U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5090

National Water-Quality Assessment Program

Trends In Nutrient and Sediment Concentrations and Loads 

In Major River Basins of the South-Central United States,

1993-2004
Trends In Nutrient and Sediment 

Concentrations and Loads In Major River 

Basins of the South-Central United States, 

1993-2004

By Richard A. Rebich and Dennis K. Demcheck

Scientific Investigations Report 2007–5090

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Rebich, R.A., and Demcheck, D.K., 2007, Trends in nutrient and sediment concentrations and loads in major river 

basins of the south-central United States, 1993-2004: U.S. Geological Survey Scientific Investigations Report 2007-

5090, 112 p.
iii

FOREWORD

The U.S. Geological Survey (USGS) is committed to providing the Nation with accurate and timely scientific information that helps 

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 suitable for industry, irrigation, and habitat for 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 ground water? How are conditions changing over 

time? How do natural features and human activities affect the quality of streams and ground water, 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 (2002–2012), a major focus is on regional assessments of water-quality conditions and trends. 

These regional assessments extend the findings in the Study Units by filling critical gaps in characterizing the flow and quality of 

surface water and ground water, and by determining 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, and establish links between sources of contaminants, the transport of those contaminants through the hydrologic system, and the 

potential effects of contaminants on humans and aquatic ecosystems. Many of the regional assessments employ modeling and other 

scientific tools, developed on the basis of data collected at individual sites, to help extrapolate and forecast conditions in 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 to predict how our 

actions, such as by adjusting nonpoint and point sources of contamination, converting land use, 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, and selected trace elements; and continuing national topical studies on the fate of agricultural chemicals, 

effects of urbanization on stream ecosystems, nutrient enrichment, bioaccumulation of mercury in aquatic organisms, and transport of 

contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to inform 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.

	 	 	 	 	 	 	 Robert M. Hirsch

	 	 	 	 	 	 	 Associate Director for Water
iv
v

Contents

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

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

Introduction ....................................................................................................................................................2

Purpose and Scope .............................................................................................................................5

Acknowledgments ...............................................................................................................................5

Approach ........................................................................................................................................................5

Sources of Data ....................................................................................................................................6

Site Selection and Water-Quality Data Screening .........................................................................7

Trend and Load Calculations ..............................................................................................................9 

Total trends in concentration and load .................................................................................10

Flow-adjusted trends in concentration and trends in flow ................................................11

Interpretation of trend results ................................................................................................11

Load calculations ......................................................................................................................11

Analysis of Source and Landscape Attributes .............................................................................12

Results ..........................................................................................................................................................13

Streamflow ..........................................................................................................................................14

Nitrogen ...............................................................................................................................................14

Ammonia trends, loads, and yields ........................................................................................14

Nitrite plus nitrate trends, loads, and yields ........................................................................19

Total nitrogen trends, loads, and yields ................................................................................24

Relation of trends in nitrogen to trends in source data and landscape attributes ........32

Overall conclusions about nitrogen trends and loads for the study area .......................37

Phosphorus .........................................................................................................................................37

Orthophosphorus trends, loads, and yields ..........................................................................37

Total phosphorus trends, loads, and yields ..........................................................................40

Relation of trends in phosphorus to trends in source data and landscape attributes.. 46

Overall conclusions about phosphorus trends and loads for the study area .................51

Suspended Sediment ........................................................................................................................51

Suspended-sediment trends, loads, and yields ...................................................................51

Relation of trends in suspended-sediment to trends in source data and landscape 

attributes .......................................................................................................................54

Overall conclusions about suspended-sediment trends and loads for the study 

area ................................................................................................................................56

Summary .......................................................................................................................................................57

References ...................................................................................................................................................58

Appendix .......................................................................................................................................................63

Tables ...........................................................................................................................................................67
vi

Figures

1-6. Maps showing:

 1. Locations of major river basin and National Water-Quality Assessment study areas .............2

 2. States, cities, and major rivers in the study area, south-central United States ........................3

 3. Physiographic provinces within the study area ..............................................................................4

 4. Land use within the study area ...........................................................................................................6

 5. Sites selected for trend analyses and load calculations in the study area for the period, 

 1993-2004 .......................................................................................................................................8

 6. Trends in streamflow at study sites, 1993-2004 ..............................................................................15

 7. Graphs showing average annual streamflow for selected study sites, 1980-2004 ..................16

8-10. Maps showing:

 8. Total trends in ammonia concentrations at study sites, 1993-2004 ............................................17

 9. Flow-adjusted trends in ammonia concentrations at study sites, 1993-2004 ...........................18

10. Trends in ammonia loads at study sites, 1993-2004 .........................................................................20

11. Graphs showing annual ammonia loads for selected study sites, 1980-2004 ............................21

12-14. Maps showing:

12. Total trends in nitrite plus nitrate concentrations at study sites, 1993-2004 ...............................22

13. Flow-adjusted trends in nitrite plus nitrate concentrations at study sites, 1993-2004 ..............23

14. Trends in nitrite plus nitrate loads at study sites, 1993-2004 .........................................................25

15. Graphs showing annual nitrite plus nitrate loads for selected study sites, 1980-2004 .............26

16-18. Maps showing:

16. Total trends in total nitrogen concentrations at study sites, 1993-2004 .......................................27

17. Flow-adjusted trends in total nitrogen concentrations at study sites, 1993-2004 ......................28

18. Trends in total nitrogen loads at study sites, 1993-2004 .................................................................30

19. Graphs showing annual total nitrogen loads for selected study sites, 1980-2004 .....................31

20-23. Maps showing:

20. Trends in population densities at study sites from 1990-2000 .......................................................33

21. Trends in nitrogen from atmospheric deposition at study sites, 1993-2004 ................................34

22. Trends in nitrogen from fertilizer at study sites, 1993-2004 ............................................................35

23. Trends in nitrogen from manure at study sites, 1992-2002 .............................................................36
vii

Figures (continued)

24-26. Maps showing:

24. Total trends in orthophosphorus concentrations at study sites, 1993-2004 ................................38

25. Flow-adjusted trends in orthophosphorus concentrations at study sites, 1993-2004 ...............39

26. Trends in orthophosphorus loads at study sites, 1993-2004 ..........................................................41

27. Graphs showing annual orthophosphorus loads for selected study sites, 1980-2004 ..............42

28-30. Maps showing:

28. Total trends in total phosphorus concentrations at study sites, 1993-2004 ................................43

29. Flow-adjusted trends in total phosphorus concentrations at study sites, 1993-2004 ...............44

30. Trends in total phosphorus loads at study sites, 1993-2004 ...........................................................47

31. Graphs showing annual total phosphorus loads for selected study sites, 1980-2004 ...............48

32-36. Maps showing:

32. Trends in phosphorus from fertilizer at study sites, 1993-2004 ......................................................49

33. Trends in phosphorus from manure at study sites, 1992-2002 .......................................................50

34. Total trends in suspended-sediment concentrations at study sites, 1993-2004 .........................52

35. Flow-adjusted trends in suspended-sediment concentrations at study sites, 1993-2004 ........53

36. Trends in suspended-sediment loads at study sites, 1993-2004 ...................................................55

37. Graphs showing annual suspended-sediment loads for selected study sites, 1980-2004 .......56

Tables

1. Stream sites in south-central United States selected for trend and load computations of 

nutrient and sediment data ......................................................................................................68

2. Analytical results for trends in daily streamflow for sites in the Lower-Mississippi-Texas 

Basin for water years 1993-2004 .............................................................................................73

3. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for ammonia data for sites in the LowerMississippi-Texas Basin, water years 1993-2004 .................................................................77

4. Ammonia annual loads, average annual load, and average annual yield for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ....................................................82

5. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for nitrite plus nitrate data for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ....................................................84

6. Nitrite plus nitrate annual loads, average annual load, and average annual yield for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004 ..............................................88
viii

CONVERSION FACTORS, ABBREVIATIONS, AND ACRONYMS

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd)

Area

square kilometer (km2) 0.3861 square mile (mi2)

Flow rate

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic kilometer per year (km3/yr) 1,119.82 cubic foot per second (ft3/s)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

metric ton (T) 1.102 ton (t)

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

°F=(1.8×°C)+32

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

Tables (continued)

7. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for total nitrogen data for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ....................................................91

8. Total nitrogen annual loads, average annual load, and average annual yield for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ....................................................94

9. Analytical results of weighted-least-squares regression comparing flow-adjusted and total 

trends in nutrient and suspended-sediment concentrations for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004 ........................................................................96

10. Sites in the Lower-Mississippi-Texas Basin where the amount of conservation practices 

increased or decreased by more than one percent of the total drainage area, water 

years 1993-2004 ..........................................................................................................................97

11. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for orthophosphorus data for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ....................................................98

12. Orthophosphorus annual loads, average annual load, and average annual yield for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004 ............................................102

13. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for total phosphorus data for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 ..................................................104

14. Total phosphorus annual loads, average annual load, and average annual yield for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004 ............................................108

15. Analytical results and reference values for trends in flow-adjusted concentrations, total 

trend in concentrations, and trends in loads for suspended-sediment data for sites in 

the Lower-Mississippi-Texas basin, water years 1993-2004 ............................................110

16. Suspended-sediment annual loads, average annual load, and average annual yield for sites 

in the Lower-Mississippi-Texas Basin, water years 1993-2004 .......................................112
ix
x
ABSTRACT

Nutrient and sediment data collected at 115 sites by 

Federal and State agencies from 1993 to 2004 were analyzed 

by the U.S. Geological Survey to determine trends in concentrations and loads for selected rivers and streams that drain 

into the northwestern Gulf of Mexico from the south-central 

United States, specifically from the Lower Mississippi, Arkansas-White-Red, and Texas-Gulf Basins. Trends observed in 

the study area were compared to determine potential regional 

patterns and to determine cause-effect relations with trends 

in hydrologic and human-induced factors such as nutrient 

sources, streamflow, and implementation of best management 

practices. Secondary objectives included calculation of loads 

and yields for the study period as a basis for comparing the 

delivery of nutrients and sediment to the northwestern Gulf of 

Mexico from the various rivers within the study area. In addition, loads were assessed at seven selected sites for the period 

1980-2004 to give hydrologic perspective to trends in loads 

observed during 1993-2004.

Most study sites (about 64 percent) either had no trends 

or decreasing trends in streamflow during the study period. 

The regional pattern of decreasing trends in streamflow during 

the study period appeared to correspond to moist conditions 

at the beginning of the study period and the influence of three 

drought periods during the study period, of which the most 

extreme was in 2000.

Trend tests were completed for ammonia at 49 sites, 

for nitrite plus nitrate at 69 sites, and for total nitrogen at 

41 sites. For all nitrogen constituents analyzed, no trends 

were observed at half or more of the sites. No regional 

trend patterns could be confirmed because there was poor 

spatial representation of the trend sites. Decreasing trends 

in flow-adjusted concentrations of ammonia were observed at 

25 sites. No increasing trends in concentrations of ammonia were noted at any sites. Flow-adjusted concentrations 

of nitrite plus nitrate decreased at 7 sites and increased at 

14 sites. Flow-adjusted concentrations of total nitrogen 

decreased at 2 sites and increased at 12 sites. Improvements 

to municipal wastewater treatment facilities contributed to the 

decline of ammonia concentrations at selected sites. Notable 

increasing trends in nitrite plus nitrate and total nitrogen at 

selected study sites were attributed to both point and nonpoint 

sources. Trend patterns in total nitrogen generally followed 

trend patterns in nitrite plus nitrate, which was understandable 

given that nitrite plus nitrate loads generally were 70-90 percent of the total nitrogen loads at most sites. Population data 

were used as a surrogate to understand the relation between 

changes in point sources and nutrient trends because data from 

wastewater treatment plants were inconsistent for this study 

area. Although population increased throughout the study 

area during the study period, there was no observed relation 

between increasing trends in nitrogen in study area streams 

and increasing trends in population. With respect to other 

nitrogen sources, statistical results did suggest that increasing trends in nitrogen could be related to increasing trends 

in nitrogen from either commercial fertilizer use and/or land 

application of manure.

Loads of ammonia, nitrite plus nitrate, and total nitrogen 

decreased during the study period, but some trends in nitrogen loads were part of long-term decreases since 1980. For 

example, ammonia loads were shown to decrease at nearly all 

sites over the past decade, but at selected sites, these decreasing trends were part of much longer trends since 1980. The 

Mississippi and Atchafalaya Rivers contributed the highest nitrogen loads to the northwestern Gulf of Mexico as 

expected; however, nitrogen yields from smaller rivers had 

similar or higher yields than yields from the Mississippi River.

Trend tests were completed for orthophosphorus at 34 

sites and for total phosphorus at 52 sites. No trends were 

observed in about 57 percent of all phosphorus trend analyses 

attempted. Similar to nitrogen, no regional patterns could be 

confirmed because there was poor spatial representation of the 

trends sites. Flow-adjusted concentrations of orthophosphorus 

decreased at 10 sites and increased at 7 sites. Flow-adjusted 

concentrations of total phosphorus decreased at 6 sites and 

increased at 17 sites. It was understandable that trend patterns 

in total phosphorus did not follow trend patterns in orthophosphorus given that orthophosphorus loads accounted for about 

only 20-30 percent of the total phosphorus load at comparable 

sites. Trends in population data were inversely related to 

trends in flow-adjusted total phosphorus; therefore, trends in 

population were not considered a controlling factor to explain 

trends in total phosphorus. No relation was observed between 

phosphorus from fertilizer use and either orthophosphorus or 

total phosphorus trends. However, statistical results did sugTrends In Nutrient and Sediment Concentrations and 

Loads In Major River Basins of the South-Central United 

States, 1993-2004

By Richard A. Rebich and Dennis K. Demcheck
gest that increasing trends in both orthophosphorus and total 

phosphorus could be related to increasing trends in phosphorus 

from land application of manure.

There were more decreasing trends than increasing trends 

in phosphorus loads during the past decade, most of which 

were unique to the recent decade and not part of long-term 

decreases since 1980. Similar to nitrogen loads, the Mississippi and Atchafalaya Rivers contributed the highest phosphorus loads to the northwestern Gulf of Mexico as expected; 

however, phosphorus yields from smaller rivers were similar 

to or higher than yields from the Mississippi River.

Trend analyses of suspended-sediment data were 

attempted at 39 sites. No trends were observed at about 71 

percent of the sites. Remaining results indicated primarily 

decreasing trends in suspended sediment data. Most of the 

decreasing trends occurred on mainstem sites for the Mississippi, Arkansas, Red, and Atchafalaya Rivers, which are all 

regulated with reservoirs, locks and dams, and other erosion 

or flood-control structures that trap and prevent sediment from 

being transported downstream. Large decreases in suspended 

sediment in the Mississippi River Basin began in the 1950s 

when large reservoirs were constructed in the Missouri and 

Arkansas Rivers, which were considered the largest sources of 

sediment at the time. Because the Mississippi River and its 

major tributaries have continued to be modified and improved 

since 1990, it is suggested that declines in suspended sediment 

observed along the mainstem sites during the study period are 

related to ongoing watershed and channel modifications.

INTRODUCTION

The U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program is conducting 

regional assessments of water-quality conditions and trends 

in 16 principal aquifers and eight major river basins (fig. 1) 

(Hamilton and others, 2005). These assessments build on the 

NAWQA studies conducted from 1991 to 2001 in 51 river 

basins (fig. 1). Regional assessments in the eight major river 

basins focus on chemicals in water, such as trends in nutrients, 

Figure 1. Locations of major river basin (MRB) and National Water-Quality Assessment (NAWQA) study areas.

   Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
sediment, and pesticides, and other relevant water-quality 

issues, such as trends in biological-response data (chlorophyll, algae). Each basin comprises more than one NAWQA 

study unit, and data used for trend testing includes data from 

NAWQA studies supplemented with data from other USGS 

studies, as well as available data collected by other agencies.

One of these regional assessments explores trends in 

nutrient and suspended-sediment concentrations and loads for 

rivers in the south-central United States, which is defined as 

the Lower Mississippi, Arkansas-White-Red, and Texas-Gulf 

Basin (hereafter referred to as the Lower-Mississippi-Texas 

Basin, or LMT Basin, fig. 2). The LMT Basin includes all of 

Arkansas and Oklahoma; nearly all of Louisiana and Texas; 

and parts of Colorado, Kansas, Kentucky, Mississippi, Missouri, New Mexico, and Tennessee. Major cities include Little 

Rock, Ark.; Tulsa and Oklahoma City, Okla.; Baton Rouge 

and New Orleans, La.; Dallas, Fort Worth, Houston, and 

San Antonio, Tex.; Colorado Springs, Colo.; Wichita, Kan.; 

Springfield, Mo.; and Memphis, Tenn. Major rivers include 

the lower Mississippi, Yazoo, Canadian, Cimarron, Arkansas, 

White, Red, Trinity, Brazos, Colorado, and Guadalupe.

The geological features of the LMT Basin vary considerably from rugged mountains to rolling hills, flat plains, 

and backwater swamps. The LMT Basin encompasses six 

physiographic provinces (from east to west): the Coastal 

Plain, which includes the East Gulf Coastal Plain, Mississippi Alluvial Plain, and West Gulf Coastal Plain sections; 

the Ozark Plateaus, which includes the Springfield-Salem 

Plateaus and the Boston “Mountains”; the Ouachita, which 

includes the Arkansas Valley and the Ouachita Mountains; 

the Osage Plains section of the Central Lowlands; the Great 

Plains, which includes the Colorado Piedmont, Raton, Pecos 

Valley, High Plains, Plains Border, Edwards Plateau, and 

Central Texas sections; and a small part of the Southern Rocky 

Mountains (U.S. Geological Survey, 2003, fig. 3). Dominating the discharge of water and nutrients into the northern Gulf 

of Mexico is the Mississippi Alluvial Plain section within the 

Coastal Plain province. The Mississippi River drains 41 perFigure 2. States, cities, and major rivers in the study area, south-central United States.

INTRODUCTION   
cent of the conterminous United States, and its two outlets (the 

lower Mississippi River and the Atchafalaya River) deliver a 

combined average of 580 km3/yr of freshwater to the Gulf of 

Mexico (Meade, 1995). Created by flow and flooding of the 

Mississippi River during the past 2 million years or more, the 

Mississippi River Alluvial Plain has an average slope of about 

9.5 cm/km towards the Gulf of Mexico (Kleiss and others, 

2000). One of the distinct features of the Mississippi Alluvial 

Plain is the formation of natural levees along the banks of the 

rivers, and the associated back-swamp deposits.

Extending to the west and south in the LMT Basin is the 

West Gulf Coastal Plain section of the Coastal Plain province 

(fig. 3). This area varies from rolling hills and prairie grass to 

piney woods and coastal prairie as one approaches the Gulf 

of Mexico in southwestern Louisiana and southeastern Texas, 

an area that is extensively cultivated for growing rice (Land 

and others, 1998). In southwestern Louisiana, the West Gulf 

Coastal Plain section is divided into a series of broad, flat 

areas separated by bottomland hardwood riparian corridors, 

which vary in width from only a few hundred meters to several 

kilometers (Demcheck and others, 2004).

To the northwest of the Coastal Plain province are the 

Ozark Plateaus and Ouachita Provinces. These areas are fairly 

rugged, mountainous areas that are predominantly pasture, 

grassland, and forest. Although mining plays a large role in 

local industry in these two provinces, they are also known 

for their beautiful and scenic landscapes that support tourism 

(Adamski and others, 1994).

Figure 3. Physiographic provinces within the study area.

   Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
To the west, the headwaters of the Trinity and other 

smaller rivers drain the southern end of the Osage Plains section of the Central Lowlands physiographic province into the 

Gulf of Mexico. The Osage Plains section is characterized by 

prairie grasses, shrubs, and some forest in southern Kansas, 

Oklahoma, and northern Texas. Much of the area supports 

corn and soybean production or has been converted to pasture 

and hay to support cattle production (Fitzgerald and others, 

2000).

Farther to the west, the remaining land area of the LMT 

Basin is in the Great Plains province, except for a small part 

in the Southern Rocky Mountains province (fig. 3). The Great 

Plains province is quite diverse. From the rugged sections of 

the Colorado Piedmont, which lies on the eastern side of the 

Rocky Mountains, to the volcanic formations of the Raton 

section, the Great Plains extends southward and eastward to 

include the flat, prairie areas of the Plains Border and High 

Plains sections (Trimble, 1980). Farther south, the Pecos Valley section of the Great Plains is dominated by karst topography, and the rugged (but picturesque) Edwards Plateau section, 

which is a fairly sparse area, is suited for oil and gas production as well as cattle farming (Trimble, 1980; Bush and others, 

2000).

Precipitation varies considerably across the LMT Basin, 

generally following a decreasing pattern from the southeast 

to northwest, from more than 152 cm per year in southeastern 

Louisiana to less than 41 cm per year in Colorado. The western part is fairly arid (annual rainfall less than about 64 cm 

total per year [Owenby and others, 2001]) and is fairly rural 

with few large cities. Land use in the western part is primarily grass and fallow land with some row and small grain crops 

(fig. 4). Water-resource issues in the western part are related 

to water use, water rights, and irrigation as much as they are 

related to water quality. The eastern part has a humid, subtropical climate with annual rainfall amounts ranging from 100 

to greater than 130 cm per year (Owenby and others, 2001); 

subsequently, water resources are fairly abundant. Land use in 

the eastern part is primarily forest and pasture land; however, row crops are abundant in the fertile Mississippi River 

Alluvial Valley. The eastern part is fairly rural with respect 

to land area but is the more populous area and contains many 

of the previously mentioned cities (fig. 2). With the extreme 

variations in geology, geography, hydrology, and land use, it 

is expected that trends in concentrations and loads of nutrients 

and sediment in surface waters within the LMT Basin will also 

vary considerably.

Several reports describe trends in nutrient and sediment 

data for the LMT Basin. For example, studies by Van Metre 

and Reutter (1995), Demcheck and others (2004), Davis and 

Bell (1998), and Coupe (2002) document trends in concentrations and loads of nutrients or sediments for statewide assessments or for selected rivers. National studies, such as work 

by Mueller and others (1995), include assessments of nutrient 

data from both ground-water wells and surface-water data-collection sites. Studies summarized by Goolsby and Battaglin 

(2000), Meade (1995), and Turner and Rabalais (2004) assess 

nutrient concentrations and loads delivered by the Mississippi 

River to the Gulf of Mexico; although these studies include 

data and results from the LMT Basin, most focus on the Upper 

Mississippi River Basin upstream from the LMT Basin. The 

U.S. Environmental Protection Agency (USEPA) recently 

released results from its Nutrient Pilot Study, which included 

an assessment of nutrient concentration and loads from 

coastal or near-coastal waters draining into the northern Gulf 

of Mexico from Louisiana, Mississippi, and Alabama (U.S. 

Environmental Protection Agency, 2004a). This assessment 

is unique and timely in that it primarily focuses on the entire 

LMT Basin, not just parts of the basin.

Purpose and Scope

This report presents trends observed in nutrient and sediment concentrations and loads during the period 1993-2004 

for selected rivers and streams in the LMT Basin that drain 

into the northwestern Gulf of Mexico. Trends are computed 

for streamflow, ammonia, nitrite plus nitrate, total nitrogen, 

orthophosphorus, total phosphorus, and suspended sediment. 

Trends observed in the LMT Basin are compared spatially 

to determine potential regional patterns and are compared 

with trends in hydrologic and human-induced factors such 

as nutrients sources, streamflow, and implementation of best 

management practices to determine potential cause and effect 

relations.

This report also presents loads and yield estimates for the 

study period as a basis for comparing the delivery of nutrients 

and sediment to the northwestern Gulf of Mexico from the 

various drainage basins within the LMT Basin. In addition, 

load estimates at a few selected sites for the period 1980-2004 

are presented to give hydrologic perspective to trends in loads 

observed during 1993-2004.

Acknowledgments

The authors wish to thank the following USGS personnel: 

Dave Lorenz, for technical input with trend testing and load 

calculations; Greg Schwarz, for help with completion of the 

non-flow-adjusted trend analyses; Naomi Nakagaki, for ancillary data compilations; Kirsten Tighe and Trent Snellings, for 

GIS-technical support; and Patty Ging and Brent Aulenbach 

for technical reviews.

APPROACH

This section documents sources of data used in this 

analysis, protocols used for site selection and data screening, 

methods used for trend and load calculation, and methods used 

for analysis of nutrient-source data and landscape attributes.

APPROACH 
Sources of Data

Two types of data were assembled for analysis in this 

study: water-chemistry and flow data used for trend and 

load analyses, and spatial data such as nutrient sources and 

landscape attributes used to explain identified trends. Data 

are stored and archived locally in the databases of the USGS 

Mississippi Water Science Center and nationally as part of the 

NAWQA Program.

The primary source of water-chemistry and flow data for 

this assessment was data collected by the USGS. Since the 

early 1970s, the USGS has collected water-quality information from major river basins throughout the United States as 

part of three national programs: the Hydrologic Benchmark 

Network (HBN), the National Stream Quality Accounting 

Network (NASQAN), and the NAWQA Program. In addition, 

other long-term water-quality monitoring stations operate as 

part of USGS cooperative projects in various States. All data 

from these USGS efforts have been compiled and are available 

to the public by means of the Internet as part of the National 

Water Information System Web Interface (NWISWeb) accessible at http://waterdata.usgs.gov/nwis/qw.

Another source of water-chemistry data was data collected by State agencies within the LMT Basin as part of 

ambient data-collection programs. Environmental agencies in 

Arkansas and Missouri have partnered in the past with USGS 

to acquire certification for their respective laboratories through 

the USGS Laboratory Evaluation Project, which is under the 

direction of the USGS Branch of Quality Systems (http://bqs.

Figure 4. Land use within the study area.

 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
usgs.gov/lep/index.html); therefore, all of the data approved 

for these two State agency laboratories have been entered into 

the national USGS database and are available to the public 

through the NWISWeb previously mentioned. In addition, 

nutrient and sediment data were requested and received from 

the States of Louisiana, Mississippi, Tennessee, and Texas. 

The final source of water-quality data considered for analysis 

was from the USEPA Legacy Data Center (LDC) and the Storage and Retrieval (STORET) database (U.S. Environmental 

Protection Agency, 2004b).

In order to explain trends in surface water-quality data, 

it is important to identify and understand temporal and spatial 

patterns in source data and landscape attributes. Geographic 

information system (GIS) software (ESRI, 2005) was used to 

automatically delineate drainage-area boundaries and create 

digital polygons for most sites included in the trend analyses. 

Sites excluded from drainage area delineation were sites that 

were extremely large, such as sites on the mainstem of the 

Mississippi River, or sites where drainage area delineations 

were indeterminate, such as marsh areas along the coast where 

flows intermingle. Once the drainage areas were delineated, 

their corresponding digital polygons were overlain on thematic maps of county-level nutrient source and landscape data 

pertinent to this study. Source data were then summed for a 

particular site’s drainage area on a temporal basis. The nutrient 

source and landscape data included in this analysis were fertilizer use for nitrogen and phosphorus (annual data for the entire 

study period), manure generation for nitrogen and phosphorus 

(available for 1992, 1997, and 2002), atmospheric deposition 

for nitrogen (annual data for the entire study period), population density (1990 and 2000 census data), and management 

practices information (including irrigation type and conservation practices for 1992 and 1997). Where drainage areas 

extended only partly into one or more counties, the source or 

landscape data were apportioned according to the amount of 

agricultural or urban land contained within the drainage area, 

as described by Nakagaki and Wolock (2005).

Site Selection and Water-Quality Data 

Screening

Sites were selected for analysis of trends from the USGS 

NWISWeb, USEPA-LDC and STORET, and State ambient databases for water years 1993-2004 (October 1, 1992, 

through September 30, 2004). A few sites were included in 

this study that had sampling periods that started after October 

1, 1992, or ended prior to September 20, 2004, because of 

their importance relative to location or land-use type. Other 

than a few exceptions, most sites were selected for trend 

analysis based on the following minimum criteria:

Period of record with a beginning year of 1993 or earlier and an ending year of 2004 or later;

At least quarterly sampling each year;

•

•

Data gaps no longer than 2 years and only during the 

middle part of the study period;

Representative coverage of samples over the complete 

range in flow for the study period to avoid bias toward 

low or high values;

Representative coverage over all seasons to avoid bias 

towards certain times of the year; and

Continuous mean daily streamflow data at the trend 

site or an alternate site nearby that could be used as 

a substitute (for example, a streamflow site located 

downstream from the trend site would be appropriate 

if no major tributaries enter the stream between the 

streamflow site and the trend site).

Based on these criteria, there were 115 sites selected 

for trend analysis that had an adequate amount of data of at 

least one of the nutrient constituents or suspended sediment 

(table 1 and fig. 5). Because site-selection criteria were based 

primarily on data availability, spatial representation of the 

selected sites within the LMT Basin was considered fair to 

poor because there were areas that were underrepresented, 

such as in southern Kansas, most of Oklahoma, and parts of 

Texas and Louisiana (fig. 5). Lack of spatial representation, as 

well as other issues such as a wide range of drainage area sizes 

and multiple sites located on the same stream (nesting), could 

cause problems when interpreting trend results in a regional 

context.

Although there were areas within the LMT Basin that 

were underrepresented spatially, nearly all of the major rivers 

and streams that drain directly into the Gulf of Mexico had 

trend sites that were included in the study. The exceptions 

were the Guadalupe River in Texas (however, sites on the San 

Antonio River, which is a major tributary of the Guadalupe 

River, were included) and the Calcasieu River in Louisiana; 

neither of these two rivers had sites with enough water-quality 

or flow data required for analysis. The Ohio River at Dam 53 

near Grand Chain, Illinois (site 1, table 1), and the Mississippi 

River at Thebes, Illinois (site 2, table 1), were outside of the 

study area but were included for analysis in order to document 

nutrient and sediment loadings entering the study area.

As a basis for comparing trend, load, and yield results, 

the trend sites were then grouped into four primary systems 

of rivers as follows: the Mississippi, Atchafalaya, LouisianaGulf/Pontchartrain, and Texas-Gulf systems (highlighted in 

yellow, green, orange, and blue, respectively, in tables and figures throughout this report). In most recent studies, the Mississippi and Atchafalaya systems typically are grouped together 

because nearly all of the water in the Atchafalaya River comes 

from diversion of about 25 percent of the flow from the Mississippi River (about 4,350 m3/s average annual flow; Goolsby 

and others, 1999). The remaining flow in the Atchafalaya 

River comes from the Red and Ouachita Rivers, which have 

combined average annual flow of about 1,020 m3/s. Because 

the Atchafalaya River is a separate entry point from the Mis-

•

•

•

•

APPROACH 
Figure 5. Sites selected for trend analyses and load calculations in the study area for the period, 1993-2004.

 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
sissippi River into the Gulf of Mexico, results were grouped 

separately for the Atchafalaya River in this study.

