U.S. Department of the Interior

U.S. Geological Survey

Response of the St. Croix River Pools,

Wisconsin and Minnesota, to

Various Phosphorus-Loading Scenarios

Water-Resources Investigations Report 02–4181

Prepared in cooperation with the

Wisconsin Department of Natural Resources

Taylors Falls, 

Minn.

MINNESOTA

WISCONSIN

St. Croix River basin

91°

46°

45°

93°

92°

St. Croix

River

WISCONSIN

Location of study area MINNESOTA

in Wisconsin and

Minnesota

Prescott, Wis.

Stillwater, Minn.

Taylors Falls, 

Minn.

Gordon, Wis.

Cable, Wis.

St. Croix Falls, Wis.

Danbury, Wis.

ST. CROIX

POLK

BARRON

BURNETT

W ASHBURN

DOUGLAS 

SAWYER

BAYFIELD

CARLTON

PINE

AITKIN

KANABEC

CHISAGO

ISANTI

W ASHINGTON

MILLE

LACS

ANOKA

PIERCE

92°50' 92°45'

45°

44°45'

PRESCOTT

STILLWATER

HUDSON

AFTON

Kinnickinnic River

Trout Creek

Valley Cr

Lake Mallalieu

Willow River

Browns Cr

12

94

Lake St. Croix and

Lake Mallalieu

Kinnickinnic Pool

Black Bass Pool

Catfish Bar 

Troy Beach Pool

Bayport Pool

EXPLANATION

Pool boundary

Sampling site

Gaging station
Response of the St. Croix River Pools, 

Wisconsin and Minnesota, to Various 

Phosphorus-Loading Scenarios

By Dale M. Robertson and Bernard N. Lenz

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4181

Prepared in cooperation with the

Wisconsin Department of Natural Resources

Middleton, Wisconsin

2002
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

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Government.
CONTENTS III

CONTENTS

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

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

Approach............................................................................................................................................................................... 4

Methods................................................................................................................................................................................. 4

Loads and concentrations............................................................................................................................................ 4

Confidence limits and errors ....................................................................................................................................... 5

Physical and chemical characteristics (model inputs)........................................................................................................... 5

Basin morphometry and water level............................................................................................................................ 5

Water quality in the St. Croix River pools .................................................................................................................. 6

Sources of nutrients to the St. Croix River pools........................................................................................................ 10

Tributaries ......................................................................................................................................................... 10

Point sources ..................................................................................................................................................... 10

Atmospheric sources ......................................................................................................................................... 13

Sediment release ............................................................................................................................................... 13

Summary of phosphorus inputs......................................................................................................................... 13

Water-residence time................................................................................................................................................... 16

Response in the water quality of St. Croix Falls Reservoir to changes in phosphorus loading............................................ 16

Modeling the response in the water quality of Lake Mallalieu and Lake St. Croix to changes in phosphorus loading....... 16

Hydrology ................................................................................................................................................................... 16

Algorithms and calibration.......................................................................................................................................... 17

Comparison between simulated and measured water quality ..................................................................................... 17

Response in water quality to changes in phosphorus loading..................................................................................... 18

Changes in phosphorus concentrations............................................................................................................. 18

Changes in chlorophyll a concentrations.......................................................................................................... 19

Changes in water clarity (Secchi depths).......................................................................................................... 21

Changes in trophic status .................................................................................................................................. 26

Changes in algal bloom response ..................................................................................................................... 26

Summary and conclusions .................................................................................................................................................... 30

References............................................................................................................................................................................. 32

Appendix:

Volumetrically weighted total phosphorus concentrations for various phosphorus loading scenarios ............................ 33

FIGURES

1–2. Maps showing:

1. St. Croix River Basin, with stream sites sampled during 1997–99 identified ......................................................... 2

2. St. Croix River pools, with pools and sampling locations identified....................................................................... 3

3–14. Graphs showing:

3. Water-quality at three locations in the St. Croix Falls Reservoir from May through September, 1999, 

and streamflow at St. Croix Falls ................................................................................................................ 8

4. Measured and simulated (from BATHTUB) water quality in Lake Mallalieu and the Lake St. Croix pools . 9

5. Phosphorus loading to the lower St. Croix River pools for the 1999 flow regime, simulated dry year, and 

simulated wet year....................................................................................................................................... 15

6. Measured and simulated chloride concentrations in Lake Mallalieu and the Lake St. Croix pools used to

determine calibration factors for dispersion between adjacent pools in the BATHTUB model ................. 17

7. Simulated changes in phosphorus concentrations in Lake Mallalieu and the Lake St. Croix pools, on the

basis of output from BATHTUB for the 1999 flow regime......................................................................... 20

8. Simulated changes in phosphorus concentrations in Lake Mallalieu and the Bayport Pool for the

1999 flow regime, simulated dry year, and simulated wet year ................................................................. 21
IV CONTENTS

CONTENTS—Continued

9. Simulated changes in chlorophyll a concentrations in Lake Mallalieu and the Lake St. Croix pools for 

the 1999 flow regime .................................................................................................................................. 23

10. Simulated changes in chlorophyll a concentrations in Lake Mallalieu and the Bayport Pool for the

1999 flow regime, a simulated dry year, and a simulated wet year ........................................................... 24

11. Simulated changes in Secchi depth in Lake Mallalieu and the Lake St. Croix pools for the 1999 flow 

regime ......................................................................................................................................................... 25

12. Simulated changes in Secchi depths in Lake Mallalieu and the Bayport Pool for the 1999 flow regime, 

a simulated dry year, and a simulated wet year .......................................................................................... 26

13. Simulated changes in the trophic state index values in Lake Mallalieu and the Lake St. Croix pools for 

the 1999 flow regime .................................................................................................................................. 27

14. Simulated changes in the frequency of algal blooms in Lake Mallalieu and the Lake St. Croix pools for 

the 1999 flow regime .................................................................................................................................. 29

15. Simulated changes in algal bloom frequency in Lake Mallalieu and the Bayport Pool for the 

1999 flow regime, a simulated dry year, and a simulated wet year ........................................................... 31

TABLES

1. Morphometry of each of the St. Croix River pools based on an average water level during May–September, 

1999, and mean depths during 1999, the simulated dry year, and the simulated wet year................................... 6

2. Average summer water quality for various locations in the St. Croix River pools, 1999 ....................................... 7

3. Tributary phosphorus loading to lower St. Croix River pools................................................................................. 10

4. Total flow and volumetrically weighted mean water quality for monitored and unmonitored areas of the study 

area for May–September 1999.............................................................................................................................. 11

5. Total flow and volumetrically-weighted mean water quality for monitored and unmonitored areas of the study

area for May–September for the simulated dry year and simulated wet year ...................................................... 12

6. Phosphorus loading to St. Croix River pools from point sources, May–September 1999...................................... 13

7. Phosphorus loading from atmospheric inputs and sediment release into Lake Mallalieu and Lake St. Croix ....... 14

8. Phosphorus release rates from the sediments of Lake St. Croix ............................................................................. 14

9. Summary of phosphorus loading to Lake Mallalieu/Lake St. Croix pools............................................................. 14

10. Water-residence time in the St. Croix River pools.................................................................................................. 16

11. Algorithms used within BATHTUB to simulate water quality in Lake Mallalieu/Lake St. Croix pools ............... 18

12. Average water quality during May–September measured at Stillwater and Prescott, 1977–2000 ......................... 22

13. Simulated algal bloom frequency in Lake Mallalieu and through Lake St. Croix in response to various 

phosphorus-loading scenarios, on the basis of output from BATHTUB .................................................... 30
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED WATER-QUALITY UNITS V

CONVERSION FACTORS AND ABBREVIATED WATER-QUALITY UNITS

Multiply By To Obtain

meter (m) 3.2808 feet

square meters (m2) 10.76391 square feet

kilometer (km) .62137 miles

square kilometer (km2) 247.105 acres

square kilometer (km2

) .3861 square miles

cubic meters (m3) 35.3147 cubic feet

cubic hectometer (hm3) 3.51347 x 107 cubic feet

liter (L) .26417 gallons

gram (g) .0022 pounds

kilogram (kg) 2.2046 pounds

cubic meters per day (m3/d) .000409 cubic feet per second

Water year: Water year is defined as the period beginning October 1 and ending September 30, designated by the calendar year in which it 

ends.

Abbreviated water-quality units used in this report: Chemical concentrations and water temperature are given in metric units. Chemical 

concentration is given in milligrams per liter (mg/L) or micrograms per liter (μg/L). Milligrams per liter is a unit expressing the concentration 

of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand micrograms per liter is 

equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts 

per million.
VI ACKNOWLEDGMENTS

ACKNOWLEDGMENTS

Technical Reviewers

William J. Rose, Hydrologist, U.S. Geological Survey, Middleton, Wis.

Patrick W. (Buzz) Sorge, Lake Management Planner, Wisconsin Department of Natural Resources, 

Eau Claire, Wis.

Patrick S. Oldenburg, Water Resources Engineer, Wisconsin Department of Natural Resources, 

Eau Claire, Wis.

Kent Johnson, Metropolitan Council-Environmental Services, St. Paul, Minn.

Catherine E. Larson, Principal Environmental Scientist, Metropolitan Council-Environmental Services, 

St. Paul, Minn.

Modeling Assistance

John C. Panuska, Water Resource Engineer, Wisconsin Department of Natural Resources, 

Madison, Wis.

Editorial and Graphics

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

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

Gail A. Moede, Technical Editor, U.S. Geological Survey, Madison, Wis.

Kathleen A. Wang, Editorial Assistant (LTE), U.S. Geological Survey, Middleton, Wis.

Chester Zenone, Hydrologist (Retired), U.S. Geological Survey, Reston, Va.

Technical Assistance

Robert M. Goldstein, Hydrologist, U.S. Geological Survey, Mounds View, Minn.

Approving Official

Dorothy H. Tepper, Reports Improvement Advisor, U.S. Geological Survey, Reston, Va.
Abstract 1

Abstract

The pools in the lower reach of the St. Croix 

National Scenic Riverway, Wisconsin and Minnesota, and the adjoining Lake Mallalieu, are 

eutrophic because of high phosphorus loading. To 

determine how changes in phosphorus loading 

would affect the trophic status of these pools, the 

water-quality model, BATHTUB, was used to simulate existing (1999) water quality and simulate the 

water quality with various phosphorus-loading scenarios. Water quality in the pools may respond differently during different flow regimes; therefore, 

sensitivity and scenario evaluations were performed not only for 1999, but also for a simulated 

period with relatively low flows throughout the 

basin (using flow data from 1988) and for a simulated period with relatively high flows throughout 

the basin (using flow data from 1996). 

On the basis of the BATHTUB simulations, 

linear increases in phosphorus loading should 

cause the following changes in water quality in 

each of the pools: linear increases in phosphorus 

concentrations, although at a smaller rate than the 

increase in loading; non-linear increases in chlorophyll a concentrations, with a smaller relative 

response with higher phosphorus loading; increase 

in the frequency of algal blooms, with a higher frequency of intense algal blooms; and slightly 

decreased water clarity.

The response in water quality to changes in 

the phosphorus loading should be relatively similar 

regardless of the flow regime. Reducing phosphorus loading by about 50 percent would be necessary 

for the Lake St. Croix pools to be classified as 

mesotrophic with respect to phosphorus and chlorophyll a concentrations, whereas a larger reduction in phosphorus loading would be needed for 

Lake Mallalieu to be classified as mesotrophic. 

Even with these reductions, water clarity will 

remain poor because of the high non-algal turbidity 

and stained water in the pools.

INTRODUCTION

The St. Croix National Scenic Riverway (hereafter 

referred to as “Riverway”) was established in 1968 as 

one of the original eight components of the National 

Wild and Scenic Rivers Act. The Riverway includes the 

St. Croix River from the Gordon Dam near Gordon, 

Wisconsin to the confluence with the Mississippi River 

at Prescott, Wisconsin, a distance of 247 km (fig. 1). 

The Riverway also includes the 158-km long Namekagon River from the Namekagon Dam near Cable, Wisconsin, to its confluence with the St. Croix River. 

