1

U.S. Department of the Interior

U.S. Geological Survey

USGS Water Resources Investigations Report 03–4334

May 2004

Analysis of Streamflow and Water-Quality Data at 

Two Long-Term Monitoring Sites on the St. Croix River, 

Wisconsin and Minnesota

Abstract

Streamflow of the St. Croix River has been monitored on 

a long-term basis by the U.S. Geological Survey (USGS) at 

streamflow-gaging stations on the St. Croix River at Wisconsin State Highway 35 near Danbury, Wisconsin and below the 

dam at St. Croix Falls, Wisconsin. Trend analysis indicates 

that annual streamflow peaks, annual mean flows, and annual 

7-day low flows have remained relatively stable at the Danbury 

(upstream) station from 1914 to 2001 but have increased by 

0.45 to 0.55 percent per year at the St. Croix Falls (downstream) 

station from 1902 to 2001. 

Water-quality sampling conducted by the USGS at two 

associated water-quality monitoring sites from 1964 to 2001 

have included analysis for nutrients, suspended sediments, metals, bacteria, and pesticides. The water-quality data at the two 

sites had considerable temporal variation and was often limited 

in extent by the data-collection needs of previous projects. 

The absence of consistent long-term water-quality data made 

detection of statistically significant trends in the water-quality data difficult. Nutrient, suspended-sediment, and major ion 

concentrations and field parameters were generally higher and 

more variable at the St. Croix Falls site than at the Danbury site. 

However, disparate sampling periods and strategies could cause 

possible biases in the comparison of water quality between sites.

Introduction

The U.S. Geological Survey (USGS) has performed long-term

continuous streamflow measurements of the St. Croix River at 

two streamflow-gaging stations: at Wisconsin State Highway 35 

near Danbury, Wisconsin and below the dam at St. Croix Falls, 

Wisconsin (fig. 1). Water-quality measurements of a variety 

of constituents such as nutrients, suspended sediments, metals, 

bacteria, and pesticides have been collected at two water-quality monitoring sites associated with these streamflow-gaging 

stations. This report will refer to both the streamflow-gaging 

stations and associated water-quality monitoring sites as sites.

This study, which was initiated in 2001, was a cooperative 

effort between the USGS, National Park Service (NPS), and the

Joint Water Quality Commission of Danbury and St. Croix 

Chippewa Indians of Wisconsin to analyze the long-term USGS 

flow data for temporal trends and provide summaries and analysis of the USGS water-quality data collected at these sites. This 

report presents trends in flow and differences in water quality 

among sites, studies, and over time. Streamflow data from 1902 

to 2001 and water-quality data from 1964 to 2001 are included 

in this study.

Differences between the long-term data sets can be used to 

determine the part of the St. Croix River Basin where change 

has occurred. Trends at the Danbury site reflect changes in 

the upper basin (1,580 square miles), trends in the difference 

between the St. Croix Falls and Danbury sites reflect changes 

that occurred in the part of the drainage basin between those 

two sites (4,660 square miles), and trends at the St. Croix Falls 

EXPLANATION

MINNESOTA

WISCONSIN

St. Croix River basin

U.S. Geological Survey long-term

monitoring site

91 ̊ 

46 ̊ 

45 ̊

City

Base map: Seaber and others, 1986.

93 ̊ 

92 ̊ 

Snake R vi er

Kettle R vi er

L. Tamarack R

U. Tamarack River

Namekagon

River

Yellow

River

Wood Riv

Clam River

er

Trade River

Wolf Creek

Valley Creek

Browns

Creek

Apple

River

Sunrise River

St. Croix River

Willow

River

Kinnickinnic River

Crooked Cr

Sand River

0 25

0 25 50 MILES

50 KILOMETERS

WISCONSIN

MINNESOTA Study area

Prescott, Wis.

Stillwater, Minn.

Taylors Falls, Minn.

Gordon, Wis.

Cable, Wis.

St. Croix Falls, Wis.

Danbury, Wis.

S T. C R O I X

P O L K

B A R R O N

B U R N E T T

W A S H B U R N

D O U G L A S 

S A W Y E R

B AY F I E L D

C A R L T O N

P I N E

A I T K I N

K A N A B E C

C H I S A G O

I S A N T I

W A S H I N G T O N

M I L L E

L A C S

A N O K A

P I E R C E

Figure 1. Long-term monitoring sites on the St. Croix River, Wisconsin 

and Minnesota.

