In cooperation with 

Maryland Department of Natural Resources 

A Revised Load Estimation Procedure for the 

Susquehanna, Potomac, Patuxent, and Choptank Rivers 

By Daniel J. Phelan, Lisa D. Olsen, Martha L. Cashel, Judith L. Tegler, and Elizabeth H. Marchand 

Water-Resources Investigations Report 00-4156 
U.S. Department of the Interior 

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

U.S. Geological Survey 

A Revised Load Estimation Procedure for the 

Susquehanna, Potomac, Patuxent, and Choptank Rivers 

By Steven E. Yochum 

Water-Resources Investigations Report 00-4156 

In cooperation with 

Maryland Department of Natural Resources 

The contents of this report have been approved for public release and unlimited distribution by the U.S. Army--

distribution number 2958-A-4 

Baltimore, Maryland 

2000 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

The use of trade, product, or firm names in this report is for 

descriptive purposes only and does not imply endorsement 

by the U.S. Geological Survey. 

For additional information contact: 

District Chief 

U.S. Geological Survey, WRD 

8987 Yellow Brick Road 

Baltimore, Maryland 21237 

Copies of this report can be purchased from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, Colorado 80225-0286 
Contents 

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

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

Purpose and scope.......................................................................................................................................................3

Description of study area ............................................................................................................................................3

Acknowledgments ......................................................................................................................................................3

Methods of load estimation ..............................................................................................................................................3

Monitoring record .......................................................................................................................................................3

Regression model........................................................................................................................................................5

Historical load estimates.............................................................................................................................................5

Revised procedure for load estimation.............................................................................................................................7

Background.................................................................................................................................................................7

Description of revised methods ..................................................................................................................................8

Comparison of historical and revised load estimates..................................................................................................9

Summary and conclusions..............................................................................................................................................11

References cited .............................................................................................................................................................11

Appendixes.....................................................................................................................................................................12

A. Annual load estimates for the River Input monitoring stations on the

Susquehanna, Potomac, Patuxent, and Choptank Rivers .................................................................................13

B. Percent differences between historical and revised load estimates.....................................................................46

Figures 

1. Map showing the Chesapeake Bay Basin study area, drainage basins and location of

Maryland River Input monitoring stations. ........................................................................................................2

2. Graphs showing sampling frequency of total phosphorus at the Maryland River Input

monitoring stations, 1978–99 at:

(A) Susquehanna River at Conowingo Dam, Maryland (Station ID 01578310); 

(B) Potomac River at Chain Bridge at Washington, D.C. (Station ID 01646580); 

(C) Patuxent River near Bowie, Maryland (Station ID 01594440); and 

(D) Choptank River near Greensboro, Maryland (Station ID 01491000).........................................................4

3. Schematic illustrating historical load computation methodology............................................................................6

4. Schematic illustrating revised load computation methodology...............................................................................8

5. Graphs showing comparison of selected revised and historical load estimates, with

standard errors of prediction:

(A) Patuxent River near Bowie, Maryland (Total Nitrogen); 

(B) Choptank River near Greensboro, Maryland (Total Kjeldahl Nitrogen); 

(C) Susquehanna River at Conowingo Dam, Maryland (Orthophosphorus); and 

(D) Potomac River at Chain Bridge at Washington, D.C. (Suspended Sediment)..........................................10

Tables 

1. Constituents monitored in the River Input Program.................................................................................................3

iii 
_____________________________________________________________________________________________________________________________________ 

_____________________________________________________________________________________________________________________________________ 

Conversion Factors and Vertical Datum 

Multiply By To obtain 

___________________________________________________________________________________

Length 

__________________________________________________ 

inch (in.) 2.54 centimeter 

inch (in.) 25.4 millimeter 

mile (mi) 1.609 kilometer 

Area 

square mile (mi2

) 

square mile (mi2) 

259.0 

2.590 

hectare 

square kilometer 

Flow rate 

cubic foot per second (ft

3/s) 0.02832 cubic meter per second 

Vertical Datum: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929–A geodetic datum 

derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum 

of 1929. 

Temperature in degrees Celsius ( °

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

°

F = 1.8 ( °C) + 32 

Temperature in degrees Fahrenheit ( °

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

°

C = ( °F - 32) / 1.8 

Chemical concentration in water is expressed in milligrams per liter (mg/L) or micrograms per liter (μg/L). 

Water year is defined as the 12-month period of October 1 through September 30 and is designated by the calendar year in 

which it ends. 

iv 
A Revised Load Estimation Procedure for the Susquehanna, 

Potomac, Patuxent, and Choptank Rivers 

By Steven E. Yochum 

Abstract 

The U.S. Geological Survey’s Chesapeake 

Bay River Input Program has updated the 

nutrient and suspended-sediment load data base 

for the Susquehanna, Potomac, Patuxent, and 

Choptank Rivers using a multiple-window, 

center-estimate regression methodology. The 

revised method optimizes the seven-parameter 

regression approach that has been used 

historically by the program. The revised method 

estimates load using the fifth or center year of a 

sliding 9-year window. Each year a new model 

is run for each site and constituent, the most 

recent year is added, and the previous 4 years of 

estimates are updated. The fifth year in the 

9-year window is considered the best estimate 

and is kept in the data base. The last year of 

estimation shows the most change from the 

previous year’s estimate and this change 

approaches a minimum at the fifth year. 

Differences between loads computed using this 

revised methodology and the loads populating 

the historical data base have been noted but the 

load estimates do not typically change 

drastically. The data base resulting from the 

application of this revised methodology is 

populated by annual and monthly load estimates 

that are known with greater certainty than in the 

previous load data base. 

Introduction 

Since 1985, the U.S. Geological Survey (USGS) 

Maryland Chesapeake Bay River Input Program, in a 

cooperative effort with the Maryland Department of Natural 

Resources and the Metropolitan Washington Council of 

Governments (MWCOG), has monitored streamflow, 

nutrients, and suspended-sediment concentrations in the 

Susquehanna, Potomac, Patuxent, and Choptank Rivers just 

above the point of tidal influence (fig. 1). These data are 

used to estimate nutrient and sediment loads entering the 

tidal waters of the Chesapeake Bay. The USGS Virginia 

Chesapeake Bay River Input Program, in cooperation with 

the Virginia Department of Environmental Quality, monitors 

concentrations of nutrients and suspended sediments of five 

Virginia rivers just above the point of tidal influence. The 

Maryland and Virginia programs jointly comprise the USGS 

River Input Program. 

Within the Maryland River Input Program, 12 nutrient 

and suspended-sediment constituents are monitored at 4 

stations. Annual and monthly loads are estimated using a 

seven-parameter log-linear regression model. This 

regression model, known as Estimator, was developed by the 

USGS and is widely used for load estimation. During this 

program, a historical data base of annual and monthly load 

estimates had been generated for each constituent from 

eleven 10- to 14-year model windows, with the first window 

providing load estimates over its entire calibration window 

length and the remaining estimates extracted from the last 

year of calibration of each subsequent model. These end-ofmodel-window estimates that have traditionally been 

provided by the monitoring program are not statistically 

preferred. The preferred load estimates for any individual 

year (those with minimum uncertainty) are the ones 

computed in the center of the regression. A new data base, 

populated by loads estimated for the fifth or center year of 

9-year calibration windows, has been developed. A 9-year 

window is preferred because it has a center (for the load 

estimation) and it is long enough to encompass a sufficient 

number of samples and a full range of wet to dry periods. 

These center estimates have less uncertainty than the load 

estimates in the historical data base. 

Introduction 1 
2 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Purpose and Scope 

The purpose of this report is to describe and provide 

justification for a revised procedure implemented in the 

computation of nutrient and suspended-sediment loads at the 

Maryland River Input monitoring stations. The revised 

annual loads at the monitoring stations are also presented. 

This report limits its discussion to the load computations at 

the four Maryland River Input monitoring stations from 

1978 through 1999 (a shorter period for some constituents 

and sites), and provides only a brief description of the 

monitoring program and study area. 

Description of Study Area 

The watersheds that drain to the four Maryland River 

Input monitoring stations (fig. 1) include a wide range of 

land cover, geographic, and geologic regions. The 

watersheds encompass such diverse areas as the forested 

Appalachian Plateau of New York and Pennsylvania, the 

Valley and Ridge of the West Virginia highlands as well as 

the agricultural regions of Pennsylvania’s Lancaster County, 

the Shenandoah Valley of Virginia, and the flat lowlands of 

the Delmarva Peninsula. 

The Susquehanna River is monitored just downstream 

from Conowingo Dam, Md., and has a drainage area of 

27,100 mi2 (square miles). The Susquehanna is the largest 

tributary to the Chesapeake Bay and is the largest single 

source of total nitrogen and phosphorus (Darrell and others, 

1999). The Potomac River is the second largest contributor 

of streamflow to the Bay and the largest contributor of 

suspended sediment. The Potomac is monitored at 

Chain Bridge and has a drainage area of 11,570 mi2

. The 

Patuxent River is monitored near Bowie, Md., at 

Governor’s Bridge and has a drainage area at the monitoring 

station of 348 mi

2

. The Choptank River is the smallest 

tributary monitored in the River Input Program. The 

Choptank monitoring station is near Greensboro, Md., on the 

Delmarva Peninsula and has a contributing drainage area of 

112 mi2

. 

Acknowledgments 

The author gratefully acknowledges the many 

technicians who collected the base-flow and stormflow 

samples used in the load computations for this study. Much 

of these data were collected during long days, weekends, 

holidays, and during adverse weather conditions. Their 

dedication and expertise is much appreciated. A special 

thanks is extended to David Brower of the USGS, the 

hydrologic technician who currently collects samples for this 

project. The author would also like to thank Joel Blomquist 

and Timothy Cohn of USGS, who both helped develop the 

revised methodology, and Brenda Majedi of USGS, who 

provided much insight into the history of the River Input 

monitoring program. The specialists and data base 

personnel in the Maryland-Delaware-D.C. District office of 

the USGS are also thanked for their advice and support. 

Harry Post of the Occoquan Watershed Monitoring 

Laboratory and Christine Becker of the Metropolitan 

Washington Council of Governments are thanked for their 

monitoring efforts at the Potomac River at Chain Bridge. 

The cooperation of the Philadelphia Electric Company in 

the Susquehanna River sampling and gaging at 

Conowingo, Md., is also appreciated. 

Methods of Load Estimation 

A detailed description of the concentration data base 

used in the load computations is provided below. The 

regression model Estimator, the model that was used to 

compute loads for both the historical and revised 

methodologies, is also discussed. A description of the 

historical load data base is also provided. 

Monitoring Record 

Twelve nutrient and suspended-sediment constituents are 

monitored at the four River Input monitoring stations. These 

constituents are listed in table 1. Annual and monthly loads 

are estimated for all 12 of these constituents. The table also 

indicates how each constituent is reported and the parameter 

numbers used for each constituent in the USGS National 

Water Information System (NWIS) water-quality data base 

and, subsequently, in the load computation process. 

Table 1. Constituents monitored in the

 River Input Program

 [n/a, not applicable] 

Parameter Reported

Constituent number as 

Total nitrogen (TN) P00600 N 

Dissolved ammonia (NH3) P00608 N 

Dissolved nitrite (NO2) P00613 N 

Dissolved kjeldahl nitrogen (dissKN) P00623 N 

Total kjeldahl ntrogen (TKN) P00625 N 

Dissolved nitrite + nitrate (NO2 +NO3) P00631 N 

Total phosphorus (TP) P00665 P 

Dissolved phosphorus (dissP) P00666 P 

Orthophosphorus (o-PO4) P00671 P 

Total organic carbon (TOC) P00680 C 

Dissolved silica (SiO2) P00955 SiO2 

Suspended sediment P80154 n/a 

Methods of Load Estimation 3 
The Maryland River Input Program began in 1985. For 

this program, 15 to 30 samples per year are collected (during

both base-flow and stormflow conditions) using an equalwidth-increment, depth-integrated sampling procedure at the 

Susquehanna, the Patuxent, and the Choptank River sites. 

The Potomac River is monitored by MWCOG through the 

Occoquan Watershed Monitoring Laboratory (OWML). 

OWML uses an automatic sampler at the Potomac River site. 

Additionally, the River Input Program periodically collects

Potomac River storm samples using the equal-widthincrement, depth-integrated sampling procedure. Nutrient

and suspended-sediment data were collected by the USGS

National Water-Quality Assessment (NAWQA) Program for 

the Susquehanna and Potomac Rivers from 1993 to the 

present and at the Choptank River from 1987–90. 

Additional samples were collected by the USGS National

Stream-Quality Accounting Network (NASQAN) at all four

of the sites until the mid–1990’s (Darrell and others, 1999), 

and a study by Lang (1982) populated the data base from

1979 through 1981 for the Potomac and Susquehanna 

Rivers. 

Except for the data collected by OWML for the 

Potomac River at Chain Bridge, all of these data are the 

result of USGS collection and analysis, and constitute the 

concentration data set used to estimate loads at the four 

Maryland River Input monitoring stations. These data are 

available on the USGS River Input Program web site at 

http://www-va.usgs.gov/chesbay/RIMP/. 

The sampling record for the Susquehanna River at 

Conowingo Dam monitoring station began with the 1979 

water year. Figure 2a shows the sampling frequency at the 

dam for total phosphorus from 1978 through 1999. (Total

phosphorus represents one of the more frequently sampled 

constituents in the record.) The sampling frequencies for all 

constituents peaked between 1979 and 1981 during the study

by Lang (1982), were at a minimum or were not analyzed for 

4 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
from 1982 through 1984, and then increased steadily to 

comparatively high levels from 1987 through 1990. 

Samples were collected less frequently in 1991 and 1992 as 

the NASQAN Program was phased out. Sampling 

frequency increased again in 1993, when the NAWQA 

program entered a high-intensity phase in this river but then 

decreased to a minimal level as NAWQA passed into a lowintensity phase. At present (2000), 25 to 35 samples per year 

are collected at this site (depending upon hydrologic 

conditions). 

The sampling frequency for total phosphorus at the 

Potomac River station is shown in figure 2b. Sampling at 

this site was also initiated at the beginning of the 1979 water 

year, with an increasing frequency through 1981 for all 

12 constituents during the Lang (1982) study. Few waterquality data were collected in 1982 as Lang’s sampling 

program ended, but the sampling frequency for most 

constituents increased again in 1983 when MWCOG and 

OWML began water-quality monitoring at this site. OWML 

does not analyze samples for nitrite and suspended 

sediment—these data sets have been populated since 1985 

by only limited (usually monthly) USGS sampling. As a 

result of this limited data availability, estimates of nitrite and 

suspended-sediment loads have greater uncertainty (larger 

standard errors) than the other constituents. Currently, 

60–80 samples per year are collected at this site (depending 

upon hydrologic conditions). 