Sites with small drainage areas were not eliminated, 

although their overall contributions of nutrient and sediment 

loads were potentially insignificant within the drainage area of 

a large river basin. These sites were important as they provided 

valuable information related to specific land-use types. For 

example, sites that were part of localized urban studies, such 

as near Colorado Springs, Colo. (sites 21-27, table 1), and near 

Austin, Tex. (sites 101-106, table 1), were included to understand trends in nutrient and sediment data from urban runoff 

as opposed to other areas in the LMT Basin that were forested 

or agricultural. Also, inclusion of sites with smaller drainage 

areas provided the opportunity to document potentially dramatic changes in water quality over the past decade that were 

due to management changes or restoration activities.

Data were compiled separately for ammonia, nitrite plus 

nitrate, total nitrogen, orthophosphorus, total phosphorus, and 

suspended sediment. For data compiled from NWISWeb, previous comparisons of paired filtered (dissolved) and unfiltered 

(total) samples for ammonia, nitrite plus nitrate, and orthophosphorus at the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colo., indicated that analytical results 

were virtually indistinguishable nationally (U.S. Geological Survey, 1992). Comparisons were made for this study 

and study period for data compiled from NWISWeb. Similar 

results were observed when filtered nitrite plus nitrate data 

were compared to unfiltered data; however, differences were 

observed when comparing filtered and unfiltered ammonia and 

orthophosphorus. Therefore, filtered and unfiltered results for 

nitrite plus nitrate were combined, but only filtered ammonia 

and orthophosphorus were used for analysis in this study for 

data compiled from NWISWeb.

When a direct measurement of total nitrogen was unavailable, it was calculated as the sum of unfiltered ammonia plus 

organic nitrogen (hereafter referred to as Kjeldahl nitrogen) 

and nitrite plus nitrate data. If either Kjeldahl nitrogen or 

nitrite plus nitrate data were missing, then total nitrogen was 

not calculated. If either Kjeldahl nitrogen or nitrite plus nitrate 

data were less than their respective reporting levels (hereafter 

referred to as censored values or results), then the value for 

total nitrogen was calculated as follows:

 If both Kjeldahl nitrogen and nitrite plus nitrate data 

were censored, then total nitrogen was censored to the 

sum of both censoring levels;

 If Kjeldahl nitrogen was not censored, but nitrite plus 

nitrate was censored, then total nitrogen was calculated 

as the sum of the Kjeldahl nitrogen value plus half of 

the censored value for nitrite plus nitrate; and

 If Kjeldahl nitrogen was censored, but nitrite plus 

nitrate was not censored, then total nitrogen was calculated as the sum of the nitrite plus nitrate value plus 

half of the censored value for Kjeldahl nitrogen.

•

•

•

Once the data sets for the sites considered in this study 

were compiled, additional censoring adjustments were necessary prior to analysis. Before the late 1990s, the NWQL 

censored data at the minimum reporting level (MRL), which 

is the smallest measurement of concentration that can be 

measured by using a particular analytical method (OblingerChildress and others, 1999). Establishment of MRLs has 

been inconsistent across methods, inadequately defined, and 

generally undocumented. In 1992, the NWQL began adopting the USEPA method detection limit (MDL) procedure for 

establishing censoring levels for two pesticide methods. The 

MDL method is described as the minimum concentration of a 

substance that can be measured when the risk of a false positive detection is no more than 1 percent (Oblinger-Childress, 

1999). Because the risk of a false negative at the MDL can 

be as much as 50 percent, the NWQL formed a team to better 

define censoring levels at higher levels. As a result, the NWQL 

began to censor data at the laboratory reporting level (LRL), 

a value generally twice the MDL (actually the LRL is twice 

the long-term MDL [LT-MDL], which is a modification of the 

USEPA MDL designed to capture more method variability). 

This practice was implemented in 1999 for Kjeldahl nitrogen, 

in 2000 for total phosphorus, and in 2001 for ammonia, nitrite 

plus nitrate, and orthophosphorus. Values measured less than 

the LT-MDL were reported as less than the LRL, and values 

measured between the LT-MDL and LRL were reported as 

estimated.

Using the LRL can result in upward bias during statistical analyses of censored data (which are used in this study) 

because the probability that an observation might fall between 

the LT-MDL and LRL is likely overstated (Helsel, 2005). The 

possibility of the occurrence of a few false negatives is less 

of a concern than the problems caused by such a bias (Mueller and Spahr, 2005). As a result, all data analyzed by the 

NWQL and used in this study were recensored from the LRL 

to the associated LT-MDL reported by the NWQL for a given 

constituent during a given time period. A small number of the 

samples had been diluted, and resulting LRL values had been 

multiplied by the dilution factor; for these samples, values 

were recensored to the MDL multiplied by the dilution factor. 

Recensoring of the NWQL data took place prior to calculation 

of total nitrogen previously discussed.

Trend and Load Calculations

A trend is defined as a systematic change in a waterquality constituent over time (D.L. Lorenz, U.S. Geological 

Survey, written commun., 2004). To complete trend tests 

for water-quality data, one must understand the complexities and processes that influence water-quality conditions in 

surface waters. Natural influences include climate, hydrology, 

precipitation, soil erosion, chemical reactions, and biological 

activities. Human influences include chemical applications, 

flow regulation, addition or removal of wastewater treatments 

plants, and land-use changes. The difficulty in interpreting 

trends in water-quality data is the ability to separate actual 

APPROACH   
trends in the data from natural variability, as well as from 

artificial trends (for example, trends resulting from changes in 

sample collection, sample processing, and laboratory analytical methods over time).

It is important to decide if flow adjustment is necessary 

in trend testing. If trends tests are used to determine effects on 

aquatic communities, then flow adjustment may not be important. For example, total ammonia (NH3 plus NH4+) exceeds 

chronic criterion for aquatic organisms for concentrations 

above about 2.1 milligrams per liter (mg/L) when pH is within 

the range of 6.5 to 9.0 and temperature from 0 to 30 degrees 

Celsius (Mueller and others, 1995). To determine the effects 

on a particular ecosystem, it is not pertinent that increases in 

streamflow were the primary cause for the increases in total 

ammonia over time, but simply that ammonia concentrations 

increased and exceeded the criterion. If, however, it is important to understand why ammonia concentrations exceeded the 

criterion over time, then the trend test should be adjusted for 

flow to determine if the trend is retained. Flow adjustment 

is a technique used to understand actual changes in a waterquality constituent without influence of trends in flow. Data 

are adjusted for flow by establishing a relation between flow 

and the water-quality constituent prior to trend testing. If the 

trend in ammonia was retained when flow adjusted, then the 

increase may have been caused by a human-related action, 

such as an increase in fertilizer usage.

For this study, it was important to estimate both unadjusted (hereafter referred to as total trends in concentration) 

and flow-adjusted trends to understand the overall picture of 

what was happening in relation to nutrient and sediment concentrations within the LMT Basin. Other important trends to 

consider were trends in load, which provide a direct measure 

of the effect of nutrients and sediment discharging on the 

northwestern Gulf of Mexico, and trends in flow, which will 

improve interpretation of water-quality trends by understanding how flow has changed over time. The following sections 

describe methods used to calculate (1) total trends in concentration and load; (2) trends in flow-adjusted concentration; (3) 

interpretation of trend results; and (4) annual load calculations.

Total trends in concentration and load

Determination of total trends in concentration and load 

was attempted for all six constituents at each site listed in table 

1 for the study period. Total trend in concentration and trend 

in load are defined as the percent changes in model-estimated, 

smoothed trend in concentration and load over the period of 

the water-quality record, divided by the length of the record 

(trend is, therefore, represented as percent change per year). 

The model-estimated trend in concentration and load is determined by combining separate trend models for streamflow and 

water-quality concentration. The streamflow model, estimated 

from all daily streamflow measurements available over the 

study period, relates the daily streamflow to an intercept, a 

linear trend term (measured by time expressed as a decimal), 

and sine and cosine seasonal factors (also functions of decimal 

time). The water-quality model is represented by the following 

equation:

(1)

where:

= the logarithm of constituent concentration in 

 period t;

= an intercept value;

= the logarithm of streamflow;

= decimal time;

= vectors of ancillary predictors such as sine

and cosine functions of decimal time to

account for seasonality;

= coefficients determined from model fitting;

= multi-element vector function consisting of

the logarithm of streamflow and the square

of the natural logarithm of streamflow;

= multi-element vector function consisting of

the second order polynomial of decimal

time; and

= random error term.

The smoothed trend in water-quality concentration is 

determined by the streamflow and time trend components of 

the water-quality model, where the smoothed trend in streamflow is substituted for the actual streamflow in the streamflow 

component (the smoothing of streamflow is a linear fit over 

logarithmic space). The smoothed trend in streamflow is given 

by the following equation (which is a form of the linear relation y = b + mx):

(2)

where:

= smoothed trend in the logarithm of flow in

the period t;

= average of the logarithm of flow over the

trend period;

= decimal time;

= average, or midpoint, of the decimal time

over the trend period; and

= coefficient determined from the model

fitting.

Total trend in concentration is obtained by transforming 

the water-quality trend from logarithm space to real space, 

computing the percent change corresponding to the first and 

last dates of the water-quality record period, and dividing by 

the decimal time length of the study period. Trend in load 

is computed similarly, except the smoothed trend in streamflow is added to the smoothed trend in water-quality prior 

to retransformation to real space. The appendix contains a 

detailed description of this method, with additional discussion 

10 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
of the estimation of the streamflow and water-quality models 

and an explanation of the associated statistical tests for trend.

Flow-adjusted trends in concentration and 

trends in flow

The estimation of flow-adjusted trend in concentration 

is similar to the estimation method for total trend. The only 

difference is that the streamflow component of the water-quality model is not included in the determination of the smoothed 

water-quality trend. The estimation of the trend in streamflow is based on the smoothed streamflow trend corresponding to the simple linear function of decimal time previously 

described. The conversion of this smoothed trend to a trend 

estimate follows the same procedure described for total trend, 

the only difference being that the period of the trend is defined 

by the beginning and ending dates for the flow record within 

the analysis period rather than the beginning and ending dates 

of the water-quality record.

Interpretation of trend results

Each trend analysis produced an associated estimate of 

probability, or p-value, which is the probability of attaining a 

specified significance level (Helsel and Hirsch, 1992). P-values were compared to a significance level, or, of 0.05 (5 percent), which meant that there was less than a 5-percent chance 

of errors in test results. Trend results presented in this report 

were considered statistically significant when p-values were 

less than 0.05. In addition, diagnostic plots (such as a plot of 

actual versus predicted values) and standard error of prediction 

(SEP) estimates were examined to determine overall model 

“fit.” Models and subsequent trend results were rejected if 

problems were observed in diagnostic plots (such as predicted 

values were much higher or lower than actual values) or if a 

model produced large, unacceptable SEP estimates.

Trend results were reported as a percent per year change; 

however, it was important to understand these percent changes 

in terms of original units, such as milligrams per liter for 

concentration data. Reference concentrations and loads were 

computed for each statistically significant trend. Reference 

concentrations and loads are best explained as the “starting point” of a trend line drawn through the data with a 

slope equal to the trend estimate. A 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 streamflow equal 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 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 (see the 

appendix for additional details). The reference load is computed similarly, except that streamflow trend as determined 

by the streamflow equation evaluated at the starting date of 

the water-quality period, is added to the reference concentration prior to transformation to real space; also, a multiplicative 

constant is applied to convert the result to appropriate load 

units.

When there is no trend in streamflow over time, total 

trend and flow-adjusted trends are basically equivalent. 

Because the water-quality model used to derive these trends 

includes streamflow as a predictor, the estimates of trend are 

immune to bias 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. Because of the possible nonlinearity of trend, as arising from nonlinear specifications of the water-quality model 

streamflow or trend components, trends within the water-quality period or trends experienced outside this period could be 

quite different from the trends reported here. It also should be 

recognized that the method used to evaluate trend is insensitive 

to changes in the variability of streamflow or to changes in the 

unexplained variability of water quality, both changes potentially resulting in trends in water quality arising from nonlinearity in the specification of the water-quality model. Accommodation of this type of uncertainty awaits future research.

Load calculations

Annual load and yield calculations were attempted for 

all six constituents at each site listed in table 1 for each water 

year in the study period. The statistical program LOADEST (Runkel and others, 2004) was used to calculate annual 

loads. The specific software used was S-LOADEST, which is 

a “USGS plug-in” version of LOADEST in S-PLUS (version 7.0), a PC-based statistical software package (Insightful, 

2005). LOADEST uses a seven-parameter linear regression 

model that incorporates flow, time, and seasonal terms to estimate loads of concentration over time for specific time periods 

(annual, monthly, or daily loads). The calibration and estimation procedures within LOADEST are based on three statistical estimation methods. The first two methods, maximum 

likelihood estimation (MLE) and adjusted maximum likelihood estimation (AMLE) are appropriate when the calibration 

model residuals are normally distributed. Of the two, AMLE 

is more appropriate when the calibration data set (time series 

of streamflow and concentration data) contains censored data; 

otherwise, MLE and AMLE give the same results when there 

are no censored data present. The third method, Least Absolute Deviation, is an alternative to AMLE when residuals are 

not normally distributed (Runkel and others, 2004).

One load model was developed for the entire study period 

for each constituent at each study site (in other words, the 

study period was not subdivided into smaller periods of time). 

Daily mean values of streamflow were used in the calibraAPPROACH 11
tion data set for each site for each sample date. A LOADEST 

option was chosen that automatically selected a “best-fit” load 

model by using the Akaike Information Criterion (Akaike, 

1981) associated with each of the eight subsets of the sevenparameter regression equation. The AMLE method was used 

to estimate the coefficients of the dependent variables in the 

best-fit regression model equation in LOADEST because 

censored data were present in nearly all data sets. The AMLE 

procedure also corrects for first-order bias in the regression 

coefficients and minimizes other biases that can occur when 

estimated logarithms of load are retransformed to original 

units (Cohn, 1988; Cohn and others, 1992).

Two fundamental assumptions of linear models are 

homoscedasticity, which means that the variance about a 

regression line is similar for all predictor variables, and normality, which is an assumption that model residuals follow a 

normal (or even) distribution. Residuals were plotted in a variety of ways such as quantile plots, residuals versus streamflow, 

residuals versus decimal time, and so forth. The plots were 

examined for homoscedasticity and normality; for example, 

residuals are considered to be normally distributed if they plot 

evenly above and below a “zero” horizontal line in a residuals 

versus decimal time plot. If either assumption was violated, 

the model was rejected. If the best-fit model did not meet the 

linearity assumptions, the next step was to explore a custom 

model with user-specified variables. Independent variables 

available for consideration in custom models were reciprocal 

transform of streamflow, reciprocal transform of streamflow 

squared, one or two breakpoints in streamflow, and seasonal 

periods that were a series of months defined by the user. 

Residuals from the resultant custom models were similarly 

examined for homoscedasticity and normality, and the optimal 

model was selected. If this step failed to produce an acceptable 

model, then the constituent at that site was excluded from load 

calculations.

Once calibrated, daily mean values of streamflow were 

used as independent variables in a prediction data set to estimate annual loads. Average annual loads were calculated by 

dividing the total load for the study period by the number of 

years of estimated loads for that site. Average annual yields at 

each site were then calculated by dividing the average annual 

load by the site’s drainage area.

An attempt was made to provide hydrologic perspective 

to selected trends in loads from this study. In other words, 

were trends in loads observed in this study part of much longer 

trends or were they more indicative of the most recent decade? 

Sites 1, 2, 48, and 53 in the Mississippi system, and sites 

64 and 68 in the Atchafalaya system are part of the USGS 

NASQAN Program and were selected to provide hydrologic 

perspective because they have long-term concentration and 

flow data. Annual nitrogen and phosphorus loads have been 

computed for these sites for their periods of record (early 

1960s to present) by using LOADEST software and a 5-year 

“moving window” approach (Aulenbach and others, 2007). 

These annual loads were plotted for the time period 1980-

2004 along with a corresponding locally weighted scatterplot 

smooth (LOWESS; Helsel and Hirsch, 1992) line drawn to 

indicate variability in the data over the longer time period. 

Although suspended sediment load calculations were not 

available in Aulenbach and others (2007), suspended sediment 

loads at these six sites were computed by using the 5-year 

moving window approach for the 1980-2004 time period and 

were graphed similarly. (Note: Annual loads of ammonia, 

orthophosphorus, and suspended sediment were not computed 

for site 64 for the period 1980-2004 by either Aulenbach and 

others (2007) or the authors of this study because of lack of 

concentration or flow data.)

Site 112 in the Texas-Gulf system was also selected 

to provide hydrologic perspective because it has long-term 

concentration and flow data, and it represents a river system 

other than the Mississippi and Atchafalaya. Annual loads were 

computed by using the 5-year moving window approach for 

the period 1980-2004 for nitrite plus nitrate, total nitrogen, 

and total phosphorus data from site 112 and then were graphed 

similarly to those previously mentioned with a corresponding 

LOWESS line. Other nutrient constituent and suspended-sediment loads were not calculated for the 1980-2004 period at 

site 112 because of lack of concentration or flow data prior 

to 1993. (Note: average annual streamflows for the period 

1980-2004 for the same seven sites mentioned here were also 

graphed with a LOWESS line to give hydrologic perspective 

to trends in streamflow calculated for the study period.)

Analysis of Source Data and Landscape 

Attributes

Trends in nonpoint and point sources of nutrients were 

related to trends in nitrogen and phosphorus data estimated for 

this study. Trends in nonpoint sources of nitrogen and phosphorus included fertilizer, manure, and atmospheric deposition (nitrogen) data, as previously mentioned. Nitrogen and 

phosphorus fertilizer data were available for both agricultural 

and urban settings. These data were combined to create a 

single nitrogen and phosphorus fertilizer data set. Similarly, 

nitrogen and phosphorus data from manure were available 

for both confined and unconfined animal feeding operations, 

which were combined to create a single nitrogen and phosphorus manure data set. Data for nitrogen in atmospheric deposition were based on data from the USGS National Atmospheric 

Deposition Program / National Trends Network, accessible at 

http://bqs.usgs.gov/acidrain/. Fertilizer, manure, and atmospheric nitrogen data were expressed, in terms of mass, as total 

kilograms of nitrogen or phosphorus summed for the drainage 

area at a particular site. More detail on fertilizer, manure, and 

atmospheric deposition data generation can be found in Ruddy 

and others (2006).

Information about point-source loadings of nutrients was 

obtained from State management agencies, with additional 

information obtained from the USEPA Permit Compliance 

System (http://www.epa.gov/compliance/data/systems/water/

pcssys.html). Unfortunately, information for point-source 

loadings from these two sources were inconsistent for the 

12 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
study area; therefore, population data were used as a “surrogate” to make inferences regarding point-source loadings as a 

means of interpreting trend results. The population data were 

derived from a 30-m resolution grid of census block groups 

and population counts based on the 1990 and 2000 Census 

of Population and Housing (U.S. Census Bureau, 1991; U.S. 

Census Bureau, 2000). Population data also were used to 

explain trends in suspended sediment; for example, increases 

in population could imply increases in urbanization, which in 

turn could cause increases in sediment in streams due to clearcutting of trees for new subdivisions, increase in impervious 

area, and so forth. In terms of density, population data were 

expressed as number of persons per square kilometer.

To relate trends in water quality to the source data, the 

source data were reduced to a single value of trend at each site 

where source data were available. If there were more than two 

points of time with data, such as the fertilizer data which was 

generated on an annual basis, a Theil slope was computed. The 

Theil slope is the median of all pairwise slopes (Helsel and 

Hirsch, 1992) and is expressed as the amount of increase or 

decrease in the source data per year. The Theil slope computed 

at each site was then normalized by dividing by the drainage 

area of the site. Because the population data had data for only 

two points in time and were already expressed in terms of 

drainage area, the trend in population was simply calculated as 

the difference in densities from 1990 to 2000.

Weighted-least-squares (WLS) regression was used to 

determine if there were statistically significant regional patterns by comparing water-quality trend results to corresponding trends in source data. The reduced results (trends) of the 

source data were the independent variables in the WLS regression. Water-quality trend values were the dependent variables 

in the WLS regression, and each was weighted so that values 

that were known with more confidence (those with lower variances) had a greater weight in the regression than values that 

were known with less confidence (Helsel and Hirsch, 1992). 

Because weights were used, all water-quality trend values 

(coefficients of time) were included in the WLS regression, 

even those that were not considered statistically significant. 

Weights were based on the inverse of the variance from the 

trend estimates for each constituent at each site. Statistically 

significant regression results were those that had p-values 

greater than 0.05 (or there was less than a 5-percent chance 

that a result was not statistically significant).

Water-quality trend results were also compared to trends 

in management practices at each site. The management practice data were derived from selected 1992 and 1997 National 

Resources Inventory (NRI) data compiled by the Natural 

Resources Conservation Service (NRCS; U.S. Department of 

Agriculture, 1995; U.S. Department of Agriculture, 2001). 

These data were categorized into two primary groups – areas 

with irrigation and areas with conservation practices. These 

data were aggregated in areas with agricultural land use at 

the county level and then summed within the drainage area 

for each site. Irrigated areas were further divided into four 

subgroups: areas that used well water as the irrigation source; 

areas that used surface water (ponds, lakes, reservoirs, streams, 

ditches and canals) as the irrigation source; areas that used 

lagoons or other wastewaters, or a combination of sources; and 

areas that used gravity, pressure, or a combination of gravity 

or pressure as a means of irrigation-water delivery. Conservation practices data were further divided into three subgroups: 

areas that used contour farming and land leveling; areas that 

used tail-water recovery; and areas that had surface drainage. 

All management practices data were expressed, in terms of 

area (square kilometers), as the amount of land in the drainage 

area of a study site that contained that particular practice.

Management practice categories used in this study were 

the only ones where data were available in both 1992 and 

1997 so that trends could be calculated. Unfortunately, several 

potentially influential data sets were not included in the 

1997 NRI series such as conservation tillage, irrigation land 

management, irrigation land leveling, and subsurface drains. 

The data that were not included, particularly the conservation 

practice data, may have had a substantial influence on waterquality trends in streams in the LMT Basin, and these possible 

influences are not reflected in the analysis of management 

practices in this report.

For these reasons, WLS regression analysis techniques 

presented previously were not used with respect to the 

management practices data. Instead, potential trends in the 

management practices data were determined by subtracting 

the amount of land in a particular practice in 1992 from the 

amount of land in that same practice in 1997. The results were 

then normalized by dividing the difference by the drainage 

area of the study site and then multiplying by 100. The resulting value represented the change in the amount of a particular 

management practice expressed as a percentage of the total 

drainage area of a study site. In most cases, the change in 

amount of management practices from 1992 to 1997 represented a very small percentage of the total drainage area of a 

particular site (most less than 1 percent); only values greater 

than 1 percent (representing an increase in the amount of management practices) or values less than -1 percent (representing a decrease in the amount of management practices) were 

reported in this study.

RESULTS

Results of this study are presented in the following four 

sections: streamflow, nitrogen, phosphorus, and suspended 

sediment. Although specific trend results are presented in 

tables, they are also plotted to provide the reader visual detail. 

Annual loads are tabled where applicable. To maintain minimal text interruptions, all tables are presented at the end of the 

report.

Explanations of detected trends on a regional scale were 

problematic due to limited nutrient source, land use, and management-practice explanatory data. However, explanations of 

notable trends at some locations are discussed in each section 

where supported by literature. These trend explanations are 

RESULTS 13
general and are not intended to be definitive. A more detailed 

scientific investigation at each site would be necessary to 

provide a complete explanation as to the cause of a particular 

trend.

Streamflow

Trend analyses of streamflow data were attempted for 

all study sites in the four primary river systems (table 2, fig. 

6). Trend results were rejected for five sites because of poor 

model fit. Of the remaining 110 sites where trend analyses 

were attempted and results were considered acceptable, there 

were 70 sites (about 64 percent) where no significant trends 

in streamflow were observed during the study period (table 

2). Decreasing trends in streamflow were observed at 38 sites 

ranging from -8.2 to -2.2 percent per year during the study 

period (table 2, fig. 6).

For the study area, moist conditions prevailed at the 

beginning of the study period through about 1996 as observed 

from monthly Palmer Hydrologic Drought Index maps 

(National Oceanic Atmospheric Administration, 2006). Severe 

to extreme drought conditions occurred three different times 

during the study period–1996, 1998, and 2000–with the most 

extreme drought conditions occurring from October 1999 

through November 2000 (National Oceanic Atmospheric 

Administration, 2006). Although there are many factors that 

could influence trends in streamflow, such as changes in land 

use, urbanization, and gain/loss of riparian zone wetlands, it 

is believed that the overall decreasing trends in streamflow for 

the study area during the study period likely were due to moist 

conditions occurring at the beginning of the study period and 

the influence of three drought periods near the middle and end 

of the study period (with the most extreme occurring in 2000).

In looking at streamflow data at selected sites for the 

period 1980-2004 (fig. 7), the decreasing trends in streamflow 

at sites 2, 48, 64, and 68 were specific to the study period and 

were not part of long-term trends. There was a slight decreasing trend (-2.2 percent per year) at site 53, which is barely 

noticeable in the streamflow data for the time period 1980-

2004 at site 53 in figure 7. There were no trends in streamflow 

in either the current study period or the period 1980-2004 at 

site 1 (table 2, fig. 7).

Only two sites had significantly increasing trends in 

streamflow. Increasing trends in streamflow of 7.9 and 10 percent per year at sites 112 and 113 (fig. 6), respectively, on the 

San Antonio River in the Texas-Gulf system were observed 

during the study period. After the drought of 2000 subsided, 

moist conditions returned to south-central Texas, and conditions have been considered extremely moist since late 2002 

with Palmer Hydrologic Drought indices consistently at or 

above about 4.00 (National Oceanic Atmospheric Administration, 2006). Moist conditions returning to southern Texas after 

the drought of 2000, coupled with recent increases in urbanization and impervious surfaces within the San Antonio River 

Basin (as suggested by Sahoo and Haan, 2005), are likely 

contributors to the increasing trends in streamflow during 

the study period at these two sites. In addition, the increasing 

trends in streamflow for the San Antonio River Basin for the 

study period were not part of long-term increases but appear to 

be part of a recent decadal trend that started about 1995 (based 

on average annual flows assembled for site 112 in figure 7).

Nitrogen

Specific details about trend, load, and yield results for 

ammonia, nitrite plus nitrate, and total nitrogen data are discussed at the beginning of this section. These results are then 

related to potential trends in nutrient sources and landscape 

attributes. Finally, some general conclusions about nitrogen 

trend and load results are discussed at the end of this section.

Ammonia trends, loads, and yields

Trend analyses of ammonia data were attempted for 93 

study sites in three of the four river systems, with the exception of the Louisiana-Gulf/Pontchartrain system, which had 

no sites with an adequate amount of ammonia data to attempt 

trend analyses (table 3). Trend results were rejected for 44 of 

the 93 sites because of poor model fit (represented as N/A in 

table 3).

Of the remaining 49 sites where trend results were 

considered acceptable, there were 25 sites (about 51 percent) 

where no total trends in concentration were observed during 

the study period (table 3). Decreasing total trends in concentration were observed at the remaining 24 sites ranging from 

-8.9 to -3.8 percent per year during the study period (table 3, 

fig. 8). There were 24 sites (about 49 percent) where no flowadjusted trends in concentration were observed during the 

study period (table 3, fig. 9). Decreasing flow-adjusted trends 

in concentration were observed at the remaining 25 sites ranging from-8.9 to -4.1 percent per year.

Either no trends or decreasing trends in ammonia data 

were evident across land-use types, physiographic regions, and 

three of the four river systems represented in the study area. 

Decreasing total trends results for ammonia were similar to 

decreasing trends in flow; however, decreasing flow-adjusted 

trend results were also observed, thus indicating that decreasing trends in ammonia data throughout the study area were 

not simply related to trends in streamflow but could be caused 

by decreases in ammonia sources or changes in management 

practices.

Improvements to municipal wastewater treatment facilities could be contributing to decreasing trends in ammonia at 

selected sites in the study area. For example, ammonia concentrations and loads decreased about 7.5 percent per year on 

average at sites 25, 26, and 27 during the study period. These 

three sites are located downstream of a large municipal wastewater treatment facility (site 25 is the nearest to the facility 

located only about 1.5 km downstream). The treatment facility 

was upgraded to advanced treatment during the mid-1990’s to 

reduce high ammonia concentrations and biochemical oxygen demand (BOD) in effluent discharged to Fountain Creek. 

14 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 6. Trends in streamflow at study sites, 1993-2004.

RESULTS 15
Figure 7. Average annual streamflow for selected study sites, 1980-2004.

16  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 8. Total trends in ammonia concentrations at study sites, 1993-2004.

RESULTS 17
Figure 9. Flow-adjusted trends in ammonia concentrations at study sites, 1993-2004.

18  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Facility upgrades included retrofitting activated sludge tanks to 

advanced waste treatment for increased nitrification (addition 

of diffuser system), adding an anoxic zone for denitrification, 

increased blower sizes for nitrification to convert ammonia, 

and conversion of primary clarifiers to secondary clarifiers 

(Ginny Johnson, Colorado Springs Utilities, written commun., 

December 5, 2007). Effluent from the facility accounts for 

40-70 percent of the total flow at site 25 (Patrick Edelmann, 

U.S. Geological Survey, written commun., October 10, 2006). 

The decreasing trends in ammonia at sites 25, 26, and 27 were 

likely due to the improvements in the wastewater treatment 

facility located upstream.

Similar results were observed for trends in ammonia 

loads in which 23 sites (about 47 percent) indicated no trends 

in load, but decreasing trends in load were observed at the 

remaining 26 sites, ranging from -9.4 to -5.3 percent per year 

(table 3, fig. 10). These trends in ammonia loads appear to be 

part of a longer trend in ammonia loads as seen in figure 11, 

which shows decreasing trends at sites 1, 2, 48, 53, and 68 

since the early 1980s.

Ammonia load calculations were attempted for 37 study 

sites in three of the four river systems with the exception of 

the Louisiana-Gulf/Pontchartrain system, which had no sites 

with an adequate amount of ammonia or flow data to attempt 

load calculations (table 4). As expected, average annual 

ammonia loads were highest for the Mississippi system sites 

when compared to loads from the other two systems. Average annual ammonia loads for some of the major drainages 

(not necessarily the most downstream) into the northwestern 

Gulf of Mexico were as follows: 13,600 metric tons (T) for 

the Mississippi River (site 53); 7,100 T for the Atchafalaya 

River (site 68); and 23 T for the Neches (site 76), 317 T for 

the Trinity (site 86), and 115 T for the Colorado Rivers (site 

108) in the Texas-Gulf system (table 4). For these same sites, 

however, the highest yield was 0.0294 metric tons per square 

kilometer per year (T∙km-2∙yr-1) for the Atchafalaya River, 

followed by yields for the Trinity, Neches, Mississippi, and 

Colorado Rivers, which were 0.00955, 0.00761, 0.00465, and 

0.00105 T∙km-2∙yr-1, respectively (table 4). Load and yield 

data indicate that although loads are greatest from the Mississippi River, other smaller river systems can yield as much, 

if not more, ammonia on a per-square-kilometer basis as the 

Mississippi River.

