U.S. Department of the Interior

U.S. Geological Survey

An Alternative Regionalization Scheme

for Defining Nutrient Criteria for

Rivers and Streams

Water-Resources Investigations Report 01–4073

Prepared in cooperation with the

U.S. Environmental Protection Agency,

Regions V and VII

Till ≥ 59 percent

Yes (151 sites)

No (83 sites)

All

Data

No (52)

Yes (31)

Runoff ≥ 10.3 inches per year

No (55)

Yes (96)

Clay ≥ 26 percent

Group 2

Group 1

Group 3

Group 4

Environmental phosphorus zone

(analysis excluded land-use

characteristics)

Study-area boundary

EXPLANATION

1

2

3

4
An Alternative Regionalization Scheme for 

Defining Nutrient Criteria for Rivers and 

Streams

By Dale M. Robertson, David A. Saad, and Ann M. Wieben

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4073

Prepared in cooperation with the

U.S. Environmental Protection Agency, 

Regions 5 and 7

Middleton, Wisconsin

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information write to: Copies of this report can be purchased from:

District Chief U.S. Geological Survey 

U.S. Geological Survey Branch of Information Services

8505 Research Way Box 25286

Middleton, WI 53562-3586 Denver, CO 80225-0286

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

by the U.S. Government.
CONTENTS III

CONTENTS

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

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

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

Establishing specific nutrient criteria.......................................................................................................................... 4

Approach, data, and study methods ...................................................................................................................................... 5

Water-quality data ....................................................................................................................................................... 5

Environmental-characteristic data............................................................................................................................... 8

Data summaries and statistical methods ..................................................................................................................... 11

Development of nutrient zones—an alternative regionalization scheme.............................................................................. 16

Distribution of nutrient concentrations ....................................................................................................................... 16

Most statistically significant environmental factors ................................................................................................... 18

Delineation of nutrient zones ...................................................................................................................................... 24

Comparison of environmental nutrient zones to nutrient ecoregions ................................................................................... 25

Defining nutrient criteria for environmental nutrient zones ................................................................................................. 28

Refining environmental nutrient zones ................................................................................................................................. 29

Further subdivision of the environmental nutrient zones............................................................................................ 29

Refinement for smaller geographical areas................................................................................................................. 32

Watershed-specific approach ................................................................................................................................................ 33

Summary and conclusions .................................................................................................................................................... 34

References............................................................................................................................................................................. 35

Appendixes:

1. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site ....................... 38

2. Median, midmonthly nutrient concentrations of 234 sites sampled in the Upper Midwest, by site ............................ 53

FIGURES

1–2. Maps showing:

1. Fourteen nutrient ecoregions delineated for the U.S. Environmental Protection Agency National 

Nutrient Strategy............................................................................................................................................... 2

2. Nutrient ecoregions in the study area .................................................................................................................. 3

3. Reference-condition approach for selection of nutrient criteria on the basis of the frequency distribution of

water quality for an entire data set and that for reference streams ................................................................... 4

4–18. Maps showing:

4. Watersheds of streams included in study, by sampling agency........................................................................... 7

5. Total annual runoff in the study area ................................................................................................................... 9

6. Average annual air temperatures in the study area .............................................................................................. 9

7. Average total annual precipitation in the study area............................................................................................ 10

8. Land use/land cover for Illinois and vicinity....................................................................................................... 10

9. Quaternary sediment types in the study area....................................................................................................... 12

10. Quaternary sediment thickness in the study area ................................................................................................ 12

11. Soil clay content in the study area....................................................................................................................... 13

12. Soil organic-matter content in the study area ...................................................................................................... 13

13. Soil erodibility in the study area.......................................................................................................................... 14

14. Soil permeability in the study area ...................................................................................................................... 14

15. Soil slope in the study area.................................................................................................................................. 15

16. Principal aquifer types in the study area.............................................................................................................. 15

17. Median, midmonthly total phosphorus concentrations for study sites ................................................................ 17

18. Median, midmonthly total nitrogen concentrations for study sites ..................................................................... 17

19. Environmental phosphorus groups from regression-tree analysis (A) including all environmental 

characteristics and (B) excluding land-use characteristics................................................................................. 21
IV CONTENTS

CONTENTS—Continued

20. Environmental nitrogen groups from regression-tree analysis including (A) all environmental characteristics 

and (B) excluding land-use characteristics........................................................................................................... 23

21–22. Maps showing:

21. Environmental phosphorus zones and environmental nitrogen zones for study area when 

land-use characteristics were included in regression-tree analyses........................................................... 26

22. Environmental phosphorus zones and environmental nitrogen zones for study area when 

land-use characteristics were excluded from regression-tree analyses...................................................... 27

23. Detailed environmental phosphorus groups from regression-tree analysis results excluding land-use 

characteristics, subdividing the data into eight groups......................................................................................... 31

24–26. Maps showing:

24. Location of the watersheds for the eight environmental phosphorus groups when land-use 

characteristics were excluded from regression-tree analysis..................................................................... 32

25. Environmental phosphorus groups for Illinois Environmental Protection Agency watersheds 

when land-use characteristics were excluded from regression-tree analysis............................................. 33

26. Environmental phosphorus groups for Nutrient Ecoregion VI watersheds when land-use 

characteristics were excluded from regression-tree analysis..................................................................... 34

TABLES

1. Summary statistics for water-quality constituents and environmental characteristics of the watersheds 

of 234 sites sampled in the Upper Midwest, 1961–99................................................................................... 6

2. Pearson correlation coefficients between water-quality constituents (total phosphorus and total 

nitrogen) and the environmental characteristics of their respective watersheds. .......................................... 19

3. Pearson correlation coefficients between environmental characteristics for sites in the study ........................ 20

4. Forward stepwise-regression models to explain variability in total phosphorus and total nitrogen 

concentrations including and excluding land-use characteristics in the analyses ......................................... 20

5. Comparison of water quality and variability in water quality within nutrient ecoregions and within 

environmental nutrient zones (delineated with results from regression-tree analysis).................................. 28

6. Percentiles of nutrient concentrations, by the percentage of agriculture in the watershed, for each 

environmental nutrient zone (created on the basis of results of regression-tree analyses when land-use

characteristics were excluded from the analyses).......................................................................................... 30
CONVERSION FACTORS, ABBREVIATIONS, AND ACKNOWLEDGMENTS V

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To Obtain

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

Temperature, in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) by use of the following equation:

°C = [°F - 32] / 1.8.

Other Abbreviations

mg/L milligrams per liter

in/hr inches per hour

in/yr inches per year

ACKNOWLEDGMENTS

Technical Reviewers

David Pfeifer, Water Quality Standards Coordinator, U.S. Environmental Protection Agency, 

Chicago, Ill.

Gregory S. Searle, Environmental Toxicologist, Wisconsin Department of Natural Resources, Madison,

Wis.

David M. Soballe, Limnologist, U.S. Geological Survey, Biological Resources Division, LaCrosse, Wis.

Danielle Tillman, Regional Nutrient Coordinator for Region 5, U.S. Environmental Protection Agency,

Chicago, Ill.

Statistical Support

Dennis M. Heisey, Statistician, Department of Surgery, University of Wisconsin, Madison, Wis.

Paul W. Rasmussen, Statistician, Wisconsin Department of Natural Resources, Monona, Wis.

Editorial and Graphics

Michael Eberle, Technical Publications Editor, U.S. Geological Survey, Columbus, Ohio

Michelle M. Greenwood, Cartographer, U.S. Geological Survey, Middleton, Wis.

Leah Hout, Editor, U.S. Geological Survey, Columbus, Ohio

Kathleen A. Hueschen, Student Trainee (Editor), U.S. Geological Survey, Middleton, Wis.

Susan Z. Jones, Editorial Assistant, U.S. Geological Survey, Middleton, Wis.

Approving Official

Chester Zenone, Reports Improvement Advisor, U.S. Geological Survey, Reston, Va.
Abstract 1

Abstract

To protect and manage rivers and streams 

(hereafter, collectively referred to as streams) in the 

United States, the U.S. Environmental Protection 

Agency (USEPA) is establishing regionally based 

nutrient criteria that reflect the natural variability in 

water quality. As a basic approach to establish 

these criteria, the USEPA has divided the country 

into nutrient ecoregions (delineated on the basis of 

natural and anthropogenic factors) to minimize 

variability within regions and maximize variability 

among regions. The USEPA has allowed states and 

tribes flexibility to modify or improve on this basic 

approach. As part of activities of a Regional Technical Assistance Group, whose role it is to examine 

and refine this basic approach, an alternative 

regionalization scheme was developed for the 

Upper Midwest. In this refined approach, the relative importance of various environmental characteristics affecting nutrient concentrations are 

determined by use of regression-tree analysis. The 

area is then subdivided into relatively homogeneous areas called “environmental nutrient zones”

on the basis of distributions of only the most statistically significant environmental characteristics. 

On the basis of data from 234 sites, the most 

statistically significant environmental characteristics affecting nutrient concentrations were the percentage of agriculture (or absence of forest) and 

factors describing the climate and geology in the 

watershed. Environmental nutrient zones were then 

delineated that incorporated distributions in land 

use (similar to the ecoregion approach) and also 

delineated with land-use information excluded so 

the criteria should reflect only the naturally occurring variability in water quality. With the environmental nutrient zone stratification scheme, the 

variability in total phosphorus concentrations 

among zones was reduced by approximately 

50 percent compared to that among nutrient 

ecoregions, whereas the variability in total 

nitrogen concentrations was reduced only slightly. 

Frequency distributions of data from each zone 

were then used to define the potential water quality 

of each zone.

The environmental nutrient zone approach 

can be applied to specific states or nutrient ecoregions and used to develop criteria as a function of 

stream type. This approach can also be applied on 

the basis of environmental characteristics of the 

watershed alone rather than the general environmental characteristics from the region in which the 

site is located. The environmental nutrient zone 

approach will enable states to refine the basic nutrient criteria established by the USEPA by developing attainable criteria given the environmental 

characteristics where the streams are located. 

INTRODUCTION

High concentrations of nutrients in surface waters 

is not a new problem, but it is among the most persistent 

water-quality problems in the Nation. According to the 

U.S. Environmental Protection Agency (USEPA), 50 

states, tribes, and other jurisdictions surveyed waterquality conditions in 19 percent of the Nation’s 3.6 million miles of rivers and streams (hereafter, collectively 

referred to as streams) and found nutrient enrichment to 

be the second most significant cause of water-quality 

impairment (U.S. Environmental Protection Agency, 

1996). Excessive concentrations of nutrients can cause 

nuisance levels of algae and aquatic vegetation, and 

they have been linked to eutrophication of downstream 

impoundments, outbreaks of Pfiesteria in several Gulf 

and Mid-Atlantic states, and hypoxia in the Gulf of 

Mexico. Under the recommendations of the Clean 

Water Action Plan released in 1998, USEPA is implementing a national strategy to develop waterbody-specific nutrient criteria (U.S. Environmental Protection 

An Alternative Regionalization Scheme for Defining 

Nutrient Criteria for Rivers and Streams

By Dale M. Robertson, David A. Saad, and Ann M. Wieben
2 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Ecoregions from Omernick, 2000

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

XIII

XIV

Nutrient ecoregion

EXPLANATION

Agency, 1998) for lakes and reservoirs, streams, wetlands, and estuaries; this report is concerned with those 

criteria for streams. The intent of this strategy was to set 

out a plan for developing water-quality criteria protective of the uses designated for the surface waters of the 

Nation. The most effective way to attain the water quality required for the designated uses of surface waters, is 

to reduce the nutrient contributions due to human activities rather than that from natural loadings. Thus, appropriate nutrient criteria will not be identical for all areas 

of the Nation, but they will differ regionally and reflect 

natural nutrient sources.

Factors such as land use, geology, climate, and 

hydrology play significant roles in water quality. 

Because these factors vary greatly across the Nation, 

regional nutrient criteria make sense scientifically. Various frameworks have been used to divide the country 

into areas of relatively similar environmental characteristics in order to minimize the natural variability in 

water quality within these areas and maximize the differences among areas. One such framework is the 

ecoregion scheme developed and refined by Omernik 

(1987, 1995, and 2000). The ecoregion scheme is a 

mapped classification system of “ecological regions”; 

that is, regions with assumed relative homogeneity of 

ecological characteristics. These regions were defined 

on the basis of relative differences in land use/land 

cover, land-surface form, geology, physiography, climate, soils, potential natural vegetation, and other environmental characteristics. The USEPA has taken the 

initial step in developing regional nutrient criteria based 

on a national nutrient ecoregion map constructed by 

combining Omernik’s 84 Level III ecoregions into 14 

subdivisions for the conterminous United States (Omernik, 2000; fig. 1). 

The use of ecoregions in developing regional nutrient criteria has several inherent problems. First, the relative weighting of each environmental characteristic is 

Figure 1. Fourteen nutrient ecoregions delineated for the U.S. Environmental Protection Agency National Nutrient Strategy.
INTRODUCTION 3

Nutrient ecoregion

Study-area boundary

EXPLANATION

V

VI

VII

VIII

IX

X

XI

Base from USGS digital line graphs Modified from Omernick, 2000

0 100 200 MILES

0 100 200 KILOMETERS

102° 84°

45°

36°

unknown and varies from boundary to boundary in an 

unknown way. Therefore, divisions among ecoregions 

can be rather arbitrary, and differences in water quality 

among ecoregions can be difficult to attribute to any 

specific environmental factor. In addition, because the 

most important environmental characteristic used to 

delineate the nutrient ecoregions may not be the primary factor affecting water quality, greater variations in 

water quality may occur within an ecoregion than 

among ecoregions. Finally, the USEPA has stated that, 

to the extent possible, classification of environmental 

characteristics should be restricted to those that are 

intrinsic, or natural, and are not the result of human 

activities (U.S. Environmental Protection Agency, 

2000a). Nutrient criteria for relatively homogeneous 

areas should reflect differences in water quality caused 

by natural factors. Land use, however, was commonly 

the most important characteristic in subdividing various 

ecoregions. Differences in land use was the primary factor used to subdivide the Upper Midwest part of the 

United States in the 14-ecoregion scheme. Because of 

these inherent problems with ecoregions, subdivision or 

refinement of the basic delineation may be necessary in 

some or all of them.

Purpose and Scope

The USEPA has provided its administrative regions 

flexibility to refine this approach by establishing 

Regional Technical Assistance Groups (RTAGs), whose 

roles are to evaluate and possibly refine the boundaries 

of these relatively homogeneous areas and corresponding nutrient criteria (U.S. Environmental Protection 

Agency, 2000b). As part of RTAG efforts, the U.S. Geological Survey (USGS), in cooperation with the 

USEPA, assessed and refined the basic nutrient ecoregion approach for the Upper Midwest area of the United 

States (fig. 2). This report describes the results of this 

Figure 2. Nutrient ecoregions in the study area. (Black lines within nutrient ecoregions delineate Omernik’s Level III 

ecoregions.)
4 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Total phosphorus, in milligrams per liter (mg/L)

75%

Better water quality Worse water quality

25%

The 75th percentile of a

measured constituent is 

selected from a distribution

of reference sites (0.039 mg/L)

Reference-site

distribution

The 25th percentile of a

measured constituent is 

selected from a distribution

of all sites (0.037 mg/L)

All-site

distribution

0 0.01 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090

Reference-site approach All-site approach

effort and describes an alternative regionalization 

scheme developed by use of regression-tree analyses 

and a geographic information system (GIS). Various 

modifications to the method are also presented that 

would enable states or USEPA administrative regions to 

apply the method at different geographic scales and 

locations.

Establishing Specific Nutrient Criteria

The primary factors that determine the productivity 

of streams are nutrient concentrations (usually phosphorus and nitrogen), light availability, and flow regime 

(sufficient time to respond to nutrient concentrations). 

Four variables have been chosen to define specific 

nutrient criteria: two causal variables, total phosphorus 

and total nitrogen concentrations; and two response 

variables, chlorophyll a concentrations and a measure 

of turbidity. In this report, the causal variables, concentrations of total phosphorus and total nitrogen are examined.

Several approaches have been suggested to define 

quantitative nutrient criteria after relatively homogenous geographic areas are chosen. The basic approach 

currently used by the USEPA to define criteria involves 

assigning a concentration value to reference streams 

from each specific area; in other words, the conditions 

that are attainable given the geographic location of the 

site (U.S. Environmental Protection Agency, 2000b). 

This concentration value can be defined from the frequency distribution of all available data for each area. 

(On a national basis, these data have been obtained primarily from USEPA Storage and Retrieval, STORET, 

database plus data from other sources including the 

USGS, universities, etc.) The lower 25th percentile of 

all the data has been suggested as representing this reference, or minimally impacted, condition (fig. 3). 

Another statistical approach to define reference condiFigure 3. Reference-condition approach for selection of nutrient criteria on the basis of the frequency distribution of water 

quality for an entire data set (25th percentile of all sites) and that for reference streams (75th percentile of reference sites). 

(Total phosphorus is shown as an example. The final criteria could be between 0.037 and 0.039 milligrams per liter, if a 

combination of these two approaches is used.)
APPROACH, DATA, AND STUDY METHODS 5

tions is to choose the upper 75th percentile of a subset 

of streams thought to be the least impacted streams for 

a defined area (fig. 3). The final criteria could be 

between these two concentrations, if a combination of 

these two approaches is used. States or tribes also may 

consider analyzing stream data based on designated-use 

classifications. Using this approach, frequency distributions for specific designated uses could be examined 

and criteria proposed based on maintenance of highquality streams that are representative of each designated use (U.S. Environmental Protection Agency, 

2000b).

An alternative approach to define the nutrient criteria is to base them on nutrient concentrations associated 

with specific thresholds of algal productivity found in 

previous studies (U.S. Environmental Protection 

Agency, 2000b). In most studies, however, productivity 

thresholds were at very low nutrient concentrations, 

lower than those that occur naturally in many areas. 

Another approach is to base the criteria on the response 

variables (chlorophyll a concentrations or turbidity).

Whatever approach is used, the final criteria must 

be stringent enough to protect not only the specific site 

but also downstream waters. 

APPROACH, DATA, AND STUDY 

METHODS

To evaluate the basic nutrient ecoregion classification scheme and understand the relation between waterquality and watershed characteristics, a reliable set of 

water-quality data was needed with corresponding environmental-characteristic information for each site. Use 

of the vast body of historically collected water-quality 

data is appealing; however, the data represent samples 

collected in many different ways and for many different 

reasons. The quality of some of these data is suspect, 

and exactly what these data represent is difficult or 

impossible to ascertain without a detailed evaluation. 

Therefore, a database was established with data from 

234 sites, all of which were sampled for defined reasons 

by means of acceptable techniques. For each site, the 

watershed was delineated and the environmental characteristics were determined by use of a GIS.

Correlations, stepwise linear regressions, and 

regression-tree analyses (including and excluding landuse variables) were used to determine the most statistically significant environmental characteristics affecting 

nutrient (phosphorus and nitrogen) concentrations in 

the Upper Midwest. Results of the regression-tree analyses, which included only the most statistically significant environmental variables affecting total phosphorus 

or nitrogen concentrations, were then used to subdivide 

the Upper Midwest into discrete environmental nutrient 

zones. Water-quality variability within the ecoregions 

were compared with that within the environmental 

nutrient zones to determine whether the refined 

approach better defines the measured distribution in 

water quality. For each environmental nutrient zone, a 

range in possible nutrient criteria were developed from 

the 25th percentile of nutrient concentrations for all of 

the streams in that zone and the 75th percentile of the 

minimally impacted streams, on the basis of the percentage of agriculture in the watershed, from the nutrient zone. As an example of how different criteria can be 

established for specific types of streams within a specific nutrient zone, the streams were subdivided on the 

basis of how the land around the stream was being used. 

Four groups were created on the basis of the percentage 

of agriculture in the watershed.

Water-Quality Data

Water-quality data for this analysis were limited to 

total phosphorus and total nitrogen concentrations measured in water samples collected from 234 streams in 

the study area during 1961–99. Concentrations of total 

nitrogen were either measured directly or computed 

from the sum of concentrations of individual nitrogen 

species. Concentrations of total phosphorus were determined for all 234 streams, but total nitrogen could be 

determined for only 152 streams. A summary of the 

basic statistics for total phosphorus and total nitrogen 

are given in table 1.

