U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5213 

National Water-Quality Assessment Program 

Trends in Streamflow and Nutrient and Suspended-Sediment 

Concentrations and Loads in the Upper Mississippi, Ohio, 

Red, and Great Lakes River Basins, 1975–2004 
Cover: Contrast in sediment loads in the Mississippi and St. Croix Rivers at their confluence. (Photograph by David Morrison, Minnesota 

Pollution Control Agency.) 
Trends in Streamflow and Nutrient and 

Suspended-Sediment Concentrations and 

Loads in the Upper Mississippi, Ohio, Red, 

and Great Lakes River Basins, 1975–2004 

By David L. Lorenz, Dale M. Robertson, David W. Hall, and David A. Saad 

National Water-Quality Assessment Program 

Scientific Investigations Report 2008–5213 

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Lorenz, D.L., Robertson, D.M., Hall, D.W., and Saad, D.A., 2009, Trends in streamflow and nutrient and suspendedsediment concentrations and loads in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004: 

U.S. Geological Survey Scientific Investigations Report 2008–5213, 81 p. 
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific information that helps to enhance and protect the overall quality of life and that facilitates effective management 

of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation’s water 

resources is critical to ensuring long-term availability of water that is safe for drinking and recreation and is 

suitable for industry, irrigation, and fish and wildlife. Population growth and increasing demands for water make 

the availability of that water, now measured in terms of quantity and quality, even more essential to the longterm sustainability of our communities and ecosystems. 

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support 

national, regional, State, and local information needs and decisions related to water-quality management and 

policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of 

our Nation’s streams and groundwater? How are conditions changing over time? How do natural features and 

human activities affect the quality of streams and groundwater, and where are those effects most pronounced? 

By combining information on water chemistry, physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights for current and emerging water issues and priorities. 

From 1991-2001, the NAWQA Program completed interdisciplinary assessments and established a baseline 

understanding of water-quality conditions in 51 of the Nation’s river basins and aquifers, referred to as Study 

Units (http://water.usgs.gov/nawqa/studyu.html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments of waterquality conditions and trends. These regional assessments are based on major river basins and principal aquifers, 

which encompass larger regions of the country than the Study Units. Regional assessments extend the findings 

in the Study Units by filling critical gaps in characterizing the quality of surface water and groundwater, and by 

determining status and trends at sites that have been consistently monitored for more than a decade. In addition, 

the regional assessments continue to build an understanding of how natural features and human activities affect 

water quality. Many of the regional assessments employ modeling and other scientific tools, developed on the 

basis of data collected at individual sites, to help extend knowledge of water quality to unmonitored, yet comparable areas within the regions. The models thereby enhance the value of our existing data and our understanding 

of the hydrologic system. In addition, the models are useful in evaluating various resource-management scenarios and in predicting how our actions, such as reducing or managing nonpoint and point sources of contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect water conditions within 

a region. 

Other activities planned during the second decade include continuing national syntheses of information 

on pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic ecology; 

and continuing national topical studies on the fate of agricultural chemicals, effects of urbanization on stream 

ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants to public-supply wells. 

The USGS aims to disseminate credible, timely, and relevant science information to address practical and effective water-resource management and strategies that protect and restore water quality. We hope 

this NAWQA publication will provide you with insights and information to meet your needs, and will foster 

increased citizen awareness and involvement in the protection and restoration of our Nation’s waters.

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and 

conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice and information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as nongovernmental 

organizations, industry, academia, and other stakeholder groups. Your assistance and suggestions are greatly 

appreciated. 

Matthew C. Larsen

Associate Director for Water 
iv

Contents

Foreword ........................................................................................................................................................iii

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

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

Study Area and Environmental Setting ......................................................................................................3

Factors Affecting Nutrient and Suspended-Sediment Concentrations and Loads ...................3

Precipitation and Runoff.............................................................................................................3

Land Use........................................................................................................................................7

Fertilizers and Manure................................................................................................................7

Atmospheric Deposition .............................................................................................................7

Total Nonpoint Sources of Nutrients ......................................................................................13

Methods.........................................................................................................................................................13

Water-Quality Data....................................................................................................................13

Streamflow Data ........................................................................................................................15

Final Site and Sample Selection..............................................................................................15

Data Manipulations ...................................................................................................................19

Basin Boundaries and Land-Use Classification ...................................................................20

Trend and Load Computations ..........................................................................................................20

Computation of Loads for General Spatial Patterns ............................................................21

Computations of Trends and Detailed Loads for 1993 to 2004 ............................................21

Computations of Trends and Detailed Loads for 1975 to 2004 ............................................22

Explanation of Observed Trends..............................................................................................23

Trends in Streamflow ..................................................................................................................................23

Trends in Nutrient and Suspended-Sediment Concentrations and Loads.........................................26

Total Nitrogen ......................................................................................................................................26

General Patterns in Total Nitrogen Concentrations and Yields .........................................26

Trends in Total Nitrogen Concentrations ...............................................................................26

Trends in Total Nitrogen Loads................................................................................................32

Dissolved Ammonia............................................................................................................................34

General Patterns in Dissolved Ammonia Concentrations and Yields ...............................34

Trends in Dissolved Ammonia Concentrations .....................................................................34

Trends in Dissolved Ammonia Loads......................................................................................34

Total Kjeldahl Nitrogen.......................................................................................................................39

General Patterns in Total Kjeldahl Nitrogen Concentrations and Yields..........................39

Trends in Total Kjeldahl Nitrogen Concentrations................................................................39

Trends in Total Kjeldahl Nitrogen Loads ................................................................................39

Dissolved Nitrite plus Nitrate Nitrogen...........................................................................................39

General Patterns in Dissolved Nitrite plus Nitrate Nitrogen Concentrations 

and Yields.......................................................................................................................39

Trends in Dissolved Nitrite plus Nitrate Nitrogen Concentrations....................................39

Trends in Dissolved Nitrite plus Nitrate Loads......................................................................46
v

Total Phosphorus ................................................................................................................................46

General Patterns in Total Phosphorus Concentrations and Yields....................................46

Trends in Total Phosphorus Concentrations .........................................................................46

Trends in Total Phosphorus Loads ..........................................................................................52

Dissolved Phosphorus........................................................................................................................52

General Patterns in Dissolved Phosphorus Concentrations and Yields...........................52

Trends in Dissolved Phosphorus Concentrations.................................................................56

Trends in Dissolved Phosphorus Loads .................................................................................56

Total Suspended Materials ...............................................................................................................56

General Patterns in Concentrations and Yields of Total Suspended Materials ..............56

Trends in Total Suspended-Material Concentrations..........................................................56

Trends in Total Suspended-Material Loads...........................................................................62

Total Suspended Sediment................................................................................................................65

Trends in Total Suspended-Sediment Concentrations ........................................................65

Trends in Total Suspended-Sediment Loads.........................................................................65

Understanding Water-Quality Trends in the Context of Human Activity ............................................65

Total Nitrogen ......................................................................................................................................68

Dissolved Ammonia............................................................................................................................68

Total Kjeldahl Nitrogen.......................................................................................................................69

Dissolved Nitrite Plus Nitrate ...........................................................................................................69

Total Phosphorus ................................................................................................................................69

Dissolved Phosphorus........................................................................................................................70

Total Suspended Materials ...............................................................................................................71

Changes in Loads from 1993 to 2004 in a Longer Term Context ...........................................................71

Understanding Trends in Loads in the Context of Climatic Variability ................................................72

Comparison between Nutrient Loads from This Study and Loads Contributing to the 

Gulf of Mexico Hypoxic Zone ......................................................................................................72

Summary........................................................................................................................................................75

References Cited..........................................................................................................................................78 

Figures

1–3. Maps showing:

1. Study area, major river basins, and sites ........................................................................4

2. Average annual precipitation across the study area, 1971 to 2000.............................5

3. Average annual runoff across the study area, 1951 to 1980.........................................6

4. Graph showing annual precipitation from 1975 to 2005 at five sites in the study area.....6

5–12. Maps showing:

5. Land use and land cover across the study area, with the 10 largest 

metropolitan areas identified.............................................................................................8

6. Study area population by county in 2000 and county-level change 

from 1990 to 2000..................................................................................................................9

7. Fertilizer nitrogen application by county in 1997 and changes in 

application rates from 1993 to 2004 for nitrogen and phosphorus ............................10
vi

8. Manure nitrogen application rates by county in 1997 and changes in 

application rates from 1992 to 2002 for nitrogen and phosphorus ............................11

9. Rates of atmospheric deposition of nitrogen by county in 1997 and 

changes in atmospheric deposition rates from 1993 to 2004 .....................................12

10. Total annual input rates of nitrogen from nonpoint sources (fertilizers, 

manure, and atmospheric deposition) by county in 1997 and changes in 

the total input rates from 1993 to 2004 for nitrogen and phosphorus; 

atmospheric deposition of phosphorus was assumed to be negligible...................14

11. Sites used in the study......................................................................................................18

12. Trends in streamflow from 1993 to 2004 throughout the study area .........................25

13. Graphs showing long-term trends in streamflow from 1975 to 2003 at six 

selected sites in the study area ...............................................................................................27

14. Maps showing spatial patterns from 1970 to 2004 in median total nitrogen 

concentrations and median annual total nitrogen yields throughout the 

study area ....................................................................................................................................28

15. Maps showing trends in total nitrogen from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout the 

study area ....................................................................................................................................31

16. Graphs showing long-term flow-adjusted trends in total nitrogen concentrations 

from 1975 to 2004 at six selected sites in the study area .....................................................33

17. Graphs showing long-term trends in total nitrogen loads from 1975 to 2003 at 

six selected sites in the study area .........................................................................................35

18–22. Maps showing:

18. Trends in dissolved ammonia from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................38

19. Trends in total Kjeldahl nitrogen from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................42

20. Trends in dissolved nitrite plus nitrate from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................45

21. Spatial patterns from 1970 to 2004 in median total phosphorus 

concentrations and median annual total phosphorus yields throughout 

the study area.....................................................................................................................47

22. Trends in total phosphorus from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................50

23. Graphs showing long-term flow-adjusted trends in total phosphorus 

concentrations from 1975 to 2004 at six selected sites in the study area.........................51

24. Graphs showing long-term trends in total phosphorus loads from 1975 to 2003 

at six selected sites in the study area.....................................................................................53

25–27. Maps showing:

25. Trends in dissolved phosphorus from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................57

26. Spatial patterns from 1970 to 2004 in median total suspended-material 

concentrations and median annual total suspended-material yields 

throughout the study area................................................................................................58
vii

27. Trends in total suspended materials from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................61

28–32. Graphs showing:

28. Long-term flow-adjusted trends in total suspended-material 

concentrations from 1975 to 2004 at six selected long-term sites ............................63

29. Long-term trends in total suspended-material loads from 1975 to 2003 at 

six sites in the study area.................................................................................................64

30. Trends in total suspended sediment from 1993 to 2004 for flow-adjusted 

concentrations, overall concentrations, and annual loads throughout 

the study area.....................................................................................................................67

31. Relation between trends in flow-adjusted dissolved phosphorus 

concentrations and trends in phosphorus inputs from farm fertilizers for 

sites with varying types of land use in the basin, 1993 to 2004 ..................................70

32. Total annual nitrogen and phosphorus loads at the Mississippi River at 

Clinton, Iowa (site with the largest drainage area in this study) and from the 

entire Mississippi/Atchafalaya River (MAR) Basin from 1975 to 2005 as 

estimated in this study and by Aulenbach and others (2007).....................................74 

Tables

1. Description of sites used in trend analyses...........................................................................16

2. Adjustments applied to censored data values retrieved from the National 

Water Quality Laboratory ..........................................................................................................20

3. Nine predefined regression models for estimating loads ...................................................20

4. Summary of results for analyses of trends in streamflow at selected sites in 

the study area, 1993 to 2004 ......................................................................................................24

5. Summary of results for analyses of flow-adjusted trends, overall trends, 

and trends in loads for total nitrogen (TN, USGS parameter code 00600), 

1993 to 2004..................................................................................................................................29

6. Summary of flow-adjusted trends in total nitrogen concentrations (TN, 

USGS parameter code 00600) estimated by QWTREND for long-term evaluation 

sites, 1975 to 2004 .......................................................................................................................32

7. Summary of results for analyses of flow-adjusted trends, overall trends, and 

trends in loads for dissolved ammonia (NH4, USGS parameter code 00608), 

1933 to 2004..................................................................................................................................36

8. Summary of results for analyses of flow-adjusted trends, overall trends, and 

trends in loads for total Kjeldahl nitrogen (KJN, USGS parameter code 00625), 

1993 to 2004..................................................................................................................................40

9. Summary of results for analyses of flow-adjusted trends, overall trends, and 

trends in loads for dissolved nitrite plus nitrate nitrogen (NO3, USGS parameter 

code 00631), 1993 to 2004...........................................................................................................43

10. Summary of results for analyses of flow-adjusted trends, overall trends, 

and trends in loads for total phosphorus (TP, USGS parameter code 00665), 

1993 to 2004..................................................................................................................................48

11. Summary of flow-adjusted trends in total phosphorus concentrations (TP, 

USGS parameter code 00665) estimated by QWTREND for long-term evaluation 

sites, 1975 to 2004 .......................................................................................................................52
viii

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

meter (m) 1.094 yard (yd)

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi)

Area

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

Flow rate

cubic meter per second (m3

/s) 22.83 million gallons per day (Mgal/d)

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d)

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

kilogram per year (kg/yr) 2.205 pound per year (lb/yr)

Application rate

kilograms per square kilometer (kg/km2

) 0.8514 pounds per square mile (lb/mi2)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Water year is the 12-month period, October 1 through September 30, and is designated by the 

calendar year in which it ends. Thus, the water year ending September 30, 2003 is called “water 

year 2003.”

Conversion Factors and Datum

12. Summary of results for analyses of flow-adjusted trends, overall trends, and 

trends in loads for dissolved phosphorus (DP, USGS parameter code 00666), 

1993 to 2004..................................................................................................................................54

13. Summary of results for analyses of flow-adjusted trends, overall trends, 

and trends in loads for total suspended material (SMAT, USGS parameter 

codes 80154 and 00530 combined), 1993 to 2004 ...................................................................59

14. Summary of flow-adjusted trends in total suspended-material concentrations 

estimated by QWTREND for long-term evaluation sites, 1975 to 2004...............................62

15. Summary of results for analyses of flow-adjusted trends, overall trends, 

and trends in loads for total suspended sediment (SS, USGS parameter 

code 80154), 1993 to 2004...........................................................................................................66 
ix

Abbreviations used in report

Abbreviation Description

River basins

GL Great Lakes 

MAR Mississippi-Atchafalaya River

OH Ohio River, exclusive of the Tennessee River

RR Red River including the Red, Rainy, and Souris Rivers

UM Mississippi River upstream from confluence with the Ohio River, 

exclusive of the Missouri River 

Constituents

DP Dissolved phosphorus

KJN Total (whole water) organic plus ammonia nitrogen (Kjeldahl)

NH4 Dissolved ammonia

NO2NO3 Dissolved nitrite plus nitrate

SMAT Total suspended material

SS Total suspended sediment

TN Total (whole water) nitrogen

TP Total (whole water) phosphorus

Equation symbols

β estimated coefficient

C log concentration

e base of natural logarithms

F unit conversion factor

FAC log of the flow-adjusted concentration

HFV high-frequency variability component

L daily load

Q log streamflow

Qa center value for streamflow

Q* centered log of streamflow

T m midpoint of the period of record

T a centering value of time

% / FAC yr percent change in flow-adjusted trend per year
x

Abbreviation Description

Other abbreviations

< less than

≤ less than or equal to

> greater than

≥ greater than or equal to

AIC Akaike information criterion

CAFO confined animal feeding operations

FAC flow-adjusted concentration

FA trend flow-adjusted trend

MDL method detection limit

NAWQA National Water-Quality Assessment

NWIS National Water Information System

NWQL National Water Quality Laboratory

OA trend overall trend in concentration

QL quantitation limit

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Abbreviations used in report—continued 
Trends in Streamflow and Nutrient and SuspendedSediment Concentrations and Loads in the Upper 

Mississippi, Ohio, Red, and Great Lakes River Basins, 

1975–2004 

By David L. Lorenz, Dale M. Robertson, David W. Hall, and David A. Saad 

Abstract

Many actions have been taken to reduce nutrient and 

suspended-sediment concentrations and the amount of 

nutrients and sediment transported in streams as a result of 

the Clean Water Act and subsequent regulations. This report 

assesses how nutrient and suspended-sediment concentrations 

and loads in selected streams have changed during recent 

years to determine if these actions have been successful.

Flow-adjusted and overall trends in concentrations and 

trends in loads from 1993 to 2004 were computed for total 

nitrogen, dissolved ammonia, total organic nitrogen plus 

ammonia, dissolved nitrite plus nitrate, total phosphorus, dissolved phosphorus, total suspended material (total suspended 

solids or suspended sediment), and total suspended sediment 

for 49 sites in the Upper Mississippi, Ohio, Red, and Great 

Lakes Basins. Changes in total nitrogen, total phosphorus, and 

total suspended-material loads were examined from 1975 to 

2003 at six sites to provide a longer term context for the data 

examined from 1993 to 2004.

Flow-adjusted trends in total nitrogen concentrations at 

19 of 24 sites showed tendency toward increasing concentrations, and overall trends in total nitrogen concentrations at 16 

of the 24 sites showed a general tendency toward increasing 

concentrations. The trends in these flow-adjusted total nitrogen 

concentrations are related to the changes in fertilizer nitrogen 

applications. Flow-adjusted trends in dissolved ammonia concentrations from 1993 to 2004 showed a widespread tendency 

toward decreasing concentrations. The widespread, downward 

trends in dissolved ammonia concentrations indicate that some 

of the ammonia reduction goals of the Clean Water Act are 

being met. Flow-adjusted and overall trends in total organic 

plus ammonia nitrogen concentrations from 1993 to 2004 did 

not show a distinct spatial pattern. Flow-adjusted and overall 

trends in dissolved nitrite plus nitrate concentrations from 

1993 to 2004 also did not show a distinct spatial pattern. 

Flow-adjusted trends in total phosphorus concentrations 

were upward at 24 of 40 sites. Overall trends in total phosphorus concentrations were mixed and showed no spatial pattern. 

Flow-adjusted and overall trends in dissolved phosphorus 

concentrations were consistently downward at all of the sites 

in the eastern part of the basins studied. The reduction in 

phosphorus fertilizer use and manure production east of the 

Mississippi River could explain most of the observed trends in 

dissolved phosphorus. 

Flow-adjusted trends in total suspended-material concentrations showed distinct spatial patterns of increasing tendencies throughout the western part of the basins studied and in 

Illinois and decreasing concentrations throughout most of 

Wisconsin, Iowa, and in the eastern part of the basins studied. 

Flow-adjusted trends in total phosphorus were strongly related 

to the flow-adjusted trends in suspended materials. The trends 

in the flow-adjusted suspended-sediment concentrations from 

1993 to 2004 resembled those for suspended materials.

The long-term, nonmonotonic trends in total nitrogen, 

total phosphorus, and suspended-material loads for 1975 to 

2003 were described by local regression, LOESS, smoothing 

for six sites. The statistical significance of those trends cannot 

be determined; however, the long-term changes found for 

annual streamflow and load data indicate that the monotonic 

trends from 1993 to 2004 should not be extrapolated backward 

in time.

Introduction

Elevated nutrient and suspended-sediment concentrations are the two of the most common contaminant stressors 

affecting streams throughout the United States (U.S. Environmental Protection Agency, 1996). Excessive nutrients in rivers 

and streams can result in the overgrowth of benthic algae in 

shallow areas and areas with fast current and in an overabundance of phytoplankton and macrophytes in deep areas with 

slow current. High algal and macrophyte biomass, in turn, 

can cause severe diurnal fluctuations in dissolved oxygen and 

pH because of biotic production and respiration. Very low 

dissolved oxygen concentrations often occur when part of the 

algal or macrophyte population dies and decomposes, which 
2 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

in turn can generate harmful organic materials (Welch and others, 1992). Therefore, high nutrient concentration conditions 

can lead to an increase in the availability of toxic substances, 

a decrease in available aquatic habitat, modifications to the 

composition of the biotic communities, and a decrease in the 

overall usefulness of the stream (Miltner and Rankin, 1998; 

Dodds and Welch, 2000). Excessive transport of nutrients has 

also been linked to eutrophication of downstream lakes and 

impoundments, outbreaks of Pfiesteria in bays and estuaries in 

various Gulf and Mid-Atlantic States, and hypoxia in the Gulf 

of Mexico (U.S. Environmental Protection Agency, 2000). 

Suspended sediment reduces clarity in streams and affects 

sight-feeding fish. Suspended sediment also interferes with 

water-treatment processes and recreational uses of streams. 

Excessive siltation can bury and suffocate fish eggs and 

bottom-dwelling organisms. In addition to in-stream effects, 

excessive sediment loading causes sedimentation problems in 

many downstream lakes and harbors and water-clarity problems in nearshore areas. 

Many actions have been taken to reduce nutrient and 

suspended-sediment concentrations and the amount or load 

of nutrients and sediment transported in streams. In 1972, the 

Federal Water Pollution Control Act Amendments, commonly 

known as the Clean Water Act, were passed, and they established the basic structure for regulating discharges of pollutants into the waters of the United States (U.S. Environmental 

Protection Agency, 2006a). As result of the Clean Water Act, 

governmental efforts in the 1970s and 1980s primarily focused 

on regulating discharges from traditional “point source” facilities, such as municipal sewage plants and industrial facilities. In the 1980s and 1990s, the focus expanded to address 

nonpoint runoff, in other words, losses from agricultural areas, 

such as feedlots and fields, and losses from urban areas, such 

as construction sites and urban storm sewers. Under recommendations of the Clean Water Action Plan released in 1998, 

the U.S. Environmental Protection Agency (USEPA) developed a national strategy to develop waterbody-specific nutrient 

criteria for lakes and reservoirs, rivers and streams, wetlands, 

and estuaries (U.S. Environmental Protection Agency, 1998). 

The intent of this strategy is to require all States and tribes 

to establish nutrient standards that, if enforced, will continue 

to reduce nutrient concentrations and improve the beneficial 

ecological uses of surface waters. To determine if these actions 

have been successful, it is important to assess how nutrient 

and suspended-sediment concentrations and loads in streams 

have changed during recent years.

Changes in the concentrations of nutrients and suspended 

sediment in streams are the result of anthropogenic changes 

in the watershed, such as changes in the levels of sewage 

treatment or implementation of best management practices, 

and the result of changes in natural factors, such as streamflow 

or climatic change. To better detect changes as the result of 

anthropogenic factors, it is best to remove or factor out the 

variability associated with the natural factors. One of the most 

dominant natural factors affecting nutrient and suspended 

sediment in streams is streamflow. Concentrations of many 

water-quality constituents increase with increasing streamflow, especially those associated with particulates, whereas 

concentrations of others can decrease with increasing streamflow because of dilution. Therefore, when data are analyzed 

for trends, it is important to compensate for variability in 

streamflow. 

Identification and analysis of trends in streamflow is of 

great interest to water-resource managers, scientists, and the 

public for a variety of important reasons. Long-term trends in 

streamflow potentially affect water levels in lakes and reservoirs; the availability of habitat for aquatic organisms; the 

availability of water for anthropogenic uses including recreation, irrigation, water supply, and waste disposal; aspects 

of bridge and dam construction; and flooding of low lying 

land. Trends in streamflow caused by climate may magnify 

the effects of land-use changes associated with anthropogenic 

activities, such as when increased precipitation causes flooding that is exacerbated by the increased amount of impervious 

areas associated with increased urbanization. Therefore, it is 

important to identify the trends in streamflow in addition to 

the trends in water quality.

Although many actions have been taken to reduce the 

concentrations and loads of nutrients and suspended sediments 

in streams across the country, most studies that have tried to 

evaluate whether or not these actions have been successful 

have examined the water quality of only a few streams from a 

specific area (such as Corsi and others, 2005) or within a specific State (such as Ruppel, 2006, for Wisconsin or Zipper and 

others, 1998, for Virginia). Only a few studies have examined 

trends over large geographical areas. For example, Stoddard 

and others (2003) examined water quality in streams from 

selected regions of the eastern and midwestern areas of the 

United States to assess changes in water quality as the result of 

recent reductions in nitrogen emissions. 

To quantify how streamflow and nutrient and suspendedsediment concentrations and loads in streams across the country have changed as result of changes in land use and management practices, and the many actions taken to improve water 

quality, regional studies were initiated by the National WaterQuality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS). The objectives of the regional studies are 

to describe changes or trends in streamflow and the associated 

trends in nutrient and suspended-sediment concentrations and 

loads that have occurred during a 1993–2004 base period and 

to try to understand what may have caused any of the substantial changes in water quality. Changes in total nitrogen, total 

phosphorus, and total suspended-material loads were examined from 1975 to 2003 at six sites to provide a longer term 

context for the data examined from 1993 to 2004.

The purpose of this report is to describe trends in 

streamflow and concentrations and loads of selected nutrients 

including total nitrogen, dissolved ammonia, dissolved nitrite 

plus nitrate, total organic plus ammonia nitrogen (Kjeldahl), 

total phosphorus, and dissolved phosphorus; total suspended 

materials; and total suspended sediment from 1993 to 2004 in 

streams at 49 selected sites within the north-central part of the 
Study Area and Environmental Setting 3

United States. The report describes what may have caused the 

changes in water quality by examining changes in the major 

sources of nutrients in this area. Changes in concentrations 

and loads from 1975 to 2003 for six of these sites, which are 

distributed throughout the basins studied, are examined to 

place the trends observed for the 1993–2004 base period into a 

longer term context.

Study Area and Environmental Setting

The study area comprises four major river basins: the 

Upper Mississippi, Ohio, Red, and Great Lakes Basins (fig. 1). 

The Upper Mississippi River (UM) Basin is defined as the 

Mississippi River and its tributaries upstream from the confluence with the Ohio River, excluding the Missouri River. The 

Ohio River begins in eastern Pennsylvania at the confluence of 

the Allegheny and Monongahela Rivers and flows westward 

until it joins the Mississippi at Cairo, Ill. The Ohio River (OH) 

Basin includes the Ohio River and its tributaries, excluding 

the Tennessee River. The Red River (RR) Basin includes all 

rivers in the United States that drain northward to the southern 

basin of Lake Winnipeg, including the Red River of the North, 

the Souris River, and the Rainy River. The Great Lakes (GL) 

Basin includes all streams in the United States draining into 

the Great Lakes upstream from Montreal, Canada. Lake Michigan was joined with the Illinois River (UM Basin) through 

construction of the Chicago Sanitary and Ship Canal in 1900, 

a project engineered to reverse the flow of the Chicago River 

in order to divert Chicago wastewaters into the Upper Illinois 

River and away from the historic receiving water of Lake 

Michigan. 

Factors Affecting Nutrient and SuspendedSediment Concentrations and Loads

Various factors may cause changes in water-quality 

concentrations and yields such as climatic factors (precipitation and resulting runoff), land-use practices, and changes in 

the inputs of nutrients to the basin. To determine how these 

factors may have affected water quality, the distribution of 

each of these factors throughout the study area and how they 

have changed from 1993 to 2004 are described in the following sections.

Precipitation and Runoff

Precipitation and resulting runoff are the major factors 

that influence transport of nutrients from the land surface to 

streams. Precipitation ranges from about 38 cm/yr in the western part of the study area to about 150 cm/yr in the eastern part 

(fig. 2). The largest relative differences in precipitation are in 

the western part of the study area, and the smallest differences 

are in the eastern part. In the western part of the area, average 

precipitation ranges from about 38 to 80 cm/yr over about 

400 km (Winterstein and Lorenz, 2007). Evaporation ranges 

from about 97 to 102 cm/yr in the southwestern part of the 

study area to about 71 cm/yr in the north-central and southeastern parts of the study area (Farnsworth and others, 1982). 