Nitrite plus nitrate trends, loads, and yields

Trend analyses of nitrite plus nitrate data were attempted 

for 90 study sites in all four river systems (table 5). Trend 

results were rejected for 21 sites because of poor model fit 

(represented as N/A in table 5). Of the remaining 69 sites 

where trend results were considered acceptable, there were 47 

sites (about 68 percent) where no total trends in concentration 

were observed during the study period (table 5). Decreasing 

total trends in concentration were observed at 12 sites ranging from -9.3 to -1.7 percent per year during the study period 

(table 5, fig. 12). Increasing total trends in concentration were 

observed at 10 sites, ranging from 4.2 to 39 percent per year 

during the study period (table 5, fig. 12). There were 48 sites 

(about 70 percent) where no flow-adjusted trends in concentration were observed during the study period (table 5). Decreasing flow-adjusted trends in concentration were observed at 

seven sites, ranging from -8.8 to -3.0 percent per year (table 5, 

fig. 13). Increasing flow-adjusted trends in concentration were 

observed at 14 sites, ranging from 2.4 to 57 percent per year 

during the study period (table 5, fig. 13).

Sites 16, 38, and 39 in the Mississippi system, sites 

59, 61, and 64 in the Atchafalaya system, and site 71 in the 

Louisiana-Gulf/Pontchartrain system indicated decreases in 

the total trends in nitrite plus nitrate concentration results. 

National “background” concentration estimates of selected 

nutrient constituents are reported in a U.S. Geological Survey Circular (1999). Waters with concentrations of nutrients 

that exceeded these background levels were considered to be 

affected by human activities from a variety of land uses. The 

national background concentration for nitrite plus nitrate is 

about 0.6 mg/L (U.S. Geological Survey, 1999); because all 

of the reference concentrations at these seven sites were less 

than 0.6 mg/L, decreasing trends at these sites were considered 

negligible. Also, trends at most of these seven sites were not 

retained when the effects of streamflow were removed (no 

flow-adjusted trends found except for sites 39 and 64), which 

indicated that trends likely were related to decreasing trends in 

streamflow (table 2).

Sites 2, 26, 27, and 53 in the Mississippi system and site 

98 in the Texas-Gulf system indicated decreases in total trends 

in nitrite plus nitrate concentration results for the study period, 

and reference concentrations were all greater than about 1.5 

mg/L. Sites 2 and 53 did not retain trends when the effects of 

streamflow were removed; thus, the decreasing total trends 

at these two sites were likely related to decreasing trends in 

streamflow. Decreasing trends were retained when adjusted 

for streamflow at sites 26, 27, and 98, potentially indicating 

that management practices improved water quality or sources 

of nitrite plus nitrate decreased at these sites during the study 

period. The decreasing trends at sites 26, 27, and 98 could not 

be explained at this time.

Site 50 in the Mississippi system and site 75 in the TexasGulf system indicated no total trends in nitrite plus nitrate 

concentrations for the study period, but decreasing trends were 

observed in flow-adjusted concentrations (table 5). Reference concentrations were 0.53 and 0.08 mg/L, respectively, 

and consequently, less than the 0.6 mg/L national background 

concentration for nitrite plus nitrate (U.S. Geological Survey, 

1999); thus, decreasing trends in nitrite plus nitrate at sites 50 

and 75 were considered negligible.

Sites 19, 36, and 49 in the Mississippi system and site 

103 in the Texas-Gulf system indicated an increase in nitrite 

plus nitrate for the study period in both the total and flowadjusted trends in concentration (table 5). Reference concentrations at these four sites were all less than about 0.2 mg/L. 

If trends are applied to the reference concentrations at these 

four sites, nitrite plus nitrate concentrations would exceed the 

RESULTS 19
Figure 10. Trends in ammonia loads at study sites, 1993-2004.

20 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
national background concentration of 0.6 mg/L for nitrite plus 

nitrate (U.S. Geological Survey, 1999) at only site 36 during 

the study period (the reference concentration at site 36 was 

0.16 mg/L; with about a 40 percent per year increase over 

the 12-year study period, the reference concentration would 

increase to about 0.9 mg/L in 2004). Increasing trends at these 

four sites were thus considered negligible during the study 

period.

Sites 9, 25, 32, and 41 in the Mississippi system and 

site 83 in the Texas-Gulf system also indicated an increase in 

nitrite plus nitrate for the study period in both total and flowadjusted trends (except for site 41, which did not indicate a 

flow-adjusted trend), and reference concentrations for these 

five sites were all greater than 1 mg/L (table 5). Because 

trends remained when the effect of streamflow was removed 

at sites 9, 25, 32, and 83, the trends were likely caused by 

influences other than streamflow, such as changes in management practices or in nitrogen sources in these basins during the 

study period.

Site 9 is located on Yocum Creek near Oak Grove, Ark. 

No recent references were available to explain the increasing 

trend in nitrite plus nitrate at site 9. Davis and Bell (1998) 

reported in their study that nitrite plus nitrate concentrations 

were higher in the Yocum Creek Basin than in surrounding 

basins due to a higher percentage of agricultural land use in 

the basin, and the type of land use included intensive poultry 

farming and application of poultry wastes to pastures. The 

increase in nitrite plus nitrate at site 25 (Fountain Creek below 

Janitell Road below Colorado Springs, Colo.) could be related 

to advanced treatment at the wastewater treatment plant previously mentioned. In typical advanced treatment systems, 

most of the conversion of nitrogen occurs in the aerobic zone 

(for example, conversion of ammonia to nitrate); however, 

reduction of nitrate is limited to that which is returned to the 

Figure 11. Annual ammonia loads for selected study sites, 1980-2004.

RESULTS 21
Figure 12. Total trends in nitrite plus nitrate concentrations at study sites, 1993-2004.

22 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 13. Flow-adjusted trends in nitrite plus nitrate concentrations at study sites, 1993-2004.

RESULTS 23
process influent and denitrified in the anoxic zone (Viessman 

and Hammer, 1985). The increase in nitrate in the plant’s 

effluent, and ultimately Fountain Creek at site 25, could be a 

by-product of the advanced waste treatment process to reduce 

ammonia. Site 32 is located on Flint Creek in Oklahoma and 

is part of the Oklahoma Scenic Rivers Program (Oklahoma 

Office of the Secretary of the Environment, 2003). A segment 

of Flint Creek is listed as impaired due, in part, to “nitrates;” 

however, the official source of the impairment is unknown 

(Oklahoma Department of Environmental Quality, 2004). No 

references were found in the literature to explain increases in 

nitrite plus nitrate at site 83.

There were no trends observed in nitrite plus nitrate loads 

at 42 sites (about 61 percent) during the study period (table 5, 

fig. 14). Decreasing trends in loads were observed at 23 sites 

during the study period, ranging from -9.4 to -3.2 percent per 

year (table 5, fig. 14). Increasing trends in loads were observed 

at only four sites during the study period, ranging from 5.5 to 

52 percent per year (table 5, fig. 14).

In looking at trends in nitrite plus nitrate loads for the 

period 1980-2004 for sites 2, 48, and 64 (fig. 15), it appears 

that loads peaked near the beginning of the study period in 

the early to mid-1990s [as also seen for sites 2 and 48 in the 

report by Goolsby and others (1999)]. Similar decreasing 

trends since the mid 1990s were also observed at sites 53 and 

68, although the magnitudes of the recent trend at these two 

sites were lower than sites 2, 48, and 64 (table 5, fig. 15). 

Therefore, decreasing trends reported in this study for the 

past decade at these five sites likely reflect decreases in loads 

unique to only the last decade and do not appear to be part 

of a longer trend in load, unlike that observed in long-term 

ammonia loads.

No trends in nitrite plus nitrate loads were observed at 

site 1 (table 5) for the study period. No trend is apparent in 

the annual load data for the period 1980-2004 (fig. 15) at 

site 1 either, although there appears to be a slight “bump” at 

about 1993 in the LOWESS line (which is attributed to scale 

in figure 15). The increasing trend in nitrite plus nitrate loads 

observed at site 112 during this study (table 5) is part of a 

longer term increasing trend as seen in figure 15.

Annual loads, average annual load, and yield calculations 

for nitrite plus nitrate were attempted for 56 study sites in all 

four river systems (table 6). Nitrite plus nitrate loads were 

at least one order of magnitude greater than ammonia loads 

where site-by-site comparisons could be made. Average annual 

nitrite plus nitrate loads for some of the major drainages (not 

necessarily the most downstream) into the northwestern Gulf 

of Mexico were as follows: 707,000 T for the Mississippi 

River (site 53); 224,000 T for the Atchafalaya River (site 68); 

322 and 68 T for the Tangipahoa River (site 70) and Tickfaw 

River (site 71), respectively, which empty into Lake Pontchartrain; and 220 T for the Neches River (site 77), 11,700 T for 

the Trinity River (site 87), 2,970 T for the San Antonio River 

(site 112), and 111 T for the Nueces River (site 115) from the 

Texas-Gulf system. If loads were summed for these eight sites, 

the Mississippi River would account for about 75 percent and 

the Atchafalaya River would account for about 24 percent of 

the total nitrite plus nitrate load entering the northwestern Gulf 

of Mexico. For these same sites, however, the top four highest yields were 0.927 T∙km-2∙yr-1 for the Atchafalaya River 

followed by 0.658, 0.325, and 0.242 T∙km-2∙yr-1 for the San 

Antonio, the Trinity, and the Mississippi Rivers, respectively. 

Nitrite plus nitrate loads indicated that although loads were 

greatest from the Mississippi River, smaller rivers yielded as 

much nitrite plus nitrate on a per-square-kilometer basis as the 

Mississippi River, similar to ammonia yield results.

Nitrite plus nitrate loads and yields for selected sites on 

the Mississippi, Arkansas, and Atchafalaya Rivers from this 

study for the period 1993-2004 were compared to loads and 

yields calculated by Goolsby and others (1999) for the period 

1980-96. The mean annual nitrate loads for sites 1, 2, 48, 53, 

and 68 were 324,000; 537,000; 18,800; 732,000; and 221,000 

T, respectively (Goolsby and others, 1999). Nitrate loads 

calculated from this study for the same sites were 337,000; 

534,000; 27,100; 707,000; and 224,000 T (table 6), respectively, and thus were comparable to the previous study results. 

Nitrate yields from the study by Goolsby and others (1999) 

were 0.62, 0.29, 0.05, and 0.3 T∙km-2∙yr-1 for sites 1, 2, 48, 

and the combined Mississippi and Atchafalaya Rivers (sites 

53 and 68). For this study, yields were 0.640 (site 1), 0.289 

(site 2), 0.0662 (site 48), and 0.294 T∙km-2∙yr-1 (sites 53 and 

68 combined), which were comparable to the previous study 

results.

Total nitrogen trends, loads, and yields

Trend analyses of total nitrogen data were attempted for 

61 study sites (table 7). Trend results were rejected for 20 sites 

because of poor model fit (represented as N/A in table 7). Of 

the remaining 41 sites where trend results were considered 

acceptable, there were 35 sites (about 85 percent) where no 

total trends in concentration were observed during the study 

period (table 7). Decreasing total trends in concentration were 

observed at two sites, which were -2.3 and -1.8 percent per 

year for site 16 in the Mississippi system and site 73 in the 

Louisiana-Gulf/Pontchartrain system, respectively, during the 

study period (table 7, fig. 16). Increasing total trends in concentration were observed at four sites, which were 7.0 and 4.1 

percent per year for sites 9 and 32, respectively, in the Mississippi system, and 6.8 and 3.8 percent per year for sites 61 and 

65, respectively, in the Atchafalaya system during the study 

period (table 7, fig. 16). There were 27 sites (about 66 percent) 

where no flow-adjusted trends in concentration were observed 

during the study period (table 7). Decreasing flow-adjusted 

trends in concentration were observed at two sites, which were 

-1.5 and -1.8 percent per year for sites 16 and 73, respectively, 

for the study period (table 7, fig. 17). Increasing flow-adjusted 

trends in concentration were observed at 12 sites, ranging from 

1.8 to 14 percent per year during the study period (table 7, fig. 

17).

Decreasing total and flow-adjusted trends in total nitrogen concentrations were observed at site 16 in the Mississippi 

24 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 14. Trends in nitrite plus nitrate loads at study sites, 1993-2004.

RESULTS 25
Figure 15. Annual nitrite plus nitrate loads for selected study sites, 1980-2004.

26  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 16. Total trends in total nitrogen concentrations at study sties, 1993-2004.

RESULTS 27
Figure 17. Flow-adjusted trends in total nitrogen concentrations at study sites, 1993-2004.

28  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
system and site 73 in the Louisiana-Gulf/Pontchartrain system 

indicating that the trends were not simply related to decreasing trends in streamflow, but could be caused by management 

changes or changes in nitrogen sources within the drainage 

areas at these two sites. For site 16, the decreasing trend 

was slight (about 2 percent), and the reference total nitrogen 

concentration was low (0.41 mg/L, table 7). The national 

background concentration for total nitrogen is 1 mg/L (U.S. 

Geological Survey, 1999). Also, the USEPA has developed 

nutrient criteria recommendations for rivers and streams in 14 

nutrient “ecoregions” (U.S. Environmental Protection Agency, 

2007). Site 16 is located in Nutrient Ecoregion XI (central 

and eastern forested uplands), and the USEPA recommendation for a total nitrogen criteria aggregated for this ecoregion is 0.31 mg/L (U.S. Environmental Protection Agency, 

2000b). The reference concentration for total nitrogen at site 

16 was less than the national background concentration, and 

total nitrogen concentrations should approach or fall below the 

recommended USEPA ecoregion criteria by 2004 if the trend 

is applied. Thus, the decreasing trend at site 16 was considered negligible. No references were found in the literature to 

explain the decrease in total nitrogen at site 73.

Sites 9 and 32 in the Mississippi system and sites 61 

and 65 in the Atchafalaya system indicated increases in total 

nitrogen in both the total and flow-adjusted trends in concentration; therefore, trends were not influenced by streamflow 

patterns, but could indicate changes in management practices 

or increases in nitrogen sources in these watersheds. Increases 

in total nitrogen were similar to increases in nitrite plus nitrate 

at site 9, which is located on Yocum Creek in Arkansas. As 

previously mentioned, site 9 was included in a study by Davis 

and Bell (1998), however, their study did not include analysis of total nitrogen. No other references were found in the 

literature to explain the increase in total nitrogen at site 9. 

Similarly, no references were found in the literature to explain 

the increases in total nitrogen at sites 32 or 61 [however, as 

previously stated, site 32 is located on Flint Creek in Oklahoma, and a segment of Flint Creek is listed as impaired due 

to nitrates (Oklahoma Department of Environmental Quality, 

2004)]. The reference concentration at site 65 was less than the 

national background concentration of 1 mg/L for total nitrogen (U.S. Geological Survey, 1999). Site 65 is located in 

Nutrient Ecoregion IX (southeastern temperate forested plains 

and hills), and the USEPA recommendation for a total nitrogen criteria aggregated for this ecoregion is 0.69 mg/L (U.S. 

Environmental Protection Agency, 2000a). Because the total 

nitrogen reference concentration at site 65 was less than the 

national background concentration, and concentrations of total 

nitrogen are not projected to exceed the recommended USEPA 

ecoregion criteria in 2004 if the trend is applied, the trend at 

site 65 is considered negligible.

Increasing flow-adjusted trends in total nitrogen concentration were observed at sites 2, 28, 33, and 35 in the Mississippi system, sites 56 and 59 in the Atchafalaya system, and 

sites 81 and 104 in the Texas-Gulf system; therefore, these 

trends could indicate changes in management practices or 

increases in nitrogen sources at these sites during the study 

period. Reference concentrations at sites 81 and 104 were low 

(less than 0.3 mg/L). USEPA recommended ecoregion total 

nitrogen criteria for sites 81 and 104 are 0.69 and 0.56 mg/L, 

respectively (U.S. Environmental Protection Agency, 2000a 

and 2001). If trends were applied to reference concentrations 

at sites 81 and 104, total nitrogen concentrations likely would 

not exceed the national background concentration nor associated ecoregion criteria by 2004; thus, increasing trends at 

these two sites were considered negligible. The drainage area 

for site 2 includes the upper Mississippi and Missouri Rivers; 

thus, an explanation for the increasing trend in total nitrogen at 

this site is beyond the scope of this report.

The flow-adjusted trend for total nitrogen increased at site 

28 along the Elk River in Missouri during the study period. 

The Elk River is listed as an impaired waterbody because of 

nitrogen concentrations that were shown to be increasing over 

the past 35 years (Missouri Department of Natural Resources, 

2004). Although there could be other factors that contribute to 

increasing nitrogen, recent TMDL and watershed restoration 

proposals indicate the most significant contribution of nitrogen 

is attributed to the poultry industry as there are approximately 

275 poultry AFOs within the Elk River watershed (Missouri 

Department of Natural Resources, 2004 and 2006). These 

reports state that the increase is related to point sources, due to 

population increases caused by availability of more jobs in the 

poultry industry, and to nonpoint sources, due to application of 

poultry litter on agricultural fields. No references were found 

in the literature to explain the increases in total nitrogen at 

sites 33, 35, 56, and 59.

There were no trends observed in total nitrogen loads at 

28 sites (about 68 percent) during the study period (table 7). 

Decreasing trends in total nitrogen loads were observed at 

12 sites during the study period and ranged from -7.3 to -2.7 

(2 sites) percent per year (fig. 18). Increasing trends in total 

nitrogen loads were observed only at site 112 located on the 

San Antonio River in the Texas-Gulf system during the study 

period (about a 5-percent per year increase in total nitrogen 

load).

The recent decreasing trends at sites 2, 48, 53, and 

68 (table 7) appear to be part of longer decreasing trends 

observed since the early 1980s at these four sites (fig. 19). No 

trends in total nitrogen loads were observed at site 1 in both 

the current study period (table 7) and the 1980-2004 time 

period (fig. 19). No trend in total nitrogen loads was observed 

at site 64 for the study period, which is in contrast to annual 

loads plotted since 1980 that seem to indicate a slight decreasing over time (fig. 19); however, annual loads plotted in figure 

19 since 1993 are scattered, thus validating the no-trend result 

for the study period.

The recent increasing trend in load at site 112 (table 7) 

is a recent trend, as total nitrogen loads at this site have been 

stable from 1980 to about 1997 (fig. 19). There were no total 

or flow-adjusted trends in concentration observed at site 112 

for the study period; however, there was a 7.9 percent per year 

increase in flow observed at site 112 for the study period. The 

RESULTS 29
Figure 18. Trends in total nitrogen loads at study sites, 1993-2004

30 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 19. Annual total nitrogen loads for selected study sites, 1980-2004.

RESULTS 31
recent increasing trend in total nitrogen load at site 112 likely 

is related to an increase in streamflow at this site.

Annual loads, average annual load, and yield calculations for total nitrogen were attempted for 35 study sites in all 

four river systems (table 8). When total nitrogen, ammonia, 

and nitrite plus nitrate loads were compared at key sites in the 

four river systems, the nitrite plus nitrate loads accounted for 

about 70-90 percent of the total nitrogen loads (tables 4, 6, and 

8). Average annual total nitrogen loads for some of the major 

drainages (not necessarily the most downstream) into the 

northwestern Gulf of Mexico were as follows: 992,000 T for 

the Mississippi River (site 53); 355,000 T for the Atchafalaya 

River (site 68); 1,150 T for the Tangipahoa River (site 70) and 

300 T for Tickfaw River (site 71), both of which empty into 

Lake Pontchartrain; and 507 T for the Colorado River (site 

100) and 3,450 T for the San Antonio River (site 112) from 

the Texas-Gulf system. Again, the Mississippi and Atchafalaya 

systems account for nearly all of the total nitrogen load into 

the northwestern Gulf of Mexico. Yields for these same sites, 

in order of magnitude, were 1.47, 0.764, 0.690, 0.469, 0.340, 

and 0.00627 T∙km-2∙yr-1 for the Atchafalaya, San Antonio, 

Tangipahoa, Tickfaw, Mississippi, and Colorado Rivers, 

respectively. Similar to ammonia and nitrite plus nitrate yield 

observations, total nitrogen yields from smaller rivers generally were as large or larger than yields from the Mississippi 

River.

Total nitrogen loads and yields for selected sites on the 

Mississippi, Arkansas, and Atchafalaya Rivers from this study 

for the period 1993 to 2004 were compared to loads and yields 

calculated by Goolsby and others (1999) for the period 1980 

to 1996. The mean annual total nitrogen loads from sites 1, 2, 

48, 53, and 68 from Goolsby and others (1999) were 496,000, 

841,000, 54,900, 1,180,000, and 386,000 T, respectively. Total 

nitrogen loads for the same sites calculated from this study 

were 480,000, 770,000, 45,900, 992,000, and 355,000 T (table 

8), respectively, which were all slightly lower than the previous study results, reflecting decreasing trends in streamflow at 

all of these sites during the last decade. Total nitrogen yields 

from Goolsby and others (1999) were 0.94, 0.46, 0.13, and 

0.49 T∙km-2∙yr-1 for sites 1, 2, 48, and the combined Mississippi and Atchafalaya Rivers (sites 53 and 68). For this study, 

total nitrogen yields were 0.912, 0.417, 0.112, and 0.427 

T∙km-2∙yr-1 for the same sites, respectively, which were comparable to the previous study results.

Relation of trends in nitrogen to trends in source 

data and landscape attributes

There were some statistically significant results from the 

WLS regression analyses where trends in nitrogen-source data 

and landscape attributes were compared to trends in nitrogen 

constituents from this study (table 9). Coefficients of determination (R2) for the statistically significant results were all 

less than about 0.3 indicating that very little of the variance 

was explained, and relations were considered poor. Therefore, 

statistically significant results of the WLS regression analyses 

are presented in this section relative to nitrogen trends, but the 

reader is cautioned against over-interpretation of the results.

Population throughout most of the study area either 

remained the same or increased during the study period (fig. 

20). Such increasing trends could explain increasing trends in 

nitrite plus nitrate and total nitrogen observed near urban areas 

(point sources) or where drainages included a combination of 

urban and agricultural areas. Results of the WLS regression 

analyses did not indicate any statistically significant results 

where trends in population were compared to trends in nitrogen constituents observed at study sites (table 9).

Nitrogen from atmospheric deposition generally 

increased for locations in the center part of the study area from 

southeastern Colorado through central Oklahoma and northern 

and eastern Texas during the study period (fig. 21). Nitrogen 

from atmospheric deposition decreased in south-central Texas, 

eastern Oklahoma, northwestern Arkansas, and southwestern 

Missouri during the study period (fig. 21). Weighted-leastsquares regression results indicated a potential inverse relation 

between trends in nitrogen from atmospheric deposition and 

flow-adjusted trends in nitrite plus nitrate at study sites (table 

9), which indicated that at locations where atmospheric deposition increased, there were decreasing trends in flow-adjusted 

nitrite plus nitrate during the study period. Trends in atmospheric deposition were, therefore, not the controlling factor 

on trends in nitrite plus nitrate.

Increased trends in nitrogen from fertilizer were observed 

at sites in north-central Texas, eastern Oklahoma, northern 

Arkansas, and southern Missouri (fig. 22). Weighted-leastsquares regression results suggest that increasing trends in 

nitrogen from fertilizer could be related to increasing flowadjusted trends in nitrite plus nitrate at study sites (table 9). 

Increasing trends in nitrogen from manure were observed at 

some of the same sites as were trends in fertilizer (figs. 22 and 

23). Weighted-least-squares regression results suggest that 

increasing trends in nitrogen from land application of manure 

could be related to increasing trends in both total and flowadjusted trends in nitrite plus nitrate and total nitrogen at study 

sites (table 9).

There were only seven sites where management practices 

increased or decreased more than 1 percent of their total drainage areas from 1992 to 1997 (table 10). Of these seven sites, 

only one corresponded to any trends in nitrogen data for the 

study period: there was a slight increase in both total and flowadjusted trends in nitrite plus nitrate for the study period at 

site 103. During the period 1992-1997, conservation practices 

(contour farming or terracing) decreased by 9.6 percent of 

the total drainage area for site 103. Because WLS regression 

analyses could not be completed by using the conservation 

practices data and trends in nitrite plus nitrate, it is unknown if 

the increasing trend in nitrite plus nitrate actually was affected 

by the decrease in conservation practices at site 103.

32 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 20. Trends in population densities at study sites from 1990-2000.

RESULTS 33
Figure 21. Trends in nitrogen from atmospheric deposition at study sites, 1993-2004.

34 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 22. Trends in nitrogen from fertilizer at study sites, 1993-2004.

RESULTS 35
Figure 23. Trends in nitrogen from manure at study sites, 1992-2002.

36  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Overall conclusions about nitrogen trends and 

loads for the study area

In general, there were few trends observed in the nitrogen data at study sites during the study period; no trends were 

observed in about 63 percent of all nitrogen trend analyses 

attempted. Although some patterns in the nitrogen data did 

exist where trends were attempted, no regional patterns could 

be confirmed because of poor spatial representation of the 

trends sites.

Decreasing trends in flow-adjusted concentrations of 

ammonia were observed at 25 sites. No increasing trends in 

concentrations of ammonia were noted at any sites. Flowadjusted concentrations of nitrite plus nitrate decreased at 7 

sites and increased at 14 sites. Flow-adjusted concentrations of total nitrogen decreased at 2 sites and increased at 

12 sites. Improvements to municipal wastewater treatment 

facilities contributed to the decline of ammonia concentrations at selected sites. Notable increasing trends in nitrite 

plus nitrate and total nitrogen at selected study sites were 

attributed to both point and nonpoint sources. Trend patterns 

in total nitrogen generally followed trend patterns in nitrite 

plus nitrate, which was understandable given that nitrite plus 

nitrate loads generally were 70-90 percent of the total nitrogen 

loads at most sites. Although population increased throughout 

the study area during the study period, there was no observed 

relation between increasing trends in nitrogen in study area 

streams and increasing trends in population. With respect 

to other nitrogen sources, statistical results did suggest that 

increasing trends in nitrogen could be related to increasing 

trends in nitrogen from either commercial fertilizer use and/or 

land application of manure.

Loads of ammonia, nitrite plus nitrate, and total nitrogen 

decreased during the study period, but some trends in nitrogen loads were part of long-term decreases since 1980. For 

example, ammonia loads were shown to decrease at nearly all 

sites over the past decade, but at selected sites, these decreasing trends were part of much longer trends since 1980. The 

Mississippi and Atchafalaya Rivers contributed the highest nitrogen loads to the northwestern Gulf of Mexico as 

expected; however, nitrogen yields from smaller rivers had 

similar or higher yields than from the Mississippi River.

Phosphorus

Specific details about trend, load, and yield results for 

orthophosphorus and total phosphorus data are discussed at 

the beginning of this section. These results are then related 

to potential trends in source and landscape attributes. Finally, 

some general conclusions about phosphorus trend and load 

results are discussed at the end of this section.

Orthophosphorus trends, loads, and yields

Trend analyses of orthophosphorus data were attempted 

for 68 study sites in three of the four river systems, with the 

exception of the Louisiana-Gulf/Pontchartrain system, which 

had no sites with an adequate amount of orthophosphorus 

data to attempt trend analyses (table 11). Trend results were 

rejected for 34 sites because of poor model fit (represented as 

N/A in table 11). Of the remaining 34 sites where trend results 

were considered acceptable, there were 17 sites (50 percent) 

where no total trends in concentration were observed during 

the study period (table 11). Decreasing total trends in concentration were observed at 11 sites ranging from -7.2 to -2.6 

percent per year during the study period (table 11, fig. 24). 

Increasing total trends in concentration were observed at six 

sites, ranging from 2.5 to 15 percent per year during the study 

period (table 11, fig. 24). There were 17 sites (50 percent) 

where no flow-adjusted trends in concentration were observed 

during the study period (table 11). Decreasing flow-adjusted 

trends in concentration were observed at 10 sites, ranging 

from -7.3 to -2.5 percent per year (table 11, fig. 25). Increasing flow-adjusted trends in concentration occurred at seven 

sites, ranging from 3.0 to 13 percent per year during the study 

period (table 11, fig. 25).

Decreasing total trends in orthophosphorus concentrations were observed at sites 1, 38, 39, 42, 45 in the Mississippi 

system, and sites 59 and 61 in the Atchafalaya system during the study period; reference concentrations at these seven 

sites were less than about 0.1 mg/L. Although there are no 

secondary water-quality standards for orthophosphorus, the 

USEPA recommends that total phosphates should not exceed 

0.05 mg/L in a stream where it enters a lake or reservoir (U.S. 

Environmental Protection Agency, 1986). If trends are applied 

to the reference concentrations at these seven sites, orthophosphorus concentrations would likely approach or fall below the 

recommended USEPA concentration during the study period. 

Decreasing trends at these seven sites were, therefore, considered negligible. In addition, trends at site 38 and 59 were 

not retained when the effects of streamflow were removed; 

therefore, total trends at these two sites likely were related to 

decreasing trends in streamflow.

Decreasing total trends in orthophosphorus concentration 

were also observed at sites 25, 26, 27 in the upper Mississippi system, and 112 in the Texas-Gulf system, and reference concentrations were all greater than about 1 mg/L. In 

addition, decreasing trends were retained when the effects of 

streamflow were removed at these four sites (table 11), indicating that the trends were not simply influenced by decreasing trends in streamflow. Sites 25, 26, and 27 were located 

downstream from the wastewater treatment plant previously 

mentioned. Although upgrades installed in the mid-1990’s 

at this particular plant targeted ammonia and BOD removal, 

decreases in orthophosphorus at these three sites could be a 

secondary benefit of advanced waste treatment and clarifier 

improvements, which are also processes that can be used for 

phosphorus removal (Viessman and Hammer, 1985). Site 112 

is located on the San Antonio River. The San Antonio River 

Authority (2003) reported that reductions in phosphorus levels 

and improvements in water quality in the San Antonio River 

RESULTS 37
Figure 24. Total trends in orthophosphorus concentrations at study sites, 1993-2004.

38  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 25. Flow-adjusted trends in orthophosphorus concentrations at study sites, 1993-2004.

RESULTS 39
were related to improvements in municipal wastewater treatment.

Decreasing trends in flow-adjusted orthophosphorus 

were observed at site 87 on the Trinity River in the Texas-Gulf 

system (although there were no total trends in concentration). 

In the mid-1990’s, Van Metre and Reutter (1995) reported a 

decreasing trend in phosphorus loads at the same site (located 

about 160 miles downstream from the City of Dallas) for data 

collected prior to 1991, but the cause of the decreasing trend 

was not determined.

Increasing total and flow-adjusted trends in orthophosphorus were observed at sites 24, 28, 33, and 48 – all in the 

Mississippi system (table 11). Reference concentrations at 

these four sites were all lower than the USEPA recommendation of 0.05 mg/L orthophosphorus. If trends were applied to 

reference concentrations at these four sites, orthophosphorus 

concentrations would remain below or barely exceed 0.05 

mg/L; thus, trends at these four sites were considered negligible. An increasing flow-adjusted trend in orthophosphorus 

occurred at site 9 in the Mississippi system (although there 

were no total trends in concentration). The reference concentration was 0.03 mg/L and would not exceed the USEPA 

recommendation of 0.05 mg/L during the study period; thus, 

the trend at site 9 was also considered negligible.