Several pools are in the lower 100 km of the Riverway 

and include the St. Croix Falls Reservoir behind the 

hydropower dam at St. Croix Falls, and a series of natural pools downstream of Stillwater collectively called 

Lake St. Croix (fig. 2). The Willow River enters into the 

Lake St. Croix, near Hudson, Wisconsin. Lake Mallalieu is located behind the dam at the mouth of the Willow River (fig. 2). In this report, we refer to all of these 

pools as the “St. Croix River pools”. Due to the proximity of this area to the Minneapolis/St. Paul, Minnesota, 

metropolitan area, it has experienced increased use and 

developmental pressure. Recreational use on the river 

has doubled since 1973 to nearly one million visitors 

per year (National Park Service, 1995).

In 1994, the National Park Service (NPS), Wisconsin Department of Natural Resources (WDNR), Minnesota Department of Natural Resources (MDNR), 

Metropolitan Council-Environmental Services 

(MCES), Minnesota Pollution Control Agency 

(MPCA), and Minnesota-Wisconsin Boundary Area 

Commission signed a Memorandum of Understanding 

that formed the interagency St. Croix Basin Water 

Resources Planning Team (hereafter referred to as 

Response of the St. Croix River Pools, Wisconsin and 

Minnesota, to Various Phosphorus-Loading Scenarios 

By Dale M. Robertson and Bernard N. Lenz
2 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Figure 1. St. Croix River Basin, with stream sites sampled during 1997–99 identified.

EXPLANATION

MINNESOTA

WISCONSIN

St. Croix River basin

Minnesota Pollution Control Agency 

monitoring site

Metropolitan Council - Environmental

Services monitoring site

U.S. Geological Survey 

tributary-monitoring site

U.S. Geological Survey 1998 

synoptic site

U.S. Geological Survey long-term

monitoring site

91°

46°

45°

City Base map: Seaber and others, 1986.

93°

92°

Sna

ke

River

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

Location of study area MINNESOTA

in Wisconsin and

Minnesota

Prescott, Wis.

Stillwater, Minn.

Taylors Falls, 

Minn.

Gordon, Wis.

Cable, Wis.

St. Croix Falls, Wis.

Danbury, Wis.

ST. CROIX

POLK

BARRON

BURNETT

WASHBURN

DOUGLAS 

SAWYER

BAYFIELD

CARLTON

PINE

AITKIN

KANABEC

CHISAGO

ISANTI

WASHINGTON

MILLE

LACS

ANOKA

PIERCE
INTRODUCTION 3

92°45' 92°40'

45°30'

45°25'

92°50' 92°45'

45°

44°45'

Nevers Dam Road

Dry

Creek

ST. CROIX

FALLS

PRESCOTT

STILLWATER

HUDSON

AFTON

Kinnickinnic River

Trout Cr

Valley Cr

Lake Mallalieu

Willow River

Browns Cr

12

94

St. Croix Falls Reservoir Lake St. Croix and

Lake Mallalieu

Nevers Dam site

St. Croix Falls Dam site

Kinnickinnic Pool

Black Bass Pool

Catfish Bar 

Troy Beach Pool

Bayport Pool

EXPLANATION

Pool boundary

Sub-pool boundary

Sampling site

Gaging station 0 2.5

0 2.5 5 MILES

5 KILOMETERS

0 2.5

0 2.5 5 MILES

5 KILOMETERS

Figure 2. St. Croix River Pools, with pools and sampling locations identified.
4 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

“Basin Team”) and agreed to investigate water-resource 

issues throughout the basin. Issues initially identified 

included effects from point and non-point sources of 

nutrient and suspended sediment loading, ground-water 

contamination, stormwater discharge, toxic contaminants, exotic species, impoundment, and recreational 

use.

The Basin Team developed a Water Resources 

Management Plan for the Riverway (Holmberg and others, 1997) that has been instrumental in securing funds 

for water-quality studies including an interagency nutrient and suspended sediment monitoring program with 

more than 25 sites monitored by six agencies during 

1997–99 (Lenz and others, 2001). The Basin Team in 

collaboration with the U.S. Geological Survey (USGS) 

collected flow and water-quality data on the Riverway 

and its tributaries (fig. 1), especially in 1999. In this 

cooperative study between the USGS and the WDNR, 

we use the data collected from 1997–99 as part of other 

studies to calibrate and apply the water-quality model, 

BATHTUB (Walker, 1996), to the St. Croix River pools 

(fig. 2) to provide a better understanding of the sensitivity and anticipated trophic response of the system to 

changes in phosphorus loading from point and nonpoint sources. In future studies, the calibrated model 

can be used to evaluate the effects of specific management scenarios supplied by the Basin Team. This information will be used to update the Management Plan that 

will be used to help make nutrient-management decisions throughout the basin.

APPROACH

To determine the sensitivity of each of the St. Croix 

River pools to decreases and increases in phosphorus 

loading, the BATHTUB model was first calibrated 

using pool and tributary data collected in 1999 by the 

USGS, WDNR, MCES, MPCA, and NPS. The model 

was used to simulate the average water quality for the 

period from May 1 to September 30 rather than a full 

year because of the short water-residence time in each 

pool and this is when most vegetative growth occurs in 

most lakes in the area. Because of the distance between 

St. Croix Falls Reservoir and Lake St. Croix, and 

because several tributaries enter the St. Croix River 

downstream of St. Croix Falls, an attempt was made to 

develop two separate BATHTUB models—one model 

for St. Croix Falls Reservoir, and one for Lake Mallalieu/Lake St. Croix. An independent BATHTUB model 

was not developed for Lake Mallalieu because flow and 

water quality were not measured between Lake Mallalieu and Lake St. Croix.

Variability in rainfall intensity throughout the area 

in 1999 resulted in relatively high flows and loading 

from the northern part of the basin compared to that 

from the southern part (Lenz and others, 2001). Water 

quality in the various pools responds differently during 

different flow regimes. Therefore, sensitivity and scenario evaluations were performed not only for 1999, but 

also for a simulated period with relatively low flows 

throughout the basin (using flow data from 1988) and 

for a simulated period with relatively high flows 

throughout the basin (using flow data from 1996). During 1999, summer-average (May–September) flow at 

St. Croix Falls was exceeded in 31 of the past 99 years. 

In 1988, summer-average flow was exceeded in 92 of 

the past 99 years, and in 1996, summer-average flow 

was exceeded in 28 of the past 99 years. During each of 

these flow regimes, phosphorus loading from each tributary was estimated by use of flow-to-load relations 

based on 1997–99 data, and loading from other sources 

was assumed to be similar to that estimated for 1999.

METHODS

Loads and Concentrations

Seasonal loads of total and dissolved ortho-phosphorus from most monitored tributaries were estimated 

with a regression approach by use of the Estimator program (Cohn and others, 1989). Estimated daily loads 

(L) were computed on the basis of relations determined 

between constituent load (in kilograms) and two variables: flow (Q, in cubic meters per day) and time of the 

year (T, in radians). The general form of the model was

log (L) = a + b [log (Q) - c] + d [log (Q) - c]2 +

e [sin (T)] + f [cos (T)] (1)

Values for the regression coefficients (a, b, c, d, e, 

and f) in Equation 1 were computed for each site by the 

use of a multiple regression analysis between daily 

loads (daily average flows multiplied by instantaneous 

measured concentrations), daily flows (Q), and time of 

the year (T). The flow-to-load relation in Equation 1 

was calibrated for each site using all available data from 

1997–99 in the regression analysis. Only those terms 

that were significant at p < 0.05 were included in the 

regression. Because a logarithmic transformation was 
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 5

used in Equation 1, estimated daily loads were adjusted 

to account for a re-transformation bias by use of the 

minimum variance unbiased estimate (MVUE) procedure (see Cohn and others, 1989, for a complete discussion). 

The flow-to-load relations developed by use of 

Equation 1 and the daily flow data from 1999 were used 

to compute tributary loads of total and dissolved orthophosphorus for May through September for all monitored tributaries, except Browns and Valley Creeks. 

Total phosphorus loads for Browns and Valley Creeks 

for 1999 were computed by the MCES with the use of 

the model FLUX (Walker, 1996). 

Tributary loading for the simulated low- and highflow regimes were calculated by use of the same flowto-load relations (developed by use of Eq. 1) with the 

actual or estimated flows for 1988 (low-flow regime) 

and 1996 (high-flow regime). Actual flows for these 

years from the St. Croix River (at St. Croix Falls) and 

Apple River were used. Flows for these years were not 

available for the Willow and Kinnickinnic Rivers and, 

therefore, were estimated by adding or removing high 

flows (peaks) in the 1999 daily flow records for those 

sites. All significant peaks in flow were removed for the 

low-flow estimates, while for the high-flow estimates, 

event peaks were added on the basis of the relative 

increases in flow at nearby sites with actual continuous 

or peak-flow data, the magnitude of known peaks that 

have occurred at the site in the past, the percent increase 

over 1999 flows in the actual Apple River and St. Croix 

River flow records, and professional judgment. Baseflow during the low-flow and high-flow regimes was 

assumed to be similar to that in 1999. Because the flowto-load relations were based only on available 1997–99 

water-quality data, the estimated loads during the simulated low- and high-flow regimes may differ from those 

from 1988 and 1996.

Flow-to-load relations were not available for 

Browns and Valley Creeks; therefore, the percent 

changes in the flow and phosphorus concentrations in 

the Apple River between the simulated wet and dry 

years and those measured for 1999 were used to adjust 

the phosphorus loading for these two sites. In other 

words, similar percent changes in flow and concentration were assumed to take place in Browns and Valley 

Creeks as those measured in the Apple River. 

Volumetrically weighted mean concentrations of 

total and dissolved ortho-phosphorus for May through 

September were then computed by dividing the total 

load by the total flow during this period. In tributaries 

where total phosphorus concentrations were measured 

but dissolved ortho-phosphorus concentrations were 

not, volumetrically weighted mean concentrations of 

dissolved ortho-phosphorus were set to be one-third of 

the total phosphorus concentration, which is approximately the fraction observed in most other monitored 

tributaries. 

Volumetrically weighted mean chloride concentrations were computed for May through September, 1999, 

in a similar manner to that for total phosphorus. Average 

total and inorganic nitrogen concentrations for 1999 

were computed by averaging the concentrations measured in each tributary from May through September, 

1999. The concentrations measured for total and inorganic nitrogen in 1999 were also used for dry- and wetyear simulations. In other words, no adjustments in 

nitrogen concentrations were made for the various flow 

regimes. 

Confidence Limits and Errors

The coefficient of variation was computed for each 

of the inputs to BATHTUB (water-quality concentrations and flow estimates). These values were used 

within BATHTUB to calculate an estimate of the certainty (plus and minus one standard error of the estimate) for each simulated value.

PHYSICAL AND CHEMICAL 

CHARACTERISTICS (MODEL INPUTS)

Three types of data are required as input to the 

BATHTUB model: morphometric information for each 

pool or basin in the model, water-quality data for each 

pool, and nutrient loading to each pool from all sources. 

The water-quality and nutrient-loading data were averaged over the period from May 1 through September 30, 

1999. This period was chosen rather than a full year 

because of the short water residence time in each pool 

and because May through September represents the 

period when most vegetative growth occurs in most 

lakes in the area.

Basin Morphometry and Water Level

The surface area and volume of each of the pools, 

as delineated in figure 2, are based on an average water 

level during May–September, 1999 (table 1). The aver-
6 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

age water level during the simulation period (May 1 

through September 30) for each of the St. Croix River 

pools was estimated for each of the three flow regimes 

from water levels measured at Prescott and Stillwater 

during 1988, 1996, and 1999 (the average water level of 

St. Croix Falls Reservoir was only estimated for 1999). 

Water levels in the simulated dry year (1988) were 

0.6 m lower than in 1999. Water levels during the simulated wet year (1996) were 0.5 m higher than in 1999. 

The mean depth of each pool for each flow regime 

(table 1) was computed by adding or subtracting the 

average change in water level. The water level in Lake 

Mallalieu is maintained at a near constant water level; 

therefore, its mean water depth was kept constant at 

1.6 m in all simulations.

Water Quality in the St. Croix River Pools

Water-quality data for each of the St. Croix River 

pools (sampling sites are shown in fig. 2) were collected 

from May through September, 1999. Water-quality data 

were collected bi-weekly at three locations in and above 

the St. Croix Falls Reservoir (by the NPS), one location 

in Lake Mallalieu (by the WDNR), and eight locations 

in Lake St. Croix (water quality by the MCES and 

WDNR, and Secchi depths by volunteers-only sites for 

which data were used are identified in fig. 2). Waterquality data were not measured in the Black Bass Pool; 

therefore, the water-quality data for this pool were calculated by averaging data collected at Catfish Bar (just 

upstream) and in the Kinnickinnic Pool (just downstream). These data were summarized into mean 

monthly and seasonal (May–September) concentrations. The average summer data are given in table 2. 

