Cover photograph: Image from the Multi-angle Imaging SpectroRadiometer (MISR) highlights 

coastal areas of three states along the Gulf of Mexico: Louisiana, Mississippi, and Alabama—

including the Mississippi River Delta. 

Photograph credit: NASA/Goddard Space Flight Center/Langley Research Center/Jet Propulsion 

Laboratory, MISR Team, October 2001
Annual Dissolved Nitrite Plus Nitrate and 

Total Phosphorous Loads for the Susquehanna, 

St. Lawrence, Mississippi–Atchafalaya, and 

Columbia River Basins, 1968–2004

By Brent T. Aulenbach

Open File Report 2006–1087

U.S. Department of the Interior

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

P. Lynn Scarlett, Acting Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

This report is a Web-only publication: pubs.usgs.gov/ofr/2006/1087/

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Aulenbach, B.T., 2006, Annual dissolved nitrite plus nitrate and total phosphorus loads for the Susquehanna, 

St. Lawrence, Mississippi–Atchafalaya, and Columbia River Basins, 1968–2004: U.S. Geological Survey Open-File 

Report 2006-1087, 16 p.
iii

Contents

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

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

Stream-Water Nutrient Loads ........................................................................................................ 1

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

Data.............................................................................................................................................................. 2

Methods....................................................................................................................................................... 4

Load Estimation................................................................................................................................. 4

River Basin Specific Load Methodology....................................................................................... 5

Results ................................................................................................................................................ 7

Discussion................................................................................................................................................... 7

Comparison of Overlapping Annual Loads from Different Station Configurations ................ 7

Previous NO2 + NO3 Loads............................................................................................................... 11

Selected References................................................................................................................................. 12

Figures

1. River basins and locations of stream-water flow stations................................................ 2

2–3. Schematic diagrams showing—

2. Columbia River Basin and sampling stations.............................................................. 6

3. Mississippi–Atchafalaya River Basin and sampling stations.................................. 6

Tables

1. Summary of data sources used for load estimation........................................................... 3

2. Summary of water-quality parameters used ....................................................................... 3

3. Summary of water-quality data used for calibrating regression models........................ 4

4. Summary of station data and time periods used for load estimation .............................. 6

5. Annual (calendar year) stream-water dissolved nitrite plus nitrate loads in 

metric tons in four major United States river basins .............................................................. 8

6. Annual (calendar year) stream-water total phosphorous loads in metric tons in 

four major United States river basins.................................................................................... 9

7. Annual (calendar year) average streamflow in cubic feet per second in four 

major United States river basins............................................................................................ 10

Annual Dissolved Nitrite Plus Nitrate and 

Total Phosphorous Loads for the Susquehanna, 

St. Lawrence, Mississippi–Atchafalaya, and 

Columbia River Basins, 1968–2004

By Brent T. Aulenbach

Abstract

Annual stream-water loads were calculated near the outlet 

of four of the larger river basins (Susquehanna, St. Lawrence, 

Mississippi–Atchafalaya, and Columbia) in the United States 

for dissolved nitrite plus nitrate (NO2 + NO3) and total phosphorus using LOADEST load estimation software. Loads were 

estimated for the period 1968–2004; although loads estimated 

for individual river basins and chemical constituent combinations typically were for shorter time periods due to limitations in 

data availability. Stream discharge and water-quality data for load 

estimates were obtained from the U.S. Geological Survey (USGS) 

with additional stream discharge data for the Mississippi–

Atchafalaya River Basin from the U.S. Army Corps of Engineers. The loads were estimated to support national assessments 

of changes in stream nutrient loads that are periodically conducted by Federal agencies (for example, U.S. Environmental 

Protection Agency) and other water- and land-resource organizations. Data, methods, and results of load estimates are summarized herein; including World Wide Web links to electronic 

ASCII text files containing the raw data. The load estimates are 

compared to dissolved NO2 + NO3 loads for three of the large 

river basins from 1971 to 1998 that the USGS provided during 

2001 to The H. John Heinz III Center for Science, Economics 

and the Environment (The Heinz Center) for a report The Heinz 

Center published during 2002. Differences in the load estimates 

are the result of using the most up-to-date monitoring data since 

the 2001 analysis, differences in how concentrations less than 

the reporting limit were handled by the load estimation models, 

and some errors and exclusions in the 2001 analysis datasets 

(which resulted in some inaccurate load estimates).

Introduction

Stream-water constituent load, often referred to as 

mass flux, is the mass of a chemical constituent or sediment 

transported past a point in a stream during a set period. From 

a watershed perspective, loads serve as an integrated measure 

of all processes within the watershed that affect water quality 

(Semkin and others, 1994). From a downstream perspective, 

loads quantify the transport of chemical constituents to a 

receiving water body such as a bay, an estuary, a lake, an 

ocean, or a reservoir. Loads can be used to assess the effectiveness of controls of nonpoint-source pollutants on water quality.

Stream-Water Nutrient Loads

Nutrient loads such as nitrogen and phosphorus play an 

important role in ecosystems. While nutrients are vital for 

ecosystem functioning, increased nutrient loads have led to 

excessive algal growth and eutrophication of surface waters, 

resulting in reduced sunlight, loss of aquatic habitat, and the 

degradation of water quality for drinking and recreational use. 

When algae dies, it sinks to the bottom of the water column, 

decays, and consumes oxygen. If the water column is stratified, the oxygen levels at depth are not replenished, resulting 

in low oxygen conditions (hypoxia) or a complete lack of 

oxygen. Levels of oxygen can become too low to support fish 

and other aquatic species. 

The effects of excess nutrients on ecosystems have been 

documented. One example is the eutrophication of many 

surface waters in the United States and Canada during the mid1960s as the result of high phosphorus loads attributed largely 

to the use of phosphate detergents (Knud-Hansen, 1994). 

This resulted in the regulation and/or banning of phosphate 

in detergents in Canada and in many cities and states in the 

United States during the 1970s and 1980s, effectively reducing 

phosphorous loads in some surface waters. Another example is 

the development of a hypoxic zone in the Gulf of Mexico during some summers (for example, Committee on Environmental 

and Natural Resources, 2000; Rabalais and Turner, 2001). 