Printed on recycled paper

Access data online at http://wi.waterdata.usgs.gov/nwis
2

site reflect changes in the entire drainage basin upstream of that 

site (6,240 square miles). U.S. Census data were used to depict 

population change by decade in the St. Croix Basin.

Streamflow

St. Croix River streamflow monitoring by the USGS has 

taken place at the streamflow-gaging station at Wisconsin State 

Highway 35 near Danbury since 1914 (with the exception of 

October 1981 to September 1984 when the site was not maintained) and at the streamflow-gaging station below the dam 

at St. Croix Falls since 1902. Flows at both sites are affected 

by regulation. The statistics and trends presented in this report 

include the impacts of that regulation.

Individual flow rates in cubic feet per second (ft3/s) are determined at 15-minute or 1-hour intervals. The highest 15-minute 

flow value (peak instantaneous) at each site occurred in April 

2001 with the river flowing at 11,000 ft3/s at the Danbury site 

and 60,900 ft3/s at the St. Croix Falls site. Individual 15-minute 

or 1-hour flow values are averaged over each day to determine 

the daily mean flow. Daily mean flow statistics for the Danbury 

and St. Croix Falls sites over the periods of record are presented 

in table 1. Daily mean flows at the Danbury site ranged from 

405 to 10,600 ft3/s. Daily mean flows were more variable at the 

St. Croix Falls site, ranging from 75 to 59,500 ft3/s. 

Annual flow statistics for the two sites are presented in table 

2. Annual mean flow was computed by averaging mean daily 

flows for the period from October 1st to September 30th of each 

water year of record. Annual mean flows ranged from 795 to 

1,982 ft3/s at the Danbury site and were more variable at the St. 

Croix Falls site, ranging from 1,754 to 8,569 ft3/s. The average annual mean flows for the periods of record differ by 3,067 

ft3/s between sites. Annual mean flows differ by as little as 959 

ft3/s in the driest year (1934) and as much as 6,587 ft3/s in the 

wettest year (1984). The highest mean daily flow calculated 

annually (peak daily) and averaged for the periods of record was 

about five times lower at the Danbury site (4,898 ft3/s) than at 

the St. Croix Falls site (24,000 ft3/s).

Low flows were defined as the minimum average mean daily 

flow for 7 consecutive days within a water year (7-day low 

flow). The minimum 7-day low flow for the period of record 

occurred in July 1934 at the Danbury site (417 ft3/s) and in 

August 1934 at the St. Croix Falls site (754 ft3/s). The average annual 7-day low flow at the Danbury site (731 ft3

/s) was 

about one-half that at the St. Croix Falls site (1,620 ft3/s) for the 

periods of record. 

Trend analysis was performed on the annual long-term 

USGS flow data from each site, and on the difference in the 

flow between the two sites using a Kendall’s Tau test (Kendall, 1975; Conover, 1971) and linear regression analysis. The 

Kendall test is a nonparametric test of the strength of a trend and 

is appropriate for data that are not normally distributed. Linear 

regression analyses were performed to quantify the amount of 

change, if a statistically significant trend existed. Ten-year moving averages were calculated by taking the mean of the previous ten years of data; thus, each new year’s numbers are added 

to the average and the oldest numbers are dropped so that the 

average “moves” over time. The moving average demonstrates 

shorter-term changes in flow over the period of record. Trend 

analyses were performed on four annual streamflow statistics: 

peak instantaneous, peak daily, mean, and 7-day low flow (table 

3, fig. 2). 