Water quality was relatively uniform throughout 

the St. Croix Falls Reservoir. The quality of the water 

within the reservoir was similar to that which entered 

and exited the reservoir (fig. 3). Nutrient concentrations 

were high; the average total phosphorus concentration 

was almost 100 μg/L, and average total nitrogen concentration was about 1,000 μg/L. The average dissolved 

ortho-phosphorus concentration was about 20 μg/L. 

Chlorophyll a concentrations, however, were relatively 

low and averaged only about 13 μg/L. The average Secchi depth was about 1 m. 

The ratio of near-surface concentrations of total 

nitrogen to total phosphorus is often used to determine 

the potential limiting nutrient in a lake. The specific 

value of when this ratio determines which nutrient is 

potentially limiting, however, varies under different 

conditions (Correll, 1998). Therefore, a range in the 

ratio is often used to determine the potential limiting 

nutrient. A ratio greater than about 15:1, by weight, 

indicates that phosphorus is potentially the limiting 

nutrient, whereas a ratio less than 5:1, by weight, indiTable 1. Morphometry of each of the St. Croix River pools based on an average water level during 

May–September, 1999, and mean depths during 1999, the simulated dry year (1988), and the simulated wet 

year (1996)

[km, kilometers, km2, square kilometers; m3, cubic meters, m, meters; --, not determined]

Average 1999 water level Mean depth (m)

Pool name Length

(km)

Area 

(km2)

Volume

(m3 x106) 1999 Dry year

(1988)

Wet year

(1996)

St. Croix Falls Reservoir 10.1 2.414a

a

Area computed based on the mean width being assumed to be similar to that from St. Croix Falls Dam to the sub-pool boundary (fig. 2).

9.6 4.0b

b

Mean depth assumed to be similar to that from St. Croix Falls Dam to sub-pool boundary (fig. 2).

-- --

Lake Mallalieu 2.4 1.093 1.8 1.6 1.6 1.6

Bayport Pool 9.7 11.340 77.1 6.8 6.2 7.3

Troy Beach Pool 9.7 12.660 133.0 10.5 9.9 11.0

Black Bass Pool 11.3 5.450 73.6 13.5 12.9 14.0

Kinnickinnic Pool 8.1 5.830 57.1 9.8 9.2 10.3
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 7

Table 2. Average summer (May–September) water quality for various locations (identified in fig. 2) in the

St. Croix River pools, 1999

[N, nitrogen; P, phosphorus; Chl a, chlorophyll a; Cl, chloride; TSS, total suspended sediment; μg/L, micrograms per liter; m, meters; 

cv, coefficient of variation; --, not measured; data within St. Croix Falls Reservoir were collected by the National Park Service, within Lake 

Mallalieu and the Bayport, Troy Beach, Catfish Bar, and Kinnickinnic Pools by the Wisconsin Department of Natural Resources, at

Stillwater and Prescott by the Metropolitan Council-Environmental Services, and Secchi depth data in Lake St. Croix by volunteers; 

data for Black Bass Pool were computed as the average of concentrations measured at Catfish Bar and in the Kinnickinnic Pool]

Total N

(μg/L)

Total P

(μg/L)

Ortho-P

[μg/L)

N : P 

ratio

Chl a

(μg/L)

Secchi

(m)

Cl

(μg/L)

TSS

(μg/L)

St. Croix Falls Reservoir

Nevers Dam Road 996 100 18 10.0 11 1.0 2,900 9,100

cv 0.14 0.75 0.23 -- 0.44 0.21 0.38 0.31

Dry Creek 859 96 17 8.9 14 1.1 2,900 8,500

cv .34 .73 .37 -- .24 .17 .25 .33

St. Croix Falls 875 84 17 10.4 13 1.0 2,900 7,270

cv .24 .59 .30 -- .28 .25 .25 .30

Lake Mallalieu

Lake Mallalieu 2,262 111 14 20.4 79 .7 11,530 19,600

cv .13 .50 1.01 -- .35 .26 .08 .51

Lake St. Croix

Stillwater 908 72 17 12.6 19 -- 4,000 8,800

cv .11 1.03 .32 -- .34 .39 .22

Bayport 1,027 52 14 19.6 15 1.2 4,520 4,780

cv .12 .18 .50 -- .55 .21 .11 .29

Hudson 1,041 52 16 20.2 12 1.8 7,580 4,720

cv .15 .22 .58 -- .64 1.19 1.27 .43

Troy Beach 1,161 50 15 23.1 9 1.4 4,790 3,740

cv .08 .19 .63 -- .91 .31 .10 .40

Catfish Bar 1,183 47 23 25.2 9 1.6 6,380 3,020

cv .24 .19 .77 -- 1.06 .23 .60 .31

Black Bass 1,215 45 20 26.8 8 1.6 6,165 3,365

cv .18 .21 .71 -- 1.10 .23 .46 .37

Kinnickinnic 1,247 44 18 28.5 8 -- 5,950 3,710

cv .12 .23 .65 -- 1.15 -- .33 .43

Prescott 1,188 50 16 23.8 17 -- 4,200 4,500

cv .26 .74 .73 -- 1.22 -- .22 .40
8 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Figure 3. Water quality at three locations in St. Croix Falls Reservoir (upstream of reservoir near Nevers Dam 

Road, mid reservoir near Dry Creek, and downstream end of reservoir at St. Croix Falls) from May through

September, 1999, and streamflow at St. Croix Falls, Wis.

0

50

100

150

200

250

300

TOTAL PHOSPHORUS,

IN MICROGRAMS PER LITER

TOTAL NITROGEN,

IN MICROGRAMS PER LITER

CHLOROPHYLL 

A,

IN MICROGRAMS PER LITER

SECCHI DEPTH, IN METERS

0

50

100

150

200

250

300

350 Upstream (Nevers Dam site)

Mid reservoir (Dry Creek site)

Downstream (St. Croix Falls site)

Flow (St. Croix Falls)

0

400

800

1,200

1,600

0

5

10

15

20

0.0

0.4

0.8

1.2

1.6

FLOW, IN CUBIC METERS PER SECOND

1999

MAY JUNE JULY AUG SEPT

MAY JUNE JULY AUG SEPT

MAY JUNE JULY AUG SEPT

MAY JUNE JULY AUG SEPT
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 9

cates that nitrogen should be the limiting nutrient. A 

value between these two ratios indicates the two nutrients can be co-limiting. The average nitrogen-to-phosphorus ratio for the St. Croix Falls Reservoir was about 

10:1 (table 2); therefore, nitrogen and phosphorus can 

both limit potential algal growth.

Variations in water quality throughout Lake Mallalieu and Lake St. Croix are shown in figure 4. In Lake 

Mallalieu, average summer nutrient and chlorophyll a

concentrations were very high (concentrations of total 

phosphorus and chlorophyll a were 111 μg/L and 

79 μg/L, respectively) and Secchi depths were low 

(average depth of 0.7 m) compared to Lake St. Croix, 

which had average total phosphorus concentrations of 

45 to 52 μg/L and chlorophyll a concentrations of 8 to 

15 μg/L. Dissolved ortho-phosphorus concentrations 

ranged from 14 to 23 μg/L, with no consistent trend 

throughout the system. Phosphorus and chlorophyll a

concentrations decreased slightly from the Bayport 

Pool (upstream) to the Kinnickinnic Pool (downstream), whereas water clarity (Secchi depth) improved 

slightly from the Bayport Pool (1.2 m) to the Kinnickinnic Pool (1.6m).

The average nitrogen-to-phosphorus ratio in Lake 

Mallalieu and each of the pools of Lake St. Croix was 

about 20:1 (table 2). This ratio dropped below 10:1 only 

once in Lake Mallalieu during 1999 and was never measured less than 17:1 in any Lake St. Croix pool. Therefore, if just nitrogen and phosphorus are considered, 

phosphorus is the nutrient in shortest supply and will 

almost always be the nutrient limiting algal growth in 

these pools.

Figure 4. Measured and simulated (from BATHTUB) water quality in Lake Mallalieu and Lake St. Croix pools (summer average, 

May–September, 1999). Vertical lines on each data point represent plus and minus one standard error in the values [LM, Lake 

Mallalieu; BP, Bayport Pool; TB, Troy Beach Pool; BB, Black Bass Pool; KP, Kinnickinnic Pool].

0

30

60

90

120

150

Simulated

Measured

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Simulated

Measured

0

25

50

75

100 Simulated

Measured

0.0

0.5

1.0

1.5

2.0

Simulated

Measured

TOTAL PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

TOTAL NITROGEN CONCENTRATION,

IN MICROGRAMS PER LITER SECCHI DEPTH, IN METERS

CHLOROPHYLL A CONCENTRATION,

IN MICROGRAMS PER LITER

LM BP TB BB KP LM BP TB BB KP

LM BP TB BB KP LM BP TB BB KP
10 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Sources of Nutrients to the St. Croix River 

Pools

Tributaries

Daily average flow data for May through September for 1999, a simulated dry year (using flows measured in 1988), and a simulated wet year (using flows 

measured in 1996) were input into the flow-to-load relations (developed based on 1997–99 data) for each monitored tributary to calculate daily loads for total and 

dissolved ortho-phosphorus from all monitored tributaries to the St. Croix River pools (table 3). Loads from 

selected tributaries were then used to estimate the loading from unmonitored areas by use of drainage-area 

ratios with monitored streams. The stream used to estimate loading from each unmonitored area is given in 

parentheses in table 3. Loads from most unmonitored 

areas were extrapolated from the nearest monitored 

stream, except for tributaries to the Troy Beach Pool and 

the Black Bass Pool. Unmonitored areas of the Bayport 

Pool were extrapolated from the St. Croix River at St. 

Croix Falls and the Apple River. Unmonitored areas of 

Lake Mallalieu and the Black Bass Pool were extrapolated from the Willow River. Unmonitored areas of the 

Troy Beach Pool were extrapolated from the Apple 

River. These sites were chosen because their watersheds 

had similar land use as the unmonitored areas. 

Volumetrically weighted mean concentrations for 

total and dissolved ortho-phosphorus were computed by 

dividing the sum of the daily loads by the sum of the 

daily flows for May–September. Flows and mean concentrations for total and dissolved ortho-phosphorus for 

1999 are listed in table 4 (along with concentrations for 

the nitrogen species and chloride). Concentrations 

based on flows for the simulated dry year (flows from 

1988) and simulated wet year (flows from 1996) are 

listed in table 5. Average flow and volumetrically 

weighted mean concentrations for the unmonitored 

areas were extrapolated in a similar manner to that 

described for loads. The tributaries used for each 

unmonitored area are given in tables 4 and 5.

