U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5093

Nutrient and Suspended-Sediment Trends, Loads, and Yields 

and Development of an Indicator of Streamwater Quality at 

Nontidal Sites in the Chesapeake Bay Watershed, 1985–2010

Nutrient and Suspended-Sediment Trends, 

Loads, and Yields and Development of 

an Indicator of Streamwater Quality at 

Nontidal Sites in the Chesapeake Bay 

Watershed, 1985–2010

By Michael Langland, Joel Blomquist, Douglas Moyer, and Kenneth Hyer

Scientific Investigations Report 2012–5093

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:

Langland, Michael, Blomquist, Joel, Moyer, Douglas, and Hyer, Kenneth, 2012, Nutrient and suspended-sediment 

trends, loads, and yields and development of an indicator of streamwater quality at nontidal sites in the Chesapeake 

Bay watershed, 1985–2010: U.S. Geological Survey Scientific Investigations Report 2012–5093, 26 p.
iii

Contents

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

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

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

Previous Investigations........................................................................................................................2

Methods...........................................................................................................................................................2

Data-Set Construction .........................................................................................................................2

Streamflow....................................................................................................................................2

Water Quality................................................................................................................................3

Water-Quality Model............................................................................................................................8

Load Estimation............................................................................................................................8

Trends in Water Quality..............................................................................................................9

Changes in Streamflow...............................................................................................................................10

Annual Mean Streamflow to Chesapeake Bay..............................................................................10

Daily Mean Streamflow .....................................................................................................................11

Changes in Water Quality–Primary Sites ................................................................................................11

Load.......................................................................................................................................................11

Flow-Adjusted Trends........................................................................................................................12

Indicator Development................................................................................................................................18

Trend Indicator....................................................................................................................................18

Yield Indicator......................................................................................................................................18

Combined Indicator............................................................................................................................18

Changes in Water Quality–Secondary Sites...........................................................................................20

Future Directions..........................................................................................................................................20

Summary........................................................................................................................................................25

References Cited..........................................................................................................................................26
iv

Figures

1. Map showing current (2010) Chesapeake Bay nontidal monitoring network with 

sites having an adequate period of record for trend and (or) load computation...............4

 2. Graph showing estimated annual mean streamflow to Chesapeake Bay from 

1937 to 2010..................................................................................................................................10

3. Graph showing annual combined total nitrogen loads and streamflow at the 

nine River Input Monitoring sites, 1990–2010.........................................................................11

4. Graph showing annual combined total phosphorus loads and streamflow at the 

nine River Input Monitoring sites, 1990–2010.........................................................................11

 5. Graph showing annual combined total sediment loads and streamflow at the 

nine River Input Monitoring sites, 1990–2010.........................................................................12

6. Map showing flow-adjusted trends in total nitrogen concentration for 31 nontidal 

sites, 1985–2010...........................................................................................................................14

7. Map showing flow-adjusted trends in total phosphorus concentration for 31 

nontidal sites, 1985–2010 ...........................................................................................................16

 8. Map showing flow-adjusted trends in sediment concentration for 31 nontidal 

sites, 1985–2010...........................................................................................................................17

9. Map showing combined nitrogen trends (WY2001–10) and yields (WY2006–10) at 

64 stream monitoring sites in the Chesapeake Bay watershed..........................................21

10. Map showing combined phosphorus trends (WY2001–10) and yields (WY2006–10) 

at 64 stream monitoring sites in the Chesapeake Bay watershed.....................................22

11. Map showing combined sediment trends (WY2001–10) and yields (WY2006–10) 

at 64 stream monitoring sites in the Chesapeake Bay watershed.....................................23
v

Tables

1. Chesapeake Bay Nontidal Monitoring Program sites, indicating site group and 

type, type of analysis, and number of sites included in the analysis, 1985–2010...............3

2. Chesapeake Bay Nontidal Monitoring Network stations, grouped by type of 

analysis reported ..........................................................................................................................5

3. Nitrogen, phosphorus, and sediment species tested for trend ............................................8

4. Flow-adjusted trends for the nine River Input Monitoring sites, 1990–2010.....................13

5. Flow-adjusted trends for water years 1985–2010 and 2001–10 at 33 streammonitoring sites in the Chespeake Bay watershed ..............................................................15

6. Mean yields of nitrogen, phosphorus, and sediment at 64 stream-monitoring sites 

in the Chesapeake Bay watershed, 2006–10..........................................................................19

7. Combined trend and yield indicator counts by classification for the 64 sites in the 

Chesapeake Bay watershed.....................................................................................................20

8. Trend results for the 16 Virginia secondary sites in the Chesapeake Bay Nontidal 

Monitoring Program...................................................................................................................24

Abstract

The U.S. Geological Survey (USGS) updates information on loads of, and trends in, nutrients and sediment annually to help the Chesapeake Bay Program (CBP) investigators 

assess progress toward improving water-quality conditions 

in the Chesapeake Bay and its watershed. CBP scientists and 

managers have worked since 1983 to improve water quality in 

the bay. In 2010, the U.S. Environmental Protection Agency 

(USEPA) established a Total Maximum Daily Load (TMDL) 

for the Chesapeake Bay. The TMDL specifies nutrient and 

sediment load allocations that need to be achieved in the 

watershed to improve dissolved oxygen, water-clarity, and 

chlorophyll conditions in the bay. The USEPA, USGS, and 

state and local jurisdictions in the watershed operate a CBP 

nontidal water-quality monitoring network and associated 

database that are used to update load and trend information to 

help assess progress toward reducing nutrient and sediment 

inputs to the bay.

Data collected from the CBP nontidal network were used 

to estimate loads and trends for two time periods: a long-term 

period (1985–2010) at 31 “primary” sites (with storm sampling) and a 10-year period (2001–10) at 33 primary sites and 

16 “secondary” sites (without storm sampling). In addition, 

loads at 64 primary sites were estimated for the period 

2006 to 2010.

Results indicate improving flow-adjusted trends for nitrogen and phosphorus for 1985 to 2010 at most of the sites in the 

network. For nitrogen, 21 of the 31 sites showed downward 

(improving) trends, whereas 2 sites showed upward (degrading) trends, and 8 sites showed no trends. The results for phosphorus were similar: 22 sites showed improving trends, 4 sites 

showed degrading trends, and 5 sites indicated no trends. For 

sediment, no trend was found at 40 percent of the sites, with 

10 sites showing improving trends and 8 sites showing degrading trends.

The USGS, working with CBP partners, developed a new 

water-quality indicator that combines the results of the 10-year 

trend analysis with results from a greater number of sites 

(64 primary sites) where loads and yields of total nitrogen 

and phosphorus and sediment could be calculated. The new 

indicator shows fewer significant trends for the 10-year time 

period than for the long-term time period (1985–2010). For 

2001–10, total nitrogen trends were downward (improving) at 

14 sites and upward (degrading) at 2 sites; no trend was found 

at 17 sites. For total phosphorus, 12 sites showed improving 

trends, 4 sites showed degrading trends, and 17 sites showed 

no trend. For total sediment, most sites (21) did not exhibit a 

significant trend; 3 sites showed improving trends, and 10 sites 

showed degrading trends. Few significant trends were seen at 

the 16 secondary sites: improving trends for total nitrogen at 

4 sites, improving trends for total phosphorus at 2 sites, and a 

degrading trend for sediment at 1 site.

Total streamflow to the Chesapeake Bay was 20 percent 

higher in 2010 than in 2009 and is considered to be within the 

normal range of flow, whereas annual streamflow at 28 sites 

was greater in 2010 than in 2009. No trends in daily streamflow were detected at the 31 long-term sites. Combined loads 

for the farthest downstream nontidal monitoring sites (called 

“River Input Monitoring sites”) increased 33 percent for total 

nitrogen, 120 percent for total phosphorus, and 330 percent 

for total sediment from 2009 to 2010. The large increase in 

phosphorus and sediment loads in 2010 was caused in large 

part by two large storm events that occurred during the spring 

in the Potomac River Basin. Yields (load per watershed area) 

of total nitrogen in the Chesapeake Bay watershed decreased 

from north to south (New York to Virginia). No spatial patterns were discernible for total phosphorus or sediment.

Introduction

The Chesapeake Bay has been adversely affected by 

nutrient and sediment enrichment. Excess nutrients stimulate algal blooms that decay and consume dissolved oxygen, 

causing areas of low dissolved-oxygen concentration in the 

Nutrient and Suspended-Sediment Trends, Loads, and 

Yields and Development of an Indicator of Streamwater

Quality at Nontidal Sites in the Chesapeake Bay 

Watershed, 1985–2010

By Michael Langland, Joel Blomquist, Douglas Moyer, and Kenneth Hyer
2 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

bay. Algal blooms and sediment reduce sunlight needed by 

underwater grasses. In the mid-1980s, the Chesapeake Bay 

Program (CBP), a partnership among the Commonwealths of 

Pennsylvania and Virginia, the State of Maryland, the District 

of Columbia, the Federal Government, and the Chesapeake 

Bay Commission, began efforts to reduce inputs of nutrients 

and sediment to the bay. In the 1990s, the States of New York, 

West Virginia, and Delaware joined in the water-quality activities of the CBP. In 2010, the U.S. Environmental Protection 

Agency (USEPA) established a Total Maximum Daily Load 

(TMDL) requiring all jurisdictions in the bay watershed to 

develop and implement Watershed Implementation Plans 

(WIPs) to have practices in place to reduce the loads of nutrients and sediment entering the bay by 2025.

The U.S. Geological Survey (USGS) routinely reports 

long-term trends and monthly and annual nutrient and sediment loads for stream-quality monitoring stations across the 

Chesapeake Bay watershed. The USGS first reported trends 

in the late 1980s at nine locations in Maryland and Virginia. 

Water-quality activities and load and trend analyses at these 

nine sites are referred to collectively as the “River Input Monitoring (RIM) program.” Multivariate regression techniques 

have been developed for load and trend data analysis. The 

trend technique attempts to adjust for the influences of streamflow and season (flow-adjusted trend) to help water managers 

understand how trends in nutrient and sediment concentrations 

and loads relate to management actions that aim to restore and 

protect water quality.

In 2004, new analysis techniques were applied, and the 

number of sites was expanded to include additional nontidal 

sites upstream from the RIM sites (Langland and others, 

2006). Annual updates of loads and flow-adjusted trends for 

these 33 sites are made public both in print and on the Internet. The methods used to estimate trends in streamflow, load, 

and flow-weighted concentration are documented. In 2004, 

the CBP partners formalized the CBP Nontidal Water-Quality 

Network (U.S. Environmental Protection Agency, 2004). 

Common sampling frequencies and protocols and nutrient and 

sediment analyses were established for all sites in the network. 

USGS and USEPA, in cooperation with the States, river basin 

commissions, and other partners, have worked to expand the 

network since 2004.