Table 2. Annual flow statistics for the Danbury and St. Croix Falls, Wis., streamflow-gaging stations on the St. Croix River, Wis. and Minn., 

1902–2001

[ft3/s, cubic feet per second]

Site Drainage

area

Years of

record

Peak

instantaneous

(ft3/s)

Peak daily

(ft3/s)

Mean

(ft3/s)

7-day 

low flow

(ft3/s)

Danbury 1,580 1914 –

 2001

Mean:

Maximum:

Minimum:

 5,055

 11,000 (on 4/24/2001)

 2,030 (on 4/17/1926)

 4,898

 10,600 (in 2001)

 2,030 (in 1926)

 1,320

 1,982 (in 1986)

 795 (in 1934)

 731

 1,171 (in Feb. 1986) 

 417 (in July 1934)

St. Croix Falls 6,240 1902 –

2001

Mean:

Maximum:

Minimum:

 24,400

 60,900 (on 4/24/2001)

 6,820 (on 10/01/1924)

 24,000

 59,500 (in 2001)

 6,820 (in 1924)

 4,387

 8,569 (in 1986)

 1,754 (in 1934)

 1,620

 2,806 (in Feb. 1983)

 754 (in Aug. 1934)

Difference 

between sites

4,660 1914 –

2001

Mean:

Maximum:

Minimum:

 19,800

 49,900 

 2,890 

 19,255

 48,900

 4,790

 3,067

 6,587

 959

 889

 1,513

 337

Table 1. Daily mean flow statistics for the Danbury and St. Croix Falls, 

Wis., streamflow-gaging stations on the St. Croix River, Wis. and Minn., 

1902–2001

[ft3/s, cubic feet per second]

Danbury (ft3/s) St. Croix Falls (ft3/s)

Maximum 10,600 59,500

10 percent exceeds 2,190 9,020

50 percent exceeds 1,080 2,800

90 percent exceeds 730 1,580

Minimum 405 75
3

Table 3. Res 

on the St. Croix River, Wis. and Minn., 1902–2001

[Highlighted data indicate values that are statistically significant at the 95-percent confidence interval, p < 0.05; ft3/s/yr, cubic feet per second per year]

Site Annual flow

value

Slope

(ft3/s/yr)

Percent 

change

per year

Kendall’s 

Tau Probability

Danbury Peak instantaneous 15.0 0.30 0.145 0.070

Peak daily 15.1 .31 .154 .058

Mean 2.7 .20 .163 .017

7-day low flow 1.3 .68 .133 .052

St. Croix Falls Peak instantaneous 129.4 .53 .193 < .010

Peak daily 109.4 .45 .172 .013

Mean 24.1 .55 .308 < .010

7-day low flow 8.1 .50 .353 < .010

Difference 

between 

two sites

Peak instantaneous 101.9 .51 .178 .016

Peak daily 81.8 .42 .137 .049

Mean 17.1 .56 .258 < .001

7-day low flow 5.0 .56 .326 < .001

Mean annual flow was the only flow characteristic at the 

Danbury site that had a statistically significant (at the 95-

percent confidence level; p<0.05) trend. Although significant, it 

only increased slowly (0.2 percent per year). All four annual 

streamflow characteristics at the St. Croix Falls site had 

significantly increasing trends of 0.45 to 0.55 percent per year. 

Trends in the difference in flow between the two sites were 

similar to the trends at the St. Croix Falls site. This is to be 

expected because the flow characteristics at the Danbury site 

remained relatively stable while the flow characteristics at the 

St. Croix Falls site increased. The trends in the difference 

between the two sites were also significant for all four annual 

streamflow characteristics.

Annual mean flows were further analyzed for trends during 

spring, summer, fall, and winter. No significant trends were 

detected in seasonal flows at the Danbury site. Significant 

increasing trends were detected at the St. Croix Falls site during 

spring, fall, and winter. The slope of the annual summer mean 

flow at the St. Croix Falls site was positive, but not significant 

at the 95-percent confidence level. Summer flows are affected 

by rainfall-runoff events, and thus more variable than flows during the rest of the year. This variability may mask subtle trends.

Analysis of Streamflow Trends

The trend analysis of St. Croix River annual flow data over 

the period of record shows peak instantaneous, peak daily, 

mean, and 7-day low flows have no trend or only slightly 

increased at the Danbury site; whereas, flows have increased 

at the St. Croix Falls site. Often, streamflow trends can be 

attributed to changes in climate; however, the St. Croix River 

sites are in such close proximity that climate should similarly 

affect flow at both sites. It is therefore likely that the increasing 

flows at St. Croix Falls could be attributed to factors other than 

climate. 