Point Sources

Contributions of nutrients from point sources that 

discharge directly into the St. Croix River pools (direct 

point sources) and from point sources that discharge 

into the tributaries and incorporated into the loads in 

table 3 (indirect point sources) were determined for 

May–September, 1999. Contributions from each point 

source were computed based on water-quality and flow 

Table 3. Tributary phosphorus loading (May–September) to lower St. Croix River pools

[Names in parentheses are the streams used for extrapolation to unmonitored areas; kg, kilograms; monitored, monitored area during 1999]

1999 Simulated dry year Simulated wet year

Tributary kg Percent kg Percent kg Percent

Willow River monitored 4,485 3.8 2,952 8.7 5,910 4.0

Lake Mallalieu unmonitored (Willow) 123 .1 81 .2 162 .1

St. Croix River monitored 85,059 71.5 18,472 54.7 101,802 69.3

St. Croix River unmonitored 2,579 2.2 560 1.7 3,087 2.1

Apple River monitored 11,595 9.7 2,776 8.2 14,738 10.0

Apple River unmonitored 621 .5 149 .4 789 0.5

Browns Creek monitored 1,508 1.3 316 .9 1,916 1.3

Valley Creek monitored 451 .4 108 .3 573 .4

Troy Beach Pool unmonitored (Apple) 2,945 2.5 705 2.1 3,742 2.5

Black Bass Pool unmonitored (Willow) 292 .2 192 .6 385 .3

Kinnickinnic River monitored 3,275 2.8 1,841 5.4 6,850 4.7

Kinnickinnic Pool unmonitored (Kinnickinnic) 913 .8 513 1.5 1,910 1.3

Total Tributary Load 113,845 100.0 28,667 100.0 141,862 100.0
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 11

Table 4. Total flow and volumetrically weighted mean water quality for monitored and unmonitored areas of the study area for May–September 1999[Names in parentheses are the streams used for extrapolation to unmonitored areas; P, phosphorus; N, nitrogen; km2, square kilometers; hm3, cubic hectometers (millions of cubic meters); μg/L, 

micrograms per liter; cv, coefficient of variation; monitored, monitored area in 1999; WWTP, wastewater treatment plant; POTW, publicly owned treatment works; --, not applicable; chloride 

concentrations from WWTPs and hatcheries were assumed to be 50,000 and 4,000 μg/L, respectively; cv for point sources assumed to be 0.25]

Area Flow Total P Ortho-P Total N Inorganic N Chloride

Tributary km2 hm3 cv μg/L cv μg/L cv μg/L cv μg/L cv μg/L cv

Willow River monitored 756.0 52.37 0.04 86 0.45 29 0.46 2,203 0.50 1,490 0.50 12,191 0.45

Lake Mallalieu unmonitored (Willow) 20.7 1.44 .04 86 .45 29 .46 2,203 .50 1,490 .50 12,191 .45

St. Croix River monitored 16,162.0 2,079.17 .06 41 .51 13 .48 971 .50 157 .50 4,410 .47St. Croix River unmonitored 490.0 63.04 .06 41 .51 13 .48 971 .50 157 .50 4,410 .47Apple River monitored 1,500.0 180.33 .05 64 .48 18 .46 1,332 .50 802 .50 7,492 .49Apple River unmonitored 80.3 9.66 .05 64 .48 18 .46 1,332 .50 802 .50 7,492 .49Browns Creek monitored 79.8 6.34 .09 238 .45 79 .50 2,203 .50 1,490 .50 12,191 .45Valley Creek monitored 33.7 5.58 .04 81 .50 27 .50 2,203 .50 1,490 .50 12,191 .50Troy Beach Pool unmonitored (Apple) 380.7 45.80 .05 64 .48 18 .46 1,332 .50 802 .50 7,492 .49Black Bass Pool unmonitored (Willow) 49.2 3.41 .04 86 .45 29 .46 2,203 .50 1,490 .50 12,191 .45Kinnickinnic River monitored 427.4 38.21 .02 86 .45 60 .45 4,767 .50 4,447 .50 16,612 .45Kinnickinnic Pool unmonitored 

(Kinnickinnic)

119.1 10.65 .02 86 .45 60 .45 4,767 .50 4,447 .50 16,612 .45

Osceola Hatchery -- 1.10 .00 70 .25 70 .25 2,202 .25 1,900 .25 4,000 .25Osceola WWTP -- .17 .01 4,696 .25 4,604 .25 14,237 .25 10,517 .25 50,000 .25St. Croix Falls Hatchery -- 1.74 .20 117 .25 83 .25 2,438 .25 2,038 .25 4,000 .25St. Croix Falls POTW -- .16 .02 3,418 .25 2,760 .25 13,981 .25 11,193 .25 50,000 .25Stillwater POTW -- 2.04 .01 341 .25 307 .25 17,900 .25 13,900 .25 50,000 .25Taylor POTW -- .07 .36 2,162 .25 1,946 .25 17,900 .25 13,900 .25 50,000 .25Hudson WWTP -- .66 .01 3,953 .25 3,486 .25 25,351 .25 20,080 .25 50,000 .25Amani POTW -- .01 .24 2,001 .25 2,001 .25 17,900 .25 13,900 .25 50,000 .25
12 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios Table 5. Total flow and volumetrically weighted mean water quality for monitored and unmonitored areas of the study area for May–September for the simulated dry year (based on 1988 flow data) and simulated wet year (based on 1996 flow data) [Names in parentheses are the streams used for extrapolation to unmonitored areas; P, phosphorus; N, nitrogen; hm3, cubic hectometers (millions of cubic meters); μg/L, micrograms per liter; cv, coefficient of variation; monitored, monitored area in 1999; WWTP, wastewater treatment plant; POTW, publicly owned treatment works] Simulated dry year Simulated wet year Total Flow Total P Ortho-P Total Flow Total P Ortho-P Tributary hm3 cv μg/L cv μg/L cv hm3 cv μg/L cv μg/L cv Willow River monitored 39.3 0.02 75 0.45 24 0.45 63.2 0.06 94 0.49 34 0.51 Lake Mallalieu unmonitored (Willow) 1.1 .02 75 .45 24 .45 1.7 .06 94 .49 34 .51 St. Croix River monitored 838.6 .10 22 .56 11 .47 2,154.8 .10 47 .62 14 .56 St. Croix River unmonitored 25.4 .10 22 .56 11 .47 65.3 .10 47 .62 14 .56 Apple River monitored 65.7 .06 42 .45 21 .45 205.7 .09 72 .56 18 .51 Apple River unmonitored 3.5 .06 42 .45 21 .45 11.0 .09 72 .56 18 .51 Browns Creek monitored 2.0 .09 156 .45 89 .50 7.2 .09 265 .56 76 .50 Valley Creek monitored 2.0 .06 53 .45 30 .50 6.4 .09 90 .56 26 .50 Troy Beach Pool unmonitored (Apple) 16.7 .04 42 .45 21 .45 52.2 .04 72 .56 18 .51 Black Bass Pool unmonitored (Willow) 2.6 .02 75 .45 24 .45 4.1 .06 94 .49 34 .51 Kinnickinnic River monitored 29.2 .02 63 .45 48 .45 48.6 .05 141 .51 85 .49 Kinnickinnic Pool unmonitored (Kinnickinnic) 8.1 .02 63 .45 48 .45 13.5 .05 141 .51 85 .49 Osceola Hatchery 1.1 .00 70 .25 70 .25 1.1 .00 70 .25 70 .25 Osceola WWTP .2 .01 4,696 .25 4,604 .25 .2 .01 4,696 .25 4,604 .25 St. Croix Falls Hatchery 1.7 .20 117 .25 83 .25 1.7 .20 117 .25 83 .25 St. Croix Falls POTW .2 .02 3,418 .25 2,760 .25 .2 .02 3,418 .25 2,760 .25 Stillwater POTW 2.0 .01 341 .25 307 .25 2.0 .01 341 .25 307 .25 Taylor POTW .1 .36 2,162 .25 1,946 .25 .1 .36 2,162 .25 1,946 .25 Hudson WWTP .7 .01 3,953 .25 3,486 .25 .7 .01 3,953 .25 3,486 .25 Amani POTW .0 .24 2,001 .25 2,001 .25 .0 .24 2,001 .25 2,001 .25
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 13

data obtained from the WDNR, MCES, and MPCA. 

The total load of phosphorus from direct point sources 

was 5,125 kg (table 6). The total input from indirect 

point sources within the tributary basins was about 

7,000 kg. Point-source loads are generally stable from 

year to year; therefore, 1999 direct point-source loads 

were applied unchanged to the simulated dry and wet 

years. Mean concentrations for total and dissolved 

ortho-phosphorus for the direct point sources were computed by dividing the seasonal load by the total seasonal 

flow from May–September (tables 4 and 5). 

Atmospheric Sources

The total precipitation to the pools was obtained by 

averaging that measured at Baldwin, Wis., and River 

Falls, Wis. from May–September, 1999. Atmospheric 

input of phosphorus to the surface of each pool (table 7) 

was estimated by multiplying a typical atmospheric 

deposition rate of phosphorus (30 kg/km2

 per year; 

Walker, 1996) by the surface area of each pool (table 1) 

and the fraction of year during the simulation period 

(0.417). The total estimated input of phosphorus from 

precipitation was 455 kg.

Sediment Release

Phosphorus release rates from the sediments was 

previously estimated at four locations in Lake St. Croix 

by William James (U.S. Army Corps of Engineers, Eau 

Galle Aquatic Ecology Laboratory, written commun., 

1999; table 8). Dissolved oxygen concentrations measured in the pools demonstrated only limited periods of 

anoxia. Therefore, based on the release rates in table 8 

and the calibration process for the BATHTUB model, 

the internal sediment release rates were set to 

5 mg/m2

/day in Lake Mallalieu, 2 mg/m2/day in the 

Bayport Pool, and 1 mg/m2

/day in the Troy Beach, 

Black Bass, and Kinnickinnic Pools. Based on these 

rates, the total internal sediment release was about 

8,000 kg (table 7). Sediment release rates were not 

adjusted for wet and dry years.

Summary of Phosphorus Inputs

For all flow regimes, tributary input was the major 

source of phosphorus to the Lake Mallalieu/Lake St. 

Croix pools (ranging from about 68 percent of the total 

in the simulated dry year to about 91 percent in the simulated wet year, table 9 and fig. 5). Point sources that 

discharge directly into the pools represent about 3 to 

4 percent of the total phosphorus loading in 1999 and 

the simulated wet year, and about 12 percent of the total 

loading in the simulated dry year. However, part of the 

total estimated tributary load is actually from other indirect point sources in the tributary basins and incorporated into the tributary estimates in table 3. If it is 

assumed that all tributary point sources reached the 

pools–an extreme assumption–all point sources of 

phosphorus would contribute 9.4 percent (4 percent 

from direct input and 5.4 percent from indirect point 

sources, table 9) of the total phosphorus load in 1999, 

28.7 percent of the total in the simulated dry year, and 

7.8 percent in the simulated wet year. Atmospheric 

input of phosphorus contributed only 0.4 to 1.1 percent 

of the total input. Input from sediment release is 

estimated to range from about 5 percent of the total 

input in 1999 and other wet years to about 19 percent in 

dry years. 

Table 6. Phosphorus loading to St. Croix 

River pools from point sources, May–

September 1999

[WWTP, wastewater treatment plant; POTW,

publicly owned treatment works]

Direct point sources to lower pools

Source Kilograms

St. Croix Falls Hatchery 204

St. Croix Falls POTW 540

Osceola Hatchery 77

Osceola WWTP 812

Stillwater POTW 697

Taylor POTW 160

Hudson WWTP 2,621

Amani POTW 14

Total 5,125

Indirect point sources in tributary basin

Basin Kilograms

Yellow River 81

Clam River 477

Kettle River 98

Snake River 852

Wood River 437

Apple River 859

Willow River 1,374

Kinnickinnic River 2,760

Total 6,938
14 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Table 7. Phosphorus loading from atmospheric inputs and

sediment release into Lake Mallalieu and Lake St. Croix, 

during May–September 

Pool

Atmospheric 

input

kilograms

Sediment 

release

kilograms

Lake Mallalieu 14 832

Bayport Pool 142 3,452

Troy Beach Pool 158 1,927

Black Bass Pool 68 829

Kinnickinnic Pool 73 887

Total 455 7,927

Table 8. Phosphorus release rates from the sediments of Lake 

St. Croix, by W. James (U.S. Army Corps of Engineers, Eau Galle

Aquatic Ecology Laboratory, written commun., 1999)

[mg/m2/d, milligrams per square meter per day]

Station Oxic conditions

mg/m2/d

Anoxic conditions

mg/m2/d

Bayport Pool - Site 1 0.7 26.1

Bayport Pool - Site 2 .7 17.8

Troy Beach Pool - Site 1 .3 8.6

Troy Beach Pool - Site 2 .2 10.9

Table 9. Summary of phosphorus loading to Lake Mallalieu/Lake St. Croix pools

1999 Dry year (1988) Wet year (1996)

Source Kilograms Percent Kilograms Percent Kilograms Percent

Total tributary load 113,845 89.4 28,667 68.0 141,862 91.3

 Point sources in basins 6,938 5.4 6,938 16.5 6,938 4.5

Direct point sources 5,125 4.0 5,125 12.2 5,125 3.3

Atmospheric input 455 .4 455 1.1 455 .3

Sediment input 7,927 6.2 7,927 18.8 7,927 5.1

Total phosphorus input 127,352 100.0 42,173 100.0 155,369 100.0
PHYSICAL AND CHEMICAL CHARACTERISTICS (MODEL INPUTS) 15

Direct point sources

Tributaries

Indirect point sources

Precipitation

Sediment 

release

Sediment 

release

Indirect point 

sources

Tributaries

Direct point 

sources

Precipitation

Phosphorus loading to Lower St. Croix pools 

Tributaries

Indirect point 

sources

Direct point 

sources

Sediment Precipitation release

Dry Year - 42,000 Kg

Wet Year - 155,000 Kg

1999 - 127,000 Kg

Figure 5. Phosphorus loading to lower St. Croix River pools (Lake Mallalieu and Lake St. Croix 

pools) for the 1999 flow regime, simulated dry year (flows based on 1988 data), and simulated 

wet year (flows base on 1996 data).
16 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Water-Residence Time

Flow data from May–September, 1999, and the 

simulated dry and wet years were used to determine the 

average water-residence time in each St. Croix River 

pool (table 10). In 1999 and the simulated wet year, the 

average water-residence time in each pool was very 

short, ranging from 0.4 days to 8.2 days. During these 

years, St. Croix Falls Reservoir had an average waterresidence time of 0.4 days, which explains the similarity 

in the quality of the water entering, within, and exiting 

the reservoir. In Lake Mallalieu, the average water-residence time was 5 days. The average water-residence 

times in the Lake St. Croix pools ranged from 3.4 days 

in the Kinnickinnic Pool to 8.2 days in the Troy Beach 

Pool, and for all four pools of Lake St. Croix, combined, 

was 21 days. 