Of the 234 streams included in this study, 75 

streams were sampled by the Illinois Environmental 

Protection Agency (IEPA) between 1980 and 1995; 48 

streams were sampled by the Wisconsin Department of 

Natural Resources (WDNR) between 1961 and 1998; 

and 111 streams were sampled by the USGS between 

1964 and 1999. Of those sampled by the USGS, 74 

streams were sampled from 11 study units in the 

National Water-Quality Assessment (NAWQA) program (Hirsch and others, 1988) between 1992 and 1999; 

15 streams that are part of the Upper Mississippi River 

System study (U.S. Geological Survey, 1999) were 

sampled between 1973 and 1998; 7 streams in Wisconsin and Minnesota were sampled from 1997 to 1999 as 

part of a study of the St. Croix River Watershed 
6 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Table 1. Summary statistics for water-quality constituents and environmental characteristics of the watersheds of 234 sites 

sampled in the Upper Midwest, 1961–99

[°F, degrees Fahrenheit; ft, feet; Kf, K factor; in/hr, inches per hour; in/yr, inches per year; mg/L, milligrams per liter; mi2, square miles; %, percent]

Constituent or characteristic Abbreviation Units Number Mean Standard 

deviation Maximum Median Minimum

Water-quality constituents

Total phosphorus Total P mg/L 234 0.17 0.25 2.16 0.11 0.01

Total nitrogen Total N mg/L 152 3.61 3.08 13.18 2.26 .34

Basin characteristics

Area Area mi2 234 429.8 1,032.4 11,628.9 157.5 1.5

Runoff Roff in/yr 234 10.28 5.01 40.00 9.00 .30

Climatic characteristics

Precipitation Prec in/yr 234 35.6 5.3 55.0 37.0 17.5

Air temperature Temp oF 234 48.1 4.6 56.3 48.8 38.5

Land-use characteristics

Urban Urbn % 234 5.7 14.8 96.1 1.2 .0

Agriculture Agri % 234 70.4 28.6 99.7 83.2 .0

Rangeland Rang % 234 .1 .7 10.1 .0 .0

Forest Fors % 234 20.2 25.0 99.3 8.0 .0

Water Watr % 234 .6 1.5 13.1 .2 .0

Forested wetland Fwet % 234 1.7 4.3 33.6 .0 .0

Nonforested wetland Nwet % 234 .6 1.7 15.6 .0 .0

Barren land Barr % 234 .7 2.5 28.5 .1 .0

Surficial-deposit characteristics

Coarse-grained stratified sediments Coar % 234 11.8 17.9 100.0 4.2 .0

Fine-grained stratified sediments Fine % 234 3.2 9.6 90.0 .0 .0

Till Till % 234 64.2 36.7 100.0 81.5 .0

Patchy Quaternary sediments Patc % 234 2.3 10.9 100.0 .0 .0

Exposed bedrock or nonglacial sediments Xbed % 234 18.2 36.1 100.0 .0 .0

Organic-rich sediments Orga % 234 .2 1.3 14.9 .0 .0

Thickness (depth to bedrock) Dbed ft 234 92.0 79.2 427.7 63.8 25.0

Soil characteristics

Clay content Clay % 234 24.1 7.5 41.9 25.7 3.2

Erodibility factor Kf 234 .31 .06 .42 .32 .13

Organic-matter content Omat % 234 2.74 3.51 27.03 1.27 .23

Permeability Perm in/hr 234 2.22 1.83 11.39 1.58 .41

Soil slope Slop % 234 6.90 6.42 45.52 4.88 .64

Principal aquifer types 

Sandstone Sdst % 234 49.2 44.6 100.0 43.9 .0

Carbonate Carb % 234 18.0 35.3 100.0 .0 .0

Sandstone and carbonate Sscb % 234 5.8 17.6 100.0 .0 .0

No principal aquifer Noaq % 234 27.0 37.4 100.0 .0 .0
APPROACH, DATA, AND STUDY METHODS 7

Allegheny and Monongahela Basins NAWQA study

Eastern Iowa Basins NAWQA study

Great and Little Miami River Basins NAWQA study

Illinois Environmental Protection Agency

Kanawha-New River Basins NAWQA study

Lake Erie-Lake St. Claire Drainages NAWQA study

Lower Illinois River Basin NAWQA study

Red River of the North Basin NAWQA study

Upper Illinois River Basin NAWQA study

Upper Mississippi River Basin NAWQA study

U.S. Geological Survey Upper Mississippi River System study

U.S. Geological Survey St. Croix River study

Western Lake Michigan Drainages NAWQA study

White River Basin NAWQA study

Wisconsin Department of Natural Resources

EXPLANATION

Sampling agency or U.S. Geological Survey study name

Study-area boundary

Base from USGS digital line graphs

102° 84°

45°

36°

(Bernard Lenz, U.S. Geological Survey, written commun., 2000); and 15 streams were sampled as part of 

USGS Wisconsin District cooperative projects with the 

WDNR. The watersheds of the 234 streams are shown 

in figure 4.

Historical IEPA water-quality data were retrieved 

from the USGS National Water Information System 

(NWIS) database maintained by the USGS, Illinois District. WDNR data were retrieved from the USEPA 

STORET database. USGS water-quality data collected 

as part of the NAWQA program were retrieved from the 

NAWQA Water-Quality Data Warehouse (U.S. Geological Survey, 2000a). Data from the Upper Mississippi 

River System study were retrieved from the Upper Mississippi Basin Loading database (U.S. Geological Survey, 2000b). Data from the St. Croix River study were 

Figure 4. Watersheds of streams included in study, by sampling agency. (Sites from the U.S. Geological Survey National WaterQuality Assessment (NAWQA) program are shown by the study unit that did the sampling.)
8 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

obtained from Bernard Lenz (U.S. Geological Survey, 

written commun., 2000).

Environmental-Characteristic Data

Environmental characteristics that were thought to 

affect or be related to nutrient concentrations in streams 

of the Upper Midwest were compiled for this study. The 

environmental characteristics include watershed area, 

runoff, climate (annual air temperature and precipitation), land-use types, surficial-deposit types and thickness, soil characteristics, and principal aquifer types. 

All characteristics were compiled in digital form by use 

of a GIS. A summary of the environmental characteristics for all of the watersheds used in this study is given 

in table 1. (Environmental characteristics for each 

watershed are provided in appendix 1.) The source of 

the data and method of compilation for each characteristic are described below.

Watershed Boundaries and Areas. Watershed boundaries for the 234 streams (fig. 4) were obtained from 

several sources and delineated by means of various 

methods. Watersheds of the IEPA and USGS St. Croix 

River studies were manually digitized on the basis of 

known or published sampling-location information 

(Illinois Environmental Protection Agency, 1996; 

Bernard Lenz, U.S. Geological Survey, written commun., 2000). Watershed boundaries for these two studies were initially based on a 1:100,000-scale digital 

coverage of the USGS Hydrologic Unit maps (Seaber 

and others, 1984) and refined with digital RF3 stream 

coverages (U.S. Environmental Protection Agency, 

2000c). Watershed boundaries of the USGS Upper Mississippi River System study were obtained from the 

USGS Upper Midwest Environmental Sciences Center 

(Hank DeHaan, U.S. Geological Survey, written commun., 2000) and refined to sampling locations with 

1:100,000-scale digital stream coverages from the 

National Hydrography Dataset (U.S. Geological Survey, 2000c). Watershed boundaries of the NAWQA 

studies were delineated from 1:24,000-scale USGS 

topographic quadrangle maps. The WDNR watersheds 

of the Montreal, Oconto, and Peshtigo Rivers were 

compiled from a 1:100,000-scale digital coverage of the 

USGS Hydrologic Unit maps. The watersheds of the 

remaining WDNR and USGS sites were digitized as 

part of an earlier study and were obtained directly from 

the WDNR (Gregory Searle, Wisconsin Department of 

Natural Resources, written commun., 1999). 

Runoff. Runoff data (fig. 5) were obtained from a digital linear coverage of average annual runoff (inches per 

year) for the conterminous United States for the years 

1951–80 (Gebert and others, 1987). Originally, this 

coverage was prepared to represent the runoff of tributary streams rather than major streams, to link smallscale variations in runoff with precipitation, and to 

show other geographical characteristics. A runoff value 

for each watershed was estimated by the line of equal 

runoff closest to the center of the watershed. If the center of a watershed fell between two lines, the average of 

those two values was used for that watershed. 

Climate. Climatic characteristics included in this study 

were air temperature and precipitation. Average annual 

air temperature data (fig. 6) were obtained as a digital 

coverage from data compiled by the National Climatic 

Data Center (U.S. Geological Survey, 2000d). The digital coverage is generalized and was intended to display 

air temperatures in the USGS National Water Summary 

reports. The digital coverage was intersected with 

watershed boundaries, and an area-weighted average 

mean annual temperature for each watershed was computed. 

Precipitation data (fig. 7) were obtained as a digital 

coverage from the PRISM (Parameter-elevation 

Regressions on Independent Slopes Model) of the Climate Mapping Program (Oregon Climate Service, 

2000). Area-weighted average total annual precipitation 

(inches per year) for each watershed was computed.

Land Use. Land-use/land-cover information for the 

study area was summarized from high-altitude aerial 

photographs compiled by the USGS (Feagus and others, 

1983). This information was manually interpreted on 

the basis of the land-use classification of Anderson and 

others (1976). Land-use/land-cover maps at 1:250,000-

scale were produced from the interpreted data and digitized into a GIS (U.S. Geological Survey, 2000e). An 

example of land-use/land-cover data for part of the 

study area (Illinois) is shown in figure 8. Land use/land 

cover for watersheds of the NAWQA, St. Croix River, 

and Upper Mississippi River System studies were 

updated with urban population information obtained 

from the 1990 census (Hitt, 1994). Percentages of the 

land-use types in each watershed were computed and 

represent the following Anderson’s Level I categories 
APPROACH, DATA, AND STUDY METHODS 9

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from Gebert and others, 1987

Total annual runoff, in inches—

contour interval variable

Line used to define part of an

environmental nutrient zone

Study-area boundary

EXPLANATION

14

14

16 22

12

10

8

6

16

14

8

8

10

8

10

12

5

4

2

1

8

9

10

0.2

0.5

0.5

1

12

14

16

18

18

22

26

16

18

16

30

22

26

16

30

18

18

26

30

22

18

16

14

1618

26

14

16

22

12

10

8

6

16

14

8

8

10

8

10

12

5

4

2

1

8

9

10

0.2

0.5

0.5

1

12

14

16

18

18

22

26

16

18

16

30

22

26

16

30

18

18

26

30

22

18

16

14

16

18

26

45°

36°

102° 84°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000d

Average annual air temperature,

in degrees Fahrenheit

Line used to define part of an

environmental nutrient zone

Study-area boundary

EXPLANATION

38.2–42

42–46

46–50

50–54

54–58.4

102° 84°

45°

36°

Figure 5. Total annual runoff in the study area.

Figure 6. Average annual air temperatures in the study area.
10 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from Oregon Climate Service, 2000

Precipitation, in inches per year

Line used to define part of an

environmental nutrient zone

Study-area boundary

EXPLANATION

12.5–25

25–32.5

32.5–37.5

37.5–50

50–75

102° 84°

45°

36°

0 50 100 MILES

0 50 100 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000e

Land-use/land-cover

classification

Lines used to define part of an

environmental nutrient zone

EXPLANATION

Urban

Agriculture

Rangeland

Forest

Water

Wetland

Barren

42°

90° 84°

39°

Figure 7. Average total annual precipitation in the study area.

Figure 8. Land use/land cover for Illinois and vicinity.
APPROACH, DATA, AND STUDY METHODS 11

(Anderson and others, 1976): urban, agriculture, rangeland, forest, water, wetland (further subdivided into forested and nonforested categories), and barren land. 

Surficial Deposits. Surficial-deposit type and thickness 

were compiled from a digital coverage of Quaternary 

sediments for the glaciated United States east of the 

Rocky Mountains (Soller and Packard, 1998). The percentage of the following Quaternary sediment types was 

computed for each watershed: coarse-grained sediments, fine-grained sediments, till, patchy Quaternary 

sediments, organic-rich sediments, and exposed bedrock or nonglacial sediments (fig. 9). Quaternary sediment thickness (depth to bedrock) is described in this 

digital coverage by ranges. Those ranges were generalized (fig. 10) by assigning a single thickness equal to the 

average of the range of values. An area-weighted average depth to bedrock was computed for each watershed 

by use of these averaged values.

Soil Characteristics. Soil-characteristic data were compiled from the USSOILS digital coverage of the State 

Soil Geographic (STATSGO) database (Schwarz and 

Alexander, 1995; U.S. Geological Survey, 2000f). Soil 

characteristics summarized were clay content (percentage of soil less than 2 micrometers in size; fig. 11), 

organic-matter content (percentage by weight, fig. 12), 

soil erodibility factor (Kf; fig. 13), permeability rates 

(inches per hour; fig. 14), and slope (percent; fig. 15). 

Soil characteristics were computed as area-weighted 

averages for each watershed. 

Principal Aquifer Type. Principal aquifer types underlying the study area were identified from a digital coverage of the principal aquifers of the 48 contiguous 

states (Miller, 1998; U.S. Geological Survey, 2000g) 

(fig. 16). The most common principal aquifer types in 

the study area are sandstone, carbonate, and sandstone 

and carbonate aquifers. Parts of the study area are not 

underlain by a principal aquifer. The percentage of each 

principal aquifer type or area not underlain by a principal aquifer was computed for each watershed. 

Data Summaries and Statistical Methods

Water-Quality Data. One of the selection requirements 

for each of the 234 sites used in this study was at least 

15 total phosphorus samples collected over a period of 

more than 1 year. The number of water-quality samples 

collected from each site was highly variable, and the 

period of record ranged from 2 years to decades. The 

number of samples collected in any given period was 

also variable. Therefore, to obtain representative statistical summaries that were not biased by intensively 

sampled periods, the data were subsampled to monthly 

intervals. For each site, only one sample per constituent 

per month per year was used. The sample included in 

statistical summaries was the one collected closest to 

the middle of the month. All data reported at less than 

the detection limit were set to the detection limit. 

Median values of all midmonthly concentrations of total 

phosphorus and total nitrogen for each site are given in 

appendix 2. The median concentrations were log transformed and used in all subsequent statistical analyses 

(correlations, stepwise regressions, and regression-tree 

analyses). Log transformation improved the normality 

of the dependent variables (concentration data) 

although not always below the 5-percent critical level.

Correlations and Regressions. To determine linear 

relations between each water-quality characteristic and 

the environmental factors, Pearson correlation analyses 

were done, followed by forward stepwise-regression 

analyses. Correlation analyses were used to describe 

how much of the linear variability in each water-quality 

characteristic was explained by each environmental factor. Forward stepwise-regression analyses (with a 5-percent critical level for entry) were then used to determine 

the direction and magnitude of the interaction between 

several environmental factors and individual waterquality characteristics, as well as to determine the best 

multivariate relation to predict total phosphorus and 

total nitrogen concentrations at a specific site as a function of the environmental characteristics in its watershed. Forward stepwise regressions were also done with 

land-use characteristics excluded from the environmental factors to determine which natural environmental 

variables could best describe the distribution of total 

phosphorus and total nitrogen.

In addition, correlation analyses were used to 

describe the relation among environmental factors. 

Sometimes one environmental variable was strongly 

correlated with one or more other environmental factors, making the actual factor causing variations in 

water quality difficult to ascertain. For example, the 

percentage of forest was highly (and negatively) correlated to the percentage of agriculture. Understanding 

the relations among environmental factors is important 
12 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from Soller and Packard, 1998

Quaternary sediment type

Lines used to define part of an

environmental nutrient zone

Study-area boundary

EXPLANATION

Coarse-grained sediment

Fine-grained sediment

Till

Patchy Quaternary sediment

Organic-rich sediment

Exposed bedrock

or nonglacial sediment

No data

102° 84°

45°

36°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from Soller and Packard, 1998

Quaternary sediment thickness,

in feet

Study-area boundary

EXPLANATION

0–50

50–100

100–400

400–800

800–1200

No data

102° 84°

45°

36°

Figure 10. Quaternary sediment thickness in the study area.

Figure 9. Quaternary sediment types in the study area.
APPROACH, DATA, AND STUDY METHODS 13

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000f

Soil clay content (percent)

Line used to define part of an

environmental nutrient zone

Study-area boundary

EXPLANATION

0.001–14.4

14.4–21.3

21.3–25.7

25.7–30.1

30.1–69

Water

102° 84°

45°

36°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000f

Soil organic-matter

content (percent)

Study-area boundary

EXPLANATION

0.001–0.7

0.7–1.1

1.1–1.8

1.8–5.3

5.3–68.1

Water

102° 84°

45°

36°

Figure 11. Soil clay content in the study area (gradations represent quantile distributions).

Figure 12. Soil organic-matter content in the study area (gradations represent quantile distributions).
14 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000f

Soil erodibility (K factor)

Study-area boundary

EXPLANATION

0–0.24

0.24–0.3

0.3–0.34

0.34–0.38

0.38–0.59

Water

102° 84°

45°

36°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000f

Soil permeability

(inches per hour)

Study-area boundary

EXPLANATION

0.001–1.13

1.13–1.67

1.67–3.19

3.19–7.01

7.01–17.78

Water

102° 84°

45°

36°

Figure 13. Soil erodibility in the study area (gradations represent quantile distributions).

Figure 14. Soil permeability in the study area (gradations represent quantile distributions).
APPROACH, DATA, AND STUDY METHODS 15

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000f

Soil slope (percent)

Study-area boundary

Line used to define part of

an environmental nutrient zone

EXPLANATION

0–2.1

2.1–3.8

3.8–6

6–11.4

11.4–51.6

Water

102° 84°

45°

36°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs Modified from USGS, 2000g

Principal aquifer type

Study-area boundary

EXPLANATION

Carbonate rock

Sandstone and carbonate rock

Sandstone

Semiconsolidated sand

Unconsolidated sand and gravel

Not a principal aquifer

102° 84°

45°

36°

Figure 15. Soil slope in the study area (gradations represent quantile distributions).

Figure 16. Principal aquifer types in the study area.
16 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

to understanding the relations between nutrient concentrations and environmental factors.

The SAS statistical software package (SAS Institute, 1989) was used for all statistical summaries, correlations, and linear regressions.

Regression-Tree Analysis. Regression-tree analysis 

(Breiman and others, 1984), like forward stepwise 

regression, is a statistical technique used to explore the 

relations between a single dependent variable and several independent variables. However, instead of trying to 

fit a single regression to a large multivariate data set, one 

can use this method to divide the dependent data into 

groups with similar statistical relations. The method 

involves repeated partitioning of a data set into subgroups based on regressions between the dependent 

variable (for example, total phosphorus concentrations) 

and one independent variable (for example, percentage 

agriculture) at a time. A causal relation between the 

dependent variable and the independent variables is 

assumed. The partitioning of the data results in smaller 

and smaller subgroups and graphically resembles the 

branches of a tree. The choice of independent variable in 

the regression that results in a “branching” of the data is 

guided by a least-squares-error criterion. The value of 

the independent variable at which the data are partitioned is the one that minimizes the total sum of squared 

residuals of each resulting subgroup. Branching continues until the number of observations in each subgroup is 

small or the total sum of squared residuals is small.

In this study, regression-tree analyses were used to 

separate the sites into groups based on relations between 

median total phosphorus or median total nitrogen concentrations and environmental characteristics of the 

watershed, except watershed area. Watershed area was 

not chosen as an environmental characteristic because it 

would not enable large areas of relatively similar environmental characteristics to be defined. Regression-tree 

analyses were done with all environmental information 

and with the land-use characteristics excluded to determine which natural environmental characteristics were 

most statistically significant in describing the distribution of total phosphorus or total nitrogen. Regressiontree analyses were performed by use of the computer 

program S-PLUS (MathSoft, 1999). 

Comparison of Variability Among Classification 

Schemes. To determine whether the environmental 

nutrient zones with relatively similar environmental 

characteristics defined by the regression-tree analyses 

had less variance in water quality than that found by use 

of the nutrient ecoregions, the weighted mean coefficient of variation (MCV) was computed with equation 1 

for each scheme, and the percentage of reduction in the 

MCV was computed. MCVs were computed from 

water-quality data for all of the sites in the defined areas 

(nutrient ecoregions and environmental nutrient zones) 

and also for each group as defined directly from regression-tree results (based on individual watershed characteristics). 

, (1)

,

where

CV is the coefficient of variation of each

group (or area),

n is the number of observations in each

group,

N is the total number of observations in all

of the groups,

StDev is the standard deviation of each group, and

X is the mean concentration of each group.

DEVELOPMENT OF NUTRIENT ZONES—

AN ALTERNATIVE REGIONALIZATION 

SCHEME

Distribution of Nutrient Concentrations

Median, midmonthly total phosphorus concentrations ranged from 0.01 to 2.16 mg/L. The overall mean 

and median of these concentrations were 0.17 and 

0.11 mg/L, respectively (table 1). High concentrations 

were found throughout the study area, especially in Illinois, Iowa, and southeastern Wisconsin (fig. 17). The 

lowest concentrations were found in northern areas of 

Wisconsin and Minnesota, and along the eastern edge of 

the study area. 

Median, midmonthly total nitrogen concentrations ranged from 0.34 to 13.18 mg/L. The overall mean 

and median of these concentrations were 3.61 and 

2.26 mg/L, respectively (table 1). The highest total 

nitrogen concentrations were generally found throughout the central part of the study area (fig. 18). The 

lowest concentrations were found in northern areas of 

MCV Σ CV2 ( ) × n

N = ----------------------------

CV StDev

X = ---------------

to understanding the relations between nutrient concentrations and environmental factors.

The SAS statistical software package (SAS Institute, 1989) was used for all statistical summaries, correlations, and linear regressions.

Regression-Tree Analysis. Regression-tree analysis 

(Breiman and others, 1984), like forward stepwise 

regression, is a statistical technique used to explore the 

relations between a single dependent variable and several independent variables. However, instead of trying to 

fit a single regression to a large multivariate data set, one 

can use this method to divide the dependent data into 

groups with similar statistical relations. The method 

involves repeated partitioning of a data set into subgroups based on regressions between the dependent 

variable (for example, total phosphorus concentrations) 

and one independent variable (for example, percentage 

agriculture) at a time. A causal relation between the 

dependent variable and the independent variables is 

assumed. The partitioning of the data results in smaller 

and smaller subgroups and graphically resembles the 

branches of a tree. The choice of independent variable in 

the regression that results in a “branching” of the data is 

guided by a least-squares-error criterion. The value of 

the independent variable at which the data are partitioned is the one that minimizes the total sum of squared 

residuals of each resulting subgroup. Branching continues until the number of observations in each subgroup is 

small or the total sum of squared residuals is small.

In this study, regression-tree analyses were used to 

separate the sites into groups based on relations between 

median total phosphorus or median total nitrogen concentrations and environmental characteristics of the 

watershed, except watershed area. Watershed area was 

not chosen as an environmental characteristic because it 

would not enable large areas of relatively similar environmental characteristics to be defined. Regression-tree 

analyses were done with all environmental information 

and with the land-use characteristics excluded to determine which natural environmental characteristics were 

most statistically significant in describing the distribution of total phosphorus or total nitrogen. Regressiontree analyses were performed by use of the computer 

program S-PLUS (MathSoft, 1999). 

Comparison of Variability Among Classification 

Schemes. To determine whether the environmental 

nutrient zones with relatively similar environmental 

characteristics defined by the regression-tree analyses 

had less variance in water quality than that found by use 

of the nutrient ecoregions, the weighted mean coefficient of variation (MCV) was computed with equation 1 

for each scheme, and the percentage of reduction in the 

MCV was computed. MCVs were computed from 

water-quality data for all of the sites in the defined areas 

(nutrient ecoregions and environmental nutrient zones) 

and also for each group as defined directly from regression-tree results (based on individual watershed characteristics). 