Increasing total and flow-adjusted trends were observed 

at sites 22 and 32 in the Mississippi system (table 11) indicating that trends were not influenced by streamflow but could 

be caused by changes in management practices or sources 

of phosphorus at this site. Site 22 is located on Monument 

Creek near the United States Air Force Academy in Colorado. 

The increasing trends in orthophosphorus at site 22 during 

this study period could not be explained at this time. Results 

from a study by Edelmann and others (2002) indicated that 

orthophosphorus concentrations tended to be higher during 

storm flows than during base flows for a location near site 

22, but their results did not provide an overall trend for their 

study period, which was 1981 through 2001. Site 32 is located 

on Flint Creek in Oklahoma, and a segment of Flint Creek is 

listed as impaired due, in part, to total phosphorus; however, 

the official source of the impairment is unknown (Oklahoma 

Department of Environmental Quality, 2004).

There were no trends observed in orthophosphorus 

loads at 15 sites (about 44 percent) during the study period 

(table 11). Decreasing trends in orthophosphorus loads were 

observed at 17 sites for the study period (fig. 26), ranging from 

-7.9 to -2.5 percent per year (table 11). There were increasing trends in orthophosphorus loads observed at two sites. 

An increase of 37 percent per year was observed at site 19, 

and an increase of 7.6 percent per year was observed at site 

22; however, reference loads at these two sites were 12 and 8 

kg/day, respectively, and trends in orthophosphorus loads were 

considered negligible.

Decreasing trends in orthophosphorus loads at sites 2, 53, 

and 68 during the study period cannot be completely confirmed in looking at annual load data for the time period 1980-

2004 plotted in figure 27. There is a lengthy gap in the annual 

load data at site 2 from about 1987 to 1995; load data plotted 

after 1995 appear to be decreasing but are also scattered (fig. 

27). The decreasing trends in orthophosphorus loads at sites 

53 and 68 appear to be decreasing since 1980, but the decrease 

is very slight (fig. 27). No trends were observed in orthophosphorus loads at sites 1 and 48 during the study period. These 

results appear to be confirmed in looking at annual loads plotted at site 1 for the period 1980-2004 in figure 27. Although 

there appears to be a decreasing trend in annual loads plotted 

for site 48 since 1980 (LOWESS line, site 48, fig. 27), the data 

are scattered, likely indicating no trend since 1980.

Annual loads, average annual load, and yield calculations 

for orthophosphorus were attempted for 29 study sites in three 

of the four river systems, with the exception being the Louisiana-Gulf/Pontchartrain system, which had no sites that had an 

adequate amount of orthophosphorus data to attempt load and 

yield calculations (table 12). Average annual orthophosphorus 

loads for some of the major drainages (not necessarily the 

most downstream) into the northwestern Gulf of Mexico were 

as follows: 30,100 T for the Mississippi River (site 53); 11,900 

T for the Atchafalaya River (site 68); and 1,290 T and 323 T 

for the Trinity River (site 87) and the San Antonio River (site 

112), respectively, in the Texas-Gulf system. Again, the Mississippi and Atchafalaya systems account for nearly all of the 

orthophosphorus load into the northwestern Gulf of Mexico. 

Yields for these same sites, in order of magnitude, were 

0.0716, 0.0491, 0.0357, and 0.0103 T∙km-2∙yr-1 for the San 

Antonio, Atchafalaya, Trinity, and Mississippi Rivers, respectively. Similar to other yield observations, orthophosphorus 

yields from smaller rivers were equal to or greater than yields 

from the Mississippi River.

Orthophosphorus loads and yields for selected sites on 

the Mississippi, Arkansas, and Atchafalaya Rivers from this 

study for the period 1993-2004 were compared to loads and 

yields calculated by Goolsby and others (1999) for the period 

1980-96. The mean annual orthophosphorus loads from sites 

1, 2, 48, 53, and 68 from Goolsby and others (1999) were 

11,200; 19,100; 1,900; 30,800; and 11,000 T, respectively. 

Orthophosphorus loads for the same sites calculated from 

this study were 10,800; 21,900; 1,980; 30,100; and 11,900 T 

(table 12), respectively, which were comparable to the previous study. Orthophosphorus yields from Goolsby and others 

(1999) were 0.021, 0.01, 0.005, and 0.013 T∙km-2∙yr-1 for 

sites 1, 2, 48, and the combined Mississippi and Atchafalaya 

Rivers (sites 53 and 68). For this study, orthophosphorus 

yields were nearly identical to the previous study results (table 

12).

Total phosphorus trends, loads, and yields

Trend analyses of total phosphorus data were attempted 

for 80 study sites in all four river systems (table 13). Trend 

results were rejected for 28 sites because of poor model fit 

(represented as N/A in table 13). Of the remaining 52 sites 

where trend results were considered acceptable, there were 32 

sites (about 62 percent) where no total trends in concentration 

40 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 26. Trends in orthophosphorus loads at study sites, 1993-2004.

RESULTS 41
were observed during the study period (table 13). Decreasing total trends in concentration were observed at six sites 

ranging from -8.8 to -5.0 percent per year during the study 

period (table 13, fig. 28). Increasing total trends in concentration occurred at 14 sites ranging from 4.1 to 13 percent per 

year during the study period (table 13, fig. 28). There were 

29 sites (about 56 percent) where no flow-adjusted trends in 

concentration were observed during the study period (table 

13). Decreasing flow-adjusted trends in concentration were 

observed at six sites ranging from -9.0 to -3.9 percent per year 

(table 13, fig. 29). Increasing flow-adjusted trends in concentration occurred at 17 sites, ranging from 3.0 to 56 percent per 

year during the study period (table 13, fig. 29).

Decreasing total and flow-adjusted trends in total phosphorus occurred at sites 34 and 46 in the Mississippi system 

and sites 89, 106, 112, and 113 in the Texas-Gulf system (table 

13, figs. 28 and 29), indicating that the decreasing trends 

were not related to decreasing trends in streamflow but could 

be caused by changes in management practices or sources of 

phosphorus within the drainage areas of these sites. Reference concentrations at sites 34, 46, and 106 were less than 

0.1 mg/L (table 13). The national background concentration 

for total phosphorus is 0.1 mg/L (U.S. Geological Survey, 

1999). Site 34 is located in nutrient ecoregion XI (central and 

eastern forested uplands), and the USEPA recommendation for 

total phosphorus criteria aggregated for this ecoregion is 0.01 

mg/L (U.S. Environmental Protection Agency, 2000b). Site 

46 is located in nutrient ecoregion IX (southeastern temperate 

forested plains and hills), and the USEPA recommendation 

for total phosphorus criteria aggregated for this ecoregion is 

0.037 mg/L (U.S. Environmental Protection Agency, 2000a). 

Site 106 is located in nutrient ecoregion IV (Great Plains grass 

and shrublands), and the USEPA recommendation for total 

phosphorus criteria aggregated for this ecoregion is 0.023 mg/

Figure 27. Annual orthophosphorus loads for selected study sites, 1980-2004.

42 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 28. Total trends in total phosphorus concentrations at study sites, 1993-2004.

RESULTS 43
Figure 29. Flow-adjusted trends in total phosphorus concentrations at study sites, 1993-2004.

44 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
L (U.S. Environmental Protection Agency, 2001). Because 

reference concentrations for these three sites were less than the 

national background concentration, and concentrations were 

projected to be lower or approach associated ecoregion criteria 

recommendations in 2004 if trends are applied, the decreasing 

trends at sites 34, 46, and 106 were considered negligible.

Reference concentrations were about 1 mg/L or greater at 

sites 89, 112, and 113. Decreasing trends in total phosphorus 

at site 89 could not be explained at this time. The magnitude 

of decreasing trends in total phosphorus at site 112 were 

similar to the magnitude of decreasing orthophosphorus trends 

at this site (tables 11 and 13; note: site 113 is also located on 

the San Antonio River downstream of site 112). As discussed 

earlier, the San Antonio River Authority (2003) reported that 

reductions in phosphorus levels and improvements in water 

quality in the San Antonio River were related to improvements 

in municipal wastewater treatment.

Increasing total and flow-adjusted trends in total phosphorus occurred at sites 1, 28, 30, 31, 32, 33, 48, 53 in the 

Mississippi system, sites 56 and 68 in the Atchafalaya system, 

site 71 in the Louisiana-Gulf/Pontchartrain system, and sites 

78 and 90 in the Texas-Gulf system (table 13, figs. 28 and 29) 

indicating that trends were not influenced by streamflow but 

could be caused by changes in phosphorus sources or management practices. Reference concentrations at most of these 

13 sites were less than or only slightly greater than 0.1 mg/L 

(table 13, except for site 31, which had a reference concentration of about 0.5 mg/L). If trends are applied to the reference 

concentrations at these 13 sites, then total phosphorus concentrations in 2004 are projected to approach or exceed the 

national background concentration (U.S. Geological Survey, 

1999) and associated USEPA ecoregion criteria recommendations for total phosphorus at each site (U.S. Environmental 

Protection Agency, 2007).

It is important to note that total phosphorus increased 

at sites 1, 48, 53, 56, and 68, which are mainstem sites on 

the Mississippi, Arkansas, Red, and Atchafalaya Rivers. The 

drainage areas for these five sites are extremely large and complex, thus, an explanation as to the increase in total phosphorus at these five sites is beyond the scope of this report.

Sites 30 and 33 are located on the Illinois River, and site 

32 is located on Flint Creek in Oklahoma. The Illinois River 

and Flint Creek are part of the Oklahoma Scenic Rivers Program (Oklahoma Office of the Secretary of the Environment, 

2003). Segments of these two streams are listed as impaired 

because of phosphorous (as well as pathogens), but the official 

sources of those impairments are listed as unknown (Oklahoma Department of Environmental Quality, 2004). Site 

31 is located on Sager Creek, which is a tributary of Flint 

Creek in the Illinois River Basin. Sager Creek is not listed 

as impaired due to phosphorus (Oklahoma Department of 

Environmental Quality, 2004), but is part of the Scenic Rivers 

monitoring program that includes the Illinois River and Flint 

Creek (Oklahoma Office of the Secretary of the Environment, 

2003).

Increasing trends in total phosphorus at sites 28 and 78 

could not be explained at this time. Site 71 is located on the 

Tickfaw River in Louisiana. Although the increasing trends 

in total phosphorus at site 71 could not be explained at this 

time, phosphorus is listed as an impairment in parts of this 

watershed because of infiltration and outflows from failing 

wastewater collection systems, according to a report by the 

Louisiana Department of Environmental Quality (2000).

No explanation for an increasing trend in total phosphorus at site 90, located on the East Fork San Jacinto River in 

Texas, was found in the literature. Sneck-Fahrer and others 

(2005) sampled inflows into Lake Houston near Houston, 

Tex., for the period 2000-04. In their report, the East Fork 

San Jacinto River represented the eastern part of the Lake 

Houston watershed, which is less densely populated than the 

western part of the watershed. Sneck-Fahrer and others 

(2005) reported a decreasing trend in dissolved phosphorus 

data for a sampling site on the eastern part of Lake Houston 

itself, but not specifically at site 90.

Site 51 indicated an increasing trend in total phosphorus 

for the study period (total trend only, table 13). Site 51 is 

located near the mouth of the Yazoo River in the Mississippi 

system downstream from some of the most intense row-crop 

agricultural production areas in the United States. Although 

the increasing trend in total phosphorus at site 51 could be 

related to agriculture, Kleiss and others (2000) pointed out that 

phosphorus is used less in the Yazoo River Basin than in many 

parts of the Midwest. Another possible explanation for the 

increasing trend in total phosphorus is related to sediment in 

the Yazoo River (Coupe, 2002). Phosphorus binds to sediment, 

especially to fine clays, which are prevalent in streams of the 

Yazoo River Basin; however, an increasing trend in suspended 

sediment was not observed at site 51 during the study period 

(sediment trend results are presented in the next section). 

Thus, the increasing trend in total phosphorus at site 51 for the 

study period cannot be explained at this time.

Although there were no total trends in total phosphorus 

concentrations at sites 2, 9, and 35 in the Mississippi system 

and site 59 in the Atchafalaya system, there were increasing 

flow-adjusted trends observed at these four sites, possibly 

indicating changes in management practices or increases in 

phosphorus sources during the study period. The drainage 

area at site 2 is large and complex; thus, an explanation for an 

increasing flow-adjusted trend at this site is beyond the scope 

of this report. The increasing trend in total phosphorus at 

site 9 is not entirely known at this time; however, this site was 

included in a study by Davis and Bell (1998) that reported 

total phosphorus concentrations were slightly higher for site 

9 than for surrounding basins due to the amount of and type 

of agricultural land use (poultry farming). Site 35 is located 

on the Baron Fork River in Oklahoma; no stream segments on 

the Baron Fork River are currently listed (2007) as impaired 

due to phosphorus (Oklahoma Department of Environmental 

Quality, 2004). However, a segment of the Baron Fork River 

has been included in the 2006 Draft 303d list for streams in 

Oklahoma as impaired due, in part, to phosphorus [Oklahoma 

RESULTS 45
Department of Environmental Quality, 2006 (draft)]. Site 59 is 

located on the Washita River in Oklahoma, and the increasing 

trends in total phosphorus at site 59 cannot be explained at this 

time.

There were no trends observed in total phosphorus 

loads at 42 sites (about 81 percent) during the study period 

(table 13). Decreasing trends in total phosphorus loads were 

observed at nine sites for the study period, ranging from 

-7.8 to -3.9 percent per year (table 13, fig. 30). There was an 

increasing trend in total phosphorus loads observed at one site; 

an increase of 11 percent per year at site 31 on Sager Creek, 

which is a tributary of Flint Creek in the Illinois River Basin.

No trends in total phosphorus loads were observed at 

sites 1, 2, 48, 53, 64, and 68 during the study period (table 13). 

Similarly, no trends in total phosphorus loads were observed 

during the period 1980-2004 at sites 2, 48, 53, and 68 as plotted in figure 31. Although LOWESS lines for sites 1 and 64 

indicate increasing and decreasing trends, respectively, for the 

period 1980-2004 (fig. 31), the scatter of the data in these plots 

clearly supports no trends in total phosphorus loads at these 

two sites (table 13, fig. 31). The decreasing trend in total phosphorus loads at site 112 on the San Antonio River is part of a 

much longer decreasing trend in loads that has been occurring 

since 1980 (fig. 31), likely due to improvements in municipal 

wastewater treatment facilities as previously presented (San 

Antonio River Authority, 2003).

Annual loads, average annual load, and yield calculations 

for total phosphorus were attempted for 35 study sites in all 

four river systems (table 14). When orthophosphorus and total 

phosphorus loads were compared at key sites in the four river 

systems, the orthophosphorus loads accounted for only about 

20-30 percent of the total phosphorus loads (tables 12 and 

14). Average annual total phosphorus loads for some of the 

major drainages (not necessarily the most downstream) into 

the northwestern Gulf of Mexico were as follows: 101,000 T 

for the Mississippi River (site 53); 40,200 T for the Atchafalaya River (site 68); 206 T for the Tangipahoa River (site 70) 

in the Louisiana-Gulf/Pontchartrain system; and 2,870 T and 

998 T for the Trinity River (site 86) and the Colorado River 

(site 108), respectively, in the Texas-Gulf system. Again, the 

Mississippi and Atchafalaya systems accounted for most of the 

total phosphorus load into the northwestern Gulf of Mexico. 

Yields for these same sites, in order of magnitude, were 

0.166, 0.123, 0.0862, 0.0347, and 0.00912 T∙km-2∙yr-1 for the 

Atchafalaya, Tangipahoa, Trinity, Mississippi, and Colorado 

Rivers, respectively. Similar to other yield observations, total 

phosphorus yields from smaller rivers were equal to or greater 

than yields from the Mississippi River.

Total phosphorus loads and yields for selected sites on 

the Mississippi, Arkansas, and Atchafalaya Rivers from this 

study for the period 1993-2004 were compared to loads and 

yields calculated by Goolsby and others (1999) for the period 

1980-96. The mean annual total phosphorus loads from sites 

1, 2, 48, 53, and 68 from Goolsby and others (1999) were 

39,400; 68,700; 5,100; 97,000; and 39,500 T, respectively. 

Total phosphorus loads for the same sites calculated from 

this study were 48,800; 78,600; 5,020; 101,000; and 40,200 

T (table 14), respectively, indicating higher loads of total 

phosphorus in the Ohio and Upper Mississippi Rivers (sites 1 

and 2, respectively) calculated for this study than in the previous study. Total phosphorus yields from Goolsby and others 

(1999) were 0.075, 0.037, 0.013, and 0.042 T∙km-2∙yr-1 for 

sites 1, 2, 48, and the combined Mississippi and Atchafalaya 

Rivers (sites 53 and 68). For this study, total phosphorus yields 

for the same sites were 0.0928, 0.0425, 0.0122, and 0.0447, 

T∙km-2∙yr-1 indicating higher yields from the Ohio and Upper 

Mississippi Rivers (sites 1 and 2, respectively) calculated for 

this study than in the previous study (table 14).

Relation of trends in phosphorus to trends in 

source data and landscape attributes

There were some statistically significant results from 

the WLS regression analyses where trends in phosphorussource data and landscape attributes were compared to trends 

in phosphorus constituents from this study (table 9). Coefficients of determination (R2) for the statistically significant 

results were all less than about 0.3, indicating that little of the 

variance was explained, and relations were considered poor. 

Statistically significant results of the WLS regression analyses 

are presented in this section relative to phosphorus trends; 

however, the reader is cautioned against over-interpretation of 

the results.

Population throughout most of the study area either 

remained the same or increased during the study period (fig. 

20). Such increasing trends could explain increasing trends in 

orthophosphorus and total phosphorus observed near urban 

areas (point sources) or where drainages included a combination of urban and agricultural areas. However, results of the 

WLS regression analyses indicated an inverse relation between 

population and flow-adjusted trends in total phosphorus 

(table 9), which was a result that was unexpected. Therefore, 

trends in population were not considered a controlling factor 

to explain trends in total phosphorus (nor could trends in 

population be used to explain trends in phosphorus from point 

sources).

Increasing trends in phosphorus from fertilizer were 

observed in southeastern Colorado, eastern Oklahoma, 

Arkansas, and southern Missouri (fig. 32). Results of the WLS 

regression analyses did not indicate any statistically significant 

results between trends in phosphorus from fertilizer and trends 

in orthophosphorus or total phosphorus observed at study sites 

(table 9). There were more increasing trends in phosphorus 

from manure than were increasing trends in phosphorus from 

fertilizer (figs. 32 and 33). Weighted-least-squares regression 

results did suggest that increasing trends in phosphorus from 

land application of manure could be related to increasing total 

and flow-adjusted trends in both orthophosphorus and total 

phosphorus at study sites (table 9).

There were only seven sites where management practices increased or decreased more than 1 percent of their total 

drainage areas from 1992 to 1997 (table 10). Of these seven 

46  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 30. Trends in total phosphorus loads at study sites, 1993-2004.

RESULTS 47
Figure 31. Annual total phosphorus loads for selected study sites, 1980-2004.

48  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 32. Trends in phosphorus from fertilizer at study sites, 1993-2004.

RESULTS 49
Figure 33. Trends in phosphorus from manure at study sites, 1992-2002.

50 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
sites, only one corresponded to any trends in phosphorus data 

for the study period. There was a decrease in both total and 

flow-adjusted trends in total phosphorus for the study period 

at site 89. During the period 1992-97, conservation practices 

(contour farming or terracing) decreased by 1.3 percent of the 

total drainage area for site 89. It is unlikely that the decrease 

in total phosphorus was related to the decrease in the amount 

of land in contour farming or terracing; rather, the decrease 

in total phosphorus at site 89 was affected more by changes 

in the wastewater treatment plant located near this site (Texas 

Natural Resource Conservation Commission, 2000).

Overall conclusions about phosphorus trends 

and loads for the study area

Similar to results for nitrogen, there were few trends 

observed in the phosphorus data at study sites during the study 

period; no trends were observed in about 57 percent of all 

phosphorus trend analyses attempted. Although some patterns 

in the phosphorus data did exist where trend analyses were 

attempted, no regional patterns could be confirmed because of 

poor spatial representation of the trends sites.

Flow-adjusted concentrations of orthophosphorus 

decreased at 10 sites and increased at 7 sites. Flow-adjusted 

concentrations of total phosphorus decreased at 6 sites and 

increased at 17 sites. It was understandable that trend patterns 

in total phosphorus did not follow trend patterns in orthophosphorus given that orthophosphorus loads accounted for only 

about 20-30 percent of the total phosphorus load at comparable sites. Trends in population data were inversely related to 

trends in flow-adjusted total phosphorus; therefore, trends in 

population were not considered a controlling factor to explain 

trends in total phosphorus. No relation was observed between 

phosphorus from fertilizer use and either orthophosphorus or 

total phosphorus trends. However, statistical results did suggest that increasing trends in both orthophosphorus and total 

phosphorus could be related to increasing trends in phosphorus 

from land application of manure.

There were more decreasing trends than increasing trends 

in phosphorus loads during the past decade, most of which 

were unique to the recent decade and not part of long-term 

decreases since 1980. Similar to nitrogen loads, the Mississippi and Atchafalaya Rivers contributed the highest phosphorus loads to the northwestern Gulf of Mexico as expected; 

however, phosphorus yields from smaller rivers were similar 

to or higher than yields from the Mississippi River.

Suspended Sediment

Specific details about suspended-sediment trend, load, 

and yield results are discussed at the beginning of this section. 

These results are then related to potential trends in source and 

landscape attributes. Finally, some general conclusions about 

suspended-sediment trend and load results are discussed at the 

end of this section.

Suspended-sediment trends, loads, and yields

Trend analyses in suspended-sediment data were 

attempted for 39 study sites in three of the four river systems, with the exception of the Louisiana-Gulf/Pontchartrain system, which had no sites with an adequate amount 

of suspended-sediment data to attempt trend analyses (table 

15). Trend results were rejected for 11 sites because of poor 

model fit (represented as N/A in table 15). Of the remaining 28 sites where trend results were considered acceptable, 

there were 19 sites (about 68 percent) where no total trends 

in concentration were observed during the study period (table 

15). Decreasing total trends in concentration were observed 

at eight sites ranging from -7.6 to -3.0 percent per year during 

the study period (table 15, fig. 34). Increasing total trends in 

concentration occurred at only site 9, which had a 9.5 percent per year increase during the study period (table 15, fig. 

34). There were 21 sites (75 percent) where no flow-adjusted 

trends in concentration were observed during the study period 

(table 15). Decreasing flow-adjusted trends in concentration 

were observed at five sites, ranging from -7.6 to -2.2 percent 

per year (table 15, fig. 35). Increasing flow-adjusted trends in 

concentration occurred at two sites, sites 1 and 9, which had 

increases of 5.7 and a 10 percent per year, respectively, during 

the study period (table 15, fig. 35).

Decreasing total trends in suspended-sediment concentration were observed at sites 2, 5, 10, 46, 48, and 53 in the Mississippi system, and sites 61 and 68 in the Atchafalaya system. 

Decreasing trends were not retained at sites 2, 46, 61, and 

68 when the effects of streamflow were removed, indicating 

that trends at these four sites were likely related to decreasing 

trends in streamflow. Decreasing trends were not retained at 

site 10 either, but there was no trend in streamflow at this site; 

therefore, the decreasing total trend in suspended sediment at 

site 10 cannot be explained at this time because of multiple 

factors that may be occurring.

Decreasing trends were retained at sites 5, 48, and 53 

when the effects of streamflow were removed, indicating 

possible improvements in management practices or decreases 

in sediment sources during the study period at these three 

sites. The decrease in suspended sediment at site 5 cannot be 

explained at this time. Sites 48 and 53 are mainstem sites on 

the Mississippi and Arkansas Rivers. An explanation as to the 

decreases in suspended sediment at these two sites is beyond 

the scope of this report because of the size and complexity of 

their associated drainage areas; however, these sites are highly 

regulated throughout their drainage areas for navigation, 

hydroelectric power, and other erosion-control or flood-control 

purposes – the majority of which were built after the 1930s. 

Meade (1995) reported that reservoirs built during the 1950s 

on the Missouri and Arkansas Rivers, which were the largest 

sources of sediment to the Mississippi River Basin at the time, 

caused large decreases in suspended sediment because of their 

trapping and settling effects. Decreasing trends in sediment 

continued to be observed through the early 1990s (Meade, 

1995).

RESULTS  51
Figure 34. Total trends in suspended-sediment concentrations at study sites, 1993-2004.

52 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 35. Flow-adjusted trends in suspended-sediment concentrations at study sites, 1993-2004.

RESULTS  53
Decreasing total trends in suspended sediment were 

not observed at sites 50 and 112; however, decreasing flowadjusted trends were observed at these two sites, indicating 

possible decreases in sediment sources within the drainage 

areas of these sites during the study period. Site 50 is located 

on the Bogue Phalia in the Yazoo River Basin in Mississippi. 

Land use within the drainage area of site 50 primarily is row 

crop production (cotton, soybean, and corn). Recent trends 

in tillage practices have indicated an increase in no-till or 

reduced tillage practices within Midsouth States since 1998 

(Delta Farm Press, 2003). Although the decreasing flowadjusted trends in suspended sediment at site 50 cannot be 

explained at this time, the decreasing trend could be influenced by recent shifts in row-crop agricultural practices from 

conventional tillage to no-till or reduced tillage.

Site 112 is located on the San Antonio River where 

multiple restoration and channel improvement projects have 

been completed since the mid- to late-1990s (for example, as 

documented in U.S. Army Corps of Engineers, 2004). These 

projects include flood control, ecological restorations, and 

recreational improvements. Decreases in suspended sediment 

at site 112 could, therefore, be a result of these restoration and 

improvement projects.

Increasing total and flow-adjusted trends in suspendedsediment concentration occurred at site 9 in the Mississippi 

system. Again, there were no recent references in literature 

explaining the increase at site 9. Davis and Bell (1998) state 

that a large percentage of the drainage area for site 9 is agricultural land use. The median suspended-sediment concentration listed in their report for site 9 was about 20 mg/L, 

which was similar in magnitude to the reference concentration 

listed in this study (about 12 mg/L, table 15).

Although increasing total trends in suspended sediment 

were not observed at site 1, increasing flow-adjusted trends 

were observed. Because of the size and complexity of the 

drainage area for site 1, an explanation as to the increase in 

flow-adjusted suspended sediment is beyond the scope of this 

report.

There were no trends observed in suspended-sediment 

loads at 20 sites (about 71 percent) during the study period 

(table 15). There were decreasing trends in suspended-sediment loads observed at eight sites for the study period, ranging 

from -8.8 to -4.4 percent per year (table 15, fig. 36). There 

were no increasing trends in suspended-sediment loads at 

study sites during the study period (table 15, fig. 36).

No trends in suspended-sediment loads were observed at 

site 1 for the study period (table 15) or for the period 1980-

2004 (fig. 37). The decreasing trends in suspended-sediment 

loads at sites 2 and 48 in the Mississippi system and at site 61 

in the Atchafalaya system appear to be part of recent trends 

during the last decade that began in the mid-1990s (fig. 37). 

The decreasing trends in suspended-sediment loads at site 53 

in the Mississippi system and at site 68 in the Atchafalaya 

system are part of a much longer decreasing trend in loads that 

has been occurring since 1980 (fig. 37).

Annual loads, average annual load, and yield calculations 

for suspended sediment were attempted for 24 study sites in 

two of the four river systems, the Mississippi and the Atchafalaya systems (table 16). Load calculations were not attempted 

for sites in the Louisiana-Gulf/Pontchartrain or the Texas-Gulf 

systems because there was an inadequate amount of data to 

attempt calculations, or poor model fit associated with the 

results. Suspended-sediment loads and yields for selected 

sites on the Mississippi, Arkansas, and Atchafalaya Rivers 

from this study for the period 1993-2004 were compared to 

loads and yields calculated by Kelly and others (2001) for the 

period 1996-2000 [suspended-sediment loads and yields from 

Kelly and others (2001) were used here as a basis for comparison because Goolsby and others (1999) did not include 

sediment data]. The mean annual suspended-sediment loads 

from sites 1, 2, 48, 53, and 68 from Kelly and others (2001) 

were 28,200,000; 89,400,000; 4,470,000; 94,400,000; and 

41,400,000 T, respectively. Suspended-sediment loads for 

the same sites calculated from this study were 29,200,000; 

110,000,000; 5,890,000; 100,000,000; and 48,500,000 T 

(table 16), respectively, indicating higher loads of suspended 

sediment for this study, which had a study period that was 

longer, more recent, and included the previous study period. 

Suspended-sediment yields calculated from Kelly and others 

(2001) were 53.5, 48.4, 10.9, 32.4, and 171 T∙km-2∙yr-1 for 

sites 1, 2, 48, 53, and 68. For this study, suspended-sediment 

yields for the same sites were 55.6, 59.8, 14.4, 34.3, and 201 

T∙km-2∙yr-1 indicating higher yields calculated for this study 

than those calculated in the previous study (table 4).

Relation of trends in suspended-sediment to 

trends in source data and landscape attributes

Population throughout most of the study area either 

remained the same or increased during the study period (fig. 

20). Such increasing trends would imply increasing trends 

in suspended-sediment data as forested areas are converted 

to urban areas to accommodate suburban expansion; however, results of the WLS regression indicated no statistically 

significant relation between trends in suspended-sediment data 

and regional patterns in population at the study sites (table 9). 

Results of the WLS regression likely are influenced more by 

the lack of sites analyzed for suspended-sediment trends than 

any other factor, as most of the trends in suspended sediment 

were observed at large sites that could mask any trends in 

population or other landscape changes.

There were no sites in which trends in conservation-practices data could be compared to trends in suspended sediment (table 10). Specifically, the lack of conservation tillage 

information for 1997 was especially critical because increases 

in acreages of conservation tillage in agricultural areas could 

cause decreasing trends in suspended sediment at those locations as is implied by the decreasing flow-adjusted trend at site 

50, previously mentioned.

54 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Figure 36. Trends in suspended-sediment loads at study sites, 1993-2004.

RESULTS  55
Overall conclusions about suspended-sediment 

trends and loads for the study area

Trend analyses of suspended-sediment data were 

attempted at 39 sites. No trends were observed at about 71 

percent of the sites. Remaining results indicated primarily 

decreasing trends in suspended sediment data. Most of the 

decreasing trends occurred on mainstem sites for the Mississippi, Arkansas, Red, and Atchafalaya Rivers, which are all 

regulated with reservoirs, locks and dams, and other erosion 

or flood-control structures that trap and prevent sediment from 

being transported downstream. Large decreases in suspended 

sediment in the Mississippi River basin began in the 1950s 

when large reservoirs were constructed in the Missouri and 

Arkansas Rivers, which were considered the largest sources of 

sediment at the time. Because the Mississippi River and its 

major tributaries have continued to be modified and improved 

since 1990, it is suggested that declines in suspended sediment 

observed along the mainstem sites during the study period are 

related to ongoing watershed and channel modifications.