Dam regulation impacts flow at both sites. The St. Croix 

Falls site is affected by the St. Croix Falls Dam and the Danbury site is affected by the Trego Dam. Both dams are used 

for electrical power generation. Permit requirements of dam 

operation require minimum flows be maintained during periods 

of regulation. When river flows drop below these minimums, 

regulation ceases and the river flows at run of the river conditions. Additionally, when flows are high there is more water 

flowing than can be used for power generation; turbines are at 

full capacity and excessive water still runs over the spillway, 

thus no regulation impacts occur. However, during periods of 

moderate flow, storage and release for power generation cause 

low flows to be lower (during storage) and high flows to be 

higher (during generation) than without regulation. Historical 

changes in reservoir operation such as required minimum flows 

or gate-changing procedures may have impacts on flow trends. 

In addition to dam regulation, other factors have been shown 

to impact flow trends in Wisconsin. A study by Potter (1991) 

attributed increased baseflow in a stream in eastern Wisconsin to advances in farming practices that resulted in increased 

recharge. A streamflow-trend study by Gebert and Krug (1996) 

concluded that decreased flood peaks and increased annual low 

flows in streams from the Wisconsin Driftless Area were caused 

by changes in agricultural practices and land use. It has been 

shown that runoff characteristics are different in urban areas of 

Wisconsin than in agricultural or forested areas, or areas with 

native grassland (Hunt, 2001).

Additional study of changes in climate, operation of hydropower dams, urban growth, agricultural practices, and land use 
0

500

1,000

1,500

2,000

2,500

3,000

1910192019301940195019601970198019902000

DISCHARGE, IN CUBIC FEET PER SECOND

ST. CROIX FALLS

DANBURY

Linear regression 

line

Linear regression 

line

10-year moving 

average

10-year moving 

average

would be needed to substantiate the causes of 

the observed flow trends.

Water Quality

The USGS water-quality database 

QWDATA contains 226 analyses of various 

water-quality constituents at the Danbury site 

and 343 analyses at the St. Croix Falls site. The 

earliest samples were suspended-sediment samples from the Danbury site in 1964 and nitrate 

samples from the St. Croix Falls site in 1966. 

Most of the data were collected in the mid1970s and early 1980s, and from 1995 to 2001. 

The major water-quality studies conducted by 

the USGS at these sites include: a 1975–83 

water-quality study commissioned by the NPS 

(Graczyk, 1986), a 1974–1986 USGS National 

Stream Quality Accounting Network study 

(St. Croix Falls site only) (U.S. Environmental 

Protection Agency, 1996), a 1995–98 USGS 

National Water-Quality Assessment study 

(Stark and others, 1996 and 1999), and from 

1997–99, a series of nutrient- and suspendedsediment-loading studies conducted in cooperation with the NPS and Wisconsin Department 

of Natural Resources (WDNR) (Fallon and 

McNellis, 2000; Lenz and others, 2001). Available water-quality data for the Danbury and St. 

Croix Falls monitoring sites are summarized 

in table 4. Values less than analytical detection 

limits were treated as zero for calculation of the 

statistics in table 4.

Field Water-Quality Constituents

Field water-quality constituents, such as 

water temperature, specific conductance, pH, 

and dissolved oxygen, were usually measured 

with the collection of USGS water-quality 

samples. Water temperatures of 0o Celsius (C) 

along with ice cover were common in the winter. Maximum river temperatures exceeding 

25o C were common in the summer. Water 

temperature values were slightly colder and 

less variable upstream at the Danbury site than 

at the St. Croix Falls site. Specific conductance 

values were lower and less variable at the 

Danbury site than at the St. Croix Falls site, 

averaging 122 and 169 microsiemens per centimeter at 25o

 C, respectively. Total and bicarbonate alkalinity values were also lower at the 

Danbury site (averaging 64 and 52 mg/L) than 

at the St. Croix Falls site (averaging 90 and 

Figure 2b. Annual mean flows for period 1902–2001 at Danbury and St. Croix Falls, Wis. on 

the St. Croix River, Wis. and Minn.

Figure 2c. Annual 7-day low flows for period 1912–2001 at Danbury and St. Croix Falls, Wis. 

on the St. Croix River, Wis. and Minn.

Figure 2a. Annual peak daily flows for period 1912–2001 at Danbury and St. Croix Falls, Wis. 

on the St. Croix River, Wis. and Minn.

