U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5240

In cooperation with the Iowa Department of Natural Resources

Concentrations, Loads, and Yields of Select Constituents 

from Major Tributaries of the Mississippi and Missouri Rivers 

in Iowa, Water Years 2004–2008
Cover. Little Sioux River, looking downstream from County Highway E54 near Turin, Iowa. Photograph by Richard L. Kopish, 

U.S. Geological Survey.
Concentrations, Loads, and Yields of Select 

Constituents from Major Tributaries of the 

Mississippi and Missouri Rivers in Iowa, 

Water Years 2004–2008

By Jessica D. Garrett

In cooperation with the Iowa Department of Natural Resources

Scientific Investigations Report 2012–5240

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Garrett, J.D., 2012, Concentrations, loads, and yields of select constituents from major tributaries of the 

Mississippi and Missouri Rivers in Iowa, water years 2004–2008: U.S. Geological Survey Scientific 

Investigations Report 2012–5240, 61 p.
iii

Contents

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

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

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

Study Area Description........................................................................................................................2

Methods...........................................................................................................................................................3

Water-Quality Data Collection............................................................................................................3

Statistical Data Summary....................................................................................................................7

Load and Yield Estimation....................................................................................................................7

Chemical Concentration, Loads, and Yields in Major Iowa Rivers........................................................8

Concentrations......................................................................................................................................9

Major Ions.....................................................................................................................................9

Carbon .........................................................................................................................................21

Nitrogen.......................................................................................................................................21

Phosphorus.................................................................................................................................30

Suspended Sediment and Turbidity ........................................................................................30

Algal Pigments............................................................................................................................31

Pesticides....................................................................................................................................31

Estimated Loads and Yields of Ions, Nutrients, and Suspended Sediment...............................31

Major Ions...................................................................................................................................32

Nitrogen.......................................................................................................................................32

Phosphorus.................................................................................................................................56

Suspended Sediment ................................................................................................................56

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

Acknowledgments..............................................................................................................................59

References Cited..........................................................................................................................................59

Figures

1. Map showing study sites and land use of river basins..........................................................4

2. Map showing study sites and ecoregions of river basins.....................................................5

3. Graphs showing physical properties and concentrations related to streamflow for

10 major Iowa rivers with map identifiers and locally weighted scatterplot smooth

(LOWESS) line, water years 2004–2008...................................................................................22

4. Graphs showing seasonal values for alkalinity and pH for 10 major Iowa rivers,

chlorophyll-a concentrations, atrazine concentrations, and prometon concentrations for selected major Iowa rivers, water years 2004–2008 with locally

weighted scatterplot smooth (LOWESS) line.........................................................................28

5. Graph showing median carbon concentrations as bicarbonate, total particulate

carbon, and dissolved organic carbon for 10 major Iowa rivers showing map

identifiers, water years 2004–2008...........................................................................................30

6. Graph showing relations between suspended-sediment concentrations and

turbidity for 10 major Iowa rivers, water years 2004–2008 ..................................................32
iv

7. Graphs showing total nitrate and total phosphorus load and combined streamflow

for 10 major Iowa rivers, water years 2004–2008 ..................................................................52

8. Graphs showing comparison of annual load estimates and 95-percent confidence

limits for select constituents at selected sites, water years 2004–2008 ...........................53

9. Graphs showing relations (log-fit) between annual constituent yield and average

streamflow relative to long-term (1979–2008) average streamflow for 10 major Iowa

rivers showing map identifier, water years 2004–2008.........................................................54

10. Graphs showing fertilizer distribution in Iowa and constituent loads combined for

10 major Iowa rivers, 2004–2008...............................................................................................56

Tables

1. Site information and streamflow summary for selected major Iowa rivers, water

years 2004–2008 ............................................................................................................................3

2. Constituents analyzed at selected major Iowa rivers, water years 2004-2008 ..................6

3. Statistical summary of select constituents at selected major Iowa rivers, water

years 2004–2008 ..........................................................................................................................10

4. Statistical summary of pesticides at selected major Iowa rivers, water years

2004–2008 .....................................................................................................................................19

5. Pesticides analyzed but not detected at selected major Iowa rivers, water years

2004–2008 .....................................................................................................................................20

6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and

yields from selected major Iowa rivers, water years 2004–2008........................................33

7. Regression models for estimating major ion, nutrient, and suspended-sediment

loads from selected major Iowa rivers, water years 2004–2008.........................................48
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Mass

ton per day (ton/d) 0.9072 metric ton per day

ton per day (ton/d) 0.9072 megagram per day (Mg/d)

ton per day per square mile 

[(ton/d)/mi2

]

 0.3503 megagram per day per square 

kilometer [(Mg/d)/km2]

ton per year (ton/yr) 0.9072 megagram per year (Mg/yr)

ton per year (ton/yr) 0.9072 metric ton per year

Pressure

inch of mercury at 32°F (in Hg) 0.1333 kilopascal (kPa) 

Application rate

pounds per acre per year 

[(lb/acre)/yr]

 1.121 kilograms per hectare per year 

[(kg/ha)/yr]

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

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

or micrograms per liter (μg/L).

Concentrations, Loads, and Yields of Select Constituents 

from Major Tributaries of the Mississippi and Missouri 

Rivers in Iowa, Water Years 2004–2008

By Jessica D. Garrett

Abstract

Excess nutrients, suspended-sediment loads, and the presence of pesticides in Iowa rivers can have deleterious effects 

on water quality in State streams, downstream major rivers, 

and the Gulf of Mexico. Fertilizer and pesticides are used 

to support crop growth on Iowa’s highly productive agricultural landscape and for household and commercial lawns and 

gardens. Water quality was characterized near the mouths 

of 10 major Iowa tributaries to the Mississippi and Missouri 

Rivers from March 2004 through September 2008. Stream 

loads were calculated for select ions, nutrients, and sediment 

using approximately monthly samples, and samples from 

storm and snowmelt events.

Water-quality samples collected using standard 

streamflow-integrated protocols were analyzed for major 

ions, nutrients, carbon, pesticides, and suspended sediment. 

Statistical data summaries of sample data used parametric and 

nonparametric techniques to address potential bias related to 

censored data and multiple levels of censoring of data below 

analytical detection limits. Constituent stream loads were 

computed using standard pre-defined models in S-LOADEST 

that include streamflow and time terms plus additional terms 

for streamflow variability and streamflow anomalies. Streamflow variability terms describe the difference in streamflow 

from recent average conditions, whereas streamflow anomaly 

terms account for deviations from average conditions from 

long- to short-term sequentially. Streamflow variability or 

anomaly terms were included in 44 of 80 site/constituent individual models, demonstrating the usefulness of these terms in 

increasing accuracy of the load estimates.

Constituent concentrations in Iowa streams exhibit 

streamflow, seasonal, and spatial patterns related to the 

landform and climate gradients across the studied basins. 

The streamflow-concentration relation indicated dilution for 

ions such as chloride and sulfate. Other constituent concentrations, such as dissolved organic carbon and suspended 

sediment, increased with streamflow. Nitrogen concentrations 

(total nitrogen and nitrate plus nitrite) increased with low and 

moderate streamflows, but decreased with high streamflows. 

Seasonal patterns observed in constituent concentrations were 

affected by streamflow, algae blooms, and pesticide application. The various landform regions produced different waterquality responses across the study basins; for example, total 

phosphorus, suspended sediment, and turbidity were greatest 

from the steep, loess-dominated southwestern Iowa basins.

Nutrient concentrations, though not regulated for drinking 

water at the study sites, were high compared to drinking-water 

limits and criteria for protection of aquatic life proposed for 

other Midwestern states (Iowa criteria for aquatic life have not 

been proposed). Nitrate plus nitrite concentrations exceeded 

the drinking-water limit [10 milligrams per liter (mg/L)] in 

11 percent of all samples at the 10 sites, and exceeded Minnesota’s proposed aquatic life criteria (4.9 mg/L) in 68 percent 

of samples. The Wisconsin standard for total phosphorus 

(0.1 mg/L) was exceeded in 92 percent of samples. Ammonia 

standards, current during sample collection and at publication 

of this report, for protection of aquatic life were met for all 

samples, but draft criteria proposed in 2009 to protect more 

sensitive species like mussels, were exceeded at three sites.

Loads and yields also differed among sites and years. The 

Big Sioux, Little Sioux, and Des Moines Rivers produced the 

greatest sulfate yields. Mississippi River tributaries had greater 

chloride yields than Missouri River tributaries. The Big Sioux 

River also had the lowest silica yields and total nitrogen and 

nitrate yields, whereas nitrogen yields were greater in the 

northeastern rivers. The Boyer and Nishnabotna River total 

phosphorus yields were the greatest in the study. The Boyer 

River orthophosphate yields were greatest except in 2008, 

when the Maquoketa River produced the greatest yield. Rivers 

in southwestern Iowa’s Western Loess Hills and Steeply 

Rolling Loess Prairie ecoregions had the greatest suspendedsediment yields, whereas the smallest yields were in the Big 

Sioux and Wapsipinicon Rivers. In the 10 Iowa rivers studied, 

combined annual total nitrogen stream transport ranged from 

3.68 to 9.95 tons per square mile per year, and total phosphorus transport ranged from 0.138 to 0.570 tons per square mile 

per year. Six-month loads relative to fertilizer use ranged from 

8 to 56 percent for nitrogen, and 1.0 to 11.1 percent for phosphorus. The smallest loads relative to fertilizer use for both 

nitrogen and phosphorus occurred in July-December of dry 
2 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

years, and the largest nitrogen and phosphorus loads relative to 

use were in wet years from January-June.

Introduction

Midwestern agricultural watersheds have been highlighted as major contributors of nutrients to the Gulf of 

Mexico, contributing to annual algal blooms and subsequent 

hypoxia zones in the Gulf (Alexander and others, 2008; 

Vitousek and others, 1997). Iowa is one of the most productive agricultural areas of the Nation, particularly for corn, 

soybeans, and swine (U.S. Department of Agriculture, 2009). 

Fertilizers and pesticides are used to support the substantial 

crop yields attained in the State. Urban and residential fertilizer and pesticides also are used in Iowa cities and towns, 

which are generally small with dwellings surrounded by lawns 

and gardens. Livestock wastes also are applied to crops as an 

important source of nutrients. Iowa has the largest inventory 

and sales for swine of any state in the Nation (U.S. Department of Agriculture, 2009), with much of this production in 

confined animal feeding operations (CAFOs).

The U.S. Geological Survey (USGS) began a project in 

spring 2004 in cooperation with the Iowa Department of Natural Resources (IDNR) to assess water quality of major rivers in 

Iowa and to allow estimation of select constituent loads. Other 

research to estimate loads in Midwest streams has focused on 

delivery to the Gulf of Mexico, but that research suggests that 

for many constituents like nitrogen, in-stream processing is 

minimal for large rivers (Aulenbach and others, 2007; Alexander and others, 2000); thus, transport from the State’s major 

rivers provides a good indication of potential effects to downstream areas. Prior nationwide or Mississippi River sampling 

networks used to estimate loads have included limited areas of 

the State, or loads were estimated for unmonitored sites from 

landscape attributes. For example, The SPAtially Referenced 

Regression On Watershed attributes (SPARROW) model 

provides valuable information on nutrient loads as relative 

subbasin contributions delivered to the Gulf of Mexico (Alexander and others, 2008). SPARROW, however, is based on 

long-term average conditions, and is not intended to describe 

temporal patterns or the effects of ongoing and changing landuse practices.

Purpose and Scope

Water-quality data summarized in this report were collected at 10 major rivers draining Iowa from March 2004 

through September 2008. The purpose of this report is twofold: (1) summarize nitrogen, phosphorus, organic carbon, 

suspended sediment, select ions, and select pesticides data 

in major Iowa streams; and (2) present estimated loads and 

yields for select ions, nutrients, and suspended sediment being 

transported to the Mississippi and Missouri Rivers from major 

streams in Iowa.

Results presented in this report on stream nutrient delivery can be used to evaluate water-quality conditions during a 

specific time and to identify emerging water-quality trends. 

This knowledge will contribute to continuing research into the 

effects of landscape, land use, and climate on local water quality and downstream delivery of nutrients affecting downstream 

rivers and the Gulf of Mexico.

Study Area Description

The major basins of Iowa drain into the Mississippi and 

Missouri Rivers bordering the east and west respective sides 

of the State, with 6 of the 10 basins studied flowing directly 

into the Mississippi River. The basins total 50,562 square 

miles (mi2

) and range in size from 871 to 14,038 mi2 (table 1, 

fig. 1). These basins include 75.0 percent of Iowa’s total land 

area, and 17.1 percent of the study basin area extends beyond 

Iowa into eastern South Dakota and southern Minnesota (U.S. 

Geological Survey, 2009). Annual average precipitation varies from 22 inches at the northwest extent of the study area 

to 38 inches toward southeast Iowa (National Oceanic and 

Atmospheric Administration, 2003). Mean annual streamflows 

(1979–2008) near the mouths of the 10 basins range from 

461 to 10,293 cubic feet per second (ft3/s) (table 1; U.S. Geological Survey, 2009).

Land use in the study basins is predominately agriculture, 

and cities and towns are generally small with low population 

densities. Widespread row-crop agriculture in Iowa accounts 

for 73 percent of the State, with 86 percent of crop production 

in corn and soybeans (U.S. Department of Agriculture, 2009, 

fig. 1). Median and mean 2008 estimated population densities 

in Iowa cities and towns were 740 and 841 people per square 

mile (people/mi2

), respectively. County level densities ranged 

from 9.5 to 746 people/mi2

, averaging 54 people/mi2 statewide 

(U.S. Census Bureau, Population Estimates Program, http://

www.census.gov/popest/estimates.php, accessed January 27, 

2011).

The landforms of the study basins (fig. 2) typify glacial 

plains and karst landscapes, including alluvial valleys, glacial 

outwash plains, loess hills, and recent and well-weathered 

glacial areas. Most of the study basins are in the Western Corn 

Belt Plains ecoregion, defined by U.S. Environmental Protection Agency and characterized by fertile, moist, glacial, commonly calcareous soils formerly covered with tallgrass prairie, 

now one of the most productive areas for corn and soybeans 

in the world (U.S. Environmental Protection Agency, 2009b). 

Within the Western Corn Belt Plains, the Western Loess Hills 

and the Steeply Rolling Loess Prairies are noteworthy for 

highly erodible soils and steep slopes. Basins also include 

areas of the Northern Glaciated Plains, Driftless Area, Central 

Irregular Plains, and Missouri and Upper Mississippi Alluvial Plains ecoregions. The parts of the Northern Glaciated 

Plains in the study basins are flat to gently rolling, subhumid 

grasslands and wetlands with fertile soil, but with climatic 

limitations on agriculture. The Driftless Area of northeast 
Methods 3

Table 1. Site information and streamflow summary for selected major Iowa rivers, water years 2004–2008.

[Water year from October 1 to September 30; ID, identifier; mi2, square miles; ft3/s, cubic feet per second; --, not available]

Map ID 

(fig. 2)

Station

number Station name

Contributing 

area 

(mi2

)

Mean annual streamflow (ft3

/s)

1979–2008 2004 2005 2006 2007 2008

1 06485500 Big Sioux River at 

Akron, Iowa

6,996 2,023 1,209 1,435 2,395 2,189 2,183

2 06607500 Little Sioux River at 

Turin, Iowa

3,526 1,895 1,338 1,666 1,618 2,184 2,714

3 06609500 Boyer River at Logan, 

Iowa

871 461 320 266 138 691 941

4 06810000 Nishnabotna River 

above Hamburg, Iowa

2,806 1,740 1,355 1,047 539 2,665 3,617

5 05490500 Des Moines River at 

Keosauqua, Iowa

14,038 9,111 7,500 6,447 5,002 12,490 18,680

6 05474000 Skunk River at Augusta, 

Iowa

4,312 3,175 2,491 2,002 1,046 4,425 7,009

7 05465500 Iowa River at Wapello, 

Iowa

12,500 10,293 9,386 7,211 5,970 12,700 21,740

8 05422000 Wapsipinicon River near 

De Witt, Iowa

2,336 2,079 2,288 1,055 1,276 2,584 4,503

9 05418600 Maquoketa River near 

Spragueville, Iowa

1,632 -- -- -- -- -- --

05418500 1

Maquoketa River at 

Maquoketa, Iowa

1,553 1,251 1,366 673 680 1,254 3,191

10 05412500 Turkey River at Garber, 

Iowa

1,545 1,276 1,534 733 874 1,562 2,788

1

Samples for Maquoketa River collected downstream of the steamflow gage.

Iowa, distinct from the corn belt regions, is characterized by 

limestone and dolomite karst, steep hills, and exposed bedrock 

bluffs, with little glacial deposits only on ridge tops. Dairy 

farming is common, with crops grown in patches where slopes 

permit and pastureland and dense woodlands on steeper terrain. The Central Irregular Plains in southern Iowa are less 

uniform in topography and land use than the Western Corn 

Belt Plains to the north; this southern Iowa region includes 

broader forested riparian areas and areas of previous coal mining. Missouri and Upper Mississippi Alluvial Plains include 

recent alluvial deposits, as well as broader plains of glacial 

outwash.

Methods

This section describes protocols and methods for collection and analysis of water-quality data. Continuous streamflow 

data and results of individual sample measurements and chemical analyses were published annually in the USGS water-data 

reports (Nalley and others, 2005a, b; U.S. Geological Survey, 

2007–2009) and also are available on the USGS National 

Water Information System website (NWISWeb) at http://

waterdata.usgs.gov/nwis (U.S. Geological Survey, 2009).

Water-Quality Data Collection

Water-quality samples were collected and field properties 

were measured approximately monthly at 10 sites, which were 

at or near previously established streamflow-gaging stations, 

near the mouths of basins representing large parts of the State 

of Iowa. Samples were collected during all seasons beginning March 2004 with an emphasis on storm and snowmelt 

events to support load and yield estimation. Samples were 

collected using isokinetic, streamflow-weighted sampling 

techniques (equal width increment, EWI), except where low 

streamflow velocities (less than 1.5 ft3

/s) or safety considerations (for example, flooding or ice) necessitated adjustments to these protocols to obtain a representative sample 

of the stream. Maquoketa River samples (map ID 9, station 

number 05418600) routinely were collected downstream of 

the streamflow-gaging station at Maquoketa (station number 

05418500), due to bridge safety considerations.

Protocols and equipment used for sample preparation, 

collection, processing, and quality assurance are described 
95° 90°

46°

44°

42°

40°

IOWA

MINNESOTA

ILLINOIS

NEBRASKA

MISSOURI

SOUTH DAKOTA

WISCONSIN

55

6

7

8

9

10

4

3

2

1

6

8

9

10

4

3

2

1

Base from U.S. Department of Agriculture, 2009

Albers Equal-area Conic projection

River

River

EXPLANATION

Surface-water-quality monitoring 

site with map identifier

Basin contributing area boundary

Cropped area (in percent)

30–60

60–70

70–80

80–90

90–95

6

0 50 100 150 MILES

0 50 100 150 KILOMETERS

Dubuque Sioux City

Council Bluffs

Ames

Waterloo

Iowa City Des Moines

Cedar Rapids

Ames

WaterlooWaterloo

Iowa CityIowa City Des MoinesDes Moines

Cedar Rapids

DavenportDavenport

Mississippi

Missouri

Council Bluffs

Figure 1. Study sites and land use of river basins.

4 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

in the USGS National field manual for the collection of 

water-quality data (U.S. Geological Survey, variously dated). 

Methanol, as used to remove residual organic compounds such 

as pesticides, was omitted from cleaning protocols to avoid 

erroneously high dissolved organic carbon (DOC) results. 

Blank, replicate, and spike quality control (QC) samples 

were routinely collected and analyzed for bias, variability, 

and analytical recovery. For every 11 environmental samples 

collected, 1 QC sample was collected, totaling 16 blanks, 

28 replicates, and 9 pesticide spikes. The ratio for replicate 

sample QC was 20:1. With the omission of methanol, qualitycontrol field blank samples indicated reduced occurrence of 

DOC without carryover of pesticides.

Samples were analyzed by the USGS National WaterQuality Laboratory in Denver, Colorado and the USGS 

Sediment Laboratory in Iowa City, Iowa. Field properties, 
95° 90°

44°

42°

40°

Base from U.S. Environmental Protection Agency digital

data, 2006, 1:250,000

Albers Equal-area Conic projection

0 50 100 150 MILES

0 50 100 150 KILOMETERS

River

River

Mississippi

Missouri

Big Sioux River (1)

Little Sioux River (2)

Boyer River (3)

Nishnabotna River (4)

Turkey River (10)

Maquoketa River (9)

Wapsipinicon River (8)

Iowa River (7)

Des Moines River (5) Skunk River (6)

EXPLANATION

Surface-water-quality monitoring site with 

map identifier

Basin contributing area boundary

Ecoregions

Northern Glaciated Plains

Western Corn Belt Plains

Western Loess Hills and Steeply Rolling 

Loess Prairie

Missouri and Upper Mississippi Alluvial 

Plains

Driftless Area

Central Irregular Plains

Skunk River (6)

Figure 2. Study sites and ecoregions of river basins.

Methods 5

analytical constituents, and field or analytical methods are outlined in table 2. Analytical results are available in the USGS 

National Water Information System database (U.S. Geological 

Survey, 2009). This report contains CAS Registry Numbers®, 

which is a registered trademark of the American Chemical 

Society. CAS recommends the verification of the CASRNs 

through CAS Client ServicesSM.
6 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Table 2. Constituents analyzed at selected major Iowa rivers, water years 2004-2008.

[Water year from October 1 to September 30; --, not available; mm Hg, millimeters of mercury; USGS, U.S. Geological Survey; μS/cm, microsiemens per centimeter; °C, degrees celsius; NTRU, nephelometric turbidity ratio units; mg/L, miligrams per liter; CaCO3

, calcium carbonate; SiO2, silicon dioxide; N, nitrogen; 

P, phosphorus; μg/L, micrograms per liter; DOC, dissolved organic carbon]

Analyte Parameter 

code CAS1

 number Units Reference for field or 

analytical method

Physical properties and sediment

Barometric pressure 00025 -- mm Hg USGS, variously dated.

pH 00400 -- standard units USGS, variously dated.

Specific conductance 00095 -- μS/cm at 25oC USGS, variously dated.

Water temperature 00010 -- °C USGS, variously dated.