It is important to note that, for this report, only suspended-sediment data were used in the analyses, and that few 

trend analyses were attempted because of lack of available 

data. Many agencies do not presently collect nor have historically collected suspended-sediment samples, but analyze water 

samples for total suspended solids (TSS). As pointed out by 

Figure 37. Annual suspended-sediment loads for selected study sites, 1980-2004.

56  Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Gray and others (2000), suspended sediment and TSS data 

are not comparable and should not be used interchangeably. 

Thus, additional sites where only TSS data were available 

were not included in this study. Sediment is considered one 

of the top three pollutants in streams and rivers in the United 

States (U.S. Environmental Protection Agency, 2006). Adoption of suspended sediment or TSS as the “analysis-of-choice” 

by agencies collecting sediment-related data would provide a 

consistent dataset to assess issues such as continued degradation or improvements due to restoration activities.

SUMMARY

The USGS-NAWQA Program is conducting regional 

assessments in eight major river basins focusing on chemicals 

in water, such as trends in nutrients, sediment, and pesticides, 

and other relevant water-quality issues, such as trends in 

biological-response data (chlorophyll, algae). This regional 

assessment explores trends in nutrient and suspended-sediment 

concentrations and loads for rivers in the south-central United 

States, which is the Lower Mississippi, Arkansas-White-Red, 

and Texas-Gulf Basin.

The primary source of water-chemistry and flow data for 

this study was data collected by the USGS. Since the early 

1970s, the USGS has collected water-quality information from 

major river basins throughout the United States as part of three 

national programs. In addition, other long-term water-quality monitoring stations operate as part of USGS cooperative 

projects in the various States. Other sources of water-chemistry data were data collected by State agencies within the 

study area as part of ambient data-collection programs. The 

final source of water-quality data considered for analysis was 

from the U.S. Environmental Protection Agency Legacy Data 

Center and the Storage and Retrieval database.

To explain trends in surface water-quality data, it was 

important to identify and understand temporal and spatial 

patterns in source data and landscape attributes. The nutrient 

source and landscape data included in this study were annual 

fertilizer-use data for nitrogen and phosphorus, 5-year compilations of manure-generation data for nitrogen and phosphorus, annual data for atmospheric deposition of nitrogen, 

population density data from the 1990 and 2000 census, and 

management practice information (including irrigation type 

and conservation practices for 1992 and 1997).

Based on specific selection criteria, 115 sites were 

selected for trend analysis for water years 1993-2004 (water 

year begins October 1 and ends September 30). There were 

sites that were included in this study that had sampling periods 

that started after October 1, 1992, or ended prior to September 

20, 2004. These sites were included in this study because of 

their importance relative to location or land-use type. Because 

site-selection criteria were primarily based on data availability, 

spatial representation of the selected sites was considered fair 

to poor in that there were areas that were under-represented 

such as in southern Kansas, most of Oklahoma, and parts of 

Texas and Louisiana. The selected sites were then grouped 

according to the four primary river systems in this study: Mississippi, Atchafalaya, Louisiana-Gulf/Pontchartrain, and the 

Texas-Gulf systems. These groupings were used as a basis for 

comparing trend, load, and yield results for major drainages 

entering the northwestern Gulf of Mexico.

Most rivers that empty into the Gulf of Mexico had 

sites that were included in this study with the exceptions of 

the Guadalupe River in Texas and the Calcasieu River in 

Louisiana. Two sites, one on the upper Mississippi River that 

included the Missouri River and another on the Ohio River, 

were outside of the study area but were included for analysis 

in order to document nutrient and sediment loadings entering 

the study area. Sites with small drainage areas were not discarded, although their overall contribution of nutrient and sediment loads was potentially insignificant within the drainage 

area of a large river basin. These sites were important because 

they provided valuable information related to specific land-use 

types. Also, sites with smaller drainage areas would provide 

the opportunity to document potentially dramatic changes in 

water quality over the past decade due to management changes 

or restoration activities.

For this study, both the total trend (not adjusted for flow) 

and a flow-adjusted trend were estimated to understand the 

overall picture of what was happening in relation to nutrient 

and sediment concentrations within the study area. Total trends 

could be used to determine impacts to aquatic communities. 

Flow-adjusted trends were estimated by removing the effects 

of streamflow on the trends in order to determine if changes in 

water quality were caused by something other than flow, such 

as landscape changes or changes in source. Other trend analyses completed were trends in load, which provided a direct 

measure of the effect of nutrients and sediment discharging to 

the northwestern Gulf of Mexico, and trends in flow, which 

improved interpretation of water-quality trends by understanding how flow has changed over time. Reference concentrations 

and loads were also computed for each statistically significant 

trend (reference concentrations and loads are best explained as 

the “starting point” of a trend line drawn through the data with 

a slope equal to the trend estimate).

The majority of study sites either had no trends (about 64 

percent of all trend analyses attempted) or decreasing trends 

in streamflow during the study period. The regional pattern 

of decreasing trends in streamflow during the study period 

appeared to correspond to moist conditions at the beginning 

of the study period and the influence of three drought periods during the study period, with the most extreme in 2000. 

Decreasing trends in streamflow at mainstem sites on the 

Mississippi River, Arkansas, Red, and Atchafalaya Rivers 

were specific to the study period and were not part of longterm trends. Increasing trends in streamflow at sites on the San 

Antonio River in the Texas-Gulf system were observed and 

likely were caused by moist conditions returning to southern Texas after the drought of 2000, coupled with increased 

urbanization and impervious surfaces during the study period. 

The increase in streamflow for the San Antonio River during 

SUMMARY  57
the study period appears to part of a recent decadal trend that 

started about 1997.

In general, there were few trends observed in the nitrogen data at study sites during the study period; no trends were 

observed in about 63 percent of all nitrogen trend analyses 

attempted. Although some patterns in the nitrogen data did 

exist where trends were attempted, no regional patterns could 

be confirmed because of poor spatial representation of the 

trends sites.

Decreasing trends in flow-adjusted concentrations of 

ammonia were observed at 25 sites. No increasing trends in 

concentrations of ammonia were noted at any sites. Flowadjusted concentrations of nitrite plus nitrate decreased at 7 

sites and increased at 14 sites. Flow-adjusted concentrations of total nitrogen decreased at 2 sites and increased at 

12 sites. Improvements to municipal wastewater treatment 

facilities contributed to the decline of ammonia concentrations at selected sites. Notable increasing trends in nitrite 

plus nitrate and total nitrogen at selected study sites were 

attributed to both point and nonpoint sources. Trend patterns 

in total nitrogen generally followed trend patterns in nitrite 

plus nitrate, which was understandable given that nitrite plus 

nitrate loads generally were 70-90 percent of the total nitrogen 

loads at most sites. Although population increased throughout 

the study area during the study period, there was no observed 

relation between increasing trends in nitrogen in study area 

streams and increasing trends in population. With respect 

to other nitrogen sources, statistical results did suggest that 

increasing trends in nitrogen could be related to increasing 

trends in nitrogen from either commercial fertilizer use or land 

application of manure.

Loads of ammonia, nitrite plus nitrate, and total nitrogen 

decreased during the study period, but some trends in nitrogen loads were part of long-term decreases since 1980. For 

example, ammonia loads were shown to decrease at nearly all 

sites over the past decade, but at selected sites, these decreasing trends were part of much longer trends since 1980. The 

Mississippi and Atchafalaya Rivers contributed the highest nitrogen loads to the northwestern Gulf of Mexico as 

expected; however, nitrogen yields from smaller rivers had 

similar or higher yields than from the Mississippi River.

Similar to results for nitrogen, there were few trends 

observed in the phosphorus data at study sites during the study 

period; no trends were observed in about 57 percent of all 

phosphorus trend analyses attempted. Although some patterns 

in the phosphorus data did exist where trend analyses were 

attempted, no regional patterns could be confirmed because of 

poor spatial representation of the trends sites.

Flow-adjusted concentrations of orthophosphorus 

decreased at 10 sites and increased at 7 sites. Flow-adjusted 

concentrations of total phosphorus decreased at 6 sites and 

increased at 17 sites. It was understandable that trend patterns 

in total phosphorus did not follow trend patterns in orthophosphorus given that orthophosphorus loads accounted for only 

about 20-30 percent of the total phosphorus load at comparable sites. Trends in population data were inversely related to 

trends in flow-adjusted total phosphorus; therefore, trends in 

population were not considered a controlling factor to explain 

trends in total phosphorus. No relation was observed between 

phosphorus from fertilizer use and either orthophosphorus or 

total phosphorus trends. However, statistical results did suggest that increasing trends in both orthophosphorus and total 

phosphorus could be related to increasing trends in phosphorus 

from land application of manure.

There were more decreasing trends than increasing trends 

in phosphorus loads during the past decade, most of which 

were unique to the recent decade and not part of long-term 

decreases since 1980. Similar to nitrogen loads, the Mississippi and Atchafalaya Rivers contributed the highest phosphorus loads to the northwestern Gulf of Mexico as expected; 

however, phosphorus yields from smaller rivers were similar 

to or higher than yields from the Mississippi River.

Trend analyses of suspended-sediment data were 

attempted for 39 sites. No trends were observed at about 71 

percent of the sites. Remaining results indicated primarily 

decreasing trends in suspended sediment data. Most of the 

decreasing trends occurred on mainstem sites for the Mississippi, Arkansas, Red, and Atchafalaya Rivers, which are all 

regulated with reservoirs, locks and dams, and other erosion 

or flood-control structures that trap and prevent sediment from 

being transported downstream. Large decreases in suspended 

sediment in the Mississippi River basin began in the 1950s 

when large reservoirs were constructed in the Missouri and 

Arkansas Rivers, which were considered the largest sources of 

sediment at the time. Because the Mississippi River and its 

major tributaries have continued to be modified and improved 

since 1990, it is suggested that declines in suspended sediment 

observed along the mainstem sites during the study period are 

related to ongoing watershed and channel modifications.

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Report 01-4255, available at URL http://pubs.usgs.gov/wri/

wri014255/.

Kleiss, B.A., Coupe, R.H., Gonthier, G.J., and Justus, B.G., 

2000, Water quality in the Mississippi Embayment, Mississippi, Louisiana, Arkansas, Missouri, Tennessee, and 

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1208, 36 p.

Land, L.F., Moring, J.B., Van Metre, P.C., Reutter, D.C., 

Mahler, B.J., Shipp, A.A., and Ulery, R.L., 1998, Water 

quality in the Trinity River Basin, 1992-2995: U.S. Geological Survey Circular 1171, 39 p.

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Watershed Protection Programs – Lake Pontchartrain Basin, 

accessed on October 16, 2006 at URL http://nps.ldeq.

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Meade, R.H., ed., 1995, Contaminants in the Mississippi 

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p.

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maximum daily load information sheet: Elk River Basin, 

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nonpoint source (NPS) implementation plan for MOA-2002 

Elk River watershed poultry manure composting project, 

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Mueller, D.K., and Spahr, N.E., 2005, Water-quality, streamflow, and ancillary data for nutrients in streams and ricers 

across the Nation, 1992-2001: U.S. Geological Survey Data 

Series 152, accessed on October 16, 2006 at URL http://

pubs.usgs.gov/ds/2005/152/.

Mueller, D.K., Hamilton, P.A., Helsel, D.R, Hitt, K.J., and 

Ruddy, B.C., 1995, Nutrients in ground water and surface 

water of the United States – an analysis of data through 

1992: U.S. Geological Survey Water-Resources Investigations Report 95-4031, 74 p. 

Nakagaki, Naomi, and Wolock, D.M., 2005, Estimation of 

agricultural pesticide use in drainage basins using land 

cover maps and county pesticide data: U.S. Geological 

Survey Open-File Report 2005-1188, 46 p.

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Historic Palmer drought indices: Accessed on January 4, 

2007, at URL http://lwf.ncdc.noaa.gov/oa/climate/research/

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Oblinger-Childress, C.J., Foreman, W.T., Connor, B.F., and 

Maloney, T.J., 1999, New reporting procedures based on 

long-term method detection levels and some considerations 

for interpretations of water-quality data provided by the U.S 

Geological Survey National Water Quality Laboratory: U.S. 

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The State of Oklahoma 2004 Water Quality Assessment 

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Protection Agency on October 10, 2006, accessed at URL: 

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for Oklahoma’s impaired scenic rivers, accessed on October 

16, 2006, at URL http://www.ose.state.ok.us/documents/

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degree days: National Oceanic and Atmospheric Administration, National Climatic Data Center, accessed April 2005 

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60 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
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REFERENCES  61
62 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 APPENDIX 

APPENDIX 63
 Total trend in concentration and trends in streamflow. 

The proposed estimates for total trend in concentration were 

derived from parameter estimates, and associated co-variances, 

obtained from a linear model of streamflow and an optimal 

model of water-quality concentration with both models being 

estimated in natural logarithm space. 

 The model of streamflow consisted of an intercept, a 

linear trend term (decimal time), sine and cosine functions 

of decimal time (the seasonal component of streamflow), 

and a serially correlated error term; the streamflow residual 

was assumed to follow an autoregressive process of order 

20 [AR(20)]. The streamflow model was estimated by using 

maximum likelihood methods as employed by the AUTOREG 

procedure in SAS 9, version 1, release 2 (SAS institute Inc., 

2004). For some sites, serial correlation in the residuals was 

not fully removed by the AR(20) model. Significant residual 

serial correlation could invalidate the standard error of the 

streamflow trend coefficient, although the practical importance 

of this effect for the statistical significance of the total trend 

was likely to be small.

 The water-quality model related the logarithm of nutrient 

or suspended sediment concentrations to various functions of 

streamflow, decimal time, and season as previously described. 

An abstract representation of the model is given by

 (1)

 where 

 c t is the natural logarithm of contaminant concentration in 

period t;

 q t is natural logarithm of streamflow;

 T t is decimal time;

 x t is a vector of ancillary predictors such as the sine and cosine 

functions of decimal time;

 e t is an independent and identically normally distributed random error;

 m ( . ) and h ( . ) are multi-element vector functions of q and T; and

 b 0 , b q , b T , and b x  are associated coefficients to be estimated.

 The multi-element vector function of the logarithm of 

streamflow, m ( . ), consisted of the logarithm of streamflow and 

the square of the logarithm of streamflow; h ( . ) the multi-element vector function of decimal time consisted of secondorder polynomial terms.

 The water-quality model was estimated by using either 

ordinary least squares if the water-quality data contained no 

censored observations or the maximum likelihood method if 

censored observations were present. The maximum likelihood 

bias adjustment required estimates of the detection level even 

for uncensored observations; the detection level was set equal 

to the maximum of the median detection level for all censored 

observations across all stations or the reported uncensored 

value.

 The estimate of total trend in concentration was based on 

the streamflow and time-trend coefficients from the waterquality model ( b q 

 and b T ), and the coefficient on decimal time 

in the streamflow model, subsequently denoted as a. The trend 

in the logarithm of streamflow during period t (between t 1 , the 

beginning of the streamflow analysis period in 1993, and t 2 , 

the end of the streamflow analysis period in 2004), , was 

defined as: 

 (2)

 where and were the means of the logarithm of 

streamflow and decimal time over the analysis period. If 

streamflow was upward trending, then a was positive and 

trend in the logarithm of streamflow was below the mean 

value of the logarithm of streamflow for the first half of the 

analysis period and above the mean value thereafter. Note 

that the average of the logarithm of streamflow, , implicitly 

accounted for the intercept and average of the seasonal terms 

that are included in the streamflow model but not otherwise 

apparent in the formulation of equation (2).

 Total trend in water-quality concentrations in period t, 

(between t 1 , the beginning of the streamflow analysis period 

in 1993, and t 2 , the end of the streamflow analysis period in 

2004), , was defined as:

 (3)

 Only terms involving trend were included in equation (3). 

Note that in forming this estimate, the trend in the logarithm 

of streamflow was substituted for the actual logarithm of flow 

in the function m ( . ). This substitution implies that variations 

in streamflow not reflected in the trend did not determine the 

proposed measure of total trend in water quality. Because of 

the nonlinearity of the function m ( . ), this might have led to a 

bias in the evaluation of full water-quality trend if flows were 

becoming more or less variable over time. The streamflow 

and concentration trends may have been calculated for slightly 

different periods because these analyses were completed as 

part of a larger national analysis that required flexibility in 

the streamflow period. The difference in the final total trend 

results from different streamflow periods and the streamflow 

period 1993 to 2004 likely is small.

 Appendix. Methodology for estimation of total trend and flow-adjusted trends in 

concentration, trends in streamflow, and trends in load 

 Based on written communication from Greg Schwarz, U.S. Geological Survey, July 24, 2006 

64 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 The total trend in water-quality concentration over the 

analysis period, , is given by

 (4)

 The total trend in water-quality concentration depends on 

the trend and streamflow coefficients from the water-quality 

model, bq and bT , as well as the trend coefficient a from the 

streamflow model. 

 The total trend, expressed as the average percent change 

per year, is given by

 (5)

 where is .

 The estimate of total trend was obtained by substituting sample estimates for the population values of a, b q 

 and b T 

in equation (4). The standard error of the resulting estimate 

was complicated to derive owing to the nonlinear manner in 

which the streamflow trend coefficient and the water-quality 

and streamflow coefficients interact in the determination of 

total trend. An approximation to the standard error suitable 

for large samples was obtained by taking a first-order Taylor approximation of the total trend estimate from equation 

(4) with respect to the streamflow and water-quality model 

coefficients. The vector of combined streamflow and trend 

coefficients from the water-quality model was represented 

by , and V b represented the covariance matrix 

of this vector. Under the plausible assumption that streamflow 

was exogenous with respect to water quality, meaning that 

changes in streamflow caused changes in water quality but 

changes in water quality did not cause changes in streamflow, 

the covariance between the estimated values of a and b was 

zero. Consequently, the standard error of , denoted , was 

defined as:

 (6)

 where 

 V a  is the variance of the estimated flow trend coefficient, a;

 (7)

 (8)

 In large samples, the t-statistics / was distributed 

standard normal; therefore, the two-sided p-value for significance of trend is given by

 (9)

 where 

 was , and 

 is the standard normal cumulative distribution.

 Trend results were considered statistically significant if 

the p-value was less than or equal to 0.05.

 Trends in flow-adjusted concentration. The estimation 

of flow-adjusted trend in concentration is similar to total trend, 

the only difference being that the streamflow component of 

the water-quality model in equation (2) is not included in the 

determination of the smoothed water-quality trend; otherwise, 

the estimation methods are the same.

 Trends in load. The trend in load, , is defined similarly to total trend in concentration as seen in equation (4) but 

includes an additional term to reflect the direct effect streamflow has on the determination of load as follows:

 (10)

 The full trend in load, expressed in percent per year, is 

given by equation (5) where is .

 The standard error of , denoted by , was defined as:

 (11)

 where is the variance of the estimated flow trend coefficient, a, from equation (7), and A is estimated from equation 

(8).

 The two-sided p-value for significance of trend in load is 

defined from equation (9) where is , and is the 

standard normal cumulative distribution.

APPENDIX 65
66 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 TABLES 

TABLES 67
 Map

 number

 (fig. 5) 

 Water- quality station

 number 

 Streamflow 

station 

number 

 Station name 

 Drainage

 area, square

 kilometers 

 Trends

 calculated, 

1993-2004 

 Loads and 

yields, 

calculated 

and tabled, 

1993-2004 

 Loads,

 calculated

 and 

graphed, 

1980-2004 

 Mississippi system 

1 03612500 03611500 Ohio River at Dam 53 near Grand Chain, Ill. 526,000 x x x2 07022000 07022000 Mississippi River at Thebes, Ill. 1,847,000 x x x3 07031740 07031740 Wolf River at Hollywood Street at Memphis, Tenn. 2,041 x

 St. Francis River 

4 07047810 07047810 St. Francis River Floodway near Marked Tree, Ark. 12,050 x x5 07047942 07047942 L’Anguille River near Colt, Ark. 1,386 x x

 White River 

6 07050500 07050500 Kings River near Berryville, Ark. 1,365 x

7 07052152 07052152 Wilson Creek near Brookline, Mo. 102 x8 07052250 07052250 James River near Boaz, Mo. 1,197 x9 07053250 07053250 Yocum Creek near Oak Grove, Ark. 137 x x

Buffalo River 

10 07055646 07055646 Buffalo River near Boxley, Ark. 149 x

11 07056000 07056000 Buffalo River near St. Joe, Ark. 2,147 x x

12 07060500 07060500 White River at Calico Rock, Ark. 25,840 x x13 07060710 07060710 North Sylamore Creek near Fifty Six, Ark. 150 x x

Black River 

14 07061600 07061500 Black River below Annapolis, Mo. 1,277 x

Current River 

15 07066110 07066000 Jacks Fork above Two River, Mo. 1,101 x x16 07068000 07068000 Current River at Doniphan, Mo. 5,278 x x17 07071500 07071500 Eleven Point River near Bardley, Mo. 2,054 x x18 07077500 07077500 Cache River at Patterson, Ark. 2,686 x x19 07077700 07077700 Bayou DeView near Morton, Ark. 1,090 x x

 Arkansas River 

Arkansas River Headwater Tributaries in Colorado 

20 07093740 07093740 Badger Creek, upper station, near Howard, Colo. 275 x21 07103700 07103700 Fountain Creek near Colorado Springs, Colo. 267 x x22 07103780 07103780 Monument Creek above North Gate Boulevard at USAF Academy, Colo. 212 x x

 Table 1. Stream sites in south-central United States selected for trend and load computations of nutrient and sediment data  [Water-quality data are provided by the U.S. Geological Survey except those highlighted in bold , which are provided by State agencies; dates listed are water years]

68 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map

 number

 (fig. 5) 

 Water- quality station

 number 

 Streamflow 

station 

number 

 Station name 

 Drainage

 area, square

 kilometers 

 Trends

 calculated, 

1993-2004 

 Loads and 

yields, 

calculated 

and tabled, 

1993-2004 

 Loads,

 calculated

 and 

graphed, 

1980-2004 

23 07104905 07104905 Monument Creek at Bijou Street at Colorado Springs, Colo. 609 x24 07105500 07105500 Fountain Creek at Colorado Springs, Colo. 1,015 x x25 07105530 07105530 Fountain Creek below Janitell Road below Colorado Springs, Colo. 1,070 x x26 07106300 07106300 Fountain Creek near Pinion, Colo. 2,209 x x27 07106500 07106500 Fountain Creek at Pueblo, Colo. 2,398 x28 07189000 07189000 Elk River near Tiff City, Mo. 2,258 x x

Illinois River 

29 07195000 07195000 Osage Creek near Elm Springs, Ark. 337 x

30 07195500 07195500 Illinois River near Watts, Okla. 1,645 x x31 07195865 07195865 Sager Creek near West Siloam Springs, Okla. 49 x x32 07196000 07196000 Flint Creek near Kansas, Okla. 285 x x33 07196500 07196500 Illinois River near Tahlequah, Okla. 2,484 x x34 07196900 07196900 Baron Fork at Dutch Mills, Ark. 105 x x35 07197000 07197000 Baron Fork at Eldon, Okla. 795 x x

Canadian River 

36 07227500 07227500 Canadian River near Amarillo, Tex. 50,360 x37 07228000 07228000 Canadian River near Canadian, Tex. 59,220 x

38 07239450 07239450 North Canadian River near Calumet, Okla. 33,570 x x39 07241000 07241000 North Canadian River below Lake Overholser near Oklahoma City, Okla. 34,240 x x40 07241520 07241520 North Canadian River at Britton Road at Oklahoma City, Okla. 34,740 x41 07241550 07241550 North Canadian River near Harrah, Okla. 34,970 x

Poteau River 

42 07247015 07247015 Poteau River at Loving, Okla. 697 x x43 07247250 07247250 Black Fork below Big Creek near Page, Okla. 193 x x44 07247345 07247250 Black Fork at Hodgen, Okla. 464 x x

Table 1. Stream sites in south-central United States selected for trend and load computations of nutrient and sediment data--Continued

TABLES 69
 Map

 number

 (fig. 5) 

 Water- quality station

 number 

 Streamflow 

station 

number 

 Station name 

 Drainage

 area, square

 kilometers 

 Trends

 calculated, 

1993-2004 

 Loads and 

yields, 

calculated 

and tabled, 

1993-2004 

 Loads,

 calculated

 and 

graphed, 

1980-2004 

45 07247650 07247500 Fourche Maline near Leflore, Okla. 699 x x46 07249400 07249400 James Fork near Hackett, Ark. 381 x x47 07263295 07263295 Maumelle River at Williams Junction, Ark. 119 x48 07263620 07263450 Arkansas River at David D. Terry Lock and Dam below Little Rock, Ark. 410,000 x x x

 Yazoo River 

49 07268000 07268000 Little Tallahatchie River at Etta, Miss. 1,362 x x50 07288650 07288650 Bogue Phalia near Leland, Miss. 1,254 x x51 07288955 07288955 Yazoo River below Steele Bayou near Long Lake, Miss. 34,590 x x52 07290650 07290650 Bayou Pierre near Willows, Miss. 1,694 x x53 07373420 07295100* Mississippi River near St. Francisville, La. 2,915,000 x x x

 Atchafalaya system 

 Red River 

54 07299540 07299540 Prairie Dog Town Fork Red River near Childress, Tex. 20,010 x

55 07300000 07300000 Salt Fork Red River near Wellington, Tex. 3,165 x56 07308500 07308500 Red River near Burkburnett, Tex. 53,280 x x

Wichita River 

57 07311700 07311700 North Wichita River near Truscott, Tex. 2,427 x x58 07311800 07311800 South Wichita River near Benjamin, Tex. 1,513 x x59 07331000 07331000 Washita River near Dickson, Okla. 18,650 x x60 07335500 07335500 Red River at Arthur City, Tex. 115,300 x x

61 07337000 07337000 Red River at Index, Ark. 124,400 x x62 07340300 07340300 Cossatot River near Vandervoort, Ark. 232 x63 07343000 07343000 North Sulphur River near Cooper, Tex. 715 x64 07355500 07355500 Red River at Alexandria, La. 174,800 x x x

Ouachita River 

65 07362000 07362000 Ouachita River at Camden, Ark. 13,880 x x66 07362587 07362587 Alum Fork Saline River near Reform, Ark. 70 x67 07373000 07373000 Big Creek at Pollock, La. 132 x

 Table 1. Stream sites in south-central United States selected for trend and load computations of nutrient and sediment data--Continued 

70 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map

 number

 (fig. 5) 

 Water- quality station

 number 

 Streamflow 

station 

number 

 Station name 

 Drainage

 area, square

 kilometers 

 Trends

 calculated, 

1993-2004 

 Loads and 

yields, 

calculated 

and tabled, 

1993-2004 

 Loads,

 calculated

 and 

graphed, 

1980-2004 

68 07381495* 07381490* Atchafalaya River at Melville, La. 241,700 x x x69 07381600 07381600 Lower Atchafalaya River at Morgan City, La. indeterminate x x

 Louisiana-Gulf/Pontchartrain system 

70 07375500 07375500 Tangipahoa River at Robert, La. 1,673 x x71 07376000 07376000 Tickfaw River at Holden, La. 640 x x72 07385765 07385765 Bayou Teche at Adeline Bridge near Jeanerette, La. indeterminate x x73 07386980 07386980 Vermilion River at Perry, La. indeterminate x74 08012150 08012150 Mermentau River at Mermentau, La. 3,577 x

 Texas-Gulf system 

75 08030500 08030500 Sabine River near Ruliff, Tex. 24,160 x

 Neches River 

76 08032000 08032000 Neches River near Neches, Tex. 2,966 x x77 08033500 08033500 Neches River near Rockland, Tex. 9,417 x x78 08041000 08041000 Neches River at Evadale, Tex. 20,590 x x

 Trinity River 

79 08044500 08044500 West Fork Trinity River near Boyd, Tex. 4,468 x

80 08049500 08049500 West Fork Trinity River at Grand Prairie, Tex. 7,938 x81 08051500 08051500 Clear Creek near Sanger, Tex. 764 x x82 08057410 08057410 Trinity River below Dallas, Tex. 16,260 x83 08062500 08062500 Trinity River near Rosser, Tex. 21,100 x x84 08064100 08064100 Chambers Creek near Rice, Tex. 2,090 x x85 08064700 08064700 Tehuacana Creek near Streetman, Tex. 368 x x86 08065000 08065000 Trinity River near Oakwood, Tex. 33,240 x x87 08065350 08065350 Trinity River near Crockett, Tex. 36,030 x x88 08066500 08066500 Trinity River at Romayor, Tex. 44,510 x89 08069000 08069000 Cypress Creek near Westfield, Tex. 738 x90 08070200 08070200 East Fork San Jacinto River near New Caney, Tex. 1,005 x91 08073500 08073500 Buffalo Bayou near Addicks, Tex. 759 x

Table 1. Stream sites in south-central United States selected for trend and load computations of nutrient and sediment data--Continued

TABLES 71
 Map

 number

 (fig. 5) 

 Water- quality station

 number 

 Streamflow 

station 

number 

 Station name 

 Drainage

 area, square

 kilometers 

 Trends

 calculated, 

1993-2004 

 Loads and 

yields, 

calculated 

and tabled, 

1993-2004 

 Loads,

 calculated

 and 

graphed, 

1980-2004 

92 08074500 08074500 Whiteoak Bayou at Houston, Tex. 224 x x93 08078000 08078000 Chocolate Bayou near Alvin, Tex. 227 x x

 Brazos River 

94 08085500 08085500 Clear Fork Brazos River at Ft. Griffin, Tex. 10,330 x

95 08114000 08114000 Brazos River at Richmond, Tex. 116,800 x96 08117500 08117500 San Bernard River near Boling, Tex. 1,883 x x

 Colorado River 

97 08123850 08123850 Colorado River above Silver, Tex. 38,620 x98 08136500 08136500 Concho River at Paint Rock, Tex. 17,030 x x99 08143600 08143600 Pecan Bayou near Mullin, Tex. 5,369 x

100 08147000 08147000 Colorado River near San Saba, Tex. 80,850 x x

Colorado River tributaries near Austin, Tex. 