Scavia and others (2003) have shown that the areal extent of 

the hypoxic zone is related to the May–June total nitrogen 

loads for the Mississippi–Atchafalaya River Basin.
Tracking stream nutrient loads over time is an important 

task for assessing changes in loads in response to changes in 

natural and anthropogenic influences on water quality. Federal 

management (for example, U.S. Environmental Protection 

Agency, 2003) and water-resource agencies (for example, 

USGS; Smith and others, 1993) periodically have compiled 

information on the state of water quality in U.S. rivers to 

promote understanding of the state of water resources throughout the United States and to document changes over time. A 

previous report (The Heinz Center, 2002) identified four of the 

larger U.S. rivers with the intent of obtaining a representative 

perspective on changes in the status of stream nitrate loads 

over time in major U.S. rivers. This information is considered 

useful as an indicator of broad geographic changes that have 

occurred in nitrogen in streams of the United States.

Purpose and Scope

The purpose of this report is to estimate and document 

dissolved nitrite plus nitrate (NO2 + NO3) and total phosphorus 

(TP) loads on an annual basis for four of the larger river basins 

in the United States (Columbia, Mississippi–Atchafalaya, 

St. Lawrence, and Susquehanna; fig. 1) for data available for 

1968 through 2004. The loads are useful to support national 

assessments of changes in stream nutrient loads that are conducted periodically by Federal agencies (for example, U.S. 

Environmental Protection Agency) and other water and landresource organizations. One such organization, The Heinz 

Center, is currently updating the estimates of nutrient loads 

that were published previously for these four river basins in 

its 2002 published report “The State of the Nation’s Ecosystems.” The Heinz Center is a nonprofit institution for improving the scientific and economic knowledge used for environmental policy. The USGS previously had provided The Heinz 

Center in a 2001 analysis with NO2 + NO3 loads for three of 

the river basins for 1971 through 1998. Additional data not 

used in the 2001 analysis and some corrections to the original 

dataset are used for the analysis in this report and a discussion 

is included on the effects on the load estimates.

Data

Sampling stations were chosen as far downstream as 

possible from stations that had long-term stream discharge and 

water-quality data so that the load estimates best represent the 

delivery of nutrients to coastal waters. Sampling stations are 

summarized in table 1. In several cases, the reported nutrient 

loads result from combining records of water quality and stream 

discharge values from nearby stations on the same river. In these 

cases, stations were considered close enough to each other as to 

reflect similar chemical and flow conditions. For the Columbia 

River and Susquehanna River Basins, data were available at 

individual stations for only part of the period of interest; therefore, loads were calculated using different station configurations 

at different times. Data from the USGS were obtained from the 

National Water Information System (NWIS) Web-based database (NWISWeb) at http://waterdata.usgs.gov/usa/nwis/nwis. 

Figure 1. River basins and locations of stream-water flow stations.

Stations are labeled with identifications from table 1.

01570500

01578310

04264331

04264331

01100

03045

02600

03611500 07022000

14246900

14211720

14105700

07381495 07373420 07381495

14128910

Columbia

Stream discharge

Water quality

Stream discharge

 and water quality

St. Lawrence

Mississippi–Atchafalaya

Susquehanna

Explanation

River Basin

Station and identifier

07022000

0

0

250

250 500 MILES

500 KILOMETERS

Base from U.S. Geological 

Survey digital data

N

Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads
Data 

Table 1. Summary of data sources used for load estimation. 

[Station identifications (ID) are identifiers used by source agency. mi2, square mile; NA, not applicable. Data type: QW, water quality; Q, stream discharge; 

Q* indicates upstream discharge used for regression model load modeling. Source: USGS, U.S. Geological Survey; USACE, U.S. Army Corps of Engineers]

Station ID 

(see fig. 1) Station name River basin Drainage area (mi2)

Data 

type Source Relational database daily discharge file name

01570500 Susquehanna River at 

Harrisburg, Pennsylvania

Susquehanna 24,100 QW, Q USGS Q.Susquehanna.Harrisburg.rdb (http://ga.

water.usgs.gov/download/ofr061087/ )

01578310 Susquehanna River at 

Conowingo, Maryland

Susquehanna 27,100 QW, Q USGS Q.Susquehanna.Conowingo.rdb (http://ga.

water.usgs.gov/download/ofr061087/ )

04264331 St. Lawrence River at Cornwall, 

Ontario near Massena, New York

St. Lawrence 298,800 QW, Q USGS Q.StLawrence.Cornwall.rdb (http://ga.water.

usgs.gov/download/ofr061087/ )

07373420 Mississippi River near 

St. Francisville, Louisiana

Mississippi–

Atchafalaya

1,144,949 QW USGS NA

07381495 Atchafalaya River at 

Melville, Louisiana

Mississippi–

Atchafalaya

93,316 QW USGS NA

01100 Mississippi River at Tarbert 

Landing, Mississippi

Mississippi–

Atchafalaya

1,125,300 Q USACE Q.Mississippi.TarbertLanding.rdb (http://ga.

water.usgs.gov/download/ofr061087/ )

02600 Old River Outflow Channel near Knox 

Landing, Louisiana (total outflow)

Mississippi–

Atchafalaya

na Q USACE Q.OldRiver.KnoxLanding.rdb (http://ga.water.

usgs.gov/download/ofr061087/ )

03045 Atchafalaya River at 

Simmesport, Louisiana

Mississippi–

Atchafalaya

87,570 Q USACE Q.Atchafalaya.Simmesport.rdb (http://ga.

water.usgs.gov.download/ofr061087/ )

03611500 Ohio River at Metropolis, Illinois Mississippi–

Atchafalaya

203,000 Q* USGS Q.Ohio.Metropolis.rdb (http://ga.water.usgs.

gov/download/ofr061087/)

07022000 Mississippi River at Thebes, Illinois Mississippi–

Atchafalaya

713,200 Q* USGS Q.Mississippi.Thebes.rdb (http://ga.water.

usgs.gov/download/ofr061087/ )

14105700 Columbia River at The Dalles, Oregon Columbia 237,000 Q USGS Q.Columbia.Dalles.rdb (http://ga.water.usgs.

gov/download/ofr061087/)

14128910 Columbia River at Warrendale, Oregon Columbia 240,400 QW USGS NA

14211720 Willamette River at Portland, Oregon Columbia 11,200 QW, Q USGS Q.Willamette.Portland.rdb (http://ga.water.

usgs.gov/download/ofr061087/ )

14246900 Columbia River at Beaver Army 

Terminal near Quincy, Oregon

Columbia 256,900 QW, Q USGS Q.Columbia.Quincy.rdb (http://ga.water.usgs.

gov/download/ofr061087/)

Stream discharge data for the lower part of the Mississippi–

Atachfalaya River Basin were obtained from the U.S. Army 

Corps of Engineers (USACE) Web site at http://www.mvn.

usace.army.mil/eng/edhd/Wcontrol/discharge.htm. The USACE 

verified its data (USACE, written commun., various dates).