4

0

10,000

20,000

30,000

40,000

50,000

60,000

1910

192019301940195019601970198019902000

DISCHARGE, IN CUBIC FEET PER SECOND

ST. CROIX FALLS

DANBURY

Linear regression 

line

Linear regression 

line

10-year moving 

average

10-year moving 

average

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

19021912192219321942195219621972198219922002

DISCHARGE, IN CUBIC FEET PER SECOND

ST. CROIX FALLS

DANBURY

Linear regression 

line

Linear regression 

line

10-year moving 

average

10-year moving 

average
Table 4. Summary of selected water-quality constituents analyzed at the Danbury and St. Croix Falls, Wis. monitoring sites on the St. Croix River, Wis. 

and Minn. [μs/cm, microsiemens per centimeter; °C, degrees Celsius; cols, colonies; mL, milliliters; mg/L, milligrams per liter; dis., dissolved; μg/L, micrograms per liter; %, percent]

Constituent

St. Croix River near Danbury, Wis. St. Croix River at St. Croix Falls, Wis.

Mean

Median

Maximum

Minimum

Standard 

deviation

Number of 

samples

Number of no 

detects

Mean

Median

Maximum

Minimum

Standard 

deviation

Number of 

samples

Number of no 

detects

Specific conductance (μs/cm) 122 12 228 48 31 210 0 169 169 295 7 43 269 0

pH, water field (standard units) 7.4 7.4 8.5 6.1 0.5 112 0 7.5 7.5 8.4 6.2 0.4 177 0

Temperature, water (°C) 9.4 6.5 26.0 -0.1 8.8 214 0 10.4 9.0 27.4 -0.1 9.2 272 0

Oxygen, dissolved (mg/L) 10 10 15 5 2 105 0 10 10 26 5 3 148 0

Fecal coliform3, (cols/100mL) 36 5 600 <2 124 24 201 46 21 420 <1 73 86 471

Fecal streptococcal

4

, (cols/100mL) 176 56 770 3 239 24 51 235 65 2000 1 447 82 241

Hardness total (mg/L as CaCO3) 57 58 81 26 12 110 0 79 81 120 26 18 176 0

Calcium, dis. (mg/L as Ca) 15.5 16 22 7 3 109 0 21 21 40 7 5 174 0

Magnesium, dis. (mg/L as Mg) 4.4 4.5 6.3 1.7 0.9 109 0 6.6 6.8 10.0 1.8 1.5 174 0

Sodium, dis. (mg/L as Na) 2.3 2.3 3.3 1.2 0.4 108 0 3.1 3.2 9.2 1.5 0.8 173 0

Potassium, dis. (mg/L as K) 0.6 0.6 1.3 0.3 0.2 108 0 1.1 1.0 5.2 <0.1 0.5 173 1

Bicarbonate, dis. field 

(mg/L as HCO3)

 64 68 106 22 17 92 0 90 93 130 30 22 114 0

Alkalinity, dis. field 

(mg/L as CaCO3)

 52 56 87 18 14 98 0 73 76 110 22 18 144 0

Sulfate, dis. (mg/L as SO4) 3.8 3.4 10.0 1.5 1.5 112 0 5.1 5.1 13.0 0.1 2.2 177 0

Chloride, dis. (mg/L as Cl) 2.3 2.2 7.8 <0.1 0.9 112 1 3.8 3.3 17.1 1.0 2.0 177 0

Fluoride, dis. (mg/L as F) <0.1 <0.1 0.3 <0.1 0.05 109 91 <0.1 <0.1 2.4 <0.1 0.3 176 100 

Silica, dis. (mg/L as SiO2) 11.3 11.0 17.0 7.7 2.3 108 0 11.6 11 27 4.8 3.3 173 0

Solids, dis. (mg/L) 73 75 136 36 16 106 0 96 98 153 37 20 173 0

Nitrogen, total (mg/L as N) 0.5 0.5 1.2 0.2 0.2 79 0 0.8 0.7 2.6 0.2 0.3 165 0

Nitrogen, dis. (mg/L as N) 0.4 0.3 1.1 0.05 0.2 56 0 0.6 0.6 1.8 0.33 0.2 91 0

Nitrogen, NO2+NO3 dis. 

(mg/L as N)

 0.1 0.09 0.38 <0.05 0.08 80 23 0.22 0.15 0.87 0.01 0.15 121 0

Nitrogen, ammonia dis. 