101 08154700 08154700 Bull Creek at Loop 360 near Austin, Tex. 58 x x102 08155200 08155200 Barton Creek at State Highway 71 near Oak Hill, Tex. 232 x x103 08155240 08155240 Barton Creek at Lost Creek Boulevard near Austin, Tex. 277 x x104 08155300 08155300 Barton Creek at Loop 360, Austin, Tex. 300 x x105 08156800 08156800 Shoal Creek at West 12th Street, Austin, Tex. 32 x x106 08159000 08159000 Onion Creek at US Hwy 183, Austin, Tex. 831 x

107 08162000 08162000 Colorado River at Wharton, Tex. 108,800 x x

108 08162500 08162500 Colorado River near Bay City, Tex. 109,400 x x

109 08162600 08162600 Tres Palacios River near Midfield, Tex. 376 x

 San Antonio River 

110 08178565 08178565 San Antonio River at Loop 410, San Antonio, Tex. 324 x

111 08178800 08178800 Salado Creek at Loop 13 at San Antonio, Tex. 490 x

112 08181800 08181800 San Antonio River near Elmendorf, Tex. 4,514 x x x113 08188500 08188500 San Antonio River at Goliad, Tex. 10,160 x

114 08189500 08189500 Mission River at Refugio, Tex. 1,787 x115 08210000 08210000 Nueces River near Three Rivers, Tex. 39,960 x x

 * Site operated by U.S. Army Corps of Engineers 

 Table 1. Stream sites in south-central United States selected for trend and load computations of nutrient and sediment data--Continued 

72 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station number Trend in daily flow begin date Trend in daily flow end date Trend in flow, 

percent per year Trend in flow, p-value 

 Mississippi system 

1 03612500 10/1/1992 9/30/2004 no trend 0.80

2 07022000 10/1/1992 9/30/2004 -4.3 0.000

3 07031740 2/1/1995 9/30/2004 no trend 0.57

4 07047810 10/1/1992 9/30/2004 no trend 0.18

5 07047942 10/1/1992 9/30/2004 no trend 0.41

6 07050500 10/1/1992 9/30/2004 -5.2 0.002

9 07053250 4/15/1993 9/30/2004 -4.8 0.000

10 07055646 4/17/1993 9/26/2004 no trend 0.44

11 07056000 10/1/1992 9/30/2004 -4.0 0.043

12 07060500 10/1/1992 9/30/2004 -5.2 0.001

13 07060710 10/1/1992 9/30/2004 -4.7 0.001

14 07061600 10/1/1992 9/30/2004 -3.5 0.003

15 07066110 10/1/1992 9/30/2004 -3.8 0.003

16 07068000 10/1/1992 9/30/2004 -3.6 0.000

17 07071500 10/1/1992 9/30/2004 -4.5 0.004

18 07077500 10/1/1996 9/30/2004 no trend 0.31

20 07093740 10/1/1992 9/30/2003 -7.0 0.000

21 07103700 10/1/1992 9/30/2004 no trend 0.21

22 07103780 10/1/1992 9/30/2004 no trend 0.38

24 07105500 10/1/1992 9/30/2004 no trend 0.32

25 07105530 10/1/1992 9/30/2004 no trend 0.47

26 07106300 10/1/1992 9/30/2004 no trend 0.87

27 07106500 10/1/1992 9/30/2004 no trend 0.74

28 07189000 10/1/1992 9/30/2004 -5.0 0.001

29 07195000 7/13/1995 9/30/2004 no trend 0.82

30 07195500 10/1/1992 9/30/2004 -3.9 0.004

31 07195865 9/12/1996 9/30/2004 no trend 0.83

32 07196000 10/1/1992 9/30/2004 -3.5 0.049

33 07196500 10/1/1992 9/30/2004 -3.8 0.010

34 07196900 10/1/1992 9/30/2004 -4.5 0.046

35 07197000 10/1/1992 9/30/2004 -4.5 0.006

36 07227500 10/1/1992 9/30/2004 -6.6 0.020

37 07228000 10/1/1992 9/30/2004 no trend 0.30

38 07239450 10/1/1992 9/30/2004 no trend 0.19

39 07241000 10/1/1992 9/30/2004 -7.3 0.014

 Table 2. Analytical results for trends in daily streamflow for sites in the Lower-Mississippi-Texas Basin for water years 1993-2004 

TABLES 73
 Map 

number

 (fig. 5) 

 Station number Trend in daily flow begin date Trend in daily flow end date Trend in flow, 

percent per year Trend in flow, p-value 

40 07241520 10/1/1992 9/30/2004 -6.0 0.011

41 07241550 10/1/1992 9/30/2004 -4.8 0.008

42 07247015 10/1/1992 9/30/2004 no trend 0.088

43 07247250 10/1/1992 9/30/2004 no trend 0.12

44 07247345 10/1/1992 9/30/2004 no trend 0.12

45 07247650 10/1/1992 9/30/2004 -5.6 0.023

46 07249400 10/1/1992 9/30/2004 -5.8 0.001

47 07263295 11/11/1992 9/29/2004 -4.8 0.030

48 07263620 10/1/1992 9/30/2004 -4.9 0.019

49 07268000 10/1/1992 9/30/2004 no trend 0.96

50 07288650 10/1/1995 9/30/2004 no trend 0.18

51 07288955 10/1/1995 9/30/2004 no trend 0.17

52 07290650 10/1/1992 9/30/2004 no trend 0.41

53 07373420 10/1/1992 9/30/2004 -2.2 0.039

 Atchafalaya system 

54 07299540 10/1/1992 9/30/2004 no trend 0.44

55 07300000 10/1/1992 9/30/2004 no trend 0.40

56 07308500 10/1/1992 9/30/2004 -7.3 0.000

57 07311700 10/1/1992 9/30/2004 -6.0 0.000

58 07311800 10/1/1992 9/30/2004 no trend 0.063

59 07331000 10/1/1992 9/30/2004 -6.6 0.000

60 07335500 10/1/1992 9/30/2004 -5.9 0.000

61 07337000 10/1/1992 9/30/2004 -5.8 0.000

62 07340300 10/1/1992 9/30/2004 -3.0 0.021

63 07343000 10/1/1992 9/19/2004 no trend 0.48

64 07355500 10/1/1992 9/30/2004 no trend 0.44

65 07362000 10/1/1992 9/30/2004 no trend 0.69

66 07362587 10/1/1992 9/13/2004 no trend 0.34

67 07373000 10/1/1992 9/30/2004 no trend 0.60

68 07381495 10/1/1992 9/30/2004 -2.2 0.030

69 07381600 10/1/1995 9/30/2004 no trend 0.74

 Louisiana-Gulf/Pontchartrain system 

70 07375500 10/1/1992 9/30/2004 no trend 0.19

71 07376000 10/1/1992 9/30/2004 no trend 0.066

73 07386980 10/1/1992 9/30/2004 no trend 0.46

74 08012150 10/1/1992 9/29/2004 no trend 0.26

Table 2. Analytical results for trends in daily streamflow for sites in the Lower-Mississippi-Texas Basin for water years 1993-2004--

Continued

74 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station number Trend in daily flow begin date Trend in daily flow end date Trend in flow, 

percent per year Trend in flow, p-value 

 Texas-Gulf system 

75 08030500 10/1/1992 9/30/2004 no trend 0.86

76 08032000 10/1/1992 9/30/2004 no trend 0.39

77 08033500 10/1/1992 9/30/2004 no trend 0.93

78 08041000 10/1/1992 9/30/2004 no trend 0.82

79 08044500 10/1/1992 9/30/2004 -5.2 0.004

80 08049500 10/1/1992 9/30/2004 no trend 0.20

81 08051500 10/1/1992 9/30/2004 no trend 0.070

82 08057410 10/1/1992 9/30/2004 no trend 0.25

83 08062500 10/1/1992 9/30/2004 no trend 0.16

84 08064100 10/1/1992 9/30/2004 no trend 0.81

85 08064700 10/1/1992 9/30/2004 no trend 0.69

86 08065000 10/1/1992 9/30/2004 no trend 0.28

87 08065350 10/1/1992 9/30/2004 no trend 0.15

88 08066500 10/1/1992 9/30/2004 no trend 0.63

89 08069000 10/1/1992 9/30/2004 no trend 0.10

90 08070200 10/1/1992 9/30/2004 no trend 0.76

91 08073500 10/1/1992 9/30/2004 no trend 0.63

92 08074500 10/1/1992 9/30/2004 no trend 0.12

93 08078000 10/1/1992 9/30/2004 no trend 0.72

94 08085500 10/1/1992 9/30/2004 -7.0 0.003

95 08114000 10/1/1992 9/30/2004 no trend 0.94

96 08117500 10/1/1992 9/30/2004 no trend 0.69

97 08123850 10/1/1992 9/30/2004 -6.9 0.004

98 08136500 10/1/1992 9/30/2004 -8.2 0.0

99 08143600 10/1/1992 9/30/2004 no trend 0.59

100 08147000 10/1/1992 9/30/2004 no trend 0.15

101 08154700 10/1/1992 9/30/2004 no trend 0.61

102 08155200 10/1/1992 9/30/2004 no trend 0.26

103 08155240 10/1/1992 9/30/2004 no trend 0.29

104 08155300 11/19/1992 8/5/2004 no trend 0.36

105 08156800 10/7/1992 9/18/2004 -5.7 0.008

106 08159000 10/1/1992 9/27/2004 no trend 0.35

107 08162000 10/1/1992 9/30/2004 no trend 0.62

108 08162500 10/1/1992 9/30/2004 no trend 0.83

109 08162600 10/1/1992 9/30/2004 no trend 0.77

 Table 2. Analytical results for trends in daily streamflow for sites in the Lower-Mississippi-Texas Basin for water years 1993-2004--

Continued 

TABLES 75
 Map 

number

 (fig. 5) 

 Station number Trend in daily flow begin date Trend in daily flow end date Trend in flow, 

percent per year Trend in flow, p-value 

110 08178565 10/1/1992 9/30/2004 no trend 0.37

111 08178800 10/1/1992 9/30/2004 no trend 0.63

112 08181800 10/1/1992 9/30/2004 7.89 0.037

113 08188500 10/1/1992 9/30/2004 10 0.026

114 08189500 10/1/1992 9/30/2004 no trend 0.25

115 08210000 10/1/1992 9/30/2004 no trend 0.11

Table 2. Analytical results for trends in daily streamflow for sites in the Lower-Mississippi-Texas Basin for water years 1993-2004--

Continued

76 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Water- quality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total trend 

reference 

concentration, mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted trend, 

model pvalue 

 Flow-

 adjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 147 61 -6.3 3.9E-10 0.06 -6.3 3.5E-9 0.06 -6.4 2.5E-7 34,300 2 07022000 7/20/1993 9/1/2004 141 55 -6.9 1.3E-8 0.08 -6.6 3.3E-6 0.08 -7.9 5.2E-12 52,600 5 07047942 11/11/1992 9/1/2004 71 6 no trend 0.14 no trend 0.16 no trend 0.506 07050500 4/28/1993 8/31/2004 69 31 N/A N/A N/A9 07053250 4/26/1993 8/25/2004 136 85 N/A N/A N/A

11 07056000 5/6/1993 3/23/2004 126 97 N/A N/A N/A

12 07060500 11/17/1992 8/25/2004 71 32 -4.0 0.017 0.02 -4.3 0.021 0.02 -6.8 2.7E-5 505 13 07060710 11/17/1992 8/24/2004 120 85 N/A N/A N/A14 07061600 5/12/1993 9/7/2004 0 0 N/A N/A N/A18 07077500 11/10/1992 9/1/2004 92 10 no trend 0.076 -7.7 0.044 0.04 -9.2 0.022 57 19 07077700 11/11/1992 9/1/2004 75 5 -8.9 0.011 0.14 -8.9 0.020 0.14 no trend 0.9921 07103700 1/28/1993 8/4/2004 99 61 N/A N/A N/A22 07103780 1/27/1993 10/28/2003 89 19 no trend 0.87 no trend 0.86 no trend 0.5823 07104905 1/27/1993 8/23/2004 92 33 N/A N/A N/A24 07105500 1/28/1993 8/4/2004 116 32 N/A N/A N/A25 07105530 1/28/1993 8/4/2004 106 2 -8.6 0.00 N/A -8.6 0.00 N/A -8.6 0.00 N/A 26 07106300 3/26/1993 7/22/2004 56 18 -7.4 0.008 0.05 -7.4 0.009 0.05 -7.4 0.009 15 27 07106500 1/29/1993 7/22/2004 105 29 -6.8 0.001 0.06 -6.7 0.001 0.06 -7.0 0.007 21 28 07189000 4/27/1993 9/14/2004 111 77 N/A N/A N/A29 07195000 8/3/1995 9/1/2004 49 17 no trend 0.12 no trend 0.13 no trend 0.14

 Table 3. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for ammonia data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but results not available because of poor model fit or incomplete datasets; reference concentrations and loads are left blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A;” there were no sites in the Louisiana-Gulf/Pontchartrain river system with an adequate amount of ammonia data to complete trend analyses] 

TABLES 77
 Map 

number

 (fig. 5) 

 Station 

number 

 Water- quality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total trend 

reference 

concentration, mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted trend, 

model pvalue 

 Flow-

 adjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

30 07195500 10/7/1992 8/17/2004 111 35 -4.4 0.017 0.03 no trend 0.070 -6.3 0.001 42 31 07195865 10/6/1992 8/18/2004 87 40 N/A N/A N/A32 07196000 10/6/1992 8/18/2004 108 49 N/A N/A N/A33 07196500 10/14/1992 8/17/2004 127 66 -4.8 0.001 0.01 -4.4 0.005 0.01 -6.5 0.000 25 34 07196900 6/28/1995 9/1/2004 50 23 N/A N/A N/A35 07197000 10/13/1992 8/17/2004 114 54 -4.2 0.040 0.01 -4.9 0.011 0.01 -6.5 0.000 8 36 07227500 1/13/1993 8/5/2004 62 33 N/A N/A N/A37 07228000 11/16/1992 4/8/2003 0 0 N/A N/A N/A38 07239450 11/17/1992 9/8/2004 143 47 no trend 0.27 no trend 0.42 no trend 0.1239 07241000 10/14/1992 9/8/2004 144 19 no trend 0.33 -4.7 0.022 0.08 -7.7 0.001 54 40 07241520 10/14/1992 9/7/2004 132 20 no trend 0.43 no trend 0.15 -6.6 0.003 86 41 07241550 10/14/1992 9/7/2004 142 26 no trend 0.87 no trend 0.81 no trend 0.05842 07247015 10/13/1992 8/24/2004 91 26 -6.7 1.7E-9 0.04 -6.5 1.0E-7 0.04 -7.6 2.6E-5 7 43 07247250 10/15/1992 8/24/2004 80 36 -7.4 1.0E-8 0.03 -7.3 3.5E-7 0.03 -8.0 7.2E-5 4 44 07247345 10/14/1992 8/25/2004 78 30 N/A N/A N/A45 07247650 10/14/1992 8/25/2004 79 20 -6.0 5.1E-5 0.05 -5.8 0.000 0.05 -7.6 0.000 6 46 07249400 6/27/1995 9/7/2004 56 17 no trend 0.20 no trend 0.30 -8.2 0.003 2 47 07263295 12/15/1992 8/2/2004 57 28 N/A N/A N/A48 07263620 12/17/1992 9/7/2004 102 21 -4.1 0.018 0.06 -5.0 0.001 0.06 -6.6 4.8E-5 5,590 50 07288650 2/16/1996 9/8/2004 100 18 no trend 0.25 no trend 0.11 no trend 0.7951 07288955 2/14/1996 9/10/2004 148 34 no trend 0.68 no trend 0.63 no trend 0.4253 07373420 10/14/1992 9/14/2004 151 80 -5.3 9.3E-7 0.03 -5.4 5.3E-6 0.03 -6.1 1.5E-7 44,400 

Table 3. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for ammonia data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

78 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Water- quality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total trend 

reference 

concentration, mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted trend, 

model pvalue 

 Flow-

 adjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

 Atchafalaya system 

55 07300000 1/12/1993 8/12/2004 48 7 N/A N/A N/A56 07308500 11/18/1992 8/20/2004 103 36 N/A N/A N/A57 07311700 1/15/1993 9/14/2004 118 9 no trend 0.40 -4.0 0.015 0.08 -6.6 4.8E-7 8 58 07311800 1/15/1993 9/9/2004 96 14 no trend 0.43 no trend 0.60 no trend 0.2459 07331000 10/21/1992 9/1/2004 111 40 no trend 0.42 no trend 0.99 -7.1 0.001 163 61 07337000 11/17/1992 8/17/2004 66 9 no trend 0.56 no trend 0.79 -5.3 0.011 1,120 62 07340300 11/18/1992 8/18/2004 58 32 N/A N/A N/A63 07343000 10/27/1992 3/10/2003 0 0 N/A N/A N/A65 07362000 11/17/1992 8/17/2004 66 13 -3.7 0.048 0.03 no trend 0.056 no trend 0.1266 07362587 12/15/1992 8/2/2004 61 27 N/A N/A N/A67 07373000 10/27/1992 2/23/2004 0 0 N/A N/A N/A68 07381495 2/23/1993 9/13/2004 152 70 -6.1 4.1E-7 0.04 -6.2 1.5E-6 0.04 -6.8 3.2E-8 22,200 

 Texas-Gulf system 

76 08032000 1/21/1993 9/13/2004 70 23 -5.5 0.002 0.04 -5.4 0.003 0.04 -6.4 0.011 42 77 08033500 1/22/1993 9/2/2004 70 36 N/A N/A N/A78 08041000 6/16/1993 4/23/2003 0 0 N/A N/A N/A79 08044500 10/1/1992 2/19/2003 42 13 N/A N/A N/A80 08049500 2/9/1993 9/14/2004 64 10 -5.8 0.002 0.04 -5.9 0.002 0.04 -6.6 0.001 55 81 08051500 2/8/1993 8/18/2004 100 57 N/A N/A N/A82 08057410 4/21/1993 8/17/2004 85 14 N/A N/A N/A83 08062500 1/19/1993 9/8/2004 60 5 -4.9 0.015 0.07 -4.9 0.014 0.07 -6.1 0.006 523 84 08064100 4/20/1993 9/15/2004 126 63 -5.7 1.3E-5 0.02 -5.8 1.7E-6 0.02 no trend 0.5285 08064700 1/27/1993 9/9/2004 53 17 -5.4 0.005 0.04 -5.4 0.006 0.04 no trend 0.38

 Table 3. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for ammonia data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

TABLES 79
 Map 

number

 (fig. 5) 

 Station 

number 

 Water- quality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total trend 

reference 

concentration, mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted trend, 

model pvalue 

 Flow-

 adjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

86 08065000 11/12/1992 4/8/2003 72 23 no trend 0.44 no trend 0.39 no trend 0.8387 08065350 1/20/1993 9/13/2004 73 30 N/A N/A N/A89 08069000 11/4/1992 4/9/2003 111 9 N/A N/A N/A90 08070200 1/14/1993 4/28/2003 126 93 N/A N/A N/A91 08073500 11/19/1992 4/1/2003 98 20 -5.2 0.031 0.24 -5.1 0.038 0.24 no trend 0.2092 08074500 11/19/1992 4/29/2003 135 17 no trend 0.20 no trend 0.20 no trend 0.09393 08078000 12/7/1992 2/12/2003 38 16 no trend 0.31 no trend 0.30 no trend 0.4794 08085500 11/5/1992 2/4/2003 35 16 N/A N/A N/A95 08114000 1/26/1993 9/25/2002 44 23 N/A N/A N/A96 08117500 12/9/1992 4/9/2003 51 24 no trend 0.60 no trend 0.62 no trend 0.5297 08123850 11/3/1992 9/2/2003 55 16 N/A N/A N/A98 08136500 1/19/1993 6/24/2003 58 20 no trend 0.36 no trend 0.62 -9.4 5.2E-8 3 99 08143600 11/18/1992 4/16/2003 55 25 no trend 0.48 no trend 0.47 no trend 0.79

100 08147000 11/18/1992 4/16/2003 54 35 N/A N/A N/A101 08154700 1/14/1993 4/6/2004 45 21 no trend 0.89 no trend 0.95 no trend 0.63102 08155200 1/25/1993 4/6/2004 86 51 N/A N/A N/A103 08155240 11/20/1992 6/9/2004 77 37 N/A N/A N/A104 08155300 11/20/1992 3/4/2004 48 30 N/A N/A N/A105 08156800 12/14/1992 12/23/2002 38 10 no trend 0.61 no trend 0.58 no trend 0.36106 08159000 10/13/1992 4/24/2003 0 0 N/A N/A N/A107 08162000 10/14/1992 4/2/2003 55 35 N/A N/A N/A108 08162500 10/14/1992 2/4/2003 55 24 no trend 0.62 no trend 0.62 no trend 0.76109 08162600 6/8/1993 3/25/2003 60 18 no trend 0.22 no trend 0.34 no trend 0.28

Table 3. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for ammonia data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

80 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Water- quality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total trend 

reference 

concentration, mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted trend, 

model pvalue 

 Flow-

 adjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

110 08178565 10/20/1992 4/24/2003 75 18 N/A N/A N/A

111 08178800 1/11/1993 9/9/2004 129 48 N/A N/A N/A

112 08181800 2/9/1993 9/9/2004 134 27 -6.4 5.0E-6 0.07 -6.3 1.5E-5 0.07 no trend 0.079113 08188500 10/15/1992 4/21/2003 78 21 N/A N/A N/A114 08189500 10/14/1992 4/22/2003 37 13 N/A N/A N/A115 08210000 10/13/1992 9/23/2004 48 7 N/A N/A N/A

 Table 3. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for ammonia data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

TABLES 81
 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average annual 

yield, metric tons/

square kilometers/

year 

 Mississippi river system 

1 03612500 21,500 24,000 14,000 16,500 16,800 11,800 8,550 5,600 5,650 7,180 7,420 6,890 12,200 0.0232 07022000 43,900 26,900 20,300 14,900 19,300 17,100 15,900 6,320 9,510 7,600 4,210 5,430 15,900 0.0095 07047942 44 80 48 33 74 42 38 27 36 55 41 28 45 0.033

12 07060500 320 277 230 100 236 161 124 64 63 144 61 92 156 0.00618 07077500 9 53 40 30 30 41 27 33 0.01219 07077700 47 36 23 34 52 55 23 39 0.03522 07103780 0.5 1 1 1 1 1 1 1 0.4 1 1 0.00425 07105530 1,080 864 825 438 317 242 151 91 45 20 11 8 341 0.31926 07106300 11 11 13 10 9 9 8 7 5 3 2 2 8 0.00330 07195500 68 33 45 17 30 26 35 32 16 25 7 21 30 0.01833 07196500 53 28 45 16 34 31 38 50 20 20 5 16 30 0.01235 07197000 20 9 15 9 15 13 9 13 6 6 1 6 10 0.01338 07239450 23 11 16 8 45 36 35 21 24 3 11 11 20 0.00139 07241000 61 20 40 19 56 43 38 21 18 3 8 6 28 0.00142 07247015 18 18 22 8 25 14 15 6 9 12 2 2 13 0.01843 07247250 10 6 7 2 8 4 5 2 2 3 0.4 1 4 0.02245 07247650 35 22 26 15 22 16 24 4 18 16 2 4 17 0.02448 07263620 5,060 2,790 3,420 1,050 2,590 2,410 2,930 1,370 1,530 1,310 838 1,210 2,210 0.00550 07288650 36 97 66 52 73 98 66 66 69 0.05551 07288955 648 1,460 1,120 1,170 562 1,100 1,610 1,500 1,050 1,140 0.03353 07373420 26,200 22,000 16,100 13,600 18,400 14,200 12,500 6,440 8,790 9,510 7,610 7,400 13,600 0.005

 Atchafalaya river system 

57 07311700 5 3 6 3 4 3 3 2 2 2 2 1 3 0.00258 07311800 1 1 2 1 2 1 1 0.4 1 1 1 1 1 0.00159 07331000 271 91 202 67 169 153 59 40 145 55 35 35 110 0.006

 Table 4. Ammonia annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets; there were no sites in the Louisiana-Gulf/Pontchartrain river system that had an adequate amount of ammonia data to complete load calcuations] 

82 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average annual 

yield, metric tons/

square kilometers/

year 

61 07337000 838 546 721 296 751 686 384 218 758 559 275 215 520 0.00465 07362000 201 232 203 66 221 154 165 76 163 158 112 112 155 0.01168 07381495 14,900 12,200 8,720 7,290 9,640 7,330 6,280 3,180 4,270 4,510 3,520 3,350 7,100 0.029

 Texas-Gulf river system 

76 08032000 43 29 51 5 27 20 23 11 28 14 11 9 23 0.00883 08062500 412 397 798 151 683 554 363 445 271 148 135 396 0.01984 08064100 20 22 58 0.4 42 32 27 9 17 9 3 7 21 0.01085 08064700 2 3 3 0.1 6 2 5 1 4 1 2 1 3 0.00786 08065000 314 365 573 72 436 331 335 151 405 316 223 288 317 0.01092 08074500 40 27 40 25 50 41 37 21 55 34 44 56 39 0.17598 08136500 3 2 4 1 3 1 1 1 1 2 0.3 2 0.000

101 08154700 1 0.2 1 0.2 1 0.3 1 1 1 0.4 1 1 0.008105 08156800 0.4 0.3 1 0.4 0.5 0.3 0.2 1 0.4 1 1 0.017108 08162500 139 45 136 42 237 115 155 26 105 130 150 105 115 0.001

Table 4. Ammonia annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 83
 Table 5. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for nitrite plus nitrate data for 

sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but reults not available because of poor model fit or incomplete datasets; reference concentrations and loads are blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A”] 

 Map 

number

 (fig. 

5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted 

trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 147 0 no trend 0.42 no trend 0.43 no trend 0.542 07022000 11/11/1992 9/1/2004 158 1 -2.0 0.031 2.3 no trend 0.92 -5.4 7.5E-5 1,720,000 3 07031740 2/2/1993 6/2/2004 43 1 N/A N/A N/A5 07047942 11/11/1992 9/1/2004 71 3 no trend 0.053 no trend 0.076 no trend 0.516 07050500 10/13/1992 8/31/2004 83 2 N/A N/A N/A7 07052152 11/24/1993 7/22/2004 69 0 no trend 0.85 no trend 0.83 no trend 0.718 07052250 11/17/1992 7/21/2004 71 0 no trend 0.14 no trend 0.15 no trend 0.239 07053250 4/26/1993 8/25/2004 136 2 7.9 1.3E-5 2.3 9.6 4.5E-6 2.3 no trend 0.56

10 07055646 4/28/1993 8/25/2004 80 55 N/A N/A N/A

11 07056000 10/27/1992 3/23/2004 144 42 no trend 0.61 4.3 0.041 0.06 no trend 0.30

12 07060500 11/17/1992 8/25/2004 71 2 no trend 0.14 no trend 0.46 -6.0 0.004 6,910 13 07060710 11/17/1992 8/24/2004 120 30 no trend 0.21 no trend 0.59 -5.4 0.002 4 14 07061600 1/5/1993 9/7/2004 68 4 N/A N/A N/A15 07066110 11/12/1992 9/7/2004 83 0 no trend 0.18 no trend 0.12 -3.2 0.039 326 16 07068000 11/12/1992 9/15/2004 144 1 -1.8 0.010 0.27 no trend 0.52 -4.7 0.000 2,080 17 07071500 8/25/1993 9/15/2004 68 0 no trend 0.26 no trend 0.28 -5.1 0.013 1,220 18 07077500 11/10/1992 9/1/2004 92 5 no trend 0.45 no trend 0.38 no trend 0.1819 07077700 11/11/1992 9/1/2004 75 6 12 0.042 0.15 13 0.042 0.15 52 0.008 27 21 07103700 10/15/1992 8/4/2004 106 0 no trend 0.93 no trend 0.32 no trend 0.1422 07103780 10/14/1992 10/28/2003 92 8 39 1.4E-5 N/A 33 3.5E-5 N/A 24 0.003 2 

84 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Table 5. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for nitrite plus nitrate data for 

sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

 Map 

number

 (fig. 

5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted 

trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

23 07104905 10/14/1992 8/23/2004 100 0 N/A N/A N/A24 07105500 10/15/1992 8/4/2004 122 0 no trend 0.49 no trend 0.68 no trend 0.2825 07105530 10/15/1992 8/4/2004 111 0 4.5 0.000 1.7 3.7 1.3E-5 1.7 no trend 0.2426 07106300 12/18/1992 7/22/2004 57 0 -4.3 2.2E-9 5.1 -4.3 1.3E-7 5.1 -4.5 0.039 1,550 27 07106500 10/16/1992 7/22/2004 114 0 -5.1 0.00 5.4 -5.1 0.00 5.4 -5.5 0.001 1,770 28 07189000 11/18/1992 9/14/2004 117 0 no trend 0.44 4.7 3.4E-6 1.6 -4.7 0.040 2,620 29 07195000 10/13/1992 9/1/2004 69 1 N/A N/A N/A30 07195500 10/7/1992 8/17/2004 111 0 no trend 0.24 no trend 0.32 -4.7 0.003 3,350 31 07195865 10/6/1992 8/18/2004 87 0 no trend 0.074 no trend 0.058 no trend 0.3032 07196000 10/6/1992 8/18/2004 109 0 4.2 4.6E-11 1.9 4.7 5.5E-12 1.9 no trend 0.6733 07196500 10/14/1992 8/17/2004 127 0 no trend 0.19 no trend 0.40 -4.6 0.023 3360 34 07196900 11/3/1992 9/1/2004 67 1 no trend 0.96 no trend 0.44 no trend 0.1735 07197000 10/13/1992 8/17/2004 114 0 no trend 0.94 no trend 0.056 no trend 0.05236 07227500 11/17/1992 8/5/2004 63 10 30 0.004 0.16 57 0.000 0.16 no trend 1.038 07239450 11/17/1992 9/8/2004 143 69 -5.8 0.029 N/A no trend 0.077 -7.2 0.048 40 39 07241000 10/14/1992 9/8/2004 144 26 -4.6 0.014 0.26 -4.8 0.009 0.26 -7.9 0.001 169 40 07241520 10/14/1992 9/7/2004 132 6 no trend 0.099 no trend 0.80 -4.4 0.029 648 41 07241550 10/14/1992 9/7/2004 142 0 7.5 0.021 1.1 no trend 0.74 no trend 0.3042 07247015 10/13/1992 8/24/2004 91 17 no trend 0.16 5.9 0.048 0.09 no trend 0.5343 07247250 10/15/1992 8/24/2004 80 21 no trend 0.48 no trend 0.050 no trend 0.3744 07247345 10/14/1992 8/25/2004 78 27 no trend 0.41 no trend 0.10 no trend 0.4145 07247650 10/14/1992 8/25/2004 79 19 no trend 0.19 no trend 0.84 -6.6 0.030 9 46 07249400 10/20/1992 9/7/2004 76 33 no trend 0.62 18 0.009 0.05 no trend 0.19

TABLES 85
 Map 

number

 (fig. 