Sampling was relatively infrequent at the Patuxent River 

site from 1978–84 for most constituents (fig. 2c); only the 

NASQAN Program collected samples during this period. 

Maximum sampling frequency occurred after the River Input 

Program was initiated in 1985, and both the River Input and 

NASQAN Programs collected samples. The sampling 

frequency decreased through the early 1990’s for all 

constituents after the River Input Program became the only 

program collecting samples at this site. Currently, 20 to 

30 samples per year are collected at this site (depending 

upon hydrologic conditions). 

Relatively low frequency sampling also occurred at the 

Choptank River site from 1978 through 1984 for most 

constituents (fig. 2d). The sampling frequency was higher 

from 1985 through 1990 with both the River Input and 

NASQAN Programs sampling at this site. Sampling 

frequency at this site peaked during the high flow year of 

1989. Since 1990, the sampling frequency for all 

constituents has remained relatively constant with only the 

River Input Program sampling at this site. Currently, 20 to 

30 samples per year are collected at this site (depending 

upon hydrologic conditions). 

Regression Model 

A seven-parameter log-linear regression model is used to 

characterize concentration data and estimate annual and 

monthly loads of nutrients and suspended sediment at the 

River Input monitoring stations. This regression model 

(Estimator) incorporates a Minimum Variance Unbiased 

Estimator (MVUE) for correcting the bias induced by log 

transformation as well as an Adjusted Maximum Likelihood 

Estimator (AMLE) for data sets containing censored 

observations (Darrell and others, 1999). Descriptions of the 

Estimator model can be found in Darrell and others (1999) 

and Cohn and others (1992). The MVUE procedure is 

discussed in Gilroy and others (1990), Cohn and others 

(1989), and Bradu and Mundlak (1970). The AMLE 

procedure is discussed in Cohn and others (1992) and 

Cohn (1988). 

The load estimation procedure involves two primary 

steps—the fitting of the model to the concentration data 

using ordinary least-squares regression, and load estimation 

from the fit model (Darrell and others, 1999). The model has 

the following form: 

ln[C] = β0+ β1ln (Q/ ) Q + β2[ln(Q/ )]

2 + β3 Q (T-T ) + 

β4 (T- )2 + β5 sin (2πT) + β6 T cos (2πT) + ε , (1)

where 

ln [.] = natural logarithm function; 

C = constituent concentration in milligrams per 

liter (mg/L); 

β’s = model coefficients; 

Q = mean-daily streamflow in cubic feet per 

second (ft

3

/s); 

Q = centered streamflow; 

T = time, in decimal years; 

T = centered time; 

sin = sine function; 

cos = cosine function; and 

ε = independent, random error. 

Daily concentration and load estimates are computed 

using the MVUE algorithm and the daily loads are summed 

to provide the monthly and annual fluxes (Darrell and others, 

1999). A standard error of prediction is also generated to 

indicate how well the estimated regression model fits a 

“true” regression model. The standard error of prediction is 

a measure of the expected difference between an individual 

estimate and the estimated daily load (measured concentration multiplied by the average daily discharge). This 

standard error of prediction is a function of the variability 

between the estimated and computed loads only on the days 

of measurement; hence, the standard error will be biased if 

there is any systematic sampling bias in the concentration 

data. Also, the standard error may not account for the 

variability on days without monitoring or from any 

systematic bias incorporated into the source concentration 

data set (Robertson and Roerish, 1999). Possible sources of 

this bias include sampling techniques and designs that do not 

adequately represent the true environmental variability. 

Historical Load Estimates 

During the developmental period of these loadestimation procedures, it was known that the underlying 

principles of the model (the relation of concentration and 

Methods of Load Estimation 5 
streamflow) may change with time, and that it would be 

inappropriate to assume that this relation would be constant 

over long periods. On the basis of this observation and the 

amount of (monitoring) data available at that time, a 

moving-window approach to load estimation for the years 

following these initial efforts was implemented (T.A. Cohn, 

and J.D. Blomquist, USGS, oral and written communs., 

2000). A variation of this technique was employed and is 

described below. 

The historical load-estimation methodology is shown in 

figure 3. Each row represents a model calibration window 

and light-shaded boxes indicate years from which load 

estimates were recorded into a load data base. The first 

model was calibrated over an 11-year period, 1978 through 

1988, and loads were predicted for all years. During the 

following year, a 10-year model was calibrated for 1980 

through 1989, and the loads predicted for each constituent in 

1989, the last year of the regression, were entered into a data 

base with the 11 years of load estimates computed the year 

before. This 10-year moving window approach was used for 

6 years, for model windows 1980–89 to 1985–94, and the 

loads estimated during the last year of each of these windows 

were recorded in the data base. Starting with the 1985–95 

model window, each of the remaining Estimator models 

were calibrated using increasing window lengths, with 1985, 

the reference year for the Chesapeake Bay Program, as the 

base year for the model. This method was used in the 

1985–94 through the 1985–98 models. Loads computed in 

the last year of each of these calibration windows were 

entered into the load data base. For each constituent, the 

1978 through 1998 historical load data base comprises 

annual and monthly load estimates from eleven 10- to 

14-year model windows with the first window providing 

load estimates over its entire calibration window length and 

the remaining estimates extracted from the last year of 

calibration of each subsequent model. 

After differences between loads estimated throughout the 

1985–98 14-year models and loads in the data base were 

noted, a comparison between two overlapping but different 

regression window lengths was performed. Annual loads for 

11 constituents and all 4 Maryland River Input monitoring 

stations were computed for a 14-year, 1985–98 model 

window and compared to loads estimated using a 10-year, 

1989–98 model window. Annual loads were extracted from 

each group of models and percent differences were 

computed for the 11 constituents. The differences between 

the load predictions from the two window lengths normally 

fell within the error bars of the standard error of prediction, 

but the total phosphorus, suspended-sediment, ammonia, and 

orthophosphorus models were found to give load predictions 

that were up to 50 percent different, beyond the error bars of 

these models. The greatest differences in load estimates 

were found at the edges of the 10-year window, in 1989 and 

1998. These differences indicated that a more systematic 

load computation procedure was required. 

6 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Revised Procedure for Load Estimation 

The rationale for a more rigorous statistical approach in 

computing loads at the River Input monitoring stations is 

provided in this section. A description of the revised 

procedure and a comparison of annual loads are also 

included. 

Background 

The concentration-discharge relation for some 

constituents and rivers may change over time. The 

Estimator model does not currently include a term allowing 

such flexibility, however. The model does allow the 

concentration-discharge relation to vary with time, using the 

T and T 2 parameters, but does not allow the slope of the 

Q and Q2

 terms to change (β1 and β2 are constant), limiting 

the flexibility of the concentration-discharge relation over a 

given model window. 

After the model is fit to the raw concentration data within 

a calibration window, each daily computed concentration is 

defined by the constant, Q, and Q2 parameters. This relation 

is set by a specific decimal year value determining the T, T 2

, 

and seasonality parameters of the model—these parameters 

for any given day are constant. Thus, the concentration 

estimates for any given day are defined by the slope of the 

Q and Q2

 parameters. These slopes are the coefficients 

defined in the model fitting and are constant over a 

calibration window. A ramification of this constancy is that, 

for any given Estimator model, the concentration-discharge 

relation for any particular date cannot change. The T and T 2 

terms can shift the curve through the model window, but do 

not change the slope of the concentration-discharge relation. 

With these defined concentrations, the daily load estimates 

are computed using the MVUE algorithm (to account for the 

log-transformation bias) and summed and averaged to 

compute monthly and annual fluxes. 

Changes in the concentration-discharge relation (and 

time trends) can be caused by many things, including 

changes in land use, implementation of best management 

practices, increases in sewage-effluent discharges, improved 

sewage-treatment processes, and climatic variation and 

change. Long calibration windows have an increased chance 

of containing time periods with significantly different 

concentration-discharge relations. Crossing through 

multiple concentration-discharge periods, however, will 

decrease the quality of the model fit; therefore, it is 

important to minimize the model calibration window length. 

Analyses of samples collected by the River Input 

Program at the Patuxent River indicated a large decrease in 

the concentrations of most constituents from the middle-tolate 1980’s to the early 1990’s, which was probably due to 

the phosphorus detergent ban and sewage-treatment plant 

upgrades. Before these changes occurred, there was a strong 

inverse relation between concentration and discharge (as 

treatment-plant effluent was diluted during higher flows). 

This concentration-discharge relation likely changed, 

however, as urban and agricultural lands became more 

important sources of nutrients in this basin. Applying a 

single Estimator model to a period such as 1985 through 

1998 will force the model to fit a constant concentrationdischarge relation over a time period that has a varying 

relation. 

“Step” trends, or rapid changes in concentration of 

nutrients or sediment over time, can also present a problem 

for load estimation using the seven-parameter Estimator 

model—particularly when the model is calibrated over long 

periods. Trends in concentration over time are fit using both 

the T and T 2

 terms and offer a reasonable fit to gradual 

changes in concentration. These two model terms are 

orthogonal and fit temporal variations evenly over the entire 

calibration period. When a “step” change is encountered in 

the record, this shift is actually represented by a gradual 

curvilinear relation with time. The use of the T and T 2

 terms 

is a considerable improvement over a simple linear trend 

because together they can provide a more suitable fit to the 

observed data and better estimates of actual loads. The 

fitting of the T 2 term to a step trend, however, may cause 

inflation of estimates near the tails of the calibration 

window. For the River Input monitoring stations, the 

primary step trends are concentration and load decreases 

(from the mid 1980’s through the early 1990’s) that were 

most likely the result of the phosphorus detergent ban and 

incremental improvements in wastewater-treatment plants. 

The effect of these trends on the estimated load is of primary 

concern. Errors in load estimates are most likely to occur 

when a step trend occurs early in a long calibration window. 

In this case, loads are somewhat underestimated in early 

years and overestimated in later years. 

Several approaches to load estimation are possible where 

step trends are indicated. The method chosen for this 

analysis shortens the calibration window in order to 

minimize the error in estimates at the tail of the calibration 

period. This method is convenient because it can be 

standardized and can be applied without prior knowledge of 

changes in watershed management practices. A second 

method attempts to identify step trends and calibrate separate 

models for pre- and post-step trend periods. This method 

requires considerable knowledge of the forcing and timing of 

the step trends. In most cases observed to date, the precise 

timing of the response to management actions is unclear, and 

may actually occur over a period of months to years. It 

appears that phosphorus concentrations decreased significantly as a result of the phosphorus detergent ban for 

example; however, concentrations at some sites continued to 

decline for more than a year following the ban. A third 

method develops additional variables and statistical tools 

using the current AMLE/MVUE framework. Such tools 

may fit step trends using separate model parameters 

(intercept terms) that are determined statistically rather than 

operationally. Such tools should be investigated further as 

the period of water-quality record at the River Input 

monitoring stations expands to several decades. 

Multiple projects within the USGS have collected 

samples at the River Input monitoring stations and added 

Revised Procedure for Load Estimation 7 
data to the record. As a result, the sampling frequency 

throughout the period of record has varied considerably 

(fig. 2). For the Susquehanna, Patuxent, and Choptank sites, 

the sampling frequency reached a maximum in the late 

1980’s, by as much as two to three times the more recent 

sampling frequency, thus placing inappropriate weight upon 

these years in the load estimates from a single Estimator 

model. A moving calibration window gradually decreases 

and eventually eliminates the weight that these early 

measurements have on the load estimates. 

Two major conclusions can be drawn from this 

discussion. The first is that load estimates near the tail of a 

calibration window have the greatest uncertainty. As a 

result, the end-of-model-window estimates that have 

traditionally been provided to the Chesapeake Bay scientific 

community by the program are not preferred. The best load 

estimates for any individual year—those with minimum 

uncertainty—are those computed in the center of the 

regression. The second major point is that the model 

window length needs to be minimized. A 9-year window is 

considered optimal because the calibration data are nearly 

symmetrically distributed around the year for which 

estimated loads are needed while still providing sufficient 

length to encompass a reasonable number of samples and a 

full range of wet to dry periods. 

Description of Revised Methods 

The (revised) method consists of running multiple 9-year 

calibration windows for each site and constituent. An 

illustration of the procedure used to compute the revised 

loads is provided in figure 4. Each row represents a model 

window and the light-shaded boxes indicate the years in each 

calibration window from which load estimates were taken 

and entered into a data base. For each constituent and site, 

up to 14 models were run. Load estimates were taken from 

the fifth (center) year of the calibration window and entered 

into a data base. On the basis of the USGS concentration 

data base and the seven parameters in the model, these 

estimates will have the least uncertainty. 

The first 4 years of estimates are in a tail of the 

calibration window and have greater uncertainty, but are all 

the data allow. Higher standard errors are usually observed 

during these first few years of estimates. 

The last 4 years of each model window are also in a tail 

of the calibration window, have greater uncertainty, and are 

considered preliminary and revised each year. These 

estimates are marked by dark-shaded boxes in figure 4. 

When the last four years of load estimates are revised each 

year, the greatest change can be expected for the last year of 

estimation from the previous year. For example, the load 

estimates from the last four years of the 1990–98 model 

8 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
windows (1995 through 1998) were compared to the loads 

predicted for these years in the 1991–99 model window. For 

suspended sediment in the Choptank River, the percent 

differences between the 1999 and 1998 model estimates 

were calculated to be -25, -16, 6, and -0.4 percent for 1998 

through 1995, respectively. Likewise, for dissolved nitrite 

plus nitrate in the Patuxent River, percent differences were 

found to be 12, 6, 0.9, and 0.3 percent, respectively. This 

pattern supports the premise that the last year of load 

estimates in any calibration window has the greatest 

uncertainty. 

Comparison of Historical and Revised Load Estimates 

The annual load estimates for the four Maryland River 

Input monitoring stations are provided in Appendix A. The 

most up-to-date annual and monthly estimates can be 

accessed on the USGS River Input Program web site at 

http://www-va.usgs.gov/chesbay/RIMP/. A comparison 

between the historical and revised load estimates is provided 

in Appendix B. This table lists the percent differences 

between historical and revised annual load estimates for the 

four River Input monitoring sites and all available 

constituents. 

The historical and revised load estimates and error bars 

for a few selected constituents are shown in figure 5. These 

plots represent the full range of differences in estimates as 

well as both narrow and wide error bars. As discussed 

earlier in this report, these bars indicate the error in the 

regression, but do not represent errors associated with 

sampling and laboratory analysis. 