These geographical differences in precipitation and evaporation result in a wide range in runoff from less than 2.5 cm/yr in 

the northwest to about 75 cm/yr in the southeastern part of the 

study area (fig. 3). This wide range in runoff results in a large 

variability in average streamflow volume and in loads of various water-quality constituents. 

The regional precipitation map (fig. 2) can be used to 

divide the study area into three different geographic/climatic 

parts: the western, central, and eastern parts. The western

part of the study area includes parts of North Dakota, South 

Dakota, and northwestern Minnesota. The climate is subhumid 

continental with warm summers and cold winters, and annual 

precipitation ranges from approximately 36 to 50 cm. The 

central part of the study area includes most of the UM Basin in 

addition to areas draining to the Great Lakes, including parts 

of Minnesota, Wisconsin, Iowa, Illinois, and Indiana. The 

central part has a predominantly humid continental climate 

with annual rainfall ranging from approximately 50 cm in 

Minnesota to 115 cm in southern Indiana. The eastern part of 

the study area includes the Upper Ohio River Basin in addition 

to other drainages to the east and includes Ohio and parts of 

Pennsylvania and New York. This area also is humid continental and has annual precipitation ranging from approximately 

85 cm in Ohio to 115 cm in Pennsylvania and New York. The 

northern regions of the central and eastern parts of the study 

area have warm summers and cold winters. The southern 

regions of the central and eastern parts of the study area have 

hot summers and cool winters.

Precipitation and associated streamflow generally has 

increased during the 20th century in the central part of the 

study area bordering the Great Lakes and UM Basin (Lins, 

2005; Hodgkins and others, 2007). Many studies have predicted greater climatic variability in the future that could have 

far-reaching effects on hydrology, such as earlier and smaller 

annual snowmelt and increased evapotranspiration; on fish, 

aquatic vegetation, and agriculture; and, in the event of lower 

lake levels, disruptions of shipping and commerce (Sousounis 

and Bisanz, 2000). 

Annual precipitation from 1975 to 2005 is shown for five 

sites in the study area (one site in the western part, two sites in 

the central part, and two sites in the eastern part) in figure 4, in 

which the period from 1993 to 2004 is identified. Precipitation 

in the western part increased during the period from 1993 to 

2004 as shown by the data from Grand Forks, N. Dak. During 

1993 to 2004, precipitation was variable in the central and 

eastern parts of the study area, except for a slight decrease in 

the precipitation at LaCrosse, Wis. The changes from 1993 to 

2004 in the western part of the study area appear to be part of 

a long-term increase in precipitation; whereas, no consistent 

long-term change in precipitation is apparent in any other part 

of the study area. 
NORTH DAKOTA MINNESOTA WISCONSINILLINOIS MICHIGAN INDIANA KENTUCKY WEST VIRGINIA IOWA SOUTH DAKOTA NEBRASKAKANSAS

OKLAHOMA

TEXAS

ARKANSAS

TENNESSEE

MISSISSIPPI ALABAMA GEORGIA

SOUTH CAROLINA

NORTH CARO

VIRGINI

PENNS

MISSOURI

COLORADO

Lake Michigan Lake Huron

Lake Superior

Lake Erie

Lake Ontario

Chicago

Cairo

EXPLANATION

Major river basins

Trend site and number (table 1)CityState boundaryWaterStudy boundary

Red River BasinGreat Lakes BasinOhio River BasinUpper Mississippi River Basin

0 100 200 MILES0 100 200 KILOMETERS

1

Ohio

River

Mississippi

River

Red 

River 

of the North8 1

7

2 6

5

4

14

15

16

17

18

19

20

23

22

11

12

13

24

32

31 21 35 33

25

27

26

36

28 37

29

38 39

34 30

42 41

43

44

40

45

47 46

45 48

4

9 10

3

102° 84°

45°

36°

Base map from U.S. Ge1:2,000,000, Albers proje

Figure 1. Study area, major river basins, and sites. Sites are described in detail in table 1. 
102° 84°

45°

36°

Base map from U.S. Geologic1:2,000,000, Albers projection, 

EXPLANATION

Annual precipitation, in centimeters

32.7–46.046.1–59.359.4–72.786.2–99.5

113–126127–139140–153154–16699.6–112

72.8–86.1

CityState boundaryStudy boundary

La Crosse

Columbus

Rochester

Milwaukee

Grand Forks

0 100 200 MILES0 100 200 KILOMETERS

Figure 2. Average annual precipitation across the study area, 1971 to 2000 (National Climatic Data Center, 2002). 
6 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

0 100 200 MILES

0 100 200 KILOMETERS

Line of equal runoff, 

in centimeters

EXPLANATION

36

1

3

5 10

13

15

20

20

25

30

20

23

25

38

41 46

38

36

30

20

20

25

25

30

36

38

41

41

41 46

46

51

56

61

64

51

56

51

64

61 51

76

64

71

66

36

1

3

5 10

13

15

20

20

25

30

20

23

25

38

41 46

38

38

36

36

30

20

30

20

25

25

30

36

38

41

41

41

41 46

46

51

56

61

64

64

51

51

64

61

61

51

76

64

71

66

25

76

102° 84°

45°

36°

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

1:2,000,000, Albers projection, NAD83

Figure 3. Average annual runoff across the study area, 1951 to 1980 (modified from Gebert and others, 1987). 

20

40

60

80

100

120

140

1975 1980 1985 1990 1995

1993 2004

2000 2005

TOTAL ANNUAL PRECIPITATION, IN CENTIMETERS

Grand Forks, N. Dak.

EXPLANATION

Milwaukee, Wis.

LaCrosse, Wis.

Columbus, Ohio

Rochester, N.Y.

Study period

Figure 4. Annual precipitation from 1975 to 2005 at five sites in the study area. 
Study Area and Environmental Setting 7

Land Use

Land use across the study area ranges from almost 

completely forested areas that are relatively unaffected by 

human activities in the north-central and southeastern parts, to 

extensively agricultural areas in the central part, to intensely 

urbanized areas around Chicago, Ill.; Milwaukee, Wis.; Minneapolis, Minn.; Detroit, Mich.; Cleveland, Columbus, and 

Cincinnati, Ohio; Pittsburgh, Penn.; Indianapolis, Ind.; and 

St. Louis, Mo. (fig. 5; U.S. Geological Survey, 2000). Population density by county is shown in fig. 6A (Geolytics, 2006). 

The extensive row-crop agricultural area (fig. 5) is known as 

the “corn belt,” where the major crops are corn and soybeans. 

Small grains, primarily wheat and barley, are important crops 

in the western parts of the study area. These differences in land 

use result in differences in the amount of nutrients applied to 

the land surface and, potentially, the amount of nutrients that 

may reach and be transported down a stream and the amount 

of sediment eroded from the land. 

The largest populations are found throughout the center 

of the study area, especially near the larger cities. Human 

population censuses, a surrogate for nutrients in wastewater 

effluent, were available for only 1990 and 2000. From 1990 

to 2000, the largest increases in population have occurred 

in the counties surrounding the largest cities in the central 

parts of Minnesota, Wisconsin, in the northern part of Lower 

Michigan, and the southeastern part of the study area, whereas 

populations have decreased in the far northwestern part of the 

study area (fig. 6B). Changes in population may have caused 

changes in the magnitude of point-source discharges of nutrients from these areas; however, improvements in wastewater 

facilities may have offset some water-quality effects attributable to the increased population.

Fertilizers and Manure

Farmers apply fertilizers and manure to increase crop 

production. When those sources exceed crop needs or have not 

been incorporated into the soil before rain, nutrients can drain 

into streams (U.S. Environmental Protection Agency, 2005). 

Rates of total fertilizer nitrogen application are shown in 

fig. 7A by county for 1997, which was the middle of the study 

period (Ruddy and others, 2006). The distribution in nitrogen 

fertilizer use closely resembles the intensity of agriculture 

across the study area (fig. 5). The highest fertilizer application 

rates were in the central and western parts of the study area 

and encompass nearly all of Indiana, Illinois, and Iowa. The 

distribution of the rates of total fertilizer phosphorus application (not shown) closely resembles that for nitrogen. Changes 

in the annual fertilizer application were quite variable; 

therefore, the changes in fertilizer inputs were obtained by 

computing the change in the average inputs from 1993–95 to 

2002–04. From about 1993 to 2004, rates of nitrogen fertilizer 

use generally have increased in northwestern North Dakota, 

and northeastern Minnesota, and most of Wisconsin, whereas 

rates of nitrogen use generally have decreased in the eastern 

part of the study area and in the eastern part of North Dakota 

(fig. 7B). From 1993 to 2004, rates of fertilizer phosphorus use 

generally have increased west of the Mississippi River, except 

in eastern part of North Dakota, and generally decreased east 

of the Mississippi River (fig. 7C). 

The rates of total manure nitrogen application are shown 

for 1997 by county in figure 8A (Ruddy and others, 2006). The 

highest application rates were in Iowa, southern Minnesota, 

southern Wisconsin and central Kentucky. The distribution 

of the rates of total manure phosphorus application closely 

resembles that for nitrogen (not shown). Manure data were 

available for only 1992, 1997, and 2002. From 1992 to 2002, 

the rates of manure nitrogen application have decreased by 

more than 15 percent throughout almost all of the study area 

except near the Iowa-Minnesota border, where the rates of 

application increased more than 25 percent (fig. 8B). From 

1992 to 2002, the rates of manure phosphorus application 

decreased by more than 15 percent throughout almost all of 

the study area except near the Iowa-Minnesota border, where 

the rates of application have increased more than 25 percent 

(fig. 8C).

Atmospheric Deposition

Atmospheric deposition can be an important source of 

nutrients to the land surface and to streams, especially in 

forested areas where there are few anthropogenic sources 

(U.S. Environmental Protection Agency, 2002). Atmospheric 

deposition of nitrogen can be elevated downwind of some 

types of agricultural animal production (Langland, 1992) 

and downwind of areas with fossil fuel combustion (Mason 

and others, 2002), such as cities. The annual rate of nitrogen 

input from atmospheric deposition in 1997 ranged from less 

than 350 kg/km2

 in the northwestern part of the study area to 

more than 600 kg/km2 in southern Michigan, northwestern 

and southern Illinois, northern and southern Indiana, northern 

and southern Ohio, northeastern and western Kentucky, and 

the northeastern part of the study area (fig. 9A; Ruddy and 

others, 2006). Because the rates of atmospheric deposition are 

fairly consistent from year to year, the change from 1993 to 

2004 is based on the values for those two years. From 1993 

to 2004, the rates of deposition have decreased or changed 

very little throughout the southern two thirds of the study area 

and increased in the far northwestern part of the study area 

(fig. 9B). 
0 100 200 0 100 200 KILOMET

EXPLANATION Land use/land cover Urban Transitional Forest/shrubland Grassland Pasture/hay/small grains/ orchard/vineyard Row crops Wetlands Water

CityState boundaryStudy boundary

Milwaukee

Detroit

Chicago

St. Louis

Indianapolis

Columbus

Cleveland Pittsburg

Cincinnati

Minneapolis/

St. Paul

Milwaukee

Detroit

Chicago

St. Louis

Indianapolis

Columbus

Cleveland Pittsburg

Cincinnati

Minneapolis/

St. Paul

102° 84°

45°

36°

Base map from U.S. Geologica1:2,000,000, Albers projection, 

Figure 5. Land use and land cover across the study area (U.S. Geological Survey, 2000), with the 10 largest metropolitan areas identified. 
Study Area and Environmental Setting 9

A. Population, 2000

B. Change in population, 1990–2000

0.66 – 6.15

Population, in persons per 

square kilometer

6.16 – 9.68

9.69 – 14.3

14.4 – 20.3

20.4 – 25.6

25.7 – 34.9

35.0 – 71.9

72.0 – 2,170

EXPLANATION

Study area

State boundary

< -15

Change in population, 

in percent 

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

County boundary

County boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 6. Study area population by A, county in 2000 and B, county-level change from 1990 to 2000 (Geolytics, 2006). 
10 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

A. Fertilizer nitrogen application rate, 1997

B. Change in fertilizer nitrogen application rate, 1993–1995 to 2002–2004

C. Change in fertilizer phosphorus application rate, 1993–1995 to 2002–2004

1 – 58

Application rate, in kilograms 

per square kilometer

59 – 224

225 – 597

598 – 1,240

1,241 – 2,110

2,111 – 3,840

3,841 – 6,160

6,161 – 9,300

EXPLANATION

Study area

State boundary

County boundary

County boundary

County boundary

< -15

Change in application rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

> 25

EXPLANATION

Study area

State boundary

< -15

Change in application rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

> 25

EXPLANATION

Study area

State boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 7. Fertilizer nitrogen application by A, county in 1997 and changes in application rates from 

1993 to 2004 for B, nitrogen and C, phosphorus. (Data from Ruddy and others, 2006.) 
Study Area and Environmental Setting 11

A. Manure nitrogen application rate, 1997

B. Change in manure nitrogen application rate, 1992–2002

C. Change in manure phosphorus application rate, 1992–2002

0 – 111

Application rate, in kilograms 

per square kilometer

112 – 295

296 – 520

521 – 726

727 –1,030

1 031 –1,360

1,361 – 1,810

1,811 – 7,090

EXPLANATION

Study area

State boundary

< -15

Change in application rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

< -15

Change in application rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

County boundary

County boundary

County boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 8. Manure nitrogen application rates by A, county in 1997 and changes in application rates from 

1992 to 2002 for B, nitrogen and C, phosphorus. (Data from Ruddy and others, 2006.) 
12 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

< -15

Change in deposition rate,

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

B. Change in atmospheric deposition rate, 1993–2004

74 – 348

Deposition rate, in kilograms 

per square kilometer

349 – 419

420 – 475

476 – 533

534 – 565

566 – 589

590 – 629

630 – 921

EXPLANATION

Study area

State boundary

County boundary

County boundary

A. Atmospheric nitrogen deposition rate, 1997

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 9. Rates of atmospheric deposition of nitrogen by A, county in 1997 and B, changes in atmospheric deposition rates 

from 1993 to 2004. (Data from Ruddy and others, 2006.) 
Methods 13

Total Nonpoint Sources of Nutrients

The nonpoint sources of nitrogen to streams include fertilizers from agricultural, urban, and suburban areas; manure 

spread on cropland or from livestock pastures; and atmospheric deposition of nitrogen from fossil fuel combustion and 

other sources. The pattern of total nitrogen input from nonpoint sources by county for 1997 (fig. 10A) closely resembles 

the amount of row crop agriculture (fig. 5) in the area and 

application of fertilizer (fig. 7A) with highest inputs in the central and western parts of the study area. In general, the input 

from fertilizers is the dominant source of nitrogen, except 

in forested areas where the input from atmospheric deposition predominates. The change in the total nonpoint nitrogen 

input rate from 1993 to 2004 was computed by use of the 

1992 and 2002 rates for manure, 3-year average rates around 

1993 and 2004 for fertilizers, and rates for 1993 and 2004 for 

atmospheric deposition. From about 1993 to 2004, the total 

nonpoint input of nitrogen has increased in western Iowa, 

southern and northeastern Minnesota, northern Wisconsin, 

and the western one-half of the Upper Peninsula of Michigan, 

and decreased in the central and eastern parts of the study area 

(fig. 10B). The changes are not as consistent or widespread 

as the changes in the individual sources. Most of the total 

nonpoint-source changes were the result of the changes in fertilizer and manure inputs. In places where these changes were 

similar—such as increases in western North Dakota, eastern 

Minnesota, and northern Iowa, and decreases in Ohio—overall 

changes in nonpoint-source inputs were large. In places where 

the changes differed in sign, such as in Wisconsin and Illinois, 

overall changes in nonpoint-source inputs were small.

The distribution in the total input of phosphorus from 

fertilizers and manure closely resembles that for nitrogen; the 

input from atmospheric deposition has not been estimated, but 

is expected to be small compared to other sources of phosphorus. From 1993 to 2004, the total input rates of phosphorus 

decreased east of the Mississippi River, near the Mississippi 

River in Missouri and Iowa, and in eastern North Dakota, 

whereas the input rates increased throughout most of Minnesota, western North Dakota and Iowa, and in and near Tennessee (fig. 10C). In most places, the types of the changes in the 

phosphorus inputs were similar for fertilizers and manure. 

The trends in the section “Trends in Nutrients and 

Suspended-Sediment Concentrations and Loads” are described 

with respect to patterns shown in figures 5–10 to determine 

if the changes in the input of nitrogen and phosphorus has 

affected the water quality, indicated by concentrations and 

loads, in the streams throughout the study area.

Methods

For this report, a trend is defined as a general change 

in concentrations or loads over time. Because of the complicating effects of natural and anthropogenic influences on 

water-quality conditions, it is important to consider three 

types of trends: (1) flow-adjusted trends in concentrations (FA 

trend)—the trends in concentrations that would have occurred 

in the absence of natural variability in streamflow and seasonal effects; (2) total or overall trends in concentrations (OA 

trend)—the trends that represent changes in the overall effects 

on the stream ecosystem and include the effects of streamflow and seasonal variability and the effects of anthropogenic 

activities; and (3) trends in the load of a constituent—trends 

that directly include the changes in streamflow and the overall 

changes in concentrations. Changes in constituent loads are 

most important to downstream water bodies, such as the Gulf 

of Mexico or Lake Michigan. The following sections describe 

data, site selection, and trend and load computations.

Water-Quality Data

Water-quality data for trend analyses were limited to the 

nutrients total nitrogen, dissolved ammonia, total Kjeldahl 

nitrogen, dissolved nitrite plus nitrate, and total and dissolved 

phosphorus; and total suspended-material and suspendedsediment data measured in streams in the study area for which 

sufficient data were available between 1970 and 2004. For 

the purpose of analysis, suspended sediment was examined as 

suspended sediment (USGS parameter code 80154) alone and 

combined with total suspended solids (USGS parameter code 

00530) into one constituent, total suspended material (SMAT). 

All data were compiled from data collected by the USGS 

and the major sampling agency(s) in each State. USGS data 

were retrieved from the National Water Information System 

(NWIS) and the NAWQA Program’s Data Warehouse (http://

water.usgs.gov/nawqa/data). All of the State-agency data were 

obtained from USEPA legacy and modernized STOrage and 

RETrieval databases, known as STORET (http://www.epa.gov/

storet/), except for data from Illinois, which were collected by 

the Illinois Environmental Protection Agency and contained in 

NWIS; Indiana, which were collected by the Indiana Department of Environmental Management (C. Bell, Indiana Department of Environmental Management written commun., 2004) 

and Wisconsin, which was collected by the Wisconsin Department of Natural Resources (J. Ruppel, Wisconsin Department 

of Natural Resources written commun., 2004). 
14 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

A. Total nonpoint nitrogen input rate, 1997

B. Change in total nonpoint nitrogen input rate, 1993–2004

C. Change in total nonpoint phosphorus input rate, 1993–2004

221 – 582

Input rate, in kilograms per 

square kilometer

583 – 1,430

1,431 – 2,040

2,041 – 2,730

2,731 – 4,130

4,131 – 5,640

5,641 – 7,860

7,861 – 14,700

EXPLANATION

Study area

State boundary

< -15

Change in input rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

< -15

Change in input rate, 

in percent

-15 – -10

-10 – -5

-5 – 5

5 – 10

10 – 15

15 – 25

>25

EXPLANATION

Study area

State boundary

County boundary

County boundary

County boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 10. Total annual input rates of nitrogen from nonpoint sources (fertilizers, manure, and atmospheric 

deposition) by A, county in 1997 and changes in the total input rates from 1993 to 2004 for B, nitrogen and 

C, phosphorus; atmospheric deposition of phosphorus was assumed to be negligible. 
Methods 15

Streamflow Data

For each water-quality site, a nearby streamflow gage 

was identified. A nearby gage was defined as a gage within 

the same stream system with a drainage-area ratio, which 

is the drainage area of the water-quality station divided by 

the drainage area of the streamflow gage, between about 0.3 

and 2.0. A nearby gage with a long record was chosen over 

a nearby gage with a very short record. At each streamflow 

gage, all of the available daily streamflow data from 1970 to 

2004 were obtained from USGS’s Automated Data-Processing 

System (ADAPS) within NWIS (U.S. Geological Survey, 

1998).

Final Site and Sample Selection

After the water-quality data were retrieved and matched 

to streamflow stations, the water-quality data were reviewed 

for completeness, quality-assurance samples were removed, 

and multiple daily samples were reduced to a single sample 

per day. For all of the multiple daily samples, the value of the 

constituent reported closest to noon was retained and all others 

deleted. Final site and sample selection involved three levels 

of screening. In the first level of screening, only sites with at 

least 15 samples for at least one constituent from a period of 

at least 2 years were retained, which resulted in approximately 

1,700 sites. Data from the first level of screening were used 

to describe general patterns in median total nitrogen, total 

phosphorus, and suspended-material concentrations in streams 

throughout the entire study area. 

A second level of screening was used to identify sites 

to describe general patterns in median annual yields or loads 

per unit area. Yields were computed to remove the effects 

caused by large differences in the size of the streams. Selection requirements for a site to be used to compute median 

annual yields were that it included at least 25 samples over a 

period of at least 2 years and at least 5 years of complete daily 

streamflow records; therefore, a site needed at least 5 years of 

complete estimated loads to compute a median annual yield. 

An annual yield was computed only if there were no missing 

daily flows for that year. The requirement of having 5 years 

of estimated load data to compute median values was used to 

reduce the potentially large effects of natural climatic variability. Approximately 700 sites were retained from the second 

level of screening. Data from the second level of screening 

were used to describe general patterns in median total nitrogen, total phosphorus, and suspended-material yields throughout the entire study area.

A third level of screening was used to select sites for 

the trend analyses. In this level, only sites that had at least 

10 years of data for at least one constituent between 1993 

and 2004, had at least 60 samples, and had a gage or a nearby 

gage with a drainage-area ratio ranging from 0.9 to 1.1 and 

streamflow record from 1993 to 2004. A base period of 1993 

to 2004 was selected because it is relevant to current conditions and represents the period since the beginning of the 

USGS NAWQA Program. For the final trend analyses in the 

base period, 49 sites met the criteria. The sites selected are 

listed in table 1 and shown on figure 1. In table 1, the sites are 

divided among the western, central, and eastern parts of the 

study area. Eight of the sites were in the western part, 31 sites 

were in the central part, and 10 sites were in the eastern part of 

the study area.

A fourth level of screening was applied to each of the 

49 selected sites for each constituent analyzed. At this level, 

10 years of data and at least 60 samples were required for 

the constituent to be analyzed at the site. For total nitrogen, 

24 sites were analyzed; for dissolved ammonia, 36 sites were 

analyzed; for total Kjeldahl nitrogen, 36 sites were analyzed; 

for dissolved nitrite plus nitrate, 29 sites were analyzed; 

for total phosphorus, 41 sites were analyzed; for dissolved 

phosphorus, 35 sites were analyzed; for SMAT, 42 sites were 

analyzed; and for total suspended sediment, 22 sites were 

analyzed.

To put the trends found for 1993 to 2004 into a longer 

term context, data from a subset of the final 49 sites were 

analyzed for long-term trends in flow-adjusted concentrations 

from 1975 to 2004. The trends in the flow-adjusted concentrations at six of these sites, which are distributed throughout 

the study area (table 1 and fig. 11), are shown graphically for 

each constituent. One site was in the western part, four sites 

were in the central part, and one site was in the eastern part of 

the study area. Of the four sites in the central part, three sites 

represented relatively small basins, and one site, Mississippi 

River at Clinton, Iowa, represented a large basin that integrates 

most of this area. Long-term trends in loads also were examined and shown graphically for these six sites. A description of 

the drainage basin for each of these six sites follows. Trends 

for the six long-term sites are presented, in order, on the basis 

of west to east.

The long-term western site drains the Souris River Basin, 

which occupies approximately 7,900 km2

 in southeastern 

Saskatchewan and southwestern Manitoba in Canada and 

north-central North Dakota in the United States. The Souris 

River starts in southeastern Saskatchewan, enters the United 

States and flows southward before looping back into Canada. 

Most of the drainage basin of the Souris River near Sherwood, 

N. Dak., is in Canada and contains substantial forested, prairie, 

and agricultural (mostly grain crops) areas. Three large dams 

are upstream from the monitoring site: the Rafferty Alameda 

Dams, built in the 1990s and the Boundary Dam, built in 1957 

(The Encyclopedia of Saskatchewan, 2008). The presence of 

these dams indicates that the flow at this site is intensively 

managed. A list of publications relating to water quality, land 

use, and basin characteristics in the Souris River Basin was 

assembled by the U.S. Geological Survey (2006b).
Table 1. Description of sites used in trend analyses. [USGS, U.S. Geological Survey; km2, square kilometer; agr, agriculture; for, forest; mix, mixed urban, agriculture, and forest; sites in bold indicate long-term evaluation sites] USGS site number Site name State Map number (figure 1) Drainage area (km2) Major river basin Land use / 

Percent urban Percent agriculture 

Western part 05056100 Mauvais near Cando ND 1 977 Red River 0.59 56.52 

05056200 Edmore Coulee near Edmore ND 2 731 Red River .34 92.72 05056239 Starkweather Coulee near Webster ND 3 544 Red River .24 84.33 05058700 Sheyenne River at Lisbon ND 4 11,370 Red River .38 76.77 05064500 Red River of the North at Halstad MN 5 46,620 Red River .63 78.65 

05082500 Red River of the North at Grand Forks ND 6 68,117 Red River .65 72.22 05114000 Souris River near Sherwood1 ND 7 7,876 Red River .02 7.45 

05120000 Souris River near Verendrye1 ND 8 11,399 Red River .26 17.54 

Central part 04063700 Popple River near Fence WI 9 360 Great Lakes 0.03 3.36 

04067500 Menominee River near McAllister WI 10 10,181 Great Lakes .41 6.22 

04073468 Green Lake Inlet near Green Lake WI 11 139 Great Lakes 3.88 78.96 04085427 Manitowoc River at Manitowoc WI 12 1,363 Great Lakes 1.41 78.53 04087000 Milwaukee River at Milwaukee WI 13 1,803 Great Lakes 8.30 65.11 

05287890 Elm Creek near Champlin MN 14 223 Upper Mississippi River 8.13 55.32 

05340500 St. Croix River at St. Croix Falls WI 15 16,166 Upper Mississippi River .57 23.25 05369500 Chippewa River at Durand WI 16 23,342 Upper Mississippi River .62 33.13 

05382000 Black River near Galesville WI 17 5,389 Upper Mississippi River .61 39.95 05389400 Bloody Run Creek near Marquette IA 18 88 Upper Mississippi River 1.02 69.03 

05389500 Mississippi River at McGregor IA 19 174,870 Upper Mississippi River 1.74 53.57 

05407000 Wisconsin River at Muscoda WI 20 26,943 Upper Mississippi River 1.15 37.84 05420500 Mississippi River at Clinton IA 21 221,762 Upper Mississippi River 1.68 52.96 

05427718 Yahara River at Windsor WI 22 191 Upper Mississippi River 2.37 91.71 05427948 Pheasant Branch at Middleton WI 23 44 Upper Mississippi River 6.10 85.81 054310157 Jackson Creek Tributary near Elkhorn WI 24 11 Upper Mississippi River 19.43 72.77 

05465500 Iowa River at Wapello IA 25 32,383 Upper Mississippi River 3.12 83.63 

05474000 Skunk River at Augusta IA 26 11,171 Upper Mississippi River 2.91 81.11 

05481650 Des Moines River near Saylorville IA 27 15,132 Upper Mississippi River 2.15 88.93 

05500000 South Fabius River near Taylor MO 28 1,606 Upper Mississippi River 1.45 67.92 

05514500 Cuivre River near Troy MO 29 2,339 Upper Mississippi River .52 72.68 

05525500 Sugar Creek at Milford IL 30 1,155 Upper Mississippi River .57 97.25 
USGS

site number Site name State

Map 

number 

(figure 1)

Drainage 

area 

(km2)

Major river basin

Land use / 

Percent 

urban

Percent 

agriculture

Central part—continued

05531500 Salt Creek at Western Springs IL 31 298 Upper Mississippi River 62.87 2.20 05532500 Des Plaines River at Riverside IL 32 1,632 Upper Mississippi River 38.85 31.98 05540275 Spring Brook at 87th St. near Naperville IL 33 26 Upper Mississippi River 27.11 58.56 05543500 Illinois River at Marseilles IL 34 21,396 Upper Mississippi River 11.97 71.74 05552500 Fox River at Dayton IL 35 6,586 Upper Mississippi River 9.19 67.81 05570000 Spoon River at Seveille IL 36 4,238 Upper Mississippi River .82 84.50 05586100 Illinois River at Valley City IL 37 69,285 Upper Mississippi River 5.68 78.49 07014500 Meramec River near Sullivan MO 38 3,821 Upper Mississippi River .54 20.75 

07018100 Big River near Richwoods MO 39 1,904 Upper Mississippi River 1.41 21.22 

Eastern part

03085000 Monongahela River at Braddock PA 40 19,008 Ohio River 2.60 20.18 

03267900 Mad River at St Paris Pike at Eagle City OH 41 803 Ohio River 2.40 80.20 

03353637 Little Buck Creek near Indianapolis IN 42 44 Ohio River 44.72 41.69 

04186500 Auglaize River near Fort Jennings OH 43 860 Great Lakes 2.05 88.53 

04193500 Maumee River at Waterville OH 44 16,399 Great Lakes 2.61 87.09 

0422026250 Northrup Creek at North Greece NY 45 26 Great Lakes 11.92 54.94 

04232034 Irondequoit Creek at Railroad Mills near 

Fishers

NY 46 102 Great Lakes 2.82 58.34 

0423204920 East Branch Allen Creek at Pittsford NY 47 18 Great Lakes 18.58 53.32 

0423205025 Irondequoit Creek at Empire Boulevard in 

Rochester

NY 48 371 Great Lakes 22.62 41.35 

04237946 Onondaga Creek Tributary number 6 

downstream of mudboil area at Tully

NY 49 1 Great Lakes .17 33.96 

1Statistics on land use/land cover represent U.S. portion only. 