Loads were estimated for dissolved1 NO2 + NO3 and TP. 

For some time periods, dissolved NO2 + NO3 measurements 

were not available and other similar water-quality parameters 

were substituted (for example, “total” and “nitrate” only) so 

that loads could still be estimated (table 2). In other cases, 

additional samples analyzed only for the alternative waterquality parameters were used to supplement the existing 

dissolved NO2 + NO3 data. There was no significant difference 

between total and dissolved parameters (Rickert, 1992) and 

there was little difference between NO2 + NO3 versus just 

NO3 as almost all of the NO2 + NO3 is in the form of NO3. 

Table 3 summarizes the number of water-quality samples used 

and which water-quality parameters were used for dissolved 

NO2 + NO3.

1Dissolved is arbitrarily defined as the amount of material in the water after 

being passed through a 0.45-micrometer filter.

Table 2. Summary of water-quality parameters used.

[USGS, U.S. Geological Survey; mg/L milligram per liter; 

NO2, nitrite; NO3, nitrate; T, total; N, nitrogen; P, phosphorus]

Water-quality 

parameter name Abbreviation Unit USGS parameter code

Nitrogen nitrite plus 

nitrate, dissolved

NO2 + NO3 mg/L as N 00631

Nitrogen nitrite plus 

nitrate, total

T(NO2 + NO3) mg/L as N 00630

Nitrogen nitrate, 

dissolved

NO3 mg/L as N 00618

Nitrogen nitrate, total TNO3 mg/L as N 00620

Phosphorus, total TP mg/L as P 00665
Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads

Table 3. Summary of water-quality data used for calibrating regression models. 

[ID, identification; NO2 + NO3, dissolved nitrite plus nitrate; TP, total phosphorous; USGS, U.S. Geological Survey; number of water-quality samples below the 

reporting limit are in parentheses]

Station ID 

(see fig. 1) Station name

Number of 

NO2 + NO3

samples

Number of 

TP samples

USGS parameter 

code used for 

NO2 + NO3

Periods of nitrate 

and nitrite 

data used

01570500 Susquehanna River at Harrisburg, Pennsylvania 337 (0) 453 (7) 00630 2/74–8/79

00631 9/79–8/95

01578310 Susquehanna River at Conowingo, Maryland 643 (0) 653 (23) 00631 1/79–12/02

04264331 St. Lawrence River at Cornwall, Ontario near Massena, New York 170 (1) 184 (27) 00630 1/74–8/79

00631 9/79–9/96

07373420 Mississippi River near St. Francisville, Louisiana 509 (2) 362 (1) 00620 10/67–5/72

00618 1/73–8/73

00618, 00620, 00631 8/73–9/74

00630 10/74–9/79

00630, 00631 10/79–9/93

00631 11/93–6/04

07381495 Atchafalaya River at Melville, Louisiana 333 (0) 334 (0) 00630 8/78–9/79

00630 10/79–9/93

00631 11/93–6/04

14128910 Columbia River at Warrendale, Oregon 135 (23) 134 (2) 00630 1/75–8/79

00631 9/79–8/93

14211720 Willamette River at Portland, Oregon 156 (0) 156 (0) 00630 2/75–8/79

00631 9/79–12/93

14246900 Columbia River at Beaver Army Terminal near Quincy, Oregon 123 (8) 123 (8) 00631 1/92–12/02

When there was more than one, but fewer than four waterquality samples collected on the same day, the concentration 

for the first sample of the day was used. If there were four or 

more samples collected for a given day, that day was excluded 

from the calibration dataset because these cases typically indicate special-purpose sampling (e.g., cross-sectional, intensive 

storm hydrograph) was conducted, which is not representative 

of the daily average concentration of interest in this analysis.

Sample water-quality and daily stream-water data used 

in the analyses are available electronically for download from 

the Internet. The water-quality data file has the name QW.rdb 

and can be downloaded at the Internet address http://ga.water.

uses.gov/download/ofr061087/. Flow data file names and 

Internet addresses are listed in table 1. Data files are in RDB 

(relational database management system) format. RDB files 

are tab-delimited text (American Standard Code for Information Interchange, ASCII) files. In RDB format, comment lines 

begin with a #. The data start with a header line containing the 

variable names, followed by a line containing variable length 

and type information, followed by lines for each record with 

the data. Data are included only for years that have estimated 

loads even if data for other years are available.

Methods

The regression-model method employed to estimate loads 

herein is described in the next section. Specific details about 

station configuration and data used to estimate loads within 

each river basin are found in the successive section.

Load Estimation

Stream-water chemical constituent load (Φ) is the product 

of constituent concentration (C) and discharge (Q) integrated 

over time (t):

 (1)

Load estimation using the integral in equation 1 requires a 

continuous record of concentration and discharge. Although 

discharge can be easily measured at a sufficiently high frequency, constituent concentration typically is measured less 

frequently due to the expense of collecting and analyzing 

samples for water-quality concentration. Therefore, concentration must be estimated between relatively infrequent samples.
The regression-model method, also known as the ratingcurve method, is a standard statistical technique for estimating C(t) that uses a regression model relating concentration 

to continuous variables such as discharge and day of year 

(for example, Johnson, 1979; Cohn and others, 1992), thus 

enabling a direct calculation of equation 1. Consistent with 

many past studies, a seven-parameter regression model equation was fit with the form (Cohn and others, 1992; Runkel and 

others, 2004, LOADEST model number 9):

ln(Li) = a0 + a1lnQ + a2lnQ2 + a3 sin(2πdtime) + 

 a4cos(2πdtime) + a5dtime +a6dtime2 + ε (2)

where

ln is the natural logarithm (log base e);

Li is the calculated load for sample i; 

lnQ is ln(daily average streamflow) – center of 

ln(daily average streamflow);

dtime is decimal time – center of decimal time; 

ε is error; and

a0...a6 are the fitted parameters in the multiple 

regression model.

Note that the sample concentration has been multiplied by the 

daily average discharge so that equation 2 is a function of load 

instead of concentration, and that this modification has no effect 

on the resulting load estimates. The model in equation 2 captures the dependence of concentration on discharge and season 

(the sine and cosine terms) and any long-term trend. The load 

estimates are sensitive to the accuracy of the fitted model. All 

model terms were retained in the models, even when the model 

parameters were not significant, to simplify calculation of models across all sites and constituents. Inclusion of the insignificant 

terms does not change the load estimates appreciably, and the 

estimation of any additional parameters caused only a small proportional decrease in the degrees of freedom in the regression 

because of the large number of observations available. Average 

daily discharge was used instead of the instantaneous discharge 

when the sample was collected because instantaneous discharge 

was typically not available.