, (1)

,

where

CV is the coefficient of variation of each

group (or area),

n is the number of observations in each

group,

N is the total number of observations in all

of the groups,

StDev is the standard deviation of each group, and

X is the mean concentration of each group.

DEVELOPMENT OF NUTRIENT ZONES—

AN ALTERNATIVE REGIONALIZATION 

SCHEME

Distribution of Nutrient Concentrations

Median, midmonthly total phosphorus concentrations ranged from 0.01 to 2.16 mg/L. The overall mean 

and median of these concentrations were 0.17 and 

0.11 mg/L, respectively (table 1). High concentrations 

were found throughout the study area, especially in Illinois, Iowa, and southeastern Wisconsin (fig. 17). The 

lowest concentrations were found in northern areas of 

Wisconsin and Minnesota, and along the eastern edge of 

the study area. 

Median, midmonthly total nitrogen concentrations ranged from 0.34 to 13.18 mg/L. The overall mean 

and median of these concentrations were 3.61 and 

2.26 mg/L, respectively (table 1). The highest total 

nitrogen concentrations were generally found throughout the central part of the study area (fig. 18). The 

lowest concentrations were found in northern areas of 

MCV Σ CV2 ( ) × n

N = ----------------------------

CV StDev

X = ---------------
DEVELOPMENT OF NUTRIENT ZONES—AN ALTERNATIVE REGIONALIZATION SCHEME 17

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Total phosphorus, in milligrams

per liter

Study-area boundary

EXPLANATION

0.01–0.05

0.05–0.09

0.09–0.15

0.15–0.22

0.22–2.16

102° 84°

45°

36°

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Total nitrogen, in milligrams

per liter

Study-area boundary

EXPLANATION

0.34–1.01

1.01–1.6

1.6–3.63

3.63–6.46

6.46–13.18

102° 84°

45°

36°

Figure 17. Median, midmonthly total phosphorus concentrations for study sites (gradations represent quantile distributions).

Figure 18. Median, midmonthly total nitrogen concentrations for study sites (gradations represent quantile distributions).
18 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Minnesota and Wisconsin, and along the eastern edge of 

the study area. Total nitrogen could not be estimated for 

many sites in Wisconsin because water samples from 

most of these sites were analyzed only for dissolved 

nitrate.

Most Statistically Significant Environmental 

Factors

Correlation Results. Pearson correlation coefficients 

(r values) between nutrient concentrations (total phosphorus and total nitrogen) and each environmental factor are shown in table 2. Nutrient concentrations were 

significantly correlated with most environmental variables; however, they were most strongly correlated with 

factors describing the land use (percentages of forest 

and agriculture) and surficial deposits or soil in the 

watershed (percentage of till, percentage of clay, erodibility, permeability, and soil slope), and runoff from the 

watershed.

Total phosphorus concentrations were most 

strongly correlated with the absence of forest or amount 

of agriculture (percentage of forested area had a strong 

inverse relation with percentage of agriculture; table 3), 

percentage of till, and percentage of clay in the watershed. Total nitrogen concentrations were most strongly 

correlated with the absence of forest or presence of agriculture, percentage of till, and average soil slope of the 

watershed. Total nitrogen concentrations were more 

highly correlated with land-use characteristics than 

were total phosphorus concentrations, whereas total 

phosphorus concentrations were more correlated with 

soil characteristics than were total nitrogen concentrations. 

Stepwise-Regression Results. Forest cover, soil permeability, and runoff were the three most significant variables contributing to the forward stepwise-regression 

model for total phosphorus concentrations (table 4). 

Collectively, these three variables, each highly correlated with total phosphorus concentrations, explained 

40 percent of the variability in concentration.

Forest cover, agriculture, and rangeland were the 

three most significant variables contributing to the forward stepwise-regression model for total nitrogen concentrations (table 4). Collectively, these three land-use 

variables explained 57 percent of the variability in total 

nitrogen concentrations. The percentage of rangeland 

was not significantly correlated with total nitrogen concentrations and only minimally improved the regression 

model.

Forward stepwise-regression analyses were also 

done with land-use characteristics excluded from the 

environmental factors to determine the most statistically significant natural environmental factors contributing to variability in water quality. Percentage of till, 

percentage of clay in the soil, and runoff were the three 

most statistically significant variables in the total phosphorus model (table 4). Collectively, these three variables explained 39 percent of the variability in total 

phosphorus concentrations. Each of these variables was 

highly correlated with total phosphorus concentrations. 

The final model explained about the same amount of 

variability in total phosphorus concentrations as the 

three-parameter model including land-use characteristics.

For total nitrogen concentrations, the percentage of 

till, percentage of clay in the soil, and soil slope were 

the three most significant variables contributing to the 

forward stepwise-regression model (table 4). Collectively, these three variables explained 38 percent of the 

variability. Each of these variables was highly correlated with total nitrogen concentrations. The final 

model explained much less of the variability in total 

nitrogen concentrations than did the three-parameter 

model that included land-use characteristics, which 

demonstrates the importance of land use in explaining 

the distribution of total nitrogen.

Regression-Tree Analysis Results. Regression-tree 

analysis was used to subdivide the 234 sites into two 

initial groups, then into four derivative groups on the 

basis of relations between total phosphorus and all of 

the environmental characteristics (fig. 19A). The percentage of forested area in the watershed was the independent variable chosen for the first subdivision. 

Forested land use is strongly correlated (negatively) 

with agricultural land use (table 3); therefore, the percentage of agricultural land would probably have provided similar results if the percentage of forested land 

had been removed from the analysis. The two groups in 

the first subdivision had watersheds that were either less 

than 30 percent forested (177 sites with a mean concentration of 0.21 mg/L) or greater than or equal to 30 percent forested (57 sites with a mean concentration of 

0.06 mg/L). Sites having less than 30 percent of their 

watersheds forested were further subdivided into 

group 1, those with soil clay content less than 26 percent (70 sites with a mean concentration of 0.12 mg/L), 
DEVELOPMENT OF NUTRIENT ZONES—AN ALTERNATIVE REGIONALIZATION SCHEME 19

Table 2. Pearson correlation coefficients between water-quality 

constituents (total phosphorus and total nitrogen) and the environmental 

characteristics of their respective watersheds. The r values are color 

coded to help demonstrate the strength of the relations (red values 

indicate | r | > 0.5, green indicate 0.5 > | r | > 0.4, and orange indicate 

0.4 > | r | > 0.3)

[All values greater than 0.15 or less than -0.15 were statistically significant at P < 0.05]

Environmental characteristic Total phosphorus Total nitrogen

Basin characteristics

Area -0.06 -0.07

Runoff -.38 -.40

Climatic characteristics

Precipitation -.05 -.08

Air temperature .25 .16

Land-use characteristics

Urban .13 .01

Agriculture .48 .71

Rangeland .05 -.09

Forest -.55 -.72

Forested wetland -.29 -.30

Nonforested wetland -.18 -.23

Barren land .06 -.07

Surficial-deposit characteristics

Coarse-grained stratified sediments -.28 -.14

Fine-grained stratified sediments .12 .11

Till .47 .52

Patchy Quaternary sediments -.14 -.06

Exposed bedrock or nonglacial sediments -.32 -.45

Organic-rich sediments -.11 -.10

Soil characteristics

Percent clay .46 .37

Erodibility .41 .32

Organic-matter content -.24 -.21

Permeability -.43 -.32

Soil slope -.34 -.52

Principal aquifer types

Sandstone .08 .01

Carbonate .16 .28

Sandstone and carbonate -.11 -.08

Depth to bedrock .03 .24
20 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Table 3. Pearson correlation coefficients between environmental characteristics for sites in the study

[All values in bold are statistically significant at P < 0.05. All abbreviations are defined in table 1] Area Roff Urbn Agri Rang Fors Fwet Nwet Barr Coar Fine Till Patc Xbed Orga

Sdst

Carb

Sscb

Dbed

Clay

Kf

Omat

Perm

SlopPrec

Temp

Area

Roff -0.10

Urbn -.10 -0.04

Agri -.05 -.44 -0.34

Rang .25 -.15 -.04 0.05

Fors .04 .55 -.20 -.83 -0.06

Fwet .16 .02 -.12 -.41 -.04 0.33

Nwet .31 -.23 -.03 -.20 .03 .07 0.43

Barr -.05 .07 .13 -.14 .04 -.01 -.08 -0.03

Coar .09 -.14 .02 -.20 -.01 .09 .42 .36 -0.04

Fine -.05 .00 .06 .15 -.02 -.18 -.09 -.06 .02 -0.03

Till .01 -.34 .14 .53 .05 -.66 -.09 -.06 .02 -.23 -0.08

Patc .02 -.01 -.07 -.11 -.02 .16 .11 -.03 -.06 -.08 -.07 -0.19

Xbed -.06 .42 -.14 -.43 -.04 .62 -.14 -.11 .02 -.23 -.14 -.82 -0.05

Orga .03 .00 -.05 -.25 -.01 .25 .34 .27 -.04 -.02 -.04 -.09 .10 0.04

Sdst -.19 -.02 -.19 .08 -.01 .07 -.22 -.14 .09 -.14 -.03 -.09 -.05 .18 -0.03

Carb -.04 -.09 .32 .10 -.06 -.28 -.05 -.11 .00 -.11 -.06 .29 -.04 -.20 -.07 -0.53

Sscb .04 .39 -.06 -.10 -.03 .19 -.11 -.11 -.04 -.13 -.05 -.18 -.03 .28 -.04 -.20 -0.14

Dbed .21 -.32 .14 .17 .02 -.33 .04 .34 -.07 .30 .01 .30 -.15 -.41 -.09 -.29 .13 -0.11

Clay -.04 -.03 .20 .38 -.02 -.42 -.51 -.37 .06 -.56 .20 .36 -.16 -.08 -.20 -.10 .29 .19 -0.01

Kf -.06 -.02 .07 .53 .07 -.52 -.49 -.42 .02 -.53 .12 .52 -.09 -.26 -.26 .01 .22 .01 -.07 0.70

Omat .09 -.11 .08 -.36 -.05 .18 .74 .55 -.05 .45 -.06 .02 .09 -.27 .41 -.31 .05 -.17 .23 -.47 -0.59

Perm .09 -.03 -.07 -.35 -.02 .33 .35 .41 -.05 .71 -.12 -.47 -.03 .16 .07 -.03 -.15 -.05 .16 -.77 -.82 0.43

Slop -.06 .58 -.08 -.55 -.04 .69 -.09 -.16 .14 -.15 -.14 -.67 .04 .79 -.08 .14 -.19 .34 -.39 -.08 -.24 -.21 0.13

Prec -.23 .73 -.07 -.05 -.22 .19 -.27 -.46 .12 -.36 .02 .00 -.08 .20 -.08 .24 -.04 .29 -.36 .28 .37 -.40 -.37 0.34

Temp -.16 .27 .00 .29 -.08 -.22 -.45 -.45 .16 -.42 .09 .27 -.23 -.01 -.17 .32 -.04 .21 -.22 .57 .62 -.53 -.55 .06 0.76

Table 4. Forward stepwise-regression models to explain variability in total 

phosphorus and total nitrogen concentrations including and excluding land-use 

characteristics in the analyses

[All regressions were on log-transformed nutrient data; r, correlation coefficient with independent 

variable; Step R2, coefficient of determination for one, two, and three variable models]

Independent variables

Dependent variables First variable Second variable Third variable

Land-use characteristics included

Total phosphorus Percent forest

r = -0.55

Step R2 = 0.31

Permeability

r = -0.43

Step R2 = 0.37

Runoff

r = -0.38

Step R2 = 0.40

Total nitrogen Percent forest

r = -0.72

Step R2 = 0.52

Percent agriculture

r = 0.71

Step R2 = 0.55

Percent rangeland

r = -0.09

Step R2 = 0.57

Land-use characteristics excluded

Total phosphorus Percent till

r = 0.47

Step R2 = 0.22

Percent clay

r = 0.46

Step R2 = 0.32

Runoff

r = -0.38

Step R2 = 0.39

Total nitrogen Percent till

r = 0.52

Step R2 = 0.27

Percent clay

r = 0.37

Step R2 = 0.32

Soil slope

r = -0.52

Step R2 = 0.38
DEVELOPMENT OF NUTRIENT ZONES—AN ALTERNATIVE REGIONALIZATION SCHEME 21

A. Including all environmental factors

Forest ≥ 30 percent

Yes (57 sites)

No (177 sites)

All

Data

No (70)

Yes (107)

Clay ≥ 26 percent

No (32)

Yes (25)

Runoff ≥ 12.3 inches per year

Group 2

Group 1

Group 3

Group 4

B. Excluding land-use characteristics

Till ≥ 59 percent

Yes (151 sites)

No (83 sites)

All

Data

No (52)

Yes (31)

Runoff ≥ 10.3 inches per year

No (55)

Yes (96)

Clay ≥ 26 percent

Group 2

Group 1

Group 3

Group 4

Group 2

Group 1

Group 3

Group 4

Group 2

Group 1

Group 3

Group 4

Figure 19. Environmental phosphorus groups from regression-tree analysis (A) including all environmental characteristics and 

(B) excluding land-use characteristics. (The numbers in brackets are the number of sites in each group. Color code for qualitative concentration range is green (lowest) followed by yellow, orange, and red (highest).)
22 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

or group 2, greater than or equal to 26 percent (107 sites 

with a mean concentration of 0.27 mg/L). Most group 1 

sites were scattered throughout the central part of the 

study area (shown later in fig. 21). Group 2 sites had the 

highest phosphorus concentrations and were generally 

in the central and southern parts of the study area and in 

eastern Wisconsin. Sites with watersheds more than or 

equal to 30 percent forested were further subdivided 

into group 3, those with less than 12.3 in/yr of runoff 

(32 sites with a mean concentration of 0.08 mg/L), or 

group 4, more than or equal to 12.3 in/yr (25 sites with 

a mean concentration of 0.03 mg/L). Groups 3 and 4 had 

the lowest average phosphorus concentrations in the 

study area. Group 3 sites were mostly in central and 

northern Wisconsin, and central Minnesota. Group 4 

sites were primarily in northern Wisconsin, Upper 

Michigan, and the extreme eastern part of the study 

area. 

A similar analysis excluding land-use characteristics was done on the 234 sites to determine which natural environmental factors best explained the variability 

in phosphorus concentrations (fig. 19B). The surficialdeposit type, till, was then the independent variable 

chosen for the first subdivision. The two groups in the 

first subdivision had sites where the percentage of till in 

the watershed was less than 59 percent (83 sites with a 

mean concentration of 0.08 mg/L) or greater than or 

equal to 59 percent (151 sites with a mean concentration 

of 0.23 mg/L). The subgroup with less than 59 percent 

till was further partitioned in group 1, those with runoff 

less than 10.3 in/yr (52 sites with a mean concentration 

of 0.10 mg/L), or group 2, greater than or equal to 

10.3 in/yr (31 sites with a mean concentration 

0.04 mg/L). Group 1 sites were mainly in the Upper 

Mississippi Watershed in Minnesota and Wisconsin. 

Group 2 sites had the lowest phosphorus concentrations 

and were mainly in the southern part of the study area 

and northern Wisconsin. The subgroup with till greater 

than or equal to 59.2 percent was further partitioned into 

group 3, those with the percentage of clay less than 

26 percent (55 sites with a mean concentration of 

0.13 mg/L), or group 4, greater than or equal to 26 percent (96 sites with a mean concentration of 0.29 mg/L). 

Group 3 sites had the second highest phosphorus concentrations and were scattered throughout the study 

area. Group 4 sites had the highest phosphorus concentrations and were primarily in Illinois, Iowa, northeastern Missouri, eastern North Dakota, Ohio, and eastern 

Wisconsin. 

Regression-tree analyses with respect to nitrogen 

were done similar to that for phosphorus, except that 

total nitrogen data were available for only 152 of the 

234 sites. The first independent variable chosen when 

all the factors were used in the analysis was percentage 

of forest in the watershed, as in the analysis for phosphorus concentration (fig. 20A). The sites were divided 

in subgroups corresponding to those where the forested 

land area was less than 9 percent (81 sites with a mean 

concentration of 5.47 mg/L) or greater than or equal to 

9 percent (71 sites with a mean concentration of 

1.48 mg/L). Sites with less than 9 percent of the watershed forested were further divided into group 1, those 

with less than 30.1 in/yr of precipitation (9 sites with a 

mean concentration of 1.44 mg/L), or group 2, greater 

than or equal to 30.1 in/yr of precipitation (72 sites with 

a mean concentration of 5.98 mg/L). Precipitation is 

highly correlated with runoff; therefore, the differences 

in these subareas also represent differences in runoff. 

Group 1 sites were primarily in the northwest part of the 

study area, in Minnesota and North Dakota (shown later 

in fig. 21). Group 2 sites had the highest nitrogen concentrations and were throughout the central part of the 

study area, from southeastern Minnesota through Iowa 

and northern Illinois and into western Ohio. Sites having watersheds with more than or equal to 9 percent forest were further partitioned into group 3, those sites 

with forested land use less than 33 percent of the total 

area (39 sites with a mean concentration of 2.02 mg/L), 

or group 4, greater than or equal to 33 percent of the 

total area (32 sites with a mean concentration of 

0.83 mg/L). Group 3 sites had the second highest total 

nitrogen concentrations and were scattered throughout 

the study area, most notably in southern Illinois and 

Indiana, eastern Ohio, and southeastern Minnesota. 

Group 4 sites had the lowest concentrations and were 

primarily in extreme southern, eastern, and north-central parts of the study area.

A similar analysis was done on the 152 sites 

excluding land-use characteristics (fig. 20B). The first 

independent variable chosen was soil slope. The sites 

were separated into subgroups corresponding to those 

where the average soil slope was less than 11 percent 

(130 sites with a mean total nitrogen concentration of 

4.08 mg/L) or greater than or equal to 11 percent 

(22 sites with a mean concentration of 0.82 mg/L). 

The subgroup with soil slopes less than 11 percent 

was further partitioned into group 1, those with mean 

annual air temperature less than 43°F (20 sites with a 

mean concentration of 1.11 mg/L), or group 2, greater 
DEVELOPMENT OF NUTRIENT ZONES—AN ALTERNATIVE REGIONALIZATION SCHEME 23

A. Including all environmental factors

Forest ≥ 9 percent

Yes (71 sites)

No (81 sites)

All

Data

No (9)

Yes (72)

Precipitation ≥ 30.1 inches per year

No (39)

Yes (32)

Forest ≥ 33 percent

Group 2

Group 1

Group 3

Group 4

B. Excluding land-use characteristics

Soil slope ≥ 11 percent

Yes (22 sites)

No (130 sites)

All

Data

No (20)

Yes (110)

Air temperature ≥ 43°F

No (5)

Yes (17)

Nonglacial sediments 

or exposed bedrock ≥ 99 percent

Group 2

Group 1

Group 3

Group 4

Group 2

Group 1

Group 3

Group 4

Group 2

Group 1

Group 3

Group 4

Figure 20. Environmental nitrogen groups from regression-tree analysis (A) including all environmental characteristics and 

(B) excluding land-use characteristics. (The numbers in brackets are the number of sites in each group. Color code for 

qualitative concentration range is green (lowest) followed by yellow, orange, and red (highest).)
24 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

than or equal to 43°F (110 sites with a mean concentration of 4.62 mg/L). Group 2 sites, which had the highest 

nitrogen concentrations, were in the central part of the 

study area. The subgroup with soil slopes greater than 

or equal to 11 percent was further partitioned into group 

3, those where the amount of exposed bedrock or nonglacial sediments covered less than 99 percent of the 

watershed (5 sites with a mean concentration of 

1.28 mg/L), or group 4, more than or equal to 99 percent 

of the watershed (17 sites with a mean concentration of 

0.68 mg/L). The lowest average total nitrogen concentrations were at sites in group 4, which were mainly in 

the extreme southeastern and southwestern parts of the 

study area. 

Summary of the Most Statistically Significant Environmental Factors. All three types of statistics indicated that land use (primarily the absence of forest or 

presence of agriculture) was the primary factor related 

to high concentrations of total phosphorus and total 

nitrogen. Land-use factors were more important in 

describing the variability in nitrogen concentrations 

(top three factors in the stepwise regression) than in 

describing the variability in phosphorus concentrations. 

The second most important type of factor was one 

describing the soil or surficial deposits. In general, total 

phosphorus concentrations were more strongly correlated with factors describing the permeability and erodibility of the soil, whereas total nitrogen concentrations 

were more strongly correlated with the soil slope. The 

third most important type of factor was one describing 

the climate (precipitation or runoff). Overall, the highest total phosphorus concentrations were found in agricultural areas with till deposits, especially those with 

high clay content; the highest total nitrogen concentrations were found in agricultural areas with gently sloping soils overlying till deposits, especially those sites in 

the southern part of the study area. 

Delineation of Nutrient Zones

By understanding which environmental factors 

most strongly affect water quality, and using the distribution of only these factors to define areas of relatively 

homogeneous environmental characteristics, one 

should be able to minimize the variability in water quality within a defined area and maximize the variations in 

water quality among areas. As demonstrated in the preceding discussion, results of the regression-tree analysis 

can be used to identify the environmental factors most 

strongly related to water quality. Of additional importance is that the values used to define the branches can 

also be used as a guide in the delineation of the relatively homogeneous areas. 