Turbidity 63676 -- NTRU USGS, variously dated.

Suspended sediment 80154 -- mg/L Guy, 1969.

Major ions

Dissolved oxygen 00300 7782-44-7 mg/L USGS, variously dated.

Alkalinity, filtered 39086 -- mg/L as CaCO3 USGS, variously dated.

Bicarbonate, filtered 00453 71-52-3 mg/L USGS, variously dated.

Carbon, total, particulate 00694 7440-44-0 mg/L Zimmermann, 1997.

Carbonate, filtered 00452 3812-32-6 mg/L USGS, variously dated.

Chloride, filtered 00940 16887-00-6 mg/L Fishman and Friedman, 1989.

Carbon, inorganic, particulate 00688 -- mg/L Zimmermann, 1997.

Silica, filtered 00955 7631-86-9 mg/L as SiO2 Fishman and Friedman, 1989.

Sulfate, filtered 00945 14808-79-8 mg/L Fishman and Friedman, 1989.

Nutrients and organic carbon

Ammonia (NH3

+NH4), filtered 00608 7664-41-7 mg/L as N Fishman, 1993.

Nitrate plus nitrite, filtered 00631 -- mg/L as N Fishman, 1993.

Nitrite, filtered 00613 14797-65-0 mg/L as N Fishman, 1993.

Orthophosphate, filtered 00671 14265-44-2 mg/L as P Fishman, 1993.

Total nitrogen, particulate 49570 17778-88-0 mg/L as N Zimmermann, 1997.

Total phosphorus, unfilterd 00665 7723-14-0 mg/L as P O’Dell, 1993.

Total nitrogen (NH3

+NO2+NO3+organic), filtered 62854 17778-88-0 mg/L as N Patton and Kryskalla, 2003.

Total nitrogen (NH3

+NO2+NO3+organic), unfiltered 62855 17778-88-0 mg/L as N Patton and Kryskalla, 2003.

Organic carbon, particulate 00689 -- mg/L Zimmermann, 1997.

Organic carbon, filtered (DOC) 00681 -- mg/L Brenton and Arnett, 1993.

Algal pigments

Chlorophyll-a 70953 479-61-8 μg/L Arar and Collins, 1997.

Pheophyton-a 62360 603-17-8 μg/L Arar and Collins, 1997.

1

This report contains CAS Registry Numbers®, which is a registered trademark of the American Chemical Society. CAS recommends the verification of the 

CASRNs through CAS Client ServicesSM
Methods 7

Statistical Data Summary

Statistical and graphical summaries of water-quality 

data were computed using the TIBCO Spotfire S+® statistical 

package (TIBCO Software Inc., 2008). Because of laboratory 

reporting conventions, censored values, or values reported as 

less than the laboratory reporting level (LRL), were re-coded 

to less than the long-term method detection limit (LT-MDL, 

typically one-half the reporting level) for the purposes of 

statistical summary and plotting. This adjustment is needed to 

account for laboratory data-reporting practices of estimating 

positive detections above the long-term method detection limit 

but below the reporting level (Childress and others, 1999). 

Concentrations are reported as <LRL for samples in which 

the analyte was either not detected or did not pass identification. Analytes that are detected at concentrations between the 

LT-MDL and the LRL and that pass identification criteria are 

reported as estimated. This information-rich laboratory reporting convention retains information about positive low-level 

detections, which would otherwise be treated simply as a nondetect. Estimated concentrations are reported with a remark 

code of “E” to indicate greater uncertainty than data reported 

without the “E” remark code, including values between the 

LT-MDL and LRL and values with uncertainty affected by 

matrix effects.

Summary statistics of water-quality data were computed 

using regression on order statistics (ROS) and adjusted maximum likelihood estimation (AMLE) parametric methods. For 

handling datasets with values reported below detection limits 

and for multiple levels of detection, these methods provide 

better estimation of mean (ROS) and percentiles (AMLE) than 

either simple substitution (such as one-half detection limit or 

zero) or ignoring values below detection (Helsel and Hirsch, 

2002). Percentiles for pesticide water-quality data were computed using a modification of the Kaplan-Meier nonparametric 

method (Helsel, 2005). This rank-based nonparametric method 

is more appropriate to the less frequent detection of many 

pesticides.

Locally weighted scatterplot smooth (LOWESS) lines are 

used in figure 3 to conveys similarities and differences among 

sites, in regards to the overall distribution of the data, and 

in relation to streamflow. The LOWESS line represents the 

pattern through the middle of the data, with a weighting function applied for distance from each point along the x-axis and 

magnitude of residuals in the y-direction (Helsel and Hirsch, 

2002).

Load and Yield Estimation

Stream load is defined as the mass of a chemical constituent transported by a stream past one location during a specific 

period of time, and is expressed in units such as tons per year. 

Stream yield denotes stream load divided by watershed area 

expressed in units of mass per year per area, and can be used 

to compare the relative contributions of constituents from 

watersheds of different sizes.

Annual loads were estimated using S-LOADEST based 

on the Fortran LOADEST program described by Runkle and 

others (2004) and operated in Spotfire S+®. S-LOADEST 

generates a regression equation between load and variables 

for streamflow and additional terms specified by the user. 

Models for each site and constituent were fit using sample 

data from 2004–2008. The models were applied using daily 

values for streamflow and additional terms to compute daily 

loads, which were summed to calculate annual load estimates. 

Explanatory terms are incorporated as linear additions to the 

model. S-LOADEST uses AMLE (Cohn, 2005) to correct 

biases because of censored data and retransformation of load 

estimated as a log-transformed variable. In cases where the 

assumption of normality of model residuals could not be met, 

a least absolute deviations (LAD) method was used.

Standard explanatory terms in S-LOADEST include linear and quadratic terms for streamflow and time plus seasonal 

terms for time. Streamflow uses daily mean values to correspond to the minimum 1-day time step. Streamflow and time 

terms are centered so linear and quadratic terms are orthogonal, to eliminate problems associated with collinear explanatory variables (Runkle and others, 2004). Sine and cosine 

time terms use an annual phase to describe seasonal patterns. 

A break-point term (BpQ) divides streamflow into two linear 

segments, and is used in cases where the relation between 

constituent load and streamflow is better empirically described 

by separate models at low and high flows. 

To improve estimates of storm and snowmelt event transport, additional terms describing streamflow variability and 

anomalies were evaluated. High-flow events can account for a 

large part of the total annual load relative to the duration of the 

events, and rating curve methods tend to underestimate loads 

at high streamflows (Horowitz, 2003) because the streamflow 

and time terms in the predefined regression models do not 

account for event-level factors such as hysteresis or recent 

events. Hysteresis occurs when concentrations (and thereby 

loads) are different on rising and falling limbs of an event 

hydrograph for the same magnitude of streamflow.

Streamflow variability terms were defined as the difference between mean streamflow (Q) on day i and the mean 

streamflow of the previous k days, given as:

dQ Q Q k k i j

j i k

i

= − = − −

−

ln ln ∑ / 1

1

(1)

This variability term (dQ) with a 1-day time step (dQ1

) helps 

describe effects of hysteresis (Wang and Linker, 2008). A term 

with a 30-day step (dQ30) helps describe effects of sequential 

events or prolonged event peaks. In some instances, the absolute value of dQ1

 (|dQ1|) better describes loads in the regression model than dQ1

, representing cases where the degree of 

flashiness of the event was the critical element, rather than 

hysteresis.
8 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

An alternate approach to incorporating streamflow history into load estimation uses time-series terms for streamflow 

anomalies to describe deviations from average conditions 

(Vecchia, 2003). The form of the load regression equation with 

anomalies is given:

ln L b = +o b A1 2 5 1 yr + + b A yr b A3 4 3mo + b HFV (2)

where A5yr, A1yr, A3mo, and HFV sequentially account for 

variability from long-term average streamflow over different 

time scales (5-year, 1-year, 3-month, and high-frequency variability, respectively), with regression coefficients, b. Anomalies were computed in Spotfire S+® using daily streamflow 

data beginning 10 years before the load estimation period. 

A5yr is the average over a 5-year interval of daily deviations 

from the long-term average streamflow. A1yr is the average 

over a 1-year interval of daily deviations between deviations 

from the long-term average streamflow and A5yr. A3mo and 

HFV sequentially account for additional variability at finer 

time scales. Thus, high values for A1yr indicate a wetter year 

than the previous 5 years; similarly, low values for A3mo 

indicate a drier season than the previous year. Vecchia (2003) 

further describes streamflow anomalies.

The process of building and selecting candidate load 

models using streamflow, time, seasonality, hysteresis, and 

anomaly terms included automated selection procedures and 

evaluation of model fit, assumptions on residuals, and correlation of the explanatory variables. First, several procedures 

were used in tandem to generate a list of candidate models. 

S-LOADEST models with standard terms were ranked based 

on Akaike Information Content (AIC) and Schwarz Posterior 

Probability Criterion (SPPC). Hysteresis terms and streamflow 

anomalies were evaluated separately with stepwise selection to 

identify terms that contributed to the standard models. Second, 

diagnostic tests and plots were considered in selection of the 

candidate models. Preferred models had low residual variance, 

residual plots indicating normality and homoskedasticity, and 

low correlation among explanatory variables indicated by low 

pairwise correlation and a low multicollinearity statistic (variance inflation factor, VIF). Finally, fit of the best candidate 

model was verified by comparing observed and predicted daily 

loads (average measured load from sampled days divided by 

average estimated load from sampled days) (Stenback and others, 2011). Models for which this ratio was less than one-half 

or greater than two were not used, and an alternate candidate 

model was evaluated. Therefore, in this study, models that did 

not meet these selection criteria were not considered appropriate for estimating loads on unsampled days. For example, of 

two potential models with different variable selections, the 

model with a greater residual variance still may have been 

preferred if diagnostics from the other model indicated a 

problem with non-normal residuals or an unacceptable ratio of 

measured/estimated loads.

Because the goal of the modeling was to compute annual 

loads, models calibrated using certain outlier data points also 

were not considered appropriate for estimating loads. Outliers 

were removed from the calibration dataset only if individual 

data points exhibited undue influence on model parameter 

estimates and models with an alternate selection of variables 

could not be used that included all data points. The undue 

influence of outliers can “pull” the model in one area of the 

data, resulting in poor model fit in areas critical for the annual 

load estimates. Exclusion of data does not indicate “bad” data, 

but rather can indicate an environmental response to something not accounted for in the model (for example, upstream 

chemical spill). Overall, 1 percent of data were excluded as 

outliers, with no more than three outliers for any one site and 

constituent model. Outliers were generally of two categories—

high-streamflow event samples and low-streamflow samples 

with evidence of algae blooms (noted green water color, 

supersaturated oxygen concentration, for example). Because 

high streamflow events contribute far more to annual transport 

than low streamflow, influential outliers at high streamflows 

were removed only as final course, and the highest sampled 

flow was never removed from a model calibration set. Lowstreamflow influential outliers resulted in models with greater 

residual errors at high streamflows. Again, high streamflow 

periods were considered more critical to the objective of 

estimating annual loads. The potential risk of overestimating concentrations at low streamflow was acknowledged and 

accepted, as the low-streamflow load contributions to the 

annual totals were less critical than the high-streamflow load 

estimates.

Load models calibrated for each site and constituent with 

the 5-year sample dataset were used to estimate loads for all 

of 2004 through 2008 water years (WY, defined as October 1 

of the previous calendar year through September 30 of the 

specified year), including standard errors of prediction (SEP) 

and upper and lower 95-percent confidence limits (U95, L95). 

Routine sampling began in March 2004. Load estimates for 

the first one-half of WY2004 required extrapolation below the 

range of sampled streamflows for all sites except the Maquoketa River near Spragueville (map identifier [ID] 9); the 

amount of extrapolation needed ranged from a few days when 

streamflows dipped below the range of sampled flows to the 

first 5 months at the Big Sioux River at Akron (map ID 1) and 

Little Sioux River near Turin (map ID 2).

Chemical Concentration, Loads, and 

Yields in Major Iowa Rivers

Summaries of concentration data, estimated loads, and 

basin yields are presented in the following section. Concentrations, loads, and yields were analyzed in various ways to 

describe the data within the context of the landscape and environmental gradients across the State and through time.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 9

Concentrations

Data from routine and high-flow event water samples 

indicate the effect of streamflow, seasonality, spatial gradients, 

and basin size. Summary statistics are presented in table 3 for 

physical properties and concentrations of ions, carbon, nutrients, and suspended sediment. Pesticide summary statistics 

are presented in table 4 for compounds detected in at least 

one sample. Pesticides analyzed but not detected through the 

5-year study period are listed in table 5.

Streamflow effects on concentrations varied by constituent, but patterns were generally consistent among sites. 

Streamflow was inversely related to some concentrations, indicating a general dilution effect for pH, alkalinity, specific conductance, chloride, and sulfate (fig. 3A–B). Other constituents 

such as particulate organic carbon (POC), dissolved organic 

carbon (DOC), total phosphorus, and suspended sediment had 

increased concentrations with streamflow (fig. 3C–F). Nitrogen, in the forms of total nitrogen and nitrate plus nitrite, had 

more complex relations to streamflow, with increasing concentrations through low to average streamflows but decreasing 

concentrations with streamflows above the 90th percentile for 

each site. Concentration showed little or no relation to streamflow for silica, particulate inorganic carbon (PIC), or chlorophyll-a. Orthophosphate concentrations also were generally 

unaffected by or slightly increasing with streamflow, except in 

the Boyer River (map ID 3), where orthophosphate indicated a 

strong inverse relation to streamflow (fig. 3E–F).

Because streamflow patterns are seasonal, generally 

peaking in May and June, patterns in concentration related 

to streamflow also are seasonal. Seasonal patterns for pH 

and alkalinity were related to streamflow, with low values 

evidence of dilution from events common in spring, and high 

values typical of low stable flows in late summer through 

winter (fig. 4A). A few constituents, however, showed seasonal 

patterns not entirely related to streamflow. Algal pigments 

(chlorophyll-a and pheophytin-a) were affected by late summer algal blooms (chlorophyll-a; fig. 4B). Occasional high 

concentrations of algal pigments indicated algae blooms in late 

winter to early spring, particularly in rivers such as the Little 

Sioux and Des Moines (map ID 5) with upstream lakes or 

reservoirs. Most detected pesticides tended to have peak concentrations in early spring, shortly after agricultural application (fig. 4C), but concentrations of the nonselective herbicide 

prometon were greatest from summer through fall (fig. 4D).

Spatial variability of concentrations reflected landform 

and climate gradients across the State for some constituents. 

Alkalinity was greater at the northwestern and northeastern 

sites. Chloride and dissolved organic carbon indicated spatial 

trends along the border rivers, with increasing concentrations 

upstream along the Missouri River tributaries (map IDs 4 to 

1) and downstream along the Mississippi River tributaries 

(map IDs 10 to 5). Turbidity, suspended-sediment, and total 

phosphorus concentrations were greatest in southwestern Iowa 

streams (map ID 3–4) in basins draining the highly erodible Western Loess Hills. Specific conductance and sulfate 

concentrations in the Big Sioux River (map ID 1) were the 

greatest and most variable of all the sites. Several constituents did not show a pronounced spatial trend, including pH, 

silica, particulate organic carbon, particulate inorganic carbon, 

nitrogen (all forms), and algal pigments. The relation between 

streamflow and concentration also varied by site for POC, 

orthophosphate, suspended sediment, and turbidity.

Major Ions

Major Iowa rivers are generally alkaline and wellbuffered, commonly with calcareous soils and underlying and 

exposed limestone and dolomite bedrock. For samples from all 

10 rivers collected from March 2004 through September 2008, 

pH was rarely measured below 7.0, and was occasionally 

measured above 9.0. Mean and median pH were both about 

8.1 (table 3). The lowest pH levels occurred during storm and 

snowmelt events when streamwaters were diluted by rainfall or snowmelt (most common in the spring), and greatest 

pH levels occurred during long periods of stable streamflow 

without major precipitation (most common in the summer 

and fall) (figs. 3A–B, 4A). Sample pH summary statistics were 

similar for all 10 sites, indicating no major spatial gradients, 

though discrete sampling data did not allow for analysis of 

site-specific diurnal patterns. Sample alkalinities commonly 

ranged from 159 to 242 milligrams per liter as calcium carbonate (mg/L as CaCO3

), summarized as the 25th to 75th percentiles for all sites, with an overall range of alkalinities from 

42.5 to 356 mg/L as CaCO3 (table 3). Similar to pH, alkalinity 

measurements were lower from rainfall dilution and greater 

during long periods of stable streamflow, with greatest alkalinity values in the winter (figs. 3A–B, 4A). Alkalinity exhibited 

spatial trends among sites, with greater alkalinities in northern 

sites (map IDs 1, 2, 3, 9, and 10) and rivers draining central 

basins with alkalinities trending lower from west to east basins 

(map IDs 4 to 8) (fig. 3A–B).

Specific conductance, chloride, sulfate, and silica concentrations reflect the extent of ion-leaching of natural minerals 

from the landscape and anthropogenic sources such as point 

discharges, winter de-icing salts, and agricultural application. 

Silica concentrations also are affected by biological uptake 

of diatoms (Wetzel, 1983). For all sites, specific conductance varied from 168 to 1,220 microsiemens per centimeter 

at 25 degrees Celsius (μS/cm at 25°C) with a median of 

574 μS/cm at 25°C and mean of 586 μS/cm at 25°C (table 3). 

High concentrations generally were observed during low or 

stable streamflows and low concentrations were the result of 

rainfall or snowmelt dilution during high streamflows. The Big 

Sioux River at Akron (site/map ID 1) in the northwest had the 

greatest and most variable specific conductance values compared with other sites. Chloride concentrations ranged from 

2.25 to 86.2 mg/L with a median of 21.6 mg/L and a mean 

of 23.4 mg/L (table 3). The inverse relation between chloride concentration and streamflow evident at every site was 

pronounced at some sites, with a coefficient of determination 

(R2

) from linear regression (Helsel and Hirsch, 2002) between 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008. [Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units] Map ID (fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75 pH, standard units [00400] 1 Big Sioux River at Akron, Iowa 58 0 7.2 7.9 8.1 8.3 0.5 

2 Little Sioux River at Turin, Iowa 58 0 6.9 7.8 8.1 8.3 0.5 3 Boyer River at Logan, Iowa 56 0 6.6 7.9 8.2 8.3 0.4 4 Nishnabotna River above Hamburg, Iowa 57 0 6.9 7.7 8.0 8.2 0.5 5 Des Moines River at Keosauqua, Iowa 55 0 7.0 8.0 8.4 8.6 0.6 6 Skunk River at Augusta, Iowa 56 0 7.2 7.8 8.2 8.5 0.7 7 Iowa River at Wapello, Iowa 61 0 6.8 7.9 8.2 8.5 0.6 8 Wapsipinicon River near De Witt, Iowa 57 0 7.3 7.8 8.1 8.4 0.6 9 Maquoketa River near Spragueville, Iowa 56 0 7.3 7.9 8.1 8.3 0.4 

10 Turkey River at Garber, Iowa 54 0 7.6 7.9 8.1 8.2 0.3 

All samples 568 0 6.6 7.9 8.1 8.4 0.5 

Alkalinity, mg/L as CaCO3 [39086] 1 Big Sioux River at Akron, Iowa 57 0 108 192 242 289 97 

2 Little Sioux River at Turin, Iowa 57 0 76.4 187 245 277 90 

3 Boyer River at Logan, Iowa 56 0 83.1 228 255 285 57 

4 Nishnabotna River above Hamburg, Iowa 57 0 73.4 172 212 230 58 

5 Des Moines River at Keosauqua, Iowa 55 0 102 153 179 212 59 

6 Skunk River at Augusta, Iowa 56 0 85.9 145 180 218 73 

7 Iowa River at Wapello, Iowa 59 0 100 130 173 200 70 

8 Wapsipinicon River near De Witt, Iowa 57 0 84.9 119 140 182 63 

9 Maquoketa River near Spragueville, Iowa 54 0 61.7 204 230 251 47 

10 Turkey River at Garber, Iowa 52 0 42.5 204 228 240 36 

All samples 560 0 42.5 159 206 242 83 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued[Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units]

Map ID 

(fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75

Specific conductance, S/cm [00095] μ

1 Big Sioux River at Akron, Iowa 58 0 317 804 844 1,004 200 

2 Little Sioux River at Turin, Iowa 58 0 168 590 692 731 141 3 Boyer River at Logan, Iowa 56 0 198 631 684 726 95 4 Nishnabotna River above Hamburg, Iowa 57 0 237 490 535 560 70 5 Des Moines River at Keosauqua, Iowa 55 0 348 512 580 656 144 6 Skunk River at Augusta, Iowa 56 0 252 442 536 620 178 7 Iowa River at Wapello, Iowa 62 0 312 447 540 584 137 8 Wapsipinicon River near De Witt, Iowa 57 0 290 373 442 490 117 9 Maquoketa River near Spragueville, Iowa 56 0 189 544 580 607 63 

10 Turkey River at Garber, Iowa 53 0 228 534 577 590 56 

All samples 568 0 168 490 574 666 176 

Chloride, mg/L [00940] 

1 Big Sioux River at Akron, Iowa 58 0 10.8 27.8 32.7 38.9 11.1 

2 Little Sioux River at Turin, Iowa 58 0 5.17 19.6 23.2 25.9 6.3 3 Boyer River at Logan, Iowa 57 0 2.25 18.3 22.2 28.2 9.9 4 Nishnabotna River above Hamburg, Iowa 58 0 4.28 12.8 15.1 17.8 5.0 5 Des Moines River at Keosauqua, Iowa 56 0 10.9 21.5 27.7 36.1 14.6 

6 Skunk River at Augusta, Iowa 56 0 7.10 19.2 24.8 29.1 9.9 7 Iowa River at Wapello, Iowa 60 0 7.84 22.0 29.3 35.7 13.7 8 Wapsipinicon River near De Witt, Iowa 57 0 7.32 18.4 20.8 22.6 4.2 9 Maquoketa River near Spragueville, Iowa 56 0 4.87 16.4 18.1 20.0 3.6 

10 Turkey River at Garber, Iowa 55 0 5.77 16.8 19.0 20.2 3.4 

All samples 571 0 2.25 17.4 21.6 28.2 10.8 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued [Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units] Map ID (fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75 Sulfate, mg/L [00945] 1 Big Sioux River at Akron, Iowa 58 0 34.6 159 189 204 45 