The integral in equation 1 for load was estimated in a discrete manner using the model in equation 2 with a daily time 

step, which was then summed for the period of interest:

 (3)

where

LT is the total load; 

Li is the predicted load for day i from equation 2; 

n is the number of days;

∆t is the daily time step; and

Σ is a summation.

The average daily discharge was used to estimate Li. The use 

of a daily time-step should be adequate for the calculation of 

annual loads of large rivers because stream-water concentration and discharge do not change radically within a given day.

Loads were estimated using Load Estimator (LOADEST), 

a FORTRAN based load estimation program (Runkel and others, 2004). LOADEST estimates loads using various algorithms 

for different statistical distributions of the data and correction 

factors for back-transformation bias corrections from a log 

model back to linear space. Results from the adjusted maximum 

likelihood estimates (AMLE) are used in this report, which 

modifies equation 2 to correct for transformation bias. LOADEST also handles censored (concentrations below the reporting 

limit) water-quality data by inferring the censored sample concentrations from the statistical distribution of sample concentrations above the reporting limit. 

River Basin Specific Load Methodology

Details on station data and time periods used for each river 

basin are summarized in table 4. Due to changes in the availability of water-quality sampling and streamflow measurements 

from different stations, two different combinations of water 

quality and stream discharge stations are used during different 

time periods in the Susquehanna and Columbia River Basins. 

These periods overlap by 17 years for the Susquehanna River 

and 2 years for the Columbia River, providing two annual load 

estimates for each overlapping year which can be used for comparisons. In the Susquehanna River Basin, the drainage area of 

the downstream station at Conowingo, Maryland, was 12.4 percent larger than the upstream station at Harrisburg, Pennsylvania. 

Flows were 16.7 percent higher at Conowingo than at Harrisburg 

for the 17-year overlapping period. In the Columbia River Basin 

(fig. 2), the drainage area of the downstream station at Beaver 

Army Terminal near Quincy, Oregon, was 3.5 percent larger 

than the upstream stations Columbia River at The Dalles and 

Willamette River at Portland, Oregon. Flows were 12.7 percent 

higher at Beaver Army Terminal than at the upstream stations 

for the 2-year overlapping period. If yields (loads per unit area) 

in these river basins are similar for the upstream basin and the 

intervening drainage area between the upstream and downstream 

stations, one would expect loads from the downstream stations to 

be proportionally higher due to the increase in drainage areas.

The model structure in equation 2 was modified for 

the Mississippi–Atchafalaya River Basin. These models used 

only a linear decimal time term (Runkel and others, 2004, 

LOADEST model number 8). The Mississippi River models 

also included flow terms (both flow and flow-squared terms) 

from two upstream stations (Mississippi River at Thebes 

and Ohio River at Metropolis, Illinois; tables 1 and 4) in addition to the flow from the sampling station. Upstream flows were 

lagged 10 days to account for travel time between the streamflow stations and the sampling station. These upstream flow 

terms improved model predictions because of the different 

nutrient characteristics that exist between the Ohio River and 

the Upper Mississippi River Basins. 

Methods 
Table 4. Summary of station data and time periods used for load estimation.

River basin Time period Water-quality station name and identification 

(see fig. 1)

Discharge station name and identification

 (see fig. 1)

Susquehanna 1971–19951 Susquehanna River at Harrisburg, Pennsylvania 

(01570500)

Susquehanna River at Harrisburg, Pennsylvania 

(01570500)

Susquehanna 1979–2002 Susquehanna River at Conowingo, Maryland 

(01578310)

Susquehanna River at Conowingo, Maryland 

(01578310)

St. Lawrence 1974–1996 St. Lawrence River at Cornwall, Ontario near 

Massena, New York (04264331)

St. Lawrence River at Cornwall, Ontario near 

Massena, New York (04264331)

Mississippi–

Atchafalaya

1968–20042 Mississippi River near St. Francisville, 

Louisiana (07373420)

Atchafalaya River at Melville, 

Louisiana (07381495)

Mississippi River at Tarbert Landing, 

Mississippi (01100)

Old River Outflow Channel near Knox Landing, 

Louisiana (total outflow) (02600)

Atchafalaya River at Simmesport, Louisiana (03045)

Ohio River at Metropolis, Illinois3 (03611500)

Mississippi River at Thebes, Illinois3 (07022000)

Columbia 1975–1993 Columbia River at Warrendale, Oregon (14128910) Columbia River at The Dalles, Oregon (14105700)

Columbia 1975–1993 Willamette River at Portland, Oregon (14211720) Willamette River at Portland, Oregon (14211720)

Columbia 1992–2002 Columbia River at Beaver Army Terminal near 

Quincy, Oregon (14246900)

Columbia River at Beaver Army Terminal near 

Quincy, Oregon (14246900)

1NO2

 + NO3 (nitrite plus nitrate); loads 1974–1995.

2Total phosphorus loads for Mississippi River 1975–2004; Atchafalaya River loads 1979–2004.

3Indicates an upstream stream discharge that is included in the regression model for modeling load.

Columbia River

Willamette River

Portland, Ore.

Warrendale, Ore. The Dalles, Ore.

Beaver Army Terminal near Quincy, Ore.

Pacific

Ocean

Figure 2. Schematic diagram of Columbia River Basin and 

sampling stations.

Gulf of Mexico

Mississippi River

Atchafalaya River

Old River Outflow Channel

Tarbert Landing, Miss.

near St. Francisville, La.

Simmesport, La.

Melville, La.

near Knox Landing, La.

Red River

Mississippi River

Ohio River

Thebes, Ill. Metropolis, Ill.

Figure 3. Schematic diagram of Mississippi–Atchafalaya 

River Basin and sampling stations.

Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads
For modeling purposes, the at-site flow used for the Mississippi River models was a combination of flows from the Mississippi River at Tarbert Landing, Mississippi, and the Mississippi River flows diverted upstream of Tarbert Landing by the 

Old River Diversion to the Atchafalaya, because this combined 

flow would be the natural flow associated with the variations 

in nutrient concentrations (fig. 3). After the Mississippi River 

load was estimated using this combined at-site flow, the portion of the load transported in the Mississippi River at Tarbert 

Landing to the Gulf of Mexico is determined by multiplying 

the load by the portion of flow transported in the Mississippi 

River at Tarbert Landing. This partitioning was done on a daily 

basis. The flow and loads diverted by the Old River Diversion 

are included in the Atchafalaya River load estimates. 