Table 1. Description of sites used in the analyses.—Continued [USGS, U.S. Geological Survey; km2, square kilometer; agr, agriculture; for, forest; mix, mixed urban, agriculture, and forest; sites in bold indicate long-term evaluation sites]
8 1 7 2 6 5 4 14 1516 17 18 19 20 23 2211 12 13 24 3231 33 35 21 25 27 2636 28 37 2938 39 34 30 42 41 4344 40 45474648 

10 9 3 EXPLANATION Trend site and number State boundary Study boundary Watershed boundary Water Long-term integrator site basin Long-term trends basin

0 100 200 MILES0 100 200 KILOMETERS

12

102° 84°

45°

36°

Base map from U.S. Geo1:2,000,000, Albers proje

Figure 11. Sites used in the study. Drainage basins of the long-term trend sites are filled in red; the basin of the long-term integrator site on the Mi

in red. Sites are described in detail in table 1. 
Methods 19

The four long-term central sites are the Chippewa River 

at Durand, Wis., the Iowa River at Wapello, Iowa, the Milwaukee River at Milwaukee, Wis., and the Mississippi River at 

Clinton, Iowa. The Chippewa River Basin is in northwestern 

Wisconsin and has an area of about 23,300 km2. The upper 

part of this basin is largely undeveloped, contains several 

major parks and many State wildlife areas, and is one of the 

least-populated parts of the State. Major tributaries to the 

upper part of the Chippewa River include the Flambeau, Jump, 

Thornapple, Manitowish, Couderay, and Elk Rivers. The lower 

part of the basin has areas of agriculture and several moderate sized cities, such as Eau Claire. Major tributaries to the 

lower part of the Chippewa River include the Red Cedar and 

Eau Claire Rivers. The lower Chippewa River Basin contains 

many large areas of undeveloped native prairie and forest with 

numerous rare and endangered species of plants of animals 

(Wisconsin Department of Natural Resources, 2006). The 

site described in this report is located approximately 25 km 

upstream from the confluence of the Chippewa River and the 

Mississippi River. The characteristics of the downstream part 

of basin are described by Voss and Beaster (2001). 

The Iowa River at Wapello, Iowa, is located near the confluence of the Iowa River and the Mississippi River. The drainage area at the monitoring site is approximately 32,400 km2

and extends from east-central Iowa to the Mississippi River. 

The Cedar River is a major tributary. The population of the 

basin was approximately 772,000 with 22 percent agricultural land use in 1960 and is expected to reach a population 

of nearly 2 million with only 4 percent agricultural land use 

by 2020 as farmland and forest lands are converted to urban, 

industrial, and residential uses (U.S. Army Corps of Engineers, 

2004). Flow at this site is influenced by management of the 

Coralville Dam and reservoir, substantial irrigation demands, 

periodic flooding and flood plain management, and construction of agricultural levees. The basin has numerous waterquality problems including habitat degradation throughout the 

basin and severe problems with sedimentation in the lower 

parts of this basin (U.S. Army Corps of Engineers, 2004).

The Milwaukee River Basin comprises six basins that 

collectively cover approximately 1,800 km2

. Grasslands, 

wetlands, forested lands, and agricultural land account for a 

large percentage of land in the upper Milwaukee River Basin; 

however, many of these areas are facing pressure from urban 

development as the greater Milwaukee metropolitan area continually expands to the northwest. The Milwaukee River and 

Estuary, downstream from the monitoring site, are listed by 

the USEPA as being an Area of Concern (U.S. Environmental 

Protection Agency, 2006b), where 11 water-quality impairments exist. Water-quality problems in the intensely urbanized 

lower Milwaukee River have caused beach closings, restrictions on fish consumption, restrictions on dredging activities, 

degradation of phytoplankton and zooplankton, restrictions 

on drinking-water supplies, extensive eutrophication, and 

disruption of animal reproductive cycles and health. Recently 

published reports documenting the water quality in the basin 

include Schneider and others (2004) and Hall (2006). 

The Upper Mississippi River drains parts of Minnesota, 

Wisconsin, Iowa, and Illinois, and represents an integration of most of the central part of the study area. The Upper 

Mississippi River is one of the most regulated waterways in 

the world, with numerous dams, irrigation channels, locks, 

dredged waterways, and artificial channels. The site at Clinton, 

Iowa, has a drainage area of approximately 222,000 km2

. The 

Upper Mississippi River Basin includes all types of land use, 

including a diverse array of habitats for birds, terrestrial animals, and aquatic species. A summary of water quality in the 

Upper Mississippi River was published by Sullivan and others 

(2002). 

The long-term site in the eastern part of the study area 

is in the Maumee River Basin. The river flows through parts 

of Indiana, Ohio, and Michigan and has the largest drainage 

area of any Great Lakes river, with approximately 16,400 km2. 

Approximately 90 percent of the basin is agricultural, with 

the remaining area consisting of forest, urban, and industrial 

land use. The Maumee River Basin is the largest single source 

of sediment to Lake Erie and is one of the USEPA’s “Areas 

of Concern” because of water-quality problems attributed to 

agricultural nonpoint-source pollution, contaminated industrial 

sites, combined sewer overflows, periodic dredging of the 

shipping channel sediments, and disposal of dredged materials 

(U.S. Environmental Protection Agency, 2006c). 

Data Manipulations

Beginning in 1999, the USGS National Water Quality 

Laboratory (NWQL) began reporting the quantitation limit 

(QL) in addition to the method detection limit (MDL) for 

reporting censored data. The QL is larger than the MDL to 

reduce the chance of the actual concentration being greater 

than the MDL and reporting the data as censored at the MDL. 

Helsel (2005) reported biased results when using censored 

values at the QL instead of the MDL and offered three methods for modifying the data to avoid biased results. Method 1, 

which used the MDL instead of the QL, was used for all trend 

analyses. Table 2 summarizes the adjustments to censored 

data retrieved from the NWQL. All of the censored data used 

to describe the general spatial patterns in concentrations and 

yields were set to one-half of the detection limit.

Other adjustments to the reported concentrations also 

were made to data retrieved from the NWQL (David K. Mueller, U.S. Geological Survey, electronic commun., November 9, 

2004). For dissolved ammonia, all concentrations reported 

with a value less than 0.02 mg/L prior to September 30, 

1997, were set to less than 0.02 mg/L. For total and dissolved 

phosphorus, all concentrations reported as a value less than 

0.03 mg/L prior to September 30, 1998, were set to less than 

0.05 mg/L. Total nitrogen concentrations were not always 

reported in the databases, but individual nitrogen constituents 

were reported. Therefore, if dissolved nitrite plus nitrate and 

total Kjeldahl nitrogen concentrations were reported, total 

nitrogen concentrations were computed from the sum of these 

two constituents. 
20 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Basin Boundaries and Land-Use Classification

Basin boundaries for all of the streams selected in the 

first level of screening to describe general spatial patterns were 

delineated and digitized from 1:24,000-scale USGS topographic quadrangle maps or 1:100,000-scale digital coverage 

of the USGS Hydrologic Unit maps (Seaber and others, 1984) 

refined with digital RF3 stream coverages (Clifford and others, 

1993). 

Each of the basins of the final 49 sites was classified into 

one of four land-use categories: urban, agriculture, forest, or 

mixed on the basis of the dominant land use from the NLCD 

land use and land cover data in figure 5 (table 1). If urban land 

use exceeded 10 percent of the basin, but was not the dominant land use, then the class “mixed” was used for that basin.

Trend and Load Computations 

A generalized least-squares regression (GLS) method 

was used to model trends in streamflow. The model used all 

daily streamflow measurements available over the analysis 

period. It relates the log of daily streamflow to an intercept, a 

linear trend term that is measured by decimal time, and sine 

and cosine seasonal factors (also functions of decimal time). 

The GLS method accounts for long-term serial correlation 

of the streamflow data. The results are expressed as percent 

change per year, and the reference flow is the mean flow for 

the analysis period.

The regression method, which also is referred to as the 

rating-curve method, was used to quantify loads and evaluate trends in water quality. This method commonly is used 

for load estimation when infrequent water-quality data are 

available over long periods of time (Cohn, 2005). Daily 

loads (L) were computed on the basis of relations between 

constituent load and two explanatory variables—streamflow 

and time. The regression models use the log-transform of load 

and streamflow to improve the linear relation and account 

for multiplicative errors. Nine different predefined regression 

models were used to estimate loads (table 3). Streamflow and 

time are transformed to produce variables that are statistically independent when squared terms are included, a process 

called centering. Because a log transformation was used in 

the models, computed daily loads were adjusted to account 

for a retransformation bias by use of the minimum variance 

unbiased estimate procedure (Cohn and others, 1989) or the 

adjusted maximum likelihood method (Cohn, 2005). 

Table 2. Adjustments applied to censored data values retrieved from the National Water Quality Laboratory. 

[All values in milligrams per liter; <, less than]

Constituent Time frame Description of adjustment

Total nitrogen No adjustments

Dissolved ammonia Prior to Sept. 30, 1997 Multiple censored values less than 0.02 were set to <0.02

After Sept. 31, 2000 Censored values (<0.041) were set to <0.021

Kjeldahl nitrogen Sept. 30, 1998, to Sept. 30, 2000, 

and after Sept. 30, 2001

Censored values (<0.1) were set to <0.05

Sept. 31, 2000, to Sept. 30, 2001 Censored values (<0.05) were set to <0.04

Dissolved nitrite plus nitrate Sept. 30, 2000, to Sept. 30, 2002 Censored values (<0.05) were set to <0.023

After Sept. 31, 2002 Censored values (<0.05 and <0.06) were set to <0.03

Total phosphorus Prior to Sept. 30, 1998 Values less than 0.03 were set to <0.05

Sept. 30, 1999, to Sept. 30, 2000 Censored values (<0.008) set to <0.004

After Sept. 30, 2000 Censored values (<0.0037) set to <0.0019

Dissolved phosphorus Prior to Sept. 30, 1998 Values less than 0.03 were set to <0.05

Sept. 30, 1999, to Sept. 30, 2000 Censored values (<0.008) set to <0.004

After Sept. 30, 2000 Censored values (<0.0037) set to <0.0019

Table 3. Nine predefined regression models for estimating loads. 

[L, natural logarithm (log) of the estimated load; Q*, centered log of 

streamflow; T 

*

, centered time (in decimal years); T, fraction of the year 

(expressed in decimal years); ßi, estimated coefficients]

Model

number Model description

1 L = ß0

 + ß1Q*

2 L = ß0

 + ß1Q* + ß2Q*2

3 L = ß0

 + ß1Q* + ß2T *

4 L = ß0

 + ß1Q* + ß2 sin(2πT) + ß3cos(2T)

5 L = ß0

 + ß1Q* + ß2Q*2 + ß3T*

6 L = ß0

 + ß1Q* + ß2Q*2 + ß3sin(2πT) + ß4cos(2πT)

7 L = ß0

 + ß1Q* + ß2T * + ß3sin(2πT) + ß4cos(2πT)

8 L = ß0

 + ß1Q* + ß2Q*2 + ß3T* + ß4sin(2πT) + ß5cos(2πT)

9 L = ß0

 + ß1Q* + ß2Q*2 + ß3T* + ß4T 

*2+ ß5

sin(2πT)

 + ß6cos(2πT)
Methods 21

Computation of Loads for General Spatial 

Patterns

To describe the general patterns in yields or loads per 

unit area, annual loads, which are calculated by summing 

daily loads, were estimated by use of the Fluxmaster program 

(Schwarz and others, 2006). To simplify processing for this 

part of the analysis, model 7 (table 3) was used to compute 

loads for all constituents at all sites rather than calibrating a 

model for each basin. All of the water-quality data from 1970 

to 2004 with available corresponding daily streamflow values 

were used in the analysis to estimate the regression coefficients. Daily loads were estimated for each site from 1970 to 

2004. Total annual loads for general spatial patterns were computed for all water years, defined as the period from October 1 

of the previous calendar year through September 30 of the current year, that had no missing daily streamflow values. Median 

annual loads were then computed for each site that had at least 

5 years of data. Yields were then computed by dividing the 

median annual load by the drainage area of the basin.

The general spatial patterns of concentrations are 

expressed as median midmonthly concentrations, which are 

intended to reduce the variability of the random sampling 

of the constituent. The number of samples collected at each 

site was quite variable and ranged from quarterly to over a 

hundred in a year. Therefore, to obtain representative statistical summaries that were not biased by intensively sampled 

periods, the data were subsampled to monthly intervals. For 

each site, only one sample per constituent per month per year 

was used. The value included in the statistical summaries was 

the one collected closest to the middle of the month (midmonthly value). The midmonthly median is the median of all 

midmonthly values at a site.

Computations of Trends and Detailed Loads for 

1993 to 2004

A version of LOADEST (Runkel and others, 2004) 

converted to run in S-PLUS (Insightful, 2005) was used to 

compute FA trends and loads. For each constituent at each site, 

the best model (table 3) identified by the Akaike information 

criterion (AIC) (Akaike, 1974) was selected and validated 

using all of the data from water year 1993 through water year 

2004 (October 1, 1992, through September 30, 2004). The validation included checking the residuals for a linear fit, uniform 

scatter around the fit, normality of distribution, and linearity 

with all explanatory variables. If the model failed validation, 

then regression models were examined individually until a 

valid model was identified, or if no valid model was found, the 

analysis was dropped. For the analysis of trends, a time term 

was added to the model if it was not in the selected model.

The FA trend in concentrations at a site can be estimated 

from the coefficients in the equation chosen in the regression method. Consider the seven-parameter model shown 

by model 9 in table 3. With adjustment to the estimated 

parameters, this could be a concentration model because 

L = C + Q + F, where C is the log concentration, Q is the log 

streamflow, and F is a units conversion factor. Only β0 and β1

need adjustment, as follows:

β’0 = β0 – F – Qa, (1)

and

β’1 = β1 – 1, (2)

where

Qa is the center value for streamflow and

β2 in model 9 is not changed because Q is 

orthogonal (uncorrelated) to Q*2. 

No other terms need adjustment because there is no streamflow component in them. The linear time terms ( β3

 and β4 ) in 

model 9 describe the FA trend. This assumes that the “true” 

model for FA is quadratic in log streamflow and first order 

Fourier seasonal component. 

The FA trend in concentrations expressed as an average 

percentage change per year is computed from the coefficients 

from the linear time terms, as follows:

% / ( ) ( ( ) ) FAC yr e Tm Ta= − − + 100 1 2 4 3 b b

, (3)

where

is the percent change in FA trend per year,

is the base of the natural logarithms,

is the coefficient for the second-order linear 

time term in model 9 and zero otherwise,

is the midpoint of the period of record,

is the centering value of time, and

is the coefficient for linear time.

OA trends in concentrations and trends in loads are 

defined as the changes in model-estimated concentrations and 

loads over the period of the water-quality record, divided by 

the length of that record. The model-estimated concentrations 

and loads are determined by fitting separate trend models for 

streamflow and for the water-quality constituent. The streamflow model used the GLS method. The water-quality model, 

estimated from all water-quality measurements collected 

during the analysis period, is one of the models in table 3 

that include a linear decimal time term; these model numbers 

are 3, 5, 7, 8, or 9. The OA trend in the log of water-quality 

concentration is determined by streamflow and time trend 

components of the water-quality model, where the effect of the 

trend of streamflow is based on the coefficient for streamflow 

in the water-quality model. The OA trends in concentrations 

and loads are obtained by transforming the model-estimated, 

water-quality trend from log space to real space, computing 

the percentage of change corresponding to the first and last 

dates of the water-quality period of record, and normalizing 

by the length of this period. The significance of the trend was 

estimated by the p-value of a t-test using standard errors of 

estimated typical values. A more complete description of the 

methods used to compute trends is given in Sprague and others 

(2006), where the OA trend is referred to as nonflow-adjusted 

trend.

% / FAC yr

e

b4

T m

Ta

b3
22 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

The results for nutrients and suspended materials are 

reported in estimates of unit trends—trends expressed in the 

units of concentration or load; for example, milligrams per 

liter per year (mg/L/yr) or kilograms per year (kg/yr). The 

unit trends in concentrations and loads are determined by 

multiplying the estimated trends in concentrations and loads 

by the appropriate reference value for concentration or load. 

For example, if the estimated trend was 10 percent per year 

and the reference concentration was 3 mg/L, then the unit 

trend would be 0.10/yr * 3 mg/L = 0.3 mg/L/yr. The reference concentration is obtained by evaluating the water-quality 

model at reference conditions consistent with the trend in 

water quality at the beginning of the water-quality period of 

record. These conditions include setting the log of streamflow 

to its smoothed trend value corresponding to the first day of 

the water-quality period, setting the trend term to the decimal 

equivalent of the first day of the water-quality period, and 

setting the sine and cosine seasonal factors to their average 

values over the full water-quality period. The log value of 

the reference concentration is transformed to real space and a 

multiplicative retransformation factor is applied to correct for 

statistical bias arising from sample error in the water-quality 

model coefficients. The reference load is computed similarly, 

except the logarithm of streamflow trend, as determined by 

the linear streamflow equation evaluated at the starting date 

of the water-quality period, is added to the log value of the 

reference concentration prior to transformation to real space; 

in addition, a multiplicative constant is applied to convert the 

result to appropriate load units. The same reference concentration is used to derive the FA and OA trends in concentrations; 

the same reference load is used in the evaluation of trends in 

loads.

The FA trends in concentrations, being independent of 

streamflow conditions, are best used to evaluate changes in 

water quality arising from changes in pollution sources or 

management activities within a basin. Conversely, the OA 

trend is indicative of the water-quality changes that riverine habitats have actually experienced. If there is no trend 

in streamflow over time, then the two estimates of trend are 

equivalent. Because the water-quality model used to derive 

these trends includes streamflow as a predictor, the estimates 

of trend are immune to potential biases arising from preferential water-quality sampling during high-streamflow events. 

Care should be taken, however, in interpolating or extrapolating these trend estimates within or beyond a site’s period of 

record or in making comparisons of trend across sites that 

have different periods of record. Due to the possible nonlinearity of trends, such as arising from nonlinear specifications of 

the water-quality model’s streamflow or trend components, 

trends within the water-quality period, or trends experienced 

outside this period, could be quite different from the trends 

reported for this study. 

The attained p-values, the probability of obtaining a 

statistic as likely as the computed statistic when the null 

hypothesis is true, associated with each observation are based 

on a two-sided test, which means that either an upward or a 

downward trend could be identified (Helsel and Hirsch, 2002). 

The null hypothesis for a two-tailed trend test is that the trend 

is zero. The alternative hypothesis is that the trend is not zero. 

For a particular criterion for rejecting the null hypothesis, the 

critical region of the test statistic is in the tails of the distribution. For example, if the criterion is set to 0.05, a value in the 

inner 95 percent of the test statistic would not result in rejecting the null hypothesis, but a value in either the upper or lower 

2.5 percent would result in rejecting the null hypothesis and 

accepting the alternative hypothesis that there is a trend in the 

data. To characterize regional patterns in trends, a 0.1 criterion 

was used to reject the null hypothesis. 

When making multiple tests of trends, the effects of those 

multiple tests on accepting or rejecting the null hypothesis 

must be considered. For example, use of a criterion of 0.05 for 

rejecting the null hypothesis would result in an incorrect conclusion in 5 percent of the tests. For a single test, that could be 

an acceptable risk. For the 49 sites that were analyzed in this 

study, it could be expected at two or three sites where a trend 

was identified (the null hypothesis of no trend is rejected) that 

no real trend would exist. The reverse case also is possible: for 

several sites, there could be a real trend but the null hypothesis 

was not rejected because the trend was not large enough to 

detect given the noise of the observed data. The probabilities 

for the reverse case were not determined for this study.

Computations of Trends and Detailed Loads for 

1975 to 2004

To place trends found for 1993–2004 into a longer term 

context, all of the sites with sufficient data, defined as sites 

with a minimum of approximately 200 observations from 1975 

to 2004 and that had data that covered most of the extended 

period with some gaps, were analyzed for long-term changes 

in flow-adjusted concentrations (FAC). In addition, data 

from the six long-term site subset described in detail previously in the section “Final Site and Sample Selection” were 

analyzed for long-term changes in loads from 1975 to 2004. 

Long-term changes in FAC and loads were assessed using the 

QWTREND program (Vecchia, 2003a, 2005) and the LOADEST program (Runkel and others, 2004). QWTREND uses 

a time-series model for estimating trends in FAC. The basic 

form of the model is

FAC = Intercept + Annual + Seasonal + Trend + HFV (4)

where

FAC is the log of the flow-adjusted concentration; 

Intercept is the intercept term; 

Annual is the annual streamflow-related anomaly 

composed of 5-year and 1-year moving 

average streamflow anomalies; 
Trends in Streamflow 23

Seasonal is the seasonal streamflow-related anomalies, 

composed of a 3-month moving average 

streamflow anomaly and second-order 

Fourier terms that describe seasonal 

variation; 

Trend is the trend component that can include trend 

terms based on time or ancillary variables 

that explain the trend; and 

HFV is the high-frequency variability component 

that includes the high-frequency 

component of streamflow, which is the 

daily fluctuations after the longer term 

anomalies have been removed, and 

periodic autoregressive coefficients to 

remove the non-random fluctuation in the 

observed concentration data. 

Vecchia (2000) describes the estimation of the time-series 

parameters, and Vecchia (2003b) describes the computation of 

the anomalies.

The suggested minimum data requirements for 

QWTREND (Vecchia, 2000) are (1) minimum water-quality 

record length of 15 years, (2) average of at least four samples 

per year, (3) at least 10 samples within each quarter of the 

year, (4) less than 10 percent censored data, and (5) complete 

streamflow record for the water-quality record for the period 

of interest plus the preceding 5 years. These minimum data 

requirements do not guarantee convergence of the program, 

therefore, an average of at least 6.5 samples per year were 

required for this analysis. The long-term analysis covered 

30 years; therefore, about 200 samples were required for each 

site. For some sites, slightly less than 200 samples were used 

because not all constituents were quantified on any given 

sampling day.

QWTREND was used to determine when changes in the 

trend during the 1975–2003 period were statistically significant. Once these changes were identified, the nine different 

predefined regression models (table 3) were examined for each 

trend period and the best load model identified by AIC was 

computed using LOADEST. The time variable in the model 

was then used to identify changes in the FAC. Total annual 

loads, computed by summing daily loads, were then computed 

for 1975–2003 by use of the various models specified for FAC 

and the use of LOADEST. Annual loads were calculated for 

each calendar year. Because streamflow data were not assembled for the period from October through December 2004, 

the annual loads for 2004 were not estimated. The LOESS 

smoothing procedure in S-PLUS (Insightful, 2005) was then 

used to identify the time-weighted central tendency, or moving 

average, of the total annual loads. 

It should be noted that the methodology used to evaluate 

long-term trends is insensitive to changes in the variability 

of streamflow or to changes in the unexplained variability of 

water quality. Both of these changes potentially result in trends 

in water quality arising from nonlinearity in the specification 

of the water-quality model. Accommodation of the effects of 

these uncertainties awaits future research.

Explanation of Observed Trends

Implications from the analysis of the influence of human 

activity on trends in constituents were generally based on 

broad regional patterns rather than the results of specific 

analyses. These general tendencies were not analyzed on a 

statistical basis—no test for regional trend was made.

In addition to comparing patterns in the changes in 

concentrations and loads with the changes in the inputs, 

a regression approach was used to relate likely sources of 

nutrients to the observed trends. Changes in fertilizer inputs 

(fig. 7) were used as an ancillary variable in the trend analysis 

of the agricultural sites, and annual inputs from fertilizers and 

atmospheric deposition (figs. 7 and 9) were used as ancillary variables in the analysis of trends of the forested sites. 

For these ancillary variables, the data were log-transformed 

and included in the regression models as a substitute for the 

time term. The model used for each site was similar to the 

water-quality model and is shown in tables presented for each 

constituent in the section “Trends in Nutrient and SuspendedSediment Concentrations and Loads.” If the p-value of the 

ancillary variable was less than 0.10, then it was deemed 

significant and a plausible candidate to explain the trend in 

the context of human activity. Because manure input data 

were available only at 5-year intervals, those data could not be 

used in the regression model approach. To assess the change 

in concentration with the change in manure input for agricultural sites, the changes in manure input were compared to the 

changes in concentration at each site, and the significance of 

the relation was assessed using the p-value from the Spearman 

rank correlation test (Helsel and Hirsch, 2002). 

Trends in Streamflow

Daily streamflows were obtained for the USGS gaging 

stations at the 49 sites and were analyzed for upward or 

downward monotonic trends from 1993 to 2004. The p-values 

in table 4 indicate the significance of the trend term and 

are used in figure 12 to demonstrate the significance of the 

trend (p < 0.05, 0.05 < p < 0.10, and p > 0.10). The trends 

in streamflow throughout the study area showed a general 

pattern: small changes in streamflow in the western part, 

significant decreases in streamflow throughout most of 

the central part, and either no change or slight increases in 

streamflow in the eastern part of the study area (fig. 12 and 

summarized in table 4). In the western part of the study area, 

only one of the eight stations, the most western site, had a 

significant trend. In the central part, 18 of 31 stations had 

significant decreases in streamflow. Two of the 10 stations in 

the eastern part of the study area had significant increases in 

streamflow. The changes in streamflow are consistent with 

the changes in precipitation that occurred throughout this area 

during this period (fig. 4). 
24 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Table 4. Summary of results for analyses of trends in streamflow at selected sites in the study 

area, 1993 to 2004. 