2 Little Sioux River at Turin, Iowa 58 0 13.6 58.2 70.7 80.5 22.3 3 Boyer River at Logan, Iowa 57 0 7.26 37.5 42.7 50.7 13.2 4 Nishnabotna River above Hamburg, Iowa 58 0 8.50 24.8 29.0 32.1 7.3 5 Des Moines River at Keosauqua, Iowa 56 0 20.0 37.5 53.2 71.2 33.7 6 Skunk River at Augusta, Iowa 56 0 9.38 28.1 33.1 42.6 14.5 7 Iowa River at Wapello, Iowa 60 0 8.86 24.3 31.7 36.5 12.2 8 Wapsipinicon River near De Witt, Iowa 57 0 7.72 20.7 24.5 27.3 6.6 9 Maquoketa River near Spragueville, Iowa 56 0 5.67 23.7 25.3 26.6 2.9 

10 Turkey River at Garber, Iowa 55 0 7.27 22.6 25.0 27.2 4.6 

All samples 571 0 5.67 25.2 32.9 54.5 29.3 

Silica, mg/L [00955] 1 Big Sioux River at Akron, Iowa 58 0 1.5 10.5 13.5 16.0 5.5 

2 Little Sioux River at Turin, Iowa 58 0 2.9 11.1 15.7 18.1 7.0 

3 Boyer River at Logan, Iowa 56 0 4.5 11.3 13.9 16.6 5.3 

4 Nishnabotna River above Hamburg, Iowa 57 0 6.8 11.6 16.5 15.8 4.2 

5 Des Moines River at Keosauqua, Iowa 54 0 3.2 10.4 13.4 15.4 5.0 6 Skunk River at Augusta, Iowa 54 3 0.2 7.2 12.2 15.1 7.9 

7 Iowa River at Wapello, Iowa 29 1 1.5 8.0 11.1 13.3 5.3 8 Wapsipinicon River near De Witt, Iowa 55 1 0.1 2.6 7.62 10.3 7.7 9 Maquoketa River near Spragueville, Iowa 56 0 4.0 4.8 9.96 11.7 6.9 

10 Turkey River at Garber, Iowa 55 0 2.1 5.4 9.23 10.9 5.5 

All samples 532 5 0.1 8.8 11.6 14.8 6.0 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued[Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units]

Map ID 

(fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75

Organic carbon, particulate, mg/L [00689] 

1 Big Sioux River at Akron, Iowa 58 0 0.38 5.31 7.87 11.5 6.2 

2 Little Sioux River at Turin, Iowa 58 0 0.36 3.72 7.52 12.2 8.5 3 Boyer River at Logan, Iowa 56 0 0.30 1.42 3.41 9.41 7.99 4 Nishnabotna River above Hamburg, Iowa 57 0 0.43 1.51 5.24 12.3 10.8 5 Des Moines River at Keosauqua, Iowa 54 0 0.41 1.52 2.38 3.83 2.31 6 Skunk River at Augusta, Iowa 56 0 0.44 3.11 4.35 6.88 3.77 7 Iowa River at Wapello, Iowa 41 0 0.48 2.66 4.71 8.06 5.4 8 Wapsipinicon River near De Witt, Iowa 55 0 0.27 1.76 4.64 9.05 7.29 9 Maquoketa River near Spragueville, Iowa 56 0 0.28 1.16 2.48 4.30 3.14 

10 Turkey River at Garber, Iowa 55 1 0.27 0.72 1.57 3.82 3.11 

All samples 543 1 0.27 1.57 4.00 8.53 6.96 

Inorganic carbon, particulate, mg/L [00688] 

1 Big Sioux River at Akron, Iowa 58 17 0.04 0.05 0.27 1.04 0.99 2 Little Sioux River at Turin, Iowa 58 21 0.02 <0.04 0.25 0.72 0.69 

3 Boyer River at Logan, Iowa 56 30 0.03 <0.04 0.04 0.18 0.17 

4 Nishnabotna River above Hamburg, Iowa 57 31 0.03 <0.04 0.04 0.19 0.17 

5 Des Moines River at Keosauqua, Iowa 54 42 0.03 <0.04 <0.04 0.03 0.03 6 Skunk River at Augusta, Iowa 53 34 0.02 <0.04 0.02 0.12 0.11 

7 Iowa River at Wapello, Iowa 41 24 0.02 0.03 0.13 0.67 0.65 8 Wapsipinicon River near De Witt, Iowa 55 21 0.02 <0.04 0.15 0.56 0.54 

9 Maquoketa River near Spragueville, Iowa 56 40 0.03 <0.04 0.02 0.08 0.07 

10 Turkey River at Garber, Iowa 55 33 0.02 <0.04 0.02 0.24 0.23 

All samples 543 283 0.02 0.01 0.05 0.29 0.28 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued [Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units] Map ID (fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75 Organic carbon, dissolved, mg/L [00681] 1 Big Sioux River at Akron, Iowa 58 0 2.4 3.8 4.6 6.4 2.6 

2 Little Sioux River at Turin, Iowa 58 0 2.3 2.9 3.4 4.2 1.4 3 Boyer River at Logan, Iowa 56 0 1.8 2.4 2.8 4.3 1.9 4 Nishnabotna River above Hamburg, Iowa 57 0 1.6 2.2 2.6 3.9 1.6 5 Des Moines River at Keosauqua, Iowa 54 0 3.3 4.1 4.4 5.1 1.0 6 Skunk River at Augusta, Iowa 53 0 2.4 3.0 3.9 5.2 2.2 7 Iowa River at Wapello, Iowa 42 0 2.5 3.5 4.0 4.9 1.4 8 Wapsipinicon River near De Witt, Iowa 55 0 1.7 2.4 2.9 3.9 1.5 9 Maquoketa River near Spragueville, Iowa 56 0 9.4 1.7 2.3 3.0 1.3 

10 Turkey River at Garber, Iowa 55 0 0.9 1.7 2.3 3.1 1.5 

All samples 544 0 0.9 2.5 3.5 4.5 2.1 

Total nitrogen, mg/L [49570 plus 62854, or 62855] 1 Big Sioux River at Akron, Iowa 56 0 0.39 6.03 7.31 9.07 3.04 

2 Little Sioux River at Turin, Iowa 58 0 2.06 6.22 8.95 10.3 4.1 3 Boyer River at Logan, Iowa 57 0 3.42 7.48 9.34 11.5 4.0 

4 Nishnabotna River above Hamburg, Iowa 58 0 2.22 4.93 7.38 9.25 4.32 5 Des Moines River at Keosauqua, Iowa 56 0 2.40 4.72 6.42 9.07 4.35 6 Skunk River at Augusta, Iowa 56 0 0.71 4.41 7.12 9.87 5.46 7 Iowa River at Wapello, Iowa 60 0 2.25 5.58 6.64 9.16 3.58 8 Wapsipinicon River near De Witt, Iowa 55 0 1.40 5.01 7.21 8.76 3.75 9 Maquoketa River near Spragueville, Iowa 56 0 3.79 6.29 7.91 9.51 3.22 

10 Turkey River at Garber, Iowa 55 0 4.01 5.99 7.39 10.0 4.0 

All samples 569 0 0.39 5.67 7.49 9.79 4.12 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued[Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units]

Map ID 

(fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75

Nitrate plus nitrite, mg/L [00631] 

1 Big Sioux River at Akron, Iowa 58 0 0.86 4.45 5.84 7.69 3.24 

2 Little Sioux River at Turin, Iowa 58 0 0.30 4.74 7.42 8.70 3.96 3 Boyer River at Logan, Iowa 56 0 1.50 5.45 7.56 9.65 4.20 4 Nishnabotna River above Hamburg, Iowa 58 0 1.66 3.39 5.40 7.46 4.07 5 Des Moines River at Keosauqua, Iowa 56 0 1.02 3.80 4.85 7.40 3.60 6 Skunk River at Augusta, Iowa 56 6 0.15 3.32 5.81 8.41 5.09 7 Iowa River at Wapello, Iowa 60 0 0.23 4.42 5.49 7.88 3.46 8 Wapsipinicon River near De Witt, Iowa 57 1 0.12 4.19 6.18 7.57 3.38 9 Maquoketa River near Spragueville, Iowa 56 0 2.40 5.06 6.80 8.20 3.14 

10 Turkey River at Garber, Iowa 55 0 2.33 5.07 6.78 7.90 2.83 

All samples 571 7 0.12 4.32 6.15 8.23 3.91 

Ammonia, mg/L as N [00608] 

1 Big Sioux River at Akron, Iowa 58 22 0.006 <0.01 0.021 0.094 0.088 

2 Little Sioux River at Turin, Iowa 58 23 0.006 <0.01 0.019 0.058 0.052 

3 Boyer River at Logan, Iowa 57 23 0.009 <0.01 0.022 0.068 0.061 

4 Nishnabotna River above Hamburg, Iowa 58 25 0.007 <0.01 0.020 0.098 0.092 

5 Des Moines River at Keosauqua, Iowa 56 30 0.006 <0.01 0.012 0.044 0.040 

6 Skunk River at Augusta, Iowa 56 27 0.005 <0.01 0.013 0.046 0.042 

7 Iowa River at Wapello, Iowa 60 34 0.006 <0.01 <0.01 0.031 0.028 8 Wapsipinicon River near De Witt, Iowa 57 28 0.007 <0.01 0.013 0.037 0.032 

9 Maquoketa River near Spragueville, Iowa 56 28 0.006 <0.01 0.014 0.046 0.042 

10 Turkey River at Garber, Iowa 55 32 0.009 <0.01 <0.01 0.040 0.038 

All samples 571 270 0.005 <0.01 0.015 0.052 0.048 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued [Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units] Map ID (fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75 Total phosphorus, mg/L [00665] 1 Big Sioux River at Akron, Iowa 58 0 0.159 0.305 0.362 0.499 0.194 

2 Little Sioux River at Turin, Iowa 58 0 0.060 0.184 0.286 0.515 0.331 3 Boyer River at Logan, Iowa 57 0 0.368 0.496 0.688 1.00 0.50 4 Nishnabotna River above Hamburg, Iowa 58 0 0.118 0.236 0.452 0.875 0.639 5 Des Moines River at Keosauqua, Iowa 55 0 0.124 0.208 0.278 0.345 0.137 6 Skunk River at Augusta, Iowa 55 0 0.054 0.197 0.327 0.459 0.262 7 Iowa River at Wapello, Iowa 60 0 0.035 0.267 0.312 0.372 0.105 8 Wapsipinicon River near De Witt, Iowa 57 0 0.055 0.143 0.209 0.265 0.122 9 Maquoketa River near Spragueville, Iowa 56 0 0.081 0.145 0.195 0.308 0.163 

10 Turkey River at Garber, Iowa 55 0 0.023 0.065 0.112 0.208 0.143 

All samples 569 0 0.023 0.191 0.303 0.486 0.295 

Orthophosphate, mg/L [00671] 1 Big Sioux River at Akron, Iowa 58 0 0.003 0.037 0.153 0.218 0.181 

2 Little Sioux River at Turin, Iowa 58 7 0.004 0.050 0.071 0.120 0.071 3 Boyer River at Logan, Iowa 57 0 0.054 0.260 0.347 0.570 0.310 4 Nishnabotna River above Hamburg, Iowa 58 0 0.057 0.098 0.130 0.158 0.060 5 Des Moines River at Keosauqua, Iowa 56 1 0.005 0.086 0.147 0.175 0.089 6 Skunk River at Augusta, Iowa 56 2 0.005 0.058 0.112 0.170 0.112 7 Iowa River at Wapello, Iowa 60 3 0.004 0.054 0.123 0.166 0.112 8 Wapsipinicon River near De Witt, Iowa 57 12 0.003 0.007 0.036 0.074 0.067 9 Maquoketa River near Spragueville, Iowa 56 2 0.008 0.049 0.078 0.134 0.086 

10 Turkey River at Garber, Iowa 55 6 0.006 0.016 0.042 0.090 0.074 

All samples 571 33 0.003 0.050 0.104 0.172 0.123 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued[Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units]

Map ID 

(fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75

Suspended sediment, mg/L [80154] 

1 Big Sioux River at Akron, Iowa 57 0 32 106 181 322 216 2 Little Sioux River at Turin, Iowa 56 0 26 125 247 588 463 3 Boyer River at Logan, Iowa 56 0 5 57.5 260 701 644 4 Nishnabotna River above Hamburg, Iowa 57 0 19 126 321 1,400 1,270 

5 Des Moines River at Keosauqua, Iowa 56 0 3 24.5 73.5 149 125 6 Skunk River at Augusta, Iowa 55 0 9 63 128 380 317 7 Iowa River at Wapello, Iowa 58 0 12 67 130 201 134 8 Wapsipinicon River near De Witt, Iowa 55 0 17 64 113 221 157 9 Maquoketa River near Spragueville, Iowa 54 0 24 85 126 261 176 

10 Turkey River at Garber, Iowa 54 0 29 74 116 191 117 

All samples 558 0 3 74 138 348 274 

Turbidity, NTRU [63676] 

1 Big Sioux River at Akron, Iowa 47 0 2.8 19 52 69 49 2 Little Sioux River at Turin, Iowa 46 0 2.5 25 60 120 95 3 Boyer River at Logan, Iowa 45 0 2.9 17 55 140 120 4 Nishnabotna River above Hamburg, Iowa 46 0 4.3 19 99 240 220 5 Des Moines River at Keosauqua, Iowa 45 0 2.3 9.9 22 39 29 6 Skunk River at Augusta, Iowa 45 0 2.2 17 60 110 93 

7 Iowa River at Wapello, Iowa 29 0 3.9 18 37 61 43 8 Wapsipinicon River near De Witt, Iowa 45 0 2.6 14 31 49 35 9 Maquoketa River near Spragueville, Iowa 45 0 3.6 8.7 24 44 35 

10 Turkey River at Garber, Iowa 45 0 2.3 4.1 12 23 18 

All samples 438 0 2.2 13 33 90 77 
Table 3. Statistical summary of select constituents at selected major Iowa rivers, water years 2004–2008.—Continued [Water year from October 1 to September 30; ID, identifier; IQR, interquartile range; parameter code given in brackets, see also table 2; mg/L, milligrams per liter; CaCO3, calci

mens per centimeter; <, less than; NTRU, nephlometric turbidity ratio units] Map ID (fig. 2) Station name Number of samples Number censored Minimum uncensored Percentile IQR 

25 50 (median) 75 Chlorophyll-a, g/L [70953] μ 1 Big Sioux River at Akron, Iowa 57 0 0.9 10.6 42.3 139 128 

2 Little Sioux River at Turin, Iowa 58 0 0.6 7.7 19.6 62.3 54.6 3 Boyer River at Logan, Iowa 56 1 1.3 5.6 9.5 16.3 10.8 4 Nishnabotna River above Hamburg, Iowa 57 0 1.0 3.9 7.1 16.4 12.5 5 Des Moines River at Keosauqua, Iowa 54 0 1.3 7.0 21.3 36.2 29.2 6 Skunk River at Augusta, Iowa 54 0 1.1 4.9 14.5 51.7 46.8 7 Iowa River at Wapello, Iowa 35 0 1.6 10.8 35.8 99.3 88.5 8 Wapsipinicon River near De Witt, Iowa 55 0 0.9 4.8 26.8 106 101 9 Maquoketa River near Spragueville, Iowa 56 0 0.3 4.0 11.6 32.3 28.3 

10 Turkey River at Garber, Iowa 55 0 0.7 3.0 6.3 16.8 13.8 

All samples 537 1 0.3 5.4 13.3 43.0 37.6 

Pheophytin-a, g/L [62360] μ 1 Big Sioux River at Akron, Iowa 57 0 0.9 7.4 16.8 55.3 47.9 

2 Little Sioux River at Turin, Iowa 58 0 0.4 4.0 8.5 24.7 20.8 3 Boyer River at Logan, Iowa 56 1 0.8 2.9 5.4 9.9 7.0 

4 Nishnabotna River above Hamburg, Iowa 57 0 0.6 3.1 4.6 9.4 6.3 

5 Des Moines River at Keosauqua, Iowa 54 0 1.4 3.6 8.6 17.6 14.1 6 Skunk River at Augusta, Iowa 54 0 0.9 3.7 11.4 31.9 28.2 

7 Iowa River at Wapello, Iowa 35 0 1.1 6.4 18.1 47.8 41.4 8 Wapsipinicon River near De Witt, Iowa 55 0 0.4 4.3 12.0 46.3 42.0 

9 Maquoketa River near Spragueville, Iowa 56 0 0.5 2.5 6.6 17.1 14.6 

10 Turkey River at Garber, Iowa 55 0 0.8 2.5 5.0 12.6 10.1 

All samples 537 1 0.4 3.5 8.3 22.5 19.1 
Table 4. Statistical summary of pesticides at selected major Iowa rivers, water years 2004–2008.[Water year from October 1 to September 30; μg/L, micrograms per liter; lrl, lab reporting limit; --, not applicable]

Analyte Parameter code CAS number Number of samples Number censored Percentile (μg/L) Ma

(

25 50 (median) 75 90 

Acetochlor 49260 34256-82-1 564 77 0.006 0.015 0.062 0.418 6

Alachlor 46342 15972-60-8 564 447 0.002 0.002 0.002 0.007 0

2,6-Diethylaniline 82660 579-66-8 560 558 -- -- -- -- 0

Atrazine 39632 1912-24-9 560 0 0.061 0.101 0.266 1.38 4

2-Chloro-4-isopropylamino6-amino-s-triazine {CIAT} 04040 6190-65-4 560 0 0.027 0.048 0.077 0.132 0

Butylate 04028 2008-41-5 536 534 -- -- -- -- 0

Carbaryl 82680 63-25-2 560 551 -- -- -- -- 0

Carbofuran 82674 1563-66-2 556 542 -- -- -- -- 0

Chlorpyrifos 38933 2921-88-2 560 507 0.001 0.002 0.002 0.002 0

Cyanazine 04041 21725-46-2 556 506 0.01 0.01 0.01 0.01 0

Dacthal 82682 1861-32-1 560 554 -- -- -- -- 0

Diazinon 39572 333-41-5 560 554 -- -- -- 0.002 0

Dieldrin 39381 60-57-1 560 554 -- 0.002 0.003 0.003 0

EPTC 82668 759-94-4 556 528 -- -- -- 0.001 0

Desulfinylfipronil amide 62169 -- 560 558 -- -- -- -- 0

Fipronil sulfide 62167 120067-83-6 560 553 -- -- -- -- 0

Fipronil sulfone 62168 120068-36-2 560 545 -- -- -- -- 0

Desulfinylfipronil 62170 -- 560 536 -- -- -- -- 0

Fipronil 62166 120068-37-3 560 508 -- -- -- -- 0

Fonofos 04095 944-22-9 560 559 -- -- -- -- 0

Metolachlor 39415 51218-45-2 564 0 0.026 0.052 0.123 0.451 6

Metribuzin 82630 21087-64-9 560 494 -- -- 0.003 0.006 0

Napropamide 82684 15299-99-7 536 535 -- -- -- -- 0

p,p’-DDE 34653 72-55-9 536 535 -- -- -- -- 0

Pendimethalin 82683 40487-42-1 560 540 -- 0.006 0.006 0.006 0

Prometon 04037 1610-18-0 560 241 0.003 0.005 0.01 0.02 0

Propachlor 04024 1918-16-7 553 551 -- -- -- -- 0

Propanil 82679 709-98-8 556 555 -- -- -- -- 0

Simazine 04035 122-34-9 560 327 0.002 0.002 0.008 0.018 0

Tebuthiuron 82670 34014-18-1 560 549 -- -- -- -- 0

Terbacil 82665 5902-51-2 536 533 -- -- -- -- 0

Trifluralin 82661 1582-09-8 560 531 -- -- -- 0.003 0

1This report contains CAS Registry Numbers®, which is a registered trademark of the American Chemical Society. CAS recommends the verification of the CASRNs through 
20 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

chloride concentration and log-streamflow as high as 0.79. 

Observed chloride concentrations tended to be greater and 

more variable at some sites, with a general increase in concentrations upstream among tributaries along the Missouri River 

(map IDs 4 to 1) and downstream among tributaries along 

the Mississippi River (map IDs 10 to 5) (fig. 3A–B). Sulfate 

concentrations varied from 5.67 to 245 mg/L with a median 

of 32.9 mg/L and a mean of 50.9 mg/L (table 3). An inverse 

relation between sulfate and streamflow was observed for each 

site, with the greatest concentrations and widest range measured at the Big Sioux River at Akron (map ID 1, fig. 3A–B). 

Silica concentrations ranged from below detection (less than 

0.2) to 25.8 mg/L with a median and mean of 11.6 mg/L 

(table 3). Unlike chloride and sulfate, silica concentrations 

were not correlated with streamflow and did not exhibit spatial 

patterns among the 10 sites (fig. 3A–B).

Table 5. Pesticides analyzed but not detected at selected major Iowa rivers, water years 2004–2008.

[Water year from October 1 to September 30; μg/L, micrograms per liter; lrl, lab reporting level]

Analyte Parameter code CAS number Number of 

samples

Censoring levels 

(1/2 lrl, in μg/L)

alpha-HCH 34253 319-84-6 536 0.001 – 0.002

Azinphos-methyl 82686 86-50-0 560 0.025 – 0.060

Benfluralin 82673 1861-40-1 560 0.002 – 0.005

cis-Permethrin 82687 61949-76-6 560 0.003 – 0.005

Disulfoton 82677 298-04-4 556 0.01 – 0.02

Ethalfluralin 82663 55283-68-6 536 0.004

Ethoprophos 82672 13194-48-4 556 0.002 – 0.018

Lindane 39341 58-89-9 536 0.002 – 0.003

Linuron 82666 330-55-2 536 0.018 – 0.030

Malathion 39532 121-75-5 560 0.008 – 0.014

Parathion-methyl 82667 298-00-0 560 0.004 – 0.008

Molinate 82671 2212-67-1 556 0.001 – 0.002

Parathion 39542 56-38-2 536 0.005

Pebulate 82669 1114-71-2 536 0.002 – 0.004

Phorate 82664 298-02-2 560 0.006 – 0.028

Propyzamide 82676 23950-58-5 560 0.002 – 0.005

Propargite 82685 2312-35-8 556 0.01 – 0.02

Terbufos 82675 13071-79-9 560 0.006 – 0.009

Thiobencarb 82681 28249-77-6 556 0.005

Tri-allate 82678 2303-17-5 536 0.001 – 0.003

1

This report contains CAS Registry Numbers®, which is a registered trademark of the American Chemical Society. 