In general, it was found that the annual load estimates of 

total nitrogen at all four sites do not change considerably 

between methodologies. As an example, figure 5a shows a 

comparison of the revised and historical load estimates for 

total nitrogen (TN) at the Patuxent River near Bowie, Md., 

monitoring station. The solid point and error bars represent 

the load value and standard error of the revised annual load 

estimate. The error bars of the two sets of estimates usually 

overlap for this site and constituent despite the relatively 

narrow bar width. For 1993 to 1995, however, the revised 

method predicts loads with error bars that do not overlap the 

error bars of the historical values. The load estimates for 

these 3 years were 10 to 12 percent different. The error bars 

also decreased in size from 1978 to 1984 as estimates are 

taken progressively closer to the model centers and include 

additional data points. Finally, the Patuxent River has shown 

significant reductions in concentrations of most constituents 

during the study period, due predominantly to sewagetreatment plant upgrades. The result of this point-source 

reduction effort can be seen in this plot as a downward trend 

in annual total nitrogen load estimates. 

Between methodologies, estimates of annual loads of 

total kjeldahl nitrogen (TKN) typically show larger 

differences than estimates for TN. Figure 5b shows a 

comparison of the load estimates and error bars for TKN in 

the Choptank River. Differences between annual load 

estimates for TKN in the Choptank ranged from less than 1 

to nearly 23 percent (see Appendix B). This consistency is 

typical at all four of the monitoring sites for this constituent. 

The error bars for TKN tend to be wide and overlap for all 

but two of the years. The error bars for the updated 

estimates become finer through the record then widen 

slightly during the last four years of estimates (the 

preliminary, non-window-centered estimates). 

The two methodologies estimated orthophosphorus 

(o-PO4) loads with the greatest differences among the 

12 constituents. Figure 5c shows the load estimates and 

error bars for o-PO4 at the Susquehanna River at Conwingo 

Dam monitoring station. Differences between the load 

estimates vary from 3 to 77 percent, with the greatest 

differences in 1994 and 1995. The error bars normally 

overlap, but the variability may indicate limitations of this 

model in predicting o-PO4 loads, especially o-PO4 loads 

discharging from the Susquehanna River at Conowingo 

Dam. 

Figure 5d shows a comparison of suspended-sediment 

load estimates for the Potomac River at Chain Bridge 

monitoring station. This plot shows the large error bars that 

are typically computed for suspended-sediment estimates. 

The largest estimate and bracket size was in 1985, a high 

discharge year. Suspended-sediment concentrations vary a 

great deal in any river and a large number of samples are 

required for accurate estimates of load. The current 

sampling program includes relatively few suspendedsediment samples from the Potomac River at Chain Bridge. 

Considering that the Potomac River has been shown to 

provide the largest flux of sediment to the Chesapeake Bay 

(Darrell and others, 1999), these large error bars are cause 

for concern. 

Revised Procedure for Load Estimation 9 
10 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Summary and Conclusions 

The U.S. Geological Survey Chesapeake Bay River Input 

monitoring program has updated its data base for nutrient 

and suspended-sediment loads using an optimized 

methodology implementing the same seven-parameter loglinear regression model that has historically been used in the 

project. This approach, which uses a multiple-window, 

center-estimate methodology, was applied to the monitoring 

record of the River Input monitoring stations on the 

Susquehanna, Potomac, Patuxent, and Choptank Rivers. 

These four rivers have a wide range of watershed 

characteristics and include both the largest and smallest 

watersheds monitored in the program. Concentrations of 

12 nutrient and suspended-sediment constituents were 

included in the analysis and annual and monthly load 

estimates were revised. 

The regression model Estimator was used to compute 

loads in both the historical and revised methodologies—the 

new method merely optimizes the use of this model. The 

historical load data base is composed of annual and monthly 

load estimates from eleven 10- to 14-year model windows 

with the first window providing load estimates over its entire 

calibration window length, and the remaining estimates 

extracted from the last year of calibration of each subsequent 

model. The revised methodology consists of multiple 9-year 

calibration windows. For each site and constituent, up to 

14 models were computed. Load estimates were taken from 

the fifth (center) year of each calibration window, and 

entered into a data base. The final four years of load 

estimates are preliminary and will be updated each year. A 

new Estimator model is run annually for each site and 

constituent, the most recent year is added, and the previous 

four years of load estimates are revised. The fifth year is 

considered the best estimate and kept in the data base. The 

resulting data base is populated by load estimates that are 

known with more certainty than the historical load estimates. 

The new procedure was developed to minimize the 

effects that changing concentration-discharge relations in a 

river have on the regression. Additionally, the new 

methodology eliminates the use of end-of-regression load 

estimates, which have been found to be overly sensitive to 

the fit of the time and time squared parameters, and 

minimizes the effect of changing sampling frequencies in the 

monitoring record. 

It has been shown that the last year of estimates 

undergoes the most change from a previous year, and this 

change usually decreases until made “final” at the fifth year. 

This finding is consistent with the premise that the last year 

of load estimates in any calibration window has the greatest 

uncertainty. 

A comparison between all of the historical and revised 

annual load estimates showed variable consistency. The 

estimates for total nitrogen at all four sites are typically 

consistent and are within the error bars. Total kjeldahl 

nitrogen and nitrite plus nitrate are also reasonably 

consistent. Annual load estimates for suspended sediment, 

the three phosphorus species, and ammonia have larger 

differences but these constituents also have wider error bars 

associated with them. In general, for some select years and 

monitoring stations, the differences between load estimates 

are relatively large and are beyond the error bars, but this is 

not common—load estimates for these rivers do not change 

drastically when the new methodology is applied. 

This revised load computation methodology provides 

greater confidence in load estimates at the River Input 

Program monitoring stations. Load estimation at these 

monitoring stations is an evolving process, however. This 

report documents various processes and shifting relations 

during the study period. Load-estimation procedures will 

likely change in the future as the in-stream processes of these 

diverse rivers are more clearly understood, and new 

statistical tools are implemented. 

References Cited 

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

lognormal linear models: Journal of the American 

Statistical Association, 65 (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. 2,353–2,364. 

Darrell, L.C., Majedi, B.F., Lizarraga, J.S., and 

Blomquist, J.D., 1999, Nutrient and suspendedsediment concentrations, trends, loads, and yields from 

the nontidal part of the Susquehanna, Potomac, Patuxent 

and Choptank Rivers, 1985–96: U.S. Geological Survey 

Water-Resources Investigations Report 98–4177, 38 p. 

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

square error of regression-based constituent transport 

estimates: Water Resources Research, v. 26, no. 9, 

p. 2,069–2,077. 

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

to the Chesapeake Bay, the Susquehanna, Potomac, and 

James Rivers, January 1979–April 1981: U.S. 

Geological Survey Water-Resources Investigations 

Report 82–32, 64 p. 

Robertson D.M., and Roerish, E.D., 1999, Influence of 

various water quality sampling strategies on load 

estimates for small streams: Water Resources Research, 

v. 35, no. 12, December, p. 3,747–3,759. 

Summary and Conclusions 11 
APPENDIXES A AND B FOLLOW

12 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers 

[kg/yr, kilograms per year; +/-, plus/minus; the standard errors of prediction are also provided. The standard error of prediction is a 

measure of the expected difference between an individual estimate and the estimated daily load. Bold numbers indicate preliminary 

load estimates that will be revised. For the most up-to-date annual and monthly load estimates, see the USGS River Input 

Monitoring Program web site at http://www-va.usgs.gov/chesbay/RIMP] 

Parameter Constituent Reported as

 P00600 Total nitrogen N

 P00608 Dissolved ammonia N

 P00613 Dissolved nitrite N

 P00623 Dissolved kjeldahl nitrogen N 

P00625 Total kjeldahl nitrogen N

 P00631 Dissolved nitrite + nitrate N

 P00665 Total phosphorus P

 P00666 Dissolved phosphorus P

 P00671 Orthophosphorus P 

P00680 Total organic carbon C

 P00955 Dissolved silica Si and SiO2

 P80154 Suspended sediment not applicable 

Appendix A 13 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310) 