For the Columbia, St. Lawrence, and Susquehanna River 

Basins, all water-quality samples were included to calibrate 

the regression models for the entire period of estimation. A 

different sample/model calibration scheme was used for the 

Mississippi–Atchafalaya River Basin. The first 10 years of 

loads are calculated using the first 10 years of available waterquality data. Each subsequent year of load estimates were 

calculated using samples from the current year and the previous 

9 years. This “moving window” approach allows a sufficient 

number of samples in each model run to represent the full range 

of flow and nutrient concentration conditions. A July through 

June annual calculation period was used for Mississippi–

Atchafalaya River Basin loads to coordinate with surveys of 

the Gulf of Mexico hypoxic zone (Rabalais and others, 2001), 

which annually requires spring loads to be estimated by midJuly; but calendar year load estimates are reported herein.

Results

Annual loads are summarized in table 5 for NO2 + NO3

and in table 6 for TP. Note that a dominant factor in controlling year-to-year variability in loads is the amount of annual 

runoff. Annual average concentrations can be computed using 

the annual loads with the annual average stream discharges 

reported in table 7. Basin yields, loads per unit area, also can 

be computed using the basin drainage areas in table 1. 

Discussion

Comparisons of loads from different station configurations for periods with overlapping annual loads are discussed 

in the next section. Comparisons of NO2 + NO3 annual loads 

estimated herein with previously estimated NO2 + NO3 loads 

are found in the successive section.

Comparison of Overlapping Annual Loads 

from Different Station Configurations

On the Susquehanna River, the downstream station at 

Conowingo, Maryland, had lower TP loads (14 percent) and 

higher NO2 + NO3 loads (26 percent) than the upstream station at Harrisburg, Pennsylvania, for the 17 overlapping years 

(1979–1995). The decrease in TP loads indicates a loss of TP 

between these stations. The increase in NO2 + NO3 is greater 

than the increase in drainage area (12 percent) and flows (17 

percent) along this stream segment, indicating that yields of 

NO2 + NO3 are higher for this intervening basin than for the 

basin upstream of the upstream station. 

On the Columbia River, the downstream station at Beaver Army Terminal near Quincy, Oregon, had much greater 

TP loads (57 percent) and higher NO2 + NO3 loads (8.6 percent) than the upstream station configuration (Columbia River 

at Warrendale and The Dalles, and Willamette River at Portland, Oregon) for the two overlapping years (1992 and 1993). 

These differences cannot be explained fully from the increase 

in drainage area of the downstream station (3.5 percent). The 

increase in NO2 + NO3 loads is not too different compared 

to the increase in flow between the upstream stations and the 

downstream station of 12.7 percent. The much larger increase 

TP loads indicates a much higher yield of TP in the intervening basin between the upstream and downstream stations.

The above observations need to be considered when 

choosing how to combine load estimates from different stations to make a longer time series of loads; scaling would be 

necessary. For years in which multiple loads are estimated 

within a single basin, the downstream loads generally are 

preferable because these better represent the entire transport 

of nutrients from that basin. A recommendation for use of 

loads for the Susquehanna River is to use the loads from Harrisburg through 1978 and then use the loads from the downstream station at Conowingo from 1979 through 2002. 

A recommendation for use of loads for the Columbia River is to 

use the loads from the upstream station configuration (Columbia 

River at Warrendale and The Dalles, and Willamette River at 

Portland) through 1993 and then use the loads from the downstream station at Beaver Army Terminal for 1994 through 2002. 

The upstream station configuration was chosen for the two overlapping years (1992 and 1993) because the record was so much 

longer for that station configuration. Note that the differences in 

the loads between stations also may be indicative of the possible 

differences one might expect to observe between the stations 

used to estimate loads and the actual bay, gulf, or ocean.

Discussion 
Table 5. Annual (calendar year) stream-water dissolved nitrite plus nitrate loads in metric tons in four major United States river basins. 

[—, data unavailable; see figure 1 for locations]

Year

Susquehanna River Basin St. Lawrence 

River Basin

Mississippi–Atchafalaya

River Basin Columbia River Basin

Susquehanna River at 

Harrisburg, Pennsylvania

Susquehanna River at 

Conowingo, Maryland

St. Lawrence River at 

Cornwall, Ontario near 

Massena, New York

Mississippi River near 

St. Francisville, Louisiana

Atchafalaya River at 

Melville, Louisiana

Total Mississippi–

Atchafalaya River Basin

Columbia River at 

Warrendale, Oregon

Willamette River 

at Portland, Oregon

Total Columbia River at 

Warrendale and 

Willamette River at 

Portland, Oregon

Columbia River at 

Beaver Army Terminal 

near Quincy, Oregon

1968 — — — 204,000 — — — — — —

1969 — — — 256,000 — — — — — —

1970 — — — 268,000 — — — — — —

1971 — — — 255,000 — — — — — —

1972 — — — 395,000 — — — — — —

1973 — — — 573,000 — — — — — —

1974 29,600 — 46,800 560,000 — — — — — —

1975 36,600 — 45,000 593,000 — — 25,400 22,700 48,100 —

1976 34,500 — 51,200 368,000 — — 30,800 16,600 47,500 —

1977 41,000 — 44,200 429,000 — — 21,500 16,700 38,200 —

1978 35,400 — 49,800 579,000 — — 31,900 12,200 44,000 —

1979 44,200 50,600 49,600 867,000 336,000 1,200,000 31,500 15,000 46,500 —

1980 23,000 27,500 51,300 638,000 193,000 831,000 34,300 19,100 53,500 —

1981 27,200 31,500 51,200 621,000 161,000 781,000 42,100 17,300 59,400 —

1982 31,300 36,600 53,000 1,010,000 235,000 1,240,000 53,800 25,400 79,200 —

1983 38,400 46,200 54,700 1,190,000 310,000 1,500,000 57,100 25,400 82,500 —

1984 43,800 55,200 59,500 1,070,000 269,000 1,340,000 53,200 20,300 73,500 —

1985 31,500 37,400 61,500 926,000 241,000 1,170,000 48,700 9,890 58,600 —

1986 41,400 50,500 75,000 774,000 205,000 980,000 49,600 19,500 69,100 —

1987 31,800 39,700 71,700 508,000 159,000 666,000 38,100 13,800 51,900 —

1988 26,500 33,800 58,100 420,000 130,000 550,000 36,000 14,800 50,800 —

1989 33,900 45,700 59,500 494,000 183,000 677,000 38,700 14,100 52,800 —

1990 49,000 61,100 64,900 700,000 183,000 882,000 42,800 16,300 59,000 —

1991 30,300 38,900 69,500 817,000 191,000 1,010,000 42,700 15,400 58,100 —

1992 33,900 43,900 67,300 710,000 150,000 860,000 28,500 11,600 40,100 41,700

1993 43,300 62,200 80,900 1,390,000 292,000 1,680,000 25,700 18,100 43,800 49,500