Purpose and Scope

This report presents results of load and trend analyses 

for nutrients and sediment at sites in the Chesapeake Bay 

Nontidal Water-Quality Network (CB-NTN) for 1985–2010 

and documents (1) changes in the methods used to calculate 

loads and trends since 2004 and (2) the development of a 

new water-quality indicator that can be used to assess current 

streamwater-quality conditions at the sites, as well as progress 

toward meeting the TMDL.

Loads and trends were evaluated for the expanded number of sites in the network for three time periods, and the following results are presented in this report: (1) loads and trends 

for the period of record (generally 1985–2010) at 31 sites; (2) 

loads and trends for a 10-year period (2001–10) at 33 primary 

sites and 16 secondary sites; and (3) loads for a 5-year period 

(2006–10) at 64 sites. The loads and trends from 33 10-year 

time period sites and the yields from the 64 5-year time period 

sites are used in the development of a new water-quality indicator for the Chesapeake Bay.

Previous Investigations

In the late 1980s, the USGS first reported loads and 

analyzed for trends at nine locations in Maryland and Virginia, 

collectively referred to as the River Input Monitoring (RIM) 

program. In 1995, the network was expanded to include waterquality data from more than 1,000 new sites, resulting in loads 

and trend results from 127 sites (Langland and others, 1995). 

Annual updates of loads and trends were completed each year 

from 1995 to 2003 with minimal change in techniques. In 

2004, when the CBP partners formalized the CBP Nontidal 

Water-Quality Network, new analysis techniques were applied, 

and the number of sites was expanded to include an additional 

24 nontidal sites upstream from the 9 RIM sites (Langland and 

others, 2006).

Methods

This section describes (1) the methods used to construct 

the data sets that were analyzed to assess streamflow and water 

quality, (2) the methods used to analyze the streamflow and 

water-quality data sets, and (3) the development of the new 

streamwater trend and yield water-quality indicator.

Data-Set Construction

This section discusses the construction of streamflow 

and water-quality data sets for input into various models to 

determine statistical results. In addition, construction of a 

water-quality database is described, specific site information is 

presented, and methods used to sum nitrogen and phosphorus 

species are discussed.

Streamflow

Daily mean streamflow data are retrieved annually 

from the USGS National Water Information System (NWIS) 

database (http://waterdata.usgs.gov/nwis/). For this study, the 

data were retrieved once for the period of record (POR); the 

10- and 5-year periods of record analyzed were subsets of the 

same file.

The streamflow files were analyzed to detect and evaluate any significant water-quality trend(s). In previous studies, streamflow was analyzed by using linear regression to 

determine the trend in streamflow. Evaluation of time series 
Methods 3

of daily mean streamflows for this study showed that the data 

residuals generally were autocorrelated, which may lead to 

significant loss of power in trend testing. The approach used to 

overcome autocorrelation was to increase the averaging period 

(Helsel and Hirsch, 2002).

Time series of seasonal mean streamflow were constructed by using an AR60 (autoregressive with 60 daily lags) 

model. Time series were constructed for daily mean streamflow at each site as well as for annual streamflow to determine 

total freshwater flow to the bay. The annual streamflow time 

series provide a basis for evaluating interannual variability; the 

daily time series allow the analysis of records for long-term 

trends.

Water Quality

In 1992, the USGS built a “nontidal database” containing selected water-quality and biological data (Langland and 

others, 1995). Fifty-eight physical, biological, and chemical 

water-quality constituents, properties, and characteristics are 

stored in this database. The USGS requests water-quality data 

annually from the cooperating agencies in the Chesapeake Bay 

watershed that collect nontidal monitoring data. Data typically are received in ASCII files. The data are then read into a 

Statistical Analysis System (SAS) database. Data are appended 

if the site is a previously existing site or, if the site is new, it 

is added to the database. When the water-quality files are read 

into the SAS database, a series of programs performs an initial 

quality assurance/quality control check on the data by searching for outliers, missing dates or times, and suspect remark 

codes. Currently (2010), there are approximately 1,400 sites 

in the Chesapeake Bay watershed. The database consists of 

water-quality and streamflow data from sites with a minimum 

of 3 consecutive years of sampling from 1972 through the 

current year. New sites are added to the database if at least 

12 samples have been collected over 3 continuous years and 

at least 1 sample has been collected in each season in the 

3 years (spring, summer, fall, and winter). Although many 

sites are sampled on a routine (usually monthly or bimonthly) 

basis, many do not have a continuous streamflow record and, 

therefore, cannot be used in the computation of annual loads 

and trends. The CB-NTN currently contains approximately 

120 sites. A subset of 80 of sites having adequate data for estimation of loads and trends was used in this study.

For data-analysis and -reporting purposes, the CB-NTN 

sites were organized into two groups: primary sites, consisting of (1) the RIM program sites and (2) the Multi-Agency 

Nontidal Monitoring Program sites, and secondary sites. Both 

primary and secondary sites are used to provide information 

from the nontidal areas of the bay (table 1). Primary sites are 

sampled monthly, supplemented with stormflow samples; secondary sites also are sampled monthly, but no targeted stormflow samples are collected. Therefore, only trends (not loads) 

are estimated. All sites must conform to the sampling protocols established by the CB-NTN. A subset of approximately 

31 primary sites with long-term (20 or more years) waterquality and streamflow data are used to determine annual and 

seasonal changes in streamflow and constituent concentrations 

and to estimate POR or long-term trends. As part of the RIM 

program, data from nine sites with streamwater-quality and 

streamflow data near the farthest downstream limit of nontidal 

waters are analyzed. The Multi-Agency Nontidal Monitoring Program added 22 sites with long-term water-quality and 

streamflow data (long-term monitoring in figure1 and table 2). 

In addition to the POR sites, primary-site lists were developed 

for two other time periods. Defining a 10-year time period 

resulted in two additional sites, whereas defining a 5-year time 

period resulted in 31 additional sites (short-term monitoring 

in figure 1 and table 2). A 10-year timeframe was used for the 

secondary sites and was not used in the indicator development; therefore, the discussion of secondary sites follows the 

discussion of indicator development in this report (secondary 

monitoring in figure 1 and table 2). The primary goal of the 

network is to provide CBP managers with water-quality monitoring information to help assess progress toward nutrient and 

sediment reductions required to meet the bay TMDL.

The species (forms) of nitrogen, phosphorus, and sediment evaluated for trends are shown in table 3. Because the 

analytical methods used to determine concentrations of 

Table 1. Chesapeake Bay Nontidal Monitoring Program sites, indicating site group and type, type of analysis, and number of sites 

included in the analysis, 1985–2010.

Site group Site type Type of analysis Number of sites

Primary River Input Monitoring (RIM) Long- (1985–2010) and short-term (2001–10) trends and loads 9

Multi-Agency Nontidal Monitoring Long- and short-term trends and loads 22

Short-term trends and loads 2

Short-term loads (2006–10) 31

Secondary Multi-Agency Nontidal Monitoring Short-term trends and loads 16
4 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

8

7

6

5

4 3

2

1

9

64

63

62

61

60

59

58

56

57

55

54

53

52

51

50

49

48

47

46 45

44

43

42

41 40 39

38

37

36

35 34

33

32

31

30

29

28

27

26

25

24

23 22

21

20

19

18

17

16

15

14

13

12

11 10

S8 S9

S7

S6

S5

S4

S3

S2

S1

S16

S15

S14

S13

S12

S11

S10

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W. 0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Secondary sites

Stream

Susquehanna 

River basin

Site and site number

Chesapeake Bay

Chesapeake Bay watershed 

boundary

Western Shore

Choptank 

Patuxent 

Potomac 

Rappahannock 

Mattaponi 

Pamunkey 

James 

Appomattox 

EXPLANATION

River Input Monitoring

Long-term Monitoring

Short-term Monitoring

S16

38

19

48

Figure 1. Current (2010) Chesapeake Bay nontidal monitoring network with sites having an adequate period of record for trend and (or) 

load computation. (Corresponding station information is included in table 2)
Methods 

5

Table 2. Chesapeake Bay Nontidal Monitoring Network stations, grouped by type of analysis reported.—Continued[bold, River Input Monitoring stations; ID, identifier; USGS, U.S. Geological Survey]

Map ID 

(figure 1)

USGS 

gage

Multi-collector 

site name Site name Years active Drainage area (square miles) Latitude Longitude

These 31 indicator sites have trends for period of record (POR) and 10 years, and 5-year yields.

1 01531500 WQN0305 Susquehanna River at Towanda, Pa. 24 7,797 414555 7626282 01536500 WQN0302 Susquehanna River at Wilkes-Barre, Pa. 24 9,960 411503 7552523 01540500 WQN0301 Susquehanna River at Danville, Pa. 24 11,220 405729 7637104 01553500 WQN0401 West Branch Susquehanna River at Lewisburg, Pa. 24 6,859 405803 7652365 01567000 WQN0214 Juniata River at Newport, Pa. 24 3,354 402842 7707466 01576000 WQN0201 Susquehanna River at Marietta, Pa. 22 25,990 400316 7631527 01576754 WQN0273 Conestoga River at Conestoga, Pa. 24 470 395647 7622058 01578310 01578310 Susquehanna River at Conowingo, Md. 24 27,100 393928 7610299 01491000 01491000 Choptank River near Greensboro, Md. 24 113 385950 754710

10 01582500 GUN0258 Gunpowder Falls at Glencoe, Md 24 160 393302 763809

11 01586000 NPA0165 Patapsco River at Cedarhurst, Md. 24 56.6 393000 765300

12 01591000 PXT0972 Patuxent River near Unity, Md 24 34.8 391418 77032113 01594440 01594440 Patuxent River near Bowie, Md. 24 348 385721 76413614 01599000 GEO0009 Georges Creek near Franklin, Md. 24 72.4 392938 79024115 01601500 WIL0013 Wills Creek near Cumberland, Md. 24 247 393941 78465016 01614500 CONO0180 / WQN0501 Conococheague Creek at Fairview, Md. 24 501 394256 77493117 01619500 ANT0044, ANT0047 Antietem Creek near Sharpsburg, Md. 24 281 392701 77435218 01631000 1BSSF003.56 S F Shenandoah River at Front Royal, Va. 24 1,642 385449 78124019 01634000 1BNFS010.34 N F Shenandoah River near Strasburg, Va. 24 768 385836 78201120 01637500 CAC0148 Catoctin Creek near Middletown, Md 24 66.9 392538 77332221 01639000 MON0528 / MON0546 Monocacy River at Bridgeport, Md. 24 173 394043 77140622 01646580 01646580 Potomac River at Chain Bridge, Md. 24 11,600 385546 77070123 01651000 ANA0082, A4 NW Branch Anaacostia River nr Hyattsville, Md. 24 49.4 385708 76577824 01668000 - Rappahannock River near Fredericksburg, Va. 21 1,596 381920 77310525 01671020 8-NAR005.42 North Anna at Hart Corner near Doswel, Va. 24 463 375100 77254126 01673000 - Pamunkey River near Hanover, Va. 20 1,081 374603 77195727 01674500 - Mattaponi River near Beulahville, Va. 20 601 375316 77094828 02035000 - James River at Cartersville, Va. 22 6,257 374015 78051029 02037500 2-JMS117.35 / 2-JMS113.20 James River at Richmond, Va. 24 6,758 373347 77325030 02041650 - Appomattox River at Matoaca, Va. 20 1,344 371330 77283231 02042500 2-CHK035.26 Chickahominy River near Providence Forge, Va 24 252 372610 770340
6 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed Table 2. Chesapeake Bay Nontidal Monitoring Network stations, grouped by type of analysis reported.—Continued