01578310 P00600 1979 76,200,000 3,210,000 4 79 - 87 

01578310 P00600 1980 43,400,000 1,560,000 4 79 - 87 

01578310 P00600 1981 50,400,000 1,880,000 4 79 - 87 

01578310 P00600 1982 60,200,000 2,480,000 4 79 - 87 

01578310 P00600 1983 77,300,000 3,400,000 4 79 - 87 

01578310 P00600 1984 94,500,000 4,070,000 4 80 - 88 

01578310 P00600 1985 58,300,000 2,190,000 4 81 - 89 

01578310 P00600 1986 75,700,000 2,360,000 3 82 - 90 

01578310 P00600 1987 57,200,000 1,580,000 3 83 - 91 

01578310 P00600 1988 46,700,000 1,360,000 3 84 - 92 

01578310 P00600 1989 64,000,000 1,830,000 3 85 - 93 

01578310 P00600 1990 78,300,000 2,210,000 3 86 - 94 

01578310 P00600 1991 48,200,000 1,540,000 3 87 - 95 

01578310 P00600 1992 54,100,000 1,470,000 3 88 - 96 

01578310 P00600 1993 82,700,000 2,330,000 3 89 - 97 

01578310 P00600 1994 76,100,000 2,060,000 3 90 - 98 

01578310 P00600 1995 41,300,000 1,460,000 4 91 - 99 

01578310 P00600 1996 94,500,000 2,810,000 3 91 - 99 

01578310 P00600 1997 43,400,000 1,430,000 3 91 - 99 

01578310 P00600 1998 61,300,000 2,160,000 4 91 - 99 

01578310 P00600 1999 40,200,000 1,910,000 5 91 - 99 

01578310 P00608 1979 3,320,000 470,000 14 79 - 87 

01578310 P00608 1980 2,450,000 276,000 11 79 - 87 

01578310 P00608 1981 3,160,000 345,000 11 79 - 87 

01578310 P00608 1982 4,150,000 503,000 12 79 - 87 

01578310 P00608 1983 5,490,000 719,000 13 79 - 87 

01578310 P00608 1984 5,980,000 739,000 12 80 - 88 

01578310 P00608 1985 3,210,000 315,000 10 81 - 89 

01578310 P00608 1986 3,970,000 346,000 9 82 - 90 

01578310 P00608 1987 3,010,000 237,000 8 83 - 91 

01578310 P00608 1988 2,470,000 190,000 8 84 - 92 

01578310 P00608 1989 3,360,000 256,000 8 85 - 93 

01578310 P00608 1990 3,570,000 243,000 7 86 - 94 

01578310 P00608 1991 2,520,000 201,000 8 87 - 95 

01578310 P00608 1992 2,540,000 197,000 8 88 - 96 

01578310 P00608 1993 4,420,000 362,000 8 89 - 97 

14 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)—Continued 

01578310 P00608 1994 3,610,000 283,000 8 90 - 98 

01578310 P00608 1995 1,660,000 151,000 9 91 - 99 

01578310 P00608 1996 3,820,000 325,000 9 91 - 99 

01578310 P00608 1997 1,710,000 148,000 9 91 - 99 

01578310 P00608 1998 2,570,000 247,000 10 91 - 99 

01578310 P00608 1999 1,590,000 202,000 13 91 - 99 

01578310 P00613 1985 661,000 74,900 11 85 - 93 

01578310 P00613 1986 781,000 62,100 8 85 - 93 

01578310 P00613 1987 677,000 44,700 7 85 - 93 

01578310 P00613 1988 592,000 41,400 7 85 - 93 

01578310 P00613 1989 774,000 58,500 8 85 - 93 

01578310 P00613 1990 806,000 58,800 7 86 - 94 

01578310 P00613 1991 554,000 47,900 9 87 - 95 

01578310 P00613 1992 668,000 60,400 9 88 - 96 

01578310 P00613 1993 694,000 62,000 9 89 - 97 

01578310 P00613 1994 649,000 65,800 10 90 - 98 

01578310 P00613 1995 407,000 49,100 12 91 - 99 

01578310 P00613 1996 604,000 64,300 11 91 - 99 

01578310 P00613 1997 414,000 45,800 11 91 - 99 

01578310 P00613 1998 517,000 64,700 13 91 - 99 

01578310 P00613 1999 479,000 85,200 18 91 - 99 

01578310 P00623 1985 14,700,000 1,200,000 8 85 - 93 

01578310 P00623 1986 19,300,000 1,170,000 6 85 - 93 

01578310 P00623 1987 13,800,000 662,000 5 85 - 93 

01578310 P00623 1988 10,800,000 529,000 5 85 - 93 

01578310 P00623 1989 14,200,000 738,000 5 85 - 93 

01578310 P00623 1990 14,500,000 722,000 5 86 - 94 

01578310 P00623 1991 8,000,000 470,000 6 87 - 95 

01578310 P00623 1992 8,050,000 407,000 5 88 - 96 

01578310 P00623 1993 11,900,000 682,000 6 89 - 97 

01578310 P00623 1994 9,770,000 552,000 6 90 - 98 

01578310 P00623 1995 4,730,000 319,000 7 91 - 99 

01578310 P00623 1996 12,100,000 740,000 6 91 - 99 

01578310 P00623 1997 5,010,000 310,000 6 91 - 99 

01578310 P00623 1998 7,740,000 530,000 7 91 - 99 

01578310 P00623 1999 5,350,000 491,000 9 91 - 99 

Appendix A 15 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)—Continued 

01578310 P00625 1979 23,100,000 1,870,000 8 79 - 87 

01578310 P00625 1980 14,000,000 984,000 7 79 - 87 

01578310 P00625 1981 17,100,000 1,230,000 7 79 - 87 

01578310 P00625 1982 21,200,000 1,680,000 8 79 - 87 

01578310 P00625 1983 28,900,000 2,470,000 9 79 - 87 

01578310 P00625 1984 41,100,000 3,280,000 8 80 - 88 

01578310 P00625 1985 21,400,000 1,350,000 6 81 - 89 

01578310 P00625 1986 26,200,000 1,450,000 6 82 - 90 

01578310 P00625 1987 18,300,000 912,000 5 83 - 91 

01578310 P00625 1988 14,200,000 709,000 5 84 - 92 

01578310 P00625 1989 19,700,000 1,050,000 5 85 - 93 

01578310 P00625 1990 20,100,000 1,020,000 5 86 - 94 

01578310 P00625 1991 10,700,000 610,000 6 87 - 95 

01578310 P00625 1992 11,600,000 540,000 5 88 - 96 

01578310 P00625 1993 19,900,000 1,100,000 6 89 - 97 

01578310 P00625 1994 16,300,000 831,000 5 90 - 98 

01578310 P00625 1995 7,230,000 439,000 6 91 - 99 

01578310 P00625 1996 21,700,000 1,290,000 6 91 - 99 

01578310 P00625 1997 8,030,000 453,000 6 91 - 99 

01578310 P00625 1998 13,800,000 902,000 7 91 - 99 

01578310 P00625 1999 9,370,000 814,000 9 91 - 99 

01578310 P00631 1979 54,500,000 2,580,000 5 79 - 87 

01578310 P00631 1980 30,200,000 1,230,000 4 79 - 87 

01578310 P00631 1981 34,400,000 1,420,000 4 79 - 87 

01578310 P00631 1982 40,200,000 1,830,000 5 79 - 87 

01578310 P00631 1983 49,500,000 2,390,000 5 79 - 87 

01578310 P00631 1984 53,700,000 2,510,000 5 80 - 88 

01578310 P00631 1985 35,900,000 1,570,000 4 81 - 89 

01578310 P00631 1986 48,600,000 1,840,000 4 82 - 90 

01578310 P00631 1987 38,400,000 1,280,000 3 83 - 91 

01578310 P00631 1988 32,600,000 1,140,000 3 84 - 92 

01578310 P00631 1989 44,600,000 1,470,000 3 85 - 93 

01578310 P00631 1990 58,900,000 1,830,000 3 86 - 94 

01578310 P00631 1991 38,300,000 1,310,000 3 87 - 95 

01578310 P00631 1992 42,300,000 1,240,000 3 88 - 96 

01578310 P00631 1993 61,600,000 1,790,000 3 89 - 97 

01578310 P00631 1994 59,100,000 1,640,000 3 90 - 98 

16 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)—Continued 

01578310 P00631 1995 35,400,000 1,340,000 4 91 - 99 

01578310 P00631 1996 74,000,000 2,350,000 3 91 - 99 

01578310 P00631 1997 35,800,000 1,280,000 4 91 - 99 

01578310 P00631 1998 46,400,000 1,810,000 4 91 - 99 

01578310 P00631 1999 29,500,000 1,560,000 5 91 - 99 

01578310 P00665 1979 4,480,000 551,000 12 79 - 87 

01578310 P00665 1980 1,960,000 195,000 10 79 - 87 

01578310 P00665 1981 2,050,000 230,000 11 79 - 87 

01578310 P00665 1982 2,150,000 220,000 10 79 - 87 

01578310 P00665 1983 2,980,000 353,000 12 79 - 87 

01578310 P00665 1984 3,490,000 424,000 12 80 - 88 

01578310 P00665 1985 1,440,000 127,000 9 81 - 89 

01578310 P00665 1986 2,110,000 178,000 8 82 - 90 

01578310 P00665 1987 1,460,000 104,000 7 83 - 91 

01578310 P00665 1988 1,170,000 86,500 7 84 - 92 

01578310 P00665 1989 1,920,000 155,000 8 85 - 93 

01578310 P00665 1990 2,210,000 172,000 8 86 - 94 

01578310 P00665 1991 1,140,000 95,700 8 87 - 95 

01578310 P00665 1992 1,130,000 83,900 7 88 - 96 

01578310 P00665 1993 3,330,000 380,000 11 89 - 97 

01578310 P00665 1994 2,470,000 256,000 10 90 - 98 

01578310 P00665 1995 840,000 86,300 10 91 - 99 

01578310 P00665 1996 4,160,000 555,000 13 91 - 99 

01578310 P00665 1997 870,000 78,700 9 91 - 99 

01578310 P00665 1998 1,920,000 231,000 12 91 - 99 

01578310 P00665 1999 1,060,000 152,000 14 91 - 99 

01578310 P00666 1979 1,200,000 135,000 11 79 - 87 

01578310 P00666 1980 524,000 50,600 10 79 - 87 

01578310 P00666 1981 558,000 56,700 10 79 - 87 

01578310 P00666 1982 571,000 62,500 11 79 - 87 

01578310 P00666 1983 680,000 80,200 12 79 - 87 

01578310 P00666 1984 764,000 87,200 11 80 - 88 

01578310 P00666 1985 495,000 52,900 11 81 - 89 

01578310 P00666 1986 660,000 65,800 10 82 - 90 

01578310 P00666 1987 471,000 42,600 9 83 - 91 

01578310 P00666 1988 345,000 31,700 9 84 - 92 

Appendix A 17 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)—Continued 

01578310 P00666 1989 489,000 42,600 9 85 - 93 

01578310 P00666 1990 697,000 61,900 9 86 - 94 

01578310 P00666 1991 323,000 30,000 9 87 - 95 

01578310 P00666 1992 425,000 34,400 8 88 - 96 

01578310 P00666 1993 698,000 57,200 8 89 - 97 

01578310 P00666 1994 675,000 58,700 9 90 - 98 

01578310 P00666 1995 350,000 34,300 10 91 - 99 

01578310 P00666 1996 994,000 87,700 9 91 - 99 

01578310 P00666 1997 370,000 33,800 9 91 - 99 

01578310 P00666 1998 552,000 57,200 10 91 - 99 

01578310 P00666 1999 350,000 48,200 14 91 - 99 

01578310 P00671 1979 1,070,000 166,000 16 79 - 87 

01578310 P00671 1980 478,000 65,100 14 79 - 87 

01578310 P00671 1981 524,000 73,200 14 79 - 87 

01578310 P00671 1982 544,000 81,500 15 79 - 87 

01578310 P00671 1983 617,000 96,900 16 79 - 87 

01578310 P00671 1984 646,000 103,000 16 80 - 88 

01578310 P00671 1985 348,000 51,300 15 81 - 89 

01578310 P00671 1986 432,000 56,700 13 82 - 90 

01578310 P00671 1987 337,000 42,900 13 83 - 91 

01578310 P00671 1988 222,000 29,600 13 84 - 92 

01578310 P00671 1989 291,000 37,000 13 85 - 93 

01578310 P00671 1990 425,000 55,500 13 86 - 94 

01578310 P00671 1991 202,000 30,800 15 87 - 95 

01578310 P00671 1992 260,000 34,300 13 88 - 96 

01578310 P00671 1993 424,000 55,500 13 89 - 97 

01578310 P00671 1994 404,000 55,300 14 90 - 98 

01578310 P00671 1995 255,000 43,800 17 91 - 99 

01578310 P00671 1996 669,000 103,000 15 91 - 99 

01578310 P00671 1997 259,000 42,000 16 91 - 99 

01578310 P00671 1998 365,000 65,800 18 91 - 99 

01578310 P00671 1999 180,000 41,200 23 91 - 99 

01578310 P00680 1985 92,400,000 4,900,000 5 85 - 93 

01578310 P00680 1986 135,000,000 5,830,000 4 85 - 93 

01578310 P00680 1987 93,900,000 3,170,000 3 85 - 93 

01578310 P00680 1988 75,900,000 2,630,000 3 85 - 93 

18 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)—Continued 

01578310 P00680 1989 134,000,000 5,180,000 4 85 - 93 

01578310 P00680 1990 149,000,000 5,320,000 4 86 - 94 

01578310 P00680 1991 80,400,000 2,660,000 3 87 - 95 

01578310 P00680 1992 102,000,000 2,970,000 3 88 - 96 

01578310 P00680 1993 218,000,000 8,000,000 4 89 - 97 

01578310 P00680 1994 198,000,000 6,730,000 3 90 - 98 

01578310 P00680 1995 83,600,000 3,260,000 4 91 - 99 

01578310 P00680 1996 267,000,000 10,600,000 4 91 - 99 

01578310 P00680 1997 85,300,000 3,110,000 4 91 - 99 

01578310 P00680 1998 140,000,000 5,980,000 4 91 - 99 

01578310 P00680 1999 76,300,000 4,300,000 6 91 - 99 

Appendix A 19 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr SiO2) (kg/yr Si) (kg/yr SiO2) Percent window 

Susquehanna River at Conowingo Dam (01578310)–Continued 

01578310 P00955 1979 194,000,000 90,700,000 16,800,000 9 79 - 87 

01578310 P00955 1980 87,600,000 41,000,000 6,580,000 8 79 - 87 

01578310 P00955 1981 89,400,000 41,800,000 6,830,000 8 79 - 87 

01578310 P00955 1982 100,000,000 47,000,000 7,910,000 8 79 - 87 

01578310 P00955 1983 130,000,000 60,900,000 11,000,000 8 79 - 87 

01578310 P00955 1984 175,000,000 81,700,000 15,300,000 9 80 - 88 

01578310 P00955 1985 107,000,000 50,200,000 8,200,000 8 81 - 89 

01578310 P00955 1986 160,000,000 74,900,000 11,600,000 7 82 - 90 

01578310 P00955 1987 112,000,000 52,300,000 6,750,000 6 83 - 91 

01578310 P00955 1988 93,700,000 43,800,000 5,860,000 6 84 - 92 

01578310 P00955 1989 139,000,000 64,900,000 8,290,000 6 85 - 93 

01578310 P00955 1990 189,000,000 88,200,000 11,600,000 6 86 - 94 

01578310 P00955 1991 101,000,000 47,200,000 7,380,000 7 87 - 95 

01578310 P00955 1992 116,000,000 54,100,000 7,280,000 6 88 - 96 

01578310 P00955 1993 204,000,000 95,500,000 15,200,000 7 89 - 97 

01578310 P00955 1994 186,000,000 87,100,000 12,900,000 7 90 - 98 

01578310 P00955 1995 94,500,000 44,200,000 8,110,000 9 91 - 99 

01578310 P00955 1996 259,000,000 121,000,000 20,400,000 8 91 - 99 

01578310 P00955 1997 91,600,000 42,800,000 7,470,000 8 91 - 99 

01578310 P00955 1998 135,000,000 63,300,000 12,600,000 9 91 - 99 

01578310 P00955 1999 72,600,000 33,900,000 8,920,000 12 91 - 99 

20 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Susquehanna River at Conowingo Dam (01578310)–Continued 

01578310 P80154 1979 2,700,000,000 458,000,000 17 79 - 87 

01578310 P80154 1980 878,000,000 95,500,000 11 79 - 87 

01578310 P80154 1981 942,000,000 140,000,000 15 79 - 87 

01578310 P80154 1982 938,000,000 107,000,000 11 79 - 87 

01578310 P80154 1983 1,730,000,000 236,000,000 14 79 - 87 

01578310 P80154 1984 2,850,000,000 467,000,000 16 80 - 88 

01578310 P80154 1985 516,000,000 40,700,000 8 81 - 89 

01578310 P80154 1986 1,110,000,000 122,000,000 11 82 - 90 

01578310 P80154 1987 535,000,000 43,400,000 8 83 - 91 

01578310 P80154 1988 397,000,000 30,100,000 8 84 - 92 

01578310 P80154 1989 942,000,000 67,700,000 7 85 - 93 

01578310 P80154 1990 861,000,000 49,800,000 6 86 - 94 

01578310 P80154 1991 419,000,000 27,400,000 7 87 - 95 

01578310 P80154 1992 422,000,000 25,300,000 6 88 - 96 

01578310 P80154 1993 3,080,000,000 340,000,000 11 89 - 97 

01578310 P80154 1994 1,970,000,000 192,000,000 10 90 - 98 

01578310 P80154 1995 397,000,000 34,600,000 9 91 - 99 

01578310 P80154 1996 3,680,000,000 584,000,000 16 91 - 99 

01578310 P80154 1997 364,000,000 26,400,000 7 91 - 99 

01578310 P80154 1998 1,110,000,000 123,000,000 11 91 - 99 

01578310 P80154 1999 349,000,000 44,600,000 13 91 - 99 

Appendix A 21 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580) 

01646580 P00600 1979 41,100,000 2,220,000 5 79 - 87 

01646580 P00600 1980 20,200,000 812,000 4 79 - 87 

01646580 P00600 1981 10,600,000 386,000 4 79 - 87 

01646580 P00600 1982 22,200,000 842,000 4 79 - 87 

01646580 P00600 1983 34,200,000 1,340,000 4 79 - 87 

01646580 P00600 1984 40,300,000 1,780,000 4 80 - 88 

01646580 P00600 1985 27,700,000 1,500,000 5 81 - 89 

01646580 P00600 1986 16,400,000 622,000 4 82 - 90 

01646580 P00600 1987 23,200,000 792,000 3 83 - 91 

01646580 P00600 1988 17,200,000 657,000 4 84 - 92 

01646580 P00600 1989 23,500,000 711,000 3 85 - 93 

01646580 P00600 1990 19,400,000 475,000 2 86 - 94 

01646580 P00600 1991 16,600,000 503,000 3 87 - 95 

01646580 P00600 1992 17,700,000 496,000 3 88 - 96 

01646580 P00600 1993 36,700,000 1,200,000 3 89 - 97 

01646580 P00600 1994 33,900,000 992,000 3 90 - 98 

01646580 P00600 1995 17,800,000 443,000 2 91 - 99 

01646580 P00600 1996 60,900,000 1,580,000 3 91 - 99 

01646580 P00600 1997 18,700,000 469,000 3 91 - 99 

01646580 P00600 1998 34,700,000 993,000 3 91 - 99 

01646580 P00600 1999 9,770,000 284,000 3 91 - 99 

01646580 P00608 1979 1,480,000 352,000 24 79 - 87 

01646580 P00608 1980 686,000 104,000 15 79 - 87 

01646580 P00608 1981 353,000 46,800 13 79 - 87 

01646580 P00608 1982 728,000 104,000 14 79 - 87 

01646580 P00608 1983 1,080,000 165,000 15 79 - 87 

01646580 P00608 1984 1,320,000 240,000 20 80 - 88 

01646580 P00608 1985 863,000 322,000 37 81 - 89 

01646580 P00608 1986 375,000 79,700 21 82 - 90 

01646580 P00608 1987 586,000 125,000 21 83 - 91 

01646580 P00608 1988 473,000 115,000 24 84 - 92 

01646580 P00608 1989 484,000 73,100 15 85 - 93 

01646580 P00608 1990 323,000 51,400 16 86 - 94 

01646580 P00608 1991 288,000 54,000 19 87 - 95 

01646580 P00608 1992 290,000 44,300 15 88 - 96 

01646580 P00608 1993 829,000 156,000 19 89 - 97 

22 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00608 1994 736,000 114,000 16 90 - 98 