CAS recommends the verification of the CASRNs through CAS Client ServicesSM.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 21

Carbon

Carbon in streamwater was dominated by inorganic 

ions such as bicarbonate, discussed previously; and particulate carbon concentrations were generally greater than DOC 

concentrations (fig. 5). POC concentrations ranged from 

below detection (less than 0.12) to 285 mg/L with a median 

of 4.00 mg/L and mean of 8.77 mg/L (table 3). At some sites 

POC concentration and streamflow were not correlated, 

whereas other sites were strongly correlated but only through 

a range of flows (fig. 3C–D). For example, POC concentrations at the Little Sioux River near Turin (map ID 2) and 

the Maquoketa River near Spragueville (map ID 9) indicate 

no correlation with streamflow less than 1,500 ft3

/s, and a 

strong positive relation for streamflow greater than 1,500 ft3

/s, 

whereas POC concentrations in the Iowa River (map ID 7) 

indicated no relation to streamflow. The differences in POC 

concentrations among sites did not reveal any distinct spatial 

trends. Particulate inorganic carbon (PIC) was frequently 

observed below detection (less than 0.04 or 0.12 mg/L) with a 

maximum concentration of 26.3 mg/L, a median of 0.05 mg/L, 

and a mean of 0.54 mg/L (table 3). No consistent pattern was 

observed between PIC concentration and streamflow or spatially among the sites (fig. 3C–D). DOC concentrations ranged 

from 0.9 to 22.9 mg/L with a median of 3.5 mg/L and mean 

of 3.9 mg/L (table 3). The correlation between DOC concentration and streamflow was positive for most sites and most 

ranges of streamflow (fig. 3C–D). Spatially, DOC concentrations trended downward from upstream to downstream Missouri River tributaries and upward in upstream to downstream 

Mississippi River tributaries.

Nitrogen

Nitrogen in Iowa streams affects in-stream aquatic 

environments, designated human uses, and the water quality 

of downstream rivers and the Gulf of Mexico. Total nitrogen 

(TN), was computed as the sum of total dissolved nitrogen (filtered, by alkaline persulfate digestion, parameter code 62854; 

table 2) plus particulate nitrogen (parameter code 49570); total 

nitrogen by alkaline persulfate digestion of unfiltered water 

(parameter code 62855) was used where the separate summation could not be made. All nitrogen species concentrations are 

reported as milligrams per liter as nitrogen. Nitrate is typically 

a large component of the total nitrogen in Iowa rivers (median 

about 85 percent). Nitrite and ammonia concentrations account 

for very little of the total, however, the in-stream deleterious 

consequences of these dissolved species occur at different 

levels.

Nitrate and nitrite numerical regulations do not apply 

to the Iowa rivers selected for this study, because the stream 

reaches are not used to supply drinking water, and stream 

nutrient criteria have not yet been proposed for the protection of aquatic life in Iowa. The maximum contaminant level 

(MCL) for surface waters designated for Iowa public drinking 

supplies is 10 mg/L for nitrate and 1 mg/L for nitrite (Iowa 

Environmental Protection Commission [567], 2002). The draft 

nitrate criteria for the protection of aquatic life in warm-water 

lakes and streams in Minnesota is 4.9 mg/L as a 4-day chronic 

criteria (Monson, 2010). Concentrations lower than these 

criteria also can contribute to nuisance algae growth, substantial nitrogen delivery to the Gulf of Mexico, and toxicity 

for sensitive aquatic life. Nitrate concentrations were equal 

to or greater than the nitrate MCL in at least one sample at 

all sites except the Big Sioux River at Akron (map ID 1), and 

11 percent of all samples had at least 10 mg/L nitrate. Nitrate 

concentrations at or above the proposed Minnesota aquatic 

life criteria occurred in 68 percent of samples at all sites, 

with 50 to 84 percent of samples at individual sites exceeding 

4.9 mg/L. The maximum observed nitrite concentration at the 

study sites was 0.164 mg/L, well below the MCL.

Criteria for ammonia vary with temperature and pH, and 

all observed concentrations met the acute and chronic criteria. Though the chronic criterion applies to a 30-day average 

concentration, the 30-day criterion is unlikely to be exceeded 

if 95 percent of discrete samples do not exceed the chronic criteria (U.S. Environmental Protection Agency, 1999). Ammonia 

concentrations also met the 2009 proposed acute ammonia 

criteria, which accounts for more sensitive mussel tolerances 

along with early life stages of fish (U.S. Environmental Protection Agency, 2009a). Though the 30-day average could not 

be evaluated directly for the proposed chronic criteria for these 

data, one sample in each of the Big Sioux (map ID 1), Boyer 

(map ID 3), and Maquoketa (map ID 9) Rivers exceeded the 

more sensitive proposed chronic criteria, indicating the possibility of exceeding the proposed chronic criteria.

Total nitrogen concentrations ranged from 0.39 mg/L 

to 37.8 mg/L distributed around a median 7.49 mg/L and 

a mean 7.79 mg/L (table 3). The relation between TN and 

streamflow varied across ranges of streamflow; increasing 

from low to average streamflows, then plateauing at many 

sites before showing effects of dilution at high streamflows 

(above the 90th percentile, fig. 3C–D, one outlier [Boyer 

River, 37.8 mg/L] is above the plotted scale for total nitrogen). TN concentration statistics did not vary substantially 

among the 10 study sites. Nitrate concentrations ranged from 

below detection (less than 0.06 mg/L) to 15.0 mg/L, with a 

median 6.15 mg/L and a mean 6.24 mg/L (table 3). Because 

nitrate is commonly a large portion of TN, both constituents 

relate similarly to streamflow and among sites (fig. 3C–D). 

Nitrite was not measured to be a substantial component of 

nitrate, (measured as nitrate plus nitrite), with nitrite concentrations exceeding the laboratory reporting level for nitrate 

plus nitrite of 0.04 mg/L in only 18.6 percent of samples. 

The maximum observed value for nitrite was 0.164 mg/L, 

well below the nitrate concentrations. Measurable ammonia 

concentrations ranged from 0.0051 (estimated value above 

the long-term method detection limit but below the laboratory 

reporting level, [Childress and others, 2009]) to 1.31 mg/L, 

which accounts for ammonia (NH3

) and the more prevalent 

and ecologically benign ammonium ion (NH4

+

). Ammonia 

was undetected in nearly 50 percent of samples, with reporting 
22 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

EXPLANATION

1,000 10,000

0

5

10

15

20

25

Silica,

milligrams per liter

1,000 10,000 100 1,000 10,000

Streamflow, cubic feet per second

1,000 10,000 1,000 10,000 100,000

50

100

150

200

250

Sulfate,

milligrams per liter

6.5

7.0

7.5

8.0

8.5

9.0

pH, standard units

Big Sioux River (1)

(2)

100

200

300

Alkalinity, milligrams

per liter as calcium

carbonate

200

400

600

800

1,000

1,200

Specific conductance,

microseimens per

centimeter at

25 degrees Celsius

20

40

60

80

Chloride,

milligrams per liter

Little Sioux River (2) Boyer River (3) Nishnabotna River (4) Des Moines River (5)

Figure 3A. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers 

and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 23

0

5

10

15

20

25

50

100

150

200

250

6.5

7.0

7.5

8.0

8.5

9.0

Skunk River (6)

100

200

300

200

400

600

800

1,000

1,200

20

40

60

80

Iowa River (7)

Wapsipinicon 

River (8)

Maquoketa 

River (9) Turkey River (10)

Silica,

milligrams per liter

Sulfate,

milligrams per liter pH, standard units

Alkalinity, milligrams

per liter as calcium

carbonate

Specific conductance,

microseimens per

centimeter at

25 degrees Celsius

Chloride,

milligrams per liter

Streamflow, cubic feet per second

100 1,000 10,000 10,000 100,000 1,000 10,000 1,000 10,000 1,000 10,000

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

Value below detection level

EXPLANATION

(7)

Figure 3B. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers 

and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.—Continued
24 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

0.01

0.1

1

0

4

8

12

0.1

1

10

100

Big Sioux River (1)

0.01

0.1

1

10

5

10

15

20

5

10

15

20

Little Sioux River (2) Boyer River (3) Nishnabotna River (4) Des Moines River (5)

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

Value below detection level

EXPLANATION

(2)

Ammonia,

milligrams per liter

Nitrate plus nitrite,

milligrams per liter

Particulate organic

carbon, milligrams 

per liter

Particulate inorganic

carbon, milligrams 

per liter

Dissolved organic

carbon, milligrams 

per liter

Total nitrogen,

milligrams per liter

Streamflow, cubic feet per second

1,000 10,000 1,000 10,000 100 1,000 10,000 1,000 10,000 1,000 10,000 100,000

Figure 3C. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers 

and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.—Continued
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 25

0.01

0.1

1

0

4

8

12

0.1

1

10

100

0.01

0.1

1

10

5

10

15

20

5

10

15

20

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

Value below detection level

EXPLANATION

(7)

Ammonia,

milligrams per liter

Nitrate plus nitrite,

milligrams per liter

Particulate organic

carbon, milligrams 

per liter

Particulate inorganic

carbon, milligrams 

per liter

Dissolved organic

carbon, milligrams 

per liter

Total nitrogen,

milligrams per liter

Streamflow, cubic feet per second

100 1,000 10,000 10,000 100,000 1,000 10,000 1,000 10,000 1,000 10,000

Skunk River (6) Iowa River (7)

Wapsipinicon 

River (8)

Maquoketa 

River (9) Turkey River (10)

Figure 3D. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers 

and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.—Continued
26 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Big Sioux River (1) Little Sioux River (2) Boyer River (3) Nishnabotna River (4) Des Moines River (5)

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

Value below detection level

EXPLANATION

(2)

Streamflow, cubic feet per second

1,000 10,000 1,000 10,000 100 1,000 10,000 1,000 10,000 1,000 10,000 100,000

1

10

100

Pheophytin-a,

micrograms per liter

1

10

100

Chlorophyll-a,

micrograms per liter

0.1

1

10

Total phosphorus,

milligrams per liter

0.01

0.1

1

Orthophosphate,

milligrams per liter

10

100

1,000

10,000

Suspended sediment,

milligrams per liter

10

100

1,000

Turbidity, nephelometric

turbidity ratio units

Figure 3E. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.—Continued
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 27

Locally weighted scatterplot smooth (LOWESS) line

Map identifier

Physical property or concentration value

Value below detection level

EXPLANATION

(7)

Streamflow, cubic feet per second

100 1,000 10,000 10,000 100,000 1,000 10,000 1,000 10,000 1,000 10,000

Skunk River (6) Iowa River (7)

Wapsipinicon 

River (8)

Maquoketa 

River (9) Turkey River (10)

1

10

100

Pheophytin-a,

micrograms per liter

1

10

100

Chlorophyll-a,

micrograms per liter

0.1

1

10

Total phosphorus,

milligrams per liter

0.01

0.1

1

Orthophosphate,

milligrams per liter

10

100

1,000

10,000

Suspended sediment,

milligrams per liter

10

100

1,000

Turbidity, nephelometric

turbidity ratio units

Figure 3F. Physical properties and concentrations related to streamflow for 10 major Iowa rivers with map identifiers and locally weighted scatterplot smooth (LOWESS) line, water years 2004–2008.—Continued
28 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Turkey River (map identifier 10)

Iowa River (map identifier 7)

Des Moines River (map identifier 5)

EXPLANATION

0

100

200

300

Alkalinity, milligrams per liter as calcium carbonate

6.5

7.0

7.5

8.0

8.5

9.0

pH, standard units

A

Month

0

1

10

100

Chlorophyll-a, micrograms per liter

B

J F M A M J J A S O N D

Alkalinity

pH

EXPLANATION

Figure 4. Seasonal values for A, Alkalinity and pH for 10 major Iowa rivers; B, chlorophyll-a concentrations; C, atrazine concentrations; and D, prometon concentrations for selected major Iowa rivers, water years 2004–2008 with locally weighted scatterplot smooth 

(LOWESS) line.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 29

Turkey River (map identifier 10)

Des Moines River (map identifier 5)

Value below detection level

EXPLANATION

Month

J F M A M J J A S O N D

0.1

1.0

10

Atrazine, micrograms per liter

C

0.001

0.01

Prometon, micrograms per liter

D

Turkey River (map identifier 10)

Des Moines River (map identifier 5)

EXPLANATION

Figure 4. Seasonal values for A, Alkalinity and pH for 10 major Iowa rivers; B, chlorophyll-a concentrations; C, atrazine concentrations; and D, prometon concentrations for selected major Iowa rivers, water years 2004–2008 with locally weighted scatterplot 

smooth (LOWESS) line.—Continued
30 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Big Sioux River (1)

Little Sioux River (2)

Boyer River (3)

Nishnabotna River (4)

Des Moines River (5)

Skunk River (6)

Iowa River (7)

Wapsipinicon River (8)

Maquoketa River (9)

Turkey River (10)

0

2

4

6

8

30

40

50

60

70

Carbon, milligrams per liter

EXPLANATION

Bicarbonate

Total particulate carbon

Dissolved organic carbon

Figure 5. Median carbon concentrations as bicarbonate, total particulate carbon, and dissolved organic carbon for 10 major Iowa 

rivers showing map identifiers, water years 2004–2008.

levels ranging from 0.01 to 0.04 mg/L (table 3). Sample 

concentrations for ammonia at high streamflows tended to be 

greater relative to low flows, with fewer results below detection (fig. 3C–D).

Phosphorus

Concentrations for phosphorus species are reported in 

milligrams per liter as phosphorus. Like nitrogen, phosphorus water-quality standards for the protection of aquatic life 

are still being developed in Iowa, though the Wisconsin total 

phosphorus (TP) criteria for large rivers is 0.1 mg/L (Wisconsin Department of Natural Resources, 2010). TP concentrations were equal to or greater than the Wisconsin criteria in 

92 percent of samples, with four sites exceeding the criteria 

with every sample (Big Sioux, Boyer, Nishnabotna, and Des 

Moines Rivers; map IDs 1, 3, 4, and 5). TP concentrations 

ranging from 0.023 to 9.41 mg/L had a median 0.303 mg/L 

and a mean 0.507 mg/L (table 3). The correlation between 

TP concentrations and streamflow was generally positive, 

but varied across ranges of streamflow and among the sites 

(fig. 3E–F). TP concentrations were greatest and most variable 

in the southwestern Iowa basins of the Boyer and Nishnabotna Rivers (map IDs 3–4), which are basins including steep 

regions of loess soils. TP concentrations indicated an overall downstream increase from Mississippi River tributaries. 

Orthophosphate concentrations ranged from below detection (reporting levels ranging from 0.006 to 0.036 mg/L) to 

1.22 mg/L (table 3). The relation between orthophosphate 

concentrations and streamflow was generally flat to slightly 

positive, with the Boyer River (map ID 3) a notable exception 

having a strongly inverse relation to streamflow (fig. 3E–F).

Suspended Sediment and Turbidity

Suspended-sediment concentrations (SSC) ranged from 

3 to 22,600 mg/L with a log-normal distribution around 

a median of 138 mg/L and a mean of 512 mg/L (table 3). 

SSC increased with streamflow, though the relation varied 

by site (fig. 3E–F ). Sites in southwestern Iowa, Boyer and 

Nishnabotna Rivers, map IDs 3–4, had the greatest and most 

variable SSC; these basins include the highly erodible loess 

soils and steep slopes characteristic of the Western Loess 

Hills and Steeply Rolling Loess Prairie Ecoregions (fig. 2). 
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 31

Turbidity values during sample collection ranged from 2.2 

to 8,050 nephelometric turbidity ratio units (NTRU), with 

a median of 33 NTRU and a mean of 140 NTRU (table 3). 

Turbidity is related to suspended sediment, with a similar relation with streamflow among different sites, though the strength 

of the correlation is weaker at low turbidity and SSC values 

(figs. 3E–F, 6). Though the overall correlation R2

 is 0.68 

between the log transform of these variables, sites with typically low turbidity and SSC values, such as the Wapsipinicon 

River (map ID 8), have a much weaker relation (R2

 = 0.37).

Algal Pigments

Algal pigments such as chlorophyll-a and pheophytin-a

varied more by season than by streamflow, peaking in late 

summer (figs. 3E–F, 4B). Chlorophyll-a concentrations ranged 

from below detection (less than 0.1) to 384 μg/L with a lognormal distribution around a median 13.3 μg/L and a mean 

38.5 μg/L (table 3). Pheophytin-a was similarly distributed, 

ranging from less than 0.1 to 181 μg/L with a median 8.3 μg/L 

and a mean 18.1 μg/L (table 3). The two algal pigments were 

strongly related to each other, with a linear correlation R2

 of 

0.83 between the log transform of each variable.

Pesticides

Pesticide concentrations also varied by season, with 

greatest concentrations and most common detections occurring 

during typical application times. Two herbicides were found in 

every sample; atrazine and metolachlor, as well as the atrazine 

breakdown product 2-Chloro-4-isopropylamino-6-amino-striazine (CIAT; table 4). Atrazine concentrations exceeded 

the drinking-water MCL of 3.0 μg/L (U.S. Environmental 

Protection Agency, 2002) in 4 percent of the samples with the 

greatest concentration estimated at 41 μg/L, though none of 

the studied rivers are used directly for drinking-water sources 

near the sample collection sites. Atrazine and other commonly 

detected herbicides, such as acetochlor, alachlor, metribuzin, 

and simazine, had peak concentrations and most common 

detections from May through July (fig. 4C). Concentrations of 

prometon, a nonselective herbicide used frequently to control 

weeds in asphalt areas, were greatest from June through September, but occurrence varied among sites (fig. 4D). Insecticides were less commonly detected in samples; chlorpyrifos 

and fipronil were detected in 9 percent of all samples. Pesticides detected in water samples are summarized in table 4, 

and other pesticide compounds analyzed for but not detected 

during the study are listed in table 5.

Estimated Loads and Yields of Ions, Nutrients, 

and Suspended Sediment

The load and yield estimates described in this section 

for major ions (chloride, silica, and sulfate), nutrients (nitrate, 

total nitrogen, orthophosphate, and total phosphorus), and 

suspended sediment in 10 major Iowa tributaries to the Mississippi and Missouri Rivers include annual load estimates with 

standard error of prediction (SEP), upper and lower confidence 

limits, and basin yields (table 6). Models used for load estimates are described in table 7.

Streamflow and loads are positively correlated, even 

for sites and constituents with an inverse relation between 

flow and concentration. The year-to-year variations in loads 

(or yields) for any given site are largely because of streamflow variation. In general, largest calculated loads were for 

water years 2007 and 2008, which were the wettest years in 

the study period with the largest flows (table 1). Statewide 

monthly nutrient loads, for example, rise and fall with streamflow, peaking annually from April to June, with the greatest 

loads during June of 2008 when historic flooding occurred 

throughout much of the State (fig. 7, Buchmiller and Eash, 

2010). Of the 10 sites, the Iowa River (map ID 7) and Des 

Moines River (map ID 5) Basins produced the largest loads for 

most constituents, which is exactly what would be expected 

given that their watershed areas are the two largest of all of the 

sites in the study, almost twice the watershed area of the next 

largest and more than an order of magnitude larger than the 

smallest watershed in the study. 

The load estimates presented with this study tend to have 

narrower confidence limits than previously reported load estimates, where the same sites and constituents were analyzed, 

though results are generally comparable. Nitrate, orthophosphate, and total phosphorus loads presented by Aulenbach and 

others (2007) were derived from similar rating-curve equations in LOADEST for the Iowa River at Wapello (map ID 7). 

A different period of record was used for model calibration, 

and additional terms to describe streamflow variability were 

included for load estimates presented in this report. The differences in reported loads are not contradictory; rather, because 

the confidence limits overlap in all cases of comparable sites 

and constituents, the results are corroborated for both methods (fig. 8A–C). In the case of orthophosphate, confidence 

limits are not reported for estimates from this report because 

prediction intervals cannot be calculated using the LAD 

method, which was used where the assumption of normality of 

model residuals could not be met. Furthermore, the increased 

accuracy of estimates presented in this study demonstrates the 

usefulness of the streamflow variability terms. Overall, 38 of 

the individual models for constituents at each site incorporated streamflow variability terms (dQ1

, |dQ1|, or dQ30), and an 

additional six models used streamflow anomalies (A5yr, A1yr, 

A3mo, HFV). Because model selection procedures emphasized low residual variance (good model fit), the inclusion 

of these terms indicates improvements in the accuracy of the 

predictions, as well.

The model residuals for suspended sediment tended to 

be greater than other constituents, because of the high variability in SSC. The SEP for annual suspended-sediment loads 

for each site and year ranged from 10 to 49 percent (table 6), 

with an average of 21 percent. Among the other constituents, 

only orthophosphate and total phosphorus average annual SEP 
32 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

1 10 100 1,000 10,000

Turbidity, nephelometric turbidity ratio units

1

10

100

1,000

10,000

Suspended sediment, milligrams per liter

All sites

Wapsipinicon River

(map identifier 8)

EXPLANATION

Figure 6. Relations between suspended-sediment concentrations and turbidity for 10 major Iowa rivers, water years 2004–2008.

exceeded 10 percent (table 6). One site with a daily suspended-sediment record was used to validate the suspendedsediment loads estimated by LOADEST. Load estimates are 

published annually (Nalley and others, 2005a, b; U.S. Geological Survey, 2007–2009) for the Skunk River at Augusta (map 

ID 6) based on suspended sediment and streamflow time-series 

daily observations and using the USGS Graphical Constituent 

Loading Analysis System (GCLAS; Koltun and others, 2006). 

The SEP for annual suspended-sediment load by rating curve 

method (LOADEST) from the Skunk River ranged from 19 to 

32 percent (table 6). The relative percent difference (RPD, calculated as the difference divided by the mean) between annual 

LOADEST and GCLAS estimates ranged from 4 to 32 percent 

(fig. 8D). Predictive errors cannot be calculated for GCLAS 

load estimates because it is not a regression technique, and 

instead fits a smoothed time series for load through every 

sampled concentration.