[bold, River Input Monitoring stations; ID, identifier; USGS, U.S. Geological Survey]

Map ID 

(figure 1)

USGS 

gage

Multi-collector 

site name Site name Years active Drainage area (square miles) Latitude Longitude

These 2 indicator sites have trends for 10 years and yields for 5 years

32 01589300 GWN0115 Gwynns Falls at Villa Nova, Md 24 32.5 392045 76435933 01658000 - Mattawoman Creek near Pomonkey, Md 10 54.8 383514 772544

These 31 indicator sites have 5-year yields

34 01515000 01515000 Susquehanna River near Waverly, NY 5 4,773 415905 76300535 01531000 01531000 Chemaung River at Chemung, NY 5 2,506 420008 76380636 01542500 01542500 WB Susquehanna River at Karthaus, Pa 5 1,462 410703 78063337 01562000 WQN0223 Raystown Branch Juniata River at Saxton, Pa 5 756 401257 78155638 01568000 WQN0243 Sherman Creek at Shermans Dale, Pa 5 200 401924 77100939 01570000 WQN0271 Conodoguinet Creek near Hogestown, Pa 5 470 401508 77011740 01571500 WQN0212 / 01571505 Yellow Breeches Creek near Camp Hill, Pa 5 216 401329 76535441 01573560 WQN0272 Swatara Creek near Hershey, Pa 5 483 401754 76400542 01574000 WQN0210 West Coneago Creek near Manchester, Pa 5 510 400456 76431343 01576787 WQN0204 Pequea Creek near MarticForge, Pa 5 148 395421 79194344 01667500 3-RAP030.21 / 01667500 Rapidan River near Culpeper, Va 5 472 382132 77582445 01487000 - Nanticoke River Near Bridgeville, De 5 75.4 384345 75334146 01488500 - Marshyhope Creek Near Adamsville, De 5 46.8 385059 75402447 01491500 TUK0181 Tuckahoe Creek Near Ruthsburg, Md 5 85.2 385802 75563548 01495000 BEL0053 Big Elk Creek At Elk Mills, Md 5 52.6 393644 75490149 01502500 - Unadilla River At Rockdale, NY 5 520 422240 75242350 01503000 - Susquehanna River At Conklin, NY 5 2232 420207 75481251 01529500 - Cohocton River Near Campbell NY 5 470 421509 77130152 01549760 WQN0410 West Branch Susquehanna River Near Jersey Shore 5 5225 411659 77192053 01578475 WQN0263 Octoraro Creek At Richardsmere, Md 5 177 394125 76074154 01580520 DER0015 Deer Creek Near Darlington, Md 5 164 393725 76095355 01594526 TF1.2 Western Branch At Upper Marlboro. Md 5 89.7 384851 76450356 01604500 - Patterson Creek Near Headsville, Wv 5 221 392635 78492057 01608500 - South Branch Potomac River Near Springfield, Wva 5 1461 392649 78391658 01610155 WQN0512 Sideling Hill Creek Near Bellegrove, Md 5 102 393858 78203959 01611500 - Cacapon River Near Great Cacapon, Wva 5 675 393456 78183660 01613095 WQN0510 Tonoloway Creek Near Hancock, Md 5 111 394223 78091061 01613525 WQN0509 Licking Creek Near Pectonville, Md 5 193 394040 780212
Methods 

7

Table 2. Chesapeake Bay Nontidal Monitoring Network stations, grouped by type of analysis reported.—Continued[bold, River Input Monitoring stations; ID, identifier; USGS, U.S. Geological Survey]

Map ID 

(figure 1)

USGS 

gage

Multi-collector 

site name Site name Years active Drainage area (square miles) Latitude Longitude

62 01616500 - Opequon Creek Near Martinsburg, Wva 5 272 392525 77562063 01619000 WQN0511 Antietam Creek Near Waynesboro, Pa 5 93.5 394260 77362964 02024752 2-JMS279.41 James River At Blue Ridge Pkwy Bridge Nr Big Island, Va 5 3076 373320 792201

These 16 Secondary sites have 10-year trends

S1 01626000 1BSTH027.85 South River neat Waynesboro, Va 10+ 127 300327 785430S2 01628500 1BSSF100.10 South Fork Shenandoah River near Dooms, Va 10+ 1083 300519 785238S3 01638480 1ACAX004.57 Catoctin Cr. at Taylorstown, Va 10+ 89.5 391518 773436S4 01654000 1AACO014.57 Accotink Cr. near Annandale, Va 10+ 23.9 384846 771343S5 01664000 3-RPP147.10 Rappahannock River at Remington, Va 10+ 252 383150 774850S6 01665500 3-RAP066.54 Rapidan River at Ruckersville, Va 10+ 115 381650 782025S7 01671100 8-LTL009.54 Little River near Doswell, Va 10+ 107 375221 773048S8 01673800 8-POR008.97 Po River near Spotsylvania, Va 10+ 77.6 381017 773542S9 01674000 8-MPN094.79 Mattaponi River near Bowling Green, Va 10+ 256 380342 772310

S10 02011500 2-BCC004.71 Back Cr. near Mountain Grove, Va 10+ 134 380410 795350

S11 02015700 2-BLP000.79 Bullpasture River at Williamsville, Va 10+ 110 381143 793414

S12 02020500 2-CFP004.67 Calfpasture River above Mill Cr. at Goshen, Va 10+ 141 375916 792938S13 02027500 2-PNY005.29 Piney River At Piney River, Va 10+ 49.4 374208 790140S14 02031000 2-MCM005.12 Mechums River near White Hall, Va 10+ 95.3 380609 783535S15 02039500 2-APP110.93 Appomattox River at Farmville, Va 10+ 303 371825 772320S16 02041000 2-DPC005.20 Deep Cr. near Mannboro, Va 10+ 177 371659 775212
8 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

suspended sediment (SSC) and total suspended solids (TSS) 

differ, concentrations of SSC tend to be higher, and measurements tend to be more accurate than those of TSS, particularly at higher flows (Kammerer and others, 1998) and where 

sediments are fine grained. Since February 2001, USGS policy 

has mandated the use of SSC rather than TSS analysis for 

suspended sediment. A recent (2006) enhancement to the data 

network (targeted collection of storm samples) is that the SSC 

analysis for sediment is used for samples collected at higher 

flows at many of the non-USGS sampling locations to improve 

comparisons with USGS data and provide more accurate estimates of sediment loads. Computing sediment loads at many 

non-USGS sites is nearly impossible as a result of the lack 

of SSC samples. Therefore, when data files were generated 

from the database, if sediment samples had been analyzed for 

both SSC and TSS, precedence was given to the SSC analysis 

value, and missing values were populated with TSS values. 

This procedure resulted in the combining of the two parameters into a single parameter named “SEDIMENT” (table 3).

To calculate trends, missing values were estimated. Many 

were calculated as the sum of reported analytes; for example, 

total nitrogen (TN) can be calculated as the sum of total dissolved and total particulate nitrogen. Missing values were 

estimated only for the input data files used to estimate trends 

and were not populated in the nontidal database. If the concentration of one or more of the nitrogen or phosphorus species 

used in calculating TN or total phosphorus (TP) was below the 

detection limit and greater than 5 percent of the summed total, 

then the remark code was coded as “less than” (<).

As previously mentioned, the data analysis for loads and 

trends was performed on data from two time periods—the 

POR and 10 years–and loads were also calculated for the last 

5 years (2006–10). Because the data for these time periods 

are subsets, master water-quality files were constructed from 

the nontidal database. Control files dictated to the program 

which time period to use. The longest period for reporting 

trend results for this study was October 1984 through September 2010. The shorter time-series data were used if they met 

certain criteria. For the 10-year period to be used, the database 

must include approximately 80 samples representing both 

monthly and storm samples. For the 5-year period to be used, 

the database must include 40 monthly and storm samples. The 

samples collected must represent the full range of the hydrograph during the estimation time period. Daily streamflow 

frequencies were developed for each site and the samplecollection times were plotted on a hydrograph to confirm that 

sampling occurred over the entire range of flows.

Water-Quality Model

Concentration data retrieved from the nontidal database 

were the basis for the load and trend test analyses in this study. 

Annual loads and flow-adjusted trends were calculated using 

the USGS water-quality model ESTIMATOR (Cohn and others, 1989, eq. 1) for all POR (long-term) and 10-year sites, 

and annual loads were calculated using ESTIMATOR for the 

5-year sites.

Load Estimation

Water-quality models were developed to estimate loads 

and flow-adjusted trends in water quality. The models use 

multiple linear regressions to relate observed concentration 

to predictor variables of streamflow and time. Models were 

developed for 64 sites, for all species given in table 2, and for 

each of the three time periods of interest. A total of 640 models were developed. Diagnostic measures used to select the 

best regression model included residual non-normality, heteroscedasticity (nonconstancy of the variance of the residuals 

over the levels of the predictor variables), low coefficient of 

determination (R2) values, and high model mean square errors. 

Table 3. Nitrogen, phosphorus, and sediment species tested for trend.

[N, nitrogen; mg/L, milligrams per liter]

Constituent Species

(parameter code) Units Abbreviation

Nitrogen Total nitrogen (00600) mg/L TN

Total or dissolved nitrate, or, total or dissolved nitrite plus nitrate

(00618, 00620, 00630, or 00631) as N mg/L NO23

Phosphorus Total phosphorus (00665) mg/L TP

Dissolved inorganic phosphorus (00671) mg/L DIP

Sediment Suspended sediment (80154) mg/L SSC

Total suspended solids (00530) mg/L TSS

Combined SSC and TSS mg/L Sediment
Methods 9

Model results that failed to meet the diagnostic measures were 

not reported. Estimates of constituent loading provide critical 

information on the amount and timing of material reaching 

downstream water bodies. A load is an integrated mass flux 

over some time interval {t

a

,tb}:

L = tbʃ

ta

ɭ (t) dt = tb

ʃ

ta

kc (t)q(t) dt , (1)

where

L is the total load,

l is the instantaneous load,

k is a unit conversion factor,

c is the instantaneous observed concentration, 

 and

q is the instantaneous streamflow.

The load represents the mass transported and delivered 

downstream from the point at which c and q are measured.

ESTIMATOR computes loads in two steps. First, a 

center-estimate linear model is fit to the logarithm of the 

concentration. The model corrects for bias from transforming logs back to “normal” space using the Minimum Variance 

Unbiased Estimator (MVUE) developed by Bradu and Mundlak (1970). The Adjusted Maximum Likelihood Estimator 

(AMLE) (Cohn, 1988) is used to estimate the log-linear model 

for sites having censored (below the detection limit) observations, and ordinary least squares is used for sites without 

censored data. AMLE is identical to ordinary least squares for 

the cases where no observations are censored. More detailed 

information on this procedure is included in Langland and 

others (2006).