01646580 P00608 1995 261,000 28,900 11 91 - 99 

01646580 P00608 1996 1,470,000 263,000 18 91 - 99 

01646580 P00608 1997 359,000 43,300 12 91 - 99 

01646580 P00608 1998 1,040,000 138,000 13 91 - 99 

01646580 P00608 1999 247,000 29,800 12 91 - 99 

01646580 P00613 1985 135,000 24,400 18 85 - 93 

01646580 P00613 1986 110,000 15,300 14 85 - 93 

01646580 P00613 1987 157,000 18,600 12 85 - 93 

01646580 P00613 1988 120,000 15,100 13 85 - 93 

01646580 P00613 1989 154,000 18,500 12 85 - 93 

01646580 P00613 1990 119,000 12,700 11 86 - 94 

01646580 P00613 1991 123,000 15,600 13 87 - 95 

01646580 P00613 1992 111,000 9,640 9 88 - 96 

01646580 P00613 1993 263,000 28,600 11 89 - 97 

01646580 P00613 1994 245,000 23,600 10 90 - 98 

01646580 P00613 1995 134,000 11,800 9 91 - 99 

01646580 P00613 1996 467,000 41,100 9 91 - 99 

01646580 P00613 1997 147,000 12,600 9 91 - 99 

01646580 P00613 1998 278,000 270,000 10 91 - 99 

01646580 P00613 1999 82,100 9,260 11 91 - 99 

01646580 P00623 1983 7,490,000 575,000 8 83 - 91 

01646580 P00623 1984 8,180,000 549,000 7 83 - 91 

01646580 P00623 1985 5,150,000 431,000 8 83 - 91 

01646580 P00623 1986 3,090,000 190,000 6 83 - 91 

01646580 P00623 1987 4,230,000 271,000 6 83 - 91 

01646580 P00623 1988 3,030,000 223,000 7 84 - 92 

01646580 P00623 1989 3,590,000 233,000 6 85 - 93 

01646580 P00623 1990 2,550,000 149,000 6 86 - 94 

01646580 P00623 1991 1,860,000 151,000 8 87 - 95 

01646580 P00623 1992 2,020,000 129,000 6 88 - 96 

01646580 P00623 1993 4,210,000 327,000 8 89 - 97 

01646580 P00623 1994 3,890,000 246,000 6 90 - 98 

01646580 P00623 1995 2,020,000 97,500 5 91 - 99 

01646580 P00623 1996 9,140,000 628,000 7 91 - 99 

01646580 P00623 1997 2,480,000 129,000 5 91 - 99 

Appendix A 23 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00623 1998 5,580,000 318,000 6 91 - 99 

01646580 P00623 1999 1,900,000 103,000 5 91 - 99 

01646580 P00625 1979 15,000,000 1,420,000 9 79 - 87 

01646580 P00625 1980 7,170,000 485,000 7 79 - 87 

01646580 P00625 1981 3,850,000 229,000 6 79 - 87 

01646580 P00625 1982 8,810,000 611,000 7 79 - 87 

01646580 P00625 1983 14,900,000 1,060,000 7 79 - 87 

01646580 P00625 1984 19,800,000 1,780,000 9 80 - 88 

01646580 P00625 1985 13,500,000 2,170,000 16 81 - 89 

01646580 P00625 1986 5,780,000 531,000 9 82 - 90 

01646580 P00625 1987 8,870,000 839,000 9 83 - 91 

01646580 P00625 1988 6,710,000 697,000 10 84 - 92 

01646580 P00625 1989 7,130,000 513,000 7 85 - 93 

01646580 P00625 1990 4,000,000 239,000 6 86 - 94 

01646580 P00625 1991 3,300,000 254,000 8 87 - 95 

01646580 P00625 1992 3,700,000 270,000 7 88 - 96 

01646580 P00625 1993 9,720,000 862,000 9 89 - 97 

01646580 P00625 1994 8,160,000 623,000 8 90 - 98 

01646580 P00625 1995 3,680,000 217,000 6 91 - 99 

01646580 P00625 1996 21,700,000 2,120,000 10 91 - 99 

01646580 P00625 1997 4,510,000 304,000 7 91 - 99 

01646580 P00625 1998 11,600,000 789,000 7 91 - 99 

01646580 P00625 1999 2,870,000 180,000 6 91 - 99 

01646580 P00631 1979 27,400,000 2,660,000 10 79 - 87 

01646580 P00631 1980 13,800,000 1,040,000 8 79 - 87 

01646580 P00631 1981 7,110,000 479,000 7 79 - 87 

01646580 P00631 1982 14,000,000 950,000 7 79 - 87 

01646580 P00631 1983 20,100,000 1,360,000 7 79 - 87 

01646580 P00631 1984 21,200,000 1,460,000 7 80 - 88 

01646580 P00631 1985 16,600,000 1,230,000 7 81 - 89 

01646580 P00631 1986 10,600,000 682,000 6 82 - 90 

01646580 P00631 1987 13,900,000 741,000 5 83 - 91 

01646580 P00631 1988 10,300,000 600,000 6 84 - 92 

01646580 P00631 1989 16,700,000 810,000 5 85 - 93 

01646580 P00631 1990 17,300,000 781,000 5 86 - 94 

01646580 P00631 1991 13,500,000 603,000 4 87 - 95 

24 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00631 1992 14,400,000 541,000 4 88 - 96 

01646580 P00631 1993 26,400,000 996,000 4 89 - 97 

01646580 P00631 1994 24,800,000 836,000 3 90 - 98 

01646580 P00631 1995 15,000,000 490,000 3 91 - 99 

01646580 P00631 1996 41,500,000 1,220,000 3 91 - 99 

01646580 P00631 1997 14,300,000 458,000 3 91 - 99 

01646580 P00631 1998 21,600,000 790,000 4 91 - 99 

01646580 P00631 1999 6,730,000 262,000 4 91 - 99 

01646580 P00665 1979 4,010,000 499,000 12 79 - 87 

01646580 P00665 1980 1,410,000 124,000 9 79 - 87 

01646580 P00665 1981 680,000 53,900 8 79 - 87 

01646580 P00665 1982 1,740,000 167,000 10 79 - 87 

01646580 P00665 1983 3,020,000 286,000 9 79 - 87 

01646580 P00665 1984 3,780,000 456,000 12 80 - 88 

01646580 P00665 1985 3,690,000 1,010,000 27 81 - 89 

01646580 P00665 1986 877,000 111,000 13 82 - 90 

01646580 P00665 1987 1,560,000 213,000 14 83 - 91 

01646580 P00665 1988 1,350,000 228,000 17 84 - 92 

01646580 P00665 1989 1,480,000 154,000 10 85 - 93 

01646580 P00665 1990 954,000 110,000 11 86 - 94 

01646580 P00665 1991 654,000 82,600 13 87 - 95 

01646580 P00665 1992 800,000 97,400 12 88 - 96 

01646580 P00665 1993 2,390,000 343,000 14 89 - 97 

01646580 P00665 1994 1,800,000 215,000 12 90 - 98 

01646580 P00665 1995 680,000 67,800 10 91 - 99 

01646580 P00665 1996 7,110,000 1,490,000 21 91 - 99 

01646580 P00665 1997 954,000 140,000 15 91 - 99 

01646580 P00665 1998 3,020,000 328,000 11 91 - 99 

01646580 P00665 1999 659,000 62,400 9 91 - 99 

01646580 P00666 1979 976,000 116,000 12 79 - 87 

01646580 P00666 1980 424,000 36,400 9 79 - 87 

01646580 P00666 1981 263,000 20,100 8 79 - 87 

01646580 P00666 1982 547,000 45,100 8 79 - 87 

01646580 P00666 1983 878,000 79,300 9 79 - 87 

01646580 P00666 1984 955,000 91,900 10 80 - 88 

01646580 P00666 1985 808,000 139,000 17 81 - 89 

Appendix A 25 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00666 1986 342,000 35,400 10 82 - 90 

01646580 P00666 1987 482,000 50,300 10 83 - 91 

01646580 P00666 1988 363,000 52,000 14 84 - 92 

01646580 P00666 1989 555,000 55,800 10 85 - 93 

01646580 P00666 1990 541,000 68,100 13 86 - 94 

01646580 P00666 1991 298,000 41,900 14 87 - 95 

01646580 P00666 1992 371,000 42,700 12 88 - 96 

01646580 P00666 1993 645,000 88,200 14 89 - 97 

01646580 P00666 1994 530,000 58,600 11 90 - 98 

01646580 P00666 1995 251,000 23,600 9 91 - 99 

01646580 P00666 1996 1,510,000 223,000 15 91 - 99 

01646580 P00666 1997 353,000 43,000 12 91 - 99 

01646580 P00666 1998 917,000 95,600 10 91 - 99 

01646580 P00666 1999 397,000 40,000 10 91 - 99 

01646580 P00671 1979 659,000 106,000 16 79 - 87 

01646580 P00671 1980 291,000 36,100 12 79 - 87 

01646580 P00671 1981 183,000 19,700 11 79 - 87 

01646580 P00671 1982 382,000 40,400 11 79 - 87 

01646580 P00671 1983 613,000 66,200 11 79 - 87 

01646580 P00671 1984 633,000 68,300 11 80 - 88 

01646580 P00671 1985 542,000 99,500 18 81 - 89 

01646580 P00671 1986 218,000 26,200 12 82 - 90 

01646580 P00671 1987 311,000 34,100 11 83 - 91 

01646580 P00671 1988 238,000 36,300 15 84 - 92 

01646580 P00671 1989 445,000 47,800 11 85 - 93 

01646580 P00671 1990 587,000 108,000 18 86 - 94 

01646580 P00671 1991 254,000 41,600 16 87 - 95 

01646580 P00671 1992 308,000 37,200 12 88 - 96 

01646580 P00671 1993 543,000 76,300 14 89 - 97 

01646580 P00671 1994 455,000 54,500 12 90 - 98 

01646580 P00671 1995 187,000 18,900 10 91 - 99 

01646580 P00671 1996 1,160,000 196,000 17 91 - 99 

01646580 P00671 1997 244,000 32,000 13 91 - 99 

01646580 P00671 1998 643,000 72,100 11 91 - 99 

01646580 P00671 1999 242,000 25,700 11 91 - 99 

26 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00680 1988 38,300,000 3,820,000 10 88 - 96 

01646580 P00680 1989 55,100,000 3,470,000 6 88 - 96 

01646580 P00680 1990 48,400,000 2,760,000 6 88 - 96 

01646580 P00680 1991 44,300,000 2,920,000 7 88 - 96 

01646580 P00680 1992 44,900,000 2,540,000 6 88 - 96 

01646580 P00680 1993 104,000,000 7,250,000 7 89 - 97 

01646580 P00680 1994 81,300,000 4,330,000 5 90 - 98 

01646580 P00680 1995 32,300,000 1,370,000 4 91 - 99 

01646580 P00680 1996 166,000,000 11,500,000 7 91 - 99 

01646580 P00680 1997 34,600,000 1,710,000 5 91 - 99 

01646580 P00680 1998 78,400,000 3,950,000 5 91 - 99 

01646580 P00680 1999 21,500,000 1,030,000 5 91 - 99 

Appendix A 27 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr SiO2) (kg/yr Si) (kg/yr SiO2) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P00955 1979 155,000,000 72,300,000 30,000,000 19 79 - 87 

01646580 P00955 1980 57,000,000 26,700,000 8,700,000 15 79 - 87 

01646580 P00955 1981 29,100,000 13,600,000 4,070,000 14 79 - 87 

01646580 P00955 1982 65,000,000 30,400,000 11,100,000 17 79 - 87 

01646580 P00955 1983 107,000,000 49,900,000 18,100,000 17 79 - 87 

01646580 P00955 1984 140,000,000 65,300,000 29,400,000 21 80 - 88 

01646580 P00955 1985 140,000,000 65,200,000 62,900,000 45 81 - 89 

01646580 P00955 1986 39,700,000 18,600,000 8,330,000 21 82 - 90 

01646580 P00955 1987 68,500,000 32,000,000 13,500,000 20 83 - 91 

01646580 P00955 1988 67,800,000 31,700,000 18,800,000 28 84 - 92 

01646580 P00955 1989 95,700,000 44,700,000 15,400,000 16 85 - 93 

01646580 P00955 1990 79,200,000 37,000,000 15,000,000 19 86 - 94 

01646580 P00955 1991 49,500,000 23,200,000 8,340,000 17 87 - 95 

01646580 P00955 1992 56,900,000 26,600,000 6,730,000 12 88 - 96 

01646580 P00955 1993 104,000,000 48,700,000 15,600,000 15 89 - 97 

01646580 P00955 1994 95,700,000 44,700,000 12,800,000 13 90 - 98 

01646580 P00955 1995 46,500,000 21,700,000 5,760,000 12 91 - 99 

01646580 P00955 1996 265,000,000 124,000,000 54,600,000 21 91 - 99 

01646580 P00955 1997 50,200,000 23,500,000 7,080,000 14 91 - 99 

01646580 P00955 1998 127,000,000 59,500,000 19,400,000 15 91 - 99 

01646580 P00955 1999 29,600,000 13,800,000 4,200,000 14 91 - 99 

28 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued

 Standard Error of 

Prediction 

Annual load +/- +/- Model 

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Potomac River at Chain Bridge (01646580)—Continued 

01646580 P80154 1979 4,130,000,000 833,000,000 20 79 - 87 

01646580 P80154 1980 1,090,000,000 148,000,000 14 79 - 87 

01646580 P80154 1981 369,000,000 56,900,000 15 79 - 87 

01646580 P80154 1982 1,480,000,000 322,000,000 22 79 - 87 

01646580 P80154 1983 2,980,000,000 564,000,000 19 79 - 87 

01646580 P80154 1984 7,450,000,000 3,110,000,000 42 80 - 88 

01646580 P80154 1985 7,530,000,000 5,480,000,000 73 81 - 89 

01646580 P80154 1986 684,000,000 229,000,000 34 82 - 90 

01646580 P80154 1987 2,100,000,000 737,000,000 35 83 - 91 

01646580 P80154 1988 3,050,000,000 1,080,000,000 35 84 - 92 

01646580 P80154 1989 1,880,000,000 435,000,000 23 85 - 93 

01646580 P80154 1990 876,000,000 228,000,000 26 86 - 94 

01646580 P80154 1991 534,000,000 115,000,000 22 87 - 95 

01646580 P80154 1992 378,000,000 65,800,000 17 88 - 96 

01646580 P80154 1993 2,080,000,000 382,000,000 18 89 - 97 

01646580 P80154 1994 1,300,000,000 224,000,000 17 90 - 98 

01646580 P80154 1995 284,000,000 43,600,000 15 91 - 99 

01646580 P80154 1996 5,420,000,000 1,650,000,000 30 91 - 99 

01646580 P80154 1997 312,000,000 59,300,000 19 91 - 99 

01646580 P80154 1998 1,240,000,000 219,000,000 18 91 - 99 

01646580 P80154 1999 73,500,000 12,400,000 17 91 - 99 

Appendix A 29 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440) 

