-09-20.

U.S. Environmental Protection Agency, 2012, Chesapeake 

Bay TMDL—Executive summary and appendices, accessed 

August 13, 2012, at http://www.epa.gov/reg3wapd/tmdl/

ChesapeakeBay/tmdlexec.html.
Appendixes 1–3 15

Appendixes 1–3
16 Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay, 2011

Appendix 1. Daily-mean discharge, average flux, and flownormalized annual flux values from the Weighted Regressions 

on Time, Discharge, and Season (WRTDS) model for total 

nitrogen in the Susquehanna River at Conowingo, Maryland, 

water years 1978–2011.

[ft3/s, cubic feet per second; tons/yr, tons per year]

Water 

year

Discharge

(103 ft3/s)

Flux

(103 tons/yr)

Flow-normalized flux

(103 tons/yr)

1978 61.1 95 62

1979 45.3 76 66

1980 37.5 62 69

1981 26.6 46 72

1982 37.8 66 73

1983 35.1 61 74

1984 53.4 99 75

1985 26.9 50 76

1986 40.3 76 77

1987 36.1 68 77

1988 29.9 56 77

1989 38.0 69 77

1990 37.5 68 75

1991 43.1 79 74

1992 29.0 50 72

1993 52.2 91 71

1994 54.1 92 69

1995 27.9 47 68

1996 51.9 87 67

1997 42.9 72 67

1998 46.2 76 67

1999 22.8 36 67

2000 35.6 55 67

2001 24.5 37 68

2002 28.2 42 68

2003 51.1 78 68

2004 66.6 135 68

2005 50.9 87 69

2006 46.6 78 68

2007 40.4 65 68

2008 40.8 65 67

2009 31.2 47 67

2010 34.0 53 66

2011 65.7 135 65

Appendix 2. Daily-mean discharge, average flux, and 

flow-normalized annual flux values from the Weighted 

Regressions on Time, Discharge, and Season (WRTDS) model 

for total phosphorus in the Susquehanna River at Conowingo, 

Maryland, water years 1978 –2011.

[ft3/s, cubic feet per second; tons/yr, tons per year]

Water 

year

Discharge

(103 ft3/s)

Flux

(103 tons/yr)

Flow-normalized flux

(103 tons/yr)

1978 61.1 6.8 3.8

1979 45.3 5.1 3.7

1980 37.5 3.0 3.6

1981 26.6 2.2 3.5

1982 37.8 2.7 3.4

1983 35.1 2.5 3.3

1984 53.4 4.9 3.1

1985 26.9 1.5 3.1

1986 40.3 2.9 3.0

1987 36.1 2.2 2.9

1988 29.9 1.6 2.8

1989 38.0 2.4 2.7

1990 37.5 1.9 2.6

1991 43.1 2.5 2.5

1992 29.0 1.3 2.5

1993 52.2 4.0 2.5

1994 54.1 3.8 2.5

1995 27.9 1.3 2.5

1996 51.9 3.9 2.5

1997 42.9 2.8 2.6

1998 46.2 2.8 2.7

1999 22.8 1.0 2.7

2000 35.6 1.7 2.8

2001 24.5 1.0 2.9

2002 28.2 1.4 3.0

2003 51.1 3.3 3.1

2004 66.6 8.2 3.2

2005 50.9 4.6 3.4

2006 46.6 4.0 3.4

2007 40.4 2.7 3.5

2008 40.8 2.8 3.6

2009 31.2 1.4 3.7

2010 34.0 2.1 3.8

2011 65.7 17.4 3.9
Appendixes 1–3 17

Appendix 3. Daily-mean discharge, average flux, and flownormalized annual flux values from the Weighted Regressions 

on Time, Discharge, and Season (WRTDS) model for 

suspended sediment in the Susquehanna River at Conowingo, 

Maryland, water years 1978–2011.

[ft3/s, cubic feet per second; tons/yr, tons per year]

Water 

year

Discharge

(103 ft3/s)

Flux

(106 tons/yr)

Flow-normalized flux

(106 tons/yr)

1978 61.1 3.6 1.9

1979 45.3 3.0 1.9

1980 37.5 1.4 2.0

1981 26.6 1.1 2.0

1982 37.8 1.2 1.9

1983 35.1 1.2 1.9

1984 53.4 3.5 1.8

1985 26.9 0.5 1.7

1986 40.3 1.5 1.6

1987 36.1 0.9 1.6

1988 29.9 0.5 1.5

1989 38.0 1.1 1.5

1990 37.5 0.7 1.5

1991 43.1 1.2 1.5

1992 29.0 0.4 1.5

1993 52.2 3.3 1.5

1994 54.1 2.6 1.5

1995 27.9 0.5 1.5

1996 51.9 3.8 1.6

1997 42.9 1.6 1.7

1998 46.2 1.5 1.8

1999 22.8 0.4 2.0

2000 35.6 0.7 2.2

2001 24.5 0.3 2.4

2002 28.2 0.5 2.6

2003 51.1 1.5 2.8

2004 66.6 11.7 2.8

2005 50.9 3.2 2.9

2006 46.6 2.7 2.8

2007 40.4 1.2 2.8

2008 40.8 1.5 2.8

2009 31.2 0.4 2.9

2010 34.0 1.1 3.0

2011 65.7 24.3 3.1

Prepared by USGS West Trenton Publishing Service Center.

Edited by Dale L. Simmons.

Graphics and layout by Timothy W. Auer.

For additional information, contact:

Robert Hirsch

Research Hydrologist

U.S. Geological Survey

12201 Sunrise Valley Drive

Reston, VA 20192

rhirsch@usgs.gov

or visit our Web site at:

http://chesapeake.usgs.gov
Hirsch, R.M.—Flux of Nitrogen, Phosphorus, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay during Tropical Storm Lee, 2011—Scientific Investigations Report 2012–5185

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