[USGS, U.S. Geological Survey; ID, identification; m3/s, cubic meter per second; ft

3

/s, cubic foot per second; 

%/yr, percent per year; sites in bold indicate long-term evaluation sites] 

USGS site 

number

StateID number

(figure 1)

Reference flow

Trend

(%/yr) p-value (m3

/s) (ft

3

/s)

Western part

05056100 ND-1 0.30 10.6 -4.90 0.435

05056200 ND-2 .53 18.7 -6.87 .169

05056239 ND-3 .56 19.9 -6.50 .169

05058700 ND-4 7.22 255 -2.47 .387

05064500 MN-5 62.8 2,220 -1.70 .406

05082500 ND-6 120 4,250 -2.63 .173

05114000 ND-7 .17 5.89 45.28 .085

05120000 ND-8 1.17 41.4 5.35 .496

Central part

04063700 WI-9 2.64 93.3 -2.99 0.004

04067500 WI-10 75.2 2,660 -.03 .977

04073468 WI-11 .85 29.9 -2.29 .321

04085427 WI-12 4.19 148 -2.71 .256

04087000 WI-13 10.2 362 -1.44 .386

05287890 MN-14 .71 25.2 -5.44 .028

05340500 WI-15 116 4,080 -1.09 .282

05369500 WI-16 206 7,270 -1.25 .285

05382000 WI-17 45.9 1,620 -3.41 .002

05389400 IA-18 .79 27.8 -4.36 .000

05389500 IA-19 1,320 46,600 -2.97 .001

05407000 WI-20 264 9,330 -2.26 .006

05420500 IA-21 1,860 65,800 -3.22 .000

05427718 WI-22 .73 25.7 -2.73 .001

05427948 WI-23 .09 3.18 -.51 .677

054310157 WI-24 .04 1.38 .26 .917

05465500 IA-25 286 10,100 -4.84 .005

05474000 IA-26 63.8 2,260 -5.89 .012

05481650 IA-27 99.5 3,520 -6.63 .000

05500000 MO-28 4.05 143 -6.27 .020

05514500 MO-29 3.53 125 -.51 .924

05525500 IL-30 4.56 161 -2.94 .440

05531500 IL-31 3.84 136 -.48 .676

05532500 IL-32 16.3 576 -1.56 .261

05540275 IL-33 .18 6.40 -3.43 .119

05543500 IL-34 302 10,700 -3.84 .001

05552500 IL-35 60.0 2,120 -3.65 .008

05570000 IL-36 30.1 1,060 -5.68 .008

05586100 IL-37 785 27,800 -4.71 .001

07014500 MO-38 35.4 1,250 -4.89 .000

07018100 MO-39 14.7 521 -4.61 .002
Methods 25

USGS site 

number

StateID number

(figure 1)

Reference flow

Trend

(%/yr) p-value (m3

/s) (ft

3

/s)

Eastern part

03085000 PA-40 210 7,410 2.48 0.275

03267900 OH-41 4.76 168 26.87 .000

03353637 IN-42 .27 9.42 -2.79 .364

04186500 OH-43 2.37 83.9 -.34 .928

04193500 OH-44 59.7 2,110 .57 .847

0422026250 NY-45 .20 7.00 5.90 .031

04232034 NY-46 .83 29.4 -.91 .464

0423204920 NY-47 .15 5.28 -1.99 .274

0423205025 NY-48 3.04 108 -2.04 .153

04237946 NY-49 .02 .69 .31 .841

Table 4. Summary of results for analyses of trends in streamflow at selected sites in the study 

area, 1993 to 2004.—Continued 

[USGS, U.S. Geological Survey; ID, identification; km2, square kilometer; m3/s, cubic meter per second; ft

3

/s, 

cubic foot per second; %/yr, percent per year; sites in bold indicate long-term evaluation sites]

EXPLANATION

Trends in streamflow

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 100 200 MILES

0 100 200 KILOMETERS

Figure 12. Trends in streamflow from 1993 to 2004 throughout the study area. 
26 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Long-term records in streamflow from 1975 to 2003 can 

be used to understand trends in streamflow from 1993 to 2004. 

The shorter term trends can be compared to the longer term 

trends and variability. Long-term trends in streamflow were 

examined at 6 of the 49 sites (fig. 13). For each site, mean 

annual streamflows were computed as the average of the mean 

daily streamflows, and then a LOESS smoothing procedure 

was used to indicate the long-term trends in mean annual 

streamflows. 

Streamflow trends from 1993 to 2004 observed at each 

of the six long-term sites agree with the general trend for that 

period as shown in figure 13. For all sites except the Souris 

River, the range and variability of streamflow during the shortterm period matched the range and variability of streamflow 

during the long-term period. The Souris River shows a period 

from 1984 to 1995 when the annual mean daily streamflow 

never exceeded 4 m3/s. There could be some effect from that 

period on the trends estimated at that site.

Trends in Nutrient and SuspendedSediment Concentrations and Loads

For each constituent, the trends in concentrations and 

loads are summarized in a table and shown spatially in a 

figure. The FA and OA trends in concentrations and trends in 

loads represent the monotonic trend from 1993 to 2004 and 

are expressed as a change in percentage per year and as a 

change in milligrams per liter per year or kilograms per year. 

The p-values in the tables indicate the significance of the trend 

term and are used in the figure to demonstrate the significance 

of the trend (p < 0.05, 0.05 < p < 0.10, and p > 0.10). The reference concentration and load in the tables are typical values 

for that site in the beginning of the period and do not represent 

a summary statistic such as a mean or median value. 

Total Nitrogen

General Patterns in Total Nitrogen 

Concentrations and Yields

From 1970 to 2004, median midmonthly total nitrogen 

(TN) concentrations across the study area ranged from 0.102 

to 206 mg/L. The overall mean and median values of the 

median concentrations at these sites were 2.65 and 1.70 mg/L, 

respectively. The highest concentrations were in the western, southwestern, and south-central parts of the study area 

(fig. 14A). The lowest concentrations were mostly in northern 

Wisconsin and Michigan and in the northeastern and southeastern parts of the study area. The highest concentrations 

were in areas with intensive agriculture, and the lowest concentrations were in forested areas (fig. 5). Median midmonthly 

TN concentrations at the 24 trend sites, based on water-quality 

data from 1993 to 2004, ranged from 0.314 to 6.22 mg/L. The 

distribution in concentrations at these sites was consistent with 

that in figure 14A. The mean and median values of the median 

concentrations at the trend sites were 2.07 and 1.41 mg/L, 

respectively.

Median annual TN yields, expressed as loads per unit 

area, ranged from about 2.36 to 18,300 kg/km2

 based on 

water-quality and streamflow data from 1970 to 2004. The 

overall mean and median values of the median yields at these 

sites were 1,280 and 771 kg/km2, respectively. The highest 

yields were throughout the middle of the study area (fig. 14B). 

The lowest yields were throughout the northern part of the 

study area. The highest yields were in areas with intense agriculture and moderate runoff. The areas with the highest runoff 

had relatively low TN concentrations, such as in northern Wisconsin and southeastern parts of the study area. The major difference in the distributions between concentrations and yields 

was in the northwestern part of the study area, which had high 

concentrations but low yields. Median annual TN yields at the 

trend sites ranged from 12 to 3,310 kg/km2 (table 5), based 

on water-quality and streamflow data from 1993 to 2004. The 

mean and median values of the median yields from each site 

were 753 and 446 kg/km2, respectively. The distribution in 

yields was consistent with yields shown in figure 14B. 

Trends in Total Nitrogen Concentrations

From 1993 to 2004, the FA trends in TN concentrations 

at 19 of the 24 sites that had sufficient data for analysis show a 

tendency toward increasing concentrations (table 5; fig. 15A). 

The 19 sites with increasing TN concentrations were distributed throughout the entire study area; however, only 9 of these 

sites had significant upward trends (p < 0.10) and were all 

located in the central part of the study area. These upward FA 

trends indicate that the effects of anthropogenic activities are 

continuing to increase in these areas and add additional TN 

to the streams. Only two of the five sites that had downward 

trends in concentrations had significant trends; those sites were 

Spring Brook in northern Illinois in the central part, and Red 

River of the North in the western part of the study area. 

When the OA trends in TN concentrations were examined 

for the period 1993–2004, 16 of the 24 sites showed a general 

tendency toward increasing concentrations (table 5; fig. 15B); 

however, only three of these sites had significant upward 

trends—two sites in southern Wisconsin in the central part of 

the study area and one site in North Dakota the western part of 

the study area. Of the eight sites with a downward trend, only 

two sites had significant trends—the same two sites that had 

downward FA trends. The sites with downward trends in concentrations were scattered throughout the study area, indicating no regional trend. 
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 27

A. B.

C. D.

E. F. ANNUAL MEAN DAILY STREAMFLOW, IN CUBIC METERS PER SECOND ANNUAL MEAN DAILY STREAMFLOW,

IN CUBIC METERS PER SECOND

ANNUAL MEAN DAILY STREAMFLOW,

IN CUBIC METERS PER SECOND

ANNUAL MEAN DAILY STREAMFLOW,

IN CUBIC METERS PER SECOND

ANNUAL MEAN DAILY STREAMFLOW,

IN CUBIC METERS PER SECOND

ANNUAL MEAN DAILY STREAMFLOW,

IN CUBIC METERS PER SECOND

1970 1975 1980 1985 1990 1995 2000 2005

0

5

10

15

20

Souris River near Sherwood, North Dakota

USGS station number 05114000

1970 1975 1980 1985 1990 1995 2000 2005

100

150

200

250

300

Chippewa River at Durand, Wisconsin

USGS station number 05369500

1970 1975 1980 1985 1990 1995 2000 2005

200

0

400

600

800

Iowa River at Wapello, Iowa

USGS station number 05465500

1970 1975 1980 1985 1990 1995 2000 2005

5

10

15

20

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

1970 1975 1980 1985 1990 1995 2000 2005

YEAR

200

700

1,200

1,700

2,200

Mississippi River at Clinton, Iowa

USGS station number 05420500

1970 1975 1980 1985 1990 1995 2000 2005

YEAR

50

100

150

200

Maumee River at Waterville, Ohio

USGS station number 04193500

LOESS smoothing line

Annual flow

EXPLANATION

Figure 13. Long-term trends in streamflow from 1975 to 2003 at six selected sites in the study area. 
28 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Median annual total nitrogen

yield, in kilograms per square

kilometer

EXPLANATION

B. Median annual total nitrogen yield

2.36 – 302

303 – 555

556 – 947

948 – 1,970

1,971 – 18,300

A. Median total nitrogen concentration

0.102 – 0.774

Median total nitrogen

concentration, in milligrams

per liter

0.775 – 1.34

1.35 – 2.20

2.21 – 4.21

4.22 – 206

EXPLANATION

Study area

State boundary

Basin boundary

Study area

State boundary

Basin boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 14. Spatial patterns from 1970 to 2004 in A, median total nitrogen concentrations and B, median annual total nitrogen 

yields throughout the study area. 
Table 5. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total nitrogen (TN, USGS parameter code 00[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites]

USGS

site

number

State-ID 

number 

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) T

(k

Western part

05056100 ND-1 9 1.284 0.061 4.76 0.110 0.086 6.71 0.080 NA NA 12,500 

05056200 ND-2 7 1.727 .037 2.15 .325 .046 2.66 .223 NA NA 28,800 

05056239 ND-3 7 1.749 .025 1.45 .637 .032 1.81 .570 NA NA 31,800 

05058700 ND-4 3 1.233 .025 2.05 .499 .015 1.18 .689 747,000 116 281,000 

05082500 ND-6 7 1.513 -.062 -4.08 .075 -.067 -4.43 .038 8,460,000 860 5,940,000 

05114000 ND-7 9 2.012 -.024 -1.20 .298 -.029 -1.42 .256 106,000 13.5 11,000 

05120000 ND-8 7 1.474 .007 .45 .742 .011 .72 .623 136,000 12.0 55,600 

Central part

04063700 WI-9 9 0.447 0.006 1.43 0.061 0.001 0.14 0.860 61,400 171 37,200 

04067500 WI-10 3 .428 -.001 -.02 .991 -.001 -.03 .986 1,330,000 131 1,020,000 

04087000 WI-13 7 1.735 .045 2.61 .000 .043 2.48 .000 1,030,000 570 562,000 

05287890 MN-14 9 1.251 .024 1.96 .032 -.005 -.41 .743 72,100 323 25,700 

05340500 WI-15 9 .539 .001 .15 .943 -.001 -.28 .893 3,180,000 197 1,970,000 -

05369500 WI-16 7 1.213 .022 1.85 .003 .024 2.01 .002 10,000,000 428 7,900,000 

05382000 WI-17 7 1.243 .018 1.43 .061 .013 1.08 .138 2,400,000 446 1,800,000 -

05407000 WI-20 7 1.354 .023 1.67 .104 .020 1.46 .111 12,300,000 456 11,300,000 -1

05420500 IA-21 5 2.096 .054 2.56 .079 .010 .46 .730 131,000,000 591 118,000,000 -3,5

05465500 IA-25 8 5.804 .250 4.30 .008 .061 1.04 .519 63,300,000 1,950 52,700,000 -2,4

05540275 IL-33 7 1.599 -.043 -2.72 .002 -.053 -3.32 .000 20,800 810 9,200 

05586100 IL-37 9 3.978 .149 3.75 .016 .060 1.51 .292 137,000,000 1,970 89,300,000 -3,8

07018100 MO-39 3 .307 .007 2.42 .609 -.003 -.91 .826 606,000 318 125,000 
USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) T

(

Eastern part 03085000 PA-40 5 0.869 0.028 3.26 0.118 0.025 2.86 0.163 13,000,000 682 5,810,000 3

03353637 IN-42 7 1.048 -.003 -.27 .749 -.010 -.97 .386 30,200 686 8,650 

04186500 OH-43 7 3.390 .080 2.37 .421 .077 2.26 .470 2,850,000 3,310 260,000 

04193500 OH-44 9 2.908 .087 2.98 .126 .093 3.20 .145 38,400,000 2,340 5,300,000 2

Table 5. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total nitrogen (TN, USGS parameter code 002004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] 
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 31

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

TOTAL NITROGEN

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 15. Trends in total nitrogen from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
32 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

The differences between FA trends and OA trends in 

TN concentrations are attributed to the trends in streamflow 

(fig. 12). The strong downward trends in streamflow in the 

central part of the study area moderated the upward FA trends 

in that area. Of the eight sites that had significant upward FA 

trends in the central part of the study area, six sites had significant downward trends in streamflow, which generally resulted 

in insignificant OA trends or trends less than about 3 percent 

per year for those sites where the trends were significant. Sites 

in the eastern part of the study area did not have significant 

trends in streamflow and, therefore, the FA and OA trends 

were similar. The OA trends at sites in the western part of the 

study area also were not strongly affected by trends in streamflow, except for the Red River of the North (05082500), which 

had a downward trend in streamflow and a downward FA trend 

that resulted in a strong downward OA trend in concentrations, 

significant at p < 0.05.

To place the 1993 to 2004 trends in concentration 

into a longer term context, FA trends were computed using 

QWTREND from 1975 to 2004 for all of the sites with sufficient data (table 6). The FA trends in TN concentrations had 

small deviations from average conditions for the Chippewa 

River (05369500) and Iowa River (05465500) sites, both in 

the central part of the study area (fig. 16; table 6). The other 

four long-term sites had larger deviations from the longterm average flow-adjusted concentrations. The Souris River 

(05114000) site, in the western part, had a downward shift 

between 1984 and 1992, when no samples were collected. The 

Milwaukee River (04087000) had three distinct periods: an 

upward FA trend from 1975 to 1986, followed by a downward 

FA trend until 1991, and an upward FA trend from 1992 to 

2004. The only long-term site in the eastern part, the Maumee 

River (04193500), had no FA trend until 1988, after which 

the flow-adjusted concentration increased for 2 years, which 

was followed by a downward FA trend to 2004. The Mississippi River integrator site (05420500) had a significant upward 

FA trend from 1975 to 1983, after which the flow-adjusted 

concentration was nearly constant. No consistent long-term FA 

trends in TN concentrations were found at these sites; changes 

generally alternated between upward and downward trends. 

Those fluctuating trends imply that local factors are more 

important that regional factors for long-term trends.

Trends in Total Nitrogen Loads

From 1993 to 2004, TN loads at 17 of the 24 sites that 

had sufficient data for analysis (table 5; fig. 15C) indicated a 

spatial tendency toward decreasing loads. Most of the sites 

with a downward trend were in the central or western parts of 

the study area. Eight of these sites had significant downward 

trends in loads (p < 0.10); most of these sites had significant 

downward trends in streamflow as well (fig. 12). The only 

exception was the site in eastern North Dakota, which had a 

significant downward trend in load and no trend in streamflow. 

Of the seven sites with upward trends in TN loads, only one 

site, the site in Pennsylvania, had a significant trend.

Table 6. Summary of flow-adjusted trends in total nitrogen concentrations (TN, USGS parameter code 

00600) estimated by QWTREND for long-term evaluation sites, 1975 to 2004. 

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

USGS site 

number Trend period Description of flow-adjusted concentration

05114000 January 1975–August 1983 Constant value of 1.88 mg/L

September 1983–February 1993 Gap in sampling

March 1993–December 2004 Upward trend from 1.30 to 1.66 mg/L

05369500 January 1975–December 2004 Upward trend from 1.14 to 1.42 mg/L

05465500 January 1975–December 2004 Downward trend from 6.93 to 6.07 mg/L

04087000 January 1975–December 1986 Upward trend from 1.86 to 2.00 mg/L

January 1987–December 1991 Downward trend from 2.00 to 1.74 mg/L

January 1992–Dec 2004 Upward trend from 1.74 to 2.18 mg/L

04193500 January 1975–December 1987 Constant value of 5.25 mg/L

January 1988–December 1989 Upward trend from 5.25 to 6.35 mg/L

January 1990–December 2004 Downward trend from 6.35 to 4.02 mg/L

05420500 January 1975–December 1982 Upward trend from 1.60 to 2.38 mg/L

January 1983–December 2004 Downward trend from 2.38 to 2.30 mg/L

Gap in sampling from October 1986 to May 1991

Gap in sampling from August 1992 to October 1995
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 33

A. B.

C. D.

E. F.

Souris River near Sherwood, North Dakota

USGS station number 05114000

Chippewa River at Durand, Wisconsin

USGS station number 05369500

Iowa River at Wapello, Iowa

USGS station number 05465500

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

Mississippi River at Clinton, Iowa

USGS station number 05420500

Maumee River at Waterville, Ohio

USGS station number 04193500

1975 1980 1985 1990 1995 2000 2005

-0.5

0.0

0.5

1.0

1975 1980 1985 1990 1995 2000 2005

-0.5

0.0

0.5

1.0

1.5

2.0

1975 1980 1985 1990 1995 2000 2005

-1.0

-0.5

0.0

0.5

1.0

1975 1980 1985 1990 1995 2000 2005

-1.0

-0.5

0.0

0.5

1.0

1975 1980 1985 1990 1995 2000 2005

-0.5

0.0

0.5

1.0

1975 1980 1985 1990 1995 2000 2005

-0.5

0.0

0.5

1.0

1.5

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL NITROGEN CONCENTRATION,

IN MILLIGRAMS PER LITER)

YEAR YEAR

LOESS smoothing line

Concentration

EXPLANATION

Figure 16. Long-term flow-adjusted trends in total nitrogen concentrations from 1975 to 2004 at six selected sites in the study 

area. 
34 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

A comparison of the trends in loads and FA trends in 

concentrations indicates that the trends in loads are driven 

primarily by streamflow rather than changes in concentrations 

(fig. 15C). However, the effects of the trends in streamflow 

can augment the FA trends in concentration to cause a larger 

OA trend, such as the site in eastern North Dakota (fig. 15B).

Annual loads from 1975 to 2003 at the six long-term sites 

were used to place these short-term trends in load into a longer 

term context (fig. 17). The long-term changes at all six sites 

closely resembled the long-term trend in streamflow (fig. 13), 

which again indicates the importance of changes in streamflow in driving the trends in loads. This relation between 

streamflow and TN loads was consistent at all scales, from the 

relatively small Souris River, N. Dak., with a maximum mean 

annual streamflow of approximately 23.2 m3/s, to the Mississippi River at Clinton, Iowa, with a maximum mean annual 

streamflow of 2,390 m3/s. 

Total nitrogen loads at the Souris River, N. Dak., 

decreased from the mid-1970s to approximately 1990, when 

loads increased slightly until 2003. That pattern in loads could 

have been affected by the construction of dams within the 

basin. Loads at the Milwaukee River, Wisconsin, generally 

increased from 1975 to 1985, then decreased until 1991, and 

then remained relatively constant through 2004. Loads at the 

Maumee River, Ohio, increased from 1975 to about 1982, 

then decreased slightly through 2003. Loads at the remaining 

four sites generally increased from 1975 to about 1982, then 

stayed relatively constant until about 1993, and then increased 

(Chippewa River) or decreased (Iowa and Mississippi Rivers) 

slightly until 2003. 

Dissolved Ammonia

General Patterns in Dissolved Ammonia 

Concentrations and Yields

Median midmonthly dissolved ammonia (NH4) concentrations at 36 of the 49 trend sites that had sufficient data for 

analysis ranged from 0.010 to 0.150 mg/L, based on data from 

1993 to 2004. The mean and median values of the median 

NH4 concentrations at these sites were 0.044 and 0.032 mg/L, 

respectively. The highest concentrations were measured at 

urban and agricultural sites in central Wisconsin and in central 

Illinois. Sites with high NH4 concentrations did not form 

regional patterns, but rather were spatially intermixed with 

sites with substantially lower concentrations. Although the 

lowest concentrations were irregularly distributed through the 

study area, the forested eastern sites in New York had the lowest concentrations.

Median annual NH4 yields at the trend sites ranged from 

1.41 to 118 kg/km2 (table 7), based on data from 1993 to 2004. 

The mean and median values from the median yields at these 

sites were 25.4 and 18.5 kg/km2, respectively. The highest 

ammonia yields were at Salt Creek, Ill.; Des Plaines River, Ill.; 

and Northrup Creek, N.Y. The lowest yields were at the Souris 

River near Sherwood, N. Dak., the Souris River near Verendrye, N. Dak., and at Irondequoit Creek, N.Y.

Trends in Dissolved Ammonia Concentrations

The FA trends in NH4 concentrations from 1993 to 2004 

showed a widespread spatial tendency toward decreasing concentrations (table 7; fig. 18A). Of the 36 sites with sufficient 

streamflow and NH4 concentration data for trend analysis, 

32 sites had downward trends in concentrations. Of these 

32 sites, 24 sites had significant downward trends (p < 0.10). 

Only four sites had upward FA trends, and only one site, 

Northrup Creek, N.Y. (0422026250), had a significant upward 

FA trend in concentrations; however, the magnitude of this 

upward trend was only 0.002 mg/L/yr. 

When the OA trends in NH4 concentrations from 1993 to 

2004 were examined, the results were similar to those of the 

FA trends. Concentrations at 32 of the 36 sites with sufficient 

data indicated a tendency toward decreasing concentrations 

(table 7; fig. 18B), and at 23 of these sites the downward 

trends were statistically significant (p < 0.10). Of the four sites 

with upward OA trends in concentrations, the only site with a 

significant trend was again Northrup Creek, N.Y.

Unlike trends for TN, almost no differences between 

FA and OA trends in NH4 concentrations were attributed to 

trends in streamflow. In general, streamflow was not important in determining the significance in the OA trend at most 

sites; however, the significance level between the FA and OA 

trends did change at three sites with downward trends in NH4 

concentrations. 

Trends in Dissolved Ammonia Loads

From 1993 to 2004, 32 of the 36 sites with sufficient data 

for analysis showed a tendency toward decreasing NH4 loads 

(table 7; fig. 18C). These sites with a downward trend were 

distributed across all three parts of study area. Twenty-six of 

these sites had significant downward trends in loads (p < 0.10). 

Most of these sites had significant downward FA trends in both 

concentration (table 7) and streamflow (table 4).
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 35

A. B.

C. D.

E. F.

19751970 19851980 19951990 20052000

0.0

0.5

1.0

1.5

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL NITROGEN LOAD,

IN MILLIONS OF KILOGRAMS

5

7

9

11

13

15

0

50

100

150

200

250

0.2

0.7

1.2

1.7

0

50

100

150

200

250

20

40

60

80

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

Souris River near Sherwood, North Dakota

USGS station number 05114000

Chippewa River at Durand, Wisconsin

USGS station number 05369500

Iowa River at Wapello, Iowa

USGS station number 05465500

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

Mississippi River at Clinton, Iowa

USGS station number 05420500

Maumee River at Waterville, Ohio

USGS station number 04193500

YEAR YEAR

LOESS smoothing line

Annual load

EXPLANATION

Figure 17. Long-term trends in total nitrogen loads from 1975 to 2003 at six selected sites in the study area. 
Table 7. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved ammonia (NH4, USGS parameter c[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) T

(

Western part 05056100 ND-1 7 0.067 -0.004 -5.68 0.082 -0.004 -5.24 0.141 NA NA 662 

05056200 ND-2 7 .040 -.002 -4.48 .197 -.002 -4.84 .140 NA NA 654 

05056239 ND-3 7 .044 -.003 -6.75 .019 -.003 -7.07 .009 NA NA 780 

05058700 ND-4 7 .066 -.003 -3.92 .324 -.003 -4.47 .229 58,400 9.05 15,100 05082500 ND-6 7 .092 -.008 -8.20 .000 -.008 -8.29 .000 301,000 30.6 362,000 -3

05114000 ND-7 8 .093 -.004 -4.27 .197 -.006 -6.50 .053 11,100 1.41 587 05120000 ND-8 8 .090 -.004 -4.56 .166 -.005 -5.06 .124 16,200 1.42 3,550 

Central part 04063700 WI-9 9 0.038 -0.002 -4.66 0.000 -0.001 -3.68 0.003 2,660 7.38 3,160 

04067500 WI-10 9 .036 -.001 -.43 .855 -.001 -.43 .856 58,700 5.76 85,100 

04085427 WI-12 7 .071 -.004 -5.19 .098 -.003 -4.58 .144 16,900 12.4 9,440 

04087000 WI-13 8 .037 -.001 -3.17 .051 -.001 -3.17 .046 30,000 16.6 11,900 05287890 MN-14 8 .183 -.012 -6.32 .000 -.012 -6.30 .000 4,350 19.5 3,980 05340500 WI-15 7 .028 .001 .29 .931 .001 .20 .952 188,000 11.6 107,000 

05369500 WI-16 9 .052 .001 -.64 .746 -.001 -1.19 .543 507,000 21.7 338,000 -

05382000 WI-17 9 .063 -.001 -.23 .932 -.002 -2.59 .278 132,000 24.6 91,500 -

05407000 WI-20 7 .046 -.001 -2.79 .366 -.002 -3.46 .207 498,000 18.5 384,000 -1

05420500 IA-21 3 .106 -.008 -7.66 .000 -.008 -7.52 .000 4,120,000 18.6 6,250,000 -50

054310157 WI-24 9 .215 -.066 -30.70 .000 -.066 -30.7 .000 NA NA 236 

05465500 IA-25 9 .024 -.002 -7.35 .003 -.002 -7.59 .000 859,000 26.5 221,000 -1

05474000 IA-26 7 .067 -.005 -6.71 .021 -.005 -7.38 .003 254,000 22.8 138,000 -105500000 MO-28 8 .069 -.004 -5.90 .000 -.005 -6.89 .000 40,100 24.9 9,130 

05525500 IL-30 9 .033 -.002 -6.94 .000 -.002 -7.09 .000 42,000 36.3 4,800 
USGS

site

number

State-ID 

number 

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value

Median 

annual 

load 

(kg)

Median 

yield 

(kg/km2)

Reference 

load

(kg)

T

(

Central part—continued

05531500 IL-31 7 0.192 -0.011 -5.76 0.000 -0.011 -5.75 0.000 35,000 118 23,400 -

05532500 IL-32 8 .167 -.012 -6.91 .000 -.012 -6.93 .000 157,000 96.3 86,400 -05540275 IL-33 3 .080 -.004 -5.24 .000 -.004 -5.41 .000 772 30.1 461 05586100 IL-37 7 .112 -.008 -6.76 .000 -.007 -6.48 .000 2,020,000 29.1 2,780,000 -2107014500 MO-38 9 .018 -.001 -7.63 .003 -.001 -7.83 .001 24,300 6.36 20,700 -

07018100 MO-39 9 .016 -.001 -5.49 .018 -.001 -5.81 .006 20,600 10.8 7,290 

Eastern part

03085000 PA-40 7 0.103 -0.004 -3.44 0.054 -0.004 -3.67 0.036 332,000 17.5 688,000 -1

03353637 IN-42 9 .028 -.002 -7.44 .000 -.002 -7.49 .000 769 17.5 247 04186500 OH-43 7 .058 -.005 -8.53 .000 -.005 -8.55 .000 35,400 41.1 4,160 04193500 OH-44 7 .056 -.004 -6.69 .001 -.004 -6.63 .002 618,000 37.7 106,000 -0422026250 NY-45 7 .032 .002 7.75 .000 .002 7.48 .000 1,810 69.3 202 

04232034 NY-46 7 .003 .001 1.82 .687 .001 1.48 .738 249 2.45 83 0423204920 NY-47 8 .019 .001 2.52 .167 .001 2.87 .128 285 15.8 89 

0423205025 NY-48 9 .022 -.001 -5.95 .000 -.001 -5.94 .000 4,260 11.5 2,110 

Table 7. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved ammonia (NH4, USGS parameter c2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites]
38 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

DISSOLVED AMMONIA

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 18. Trends in dissolved ammonia from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 39

Total Kjeldahl Nitrogen

General Patterns in Total Kjeldahl Nitrogen 

Concentrations and Yields

Median midmonthly Kjeldahl nitrogen (KJN) concentrations at 36 of 49 trend sites that had sufficient data for analysis, based on data from 1993 to 2004, ranged from 0.200 to 

1.50 mg/L. The mean and median values of the median KJN 

concentrations at these sites were 0.88 mg/L. Concentrations 

were uniformly high throughout the western parts of the study 

area and sporadically high at isolated sites throughout the rest 

of the study area. The lowest concentrations were at sites in 

eastern Ohio and eastern Pennsylvania. The highest concentrations were in areas with intensive agriculture, and the lowest 

concentrations were in forested areas.