01594440 P00600 1978 989,000 69,800 7 78 - 86 

01594440 P00600 1979 1,560,000 102,000 7 78 - 86 

01594440 P00600 1980 952,000 38,300 4 78 - 86 

01594440 P00600 1981 615,000 27,500 4 78 - 86 

01594440 P00600 1982 812,000 37,400 5 78 - 86 

01594440 P00600 1983 1,390,000 73,100 5 79 - 87 

01594440 P00600 1984 1,330,000 48,900 4 80 - 88 

01594440 P00600 1985 805,000 23,600 3 81 - 89 

01594440 P00600 1986 778,000 21,000 3 82 - 90 

01594440 P00600 1987 941,000 23,100 2 83 - 91 

01594440 P00600 1988 887,000 22,000 2 84 - 92 

01594440 P00600 1989 1,160,000 31,700 3 85 - 93 

01594440 P00600 1990 908,000 24,100 3 86 - 94 

01594440 P00600 1991 679,000 19,700 3 87 - 95 

01594440 P00600 1992 591,000 17,200 3 88 - 96 

01594440 P00600 1993 838,000 26,000 3 89 - 97 

01594440 P00600 1994 778,000 21,300 3 90 - 98 

01594440 P00600 1995 480,000 13,300 3 91 - 99 

01594440 P00600 1996 1,040,000 25,800 2 91 - 99 

01594440 P00600 1997 638,000 17,100 3 91 - 99 

01594440 P00600 1998 676,000 21,400 3 91 - 99 

01594440 P00600 1999 484,000 20,500 4 91 - 99 

01594440 P00608 1980 210,000 34,200 16 80 - 88 

01594440 P00608 1981 157,000 18,800 12 80 - 88 

01594440 P00608 1982 162,000 15,400 10 80 - 88 

01594440 P00608 1983 202,000 20,300 10 80 - 88 

01594440 P00608 1984 172,000 15,500 9 80 - 88 

01594440 P00608 1985 122,000 8,980 7 81 - 89 

01594440 P00608 1986 106,000 7,520 7 82 - 90 

01594440 P00608 1987 99,700 6,920 7 83 - 91 

01594440 P00608 1988 76,800 5,980 8 84 - 92 

01594440 P00608 1989 73,900 5,910 8 85 - 93 

01594440 P00608 1990 57,400 4,410 8 86 - 94 

01594440 P00608 1991 48,900 4,450 9 87 - 95 

01594440 P00608 1992 40,500 3,520 9 88 - 96 

01594440 P00608 1993 73,700 7,000 10 89 - 97 

30 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00608 1994 74,300 7,570 10 90 - 98 

01594440 P00608 1995 39,600 4,090 10 91 - 99 

01594440 P00608 1996 82,900 7,650 9 91 - 99 

01594440 P00608 1997 52,800 5,370 10 91 - 99 

01594440 P00608 1998 51,100 6,020 12 91 - 99 

01594440 P00608 1999 27,500 3,960 14 91 - 99 

01594440 P00613 1985 24,900 3,110 13 85 - 93 

01594440 P00613 1986 17,500 1,560 9 85 - 93 

01594440 P00613 1987 14,300 1,060 7 85 - 93 

01594440 P00613 1988 11,700 915 8 85 - 93 

01594440 P00613 1989 12,600 1,210 10 85 - 93 

01594440 P00613 1990 8,410 680 8 86 - 94 

01594440 P00613 1991 7,120 635 9 87 - 95 

01594440 P00613 1992 5,980 470 8 88 - 96 

01594440 P00613 1993 11,200 916 8 89 - 97 

01594440 P00613 1994 11,300 973 9 90 - 98 

01594440 P00613 1995 6,820 623 9 91 - 99 

01594440 P00613 1996 15,000 1,200 8 91 - 99 

01594440 P00613 1997 9,450 850 9 91 - 99 

01594440 P00613 1998 9,310 972 10 91 - 99 

01594440 P00613 1999 5,250 700 13 91 - 99 

01594440 P00623 1985 253,000 18,100 7 85 - 93 

01594440 P00623 1986 233,000 11,700 5 85 - 93 

01594440 P00623 1987 258,000 10,600 4 85 - 93 

01594440 P00623 1988 241,000 10,200 4 85 - 93 

01594440 P00623 1989 296,000 14,800 5 85 - 93 

01594440 P00623 1990 219,000 9,610 4 86 - 94 

01594440 P00623 1991 155,000 8,100 5 87 - 95 

01594440 P00623 1992 132,000 6,640 5 88 - 96 

01594440 P00623 1993 215,000 11,800 5 89 - 97 

01594440 P00623 1994 193,000 10,300 5 90 - 98 

01594440 P00623 1995 110,000 6,140 6 91 - 99 

01594440 P00623 1996 269,000 13,800 5 91 - 99 

01594440 P00623 1997 150,000 8,110 5 91 - 99 

01594440 P00623 1998 161,000 10,400 6 91 - 99 

01594440 P00623 1999 109,000 9,310 9 91 - 99 

Appendix A 31 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00625 1978 427,000 53,300 12 78 - 86 

01594440 P00625 1979 736,000 91,200 12 78 - 86 

01594440 P00625 1980 376,000 24,400 6 78 - 86 

01594440 P00625 1981 223,000 15,800 7 78 - 86 

01594440 P00625 1982 305,000 22,500 7 78 - 86 

01594440 P00625 1983 581,000 52,700 9 79 - 87 

01594440 P00625 1984 557,000 34,600 6 80 - 88 

01594440 P00625 1985 293,000 15,300 5 81 - 89 

01594440 P00625 1986 272,000 13,200 5 82 - 90 

01594440 P00625 1987 336,000 14,200 4 83 - 91 

01594440 P00625 1988 296,000 12,300 4 84 - 92 

01594440 P00625 1989 423,000 22,300 5 85 - 93 

01594440 P00625 1990 298,000 14,000 5 86 - 94 

01594440 P00625 1991 199,000 11,300 6 87 - 95 

01594440 P00625 1992 182,000 10,100 6 88 - 96 

01594440 P00625 1993 304,000 18,400 6 89 - 97 

01594440 P00625 1994 276,000 15,400 6 90 - 98 

01594440 P00625 1995 150,000 8,580 6 91 - 99 

01594440 P00625 1996 409,000 21,500 5 91 - 99 

01594440 P00625 1997 213,000 11,600 5 91 - 99 

01594440 P00625 1998 237,000 15,600 7 91 - 99 

01594440 P00625 1999 169,000 15,500 9 91 - 99 

01594440 P00631 1980 539,000 39,000 7 80 - 88 

01594440 P00631 1981 394,000 20,600 5 80 - 88 

01594440 P00631 1982 504,000 21,000 4 80 - 88 

01594440 P00631 1983 773,000 35,200 5 80 - 88 

01594440 P00631 1984 754,000 30,400 4 80 - 88 

01594440 P00631 1985 510,000 15,900 3 81 - 89 

01594440 P00631 1986 510,000 14,600 3 82 - 90 

01594440 P00631 1987 607,000 15,800 3 83 - 91 

01594440 P00631 1988 596,000 15,800 3 84 - 92 

01594440 P00631 1989 733,000 20,800 3 85 - 93 

01594440 P00631 1990 603,000 16,700 3 86 - 94 

01594440 P00631 1991 480,000 13,800 3 87 - 95 

01594440 P00631 1992 400,000 12,600 3 88 - 96 

01594440 P00631 1993 528,000 17,700 3 89 - 97 

01594440 P00631 1994 492,000 14,800 3 90 - 98 

32 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00631 1995 332,000 10,500 3 91 - 99 

01594440 P00631 1996 635,000 17,800 3 91 - 99 

01594440 P00631 1997 424,000 12,900 3 91 - 99 

01594440 P00631 1998 439,000 15,500 4 91 - 99 

01594440 P00631 1999 320,000 15,300 5 91 - 99 

01594440 P00665 1978 165,000 21,600 13 78 - 86 

01594440 P00665 1979 332,000 40,000 12 78 - 86 

01594440 P00665 1980 182,000 14,900 8 78 - 86 

01594440 P00665 1981 103,000 9,420 9 78 - 86 

01594440 P00665 1982 135,000 12,900 9 78 - 86 

01594440 P00665 1983 265,000 31,000 12 79 - 87 

01594440 P00665 1984 177,000 14,100 8 80 - 88 

01594440 P00665 1985 72,700 5,390 7 81 - 89 

01594440 P00665 1986 55,100 3,610 7 82 - 90 

01594440 P00665 1987 65,500 4,150 6 83 - 91 

01594440 P00665 1988 49,500 2,950 6 84 - 92 

01594440 P00665 1989 81,300 6,110 8 85 - 93 

01594440 P00665 1990 53,800 3,390 6 86 - 94 

01594440 P00665 1991 33,000 2,360 7 87 - 95 

01594440 P00665 1992 35,000 2,800 8 88 - 96 

01594440 P00665 1993 55,700 4,730 9 89 - 97 

01594440 P00665 1994 51,600 3,920 8 90 - 98 

01594440 P00665 1995 29,500 2,600 9 91 - 99 

01594440 P00665 1996 85,700 7,120 8 91 - 99 

01594440 P00665 1997 36,400 2,930 8 91 - 99 

01594440 P00665 1998 38,400 3,710 10 91 - 99 

01594440 P00665 1999 32,300 5,370 17 91 - 99 

01594440 P00666 1978 89,000 10,600 12 78 - 86 

01594440 P00666 1979 142,000 12,700 9 78 - 86 

01594440 P00666 1980 119,000 9,120 8 78 - 86 

01594440 P00666 1981 90,600 8,190 9 78 - 86 

01594440 P00666 1982 94,900 8,550 9 78 - 86 

01594440 P00666 1983 108,000 10,700 10 79 - 87 

01594440 P00666 1984 61,300 5,040 8 80 - 88 

01594440 P00666 1985 29,600 1,960 7 81 - 89 

01594440 P00666 1986 21,600 1,340 6 82 - 90 

Appendix A 33 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00666 1987 18,600 1,050 6 83 - 91 

01594440 P00666 1988 14,100 805 6 84 - 92 

01594440 P00666 1989 17,200 1,160 7 85 - 93 

01594440 P00666 1990 12,900 733 6 86 - 94 

01594440 P00666 1991 9,360 575 6 87 - 95 

01594440 P00666 1992 10,500 729 7 88 - 96 

01594440 P00666 1993 15,600 1,250 8 89 - 97 

01594440 P00666 1994 15,600 1,220 8 90 - 98 

01594440 P00666 1995 9,730 854 9 91 - 99 

01594440 P00666 1996 21,600 1,800 8 91 - 99 

01594440 P00666 1997 10,900 886 8 91 - 99 

01594440 P00666 1998 10,600 1,030 10 91 - 99 

01594440 P00666 1999 8,490 1,290 15 91 - 99 

01594440 P00671 1982 109,000 19,300 18 82 - 90 

01594440 P00671 1983 96,800 12,200 13 82 - 90 

01594440 P00671 1984 59,900 5,440 9 82 - 90 

01594440 P00671 1985 26,600 1,850 7 82 - 90 

01594440 P00671 1986 17,700 1,190 7 82 - 90 

01594440 P00671 1987 15,400 961 6 83 - 91 

01594440 P00671 1988 11,000 672 6 84 - 92 

01594440 P00671 1989 12,900 857 7 85 - 93 

01594440 P00671 1990 10,300 637 6 86 - 94 

01594440 P00671 1991 7,280 424 6 87 - 95 

01594440 P00671 1992 8,620 573 7 88 - 96 

01594440 P00671 1993 12,400 912 7 89 - 97 

01594440 P00671 1994 13,000 843 6 90 - 98 

01594440 P00671 1995 9,580 686 7 91 - 99 

01594440 P00671 1996 21,000 1,440 7 91 - 99 

01594440 P00671 1997 10,200 674 7 91 - 99 

01594440 P00671 1998 9,240 722 8 91 - 99 

01594440 P00671 1999 6,850 840 12 91 - 99 

01594440 P00680 1985 1,440,000 78,300 5 85 - 93 

01594440 P00680 1986 1,230,000 47,800 4 85 - 93 

01594440 P00680 1987 1,750,000 57,200 3 85 - 93 

01594440 P00680 1988 1,720,000 57,200 3 85 - 93 

01594440 P00680 1989 3,210,000 142,000 4 85 - 93 

34 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00680 1990 1,980,000 58,400 3 86 - 94 

01594440 P00680 1991 1,260,000 42,200 3 87 - 95 

01594440 P00680 1992 1,430,000 57,700 4 88 - 96 

01594440 P00680 1993 2,740,000 104,000 4 89 - 97 

01594440 P00680 1994 2,730,000 93,600 3 90 - 98 

01594440 P00680 1995 1,560,000 62,400 4 91 - 99 

01594440 P00680 1996 4,940,000 180,000 4 91 - 99 

01594440 P00680 1997 2,200,000 81,000 4 91 - 99 

01594440 P00680 1998 2,370,000 106,000 4 91 - 99 

01594440 P00680 1999 1,640,000 116,000 7 91 - 99 

Appendix A 35 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load Annual load +/- +/- Model

Station no. Parameter Year (kg/yr SiO2) (kg/yr Si) (kg/yr SiO2) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P00955 1978 3,270,000 1,530,000 212,000 6 78 - 86 

01594440 P00955 1979 4,710,000 2,200,000 250,000 5 78 - 86 

01594440 P00955 1980 2,760,000 1,290,000 110,000 4 78 - 86 

01594440 P00955 1981 1,440,000 671,000 62,800 4 78 - 86 

01594440 P00955 1982 1,940,000 905,000 88,600 5 78 - 86 

01594440 P00955 1983 3,160,000 1,480,000 160,000 5 79 - 87 

01594440 P00955 1984 2,760,760 1,290,000 92,000 3 80 - 88 

01594440 P00955 1985 1,460,000 684,000 46,700 3 81 - 89 

01594440 P00955 1986 1,490,000 698,000 42,700 3 82 - 90 

01594440 P00955 1987 2,020,000 942,000 54,700 3 83 - 91 

01594440 P00955 1988 2,100,000 982,000 55,700 3 84 - 92 

01594440 P00955 1989 2,980,000 1,390,000 78,500 3 85 - 93 

01594440 P00955 1990 2,550,000 1,190,000 67,400 3 86 - 94 

01594440 P00955 1991 1,880,000 877,000 50,100 3 87 - 95 

01594440 P00955 1992 1,820,000 850,000 47,100 3 88 - 96 

01594440 P00955 1993 2,680,000 1,250,000 75,600 3 89 - 97 

01594440 P00955 1994 2,810,000 1,310,000 65,400 2 90 - 98 

01594440 P00955 1995 1,980,000 924,000 47,500 2 91 - 99 

01594440 P00955 1996 4,340,000 2,030,000 94,000 2 91 - 99 

01594440 P00955 1997 2,650,000 1,240,000 61,900 2 91 - 99 

01594440 P00955 1998 2,650,000 1,240,000 72,500 3 91 - 99 

01594440 P00955 1999 1,900,000 887,000 71,000 4 91 - 99 

36 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Patuxent River near Bowie (01594440)—Continued 