Constituent yields tended to be greater during wet years, 

but similarities and geographic patterns among sites varied by 

constituent (fig. 9). Yields related to annual streamflow scaled 

to the long-term average streamflow (1979–2008; table 1) 

for each site are shown on figure 9. This representation of 

the x-axis allows for easy comparison of wet (greater than 

1) and dry (less than 1) years among sites. Constituents with 

similar patterns in yields among sites suggest that transport in 

the basins is governed by similar processes. Iowa’s Missouri 

River tributaries consistently yielded less chloride and more 

sediment than Mississippi River tributaries. The Big Sioux 

River Basin yields were distinct from other sites for many constituents, including sulfate, silica, total nitrogen, nitrate, total 

phosphorus, and suspended sediment. 

Major Ions

Annual chloride loads ranged from 3,670 tons from 

the Boyer River (map ID 3) in 2006 to 348,000 tons from 

the Iowa River (map ID 7) in 2008, with SEP not exceeding 

5 percent (table 6). Smaller basins generated greater loads for 

the same streamflows as larger basins. The lowest annual yield 

was 3.29 tons per square mile (ton/mi2) from the Nishnabotna 

River (map ID 4) in 2006 and the greatest was 28.6 ton/mi2

from the Wapsipinicon River (map ID 8) in 2008. Missouri 

River tributaries tended to have lower yields than Mississippi 

River tributaries (fig. 9A). 

Annual sulfate loads ranged from 6,850 tons from the 

Boyer River (map ID 3) in 2006 to 612,000 tons from the Des 

Moines River (map ID 5) in 2008 (table 6). Annual sulfate 

yields ranged from 6.09 ton/mi2

 from the Nishnabotna River 

(map ID 4) to 57.6 tons/mi2

 from the Big Sioux River (map 

ID 1), both occurring in 2006. The Big Sioux, Little Sioux 

(map ID 2), and Des Moines Rivers, had the greatest sulfate 

loads and yields, and the yields compared to long term streamflow for these sites indicated different patterns than the other 

seven sites (fig. 9B).

Silica loads were smallest in 2006 at the Boyer River 

(map ID 3) at 1,830 tons and greatest in 2008 at the Des 

Moines River (map ID 5) at 312,000 tons (table 6). The Des 

Moines and Iowa (map ID 7) Rivers had the greatest annual 

silica loads by a factor of 2 to 3 above any other site, because 

of the greater streamflow in these rivers. The smallest and 

largest silica yields were estimated at 1.84 ton/mi2

 from the 

Big Sioux (map ID 1) in 2004 and 24.2 ton/mi2 from the Iowa 

River in 2008. The Big Sioux River had the lowest silica yield 

over the 5-year study period, and yields at this site had a distinct pattern with streamflow compared to other sites (fig. 9C).

Nitrogen

TN loads ranged from 1,200 tons in 2006 from the Boyer 

River (map ID 3) to 145,000 tons in 2008 from the Iowa River 

(map ID 7; table 6). The Boyer River had the smallest TN 

loads whereas the Des Moines (map ID 5) and Iowa Rivers 

consistently had the largest loads of the study sites, reflective of basin size and overall streamflow. The smallest annual 

yield was estimated at 1.09 ton/mi2

 in 2006 at the Nishnabotna 

River (map ID 4), and the largest yield was 19.3 ton/mi2

 in 

2008 at the Maquoketa River (map ID 9). On average, the Big 

Sioux River (map ID 1) had the smallest TN yield during the 

study period (less than one-half the yield of any other river), 

and northeastern basins (map IDs 7–10) tended to have larger 

yields than western and central basins (fig. 9D).
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 33

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2004

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Chloride, in mg/L [00940] 

1 Big Sioux River at Akron, Iowa 38,300 1,170 3.1 36,000 40,600 5.47

2 Little Sioux River at Turin, Iowa 24,700 686 2.8 23,400 26,100 6.95

3 Boyer River at Logan, Iowa 6,280 150 2.4 5,990 6,580 7.21

4 Nishnabotna River above Hamburg, Iowa 17,500 453 2.6 16,700 18,400 6.24

5 Des Moines River at Keosauqua, Iowa 194,000 8,980 4.6 177,000 212,000 13.8

6 Skunk River at Augusta, Iowa 50,200 1,260 2.5 47,800 52,700 11.6

7 Iowa River at Wapello, Iowa 261,000 6,770 2.6 248,000 275,000 20.9

8 Wapsipinicon River near De Witt, Iowa 38,500 1,360 3.5 35,900 41,200 16.5

9 Maquoketa River near Spragueville, Iowa 23,500 510 2.2 22,600 24,600 14.4

10 Turkey River at Garber, Iowa 22,100 730 3.3 20,700 23,600 14.2

Total 677,000 22,100 3.3 634,000 721,000 13.4

Sulfate, in mg/L [00945] 

1 Big Sioux River at Akron, Iowa 185,000 7,960 4.3 169,000 201,000 26.4

2 Little Sioux River at Turin, Iowa 71,000 1,600 2.3 67,900 74,200 20.0

3 Boyer River at Logan, Iowa 11,900 331 2.8 11,300 12,600 13.7

4 Nishnabotna River above Hamburg, Iowa 33,200 781 2.4 31,700 34,800 11.8

5 Des Moines River at Keosauqua, Iowa 352,000 13,400 3.8 327,000 379,000 25.1

6 Skunk River at Augusta, Iowa 69,500 1,620 2.3 66,400 72,800 16.1

7 Iowa River at Wapello, Iowa 243,000 3,980 1.6 235,000 251,000 19.4

8 Wapsipinicon River near De Witt, Iowa 41,900 1,000 2.4 40,000 43,900 17.9

9 Maquoketa River near Spragueville, Iowa 29,300 529 1.8 28,300 30,400 18.0

10 Turkey River at Garber, Iowa 27,600 474 1.7 26,600 28,500 17.7

Total 1,060,000 31,700 3.0 1,000,000 1,130,000 21.0

Silica, in mg/L [00955] 

1 Big Sioux River at Akron, Iowa 12,900 -- -- -- -- 1.84

2 Little Sioux River at Turin, Iowa 14,700 1,470 10 12,000 17,800 4.14

3 Boyer River at Logan, Iowa 4,700 225 4.8 4,270 5,150 5.40

4 Nishnabotna River above Hamburg, Iowa 17,700 -- -- -- -- 6.29

5 Des Moines River at Keosauqua, Iowa 98,000 4,420 4.5 89,600 107,000 6.98

6 Skunk River at Augusta, Iowa 31,500 -- -- -- -- 7.29

7 Iowa River at Wapello, Iowa 109,000 -- -- -- -- 8.72

8 Wapsipinicon River near De Witt, Iowa 20,100 -- -- -- -- 8.59

9 Maquoketa River near Spragueville, Iowa 12,000 1,220 10 9,830 14,600 7.37

10 Turkey River at Garber, Iowa 12,100 1,800 15 9,000 16,000 7.82

Total 333,000 -- -- -- -- 6.59
34 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2004

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

 Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux River at Akron, Iowa 9,040 422 4.7 8,240 9,890 1.29

2 Little Sioux River at Turin, Iowa 12,400 586 4.7 11,200 13,500 3.48

3 Boyer River at Logan, Iowa 3,630 203 5.6 3,250 4,050 4.18

4 Nishnabotna River above Hamburg, Iowa 12,500 569 4.6 11,400 13,600 4.44

5 Des Moines River at Keosauqua, Iowa 51,800 4,580 8.8 43,400 61,300 3.69

6 Skunk River at Augusta, Iowa 22,000 1,780 8.1 18,800 25,700 5.11

7 Iowa River at Wapello, Iowa 79,700 19,000 24 48,900 123,000 6.38

8 Wapsipinicon River near De Witt, Iowa 19,300 2,000 10 15,700 23,500 8.27

9 Maquoketa River near Spragueville, Iowa 12,500 473 3.8 11,600 13,400 7.65

10 Turkey River at Garber, Iowa 14,800 955 6.5 13,000 16,800 9.53

Total 238,000 30,600 13 185,000 305,000 4.71

 Nitrate plus nitrite, in mg/L [00631] 

1 Big Sioux River at Akron, Iowa 6,490 574 8.8 5,440 7,690 0.928

2 Little Sioux River at Turin, Iowa 7,850 569 7.2 6,800 9,030 2.21

3 Boyer River at Logan, Iowa 2,660 147 5.5 2,390 2,960.0 3.06

4 Nishnabotna River above Hamburg, Iowa 10,200 848 8.3 8,680 12,000 3.65

5 Des Moines River at Keosauqua, Iowa 43,300 4,780 11 34,700 53,400 3.08

6 Skunk River at Augusta, Iowa 16,100 1,350 8.4 13,600 18,900 3.74

7 Iowa River at Wapello, Iowa 68,500 8,690 13 53,100 87,100 5.48

8 Wapsipinicon River near De Witt, Iowa 17,200 2,140 12 13,400 21,800 7.38

9 Maquoketa River near Spragueville, Iowa 9,750 415 4.3 8,960 10,600 5.97

10 Turkey River at Garber, Iowa 10,200 624 6.1 8,980 11,400 6.53

Total 192,000 20,100 10 156,000 235,000 3.80

Total phosphorus, in mg/L [00665] 

1 Big Sioux River at Akron, Iowa 683 53.1 7.8 585 793 0.0976

2 Little Sioux River at Turin, Iowa 1,430 267 19 981 2,020 0.404

3 Boyer River at Logan, Iowa 406 69.0 17 287 558 0.466

4 Nishnabotna River above Hamburg, Iowa 2,020 272 13 1,540 2,610 0.721

5 Des Moines River at Keosauqua, Iowa 2,210 100 4.5 2,020 2,410 0.157

6 Skunk River at Augusta, Iowa 1,360 132 9.7 1,120 1,640 0.315

7 Iowa River at Wapello, Iowa 2,870 167 5.8 2,560 3,210 0.230

8 Wapsipinicon River near De Witt, Iowa 649 85.4 13 498 832 0.278

9 Maquoketa River near Spragueville, Iowa 675 74.4 11 541 832 0.413

10 Turkey River at Garber, Iowa 1,500 549 37 706 2,830 0.968

Total 13,800 1,770 13 10,800 17,700 0.273
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 35

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2004

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Orthophosphate, in mg/L [00671] 

1 Big Sioux River at Akron, Iowa 304 166 55 98.0 729 0.0435

2 Little Sioux River at Turin, Iowa 158 39.8 25 93.9 249 0.0444

3 Boyer River at Logan, Iowa 113 5.51 4.9 102 124 0.129

4 Nishnabotna River above Hamburg, Iowa 192 8.95 4.7 175 210 0.0683

5 Des Moines River at Keosauqua, Iowa 808 -- -- -- -- 0.0576

6 Skunk River at Augusta, Iowa 331 -- -- -- -- 0.0766

7 Iowa River at Wapello, Iowa 1,020 -- -- -- -- 0.0812

8 Wapsipinicon River near De Witt, Iowa 241 109 45 93.7 513 0.103

9 Maquoketa River near Spragueville, Iowa 202 35.6 18 141 280 0.123

10 Turkey River at Garber, Iowa 190 62.3 33 96.3 337 0.122

Total 3,560 -- -- -- -- 0.0705

Suspended sediment, in mg/L [80154] 

1 Big Sioux River at Akron, Iowa 427,000 61,400 14 319,000 559,000 61.0

2 Little Sioux River at Turin, Iowa 2,060,000 401,000 19 1,390,000 2,950,000 580

3 Boyer River at Logan, Iowa 587,000 168,000 29 326,000 977,000 674

4 Nishnabotna River above Hamburg, Iowa 3,620,000 898,000 25 2,180,000 5,670,000 1,290

5 Des Moines River at Keosauqua, Iowa 1,800,000 425,000 24 1,110,000 2,770,000 128

6 Skunk River at Augusta, Iowa 1,380,000 342,000 25 832,000 2,160,000 320

7 Iowa River at Wapello, Iowa 2,300,000 395,000 17 1,620,000 3,170,000 184

8 Wapsipinicon River near De Witt, Iowa 405,000 68,300 17 287,000 554,000 173

9 Maquoketa River near Spragueville, Iowa 661,000 141,000 21 428,000 978,000 405

10 Turkey River at Garber, Iowa 1,350,000 424,000 31 704,000 2,350,000 867

Total 14,600,000 3,320,000 23 9,190,000 22,100,000 289
36 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2005

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Chloride, in mg/L [00940] 

1 Big Sioux River at Akron, Iowa 44,500 796 1.8 43,000 46,100 6.36

2 Little Sioux River at Turin, Iowa 36,200 949 2.6 34,400 38,100 10.2

3 Boyer River at Logan, Iowa 5,630 132 2.3 5,370 5,890 6.47

4 Nishnabotna River above Hamburg, Iowa 15,100 380 2.5 14,400 15,800 5.37

5 Des Moines River at Keosauqua, Iowa 159,000 6,740 4.2 146,000 172,000 11.3

6 Skunk River at Augusta, Iowa 46,400 1,190 2.6 44,100 48,800 10.8

7 Iowa River at Wapello, Iowa 199,000 3,480 1.7 192,000 206,000 15.9

8 Wapsipinicon River near De Witt, Iowa 24,300 476 2.0 23,400 25,300 10.4

9 Maquoketa River near Spragueville, Iowa 12,300 261 2.1 11,800 12,800 7.52

10 Turkey River at Garber, Iowa 13,700 340 2.5 13,100 14,400 8.85

Total 556,000 14,800 2.7 528,000 586,000 11.0

Sulfate, in mg/L [00945] 

1 Big Sioux River at Akron, Iowa 235,000 7,390 3.1 221,000 250,000 33.6

2 Little Sioux River at Turin, Iowa 103,000 2,210 2.1 98,700 107,000 29.0

3 Boyer River at Logan, Iowa 10,600 292 2.8 10,100 11,200 12.2

4 Nishnabotna River above Hamburg, Iowa 28,300 647 2.3 27,000 29,600 10.1

5 Des Moines River at Keosauqua, Iowa 331,000 12,200 3.7 308,000 356,000 23.6

6 Skunk River at Augusta, Iowa 64,700 1,550 2.4 61,800 67,800 15.0

7 Iowa River at Wapello, Iowa 220,000 3,620 1.6 213,000 227,000 17.6

8 Wapsipinicon River near De Witt, Iowa 26,000 544 2.1 24,900 27,100 11.1

9 Maquoketa River near Spragueville, Iowa 16,500 281 1.7 16,000 17,100 10.1

10 Turkey River at Garber, Iowa 17,900 229 1.3 17,500 18,400 11.5

Total 1,050,000 29,000 2.8 998,000 1,110,000 20.8

Silica, in mg/L [00955] 

1 Big Sioux River at Akron, Iowa 16,600 -- -- -- -- 2.37

2 Little Sioux River at Turin, Iowa 25,700 1,810 7.0 22,400 29,500 7.25

3 Boyer River at Logan, Iowa 3,870 177 4.6 3,540 4,230 4.45

4 Nishnabotna River above Hamburg, Iowa 13,800 -- -- -- -- 4.93

5 Des Moines River at Keosauqua, Iowa 80,300 3,400 4.2 73,900 87,200 5.73

6 Skunk River at Augusta, Iowa 26,000 -- -- -- -- 6.04

7 Iowa River at Wapello, Iowa 73,200 -- -- -- -- 5.85

8 Wapsipinicon River near De Witt, Iowa 6,370 -- -- -- -- 2.73

9 Maquoketa River near Spragueville, Iowa 5,750 330 5.7 5,130 6,420 3.52

10 Turkey River at Garber, Iowa 5,630 481 8.5 4,750 6,630 3.62

Total 257,000 -- -- -- -- 5.09
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 37

Map ID 

(fig. 2) Station name

2005

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux River at Akron, Iowa 10,700 468 4.4 9,770 11,600 1.53

2 Little Sioux River at Turin, Iowa 16,600 716 4.3 15,300 18,100 4.68

3 Boyer River at Logan, Iowa 2,870 156 5.4 2,580 3,190 3.30

4 Nishnabotna River above Hamburg, Iowa 8,860 398 4.5 8,100 9,660 3.15

5 Des Moines River at Keosauqua, Iowa 58,700 3,750 6.4 51,700 66,400 4.18

6 Skunk River at Augusta, Iowa 18,700 1,500 8.0 15,900 21,800 4.33

7 Iowa River at Wapello, Iowa 81,800 12,800 16 59,500 110,000 6.54

8 Wapsipinicon River near De Witt, Iowa 7,950 493 6.2 7,030 8,960 3.41

9 Maquoketa River near Spragueville, Iowa 5,180 187 3.6 4,830 5,560 3.17

10 Turkey River at Garber, Iowa 5,680 202 3.6 5,290 6,080 3.65

Total 217,000 20,700 10 180,000 261,000 4.29

Nitrate plus nitrite, in mg/L [00631] 

1 Big Sioux River at Akron, Iowa 7,850 625 8.0 6,700 9,150 1.12

2 Little Sioux River at Turin, Iowa 13,900 891 6.4 12,200 15,700 3.91

3 Boyer River at Logan, Iowa 2,200 114 5.2 1,980 2,430 2.53

4 Nishnabotna River above Hamburg, Iowa 6,740 395 5.9 6,000 7,550 2.40

5 Des Moines River at Keosauqua, Iowa 49,800 4,040 8.1 42,300 58,200 3.55

6 Skunk River at Augusta, Iowa 14,500 1,170 8.1 12,300 16,900 3.36

7 Iowa River at Wapello, Iowa 56,000 4,150 7.4 48,300 64,500 4.48

8 Wapsipinicon River near De Witt, Iowa 6,970 733 11 5,650 8,520 2.98

9 Maquoketa River near Spragueville, Iowa 4,200 179 4.3 3,860 4,570 2.57

10 Turkey River at Garber, Iowa 4,520 194 4.3 4,150 4,910 2.91

Total 167,000 12,500 7.5 144,000 192,000 3.30

Total phosphorus, in mg/L [00665] 

1 Big Sioux River at Akron, Iowa 671 37.1 5.5 601 747 0.0959

2 Little Sioux River at Turin, Iowa 944 110 12 748 1,180 0.266

3 Boyer River at Logan, Iowa 380 46.5 12 297 479 0.436

4 Nishnabotna River above Hamburg, Iowa 1160 135 12 920 1,450 0.414

5 Des Moines River at Keosauqua, Iowa 1920 87.0 4.5 1,750 2,090 0.137

6 Skunk River at Augusta, Iowa 912 75.4 8.3 773 1,070 0.211

7 Iowa River at Wapello, Iowa 2,200 78.3 3.6 2,050 2,360 0.176

8 Wapsipinicon River near De Witt, Iowa 239 22.9 10 197 286 0.102

9 Maquoketa River near Spragueville, Iowa 171 14.8 8.7 144 202 0.105

10 Turkey River at Garber, Iowa 169 28.2 17 120 230 0.109

Total 8,770 635 7.2 7,600 10,100 0.173

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
38 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Map ID 

(fig. 2) Station name

2005

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Orthophosphate, in mg/L [00671] 

1 Big Sioux River at Akron, Iowa 291 80.3 28 165 477 0.0416

2 Little Sioux River at Turin, Iowa 258 58.1 23 163 390 0.0727

3 Boyer River at Logan, Iowa 96.0 6.88 7.2 83.3 110 0.110

4 Nishnabotna River above Hamburg, Iowa 148 6.66 4.5 135 161 0.0526

5 Des Moines River at Keosauqua, Iowa 737 -- -- -- -- 0.0525

6 Skunk River at Augusta, Iowa 233 -- -- -- -- 0.0539

7 Iowa River at Wapello, Iowa 813 -- -- -- -- 0.0650

8 Wapsipinicon River near De Witt, Iowa 49.0 14.3 29 26.9 82.4 0.0210

9 Maquoketa River near Spragueville, Iowa 69.6 15.0 22 44.8 103 0.0426

10 Turkey River at Garber, Iowa 51.3 12.2 24 31.5 79.0 0.0330

Total 2,750 -- -- -- -- 0.0544

 Suspended sediment, in mg/L [80154] 

1 Big Sioux River at Akron, Iowa 425,000 48,200 11 339,000 527,000 60.8

2 Little Sioux River at Turin, Iowa 1,100,000 123,000 11 875,000 1,360,000 308

3 Boyer River at Logan, Iowa 443,000 172,000 39 199,000 860,000 510

4 Nishnabotna River above Hamburg, Iowa 1,920,000 466,000 24 1,160,000 2,980,000 682

5 Des Moines River at Keosauqua, Iowa 1,330,000 324,000 24 810,000 2,070,000 95.1

6 Skunk River at Augusta, Iowa 744,000 141,000 19 506,000 1,060,000 172

7 Iowa River at Wapello, Iowa 1,330,000 193,000 15 995,000 1,750,000 107

8 Wapsipinicon River near De Witt, Iowa 157,000 22,900 15 117,000 207,000 67.2

9 Maquoketa River near Spragueville, Iowa 222,000 56,900 26 131,000 353,000 136

10 Turkey River at Garber, Iowa 129,000 15,600 12 101,000 162,000 83.0

Total 7,800,000 1,560,000 20 5,240,000 11,300,000 154

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 39

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2006

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Chloride, in mg/L [00940] 

1 Big Sioux River at Akron, Iowa 65,300 1,400 2.1 62,600 68,100 9.33

2 Little Sioux River at Turin, Iowa 34,600 949 2.7 32,800 36,500 9.75

3 Boyer River at Logan, Iowa 3,670 103 2.8 3,470 3,870 4.22

4 Nishnabotna River above Hamburg, Iowa 9,250 276 3.0 8,720 9,800 3.29

5 Des Moines River at Keosauqua, Iowa 161,000 5,430 3.4 151,000 172,000 11.5

6 Skunk River at Augusta, Iowa 27,000 757 2.8 25,500 28,500 6.25

7 Iowa River at Wapello, Iowa 191,000 3,750 2.0 184,000 199,000 15.3

8 Wapsipinicon River near De Witt, Iowa 30,100 685 2.3 28,700 31,400 12.9

9 Maquoketa River near Spragueville, Iowa 12,700 273 2.1 12,200 13,300 7.78

10 Turkey River at Garber, Iowa 16,100 410 2.5 15,300 16,900 10.3

Total 551,000 14,000 2.5 524,000 579,000 10.9

Sulfate, in mg/L [00945] 