Environmental Phosphorus Zones (Derived with 

Land-use Characteristics Included). Regression-tree 

results (including land-use characteristics) for total 

phosphorus were used to divide the entire study area 

into zones having environmental characteristics similar 

to the four main groups described previously. By use of 

the available GIS coverages, the entire study area was 

manually partitioned by the spatial extent of the three 

independent variables—percentage of forest, percentage of clay, and runoff (figs. 8, 11, and 5). For the continuous variables—percentage of clay and runoff—the 

study area was partitioned on the basis of “branching”

values from the regression-tree results. For discrete 

characteristics—percentage of forest—the study area 

was partitioned on the basis of generalized presence or 

absence of that characteristic. (A generalized coverage 

of the major land uses in the United States (Anderson, 

1967) was used to delineate the extent of the forested 

areas.) For total phosphorus, the resulting coverages of 

forest, runoff, and clay were overlain and combined into 

a single map representing “Environmental Phosphorus 

Zones” (EPZs) (fig. 21A). The spatial extent of EPZs 

corresponded closely to the location of the groups of 

watersheds described in the regression-tree analysis 

(environmental phosphorus groups). A few sites (typically with larger watersheds) were on the border 

between EPZs and assigned to the less-alike EPZ 

because the environmental characteristics used in 

regression-tree analysis were based on watershed averages for the environmental characteristics. Additionally, 

a few watersheds (typically smaller watersheds) were 

“islands” in an EPZ other than that for which they were 

grouped because they were in small areas with environmental conditions different from most of the surrounding EPZ.

Environmental Phosphorus Zones (Derived with 

Land-use Characteristics Excluded). The USEPA has 

stated that, to the extent possible, classification should 

be based on those characteristics that are intrinsic, or 

natural, and not the result of human activities (U.S. 

Environmental Protection Agency, 2000a). Differences 

in nutrient criteria among areas should reflect differences in water quality caused by natural factors and 

therefore reflect differences in potential water quality. 
COMPARISON OF ENVIRONMENTAL NUTRIENT ZONES TO NUTRIENT ECOREGIONS 25

Although land use was the most important characteristic 

in subdividing the area into the nutrient zones in 

figure 21 and also the primary characteristic in defining 

the ecoregions of the Midwest, developing zones based 

on the results of the tree-regression analyses excluding 

land use is appropriate and in-line with the USEPA classification philosophy.

By use of the method just described, the study area 

was partitioned by independent variables chosen from 

the results of regression-tree analysis, excluding landuse characteristics. For total phosphorus, the resulting 

coverages of percentage of till, runoff, and percentage 

of clay (figs. 9, 5, and 11) were overlain and combined 

into a single map representing EPZs (fig. 22A). As with 

the previous EPZs (incorporating land use), the extent 

of these areas also corresponded closely to the location 

of the environmental phosphorus groups described in 

the regression-tree analysis. The sites are not shown on 

figure 22 to enable the EPZs to be better illustrated.

In general, EPZs developed including and excluding land-use characteristics were similar because both 

sets of areas were based on the distribution of clay and 

runoff; and in general, agriculture is found in areas 

dominated by till. The differences in the delineation 

reflect areas that have till but are not farmed (such as 

north-central Wisconsin) and areas without till that are 

farmed (such as western Kentucky).

Environmental Nitrogen Zones. The study area was 

partitioned into “Environmental Nitrogen Zones”

(ENZs, fig. 21B) on the basis of independent variables 

chosen in the regression-tree analysis: percentage of 

forest and precipitation (figs. 8 and 7). The strong relation between nitrogen concentrations and land use was 

especially evident here, the percentage of forested land 

being used as first and secondary breaks in the regression-tree analysis. Therefore, the ENZs primarily represent low, moderate, and high amounts of agriculture.

In an attempt to remove the effects of land use, the 

study area was partitioned on the basis of independent 

variables chosen in the regression-tree analysis excluding land-use characteristics. The resulting coverages of 

soil slope, mean annual air temperature, and percentage 

of exposed bedrock or nonglacial sediment (figs. 15, 6, 

and 9) were overlain and combined into a single map 

representing ENZs excluding land-use characteristics 

(fig. 22B). 

In general, ENZs developed including and excluding land-use characteristics were substantially different. 

The ENZs, derived excluding land-use characteristics, 

divide the area primarily into three zones from northwest to southeast. A fourth zone—ENZ 3 in southwestern Wisconsin and western New York—is made up of 

areas with steeply sloping soils and at least some glacial 

sediments. The completely nonglaciated area in southwestern Wisconsin that is surrounded by ENZ 3 was 

classified as ENZ 4, the same as southeastern part of the 

study area.

COMPARISON OF ENVIRONMENTAL 

NUTRIENT ZONES TO NUTRIENT 

ECOREGIONS

The goal of subdividing the study area into various 

nutrient ecoregions or environmental nutrient zones 

was to develop areas of relatively homogeneous water 

quality based on various environmental factors. The 

ecoregion subdivision was based on relative differences 

in a suite of factors, whereas the environmental nutrient 

zone subdivision was based only on the most important 

two or three factors. This leads to the question, “Which 

approach is best in subdividing the entire area into four 

or five subareas?” In general, nutrient ecoregions (at 

this 14-subdivision level) provide larger, more contiguous subareas than do nutrient zones. But which 

approach minimizes the variability in water quality 

within the subareas? To compare the two approaches, 

the weighted mean coefficient of variation (MCV, eq. 1) 

was computed for each scheme. Land use was the primary factor influencing the distribution of phosphorus 

and nitrogen; therefore, to fairly compare the two 

approaches, the MCVs for the nutrient ecoregions are 

compared with those for environmental nutrient zones 

derived including land-use characteristics. For total 

phosphorus, the MCV for the nutrient ecoregions was 

1.96 compared to 0.98 for the EPZs (table 5). (The values in table 5 are based on the environmental nutrient 

zones in which the watersheds were located, not necessarily the environmental nutrient group in which the site 

was classified.) Therefore, the environmental nutrient 

zone approach was better than the nutrient ecoregion 

approach for total phosphorus and reduced the variability within the subareas by about 50 percent. For total 

nitrogen, the MCV for the nutrient ecoregions was 0.59 

compared to 0.58 for the ENZs. Therefore, the environmental nutrient zone approach was not much better than 

the ecoregion approach for total nitrogen and only 

reduced the MCV by about 1 percent. This similarly 

was expected because the ENZs (including land-use 

characteristics) were similar to the nutrient ecoregions. 
26 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Base from USGS digital line graphs

Environmental phosphorus zone

and watershed group

(analysis included land-use 

characteristics)

Study-area boundary

EXPLANATION

1

2

3

4

1

2

3

4

(A)

(B)

Environmental phosphorus group

Environmental phosphorus zone

Environmental nitrogen zone

and watershed group

(analysis included land-use

characteristics)

Study-area boundary

EXPLANATION

1

2

3

4

1

2

3

4

Environmental nitrogen group

Environmental nitrogen zone

102° 84°

45°

36°

102° 84°

45°

36°

Figure 21. (A) Environmental phosphorus zones and (B) environmental nitrogen zones for study area when land-use characteristics were included in the regression-tree analyses. (Watersheds are color coded by the regression-tree group to which they 

were assigned. Color code for qualitative concentration range is green (lowest) followed by yellow, orange, and red (highest).)
COMPARISON OF ENVIRONMENTAL NUTRIENT ZONES TO NUTRIENT ECOREGIONS 27

0 100 200 MILES

0 100 200 KILOMETERS

(B)

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Base from USGS digital line graphs

Environmental phosphorus zone

(analysis excluded land-use

characteristics)

Study-area boundary

EXPLANATION

1

2

3

4

Environmental nitrogen zone

(analysis excluded land-use

characteristics)

Study-area boundary

EXPLANATION

1

2

3

4

(A)

102° 84°

45°

36°

102° 84°

45°

36°

Figure 22. (A) Environmental phosphorus zones and (B) environmental nitrogen zones for study area when land-use characteristics were excluded from the regression-tree analyses. (Watersheds are color coded by the regression-tree group to which they 

were assigned. Color code for qualitative concentration range is green (lowest) followed by yellow, orange, and red (highest).)
28 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Similar results were found when comparing the 

variability within nutrient ecoregions to that within 

environmental nutrient zones derived excluding landuse characteristics. The MCV for the EPZs (excluding 

land-use characteristics) also was 50 percent less than 

that for the nutrient ecoregions, and the MCV for ENZs 

(excluding land-use characteristics) also was about the 

same as that for nutrient ecoregions (table 5). Therefore, 

for total phosphorus, environmental nutrient zones 

developed by use of the methods described in this 

report, including only natural occurring environmental 

factors, reduce the variance within each subarea and yet 

should reflect areas with similar potential water quality. 

For total nitrogen, the variance within the environmental nutrient zones was similar to that for the nutrient 

ecoregions, but should reflect areas with similar potential water quality.

DEFINING NUTRIENT CRITERIA FOR 

ENVIRONMENTAL NUTRIENT ZONES

Criteria for each nutrient zone can be defined with 

a reference-condition approach similar to that described 

earlier for nutrient ecoregions. This concentration can 

be defined from the frequency distribution of all available data for a specified area (for example, the lower 

25th percentile) or the frequency distribution of a subset 

of streams thought to be the least impacted (for example, the upper 75th percentile) (fig. 3). The final criteria 

could be between these two concentrations.

On the basis of concentrations of the lower 25th 

percentiles of all of the data from the EPZs (derived by 

Table 5. Comparison of water quality and variability in water quality within nutrient ecoregions and within environmental nutrient 

zones (delineated with results from regression-tree analyses)

Total phosphorus Total nitrogen

Ecoregions

Ecoregion Count Mean Median Standard deviation Coefficient of

variation Count Mean Median Standard deviation Coefficient of

variation

VI 83 0.24 0.15 0.37 1.51 68 5.81 5.83 3.10 0.53

VII 86 .13 .10 .12 .90 29 2.32 1.88 1.37 .59

VIII 14 .00 .03 .02 .58 10 .77 .73 .32 .42

IX 36 .23 .17 .21 .93 30 2.26 1.55 1.82 .81

XI 15 .04 .05 .02 .49 15 .73 .68 .22 .31

Total 234 Weighted mean 152 Weighted mean

coefficient of variation (MCV) 1.96 coefficient of variation (MCV) .59

Nutrient Zones - Regression tree (including land-use characteristics)

Zone Count Mean Median Standard deviation Coefficient of

variation Count Mean Median Standard deviation Coefficient of

variation

1 82 0.11 0.09 0.08 0.68 8 1.42 1.33 0.37 0.26

2 109 .27 .17 .34 1.26 70 5.88 5.76 2.94 .50

3 23 .06 .05 .04 .65 49 2.17 1.52 1.66 .76

4 20 .03 .04 .02 .55 25 .77 .68 .33 .43

Total 234 Weighted mean 152 Weighted mean

coefficient of variation (MCV) .98 coefficient of variation (MCV) .58

Nutrient Zones - Regression tree (excluding land-use characteristics)

Zone Count Mean Median Standard deviation Coefficient of

variation Count Mean Median Standard deviation Coefficient of

variation

1 44 0.10 0.08 0.06 0.60 21 1.08 1.10 0.38 0.36

2 30 .04 .03 .03 .75 110 4.62 4.01 3.05 .66

3 58 .12 .09 .08 .66 4 1.48 1.42 .29 .19

4 102 .28 .17 .35 1.26 17 .68 .66 .24 .36

Total 234 Weighted mean 152 Weighted mean

coefficient of variation (MCV) .97 coefficient of variation (MCV) .59
REFINING ENVIRONMENTAL NUTRIENT ZONES 29

excluding land-use characteristics, fig. 22a), phosphorus criteria could range from 0.02 mg/L for EPZ 2 to 

0.11 mg/L for EPZ 4 (table 6). If it is assumed that the 

least impacted streams in each zone have less than 

25 percent agriculture in their watersheds, then alternative criteria could be established. Based on the 75th percentile of this subset of streams, phosphorus criteria 

could range from 0.05 mg/L for EPZ 2 to 0.16 mg/L for 

EPZ 4 (table 6).

On the basis of concentrations of the lower 25th 

percentiles of all data from the ENZs (derived by 

excluding land-use characteristics), nitrogen criteria 

could range from 0.51 mg/L for ENZ 4 to 1.75 mg/L for 

ENZ 2 (table 6). Based on the 75th percentiles of the 

subsets of streams with less than 25 percent agriculture 

in their watersheds, nitrogen criteria could range from 

0.67 mg/L for ENZ 4 to 9.00 mg/L for ENZ 2 (table 6). 

Only two sites had data available for ENZ 2 with less 

than 25 percent agriculture. 

For many environmental nutrient zones, few sites 

had less than 25 percent agriculture in their watersheds. 

Some of these “reference” sites did not actually represent natural conditions because they were dominated by 

urban land use. Therefore, reference sites must be 

closely examined to keep atypical conditions from corrupting this approach.

Within each environmental nutrient zone, streams 

may be classified by size, stream order, or use for example. In this report, as an example of how different criteria could be established for specific types of streams 

within a specific environmental nutrient zone, the sites 

were subdivided into four types on the basis of how the 

land around the stream was being used (based on the 

percentage of agriculture in the watershed). Nutrient 

criteria for each stream type can be developed with an 

approach that is similar to that for all streams in an environmental nutrient zone; that is, by examining the frequency distribution of all available data for a specific 

type of stream. For example, in streams with more than 

75 percent of their watershed being used for agriculture 

in EPZ 4 (fig. 22a), if the 25th percentile of all of the 

data of this type of stream is used to define the criterion, 

then the criterion would be 0.12 mg/L (table 6). In 

EPZ 4, most of the streams had more than 75 percent 

of their watershed area in agriculture; therefore, the 

concentration of the 25th percentile of this subset of 

streams was similar to that for all of the streams in 

EPZ 4 (0.11 mg/L). If the 75th percentile of the reference streams (defined as streams with less than 25 percent agricultural land) from the area are used, the 

criterion would again be 0.16 mg/L. Therefore the criterio for streams with 75 to 100 percent agriculture would 

be between 0.12 and 0.16 mg/L. With this approach, a 

specific type of stream may have more or less stringent 

criteria than other types of streams within the defined 

area.

REFINING ENVIRONMENTAL NUTRIENT 

ZONES

The USEPA has taken the initial step in developing 

regional nutrient criteria based on national nutrient 

ecoregions (fig. 1); however, the criteria may be refined 

on the basis of Omernik’s original 84 Level III ecoregions (fig. 2). The environmental nutrient zones in figure 22 were developed for the entire study area; 

however, individual states or tribes may wish to refine 

the results at a smaller spatial scale or refine the basic 

USEPA approach but maintain its basic classification 

scheme. With slight modifications to the approach, the 

four environmental nutrient groups can be refined by 

further subdivision, or new zones can be defined within 

smaller geographical areas (such as specific states or 

specific nutrient ecoregions).

Further Subdivision of the Environmental 

Nutrient Zones

To refine the classification scheme found for the 

entire Midwest (fig. 22), the regression trees shown in 

figures 19 and 20 can be further branched. Further 

branching results in a larger number of groups with a 

smaller number of sites in each group (fig. 23). The 

location of the sites in each of the resulting groups for 

total phosphorus (developed from analyses excluding 

land-use characteristics) are shown in figure 24. Group 

1 sites from figure 19b were further partitioned on the 

basis of whether the soil erodibility factor was less than 

0.22 (17 sites) or greater than or equal to 0.22 (35 sites). 

Group 2 sites were further partitioned on the basis of 

whether average annual air temperature was less than 

50°F (21 sites) or greater than or equal to 50°F 

(10 sites). Group 3 sites were further partitioned on 

the basis of whether runoff was less than 11.5 in/yr 

(41 sites) or greater than or equal to 11.5 in/yr (14 sites). 

Group 4 sites were further partitioned on the basis of 

whether the surficial-deposit type “coarse-grained sediment” was not present (20 sites) or present (76 sites). 

Overall, the lowest total phosphorus concentrations 

occurred at sites with less than 59 percent till, more than 
30 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Table 6. Percentiles of nutrient concentrations, by the percentage of agriculture in the watershed, 

for each environmental nutrient zone (created on the basis of results of regression-tree analyses 

when land-use characteristics were excluded from the analyses)

[--, no data]

 Percentiles

Percent

agriculture Number Mean Standarddeviation 10th 25th 50th 75th 90th

Phosphorus Zone 1

0–100 44 0.10 0.06 0.04 0.06 0.08 0.14 0.18

0–25 2 .04 .03 .02 .02 .04 .06 .06

25–50 12 .08 .08 .03 .03 .06 .11 .15

50–75 14 .09 .04 .05 .06 .08 .12 .14

75–100 16 .13 .06 .08 .08 .12 .18 .21

Phosphorus Zone 2

0–100 30 .04 .03 .01 .02 .03 .05 .05

0–25 14 .03 .02 .01 .01 .03 .05 .05

25–50 8 .03 .02 .01 .02 .03 .05 .05

50–75 5 .05 .05 .02 .02 .02 .05 .14

75–100 3 .05 .03 .02 .02 .05 .08 .08

Phosphorus Zone 3

0–100 58 .12 .08 .03 .06 .24 .17 .09

0–25 4 .05 .01 .03 .04 .05 .06 .06

25–50 3 .12 .09 .06 .06 .07 .22 .22

50–75 11 .09 .07 .01 .03 .07 .16 .19

75–100 40 .14 .08 .06 .08 .11 .21 .25

Phosphorus Zone 4

0–100 102 .28 .35 .07 .11 .17 .27 .54

0–25 5 .04 .63 .04 .08 .13 .16 1.51

25–50 3 1.11 1.06 .05 .05 1.13 2.16 2.16

50–75 15 .23 .17 .07 .10 .19 .38 .54

75–100 79 .25 .28 .08 .12 .17 .25 .50

 Percentiles

Percent 

agriculture Number Mean Standarddeviation 10th 25th 50th 75th 90th

Nitrogen Zone 1

0–100 21 1.08 0.38 0.58 0.91 1.10 1.26 1.61

0–25 6 .80 .35 .41 .47 .84 1.10 1.15

25–50 5 .91 .19 .66 .79 .91 1.00 1.16

50–75 4 1.09 .16 .95 .95 1.08 1.22 1.26

75–100 6 1.48 .33 .99 1.30 1.48 1.72 1.90

Nitrogen Zone 2

0–100 110 4.62 3.05 1.24 1.75 4.01 6.80 9.35

0–25 2 4.97 5.71 .93 .93 4.97 9.00 9.00

25–50 4 5.04 2.56 1.35 3.32 6.02 6.77 6.80

50–75 18 1.81 1.27 .86 .95 1.24 2.40 4.30

75–100 86 5.18 3.00 1.56 2.60 4.73 7.75 9.40

Nitrogen Zone 3

0–100 4 1.48 .29 1.21 1.29 1.42 1.67 1.88

0–25 0 -- -- -- -- -- -- --

25–50 1 1.88 -- 1.88 1.88 1.88 1.88 1.88

50–75 3 1.35 .13 1.21 1.21 1.37 1.47 1.47

75–100 0 -- -- -- -- -- -- --

Nitrogen Zone 4

0–100 17 .68 .24 .37 .51 .66 .76 1.06

0–25 8 .56 .14 .37 .45 .58 .67 .74

25–50 7 .79 .27 .34 .59 .76 1.05 1.06

50–75 2 .79 .40 .51 .51 .79 1.08 1.08

75–100 0 -- -- -- -- -- -- --
REFINING ENVIRONMENTAL NUTRIENT ZONES 31

Runoff ≥ 10.3 inches per year

Yes (31)

Yes (151 sites)

No (52)

No (83 sites)

All

Data

No (17)Yes (35)

Erodibility factor ≥ 0.22

No (21)Yes (10)

Air temperature ≥ 50°F

Group 1aGroup 1bGroup 2aGroup 2b

Clay ≥ 26 percent

Till ≥ 59 percent

No (55)Yes (96)

No (41)Yes (14)

Runoff ≥ 11.5 inches per year

No (20)Yes (76)

Coarse-grained sediments present

Group 1aGroup 1bGroup 2aGroup 2bGroup 3a Group 3aGroup 3b Group 3bGroup 4a Group 4aGroup 4b Group 4b

Figure 23. Detailed environmental phosphorus groups from regression-tree analysis results excluding land-use characteristics, subdividing the data into eight 

groups. (The numbers in brackets are the number of sites in each group. Color code for qualitative concentration range is green (lowest) followed by yellow, orange, and red (highest). The lighter shade of color corresponds to the lower qualitative concentration for each of the four original groups.)) 
32 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Detailed environmental 

phosphorus groups

(analysis excluded land-use

characteristics)

Study-area boundary

EXPLANATION

1a

1b

2a

2b

3a

3b

4a

4b

102° 84°

45°

36°

10.3 in/yr of runoff, and have a mean annual air temperature of less than 50°F (group 2a, with a mean concentration of 0.03 mg/L). The highest concentrations 

occurred at sites with more than 59 percent till, more 

than 26 percent soil clay content, and coarse-grained 

surficial deposits present (group 4b, with a mean concentration of 0.32 mg/L). By use of the GIS coverages 

for these additional variables, the four environmental 

nutrient zones (fig. 22) could be further subdivided into 

eight zones corresponding to each of these groups.

Refinement for Smaller Geographical Areas

To demonstrate refinement of the approach for 

smaller geographical areas, regression-tree analyses 

(excluding land-use characteristics) were performed on 

two subregions of the study area. 

As an example of a subdivision for an individual 

state, total phosphorus for the sites in Illinois were used 

in a regression-tree analysis, excluding land-use characteristics. This subset of 75 sites was chosen because it 

provides a good spatial coverage of an entire state. An 

asymmetrical branching of the regression tree (not 

shown) resulted in three subgroups (fig. 25). The first 

independent variable chosen was surficial-deposit type, 

till (fig. 9). The sites were partitioned into subgroups 

corresponding to those with less than 66 percent till 

(group 1, 9 sites) or greater than or equal to 66 percent 

till (66 sites). Those sites with less than 66 percent till 

were not further subdivided. The sites with more than or 

equal to 66 percent till were further partitioned on the 

basis of whether the soil slope was less than 3.2 percent 

(group 2, 19 sites) or greater than or equal to 3.2 percent 

(group 3, 47 sites). Group 1, with less than 66 percent 

till, had the lowest total phosphorus concentrations 

(mean concentration of 0.10 mg/L). These sites are 

mostly found in the extreme northern and southern part 

of Illinois (fig. 25). Group 3, with more than or equal to 

66 percent till and average soil slopes greater than 

3.2 percent, had the highest concentrations (mean concentration of 0.33 mg/L) and were scattered throughout 

the state. 