01594440 P80154 1985 16,300,000 3,480,000 21 85 - 93 

01594440 P80154 1986 10,600,000 1,430,000 13 85 - 93 

01594440 P80154 1987 21,800,000 3,040,000 14 85 - 93 

01594440 P80154 1988 22,400,000 3,010,000 13 85 - 93 

01594440 P80154 1989 58,500,000 7,990,000 14 85 - 93 

01594440 P80154 1990 24,800,000 3,030,000 12 86 - 94 

01594440 P80154 1991 9,900,000 1,340,000 14 87 - 95 

01594440 P80154 1992 13,300,000 2,370,000 18 88 - 96 

01594440 P80154 1993 27,500,000 3,990,000 15 89 - 97 

01594440 P80154 1994 24,800,000 3,100,000 13 90 - 98 

01594440 P80154 1995 11,700,000 1,900,000 16 91 - 99 

01594440 P80154 1996 53,200,000 6,860,000 13 91 - 99 

01594440 P80154 1997 15,500,000 2,090,000 14 91 - 99 

01594440 P80154 1998 17,900,000 2,710,000 15 91 - 99 

01594440 P80154 1999 14,100,000 4,890,000 35 91 - 99 

Appendix A 37 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000) 

01491000 P00600 1978 237,000 17,500 7 78 - 86 

01491000 P00600 1979 314,000 19,700 6 78 - 86 

01491000 P00600 1980 188,000 7,320 4 78 - 86 

01491000 P00600 1981 107,000 4,370 4 78 - 86 

01491000 P00600 1982 166,000 7,680 5 78 - 86 

01491000 P00600 1983 329,000 19,400 6 79 - 87 

01491000 P00600 1984 276,000 14,200 5 80 - 88 

01491000 P00600 1985 113,000 3,760 3 81 - 89 

01491000 P00600 1986 151,000 5,000 3 82 - 90 

01491000 P00600 1987 167,000 4,980 3 83 - 91 

01491000 P00600 1988 126,000 3,370 3 84 - 92 

01491000 P00600 1989 355,000 8,260 2 85 - 93 

01491000 P00600 1990 188,000 4,750 3 86 - 94 

01491000 P00600 1991 156,000 4,370 3 87 - 95 

01491000 P00600 1992 132,000 3,660 3 88 - 96 

01491000 P00600 1993 177,000 5,700 3 89 - 97 

01491000 P00600 1994 284,000 9,340 3 90 - 98 

01491000 P00600 1995 133,000 3,750 3 91 - 99 

01491000 P00600 1996 380,000 10,400 3 91 - 99 

01491000 P00600 1997 201,000 5,360 3 91 - 99 

01491000 P00600 1998 230,000 7,320 3 91 - 99 

01491000 P00600 1999 183,000 7,680 4 91 - 99 

01491000 P00608 1980 12,300 3,280 27 80 - 88 

01491000 P00608 1981 6,300 1,220 19 80 - 88 

01491000 P00608 1982 10,800 1,850 17 80 - 88 

01491000 P00608 1983 29,200 7,250 25 80 - 88 

01491000 P00608 1984 21,000 5,130 24 80 - 88 

01491000 P00608 1985 3,780 579 15 81 - 89 

01491000 P00608 1986 5,340 771 14 82 - 90 

01491000 P00608 1987 6,290 823 13 83 - 91 

01491000 P00608 1988 3,920 445 11 84 - 92 

01491000 P00608 1989 17,200 1,890 11 85 - 93 

01491000 P00608 1990 8,380 1,070 13 86 - 94 

01491000 P00608 1991 7,190 817 11 87 - 95 

01491000 P00608 1992 5,610 644 11 88 - 96 

01491000 P00608 1993 10,200 1,340 13 89 - 97 

38 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00608 1994 17,000 3,100 18 90 - 98 

01491000 P00608 1995 4,440 545 12 91 - 99 

01491000 P00608 1996 18,200 2,230 12 91 - 99 

01491000 P00608 1997 8,280 1,010 12 91 - 99 

01491000 P00608 1998 10,300 1,450 14 91 - 99 

01491000 P00608 1999 7,410 1,700 23 91 - 99 

01491000 P00613 1985 698 104 15 85 - 93 

01491000 P00613 1986 928 109 12 85 - 93 

01491000 P00613 1987 1,100 104 10 85 - 93 

01491000 P00613 1988 877 79 9 85 - 93 

01491000 P00613 1989 3,100 265 9 85 - 93 

01491000 P00613 1990 1,560 143 9 86 - 94 

01491000 P00613 1991 1,380 130 9 87 - 95 

01491000 P00613 1992 1,040 110 11 88 - 96 

01491000 P00613 1993 1,320 152 12 89 - 97 

01491000 P00613 1994 1,670 199 12 90 - 98 

01491000 P00613 1995 698 86 12 91 - 99 

01491000 P00613 1996 2,090 247 12 91 - 99 

01491000 P00613 1997 1,150 139 12 91 - 99 

01491000 P00613 1998 1,200 157 13 91 - 99 

01491000 P00613 1999 921 148 16 91 - 99 

01491000 P00623 1985 37,200 3,340 9 85 - 93 

01491000 P00623 1986 49,600 3,580 7 85 - 93 

01491000 P00623 1987 54,400 3,250 6 85 - 93 

01491000 P00623 1988 37,600 1,940 5 85 - 93 

01491000 P00623 1989 129,000 6,440 5 85 - 93 

01491000 P00623 1990 54,700 3,020 6 86 - 94 

01491000 P00623 1991 41,400 2,340 6 87 - 95 

01491000 P00623 1992 32,100 1,750 5 88 - 96 

01491000 P00623 1993 48,300 3,480 7 89 - 97 

01491000 P00623 1994 84,600 6,570 8 90 - 98 

01491000 P00623 1995 26,900 1,340 5 91 - 99 

01491000 P00623 1996 104,000 5,500 5 91 - 99 

01491000 P00623 1997 46,900 2,290 5 91 - 99 

01491000 P00623 1998 60,000 3,570 6 91 - 99 

01491000 P00623 1999 52,300 5,590 11 91 - 99 

Appendix A 39 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00625 1978 125,000 23,200 19 78 - 86 

01491000 P00625 1979 159,000 29,000 18 78 - 86 

01491000 P00625 1980 75,400 7,280 10 78 - 86 

01491000 P00625 1981 35,600 3,400 10 78 - 86 

01491000 P00625 1982 58,000 6,140 11 78 - 86 

01491000 P00625 1983 143,000 20,100 14 79 - 87 

01491000 P00625 1984 141,000 18,400 13 80 - 88 

01491000 P00625 1985 49,300 4,710 10 81 - 89 

01491000 P00625 1986 65,100 5,430 8 82 - 90 

01491000 P00625 1987 70,800 4,800 7 83 - 91 

01491000 P00625 1988 45,100 2,590 6 84 - 92 

01491000 P00625 1989 167,000 8,880 5 85 - 93 

01491000 P00625 1990 65,100 3,920 6 86 - 94 

01491000 P00625 1991 44,100 2,540 6 87 - 95 

01491000 P00625 1992 33,900 1,840 5 88 - 96 

01491000 P00625 1993 59,600 4,400 7 89 - 97 

01491000 P00625 1994 113,000 9,670 9 90 - 98 

01491000 P00625 1995 31,800 1,770 6 91 - 99 

01491000 P00625 1996 149,000 9,360 6 91 - 99 

01491000 P00625 1997 63,000 3,490 6 91 - 99 

01491000 P00625 1998 88,900 6,130 7 91 - 99 

01491000 P00625 1999 90,300 13,300 15 91 - 99 

01491000 P00631 1980 113,000 9,060 8 80 - 88 

01491000 P00631 1981 69,300 3,870 6 80 - 88 

01491000 P00631 1982 101,000 5,120 5 80 - 88 

01491000 P00631 1983 161,000 9,220 6 80 - 88 

01491000 P00631 1984 132,000 7,520 6 80 - 88 

01491000 P00631 1985 64,000 2,510 4 81 - 89 

01491000 P00631 1986 83,500 3,140 4 82 - 90 

01491000 P00631 1987 95,100 3,360 4 83 - 91 

01491000 P00631 1988 81,600 2,650 3 84 - 92 

01491000 P00631 1989 194,000 5,510 3 85 - 93 

01491000 P00631 1990 121,000 3,680 3 86 - 94 

01491000 P00631 1991 114,000 3,910 3 87 - 95 

01491000 P00631 1992 99,900 3,570 4 88 - 96 

01491000 P00631 1993 118,000 4,750 4 89 - 97 

01491000 P00631 1994 172,000 7,140 4 90 - 98 

40 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00631 1995 100,000 4,070 4 91 - 99 

01491000 P00631 1996 228,000 8,620 4 91 - 99 

01491000 P00631 1997 137,000 5,170 4 91 - 99 

01491000 P00631 1998 141,000 6,080 4 91 - 99 

01491000 P00631 1999 112,000 6,020 5 91 - 99 

01491000 P00665 1978 18,200 4,580 25 78 - 86 

01491000 P00665 1979 29,000 9,580 33 78 - 86 

01491000 P00665 1980 10,800 1,370 13 78 - 86 

01491000 P00665 1981 4,930 599 12 78 - 86 

01491000 P00665 1982 7,610 990 13 78 - 86 

01491000 P00665 1983 30,300 6,090 20 79 - 87 

01491000 P00665 1984 18,900 3,500 19 80 - 88 

01491000 P00665 1985 5,410 871 16 81 - 89 

01491000 P00665 1986 6,270 768 12 82 - 90 

01491000 P00665 1987 6,950 717 10 83 - 91 

01491000 P00665 1988 4,430 363 8 84 - 92 

01491000 P00665 1989 20,900 1,670 8 85 - 93 

01491000 P00665 1990 8,380 811 10 86 - 94 

01491000 P00665 1991 6,840 592 9 87 - 95 

01491000 P00665 1992 4,870 400 8 88 - 96 

01491000 P00665 1993 9,350 1,150 12 89 - 97 

01491000 P00665 1994 19,400 3,030 16 90 - 98 

01491000 P00665 1995 4,180 434 10 91 - 99 

01491000 P00665 1996 24,700 3,200 13 91 - 99 

01491000 P00665 1997 9,060 991 11 91 - 99 

01491000 P00665 1998 13,300 1,770 13 91 - 99 

01491000 P00665 1999 20,100 7,180 36 91 - 99 

01491000 P00666 1978 4,190 1,000 24 78 - 86 

01491000 P00666 1979 8,010 2,530 32 78 - 86 

01491000 P00666 1980 3,570 427 12 78 - 86 

01491000 P00666 1981 1,980 232 12 78 - 86 

01491000 P00666 1982 2,890 368 13 78 - 86 

01491000 P00666 1983 8,580 1,810 21 79 - 87 

01491000 P00666 1984 4,590 796 17 80 - 88 

01491000 P00666 1985 1,790 297 17 81 - 89 

01491000 P00666 1986 1,640 185 11 82 - 90 

Appendix A 41 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00666 1987 1,930 183 10 83 - 91 

01491000 P00666 1988 1,570 121 8 84 - 92 

01491000 P00666 1989 8,700 700 8 85 - 93 

01491000 P00666 1990 3,250 321 10 86 - 94 

01491000 P00666 1991 2,860 268 9 87 - 95 

01491000 P00666 1992 2,680 208 8 88 - 96 

01491000 P00666 1993 4,020 379 9 89 - 97 

01491000 P00666 1994 9,650 1,110 11 90 - 98 

01491000 P00666 1995 2,480 195 8 91 - 99 

01491000 P00666 1996 13,200 1,280 10 91 - 99 

01491000 P00666 1997 4,420 379 9 91 - 99 

01491000 P00666 1998 4,830 479 10 91 - 99 

01491000 P00666 1999 5,410 1,420 26 91 - 99 

01491000 P00671 1982 2,860 772 27 82 - 90 

01491000 P00671 1983 7,140 1,580 22 82 - 90 

01491000 P00671 1984 4,090 761 19 82 - 90 

01491000 P00671 1985 1,410 304 22 82 - 90 

01491000 P00671 1986 1,350 216 16 82 - 90 

01491000 P00671 1987 1,530 201 13 83 - 91 

01491000 P00671 1988 1,160 123 11 84 - 92 

01491000 P00671 1989 6,260 685 11 85 - 93 

01491000 P00671 1990 2,500 358 14 86 - 94 

01491000 P00671 1991 2,280 292 13 87 - 95 

01491000 P00671 1992 2,110 245 12 88 - 96 

01491000 P00671 1993 2,950 419 14 89 - 97 

01491000 P00671 1994 6,290 1,090 17 90 - 98 

01491000 P00671 1995 1,690 204 12 91 - 99 

01491000 P00671 1996 7,600 1,070 14 91 - 99 

01491000 P00671 1997 2,870 368 13 91 - 99 

01491000 P00671 1998 3,200 478 15 91 - 99 

01491000 P00671 1999 3,050 1,070 35 91 - 99 

01491000 P00680 1985 430,000 25,900 6 85 - 93 

01491000 P00680 1986 622,000 30,600 5 85 - 93 

01491000 P00680 1987 680,000 27,500 4 85 - 93 

01491000 P00680 1988 426,000 13,900 3 85 - 93 

01491000 P00680 1989 1,850,000 61,300 3 85 - 93 

42 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00680 1990 759,000 29,300 4 86 - 94 

01491000 P00680 1991 596,000 21,000 4 87 - 95 

01491000 P00680 1992 461,000 16,300 4 88 - 96 

01491000 P00680 1993 903,000 44,800 5 89 - 97 

01491000 P00680 1994 1,880,000 101,000 5 90 - 98 

01491000 P00680 1995 555,000 22,000 4 91 - 99 

01491000 P00680 1996 2,590,000 114,000 4 91 - 99 

01491000 P00680 1997 1,040,000 40,400 4 91 - 99 

01491000 P00680 1998 1,370,000 66,900 5 91 - 99 

01491000 P00680 1999 1,190,000 121,000 10 91 - 99 

Appendix A 43 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load Annual load +/- +/- Model

Station no. Parameter Year (kg/yr SiO2) (kg/yr Si) (kg/yr SiO2) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P00955 1978 1,760,000 824,000 139,000 8 78 - 86 