1 Big Sioux River at Akron, Iowa 403,000 13,600 3.4 377,000 430,000 57.6

2 Little Sioux River at Turin, Iowa 98,200 2,220 2.3 93,900 103,000 27.6

3 Boyer River at Logan, Iowa 6,850 216 3.2 6,430 7,280 7.87

4 Nishnabotna River above Hamburg, Iowa 17,100 505 3.0 16,100 18,100 6.09

5 Des Moines River at Keosauqua, Iowa 298,000 11,800 4.0 276,000 322,000 21.3

6 Skunk River at Augusta, Iowa 38,400 1,000 2.6 36,400 40,400 8.89

7 Iowa River at Wapello, Iowa 190,000 3,180 1.7 184,000 197,000 15.2

8 Wapsipinicon River near De Witt, Iowa 29,800 618 2.1 28,600 31,100 12.8

9 Maquoketa River near Spragueville, Iowa 17,200 287 1.7 16,600 17,700 10.5

10 Turkey River at Garber, Iowa 20,700 272 1.3 20,100 21,200 13.3

Total 1,120,000 33,600 3.0 1,050,000 1,190,000 22.2

Silica, in mg/L [00955] 

1 Big Sioux River at Akron, Iowa 34,300 -- -- -- -- 4.90

2 Little Sioux River at Turin, Iowa 27,600 2,240 8.1 23,400 32,200 7.76

3 Boyer River at Logan, Iowa 1,830 79.5 4.3 1,680 1,990 2.10

4 Nishnabotna River above Hamburg, Iowa 6,650 -- -- -- -- 2.37

5 Des Moines River at Keosauqua, Iowa 59,000 2,460 4.2 54,300 64,000 4.21

6 Skunk River at Augusta, Iowa 12,000 -- -- -- -- 2.78

7 Iowa River at Wapello, Iowa 58,900 -- -- -- -- 4.71

8 Wapsipinicon River near De Witt, Iowa 7,940 -- -- -- -- 3.40

9 Maquoketa River near Spragueville, Iowa 5,560 343 6.2 4,920 6,260 3.40

10 Turkey River at Garber, Iowa 7,790 587 7.5 6,710 9,000 5.01

Total 222,000 -- -- -- -- 4.39
40 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Map ID 

(fig. 2) Station name

2006

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux River at Akron, Iowa 20,200 1,020 5.0 18,300 22,200 2.89

2 Little Sioux River at Turin, Iowa 17,600 856 4.9 16,000 19,300 4.95

3 Boyer River at Logan, Iowa 1,200 62.2 5.2 1,090 1,330 1.38

4 Nishnabotna River above Hamburg, Iowa 3,060 129 4.2 2,810 3,320 1.09

5 Des Moines River at Keosauqua, Iowa 43,400 2,750 6.3 38,300 49,100 3.10

6 Skunk River at Augusta, Iowa 8,930 820 9.2 7,430 10,600 2.07

7 Iowa River at Wapello, Iowa 69,000 10,200 15 51,200 91,100 5.52

8 Wapsipinicon River near De Witt, Iowa 10,600 665 6.3 9,350 12,000 4.54

9 Maquoketa River near Spragueville, Iowa 5,050 171 3.4 4,720 5,390 3.09

10 Turkey River at Garber, Iowa 7,280 282 3.9 6,740 7,850 4.69

Total 186,000 17,000 9.1 156,000 222,000 3.68

 Nitrate plus nitrite, in mg/L [00631] 

1 Big Sioux River at Akron, Iowa 18,700 1,900 10 15,200 22,700 2.67

2 Little Sioux River at Turin, Iowa 14,300 1,060 7.4 12,400 16,500 4.03

3 Boyer River at Logan, Iowa 1,040 52.6 5.1 945 1,150 1.20

4 Nishnabotna River above Hamburg, Iowa 2,030 127 6.3 1,790 2,290 0.723

5 Des Moines River at Keosauqua, Iowa 35,400 2,870 8.1 30,100 41,400 2.52

6 Skunk River at Augusta, Iowa 6,680 590 8.8 5,600 7,910 1.55

7 Iowa River at Wapello, Iowa 44,200 3,410 7.7 37,900 51,200 3.53

8 Wapsipinicon River near De Witt, Iowa 9,490 995 10 7,690 11,600 4.06

9 Maquoketa River near Spragueville, Iowa 4,030 181 4.5 3,680 4,390 2.46

10 Turkey River at Garber, Iowa 6,220 278 4.5 5,690 6,780 4.00

Total 142,000 11,500 8.1 121,000 166,000 2.81

 Total phosphorus, in mg/L [00665] 

1 Big Sioux River at Akron, Iowa 978 70.5 7.2 847 1,120 0.140

2 Little Sioux River at Turin, Iowa 984 114 12 779 1,230 0.277

3 Boyer River at Logan, Iowa 129 13.4 10 105 158 0.148

4 Nishnabotna River above Hamburg, Iowa 295 30.1 10 240 358 0.105

5 Des Moines River at Keosauqua, Iowa 1,500 68.9 4.6 1,370 1,640 0.107

6 Skunk River at Augusta, Iowa 486 51.9 11 392 595 0.113

7 Iowa River at Wapello, Iowa 1,930 61.3 3.2 1,810 2,050 0.154

8 Wapsipinicon River near De Witt, Iowa 312 30.5 10 256 376 0.133

9 Maquoketa River near Spragueville, Iowa 174 11.0 6.3 154 197 0.107

10 Turkey River at Garber, Iowa 214 30.7 14 160 280 0.138

Total 7,000 482 6.9 6,110 8,000 0.138

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 41

Map ID 

(fig. 2) Station name

2006

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

 Orthophosphate, in mg/L [00671] 

1 Big Sioux River at Akron, Iowa 689 244 35 331 1,270 0.0985

2 Little Sioux River at Turin, Iowa 319 85.7 27 184 517 0.0899

3 Boyer River at Logan, Iowa 85.1 6.65 7.8 72.8 98.9 0.0978

4 Nishnabotna River above Hamburg, Iowa 72.9 3.42 4.7 66.5 79.9 0.0260

5 Des Moines River at Keosauqua, Iowa 601 -- -- -- -- 0.0428

6 Skunk River at Augusta, Iowa 102 -- -- -- -- 0.0235

7 Iowa River at Wapello, Iowa 607 -- -- -- -- 0.0486

8 Wapsipinicon River near De Witt, Iowa 63.5 15.0 24 39.2 97.5 0.0272

9 Maquoketa River near Spragueville, Iowa 52.4 7.49 14 39.2 68.5 0.0321

10 Turkey River at Garber, Iowa 73.8 16.0 22 47.4 110 0.0475

Total 2,670 -- -- -- -- 0.0528

 Suspended sediment, in mg/L [80154] 

1 Big Sioux River at Akron, Iowa 614,000 76,700 12 478,000 778,000 87.8

2 Little Sioux River at Turin, Iowa 1,160,000 143,000 12 910,000 1,470,000 328

3 Boyer River at Logan, Iowa 53,000 13,500 25 31,400 84,100 60.9

4 Nishnabotna River above Hamburg, Iowa 306,000 66,800 22 196,000 457,000 109

5 Des Moines River at Keosauqua, Iowa 727,000 171,000 24 449,000 1,120,000 51.8

6 Skunk River at Augusta, Iowa 385,000 97,500 25 229,000 608,000 89.2

7 Iowa River at Wapello, Iowa 949,000 124,000 13 729,000 1,220,000 75.9

8 Wapsipinicon River near De Witt, Iowa 211,000 31,300 15 156,000 278,000 90.2

9 Maquoketa River near Spragueville, Iowa 86,100 12,800 15 63,700 114,000 52.7

10 Turkey River at Garber, Iowa 154,000 15,600 10 126,000 187,000 99.1

Total 4,650,000 752,000 16 3,370,000 6,300,000 92.0

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
42 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2007

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Chloride, in mg/L [00940] 

1 Big Sioux River at Akron, Iowa 57,900 1,230 2.1 55,500 60,400 8.28

2 Little Sioux River at Turin, Iowa 40,500 1,330 3.3 38,000 43,200 11.4

3 Boyer River at Logan, Iowa 10,600 301 2.8 10,000 11,200 12.2

4 Nishnabotna River above Hamburg, Iowa 29,500 791 2.7 28,000 31,100 10.5

5 Des Moines River at Keosauqua, Iowa 283,000 10,500 3.7 264,000 305,000 20.2

6 Skunk River at Augusta, Iowa 76,800 2,150 2.8 72,700 81,100 17.8

7 Iowa River at Wapello, Iowa 305,000 6,490 2.1 293,000 318,000 24.4

8 Wapsipinicon River near De Witt, Iowa 52,500 1,190 2.3 50,200 54,900 22.5

9 Maquoketa River near Spragueville, Iowa 22,500 486 2.2 21,600 23,500 13.8

10 Turkey River at Garber, Iowa 25,400 802 3.2 23,800 27,000 16.3

Total 904,000 25,200 2.8 856,000 955,000 17.9

Sulfate, in mg/L [00945] 

1 Big Sioux River at Akron, Iowa 378,000 14,700 3.9 350,000 408,000 54.0

2 Little Sioux River at Turin, Iowa 114,000 2,870 2.5 109,000 120,000 32.2

3 Boyer River at Logan, Iowa 20,400 649 3.2 19,200 21,700 23.4

4 Nishnabotna River above Hamburg, Iowa 57,900 1,660 2.9 54,700 61,200 20.6

5 Des Moines River at Keosauqua, Iowa 503,000 18,600 3.7 468,000 541,000 35.9

6 Skunk River at Augusta, Iowa 105,000 2,730 2.6 99,900 111,000 24.4

7 Iowa River at Wapello, Iowa 318,000 5,760 1.8 306,000 329,000 25.4

8 Wapsipinicon River near De Witt, Iowa 51,600 1,200 2.3 49,300 54,000 22.1

9 Maquoketa River near Spragueville, Iowa 28,300 509 1.8 27,300 29,300 17.3

10 Turkey River at Garber, Iowa 31,500 522 1.7 30,500 32,500 20.3

Total 1,610,000 49,200 3.1 1,510,000 1,710,000 31.8

Silica, in mg/L [00955] 

1 Big Sioux River at Akron, Iowa 34,300 -- -- -- -- 4.90

2 Little Sioux River at Turin, Iowa 37,300 3,090 8.3 31,600 43,700 10.5

3 Boyer River at Logan, Iowa 9,810 535 5.5 8,800 10,900 11.3

4 Nishnabotna River above Hamburg, Iowa 38,500 -- -- -- -- 13.7

5 Des Moines River at Keosauqua, Iowa 183,000 8,440 4.6 167,000 201,000 13.1

6 Skunk River at Augusta, Iowa 53,800 -- -- -- -- 12.5

7 Iowa River at Wapello, Iowa 150,000 -- -- -- -- 12.0

8 Wapsipinicon River near De Witt, Iowa 24,200 -- -- -- -- 10.4

9 Maquoketa River near Spragueville, Iowa 11,300 683 6.0 10,100 12,700 6.94

10 Turkey River at Garber, Iowa 17,300 1,700 10 14,200 20,900 11.2

Total 561,000 -- -- -- -- 11.1
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 43

Map ID 

(fig. 2) Station name

2007

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux River at Akron, Iowa 18,300 1,040 5.7 16,400 20,400 2.62

2 Little Sioux River at Turin, Iowa 25,100 1,460 5.8 22,400 28,100 7.07

3 Boyer River at Logan, Iowa 8,420 523 6.2 7,440 9,490 9.68

4 Nishnabotna River above Hamburg, Iowa 24,800 1,150 4.6 22,600 27,100 8.82

5 Des Moines River at Keosauqua, Iowa 114,000 7,270 6.4 99,900 128,000 8.09

6 Skunk River at Augusta, Iowa 36,600 3,230 8.8 30,700 43,300 8.48

7 Iowa River at Wapello, Iowa 124,000 31,500 25 73,800 196,000 9.93

8 Wapsipinicon River near De Witt, Iowa 25,200 1,830 7.3 21,800 28,900 10.8

9 Maquoketa River near Spragueville, Iowa 11,400 413 3.6 10,600 12,200 6.95

10 Turkey River at Garber, Iowa 15,200 728 4.8 13,800 16,600 9.75

Total 403,000 49,200 12 319,000 511,000 7.97

Nitrate plus nitrite, in mg/L [00631] 

1 Big Sioux River at Akron, Iowa 16,700 1,970 12 13,100 20,900 2.39

2 Little Sioux River at Turin, Iowa 19,700 1,800 9.1 16,400 23,500 5.55

3 Boyer River at Logan, Iowa 5,550 352 6.3 4,890 6,270 6.37

4 Nishnabotna River above Hamburg, Iowa 19,400 1,270 6.5 17,000 22,000 6.90

5 Des Moines River at Keosauqua, Iowa 101,000 8,320 8.2 86,000 119,000 7.22

6 Skunk River at Augusta, Iowa 27,500 2,470 9.0 22,900 32,600 6.36

7 Iowa River at Wapello, Iowa 104,000 8,250 7.9 88,900 121,000 8.33

8 Wapsipinicon River near De Witt, Iowa 24,400 2,700 11 19,600 30,100 10.5

9 Maquoketa River near Spragueville, Iowa 8,990 381 4.2 8,260 9,760 5.50

10 Turkey River at Garber, Iowa 12,500 745 6.0 11,100 14,000 8.05

Total 340,000 28,300 8.3 288,000 399,000 6.72

Total phosphorus, in mg/L [00665] 

1 Big Sioux River at Akron, Iowa 985 78.3 7.9 841 1,150 0.141

2 Little Sioux River at Turin, Iowa 1,790 288 16 1,300 2,420 0.505

3 Boyer River at Logan, Iowa 1,290 267 21 846 1,890 1.48

4 Nishnabotna River above Hamburg, Iowa 2,520 279 11 2,020 3,110 0.897

5 Des Moines River at Keosauqua, Iowa 3,980 184 4.6 3,630 4,350 0.283

6 Skunk River at Augusta, Iowa 2,320 227 9.8 1,910 2,790 0.537

7 Iowa River at Wapello, Iowa 4,090 128 3.1 3850 4,350 0.328

8 Wapsipinicon River near De Witt, Iowa 706 73.7 10 573 861 0.302

9 Maquoketa River near Spragueville, Iowa 473 35.2 7.4 408 545 0.289

10 Turkey River at Garber, Iowa 645 113 18 453 893 0.415

Total 18,800 1,670 8.9 15,800 22,400 0.372

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
44 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Map ID 

(fig. 2) Station name

2007

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Orthophosphate, in mg/L [00671] 

1 Big Sioux River at Akron, Iowa 973 421 43 397 2,010 0.139

2 Little Sioux River at Turin, Iowa 483 148 31 256 832 0.136

3 Boyer River at Logan, Iowa 155 10.7 6.9 135 177 0.178

4 Nishnabotna River above Hamburg, Iowa 329 15.1 4.6 301 360 0.117

5 Des Moines River at Keosauqua, Iowa 1,830 -- -- -- -- 0.131

6 Skunk River at Augusta, Iowa 718 -- -- -- -- 0.166

7 Iowa River at Wapello, Iowa 1,710 -- -- -- -- 0.137

8 Wapsipinicon River near De Witt, Iowa 319 84.9 27 185 515 0.137

9 Maquoketa River near Spragueville, Iowa 179 30.0 17 127 245 0.110

10 Turkey River at Garber, Iowa 216 53.2 25 130 337 0.139

Total 6,910 -- -- -- -- 0.137

Suspended sediment, in mg/L [80154] 

1 Big Sioux River at Akron, Iowa 608,000 84,500 14 459,000 790,000 86.9

2 Little Sioux River at Turin, Iowa 2,550,000 400,000 16 1,860,000 3,420,000 719

3 Boyer River at Logan, Iowa 1,890,000 792,000 42 791,000 3,830,000 2,170

4 Nishnabotna River above Hamburg, Iowa 3,320,000 631,000 19 2,260,000 4,720,000 1,180

5 Des Moines River at Keosauqua, Iowa 3,380,000 1,040,000 31 1,800,000 5,810,000 241

6 Skunk River at Augusta, Iowa 2,420,000 595,000 25 1,460,000 3,780,000 561

7 Iowa River at Wapello, Iowa 2,620,000 368,000 14 1,970,000 3,410,000 209

8 Wapsipinicon River near De Witt, Iowa 498,000 76,900 15 365,000 665,000 213

9 Maquoketa River near Spragueville, Iowa 437,000 68,400 16 318,000 586,000 267

10 Turkey River at Garber, Iowa 714,000 108,000 15 525,000 948,000 459

Total 18,400,000 4,160,000 23 11,800,000 28,000,000 364

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 45

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]

Map ID 

(fig. 2) Station name

2008

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Chloride, in mg/L [00940] 

1 Big Sioux River at Akron, Iowa 63,100 1,780 2.8 59,700 66,600 9.02

2 Little Sioux River at Turin, Iowa 52,900 1,630 3.1 49,800 56,200 14.9

3 Boyer River at Logan, Iowa 12,700 389 3.1 12,000 13,500 14.7

4 Nishnabotna River above Hamburg, Iowa 36,900 1,070 2.9 34,900 39,100 13.1

5 Des Moines River at Keosauqua, Iowa 328,000 13,300 4.1 303,000 355,000 23.4

6 Skunk River at Augusta, Iowa 103,000 3,550 3.4 96,400 110,000 23.9

7 Iowa River at Wapello, Iowa 348,000 9,210 2.6 331,000 367,000 27.9

8 Wapsipinicon River near De Witt, Iowa 66,700 1,860 2.8 63,200 70,400 28.6

9 Maquoketa River near Spragueville, Iowa 43,400 1,330 3.1 40,800 46,100 26.6

10 Turkey River at Garber, Iowa 39,100 1,460 3.7 36,400 42,100 25.2

Total 1,090,000 35,500 3.3 1,030,000 1,170,000 21.6

Sulfate, in mg/L [00945] 

1 Big Sioux River at Akron, Iowa 401,000 18,300 4.6 366,000 438,000 57.3

2 Little Sioux River at Turin, Iowa 147,000 3,820 2.6 139,000 154,000 41.3

3 Boyer River at Logan, Iowa 24,800 833 3.4 23,200 26,400 28.5

4 Nishnabotna River above Hamburg, Iowa 73,100 2,300 3.1 68,700 77,700 26.0

5 Des Moines River at Keosauqua, Iowa 612,000 27,700 4.5 560,000 668,000 43.7

6 Skunk River at Augusta, Iowa 140,000 4,510 3.2 132,000 149,000 32.5

7 Iowa River at Wapello, Iowa 438,000 9,090 2.1 420,000 456,000 35.0

8 Wapsipinicon River near De Witt, Iowa 72,700 2,210 3.0 68,400 77,100 31.1

9 Maquoketa River near Spragueville, Iowa 51,000 1,260 2.5 48,600 53,500 31.2

10 Turkey River at Garber, Iowa 47,700 952 2.0 45,800 49,600 30.7

Total 2,010,000 71,000 3.5 1,870,000 2,150,000 39.8

Silica, in mg/L [00955] 

1 Big Sioux River at Akron, Iowa 36,000 -- -- -- -- 5.15

2 Little Sioux River at Turin, Iowa 49,400 4,590 9.3 41,000 59,000 13.9

3 Boyer River at Logan, Iowa 12,500 735 5.9 11,200 14,000 14.4

4 Nishnabotna River above Hamburg, Iowa 54,800 -- -- -- -- 19.5

5 Des Moines River at Keosauqua, Iowa 312,000 18,600 6.0 277,000 350,000 22.2

6 Skunk River at Augusta, Iowa 79,700 -- -- -- -- 18.5

7 Iowa River at Wapello, Iowa 302,000 -- -- -- -- 24.2

8 Wapsipinicon River near De Witt, Iowa 54,300 -- -- -- -- 23.2

9 Maquoketa River near Spragueville, Iowa 34,600 3,080 8.9 29,000 41,100 21.2

10 Turkey River at Garber, Iowa 32,900 4,220 13 25,400 41,900 21.2

Total 968,000 -- -- -- -- 19.1
46 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Map ID 

(fig. 2) Station name

2008

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux River at Akron, Iowa 17,100 798 4.7 15,600 18,700 2.44

2 Little Sioux River at Turin, Iowa 29,400 1,380 4.7 26,800 32,200 8.29

3 Boyer River at Logan, Iowa 11,100 758 6.8 9,670 12,600 12.7

4 Nishnabotna River above Hamburg, Iowa 33,100 1,550 4.7 30,100 36,200 11.8

5 Des Moines River at Keosauqua, Iowa 114,000 7,820 6.9 99,200 130,000 8.11

6 Skunk River at Augusta, Iowa 50,000 5,100 10 40,700 60,700 11.6

7 Iowa River at Wapello, Iowa 145,000 51,800 36 69,100 269,000 11.6

8 Wapsipinicon River near De Witt, Iowa 43,000 5,590 13 33,100 54,900 18.4

9 Maquoketa River near Spragueville, Iowa 31,600 1,760 5.6 28,300 35,200 19.3

10 Turkey River at Garber, Iowa 29,300 1,760 6.0 26,000 32,900 18.9

Total 503,000 78,300 16 379,000 682,000 9.95

Nitrate plus nitrite, in mg/L [00631] 

1 Big Sioux River at Akron, Iowa 14,000 1,190 8.5 11,800 16,500 2.00

2 Little Sioux River at Turin, Iowa 24,500 1,790 7.3 21,200 28,200 6.90

3 Boyer River at Logan, Iowa 7,090 483 6.8 6,190 8,080 8.15

4 Nishnabotna River above Hamburg, Iowa 24,500 1,750 7.1 21,200 28,100 8.71

5 Des Moines River at Keosauqua, Iowa 96,600 8,270 8.6 81,400 114,000 6.89

6 Skunk River at Augusta, Iowa 39,200 4,230 10.8 31,600 48,100 9.08

7 Iowa River at Wapello, Iowa 123,000 10,700 8.7 104,000 145,000 9.85

8 Wapsipinicon River near De Witt, Iowa 41,900 6,130 15 31,200 55,100 17.9

9 Maquoketa River near Spragueville, Iowa 24,300 1,660 6.8 21,200 27,800 14.9

10 Turkey River at Garber, Iowa 22,000 1,560 7.1 19,100 25,200 14.2

Total 417,000 37,700 9.0 348,000 496,000 8.25

Total phosphorus, in mg/L [00665] 