Figure 24. Location of the watersheds for the eight environmental phosphorus groups when land-use characteristics were 

excluded from the regression-tree analysis. (Color code for qualitative concentration range is green (lowest) followed by yellow, 

orange, and red (highest). The lighter shade of color corresponds to the lower qualitative concentration for each of the four 

original groups.)
WATERSHED-SPECIFIC APPROACH 33

0 50 100 MILES

0 50 100 KILOMETERS

Base from USGS digital line graphs

Environmental phosphorus groups for

Illinois Environmental Protection Agency

watersheds

(analysis excluded land-use characteristics)

EXPLANATION

1

2

3

42°

90° 84°

39°

As an example of a subdivision for a specific nutrient ecoregion, sites in the study area with watersheds 

mostly or entirely in nutrient ecoregion VI were used in 

a regression-tree analysis, excluding land-use characteristics. This subset of 83 sites was chosen because it 

had the best spatial coverage of a nutrient ecoregion in 

the study area. Two branchings of the regression tree 

(not shown) based on three independent variables 

resulted in four subgroups. The first independent variable chosen was the soil characteristic percentage of 

clay. The sites were partitioned into subgroups corresponding to those with average soil clay less than 

26 percent (25 sites) or greater than or equal to 26 percent (58 sites). Those sites with soil clay less than 

26 percent were further subdivided on the basis of 

whether the proportion of the watershed underlain by 

till was less than 88 percent (group 1, 17 sites) or greater 

than or equal to 88 percent (group 2, 8 sites). Sites with 

average soil clay content greater than or equal to 26 percent were further partitioned on the basis of whether the 

proportion of the watershed underlain by carbonate 

aquifer was less than 80 percent (group 3, 50 sites) or 

greater than or equal to 80 percent (group 4, 8 sites). 

Sites in group 2 had the lowest total phosphorus concentrations (mean concentration of 0.06 mg/L) and were 

scattered throughout nutrient ecoregion VI (fig. 26). 

Sites in group 4 had the highest concentrations (mean 

concentration of 0.75 mg/L) and were primarily in the 

eastern part of the nutrient ecoregion in northeastern 

Illinois, western Ohio, and southeastern Wisconsin. 

WATERSHED-SPECIFIC APPROACH

Although environmental nutrient zones were 

defined to be relatively homogeneous areas, small 

watersheds within each zone may still have considerably different environmental characteristics than those 

of the generalized zone. Therefore, another refinement 

of this method may be to use a watershed approach with 

classification based only on the environmental characteristics of the watershed of a specific stream. With this 

approach, available data would be used to establish 

environmental nutrient groups (such as depicted in figures 19, 20, or 23), then only the watershed characterisFigure 25. Environmental phosphorus groups for Illinois Environmental Protection Agency watersheds when land-use 

characteristics were excluded from the regression-tree analysis. (Color code for qualitative concentration range is green 

(lowest) followed by orange and red (highest).)
34 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

0 100 200 MILES

0 100 200 KILOMETERS

Base from USGS digital line graphs

Environmental phosphorus 

groups for Nutrient Ecoregion

VI watersheds

(analysis excluded land-use

characteristics)

Nutrient Ecoregion VI

Study-area boundary

EXPLANATION

1

2

3

4

45°

102° 87°

39°

tics of the site would be used to place the site into an 

environmental nutrient group. The criteria for the site 

would thus be based on the distribution of existing data 

in that group. This approach would work best for highly 

heterogeneous areas where the specific environmental 

nutrient zone is difficult to define, for streams with large 

watersheds that span more than one environmental 

nutrient zone, and for small areas of atypical environmental characteristics.

By applying a watershed-specific approach to 

the sites used in this study, rather than the generalized 

nutrient-zone approach, the overall variability in water 

quality within the groups was found to be highly similar 

to that within the nutrient zones. For total phosphorus 

concentrations, the MCV (0.94) for the environmental 

phosphorus groups was similar to that for the EPZs 

(MCV of 0.98; table 5); both subdivisions had about 

50 percent less variability than within the nutrient 

ecoregions (MCV of 1.96). For total nitrogen concentrations, the MCV for the environmental nitrogen 

groups was about the same as that for the ENZs (MCV 

of 0.59; table 5); the variability within both subdivisions 

was about the same as within the nutrient ecoregions 

(MCV of 0.59).

In general, the watershed-specific approach should 

provide more relevant nutrient criteria than any generalized approach. The difficulty in applying the watershed approach, however, is that site-specific information is needed to classify a watershed into the appropriate nutrient group and thus determine its corresponding 

criterion. Therefore, the watershed-specific approach 

may be most appropriate where detailed environmental 

information is available at the statewide scale—or even 

more local scales—and can be used to evaluate sites 

before final criteria are set and management actions are 

implemented.

SUMMARY AND CONCLUSIONS

In order to establish regional nutrient criteria that 

reflect the influence of various environmental factors, 

the USEPA has divided the country into nutrient ecoregions (areas of relatively similar environmental characteristics) and proposed different criteria for each region. 

These regions were supposed to incorporate the effects 

Figure 26. Environmental phosphorus groups for Nutrient Ecoregion VI watersheds when land-use characteristics were 

excluded from the regression-tree analysis. (Color code for qualitative concentration range is green (lowest) followed by yellow, 

orange, and red (highest).)
REFERENCES 35

of relative differences in natural and anthropogenic factors; however, the distribution in land-use practices was 

commonly the primary factor used in to delineate the 

nutrient ecoregions, such as in the Upper Midwest part 

of the United States. Therefore, because land use is 

commonly the primary factor affecting water quality, 

the differences among ecoregions strongly reflect 

anthropogenic effects. 

As part of a Regional Technical Assistance Group 

(RTAG) for the USEPA, whose role it is to examine and 

refine this basic approach, an alternative regionalization 

scheme was developed for the Upper Midwest. Through 

the use of a regression-tree analyses and a geographic 

information system (GIS), the importance of various 

environmental characteristics were determined and then 

the distributions of only the most statistically significant 

environmental characteristics were used to subdivide 

the Midwest into relatively homogeneous environmental nutrient zones. The most statistically significant 

characteristics affecting nutrient concentrations were 

percentage of agriculture (lack of forest), and surficialdeposit and climatic characteristics of the watershed. 

Environmental nutrient zones were delineated that 

incorporated land-use distributions (similar to the basic 

approach) and also delineated excluding land-use information so that the criteria should reflect the differences 

in water quality caused by natural factors.

The environmental nutrient zones reduced the variability in water quality within the defined areas better 

than the basic nutrient ecoregions by approximately 

50 percent for total phosphorus, but only slightly 

reduced the variability for total nitrogen. Various modifications were demonstrated to this refined approach, 

including development of nutrient zones for specific 

states or nutrient ecoregions and development of criteria 

as a function of stream type (described here as a function of land use in the watershed). An additional modification would be to develop nutrient criteria based only 

on the environmental characteristics of the watershed of 

a stream rather than the general environmental characteristics from the region in which the watershed is 

located.

This alternative regionalization scheme and various modifications presented in this report provide states 

methods to refine the basic nutrient criteria proposed by 

the USEPA. This alternative scheme should enable 

streams to be better managed and protected because criteria based on this regionalization should better reflect 

concentrations that are attainable given the environmental conditions where the streams are located. 

REFERENCES

Anderson, J.R., 1967, Major land uses in the United States, in 

U.S. Geological Survey, 1970: National atlas of the 

United States of America, p. 158–159.

Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 

1976, A land use and land cover classifications system 

for use with remote sensor data: U.S. Geological Survey 

Professional Paper 964, 28 p.

Breiman, L., Friedman, J.H., Olshen, R.A., and Stone C.J., 

1984, Classification and regression trees: Belmont, 

Calif., Wadsworth International Group, 358 p.

Feagus, R.G., Claire, R.W., Guptil, S.C., Anderson, K.E., and 

Hallam, C.A., 1983, Land use and land cover data—

U.S. Geological Survey Digital Cartographic Data Standards: U.S. Geological Survey Circular 895–E, 21 p.

Gebert, W.A., Graczyk, D.J., and Krug, W.R., 1987, Average 

annual runoff in the United States, 1951–80: U.S. Geological Survey Hydrologic Investigation Atlas HA-170, 

1 sheet, scale 1:2,000,000.

Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, Concepts for a National Water-Quality Assessment program: U.S. Geological Survey Circular 1021, 42 p.

Hitt, K.J., 1994, Refining 1970’s land-use data with 1990 

population data to indicate new residential development: U.S. Geological Survey Water-Resources Investigations Report 94–4250, 15 p.

Illinois Environmental Protection Agency, 1996, Illinois 

Water Quality Report, 1994–1995, volume 2: Illinois 

Environmental Protection Agency, Bureau of Water, 

IEPA/BOW/96–060b.

MathSoft, Inc., 1999, S-PLUS 2000 Guide to statistics: Seattle, Wash., Mathsoft, Inc., 650 p.

Miller, J.A., 1998, Principal aquifers of the 48 contiguous 

United States: Ground water atlas of the United States, 

scale 1:2,500,000.

Omernik, J.A., 1987, Ecoregions of the conterminous United 

States: Annals of the Association of American Geographers, v. 77, p. 118–125.

Omernik, J.A., 1995, Ecoregions: A spatial framework for 

environmental management, in Davis, W.S., and Simon, 

T.P., eds., Biological assessment and criteria—Tools for 

water resources planning and decision making: Boca 

Raton, Fla., Lewis Publishers, p. 49–66. 

Omernik, J.A., 2000, Draft aggregations of level III Ecoregions for the National Nutrient Strategy: accessed 

November 22, 2000, at URL 

http://www.epa.gov/ost/standards/ecomap.html.

Oregon Climate Service, 2000, Climate mapping with 

PRISM: Oregon State University, accessed October 6, 

2000, at URL 

http://www.ocs.orst.edu/prism/prism_new.html.
36 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

SAS Institute, 1989, SAS/STAT User’s guide, version 6, 4th 

ed.: Cary, N.C., SAS Institute Inc. [variously paged].

Schwarz, G.E., and Alexander, R.B., 1995, State Soil Geographic (STATSGO) data base for the conterminous 

United States: U.S. Geological Survey Open-File Report 

95–449.

Seaber, P.R., Kapinos, E.P., and Knapp, G.L., 1984, State 

hydrologic unit maps: U.S. Geological Survey Open-File 

Report 84–708, 198 p.

Soller, D.R., and Packard, P.H., 1998, Digital representation of 

a map showing the thickness and character of Quaternary 

sediments in the glaciated United States east of the Rocky 

Mountains: U.S. Geological Survey Digital Data Series 

DDS–38.

U.S. Environmental Protection Agency, 1996, National water 

quality inventory report to Congress: Office of Water, 

U.S. Environmental Protection Agency, EPA–R–97–008.

____1998, National strategy for the development of regional 

nutrient criteria: Office of Water, ERP–822–R–98–002.

____2000a, Nutrient criteria technical guidance manual: 

lakes and reservoirs: EPA–822–B–00–001.

____2000b, Nutrient criteria technical guidance manual: 

rivers and streams: EPA–822–B–00–002.

____2000c, Basins data download: Accessed October 6, 2000, 

at URL http://www.epa.gov/ost/basins/gisdata.html.

U.S. Geological Survey, 1999, Ecological status and trends of 

the Upper Mississippi River System 1998—A report of 

the Long Term Resource Monitoring Program: U.S. Geological Survey, Upper Midwest Environmental Sciences 

Center, LaCrosse, Wis., LTRMP 99–T001, 236 p.

U.S. Geological Survey, 2000a, NAWQA Water Quality Data 

Warehouse: Accessed October 6, 2000, at URL 

http://infotrek.er.usgs.gov.

____2000b, Upper Mississippi Basin Loading Database (Sediment and Nutrients): Accessed October 6, 2000, at URL 

http://www.umesc.usgs.gov/data_library/sediment_

nutrients/sediment_nutrient_page.html.

____2000c, National hydrography dataset: Accessed 

October 6, 2000, at URL 

http://mapping.usgs.gov/mac/isb/pubs/factsheets/ 

fs10699. html.

____2000d, Climate divisions: Accessed October 6, 2000, at 

URL http://water.usgs.gov/GIS/metadata/usgswrd/ 

climate_div.html.

____2000e, U.S. Geological Survey land-use and land-cover 

data: Accessed October 6, 2000, at URL

http://edcwww.cr.usgs.gov/glis/hyper/guide/1_250_lulc.

____2000f, State Soil Geographic (STATSGO) data base for 

the conterminous United States: Accessed October 6, 

2000, at URL 

http://water.usgs.gov/GIS/metadata/usgswrd/ussoils. html.

____2000g, Principal Aquifers—map layer description file: 

Accessed October 6, 2000, at URL

http://www-atlas.usgs.gov/aquifersm.html.
APPENDIXES 1–2 37

APPENDIXES 1–2
3

8AnAlternativeRegionalizationSchemeforDefiningNutrientCriteriaforRiversandStreams

Appendix 1a. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

Climate Land-use characteristics (percent)

Station

number

ycnegagnilpmaS

ydutsSGSUro

eman

aerA

im(

2

)

ffonuR

)ry/ni(

noitatipicerP

)ry/ni(

erutarepmeT

)F°(

nabrU

erutlucirgA

dnalegnaR

tseroF

retaW

detseroF

dnaltew

detserofnoN

dnaltew

nerraB

dnal

03015795 almn 20 25.5 42.5 47.1 0.0 0.7 0.0 99.3 0.0 0.0 0.0 0.003024000 almn 1,067 24.0 42.5 47.1 2.8 63.3 .0 31.5 .5 1.5 .3 .203037350 almn 33 24.0 42.5 49.8 .0 50.2 .0 49.6 .0 .0 .0 .203040000 almn 452 24.0 42.5 49.8 2.9 33.3 .0 58.4 .6 .0 .0 4.803049646 almn 27 18.0 37.5 49.8 15.3 47.5 .0 36.1 .0 .0 .0 1.203070350 almn 1,128 30.0 53.6 49.1 .5 10.4 .0 86.6 .4 .0 .9 1.103072000 almn 227 18.0 42.5 50.2 .7 24.9 .0 73.8 .1 .0 .0 .503083500 almn 1,710 26.0 43.0 49.1 3.5 34.6 .0 59.4 .8 .1 .0 1.640001 umbl 337 5.5 31.0 44.1 8.4 82.3 .0 4.6 .9 2.9 .3 .640007 umbl 1,481 6.0 31.9 44.7 2.4 88.6 .0 4.3 2.4 .5 1.6 .250001 umbl 1,420 6.0 32.5 44.2 2.9 90.0 .0 6.7 .2 .0 .0 .150003 umbl 323 6.0 32.5 44.2 .8 79.6 .0 19.2 .0 .0 .3 .080001 umbl 477 8.5 32.5 44.5 8.6 39.9 .0 49.2 .2 1.3 .5 .280003 umbl 1,665 7.0 32.5 44.2 .8 77.4 .0 21.5 .0 .1 .1 .180007 umbl 141 8.5 32.5 45.7 .5 51.0 .0 48.2 .0 .0 .3 .090001 umbl 188 8.5 32.5 46.2 1.4 56.6 .0 38.9 .1 .1 2.8 .090003 umbl 992 8.0 32.5 45.9 .6 83.4 .0 16.0 .0 .0 .0 .0130001 umbl 1,860 9.0 33.7 47.7 .8 94.1 .0 4.9 .1 .0 .0 .1130003 umbl 256 9.0 32.6 48.8 .8 87.2 .0 10.5 .2 .0 .0 1.4200003 umbl 3,444 5.5 32.3 48.0 1.0 96.0 .1 2.3 .3 .0 .1 .2240001 umbl 2,484 8.5 37.6 53.9 1.0 87.4 .0 11.4 .1 .0 .0 .1260001 umbl 1,227 8.0 37.5 53.9 1.1 79.9 .1 18.7 .1 .0 .0 .1280001 umbl 3,781 9.5 41.7 55.3 1.5 30.0 .0 67.4 .3 .0 .0 .905420680 eiwa 346 7.5 32.5 46.1 1.2 92.5 .0 1.9 .1 3.5 .7 .105449500 eiwa 418 6.0 32.5 46.0 1.1 97.1 .0 .7 .2 .0 .7 .2Appendix 1a. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site[almn, Allegheny and Monongahela River Basins NAWQA study unit; eiwa, Eastern Iowa Basins NAWQA study unit; iepa, Illinois Environmental Protection Agency; kana, KanawhaNew River Basins NAWQA study unit; leri, Lake Erie-Lake Saint Clair Drainages NAWQA study unit; lirb, Lower Illinois River Basin NAWQA study unit; miam, Great and Little Miami River Basins NAWQA study unit; redn, Red River of the North Basin NAWQA study unit; stx, U.S. Geological Survey, St. Croix River Watershed study; uirb; Upper Illinois River Basin NAWQA study unit; umbl, U.S. Geological Survey, Upper Mississippi River System study; umis, Upper Mississippi River Basin NAWQA study unit; wdnr, Wisconsin Department of Natural Resources; whit, White River Basin NAWQA study unit; wmic, Western Lake Michigan Drainages NAWQA study unit; mi2, square miles; in/yr, inches per year; °F, degrees Fahrenheit]
APPENDIX 1a 39

Climate Land-use characteristics (percent)

Station

number

Sampling agency 

or USGS study 

name

Area

(mi

2

)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F)°(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1a05451210 eiwa 224 6.5 32.5 47.9 0.4 97.8 0.0 1.7 0.0 0.1 0.1 0.005455100 eiwa 201 8.0 37.5 49.1 1.4 95.8 .0 2.8 .0 .0 .0 .005461390 eiwa 124 7.0 32.5 46.0 .2 97.7 .0 1.6 .0 .0 .4 .005464220 eiwa 299 7.5 32.5 47.9 .8 98.0 .0 1.3 .0 .0 .0 .005474000 eiwa 4,311 7.5 35.1 49.6 1.5 93.1 .0 5.2 .1 .0 .0 .1BPK07 iepa 442 10.0 37.5 51.3 .8 96.7 .0 2.2 .1 .0 .0 .2BPJ07 iepa 135 10.0 37.5 51.3 4.8 95.1 .0 .0 .0 .0 .0 .0BPJC06 iepa 73 10.0 37.5 51.3 12.6 86.7 .0 .5 .0 .0 .0 .1BO07 iepa 196 10.0 37.5 51.3 1.3 96.4 .0 2.2 .0 .0 .0 .0BN01 iepa 255 11.0 41.2 53.5 .7 97.5 .0 1.9 .0 .0 .0 .0BM02 iepa 59 11.0 42.5 53.4 6.9 79.2 .0 13.3 .5 .0 .0 .0BE14 iepa 181 10.0 37.5 51.9 3.4 96.3 .0 .3 .0 .0 .0 .0BEF05 iepa 304 11.0 42.3 53.8 1.5 83.7 .0 14.5 .0 .0 .0 .2BC02 iepa 226 13.0 42.5 55.3 1.3 91.0 .0 7.6 .1 .0 .0 .0C21 iepa 237 9.5 37.6 53.8 1.3 88.3 .0 9.2 1.1 .0 .0 .0CD01 iepa 254 11.0 42.5 55.2 1.6 87.2 .0 11.1 .0 .0 .0 .0CA06 iepa 208 10.0 42.5 53.8 .2 71.0 .0 27.8 .4 .0 .0 .1ATGC01 iepa 82 13.0 42.5 56.3 1.0 64.0 .4 15.2 .4 .2 .9 17.9ATF04 iepa 249 13.0 42.5 56.3 1.2 85.6 .2 12.2 .1 .5 .0 .0AK02 iepa 45 15.0 42.5 56.3 .2 29.5 .3 66.9 3.1 .0 .0 .0AD02 iepa 242 15.0 42.5 56.3 1.4 70.2 .4 24.4 .1 2.6 .3 .8MJ01 iepa 272 9.0 36.7 49.0 .6 90.9 .0 7.3 .3 .0 .0 .9PWN01 iepa 194 9.0 32.5 49.0 1.6 96.2 .0 1.9 .0 .0 .0 .3PQ10 iepa 225 9.0 37.5 49.4 3.6 89.3 .0 5.7 .2 .2 .3 .7PQC06 iepa 396 9.5 37.5 49.4 3.9 94.3 .0 1.3 .1 .0 .0 .3PQB02 iepa 134 9.5 37.5 49.1 1.2 97.7 .0 .9 .0 .0 .1 .1PL03 iepa 196 9.5 37.5 49.0 2.2 95.7 .0 1.7 .2 .0 .0 .2PH16 iepa 146 9.0 37.0 49.0 1.1 97.1 .0 1.7 .0 .0 .0 .1PE05 iepa 241 9.0 37.5 49.0 1.2 96.6 .0 2.0 .1 .0 .0 .0PB02 iepa 147 9.0 37.5 49.0 2.4 89.7 .0 7.2 .3 .0 .1 .2LF01 iepa 434 9.0 37.2 49.0 1.3 91.6 .0 6.7 .1 .0 .0 .2LD02 iepa 432 9.0 37.5 51.1 3.6 92.3 .0 3.9 .1 .0 .0 .1

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
40 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Climate Land-use characteristics (percent) Station number