In the simulated dry year, the average water-residence time was about twice that of 1999, except in Lake 

Mallalieu where it was only slightly longer (6.7 days). 

During the simulated dry year, the average water-residence time for the Lake St. Croix pools ranged from 

8.1 days in the Kinnickinnic Pool to 19.4 days in the 

Troy Beach Pool, and for all four pools of Lake St. 

Croix, combined, was 50.1 days. 

RESPONSE IN THE WATER QUALITY OF 

ST. CROIX FALLS RESERVOIR TO 

CHANGES IN PHOSPHORUS LOADING

An attempt was made to develop a BATHTUB 

model for the St. Croix Falls Reservoir; however, 

because the average water-retention time in the reservoir was less than one day (table 10), the empirical models within BATHTUB did not work well in simulating 

its water quality. Because of this short water-retention 

time, the quality of the water entering the reservoir was 

similar to that exiting except during a few occasions 

(fig. 3). These exceptions appear to be due primarily to 

errors in the data set. 

Because the short water-retention time and the similarity of the quality of water entering and exiting the 

reservoir, a detailed reservoir model is not needed to 

simulate the water-quality response to various loading 

scenarios. The change in phosphorus concentrations in 

the reservoir should be of similar magnitude to the 

change in phosphorus loading. For example, if phosphorus loading is reduced by 50 percent, the phosphorus 

concentration in the reservoir should also decrease by 

about 50 percent. Chlorophyll a concentrations and 

Secchi depths also vary very little throughout the reservoir (fig. 3), but their response may not be of similar 

magnitude to the change in loading. Therefore, a river 

model is needed to estimate the changes that should 

occur in chlorophyll a concentrations and Secchi depths 

upstream and within St. Croix Falls Reservoir in 

response to changes in phosphorus loading.

MODELING THE RESPONSE IN WATER 

QUALITY OF LAKE MALLALIEU AND 

LAKE ST. CROIX TO CHANGES IN 

PHOSPHORUS LOADING

Because of the longer water retention time in Lake 

Mallalieu and Lake St. Croix than in St. Croix Falls 

Reservoir, the water quality within these pools differs 

from the water entering them. Therefore, the empirical 

models within BATHTUB are applicable to describe the 

changes in the water quality in these pools.

Hydrology

To determine the calibration factors within BATHTUB for dispersion or exchange of water between adjacent pools, the volumetrically weighted mean chloride 

concentrations for 1999 for each tributary were input 

into the model, and estimated and measured chloride 

concentrations were compared for each pool (fig. 6). An 

estimate of the certainty of the measured and estimated 

Table 10. Water-residence time in the 

St. Croix River pools

Average water-residence time 

(days)

Pool 1999 Dry year Wet year

St. Croix Falls Reservoir 0.4 0.9 0.4

Lake Mallalieu 5.0 6.7 4.1

Bayport Pool 4.9 11.0 5.0

Troy Beach Pool 8.2 19.4 8.1

Black Bass Pool 4.5 11.0 4.3

Kinnickinnic Pool 3.4 8.1 3.3

Lake St. Croix 

(four pools combined)

20.6 50.1 20.0
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 17

values are shown by the vertical bars (plus and minus 

one standard error) on each data point. On the basis of 

these comparisons, the calibration factor for dispersion 

between Lake Mallalieu and the Bayport Pool was set to 

0.0, which represents no upstream dispersion from the 

Bayport Pool into Lake Mallalieu. This is reasonable 

given that the dam and head difference between these 

pools prevents all upstream mixing. The calibration factors for dispersion between the Bayport Pool and Troy 

Beach Pool, and the Troy Beach Pool and the Black 

Bass Pool were set to 0.5, which represents limited 

upstream mixing between these pools. The calibration 

factor for dispersion between the Black Bass Pool and 

the Kinnickinnic Pool was set to 0.7, which represents 

slightly more upstream mixing than between the other 

pools. More mixing may occur between the Kinnickinnic and Black Bass Pools than between the other pools 

because of backwater from the Mississippi River. The 

resulting simulated and measured chloride concentrations are shown in figure 6.

Algorithms and Calibration

Within BATHTUB, several algorithms are available to simulate each water-quality constituent. The 

algorithms chosen to simulate nutrient and chlorophyll 

a concentrations in Lake Mallalieu and the Lake St. 

Croix pools are listed in table 11. All of the default algorithms within BATHTUB, except that for simulating 

total nitrogen, provided reasonable results and, therefore, were used. The only algorithm that accurately simulated total nitrogen concentrations was the first-order 

settling velocity algorithm. Within BATHTUB, calibration coefficients for each water-quality constituent may 

be applied for each pool; however, no site-specific calibrations were applied.

Comparison Between Simulated and 

Measured Water Quality

With the data from May–September, 1999 (flow 

from each tributary and that estimated for each unmonFigure 6. Measured and simulated chloride concentrations in Lake Mallalieu and Lake St. Croix pools (summer 

average, May–September, 1999) used to determine calibration factors for dispersion between adjacent pools in 

BATHTUB model [LM, Lake Mallalieu; BP, Bayport Pool; TB, Troy Beach Pool; BB, Black Bass Pool; KP, Kinnickinnic

Pool].

0

3,000

6,000

9,000

12,000

15,000

18,000

LM BP TB BB KP

Simulated

Measured

CHLORIDE, IN MICROGRAMS PER LITER
18 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

itored area, and the volumetrically weighted nutrient 

concentrations computed from the respective loads and 

flows, table 4), BATHTUB was used to simulate the 

water quality in Lake Mallalieu and the Lake St. Croix 

pools. Water-quality data measured in Lake Mallalieu 

and the Lake St. Croix pools in 1999 are compared with 

that simulated with BATHTUB in figure 4. As in 

figure 6, an estimate of the certainty of the measured 

and simulated values are shown by the vertical bars 

(plus and minus one standard error) on each data point. 

Estimates of the certainty in the simulated response in 

each water-quality constituent in the figures that follow 

had relatively similar estimates of certainty as shown in 

figure 4; however, the confidence limits are not displayed.

In general, simulated water quality in Lake Mallalieu and the Lake St. Croix pools from BATHTUB was 

similar to the measured water quality in 1999 (fig. 4). 

The only discrepancy was in chlorophyll a concentrations in Lake Mallalieu, where chlorophyll a concentrations were underestimated by BATHTUB. This 

discrepancy was probably caused by the high non-algal 

turbidity in Lake Mallalieu. A site-specific calibration 

could have been applied to adjust the simulated chlorophyll a concentrations in Lake Mallalieu; however, 

instead the response in chlorophyll a concentrations are 

described in terms of a percent change from a base condition.

Response in Water Quality to Changes in 

Phosphorus Loading

To simulate water-quality changes in Lake Mallalieu and the Lake St. Croix pools in response to basinwide changes in phosphorus loading, the average 

volumetrically weighted total and dissolved ortho-phosphorus concentrations for each tributary (monitored and 

unmonitored) and direct point source were decreased 

and increased for each of the three flow regimes. Phosphorus concentrations were decreased by 50, 25, and 10 

percent, and increased by 10, 25, 50, 75, 100, 150, and 

200 percent (the actual phosphorus concentrations are 

given in the Appendix). Nitrogen concentrations were 

not altered in the various flow regimes or the various 

phosphorus loading scenarios. Since Lake Mallalieu 

and the Lake St. Croix pools are phosphorus limited, 

altering nitrogen concentrations should have little effect 

on the simulated results. Additional simulations conducted with adjusted nitrogen concentrations confirmed 

this assumption. Decreases and increases in phosphorus 

loading were examined in these three types of years to 

determine whether the water quality in Lake Mallalieu 

and the Lake St. Croix pools responds differently to 

changes in phosphorus loading during different flow 

regimes. 

Changes in Phosphorus Concentrations

On the basis of model results for the 1999 flow 

regime, phosphorus concentrations in Lake Mallalieu 

Table 11. Algorithms used within BATHTUB to simulate water quality in Lake 

Mallalieu/Lake St. Croix pools

Process Model number 

within BATHTUB Algorithm description

Phosphorus balance 1 Second order, available phosphorus

Nitrogen balance 7 Settling velocity

Chlorophyll a concentration 2 Phosphorus, light, and flushing rate

Secchi depth 1 Chlorophyll a and turbidity

Dispersion 1 Fisher numeric

Phosphorus calibration 1 Decay rates

Nitrogen calibration 0 None
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 19

and the Lake St. Croix pools have a relatively linear 

response to a linear change in phosphorus loading 

(fig. 7); however, the changes in phosphorus concentrations in the pools are not expected to be as large as the 

changes in phosphorus loadings. Changes in phosphorus concentrations in the pools should be about one-half 

that of the changes in phosphorus loading. In other 

words, a 100-percent increase in phosphorus loading 

should result in a 40- to 65-percent increase in phosphorus concentrations. This non-linear response may be 

caused partly by the assumption that loadings from precipitation and sediment release do not change during 

different flow regimes, and partly by increased settling 

of nutrients associated with higher loading.

Changes in phosphorus concentrations in response 

to changes in phosphorus loading should vary amongst 

the pools in Lake St. Croix, with the largest response in 

the Bayport Pool and the smallest response in the Kinnickinnic Pool (fig. 7). The decreasing phosphorus concentrations from the Bayport Pool to the Kinnickinnic 

Pool observed in 1999 and simulated by BATHTUB 

(fig. 4) should still occur with increases and most 

decreases in phosphorus loading. With a decrease in 

loading by 50 percent, all of the pools should have similar phosphorus concentrations. However, as phosphorus loading increases, the downstream differences in 

concentration become more extreme because phosphorus concentrations in the Bayport Pool are more responsive than the downstream pools. 

BATHTUB simulations for wet and dry years indicate that phosphorus concentrations in Lake Mallalieu 

and the Lake St. Croix pools should respond to variable 

phosphorus loading slightly different from those simulated for the 1999 flow regime (demonstrated for Lake 

Mallalieu and the Bayport Pool in fig. 8). In dry years, 

the response should be less extreme, and in wet years, 

the response should be more extreme. This response 

was caused by phosphorus concentrations in the tributaries increasing during higher flows and decreasing 

during lower flows, which is a typical response of 

streams dominated by non-point sources of phosphorus. 

To determine if the changes simulated by the calibrated BATHTUB model in the dry and wet years were 

reasonable and to further evaluate the model, the simulated total phosphorus concentrations with 0-percent 

change in loading (figs. 7 and 8) were compared with 

data collected immediately upstream of the Bayport 

Pool (Stillwater) and downstream of the Kinnickinnic 

Pool (Prescott) during dry and wet years (table 12). 

Total phosphorus concentrations measured in 1996 (the 

simulated wet year) at both of these sites were higher 

than in 1988 (the simulated dry year), consistent with 

that simulated by the model (fig. 8); however, the 

median total phosphorus concentration measured during the five wettest years from 1977–2000 was only 

slightly higher than that measured during the five driest 

years (table 12). The median dissolved ortho-phosphorus concentration measured during the five wettest years 

from 1977–2000 also was higher at both sites than that 

measured during the five driest years. 