1994 43,900 59,800 69,500 801,000 201,000 1,000,000 — — — 46,500

1995 26,200 34,200 67,700 770,000 206,000 976,000 — — — 77,000

1996 — 73,000 76,600 712,000 248,000 960,000 — — — 117,000

1997 — 34,300 — 699,000 250,000 949,000 — — — 103,000

1998 — 45,000 — 788,000 285,000 1,070,000 — — — 69,400

1999 — 29,000 — 657,000 230,000 887,000 — — — 88,800

2000 — 33,600 — 359,000 145,000 504,000 — — — 53,600

2001 — 22,300 — 704,000 216,000 920,000 — — — 31,900

2002 — 29,500 — 653,000 213,000 866,000 — — — 40,000

2003 — — — 530,000 193,000 723,000 — — — —

2004 — — — 734,000 235,000 969,000 — — — —

Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads
Table 6. Annual (calendar year) stream-water total phosphorous loads in metric tons in four major United States 

river basins.

[—, data unavailable; see figure 1 for locations]

Year

Susquehanna River Basin St. Lawrence 

River Basin

Mississippi–Atchafalaya 

River Basin Columbia River Basin

Susquehanna River at 

Harrisburg, Pennsylvania

Susquehanna River at 

Conowingo, Maryland

St. Lawrence River at 

Cornwall, Ontario near 

Massena, New York

Mississippi River at St. 

Francisville, Louisiana

Atchafalaya River at 

Melville, Louisiana

Total Mississippi–

Atchafalaya River Basin

Columbia River at 

Warrendale, Oregon

Willamette River at 

Portland, Oregon

Total Columbia River at 

Warrendale and 

Willamette River at 

Portland, Oregon

Columbia River at 

Beaver Army Terminal 

near Quincy, Oregon

1971 2,410 — — — — — — — — —

1972 8,310 — — — — — — — — —

1973 3,220 — — — — — — — — —

1974 2,820 — 7,630 — — — — — — —

1975 5,000 — 7,410 96,300 — — 7,530 2,920 10,400 —

1976 3,630 — 7,040 70,300 — — 9,300 2,210 11,500 —

1977 4,740 — 6,490 73,600 — — 4,060 2,830 6,880 —

1978 4,100 — 6,530 89,800 — — 7,270 1,810 9,080 —

1979 4,610 4,450 6,360 123,000 62,200 185,000 6,090 2,170 8,260 —

1980 2,200 1,830 6,140 82,500 34,500 117,000 6,950 2,950 9,900 —

1981 2,440 1,900 5,790 74,000 28,500 102,000 8,780 2,830 11,600 —

1982 2,650 1,990 5,620 109,000 45,200 155,000 12,600 3,760 16,300 —

1983 3,620 2,770 5,410 112,000 56,300 169,000 11,400 3,720 15,100 —

1984 4,340 3,440 5,310 102,000 50,300 153,000 10,200 3,150 13,400 —

1985 1,960 1,470 5,090 83,600 47,400 131,000 6,960 1,580 8,540 —

1986 3,010 2,290 4,870 83,500 41,300 125,000 8,040 2,770 10,800 —

1987 1,930 1,450 4,640 57,100 29,100 86,200 4,820 1,950 6,770 —

1988 1,490 1,130 4,070 57,100 26,200 83,300 4,270 2,070 6,350 —

1989 2,580 2,020 3,750 98,400 46,400 145,000 4,830 1,810 6,640 —

1990 2,820 2,390 4,020 97,600 51,600 149,000 5,620 2,010 7,620 —

1991 1,400 1,240 3,690 115,000 51,900 167,000 5,640 1,840 7,480 —

1992 1,540 1,420 3,580 98,700 37,300 136,000 2,980 1,360 4,340 6,530

1993 3,420 3,630 3,520 160,000 61,700 222,000 3,090 1,860 4,950 8,090

1994 2,860 2,990 3,320 114,000 36,600 150,000 — — — 5,970

1995 965 1,090 2,900 96,400 31,500 128,000 — — — 11,100

1996 — 4,160 3,100 97,800 36,600 134,000 — — — 18,300

1997 — 1,110 — 94,300 36,300 131,000 — — — 17,400

1998 — 2,080 — 103,000 38,800 142,000 — — — 10,100

1999 — 1,050 — 91,900 34,600 127,000 — — — 14,200

2000 — 1,440 — 65,700 25,300 91,000 — — — 8,270

2001 — 884 — 110,000 44,400 155,000 — — — 5,210

2002 — 1,460 — 116,000 48,200 164,000 — — — 8,730

2003 — — — 109,000 46,100 155,000 — — — —

2004 — — — 137,000 56,900 194,000 — — — —

Discussion
Table 7. Annual (calendar year) average streamflow in cubic feet per second in four major United States river basins. 

[—, data unavailable; see figure 1 for locations]