Median annual KJN yields at the trend sites ranged from 

9.13 to 796 kg/km2 (table 8), based on data from 1993 to 2004. 

The mean and median values from the median yields at these 

sites were 313 and 261 kg/km2, respectively. The highest 

yields were from two sites in Illinois and one site in Missouri. 

The lowest yields were throughout the northwestern part of 

the study area, including three sites in North Dakota and three 

sites in northern Wisconsin.

Trends in Total Kjeldahl Nitrogen Concentrations

The FA trends in KJN concentrations from 1993 to 2004 

showed no distinct spatial pattern. Of the 36 sites that had sufficient streamflow and KJN concentration data for trend analysis (table 8; fig. 19A), 17 sites had tendency toward decreasing 

concentrations, and 19 sites had a tendency toward increasing 

concentrations. Seven sites had significant downward trends,

and six sites had significant upward trends (p < 0.10).

Of the 36 sites that had sufficient streamflow and KJN 

concentration data from 1993 to 2004 for analysis (table 8; 

fig. 19B), 18 sites had downward OA trends in concentrations, 

and 18 sites had upward trends. Of these, nine sites had significant downward trends, and six sites had significant upward 

trends (p < 0.10). When the OA trends in KJN concentrations were examined no distinct spatial patterns were evident; 

however, the tendencies and statistical results were slightly 

different than those of the FA trends. The differences in the 

results of the FA and OA trend analyses were attributable to 

differences in the trends in streamflow (fig. 12). 

Trends in Total Kjeldahl Nitrogen Loads

From 1993 to 2004, KJN loads at the 36 trend sites that 

had sufficient data for analysis showed a tendency toward 

decreasing loads at 30 sites and a tendency toward increasing 

loads at 6 sites (table 8; fig. 19C). The sites with downward 

trends in KJN loads were distributed across all three parts of 

study area, and three of the six sites with upward trends were 

in the eastern part of the study area. Thirteen of the sites with 

downward trends in loads had significant trends, whereas only 

one of the six sites with upward trends in loads had a significant trend (p < 0.10). 

Dissolved Nitrite plus Nitrate Nitrogen

General Patterns in Dissolved Nitrite plus Nitrate 

Nitrogen Concentrations and Yields

Median midmonthly dissolved nitrite plus nitrate nitrogen 

(NO2NO3) concentrations at 29 of the 49 trend sites ranged 

from 0.05 to 8.11 mg/L, based on data from 1993 to 2004. The 

mean and median values from the median NO2NO3 concentrations at these sites were 2.21 and 0.65 mg/L, respectively. 

The highest concentrations were in the central and eastern 

parts of the study area, and the lowest concentrations were in 

the north-central and western parts of the study area. The highest concentrations were in areas with intensive agriculture, and 

the lowest concentrations were in forested areas. 

Median annual NO2NO3 yields at the trend sites ranged 

from 1.80 to 4,190 kg/km2, based on data from 1993 to 2004 

(table 9). The mean and median values of the median yields at 

these trend sites were 774 and 364 kg/km2, respectively. The 

highest yields were at two sites in Ohio and one site each in 

Illinois and Iowa. The lowest yields were from six sites from 

throughout the northwest and north-central parts of the study 

area, three sites in North Dakota and three sites in northern 

Wisconsin. 

Trends in Dissolved Nitrite plus Nitrate Nitrogen 

Concentrations

The FA trends in NO2NO3 concentrations from 1993 

to 2004 showed no distinct spatial pattern (fig. 20A). Of the 

29 sites that had sufficient streamflow and NO2NO3 concentration data for analysis (table 9 and fig. 20A), 12 sites had 

downward trends, and 17 sites had upward trends. Of these 

sites, three sites (one in North Dakota, one in Illinois, and 

one in Indiana) had significant downward trends, and four 

sites (one in Iowa, one in Illinois, and two in Wisconsin) had 

significant upward trends (p < 0.10).

When the OA trends in NO2NO3 concentrations from 

1993 to 2004 were examined, no distinct spatial patterns were 

evident; however, tendencies and trend results were somewhat 

different than those of the FA trends. These differences in 

the results of the FA and OA trend analyses are attributed to 

differences in streamflow trends (fig. 12). Of the 29 sites that 

had sufficient streamflow and NO2NO3 concentration data for 

analysis (table 9; fig. 20B), 13 sites had downward OA trends, 

and 16 sites had upward OA trends. Of these sites, four sites 

had significant downward trends, and three sites had significant upward trends (p < 0.10). 
Table 8. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total Kjeldahl nitrogen (KJN, USGS paramete[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Western part 05056100 ND-1 9 1.266 0.017 1.37 0.525 0.036 2.84 0.328 NA NA 12,400 

05056200 ND-2 7 1.497 .039 2.61 .031 .066 4.44 .009 NA NA 25,400 

05056239 ND-3 9 1.126 -.009 -.79 .767 .005 .44 .881 NA NA 20,900 

05058700 ND-4 3 1.017 -.002 -.19 .936 -.006 -.60 .795 511,000 79.3 233,000 

05064500 MN-5 3 1.170 -.058 -4.97 .004 -.060 -5.09 .001 3,400,000 345 2,330,000 -1

05082500 ND-6 3 1.029 -.033 -3.24 .011 -.036 -3.49 .003 5,470,000 555 4,050,000 -2

05114000 ND-7 9 1.955 -.024 -1.25 .276 -.034 -1.72 .185 92,300 11.7 10,800 

05120000 ND-8 5 1.092 .017 1.55 .244 .016 1.46 .266 104,000 9.13 41,700 

Central part 04063700 WI-9 9 0.411 0.004 1.04 0.228 -0.002 -0.47 0.619 55,200 153 34,200 

04067500 WI-10 9 .323 -.003 -.87 .510 -.003 -.88 .516 1,060,000 104 769,000 

04085427 WI-12 3 1.421 .020 1.40 .460 .016 1.14 .530 337,000 248 187,000 

04087000 WI-13 8 .821 .004 .52 .424 .005 .62 .340 406,000 225 267,000 

05287890 MN-14 9 1.071 .023 2.17 .001 .003 .33 .728 104,000 467 24,400 

05340500 WI-15 9 .387 .009 2.25 .293 .006 1.51 .481 2,410,000 149 1,410,000 

05369500 WI-16 9 .544 .006 1.03 .222 .005 .96 .244 5,130,000 220 3,540,000 

05382000 WI-17 8 .657 .010 1.54 .241 -.001 -.09 .940 1,400,000 261 952,000 

05407000 WI-20 7 .673 .020 2.99 .046 .016 2.43 .064 6,610,000 245 5,610,000 

05420500 IA-21 9 1.625 -.030 -1.83 .075 -.035 -2.17 .025 53,500,000 241 95,700,000 -4,4

05465500 IA-25 7 1.076 .024 2.19 .278 .041 3.82 .066 11,800,000 366 9,860,000 -3

05500000 MO-28 8 .859 -.001 -.04 .965 -.015 -1.75 .099 900,000 560 115,000 

05514500 MO-29 7 .942 -.039 -4.17 .011 -.040 -4.25 .019 1,000,000 427 106,000 

05525500 IL-30 8 .538 -.002 -.42 .929 -.005 -1.00 .833 527,000 456 62,800 

05531500 IL-31 9 1.458 -.030 -2.08 .036 -.030 -2.05 .038 237,000 796 178,000 

05532500 IL-32 9 1.586 -.036 -2.29 .049 -.035 -2.21 .058 946,000 580 814,000 
USGS

site

number

State-ID 

number 

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value

Median 

annual 

load 

(kg)

Median 

yield 

(kg/km2)

Reference 

load

(kg)

Central part—continued

05540275 IL-33 3 0.589 0.011 1.81 0.155 0.007 1.24 0.322 7,980 311 3,400 

05586100 IL-37 8 1.150 .013 1.13 .345 .012 1.06 .346 25,300,000 365 28,000,000 -1,2

07014500 MO-38 8 .172 -.002 -1.14 .695 -.005 -2.72 .251 599,000 157 193,000 

07018100 MO-39 8 .203 -.001 -.67 .833 -.004 -1.78 .562 485,000 255 95,100 

Eastern part

03085000 PA-40 8 0.228 0.004 1.96 0.690 0.004 1.83 0.709 3,810,000 200 1,510,000 

03353637 IN-42 9 .223 .011 4.92 .003 .008 3.67 .057 14,100 319 1,840 

04186500 OH-43 8 .773 -.007 -.90 .602 -.007 -.94 .597 355,000 413 59,400 

04193500 OH-44 8 1.065 .022 2.03 .093 .022 2.09 .090 8,560,000 522 1,950,000 

0422026250 NY-45 9 1.111 .027 2.46 .001 .029 2.60 .000 12,700 484 6,990 

04232034 NY-46 9 .621 .009 1.50 .260 .005 .87 .568 22,200 218 16,300 

0423204920 NY-47 9 .781 -.010 -1.27 .129 -.011 -1.45 .090 6,590 365 3,690 

0423205025 NY-48 9 .831 -.018 -2.19 .000 -.020 -2.40 .000 84,200 227 80,600 

Table 8. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total Kjeldahl nitrogen (KJN, USGS paramete1993 to 2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] 
42 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

KJELDAHL NITROGEN

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 19. Trends in total Kjeldahl nitrogen from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
Table 9. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved nitrite plus nitrate nitrogen (NO3, Ucode 00631), 1993 to 2004.—Continued[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites]

USGS

site

number

State-ID 

number 

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value

Median 

annual 

load 

(kg)

Median 

yield 

(kg/km2)

Reference 

load 

(kg)

Western part

05056100 ND-1 8 0.018 0.003 15.00 0.272 0.003 16.76 0.251 NA NA 173 

05056200 ND-2 8 .036 -.001 -4.00 .553 -.002 -5.39 .354 NA NA 537 

05056239 ND-3 7 .043 -.002 -5.67 .249 -.003 -7.37 .103 NA NA 704 

05058700 ND-4 7 .151 .012 7.65 .456 .007 4.51 .623 279,000 43.2 34,000 

05064500 MN-5 7 .436 .046 10.60 .208 .043 9.93 .203 3,380,000 344 866,000 

05082500 ND-6 8 .421 -.014 -3.26 .474 -.018 -4.25 .293 3,990,000 405 1,640,000 

05114000 ND-7 7 .046 -.001 -.76 .834 .001 3.12 .561 14,100 1.80 237 

05120000 ND-8 7 .153 -.009 -6.10 .062 -.009 -5.86 .089 58,300 5.11 5,690 

Central part

04063700 WI-9 7 0.044 0.001 1.57 0.447 0.001 1.54 0.439 7,330 20.4 3,680 

04067500 WI-10 7 .088 -.002 -2.58 .172 -.002 -2.59 .170 397,000 39.0 210,000 

04085427 WI-12 7 .231 .027 11.51 .304 .024 10.24 .330 518,000 380 30,300 

04087000 WI-13 8 .669 .056 8.40 .000 .043 6.37 .019 827,000 458 215,000 

05287890 MN-14 7 .181 .004 2.39 .351 -.003 -1.56 .536 24,100 108 3,840 

05340500 WI-15 7 .155 .006 3.57 .144 .006 3.60 .135 919,000 56.9 598,000 

05369500 WI-16 7 .554 .013 2.32 .053 .015 2.78 .028 4,700,000 201 3,610,000 

05382000 WI-17 7 .528 .009 1.61 .223 .012 2.26 .090 969,000 180 767,000 

05407000 WI-20 7 .645 -.015 -2.26 .461 -.013 -2.04 .483 7,470,000 277 5,370,000 -

05420500 IA-21 8 2.052 -.049 -2.41 .253 -.106 -5.16 .003 111,000,000 502 117,000,000 -7,

05465500 IA-25 8 3.982 .481 12.07 .006 .040 1.00 .786 71,400,000 2,210 35,900,000 -1,

05474000 IA-26 5 5.885 .360 6.11 .195 -.328 -5.57 .174 18,600,000 1,660 11,900,000 -

05514500 MO-29 7 .293 .007 2.37 .752 .006 2.06 .794 3,510,000 1,500 32,600 

05540275 IL-33 7 .883 -.046 -5.24 .000 -.051 -5.74 .000 12,600 490 5,070 05586100 IL-37 9 3.364 .077 2.28 .038 -.009 -.27 .802 119,000,000 1,720 78,100,000 -3,07018100 MO-39 9 .013 .006 43.56 .060 .002 13.46 .317 195,000 102 5,140 
Table 9. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved nitrite plus nitrate nitrogen (NO3, Ucode 00631), 1993 to 2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Eastern Part 03085000 PA-40 5 0.672 0.016 2.45 0.282 0.013 1.97 0.380 9,320,000 490 4,500,000 

03267900 OH-41 5 3.893 -.032 -.81 .354 .056 1.43 .146 1,290,000 1,610 585,000 03353637 IN-42 8 .857 -.030 -3.50 .000 -.032 -3.77 .000 13,900 315 7,460 

04186500 OH-43 8 1.973 .020 1.04 .799 .016 .81 .863 3,600,000 4,190 141,000 

04193500 OH-44 8 3.085 -.077 -2.48 .405 -.066 -2.15 .557 46,100,000 2,810 5,760,000 
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 45

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

DISSOLVED

NITRITE PLUS NITRATE

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

0 400 MILES

0 400 KILOMETERS

Figure 20. Trends in dissolved nitrite plus nitrate from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
46 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Trends in Dissolved Nitrite plus Nitrate Loads

Of the 29 sites that had sufficient data from 1993 to 

2004 for analysis, 16 sites had a tendency toward decreasing 

NO2NO3 loads, and 13 sites had a tendency toward increasing loads (table 9; fig. 20C). Seven of the sites with downward 

trends in loads had significant downward trends, whereas 

only one of the 12 sites with upward trends had a significant 

upward trend (p < 0.10). 

Total Phosphorus

General Patterns in Total Phosphorus 

Concentrations and Yields

Median midmonthly total phosphorus (TP) concentrations ranged from 0.005 to 4.45 mg/L, based on water-quality 

data from all sites from 1970 to 2004. The overall mean 

and median of the median TP concentrations were 0.17 

and 0.11 mg/L, respectively. High concentrations occurred 

throughout the study area, but especially at sites along a belt 

that spans North Dakota, southern Minnesota, Iowa, Illinois, 

Indiana, and Ohio (fig. 21A). The lowest concentrations 

occurred in forested areas of northern Wisconsin, Michigan, 

and Minnesota and along the eastern edge of the study area 

(figs. 5 and 21). The highest concentrations generally occurred 

in areas with large amounts of agriculture and urban land use. 

The spatial pattern in TP concentrations was similar to that 

for TN; however, sporadically high and low concentrations 

were found throughout the study area. Median midmonthly TP 

concentrations ranged from 0.028 to 1.49 mg/L at the 41 trend 

sites that had sufficient data for analysis from 1993 to 2004. 

The mean and median values of the median concentrations at 

these sites were 0.22 and 0.16 mg/L, respectively. The distribution in concentrations of these sites was consistent with that 

in fig. 21A.

Median annual TP yields ranged from 0.421 to 

1,950 kg/km2, based on water-quality and streamflow data 

from all sites from 1970 to 2004. The mean and median 

values of the median TP yields at these sites were 82.2 and 

48.8 kg/km2, respectively. The highest yields were throughout 

the southern one-half of the study area, especially through the 

middle of the study area (fig. 21B). The lowest yields were 

throughout the northern one-half of the study area. Similar to 

TN, the highest TP yields were in areas with intense row crop 

agriculture and moderate runoff. The areas with the highest 

runoff (fig. 3) had relatively low TP concentrations, such as in 

northern Wisconsin and southeastern parts of the study area. 

The major difference in the distributions in TP concentrations 

and yields was in the northwestern part of the study area, 

which had high concentrations but low yields, which can be 

attributed to lower runoff than other agricultural areas. Based 

on 1993 to 2003 data, median annual TP yields at the trend 

sites ranged from 2.65 to 649 kg/km2 (table 10), which is 

within the range found for the entire data set. The distribution 

in yields was consistent with that in fig. 21B. The mean and 

median values of the median yields at these trend sites were 

86.3 and 44.8 kg/km2

, respectively.

Trends in Total Phosphorus Concentrations

From 1993 to 2004, FA trends in TP concentrations at 

24 of the 40 sites that had sufficient data for analysis indicated a tendency toward increasing concentrations (table 10; 

fig. 22A). The 24 sites with increasing TP concentrations were 

distributed throughout the entire study area; however, most 

of these sites were in the central part and the eastern side of 

the western part of the study area. Twelve of the 24 sites had 

significant upward trends (p < 0.10). These upward FA trends 

indicate that the effects of anthropogenic activities continue to 

increase in these areas and add additional TP to the streams. 

Concentrations at 7 of the 16 sites with downward trends were 

significant (p < 0.10), indicating that the effects of anthropogenic activities were decreasing. 

The OA trends in TP concentrations from 1993 to 2004 

showed a general tendency toward increasing concentrations 

at 21 of the 41 sites (table 10; fig. 22B); however, the upward 

trends were significant at only 7 of these sites (p < 0.10). Of 

the 20 sites with downward trends, only 7 sites had significant 

trends. The sites with significant upward or downward trends 

were scattered throughout the study area.

The differences between FA trends and OA trends in TP 

concentrations are attributed to the trends in streamflow. The 

major difference was in the central part of the study area—

several sites in Minnesota, Wisconsin, Illinois, Iowa, and Missouri had significant downward trends in streamflow (fig. 12). 

In this area, only four of the nine sites that had statistically 

significant FA trends in TP concentrations had statistically 

significant OA trends in concentrations. Very few changes 

occurred in the other parts of the study area. 

To place the 1993 to 2004 trends in TP concentrations 

into a longer term context, FACs were computed from 1975 

to 2004 for the six sites with sufficient data and examined for 

trends with QWTREND (table 11; fig. 23). Three sites had little or no trend from 1975 to 2004; the Iowa River (05465500) 

and Mississippi River (05420500) showed no trend, and the 

Chippewa River (05369500) had a downward trend from 1975 

to 1985 and no trend after that. The Souris River (05114000) 

site had no trend from 1975 to 1979, then a downward trend 

until 1988 and an upward trend through 2004. The Milwaukee 

River (04087000) had a downward trend from 1975 through 

1995 and then an upward trend though 2004. The Maumee 

River (04193500) had a very strong downward trend from 

1975 through 1982, followed by an upward trend until 1989, 

and another downward trend through 2004. The flow-adjusted 

concentration in the Maumee River in 2004 was less than onehalf of the flow-adjusted concentration in 1975 (table 11).
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 47

0.421 – 15.0

15.1 – 35.6

35.7 – 58.9

59.0 – 105

106 – 1,950

Median annual total phosphorus

yield, in kilograms per square

kilometer

EXPLANATION

B. Median annual total phosphorus yields

0.005 – 0.040

Median total phosphorus

concentration, in millgrams

per liter

0.041 – 0.080

0.081 – 0.145

0.146 – 0.230

0.231 – 4.450

EXPLANATION

Study area

State boundary

Basin boundary

A. Median total phosphorus concentrations

Study area

State boundary

Basin boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 21. Spatial patterns from 1970 to 2004 in A, median total phosphorus concentrations and B, median annual total 

phosphorus yields throughout the study area. 
Table 10. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total phosphorus (TP, USGS parameter cod[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) Tre

(kg

Western part 05056100 ND-1 7 0.362 0.001 0.11 0.960 0.001 0.28 0.898 NA NA 3,430 -1

05056200 ND-2 5 .302 .011 3.66 .183 .015 4.83 .095 NA NA 5,040 -3

05056239 ND-3 3 .231 .008 3.39 .370 .005 2.05 .581 NA NA 4,100 -2

05058700 ND-4 3 .187 .002 .89 .854 -.001 -.11 .980 141,000 21.8 42,600 -1,1

05064500 MN-5 7 .186 .019 10.14 .033 .017 9.15 .032 1,200,000 122 369,000 21,0

05082500 ND-6 5 .151 .012 7.83 .062 .008 5.42 .159 1,510,000 153 590,000 6,7

05114000 ND-7 9 .385 -.010 -2.70 .061 -.011 -2.91 .059 20,800 2.65 2,130 N

05120000 ND-8 9 .263 -.003 -1.09 .483 -.002 -.93 .550 30,400 2.67 9,980 3

Central part 04063700 WI-9 3 0.025 -0.001 -2.00 0.374 -0.001 -2.83 0.220 2,410 6.70 2,090 -1

04067500 WI-10 1 .027 NA NA NA -.001 -.01 .977 79,100 7.76 64,800 -

04073468 WI-11 8 .131 -.003 -2.00 .000 -.003 -2.13 .000 4,050 29.2 3,480 -1

04085427 WI-12 7 .184 -.002 -.85 .739 -.002 -1.25 .609 40,700 29.9 24,100 -8

04087000 WI-13 8 .091 .001 .63 .463 .001 .55 .519 49,500 27.5 29,400 -2

05287890 MN-14 9 .135 .004 2.68 .016 .001 .47 .713 8,420 37.8 3,080 -1

05340500 WI-15 5 .028 .003 11.71 .003 .003 9.71 .011 225,000 13.9 101,000 7,8

05369500 WI-16 7 .084 .001 .57 .510 .001 .24 .792 737,000 31.6 545,000 -5,7

05382000 WI-17 9 .124 .003 2.34 .013 .001 1.17 .198 259,000 48.0 180,000 -4,9

05407000 WI-20 7 .086 -.001 -.75 .635 -.001 -1.18 .397 709,000 26.3 713,000 -22,0

05420500 IA-21 9 .183 -.001 -.07 .952 -.002 -.99 .361 9,230,000 41.6 10,800,000 -420,0

05427718 WI-22 9 .143 .001 .02 .974 -.005 -3.34 .003 7,280 38.2 3,290 -1

05427948 WI-23 5 .377 -.020 -5.32 .000 -.021 -5.44 .000 3,700 83.5 1,070 -

054310157 WI-24 8 .130 -.002 -1.69 .054 -.002 -1.60 .228 1,000 89.3 159 05465500 IA-25 3 .227 .014 6.22 .002 .009 4.11 .022 2,570,000 79.4 2,070,000 -65,0

05474000 IA-26 9 .142 .013 9.26 .008 .005 3.53 .281 1,160,000 104 293,000 -14,0
USGS

site

number

State-ID

number

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value

Median 

annual 

load 

(kg)

Median 

yield 

(kg/km2)

Reference 

load

(kg)

Tre

(kg

Central part—continued

05500000 MO-28 8 0.098 0.004 4.43 0.011 -0.001 -0.56 0.794 243,000 151 12,900 -8

05514500 MO-29 7 .121 -.001 -.11 .972 -.001 -.34 .930 385,000 164 13,500 -1

05525500 IL-30 8 .057 .003 5.11 .024 .002 3.11 .283 209,000 181 8,160 -

05531500 IL-31 9 .887 .009 1.00 .448 .012 1.39 .387 193,000 649 108,000 8

05532500 IL-32 9 .458 .012 2.68 .040 .019 4.08 .020 439,000 269 238,000 4,2

05540275 IL-33 9 .076 -.001 -.21 .891 -.001 -1.07 .488 990 38.6 435 -

05552500 IL-35 5 .214 .006 3.00 .641 .007 3.37 .599 555,000 84.3 399,000 -6,4

05586100 IL-37 9 .258 .022 8.38 .000 .022 8.66 .000 8,580,000 124 6,410,000 -57,0

07018100 MO-39 5 .031 -.001 -.43 .805 -.001 -1.52 .340 71,100 37.4 14,600 -7

Eastern part

03085000 PA-40 5 0.018 0.002 9.08 0.134 0.002 9.66 0.113 578,000 30.4 121,000 N

03353637 IN-42 9 .024 .001 1.83 .236 .001 .26 .901 3,420 77.5 207 

04186500 OH-43 8 .132 -.002 -1.71 .383 -.002 -1.78 .398 81,900 95.3 10,100 -2

04193500 OH-44 8 .135 .001 .03 .985 .001 .26 .903 1,930,000 118 255,000 2,1

0422026250 NY-45 9 .390 -.011 -2.69 .000 -.013 -3.30 .000 3,240 124 2,470 

04232034 NY-46 7 .070 -.002 -2.35 .046 -.002 -2.71 .033 3,530 34.8 1,850 -

0423204920 NY-47 8 .090 -.001 -1.56 .052 -.002 -1.93 .027 1,210 67.1 425 -

0423205025 NY-48 8 .068 .004 5.27 .000 .003 4.17 .000 14,200 38.2 6,570 

Table 10. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total phosphorus (TP, USGS parameter cod1993 to 2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per s

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] 
50 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

TOTAL PHOSPHORUS

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 22. Trends in total phosphorus from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 51

A. B.

C. D.

E. F.

-2.0

-1.5

-1.0

-0.5

0.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

-2.0

-1.5

-1.0

-0.5

0.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

-1.5

-1.0

-0.5

0.0

0.5

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005 1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

LOG (TOTAL PHOSPHORUS CONCENTRATION,

IN MILLIGRAMS PER LITER)

Souris River near Sherwood, North Dakota

USGS station number 05114000

Chippewa River at Durand, Wisconsin

USGS station number 05369500

Iowa River at Wapello, Iowa

USGS station number 05465500

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

Mississippi River at Clinton, Iowa

USGS station number 05420500

Maumee River at Waterville, Ohio

USGS station number 04193500

YEAR YEAR

LOESS smoothing line

Concentration

EXPLANATION

Figure 23. Long-term flow-adjusted trends in total phosphorus concentrations from 1975 to 2004 at six selected sites in the study 

area.
52 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Trends in Total Phosphorus Loads

From 1993 to 2004, TP loads at 30 of the 41 sites that 

had sufficient data for analysis indicated a tendency toward 

decreasing loads (table 10; fig. 22C). Twelve of these sites, 

all within the central and eastern parts of the study area, had 

significant (p < 0.10) downward trends in loads. Most of these 

sites had significant downward trends in streamflow. The 

trends at only 2 of the 11 sites that had upward trends were 

statistically significant (p < 0.10). A comparison of the trends 

in loads and FA trends in concentrations indicates that most of 

the trends in loads were driven primarily by streamflow rather 

than by changes in concentrations, except in southern Wisconsin where trends in both streamflow and concentration were 

significantly downward.