Changes in Chlorophyll a Concentrations

On the basis of model results for the 1999 flow 

regime and variable phosphorus loading, chlorophyll a

concentrations in Lake Mallalieu and the Lake St. Croix 

pools have a non-linear response to a linear change in 

phosphorus loading, with a greater response to reductions in phosphorus loading than to increases in loading 

(fig. 9). With a 50-percent reduction in loading, chlorophyll a concentrations should decrease by about 25 to 

30 percent; however, with a 50-percent increase, chlorophyll a concentrations should increase by about 

15 percent. With a 200-percent increase in loading, 

chlorophyll a concentrations should only increase by 

about 35 percent. The non-linear response in chlorophyll a concentrations associated with the near-linear 

change in total phosphorus concentrations was expected 

based on the non-linear relation between phosphorus 

and chlorophyll a concentrations found for other Wisconsin water bodies (Lillie and others, 1993). 

Changes in chlorophyll a concentrations in 

response to variable phosphorus loading should be 

slightly different for the various Lake St. Croix pools. 

Chlorophyll a concentrations in the Bayport Pool 

should have the greatest response and those in the Kinnickinnic Pool should have the least response (fig. 9). 

The greater response in chlorophyll a concentrations in 

the Bayport Pool may be due to its greater response in 

phosphorus concentrations (fig. 7).

On the basis of model results for the simulated dry 

and wet years, chlorophyll a concentrations in Lake 

Mallalieu and the Lake St. Croix pools should respond 

to variable phosphorus loading slightly different from 

those for 1999 flow regime. In Lake Mallalieu, chlorophyll a concentrations should be slightly higher in dry 

and wet years (fig. 10), but the percent changes with 

increased and decreased phosphorus loading should to 

be similar to those simulated for the 1999 flow regime 

(fig. 9). Throughout Lake St. Croix, the response in 
20 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Figure 7. Simulated changes in phosphorus concentrations in Lake Mallalieu and Lake St. Croix pools, in

response to various phosphorus-loading scenarios, on the basis of output from BATHTUB for the 1999 flow 

regime.

-100 -50 0 50 100 150 200

PERCENT CHANGE IN PHOSPHORUS LOADING

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

-100 -50 0 50 100 150 200

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

0

25

50

75

100

125

150

175

200

225

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

PERCENT CHANGE IN PHOSPHORUS LOADING

PERCENT CHANGE 

IN PHOSPHORUS CONCENTRATION

-50

-25

0

25

50

75

100

125
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 21

chlorophyll a concentrations to variable loading in wet 

years should be similar to those for the 1999 flow 

regime, but slightly more responsive than concentrations in dry years. The lower chlorophyll a concentrations in the dry years were primarily caused by the 

lower phosphorus concentrations associated with lower 

flows. 

The lower chlorophyll a concentrations in the dry 

year are not consistent with data collected immediately 

upstream of the Bayport Pool (Stillwater) and downstream of the Kinnickinnic Pool (Prescott) (table 12). 

The measured data indicate that although total and dissolved phosphorus concentrations decrease during dry 

years, the increased water-residence time in the pools 

results in higher chlorophyll a concentrations than in 

wet years when algae are flushed more rapidly through 

the pools. Therefore, chlorophyll a concentrations 

shown for dry years in figure 10 are expected to be 

higher than in years with higher flows, but the percentage of change shown in figure 9 may still be valid. 

Changes in Water Clarity (Secchi Depths)

On the basis of model results for the 1999 flow 

regime, Secchi depths (water clarity) in Lake Mallalieu 

and the Lake St. Croix pools change very little in 

response to variable phosphorus loading (fig. 11). 

Changes in phosphorus loading from 50-percent reductions to 200-percent increases should only cause average Secchi depths to change about 0.4 m or less from 

present conditions. The limited response in Secchi 

depths may be caused by the high non-algal turbidity 

and stained water in Lake Mallalieu and Lake St. Croix. 

The small increase in water clarity from the Bayport 

Pool to the Kinnickinnic Pool still should be present 

with any change in phosphorus loading.

In the simulated dry and wet years, average summer Secchi depths in Lake Mallalieu and the Lake St. 

Croix pools are expected to respond to variable loading 

similarly to those for the 1999 flow regime (fig. 12). 

Water clarity may, however, slightly decrease during 

dry years because of the increase in chlorophyll a concentrations not simulated by BATHTUB. 

Figure 8. Simulated changes in phosphorus concentrations in Lake Mallalieu and the Bayport Pool for the 1999 flow regime, 

simulated dry year (flows based on 1988 data), and simulated wet year (flows based on 1996 data) in response to various 

phosphorus-loading scenarios, on the basis of output from BATHTUB.

Mallalieu - wet year

Mallalieu - dry year

Mallalieu - 1999

Bayport - wet year

Bayport - dry year

Bayport - 1999

-100 -50 0 50 100 150 200

PERCENT CHANGE IN PHOSPHORUS LOADING

PHOSPHORUS CONCENTRATION,

IN MICROGRAMS PER LITER

0

50

100

150

200

250
22 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Table 12. Average water quality during May–September measured at Stillwater and Prescott,

1977–2000

[Data were collected by the Metropolitan Council-Environmental Services (MCES); P, phosphorus; μg/L, micrograms 

per liter; m3/s, cubic meters per second]

Average flow Total P, μg/L Dissolved Ortho-P, μg/L Chlorophyll a, μg/L

Year m3/s Stillwater Prescott Stillwater Prescott Stillwater Prescott

1977 99.6 53 51 19 26 19 9

1978 174.9 110 55 25 38 10 2

1979 162.0 78 56 46 49 27 13

1980 80.1 55 43 14 26 48 22

1981 152.8 63 54 18 14 31 8

1982 135.6 58 33 12 16 38 24

1983 148.4 78 41 16 8 31 22

1984 213.7 65 135 16 34 35 10

1985 183.1 89 47 16 26 32 12

1986 303.9 54 41 30 46 27 11

1987 68.5 56 26 7 25 43 28

1988 63.4 63 60 8 48 34 20

1989 102.8 52 34 46 39 27 16

1990 123.4 49 55 21 29 26 13

1991 212.1 109 110 56 72 18 5

1992 110.2 55 29 18 99 28 10

1993 184.5 49 37 29 19 15 7

1994 147.5 79 85 24 29 32 18

1995 168.1 100 63 73 14 24 13

1996 162.9 129 32 11 11 22 8

1997 104.4 56 27 18 5 22 11

1998 78.9 63 52 9 15 32 17

1999 157.2 62 50 15 16 23 16

2000 95.2 73 44 16 9 36 23

Median concentration

Dry yearsa 78.9 63 44 9 25 36 22

Other years 148.0 62 51 18 21 26 13

Wet yearsb 212.1 65 47 29 34 27 10

a

Five driest years (1980, 1987, 1988, 1998, and 2000).

bFive wettest years (1984, 1985, 1986, 1991, and 1993).
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 23

Figure 9. Simulated changes in chlorophyll a concentrations in Lake Mallalieu and Lake St. Croix pools in 

response to various phosphorus-loading scenarios, on the basis of output from BATHTUB for the 1999 flow

regime.

0

10

20

30

40

50

60

70

80

90

-100 -50 0 50 100 150 200

PERCENT CHANGE IN PHOSPHORUS LOADING

PERCENT CHANGE IN PHOSPHORUS LOADING

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

-100 -50 0 50 100 150 200

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

-40

-30

-20

-10

0

10

20

30

40

PERCENT CHANGE IN

CHLOROPHYLL 

A CONCENTRATION

CHLOROPHYLL 

A CONCENTRATION,

IN MICROGRAMS PER LITER
24 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Figure 10. Simulated changes in chlorophyll a concentrations in Lake Mallalieu and the Bayport Pool for

the 1999 flow regime, a simulated dry year (flows based on 1988 data), and a simulated wet year (flows

based on 1996 data) in response to various phosphorus-loading scenarios, on the basis of output from 

BATHTUB.

-100 -50 0 50 100 150 200

PERCENT CHANGE IN PHOSPHORUS LOADING

PERCENT CHANGE IN PHOSPHORUS LOADING

Mallalieu - wet year

Mallalieu - dry year

Mallalieu - 1999

-100 -50 0 50 100 150 200 250

Bayport - wet year

Bayport - dry year

Bayport - 1999

CHLOROPHYLL A CONCENTRATION,

IN MICROGRAMS PER LITER

CHLOROPHYLL 

A CONCENTRATION,

IN MICROGRAMS PER LITER

40

50

60

70

80

90

5

8

11

14

17

20
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 25

Figure 11. Simulated changes in Secchi depth in Lake Mallalieu and the Lake St. Croix pools in 

response to various phosphorus-loading scenarios, on the basis of output from BATHTUB for the 1999

flow regime.

0.0

0.5

1.0

1.5

2.0

-100 -50 0 50 100 150 200 250

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

-100 -50 0 50 100 150 200

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

PERCENT CHANGE IN PHOSPHORUS LOADING

PERCENT CHANGE IN PHOSPHORUS LOADING

SECCHI DEPTH, IN METERS PERCENT CHANGE IN SECCHI DEPTH

-40

-30

-20

-10

0

10

20

30

40
26 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Changes in Trophic Status

Trophic state index (TSI) values, based on nearsurface concentrations of phosphorus and chlorophyll a, 

and on Secchi depths, are used to classify the water 

quality or productivity of a lake. TSI values allow these 

three water-quality characteristics to be placed on similar scales. Oligotrophic lakes (TSI values less than 40) 

have a limited supply of nutrients, and are typically 

clear with low algal populations and phosphorus concentrations. Mesotrophic lakes (TSI values between 40 

and 50) have a moderate supply of nutrients, are not 

especially clear, and are prone to moderate algal 

blooms. Eutrophic lakes (TSI values greater than 50) 

are nutrient rich with correspondingly severe waterquality problems, such as frequent seasonal algal 

blooms, and have poor water clarity. Lakes with TSI 

values greater than 60–70 are considered hypereutrophic, and have very high nutrient concentrations 

and high algal turbidity.

Trophic state index values indicate that Lake Mallalieu is hypereutrophic and each of the Lake St. Croix 

pools is eutrophic (0-percent change in phosphorus 

loading in fig. 13). On the basis of model results, even 

with 50-percent reductions in phosphorus loading, Lake 

Mallalieu will remain eutrophic, with TSI values generally greater than 65. The trophic status of the Lake St. 

Croix pools will respond to changes in phosphorus 

loading, especially phosphorus-load reductions. Phosphorus-load reductions of 25–50 percent will result in 

most of Lake St. Croix approaching the mesotrophiceutrophic range based on total phosphorus and chlorophyll a concentrations. However, due to the high nonalgal turbidity and stained water in the lake, Secchi 

depths will remain very low and result in TSI values that 

classify the lake as eutrophic.

Changes in Algal Bloom Response

Currently, algal blooms are a common occurrence 

in Lake Mallalieu, but occur less frequently in Lake St. 

Figure 12. Simulated changes in Secchi depths in Lake Mallalieu and the Bayport Pool for the 1999 

flow regime, a simulated dry year (flows based on 1988 data), and a simulated wet year (flows based 

on 1996 data) in response to various phosphorus-loading scenarios, on the basis of output from 

BATHTUB.

0.0

0.5

1.0

1.5

-100 -50 0 50 100 150 200

PERCENT CHANGE IN PHOSPHORUS LOADING

Mallalieu - wet year

Mallalieu - dry year

Mallalieu - 1999

Bayport - wet year

Bayport - dry year

Bayport - 1999

SECCHI DEPTH, IN METERS
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 27

40

50

60

70

80

90

-100 -50 0 50 100 150 200

Phosphorus

40

50

60

70

80

90

-100 -50 0 50 100 150 200

Chlorophyll a

40

50

60

70

80

90

-100 -50 0 50 100 150 200

Secchi depth

TROPHIC STATE INDEX

PERCENT CHANGE IN PHOSPHORUS LOADING

Mallalieu

Bayport

Troy Beach

Black Bass

Kinnickinnic

Eutrophic

Mesotrophic

Eutrophic

Mesotrophic

Eutrophic

Mesotrophic

Figure 13. Simulated changes in trophic state index values in Lake Mallalieu and the Lake St. Croix pools in 

response to various phosphorus-loading scenarios, on the basis of output from BATHTUB for the 1999 flow regime.
28 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

Croix. The definition of what constitutes the occurrence 

of an algal bloom can be site specific and subjective. A 

chlorophyll a concentration of 10 μg/L represents a 

small amount of algae in a water body, whereas a concentration of 60 μg/L represents an extremely severe 

algal bloom. A chlorophyll a concentration of 30 μg/L 

represents what many would consider a moderate algal 

bloom. Figure 14 demonstrates how the occurrence of 

algal blooms (percent of days from May through September with a chlorophyll a concentration exceeding a 

certain value) responds to variable phosphorus loading 

based on how an algal bloom is defined (10 to 60 μg/L 

of chlorophyll a) in Lake Mallalieu, and the Bayport, 

Troy and Kinnickinnic Pools. A subset of these data are 

summarized in table 13 for algal blooms defined as 

chlorophyll a concentrations greater than 10, 30, and

60 μg/L.