01491000 P00955 1979 2,310,000 1,080,000 126,000 5 78 - 86 

01491000 P00955 1980 1,640,000 768,000 73,200 4 78 - 86 

01491000 P00955 1981 1,060,000 495,000 51,500 5 78 - 86 

01491000 P00955 1982 1,490,000 695,000 81,500 5 78 - 86 

01491000 P00955 1983 2,280,000 1,070,000 143,000 6 79 - 87 

01491000 P00955 1984 1,870,000 876,000 112,000 6 80 - 88 

01491000 P00955 1985 903,000 422,000 36,600 4 81 - 89 

01491000 P00955 1986 1,180,000 552,000 46,100 4 82 - 90 

01491000 P00955 1987 1,350,000 631,000 48,800 4 83 - 91 

01491000 P00955 1988 1,130,000 528,000 37,300 3 84 - 92 

01491000 P00955 1989 2,610,000 1,220,000 76,200 3 85 - 93 

01491000 P00955 1990 1,560,000 727,000 50,600 3 86 - 94 

01491000 P00955 1991 1,370,000 642,000 45,700 3 87 - 95 

01491000 P00955 1992 1,190,000 554,000 38,100 3 88 - 96 

01491000 P00955 1993 1,360,000 637,000 40,700 3 89 - 97 

01491000 P00955 1994 2,120,000 992,000 63,300 3 90 - 98 

01491000 P00955 1995 1,320,000 615,000 37,600 3 91 - 99 

01491000 P00955 1996 2,910,000 1,360,000 76,600 3 91 - 99 

01491000 P00955 1997 1,790,000 835,000 47,800 3 91 - 99 

01491000 P00955 1998 1,720,000 806,000 52,000 3 91 - 99 

01491000 P00955 1999 1,420,000 661,000 54,900 4 91 - 99 

44 A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 
Appendix A. Annual load estimates for the River Input monitoring stations on the

 Susquehanna, Potomac, Patuxent, and Choptank Rivers–Continued 

Standard Error of 

Prediction 

Annual load +/- +/- Model

Station no. Parameter Year (kg/yr) (kg/yr) Percent window 

Choptank River near Greensboro (01491000)—Continued 

01491000 P80154 1978 3,540,000 1,300,000 37 78 - 86 

01491000 P80154 1979 6,680,000 3,400,000 51 78 - 86 

01491000 P80154 1980 1,950,000 362,000 19 78 - 86 

01491000 P80154 1981 730,000 129,000 18 78 - 86 

01491000 P80154 1982 1,370,000 237,000 17 78 - 86 

01491000 P80154 1983 6,790,000 1,530,000 23 79 - 87 

01491000 P80154 1984 3,560,000 729,000 20 80 - 88 

01491000 P80154 1985 901,000 237,000 26 81 - 89 

01491000 P80154 1986 1,560,000 337,000 22 82 - 90 

01491000 P80154 1987 1,770,000 299,000 17 83 - 91 

01491000 P80154 1988 874,000 119,000 14 84 - 92 

01491000 P80154 1989 5,130,000 660,000 13 85 - 93 

01491000 P80154 1990 1,950,000 334,000 17 86 - 94 

01491000 P80154 1991 1,480,000 200,000 14 87 - 95 

01491000 P80154 1992 733,000 89,300 12 88 - 96 

01491000 P80154 1993 2,200,000 433,000 20 89 - 97 

01491000 P80154 1994 5,850,000 1,580,000 27 90 - 98 

01491000 P80154 1995 742,000 109,000 15 91 - 99 

01491000 P80154 1996 5,090,000 981,000 19 91 - 99 

01491000 P80154 1997 1,420,000 180,000 13 91 - 99 

01491000 P80154 1998 2,940,000 548,000 19 91 - 99 

01491000 P80154 1999 2,820,000 1,310,000 46 91 - 99 

Appendix A 45 
----------------------------------------------------------------------------------------------------

A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers 

Appendix B. Percent differences between historical and revised load estimates

 [This appendix summarizes the percent differences between the historical load estimates from the River Input monitoring program and the revised estimates provided in Appendix A; --, estimates are not available for comparison] 

Revised estimate - Historic estimate Percent difference = × 100 % (Revised estimate + Historic estimate )/ 2 

Di ssolved Total Total

To tal Dissolved kjeldahl kjeldahl Dissolved Total Dissolved organic Suspendednitr ogen ammonia nitrogen nitrogen nitrite + nitrate phosphorus phosphorus Orthophosphorus carbon sediment

Year (P00600) (P00608) (P00623) (P00625) (P00631) (P00665) (P00666) (P00671) (P00680) (P80154) 

Susquehanna River at Conowingo Dam (01578310)

1978 -- -- -- -- -- -- -- -- -- --1979 -3.9 -18.7 -- -2.2 -1.4 -4.2 7.8 14.2 -- 3.4 1980 -1.1 -1.1 -- -0.9 2.2 -7.5 5.2 3.0 -- -3.0 1981 1.4 7.0 -- 0.9 4.9 -5.7 8.4 -4.3 -- -1.1 1982 2.8 17.0 -- 1.8 5.8 -11.4 2.0 -10.5 -- -1.9 1983 4.2 20.2 -- 3.8 5.4 -11.6 -5.3 -11.5 -- 2.5 1984 6.3 10.7 -- 17.6 -2.9 -20.9 -13.5 -12.1 -- 5.6 1985 6.2 -5.3 -2.9 12.7 -0.8 -23.1 -5.8 -16.1 -13.9 -1.8 1986 2.3 -10.2 -0.2 2.2 -0.4 -32.8 -17.9 -3.2 -7.9 -12.7 1987 3.5 -5.8 2.3 2.5 2.3 -27.7 -29.4 21.9 -6.3 -8.1 1988 4.2 -2.3 9.5 5.6 3.5 -27.3 -49.3 29.0 1.0 -3.8 1989 -6.1 19.7 -6.4 -1.8 -9.0 -29.7 -66.9 18.1 15.0 -0.8 1990 -8.2 5.8 -18.8 -7.7 -10.8 -18.2 -28.4 24.8 8.1 -3.7 1991 -8.8 16.1 -25.9 -16.5 -9.5 -10.8 2.4 44.6 -1.6 -6.1 1992 -7.7 -6.2 -13.2 -13.1 -10.0 -19.8 3.5 7.1 3.2 -15.5 1993 -1.8 4.9 -7.1 -8.6 -3.3 -0.7 13.1 4.6 1.0 7.7 1994 -1.2 -4.8 -8.1 -12.6 0.6 -0.3 8.1 -35.0 0.0 9.8 1995 -1.7 -14.6 -15.1 -21.0 2.6 -6.3 20.6 -76.5 -8.4 -2.4 1996 -4.5 -16.9 -4.8 -4.1 -4.3 16.7 12.4 -15.6 -6.7 4.0 1997 -3.2 -- -- 1.3 1.6 19.1 29.2 -5.2 -- -10.3 1998 0.0 -12.9 4.9 -1.4 0.9 17.6 11.2 -36.7 -13.8 1.0 

46 
Appendix B. Percent differences between historical and revised load estimates—Continued

 Dissolved Total Total

 Total Dissolved kjeldahl kjeldahl Dissolved Total Dissolved organic Suspended nitrogen ammonia nitrogen nitrogen nitrite + nitrate phosphorus phosphorus Orthophosphorus carbon sediment

Year (P00600) (P00608) (P00623) (P00625) (P00631) (P00665) (P00666) (P00671) (P00680) (P80154) 

Potomac River at Chain Bridge (01646580)

1978 -- -- -- -- -- -- -- -- -- --1979 1.2 49.6 -- 0.2 -3.2 3.9 15.0 27.7 -- -21.2 1980 2.0 35.6 -- 0.4 -2.8 1.4 12.7 17.2 -- 7.0 1981 1.3 24.3 -- -1.6 -2.5 -3.5 9.8 9.2 -- 29.3 1982 1.7 13.8 -- -2.4 0.6 -3.7 2.8 5.3 -- 26.5 1983 0.7 2.5 0.6 -3.0 1.4 -5.5 -4.3 0.7 -- 26.9 1984 0.7 -4.7 -7.7 0.5 -0.6 -7.7 -9.8 -6.8 -- 52.8 1985 -5.3 -5.0 -20.9 -9.6 1.2 -10.3 -15.7 -5.6 -- 41.3 1986 -9.2 -24.5 -14.1 -13.6 -11.2 -26.3 -19.1 -24.3 -- 11.9 1987 -8.8 -2.1 -12.4 -9.9 -16.1 -11.8 -14.9 -13.5 -- 34.8 1988 -3.9 24.6 -5.7 -4.1 -12.5 5.9 3.3 12.0 -32.8 57.4 1989 -0.1 -13.6 8.8 4.8 -7.0 6.4 14.1 12.5 -1.6 27.2 1990 -9.7 -36.7 6.7 -29.3 -3.8 -21.2 27.4 40.8 -20.6 17.6 1991 -16.8 -32.6 7.3 -27.5 -24.1 -28.5 -14.0 -16.0 -8.9 -6.8 1992 -13.2 -31.0 20.6 -8.0 -26.3 -21.9 -17.9 -26.4 -29.3 -32.5 1993 -8.0 -0.1 16.8 -8.0 -13.1 -7.6 -4.8 -1.4 -1.3 -14.4 1994 -3.1 22.9 13.7 -5.7 -4.4 2.1 15.9 26.0 6.1 -15.7 1995 5.2 1.1 8.8 4.3 -2.0 -2.0 -8.4 -14.2 3.3 -7.6 1996 -11.0 -8.3 -0.7 -2.9 -14.7 11.4 13.4 15.6 -2.7 -18.4 1997 -7.1 -- -- -2.0 -10.5 13.6 29.1 10.6 -- 15.2 1998 -5.5 -4.0 1.3 -2.4 -6.4 5.3 8.9 9.6 -3.3 7.1 

Appendix B 47 
A Revised Load Estimation Procedure for the Susquehanna, Potomac, Patuxent, and Choptank Rivers Appendix B. Pe

rcent differences between historical and re

vised load estimates—Continue

d

 Dissolved Total Total

 Total Dissolved kjeldahl

kjeldahl Dissolved Total Dissolved organic Suspended

 nitrogen ammonia nitrogen nitrogen nitrite + nitrate phosphorus phosphorus

Orthophosphorus carbon sediment

Year (

P00600) (

P00608) (

P00623) (P00625) (P00631) (P00665) (P00666) (P00671) (P00680) (P80154) 

Patuxent River near Bowie (01594440)

1978 -2.1 -- -- -11.7 -- -48.3 -50.4 -- -- --1979 -2.3 -- -- -10.7 -- -36.7 -24.3 -- -- --1980 -2.5 15.6 -- -5.7 -4.2 -10.9 10.6 -- -- --1981 -7.4 6.1 -- -2.1 -5.1 -4.9 31.6 -- -- --1982 -4.3 -2.1 -- -2.4 -2.2 1.9 36.2 33.6 -- --1983 -0.4 -1.0 -- -4.3 1.4 17.2 38.8 14.7 -- --1984 3.9 -6.6 -- 8.3 1.7 7.1 3.6 -8.1 -- --1985 -1.8 -3.7 4.8 2.5 -1.9 -10.0 -14.2 -31.9 6.9 28.8 1986 -3.2 -4.8 2.2 -2.2 -1.2 -13.0 -17.9 -36.8 6.6 8.2 1987 0.3 0.1 1.8 0.8 0.2 0.6 -27.4 -30.9 9.8 3.4 1988 -3.8 -5.3 2.2 -9.8 -1.0 -1.2 -27.1 -23.3 11.9 -17.2 1989 0.8 21.0 5.7 3.2 0.0 8.8 -25.3 -18.9 12.3 14.5 1990 1.2 36.1 11.0 11.4 -1.6 14.4 4.3 -1.7 11.5 17.3 1991 5.6 57.4 7.6 11.3 5.8 6.0 -18.7 -27.3 -7.4 21.7 1992 -1.3 23.9 -0.5 7.1 0.7 6.2 -18.3 -21.8 -10.0 29.3 1993 9.9 27.6 19.2 12.0 13.1 0.9 -26.4 -37.4 6.6 22.0 1994 12.0 7.6 19.3 3.6 14.5 -1.0 -27.3 -37.1 4.8 10.0 1995 12.1 -20.0 1.2 -9.9 16.9 -18.8 -44.7 -51.6 -11.0 -17.4 1996 0.4 -8.9 -3.3 -0.6 0.5 -5.7 -14.1 -22.8 -9.3 -10.4 1997 4.6 -- -- -- 6.8 -14.0 -28.6 -45.7 -- -10.0 1998 8.2 -32.3 3.5 -6.5 16.6 -31.9 -69.1 -72.8 -16.4 -15.2 

48 
Appendix B. Percent differences between historical and revised load estimates—Continued

 Dissolved Total Total

 Total Dissolved kjeldahl kjeldahl Dissolved Total Dissolved organic Suspended nitrogen ammonia nitrogen nitrogen nitrite + nitrate phosphorus phosphorus Orthophosphorus carbon sediment

Year (P00600) (P00608) (P00623) (P00625) (P00631) (P00665) (P00666) (P00671) (P00680) (P80154) 

Choptank River near Greensboro (01491000)

1978 -17.5 -- -- -2.2 -- 9.8 4.1 -- -- -27.4 1979 -15.2 -- -- -13.4 -- 16.2 21.4 -- -- -20.9 1980 -3.0 16.5 -- -5.6 -0.9 -11.6 4.6 -- -- -10.3 1981 1.9 11.8 -- -7.7 0.2 -16.3 7.5 -- -- 1.6 1982 1.0 11.1 -- -9.0 0.7 -16.1 12.3 -12.1 -- 4.0 1983 -2.2 10.2 -- -22.8 -1.5 10.8 16.8 -8.5 -- -19.9 1984 4.5 12.5 -- 4.6 -0.9 1.3 -5.4 -7.9 -- -40.3 1985 8.3 -20.2 3.7 14.3 -0.1 -11.0 -8.9 -13.0 -12.5 -54.2 1986 -0.9 -10.0 -0.6 -0.3 -4.5 -13.0 -14.6 -0.9 -5.8 -29.9 1987 -2.1 10.3 -0.7 -7.0 0.2 -12.1 -1.7 8.7 -5.1 -26.3 1988 -0.3 35.2 1.6 -17.7 9.2 -15.9 13.5 12.3 -3.4 -35.2 1989 1.0 15.7 2.3 3.2 -0.3 -1.0 -1.3 -11.1 -5.2 -28.1 1990 -3.0 12.4 -4.0 -4.1 -6.8 -7.9 -9.8 -15.7 -1.6 -14.9 1991 -9.5 8.9 -6.7 -11.7 -8.9 -4.7 3.1 -10.0 -1.3 10.6 1992 -5.8 -1.6 0.3 -9.7 -6.8 -25.9 15.4 -15.8 6.2 11.5 1993 -8.5 -11.7 8.1 1.3 -8.7 -9.1 27.9 -32.9 4.1 2.3 1994 -1.7 -8.2 8.8 3.7 -1.1 9.8 2.2 -43.9 -4.4 14.6 1995 -2.8 -1.8 0.5 -17.3 1.8 -19.6 -24.6 -61.2 -2.1 1.0 1996 -1.8 -13.7 -0.5 4.9 -4.7 -3.5 -16.0 -11.6 -5.7 9.5 1997 -4.3 -- -- 10.3 -8.5 7.1 -3.4 3.0 -- 9.0 1998 2.0 2.8 6.5 7.0 10.0 -2.2 -7.7 -30.8 -3.0 11.7 

Appendix B 49