(mg/L as N)

0.02 0.02 0.09 <0.04 0.02 107 49 0.05 0.03 0.37 <0.01 0.06 119 24

Nitrogen, ammonia + organic total 

(mg/L as N)

 0.4 0.4 1.1 <0.1 0.2 109 3 0.6 0.5 3.2 0.13 0.4 172 0

Nitrogen, ammonia + organic dis. 

(mg/L as N)

 0.3 0.3 1.5 <0.1 0.2 80 10 0.5 0.4 1.2 0.07 0.2 117 0

Phosphorus, total (mg/L as P) 0.02 0.02 0.08 <0.008 0.02 109 24 0.04 0.04 0.20 <0.01 0.04 177 11

Phosphorus, dis. (mg/L as P) <0.01 <0.01 0.06 <0.01 0.01 106 45 0.02 0.01 0.17 <0.004 0.02 141 37

Phosphorus-ortho, dis. (mg/L as P) <0.01 <0.01 0.026 <0.01 0.007 80 59 <0.010 <0.010 0.257 <0.010 0.04 98 51

Iron, dis.2 (μg/L as Fe) 227 200 790 40 132 91 0 301 280 940 20 193 115 0

Lead, total2 (μg/L as Pb) 9.3 6.0 24 <1.0 8.4 9 1 9.8 7.0 63 <1.0 11.8 32 1

Manganese, dis.2 (μg/L as Mn) 18 12 270 5.8 29 91 0 32 24 223 1.2 29 115 0

Nickel, dis.2 (μg/L as Ni) 5.2 3.0 16 <1.0 5.6 8 1 2.8 1.0 19 <1.0 4.6 28 8

Selenium, total2 (μg/L as Se) <1 <1 <1 <1 0 9 9 <1 <1 1 <1 0 31 22

Zinc, total2 (μg/L as Zn) 14 10 20 <10 5 9 2 44 20 380 <10 88 31 14

Carbon, organic, dis. (mg/L as C) 7.1 6.1 20 2.4 3.7 78 0 8.4 7.7 20 1.5 3.7 85 0

Carbon, organic suspended, total 

(mg/L as C)

 0.5 0.4 2.2 0.1 0.4 78 0 8.8 8.1 21.3 1.1 4.1 109 0

Sediment, suspended (mg/L) 9 5 94 1 14 111 0 10 6 85 1 13 165 0

Sediment, suspended, 

% finer than 0.062 mm

 71 76 100 4 23 78 0 84 89 100 0 20 125 0

 1 Values below detect determined using most probable number method (Cochran, 1950); 2 May be positively biased from sample handling prior to 1992;

 3 Values determined using mFC MF agar method (Britton and Greeson, 1987); 4 Values determined using membrane filter method (Greeson and others, 1977)

5
0

0.05

0.1

0.15

0.2

0.25

Jan-1973Jan-1977Jan-1981Jan-1985Jan-1989Jan-1993Jan-1997Jan-2001

CONCENTRATION, IN MILLIGRAMS PER LITER

Danbury

St. Croix Falls

73 mg/L). Samples at both sites had pH values 

ranging from 6.1 to 8.0 (in standard units). 

Nutrients and Suspended Sediments 

Six suspended-sediment samples were 

collected at the Danbury site from 1964 to 1966. 

Suspended-sediment and nutrient samples were 

collected quarterly from 1976 to 1981, twice a 

year in 1982 and 1983, and monthly and during 

runoff events from 1995 to 2001. Two nitrate 

samples were collected in November 1966 at the 

St. Croix Falls site. Suspended-sediment and 

nutrient samples were collected monthly in 1975-

81, bi-monthly in 1982, monthly in spring and 

summer of 1983, quarterly from 1984 to 1986, 

twice in 1994, once in 1995, and monthly and 

during several runoff events from 1996 to 2001. 

Nutrient and suspended-sediment concentration data were generally lower and less variable 

upstream at the Danbury site than at the St. 