Susquehanna River Basin St. Lawrence 

River Basin

Mississippi–Atchafalaya

River Basin Columbia River Basin

Year

Susquehanna River at 

Harrisburg, Pennsylvania

Susquehanna River at 

Conowingo, Maryland

St. Lawrence River at 

Cornwall, Ontario near 

Massena, New York

Mississippi River at 

Tarbet Landing, 

Mississippi

Atchafalaya River at 

Simmesport, Louisiana

Total Mississippi–

Atchafalaya River Basin

Columbia River at 

Warrendale, Oregon

Willamette River at 

Portland, Oregon

Total Columbia River at 

Warrendale and 

Willamette River 

at Portland, Oregon

Columbia River at 

Beaver Army Terminal 

near Quincy, Oregon

1968 — — — 433,000 — — — — — —

1969 — — — 457,000 — — — — — —

1970 — — — 437,000 — — — — — —

1971 33,200 — — 388,000 — — — — — —

1972 59,500 — — 481,000 — — — — — —

1973 40,200 — — 720,000 — — — — — —

1974 36,000 — 300,000 586,000 — — — — — —

1975 45,800 — 284,000 563,000 — — 193,000 39,800 233,000 —

1976 39,400 — 300,000 364,000 — — 215,000 28,400 244,000 —

1977 45,200 — 262,000 379,000 — — 117,000 27,300 144,000 —

1978 39,300 — 277,000 469,000 — — 174,000 25,100 199,000 —

1979 44,300 52,300 276,000 708,000 304,000 1,010,000 150,000 29,800 179,000 —

1980 24,400 28,400 277,000 437,000 186,000 623,000 161,000 31,700 193,000 —

1981 27,100 30,400 270,000 363,000 156,000 519,000 188,000 32,600 220,000 —

1982 29,900 34,600 270,000 544,000 235,000 779,000 231,000 41,700 273,000 —

1983 36,200 41,900 270,000 682,000 293,000 975,000 218,000 43,100 261,000 —

1984 41,800 49,800 282,000 616,000 264,000 879,000 210,000 41,700 251,000 —

1985 26,300 30,500 284,000 591,000 254,000 845,000 167,000 23,500 191,000 —

1986 35,400 41,200 316,000 519,000 223,000 741,000 187,000 32,400 220,000 —

1987 27,600 32,300 292,000 406,000 175,000 581,000 142,000 24,200 166,000 —

1988 23,000 27,200 246,000 375,000 160,000 535,000 138,000 29,100 167,000 —

1989 33,600 39,900 243,000 573,000 246,000 818,000 155,000 26,700 182,000 —

1990 41,400 48,300 266,000 614,000 263,000 878,000 184,000 30,200 215,000 —

1991 24,800 29,700 264,000 647,000 278,000 925,000 197,000 29,500 226,000 —

1992 30,300 35,500 264,000 470,000 201,000 671,000 139,000 21,200 160,000 179,000

1993 44,200 52,500 292,000 773,000 331,000 1,100,000 154,000 32,400 186,000 211,000

1994 45,500 51,700 264,000 556,000 238,000 794,000 — — — 178,000

1995 24,600 28,000 244,000 513,000 220,000 732,000 — — — 251,000

1996 — 63,500 278,000 562,000 241,000 803,000 — — — 323,000

1997 — 29,700 — 600,000 257,000 857,000 — — — 330,000

1998 — 41,300 — 604,000 259,000 862,000 — — — 248,000

1999 — 26,800 — 496,000 212,000 708,000 — — — 296,000

2000 — 34,300 — 337,000 144,000 481,000 — — — 216,000

2001 — 23,600 — 499,000 214,000 714,000 — — — 145,000

2002 — 33,400 — 532,000 228,000 760,000 — — — 214,000

2003 — — — 492,000 211,000 703,000 — — — —

2004 — — — 581,000 249,000 831,000 — — — —

10 Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads
Previous NO2

 + NO3 Loads

During 2001, the USGS provided dissolved NO2 + NO3

loads for the Columbia, St. Lawrence, and Susquehanna 

River Basins from 1971 through 1998 to The Heinz Center, 

which it used the 2002 report “The State of the Nation’s 

Ecosystems.” These loads are compared to the loads contained 

herein, whereas some of the differences in loads are minor, 

with the previous loads being sufficiently accurate, other loads 

are significantly different. Note that the loads from the 2001 

analysis are not included in this report.

There were several reasons for minor differences in 

the loads. Changes in the water-quality sample datasets used to 

calibrate the regression model caused some minor differences. Changes in the water-quality datasets were caused 

by (1) slight changes in the sample selection included in the 

analysis; (2) additional samples which had similar water-quality parameters available to augment the dissolved NO2

 + NO3

dataset, and; (3) additional samples related to the extension 

in the length of period analyzed for this most current analysis. The updated loads reflect the most up-to-date retrievals 

(data retrieved May 19–20, 2005) and selection of observations from the NWIS records and the use of the most current 

load estimation methods, though the loads estimated from the 

2001 analysis are not considered to be in error except for the 

specific years noted below. The load estimation method used 

here was based on the AMLE procedure, which differs from 

the method used to calculate the 2001 load estimates. The 

2001 analysis used the minimum variance unbiased estimator 

(Cohn and others, 1989), which is equivalent to the maximum 

likelihood estimation and the AMLE methods in LOADEST 

when there are no censored (concentrations less than the 

detection limit) data. The 2001 analysis estimated values 

below the detection limit by using one-half the detection 

limit, whereas the AMLE method, used in the analysis for this 

report, determines an expected distribution of sample concentrations below the detection limit when calculating the loads. 

Most stations had very few NO2 + NO3 concentrations below 

the detection limit, less than (<)1 percent, with the exception 

of the Columbia River at Beaver Army Terminal near Quincy, 

Oregon (6.5 percent), and the Columbia River at Warrendale, 

Oregon (17 percent; table 3). Hence, only the Columbia River 

loads were significantly affected by the one-half detection 

limit simplification.

For the Susquehanna River at Harrisburg, Pennsylvania, 

there were only minor differences between the 2001 and current load estimates, with differences always being less than 

3 percent on an annual basis. One cause for the difference 

was that the 2001 calibration dataset had no available dissolved NO2

 + NO3 concentrations from March 1974 through 

September 1979. For the current load estimates, an additional 101 samples were added to the calibration dataset for 

this missing period using data from the alternative parameter 

code 00630 (tables 2 and 3). Furthermore, the 2001 NO2 + NO3

dataset had no water-quality data for 1973, although loads 

were estimated from 1971 through 1994. Because of this data 

gap, the loads in this report were only estimated for 1974 

through 1994. 

For the Susquehanna River at Conowingo, Maryland, 

90 percent of the annual loads differed by <5 percent, and 

10 percent of the annual loads differed between 5 and 10 percent. The reason for the difference in annual loads was the 

length of the period of the calibration dataset; the 2001 load 

estimates spanning the period from 1979 to 1998, whereas the 

current load estimates were extended through 2002. 

For the St. Lawrence River, the differences in annual load 

estimates from 1974 through 1987 were small, with only one 

year being greater than (>) 5 percent different. But starting 

with 1988, the differences are progressively larger, with the 

earlier load estimates underestimated by as much as 49 percent 

during 1996. These large differences in loads occurred because 

of the lack of any NO2 + NO3 concentrations at the end of 

the calibration period, from November 1985 through 1996, 

for the 2001 analysis. Regression models generally cannot 

accurately predict values outside the period of the calibration 

dataset. Furthermore, the inclusion of the time-squared term 

in the model allowed the errors to become much larger as one 

gets farther out from the calibration period. The current load 

estimates include an additional 51 samples in the calibration 

dataset, and provide a more accurate description of the annual 

loads during this period than those based on the 2001 report. 