Sampling agency 

or USGS study 

name

Area

(mi2)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F) °(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1aKI02 iepa 354 8.5 37.5 51.8 .6 90.9 0.0 8.5 0.0 0.0 0.0 0.0KCA01 iepa 140 8.5 37.5 53.5 1.4 90.9 .0 7.7 .1 .0 .0 .0HBD04 iepa 102 12.0 37.5 49.5 39.2 48.1 .0 7.6 .7 1.4 .7 2.3GG02 iepa 105 11.0 37.5 49.4 16.8 72.3 .0 7.8 .3 .9 .0 1.8GB10 iepa 214 10.5 37.5 49.4 48.4 36.9 .0 5.4 .6 .0 .2 8.5DW01 iepa 177 10.0 37.5 49.4 .7 98.3 .0 .8 .0 .0 .0 .1DV04 iepa 451 9.5 37.5 50.6 1.3 95.8 .0 .4 .2 .0 .0 2.3DTD02 iepa 73 10.0 37.5 49.4 8.0 86.2 .0 2.6 .3 .0 .5 1.8DTB01 iepa 90 9.5 37.5 49.4 4.0 91.8 .0 3.4 .6 .0 .0 .3DS07 iepa 1,230 9.5 37.3 51.0 1.5 97.4 .0 .7 .1 .0 .0 .2DR01 iepa 119 9.5 37.5 49.4 2.9 92.1 .0 3.6 .2 .0 .0 .9DL01 iepa 302 9.0 37.5 51.7 9.8 73.9 .0 10.9 .1 .0 .0 5.1DJ06 iepa 196 9.0 37.5 50.6 1.9 94.0 .0 1.3 .0 .0 .0 2.8DJB18 iepa 32 8.5 37.5 51.8 9.0 51.1 .0 11.3 .1 .0 .0 28.5E29 iepa 238 9.5 37.5 51.4 1.2 98.4 .0 .3 .0 .0 .0 .1EOH01 iepa 289 9.5 37.8 53.3 1.0 97.5 .0 1.5 .0 .0 .0 .0EL01 iepa 106 9.0 37.5 53.5 6.6 91.2 .0 1.7 .0 .0 .0 .4EIG01 iepa 213 9.0 37.5 51.9 .9 98.6 .0 .1 .0 .0 .0 .3EIE04 iepa 224 9.5 37.5 51.7 2.6 96.0 .0 .9 .2 .0 .0 .3EID04 iepa 331 9.0 37.5 51.7 5.6 92.0 .0 1.6 .2 .0 .0 .3DH01 iepa 160 8.5 37.5 51.8 .3 68.6 .0 29.7 .1 .0 .0 1.3DF04 iepa 167 9.0 37.5 53.5 .8 94.1 .0 5.1 .0 .0 .0 .0DE01 iepa 339 8.5 37.5 51.9 .5 75.4 .0 24.0 .1 .0 .0 .0DD04 iepa 109 9.0 37.5 53.5 5.9 91.7 .0 2.1 .0 .0 .0 .1DB01 iepa 405 9.0 37.5 53.5 .9 86.0 .0 12.3 .1 .7 .0 .0DA04 iepa 299 9.0 37.5 53.5 1.2 87.5 .0 10.5 .4 .0 .0 .3JR02 iepa 121 9.0 37.5 53.5 10.3 68.0 .0 21.0 .3 .0 .0 .4JQ05 iepa 211 9.0 37.5 53.5 4.3 71.0 .0 23.6 .5 .0 .0 .5O31 iepa 113 9.5 37.5 51.3 11.8 87.7 .0 .0 .1 .0 .0 .3OU01 iepa 54 9.5 37.5 53.8 .0 99.7 .0 .2 .0 .0 .0 .0OT02 iepa 132 9.5 37.5 52.2 .8 98.8 .0 .4 .0 .0 .0 .0

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
APPENDIX 1a 41

Climate Land-use characteristics (percent)

Station

number

Sampling agency 

or USGS study 

name

Area

(mi

2

)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F)°(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1aOQ01 iepa 113 9.5 38.8 53.7 2.3 81.0 0.0 16.3 0.4 0.0 0.0 0.0ON01 iepa 78 9.5 42.5 53.8 .2 66.2 .0 33.6 .0 .0 .0 .0OL02 iepa 152 9.5 37.5 53.7 .6 87.3 .0 11.9 .0 .0 .0 .2OK01 iepa 111 9.5 42.5 53.8 1.4 7.5 .0 27.6 .2 .0 .0 .0OKA01 iepa 37 9.5 42.5 53.8 3.2 85.3 .0 11.3 .1 .0 .0 .0OJ08 iepa 254 10.0 42.4 54.5 5.7 73.7 .0 19.4 .7 .1 .0 .0OI09 iepa 285 9.0 38.9 53.5 2.5 85.7 .0 10.8 .9 .0 .0 .1OH01 iepa 115 9.0 37.5 54.5 3.1 89.9 .0 6.8 .1 .0 .0 .0OD06 iepa 155 9.0 37.5 53.5 2.8 83.2 .0 13.3 .1 .0 .0 .0OC04 iepa 125 9.0 37.7 56.3 10.9 78.0 .0 6.2 .4 .0 .0 4.3OZC01 iepa 61 10.0 42.5 56.3 2.3 77.6 4.0 10.2 .1 .0 .0 5.8II03 iepa 109 10.5 42.5 56.3 4.5 73.3 .0 20.8 .3 .0 .0 1.1NK01 iepa 94 10.0 42.5 56.3 .5 76.7 .0 22.5 .2 .0 .0 .1NJ07 iepa 88 10.5 42.5 56.2 9.0 64.4 .0 25.7 .8 .0 .0 .1NH06 iepa 152 11.5 42.5 56.3 1.4 85.2 .1 11.7 .1 1.1 .0 .0NE05 iepa 253 11.0 42.5 56.3 2.7 83.1 .0 11.0 .6 1.4 .0 1.1NC07 iepa 476 10.5 42.2 56.3 .8 76.8 .0 18.5 .4 .0 .0 3.303167000 kana 258 17.0 38.6 52.8 4.2 46.8 .0 48.8 .1 .0 .0 .203170000 kana 307 18.5 43.1 53.0 .5 52.4 .0 46.8 .0 .0 .0 .103178000 kana 200 17.0 38.5 52.6 6.7 14.1 .0 75.8 .2 .0 .0 3.203183000 kana 85 18.0 38.7 52.5 .3 28.0 .0 71.4 .2 .0 .0 .003186500 kana 127 40.0 55.0 49.1 .0 3.1 .0 95.9 .0 .0 .0 .903187500 kana 80 40.0 55.0 49.1 .0 .2 .0 99.3 .0 .0 .0 .503191500 kana 40 25.0 42.5 49.1 1.1 8.9 .0 88.8 .0 .0 .0 1.203198350 kana 64 20.0 42.5 49.3 1.1 1.6 .0 89.7 .1 .0 .0 7.504159492 leri 462 8.5 30.1 46.9 1.5 89.7 .0 6.8 .1 .7 .9 .304161820 leri 310 10.0 32.5 48.6 23.2 51.3 .0 14.9 4.9 1.9 .5 3.404175600 leri 128 9.0 32.5 48.0 5.3 70.0 .1 12.3 4.5 3.5 3.7 .704178000 leri 618 10.5 34.9 48.9 1.4 90.1 .0 6.8 1.0 .0 .3 .304186500 leri 331 11.0 37.0 50.1 2.8 95.4 .0 1.5 .0 .0 .0 .2

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
42 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Climate Land-use characteristics (percent) Station number

Sampling agency 

or USGS study 

name

Area

(mi2)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F) °(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1a04208504 leri 789 17.0 38.6 49.2 29.8 47.2 .0 18.0 1.5 2.3 .0 1.204211820 leri 553 20.0 39.4 49.2 2.0 72.8 .0 16.3 .3 8.5 .0 .104213500 leri 436 23.0 42.4 46.1 1.5 50.3 .0 47.0 .3 .6 .0 .205567000 lirb 94 9.5 37.5 51.6 1.0 98.8 .0 .2 .0 .0 .0 .005568800 lirb 63 8.5 37.5 50.6 2.2 96.5 .0 1.1 .0 .0 .0 .105584500 lirb 655 8.5 37.5 51.8 1.2 90.0 .0 8.5 .1 .0 .0 .203245500 miam 1,203 14.0 40.4 53.0 10.5 83.3 .0 5.1 .6 .0 .0 .403267900 miam 310 11.5 37.5 50.3 4.3 87.6 .0 7.8 .2 .0 .0 .203275000 miam 529 13.0 40.2 50.4 3.1 93.1 .0 3.6 .0 .0 .0 .1393944084120700 miam 20 13.5 37.5 53.2 52.4 44.9 .0 2.7 .0 .0 .0 .0395355084173600 miam 646 12.5 37.5 50.5 2.9 96.1 .0 .7 .1 .0 .0 .2395457084095100 miam 1,142 12.0 37.5 50.3 4.4 92.3 .0 2.1 .9 .0 .0 .205030150 redn 336 3.5 23.2 39.8 .2 25.2 .0 52.7 13.1 4.3 4.3 .105051300 redn 1,553 1.5 22.5 42.7 .4 93.9 .2 .1 2.4 .0 3.0 .005058700 redn 4,493 .3 17.6 40.0 .4 85.5 10.1 .8 1.5 .0 1.6 .005062500 redn 929 3.0 22.9 38.9 .2 50.1 .0 36.8 4.8 3.2 4.7 .205079000 redn 5,268 5.0 22.9 38.8 .3 42.6 .0 23.3 10.0 12.2 11.5 .105082625 redn 254 .8 17.5 38.5 .5 96.2 .9 1.0 .5 .0 .7 .105085900 redn 218 2.0 19.3 38.9 .6 91.5 .0 4.4 .0 1.6 1.6 .305099600 redn 3,202 .5 17.5 38.5 .3 88.8 1.6 6.0 1.3 .0 2.0 .005112000 redn 1,556 3.0 21.5 38.9 .1 54.2 .0 15.3 .3 14.4 15.6 .105333579 stx 95 12.5 32.5 41.5 .0 2.4 .0 74.8 .3 19.6 2.9 .005335151 stx 181 12.0 32.5 41.7 .0 3.7 .0 74.7 .1 16.4 5.1 .005338955 stx 84 10.0 32.5 41.4 .4 55.2 .0 37.7 4.5 1.3 .9 .005340390 stx 133 9.0 32.5 41.4 .5 47.2 .0 37.5 2.6 4.3 8.0 .005341500 stx 549 8.0 32.5 41.6 .5 62.7 .0 24.4 5.2 5.2 1.9 .105341752 stx 278 7.0 32.5 44.1 .8 89.3 .0 8.0 .9 .3 .7 .005342000 stx 173 5.5 32.5 44.5 2.3 92.9 .0 4.6 .2 .0 .0 .005526000 uirb 2,087 11.0 37.5 50.7 1.2 95.4 .0 3.0 .1 .0 .1 .105527800 uirb 123 8.5 32.8 46.8 6.1 85.7 .0 4.4 .8 .3 1.6 1.105531500 uirb 115 11.0 37.5 49.4 70.5 18.1 .0 5.8 .9 .0 .0 4.6

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
APPENDIX 1a 43

Climate Land-use characteristics (percent)

Station

number

Sampling agency 

or USGS study 

name

Area

(mi

2

)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F)°(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1a05548105 uirb 85 9.0 37.5 49.3 5.6 88.2 .0 3.4 .3 .1 1.4 1.005267000 umis 11,629 6.0 27.3 40.4 .5 25.1 .0 48.7 8.6 11.0 5.2 .905276005 umis 232 4.5 27.5 43.1 .3 90.2 .0 2.1 1.7 .8 4.8 .105288705 umis 28 5.5 27.5 41.7 38.7 45.7 .0 1.5 4.0 .0 4.0 6.205320270 umis 130 5.5 32.4 44.9 .2 94.4 .0 .5 .6 1.5 2.4 .005330902 umis 45 5.5 27.5 41.7 70.0 16.2 .0 5.1 3.7 .0 4.1 .805331833 umis 128 14.0 32.5 41.4 .7 1.8 .0 77.9 6.3 12.3 .1 1.004027595 wdnr 979 14.5 33.2 41.0 .1 9.3 .0 80.4 1.2 8.6 .4 .0040734644 wdnr 37 9.0 32.5 44.8 7.3 85.6 .0 1.1 .2 1.6 3.8 .404085463 wdnr 77 8.0 32.5 44.8 2.8 87.9 .0 7.9 .5 .5 .1 .304086500 wdnr 122 8.0 32.5 46.7 3.6 82.6 .0 8.0 1.9 3.1 .3 .5053230 wdnr 4 9.0 30.0 44.8 87.0 5.4 .0 7.7 .0 .0 .0 .0053232 wdnr 18 9.0 30.1 44.8 .6 98.4 .0 1.1 .0 .0 .0 .0053511 wdnr 20 8.0 30.2 44.8 1.1 89.8 .0 8.8 .0 .0 .0 .405368000 wdnr 420 8.0 32.5 43.1 .5 77.0 .0 19.6 1.0 1.2 .6 .005378185 wdnr 8 6.0 32.5 44.5 .0 33.0 .0 67.0 .0 .0 .0 .005379430 wdnr 8 6.0 32.5 44.5 .0 47.2 .0 52.8 .0 .0 .0 .005379472 wdnr 10 6.0 32.5 44.4 .0 34.5 .0 65.5 .0 .0 .0 .005406460 wdnr 13 8.5 32.5 46.6 2.6 67.5 .0 28.4 .2 .0 .0 1.305407500 wdnr 130 9.0 32.5 44.6 .4 62.6 .0 36.9 .0 .0 .0 .005427950 wdnr 11 8.5 32.5 46.6 6.0 93.8 .0 .2 .0 .0 .0 .005429580 wdnr 15 8.0 32.5 46.6 4.2 93.3 .0 1.0 .2 .0 .0 1.305431014 wdnr 9 8.5 37.5 46.7 2.8 97.0 .0 .0 .2 .0 .0 .005431018 wdnr 9 8.5 37.0 46.7 1.4 93.1 .0 30.4 .4 .0 1.7 .005433510 wdnr 6 9.0 32.5 46.2 .0 72.2 .0 27.8 .0 .0 .0 .0063035 wdnr 192 8.5 32.5 44.1 1.0 68.2 .0 3.8 .0 .0 .0 .0063037 wdnr 6 6.0 32.5 44.5 .0 36.1 .0 63.9 .0 .0 .0 .0103094 wdnr 682 10.0 32.5 40.7 .9 62.7 .0 31.8 .4 3.8 .1 .2103105 wdnr 61 9.5 32.5 40.7 1.5 80.1 .0 18.3 .1 .0 .0 .1113086 wdnr 154 8.0 32.5 46.6 .8 90.1 .0 3.2 .4 2.2 3.1 .1123023 wdnr 52 8.5 32.5 46.2 .1 54.4 .0 44.0 .0 1.5 .0 .0

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
44 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Climate Land-use characteristics (percent) Station number

Sampling agency 

or USGS study 

name

Area

(mi2)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F) °(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1a133024 wdnr 146 8.0 32.5 46.6 4.4 89.2 .0 2.1 .8 2.1 1.0 .3133119 wdnr 137 8.5 32.5 46.6 2.4 90.2 .0 6.5 .2 .0 .2 .5133336 wdnr 10 8.5 32.5 46.6 1.3 81.5 .0 16.7 .5 .0 .0 .0133337 wdnr 6 8.5 32.5 46.6 .0 55.4 .0 44.5 .0 .0 .0 .0143012 wdnr 3 8.0 32.5 46.6 3.1 95.1 .0 .0 .0 .0 .0 1.8163002 wdnr 180 14.0 32.5 41.4 .1 2.6 .0 85.0 1.7 1.5 .0 .0183042 wdnr 57 9.0 32.5 41.5 1.2 75.1 .0 23.6 .2 .0 .0 .0183064 wdnr 204 9.5 32.5 40.9 .9 61.0 .0 36.1 .2 1.1 .4 .3183077 wdnr 215 9.5 32.5 40.7 .1 38.7 .0 51.5 .4 7.9 1.5 .0223248 wdnr 42 9.0 32.5 46.2 .0 99.2 .0 .8 .0 .0 .0 .0263001 wdnr 269 14.5 33.6 40.5 1.8 5.2 .0 76.8 2.2 14.0 .0 .0273015 wdnr 48 9.0 32.5 44.5 .8 64.5 .0 34.7 .0 .0 .0 .0273016 wdnr 26 9.0 32.5 42.8 .8 38.1 .0 60.4 .2 .5 .0 .0273031 wdnr 5 9.0 32.5 44.5 .0 1.2 .0 96.1 .8 .0 1.9 .0273032 wdnr 298 9.0 32.5 43.1 .0 14.7 .0 68.1 .7 11.1 5.4 .0273034 wdnr 2 9.0 32.5 44.5 .0 33.2 .0 66.8 .0 .0 .0 .0313023 wdnr 25 9.0 32.5 44.8 .3 78.3 .0 4.2 .0 15.3 1.8 .0313038 wdnr 133 8.0 32.5 44.8 1.0 89.6 .0 6.3 .0 2.6 .0 .4363070 wdnr 130 8.0 30.6 44.8 1.4 82.8 .0 8.7 .2 6.8 .0 .1363071 wdnr 167 8.0 31.4 44.8 2.0 86.8 .0 9.4 .1 1.1 .0 .6363218 wdnr 108 8.0 30.8 44.8 .9 87.1 .0 6.0 .0 5.8 .0 .2373047 wdnr 55 11.0 32.5 40.7 .7 59.2 .0 36.9 .1 2.5 .0 .5373325 wdnr 220 10.5 32.5 40.7 1.1 82.1 .0 16.1 .1 .6 .0 .0383001 wdnr 1,165 11.0 32.5 42.2 1.0 20.7 .0 54.7 1.8 2.9 .8 .2413006 wdnr 11 8.5 32.5 46.7 94.3 5.3 .0 .0 .0 .0 .0 .5413008 wdnr 18 8.0 32.5 46.7 20.7 74.5 .0 3.5 .0 .0 .0 1.3413011 wdnr 2 8.0 32.5 46.7 81.3 17.5 .0 .0 .0 .0 .0 1.2433002 wdnr 1,036 11.0 32.5 42.2 .7 28.6 .0 49.4 1.8 17.8 1.2 .5453030 wdnr 185 10.0 32.4 43.5 .9 77.4 .0 7.8 .2 11.6 1.8 .3483043 wdnr 94 6.0 32.5 44.5 1.2 92.7 .0 6.1 .0 .0 .0 .0503069 wdnr 8 10.5 32.5 44.0 .0 90.0 .0 3.4 .0 1.9 3.0 1.7

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
APPENDIX 1a 45

Climate Land-use characteristics (percent)

Station

number

Sampling agency 

or USGS study 

name

Area

(mi

2

)

Runoff

(in/yr)

Precipitation

(in/yr)

Temperature

F)°(

Urban

Agriculture

Rangeland

Forest

Water

Forested 

wetland

Nonforested

wetland

Barren

land

Appendix 1a523061 wdnr 204 8.5 32.5 46.7 24.8 70.7 .0 3.2 .2 .2 .1 .9573076 wdnr 386 9.0 32.5 45.3 1.0 72.3 .0 22.8 .5 2.1 .6 .7603049 wdnr 80 8.0 34.9 44.8 4.8 74.6 .0 14.6 .8 2.6 1.8 .8603304 wdnr 94 8.0 33.7 45.0 1.1 92.7 .0 4.2 .3 1.5 .0 .2603326 wdnr 9 8.0 36.2 44.8 6.4 84.5 .0 6.6 1.6 .7 .0 .3683001 wdnr 60 8.0 32.5 46.7 30.4 64.8 .0 3.1 .0 .8 .0 .9683271 wdnr 27 8.0 32.5 46.7 2.6 86.9 .0 2.6 .6 4.5 2.8 .0693021 wdnr 516 11.0 32.5 43.5 .5 48.4 .0 35.8 1.1 12.5 1.6 .103353637 whit 17 13.5 37.5 51.3 44.5 55.1 .0 .5 .0 .0 .0 .003360895 whit 56 14.5 42.5 55.0 1.8 93.7 .0 4.2 .0 .0 .0 .303366500 whit 292 15.5 42.5 54.3 3.9 71.3 .0 24.4 .1 .0 .0 .203373530 whit 35 16.0 42.5 53.9 .6 94.2 .0 5.2 .0 .0 .0 .0391732085414401 whit 88 14.0 42.5 51.3 .7 98.4 .0 .8 .0 .0 .0 .0393306086585201 whit 318 13.0 42.5 51.5 1.2 83.2 .0 14.4 .2 .0 .0 .9394340085524601 whit 95 13.0 41.1 51.2 3.4 95.5 .0 .9 .2 .0 .0 .104062085 wmic 44 17.0 35.5 40.4 .0 .0 .0 87.7 2.3 9.9 .0 .004063700 wmic 140 13.0 32.5 42.2 .1 3.0 .0 61.1 .7 33.6 1.5 .004071795 wmic 34 10.0 32.5 42.2 .2 86.0 .0 4.4 .0 9.3 .0 .104072050 wmic 95 9.0 31.5 44.8 .6 89.3 .0 4.6 .0 4.9 .0 .604080798 wmic 44 11.0 32.5 44.0 .2 58.3 .0 31.5 1.3 6.5 2.3 .004085109 wmic 47 8.0 30.0 44.8 .8 91.5 .0 5.1 .0 2.3 .2 .1040863075 wmic 51 8.0 33.5 44.8 1.1 88.1 .0 6.4 .3 3.8 .2 .1040869415 wmic 10 8.0 32.5 46.7 96.1 3.9 .0 .0 .0 .0 .0 .0

. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
46 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Appendix 1b. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site Surficial-deposit characteristics Soil characteristics Principal aquifer type Station number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility 

)f (K

Organic-matter

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

03015795 0.0 0.0 0.0 0.0 100.0 0.0 25.0 19.0 0.24 0.5 4.53 14.7 31.2 0.0 68.8 0.003024000 17.9 13.0 69.1 .0 .0 .0 62.5 18.3 .40 .8 2.04 4.3 3.0 .0 22.0 75.003037350 .0 .0 .0 .0 100.0 .0 25.0 23.9 .30 .7 2.53 20.9 100.0 .0 .0 .003040000 .0 .0 .0 .0 100.0 .0 25.0 23.5 .26 .5 2.76 17.1 89.9 .0 10.1 .003049646 .0 .0 .0 .0 100.0 .0 25.0 25.2 .28 .5 1.65 19.1 100.0 .0 .0 .003070350 .0 .0 .0 .0 100.0 .0 25.0 20.0 .25 .6 3.88 29.2 28.1 .0 4.7 31.103072000 .0 .0 .0 .0 100.0 .0 25.0 31.9 .33 .5 1.67 21.3 100.0 .0 .0 .003083500 .0 .0 .0 .0 100.0 .0 25.0 22.3 .26 .6 3.49 17.3 74.6 .0 19.4 6.040001 6.5 .0 36.8 .0 2.7 .0 128.3 14.9 .24 2.0 6.40 5.1 100.0 .0 .0 .040007 11.5 .0 78.0 1.9 8.4 .2 120.3 23.6 .29 3.2 2.45 4.9 58.4 25.7 .0 15.950001 6.6 .0 49.3 34.3 9.8 .0 59.8 22.2 .32 1.3 2.19 6.4 42.0 40.5 .0 17.550003 2.1 .0 .0 76.3 21.6 .0 25.6 22.7 .34 .8 2.09 11.0 65.6 15.8 .0 18.780001 21.3 .0 .0 .0 78.7 .0 37.5 15.5 .22 .4 4.62 14.3 100.0 .0 .0 .080003 4.1 .0 21.4 42.9 31.6 .0 38.0 21.7 .33 1.1 2.32 10.3 45.5 40.0 .0 14.580007 1.5 .0 .0 .0 98.5 .0 25.9 22.8 .31 .4 1.95 15.5 100.0 .0 .0 .090001 2.4 .0 .0 .0 97.6 .0 25.4 24.6 .33 .5 1.71 13.6 100.0 .0 .0 .090003 3.7 .0 38.2 3.7 27.4 .0 38.5 24.2 .33 1.1 1.91 10.2 31.9 32.6 .0 35.5130001 2.8 .0 87.1 4.5 5.6 .0 47.6 23.8 .33 1.2 2.16 8.1 .0 99.4 .0 .6130003 1.5 1.2 4.9 .0 92.4 .0 25.8 29.2 .33 .8 1.21 10.0 .0 1.1 .0 98.9200003 9.8 .0 90.2 .0 .0 .0 181.0 26.8 .31 2.6 1.49 4.0 51.9 .0 2.5 45.6240001 4.9 .0 88.7 3.4 .0 .0 48.2 37.1 .34 .6 .47 7.8 .0 .0 24.4 75.6260001 4.6 .0 62.7 32.7 .0 .0 37.3 39.5 .33 .6 .46 9.1 4.9 .0 51.2 43.9280001 .0 .0 .0 .0 100.0 .0 25.0 35.9 .35 .4 1.68 12.1 .0 90.6 .0 9.405420680 21.1 .0 78.9 .0 .0 .0 121.8 21.4 .28 2.1 3.21 2.2 .0 100.0 .0 .005449500 29.6 .0 70.4 .0 .0 .0 120.2 26.3 .30 2.8 1.75 2.5 5.6 60.4 33.8 .205451210 7.9 .0 92.1 .0 .0 .0 153.8 25.6 .30 2.9 1.69 2.6 .0 .0 83.0 17.005455100 3.3 .0 96.7 .0 .0 .0 253.2 32.2 .37 1.3 1.07 6.5 .0 32.7 .0 67.305461390 .0 .0 100.0 .0 .0 .0 41.7 23.6 .31 2.0 2.30 2.1 .0 100.0 .0 .0Appendix 1b.