Croix Falls site. Mean total phosphorus and total 

nitrogen concentrations at the Danbury site 

were 0.02 and 0.5 milligrams per liter (mg/L), 

respectively, and mean concentrations at the 

St. Croix Falls site were 0.04 and 0.8 mg/L, 

respectively. The variability in total phosphorus 

concentrations at and between sites is shown 

in figure 3. Other nutrient data have similar 

variability. Mean suspended-sediment concentrations were 9 mg/L at the Danbury site and 10 

mg/L at the St. Croix Falls site. The mean percent

suspended sediment finer than 0.062 millimeter 

was 71 percent at the Danbury site compared to 

84 percent at the St. Croix Falls site. 

In an effort to account for the effects of variable flow on concentrations, flow-weighted concentrations of all nutrients and suspended sediment were calculated by water year at both sites. 

No trends were detected in statistical analysis of 

the flow-weighted data. The runoff-event-driven 

nature of nutrient and suspended-sediment loading in the St. Croix River Basin results in highly 

variable concentrations and loading rates (Lenz 

and others, 2001). Detection of trends in a basin 

with such variability is difficult without consistent, long-term data or major changes in water 

quality.

Major Ions

Major ion samples were collected at the Danbury site once in 1966, once in 1968, quarterly 

from 1976 to 1981, twice a year in 1982 and 

1983, once in 1985, and monthly and during runoff events from 1996 to 2001. At the St. Croix 

Figure 4. Dissolved sulfate concentrations at Danbury and St. Croix Falls, Wis. on the St. 

Croix River, Wis. and Minn.

Figure 5. Bacteria concentrations at Danbury and St. Croix Falls, Wis. on the St. Croix 

River, Wis. and Minn. [Bacteria sample methods after Britton and Greeson, 1987.]

Figure 3. Total phosphorus concentrations at Danbury and St. Croix Falls, Wis. on the St. 

Croix River, Wis. and Minn. [Values less than detection limits were plotted at zero.]

6

0

2

4

6

8

10

12

14

Jan-1973Jan-1977Jan-1981Jan-1985Jan-1989Jan-1993Jan-1997Jan-2001

CONCENTRATION, IN MILLIGRAMS PER LITER

Danbury

St. Croix Falls

0

500

1000

1500

2000

Jan-1976Jan-1980Jan-1984Jan-1988Jan-1992Jan-1996Jan-2000

CONCENTRATION, IN COLONES PER 100 MILLILITERS

Danbury

 Fecal coliform

 Fecal streptococci

St. Croix Falls

 Fecal coliform

 Fecal strep
Falls site, major ion samples were collected once in 1966, once 

in 1968, monthly in water years 1975–81, quarterly from 1982 

to 1985, twice in 1986, and monthly and during several runoff 

events from 1996 to 2001.

Patterns in the major ion concentration data were similar to 

those for nutrient and suspended-sediment concentration data; 

that is, there were generally lower and less variable concentrations upstream at the Danbury site than at the St. Croix Falls 

site. Sulfate was the only ion that had a statistically significant 

change in concentration over time. Sulfate concentrations were 

greater than 10 mg/L in many samples from the late 1970s and 

early 1980s, but all concentrations in samples from 1996 to 

2001 were less than 7 mg/L (fig. 4).

Metals

Metals in the water column were sampled annually from 

1975 to 1983 at the Danbury site and quarterly from 1975 to 

1986 at the St. Croix Falls site. Compared to examining concentrations in the water column, examination of streambed sediments can be more appropriate for detecting changes in metal 

levels in rivers. Brigham (2002) conducted detailed analysis 

of trace elements, including metals, in streambed sediments 

within the St. Croix River in 2000. He concluded that there did 

not appear to be large anthropogenic contributions above the 

background geochemical and atmospherically deposited levels. 

However, the study found possible occurrences of low-level 

anthropogenic contributions of several trace elements downstream of a few of the larger cities in the basin.

Bacteria

Fecal coliform and fecal streptococci samples were collected quarterly from 1976 to 1983 and once in 1985 at the 

Danbury site. Samples were collected monthly from 1975 to 

1982, quarterly from 1983 to 1986, and three times during the 

summer of 1999 at the St. Croix Falls site. Bacteria concentrations at both sites ranged from less than 1 to 2,000 colonies per 

100 milliliters and had substantial scatter with no distinct spatial 

or temporal patterns evident (fig. 5). There was no trend in the 

frequency of high bacteria counts.