A seven-coefficient parameter model is used to estimate 

loads for the Nontidal Program sites:

4 5

ˆ ˆ β β t 

ˆ ˆ ln ( c q ) = β β + ln ( / ) + [ln( / )] 0 1 2 q q q c cˆ

β 2

2

 + ( - )

+ ( - ) + sin ( )

3

ˆ

β

π

t t

 t 2 t

c

c + cos ( ) + 6

ˆ

β π2 t ε , (2)

where

ln is the natural logarithm function;

c is measured concentration, in milligrams per 

 liter;

q is measured daily mean streamflow, in cubic 

 feet per second;

t is time, in decimal years;

qc , t

c are centering variables for streamflow and 

 time;

ˆ

βi

are coefficients estimated by ordinary least 

 squares (for noncensored observations) 

 and AMLE (for censored observations);

ˆ

β 0 is a constant;

ˆ ˆ β β 1 2 , describe the relation between 

 concentration and streamflow;

ˆ ˆ β β 3 4 , describe the relation between 

 concentration and time, indpendent

 of flow; ˆ

β β 5 6 , ˆ describe seasonal variations in

 concentration data; and

	 ɛ	 is residual error, assumed to be normally 

 distributed with zero mean and 

 variance ơ2

ɛ .

Centering variables are defined such that time and time 

squared (and streamflow and streamflow squared) predictor 

variables are independent, automatically accounting for any 

bias in the sampling strategy. The ideal sampling protocol 

that targets high- and low-flow conditions reduces the error in 

load estimates that results from the log-linear relation between 

concentration and flow. 

Once model coefficients have been estimated, the second 

step in computing loads is to compute daily concentrations. 

Daily concentrations are used to estimate daily loads (and 

variances) using daily mean streamflow. Daily loads are 

summed to produce monthly and annual loads. The standard 

errors are estimated by using formulas in Gilroy and others 

(1990) and Cohn and others (1992).

Because the relation between concentration and streamflow can change over time as a result of changes in land 

use, wastewater discharges, best management practices, and 

climate change, a “moving-window” approach was developed 

to estimate loads for the nine RIM program sites (Yochum, 

2000). The moving-window method optimizes load estimates 

near the middle of the estimation time period (year 5) by running 9-year “windows” with errors that generally increase with 

distance from the center. Where possible, loads are estimated 

at the beginning to try to center 1985. Years 6 to 9 are considered provisional and will be updated in subsequent movingwindow years. The seven-parameter ESTIMATOR model with 

no moving window also is used to estimate long-term loads at 

the non-RIM sites. ESTIMATOR also is used for the 10- and 

5-year time periods, beginning and ending with the relevant 

years.

Trends in Water Quality

Concentrations commonly are correlated with streamflow and season. Ten- and 20-year trends presented in this 

report (referred to as “flow-adjusted trends”) were adjusted 

for streamflow and season to reduce the variability in concentrations and increase the likelihood of detecting an unbiased 

significant trend. These adjusted trends aid in the evaluation 

of changes in concentration resulting from changes in sources 

inputs and implementation of management practices. The relations between nutrient and sediment concentration and time 

(or discharge) are represented by both linear- and quadraticterm variables in the model form. A significant linear parameter indicates an upward (+) or downward (–) linear trend in 

nutrient and sediment concentrations. A significant quadratic 
10 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

parameter indicates a nonlinear relation between concentration 

and time (or discharge). For example, a significant positive 

quadratic parameter indicates a downward trend in early years 

followed by an upward trend in later years. A significant negative quadratic parameter indicates the opposite. If both the 

linear- and quadratic-time trend parameters were significant, 

then an acceleration (+) or deceleration (–) in trend occurred 

in later years (see Langland and others (2006) for a detailed 

description). Similarly, a significant positive linear-flow 

parameter combined with a significant negative quadratic-flow 

parameter indicates that concentration would increase with 

increasing flow, but the rate of increase in concentration would 

decelerate, or decline, at higher flows. The entire time period 

(long-term, or 10-year) is used in a single ESTIMATOR model 

run to estimate flow-adjusted trends for all nutrients and sediments at all 33 sites.

Changes in Streamflow

Variability in streamflow is one of the primary factors 

affecting water quality in the Chesapeake Bay and its watershed. Variability in streamflow can be caused by both natural 

and human-influenced factors. The spatial and temporal patterns of precipitation; evapotranspiration; and recharge, storage, and discharge of groundwater are the primary natural factors affecting streamflow. Diversions, land-use changes, and 

other anthropogenic factors in the watershed also may affect 

streamflow. Variability in streamflow affects both the observed 

concentrations and the average loads and concentrations of 

nutrients and sediment delivered to the bay and tidal parts of 

rivers.

Annual Mean Streamflow to Chesapeake Bay

Estimated total freshwater flow entering the Chesapeake 

Bay in 2010 was 79,900 cubic feet per second (ft3/s), 2 percent 

above the median for the period 1937 through 2010. Streamflow to the bay has been in the normal range (defined as the 

interquartile range, between the 25th and 75th percentiles) 

since 2005, except in 2009, when the total streamflow was 

below normal. The total streamflow entering the bay in 2010 

was 20 percent greater than in 2009. From1937 through 2010, 

the 25th, 50th, and 75th percentiles of annual streamflow were 

64,950, 78,280, and 89,000 ft3/s, respectively (fig. 2).

Upon examination of the streamflow record for 1937 to 

2010, several decadal-scale patterns emerge. During 1937 to 

1959, annual total streamflow values were within the normal range 15 of the 23 years. A dry period occurred in the 

1960s (streamflow during 5 of the 10 years was below the 

25th percentile); however, wetter conditions occurred in the 

1970s (streamflow during 5 of the 10 years was above the 

75th percentile). The long-term period used in this report 

(1985–2010) is of special interest in relating water-quality 

change to management actions and is the focus of this report. 

Annual streamflows were more variable during the last 20 

years, with 6 of the 8 highest flows and 2 of the lowest flows 

occurring in this time period. The wetter conditions observed 

from 1970 to 2010 (when streamflows during 13 of 40 years 

were above the 75th percentile), combined with the effects of 

25th percentile

75th percentile

Above normal

EXPLANATION

Normal

Below normal

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

1937 1946 1955 1964 1973 1982 1991 2000 2009

Annual mean flow, in cubic feet per second

Figure 2. Estimated annual mean streamflow to Chesapeake Bay from 1937 to 2010. (Flow calculated using methods from Bue, 1968)
Changes in Water Quality–Primary Sites 11

increased nutrients and sediment from human activities reaching the bay, have been cited as possible causes for the declines 

in dissolved-oxygen concentration and water clarity in the bay 

that were documented in the 1970s and that persist (Phillips, 

2002).

Daily Mean Streamflow

Regression models for daily mean streamflow at the 31 

long-term sites were constructed using data from 1985 to 

2010. The models included seasonal and 60-order autoregressive process terms. 

Results from these models indicated that none of the 31 

sites had significant trends in daily mean streamflow. Three 

sites in the northern Susquehanna River Basin (sites 1, 2, and 

3; table 2) indicated a nonsignificant decline in daily streamflow of approximately 10 percent, whereas the remaining 28 

sites indicated an increase in streamflow from 2009 to 2010, 

ranging from 10 to 75 percent. The median increase was 

approximately 40 percent. Daily mean streamflow at one site 

(site 31; table 2) increased from below to above normal. Daily 

and seasonal streamflows are illustrated on the Internet at 

http://md.water.usgs.gov/gis/trends/.

Changes in Water Quality–Primary 

Sites

Changes in streamflow, discussed in the previous section, 

affect streamwater quality. In this section, potential changes in 

water quality are discussed by comparing the constituent loads 

measured at the sites and flow-adjusted trends for the different 

modeled time periods.

Load

Nutrient and sediment loads have a substantial effect on 

the health of the Chesapeake Bay ecosystem and habitat in the 

rivers of the watershed. All but 3 of the 31 basins experienced 

an increase in runoff from 2009 to 2010. Nearly all sites had 

an increase in streamflow, constituent concentrations, and 

constituent loads. As a result, loads of all constituents at the 

nine RIM (farthest downstream) sites were higher in 2010 than 

in 2009. The loads at the RIM sites represent drainage from 

approximately 78 percent of the Chesapeake Bay watershed 

but do not include load estimates for the remaining 22 percent 

of the watershed, which for the most part represent the tidally 

influenced tributaries to the bay.

In 2010, combined estimated TN loads for the RIM 

sites were 178 million pounds (Mlbs), 27 Mlbs less than the 

long-term average of 205 Mlbs for 1990 to 2010 (fig. 3), the 

common time period used for the RIM program. Combined 

estimated TN loads for the RIM program sites increased by 

nearly 50 Mlbs to 178 Mlbs (33 percent) from 2009 to 2010. 

The high correspondence between annual streamflow and the 

TN combined load is apparent in figure 3.

In 2010, the combined TP loads (13 Mlbs) exceeded the 

long-term average of 12 Mlbs (fig. 4). The increase in streamflow and concentrations from 2009 to 2010 resulted in an estimated TP load increase of 7 Mlbs to 13 Mlbs, a 120-percent 

increase. Annual variability in the combined TP load is highly 

related to the variability in streamflow.

In 2010, the combined sediment load at the RIM sites, 

15,500 Mlbs, was the third highest since 1990 and was far 

greater than the long-term average of 8,300 Mlbs (fig. 5). The 

increase in sediment loads from 2009 to 2010 was caused in 

large part by two major storm events during the spring season 

in the Potomac River Basin. The combined estimated sediment 

load increased by 12,000 Mlbs to 15,500 Mlbs, a 330-percent 

increase, from 2009 to 2010.

Total nitrogen

EXPLANATION

Streamflow

0

20

40

60

80

100

120

0

50

100

150

200

250

300

350

1990 1994 1998 2002 2006 2010

Streamflow, in cubic feet per second x 1,000

Total nitrogen load, in millions of pounds

Figure 3. Annual combined total nitrogen loads and streamflow 

at the nine River Input Monitoring sites, Chesapeake Bay 

watershed, 1990–2010.

Total phosphorus

EXPLANATION

Streamflow

0

20

40

60

80

100

120

0

5

10

15

20

25

30

35

1990 1994 1998 2002 2006 2010

Streamflow, in cubic feet per second x 1,000

Total phosphorus load, in millions of pounds

Figure 4. Annual combined total phosphorus loads and streamflow at the nine River Input Monitoring sites, Chesapeake Bay 

watershed, 1990–2010.
12 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

Flow-Adjusted Trends

Observed concentrations are highly influenced by 

variability in streamflow and season. The seven-parameter 

regression model indicated in equation 1 is used to estimate a 

trend independent of the influence of streamflow and season 

to improve the understanding of water-quality changes that 

may result from human influences. Model results are used to 

determine flow-adjusted trends by partitioning variability in 

observed concentration due to season and streamflow, so that 

the coefficients from the time parameters in equation 1 are 

estimates of the amount of change over time without interference from streamflow and seasonal influences.