05464220 .0 .0 100.0 .0 .0 .0 208.3 28.9 .38 1.7 1.31 4.9 .0 65.0 20.6 14.405474000 11.3 .0 88.7 .0 .0 .0 133.2 30.4 .35 1.6 1.17 5.6 .0 .0 53.6 46.4BPK07 11.6 .0 88.4 .0 .0 .0 278.8 35.7 .32 1.7 .66 2.5 46.8 31.5 6.4 15.4BPJ07 12.7 .0 87.3 .0 .0 .0 227.3 28.4 .34 1.3 1.24 2.1 38.0 .0 33.4 28.6BPJC06 .1 .0 99.9 .0 .0 .0 275.5 26.4 .33 1.3 1.35 1.9 64.0 .0 14.8 21.3BO07 .1 .0 99.9 .0 .0 .0 95.6 26.8 .33 1.3 1.19 2.2 100.0 .0 .0 .0BN01 2.7 .0 79.3 .0 .0 .0 70.9 26.9 .34 1.2 1.18 2.0 100.0 .0 .0 .0

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

[%, percent; ft, feet, Kf, K factor; in/hr, inches per hour]
APPENDIX 1b 47

Surficial-deposit characteristics Soil characteristics Principal aquifer type

Station

number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility )f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

BM02 0.5 0.0 99.5 0.0 0.0 0.0 80.1 26.1 0.36 0.8 1.10 7.5 100.0 0.0 0.0 0.0BE14 .9 2.8 78.3 .0 .0 .0 210.1 26.6 .32 1.3 1.36 1.8 91.6 .0 6.8 1.6BEF05 7.5 .0 92.5 .0 .0 .0 41.3 27.3 .37 .6 .60 5.0 100.0 .0 .0 .0BC02 .0 26.0 74.0 .0 .0 .0 29.7 27.2 .37 .6 .62 4.9 100.0 .0 .0 .0C21 7.0 .0 93.0 .0 .0 .0 61.5 28.1 .37 .7 .68 3.7 100.0 .0 .0 .0CD01 3.7 3.8 92.5 .0 .0 .0 26.8 28.0 .37 .5 .50 5.2 100.0 .0 .0 .0CA06 4.9 .0 95.1 .0 .0 .0 25.0 28.5 .37 .5 .46 4.9 100.0 .0 .0 .0ATGC01 .0 18.5 41.6 .0 39.9 .0 27.6 24.7 .39 .6 .74 8.8 100.0 .0 .0 .0ATF04 .0 35.6 64.4 .0 .0 .0 30.5 25.8 .38 .6 .70 5.8 100.0 .0 .0 .0AK02 .0 .0 0.0 .0 100.0 .0 25.0 25.4 .39 .4 .85 11.8 89.8 .0 .0 10.2AD02 .1 8.0 0.0 .0 91.9 .0 25.0 22.5 .38 .4 .94 10.8 42.2 .5 .0 57.3MJ01 3.0 .7 50.6 .0 45.6 .0 25.0 28.7 .33 .9 1.21 8.6 .0 39.5 .0 60.5PWN01 3.5 4.5 91.6 .0 .3 .0 25.5 28.2 .36 1.2 1.32 4.3 1.0 4.7 .0 94.3PQ10 32.4 8.8 58.8 .0 .0 .0 200.9 22.1 .31 3.1 3.41 5.7 .0 18.9 .0 81.1PQC06 9.1 13.8 77.0 .0 .0 .0 184.9 25.1 .34 1.1 1.41 3.5 1.8 1.1 .0 88.0PQB02 15.6 6.5 77.0 .9 .0 .0 106.1 25.3 .35 1.2 1.47 3.2 4.6 .0 .0 59.4PL03 6.2 23.8 67.4 1.9 .7 .0 124.9 25.9 .35 1.2 1.62 3.4 73.9 .0 .0 26.1PH16 .0 .0 93.1 .0 6.9 .0 32.4 28.0 .37 1.2 1.30 4.5 14.5 .0 .0 85.5PE05 18.3 .6 81.0 .0 .1 .0 37.1 25.3 .35 1.7 1.72 4.5 .0 59.1 .0 40.9PB02 42.8 .0 57.2 .0 .0 .0 222.0 19.8 .30 1.2 3.93 3.4 37.8 2.9 .0 41.2LF01 17.3 .0 82.7 .0 .0 .0 73.2 27.5 .35 1.0 1.28 4.8 98.4 .0 .0 1.6LD02 5.6 .0 94.4 .0 .0 .0 65.5 27.9 .36 1.1 1.20 4.0 79.1 .4 9.4 11.1KI02 4.1 .7 95.2 .0 .0 .0 80.4 30.7 .34 .8 .87 5.7 26.2 .0 73.8 .0KCA01 5.0 .0 35.5 .0 59.5 .0 25.9 26.4 .34 .6 1.23 7.5 17.7 .3 82.1 .0HBD04 1.9 29.8 68.3 .0 .0 .0 64.2 34.3 .30 3.0 1.27 3.5 .0 100.0 .0 .0GG02 5.4 6.2 88.3 .0 .1 .0 94.9 37.8 .35 3.1 .54 4.7 .0 100.0 .0 .0GB10 9.7 4.8 85.6 .0 .0 .0 100.6 29.4 .29 3.7 2.19 4.5 .0 100.0 .0 .0DW01 .3 52.8 44.3 .0 2.7 .0 79.9 30.4 .34 1.3 1.12 2.3 47.5 5.8 .0 46.7DV04 .0 17.4 82.6 .0 .0 .0 111.2 33.2 .28 1.4 1.26 2.1 94.1 2.0 .0 3.9DTD02 28.9 3.4 67.7 .0 .0 .0 107.0 25.2 .34 1.5 1.62 3.5 .0 14.3 .0 85.7DTB01 .6 1.1 98.3 .0 .0 .0 127.3 26.5 .32 1.3 1.41 1.9 44.5 .0 .0 55.5DS07 7.9 9.9 82.2 .0 .0 .0 129.1 33.6 .31 1.4 1.04 1.9 99.0 1.0 .0 .0DR01 .0 .5 98.9 .0 .7 .0 113.6 28.4 .34 1.6 1.27 2.3 100.0 .0 .0 .0DL01 8.9 .3 90.7 .0 .0 .0 72.1 28.9 .34 .7 1.02 6.9 100.0 .0 .0 .0DJ06 10.7 1.2 88.2 .0 .0 .0 67.3 28.1 .35 1.2 1.26 3.4 100.0 .0 .0 .0DJB18 .0 .0 100.0 .0 .0 .0 48.3 29.1 .34 .5 .86 10.4 100.0 .0 .0 .0Appendix 1b. Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
48 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Surficial-deposit characteristics Soil characteristics Principal aquifer type Station number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility 

)

f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

E29 15.7 0.0 84.3 0.0 0.0 0.0 282.0 29.7 0.34 1.5 1.14 2.5 38.8 61.2 0.0 0.0

EOH01 9.3 1.0 89.7 .0 .0 .0 96.6 29.9 .35 1.2 .97 2.0 100.0 .0 .0 .0

EL01 .5 .0 99.5 .0 .0 .0 68.0 29.8 .35 1.1 .99 2.7 100.0 .0 .0 .0EIG01 2.1 .0 79.9 .0 .0 .0 164.8 28.3 .36 1.2 1.18 2.4 100.0 .0 .0 .0

EIE04 3.0 .0 97.0 .0 .0 .0 255.8 27.8 .35 1.2 1.09 4.2 100.0 .0 .0 .0EID04 5.9 .0 94.1 .0 .0 .0 282.6 28.9 .35 1.2 1.05 3.3 100.0 .0 .0 .0DH01 .0 .0 100.0 .0 .0 .0 29.9 29.9 .33 .6 .97 7.1 96.9 .0 3.1 .0

Appendix 1b.

DF04 .0 .0 100.0 .0 .0 .0 89.2 29.5 .34 .8 1.02 5.2 100.0 .0 .0 .0DE01 1.3 13.7 76.1 .0 8.9 .0 34.0 29.3 .33 .6 .97 7.1 67.0 .0 33.0 .0DD04 .1 .0 99.9 .0 .0 .0 82.0 29.7 .35 .9 .99 4.1 100.0 .0 .0 .0DB01 4.9 .1 94.0 1.0 .0 .0 29.6 29.7 .34 .8 1.03 5.7 84.3 .0 15.7 .0DA04 1.8 .0 98.0 .0 .1 .0 63.3 31.2 .34 .9 .80 4.2 100.0 .0 .0 .0JR02 3.2 .0 96.8 .0 .0 .0 43.4 28.7 .33 .6 1.02 7.1 99.7 .0 .3 .0JQ05 4.2 .0 95.8 .0 .0 .0 46.1 29.4 .34 .7 .90 5.5 100.0 .0 .0 .0O31 2.3 .0 97.7 .0 .0 .0 244.6 26.9 .32 1.3 1.34 1.7 81.0 .0 15.9 3.1OU01 .0 .0 100.0 .0 .0 .0 83.7 27.6 .31 1.3 1.23 1.5 100.0 .0 .0 .0OT02 .0 .0 100.0 .0 .0 .0 182.2 27.8 .33 1.3 1.15 2.0 100.0 .0 .0 .0OQ01 5.6 .0 94.4 .0 .0 .0 53.4 29.4 .37 .6 .46 4.0 100.0 .0 .0 .0ON01 1.3 .0 98.7 .0 .0 .0 25.0 28.7 .37 .5 .41 4.4 100.0 .0 .0 .0OL02 8.6 .3 91.1 .0 .0 .0 74.2 27.8 .37 .5 .45 3.9 100.0 .0 .0 .0OK01 1.5 .0 98.5 .0 .0 .0 25.0 28.7 .37 .5 .41 4.4 100.0 .0 .0 .0OKA01 .0 .0 100.0 .0 .0 .0 25.0 28.7 .37 .5 .42 4.5 100.0 .0 .0 .0OJ08 4.2 .0 95.8 .0 .0 .0 25.3 27.9 .37 .6 .46 4.2 100.0 .0 .0 .0OI09 3.5 .0 96.5 .0 .0 .0 68.4 30.1 .36 .9 .56 2.9 100.0 .0 .0 .0OH01 4.2 .0 95.8 .0 .0 .0 41.2 28.4 .36 .7 .72 4.2 100.0 .0 .0 .0OD06 2.3 .0 96.1 .0 1.6 .0 57.5 29.1 .35 .8 .88 4.8 100.0 .0 .0 .0OC04 12.8 .0 87.2 .0 .0 .0 27.2 26.0 .35 .6 1.11 5.9 100.0 .0 .0 .0OZC01 6.6 .0 93.4 .0 .0 .0 34.3 25.6 .36 .5 .59 5.8 100.0 .0 .0 .0II03 4.7 6.6 88.8 .0 .0 .0 26.5 25.0 .36 .5 .68 7.3 100.0 .0 .0 .0NK01 1.7 6.2 92.1 .0 .0 .0 25.0 28.6 .37 .5 .47 4.9 100.0 .0 .0 .0NJ07 3.3 8.0 88.7 .0 .0 .0 25.0 26.9 .37 .5 .74 8.5 100.0 .0 .0 .0NH06 3.4 12.6 84.0 .0 .0 .0 25.0 27.2 .37 .5 .65 7.2 100.0 .0 .0 .0NE05 7.1 9.4 83.5 .0 .0 .0 28.8 27.1 .37 .6 .55 4.8 100.0 .0 .0 .0NC07 6.5 2.9 90.6 .0 .0 .0 29.1 26.7 .37 .5 .54 6.4 100.0 .0 .0 .003167000 .0 .0 .0 .0 100.0 .0 25.0 36.9 .25 .5 2.56 24.4 .0 .0 100.0 .003170000 .0 .0 .0 .0 100.0 .0 25.0 26.5 .22 .6 3.42 21.1 .0 .0 23.8 76.2

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
APPENDIX 1b 49

Surficial-deposit characteristics Soil characteristics Principal aquifer type

Station

number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility )f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

Appendix 1b.03178000 0.0 0.0 0.0 0.0 100.0 0.0 25.0 29.6 0.28 0.5 2.16 34.7 26.0 0.0 74.0 0.003183000 .0 .0 .0 .0 100.0 .0 25.0 24.7 .25 .6 5.00 31.8 .0 .0 52.1 47.903186500 .0 .0 .0 .0 100.0 .0 25.0 22.5 .26 .4 2.40 31.1 29.4 .0 70.6 .003187500 .0 .0 .0 .0 100.0 .0 25.0 20.6 .25 .4 2.84 26.7 48.6 .0 51.4 .003191500 .0 .0 .0 .0 100.0 .0 25.0 23.4 .27 .5 1.73 30.3 100.0 .0 .0 .003198350 .0 .0 .0 .0 100.0 .0 25.0 19.8 .20 .4 3.32 45.5 100.0 .0 .0 .004159492 24.8 25.8 49.4 .0 .0 .0 121.9 20.6 .27 5.7 2.34 1.1 11.4 .0 .0 88.604161820 47.0 .0 53.0 .0 .0 .0 286.7 15.1 .15 7.9 4.59 6.2 1.5 .0 .0 98.504175600 5.4 .0 49.6 .0 .0 .0 122.8 11.9 .18 12.3 6.91 7.9 99.9 .0 .0 .104178000 9.0 .0 91.0 .0 .0 .0 276.2 28.3 .34 3.6 1.65 3.3 .0 .0 .0 100.004186500 3.0 .7 96.4 .0 .0 .0 79.6 35.5 .38 1.8 0.61 2.8 .0 100.0 .0 .004208504 26.9 4.6 68.6 .0 .0 .0 90.1 25.6 .38 1.4 1.43 6.7 43.2 .0 47.4 9.404211820 3.0 12.3 84.7 .0 .0 .0 53.5 30.4 .41 .5 .55 3.5 3.5 .0 35.6 60.904213500 21.8 2.8 73.1 2.3 .0 .0 54.6 19.7 .31 1.0 1.90 11.3 .0 .0 .0 100.005567000 .0 .0 100.0 .0 .0 .0 179.6 31.3 .31 1.4 .94 1.6 100.0 .0 .0 .005568800 .0 .0 100.0 .0 .0 .0 43.1 29.7 .34 0.9 1.00 4.8 100.0 .0 .0 .005584500 1.8 6.7 90.4 .0 1.2 .0 57.6 30.0 .35 0.9 .98 4.5 48.8 .0 46.8 4.403245500 10.2 0.7 89.1 .0 .0 .0 84.3 26.6 .40 0.6 1.26 5.1 .0 43.3 .0 56.703267900 45.7 0.4 53.9 .0 .0 .0 201.9 24.2 .42 0.8 2.52 4.1 .0 10.0 .0 .003275000 23.6 0.0 76.4 .0 .0 .0 173.8 24.9 .41 0.7 1.67 5.3 .0 29.8 .0 70.2393944084120700 .0 .0 100.0 .0 .0 .0 49.5 26.4 .41 0.5 .93 5.3 .0 .0 .0 100.0395355084173600 3.1 .0 96.9 .0 .0 .0 86.7 27.5 .41 1.0 .99 3.8 .0 79.5 .0 20.5395457084095100 6.1 3.0 90.9 .0 .0 .0 130.5 30.5 .40 1.3 1.02 3.3 .0 81.4 .0 18.605030150 42.3 .0 57.7 .0 .0 .0 427.7 12.2 .22 4.5 6.28 4.3 .0 .0 .0 100.005051300 5.6 .2 94.1 .0 .0 .0 245.8 28.4 .32 1.5 1.30 2.1 3.3 .0 .0 96.705058700 13.8 .2 83.5 .0 2.5 .0 147.7 20.9 .34 1.2 2.60 3.5 25.7 .0 .0 74.305062500 18.0 .0 82.0 .0 .0 .0 405.1 21.2 .27 4.3 2.65 3.5 .0 .0 .0 100.005079000 20.7 0.2 70.9 .0 .0 .0 265.0 18.7 .23 9.8 4.02 1.7 .2 .0 .0 99.805082625 27.8 7.5 62.8 .0 1.9 .0 96.4 25.4 .33 1.2 2.12 2.7 .0 .0 .0 100.005085900 42.8 16.2 41.0 .0 .0 .0 321.5 20.6 .27 2.0 4.75 .9 2.0 .0 .0 98.005099600 2.4 1.0 92.5 .0 4.1 .0 104.5 26.4 .35 1.1 1.67 3.3 5.3 .0 .0 94.705112000 28.7 0.0 71.3 .0 .0 .0 205.9 16.8 .21 14.5 4.64 .6 .0 .0 .0 100.005333579 .0 0.0 70.4 25.8 .0 3.8 27.1 11.8 .21 16.8 1.55 2.2 .0 .0 .0 100.005335151 .6 0.0 85.9 1.2 .0 12.2 28.8 11.8 .16 27.0 2.46 2.0 .0 .0 .0 100.005338955 77.5 0.0 22.5 .0 .0 .0 123.6 13.6 .22 5.7 5.40 6.2 56.8 .0 .0 43.205340390 63.4 0.0 36.6 .0 .0 .0 116.1 10.0 .20 7.8 6.41 5.2 59.2 .0 .0 40.8

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
50 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Surficial-deposit characteristics Soil characteristics Principal aquifer type Station number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility 