Annual loads at the six long-term sites from 1975 to 

2003 were used to put the short-term trends in TP loads into 

a longer term context (fig. 24). The long-term changes at all 

six sites resembled the long-term trend in streamflow (fig. 13), 

which again indicates the importance of changes in streamflow 

in driving the trends in loads. Total phosphorus loads at the 

Souris River, N. Dak., probably were affected by the construction of dams within the basin during the 1990s. Loads at the 

Souris River, N. Dak., were more variable from 1975 to 1983 

and from 1996 to 2004, producing larger average loads during 

those periods than from 1984 to 1995. Loads at the Milwaukee River, Wis., showed a decrease between 1982 and 1990. 

Loads at the remaining four sites varied slightly and showed 

no sustained trends.

Dissolved Phosphorus

General Patterns in Dissolved Phosphorus 

Concentrations and Yields

Median midmonthly dissolved phosphorus (DP) concentrations at the 35 trend sites that had sufficient data for 

analysis ranged from 0.020 to 1.29 mg/L, based on data from 

1993 to 2004. The mean and median values of the median DP 

concentrations at these trend sites were 0.149 and 0.075 mg/L, 

respectively. The highest DP concentrations were in central 

Illinois and eastern North Dakota; whereas the lowest concentrations (<0.1 mg/L) were in all three parts of the study area. 

The highest concentrations were in areas with intensive agriculture, and the lowest concentrations were in forested areas. 

Median annual DP yields ranged from 2.39 to 

496 kg/km2

, based on data from 1993 to 2003 (table 12). 

The mean and median values of the median DP yields at 

these trend sites were 48.3 and 19.0 kg/km2, respectively. 

Five of the six sites with the highest yields were in Illinois, 

and the remaining high-yield site was in North Dakota. The 

lowest yields were from sites throughout the north-central 

and eastern parts of the study area; especially low yields 

were from two sites in northern Wisconsin and one site in 

New York. 

Table 11. Summary of flow-adjusted trends in total phosphorus concentrations (TP, USGS parameter 

code 00665) estimated by QWTREND for long-term evaluation sites, 1975 to 2004. 

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

USGS site 

number Trend period Description of flow-adjusted concentration

05114000 January 1975–December 1978 Constant value of 0.19 mg/L

January 1979–December 1988 Downward trend from 0.19 to 0.16 mg/L

January 1989–December 2004 Upward trend from 0.16 to 0.22 mg/L

05369500 January 1975–December 1984 Downward trend from 0.11 to 0.09 mg/L

January 1985–December 2004 Constant value of 0.09 mg/L

05465500 November 1977–December 2004 Constant value of 0.29 mg/L

04087000 January 1975–December 1995 Downward trend from 0.15 to 0.09 mg/L

January 1996–December 2004 Upward trend from 0.09 to 0.11 mg/L

04193500 January 1975–December 1982 Downward trend from 0.32 to 0.17 mg/L

January 1983–December 1988 Upward trend from 0.17 to 0.21 mg/L

January 1989–December 2004 Downward trend from 0.21 to 0.14 mg/L

05420500 January 1975–December 2004 Constant value of 0.16 mg/L
Trends in Nutrient and Suspended-Sediment Concentrations and Loads 53

Figure 24. Long-term trends in total phosphorus loads from 1975 to 2003 at six selected sites in the study area. 

A. B.

C. D.

E. F. ANNUAL TOTAL PHOSPHORUS LOAD, IN MILLIONS OF KILOGRAMS ANNUAL TOTAL PHOSPHORUS LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL PHOSPHORUS LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL PHOSPHORUS LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL PHOSPHORUS LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL TOTAL PHOSPHORUS LOAD,

IN MILLIONS OF KILOGRAMS

1970 1975 1980 1985 1990 1995 2000 2005

Souris River near Sherwood, North Dakota

USGS station number 05114000

1970 1975 1980 1985 1990 1995 2000 2005

Chippewa River at Durand, Wisconsin

USGS station number 05369500

1970 1975 1980 1985 1990 1995 2000 2005

Iowa River at Wapello, Iowa

USGS station number 05465500

1970 1975 1980 1985 1990 1995 2000 2005

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

1970 1975 1980 1985 1990 1995 2000 2005

YEAR

Mississippi River at Clinton, Iowa

USGS station number 05420500

1970 1975 1980 1985 1990 1995 2000 2005

YEAR

Maumee River at Waterville, Ohio

USGS station number 04193500

0

0.05

0.10

0.15

0.20

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

0.01

0.03

0.05

0.07

0.09

0.11

0

1

2

3

4

0

5

10

15

LOESS smoothing line

Annual load

EXPLANATION
Table 12. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved phosphorus (DP, USGS paramete[USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

per year; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Western part 05056100 ND-1 7 0.276 -0.007 -2.54 0.345 -0.007 -2.69 0.313 NA NA 2,580 

05056200 ND-2 3 .283 .003 1.09 .743 .004 1.40 .675 NA NA 4,710 

05056239 ND-3 3 .146 -.003 -1.75 .699 -.005 -3.63 .394 NA NA 2,520 

05058700 ND-4 8 .130 .007 5.33 .375 .004 3.20 .563 60,000 9.30 29,500 

05082500 ND-6 3 .097 -.002 -2.14 .453 -.003 -2.90 .291 734,000 74.5 379,000 -

Central part 04063700 WI-9 7 0.026 -0.002 -7.70 0.000 -0.002 -7.73 0.000 860 2.39 2,170 

04067500 WI-10 13 .002 .001 .00 .977 .001 .00 .977 30,400 2.99 4,750 

04085427 WI-12 3 .077 -.001 -1.77 .605 -.002 -2.19 .498 23,200 17.0 10,100 

04087000 WI-13 7 .043 -.001 -1.36 .402 -.001 -1.62 .317 25,300 14.0 13,900 

05287890 MN-14 9 .056 .002 3.52 .025 .001 .96 .568 5,270 23.6 1,280 

05340500 WI-15 5 .016 -.001 -.14 .980 .001 -1.18 .830 116,000 7.15 62,500 

05369500 WI-16 8 .039 -.001 -3.02 .142 -.001 -3.43 .091 293,000 12.6 250,000 -

05382000 WI-17 8 .074 -.001 -.77 .419 -.001 -1.99 .035 106,000 19.7 107,000 

05407000 WI-20 3 .038 -.002 -6.35 .036 -.003 -6.78 .008 229,000 8.51 314,000 -

05420500 IA-21 7 .077 -.003 -3.29 .025 -.003 -3.76 .003 4,050,000 18.3 4,500,000 -2

05427718 WI-22 8 .097 -.001 -.28 .836 -.003 -2.81 .048 3,190 16.7 2,220 

05427948 WI-23 7 .183 -.013 -7.25 .000 -.013 -7.29 .000 1,400 31.6 520 

05465500 IA-25 9 .055 .001 1.04 .726 -.002 -2.97 .226 944,000 29.2 498,000 -

05474000 IA-26 9 .100 -.003 -2.51 .248 -.006 -5.85 .012 324,000 29.0 205,000 -

05500000 MO-28 7 .053 -.001 -.36 .914 -.001 -2.76 .375 52,200 32.5 4,080 

05514500 MO-29 5 .071 -.003 -4.80 .095 -.004 -5.00 .195 73,700 31.5 7,870 

05525500 IL-30 9 .026 .001 .58 .817 -.001 -1.15 .712 75,700 65.5 3,630 

05531500 IL-31 9 .785 .025 3.19 .006 .029 3.70 .018 148,000 496 95,600 

05532500 IL-32 3 .490 .016 3.29 .011 .025 5.03 .009 323,000 198 255,000 
USGS

site

number

State-ID

number

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Central part—continued

05540275 IL-33 9 0.039 0.001 0.98 0.706 0.001 0.69 0.786 411 16.0 227 

05543500 IL-34 8 .238 .018 7.59 .084 .031 12.98 .009 3,970,000 186 2,450,000 1

05586100 IL-37 7 .158 .005 3.00 .002 .008 5.11 .000 4,110,000 59.3 3,930,000 -

Eastern part

03085000 PA-40 5 0.013 -0.001 -1.46 0.073 -0.001 -1.61 0.059 110,000 5.77 89,500 

03353637 IN-42 9 .023 -.002 -6.65 .000 -.002 -6.78 .000 634 14.4 202 

04186500 OH-43 7 .074 -.003 -3.61 .056 -.003 -3.66 .062 26,900 31.3 5,360 

04193500 OH-44 7 .057 -.002 -4.23 .022 -.002 -4.10 .048 486,000 29.6 108,000 

0422026250 NY-45 9 .268 -.012 -4.34 .000 -.014 -5.18 .000 1,090 41.6 1,700 

04232034 NY-46 7 .008 -.001 .00 .464 -.001 .00 .464 345 3.40 208 

0423204920 NY-47 8 .022 -.001 -4.62 .002 -.001 -4.82 .001 228 12.6 106 

0423205025 NY-48 9 .018 -.001 -2.21 .027 -.001 -2.41 .015 2,440 6.59 1,740 

Table 12. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for dissolved phosphorus (DP, USGS paramete2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per sq

per year; NC, not calculated; sites in bold indicate long-term evaluation sites] 
56 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Trends in Dissolved Phosphorus Concentrations

The FA trends in DP concentrations from 1993 to 2004 

showed mixed regional tendencies; however, DP concentrations consistently decreased at all of the sites in the eastern 

part of the study area (table 12; fig. 25A). Of the 35 sites that 

had sufficient streamflow and DP concentration data for analysis, 23 sites had downward trends in concentrations. Twelve 

of these sites had significant downward trends (p < 0.10). 

Seven sites in the eastern part of the study area had downward 

trends. Five sites, all in the central part of the study area, had 

significant upward trends (p < 0.10). Two sites had almost no 

FA trends.

When the OA trends in DP concentrations from 1993 

to 2004 were examined, spatial trends followed the same 

general decreasing concentration pattern in the eastern areas 

with mixed tendencies in the central and western parts of the 

study area. Several differences between the FA and OA trend 

analyses occurred at specific sites in the central area, where 

many more sites had significantly downward OA trends than 

downward FA trends. These differences in the results of the FA 

and OA trend analyses are attributable to the significant trends 

in streamflow. Of the 35 sites that had sufficient streamflow 

and DP data for analysis (table 12; fig. 25B), 25 sites had 

downward trends, 8 sites had upward trends, and 2 sites had 

almost no trend in concentrations. Of these sites, 15 sites had 

significant downward trends, and 4 sites (all in Illinois) had 

significant upward trends (p < 0.10).

Trends in Dissolved Phosphorus Loads

From 1993 to 2004, dissolved phosphorus loads at 32 

of the 35 sites that had sufficient data for analysis indicated a 

tendency toward decreasing loads (table 12; fig. 25C); trends 

at 15 of these sites were significant (p < 0.10). Two of the 

three sites, all located in northern Illinois, with upward trends 

in loads had significant upward trends. 

Total Suspended Materials

General Patterns in Concentrations and Yields of 

Total Suspended Materials 

Median midmonthly total SMAT concentrations in the 

study area ranged from 0.7 to 5,240 mg/L, based on data 

for all of the sites from 1970 to 2004 (fig. 26A). The overall median and mean SMAT concentrations were 24.0 and 

112 mg/L, respectively. The highest concentrations were in 

the western, southwestern, and south-central parts of the study 

area. The lowest concentrations were mostly in northern areas 

of Wisconsin and Michigan and in the northeastern part of 

the study area. The highest concentrations were in areas with 

intensive row-crop agriculture, and the lowest concentrations 

were in forested areas. Median midmonthly SMAT concentrations ranged from 4.0 to 424 mg/L at the 42 trend sites that 

had sufficient data for analysis, based on data from 1993 to 

2004. The mean and median values of the median SMAT 

concentrations at these sites were 59.1 and 37.5 mg/L, respectively. The distribution in concentrations of these trend sites 

was consistent with that shown in fig. 26A. 

Median annual SMAT yields from 1970 to 2004 ranged 

from 22.0 to 3,370,000 kg/km2

. The overall mean and median 

yields based on all of the data in the study area from 1970 

to 2004 were 85,100 and 35,400 and kg/km2, respectively. 

The highest yields were from basins throughout the southern 

one-half of the study area, and the lowest yields were from 

basins throughout the northern one-half of the study area 

(fig. 26B). Similar to TP and TN, the major difference in the 

distributions in SMAT between concentrations and yields 

was in the northwestern part of the study area, which had 

high concentrations but low yields, primarily because of low 

mean annual runoff (fig. 3). Median annual SMAT yields at 

trend sites, based on data from 1993 to 2004, ranged from 

188 to 939,000 kg/km2 (table 13). The distribution in yields 

was consistent with that in fig. 26B. The mean and median 

yields based only on data from the trend sites were 60,100 and 

25,300 kg/km2, respectively. 

Trends in Total Suspended-Material 

Concentrations

The FA trends in SMAT concentrations from 1993 to 

2004 showed distinct spatial patterns of tendencies toward 

increasing concentrations in Illinois, and tendencies toward 

decreasing concentrations throughout most of Wisconsin, 

Iowa, and the eastern part of the study area (fig. 27A). Of the 

42 sites that had sufficient streamflow and SMAT concentration data for analysis (table 13), 25 sites had downward trends, 

and 17 sites had upward trends in concentrations. Of these 

sites, 12 sites had significant downward trends, and 5 sites had 

significant upward trends (p < 0.10).

The spatial pattern in the OA trends in SMAT concentrations generally was the same as that for the FA trends 

(fig. 27B); however, several of the sites with nonsignificant 

downward trends, especially in Iowa, had significant downward OA trends (p < 0.10). Of the 42 sites that had sufficient streamflow and SMAT concentration data for analysis 

(table 13), 30 sites had downward trends, and 12 sites had 

upward trends in concentrations. Of these sites, 18 sites 

had significant downward trends, and 5 sites had significant 

upward trends. The sites with significant increases in concentrations were in Illinois and North Dakota.
Trends in Nutrient and Suspended Sediment Concentrations and Loads 57

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

DISSOLVED

PHOSPHORUS

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 25. Trends in dissolved phosphorus from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area. 
58 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

A. Median total suspended-material concentrations

B. Median annual total suspended-material yields

22.0 – 7,925

Median annual yield, 

in kilograms per square 

kilometer

7,926 – 24,000

24,001 – 47,900

47,901 – 93,300

93,301 – 3,370,000

EXPLANATION

0.7 – 7.0

Median concentration, 

in milligrams per liter

7.1 – 13

14 – 23

24 – 41

42 – 5,240

EXPLANATION

Study area

State boundary

Basin boundary

Study area

State boundary

Basin boundary

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Figure 26. Spatial patterns from 1970 to 2004 in A, median total suspended-material concentrations and B, median annual 

total suspended-material yields throughout the study area. 
Table 13. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total suspended material (SMAT, USGS paramcombined), 1993 to 2004. [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per squ

year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites. Total suspended material, or SMAT, is total suspended sediment combined with total codes 80154 and 00530, respectively.]

USGS

site

number

State-ID 

number 

(figure 1)

Waterquality 

model

(table 3)

Concentration

Annual loads

Flow-adjusted trend Overall trend

Reference 

concentration

(mg/L)

Trend 

(mg/L/yr)

Trend 

(%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value

Median 

annual 

load 

(kg)

Median 

yield 

(kg/km2)

Reference 

load 

(kg)

Western part

05082500 ND-6 5 70.3 7.148 10.17 0.675 2.750 3.91 0.857 NA NA 267,000,000 

05114000 ND-7 9 5.0 .187 3.75 .223 .427 8.56 .077 1,480,000 188 26,200 

05120000 ND-8 9 5.6 .092 1.65 .593 .172 3.07 .420 2,770,000 243 207,000 

Central part

04063700 WI-9 7 3.8 0.208 5.48 0.132 0.156 4.12 0.221 956,000 2,660 316,000 

04067500 WI-10 8 5.0 -.149 -2.97 .164 -.150 -2.98 .182 15,800,000 1,560 12,000,000 

04073468 WI-11 9 21.9 .414 1.89 .081 .369 1.68 .128 897,000 6,470 586,000 

04085427 WI-12 3 33.7 -2.134 -6.32 .080 -2.085 -6.18 .078 5,100,000 3,740 4,460,000 

04087000 WI-13 7 30.3 .205 .68 .667 .057 .19 .907 22,400,000 12,400 9,790,000 

05287890 MN-14 7 6.9 .035 .50 .812 -.172 -2.49 .240 1,010,000 4,540 156,000 

05340500 WI-15 8 5.3 .028 .53 .901 -.020 -.38 .927 56,400,000 3,490 19,400,000 

05369500 WI-16 7 8.8 -.256 -2.91 .072 -.285 -3.24 .042 90,700,000 3,880 57,000,000 

05382000 WI-17 9 26.3 -1.221 -4.64 .001 -1.396 -5.31 .000 41,000,000 7,600 38,100,000 

05389400 IA-18 9 83.9 -.074 -.09 .977 -1.581 -1.88 .410 1,100,000 12,500 2,080,000 

05389500 IA-19 3 36.2 -1.051 -2.90 .179 -1.311 -3.62 .063 2,140,000,000 12,200 1,510,000,000 

05407000 WI-20 7 24.5 -1.067 -4.35 .005 -1.110 -4.53 .001 175,000,000 6,490 204,000,000 

05420500 IA-21 8 54.3 -1.534 -2.83 .109 -2.478 -4.57 .003 3,950,000,000 17,800 3,190,000,000 -

05427718 WI-22 9 40.4 -1.257 -3.11 .000 -1.942 -4.80 .000 2,210,000 11,600 929,000 

05427948 WI-23 8 33.7 -1.006 -2.98 .000 -1.046 -3.10 .000 1,410,000 31,900 95,800 

054310157 WI-24 8 48.1 -1.422 -2.95 .007 -1.410 -2.93 .010 447,000 39,800 59,400 

05465500 IA-25 8 181.0 -2.893 -1.60 .401 -9.416 -5.20 .008 1,860,000,000 57,300 1,640,000,000 -

05474000 IA-26 7 207.0 -4.453 -2.15 .267 -12.022 -5.81 .007 1,420,000,000 127,000 417,000,000 

05481650 IA-27 3 52.9 -1.361 -2.57 .232 -2.444 -4.62 .007 213,000,000 14,100 166,000,000 

05500000 MO-28 8 28.8 -.893 -3.10 .368 -1.794 -6.22 .030 388,000,000 242,000 3,680,000 
USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Central part—continued 05525500 IL-30 9 17.4 6.025 34.60 0.000 5.417 31.11 0.000 100,000,000 87,000 2,510,000 

05531500 IL-31 9 16.6 6.202 37.39 .000 6.071 36.60 .000 22,300,000 74,800 2,010,000 

05532500 IL-32 9 16.4 5.681 34.62 .000 4.932 30.06 .000 83,500,000 51,100 8,450,000 

05540275 IL-33 7 10.5 .328 3.13 .260 .063 .61 .827 328,000 12,800 59,700 

05543500 IL-34 8 31.0 .783 2.52 .320 -.199 -.64 .785 542,000,000 25,300 296,000,000 

05552500 IL-35 7 37.9 .214 .56 .872 -.960 -2.53 .406 164,000,000 24,900 71,800,000 

05570000 IL-36 7 85.0 10.015 11.78 .250 2.602 3.06 .696 489,000,000 115,000 80,600,000 

05586100 IL-37 9 81.5 12.415 15.23 .000 8.016 9.83 .000 3,490,000,000 50,300 2,020,000,000 

07014500 MO-38 7 6.5 .016 .25 .959 -.307 -4.71 .159 46,900,000 12,300 7,280,000 

07018100 MO-39 5 4.1 -.120 -2.96 .558 -.190 -4.68 .248 52,200,000 27,400 1,890,000 

Eastern part 03085000 PA-40 9 10.7 -0.655 -6.14 0.005 -0.601 -5.63 0.022 609,000,000 32,000 70,700,000 

03353637 IN-42 8 21.2 -.371 -1.75 .361 -.581 -2.74 .206 3,780,000 85,800 178,000 

04186500 OH-43 7 47.4 -3.272 -6.90 .001 -3.301 -6.96 .002 32,500,000 37,700 3,550,000 

04193500 OH-44 9 32.9 -1.322 -4.02 .008 -1.268 -3.86 .043 1,040,000,000 63,600 61,900,000 

0422026250 NY-45 7 91.4 -5.705 -6.24 .000 -5.109 -5.59 .000 1,600,000 61,100 572,000 

04232034 NY-46 7 132.1 -7.685 -5.82 .000 -7.818 -5.92 .000 6,020,000 59,300 3,470,000 

0423204920 NY-47 7 72.4 -.850 -1.17 .603 -1.178 -1.63 .472 978,000 54,200 341,000 

0423205025 NY-48 9 45.6 -.585 -1.28 .429 -.791 -1.73 .285 11,900,000 32,000 4,380,000 

04237946 NY-49 9 984.6 -13.718 -1.39 .288 -14.079 -1.43 .280 778,000 939,000 608,000 

Table 13. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total suspended material (SMAT, USGS param00530 combined), 1993 to 2004.—Continued [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per squ

per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites. Total suspended material, or SMAT, is total suspended sediment combined with toparameter codes 80154 and 00530, respectively.]
 Trends in Nutrient and Suspended Sediment Concentrations and Loads 61

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

SUSPENDED MATERIAL

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 27. Trends in total suspended materials from 1993 to 2004 for A, flow-adjusted concentrations, 

B, overall concentrations, and C, annual loads throughout the study area.
62 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

To place the 1993–2004 trends in SMAT concentrations 

into a longer term context, FACs were computed from 1975 

to 2004 for the six sites that had sufficient data and examined for trends with QWTREND (table 14; fig. 28). Only one 

site, the Milwaukee River (04087000), had an upward trend 

over the entire period from 1975 through 2004. The Chippewa River had an upward trend from 1975 through 1981, a 

downward trend from 1982 through 2001, and no trend from 

2002 through 2004. The Souris (05114000) and Maumee 

(05420500) Rivers had no trend through the early part of the 

period and downward trends in the later part, beginning in 

1996. Myers and others (2000) reported a relatively small, 

but significant, downward trend in FACs from 1970 to 1998. 

The Iowa River (05465500) had a downward trend from 1982 

through 2004, and the Mississippi River (05420500) showed 

downward trends throughout the period from 1975 though 

2004. Three sites (05465500, 04193500, and 05420500) had 

FACs in 2004 that were less than one-half of the FACs in 1975 

(table 14).

Trends in Total Suspended-Material Loads

From 1993 to 2004, SMAT loads indicated a tendency 

toward decreasing loads at 36 sites and a tendency toward 

increasing loads at 6 sites (table 13; fig. 27C). Downward 

trends in loads at 20 of the sites, throughout most of the study 

area, were significant, and upward trends at 3 sites in northern 

Illinois and North Dakota were significant (p < 0.10). Once 

again the pattern in the trends in loads resembles that for 

streamflow (fig. 12).

Annual SMAT loads at the six long-term sites from 1975 

to 2003 can be used to put the short-term trends in loads into 

a longer term context (fig. 29). The long-term changes at all 

six sites resembled the long-term trend in streamflow (fig. 12), 

which again indicates the importance of changes in streamflow 

in driving the trends in loads. A combination of total suspended sediment (SS) and total suspended solid data was used 

to estimate changes in the long-term loads at all of the sites 

except the Iowa and Maumee Rivers, which were estimated 

on the basis of only SS data. This combination of data from 

two different types of analysis could in itself induce long-term 

trends in the loads; however, for these sites, the major changes 

appear to be associated with changes in streamflow. 

The SMAT loads at the Souris River, N.Dak., decreased 

after a very high streamflow period (fig. 13) in the mid-1970s 

until approximately 1987, when loads remained relatively 

constant until 2003; however, very few SMAT samples were 

collected during the early years of the record, so the apparent 

large downward trend in loads may be partially attributable 

to sampling bias. For example, only two suspended-material 

samples were collected in 1975, and only five samples were 

collected in 1976—the two highest loading years (fig. 29).

Loads at the Chippewa River at Durand, Wisc., increased 

from 1975 to 1982 and then decreased at a fairly constant rate 

until 2003. Loads at the Milwaukee, Iowa, and Mississippi 

Rivers generally increased from 1975 to about 1982, then generally decreased until about 1990, and then remained approximately constant during a period of irregular wet or dry years 

until 2003. It is probable that both streamflow and the associated transport of suspended materials in these rivers were 

affected by the presence of the many dams upstream from 

the sampling sites. The SMAT loads at the Maumee River at 

Waterville, Ohio, decreased from 1975 to about 1987, and then 

generally increased through 1994, then decreased again during 

a period of wet and dry years until 2003. 

Table 14. Summary of flow-adjusted trends in total suspended-material concentrations estimated by QWTREND for longterm evaluation sites, 1975 to 2004. 

[USGS, U.S. Geological Survey; mg/L, milligram per liter. Total suspended material, or SMAT, is total suspended sediment combined with total 

suspended solids, USGS parameter codes 80154 and 00530, respectively]

USGS site number Trend period Description of flow-adjusted concentration

05114000 January 1975–December 1995 Constant value of 13.3 mg/L

January 1996–December 2004 Downward trend from 13.3 to 7.0 mg/L

05369500 January 1975–December 1981 Upward trend from 8.3 to 22.5 mg/L

January 1982–December 2001 Downward trend from 22.5 to 7.1 mg/L 

January 2002–December 2004 Constant value of 7.1 mg/L

05465500 February 1978–December 1981 Constant value of 149 mg/L

January 1982–December 2004 Downward trend from 149 to 67 mg/L

04087000 January 1975–December 2004 Upward trend from 19.8 to 28.9 mg/L

04193500 January 1975–December 1995 Constant value of 57.0 mg/L

January 1996–December 2004 Downward trend from 57.0 to 21.7 mg/L

05420500 January 1975–December 2004 Downward trend from 56.1 to 24.3 mg/L
 Trends in Nutrient and Suspended Sediment Concentrations and Loads 63

A. B.

C. D.

E. F.

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005 1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

1975 1980 1985 1990 1995 2000 2005

LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)

LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)

LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)

LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)LOG (SUSPENDED-MATERIAL CONCENTRATION, IN MILLIGRAMS PER LITER)

YEAR YEAR

0.5

0.0

1.0

1.5

2.0

2.5

3.0

0.5

0.0

1.0

1.5

2.0

2.5

3.0

0.5

0.0

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Souris River near Sherwood, North Dakota

USGS station number 05114000

Chippewa River at Durand, Wisconsin

USGS station number 05369500

Iowa River at Wapello, Iowa

USGS station number 05465500

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

Mississippi River at Clinton, Iowa

USGS station number 05420500

Maumee River at Waterville, Ohio

USGS station number 04193500

LOESS smoothing line

Concentration

EXPLANATION

Figure 28. Long-term flow-adjusted trends in total suspended-material concentrations from 1975 to 2004 at six selected long-term 

sites.
64 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

A. B.

C. D.

E. F.

19751970 19851980 19951990 20052000

ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

 ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

ANNUAL SUSPENDED-MATERIAL LOAD,

IN MILLIONS OF KILOGRAMS

0

0

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

19751970 19851980 19951990 20052000

Souris River near Sherwood, North Dakota

USGS station number 05114000

Chippewa River at Durand, Wisconsin

USGS station number 05369500

Iowa River at Wapello, Iowa

USGS station number 05465500

Milwaukee River at Milwaukee, Wisconsin

USGS station number 04087000

Mississippi River at Clinton, Iowa

USGS station number 05420500

Maumee River at Waterville, Ohio

USGS station number 04193500

0

50

100

150

200

250

0

100

200

300

400

500

600

4,000

8,000

12,000

5

15

25

35

0

1,000

2,000

3,000

4,000

4,000

8,000

12,000

YEAR YEAR

LOESS smoothing line

Annual load

EXPLANATION

Figure 29. Long-term trends in total suspended-material loads from 1975 to 2003 at six sites in the study area. 
Understanding Water-Quality Trends in the Context of Human Activity 65

Total Suspended Sediment

The USGS generally analyzes water samples for total 

suspended sediment (SS, USGS parameter code 80154), 

whereas most State agencies analyze their samples for total 

suspended solids (USGS parameter code 00530). Gray and 

others (2000) found that total suspended solid concentrations 

generally were less than SS concentrations by about 25 to 

34 percent because the analytical procedure for total suspended solids inadequately accounts for the amount of sandsized particles in the sample. This difference often is considered to be small compared to spatial differences that exist 

throughout large areas (Robertson and others, 2006); however, 

this bias may be significant when examining long-term trends. 