The flow-adjusted trend does not necessarily represent all 

the water-quality changes that result from human activities and 

management actions; it describes only those changes unrelated to flow. For example, a change in farming practices that 

reduces surface runoff but increases ground-water recharge or 

a change in atmospheric deposition may not be captured in the 

flow-adjusted trend. Therefore, although flow-adjusted trends 

are an indicator of human activities that affect water quality 

within a watershed, the relative magnitude of the trend must 

be considered in terms of the hydrologic variability.

Flow-adjusted trends in TN, TP, and sediment are discussed by major watershed (nine RIM sites) and by total number of monitoring locations for the two time periods. Trends 

at the nine RIM sites for the period 1990–2010 are shown 

in table 4. The trend results for the RIM sites are described 

because they are for the most part located on the largest tributaries to the bay and, therefore, represent overall trends for the 

basin. A common start date of 1990 is used for the nine RIM 

sites.

Total Nitrogen: The three largest monitored rivers 

(Susquehanna, Potomac, and James) and the Patuxent River 

exhibit statistically significant downward trends in TN 

(table 4). Only the Pamunkey and Choptank Rivers exhibit statistically significant upward trends in TN. The remaining RIM 

rivers (Rappahannock, Mattaponi, and Appomattox) indicate 

no significant trend.

In the 31-site network, 21 sites had improving trends, 

2 sites had degrading trends, and 8 sites had no trend. Eight 

sites in the Susquehanna River Basin, including the RIM site, 

had decreasing trends in TN over the study period (fig. 6 and 

table 5). Of the 11 sites in the Potomac River Basin, 9 showed 

decreasing flow-adjusted trends in TN, and 2 showed no 

statistically significant trend. Three sites in the lower Virginia 

basins indicated significantly decreasing flow-adjusted trends 

in TN, one site showed an increasing trend, and three sites 

showed no statistically significant trend.

Total Phosphorus: With respect to TP, the Patuxent, 

Mattaponi, and James River RIM sites have statistically significant downward trends, whereas the Choptank, Pamunkey, 

and Appomattox River RIM sites have statistically significant 

upward trends (table 4). The two RIM sites on the Susquehanna and Potomac Rivers indicate a nonsignificant downward 

trend in TP.

In the nontidal monitoring network, flow-adjusted trends 

in TP decreased significantly at 22 of the 31 sites, increased 

significantly at 4 sites, and were not significant at 5 sites 

(fig. 7 and table 5). Decreasing trends were observed at seven 

of the eight sites in the Susquehanna River Basin. In the 

Potomac River Basin, eight sites showed decreasing trends, 

and two sites showed no statistically significant trend. In the 

James River Basin, two sites had a decreasing trend, and one 

site had no significant trend. In the entire Chesapeake Bay 

Basin, 14 sites had a downward flow-adjusted TP trend greater 

than 50 percent, whereas 1 site had an upward trend greater 

than 50 percent.

Sediment: Statistically significant downward trends in 

sediment load were observed for the Susquehanna, Potomac, 

Patuxent, and Choptank River RIM sites. Only the Pamunkey 

River RIM site indicated a statistically significant upward 

trend in sediment (table 4). The James, Rappahannock, Mattaponi, and Appomattox River RIM sites showed no trend.

Significant downward flow-adjusted trends in sediment 

load were calculated at 10 of the 31 sites, whereas upward 

trends were reported at 8 sites. Increases greater than 50 

percent were estimated at 2 of the 10 downward-trend sites 

and 5 of the 7 upward-trend sites (fig. 8 and table 5). In the 

Susquehanna River Basin, downward trends were estimated 

for five sites. Results for the Potomac River Basin indicate an 

equal number of sites (two) with significant downward and 

upward trends, and no significantly detectable trend at six 

sites. In the lower Virginia river basins, there were no sites 

with downward trends, four sites with upward trends, and four 

sites with no detectable significant trend. In addition, five sites 

had downward sediment flow-adjusted trends in sediment load 

with decreases greater than 50 percent, whereas two sites had 

increasing trends with increases greater than 50 percent.

Sediment

EXPLANATION

Streamflow

0

20

40

60

80

100

120

0

5,000

10,000

15,000

20,000

25,000

30,000

1990 1994 1998 2002 2006 2010

Streamflow, in cubic feet per second x 1,000

Total sediments load, in millions of pounds

Figure 5. Annual combined total sediment loads and streamflow 

at the nine River Input Monitoring sites, Chesapeake Bay watershed, 1990–2010.
Changes in Water Quality–Primary Sites 13

Table 4. Flow-adjusted trends for the nine River Input Monitoring sites, Chesapeake Bay watershed, 1990–2010. 

[STAID: U.S. Geological Survey gaging station number; Map ID, as appears in figure 1; TN, total nitrogen; TP, total phosphorus; TSS, total suspended solids; 

Significance: indicated direction of a significant trend or ns (not significant); Tau, association between two measured quantiles, values closer to zero indicate 

more independence; p-value, <0.05 used for significance testing]

STAID Map ID Station name Constituent Trend Tau (p-value) Significance

1578310 8 Susquehanna River at Conowingo, MD TN -25.5 -0.2944 (<0.0001) IMPROVING

1491000 9 Choptank River near Greensboro, Md. TN 6.8 0.0662 (0.0333) DEGRADING

1594440 13 Patuxent River at Bowie, MD TN -57.2 -0.8492 (<0.0001) IMPROVING

1646580 22 Potomac River at Chain Bridge, MD TN -24.3 -0.2789 (<0.0001) IMPROVING

1668000 24 Rappahannock River near Fredericksburg, VA TN -9.8 -0.1036 (0.1194) ns

1673000 26 Pamunkey River near Hanover, VA TN 18.8 0.1725 (<0.0001) DEGRADING

1674500 27 Mattaponi River near Beulahville, VA TN -1 -0.0100 (0.7924) ns

2037500 29 James River near Richmond, VA TN -17.6 -0.1937 (0.0031) IMPROVING

2041650 30 Appomattox River at Matoaca, VA TN 5.7 0.0557 (0.2235) ns

1578310 8 Susquehanna River at Conowingo, MD TP -7.4 -0.0773 (0.2561) ns

1491000 9 Choptank River near Greensboro, Md. TP 47.5 0.3889 (<0.0001) DEGRADING

1594440 13 Patuxent River at Bowie, MD TP -58.9 -0.8886 (<0.0001) IMPROVING

1646580 22 Potomac River at Chain Bridge, MD TP -12.1 -0.1293 (0.0741) ns

1668000 24 Rappahannock River near Fredericksburg, VA TP -11.5 -0.1218 (0.2380) ns

1673000 26 Pamunkey River near Hanover, VA TP 99.4 0.6900 (<0.0001) DEGRADING

1674500 27 Mattaponi River near Beulahville, VA TP -11.9 -0.1265 (0.0368) IMPROVING

2037500 29 James River near Richmond, VA TP -40 -0.5111 (<0.0001) IMPROVING

2041650 30 Appomattox River at Matoaca, VA TP 27.9 0.2464 (0.0010) DEGRADING

1578310 8 Susquehanna River at Conowingo, MD Sediment -17.2 -0.1883 (0.0026) IMPROVING

1491000 9 Choptank River near Greensboro, Md. Sediment -32.9 -0.3984 (<0.0001) IMPROVING

1594440 13 Patuxent River at Bowie, MD Sediment -44.6 -0.5913 (<0.0001) IMPROVING

1646580 22 Potomac River at Chain Bridge, MD Sediment -61.9 -0.9650 (<0.0001) IMPROVING

1668000 24 Rappahannock River near Fredericksburg, Va TSS -2.7 -0.0270 (0.8655) ns

1673000 26 Pamunkey River near Hanover, Va TSS 123.7 0.8050 (<0.0001) DEGRADING

1674500 27 Mattaponi River near Beulahville, Va TSS -2.4 -0.0247 (0.8228) ns

2037500 29 James River near Richmond, Va TSS 43.9 0.3639 (0.0108) DEGRADING

2041650 30 Appomattox River at Matoaca, VA Sediment 14.5 0.1351 (0.1116) ns
14 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W. 0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Increased 0-50 percent

Increased greater than 

50 percent

Stream

Susquehanna 

River basin

Trend

Chesapeake Bay

Chesapeake Bay watershed 

boundary

Western Shore

Choptank 

Patuxent 

Potomac 

Rappahannock 

Mattaponi 

Pamunkey 

James 

Appomattox 

EXPLANATION

Not significant

Decreased 0-50 percent

Decreased less than 

50 percent

Figure 6. Flow-adjusted trends in total nitrogen concentration for 31 nontidal sites, 1985–2010.
Changes in Water Quality–Primary Sites 15

Table 5. Flow-adjusted trends for water years 1985–2010 and 2001–10 at 33 stream-monitoring sites in the Chespeake Bay watershed.[STAID: U.S. Geological Survey gaging station number; Map ID, as appears in figure 1; Bold, River Input Monitoring (RIM) site; FA, flow-adjusted significance, type of significant trend; ns, not siginificant; -, model results not reported]