)

f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

Appendix 1b.05341500 63.6 0.0 36.4 0.0 0.0 0.0 128.1 8.4 0.22 3.3 5.65 7.9 83.7 0.0 0.0 16.305341752 9.9 .0 90.1 .0 .0 .0 75.2 10.1 .26 1.7 4.21 5.3 100.0 .0 .0 .005342000 .0 .0 97.7 2.3 .0 .0 37.2 13.1 .26 1.7 3.89 5.4 100.0 .0 .0 .005526000 24.1 18.9 57.0 .0 .0 .0 147.7 26.3 .29 1.9 3.15 2.2 16.4 42.5 5.5 35.605527800 11.2 .0 88.8 .0 .0 .0 197.0 34.4 .34 5.2 .72 3.8 .0 100.0 .0 .005531500 1.6 1.2 97.2 .0 .0 .0 113.1 34.7 .26 3.9 .78 3.9 .0 97.2 .0 2.805548105 2.7 13.6 83.7 .0 .0 .0 224.6 21.8 .31 3.2 2.31 3.7 .0 53.9 .0 46.105267000 41.2 .0 53.6 .5 .0 1.5 265.7 12.1 .19 9.2 5.74 4.9 3.7 .0 .0 96.305276005 59.5 .0 40.5 .0 .0 .0 282.5 15.4 .21 2.8 6.46 2.2 69.7 .0 .0 30.305288705 45.8 .0 54.2 .0 .0 .0 181.3 14.0 .20 8.2 6.83 3.7 23.0 .0 .0 77.005320270 .0 48.7 51.3 .0 .0 .0 185.9 32.0 .31 3.0 .89 2.0 58.8 7.9 .0 33.305330902 33.1 .0 66.9 .0 .0 .0 256.8 17.2 .21 12.7 4.66 5.4 100.0 .0 .0 .005331833 74.5 .0 23.7 .0 1.0 .8 53.6 6.9 .20 11.2 5.00 10.1 .0 .0 .0 100.004027595 24.8 1.5 54.5 .3 18.8 .1 92.7 17.5 .21 8.6 2.35 8.9 .0 .0 .0 100.0040734644 4.7 .0 95.3 .0 .0 .0 51.2 21.5 .35 4.3 1.67 3.2 29.7 .0 .0 70.304085463 .0 .4 99.6 .0 .0 .0 114.7 37.5 .32 4.1 1.00 3.2 .0 100.0 .0 .004086500 2.7 .0 79.3 .0 .0 .0 99.7 19.5 .29 9.2 2.97 4.5 .0 85.8 .0 14.2053230 61.8 38.2 .0 .0 .0 .0 75.0 26.6 .29 .6 3.47 6.6 .0 .0 .0 100.0053232 8.5 90.0 1.4 .0 .0 .0 75.3 41.9 .32 4.6 .82 3.1 .0 .0 .0 100.0053511 .0 .0 100.0 .0 .0 .0 155.5 41.9 .32 4.6 .82 3.1 .0 4.8 .0 95.205368000 11.7 .0 55.4 3.5 29.5 .0 68.1 12.2 .23 1.3 3.28 8.6 100.0 .0 .0 .005378185 .0 .0 .0 .0 100.0 .0 25.0 20.4 .28 .5 2.38 18.8 100.0 .0 .0 .005379430 9.7 .0 .0 .0 90.3 .0 29.5 18.8 .26 .4 2.67 23.1 100.0 .0 .0 .005379472 .0 .0 .0 .0 100.0 .0 25.0 19.9 .27 .4 2.47 20.2 100.0 .0 .0 .005406460 27.3 .5 46.1 .0 26.1 .0 58.9 18.3 .25 3.8 4.78 9.3 100.0 .0 .0 .005407500 .0 .0 .0 .0 100.0 .0 25.0 28.6 .27 .3 1.53 16.5 100.0 .0 .0 .005427950 14.1 26.3 59.6 .0 .0 .0 89.2 15.7 .24 5.6 5.27 5.8 100.0 .0 .0 .005429580 7.1 4.9 88.0 .0 .0 .0 50.5 17.7 .27 5.2 4.00 5.3 100.0 .0 .0 .005431014 .0 .0 100.0 .0 .0 .0 159.4 20.5 .28 6.8 2.23 3.8 .0 100.0 .0 .005431018 .9 .0 99.1 .0 .0 .0 356.4 19.0 .27 6.3 2.41 4.6 .0 .0 .0 100.005433510 .0 .0 .0 .0 100.0 .0 25.0 32.6 .29 1.2 1.05 11.2 9.0 .0 .0 91.0063035 22.6 .0 .0 .0 77.4 .0 31.3 12.1 .19 .5 5.17 11.3 100.0 .0 .0 .0063037 .0 .0 .0 .0 100.0 .0 25.0 19.6 .27 .4 2.52 21.0 100.0 .0 .0 .0103094 12.1 .0 63.4 24.5 .0 .0 57.8 15.3 .31 3.6 1.71 4.0 18.2 .0 .0 81.8103105 .0 .0 53.4 46.6 .0 .0 29.9 18.5 .34 1.4 1.05 2.8 60.6 .0 .0 39.4113086 13.5 .0 86.5 .0 .0 .0 34.3 19.4 .29 4.3 2.69 5.0 95.2 .0 .0 4.8

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
APPENDIX 1b 51

Surficial-deposit characteristics Soil characteristics Principal aquifer type

Station

number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility )f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

Appendix 1b.123023 0.4 0.0 0.0 0.0 99.6 0.0 25.0 25.2 0.34 0.5 1.56 14.6 100.0 0.0 0.0 0.0133024 34.4 .0 65.6 .0 .0 .0 76.6 16.5 .25 6.1 5.24 5.7 96.1 .0 .0 3.9133119 16.0 .0 18.2 5.3 60.4 .0 33.2 26.0 .26 2.1 2.28 9.4 88.5 .0 .0 11.5133336 .0 .0 100.0 .0 .0 .0 27.6 19.0 .27 6.2 2.41 4.9 100.0 .0 .0 .0133337 11.9 .0 .0 .0 88.1 .0 27.0 23.9 .27 .6 1.99 16.5 100.0 .0 .0 .0143012 .0 .0 100.0 .0 .0 .0 25.0 21.0 .35 2.5 1.82 3.5 .0 .0 .0 100.0163002 51.6 .0 47.9 .5 .0 .0 181.0 14.0 .18 12.2 5.97 7.5 4.1 .0 .0 95.9183042 .0 .0 .0 .0 100.0 .0 25.0 12.0 .19 1.7 5.16 7.2 100.0 .0 .0 .0183064 8.7 .0 47.4 43.9 .0 .0 49.9 13.8 .28 2.6 2.50 4.2 50.5 .0 .0 49.5183077 15.6 .0 13.7 52.9 11.8 6.0 26.8 13.8 .22 3.4 3.24 2.9 94.1 .0 .0 5.9223248 .0 .0 .0 .0 100.0 .0 25.0 28.6 .39 1.0 1.30 7.5 0.0 .0 .0 100.0263001 13.7 .0 65.1 .2 20.2 .8 41.3 11.6 .23 11.2 1.74 7.2 7.8 .0 .0 92.2273015 24.3 .0 .0 .0 75.7 .0 25.0 12.7 .19 .7 4.94 8.7 100.0 .0 .0 .0273016 3.0 .0 .0 .0 97.0 .0 25.0 12.6 .16 1.5 4.72 5.3 100.0 .0 .0 .0273031 67.4 .0 .0 .0 32.6 .0 25.0 9.4 .14 1.3 6.63 6.0 62.3 .0 .0 37.7273032 9.5 .0 .5 .0 75.1 14.9 25.2 13.4 .15 7.6 4.09 2.7 63.6 .0 .0 36.4273034 .0 .0 .0 .0 100.0 .0 25.0 3.2 .13 .3 10.90 10.5 100.0 .0 .0 .0313023 .2 .0 97.1 2.8 .0 .0 40.1 15.2 .30 4.4 1.95 3.0 .0 100.0 .0 .0313038 14.5 .0 84.9 .6 .0 .0 83.4 32.1 .30 4.0 2.11 3.9 .0 100.0 .0 .0363070 27.5 3.6 68.9 .0 .0 .0 73.9 24.0 .28 2.3 4.04 5.7 .0 100.0 .0 .0363071 9.5 3.4 87.1 .0 .0 .0 80.2 33.0 .32 3.2 1.26 4.0 .0 100.0 .0 .0363218 8.2 3.6 88.2 .0 .0 .0 62.0 27.0 .31 5.3 1.49 4.0 .0 99.6 .0 .4373047 14.8 .0 85.2 .0 .0 .0 60.8 14.5 .33 1.6 1.29 3.3 .0 .0 .0 100.0373325 1.3 .0 98.7 .0 .0 .0 36.7 17.9 .35 2.0 1.21 2.0 .0 .0 .0 100.0383001 55.1 .0 44.8 .0 .0 .0 101.1 7.7 .17 11.3 7.04 6.9 28.1 .0 .0 71.9413006 .3 .0 99.7 .0 .0 .0 144.1 35.1 .35 4.3 .63 5.3 .0 100.0 .0 .0413008 .0 .0 100.0 .0 .0 .0 57.6 35.1 .35 4.3 .63 5.3 .0 100.0 .0 .0413011 .0 .0 100.0 .0 .0 .0 150.0 35.1 .35 4.3 .63 5.3 .0 100.0 .0 .0433002 49.0 .0 50.9 .0 .0 .0 135.6 9.3 .20 8.1 6.34 6.2 41.5 .0 .0 58.5453030 27.9 .0 72.1 .0 .0 .0 110.2 14.0 .27 6.5 3.25 2.6 93.3 .0 .0 6.7483043 .0 .0 88.3 .4 11.3 .0 46.7 18.6 .32 .8 1.92 6.6 100.0 .0 .0 .0

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site—Continued
52 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams Surficial-deposit characteristics Soil characteristics Principal aquifer type Station number

Coarse-grained 

stratified 

sediment

(%)

Fine-grained

stratified

sediment

(%)

Till (%)

Patchy

Quaternary

sediment (%)

Exposed 

bedrock

or non-glacial 

sediments (%)

Organic-rich

sediments

(%)

Thickness

(ft)

Clay content

(%)

Erodibility 

)

f (K

Organic-matte

content

(%)

Permeability

(in/hr)

Soil slope

(%)

Sandstone

(%)

Carbonate

(%)

Sandstone 

and carbonate

(%)

No principal

aquifer

(%)

Appendix 1b.503069 100.0 0.0 0.0 0.0 0.0 0.0 99.3 4.0 0.14 9.1 11.39 1.6 14.6 0.0 0.0 85.4523061 7.5 .0 92.5 .0 .0 .0 162.9 34.0 .34 4.7 .89 4.4 .0 100.0 .0 .0573076 5.1 .0 .0 .0 94.9 .0 25.9 24.4 .26 .3 2.03 16.4 100.0 .0 .0 .0603049 14.4 .0 85.6 .0 .0 .0 107.0 23.3 .27 6.3 4.47 4.9 .0 100.0 .0 .0603304 5.6 .0 94.4 .0 .0 .0 127.2 35.0 .31 4.2 1.78 3.7 .0 100.0 .0 .0603326 .0 .0 100.0 .0 .0 .0 116.0 28.6 .31 3.6 2.08 3.9 .0 100.0 .0 .0683001 4.0 .0 96.0 .0 .0 .0 61.6 30.3 .34 3.7 .98 4.9 .0 100.0 .0 .0683271 36.3 .0 63.7 .0 .0 .0 119.3 14.0 .23 7.6 6.82 6.1 .0 22.3 .0 77.7693021 62.4 .0 37.6 .0 .0 .0 96.1 12.4 .18 10.3 4.91 4.6 32.5 .0 .0 67.503353637 4.3 .0 95.7 .0 .0 .0 228.0 23.7 .40 1.3 1.61 3.3 .0 62.9 .0 37.103360895 .2 27.4 72.4 .0 .0 .0 63.5 23.5 .37 .5 1.39 6.0 100.0 .0 .0 .003366500 .0 .3 86.6 .0 13.2 .0 25.9 25.8 .40 .2 .59 5.6 .0 98.4 .0 1.603373530 .0 .0 .0 .0 100.0 .0 25.0 37.6 .31 .4 1.22 10.7 .0 .0 100.0 .0391732085414401 .0 .0 100.0 .0 .0 .0 42.3 25.7 .39 .8 1.08 3.9 .0 100.0 .0 .0393306086585201 4.0 5.5 90.5 .0 .0 .0 85.0 25.2 .38 .9 1.14 7.1 3.1 .0 32.4 64.5394340085524601 2.1 .0 97.9 .0 .0 .0 161.8 24.9 .42 1.5 1.23 2.2 .0 100.0 .0 .004062085 .0 .0 .0 100.0 .0 .0 25.0 9.5 .17 10.8 1.70 13.7 .0 .0 .0 100.004063700 80.3 .0 19.7 .0 .0 .0 70.6 7.4 .15 15.5 5.22 9.9 .0 .0 .0 100.004071795 .0 .0 100.0 .0 .0 .0 45.8 14.7 .31 4.6 1.91 2.7 100.0 .0 .0 .004072050 3.0 .0 97.0 .0 .0 .0 48.9 26.9 .31 5.0 1.09 3.2 37.1 .0 .0 62.904080798 50.1 .0 49.9 .0 .0 .0 143.1 8.3 .15 11.3 5.36 5.1 96.8 .0 .0 3.204085109 .3 .0 99.7 .0 .0 .0 94.0 37.3 .32 4.6 1.04 3.7 .0 28.3 .0 71.7040863075 8.2 .0 91.8 .0 .0 .0 171.1 17.3 .31 3.1 2.87 4.4 .0 100.0 .0 .0040869415 .0 .0 100.0 .0 .0 .0 126.7 35.1 .35 4.3 .63 5.3 .0 100.0 .0 .0

 Environmental characteristics of the watersheds of 234 sites sampled in the Upper Midwest, by site

—Continued
Appendix 2 53

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site

Station number Sampling agency or

USGS study name

Total nitrogen

(mg/L)

Total phosphorus

(mg/L)

03015795 almn 0.37 0.01

03024000 almn .93 .03

03037350 almn 1.08 .02

03040000 almn 1.05 .02

03049646 almn .71 .02

03070350 almn .68 .01

03072000 almn .51 .01

03083500 almn 1.06 .01

40001 umbl 3.81 .19

40007 umbl 4.70 .16

50001 umbl 4.76 .08

50003 umbl 3.80 .09

80001 umbl 1.88 .15

80003 umbl 3.99 .08

80007 umbl 1.37 .08

90001 umbl 1.47 .06

90003 umbl 3.63 .07

130001 umbl 6.46 .21

130003 umbl 4.16 .14

200003 umbl 7.20 .18

240001 umbl 1.85 .14

260001 umbl 1.60 .10

280001 umbl .76 .04

05420680 eiwa 4.89 .09

05449500 eiwa 6.33 .17

05451210 eiwa 10.00 .09

05455100 eiwa 5.10 .13

05461390 eiwa 8.55 .10

05464220 eiwa 10.09 .13

05474000 eiwa 7.06 .32

BPK07 iepa 7.73 .05

BPJ07 iepa 8.20 .28

BPJC06 iepa -- 1.85

BO07 iepa 10.40 .11

BN01 iepa -- .05

BM02 iepa -- .20

BE14 iepa -- .09

BEF05 iepa -- .16

BC02 iepa -- .19

C21 iepa 1.35 .17

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site

[mg/L, milligrams per liter; --, no data; almn, Allegheny and Monongahela River Basins 

NAWQA study unit; eiwa, Eastern Iowa Basins NAWQA study unit; iepa, Illinois 

Environmental Protection Agency; kana, Kanawha-New River Basins NAWQA study unit; 

leri, Lake Erie-Lake Saint Clair Drainages NAWQA study unit; lirb, Lower Illinois River 

Basin NAWQA study unit; miam, Great and Little Miami River Basins NAWQA study 

unit; redn, Red River of the North Basin NAWQA study unit; stx, U.S. Geological Survey, 

St. Croix River Watershed study; uirb, Upper Illinois River Basin NAWQA study unit; 

umbl, U.S. Geological Survey, Upper Mississippi River System study; umis, Upper 

Mississippi River Basin NAWQA study unit; wdnr, Wisconsin Department of Natural 

Resources; whit, White River Basin NAWQA study unit; wmic, Western Lake Michigan 

Drainages NAWQA study unit]
54 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Station number Sampling agency or

USGS study name

Total nitrogen

(mg/L)

Total phosphorus

(mg/L)

CD01 iepa 1.56 .20

CA06 iepa 1.20 .14

ATGC01 iepa 1.27 .02

ATF04 iepa 1.50 .09

AK02 iepa 0.34 .01

AD02 iepa .86 .14

MJ01 iepa 8.45 .18

PWN01 iepa -- .21

PQ10 iepa 4.25 .08

PQC06 iepa 5.50 .25

PQB02 iepa 5.86 .05

PL03 iepa 7.75 .39

PH16 iepa 8.20 .16

PE05 iepa 7.80 .23

PB02 iepa 3.20 .06

LF01 iepa -- .15

LD02 iepa -- .50

KI02 iepa 6.10 .15

KCA01 iepa -- .22

HBD04 iepa 6.73 2.16

GG02 iepa 4.30 .38

GB10 iepa 6.80 1.13

DW01 iepa 1.10 .09

DV04 iepa 9.30 .07

DTD02 iepa 3.95 .10

DTB01 iepa 5.40 .08

DS07 iepa 9.40 .22

DR01 iepa 5.80 .15

DL01 iepa -- .08

DJ06 iepa -- .17

DJB18 iepa -- .43

E29 iepa 10.10 .11

EOH01 iepa 6.40 .23

EL01 iepa 1.60 .20

EIG01 iepa 8.70 .09

EIE04 iepa 10.00 .07

EID04 iepa 9.40 .51

DH01 iepa -- .10

DF04 iepa -- .15

DE01 iepa -- .10

DD04 iepa -- 1.17

DB01 iepa -- .21

DA04 iepa -- .28

JR02 iepa 3.05 .17

JQ05 iepa 1.30 .20

O31 iepa 9.40 .15

OU01 iepa 10.17 .09

OT02 iepa 10.60 .07

OQ01 iepa 2.66 .20

ON01 iepa .96 .12

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site—Continued
Appendix 2 55

Station number Sampling agency or

USGS study name

Total nitrogen

(mg/L)

Total phosphorus

(mg/L)

OL02 iepa 1.38 .16

OK01 iepa 1.20 .19

OKA01 iepa 1.62 .29

OJ08 iepa 4.86 .54

OI09 iepa 2.00 .41

OH01 iepa 2.70 .85

OD06 iepa 2.50 .66

OC04 iepa 8.40 1.00

OZC01 iepa 1.54 .16

II03 iepa .57 .21

NK01 iepa 1.52 .14

NJ07 iepa 3.40 .25

NH06 iepa 1.75 .17

NE05 iepa 1.52 .25

NC07 iepa 1.35 .17

03167000 kana 1.02 .05

03170000 kana .51 .05

03178000 kana .66 .05

03183000 kana .59 .05

03186500 kana .42 .05

03187500 kana .48 .05

03191500 kana .65 .05

03198350 kana .74 .05

04159492 leri 4.10 .10

04161820 leri 1.84 .05

04175600 leri .92 .01

04178000 leri 2.32 .12

04186500 leri 5.71 .14

04208504 leri 5.30 .22

04211820 leri .95 .06

04213500 leri 1.21 .01

05567000 lirb 13.18 .13

05568800 lirb 8.62 .13

05584500 lirb 5.40 .17

03245500 miam 3.70 .37

03267900 miam 4.29 .08

03275000 miam 2.88 .05

393944084120700 miam 1.35 .05

395355084173600 miam 3.18 .17

395457084095100 miam 3.17 .32

05030150 redn .66 .03

05051300 redn 1.72 .20

05058700 redn 1.35 .22

05062500 redn .95 .04

05079000 redn 1.00 .07

05082625 redn 1.30 .09

05085900 redn 1.61 .18

05099600 redn .99 .23

05112000 redn 1.26 .09

05333579 stx 1.15 .03

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site—Continued
56 An Alternative Regionalization Scheme for Defining Nutrient Criteria for Rivers and Streams

Station number Sampling agency or

USGS study name

Total nitrogen

(mg/L)

Total phosphorus

(mg/L)

05335151 stx 1.10 .04

05338955 stx .96 .06

05340390 stx .91 .04

05341500 stx 1.19 .05

05341752 stx 2.95 .07

05342000 stx 5.28 .08

05526000 uirb 7.40 .08

05527800 uirb 2.64 .19

05531500 uirb 9.00 1.51

05548105 uirb 4.03 .08

05267000 umis .79 .03

05276005 umis 2.09 .04

05288705 umis 1.16 .06

05320270 umis 8.95 .16

05330902 umis 1.10 .06

05331833 umis .41 .01

04027595 wdnr -- .04

040734644 wdnr -- .24

04085463 wdnr -- .33

04086500 wdnr -- .22

053230 wdnr -- .16

053232 wdnr -- .27

053511 wdnr -- .83

05368000 wdnr -- .09

05378185 wdnr -- .09

05379430 wdnr -- .30

05379472 wdnr -- .13

05406460 wdnr -- .14

05407500 wdnr -- .12

05427950 wdnr -- .26

05429580 wdnr -- .14

05431014 wdnr -- .08

05431018 wdnr -- .27

05433510 wdnr -- .11

063035 wdnr -- .07

063037 wdnr -- .07

103094 wdnr -- .16

103105 wdnr -- .10

113086 wdnr -- .14

123023 wdnr -- .06

133024 wdnr -- .38

133119 wdnr -- .16

133336 wdnr -- .17

133337 wdnr -- .12

143012 wdnr -- .11

163002 wdnr -- .03

183042 wdnr -- .18

183064 wdnr -- .08

183077 wdnr -- .06

223248 wdnr -- .21

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site—Continued
Appendix 2 57

Station number Sampling agency or

USGS study name

Total nitrogen

(mg/L)

Total phosphorus

(mg/L)

263001 wdnr -- .05

273015 wdnr -- .10

273016 wdnr -- .06

273031 wdnr -- .02

273032 wdnr -- .06

273034 wdnr -- .02

313023 wdnr -- .05

313038 wdnr -- .11

363070 wdnr -- .12

363071 wdnr -- .10

363218 wdnr -- .14

373047 wdnr -- .11

373325 wdnr -- .22

383001 wdnr -- .03

413006 wdnr -- .13

413008 wdnr -- .07

413011 wdnr -- .08

433002 wdnr -- .04

453030 wdnr -- .28

483043 wdnr -- .02

503069 wdnr -- .02

523061 wdnr -- .20

573076 wdnr -- .17

603049 wdnr -- .19

603304 wdnr -- .38

603326 wdnr -- .06

683001 wdnr -- .56

683271 wdnr -- .09

693021 wdnr -- .05

03353637 whit 1.16 .03

03360895 whit 5.10 .10

03366500 whit 1.45 .07

03373530 whit 6.80 .05

391732085414401 whit 6.30 .07

393306086585201 whit 2.40 .07

394340085524601 whit 2.10 .06

04062085 wmic .47 .01

04063700 wmic .58 .02

04071795 wmic 1.90 .15

04072050 wmic 2.70 .15

04080798 wmic 2.40 .02

04085109 wmic 1.65 .20

040863075 wmic 2.60 .12

040869415 wmic .93 .04

Appendix 2. Median, midmonthly nutrient concentrations of 

234 sites sampled in the Upper Midwest, by site—Continued