On the basis of model results for the 1999 flow 

regime, algal blooms in Lake Mallalieu are common 

regardless of how an algal bloom is defined. If defined 

as a chlorophyll a concentration greater than 30 μg/L, 

algal blooms in Lake Mallalieu should presently occur 

on about 75 percent of the days from May through September (table 13, fig. 14). With a decrease in phosphorus loading by 50 percent, this frequency should 

decrease to about 60 percent of the time. With an 

increase in loading by 50 percent, this frequency should 

increase to about 85 percent of the time, and with an 

increase of 200 percent, this frequency should increase 

to about 90 percent of the time. Severe algal blooms, as 

defined by a chlorophyll a concentration greater than 

60 μg/L, should presently occur about 35 percent of the 

time in Lake Mallalieu. With a 50-percent reduction in 

the phosphorus load, this bloom frequency should 

decrease to 20 percent of the time. Conversely, with a 

200-percent increase in loading, the frequency of severe 

algal blooms should increase to about 55 percent of the 

time. In Lake Mallalieu, the number of days that chlorophyll a concentrations are above a given threshold 

should be more than presented here because BATHTUB 

underestimates the amount of chlorophyll a in Lake 

Mallalieu (fig. 5); however, the relative changes should 

be correct.

Algal blooms are more frequent in the Bayport 

Pool than in the three downstream pools (fig. 14 and 

table 13). On the basis of model results for 1999, algal 

blooms throughout Lake St. Croix are uncommon 

unless an algal bloom is defined as a chlorophyll a concentration greater than 10 μg/L. With this definition, 

algal blooms occur about 50 percent of the time 

between May and September in the Bayport Pool and 

30 percent of the time in the other pools. If defined as a 

chlorophyll a concentration greater than 30 μg/L, algal 

blooms should only occur about 4 percent of the time in 

the Bayport Pool and about 1 percent of the time in the 

other pools. 

With decreases in phosphorus loading for the 1999 

flow regime, the frequency of algal blooms will 

decrease (fig. 14 and table 13). If a bloom is defined as 

a chlorophyll a concentration greater than 10 μg/L, a 

decrease in phosphorus loading by 50 percent should 

decrease the frequency of algal blooms to less than 

30 percent of the time in the Bayport Pool and to less 

than 20 percent of the time in the other pools (compared 

to about 30 to 50 percent of the time with no load reduction). If defined as a chlorophyll a concentration greater 

than 30 μg/L, a 50-percent load reduction should 

decrease the occurrence of algal blooms to less than 

1 percent of the time anywhere in Lake St. Croix (compared to 1 to 4 percent of the time with no load reduction).

With increases in phosphorus loading, algal bloom 

frequency will increase throughout Lake St. Croix. If 

blooms are defined as a chlorophyll a concentration 

greater than 10 μg/L, a 50-percent increase in loading 

should cause algal blooms to occur about 60 percent of 

the time in the Bayport Pool, and about 40 percent of the 

time in the other pools (compared to about 30 to 50 percent of the time with no load reduction). With a 200-percent increase in loading, algal blooms should occur 

about 70 percent of the time in the Bayport Pool and 

about 50 percent of the time in the other pools. If 

defined as a chlorophyll a concentration greater than 

30 μg/L, a 50-percent increase in loading should cause 

algal blooms to occur about 6 percent of the time in the 

Bayport Pool and about 2 percent of the time in the 

other pools (compared to 1 to 4 percent of the time with 

no load reduction). With a 200-percent increase in loading, moderate algal blooms should occur about 10 percent of the time in the Bayport Pool and about 3 percent 

of the time in the other pools. 

On the basis of model simulations for the dry and 

wet years, the response in the frequency of algal blooms 

in Lake Mallalieu and the Lake St. Croix pools to variable phosphorus loading only differs slightly from that 

for the 1999 flow regime. In Lake Mallalieu, algal 

blooms should be slightly more frequent in dry and wet 

years, but the percent change for different loading scenarios should be similar to that for the 1999 flow regime 

(fig. 15). Throughout Lake St. Croix, model simulations 
MODELING THE RESPONSE IN WATER QUALITY OF LAKE MALLALIEU AND ST. CROIX TO CHANGES IN

PHOSPHORUS LOADING 29 Figure 14. Simulated changes in the frequency (percent of days from May through September) of algal blooms (as defined by different chlorophyll 

a

concentrations) in Lake Mallalieu and the Lake St. Croix pools in response to various phosphorus-loading scenarios, on the basis of output from

BATHTUB for the 1999 flow regime.

10

20

30

40

50

60

-50

-25

-10

0

10

25

50

75

100

150

200

20

10

0

60

50

40

30

90

80

70

100

PERCENT CHANGE IN PHOSPHORUS LOADING

Lake Mallalieu 

10

20

30

40

50

60

-50

-25

-10

0

10

25

50

75

100

150

200

20

10

0

60

50

40

30

90

80

70

100

Troy Beach Pool

10

20

30

40

50

60

-50

-25

-10

0

10

25

50

75

100

150

200

20

10

0

60

50

40

30

100

90

80

70

Kinnickinnic Pool 

10

20

30

40

50

60

-50

-25

-10

0

10

25

50

75

100

150

200

20

10

0

60

50

40

30

90

80

70

100

Bayport Pool

BLOOM FREQUENCY, IN PERCENT

CHLOROPHYLL A,

IN MICROGRAMS PER LITER

PERCENT CHANGE IN PHOSPHORUS LOADING

BLOOM FREQUENCY, IN PERCENT

CHLOROPHYLL A,

IN MICROGRAMS PER LITER

PERCENT CHANGE IN PHOSPHORUS LOADING

BLOOM FREQUENCY, IN PERCENT

CHLOROPHYLL A,

IN MICROGRAMS PER LITER

PERCENT CHANGE IN PHOSPHORUS LOADING

BLOOM FREQUENCY, IN PERCENT

CHLOROPHYLL A,

IN MICROGRAMS PER LITER

Percent changein phosphorus loading

-50-25-10010255075100150200

EXPLANATION
30 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

indicate that changes in the frequency of algal blooms in 

wet years should be similar to those estimated for the 

1999 flow regime. Results of model simulations also 

indicate a decreased frequency of algal blooms throughout Lake St. Croix in dry years; however, measured 

chlorophyll a concentrations were higher during dry 

years than in moderate and wet years (table 12). Therefore, the frequency of algal blooms during dry years are 

expected to be higher than shown in figure 15; however, 

the relative changes for different loading scenarios 

(shown in figure 14) are likely to still be valid. 

SUMMARY AND CONCLUSIONS

Each of the St. Croix River pools is currently 

eutrophic because of high nutrient loading. The waterquality model, BATHTUB, was used to better understand anticipated changes in water quality and trophic 

status of each of the pools in response to basinwide 

changes in phosphorus loading. The model was first calibrated by use of water-quality and flow data collected 

throughout the basin from 1997–99. The model was 

then applied to the pools with various basinwide 

changes in phosphorus loading during three different 

flow regimes: 1999, a simulated dry year (flows based 

on 1988 data), and a simulated wet year (flows based on 

1996 data).

On the basis of model results, the water quality of 

each of the pools will respond to changes in phosphorus 

loading. Linear increases in phosphorus loads should 

cause: linear increases in phosphorus concentrations in 

all of the St. Croix River pools (although phosphorus 

concentrations in the pools will change at a smaller rate 

than the change in loading); non-linear increases in 

chlorophyll a concentrations (with a smaller relative 

response with higher phosphorus loading); the frequency of algal blooms to increase (a greater response 

in severe blooms), and water clarity to decrease slightly.

The relative response in water quality should be very 

similar regardless of the flow regime, although phosphorus concentrations should be slightly lower in dry 

years than in wet years because phosphorus concentraTable 13. Simulated algal bloom frequency in Lake Mallalieu and throughout Lake St. Croix in response to

various phosphorus-loading scenarios, on the basis of output from BATHTUB

[Algal blooms defined as chlorophyll a concentrations greater than (>) 10, 30, and 60 micrograms per liter (μg/L)]

Percent of days (from May through September) with algal blooms in indicated pool

Percent change in 

phosphorus loading Mallalieu Bayport Troy Beach Black Bass Kinnickinnic

> 10 μg/L

-50 97.9 27.0 16.1 14.5 16.8

0 99.4 48.8 30.6 26.9 30.2

50 99.7 58.5 38.1 33.4 37.4

100 99.8 63.8 42.8 37.7 42.0

150 99.9 67.0 45.8 40.4 45.0

200 99.9 69.2 48.0 42.4 47.3

> 30 μg/L

-50 60.2 0.9 0.3 0.2 0.3

0 77.4 3.6 1.1 0.8 1.1

50 83.6 6.0 1.9 1.4 1.8

100 86.9 7.8 2.5 1.8 2.4

150 88.8 9.1 3.0 2.2 2.9

200 90.1 10.2 3.4 2.5 3.3

> 60 μg/L

-50 19.5 0.0 0.0 0.0 0.0

0 35.7 0.2 0.0 0.0 0.0

50 44.4 0.4 0.1 0.0 0.1

100 50.1 0.6 0.1 0.1 0.1

150 53.9 0.7 0.1 0.1 0.2

200 56.6 0.8 0.2 0.1 0.2
SUMMARY AND CONCLUSIONS 31

Figure 15. Simulated changes in algal bloom frequency (defined as the percent of days from May through 

September with chlorophyll a concentrations greater than 30 μg/L) in Lake Mallalieu and the Bayport Pool for the

1999 flow regime, a simulated dry year (flows based on 1988 data), and a simulated wet year (flows based on

1996 data) in response to various phosphorus-loading scenarios, on the basis of output from BATHTUB.

-100 -50 0 50 100 150 200

Mallalieu - wet year

Mallalieu - dry year

Mallalieu -1999

-100 -50 0 50 100 150 200

Bayport - wet year

Bayport - dry year

Bayport - 1999

ALGAL BLOOM FREQUENCY, IN PERCENTAGE

OF DAYS FROM MAY THROUGH SEPTEMBER

ALGAL BLOOM FREQUENCY, IN PERCENTAGE

OF DAYS FROM MAY THROUGH SEPTEMBER

50

60

70

80

90

100

0

3

6

9

12

15

PERCENT CHANGE IN PHOSPHORUS LOADING

PERCENT CHANGE IN PHOSPHORUS LOADING
32 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios

tions in most tributaries increase with increasing flow, 

and chlorophyll a concentrations should be higher in 

dry years than in wet years because of the increased 

water-residence time.

About a 50-percent reduction in phosphorus loading is required for the Lake St. Croix pools to be classified as mesotrophic with respect to phosphorus and 

chlorophyll a concentrations, whereas a larger reduction in phosphorus loading is needed for Lake Mallalieu 

to be classified as mesotrophic. Even with these reductions, however, water clarity in each of the pools will 

remain poor because of the high non-algal turbidity and 

stained water.

REFERENCES

Cohn, T.A., Doling, L.L., Gilroy, E.J., Hirsch, R.M., and 

Wells, D.K., 1989, Estimating constituent loads: Water 

Resources Research, v. 25, no. 5, p. 937–942.

Corell, D.L., 1998, The role of phosphorus in the eutrophication of receiving water— A review: Journal of Environmental Quality, v. 27, p. 26–266.1

Holmberg, K.L., Perry, J.A., Ferrin, R.S., and Sharrow, D.L., 

1997, Water resources management plan–St. Croix 

National Scenic Riverway: National Park Service, St. 

Croix Falls, Wis., 155 p.