STAID Map ID (figure 1) Station name FA trend

Significance

Total nitrogen Total phosphorus Sediment

1985–2010 2001–10 1985–2010 2001–10 1985–2010 2001–10

01531500 1 Susquehanna River At Towanda, Pa -45.4 IMPROVING IMPROVING IMPROVING ns ns ns01536500 2 Susquehanna River At Wilkes-Barre, Pa -41.9 IMPROVING IMPROVING IMPROVING ns IMPROVING ns01540500 3 Susquehanna River At Danville, Pa -44.2 IMPROVING IMPROVING IMPROVING ns IMPROVING ns01553500 4 West Branch Susquehanna River At Lewisburg, Pa -31.5 IMPROVING IMPROVING IMPROVING ns ns ns01567000 5 Juniata River At Newport, Pa -12.7 IMPROVING IMPROVING IMPROVING IMPROVING ns IMPROVING01576000 6 Susquehanna River At Marietta, Pa -23 IMPROVING IMPROVING IMPROVING IMPROVING IMPROVING ns01576754 7 Conestoga River At Conestoga, Pa - IMPROVING IMPROVING IMPROVING IMPROVING IMPROVING IMPROVING01578310 8 Susquehanna River At Conowingo, MD -25.5 IMPROVING ns ns ns IMPROVING DEGRADING01491000 9 Choptank River Near Greensboro, Md 6.8 DEGRADING ns DEGRADING ns IMPROVING DEGRADING01582500 10 Gunpowder Falls At Glencoe, Md 6.3 ns DEGRADING IMPROVING ns DEGRADING DEGRADING01586000 11 North Branch Patapsco River At Cedarhurst, Md 3.5 ns ns IMPROVING ns IMPROVING DEGRADING01591000 12 Patuxent River Near Unity, Md 6.3 ns ns IMPROVING ns ns DEGRADING01594440 13 Patuxent River At Bowie, MD -57.2 IMPROVING IMPROVING IMPROVING IMPROVING IMPROVING ns01599000 14 Georges Creek At Franklin, Md -43.1 IMPROVING ns IMPROVING ns ns DEGRADING01601500 15 Wills Creek Near Cumberland, Md -43.6 IMPROVING IMPROVING IMPROVING ns ns ns01614500 16 Conococheague Creek At Fairview, Md - IMPROVING IMPROVING IMPROVING IMPROVING ns ns01619500 17 Antietam Creek Near Sharpsburg, Md -23.4 IMPROVING DEGRADING IMPROVING ns ns ns01631000 18 S F Shenandoah River At Front Royal, Va -38 IMPROVING ns IMPROVING IMPROVING ns ns01634000 19 N F Shenandoah River Near Strasburg, Va -2.4 ns IMPROVING IMPROVING IMPROVING DEGRADING ns01637500 20 Catoctin Creek Near Middletown, Md -29.8 IMPROVING ns IMPROVING IMPROVING DEGRADING ns01639000 21 Monocacy River At Bridgeport, Md -31.6 IMPROVING IMPROVING IMPROVING ns IMPROVING ns01646580 22 Potomac River At Chain Bridge, MD -24.3 IMPROVING IMPROVING ns IMPROVING IMPROVING ns01651000 23 Nw Branch Anacostia River Near Hyattsville, Md -21.6 IMPROVING ns ns DEGRADING DEGRADING DEGRADING01668000 24 Rappahannock River Near Fredericksburg, Va -9.8 ns ns ns ns ns ns01671020 25 North Anna River At Hart Corner Near Doswell, Va 5.1 ns ns DEGRADING DEGRADING DEGRADING ns01673000 26 Pamunkey River Near Hanover, Va 18.8 DEGRADING ns DEGRADING IMPROVING DEGRADING ns01674500 27 Mattaponi River Near Beulahville, Va ns ns IMPROVING ns ns IMPROVING02035000 28 James River At Cartersville, Va -18.2 IMPROVING ns IMPROVING IMPROVING ns ns02037500 29 James River Near Richmond, Va -17.6 IMPROVING IMPROVING IMPROVING IMPROVING DEGRADING ns02041650 30 Appomattox River At Matoaca, Va 5.7 ns ns DEGRADING DEGRADING ns ns02042500 31 Chickahominy River Near Providence Forge, Va -20.2 IMPROVING ns ns ns DEGRADING ns01589300 32 Gwynns Falls At Villa Nova, Md - - ns - ns - DEGRADING01658000 33 Mattawoman Creek Near Pomonkey, Md - - ns - DEGRADING - DEGRADING
16 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

Increased 0-50 percent

Increased greater than 

50 percent

Stream

Susquehanna 

River basin

Trend

Chesapeake Bay

Chesapeake Bay watershed 

boundary

Western Shore

Choptank 

Patuxent 

Potomac 

Rappahannock 

Mattaponi 

Pamunkey 

James 

Appomattox 

EXPLANATION

Not significant

Decreased 0-50 percent

Decreased less than 

50 percent

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W.

0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Figure 7. Flow-adjusted trends in total phosphorus concentration for 31 nontidal sites, 1985–2010.
Changes in Water Quality–Primary Sites 17

Increased 0-50 percent

Increased greater than 

50 percent

Stream

Susquehanna 

River basin

Trend

Chesapeake Bay

Chesapeake Bay watershed 

boundary

Western Shore

Choptank 

Patuxent 

Potomac 

Rappahannock 

Mattaponi 

Pamunkey 

James 

Appomattox 

EXPLANATION

Not significant

Decreased 0-50 percent

Decreased less than 

50 percent

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W.

0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Figure 8. Flow-adjusted trends in sediment concentration for 31 nontidal sites, 1985–2010.
18 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

Indicator Development

The USGS worked with CBP partners to develop 

additional indicators (trend, yield, and combined) that graphically summarize water-quality data that will be placed in a 

Web-accessible format for use by the public. The indicators 

presented in this report are distinct from previous waterquality trends and loads summarized in that they (1) focus on 

recent (2001–10) data, (2) include data from a larger number 

of monitoring sites, and (3) use consistent periods of record. 

These indicators will assist water-quality managers in optimizing management actions to help meet TMDL.

Trend Indicator

Trends in concentrations were estimated using the load 

model ESTIMATOR. The USGS typically reports trends 

over the longest time period for which data are available. As 

discussed in this report, an additional 10-year time period was 

analyzed to examine changes in concentrations and loads over 

a shorter time frame. Trends in concentration were estimated 

for the period 2001–10 for 33 sites. Both flow and water-quality data are available for the 33 sites for the 10-year minimum 

time period and the data represent samples collected over the 

entire range of flow conditions that occurred at all 33 sites 

during the 10-year period.

Trend results for the two time periods (1985–2010 

and 2001–10) are presented in table 5. A comparison of the 

number of significant trends during the two time periods 

indicates that (1) fewer sites showed significantly improving 

(downward) trends in the 10-year period for all constituents 

and (2) the sites with significantly degrading (upward) trends 

in the two time periods seldom coincided. Although fewer 

trends were found in the 10-year time period than in the longer 

time period, sites with improving trends outnumbered sites 

with degrading trends in both time periods. Also, 10 of the 

13 degrading trends in 1985–2010 became either not significant or improving during 2001–10, and 4 of the 15 degrading 

trends during the 2001–10 time period represent reversals 

from improving trends during 1985–2010. These changes in 

significant trends between the two time periods may be related 

to the rate of implementation and number of best management 

practices (BMPs) instituted, and the delay in improvement (lag 

time) from the actual detection of the improvement in water 

quality.

Yield Indicator

Yield is the load for a given time period divided by 

(normalized to) the drainage area (in square miles in this 

report). The yield indicator provides additional information 

about loads so that comparisons among sites can be made 

over a common time period. Annual loads at all 64 sites for 

2006–2010 were estimated and converted to yields. The 5-year 

mean was used to represent the yield indicator. The mean 

yields at the 64 sites were sorted by magnitude, grouped into 

thirds (low, 1–22; medium, 23–43; and high, 44–64), color 

coded for display purposes, and resorted in the order in which 

they are listed in table 1 (table 6). Ranges for TN yields (in 

tons per square mile (tons/mi2)) are low (0.83–1.39), medium 

(1.4–2.5), and high (2.0–10); ranges for TP yields are low 

(0.02–0.09), medium (0.10–0.18), and high (0.19–0.56); and 

ranges for sediment yields are low (12–58), medium (64–152), 

and high (160–2,500).

Combined Indicator

A new indicator, referred to in this report as the “combined indicator,” was developed for this study. The combined 

indicator permits site-to-site comparisons of yields and trends. 

Trend results are available for 33 sites in the bay watershed for 

the most recent 10 years of record (2001–10), and yield results 

are available for 64 sites for the most recent 5 years of record 

(2006–10). After an evaluation of the data with the goal of 

efficiently displaying results from as many nontidal monitoring sites as possible, four classifications of the 10-year trends 

were combined with three magnitude groupings of the 5-year 

yields to produce a classification system consisting of 12 categories (table 7). Relative yield (indicated by color) and trend 

direction (indicated by arrows) are shown in figures 6 to 8. By 

combining information about trend and yield in one illustration, these maps show that progress has been made in response 

to recent efforts to reduce inputs of nutrients and sediment to 

the bay from the watershed.

The combined trends and yields indicator for TN for the 

64 sites is grouped by flow-adjusted-concentration trend direction (table 5) and yield (table 6) for the 64 sites in figure 9. 

Seventeen of the 33 TN sites for which trends are available 

indicate improving (downward) trends; nearly half of these 

are in the Susquehanna River Basin (fig. 9). Two sites indicate 

degrading (upward) trends. More than half (16) of the sites 

had no significant trend. Yields generally decrease from north 

to south, and are higher in the northern areas of the bay basin 

(Susquehanna River), intermediate (medium) in the middle 

bay basin (Potomac River), and lower in the southern bay 

basin (Virginia rivers). One-third of the sites indicate improving trends and low to medium yields, but two sites indicate 

degrading trends and high yields. In addition, lower yields 

upstream appear to increase downstream in the Susquehanna 

River Basin.

The combined indicator for TP shows improving trends 

at 12 of the 33 sites for which trend data are available (table 6, 

fig. 10); however, 4 sites indicate degrading trends. Fewer than 

half the sites indicate no change. No general spatial pattern 

can be observed in yields of TP, with the exception of lower 

yields in the western headwaters of the Potomac River Basin 

(Maryland and West Virginia). High and medium yields appear 

in every major basin in the monitoring network. Results for 

TP are similar to those for TN; one-third of the sites indicate 
Indicator Development 19

Table 6. Mean yields of nitrogen, phosphorus, and sediment at 64 stream-monitoring sites in the Chesapeake Bay watershed, 

2006–10. 

[blue, lower yields; yellow, medium yields; orange, higher yields; yields in tons per square mile; Map ID, as appears in figure 1] 

Map ID 

(figure 1)

Site 

number

Total

nitrogen 

yield

Total

phosphorus 

yield

Sediment 

yield

Map ID 

(figure 1)

Site

number

Total

nitrogen 

yield

Total

phosphorus 

yield

Sediment 

yield

1 01531500 4.35 0.49 414.89 33 01658000 1.04 0.15 164.01

2 01536500 1.38 0.18 167.88 34 01515000 2.05 0.23 267.32

3 01540500 1.55 0.19 167.48 35 01531000 1.31 0.15 178.95

4 01553500 1.22 0.06 35.20 36 01542500 0.96 0.06 50.67

5 01567000 1.99 0.08 46.78 37 01562000 2.04 0.06 72.18

6 01576000 2.03 0.11 96.93 38 01568000 3.71 0.15 98.08

7 01576754 9.58 0.39 160.46 39 01570000 4.91 0.08 64.76

8 01578310 2.14 0.08 46.45 40 01571500 3.05 0.10 69.51

9 01491000 2.34 0.17 27.95 41 01573560 6.03 0.23 178.26

10 01582500 3.19 0.05 58.07 42 01574000 3.62 0.27 116.67

11 01586000 4.37 0.12 130.14 43 01576787 10.01 0.57 394.70

12 01591000 3.10 0.34 2549.53 44 01667500 1.32 0.36 426.90

13 01594440 1.83 0.16 89.35 45 01487000 8.28 0.11 13.98

14 01599000 1.62 0.09 71.29 46 01488500 4.44 0.25 39.43

15 01601500 1.72 0.10 120.08 47 01491500 4.53 0.17 20.69

16 01614500 4.84 0.13 82.56 48 01495000 5.50 0.41 653.29

17 01619500 4.43 0.18 69.40 49 01502500 2.19 0.24 589.28

18 01631000 1.35 0.14 87.13 50 01503000 1.85 0.30 697.02

19 01634000 1.32 0.09 66.33 51 01529500 1.79 0.11 33.44

20 01637500 2.06 0.22 343.44 52 01549760 0.88 0.05 32.44

21 01639000 3.01 0.27 70.79 53 01578475 8.32 0.23 106.25

22 01646580 1.75 0.12 152.86 54 01580520 4.99 0.29 400.16

23 01651000 2.54 0.54 288.40 55 01594526 1.43 0.48 1229.73

24 01668000 1.07 0.25 353.41 56 01604500 0.69 0.04 33.49

25 01671020 0.39 0.04 40.87 57 01608500 0.81 0.07 52.70

26 01673000 0.54 0.07 42.38 58 01610155 1.05 0.02 47.50

27 01674500 0.52 0.05 12.10 59 01611500 0.72 0.04 35.37

28 02035000 0.66 0.14 125.90 60 01613095 1.43 0.05 81.62

29 02037500 0.70 0.18 204.58 61 01613525 1.58 0.06 90.56

30 02041650 0.48 0.05 13.78 62 01616500 2.04 0.18 57.71

31 02042500 0.69 0.13 37.01 63 01619000 4.49 0.17 55.31

32 01589300 2.17 0.35 491.43 64 02024752 0.58 0.10 97.45
20 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

improving trends and low to medium yields, whereas three 

sites indicate degrading trends and high yields. 