5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted 

trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

48 07263620 12/17/1992 9/7/2004 102 14 no trend 0.10 no trend 0.23 -6.3 0.009 32,000 49 07268000 11/2/1992 12/5/2001 59 1 21 0.002 0.08 21 0.003 0.08 20 0.044 42 50 07288650 2/16/1996 9/8/2004 100 13 no trend 0.10 -6.1 0.022 0.53 no trend 0.9251 07288955 2/14/1996 9/10/2004 149 0 no trend 0.40 no trend 0.36 no trend 0.4352 07290650 11/2/1992 12/5/2001 77 23 no trend 0.68 no trend 0.39 no trend 0.8653 07373420 10/14/1992 9/14/2004 153 0 -1.7 0.015 1.6 no trend 0.068 -3.5 0.007 2,100,000 

 Atchafalaya system 

55 07300000 11/16/1992 8/12/2004 49 0 N/A N/A N/A56 07308500 11/18/1992 8/20/2004 103 44 N/A N/A N/A57 07311700 11/13/1992 9/14/2004 119 62 N/A N/A N/A58 07311800 11/13/1992 9/9/2004 97 45 N/A N/A N/A59 07331000 10/21/1992 9/1/2004 111 21 -5.0 0.037 0.36 no trend 0.60 -7.7 0.000 2,200 61 07337000 11/17/1992 8/17/2004 66 34 -5.8 0.031 0.14 no trend 0.57 -7.7 0.003 5,110 64 07355500 10/27/1992 9/23/2004 135 36 -4.3 0.002 0.16 -4.0 0.002 0.16 -5.2 0.037 11,200 65 07362000 11/17/1992 8/17/2004 66 8 no trend 0.75 no trend 0.80 no trend 0.6467 07373000 10/27/1992 2/23/2004 27 8 N/A N/A N/A68 07381495 11/19/1992 9/13/2004 155 0 no trend 0.072 no trend 0.19 -3.3 0.011 649,000 69 07381600 10/28/1992 9/8/2004 68 0 N/A N/A N/A

 Louisiana-Gulf/Pontchartrain system 

70 07375500 10/12/1992 8/24/2004 144 3 no trend 0.055 no trend 0.17 no trend 0.09471 07376000 11/16/1992 8/24/2004 110 2 -2.8 0.009 0.3 no trend 0.067 -4.7 0.022 197 73 07386980 10/12/1992 9/1/2004 112 1 no trend 0.48 no trend 0.60 no trend 0.9774 08012150 11/17/1992 9/1/2004 53 2 N/A N/A N/A

 Texas-Gulf system 

75 08030500 10/15/1992 4/9/2003 114 45 no trend 0.088 -3.0 0.047 0.08 no trend 0.1876 08032000 1/21/1993 9/13/2004 70 10 no trend 0.84 no trend 0.71 no trend 0.36

 Table 5. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for nitrite plus nitrate data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

86 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 

5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted 

trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

77 08033500 1/22/1993 9/2/2004 70 15 no trend 0.59 no trend 0.57 no trend 0.8080 08049500 11/23/1992 9/14/2004 65 0 N/A N/A N/A81 08051500 11/2/1992 8/18/2004 102 28 no trend 0.16 no trend 0.72 no trend 0.08582 08057410 11/2/1992 8/17/2004 86 1 N/A N/A N/A83 08062500 11/4/1992 9/8/2004 61 0 6.1 0.019 2.8 2.4 0.012 2.8 no trend 0.4184 08064100 10/20/1992 9/15/2004 127 27 no trend 0.96 no trend 0.66 no trend 0.8885 08064700 10/21/1992 9/9/2004 54 36 N/A N/A N/A87 08065350 1/20/1993 9/13/2004 72 1 no trend 0.84 no trend 0.49 no trend 0.1595 08114000 1/26/1993 9/25/2002 44 16 N/A N/A N/A97 08123850 11/3/1992 9/2/2003 55 32 N/A N/A N/A98 08136500 11/5/1992 6/24/2003 59 5 -9.3 7.5E-14 11.6 -8.8 9.8E-5 11.6 -9.4 2.2E-16 1,640 99 08143600 11/18/1992 4/16/2003 50 14 no trend 0.68 no trend 0.59 no trend 0.97

100 08147000 11/18/1992 4/16/2003 48 5 no trend 0.81 no trend 0.23 no trend 0.53101 08154700 11/19/1992 4/6/2004 48 9 no trend 0.57 no trend 0.79 no trend 0.57102 08155200 1/25/1993 4/6/2004 86 31 no trend 0.083 no trend 0.19 no trend 0.17103 08155240 11/19/1992 6/9/2004 79 10 12 0.009 0.06 7.1 0.007 0.06 no trend 0.14104 08155300 11/19/1992 3/4/2004 49 2 no trend 0.15 6.4 0.042 0.11 no trend 0.68105 08156800 12/14/1992 12/23/2002 38 1 no trend 0.42 no trend 0.45 no trend 0.22106 08159000 2/15/1994 4/17/2003 40 9 no trend 0.98 no trend 0.65 no trend 0.56107 08162000 2/16/1994 4/2/2003 46 2 N/A N/A N/A108 08162500 2/16/1994 4/2/2003 47 3 N/A N/A N/A

111 08178800 10/28/1992 9/9/2004 131 2 N/A N/A N/A

112 08181800 10/21/1992 9/9/2004 137 1 no trend 0.51 no trend 0.075 5.5 0.006 5,470 115 08210000 10/13/1992 9/23/2004 48 7 no trend 0.59 no trend 0.98 no trend 0.20

Table 5. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for nitrite plus nitrate data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 87
 Table 6. Nitrite plus nitrate annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004

 [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets]

 Map 

Number

 (fig. 5) 

 Station 

Number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, metric 

tons/square 

kilometers/

year 

 Mississippi system 

1 03612500 327,000 445,000 281,000 412,000 428,000 376,000 252,000 199,000 213,000 338,000 399,000 372,000 337,000 0.6402 07022000 970,000 570,000 593,000 505,000 574,000 626,000 644,000 270,000 521,000 453,000 274,000 409,000 534,000 0.2895 07047942 131 178 107 82 134 81 81 65 79 121 113 96 106 0.0769 07053250 190 161 132 226 262 151 123 113 312 89 239 182 1.33

11 07056000 174 142 326 129 390 191 171 101 117 313 69 111 186 0.087

12 07060500 4,440 4,160 3,750 1,440 4,120 2,710 2,200 1,130 1,030 3,420 1,220 2,180 2,650 0.10313 07060710 6 4 4 2 6 3 2 3 28234 0.025

15 07066110 171 210 178 147 206 167 175 78 84 178 118 177 157 0.14316 07068000 847 1,019 828 629 811 750 694 315 302 824 444 670 678 0.12817 07071500 507 665 548 347 556 351 291 152 121 437 230 387 383 0.18618 07077500 32 356 249 233 196 373 351 256 0.09519 07077700 103 86 56 111 212 259 135 137 0.12621 07103700 8 11 20 12 20 17 23 12 8 5 6 9 13 0.04722 07103780 1 3 5 5 7 13 12 14 10 6 7 8 0.03624 07105500 56 87 161 104 154 154 223 125 95 56 55 63 111 0.10925 07105530 146 206 361 270 380 394 524 362 306 237 219 236 303 0.28426 07106300 374 498 734 563 630 646 686 507 384 251 223 261 480 0.21728 07189000 2,720 1,870 1,910 446 1,540 1,480 1,800 829 940 1,660 556 1,450 1,430 0.63430 07195500 2,400 1,680 1,720 800 1,590 1,540 1,630 931 1,180 1,530 687 1,090 1,400 0.85031 07195865 92 91 113 111 100 126 82 141 107 2.18

88 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Table 6. Nitrite plus nitrate annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

 Map 

Number

 (fig. 5) 

 Station 

Number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, metric 

tons/square 

kilometers/

year 

32 07196000 365 256 338 103 289 252 353 235 285 260 118 294 262 0.92033 07196500 3,170 1,950 2,130 755 1,930 1,810 1,920 1,290 1,600 1,730 606 1,530 1,700 0.68534 07196900 349 169 200 117 193 195 167 108 112 157 33 112 159 1.5135 07197000 901 557 709 338 749 765 693 389 553 565 139 440 566 0.71238 07239450 111 67 64 23 201 201 139 84 90 6 34 93 0.00339 07241000 237 56 129 61 237 231 139 54 69 11 33 15 106 0.00339 07241000 879 190 473 186 360 404 302 182 230 123 161 187 307 0.00939 07241000 1,010 632 912 661 950 977 868 672 759 572 596 597 767 0.02242 07247015 67 68 82 26 114 88 76 34 93 149 26 40 72 0.10348 07263620 54,200 27,300 34,700 6,570 30,800 37,000 41,300 15,900 25,800 20,300 9,990 21,700 27,100 0.06649 07268000 27 43 162 325 225 231 55 165 266 167 0.12250 07288650 350 893 501 358 469 581 362 332 481 0.38351 07288955 4,853 10,130 6,803 7,338 4,545 6,740 9,795 9,758 7,616 7,509 0.21752 07290650 115 150 102 103 121 136 75 19 105 108 126 135 108 0.06453 07373420 1,020,000 844,000 740,000 676,000 870,000 757,000 699,000 393,000 565,000 661,000 610,000 644,000 707,000 0.242

 Atchafalaya system 

56 07308500 2,700 320 1,530 1,560 4,070 6,240 685 632 2,250 327 450 1,890 0.03559 07331000 3,580 1,110 2,640 1,160 2,650 1,920 716 458 2,410 670 473 414 1,520 0.08161 07337000 7,240 3,730 5,400 902 6,240 5,950 1,780 443 6,540 3,940 926 439 3,630 0.02964 07355500 13,200 8,530 11,100 1,490 13,800 9,890 7,110 3,010 11,800 7,740 3,370 3,130 7,850 0.04565 07362000 936 1,090 1,010 301 1,240 899 988 423 1,070 1,080 767 832 886 0.06468 07381495 312,000 248,000 229,000 213,000 267,000 242,000 219,000 128,000 183,000 219,000 206,000 222,000 224,000 0.927

 Louisiana-Gulf/Pontchartrain system 

70 07375500 436 358 399 277 422 366 168 56 281 279 490 327 322 0.19271 07376000 103 85 95 59 91 69 45 12 60 48 85 64 68 0.10672 07385765 149 97 44 136 110 124 193 122 ---

TABLES 89
 Map 

Number

 (fig. 5) 

 Station 

Number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, metric 

tons/square 

kilometers/

year 

 Texas-Gulf system 

76 08032000 89 60 122 23 57 62 88 34 90 70 58 42 66 0.02277 08033500 254 243 303 86 246 185 265 104 276 209 237 232 220 0.02381 08051500 326 194 284 2 421 106 8 0.3 443 109 14 126 170 0.22283 08062500 8,100 8,660 11,200 7,420 11,700 10,800 10,500 11,700 10,400 8,900 9,170 9,870 0.46884 08064100 1,100 1,050 3,030 2,380 1,580 897 647 1,350 528 263 1,260 1,280 0.61287 08065350 9,590 11,900 17,400 7,610 15,700 13,800 15,500 8,600 14,400 10,600 8,510 7,010 11,700 0.325

102 08155200 3 0.1 5 11 5 8 0.1 7 17736 0.024

103 08155240 4 0.1 8 0.1 19 9 15 1 13 29 14 7 10 0.036104 08155300 2 9 0.1 18 7 22 0.4 10 32 13 7 11 0.034105 08156800 3 2 6 2 3 2 4 2 5 5 3 4 4 0.111

112 08181800 2,920 2,490 2,520 2,080 2,570 2,710 3,080 2,390 3,180 3,850 3,980 3,900 2,970 0.658115 08210000 70 84 93 74 123 83 157 21 72 170 269 111 0.003

 Table 6. Nitrite plus nitrate annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

90 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Table 7. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total nitrogen data for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but results not available because of poor model fit or incomplete datasets; reference concentrations and loads are blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A”] 

 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 139 0 no trend 0.12 no trend 0.093 no trend 0.712 07022000 11/11/1992 9/1/2004 156 0 no trend 0.38 1.8 0.030 3.2 -4.7 0.001 2,320,000 6 07050500 4/28/1993 8/31/2004 70 1 N/A N/A N/A7 07052152 11/24/1993 7/22/2004 64 0 no trend 0.68 no trend 0.69 no trend 0.988 07052250 11/17/1992 7/21/2004 66 0 no trend 0.11 no trend 0.13 no trend 0.219 07053250 4/26/1993 9/3/2003 125 2 7.0 1.4E-6 2.5 8.7 1.9E-7 2.5 no trend 0.44

11 07056000 5/6/1993 3/23/2004 126 61 N/A N/A N/A

14 07061600 1/5/1993 9/7/2004 63 35 N/A N/A N/A15 07066110 11/12/1992 9/7/2004 76 2 no trend 0.68 no trend 0.98 -4.0 0.005 512 16 07068000 11/12/1992 9/15/2004 130 6 -2.3 0.001 0.41 -1.5 0.039 0.41 -4.9 2.4E-5 3,140 17 07071500 11/10/1993 9/15/2004 60 0 no trend 0.18 no trend 0.95 -5.3 0.007 1,450 18 07077500 10/12/1994 9/1/2004 59 0 N/A N/A N/A28 07189000 11/18/1992 9/14/2004 108 0 no trend 0.55 3.8 3.3E-5 1.7 -4.8 0.028 2,860 29 07195000 8/3/1995 9/1/2004 49 0 N/A N/A N/A30 07195500 10/7/1992 8/17/2004 110 1 no trend 0.74 no trend 0.53 -4.1 0.014 3,520 31 07195865 10/6/1992 8/18/2004 87 0 no trend 0.062 no trend 0.053 no trend 0.3532 07196000 10/6/1992 8/18/2004 109 0 4.1 2.5E-10 1.9 4.8 8.5E-13 1.9 no trend 0.6633 07196500 10/14/1992 8/17/2004 127 0 no trend 0.75 2.0 0.007 1.7 no trend 0.05834 07196900 6/28/1995 9/1/2004 50 2 N/A N/A N/A35 07197000 10/13/1992 8/17/2004 114 0 no trend 0.72 3.1 0.017 1 no trend 0.068

TABLES 91
Table 7. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total nitrogen data for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

42 07247015 10/13/1992 8/24/2004 91 1 no trend 0.39 no trend 0.19 no trend 0.2043 07247250 10/15/1992 8/24/2004 80 26 no trend 0.29 no trend 0.45 no trend 0.09044 07247345 10/14/1992 8/25/2004 77 22 no trend 0.50 no trend 0.90 no trend 0.1245 07247650 10/14/1992 8/25/2004 78 0 no trend 0.82 no trend 0.67 -5.7 0.032 65 46 07249400 6/27/1995 9/7/2004 56 13 no trend 0.58 no trend 0.091 no trend 0.06647 07263295 12/15/1992 8/2/2004 57 24 N/A N/A N/A48 07263620 12/17/1992 9/7/2004 102 0 no trend 0.45 no trend 0.13 -4.7 0.048 81,500 49 07268000 11/2/1992 12/5/2001 73 3 no trend 0.13 no trend 0.11 no trend 0.4150 07288650 2/16/1996 9/5/2003 92 2 no trend 0.18 no trend 0.057 no trend 0.6651 07288955 2/14/1996 9/9/2003 136 0 no trend 0.66 no trend 0.99 no trend 0.1952 07290650 11/2/1992 12/5/2001 77 10 no trend 0.52 no trend 0.66 no trend 0.3653 07373420 10/14/1992 9/14/2004 149 0 no trend 0.30 no trend 0.54 -2.7 0.033 2,830,000 

 Atchafalaya system 

56 07308500 11/18/1992 8/20/2004 103 0 no trend 0.73 7.5 0.004 1.4 -7.3 0.001 4,190 57 07311700 11/13/1992 9/14/2004 96 28 N/A N/A N/A58 07311800 11/13/1992 9/9/2004 79 20 N/A N/A N/A59 07331000 10/21/1992 9/1/2004 111 1 no trend 0.37 6.0 0.000 1 -6.4 0.001 6,250 61 07337000 11/17/1992 8/17/2004 66 2 6.8 2.6E-5 0.67 8.3 4.5E-6 0.67 no trend 0.1164 07355500 10/27/1992 9/23/2004 134 0 no trend 0.39 no trend 0.10 no trend 0.6465 07362000 11/17/1992 8/17/2004 66 11 3.8 0.041 0.32 3.9 0.041 0.32 no trend 0.4966 07362587 12/15/1992 8/2/2004 61 32 N/A N/A N/A67 07373000 10/27/1992 2/23/2004 27 7 N/A N/A N/A68 07381495 2/23/1993 9/13/2004 149 0 no trend 0.32 no trend 0.70 -2.7 0.031 1,020,000 

92 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Table 7. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total nitrogen data for sites in 

the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flowadjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference load, 

kg/day 

 Louisiana-Gulf/Pontchartrain system 

70 07375500 10/12/1992 8/24/2004 143 6 no trend 0.050 no trend 0.14 no trend 0.09071 07376000 11/16/1992 8/24/2004 109 8 no trend 0.19 no trend 0.95 no trend 0.08473 07386980 10/12/1992 9/1/2004 112 0 -1.8 0.014 1.9 -1.8 0.019 1.9 no trend 0.7674 08012150 11/17/1992 9/1/2004 53 0 N/A N/A N/A

 Texas-Gulf system 

81 08051500 11/2/1992 7/15/2003 90 13 no trend 0.19 14 8.1E-6 0.3 no trend 0.2782 08057410 11/2/1992 9/8/2003 71 0 N/A N/A N/A84 08064100 10/20/1992 7/1/2003 89 0 N/A N/A N/A99 08143600 11/18/1992 4/16/2003 49 1 no trend 0.24 no trend 0.27 no trend 0.84

100 08147000 11/18/1992 4/16/2003 48 2 no trend 0.74 no trend 0.36 no trend 0.44101 08154700 11/19/1992 4/6/2004 36 7 N/A N/A N/A102 08155200 1/25/1993 4/6/2004 86 47 N/A N/A N/A103 08155240 11/19/1992 6/9/2004 78 26 N/A N/A N/A104 08155300 11/19/1992 3/4/2004 48 10 no trend 0.21 8.5 0.040 0.23 no trend 0.73105 08156800 12/14/1992 12/23/2002 34 1 no trend 0.42 no trend 0.14 no trend 0.25106 08159000 2/15/1994 4/17/2003 36 4 no trend 0.36 no trend 0.20 no trend 0.72107 08162000 2/16/1994 4/2/2003 42 0 N/A N/A N/A108 08162500 2/16/1994 4/2/2003 42 0 N/A N/A N/A

111 08178800 10/28/1992 9/10/2003 104 0 N/A N/A N/A

112 08181800 10/21/1992 9/10/2003 111 0 no trend 0.48 no trend 0.23 5.4 0.019 6,110 

TABLES 93
 Table 8. Total nitrogen annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004

 [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets]

 Map 

Number

 (fig. 5) 

 Station 

Number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, metric tons/

square 

kilometers/

year 

 Total nitrogen 

 Mississippi system 

1 03612500 432,000 606,000 380,000 572,000 609,000 532,000 354,000 278,000 304,000 503,000 610,000 579,000 480,000 0.9122 07022000 1,370,000 767,000 829,000 703,000 803,000 897,000 936,000 395,000 783,000 697,000 420,000 638,000 770,000 0.4179 07053250 195 164 134 232 268 151 123 113 312 88 237 183 1.34

15 07066110 348 417 272 222 300 232 249 98 110 299 157 302 250 0.22716 07068000 1,560 1,960 1,380 1,040 1,300 1,230 1,070 482 476 1,490 606 1,010 1,130 0.21417 07071500 634 855 678 427 687 431 352 192 157 603 278 481 481 0.23428 07189000 3,010 2,060 2,110 487 1,650 1,580 1,940 899 1,010 1,760 593 1,530 1,550 0.68730 07195500 3,320 2,100 2,290 998 2,080 1,970 2,140 1,310 1,450 1,980 766 1,440 1,820 1.1131 07195865 103 101 128 128 111 143 84 163 120 2.4532 07196000 456 300 410 112 352 293 431 343 341 289 124 359 317 1.1133 07196500 4,180 2,400 2,760 919 2,490 2,390 2,640 1,840 2,090 2,440 733 2,200 2,260 0.90935 07197000 1,240 672 943 510 1,070 1,060 886 703 708 769 147 675 782 0.98342 07247015 243 254 325 109 464 327 297 132 312 486 77 111 261 0.37543 07247250 96 69 73 22 116 63 86 43 67 109 21 30 66 0.34444 07247345 289 200 215 56 367 182 249 118 195 345 54 78 196 0.42245 07247650 336 247 301 174 317 299 367 75 378 372 65 105 253 0.36246 07249400 259 164 323 109 437 358 279 63 264 664 35 269 0.70648 07263620 85,500 42,300 65,800 13,400 51,700 52,700 76,900 30,500 38,800 34,400 20,000 38,600 45,900 0.11249 07268000 156 385 1,140 2,240 1,080 1,250 245 918 2,380 1,090 0.79950 07288650 911 2,640 1,960 1,120 2,220 3,020 1,590 1,460 1,860 1.49

94 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
Table 8. Total nitrogen annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

 Map 

Number

 (fig. 5) 

 Station 

Number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, metric tons/

square 

kilometers/

year 

51 07288955 14,800 34,800 23,800 25,200 14,500 23,700 35,700 34,700 25,300 25,800 0.74752 07290650 934 1,890 1,070 932 1,300 1,730 621 211 1,490 1,530 1,560 1,580 1,240 0.73053 07373420 1,330,000 1,120,000 986,000 919,000 1,200,000 1,070,000 992,000 573,000 834,000 990,000 914,000 982,000 992,000 0.340

 Atchafalaya system 

56 07308500 3,970 909 8,110 3,410 7,710 4,710 1,970 1,320 2,780 750 946 1,370 3,160 0.05959 07331000 7,480 3,050 9,280 3,060 7,110 5,370 2,890 2,170 6,000 2,540 1,520 1,780 4,350 0.23361 07337000 16,800 10,500 18,000 5,980 18,500 15,000 8,910 4,530 21,000 16,300 6,160 5,520 12,300 0.09964 07355500 38,500 28,300 39,400 8,180 49,900 35,300 31,000 18,500 53,300 39,300 20,300 22,100 32,000 0.13265 07362000 2,440 3,160 2,850 989 4,330 3,000 3,410 1,590 4,310 4,600 3,080 3,220 3,080 0.22268 07381495 483,000 398,000 357,000 332,000 426,000 382,000 352,000 205,000 296,000 351,000 327,000 352,000 355,000 1.47

 Louisiana-Gulf/Pontchartrain system 

70 07375500 1,550 1,150 1,490 870 1,490 1,400 564 151 1,040 943 1,910 1,290 1,150 0.69071 07376000 430 347 401 229 412 337 174 39 266 214 437 313 300 0.46972 07385765 828 395 165 610 509 573 867 564

 Texas-Gulf system 

81 08051500 244 162 247 5 358 124 16 1 553 178 40 265 183 0.239

100 08147000 409 537 568 411 1,394 452 165 137 556 535 325 600 507 0.006

112 08181800 3,850 3,010 2,990 2,260 2,990 3,050 3,630 2,450 3,580 4,720 4,540 4,290 3,450 0.764

TABLES 95
 Table 9. Analytical results of weighted-least-squares regression comparing flow-adjusted and total trends in nutrient and suspended-sediment concentrations for sites in the 

Lower-Mississippi-Texas Basin, water years 1993-2004 [P-value, probability value; items in bold are statistically significant trends (p-values less than value of significance of 0.05)]

 Population Density Atmospheric Deposition Fertilizer Manure 

 Trend 

type Parameter 

 Model 

coefficient P-value 

 Coefficient 

of determination 

(R2) 

 Model coefficient P-value 

 Coefficient 

of determination 

(R2) 

 Model coefficient P-value 

 Coefficient 

of determination 

(R2) 

 Model coefficient P-value 

 Coefficient 

of determination 

(R2) 

Total

trend

Ammonia 0.018 0.13 0.057 0.15 0.094 0.070 -0.022 0.39 0.019 -0.012 0.59 0.008

Total

trend

Nitrite plus

nitrate

-0.021 0.49 0.009 -0.19 0.11 0.048 0.056 0.18 0.032 0.031 0.011 0.11 

Total

trend

Total nitrogen 0.011 0.77 0.003 -0.036 0.70 0.006 0.027 0.38 0.028 0.023 0.030 0.16 

Total

trend

Orthophosphorus 0.021 0.70 0.006 0.18 0.20 0.066 0.13 0.059 0.14 

Total

trend

Total phosphorus -0.017 0.74 0.003 0.13 0.097 0.069 0.15 0.002 0.21 

Total

trend

Suspended

sediment

-0.046 0.55 0.020

Flowadjusted

trend

Ammonia -0.013 0.26 0.032 0.057 0.47 0.013 0.001 0.97 0.000 0.015 0.33 0.025

Flowadjusted

trend

Nitrite plus

nitrate

-0.038 0.074 0.058 -0.34 0.000 0.23 0.092 0.007 0.12 0.050 8.3E-6 0.31 

Flowadjusted

trend

Total nitrogen 0.003 0.94 0.000 0.019 0.85 0.001 0.024 0.35 0.031 0.024 0.013 0.21 

Flowadjusted

trend

Orthophosphorus -0.064 0.18 0.073 0.19 0.24 0.057 0.18 0.011 0.24 

Flowadjusted

trend

Total phosphorus -0.092 0.037 0.11 0.11 0.25 0.034 0.20 0.001 0.24 

Flowadjusted

trend

Suspended

sediment

-0.023 0.64 0.012

96 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Table 10. Sites in the Lower-Mississippi-Texas Basin where the amount of conservation practices increased or decreased by more than one percent of the total drainage area, 

water years 1993-2004  [All data are from the National Resources Inventories of 1992 and 1997 compiled by the U.S. Department of Agriculture, Natural Resources Conservation Service (U.S. Department of Agriculture, 1995; U.S. Department of Agriculture, 2001); blank cells in the table indicate less than a 1 percent change rather than missing data] 

 Map 

number

 (fig. 5) 

 Station 

number 

 Drainage

 area, 

square 

kilometers 

 Irrigation source: wells, 

difference from 1992-1997 

divided by drainage area, 

in percent 

 Irrigation type: gravity, pressure, or combination, difference from 1992-1997 divided 

by drainage area, in percent 

 Conservation practice: contour 

farming or terrace, difference from 

1992-1997 divided by drainage area, 

in percent 

 Conservation practice: surface 

drainage, difference from 1992-

1997 divided by drainage area, in 

percent 

74 08012150 3,577 -3.189 08069000 738 -1.391 08073500 759 -1.3

101 08154700 58 1.6 1.6 -11103 08155240 277 1.8 1.6 -9.6

110 08178565 324 1.4 1.2 -9.9

111 08178800 490 -6.6

TABLES 97
 Table 11. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for orthophosphorus data for 

sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but results not available because of poor model fit or incomplete datasets; reference concentrations and loads are blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A;” there were no sites in the Louisiana-Gulf/Pontchartrain river system with an adequate amount of orthophosphorus data to complete trend analyses] 

 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality data 

end date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 147 1 -2.6 0.007 0.04 -2.5 0.006 0.04 no trend 0.132 07022000 7/20/1993 9/1/2004 141 2 no trend 0.11 no trend 0.92 -5.4 0.000 72,900 5 07047942 11/11/1992 9/1/2004 71 0 no trend 0.77 no trend 0.77 no trend 0.396 07050500 4/28/1993 8/31/2004 69 2 N/A N/A N/A8 07052250 5/24/1994 7/21/2004 44 1 N/A N/A N/A9 07053250 4/26/1993 8/25/2004 136 2 no trend 0.29 4.2 0.003 0.03 -4.2 0.027 2 

11 07056000 5/6/1993 3/23/2004 126 100 N/A N/A N/A

12 07060500 11/17/1992 8/25/2004 71 53 N/A N/A N/A17 07071500 8/25/1993 9/15/2004 0 0 N/A N/A N/A18 07077500 11/10/1992 9/1/2004 92 1 no trend 0.72 no trend 0.73 no trend 0.2919 07077700 11/11/1992 9/1/2004 75 0 no trend 0.28 no trend 0.37 37 0.044 12 21 07103700 1/28/1993 8/4/2004 98 69 N/A N/A N/A22 07103780 1/27/1993 10/28/2003 85 0 15 0.004 0.39 9.1 1.3E-7 0.39 7.6 9.7E-5 8 23 07104905 1/27/1993 8/23/2004 0 0 N/A N/A N/A24 07105500 1/28/1993 8/4/2004 113 2 14 1.4E-13 0.04 13 1.4E-11 0.03 no trend 0.1825 07105530 1/28/1993 8/4/2004 104 0 -7.2 8.4E-8 2.5 -7.3 6.5E-8 2.5 -7.5 9.2E-8 735 26 07106300 3/26/1993 7/22/2004 52 0 -5.9 1.6E-9 1.2 -6.0 6.2E-8 1.2 -6.1 0.001 372 27 07106500 1/29/1993 7/22/2004 104 0 -6.2 0.00 1.1 -6.2 0.00 1.1 -6.5 0.000 349 28 07189000 4/27/1993 9/14/2004 111 2 11 0.000 0.03 6.9 0.008 0.03 no trend 0.7829 07195000 6/29/1995 9/1/2004 49 1 no trend 0.52 no trend 0.54 no trend 0.56

98 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality data 

end date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

30 07195500 10/7/1992 8/17/2004 111 2 no trend 0.51 no trend 0.82 no trend 0.1231 07195865 10/6/1992 8/18/2004 85 0 N/A N/A N/A32 07196000 10/6/1992 8/18/2004 109 0 7.1 5.4E-11 0.09 8.2 1.2E-12 0.09 no trend 0.8033 07196500 10/14/1992 8/17/2004 127 0 3.3 0.017 0.05 5.4 6.6E-5 0.05 no trend 0.3834 07196900 6/28/1995 9/1/2004 50 14 N/A N/A N/A35 07197000 10/13/1992 8/17/2004 114 3 no trend 0.38 no trend 0.51 -5.1 0.020 14 36 07227500 1/13/1993 8/5/2004 62 36 N/A N/A N/A38 07239450 11/17/1992 9/8/2004 143 20 -4.8 0.026 0.05 no trend 0.059 -6.7 0.043 20 39 07241000 10/14/1992 9/8/2004 144 2 -4.0 0.005 0.12 -4.7 0.000 0.12 -7.8 0.001 80 40 07241520 10/14/1992 9/7/2004 132 0 no trend 0.17 no trend 0.072 -4.8 0.001 208 41 07241550 10/14/1992 9/7/2004 142 0 N/A N/A N/A42 07247015 10/13/1992 8/24/2004 91 4 -5.1 0.002 0.04 -4.7 0.008 0.04 -6.8 0.004 7 43 07247250 10/15/1992 8/24/2004 80 50 N/A N/A N/A44 07247345 10/14/1992 8/25/2004 78 56 N/A N/A N/A45 07247650 10/14/1992 8/25/2004 79 41 -6.9 1.3E-7 0.01 -6.7 6.3E-6 0.01 -7.9 9.7E-6 2 46 07249400 6/27/1995 9/7/2004 56 37 N/A N/A N/A47 07263295 12/15/1992 8/2/2004 57 35 N/A N/A N/A48 07263620 12/17/1992 9/7/2004 101 0 2.5 0.049 0.03 3.0 0.025 0.03 no trend 0.1450 07288650 2/16/1996 9/8/2004 100 4 no trend 0.62 no trend 0.15 no trend 0.4251 07288955 2/14/1996 9/10/2004 148 3 no trend 0.80 no trend 0.50 no trend 0.2953 07373420 10/14/1992 9/14/2004 144 0 no trend 0.52 no trend 0.71 -2.5 0.042 80,600 