Lenz, B.N., Robertson, D.M., Fallon, J.D., and Ferrin, R., 

2001, Nutrient and suspended-sediment loads and 

benthic invertebrate data for tributaries to the St. Croix 

River, Wisconsin and Minnesota, 1997–99: U.S. Geological Survey Water-Resources Investigations Report 

01-4162, 57 p.

Lillie, R.A., Graham, S., and Rasmussen, P., 1993, Trophic 

state index equations and regional predictive equations 

for Wisconsin lakes: Research Management Findings 

No. 35. WDNR Publication No. PUBL–RS–735 93, 4p.

National Park Service, 1995, Interpretive prospectus: St. 

Croix National Scenic Riverway, Minnesota/Wisconsin.

Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1986, Hydrologic units maps: U.S. Geological Survey Water-Supply 

Paper 2294, 63 p.

Walker, W.W., 1996, Simplified procedures for eutrophication assessment and prediction: U.S Army Corps of 

Engineers, Instruction Report W–96–2, variously paginated.
APPENDIX 33

APPENDIX
34 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios Appendix. Volumetrically-weighted total phosphorus concentrations, in micrograms per liter, for various phosphorus loading scenarios for the flows of 1999, a simulated dry year (flows based on 1988 data), and simulated wet year (flows based on 1996 data) computed from loads estimated with flow-to-load relations [TP, total phosphorus; DOP, dissolved ortho-phosphorus; monitored, monitored area in 1999; unmonitored, unmonitored area; WWTP, wastewater treatment plant; POTW, publicly owned treatment works] Load reduction Load increase Base 50 percent 25 percent 10 percent 10 percent 25 percent 50 percent 75 percent 100 percent 150 percent 200 percent Stream TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP 1999 - Flow regime Willow River monitored 86 29 43 15 64 22 77 26 94 32 107 37 128 44 150 51 171 59 214 73 257 88 Lake Mallalieu unmonitored (Willow) 86 29 43 15 64 22 77 26 94 32 107 37 128 44 150 51 171 59 214 73 257 88 St. Croix River monitored 41 13 20 7 31 10 37 12 45 15 51 17 61 20 72 23 82 27 102 33 123 40 St. Croix River unmonitored 41 13 20 7 31 10 37 12 45 15 51 17 61 20 72 23 82 27 102 33 123 40 Apple River monitored 64 18 32 9 48 14 58 16 71 20 80 23 96 27 113 32 129 36 161 46 193 55 Apple River unmonitored 64 18 32 9 48 14 58 16 71 20 80 23 96 27 113 32 129 36 161 46 193 55 Browns Creek monitored 238 79 119 39 179 59 214 71 262 86 298 98 357 118 417 137 476 157 595 196 714 236 Valley Creek monitored 81 27 40 14 61 20 73 24 89 30 101 34 121 41 142 47 162 54 202 68 243 81 Troy Beach Pool unmonitored (Apple) 64 18 32 9 48 14 58 16 71 20 80 23 96 27 113 32 129 36 161 46 193 55 Black Bass Pool unmonitored (Willow) 86 29 43 15 64 22 77 26 94 32 107 37 128 44 150 51 171 59 214 73 257 88 Kinnickinnic River monitored 86 60 43 30 64 45 77 54 94 66 107 76 129 91 150 106 171 121 214 151 257 181 Kinnickinnic Pool unmonitored (Kinnickinnic) 86 60 43 30 64 45 77 54 94 66 107 76 129 91 150 106 171 121 214 151 257 181 Osceola Hatchery 70 70 35 35 53 53 63 63 77 77 88 88 105 105 123 123 140 140 175 175 210 210 Osceola WWTP 4,696 4,604 2,348 2,302 3,522 3,453 4,226 4,144 5,166 5,064 5,870 5,755 7,044 6,906 8,218 8,057 9,392 9,208 11,740 11,510 14,088 13,812 St. Croix Falls Hatchery 117 83 59 42 88 62 105 75 129 91 146 104 176 125 205 145 234 166 293 208 351 249 St. Croix Falls POTW 3,418 2,760 1,709 1,380 2,564 2,070 3,076 2,484 3,760 3,036 4,273 3,450 5,127 4,140 5,982 4,830 6,836 5,520 8,545 6,900 10,254 8,280 Stillwater POTW 341 307 171 154 256 230 307 276 375 338 426 384 512 461 597 537 682 614 853 768 1,023 921 Taylor POTW 2,162 1,946 1,081 973 1,622 1,460 1,946 1,751 2,378 2,141 2,703 2,433 3,243 2,919 3,784 3,406 4,324 3,892 5,405 4,865 6,486 5,838 Hudson WWTP 3,953 3,486 1,977 1,743 2,965 2,615 3,558 3,137 4,348 3,835 4,941 4,358 5,930 5,229 6,918 6,101 7,906 6,972 9,883 8,715 11,859 10,458
Appendix 35

Load reduction Load increase

Base 50 percent 25 percent 10 percent 10 percent 25 percent 50 percent 75 percent 100 percent 150 percent 200 percent

Stream TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP

Simulated dry year

Willow River monitored 75 24 38 12 56 18 68 22 83 26 94 30 113 36 132 42 150 48 188 60 226 72Lake Mallalieu 

unmonitored (Willow)

75 24 38 12 56 18 68 22 83 26 94 30 113 36 132 42 150 48 188 60 226 72

St. Croix River monitored 22 11 11 6 17 8 20 10 24 12 28 14 33 17 39 19 44 22 55 28 66 33St. Croix River 

unmonitored

22 11 11 6 17 8 20 10 24 12 28 14 33 17 39 19 44 22 55 28 66 33

Apple River monitored 42 21 21 10 32 15 38 19 46 23 53 26 63 31 74 36 85 41 106 52 127 62Apple River unmonitored 42 21 21 10 32 15 38 19 46 23 53 26 63 31 74 36 85 41 106 52 127 62Browns Creek monitored 156 89 78 45 117 67 141 80 172 98 195 111 234 134 274 156 313 178 391 223 469 267Valley Creek monitored 53 30 27 15 40 23 48 27 58 33 66 38 80 45 93 53 106 61 133 76 159 91Troy Beach Pool 

unmonitored (Apple)

42 21 21 10 32 15 38 19 46 23 53 26 63 31 74 36 85 41 106 52 127 62

Black Bass Pool

unmonitored(Willow)

75 24 38 12 56 18 68 22 83 26 94 30 113 36 132 42 150 48 188 60 226 72

Kinnickinnic River 

monitored

63 48 32 24 47 36 57 43 69 53 79 60 95 72 110 84 126 96 158 120 189 144

Kinnickinnic Pool 

unmonitored(Kinnickinnic)

63 48 32 24 47 36 57 43 69 53 79 60 95 72 110 84 126 96 158 120 189 144

Osceola Hatchery 70 70 35 35 53 53 63 63 77 77 88 88 105 105 123 123 140 140 175 175 210 210Osceola WWTP 4,696 4,604 2,348 2,302 3,522 3,453 4,226 4,144 5,166 5,064 5,870 5,755 7,044 6,906 8,218 8,057 9,392 9,208 11,740 11,510 14,088 13,812St. Croix Falls 

Hatchery

117 83 59 42 88 62 105 75 129 91 146 104 176 125 205 145 234 166 293 208 351 249

St. Croix Falls POTW 3,418 2,760 1,709 1,380 2,564 2,070 3,076 2,484 3,760 3,036 4,273 3,450 5,127 4,140 5,982 4,830 6,836 5,520 8,545 6,900 10,254 8,280

Stillwater POTW 341 307 171 154 256 230 307 276 375 338 426 384 512 461 597 537 682 614 853 768 1,023 921Taylor POTW 2,162 1,946 1,081 973 1,622 1,460 1,946 1,751 2,378 2,141 2,703 2,433 3,243 2,919 3,784 3,406 4,324 3,892 5,405 4,865 6,486 5,838Hudson WWTP 3,953 3,486 1,977 1,743 2,965 2,615 3,558 3,137 4,348 3,835 4,941 4,358 5,930 5,229 6,918 6,101 7,906 6,972 9,883 8,715 11,859 10,458

Appendix. Volumetrically-weighted total phosphorus concentrations, in micrograms per liter, for various phosphorus loading scenarios for the flows of 1999, a simulated dry year (flows based 

on 1988 data), and simulated wet year (flows based on 1996 data) computed from loads estimated with flow-to-load relations—Continued[TP, total phosphorus; DOP, dissolved ortho-phosphorus; monitored, monitored area in 1999; unmonitored, unmonitored area; WWTP, wastewater treatment plant; POTW, publicly owned treatment works]
36 Response of the St. Croix River Pools, Wisconsin and Minnesota, to Various Phosphorus-Loading Scenarios Load reduction Load increase Base 50 percent 25 percent 10 percent 10 percent 25 percent 50 percent 75 percent 100 percent 150 percent 200 percent Stream TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP TP DOP Simulated wet year Willow River monitored 94 34 47 17 70 25 84 30 103 37 117 42 140 51 164 59 187 68 234 84 281 101 Lake Mallalieu unmonitored (Willow) 94 34 47 17 70 25 84 30 103 37 117 42 140 51 164 59 187 68 234 84 281 101 St. Croix River monitored 47 14 24 7 35 10 43 12 52 15 59 17 71 21 83 24 94 27 118 34 142 41 St. Croix River unmonitored 47 14 24 7 35 10 43 12 52 15 59 17 71 21 83 24 94 27 118 34 142 41 Apple River monitored 72 18 36 9 54 13 64 16 79 19 90 22 107 26 125 31 143 35 179 44 215 53 Apple River unmonitored 72 18 36 9 54 13 64 16 79 19 90 22 107 26 125 31 143 35 179 44 215 53 Browns Creek monitored 265 76 133 38 199 57 239 68 292 83 331 95 398 114 464 133 530 151 663 189 795 227 Valley Creek monitored 90 26 45 13 68 19 81 23 99 28 113 32 135 39 158 45 180 51 225 64 270 77 Troy Beach Pool unmonitored (Apple) 72 18 36 9 54 13 64 16 79 19 90 22 107 26 125 31 143 35 179 44 215 53 Black Bass Pool unmonitored (Willow) 94 34 47 17 70 25 84 30 103 37 117 42 140 51 164 59 187 68 234 84 281 101 Kinnickinnic River monitored 141 85 71 43 106 64 127 77 155 94 176 107 212 128 247 149 282 170 353 213 423 256 Kinnickinnic Pool unmonitored (Kinnickinnic) 141 85 71 43 106 64 127 77 155 94 176 107 212 128 247 149 282 170 353 213 423 256 Osceola Hatchery 70 70 35 35 53 53 63 63 77 77 88 88 105 105 123 123 140 140 175 175 210 210 Osceola WWTP 4,696 4,604 2,348 2,302 3,522 3,453 4,226 4,144 5,166 5,064 5,870 5,755 7,044 6,906 8,218 8,057 9,392 9,208 11,740 11,510 14,088 13,812 St. Croix Falls Hatchery 117 83 59 42 88 62 105 75 129 91 146 104 176 125 205 145 234 166 293 208 351 249 St. Croix Falls POTW 3,418 2,760 1,709 1,380 2,564 2,070 3,076 2,484 3,760 3,036 4,273 3,450 5,127 4,140 5,982 4,830 6,836 5,520 8,545 6,900 10,254 8,280 Stillwater POTW 341 307 171 154 256 230 307 276 375 338 426 384 512 461 597 537 682 614 853 768 1,023 921 Taylor POTW 2,162 1,946 1,081 973 1,622 1,460 1,946 1,751 2,378 2,141 2,703 2,433 3,243 2,919 3,784 3,406 4,324 3,892 5,405 4,865 6,486 5,838 Hudson WWTP 3,953 3,486 1,977 1,743 2,965 2,615 3,558 3,137 4,348 3,835 4,941 4,358 5,930 5,229 6,918 6,101 7,906 6,972 9,883 8,715 11,859 10,458 Appendix. Volumetrically-weighted total phosphorus concentrations, in micrograms per liter, for various phosphorus loading scenarios for the flows of 1999, a simulated dry year (flows based on 1988 data), and simulated wet year (flows based on 1996 data) computed from loads estimated with flow-to-load relations—Continued [TP, total phosphorus; DOP, dissolved ortho-phosphorus; monitored, monitored area in 1999; unmonitored, unmonitored area; WWTP, wastewater treatment plant; POTW, publicly owned treatment works]
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