For sediment, unlike TN and TP, more sites indicate 

degrading trends (9) than improving trends (3) (table 6; 

fig. 11). Seven of the nine sites with degrading trends in sediment are in the Potomac River Basin, whereas two of the three 

improving sites are in the Susquehanna River Basin. Like 

yields of TP, sediment yields show no discernible spatial pattern. In contrast to TN and TP sites, more sediment sites indicate degrading water quality and high yield (5) than improving 

water quality and low yield (2).

A degrading water-quality trend and a high yield at a 

site might be considered the “least favorable” combination 

in table 7. In contrast, an improving water-quality trend and 

a low yield might be considered the “most favorable” combination, and the goal for all sites. Of the 33 sites, 2, 2, and 

5 sites would be classified as “least favorable” (27 percent) 

and 4, 6, and 2 sites would be classified as “most favorable” (36 percent) for TN, TP, and sediment, respectively. 

If managers and scientists wanted to consider sites near the 

middle of the range (improving or not-significant trends and 

medium yields) for economic or environmental feasibility 

reasons, then 13, 12, and 10 sites could be classified as “near 

middle” (35 percent) for TN, TP, and sediment, respectively. 

For nutrients and sediment, “more favorable” areas generally 

are located in parts of the upper and western Susquehanna and 

western Potomac River Basins and many locations in Virginia. 

“Least favorable” areas tend to be located in the middle of the 

Chesapeake Bay Basin. Locations classified as “most favorable” tend to be dominated by forested land, whereas those 

classified as “least favorable” tend to be dominated by urban 

and agricultural land uses.

Changes in Water Quality–Secondary 

Sites

As mentioned previously, the Chesapeake Bay Nontidal 

Monitoring Network was designed for a variety of sampling 

and study needs. Primary sites are sampled monthly with 

targeted stormflow sampling, and secondary sites are sampled 

monthly but with no targeted stormflow sampling, by using the 

protocols established by the monitoring network partners and 

multiple Federal, State, and basin commissions (U.S. Environmental Protection Agency, 2004.

The secondary network currently consists of 16 sites, 

all located in Virginia (table 8 and fig. 1). Ten-year trends 

(2001–10) were estimated at each site for TN, TP, and sediment. Not all results are reported as a result of a lack of data 

to calibrate the ESTIMATOR model or inadequate sample 

coverage over the hydrograph. Four of the 16 sites indicated 

improving trends for TN, and 2 of the 14 sites indicated 

improving trends for TP. A degrading trend for sediment was 

found at one site.

Future Directions

The annual evaluation of water-quality trends in the 

Chesapeake Bay watershed has been done with USGS participation since the early 1990s. Trends from the River Input 

Monitoring (RIM) sites were originally reported by USGS 

using multivariate regression techniques (ESTIMATOR 

model) developed by Cohn and others (1992) and by 2010 

Table 7. Combined trend and yield indicator counts by classification for the 64 sites in the Chesapeake Bay watershed. 

[Ten-year flow-adjusted trends are for 33 sites, and 5-year mean yields are for all 64 sites]

10-year flow-adjusted trend (2001–10)

Total nitrogen Degrading (upward) Not significant Improving (downward) Trends not available

5-year yields

(2005–10)

High 2 4 2 13

Medium 0 6 7 10

Low 0 4 8 8

Total Phosphorus Degrading (upward) Not significant Improving (downward) Trends not available

5-year yields

(2005–10)

High 2 2 6 14

Medium 1 7 5 7

Low 1 3 6 10

Total Sediment Degrading (upward) Not significant Improving (downward) Trends not available

5-year yields

(2005–10)

High 5 6 1 10

Medium 1 10 0 8

Low 3 5 2 13
Future Directions 21

Stream

Trend

(2001–10)

Chesapeake Bay

Chesapeake Bay watershed 

boundary

EXPLANATION

Degrading

Not significant

Improving

N/A*

High

Medium

Low

Yield

(2006–10)

Note:

"Yield" equals annual load 

divided by drainage area. 

High, Medium, and Low

represent the highest, middle, 

and lowest 1/3 of the yield data.

Arrows signify direction in 

environmental condition.

*N/A–Trend not computed, 

less than 10 years of 

information.

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W.

0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Figure 9. Combined nitrogen trends (WY2001–10) and yields (WY2006–10) at 64 stream monitoring sites in the Chesapeake Bay 

watershed. (WY, water years - the 12-month period from October 1 to September 30, designated by the year in which it ends)
22 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

Stream

Trend

(2001–10)

Chesapeake Bay

Chesapeake Bay watershed 

boundary

EXPLANATION

Degrading

Not significant

Improving

N/A*

High

Medium

Low

Yield

(2006–10)

Note:

"Yield" equals annual load 

divided by drainage area. 

High, Medium, and Low

represent the highest, middle, 

and lowest 1/3 of the yield data.

Arrows signify direction in 

environmental condition.

*N/A–Trend not computed, 

less than 10 years of 

information.

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W.

0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Figure 10. Combined phosphorus trends (WY2001–10) and yields (WY2006–10) at 64 stream monitoring sites in the Chesapeake 

Bay watershed. (WY, water years - the 12-month period from October 1 to September 30, designated by the year in which it ends)
Future Directions 23

Stream

Trend

(2001–10)

Chesapeake Bay

Chesapeake Bay watershed 

boundary

EXPLANATION

Degrading

Not significant

Improving

N/A*

High

Medium

Low

Yield

(2006–10)

Note:

"Yield" equals annual load 

divided by drainage area. 

High, Medium, and Low

represent the highest, middle, 

and lowest 1/3 of the yield data.

Arrows signify direction in 

environmental condition.

*N/A–Trend not computed, 

less than 10 years of 

information.

78o

80o

42o

38o

40o

76o

NEW YORK

MARYLAND

PENNSYLVANIA

DELAWARE

VIRGINIA

WEST VIRGINIA

Base from U.S. Geological Survey digital data, 1972, 

1:2,000,000 Albers Equal-Area Conic Projection. 

Standard parallels 29 degrees 30 minutes N, 

central meridian 75 degrees 00 minutes W. 0 25 50 100 MILES

0 37.5 75 150 KILOMETERS

Chesapeake Bay

Figure 11. Combined sediment trends (WY2001–10) and yields (WY2006–10) at 64 stream monitoring sites in the Chesapeake Bay 

watershed. (WY, water years - the 12-month period from October 1 to September 30, designated by the year in which it ends)
24 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

Table 8. Trend results for the 16 Virginia secondary sites in the Chesapeake Bay Nontidal Monitoring Program. 

[STAID: U.S. Geological Survey gaging station number; Map ID, identification number that appears in figure 1; TN, total nitrogen; TP, total phosphorus; FA, 

flow-adjusted; Significance: type of significant trend or ns (not significant)]

STAID Map ID Station name Constituent FA trend Significance

1626000 S1 SOUTH RIVER NEAR WAYNESBORO, VA TN -14.5 ns

1628500 S2 S.F. SHENANDOAH RIVER NEAR LYNNWOOD, VA TN -6.5 ns

1638480 S3 CATOCTIN CREEK NEAR TAYLORSTOWN, VA TN -9.1 ns

1654000 S4 ACCOTINK CREEK NEAR ANNANDALE, VA TN -46.9 IMPROVING

1664000 S5 RAPPAHANNOCK RIVER AT REMINGTON, VA TN -0.5 ns

1665500 S6 RAPIDAN RIVER AT RUCKERSVILLE, VA TN -52.8 IMPROVING

1671100 S7 LITTLE RIVER NEAR DOSWELL, VA TN -22.7 ns

1673800 S8 PO RIVER NEAR SPOTSYLVANIA, VA TN -30 IMPROVING

1674000 S9 MATTAPONI RIVER NEAR BOWLING GREEN, VA TN -42 IMPROVING

2011500 S10 BACK CREEK NEAR MOUNTAIN GROVE, VA TN -13.9 ns

2015700 S11 BULLPASTURE RIVER AT WILLIAMSVILLE, VA TN -11.2 ns

2020500 S12 CALFPASTURE RIVER ABV MILL CREEK AT GOSHEN, VA TN -12.8 ns

2027500 S13 PINEY RIVER AT PINEY RIVER, VA TN -18.6 ns

2031000 S14 MECHUMS RIVER NEAR WHITE HALL, VA TN -15.4 ns

2039500 S15 APPOMATTOX RIVER AT FARMVILLE, VA TN -19.9 ns

2041000 S16 DEEP CREEK NEAR MANNBORO, VA TN -6.8 ns

1626000 S1 SOUTH RIVER NEAR WAYNESBORO, VA TP -25.7 IMPROVING

1628500 S2 S.F. SHENANDOAH RIVER NEAR LYNNWOOD, VA TP -28.8 ns

1638480 S3 CATOCTIN CREEK NEAR TAYLORSTOWN, VA TP 20.1 ns

1654000 S4 ACCOTINK CREEK NEAR ANNANDALE, VA TP 1.3 ns

1664000 S5 RAPPAHANNOCK RIVER AT REMINGTON, VA TP -9.1 ns

1665500 S6 RAPIDAN RIVER AT RUCKERSVILLE, VA TP -64.3 IMPROVING

1671100 S7 LITTLE RIVER NEAR DOSWELL, VA TP 21.6 ns

1673800 S8 PO RIVER NEAR SPOTSYLVANIA, VA TP -0.3 ns

1674000 S9 MATTAPONI RIVER NEAR BOWLING GREEN, VA TP -18.5 ns

2020500 S12 CALFPASTURE RIVER ABV MILL CREEK AT GOSHEN, VA TP 20.8 ns

2027500 S13 PINEY RIVER AT PINEY RIVER, VA TP -8.4 ns

2031000 S14 MECHUMS RIVER NEAR WHITE HALL, VA TP 18.6 ns

2039500 S15 APPOMATTOX RIVER AT FARMVILLE, VA TP 11.7 ns

2041000 S16 DEEP CREEK NEAR MANNBORO, VA TP -3 ns

1626000 S1 SOUTH RIVER NEAR WAYNESBORO, VA Sediment 5.2 ns

1628500 S2 S.F. SHENANDOAH RIVER NEAR LYNNWOOD, VA Sediment 135.1 DEGRADING

1638480 S3 CATOCTIN CREEK NEAR TAYLORSTOWN, VA Sediment 17.2 ns

1654000 S4 ACCOTINK CREEK NEAR ANNANDALE, VA Sediment -58.4 ns

1664000 S5 RAPPAHANNOCK RIVER AT REMINGTON, VA Sediment 19.6 ns

1671100 S7 LITTLE RIVER NEAR DOSWELL, VA Sediment 49.7 ns

2031000 S14 MECHUMS RIVER NEAR WHITE HALL, VA Sediment -4.5 ns

2039500 S15 APPOMATTOX RIVER AT FARMVILLE, VA Sediment 63.7 ns

2041000 S16 DEEP CREEK NEAR MANNBORO, VA Sediment 17.4 ns
Summary 25

had expanded to the current network of 64 sites. The regression technique produces a trend adjusted for flow by adjusting 