Table 11. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for orthophosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 99
 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality data 

end date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

 Atchafalaya system 

56 07308500 11/18/1992 8/20/2004 102 63 N/A N/A N/A59 07331000 10/21/1992 9/1/2004 111 22 -5.4 0.004 0.05 no trend 0.47 -7.8 6.3E-6 282 61 07337000 11/17/1992 8/17/2004 66 20 -5.5 0.000 0.01 -4.1 0.030 0.01 -7.6 1.2E-5 546 65 07362000 11/17/1992 8/17/2004 66 39 N/A N/A N/A66 07362587 12/15/1992 8/2/2004 60 43 N/A N/A N/A68 07381495 2/23/1993 9/13/2004 148 1 no trend 0.17 no trend 0.36 -3.2 0.021 34,700 69 07381600 10/28/1992 9/8/2004 61 0 N/A N/A N/A

 Texas-Gulf system 

76 08032000 1/21/1993 9/13/2004 70 49 N/A N/A N/A77 08033500 1/22/1993 9/2/2004 70 25 N/A N/A N/A80 08049500 2/9/1993 9/14/2004 64 0 N/A N/A N/A81 08051500 2/8/1993 8/18/2004 100 66 N/A N/A N/A82 08057410 4/21/1993 8/17/2004 85 1 N/A N/A N/A83 08062500 1/19/1993 9/8/2004 60 0 N/A N/A N/A84 08064100 4/20/1993 9/15/2004 126 81 no trend 0.94 no trend 0.90 no trend 0.8485 08064700 1/27/1993 9/9/2004 53 15 no trend 0.46 no trend 0.30 no trend 0.9787 08065350 1/20/1993 9/13/2004 73 1 no trend 0.54 -3.4 0.028 0.31 -4.5 0.007 3,720 95 08114000 1/26/1993 9/25/2002 44 10 N/A N/A N/A97 08123850 11/3/1992 9/2/2003 0 0 N/A N/A N/A98 08136500 1/19/1993 6/24/2003 0 0 N/A N/A N/A

101 08154700 1/14/1993 4/6/2004 45 24 N/A N/A N/A102 08155200 1/25/1993 4/6/2004 86 67 N/A N/A N/A103 08155240 11/20/1992 6/9/2004 78 52 N/A N/A N/A

 Table 11. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for orthophosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

100 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality data 

end date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

104 08155300 11/20/1992 3/4/2004 48 31 N/A N/A N/A105 08156800 12/14/1992 12/23/2002 38 3 no trend 0.93 no trend 0.84 no trend 0.088

111 08178800 1/11/1993 9/9/2004 111 12 N/A N/A N/A

112 08181800 2/9/1993 9/9/2004 119 2 -6.3 9.0E-7 1.5 -5.0 1.7E-6 1.5 -4.2 0.000 1,010 115 08210000 10/13/1992 9/23/2004 48 3 N/A N/A N/A

Table 11. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for orthophosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 101
 Table 12. Orthophosphorus annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004 

 [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets; there were no sites in the Louisiana-Gulf/Pontchartrain system that had an adequate amount of orthophosphorus data to complete load calcuations] 

 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load, 

metric 

tons 

 Average

 annual yield, 

metric tons/

square kilometers/year 

 Mississippi system 

1 03612500 12,000 16,100 9,670 14,000 15,100 11,600 7,290 5,370 5,960 9,580 11,900 11,400 10,800 0.0212 07022000 39,000 24,700 23,900 21,400 23,000 24,600 26,900 12,800 19,200 18,000 12,100 17,200 21,900 0.0125 07047942 20 56 29 18 59 30 27 20 30 65 46 25 35 0.0269 07053250 6 5 3 10 21 3 3 1 10 1 21 8 0.056

18 07077500 27 71 52 40 45 103 73 59 0.02219 07077700 25 23 14 32 70 64 38 38 0.03522 07103780 3 3

4 445 5 5 5 5 8 4 0.021

24 07105500 1 2 5 255 11 4 4 2 3 4 4 0.00426 07106300 98 86 87 59 53 49 44 37 32 24 26 37 53 0.02426 07106300 85 82 88 51 49 46 46 35 28 20 22 32 49 0.02228 07189000 82 60 24 72 67 98 57 51 75 26 51 60 0.02730 07195500 204 116 140 71 128 115 128 119 88 121 50 107 116 0.07032 07196000 25 14 20 5 15 11 21 29 16 15 7 28 17 0.06133 07196500 150 81 115 47 109 92 128 158 87 112 33 118 102 0.04135 07197000 42 17 23 18 26 20 15 36 11 15 2 23 21 0.02638 07239450 32 14 21 9 49 37 33 18 19 3 8 7 21 0.00139 07241000 79 19 60 21 66 46 42 20 16 3 7 6 32 0.00139 07241000 166 53 116 57 94 86 79 52 57 34 40 45 73 0.00242 07247015 22 25 34 11 46 33 27 10 22 23 4 3 22 0.03148 07263620 3,440 1,680 2,880 606 2,290 2,090 3,550 1,380 1,620 1,500 949 1,770 1,980 0.005

102 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load, 

metric 

tons 

 Average

 annual yield, 

metric tons/

square kilometers/year 

50 07288650 26 73 56 27 76 115 65 54 61 0.049

51 07288955 460 1,029 679 691 406 654 1,031 1,056 771 753 0.02253 07373420 40,300 35,500 31,500 29,500 35,300 31,400 29,800 21,000 25,600 28,200 26,400 27,000 30,100 0.010

 Atchafalaya system 

59 07331000 237 85 216 107 193 113 61 42 157 52 39 39 112 0.00661 07337000 399 278 473 149 527 389 173 61 361 196 58 29 258 0.00268 07381495 17,300 13,200 12,300 11,300 14,000 12,500 11,400 6,740 9,340 11,300 11,200 12,000 11,900 0.049

 Texas-Gulf system 

87 08065350 1,720 1,650 2,080 885 1,480 1,280 1,340 808 1,310 1,040 939 914 1,290 0.036

105 08156800 0.3 0.2 1 0.3 0.4 0.3 1 0.2 1 1 0.3 0.4 0.4 0.013

112 08181800 518 415 383 301 324 311 317 234 270 288 273 243 323 0.072

Table 12. Orthophosphorus annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 103
 Table 13. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total phosphorus data for 

sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but results not available because of poor model fit or incomplete datasets; reference concentrations and loads are blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A”] 

 Map 

number

 (fig. 5) 

 Station 

Number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 139 0 5.6 0.001 0.08 6.0 1.4E-7 0.08 no trend 0.162 07022000 11/11/1992 9/1/2004 156 0 no trend 0.097 8.9 1.1E-8 0.23 no trend 0.113 07031740 5/4/1993 6/2/2004 43 1 N/A N/A N/A6 07050500 10/13/1992 8/31/2004 86 7 N/A N/A N/A7 07052152 11/24/1993 7/22/2004 64 0 no trend 0.48 no trend 0.46 no trend 0.378 07052250 11/17/1992 7/21/2004 69 1 no trend 0.74 no trend 0.73 no trend 0.739 07053250 4/26/1993 8/25/2004 136 34 no trend 0.36 3.1 0.021 0.04 -4.4 0.015 4 

10 07055646 4/28/1993 8/25/2004 79 39 N/A N/A N/A

11 07056000 10/27/1992 3/23/2004 145 55 N/A N/A N/A

13 07060710 11/17/1992 8/24/2004 120 92 N/A N/A N/A18 07077500 10/12/1994 9/1/2004 59 0 N/A N/A N/A27 07106500 10/20/1992 7/22/2004 36 0 N/A N/A N/A28 07189000 11/18/1992 9/14/2004 108 3 11 3.7E-6 0.03 8.5 0.000 0.03 no trend 0.7729 07195000 10/13/1992 9/1/2004 70 0 no trend 0.94 no trend 0.98 no trend 0.9730 07195500 10/7/1992 8/17/2004 111 1 5.4 0.012 0.12 6.5 0.003 0.12 no trend 0.6731 07195865 10/6/1992 8/18/2004 86 0 12 0.016 0.51 11 0.005 0.51 11 0.009 17 32 07196000 10/6/1992 8/18/2004 108 0 8.3 3.9E-9 0.09 10 2.1E-12 0.09 no trend 0.6433 07196500 10/14/1992 8/17/2004 127 5 5.4 0.001 0.07 9.4 4.4E-9 0.07 no trend 0.7534 07196900 11/3/1992 9/1/2004 69 9 -5.1 0.001 0.08 -4.9 0.003 0.08 -6.9 0.001 4 35 07197000 10/13/1992 8/17/2004 114 37 no trend 0.70 5.0 0.036 0.03 no trend 0.14

104 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

Number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

37 07228000 11/16/1992 4/8/2003 43 7 no trend 0.058 no trend 0.12 no trend 0.08842 07247015 11/18/1992 8/24/2004 95 0 no trend 0.50 no trend 0.81 no trend 0.09443 07247250 10/15/1992 8/24/2004 83 45 N/A N/A N/A44 07247345 1/6/1993 8/25/2004 74 37 N/A N/A N/A45 07247650 10/14/1992 8/25/2004 78 4 no trend 0.73 no trend 0.11 no trend 0.06946 07249400 10/20/1992 9/7/2004 75 25 -5.7 3.2E-5 0.06 -3.9 0.008 0.06 -7.6 1.6E-5 8 47 07263295 12/15/1992 8/2/2004 57 21 N/A N/A N/A48 07263620 12/17/1992 9/7/2004 102 0 4.0 0.000 0.08 7.4 1.6E-8 0.08 no trend 0.3849 07268000 11/2/1992 12/5/2001 60 4 N/A N/A N/A50 07288650 2/16/1996 9/8/2004 99 1 no trend 0.45 no trend 0.14 no trend 0.3851 07288955 2/14/1996 9/10/2004 146 0 4.3 0.030 0.19 no trend 0.097 no trend 0.07552 07290650 11/2/1992 12/5/2001 78 4 no trend 0.37 no trend 0.48 no trend 0.2853 07373420 10/14/1992 9/14/2004 154 0 4.1 0.000 0.18 4.4 0.000 0.18 no trend 0.65

 Atchafalaya system 

54 07299540 8/25/1993 3/4/2003 47 12 no trend 0.28 no trend 0.34 no trend 0.2156 07308500 11/18/1992 8/20/2004 104 5 13 0.032 0.07 56 1.4E-6 0.07 no trend 0.1557 07311700 11/13/1992 9/14/2004 97 68 N/A N/A N/A58 07311800 11/13/1992 9/9/2004 79 36 N/A N/A N/A59 07331000 10/21/1992 9/1/2004 110 0 no trend 0.88 12 5.1E-5 0.22 -6.5 0.005 1,310 60 07335500 8/23/1993 4/22/2003 35 0 no trend 0.64 no trend 0.84 -7.1 0.014 3,150 61 07337000 11/17/1992 8/17/2004 66 0 no trend 0.77 no trend 0.071 -5.7 0.017 4,170 63 07343000 10/27/1992 3/10/2003 37 9 no trend 0.26 no trend 0.33 no trend 0.2664 07355500 10/27/1992 9/23/2004 133 2 no trend 0.35 no trend 0.58 no trend 0.37

Table 13. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total phosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 105
 Map 

number

 (fig. 5) 

 Station 

Number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

65 07362000 11/17/1992 8/17/2004 66 22 no trend 0.55 no trend 0.59 no trend 0.5166 07362587 12/15/1992 8/2/2004 61 28 N/A N/A N/A67 07373000 10/27/1992 2/23/2004 0 0 N/A N/A N/A68 07381495 11/19/1992 9/13/2004 157 0 4.8 2.1E-6 0.15 6.0 2.80E-9 0.15 no trend 0.50

 Louisiana-Gulf/Pontchartrain system 

70 07375500 10/12/1992 8/24/2004 142 0 no trend 0.94 no trend 0.52 no trend 0.3671 07376000 11/16/1992 8/24/2004 107 0 4.5 0.010 0.07 5.2 0.004 0.07 no trend 0.9573 07386980 10/12/1992 9/1/2004 112 0 no trend 0.96 no trend 0.78 no trend 0.5674 08012150 11/17/1992 9/1/2004 55 0 N/A N/A N/A

 Texas-Gulf system 

78 08041000 1/19/1993 4/23/2003 53 1 6.0 0.028 0.05 6.2 0.024 0.05 no trend 0.1279 08044500 10/1/1992 2/19/2003 41 1 no trend 0.96 no trend 0.88 -5.6 0.043 34 81 08051500 11/2/1992 8/18/2004 97 31 N/A N/A N/A82 08057410 11/2/1992 8/17/2004 79 0 N/A N/A N/A84 08064100 10/20/1992 8/16/2004 97 13 N/A N/A N/A86 08065000 11/12/1992 4/8/2003 56 0 no trend 0.52 no trend 0.93 no trend 0.3688 08066500 10/20/1992 3/20/2003 101 5 no trend 0.89 no trend 0.93 no trend 0.6489 08069000 11/4/1992 1/22/2003 74 0 -5.5 0.006 1.5 -4.2 0.032 1.5 no trend 0.1690 08070200 6/2/1993 1/22/2003 85 0 6.6 0.010 0.07 6.4 0.011 0.07 no trend 0.1491 08073500 11/19/1992 1/30/2003 37 0 N/A N/A N/A92 08074500 11/19/1992 1/30/2003 42 0 N/A N/A N/A93 08078000 12/7/1992 2/12/2003 38 1 no trend 0.56 no trend 0.42 no trend 1.094 08085500 11/5/1992 2/4/2003 35 0 N/A N/A N/A96 08117500 12/9/1992 4/9/2003 50 0 no trend 0.70 no trend 0.79 no trend 0.6499 08143600 11/18/1992 4/16/2003 55 1 no trend 0.96 no trend 1.0 no trend 0.65

 Table 13. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total phosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued 

106 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

Number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number 

of observations 

 Number 

of censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model pvalue 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted trend, 

model pvalue 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, 

kg/day 

100 08147000 11/18/1992 4/16/2003 58 12 no trend 0.058 no trend 0.072 -7.8 0.020 40 101 08154700 11/19/1992 4/6/2004 36 15 N/A N/A N/A102 08155200 1/25/1993 4/6/2004 86 62 N/A N/A N/A103 08155240 11/19/1992 6/9/2004 79 53 N/A N/A N/A104 08155300 11/19/1992 3/4/2004 49 20 no trend 0.75 no trend 0.36 no trend 0.63105 08156800 12/14/1992 12/23/2002 34 3 N/A N/A N/A106 08159000 10/13/1992 4/24/2003 56 22 -8.9 0.000 0.08 -9.0 0.001 0.08 no trend 0.15107 08162000 10/14/1992 4/2/2003 70 0 no trend 0.85 no trend 0.79 no trend 0.76108 08162500 10/14/1992 4/2/2003 55 1 no trend 0.42 no trend 0.39 no trend 0.82109 08162600 6/8/1993 3/25/2003 56 0 no trend 0.80 no trend 0.66 no trend 0.98

110 08178565 10/20/1992 4/24/2003 93 10 N/A N/A N/A

111 08178800 10/28/1992 9/9/2004 115 11 N/A N/A N/A

112 08181800 10/21/1992 9/9/2004 121 0 -6.1 1.1E-9 1.5 -5.3 2.6E-9 1.6 -3.9 0.008 1,050 113 08188500 10/15/1992 4/21/2003 91 0 -5.0 7.4E-8 0.93 -5.0 8.2E-7 0.93 no trend 0.85114 08189500 10/14/1992 4/22/2003 40 5 no trend 0.45 no trend 0.50 no trend 0.18

Table 13. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for total phosphorus data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 107
 Table 14. Total phosphorus annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004

 [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets]

 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, 

metric 

tons/

square 

kilometers/year 

 Mississippi system 

1 03612500 34,200 60,300 30,400 54,000 68,200 50,400 34,500 21,500 25,600 59,000 75,100 72,700 48,800 0.0932 07022000 147,000 64,500 79,200 64,800 72,800 89,200 97,800 37,100 87,000 82,000 44,900 76,600 78,600 0.0439 07053250 7 5 3 9 21 3 3 2 11 1 21 8 0.058

28 07189000 99 73 116 28 84 80 120 71 64 97 32 68 78 0.03430 07195500 294 144 199 106 209 168 213 296 135 229 70 240 192 0.11731 07195865 12 11 14 38 18 18 11 29 19 0.38332 07196000 37 18 28 6 21 14 30 66 22 18 8 47 26 0.09233 07196500 323 132 202 78 181 157 205 423 161 210 48 335 205 0.08234 07196900 16675664523135 0.051

35 07197000 101 31 55 62 72 52 37 217 30 48 4 127 70 0.08842 07247015 62 60 70 21 91 67 55 24 63 99 16 24 54 0.07845 07247650 49 35 44 28 49 43 61 10 63 69 9 16 39 0.05646 07249400 47 26 49 17 54 36 33 7 20 52 2 53 33 0.08748 07263620 9,110 3,980 7,020 1,150 5,370 5,190 8,930 3,260 4,220 4,030 2,430 5,570 5,020 0.01250 07288650 133 417 370 222 421 582 314 304 345 0.27551 07288955 2,390 7,610 5,670 6,360 3,640 6,420 10,300 9,270 5,660 6,370 0.18452 07290650 248 605 297 261 370 569 165 55 473 548 499 456 379 0.22453 07373420 111,000 98,200 89,300 85,800 114,000 105,000 104,000 61,600 92,900 114,000 112,000 125,000 101,000 0.035

 Atchafalaya system 

56 07308500 654 99 4,570 725 2,490 1,050 536 384 750 211 286 619 1,030 0.01959 07331000 2,580 863 3,190 689 1,710 1,320 638 501 1,630 748 381 672 1,240 0.067

108 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

Number

 (fig. 5) 

 Station 

number 

 1993 

Water 

Year 

 1994 

Water 

Year 

 1995 

Water 

Year 

 1996 

Water 

Year 

 1997 

Water 

Year 

 1998 

Water 

Year 

 1999 

Water 

Year 

 2000 

Water 

Year 

 2001 

Water 

Year 

 2002 

Water 

Year 

 2003 

Water 

Year 

 2004 

Water 

Year 

 Average 

annual 

load 

 Average 

annual 

yield, 

metric 

tons/

square 

kilometers/year 

60 07335500 2,680 1,330 3,020 734 2,130 1,820 800 400 1,950 1,220 606 402 1,430 0.01261 07337000 3,710 2,040 3,640 943 3,570 2,860 1,380 592 3,720 2,740 759 647 2,220 0.01864 07355500 6,010 4,160 5,990 840 8,040 5,580 4,390 2,230 8,320 5,820 2,590 2,690 4,720 0.02065 07362000 290 408 336 95 485 317 346 142 422 438 273 261 318 0.02368 07381495 44,400 38,600 33,800 32,300 47,700 42,700 41,200 22,500 36,700 47,700 44,500 50,400 40,200 0.166

 Louisiana-Gulf/Pontchartrain system 

70 07375500 300 188 269 137 246 236 102 32 192 169 351 249 206 0.12371 07376000 55 40 55 29 53 47 26 8 47 35 70 64 44 0.06972 07385765 203 76 29 135 109 125 200 125

 Texas-Gulf system 

78 08041000 469 336 705 171 418 454 547 169 574 419 490 454 434 0.02186 08065000 3,180 3,020 4,560 1,220 3,670 3,000 2,950 1,700 3,610 2,810 2,150 2,530 2,870 0.08693 08078000 41 29 48 19 43 35 21 3 34 40 35 38 32 0.14196 08117500 338 104 203 78 272 176 325 34 195 169 253 213 197 0.10498 08136500 12 56 43 10 136 32 7 5 19 83 29 46 40 0.002

107 08162000 812 303 995 279 1,970 809 1,190 199 768 1,020 1,160 726 853 0.008108 08162500 1,150 295 1,250 283 2,240 943 1,530 162 823 1,110 1,320 870 998 0.009

112 08181800 907 631 574 380 462 425 478 269 376 465 395 325 474 0.105

Table 14. Total phosphorus annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 109
 Table 15. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for suspended-sediment data for 

sites in the Lower-Mississippi-Texas Basin, water years 1993-2004  [mg/L, milligrams per liter; kg/day, kilograms per day; N/A, analysis attempted but results not available because of poor model fit or incomplete datasets; reference concentrations and loads are blank where trend results are “no trend;” p-values and reference concentrations and loads are blank where trend results are “N/A;” there were no sites in the Louisiana-Gulf/Pontchartrain system with an adequate amount of suspended-sediment data to complete trend analyses] 

 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model p-value 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, kg/day 

 Mississippi system 

1 03612500 10/20/1992 9/1/2004 138 0 no trend 0.081 5.7 0.002 33 no trend 0.342 07022000 11/6/1995 9/1/2004 123 0 -6.5 9.20E-5 287 no trend 0.41 -8.5 1.30E-5 174,000,000 4 07047810 10/14/1992 6/8/2004 98 0 no trend 0.18 no trend 0.49 no trend 0.145 07047942 11/11/1992 9/1/2004 184 0 -3.9 3.80E-5 155 -3.8 6.70E-5 155 no trend 0.356 07050500 4/28/1993 8/31/2004 62 0 N/A N/A N/A9 07053250 4/26/1993 8/25/2004 137 0 9.5 0.006 12 10 0.006 12 no trend 0.85

10 07055646 4/28/1993 8/25/2004 80 0 -4 0.045 9 no trend 0.067 no trend 0.16

11 07056000 5/6/1993 3/23/2004 126 0 no trend 0.5 no trend 0.14 no trend 0.41

12 07060500 11/17/1992 8/25/2004 71 0 N/A N/A N/A13 07060710 11/17/1992 8/24/2004 120 0 no trend 0.056 no trend 0.064 no trend 0.4618 07077500 11/10/1992 9/1/2004 91 0 no trend 0.88 no trend 0.99 no trend 0.4119 07077700 11/11/1992 9/1/2004 71 0 no trend 0.94 no trend 0.75 no trend 0.1420 07093740 10/7/1992 9/11/2003 109 0 no trend 0.17 no trend 0.32 -7.7 0.001 240 21 07103700 11/12/1992 8/4/2004 156 0 no trend 0.23 no trend 0.77 no trend 0.224 07105500 10/15/1992 9/27/2004 166 0 no trend 0.4 no trend 0.98 no trend 0.3427 07106500 10/20/1992 8/31/2004 78 0 N/A N/A N/A29 07195000 8/3/1995 9/1/2004 54 0 N/A N/A N/A33 07196500 4/29/1993 8/17/2004 103 0 no trend 0.38 no trend 0.8 -4.9 0.047 34,000 35 07197000 5/17/1994 8/17/2004 75 0 N/A N/A N/A42 07247015 10/13/1992 8/24/2004 94 0 no trend 0.23 no trend 0.66 no trend 0.082

110 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004
 Map 

number

 (fig. 5) 

 Station 

number 

 Waterquality 

data start 

date 

 Waterquality 

data end 

date 

 Number of 

observations 

 Number of 

censored 

values 

 Total 

trend, 

percent 

per year 

 Total trend, 

model p-value 

 Total 

trend 

reference 

concentration, 

mg/L 

 Flowadjusted trend, 

percent 

per year 

 Flow-adjusted 

trend, model 

p-value 

 Flowadjusted 

trend 

reference 

concentration, 

mg/L 

 Trend 

in load, 

percent 

per year 

 Trend in 

load, model 

p-value 

 Reference 

load, kg/day 

43 07247250 10/15/1992 8/24/2004 82 0 no trend 0.82 no trend 0.52 no trend 0.244 07247345 10/14/1992 8/25/2004 77 0 no trend 0.98 no trend 0.5 no trend 0.2145 07247650 10/14/1992 8/25/2004 79 0 no trend 0.63 no trend 0.055 no trend 0.146 07249400 6/27/1995 9/7/2004 56 0 -5.8 0.016 41 no trend 0.094 -8.8 0.000 4,180 47 07263295 12/15/1992 8/2/2004 54 0 N/A N/A N/A48 07263620 12/17/1992 9/7/2004 99 0 -7.6 9.70E-13 87 -7 1.20E-10 87 -8.1 5.90E-7 8,370,000 50 07288650 1/23/1996 9/8/2004 99 0 no trend 0.14 -3.3 0.047 211 no trend 0.5951 07288955 2/14/1996 9/10/2004 148 0 no trend 0.41 no trend 1 no trend 0.2153 07373420 10/14/1992 9/14/2004 150 0 -3 0.001 239 -2.2 0.016 239 -4.4 0.002 322,000,000 

 Atchafalaya system 

61 07337000 11/17/1992 8/17/2004 66 0 -4.7 0.018 445 no trend 1 -7.3 0.001 16,200,000 62 07340300 11/18/1992 8/18/2004 58 0 N/A N/A N/A65 07362000 11/17/1992 8/17/2004 66 0 no trend 0.13 no trend 0.15 no trend 0.2568 07381495 2/23/1993 9/13/2004 132 0 -3.2 0.032 211 no trend 0.41 -4.6 0.021 121,000,000 69 07381600 10/28/1992 9/8/2004 200 0 no trend 0.54 no trend 0.22 no trend 0.86

 Texas-Gulf system 

81 08051500 10/21/1993 8/18/2004 74 0 N/A N/A N/A82 08057410 4/21/1993 8/17/2004 57 0 N/A N/A N/A84 08064100 4/20/1993 8/16/2004 81 0 N/A N/A N/A

111 08178800 4/22/1996 9/9/2004 87 0 N/A N/A N/A

112 08181800 4/16/1996 9/9/2004 102 0 no trend 0.16 -7.6 0.001 64 no trend 0.94

Table 15. Analytical results and reference values for trends in flow-adjusted concentrations, total trend in concentrations, and trends in loads for suspended-sediment data for sites in the Lower-Mississippi-Texas Basin, water years 1993-2004--Continued

TABLES 111
 Table 16. Suspended-sediment annual loads, average annual load, and average annual yield for sites in the Lower-Mississippi-Texas 

Basin, water years 1993-2004  [All values in table are loads in metric tons unless otherwise noted; blanks appear in table for water years at specific sites where load calculations could not be completed because of poor model fit or incomplete data sets; there were no sites in the Louisiana-Gulf/Pontchartrain and Texas-Gulf systems that had an adequate amount of suspended-sediment data to complete load calcuations] 

112 Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central U.S., 1993-2004 Map Number (fig. 5) Station number 1993 Water Year 1994 Water Year 1995 Water Year 1996 Water Year 1997 Water Year 1998 Water Year 1999 Water Year 2000 Water Year 2001 Water Year 2002 Water Year 2003 Water Year 2004 Water Year Average annual load Average annual yield, metric tons/square kilometers/ year Mississippi system 1 03612500 19,900,000 39,300,000 17,600,000 34,300,000 43,100,000 31,400,000 19,800,000 11,000,000 12,700,000 36,400,000 45,200,000 40,300,000 29,200,000 55.6 2 07022000 94,300,000 117,000,000 118,000,000 28,100,000 92,900,000 84,200,000 30,000,000 58,400,000 77,900,000 59.8 4 07047810 581,000 928,000 603,000 303,000 1,170,000 780,000 741,000 240,000 256,000 1,350,000 744,000 489,000 682,000 56.6 5 07047942 54,800 117,000 69,200 46,200 121,000 64,400 60,000 39,800 56,000 88,200 62,300 36,800 68,000 49.1 9 07053250 1,480 1,800 1,770 3,490 3,860 2,520 2,400 1,510 3,760 856 1,620 2,280 16.7 11 07056000 138,000 72,000 294,000 259,000 90,000 54,900 118,000 70,400 445,000 48,100 159,000 74.0 13 07060710 900 889 1,390 999 2,890 1,620 1,380 912 3,590 729 1,030 1,480 9.86 18 07077500 32,200 145,000 107,000 84,000 93,300 165,000 134,000 109,000 40.4 19 07077700 84,100 67,700 32,800 75,100 134,000 113,000 54,900 80,200 73.6 24 07105500 11,400 52,200 295,000 23,500 207,000 63,100 1,130,000 29,600 30,600 12,700 12,600 34,200 158,000 156 33 07196500 198,000 69,300 151,000 58,800 135,000 124,000 147,000 505,000 112,000 109,000 13,600 161,000 149,000 59.9 42 07247015 36,900 34,300 45,300 11,900 66,300 37,100 32,200 14,400 35,300 74,600 5,620 13,600 34,000 48.7 43 07247250 6,250 3,980 4,490 957 8,190 3,580 5,330 2,480 3,920 7,980 901 1,540 4,130 21.4 44 07247345 13,400 8,710 9,620 2,250 15,900 8,680 10,100 4,720 9,520 17,200 2,270 3,130 8,790 18.9 45 07247650 50,800 35,000 45,000 29,000 52,200 46,000 62,500 8,010 69,400 85,100 8,620 15,100 42,200 60.4 46 07249400 29,000 19,500 27,500 11,100 27,200 20,200 21,900 4,790 14,200 25,700 2,260 20,100 18,600 48.9 48 07263620 23,100,000 7,460,000 12,800,000 971,000 6,060,000 5,670,000 7,540,000 1,770,000 2,150,000 1,500,000 498,000 1,240,000 5,890,000 14.4 50 07288650 118,000 359,000 296,000 181,000 306,000 396,000 210,000 200,000 258,000 206 51 07288955 2,630,000 8,160,000 5,700,000 5,750,000 2,580,000 5,500,000 10,200,000 9,440,000 5,380,000 6,150,000 178 53 07373420 157,000,000 131,000,000 105,000,000 92,800,000 128,000,000 108,000,000 100,000,000 46,900,000 75,600,000 90,900,000 79,900,000 86,500,000 100,000,000 34.3 Atchafalaya system 61 07337000 23,400,000 10,400,000 19,600,000 3,260,000 18,000,000 14,800,000 4,670,000 1,500,000 17,100,000 11,900,000 1,770,000 1,390,000 10,600,000 85.5 65 07362000 374,000 487,000 396,000 117,000 578,000 337,000 370,000 167,000 429,000 442,000 274,000 257,000 352,000 25.4 68 07381495 75,900,000 63,500,000 43,700,000 37,400,000 70,300,000 54,100,000 49,300,000 16,700,000 35,900,000 51,100,000 39,700,000 44,300,000 48,500,000 201 69 07381600 25,500,000 59,200,000 34,200,000 38,900,000 21,700,000 35,900,000
Rebich, R.A., and Demcheck, D.K.—Trends In Nutrient and Sediment Concentrations and Loads In Major River Basins of the South-Central United States, 

1993-2004—USGS Scientific Investigations Report 2007-5090

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