Therefore, SS data were analyzed separately to note potential 

bias in SMAT trends. 

Median midmonthly SS concentrations from 22 trend 

sites ranged from 5.50 to 424 mg/L, based on data from 1993 

to 2004. The mean and median values of the median SS 

concentrations at these trend sites were 65.2 and 40.5 mg/L, 

respectively. The highest concentrations again were in the 

western, southwestern, and south-central parts of the study 

area. The lowest concentrations were mostly in northern parts 

of Wisconsin and Michigan and in the northeastern part of the 

study area. The mean and median values based only on the SS 

data were slightly higher than those for SMAT when all of the 

sites that had only total suspended solids data were removed. 

This again indicates that SMAT concentrations may be biased 

slightly low compared to SS concentrations.

Median annual SS yields ranged from 1,620 to 

939,000 kg/km2 (table 15). The mean and median values of 

the median SS yields at these sites were 75,500 and 

25,200 kg/km2, respectively. Similar to SMAT yields, the 

highest SS yields were throughout the southern and eastern 

parts of the study area. The lowest SS yields were throughout 

the northern part of the study area. 

Trends in Total Suspended-Sediment 

Concentrations

The spatial tendencies in the FA trends in SS concentrations from 1993 to 2004 resembled those for SMAT, but with 

several differences (table 15; fig. 30). The main difference 

was that some of the downward trends observed for the SMAT 

data in Wisconsin and New York and upward trends in Illinois 

were not observed for SS data because most of these sites only 

had total suspended-solids data. In addition, when the total 

suspended-solids data were removed from the Popple River 

data in northern Wisconsin, the upward trend in SS became 

significant. Of the 22 sites that had sufficient streamflow and 

SS concentration data for analysis (table 15), 15 sites had 

downward trends, of which 7 trends were statistically significant at p < 0.10, and 7 sites had upward trends, of which 

3 trends were statistically significant. 

When the OA trends in SS concentrations from 1993 to 

2004 were examined, a few of the downward FA trends in 

Iowa and Illinois became statistically significant (p < 0.10; 

fig. 30). The differences between FA and OA trends in SS 

concentrations are again attributed to the trends in streamflow 

(fig. 12). Of the 22 trend sites, 16 sites had downward trends, 

of which 10 trends were significant, and 6 sites had upward 

trends, of which 1 trend was significant.

Trends in Total Suspended-Sediment Loads

From 1993 to 2004, most of the SS data showed a tendency toward decreasing loads. Nineteen of the 22 sites had 

a downward trend in SS loads (table 15; fig. 30C). Trends at 

11 of the 19 sites with downward trends in loads were significant, but the trend at only one site with an upward trend was 

significant (p < 0.10). The only site with a significant increase 

in loads was in northern Wisconsin. The slight upward trend in 

SMAT loads in the western part of the study area could not be 

confirmed because the western part of the study area could not 

be analyzed for SS trends. 

Understanding Water-Quality Trends in 

the Context of Human Activity

Water-quality studies that summarize and analyze concentration or load data from many monitoring sites provide 

valuable insights into statewide, regional, or in some cases 

national trends. A monitoring site may have a downward 

trend in, for example TN concentrations, that is not statistically significant; however, when that trend is analyzed in a 

regional context where proximate sites have similar environmental characteristics, especially for the same type of land 

use, and significantly decreasing TN concentrations, then the 

downward trend at the original site becomes a candidate for 

inclusion as part of the broad regional downward trend. In 

this report, no statistical test was applied to the regional trend. 

Additional information is thus obtained by analyzing data in a 

regional context.
Table 15. Summary of results for analyses of flow-adjusted trends, overall trends, and trends in loads for total suspended sediment (SS, USGS param1993 to 2004. [USGS, U.S. Geological Survey; ID, identification; mg/L, milligram per liter; mg/L/yr, milligram per liter per year; %/yr, percent per year; kg, kilogram; kg/km2, kilogram per squ

kg/yr, kilogram per year; NA, not available; NC, not calculated; sites in bold indicate long-term evaluation sites] USGS site number State-ID number (figure 1) Water- quality model (table 3) Concentration Annual loads Flow-adjusted trend Overall trend Reference concentration (mg/L) Trend (mg/L/ yr) Trend (%/yr) p-value Trend (mg/L/yr) Trend (%/yr) p-value Median annual load (kg) Median yield (kg/km2) Reference load (kg) 

Central part 04063700 WI-9 3 4.0 0.823 2.78 0.000 0.727 18.35 0.000 1,130,000 3,130 330,000 

04067500 WI-10 5 4.6 .033 .72 .721 .031 .67 .789 16,500,000 1,620 10,900,000 

04073468 WI-11 9 21.9 .414 1.89 .081 .369 1.68 .128 897,000 6,470 586,000 

04087000 WI-13 7 32.5 .549 1.69 .308 .403 1.24 .464 23,800,000 13,200 10,500,000 

05340500 WI-15 3 5.6 .159 2.83 .369 .086 1.54 .631 59,400,000 3,680 20,400,000 

05389400 IA-18 9 83.9 -.074 -.09 .977 -1.581 -1.88 .410 1,100,000 12,500 2,080,000 

05389500 IA-19 3 30.9 -.445 -1.44 .602 -.706 -2.28 .364 2,140,000,000 12,200 1,290,000,000 -

05420500 IA-21 8 51.8 -2.075 -4.00 .067 -2.829 -5.46 .005 4,280,000,000 19,300 2,790,000,000 -2

05427718 WI-22 9 40.4 -1.257 -3.11 .000 -1.942 -4.80 .000 2,210,000 11,600 929,000 

05427948 WI-23 8 33.7 -1.006 -2.98 .000 -1.046 -3.10 .000 1,410,000 31,900 95,800 

054310157 WI-24 8 48.1 -1.422 -2.95 .007 -1.410 -2.93 .010 447,000 39,800 59,400 

05465500 IA-25 8 181.0 -2.893 -1.60 .401 -9.416 -5.20 .008 1,860,000,000 57,300 1,640,000,000 -12

05474000 IA-26 7 207.0 -4.453 -2.15 .267 -12.022 -5.81 .007 1,420,000,000 127,000 417,000,000 -

05481650 IA-27 3 52.9 -1.361 -2.57 .232 -2.444 -4.62 .007 213,000,000 14,100 166,000,000 -

05532500 IL-32 3 129.0 -11.148 -8.64 .038 -11.653 -9.03 .025 178,000,000 109,000 61,900,000 

05543500 IL-34 8 30.1 .124 .41 .876 -.501 -1.66 .505 540,000,000 25,200 283,000,000 -

05586100 IL-37 9 140.9 7.302 5.18 .068 2.130 1.51 .552 3,260,000,000 47,000 2,830,000,000 -1

Eastern part

03085000 PA-40 8 12.6 -0.485 -3.86 0.331 -0.351 -2.79 0.517 630,000,000 33,100 83,000,000 

03353637 IN-42 8 20.1 -.267 -1.32 .531 -.471 -2.34 .309 3,780,000 85,800 170,000 

04186500 OH-43 7 47.4 -3.272 -6.90 .001 -3.301 -6.96 .002 32,500,000 37,700 3,550,000 

04193500 OH-44 9 32.9 -1.322 -4.02 .008 -1.268 -3.86 .043 1,040,000,000 63,600 61,900,000 

04237946 NY-49 9 984.6 -13.718 -1.39 .288 -14.079 -1.43 .280 778,000 939,000 608,000 
Understanding Water-Quality Trends in the Context of Human Activity 67

EXPLANATION

Trends

Upward (p < 0.05)

Upward (0.05 ≤ p < 0.1)

Upward (p ≥ 0.1)

Downward (p ≥ 0.1)

Downward (0.05 ≤ p < 0.1)

Downward (p < 0.05)

0 400 MILES

0 400 KILOMETERS

Total

Suspended Sediment

A. Flow-adjusted trend in concentrations

B. Overall trend in concentrations

C. Trend in annual loads

Figure 30. Trends in total suspended sediment from 1993 to 2004 for A, flow-adjusted 

concentrations, B, overall concentrations, and C, annual loads throughout the study area. 
68 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Ideally, any regional trends in concentrations or loads 

would be linked to changes in sources in a cause-and-effect 

relation. Mosteller and Tukey (1977) proposed that four conditions should be satisfied to define a cause-and-effect relation: 

(1) an association, such as a statistically significant correlation between the source variable and the response variable; 

(2) responsiveness, such as both proportional increases and 

decreases in response to the correlated changes in the source 

variable; (3) a mechanism to explain the relation; and (4) consistency, which requires that the relation should be repeatable 

at more than one location. Defining or proving cause-andeffect relations can be time-consuming, data-intensive, and 

costly. Few programs or studies have sufficient resources to 

establish such relations at statewide, regional, or national levels. One type of analysis used in this report to indicate causeand-effect relations is the consistency in the patterns in the 

trends in nutrient inputs and the patterns in the trends of the 

water-quality data. These spatial associations are qualitative, 

but they are strengthened by the consistency of a large number 

of sites. A second type of analysis used to derive possible 

cause-and-effect relations is a regression approach between 

the changes in concentrations and yields at selected sites and 

the changes in land use and the amount of a specific source of 

nutrients or sediment. Flow-adjusted trends in concentrations, 

being independent of streamflow conditions, are best used 

to evaluate changes in water quality arising from changes in 

management activities within a basin. Flow-adjusted trends are 

therefore examined in this section to try to describe potential 

cause-and-effect relations in the context of human activities.

Total Nitrogen 

Of the 24 sites examined for FA trends in TN concentrations, 19 sites had upward trends, and 5 sites had downward 

trends (fig. 15A). The FA trend patterns for TN are similar 

to the trends in fertilizer nitrogen applications (fig. 7B). The 

similarity in the patterns of these trends indicates a broad 

regional association between FA trends in TN and fertilizer 

applications. 

The FA trends in TN concentrations at 10 of these sites 

had significant (p < 0.10) trends, most of which were in the 

central part of the study area (fig. 15A). Six of these sites 

(05082500, 04087000, 05287890, 05420500, 05465500, 

and 05586100) were classed as agricultural, and FA trends 

were upward at all these sites except 05082500. Three sites 

with significant upward trends (04063700, 05369500, and 

05382000) were classed as forested. One site with a significant downward trend (05540275) was classed as mixed. The 

forested sites in northern Wisconsin and the Upper Peninsula 

of Michigan also may be affected by the increase in atmospheric deposition of nitrogen (fig. 9B). Because nitrogen 

input from fertilizers is the dominant source of nitrogen in 

agricultural areas, the annual value in each basin was analyzed in a regression analysis to put the trend into the context 

of human activity. Atmospheric deposition was used as an 

ancillary variable in the analysis of trends at the forested sites 

because that is the major source of nitrogen in those basins. 

Nitrogen from fertilizer applications was not significant at 

p < 0.10 for any agricultural site. The lack of significance 

indicates that many factors affect TN in streams, such as agricultural practices including artificial drainage, land-retirement 

plans, tillage practices, and animal husbandry. The use of 

nitrogenous fertilizers has remained relatively stable over the 

12-year period, increasing in some areas and decreasing in 

others (fig. 7B). The significant upward trends in TN concentrations at the five agricultural sites and upward trends at 

several of the other sites indicate that efforts to reduce runoff 

from agricultural lands have not been universally successful. 

At least two reasons account for that lack of success—the first 

is that local benefits do not necessarily translate to basinwide 

benefits, and the second is that unregulated activities such as 

tile drainage may counteract any benefit from efforts to reduce 

agricultural runoff. Nitrogen as nitrate moves easily through 

the soils to tile drains and into streams. In tile-drained areas, 

nitrate makes up most of the TN in the streams (Baker and 

others, 2006). Because much of the row cropland in the study 

area is heavily tiled (Strock and others, 2007), much of the 

nitrate enters streams by bypassing best management practices 

such as filter strips (Baker and others, 2007; Strock and others, 

2007). Another confounding factor is the growth of confined 

animal feeding operations (CAFOs) in the study area (Horne, 

2000). These CAFOs are highly regulated—the Clean Water 

Act (U.S. Environmental Protection Agency, 2006a) requires 

operators to carry a national pollutant discharge elimination 

system permit; the concentration of animal waste requires 

proper disposal.

Annual atmospheric deposition inputs were significant at 

p < 0.10 for two of the three forested sites, and fertilizer input 

was significant at one site (04063700). The results for the 

atmospheric deposition analysis indicate that the atmospheric 

deposition of nitrogen may have some effect on TN concentrations in streams.

The mixed site had a downward trend, which indicates 

that urban controls on nitrogen could have been effective in 

reducing total nitrogen in that stream. Unfortunately, no ancillary data pertaining to urban controls are available to test that 

conclusion.

Dissolved Ammonia

One of the goals of the Clean Water Act (U.S. Environmental Protection Agency, 2006a) is to reduce ammonia 

concentrations in streams through management of both animal 

and human wastes. The widespread, downward trends in 

NH4 concentrations for the 36 trend sites indicate that those 

goals are being met; 24 of the 36 trend sites had significant 

downward trends (p < 0.10). Regionally, good agreement 

exists between FA trends in NH4 concentrations (fig. 18A) and 

change in manure nitrogen application rates (fig. 8B), except 

in the farthest western part of the study area.

The strength of the relation between FA trends in NH4 

concentrations and the percent change in manure nitrogen was 
Understanding Water-Quality Trends in the Context of Human Activity 69

evaluated for agricultural sites. The p-value of the Spearman 

rank correlation estimate was 0.13, which indicates that the 

relation is not statistically significant at p < 0.10. This finding 

implies that although reduction in manure input may partially 

account for the reduction in FACs for NH4 , other efforts, such 

as improved wastewater treatment, have contributed to the 

observed downward trends.

The median values of the reference concentrations were 

about 0.05 mg/L for sites with no significant trends and about 

0.06 mg/L for sites with significant trends. Those two medians 

are not substantially different. However, the range in concentrations was smaller for those sites that did not have significant 

trends than those that did, which indicates for sites without 

significant trends that either NH4 was never an issue or that 

controls on waste had been effective earlier. The site that had a 

significant upward trend, Northrup Creek, N.Y. (0422026250), 

had a reference concentration of 0.032 mg/L. Land use at that 

site was classified as mixed, and the site has a relatively small 

drainage area of 26 km2, which implies that a very small, possibly local, source may be responsible for the trend observed 

at that site.

Total Kjeldahl Nitrogen

The FA trends in KJN concentrations from 1993 to 2004 

showed no distinct spatial pattern. Of the 36 sites that had 

sufficient streamflow and KJN concentration data for analysis (table 8; fig. 19A), 17 sites had downward trends, and 19 

sites had upward trends in concentrations. Seven sites had 

significant downward trends, and six sites had significant 

upward FA trends (p < 0.10). In general, human activities that 

affect KJN are the processing of human and animal waste, 

which can affect both NH4 and organic nitrogen in streams. 

A comparison between the change in TN from manure from 

1992 to 2002 (fig. 8B) and the FA trends in KJN concentrations (fig. 19) indicates that less manure was produced in all of 

the basins in 2002 than in 1992, except for the Maumee River, 

Ohio (04193500), which had a slight increase in manure production and a significant upward trend in FA concentrations. 

The broad decrease in manure production in most basins does 

not explain the significant upward trends in KJN at the remaining five sites. The p-value of the Spearman rank correlation 

test was 0.49. Human population was constant or increased 

in all but three basins from 1990 to 2000. The p-value of the 

Spearman rank correlation between KJN trends and changes in 

human population was 0.56. The increase in human population 

and apparent decrease in animal manure production are competing factors that confound the interpretation of trend in KJN 

concentrations. Other confounding factors are the changes in 

treatments of those human and animal wastes.

Dissolved Nitrite Plus Nitrate

The FA trends in NO2NO3 concentrations from 1993 

to 2004 showed no distinct spatial pattern (fig. 20A). Of the 

29 sites that had sufficient data for analysis, 12 sites had 

downward trends, and 17 sites had upward trends in concentrations. Some general agreement exists between the FA 

trends in NO2NO3 concentrations (fig. 20A) and the change in 

nitrogen fertilizer application (fig. 7B). The small changes in 

the forested sites in northern Wisconsin are consistent with the 

changes in atmospheric deposition of nitrogen (fig. 9).

Only two sites—Milwaukee River, Wis. (04087000) and 

Chippewa River, Wis. (05369500)—had significant (p < 0.10) 

positive relations between FA trends in NO2NO3 concentrations and the logarithm of nitrogen fertilizer application as an 

ancillary variable for trend. Another site, Little Buck Creek, 

Ind. (03353637), had a significant negative relation between 

FA trends in NO2NO3 concentrations and the logarithm of 

nitrogen fertilizer application; that is, increasing fertilizer 

application resulted in a decrease in concentrations, which 

contradicts a reasonable mechanism, although the substantial 

decrease in manure applications, about 80 percent decrease, 

could explain the downward trend.

An issue related to downward trends in NH4 concentrations often is an associated upward trend in NO2NO3 concentrations, which could be a product of the microbiologically 

mediated oxidation of NH4. Three sites—Milwaukee River, 

Wis. (04087000), Iowa River, Iowa (05465500), and Illinois 

River, Ill. (05586100)—had significant upward FA trends in 

NO2NO3 concentrations; however, none of these sites had 

significant downward FA trends in NH4 concentrations. Two 

sites—Little Buck Creek, Ind. (03353637) and Spring Brook, 

Ill. (05540275)—had significant downward FA trends in 

NO2NO3 concentrations. Both of these sites had significant 

downward FA trends in NH4 concentrations, and both sites are 

very urbanized and have 45 and 27 percent urban land cover, 

respectively.

Total Phosphorus

From 1993 to 2004, FA trends in TP concentrations were 

upward at 24 of the 40 sites (fig. 22A). The 24 sites with 

upward trends were distributed throughout the entire study 

area; however, most of these sites were in the central part and 

the eastern side of the western part of the study area. Twelve 

of these sites, mostly in the central part of the study area, 

had significant upward trends (p < 0.10). Downward trends 

were significant at 7 of the 16 sites, mostly in Wisconsin and 

the eastern part of the study area. A weak agreement exists 

between these trends and the trends in phosphorus fertilizer 

application (fig. 7C). Both trends indicate increases west of 

the Mississippi River and decreases in Wisconsin and eastern 

part of the study area. However, several sites in central and 

northern Illinois and in western Wisconsin are not consistent 

with this relation and had significant upward trends in TP concentrations (p < 0.05). These concentration increases may be 

associated with increases from local point sources. 
70 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Changes in the amount of phosphorus fertilizer use are 

consistent with the trend in FA trend in TP concentrations 

at four sites—Green Lake Inlet, Wis. (04073468), Pheasant Branch, Wis. (05427948), Jackson Creek Tributary, Wis. 

(054310157), and Northrup Creek, N.Y. (0422026250)—all of 

which had significant downward trends. However, phosphorus 

fertilizer use was negatively associated with the FA trend in 

TP concentrations at six sites (p < 0.05), which contradicts its 

use in explaining the observed trend at these sites. The lack 

of a strong association between phosphorus fertilizer use and 

FA trends in TP concentrations indicates that other factors are 

important in determining TP concentrations in streams.

Dissolved Phosphorus

The FA trends in DP concentrations from 1993 to 2004 

showed mixed regional tendencies; however, DP concentrations consistently decreased at all of the sites in the eastern 

part of the study area (fig. 25A). Many of the sites with significantly decreasing TP concentrations had little change in DP 

concentrations. Of the 35 sites analyzed, 23 sites had downward trends in DP concentrations; most of these sites were east 

of the Mississippi River, and they included 10 of the 12 sites 

with significant (p < 0.10) downward trends. A comparison 

of these patterns with those of nonpoint phosphorus inputs 

(fig. 10C) and those for fertilizers and manure indicate fairly 

good regional agreement indicating that reduction in fertilizers 

and manure east of the Mississippi River could explain most 

of the observed trends in DP concentrations. Some discrepancies occur in Illinois, but those could be caused by nonagricultural influences. 

Excluding the sites classified as urban in part of the study 

area east of the Mississippi River, 7 of the remaining 11 sites 

have a significant positive relation between the FA trends in 

DP concentrations and the logarithm of phosphorus fertilizer application rates as an ancillary variable in a regression 

analysis. No sites west of the Mississippi River had a significant relation between the FA trends in DP concentrations and 

phosphorus fertilizer application rates. This again indicates 

that the reductions in phosphorus fertilizer application rates 

possibly have had some effect on the concentration of phosphorus in the streams.

With the exception of one site, Spring Brook, Ill. 

(05540275), the correlation between trends in phosphorus 

fertilizer use and FA trends in DP concentrations was significant (fig. 31; Spearman correlation coefficient p-value was less 

than 0.05). Although most sites had downward trends in farm 

phosphorus use and FACs for DP—two sites Salt Creek, Ill. 

(05531500), and Des Plaines River, Ill. (05532500)—had the 

opposite relation. Land use at both of these sites was classified as predominantly urban. The urban and mixed sites had 

no significant relation between trends in phosphorus fertilizer 

use and FA trends in DP concentrations (fig. 31). All of the 

urban and mixed sites were well above the 1:1 reference line 

in figure 31, indicating that sources other than farm fertilizers 

affect the FA trends at those sites. In addition, all urban and 

mixed sites, except 05531500, had upward trends in nonfarm 

phosphorus fertilizer use, but FA trends in DP concentrations 

were mixed—four sites had downward trends and four sites 

had upward trends.

-12

-8

-4

0

4

8

12

-25 -20 -15 -10 -5 0 5

TREND IN FARM FERTILIZER (PHOSPHORUS) APPLICATION RATE,

IN PERCENT PER YEAR

TREND IN FLOW-ADJUSTED DISSOLVED PHOSPHORUS

CONCENTRATION, IN PERCENT PER YEAR

Agriculture

Urban

Mixed

Forest

1-to-1 line

LAND USE

Figure 31. Relation 

between trends 

in flow-adjusted 

dissolved 

phosphorus 

concentrations 

and trends in 

phosphorus 

inputs from farm 

fertilizers for sites 

with varying types 

of land use in the 

basin, 1993 to 2004. 
Changes in Loads from 1993 to 2004 in a Longer Term Context 71

Total Suspended Materials

The FA trends in SMAT concentrations from 1993 to 

2004 showed distinct spatial patterns (fig. 27), with significant 

upward trends in Illinois, and downward trends throughout most of Wisconsin, Iowa, and in the eastern part of the 

study area (fig. 27A). Of the 42 sites, 12 sites, mostly east of 

the Mississippi but not in Illinois, had significant (p < 0.10) 

downward trends. All but one of the five sites with statistically 

significant increases in SMAT were in Illinois. A comparison 

of these patterns with those of FA trends in TP concentrations 

(fig. 22A) indicates fairly good regional agreement.

To better determine if the FA trends in SMAT concentrations (fig. 27A) were related to the FA trends in TP concentrations (fig. 22A), the log-transform of FACs for SMAT was 

included as an ancillary variable in the regression models 

that showed significant FA trends in TP concentrations. The 

log-transform of SMAT was significant (p < 0.10) at 16 of the 

19 sites. The log-transform of SMAT concentrations was not 

significant at one site (05114000), and data for two Red River 

of the North sites—05064500 in Minnesota and 05082500 in 

North Dakota—were insufficient to evaluate the relation. This 

indicates that the FA trends in TP concentrations are strongly 

related to the FA trends in SMAT concentrations, which is 

very reasonable because TP includes phosphorus bound to 

suspended material.

FA trends in SMAT can be related to agricultural practices, urbanization, and forest management. The widespread 

downward FA trends in SMAT concentrations indicate that 

best management practices may have had some effect on 

reducing suspended material in many of the streams. The 

upward trends in Illinois are in urban and heavily agricultural 

areas and indicate that there may be confounding factors in the 

Illinois River Basin. Many types of human activity in streams, 

such as construction or removal of dams or bridges, can affect 

suspended material in streams. Natural changes, such as 

degradation or aggradation of the stream bed, also can affect 

suspended material. Ancillary data about these activities and 

changes are not broadly available and complicate the interpretation of trends in suspended material.

Changes in Loads from 1993 to 2004 in a 

Longer Term Context 

Changes in TN, TP, and SMAT loads were examined 

from 1975 to 2003 at six sites (figs. 17, 24, and 29). These 

changes were examined to provide a longer term perspective 

on the data examined for the 1993 to 2004 period. The longterm, nonmonotonic trends, which were shown as curves on 

the 1975–2003 graphs, were generated by LOESS smoothing, 

a type of moving average function. The statistical significance 

of those trends over the 1975–2003 period cannot be determined; however, the long-term changes for annual streamflow 

and load data indicate that the monotonic trends from 1993 to 

2004 should not be extrapolated backward in time. 

Examination of the longer term (1975–2003) mean 

annual streamflow data (fig. 13) and associated loads of TN 

(fig. 17), TP (fig. 24), and SMAT (fig. 29) at the six long-term 

sites indicates that the trends in streamflow and associated 

loads from 1993 to 2004 are not, in most cases, simple continuations of longer term linear trends; rather, the changes consist 

of a series of increases and decreases in streamflow and the 

associated loads. Most of the changes in long-term loads are 

directly caused by changes in streamflow rather than changes 

in nutrient or sediment concentrations resulting in longterm changes in loads resembling the long-term changes in 

streamflow. The most prominent example of this is in data for 

streamflow, and TN, TP, and SMAT loads at the Souris River, 

N. Dak., from 1975 to 2003. During this time, both streamflow 

and the associated loads, although trending downward, were 

all substantially larger during the 1970s and early 1980s than 

during 1993–2004 (figs. 13A, 17A, 24A, and 29A). Streamflow 

and associated TN, TP, and SMAT loads in the Souris River 

are to some extent artificially controlled by the large Canadian 

dams located upstream, especially in the 1993–2004 period 

when two of the dams were constructed.

In addition, a few of the sites are in the same basin and, 

therefore, trends should resemble one another. For example, 

the Chippewa River (site 16 in fig. 11) is in the UM Basin 

upstream from the Mississippi River at Clinton, Iowa 

(site 21 in fig. 11), which partially accounts for the similarities in trends in the streamflow and loads at these two sites 

(figs. 13B,F; 17B,F; 24B,F; and 29B,F). The largest exception to the general bimodal similarity, which was caused by 

similarities in climate over the UM region, is a local wet year 

recorded at the Chippewa Basin during 2002 that was not 

representative of conditions in the larger UM Basin upstream 

from Clinton. This difference resulted in the LOESS curves 

for streamflow, TN, and TP at the Chippewa River site trending upward during 2000–2003, whereas those for the Mississippi River at Clinton trended downward. 