for the influences of streamflow and season. This technique 

is useful in assessing the water-quality changes resulting 

primarily from resource-management actions. Evaluations 

of methods and test results indicate the need for continuing improvement in statistical methods to help answer new 

technical questions about water-quality change. In particular, 

improvements have been identified to (1) construct sample 

data sets to more accurately represent concentrations and loads 

at water-quality monitoring locations, (2) improve the statistical analysis to assess changes in trends over shorter periods of 

time, (3) better describe the patterns of long-term trends, and 

(4) improve ways to compare trends and additional watershed 

information to trends in the tidal waters. New approaches to 

and improvements in data analysis will be a continual process 

as data from additional sites are added to the Nontidal Monitoring Network database and new methods are developed to 

aid in the interpretation of environmental data. A recently 

developed method to examine concentration, load, and trend 

over the long term (greater than 20 years) is the “Weighted 

Regressions on Discharge Time and Season” (WRTDS) 

(Hirsch and others, 2010). This method allows for consistent 

estimates of the actual history of concentrations and loads, as 

well as histories that eliminate the influence of year-to-year 

variations in streamflow. Application of WRTDS to selected 

sites in the network in future load and trend evaluations will 

further enhance the understanding of management actions and 

changes in water quality, ultimately leading to meeting the 

load allocations to meet the TMDL for the bay. 

Summary

In 2011, the U.S. Geological Survey completed analyses 

of loads of, and trends in, nutrients and sediment at 31 and 

33 “primary sites” (sites where stormflow was sampled) for 

two time periods (1985–2010 and 2001–10, respectively), 

loads of nutrients and sediment at 64 sites for a 5-year time 

period (2006–10); and 10-year nutrient and sediment trends at 

“secondary” sites (16 sites where stormflow was not sampled), 

as part of the effort to evaluate progress, on an annual basis, 

toward meeting nutrient and sediment allocation goals. The 

64 sites represent an increase of 19 sites from those evaluated 

in the 2009 data analysis update. The total estimated freshwater streamflow entering the bay in 2010 was 78,280 cubic feet 

per second, an amount that was defined as “normal” and as 

being between the 25th- and 75th-percentile values, as determined by using data from 1937. Regression models indicated 

no significant trends in streamflow (1985–2010) at the 31 

long-term sites.

Streamflow increased from 2009 to 2010 at 28 of the 

31 sites where long-term loads and trends were estimated. As a 

result, combined loads for total nitrogen, total phosphorus, and 

sediment at the nine River Input Monitoring sites increased 

33, 120, and 330 percent, respectively. Total nitrogen loads 

(178 million pounds) were below the long-term average, total 

phosphorus loads (13 million pounds) were slightly above 

the long-term average, and sediment loads (15,500 million 

pounds) were the third highest since 1990 and well above the 

long-term average.

Long-term flow-adjusted trends were computed at 31 primary sites in the bay watershed. The three largest monitored 

rivers (Susquehanna, Potomac, and James) exhibited statistically significant downward (improving) trends in total nitrogen. Twenty-one of the 31 sites indicated improving trends in 

total nitrogen. For total phosphorus, the farthest downstream 

monitoring sites on the two largest rivers, the Susquehanna 

and the Potomac, indicated a nonsignificant trend. Twenty-two 

of the 31 sites indicated improving trends in total phosphorus. 

Statistically significant downward (improving) trends in sediment were observed for the Susquehanna and Potomac River 

Basins. There were nine improving trends and seven degrading trends for sediment.

The USGS, working with its Chesapeake Bay Program 

partners, developed new trend indicators to provide additional information on all 64 primary sites and 16 secondary 

sites currently in the Chesapeake Bay Nontidal Water-Quality 

Monitoring Network, which will assist managers to determine 

optimal management actions to improve water quality in the 

Chesapeake Bay watershed to help reach bay-wide standards 

for water quality and, ultimately, to meet the Total Maximum 

Daily Load mandated by the U.S. Environmental Protection Agency. Trends were evaluated for a shorter time period 

(2001–10) at 33 primary sites, and loads and yields were estimated for all 64 primary sites. A comparison between trends 

computed for 1985 to 2010 and those computed for 2001 to 

2010 revealed a reduction in the number of sites exhibiting 

significant trends from the longer to the shorter time period, 

and sites with significant degrading trends were seldom the 

same for the two time periods. The 2001–10 time period indicated more improving trends than degrading trends.

Trends for the shorter time period were combined with 

the yields to produce a classification system consisting of 12 

categories. The combined indicator revealed that 17 of the 

33 primary sites for which the appropriate data were available showed improving water quality for total nitrogen (TN), 

and 2 sites showed degrading water quality with respect to 

TN. A general spatial pattern was noted of decreasing yields 

from north (Susquehanna River Basin) to south (James River 

Basin). With respect to TN, six sites had both improving 

trends and low yields, whereas two sites had both degrading 

trends and high yields. Combined-indicator results for total 

phosphorus (TP) show that 17 of the 33 primary sites for 

which the appropriate data were available showed improving 

trends, and 3 sites showed degrading trends. No general spatial 

pattern was demonstrated in TP yields, except that of lower 

yields in the western Potomac River Basin. With respect to 

TP, two sites showed both improving trends and low yields, 

whereas five sites showed degrading trends and high yields. 

Degrading trends were found at more sites than improving 
26 Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed

trends for sediment. Seven of the nine sites with degrading 

trends in sediment were in the Potomac River Basin, whereas 

two of the three sites with improving trends were in the 

Susquehanna River Basin. No discernible spatial pattern was 

observed for sediment yields. Few significant trends were 

indicated for the 16 secondary sites. Only improving trends 

in TN at four sites, improving trends in TP at two sites, and a 

degrading trend in sediment at one site were reported.

References Cited

Bue, C.D., 1968, Monthly surface-water input to Chesapeake 

Bay: U.S. Geological Survey Open-File Report 68–29, 45 p. 

Bradu, D., and Mundlak, Y., 1970, Estimation of lognormal 

linear models: Journal of the American Statistical Association, v. 65, no. 329, p. 198–211. 

Cohn, T.A., 1988, Adjusted maximum likelihood estimation of 

the moments of lognormal populations from type I censored 

samples: U.S. Geological Survey Open-File Report 88–350, 

34 p. 

Cohn, T.A., Delong, L.L., Gilroy, E.J., Hirsch, R.M., and 

Wells, D., 1989, Estimating constituent loads: Water 

Resources Research, v. 25, no. 5, p. 937–942. 

Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and 

Summers, R.M., 1992, The validity of a simple log-linear 

model for estimating fluvial constituent loads: An empirical 

study involving nutrient loads entering Chesapeake Bay: 

Water Resources Research, v. 28, no. 9, p. 2353–2364.

Gilroy, E.J., Hirsch, R.M., and Cohn, T., 1990, Mean square 

error of regression-based constituent transport estimates: 

Water Resources Research, v. 276, no. 9, p. 2069–2077.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 4, chap. A3, 522 p. (Also available 

online at http://pubs.usgs.gov/twri/twri4a3/.)

Hirsch, R.M., Moyer, D.L., and Archfield, S.A., 2010, 

Weighted regressions on time, discharge, and season 

(WRTDS), with an application to Chesapeake Bay river 

inputs: Journal of the American Water Resources Association, v. 46, iss. 5, 5 p.

Kammerer, P.A., Garn, H.S., Rasmussen, P.W., and Ball, 

J.R., 1998, A comparison of water-quality sample collection methods used by the U.S. Geological Survey and the 

Wisconsin Department of Natural Resources in Proceedings 

of the National Water-Quality Monitoring Council, National 

Conference on Monitoring, Reno, Nevada, July 7–9, 1998: 

Washington, D.C., U.S. Environmental Protection Agency, 

p. III-259–269.

Langland, M.J., Lietman, L.P., and Hoffman, S., 1995, Synthesis of nutrient and sediment data for watersheds within 

the Chesapeake Bay drainage basin: U.S. Geological Survey 

Water-Resources Investigations Report 95–4233, 121 p.

Langland, M.J., Edwards, R.E., and Darrell, L.C., 1998, 

Yields and trends of nutrients and total suspended solids in 

nontidal areas of the Chesapeake Bay basin, 1985–96: U.S. 

Geological Survey Water-Resources Investigations Report 

98–17, 60 p.

Langland, M.J., Phillips, S.W., Raffensperger, J.P., Moyer, 

D.L., Landwehr, J.M., and Schwarz, G.E., 2006, Changes in 

streamflow and water quality in selected nontidal sites in the 

Chesapeake Bay Basin, 1985–2004: U.S. Geological Survey 

Scientific Investigations Report 2006–5178, 75 p. 

Phillips, S.W., ed., 2002, The U.S. Geological Survey and the 

Chesapeake Bay—The role of science in environmental 

restoration: U.S. Geological Survey Circular 1220, 32 p. 

U.S. Environmental Protection Agency, 2003, Ambient water 

quality criteria for dissolved oxygen, water clarity, and 

chlorophyll-a for the Chesapeake Bay and its tidal tributaries: 903-R-03-002, 231 p. 

U.S. Environmental Protection Agency, 2004, Establishing a 

Chesapeake Bay nontidal watershed water-quality network: 

prepared by the Chesapeake Bay Program’s Nontidal WaterQuality Monitoring Workgroup, September 2004, 28 p.

Yochum, S.E., 2000, A revised Load Estimation Procedure for 

the Susquehanna, Potomac, Patuxent, and Choptank Rivers: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4156, 49 p.
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Director

U.S. Geological Survey

Pennsylvania Water Science Center

215 Limekiln Road

New Cumberland, Pa. 17070

or visit our Web site at:

http://pa.water.usgs.gov/
Langland and others—Nutrient and Sediment Trends, Loads, and Yields and Indicator of Streamwater Quality, Chesapeake Bay Watershed—Scientific Investigations Report 

2012–5093

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