Pesticides

Two pesticide samples were collected at the Danbury site 

in 1997. Compounds detected in one or both samples included 

atrazine (0.016 and 0.012 micrograms per liter (μg/L)), deethylatrazine (E1

0.005 μg/L), and diazinon (0.009 μg/L). Pesticides 

were sampled at the St. Croix Falls site once in 1995, monthly 

and during two rainfall-runoff events in 1997, twice in 1998, 

and once in 1999. Compounds detected included acetochlor 

(n = 3 at 0.006 to 0.026 μg/L), alachlor (n = 1 at 0.011 μg/L), 

atrazine (n = 16 at 0.007 to 0.24 μg/L), cyanazine (n = 1 at 

0.007 μg/L), diazinon (n = 1 at 0.007 μg/L), lindane (n = 1 at 

0.017 μg/L), metolachlor (n = 14 at E10.002 to 0.16 μg/L), and 

metribuzin (n = 1 at 0.007 μg/L). 

Analysis of Water Quality

The absence of consistent long-term water-quality data made 

trend analysis and comparison of water-quality between sites 

difficult. The water-quality data at these sites had considerable 

temporal variation and were often limited in extent by the datacollection needs of specific projects. Trends in water quality 

were not detected using the limited data available. Comparison 

of water quality between the upstream and downstream sites 

could contain possible biases caused by disparate sampling 

periods and strategies.

Population

Decadal census data from 1900 to 2000 (U.S. Census 

Bureau, 1995, 2002) for all counties mostly or entirely within 

the St. Croix River Basin above St. Croix Falls are shown in 

figure 6. Anoka, Mil Lacs, and Isanti Counties in Minnesota and 

Barron County in Wisconsin were not included in the population summaries because only a small portion of each of these 

counties is within the basin. The data are grouped into three categories: counties in Wisconsin above the Danbury site, counties 

in Wisconsin below the Danbury site, and counties in Minnesota below the Danbury site. Population in the basin increased 

from 1900 to 1920 and then remained relatively stable through 

1970. Starting in 1970, the population has greatly increased in 

the basin, with counties in Minnesota below the Danbury site 

showing the largest increase. U. S. Census Bureau data (2002) 

show the population in the St. Croix River Basin has increased 

0

50,000

100,000

150,000

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

POPULATION

Wisconsin counties above Danbury

Wisconsin counties below Danbury

Minnesota counties below Danbury

Figure 6. Population estimates for Wisconsin and Minnesota counties entirely or partially within the St. Croix Basin 

above St. Croix Falls, Wis. (U.S. Census Bureau, 2002).

1

 E, value is estimated

7
8

16 percent above Danbury and 98 percent between Danbury and 

St. Croix Falls from 1910 to 2000. Substantial land-use changes 

are likely associated with the population increase.

Summary and Conclusions

Trend analysis of long-term streamflow data from streamflow-gaging stations on the St. Croix River near Danbury (85 

years of record) and at St. Croix Falls (95 years of record) indicates that annual peaks, annual mean flows, and low flows have 

remained relatively stable at the Danbury (upstream) site while 

increasing by 0.45 to 0.55 percent per year at the St. Croix Falls 

(downstream) site. 

The St. Croix River sites are in such close proximity that 

climate should similarly affect flow at both sites. The trends 

of increased flow between Danbury and St. Croix Falls may 

be related to factors other than climate, such as hydropower 

operation, population growth, changes in agricultural practices, 

or changes in land use. Additional study would be needed to 

substantiate and quantify such relations.

 Water-quality data collected at these two sites from 1964 

to 2001 have high temporal variation due to the runoff-eventdriven nature of the St. Croix River and are often limited in 

extent by the data-collection needs of previous studies. Nutrient, suspended-sediment, and major ion concentrations were 

generally lower and less variable at the Danbury site than at 

the St. Croix Falls site. However, disparate sampling periods 

and strategies could cause possible biases in the comparison of 

water quality between sites. The absence of consistent longterm water-quality data made statistical trend analysis of waterquality data impractical. 

Starting in 1970, population has greatly increased in the 

basin, with the counties in Minnesota below Danbury showing 

the largest increase. 

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For Additional Information:

U.S. Geological Survey, 

Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

Telephone: 1–888–ASK–USGS

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

By Bernard N. Lenz

In cooperation with the National Park Service and the Joint 

Water Quality Commission of Danbury and St. Croix Chippewa 

Indians of Wisconsin.