Annual precipitation across the study area (fig. 4) 

and associated streamflow and loading at the Iowa River 

at Wapello, Iowa, and at the Maumee River, Ohio, varied 

substantially during 1975–2003 (figs. 13C,E; 17C,E; 24C,E; 

and 29C,E). The wet years generally averaged out with the 

dry years because the LOESS procedure at these two sites do 

not show any substantial or sustained upward or downward 

trends during this period. With the exceptions of the unusually 

dry year in 1989 and the unusually wet year in 1993 for the 

Iowa River at Wapello, Iowa, the short-term trends in streamflow, TP, TN, and SMAT during 1993–2004 generally were 

representative of conditions during the longer term period 

1974–2003. 

Unlike streamflow and TP loads, SMAT loads in the 

Maumee River decreased during 1996–2003 relative to 

1975–1995 (fig. 29). Such a decrease in SMAT loads relative to streamflow is of interest in the Maumee River Basin, 
72 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

because the Maumee River has been identified as being the 

single greatest contributor of sediment to Lake Erie and is an 

official USEPA “Area of Concern” for this and other waterquality problems (U.S. Environmental Protection Agency, 

2007). Because the FA trend in SMAT concentrations during 

1993–2004 is significantly downward (fig. 27, table 13) and 

although the trend in streamflow is slightly upward (fig. 12, 

table 4), the decrease in SMAT loads could be attributed to 

implementation of sediment control practices in the Maumee 

River Basin, which began in 1987 (U.S. Environmental Protection Agency, 2007).

The Milwaukee River at Milwaukee, Wis., drains to 

Lake Michigan and has the smallest drainage area of the six 

sites selected for the long-term analysis. Annual TN and TP 

loads for the Milwaukee River during 1975–1989 (figs. 17D; 

24D) were larger than those measured during 1993–2004 

because of trends in annual precipitation (fig. 4) and associated 

streamflow (fig. 13D). The SMAT loads during 1975–1984 

showed an upward trend, but not substantially different in 

magnitude than during 1985–2004 (fig. 29D).

Understanding Trends in Loads in the 

Context of Climatic Variability 

The insights provided by data in figures 13, 17, 24, and 

29 from sites that have a long history of streamflow and waterquality monitoring demonstrate the value of establishing and 

maintaining a long-term streamflow and water-quality monitoring network for planning purposes in the United States, 

especially as current thought (Solomon and others, 2007) indicates that global warming could lead to large-scale changes 

in the climate of North America during the 21st century. In 

general, such large-scale regional changes in climate and 

associated hydrology could necessitate modifications to urban 

infrastructure if changes occur in lake levels, flood crests, or 

flood frequency, or if lesser rainfall in some areas places unanticipated strains on water use and sewerages.

The nutrient and sediment loads described in this report 

primarily are controlled by changes in streamflow rather than 

changes in concentration, as governed by the substantially 

larger magnitude of the streamflow coefficients in all load 

computations relative to the much smaller concentration 

coefficients. In almost all cases for both the short-term loads 

(1993–2004) and the long-term loads (1975–2003), changes 

in loading closely mirror changes in streamflow because of 

dominance of streamflow coefficients. Sources of nitrogen, 

phosphorus, and sediment were relatively steady over the 

study period. Therefore, changes and trends in annual loading 

primarily were attributed to long-term trends and the regional 

distribution of precipitation (fig. 4) or water use, such as dam 

management or irrigation, and associated streamflow rather 

than the magnitude of trends in TP, TN, and SMAT concentrations. The implication of streamflow dominance in the load 

computations is that the 12-year loads presented in this report 

could be at least crudely estimated backward to the mid-1970s 

if a streamflow record exists for the period of interest and an 

assumption is made that nutrient and sediment sources did not 

change substantially in the basin during the period of interest. 

The strong influence of streamflow on nutrient and 

sediment loads implies that long-term trends in loads can 

be more sensitive to changes in climate rather than anthropogenic changes. For example, SMAT data for the Souris 

River, N. Dak., shown on figure 29A, might indicate that the 

substantial reduction in loads during the period 1975–1983 is 

attributed to some widespread implementation of an effective agricultural best-management practice for sediment 

control or improvements in industrial or sewage treatment 

plant processes; however, further consideration is required to 

understand the loads. As previously described, the streamflow 

in the Souris River decreased dramatically through time and, 

in addition, the Souris River is downstream from major dams 

in Canada, so streamflow in the Souris River is artificially 

controlled. Therefore, examination of FA trends in concentrations provides much more information regarding the effects of 

anthropogenic practices. 

Comparison between Nutrient Loads 

from This Study and Loads Contributing 

to the Gulf of Mexico Hypoxic Zone 

In addition to degrading the local quality of the streams, 

the nutrients and sediments transported in the streams can 

cause problems in downstream reservoirs, lakes, and coastal 

ecosystems. An increasing supply of nutrients, primarily nitrogen, from the Mississippi River Basin has been implicated 

as one of the main causes for the expanding hypoxic zone in 

the Gulf of Mexico (Rabalais and others, 2002; Goolsby and 

Battaglin, 2000). The Mississippi River and its distributary, 

the Atchafalaya River, drain an area of about 3,200,000 km2, 

or about 41 percent of the conterminous United States. In 

an average year, the Mississippi-Atchafalaya River (MAR) 

Basin contributes about 1,568,000,000 kg of TN to the Gulf of 

Mexico (Goolsby and others, 1999). 

Relative loading (yields) of nitrogen from the individual 

tributaries of the Mississippi River are not uniform. To 

better define where these nutrients originate, Goolsby and 

Battaglin (2000) quantified the TN loading from 42 large 

drainages within the Mississippi River Basin, including the 

Mississippi River at Clinton, Iowa, by use of a multiple 

regression approach, which is a method similar to that used 

in this study to compute loads. They then related these data 

to various nitrogen sources to estimate yields from similar 

sized drainages throughout the entire basin. They found 

that the highest TN yields were from areas with the highest 

streamflow, precipitation, and nitrogen inputs, primarily 

fertilizers; the highest yields were from drainages in the 

Ohio and middle part of the Mississippi River Basins. For 
the period from 1980 to 1996, they found that the UM Basin 

contributed 9.6 percent or 149,800,000 kg of the TN load of 

1,568,000,000 kg to the Gulf of Mexico from the MAR Basin, 

whereas the UM Basin represented only about 6.9 percent of 

the entire area. The load computed at the Mississippi River 

by Goolsby and Battaglin (2000) at Clinton, Iowa, is similar 

to that computed in this study of 149,300,000 kg for the 

same years. Only the Ohio River Basin and middle part of 

the Mississippi River Basins contributed a relatively higher 

percentage of the total load than the UM Basin. Goolsby and 

Battaglin (2000) found that streams draining Iowa and Illinois 

contribute as much as 35 percent of the TN flux during years 

with average and high rainfall, although they represent only 

9 percent of the total area. The UM Basin contributed about 

6 percent or 8,000,000 kg of the TP load of 136,500,000 kg 

from the MAR Basin, which is similar to the percentage of 

the area it represents. The TP load computed by Goolsby and 

Battaglin (2000) at the Mississippi River at Clinton, Iowa, is 

slightly less than that computed in this study of 9,030,000 kg 

for the same years.

The size of the hypoxic zone in the Gulf has been 

shown to be directly related to the TN flux from the MAR 

Basin (Scavia and others, 2003). Rabalais and others (2002) 

concluded that almost a three-fold increase in TN load to the 

Gulf has driven the long-term increase in hypoxia since the 

middle of the last century. To determine how the changes in 

the annual loads of TN and TP computed from the study area 

are related to changes in annual loads entering the Gulf, the 

long-term annual loadings at the Mississippi River at Clinton, 

Iowa, from this study were compared to total load estimated 

from the MAR Basin by the Aulenbach and others (2007).

The long-term annual TN and TP loads at the Mississippi 

River at Clinton, Iowa, are compared with those estimated 

from the MAR Basin in figure 32. Data from Aulenbach and 

others (2007) demonstrate a slight decrease in TN loads from 

the MAR Basin from 1980 to 2004. Although the TN load at 

Clinton, Iowa, represents only about 10 percent of the total 

load from the MAR Basin, the loads at Clinton, Iowa, are 

strongly correlated to those estimated from the MAR Basin. 

Annual variability in the TN loads at Clinton, Iowa, describe 

about 57 percent of the variance in the loads estimated by 

simple correlation analysis (Aulenbach and others, 2007). The 

loads estimated at Clinton, Iowa, indicate that the decrease in 

TN loads from 1980 to 2004 in the data from Aulenbach and 

others (2007) is not a long-term trend but rather a short-term 

decline after increasing from 1975 to about 1985.

Data from Aulenbach and others (2007) demonstrate no 

trends in TP loads from the MAR Basin from 1980 to 2004. 

The TP loads from Clinton, Iowa, represent a smaller proportion of the loads from the MAR Basin than for TN and result 

in poorer correlation between TP loads than between TN 

loads. Annual variability in the TP loads at Clinton, Iowa, 

describe only about 10 percent of the variance in the loads 

estimated by Aulenbach and others (2007), estimated by 

simple correlation analysis. The loads estimated at Clinton, 

Iowa, also indicate no long-term trend in TP loads.

Comparison between Nutrient Loads from This Study and Loads Contributing to the Gulf of Mexico Hypoxic Zone 73
74 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

0

50

100

150

200

250

300

1975 1980 1985 1990 1995 2000 2005

500

1,000

1,500

2,000

2,500

TOTAL NITROGEN LOAD FROM THE MAR BASIN,

IN MILLIONS OF KILOGRAMS PER YEAR

TOTAL NITROGEN LOAD AT CLINTON, IOWA,

IN MILLIONS OF KILOGRAMS PER YEAR 

TOTAL PHOSPHORUS LOAD AT CLINTON, IOWA,

IN MILLIONS OF KILOGRAMS PER YEAR

TOTAL PHOSPHORUS LOAD FROM THE MAR BASIN,

IN MILLIONS OF KILOGRAMS PER YEAR

0

2

4

6

8

10

12

14

16

18

20

0

20

40

60

80

100

120

140

160

180

200

1975 1980 1985 1990 1995 2000 2005

A. Total annual nitrogen loads

B. Total annual phosphorus loads

Clinton, Iowa (present study)

MAR Basin (Aulenbach and others, 2007)

Figure 32. Total annual A, nitrogen and B, phosphorus loads at the Mississippi River at Clinton, Iowa (site with the 

largest drainage area in this study) and from the entire Mississippi/Atchafalaya River (MAR) Basin from 1975 to 2005 

as estimated in this study and by Aulenbach and others (2007).
Summary 75

Summary

Elevated nutrient and suspended-sediment concentrations 

are two of the most common stressors or contaminants affecting streams throughout the United States. The elevated concentrations in rivers and streams can result in the overgrowth 

of benthic algae in shallow areas and areas with fast current 

and in an overabundance of phytoplankton and macrophytes 

in deep areas with slow current. High algal and macrophyte 

biomass, in turn, can cause severe diurnal fluctuations in 

dissolved oxygen and pH because of biotic production and 

respiration and can generate harmful organic materials when 

they die. Excessive transport of nutrients also has been linked 

to eutrophication of downstream lakes and impoundments, 

outbreaks of Pfiesteria in bays and estuaries in various Gulf 

and Mid-Atlantic States, and hypoxia in the Gulf of Mexico. 

Suspended sediment reduces clarity in streams and affects 

sight-feeding fish. Suspended sediment also interferes with 

water-treatment processes and recreational uses of streams. 

Excessive siltation can bury and suffocate fish eggs and 

bottom-dwelling organisms. In addition to instream effects, 

excessive sediment loading causes sedimentation problems in 

many downstream lakes and harbors and water-clarity problems in nearshore areas. 

As a result of the Clean Water Act and subsequent 

regulations, many actions have been taken to reduce nutrient 

and suspended-sediment concentrations and the amount of 

nutrients and sediment transported in streams. It is important 

to assess how nutrient and suspended-sediment concentrations 

and loads in streams have changed in recent years to determine 

if these actions have been successful. A focus of this study 

was on flow-adjusted trends in concentrations, which reflect 

anthropogenic changes in the basin and are not the result of 

changes in climatic factors.

The study area comprises four major river basins: Upper 

Mississippi, Ohio, Red, and Great Lakes River Basins. The 

Upper Mississippi River Basin is defined as the Mississippi 

River and its tributaries upstream from the confluence with 

the Ohio River, excluding the Missouri River. The Ohio River 

Basin includes the Allegheny and Monongahela Rivers, and 

the Ohio River and its tributaries except the Tennessee River. 

The Red River Basin includes all rivers in the United States 

that drain northward to Lake Winnipeg and Hudson Bay, 

including the Red River of the North, the Souris River, and the 

Rainy River. The Great Lakes Basin includes all streams in 

the United States draining into the Great Lakes upstream from 

Montreal, Canada. Lake Michigan was joined with the Illinois 

River (Upper Mississippi River Basin) through construction of 

the Chicago Sanitary and Ship Canal in 1900, a project engineered to reverse the streamflow of the Chicago River in order 

to divert Chicago wastewaters into the Upper Illinois River 

and away from the historic receiving water of Lake Michigan.

Flow-adjusted (FA) and overall (OA) trends in concentrations and trends in loads were computed for total nitrogen 

(TN), dissolved ammonia (NH4), total organic plus ammonia 

(Kjeldahl; KJN), dissolved nitrite plus nitrate (NO2NO3), total 

phosphorus (TP), dissolved phosphorus (DP), total suspended 

material (SMAT), and total suspended sediment (SS) at 

selected sites in the study area. Only sites that had at least 10 

years of data for at least one constituent from 1993 to 2004, at 

least 60 samples, and streamflow data from a gage or a nearby 

gage with a drainage-area ratio ranging from 0.9 to 1.1 were 

selected for trend analyses. The base period from 1993 to 

2004 was selected because it is relevant to current conditions 

and represents the period since the beginning of the National 

Water-Quality Assessment Program of the U.S. Geological 

Survey. For the final trend analysis in the base period, only 

49 sites met the criteria. The sites are divided among the three 

different climatic parts of the study area, the western, central, 

and eastern parts. Eight of the sites were in the western part, 

31 sites were in the central part, and 10 sites were in the eastern part of the study area.

From 1993 to 2004, FA trends in TN at 19 of the 24 sites 

that had sufficient data for analysis showed a tendency toward 

increasing concentrations. The 19 sites with increasing TN 

concentrations were distributed throughout the entire study 

area; however, only 8 of these sites had significant upward 

trends, and they were in the central part of the study area. 

These upward FA trends indicate that the effects of anthropogenic activities are continuing to increase in these areas 

and are contributing additional TN to the streams. Only two 

of the five sites with a downward tendency in concentrations 

had significant trends. The patterns in these FA trends in TN 

concentrations are similar to the changes in fertilizer nitrogen 

applications. The similarity in the patterns of these changes 

indicates a broad regional association between FA trends in 

TN concentrations and fertilizer applications. However, the 

significant upward FA trends in TN concentrations at five agricultural sites and upward trends at several of the others indicate that efforts to reduce runoff from agricultural lands have 

not been universally successful. At least two reasons account 

for that lack of success—the first is that local benefits do not 

necessarily translate to basinwide benefits, and the second is 

that unregulated activities such as tile drainage may counteract 

any benefit from efforts to reduce agricultural runoff.

The OA trends in TN concentrations from 1993 to 2004 

indicated a general tendency toward increasing concentrations at 16 of the 24 sites; however, only 3 of these sites had 

significant upward trends; 2 sites were in southern Wisconsin 

in the central part of the study area, and 1 site was in the western part of the study area. Of the eight sites with a tendency 

toward decreasing concentrations, only two sites had significant downward trends and they are the same two sites that 

had downward FA trends. The sites with downward trends in 

concentrations were scattered throughout the study area. 

From 1993 to 2004, TN loads at 17 of the 24 sites that 

had sufficient data for analysis indicated a tendency toward 

decreasing loads. Most of the sites with a downward trend 

were in the central or western parts of the study area. Eight 

of these sites had significant downward trends in loads; most 

of those sites had significant downward trends in streamflow. 
76 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

The only exception was the site in the eastern North Dakota, 

which had a significant downward trend in loads and no trend 

in streamflow. Only one of the seven sites with an upward 

trend in TN loads had a significant trend.

The FA trends in NH4 concentrations had a widespread 

downward tendency from 1993 to 2004. Of the 36 sites that 

had sufficient streamflow and NH4 concentration data for 

trend analysis, 32 sites had a downward trend in concentrations. Of these 32 sites, 24 sites had significant downward 

trends (at p < 0.10). Only four sites had an upward FA trend in 

NH4 concentrations, of which only one site, Northrup Creek, 

N.Y. (0422026250), had a significant upward FA trend in 

concentrations; however, the magnitude of this upward trend 

was only 0.002 mg/L/yr. The widespread, downward trends 

in NH4 concentrations for the trend sites indicate that some 

of the goals of the Clean Water Act are being met. There is, 

regionally, good agreement between FA trends in NH4 concentrations and changes in manure nitrogen, except in the farthest 

western part of the study area.

The OA trends in NH4 concentrations from 1993 to 

2004 were similar to those of the FA trends. The OA trends 

in NH4 concentrations at 32 of the 36 sites that had sufficient 

data showed a general tendency toward decreasing concentrations, and the downward trends were statistically significant 

(p < 0.10) at 23 of these sites. Of the remaining four sites 

with upward OA trends in concentrations, the only site with a 

significant trend was again Northrup Creek, N.Y.

From 1993 to 2004, NH4 loads at 32 of the 36 sites that 

had sufficient data for analysis indicated a tendency toward 

decreasing loads. These sites were distributed across all three 

parts of study area. Twenty-six of these sites had significant 

downward trends in loads (p < 0.10), and most of these sites 

had significant downward trends in both FA concentrations 

and streamflow.

The FA trends in KJN concentrations from 1993 to 2004 

showed no distinct spatial pattern. Of the 36 sites that had 

sufficient streamflow and KJN concentration data for analysis, 

17 sites had downward trends, and 19 sites had upward trends 

in FA concentrations. Seven sites had significant downward 

trends, and six sites had significant upward FA trends. The 

increase in human population and apparent decrease in animal 

manure production are competing factors that confound the 

interpretation of trend in KJN. Other confounding factors are 

the changes in treatments of those human and animal wastes.

When the OA trends in KJN concentrations from 1993 to 

2004 were examined, no distinct spatial patterns were evident; 

however, the tendencies and statistical results were somewhat 

different than those of the FA trends. The differences in the 

results of the FA and OA trend analyses were attributable to 

differences in the trends in streamflow. Of the 36 sites that had 

sufficient streamflow and KJN concentration data for analysis, 

18 sites had downward trends, and 18 sites had upward trends 

in KJN concentrations. Of these, nine sites had significant 

downward trends and six sites had significant upward trends 

(p < 0.10).

The KJN loads from 1993 to 2004 had a downward trend 

at 30 sites and an upward trend at 6 sites, of the 36 sites that 

had sufficient data for analysis. The sites with downward 

trends in KJN loads were distributed across all three parts of 

study area; three of the six sites with upward trends were in 

the eastern part of the study area. Thirteen of the sites with 

downward trends in loads had significant trends, whereas only 

one of the six sites with upward trends in loads had a significant trend (p < 0.10). 

The FA trends in NO2NO3 concentrations from 1993 to 

2004 showed no distinct spatial pattern. Of the 29 sites that 

had sufficient streamflow and NO2NO3 concentration data 

for analysis, 12 sites had downward trends, and 17 sites had 

upward trends in concentrations. Of these sites, three sites—

one in North Dakota, one in Illinois, and one in Indiana—had 

significant downward trends, and four sites—one in Iowa, 

one in Illinois, and two in Wisconsin—had significant upward 

trends (p < 0.10). Some general agreement exists between the 

FA trends in NO2NO3 concentrations and the change in nitrogen fertilizer application. The small changes in the forested 

sites in northern Wisconsin are consistent with the changes in 

atmospheric deposition of nitrogen.

When the OA trends in NO2NO3 concentrations from 

1993 to 2004 were examined, no distinct spatial patterns were 

evident; however, tendencies and trend results were somewhat 

different than those of the FA trends. These differences in the 

results of the FA and OA trend analyses are attributable to differences in streamflow trends. Of the 29 sites that had sufficient streamflow and NO2NO3 concentration data for analysis, 

13 sites had downward trends, and 16 sites had upward trends 

in concentrations. Of these sites, four sites had significant 

downward trends, and three sites had significant upward trends 

(p < 0.10).

Of the 29 sites that had sufficient data from 1993 to 2004 

for analysis, 16 sites had downward trends, and 13 sites had 

upward trends in NO2NO3 loads. Seven of the sites with 

downward trends in loads had significant trends, whereas only 

one of the 12 sites with upward trends in loads had a significant trend (p < 0.10). 

From 1993 to 2004, FA trends in TP concentrations at 

24 of the 40 sites that had sufficient data for analysis were 

upward. The 24 sites that had increasing TP concentrations 

were distributed throughout the entire study area; however, 

most of these sites were in the central part of the study area 

and the eastern side of the western part. Twelve of these 

24 sites had significant upward trends (p < 0.10). These 

upward FA trends indicate that the effects of anthropogenic 

activities are continuing to increase in these areas and are contributing additional TP to the streams. Downward FA trends in 

concentrations at 7 of the 16 sites were significant (p < 0.10) 

and indicate that the effects of anthropogenic activities are 

decreasing. Changes in the amount of phosphorus fertilizer 

use are consistent with FA trends in TP concentrations at four 

sites (04073468, 05427948, 054310157, and 0422026250), 

all of which had downward trends. However, phosphorus 

fertilizer use was negatively associated with the FA trends in 
Summary 77

TP concentrations at six sites (p < 0.05), which contradicts its 

use in explaining the observed trend at these sites. The lack 

of a strong association between phosphorus fertilizer use and 

FA trends in TP concentrations indicates that other factors are 

important in determining TP concentrations in streams.

The OA trends in TP concentrations from 1993 to 2004 

showed a general tendency toward increasing concentrations 

at 21 of the 41 sites; however, the upward trends were significant at only 7 of these sites (p < 0.10). Of the 20 sites with a 

downward trend, only 7 sites had significant trends. The sites 

with upward or downward trends were scattered throughout 

the study area. 

From 1993 to 2004, trends in TP loads at 30 of the 

41 sites that had sufficient data for analysis were downward. 

Twelve of these sites, all within the central and eastern parts 

of the study area, had significant (p < 0.10) downward trends 

in loads. Most of these sites had significant downward trends 

in streamflow. The upward trends were statistically significant 

(p < 0.10) at only 2 of the 11 sites. A comparison of the trends 

in loads and FA trends in concentrations indicates that most of 

the trends in loads were driven primarily by streamflow rather 

than changes in concentration, except in southern Wisconsin 

where trends in both streamflow and concentrations were 

significantly downward.

The FA trends in DP concentrations from 1993 to 2004 

showed mixed regional tendencies; however, DP concentrations consistently decreased at all of sites in the eastern part of 

the study area. Of the 35 sites that had sufficient streamflow 

and DP concentration data for analysis, 23 sites had downward 

trends in concentrations. Of these sites, 12 sites had significant downward trends, and the trends at all 7 of the sites in 

the eastern part of the study area were significant. Five sites, 

all in the central part of the study area, had significant upward 

trends (p < 0.10). A comparison of the patterns of FA trends in 

DP concentrations with those of nonpoint phosphorus inputs 

indicates fairly good regional agreement indicating that the 

reduction in fertilizers and manure east of the Mississippi 

River influences observed trends in DP concentrations. Some 

discrepancies exist in Illinois, but those probably are caused 

by nonagricultural influences.

When the OA trends in DP concentrations from 1993 to 

2004 were examined, spatial trends followed the same general 

pattern of decreasing concentrations in the eastern areas and 

increasing and decreasing concentrations in the central and 

western parts of the study area. Several differences between 

the FA and OA trend analyses were found for specific sites 

in the central part of the study area, where many more sites 

had significant downward OA trends than downward FA 

trends. These differences in the results of the FA and OA trend 

analyses are attributable to the significant trends in streamflow. Of the 35 sites that had sufficient streamflow and DP 

data for analysis, 26 sites had downward trends, and 8 sites 

had upward trends in concentrations. Of these sites, 15 sites 

had significantly downward trends, and 4 sites in Illinois had 

significant upward trends (p < 0.10).

From 1993 to 2004, trends in DP loads at 32 of the 

35 sites that had sufficient data for analysis were downward; 

trends at 15 of these sites were significant (p < 0.10). Upward 

trends in DP loads at two of the three sites, all located in 

northern Illinois, were significant.

The FA trends in SMAT and SS concentrations from 1993 

to 2004 showed distinct spatial patterns; concentrations tended 

to increase in Illinois and to decrease throughout most of Wisconsin, Iowa, and the eastern part of the study area. The main 

difference was that some of the downward trends observed 

for the SMAT data in Wisconsin and New York and upward 

trends in Illinois were not observed for SS data because most 

of these sites only had total suspended solids data. In addition, when the total suspended solids data were removed 

from the data from the Popple River in northern Wisconsin, 

the upward trend in SS became significant. Of the 42 sites 

that had sufficient streamflow and SMAT concentration data 

for analysis, 25 sites had downward trends, and 17 sites had 

upward trends in concentrations. Of these sites, 12 sites had 

significant downward trends and 5 sites had significant upward 

trends (p < 0.10). Of the 22 sites that had sufficient streamflow 

and SS concentration data for analysis, 15 sites had downward 

trends, of which 7 trends were statistically significant, and 

7 sites had upward trends in concentrations, of which 3 trends 

were statistically significant (p < 0.10).

The spatial pattern in the OA trends in SMAT and 

SS concentrations generally was the same as that for the 

FA trends; however, several of the sites with non-significant trends, especially in Iowa, had significant downward 

OA trends (p < 0.10). Of the 42 sites that had sufficient 

streamflow and SMAT concentration data for trend analysis, 

30 sites had downward trends, and 12 sites had upward trends 

in concentrations. Of these sites, 18 sites had significant downward trends, and 5 sites had significant upward trends. The 

sites with significant upward trends in concentrations were in 

Illinois and North Dakota.

From 1993 to 2004, SMAT and SS loads indicated a 

tendency toward decreasing loads. SMAT loads had downward trends at 36 sites and upward trends at 6 sites. Nineteen 

of the 22 sites had a downward trend in SS loads. Twenty of 

the sites, throughout most of the study area, with a downward 

trend in SMAT loads had significant trends, and three sites in 

northern Illinois and North Dakota had a significant upward 

trend in loads. Once again the pattern in the load trends 

resembles that for streamflow.

Changes in TN, TP, and SMAT loads were examined 

from 1975 to 2003 at six sites. These changes were examined 

to provide a longer term perspective on the data examined 

from 1993 to 2004. The long-term, nonmonotonic trends were 

described by LOESS smoothing. The statistical significance 

of those trends cannot be determined; however, the long-term 

changes for annual streamflow and load data indicate that the 

monotonic trends from 1993 to 2004 should not be extrapolated backward in time.
78 Trends in the Upper Mississippi, Ohio, Red, and Great Lakes River Basins, 1975–2004

Most of the changes in long-term loads are directly 

caused by the change in streamflow rather than a change in 

nutrient or sediment concentrations. The long-term changes in 

concentrations were much smaller than the long-term changes 

in streamflow. This results in the long-term changes in loads 

directly resembling the long-term changes in streamflow. The 

most prominent example of this is in data for streamflow and 

TN, TP, and SMAT loads at the Souris River, N. Dak., from 

1975 to 2003.

The insights provided by examining trends at sites that 

have a long history of streamflow and water-quality monitoring demonstrate the value of establishing and maintaining a 

long-term streamflow and water-quality monitoring network 

for planning purposes in the United States, especially because 

current thought (2007) is that global warming could lead to 

large-scale changes in the climate of North America during the 

21st century. The strong influence of streamflow on nutrient 

and sediment loads can result in long-term trends caused by 

changes in climate rather than changes caused by anthropogenic changes. In general, such large-scale regional changes in 

climate and associated hydrology could necessitate modifications to infrastructure if changes occur in lake levels, flood 

crests, and flood frequency, or if lesser future rainfall in some 

areas places unanticipated strains on water use and sewerages.

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