Load estimates from the 2001 analysis should not be used 

from 1988 through 1996. 

For the Columbia River at Warrendale, there were significant differences between the 2001 and current annual load 

estimates from 1975 to 1993; 5 percent of the loads differed 

by < 5 percent; 47 percent of the loads differed between 5 and 

10 percent, and; 47 percent, of the loads differed by >10 percent. One reason for the difference was the length of the period 

of the calibration dataset; the 2001 load estimates used data 

from 1975 to 1993, whereas the current load estimates were 

extended through 1998. The effect of using the simplification 

of setting censored values to one-half the detection limit in the 

2001 analysis likely caused much of the differences observed. 

The Columbia River at Warrendale NO2 + NO3 dataset had 

17 percent censored values along with much larger differences in loads compared to the differences in the Susquehanna 

River at Conowingo loads for which NO2 + NO3 dataset had 

no censored values.

For the Willamette River at Portland, Oregon, the differences in annual load estimates from 1975 through 1993 

were generally small, with 74 percent of the loads differing by <5 percent and 26 percent of the loads differing by 

between 5 and 10 percent. However, the load estimates reported 

herein are recommended for use in place of those published 

in the 2001 analysis. Loads estimated in the 2001 analysis for 

1994 through 1997 are incorrect—the flow data from water 

years (WY; defined as October 1 of previous year through 

September 30 of water year) 1996 to 1999 were mistakenly 

assigned the dates of WYs 1995 through 1998. This affected 

all load estimates from the 2001 analysis because the regression 

model calibration had the wrong flows assigned to the waterDiscussion 11
quality samples from October 1994 through December 1997. 

Therefore, the 2001 analysis loads should not be used for the 

Willamette River. The Willamette River accounted for a significant portion (31 percent) of the total load from the sum of 

the Columbia River at The Dalles and the Willamette River 

at Portland for the period 1975–1993. Flow data were not 

available for the Willamette River at Portland for WY 1995; 

hence, the current analysis loads are for the 1975–1993 period. 

Loads for the Columbia River at Beaver Army Terminal from 

1992 through 2002 in the current analysis were estimated for 

and can be used to extend the time period of loads for the 

Columbia River Basin.

Selected References

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

Wells, D.K., 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 

Sommers, R.M., 1992, The validity of a simple statistical 

model for estimating fluvial constituent loads: An empirical 

study involving nutrient loads entering Chesapeake Bay: 

Water Resources Research, v. 28, no.9, p. 2,352–2,363.

Committee on Environmental and Natural Resources, 

National Science and Technology Council, 2000, Integrated 

assessment of hypoxia in the northern Gulf of Mexico: 

Washington, D.C., May 2000, 58 p.

Crawford, C.G., 1996, Estimating mean constituent loads 

in rivers by the rating-curve and flow-duration, ratingcurve methods: Ph.D. dissertation, Indiana University, 

Bloomington, Indiana, 245 p.

Gilroy, E.J., Hirsh, 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.

The H. John Heinz III Center for Science, Economics and 

the Environment, 2002, The state of the nation’s ecosystems—Measuring the lands, waters, and living resources 

of the United States: The Heinz Center, 270 p.

Johnson, A.H., 1979, Estimating solute transport in streams 

from grab samples: Water Resources Research, v. 15, no. 5, 

p. 1,224–1,228.

Knud-Hansen, Chris, 1994, Historical perspective of the 

phosphate detergent conflict, in Fall 1993 Natural Resources 

and Environmental Policy Seminar of the University of 

Colorado Interdisciplinary Graduate Certificate Program 

in Environmental Policy, Conflict Research Consortium, 

University of Colorado, working paper 94-54.

Rabalais, N.N., and Turner, R.E., 2001, Hypoxia in the 

northern Gulf of Mexico: Description, causes, and change, 

in Rabalais, N.N., and Turner, R.E., eds., Coastal hypoxia: 

Consequences for living resources and ecosystems: 

American Geophysical Union, Coastal and estuarine 

studies series 58, p. 1–36.

Rabalais, N.N., Turner, R.E., and Weisman, W.J., Jr., 2001, 

Hypoxia in the Gulf of Mexico: Journal of Environmental 

Quality, v. 30, p. 320–329.

Rickert, D.A., 1992, Analytical methods – Discontinuation of 

the National Water Quality Laboratory determinations for 

“total” nitrate, “total” nitrite plus nitrate, “total” ammonia, and “total” orthophosphate (using the four-channel 

analyzer): U.S. Geological Survey Office of Water Quality 

Technical Memorandum 93.04, December 2, 1992.

Runkel, R.L., Crawford, C.G., and Cohn, T.A., 2004, Load 

estimator (LOADEST): A FORTRAN program for estimating constituent loads in streams and rivers: U.S. Geological 

Survey Techniques and Methods, book 4, chap. A5, 69 p.

Scavia, Donald, Rabalais, N.N., Turner, R.E., Dubravko, Justic, 

and Wiseman, W.J., Jr., 2003, Predicting the response of 

Gulf of Mexico hypoxia to variations in Mississippi River 

nitrogen load: Limnology and Oceanography, v. 48, no. 3, 

p. 951–956.

Semkin R.G., Jefferies, D.S., and Clair, T.A., 1994, Hydrochemical methods and relationships for study of stream 

output from small catchments, in Moldan, Bedřich, and 

Černý, Jiří, eds., Biogeochemistry of small catchments: 

A tool for environmental research: John Wiley and Sons, 

New York, p. 163–187.

Smith, R.A., Alexander, R.A., and Lanfear, K.J., 1993, Stream 

water quality in the conterminous United States—Status 

and trends of selected indicators during the 1980’s, in

National water summary 1990–91—Stream water quality, 

U.S. Geological Survey Water-Supply Paper 2400, 

p. 111–140.

U.S. Environmental Protection Agency, 2003, Draft report on 

the environment 2003: EPA 600-R-03-050, 167 p.

12 Annual Dissolved Nitrite Plus Nitrate and Total Phosphorous Loads
Manuscript approved for publication, March 3, 2006

Prepared by USGS Georgia Water Science Center

Edited by Patricia L. Nobles

Graphics by Caryl J. Wipperfurth

For more information concerning the research in this report, contact 

USGS Georgia Water Science Center, Atlanta, 

telephone: 770-903-9100