1 Big Sioux River at Akron, Iowa 951 59.3 6.2 840 1,070 0.136

2 Little Sioux River at Turin, Iowa 1,530 231 15 1,130 2,030 0.431

3 Boyer River at Logan, Iowa 1,190 314 26 694 1,910 1.37

4 Nishnabotna River above Hamburg, Iowa 3,230 355 11 2,590 3,980 1.15

5 Des Moines River at Keosauqua, Iowa 5,880 329 5.6 5,260 6,550 0.419

6 Skunk River at Augusta, Iowa 3,800 484 13 2,940 4,830 0.880

7 Iowa River at Wapello, Iowa 7,190 330 4.6 6,570 7,860 0.575

8 Wapsipinicon River near De Witt, Iowa 1,230 187 15 909 1,640 0.528

9 Maquoketa River near Spragueville, Iowa 2,260 280 12 1,760 2,860 1.38

10 Turkey River at Garber, Iowa 1,570 430 27 894 2,560 1.01

Total 28,800 3,000 10 23,600 35,300 0.570

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 47

Map ID 

(fig. 2) Station name

2008

Load SEP L95 U95 Yield

(tons) (tons) (percent) (tons) (tons) (ton/mi2

)

Orthophosphate, in mg/L [00671] 

1 Big Sioux River at Akron, Iowa 326 106 33 167 577 0.0466

2 Little Sioux River at Turin, Iowa 517 120 23 322 788 0.146

3 Boyer River at Logan, Iowa 201 16.5 8.2 170 235 0.231

4 Nishnabotna River above Hamburg, Iowa 450 22.5 5.0 408 496 0.160

5 Des Moines River at Keosauqua, Iowa 3,030 -- -- -- -- 0.216

6 Skunk River at Augusta, Iowa 1,380 -- -- -- -- 0.319

7 Iowa River at Wapello, Iowa 3,300 -- -- -- -- 0.264

8 Wapsipinicon River near De Witt, Iowa 859 332 39 386 1,660 0.368

9 Maquoketa River near Spragueville, Iowa 651 158 24 396 1,010 0.398

10 Turkey River at Garber, Iowa 447 128 29 247 746 0.287

Total 11,200 -- -- -- -- 0.221

Suspended sediment, in mg/L [80154] 

1 Big Sioux River at Akron, Iowa 663,000 75,700 11 527,000 824,000 94.8

2 Little Sioux River at Turin, Iowa 2,150,000 323,000 15 1,590,000 2,850,000 605

3 Boyer River at Logan, Iowa 3,850,000 1,870,000 49 1,410,000 8,530,000 4,430

4 Nishnabotna River above Hamburg, Iowa 4,550,000 873,000 19 3,070,000 6,480,000 1,620

5 Des Moines River at Keosauqua, Iowa 7,310,000 2,420,000 33 3,700,000 13,100,000 521

6 Skunk River at Augusta, Iowa 4,170,000 1,320,000 32 2,160,000 7,280,000 965

7 Iowa River at Wapello, Iowa 3,960,000 637,000 16 2,860,000 5,340,000 317

8 Wapsipinicon River near De Witt, Iowa 754,000 145,000 19 511,000 1,080,000 323

9 Maquoketa River near Spragueville, Iowa 3,060,000 696,000 23 1,920,000 4,640,000 1,870

10 Turkey River at Garber, Iowa 2,840,000 643,000 23 1,790,000 4,290,000 1,830

Total 33,300,000 9,000,000 27 19,500,000 54,400,000 659

Table 6. Estimated major ions, nitrogen, phosphorus, and suspended-sediment loads and yields from selected major Iowa rivers, water 

years 2004–2008.—Continued

[Water year from October 1 to September 30; ID, identifier; SEP, standard error of prediction; L95 and U95, lower (L) and upper (U) limits of the 95-percent 

confidence interval of estimated load; ton/mi2, tons per square mile; mg/L, milligrams per liter; parameter code given in brackets, see also table 2; μg/L, micrograms per liter; --, not available]
Table 7. Regression models for estimating major ion, nutrient, and suspended-sediment loads from selected major Iowa rivers, water years 2004–20[Water year from October 1 to September 30; ID, identifier; N., number; obs., observations; R2, coefficient of determination; %, percent; mg/L, milligrams per liter; parameter coalso table 2; ln, natural logarithm; L, daily load in tons per day; Q, centered mean daily streamflow in cubic feet per second; T, centered time in decimal years; SS, seasonality paAMLE, adjusted maximum likelihood estimation; dQi, change in flow relative to average flow of previous i number of days; A5yr, 5-year flow anomaly; A1yr, 1-year flow anomanomaly; HFV, high-frequency flow anomaly; LAD, least absolute deviation; BpQ, streamflow breakpoint term] Map ID (fig. 2) River N. obs. N. censored Regression model Es

r

v

Chloride, in mg/L [00940] 1 Big Sioux 56 0 ln(L) = 5.19 + 0.711*lnQ − 0.0385*lnQ2 + 0.00820*T + 0.0161*T2 + 0.115*sin(SS) + 0.0970*cos(SS) 

2 Little Sioux 58 0 ln(L) = 4.77 + 0.865*lnQ − 0.0505*lnQ2 + 0.0766*sin(SS) + 0.108*cos(SS) − 0.471*|dQ1| − 0.101*dQ30 

3 Boyer 54 0 ln(L) = 3.62 + 0.676*lnQ − 0.0186*lnQ2 − 0.253*dQ1 4 Nishnabotna 58 0 ln(L) = 4.43 + 0.734*lnQ − 0.0394*lnQ2 + 0.103*sin(SS) + 0.0382*cos(SS) 

5 Des Moines 55 0 ln(L) = 6.13 + 0.117*sin(SS) + 0.0524*cos(SS) + (0.924*A5yr + 0.415*A1yr + 0.659*A3mo + 0.753*HFV) 

6 Skunk 55 0 ln(L) = 4.87 + 0.735*lnQ − 0.0335*lnQ2 + 0.146*sin(SS) + 0.0957*cos(SS) 

7 Iowa 60 0 ln(L) = 6.31 + 0.0718*sin(SS) + 0.0798*cos(SS) + (0.404*A1yr + 0.711*A3mo + 0.699*HFV) 

8 Wapsipinicon 55 0 ln(L) = 4.97 + 0.939*lnQ − 0.0691*lnQ2 − 0.0409*T − 0.0571*T2 + 0.0426*sin(SS) + 0.0822*cos(SS) 

9 Maquoketa 56 0 ln(L) = 4.56 + 0.837*lnQ − 0.135*lnQ2 + 0.0632*sin(SS) + 0.0403*cos(SS) 

10 Turkey 55 0 ln(L) = 4.68 + 0.827 *lnQ − 0.0639*lnQ2 

Sulfate, in mg/L [00945] 1 Big Sioux 57 0 ln(L) = 6.94 + 0.974*lnQ + 0.0428*T − 0.227*dQ30 

2 Little Sioux 58 0 ln(L) = 5.79 + 0.791*lnQ + 0.0666*sin(SS) + 0.0890*cos(SS) − 0.523*dQ30 − 0.195*|dQ1| 

3 Boyer 57 0 ln(L) = 4.26 + 0.739*lnQ − 0.225*dQ1 

4 Nishnabotna 58 0 ln(L) = 5.04 + 0.784*lnQ + 0.105*sin(SS) + 0.0188*cos(SS) − 0.165*dQ30 5 Des Moines 56 0 ln(L) = 7.07 + 0.608*lnQ − 0.0481*lnQ2 + 0.0964*sin(SS) + 0.0217*cos(SS) + 0.223*dQ30 6 Skunk 56 0 ln(L) = 5.22 + 0.704*lnQ − 0.0300*lnQ2 + 0.107*sin(SS) + 0.0971*cos(SS) 

7 Iowa 60 0 ln(L) = 6.97 + 0.673*lnQ − 0.0447*lnQ2 − 0.668*|dQ1| 

8 Wapsipinicon 57 0 ln(L) = 4.92 + 0.801*lnQ − 0.0722*lnQ2 

9 Maquoketa 56 0 ln(L) = 4.76 + 0.741*lnQ − 0.112*lnQ2 

10 Turkey 56 0 ln(L) = 4.56 + 0.837*lnQ − 0.135*lnQ2 + 0.0632*sin(SS) + 0.0403*cos(SS) 
Table 7. Regression models for estimating major ion, nutrient, and suspended-sediment loads from selected major Iowa rivers, water years 2004–20[Water year from October 1 to September 30; ID, identifier; N., number; obs., observations; R2, coefficient of determination; %, percent; mg/L, milligrams per liter; parameter coalso table 2; ln, natural logarithm; L, daily load in tons per day; Q, centered mean daily streamflow in cubic feet per second; T, centered time in decimal years; SS, seasonality paAMLE, adjusted maximum likelihood estimation; dQi, change in flow relative to average flow of previous i number of days; A5yr, 5-year flow anomaly; A1yr, 1-year flow anomanomaly; HFV, high-frequency flow anomaly; LAD, least absolute deviation; BpQ, streamflow breakpoint term]

Map ID 

(fig. 2) River N. obs. N. censored Regression model Es

r

v

Silica, in mg/L [00955] 

1 Big Sioux 58 0 ln(L) = 4.49 +1.21*lnQ + 0.0852*T − 0.126*sin(SS) + 0.159*cos(SS) 2 Little Sioux 58 0 ln(L) = 4.64 +1.24*lnQ − 0.225*lnQ2+ 0.0888*T − 0.0699*T2 − 0.125*sin(SS) + 0.189*cos(SS) 

3 Boyer 56 0 ln(L) = 3.26 + 1.03*lnQ − 0.0754*lnQ2 − 0.338*dQ1 

4 Nishnabotna 57 0 ln(L) = 4.51 + 1.07*lnQ − 0.0401*lnQ2 + 0.0309*T − 0.196*dQ30 

5 Des Moines 53 0 ln(L) = 5.63 + 1.26*lnQ − 0.132*sin(SS) + 0.209*cos(SS) − 0.221*dQ30 

6 Skunk 51 0 ln(L) = 4.12 + 1.36*lnQ − 0.205*lnQ2 − 0.271*sin(SS) + 0.0256*cos(SS) 

7 Iowa 28 0 ln(L) = 5.89 + 1.24*lnQ 

8 Wapsipinicon 54 0 ln(L) = 3.58 + 1.52*lnQ − 0.239*sin(SS) + 0.585*cos(SS) 

9 Maquoketa 56 0 ln(L) = 3.68 + 0.923*lnQ + 0.0901*T + 0.0734*T2 − 0.0511*sin(SS) + 0.142*cos(SS) 

10 Turkey 55 0 ln(L) = 4.15 + 1.15*lnQ − 0.105*lnQ2 + 0.0846*T 

Total nitrogen, in mg/L [49570 plus 62854, or 62855] 

1 Big Sioux 58 0 ln(L) = 3.92 + 1.09*lnQ − 0.0817*lnQ2 + 0.174*sin(SS) + 0.172*cos(SS) 

2 Little Sioux 58 0 ln(L) = 3.87 + 1.31*lnQ − 0.111*lnQ2 + 0.221*sin(SS) + 0.114*cos(SS) 

3 Boyer 57 0 ln(L) = 3.17 + 1.12*lnQ − 0.0513*lnQ2 + 0.128*sin(SS) − 0.0142*cos(SS) 

4 Nishnabotna 58 0 ln(L) = 4.09 + 1.22*lnQ − 0.159*lnQ2 + 0.158*sin(SS) − 0.0403*cos(SS) 5 Des Moines 55 0 ln(L) = 5.25 + 1.35*lnQ − 0.124*T − 0.132*T2 + 0.315*sin(SS) + 0.0704*cos(SS) − 0.345*dQ30 

6 Skunk 56 0 ln(L) = 3.70 + 1.22*lnQ − 0.181*lnQ2 + 0.359*sin(SS) − 0.0153*cos(SS) 

7 Iowa 59 0 ln(L) = 5.99 + 1.33*lnQ − 0.142*lnQ2 − 0.121*T − 0.0841*T2 + 0.114*sin(SS) + 0.121*cos(SS) − 

0.556*dQ1

8 Wapsipinicon 55 0 ln(L) = 3.66 + 1.41*lnQ − 0.151*lnQ2 + 0.147*sin(SS) + 0.224*cos(SS) 

9 Maquoketa 56 0 ln(L) = 3.89 + 1.20*lnQ − 0.0935*lnQ2 + 0.0904*sin(SS) + 0.0902*cos(SS) 

10 Turkey 55 0 ln(L) = 4.09 + 1.21*lnQ − 0.0528*lnQ2 + 0.0697*sin(SS) + 0.0798*cos(SS) 
Table 7. Regression models for estimating major ion, nutrient, and suspended-sediment loads from selected major Iowa rivers, water years 2004–20[Water year from October 1 to September 30; ID, identifier; N., number; obs., observations; R2, coefficient of determination; %, percent; mg/L, milligrams per liter; parameter coalso table 2; ln, natural logarithm; L, daily load in tons per day; Q, centered mean daily streamflow in cubic feet per second; T, centered time in decimal years; SS, seasonality paAMLE, adjusted maximum likelihood estimation; dQi, change in flow relative to average flow of previous i number of days; A5yr, 5-year flow anomaly; A1yr, 1-year flow anomanomaly; HFV, high-frequency flow anomaly; LAD, least absolute deviation; BpQ, streamflow breakpoint term] Map ID (fig. 2) River N. obs. N. censored Regression model Es

r

v

Nitrate plus nitrite, in mg/L [00631] 1 Big Sioux 58 0 ln(L) = 3.72 + 1.24*lnQ − 0.122*lnQ2 + 0.300*sin(SS) + 0.439*cos(SS) − 0.301*dQ30 

2 Little Sioux 57 0 ln(L) = 3.73 + 1.45*lnQ − 0.179*lnQ2 + 0.269*sin(SS) + 0.362*cos(SS) − 0.390*dQ30 3 Boyer 57 0 ln(L) = 2.92 + 0.985*lnQ − 0.0731*lnQ2 − 0.398*dQ1d 

4 Nishnabotna 58 0 ln(L) = 3.88 + 1.32*lnQ − 0.125*lnQ2 − 0.0632*T + 0.166*sin(SS) + 0.0867*cos(SS) − 0.397*dQ30 

5 Des Moines 56 0 ln(L) = 5.12 + 1.56*lnQ − 0.171*T − 0.160*T2 + 0.286*sin(SS) + 0.236*cos(SS) − 0.657*dQ30 6 Skunk 53 6 ln(L) = 8.47 + 6.37*lnQ − 5.63*BpQ 

7 Iowa 57 0 ln(L) = 6.08 + 1.45*lnQ − 0.147*lnQ2 − 0.153*T − 0.101*T2 + 0.0925*sin(SS) + 0.264*cos(SS) − 1.42*|dQ1| 

8 Wapsipinicon 55 0 ln(L) = 3.81 + 1.52*lnQ − 0.212*lnQ2 + 0.210*sin(SS) + 0.437*cos(SS) 

9 Maquoketa 55 0 ln(L) = 3.73 + 1.20*lnQ − 0.104*lnQ2 + 0.0757*sin(SS) + 0.189*cos(SS) − 0.293*dQ30 

10 Turkey 54 0 ln(L) = 4.10 + 1.21*lnQ − 0.0628*lnQ2 − 1.15*|dQ1| 

Total phosphorus, in mg/L [00665] 1 Big Sioux 58 0 ln(L) = 0.787 + 0.831*lnQ + 0.0970*sin(SS) + 0.215*cos(SS) + 0.596*dQ30 

2 Little Sioux 57 0 ln(L) = 0.542 + 1.55*lnQ − 0.108*T + 0.459*dQ30 3 Boyer 57 0 ln(L) = 1.066 + 1.47*lnQ + 0.0700*lnQ2 − 0.270*T − 0.177*T2 4 Nishnabotna 57 0 ln(L) = 1.41 + 1.31*lnQ − 0.140+lnQ2 − 0.115*T − 0.00113*sin(SS) − 0.385*cos(SS) + 0.638*dQ30 5 Des Moines 54 0 ln(L) = 1.88 + 1.10*lnQ + 0.123*sin(SS) + 0.208*cos(SS) + 0.221*dQ30 6 Skunk 54 0 ln(L) = 0.659 + 1.17*lnQ − 0.0685+lnQ2 + 0.00225*sin(SS) − 0.344*cos(SS) + 0.395*dQ30 7 Iowa 59 0 ln(L) = 2.41 + 0.914*lnQ + 0.0426+T + 0.0214*sin(SS) − 0.174*cos(SS) + 0.244*dQ30 8 Wapsipinicon 56 0 ln(L) = 0.331 + 0.999*lnQ − 0.175*lnQ2 − 0.0940*sin(SS) − 0.626*cos(SS) + 0.551*dQ30 9 Maquoketa 55 0 ln(L) = 0.377 + 1.41*lnQ + 0.0609*sin(SS) − 0.189*cos(SS) − 0.293*dQ30 

10 Turkey 55 0 ln(L) = 0.651 + 1.85*lnQ − 0.0948*lnQ2 − 0.163*T 
Table 7. Regression models for estimating major ion, nutrient, and suspended-sediment loads from selected major Iowa rivers, water years 2004–20[Water year from October 1 to September 30; ID, identifier; N., number; obs., observations; R2, coefficient of determination; %, percent; mg/L, milligrams per liter; parameter coalso table 2; ln, natural logarithm; L, daily load in tons per day; Q, centered mean daily streamflow in cubic feet per second; T, centered time in decimal years; SS, seasonality paAMLE, adjusted maximum likelihood estimation; dQi, change in flow relative to average flow of previous i number of days; A5yr, 5-year flow anomaly; A1yr, 1-year flow anomanomaly; HFV, high-frequency flow anomaly; LAD, least absolute deviation; BpQ, streamflow breakpoint term]

Map ID 

(fig. 2) River N. obs. N. censored Regression model Es

r

v

 Orthophosphate, in mg/L [00671] 

1 Big Sioux 57 0 ln(L) = -2.02 + 0.209*sin(SS) + 0.935*cos(SS) + (-0.574*A5yr + 1.77*A1yr + 1.20*A3mo + 

1.69*HFV)

2 Little Sioux 58 7 ln(L) = − 0.759 + 2.03*lnQ − 0.437*lnQ2 + 0.460*sin(SS)+ 0.695*cos(SS) 

3 Boyer 57 0 ln(L) = -1.01 − 0..228*sin(SS) − 0.0172*cos(SS) + (1.22*A5yr + 0.358*A1yr + 0.485*A3mo + 

0.732*HFV)

4 Nishnabotna 58 0 ln(L) = − 0.143 + 0.938*lnQ − 0.0860*lnQ2 − 0.0485*sin(SS) − 0.226*cos(SS) 

5 Des Moines 55 0 ln(L) = 1.25 + 1.37*lnQ + 0.0333*sin(SS) + 0.810*cos(SS) − 0.399*dQ30 

6 Skunk 54 0 ln(L) = − 0.486 + 1.38*lnQ − 0.268*sin(SS) + 0.146*cos(SS) 7 Iowa 57 0 ln(L) = 1.63 + 1.40*lnQ + 0.00372*sin(SS) + 0.746*cos(SS) 8 Wapsipinicon 57 12 ln(L) = -1.38 + 2.41*lnQ − 0.158*lnQ2 − 0.250*sin(SS) + 1.066*cos(SS) 

9 Maquoketa 56 2 ln(L) = − 0.325 + 1.68*lnQ − 0.203*lnQ2 − 0.181*sin(SS) + 0.343*cos(SS) 

10 Turkey 55 6 ln(L) = -0.531 + 1.75*lnQ − 0.197*lnQ2 + 0.127*sin(SS) + 0.334*cos(SS) 

 Suspended sediment, in mg/L [80154] 

1 Big Sioux 57 0 ln(L) = 7.16 + 1.18*lnQ − 0.134*lnQ2 − 0.0255*sin(SS) − 0.417*cos(SS) + 0.521*dQ30 

2 Little Sioux 56 0 ln(L) = 7.50 + 1.90*lnQ − 0.110*T − 0.0938*sin(SS) − 0.250*cos(SS) + 0.383*dQ30 

3 Boyer 55 0 ln(L) = 7.43 + 2.12*lnQ − 0.160*lnQ2 + 0.734*dQ1 

4 Nishnabotna 57 0 ln(L) = 8.41 + 1.65*lnQ − 0.316*lnQ2 − 0.157*T − 0.117*sin(SS) − 0.548*cos(SS) + 0.907*dQ30 5 Des Moines 55 0 ln(L) = 7.38 + 1.50*lnQ + 0.0732*sin(SS) − 0.534*cos(SS) + 0.904*dQ30 6 Skunk 55 0 ln(L) = 6.91 + 1.50*lnQ − 0.144*lnQ2 − 0.0643*sin(SS) − 0.544*cos(SS) + 0.662*dQ30 7 Iowa 57 0 ln(L) = 8.60 + 1.11*lnQ − 0.276*lnQ2 − 0.0736*sin(SS) − 0.561*cos(SS) + 1.61*dQ1 + 0.625*dQ30 8 Wapsipinicon 53 0 ln(L) = 6.82 + 1.35*lnQ − 0.258*lnQ2 − 0.298*sin(SS) − 0.363*cos(SS) + 3.03*dQ1 9 Maquoketa 54 0 ln(L) = 5.93 − 0.0611*sin(SS) − 0.568*cos(SS) + (2.21*A1yr + 0.946*A3mo + 2.16*HFV) 

10 Turkey 53 0 ln(L) = 4.90 − 0.284*sin(SS) − 0.269*cos(SS) + (2.53*A1yr + 1.67*A3mo + 1.95*HFV) 

1LAD method does not produce estimated residual variance or R2; values are based on similar models using AMLE.
52 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Phosphorus load

Streamflow

EXPLANATION

0

100

200

300

400

Streamflow, thousand cubic feet per second

0

20

40

60

80

100

Nitrate load, thousand ton/day

A

Water year

0

100

200

300

400

Streamflow, thousand cubic feet per second

0

2

4

6

8

10

Phosphorus load, thousand ton/day

2004 2005 2006 2007 2008

B

Nitrate load

Streamflow

EXPLANATION

Figure 7. Total nitrate and total phosphorus load and combined streamflow for 10 major Iowa rivers, water years 2004–2008.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 53

EXPLANATION

1,000

100

10,000

Suspended-sediment load, thousand tons

D Suspended sediment for Skunk River at Augusta

2004 2005 2006 2007 2008

Year

2004 2005 2006 2007 2008

Year

2004 2005 2006 2007 2008

Year

2004 2005 2006 2007 2008

Year

10,000

100,000

Nitrate plus nitrite load, tons

A Nitrate plus nitrite for Iowa River at Wapello

1,000

10,000

Orthophosphate load, tons

B Orthophosphate for Iowa River at Wapello

1,000

10,000

Total phosphorus load, tons

C Total phosphorus for Iowa River at Wapello

Aulenbach and others, 2007

Current report

Nalley and others, 2005a,b; U.S. Geological Survey, 2007–2009

Figure 8. Comparison of annual load estimates and 95-percent confidence limits for select constituents at selected sites, water years 

2004–2008.

Because TN concentrations in Iowa streams are dominated by nitrate, patterns in nitrate loads and yields are similar 

to TN. The smallest nitrate load was estimated at 1,040 tons 

in 2006 from the Boyer River (map ID 3), and the largest 

load was 123,000 tons in 2008 from the Iowa River (map 

ID 7; table 6). The Des Moines (map ID 5) and Iowa Rivers 

consistently had the largest nitrate loads of the studied rivers, 

and the Boyer River the smallest. Nitrate yields ranged from 

0.723 ton/mi2 in 2006 from the Nishnabotna River (map ID 4) 

to 17.9 ton/mi2 in 2008 from the Iowa River. Similar to total 

nitrogen, average nitrate yields for the 5-year study period 

were smallest at the Big Sioux River and largest in northeastern basins (map IDs 7–10; fig. 9E). Total nitrogen and nitrate 

yields relative to a long-term average streamflow indicated 

that Mississippi tributaries yielded only slightly more nitrogen 

than Missouri tributaries, with the greatest yields related less 

to site differences than to exceptionally wet conditions (years 

with greater than twice the long-term average streamflow).

The patterns in nitrogen loads are largely reflective of the 

relation with streamflow, but also demonstrate the seasonal 

effect of agricultural practices on the landscape. Nitrogen 

fertilizer application is greatest in the spring, coincident with 

planting and typical spring increases in precipitation and 

streamflow. Fertilizer distribution data, available biannually 

at the statewide level for the State of Iowa (Iowa Department of Agriculture and Land Stewardship, Fertilizer Tonnage Distribution in Iowa, http://www.iowaagriculture.gov/

feedAndFertilizer/fertilizerDistributionReport.asp, accessed 

February 18, 2009), compared with combined basin-wide 

loads for the 10 study basins (fig. 10), provides a useful, albeit 

incomplete, comparison of nutrient application and stream 

loads. Figure 10A shows that although nitrogen fertilizer 
54 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

Missouri River tributaries

Selected tributaries

Big Sioux River (1)

Little Sioux River (2)

Boyer River (3)

Nishnabotna River (4)

Mississippi River tributaries

Selected tributaries

Des Moines River (5)

Skunk River (6)

Iowa River (7)

Wapsipinicon River (8)

Maquoketa River (9)

Turkey River (10)

EXPLANATION

10

2

3

4

5

6

8

20

30

Chloride yield, ton/square mile

A

10

5

6

7

20

30

40

50

60

Sulfate yield, ton/square mile

B

1

10

2

3

4

5

7

20

30

Silica yield, ton/square mile

C

0.3 0.4 0.6 1.0 2.0 3.0

Relative streamflow, percent

1

10

2

3

4

5

7

20

Total nitrogen yield, ton/square mile

D

0.3 0.4 0.6 1.0 2.0 3.0

Relative streamflow, percent

1

10

2

3

4

5

7

20

Nitrate plus nitrite yield, ton/square mile

E

Figure 9. Relations (log-fit) between annual constituent yield and average streamflow relative to long-term (1979–2008) average 

streamflow for 10 major Iowa rivers showing map identifier, water years 2004–2008.
Chemical Concentration, Loads, and Yields in Major Iowa Rivers 55

Missouri River tributaries

Selected tributaries

Big Sioux River (1)

Little Sioux River (2)

Boyer River (3)

Nishnabotna River (4)

Mississippi River tributaries

Selected tributaries

Des Moines River (5)

Skunk River (6)

Iowa River (7)

Wapsipinicon River (8)

Maquoketa River (9)

Turkey River (10)

EXPLANATION

Total phosphorus yield, ton/square mile

F

Orthophosphate yield, ton/square mile

G

Suspended-sediment yield, ton/square mile

H

0.3 0.4 0.6 1.0 2.0 3.0

Relative streamflow, percent

0.3 0.4 0.6 1.0 2.0 3.0

Relative streamflow, percent

1,000

100

0.1

1.0

0.01

0.10

Figure 9. Relations (log-fit) between annual constituent yield and average streamflow relative to long-term (1979–2008) average 

streamflow for 10 major Iowa rivers showing map identifier, water years 2004–2008.—Continued

application (expressed as statewide distribution) and nitrogen 

loads are greater in the first one-half of the year, loads relative 

to fertilizer application are greater during wet periods. This 

pattern was evident in the dry years of 2004 through 2006, but 

was more dramatic in 2007 and 2008, when nitrogen stream 

loads were above 40 percent relative to the amount of nitrogen fertilizer application and above-average precipitation and 

streamflow was recorded. Nitrogen loads relative to fertilizer 

use for January to June (including the critical period of spring 

application and high streamflow) rise from about 30 percent in 

2004–2006 to an average 52 percent in the wet 2007 and 2008 

years. Loads relative to use were much smaller for nitrogen in 

the second one-half of the year, with July-to-December loads 

ranging from 8 to 27 percent.

Nutrient loads are presented relative to fertilizer distribution to provide perspective. Only 75 percent of the State of 

Iowa is included in the study basins and areas of Minnesota 

and South Dakota account for 17 percent of the study basins. 

Agricultural fertilizer certainly is not the only source of nutrients in streams, but the mass of nitrogen exported from the 

State of Iowa from the 10 study basins follows similar temporal patterns to distribution, and this load represents a large part 

of the statewide nitrogen use.
56 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

0

100

300

200

400

500

600

700

800

900

Nitrogen, thousand tons

31

20

28

11

28

8

47

27

56

A

 Jan. 2004

July 2004

 Jan. 2005

July 2005

 Jan. 2006

July 2006

 Jan. 2007

July 2007

 Jan. 2008

July 2008

0

50

100

150

200

250

300

Phosphorus, thousand tons

6.6 2.2 4.3 1.4 3.4 1.0

8.1 3.4

11.1

B

 Jan. 2004

July 2004

 Jan. 2005

July 2005

 Jan. 2006

July 2006

 Jan. 2007

July 2007

 Jan. 2008

July 2008

31

Fertilizer

Stream load

Load as percent fertilizer

EXPLANATION

2.2

Fertilizer

Stream load

Load as percent fertilizer

EXPLANATION

Figure 10. Fertilizer distribution in Iowa and constituent loads 

combined for 10 major Iowa rivers, 2004–2008.

Phosphorus

Total phosphorus loads ranged from 129 tons in 2006 

from the Boyer River (map ID 3) to 7,190 tons in 2008 from 

the Iowa River (map ID 7; table 6). The effect of streamflow 

on loads resulted in larger TP loads from large central Iowa 

basins, particularly the Iowa and Des Moines (map ID 5) 

Rivers, than from smaller basins. The smallest annual TP 

yield was in 2005 from the Big Sioux River (map ID 1) at 

0.0959 ton/mi2, and the largest annual TP yield was in 2007 

at 1.48 ton/mi2 from the Boyer River (map ID 3). The Boyer 

and Nishnabotna Rivers (map IDs 3 and 4) reported results of 

consistently high TP yields each year, with an exception for 

the Nishnabotna River in 2006, which was a dry year across 

much of the State, as indicated by relatively lower streamflow 

compared to the long-term average for 1979–2008. Excepting the Nishnabotna River in 2006, the Big Sioux River had 

the lowest annual TP yields through the 5-year study period 

(fig. 9F).

Annual orthophosphate loads ranged from 49.0 tons in 

2005 from the Wapsipinicon River (map ID 8) to 3,300 tons 

in 2008 from the Iowa River (map ID 7; table 6). The Iowa 

and Des Moines Rivers (map ID 5) had the greatest annual 

orthophosphate loads, generally twice the annual load of 

any other site, because of greater streamflow. The Big Sioux 

River also had high orthophosphate loads in 2006 and 2007. 

The annual orthophosphate yields among all 10 sites ranged 

from 0.0210 ton/mi2 in 2005 from the Wapsipinicon River to 

0.398 ton/mi2 in 2008 from the Maquoketa River. The Boyer 

River was consistently one of the greatest orthophosphateyielding basins relative to the other sites, particularly during 

dry years (fig. 9G).

The seasonal patterns in phosphorus loads, similar to 

nitrogen, are largely reflective of the relation with streamflow, 

but the effects of agricultural practices differ for nitrogen and 

phosphorus. Phosphorus commercial fertilizer applications 

are typically greatest in the fall after harvest when streamflows are generally low. Fertilizer application expressed as 

statewide distribution, along with combined stream total 

phosphorus loads with an indication of percent loads relative to application is shown in figure 10B. As with nitrogen, 

loads are greater during wet periods than dry, with about a 

fourfold increase in loads between the driest (2006) and wettest water years (2008). Phosphorus January-to-June relative 

loads for 2004–2006 average 4.8 percent, and 9.6 percent 

for 2007–2008. Loads relative to use were much smaller for 

phosphorus in the second one-half of the year, with July-toDecember loads ranging from 1.0 to 3.4 percent.

Suspended Sediment

Annual suspended-sediment loads ranged from 

53,000 tons from the Boyer River (map ID 3) in 2006 to 

7,310,000 tons from the Des Moines River (map ID 5) in 

2008 (table 6). The general relation between basin size and 

constituent load does not hold well for suspended sediment. 
Summary 57

The Nishnabotna River (map ID 4) loads are similar to the 

Des Moines River, although the Des Moines River basin is 

five times larger. Conversely, the Big Sioux (map ID 1) and 

Wapsipinicon Rivers (map ID 8), ranked three and seven 

by basin size, had consistently lower loads and yields of 

suspended sediment than other rivers. Annual yields ranged 

from 51.8 ton/mi2 from the Des Moines River in 2006 to 

4,430 ton/mi2 from the Boyer River in 2008. The Big Sioux 

River generally had the lowest suspended-sediment yields, 

whereas other Missouri River tributaries, particularly the 

two southwestern Iowa basins of the Boyer and Nishnabotna 

Rivers tended to have greater suspended-sediment yields than 

Mississippi River tributaries (fig. 9H).

Summary

Concentrations, loads, and yields of streamflow constituents were assessed for 10 large Iowa rivers for the 2004 

through 2008 water years including analysis of major ions, 

carbon, nutrients, suspended sediment, and pesticides. Fertilizers and pesticides are commonly used on agricultural, 

residential, and urban lands to sustain crop yields and support 

the desired urban landscape with lawns. Fertilizer and pesticide application and variable climate conditions can lead to 

precipitation washing some of these chemicals into streams 

and rivers. Pesticides and high levels of nutrients can have 

deleterious effects on aquatic health of streams, and nutrients 

transported from Iowa and other Midwest agricultural states 

have been linked to hypoxia in the Gulf of Mexico.

Samples were collected from March 2004 through September 2008 as part of a project in cooperation with the Iowa 

Department of Natural Resources. Sampling sites are located 

near the mouths of Iowa rivers in basins, which cover large 

parts of the State, ranging in size from 871 to 14,038 square 

miles, covering a total 50,562 square miles. Basins include 

75.0 percent of Iowa and additional areas in eastern South 

Dakota and southern Minnesota accounting for 17.1 percent 

of the study basins. The average annual precipitation gradient 

increases across the basins from 22 to 38 inches from northwest to southeast. The variations in basin size and precipitation resulted in long-term (1979–2008) average annual 

streamflows at sampling sites from 461 to 10,293 cubic feet 

per second.

Iowa land use is largely agricultural, with 73 percent of 

the State used for crop production, 86 percent of which is corn 

and soybeans. Iowa is one of the most productive areas for 

corn and soybeans in the world. The glacial and alluvial landforms vary across the study basins, though most of this region 

of fertile, moist, glacial, commonly calcareous soils, and former prairies is well suited to extensive agricultural land uses. 

The thick loess deposits and steep to rolling hills of western 

and southern Iowa produce highly erodible slopes. Population 

density in this rural state averages 54 people per square mile.

Water-quality samples were collected using standard 

protocols to obtain streamflow-integrated samples, typically 

using equal-width increment techniques. The exclusion of 

methanol from cleaning procedures reduced occurrence of dissolved organic carbon from field blank quality-control samples 

without increased incident of pesticide carry-over. Samples 

sent to USGS laboratories for analysis of major ions, nutrients, 

carbon, pesticides, and suspended sediment.

Statistical summaries of sample data computed in TIBCO 

Spotfire S+® used nonparametric regression on order statistics, 

parametric adjusted maximum likelihood estimation methods, 

and a modification of the Kaplan-Meier nonparametric 

method. These methods provide correct handling of datasets 

with values below analytical detection limits and with changing levels of detection.

Stream loads, the chemical mass transported by a stream 

past a location during a specified time period, were estimated for water years 2004 through 2008 by a rating-curve 

method using S-LOADEST. Stream yields (loads divided 

by watershed area) were computed to compare constituent 

contributions from watersheds of different sizes. In addition to 

predefined models using linear and quadratic streamflow and 

time terms with sine and cosine to describe seasonal patterns, 

additional terms describing streamflow variability and anomalies were evaluated. Streamflow variability terms describe 

the difference in streamflow from recent average conditions, 

on a 1-day or 30-day time step. Streamflow anomaly terms 

account for deviations from average conditions sequentially 

from long- to short-term, using 5-year, 1-year, 3-month, and 

high frequency variation terms. Candidate regression load 

models were evaluated for model fit, distribution assumptions 

on the residuals, and correlation of the explanatory variables, 

with preferred models with low residual variance, normal and 

homoskedastic residual distributions, low correlation among 

explanatory variables, and good empirical agreement with 

measured data.

Constituent concentrations vary by streamflow and 

season in Iowa. Constituent concentrations decreased with 

streamflow for pH, alkalinity, specific conductance, chloride, 

and sulfate, whereas concentrations increased with streamflow 

for particulate and dissolved organic carbon, total phosphorus, 

and suspended sediment. Silica, particulate inorganic carbon, 

and chlorophyll-a concentrations did not correlate directly 

with streamflow. Nitrogen concentrations (total nitrogen and 

nitrate plus nitrite) increased with low and moderate streamflows, but decreased with high streamflows. The seasonality 

of streamflow affected concentrations, but additional seasonal patterns were defined by algae blooms and pesticide 

application.

Climate and landscape gradients resulted in spatial patterns across Iowa in specific conductance, alkalinity, chloride, 

total phosphorus, dissolved organic carbon, suspended sediment, and turbidity. Alkalinity was greatest in northern Iowa 

rivers. Chloride and dissolved organic carbon increased among 

tributaries upstream along the Missouri River and downstream 

among tributaries along the Mississippi River. Turbidity, 
58 Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008

suspended sediment, and total phosphorus were greatest from 

southwestern Iowa rivers draining loess landscapes. Specific 

conductance and sulfate concentrations were greatest and 

most variable in the Big Sioux River in northwestern Iowa. 

Spatial variability also was evident in different streamflowconcentration relations among sites for particulate organic 

carbon, orthophosphate, suspended sediment and turbidity. 

Spatial patterns were not evident for pH, silica, nitrogen (all 

forms), or algal pigments.

Major ion and carbon concentrations were largely reflective of the calcareous, glacial soils. In general, the rivers 

sampled in Iowa were alkaline and well-buffered, with pH 

and alkalinity inversely related to streamflow with the greatest 

alkalinities during long periods of stable streamflow. Alkalinities were greatest in northern Iowa streams, though no spatial 

pattern was observed for pH levels. Specific conductance and 

ion concentrations were also inversely related to streamflow, 

except silica, which was not related to streamflow. Specific 

conductance and sulfate concentrations were greatest in the 

Big Sioux River. Carbon in streamwater was dominated by 

bicarbonate, followed by particulate organic carbon and dissolved organic carbon, and generally much lower concentrations of particulate inorganic carbon.

Total nitrogen concentrations were dominated by nitrate 

plus nitrite (average about 85 percent) and both exhibited 

similar patterns with streamflow for each site. Total nitrogen 

and nitrate plus nitrite concentrations had a positive relation 

to streamflow at low to moderate streamflows and a negative 

relation at high streamflows. The studied rivers had nitrate 

concentrations greater than the drinking-water criteria, or 

maximum contaminant level, of 10 milligrams per liter in 

11 percent of samples, and the draft criteria for aquatic life 

of 4.9 milligrams per liter (Minnesota proposed limit) was 

exceeded in 68 percent of samples. Nitrite did not exceed 

the drinking-water criteria of 1.0 milligrams per liter in any 

sample. Ammonia was not detected in nearly one-half of all 

samples, and met the criteria for ammonia, which vary by 

temperature. Proposed ammonia criteria with more sensitive 

standards, however, were exceeded in three samples.

Total phosphorus concentrations generally had a positive 

relation with streamflow, though correlations were site-specific 

and differed across ranges of streamflow. Total phosphorus 

concentrations generally increased downstream among Mississippi River tributaries, but were greatest and most variable in 

southwestern Iowa rivers. Compared to the Wisconsin criteria 

for phosphorus in large rivers, the studied rivers exceeded 

the 0.1 milligram per liter standard in 92 percent of samples. 

Orthophosphate concentrations were flat to slightly positive with relation to streamflow, except the strongly negative 

orthophosphate-streamflow relation observed in the Boyer 

River, which also had the greatest concentrations overall.

Suspended-sediment concentrations ranged five orders of 

magnitude and were positively related to turbidity and streamflow. The greatest concentrations were in the southwestern 

Iowa basins, which contain extensive loess and steep hills. The 

positive relation between suspended-sediment concentrations 

and turbidity is distinct, but less defined for low values, such 

that the site-specific relation can be weak [coefficient of determination (R2

) = 0.37] in rivers with typically low suspendedsediment concentrations and turbidity.

Algal pigments and pesticides exhibited strong seasonal patterns. Late-summer algal blooms were evident with 

peaks in concentrations for chlorophyll-a and pheophytin-a. 

Spring concentrations and detections peak for pesticides were 

coincident with application times. Atrazine, metolachlor, and 

the atrazine breakdown product 2-Chloro-4-isopropylamino6-amino-s-triazine were detected in all samples. Other commonly detected herbicides included acetochlor, prometon, 

simazine, alachlor, and metribuzin. Insecticides were less 

commonly detected, with chlorpyrifos and fipronil detected in 

9 percent of samples.

Stream loads are presented for chloride, sulfate, silica, 

total nitrogen, nitrate plus nitrite, total phosphorus, orthophosphate, and suspended sediment. Because constituent 

loads in streams are largely determined by streamflow, loads 

were greatest for all constituents in larger basins and during 

periods of increased streamflow. For most constituents, the 

Des Moines River and Iowa River, the largest basins in the 

study, had the greatest loads of the studied rivers. Constituent yields also were positively related to streamflow, but 

yields revealed additional spatial patterns in ion, nutrient, and 

suspended-sediment transport. Chloride yields were greater in 

the eastern Iowa tributaries to the Mississippi River than in the 

western Iowa tributaries to the Missouri River. Sulfate yields 

were greatest in the Big Sioux, Little Sioux, and Des Moines 

Rivers. Silica yields were lowest in the Big Sioux River. Total 

nitrogen and nitrate yields were low in the Big Sioux River 

and greater in the northeastern rivers. Total phosphorus yields 

were greatest in the Boyer and Nishnabotna Rivers. Orthophosphate yields were greatest in the Boyer River, except in 

2008, when the Maquoketa River produced the greatest yield. 

Suspended-sediment yields were greatest from the Boyer and 

Nishnabotna Rivers in the southwestern Western Loess Hills 

region, whereas the Big Sioux and Wapsipinicon Rivers produced the lowest suspended-sediment yields.

Loads presented in this report corroborate previously 

reported loads for nitrate, orthophosphate, and total phosphorus for the Iowa River at Wapello and suspended sediment for 

the Skunk River at Augusta. Ranges of predictive errors at the 

95-percent confidence limit overlapped for all four instances 

of comparison. The two instances where predictive errors 

were presented for both reported loads, the Iowa River at 

Wapello nitrate plus nitrate and total phosphorus, confidence 

limits presented in this report were narrower than in previous 

estimates. This increased accuracy demonstrates the usefulness of the additional streamflow variability terms used in 

the models. Of the 80 individual site/constituent models, 44 

models included streamflow variability terms or streamflow 

anomalies to improve load estimates. Overall, predictive 

errors for suspended-sediment loads were greater than most 

other constituents, with average standard errors of prediction of 21 percent for suspended-sediment loads. Of the other 
References Cited 59

constituents, orthophosphate and total phosphorus were the 

only others with average standard errors of prediction above 

10 percent.

Nutrient loads presented relative to fertilizer distribution 

indicates that in wet years, fertilizer use and the proportion of 

nitrogen and phosphorus in the streams relative to use goes up 

compared to dry years. Loads relative to use also were smaller 

for nitrogen and phosphorus in the second one-half of the year. 

Nitrogen loads relative to fertilizer use for January to June 

(including the critical period of spring application and high 

streamflow) in the wet 2007–2008 years averaged 52 percent. 

Phosphorus relative loads for the same periods averaged 

9.6 percent.

Acknowledgments

The author would like to thank the numerous U.S. Geological Survey field personnel who collected surface-water 

data throughout this study and Stephen J. Kalkhoff for help in 

project development and support. Additionally, three reviewers contributed valuable comments to the betterment of this 

report.

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Publishing support provided by the: 

Rolla and Denver Publishing Service Centers 

For more information concerning this publication, contact:

Director, USGS Iowa Water Science Center

P.O. Box 1230

Iowa City, IA 52244

(319) 337–4191

Or visit the Iowa Water Science Center Web site at:

http://ia.water.usgs.gov

Garrett—Concentrations, Loads, and Yields of Select Constituents from Selected Major Iowa Rivers, Water Years 2004–2008—SIR 2012–5240