Prepared in cooperation with the Little St. Germain Lake Protection and

Rehabilitation District

Evaluation of the Effects of Changes in the Timing 

of Water-Level Drawdowns on the Export of 

Phosphorus from Little St. Germain Lake, Wisconsin

Scientific Investigations Report 2018–5078

U.S. Department of the Interior

U.S. Geological Survey
Cover. At the outlet of Little St. Germain Lake, a sluice gate drains water from the lake into Little St. Germain Creek. Photograph by Eric 

D. Dantoin, U.S. Geological Survey.
Evaluation of the Effects of Changes in 

the Timing of Water-Level Drawdowns 

on the Export of Phosphorus from Little 

St. Germain Lake, Wisconsin

By Eric D. Dantoin and Dale M. Robertson

Prepared in cooperation with the Little St. Germain Lake Protection and 

Rehabilitation District

Scientific Investigations Report 2018–5078

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Dantoin, E.D., and Robertson, D.M., 2018. Evaluation of the effects of changes in the timing of water-level drawdowns on the export of phosphorus from Little St. Germain Lake, Wisconsin: U.S. Geological Survey Scientific 

Investigations Report 2018–5078, 14 p., https://doi.org/10.3133/sir20185078.

ISSN 2328-0328 (online)
iii

Acknowledgments

We would like to acknowledge the help of Tom Best, the local data collector, and the Wisconsin 

Valley Improvement Company (WVIC) who provided streamflow data and information on managing the water level of Little St. Germain Lake. We would also like to thank WVIC, Paul Reneau 

(USGS), and Bill Rose (USGS Emeritus) for helpful comments on earlier versions of this report. 

Financial support for this study was provided by the Little St. Germain Lake Protection and 

Rehabilitation District.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Description of the Study Area ............................................................................................................1

Previous Studies ...................................................................................................................................2

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

Purpose and Approach........................................................................................................................3

Data Collection......................................................................................................................................3

Load and Daily Average Concentration Computations...................................................................4

Quantifying Effects of Changes in the Timing of Water Releases ................................................4

Results .............................................................................................................................................................4

Streamflow.............................................................................................................................................4

Stream Water Quality...........................................................................................................................6

Output of Phosphorus from Little St. Germain Lake........................................................................6

Input of Phosphorus from Muskellunge Creek to Little St. Germain Lake ..................................6

Effects Of Changes in the Timing of the Water Releases from Little St. Germain Lake............6

Relative Importance of Focused Water Withdrawals ....................................................................8

Discussion.......................................................................................................................................................8

Factors Causing Focused Drawdowns To Affect Phosphorus Export .........................................8

Effects of Drawdowns on Lake Water Quality ...............................................................................10

Summary........................................................................................................................................................13

References Cited..........................................................................................................................................13

Figures

1. Map showing the watershed of Little St. Germain Lake, Wisconsin; water-quality

sampling locations are also identified ......................................................................................2

2. Graph showing A, streamflow, B, total phosphorus concentrations, and C, daily

loads in Muskellunge Creek and Little St. Germain Creek into and out from Little St.

Germain Lake, Wisconsin. ..........................................................................................................5

3. Graph showing flow releases from Little St. Germain Lake, Wisconsin, during

September 1, 2014 through February 28, 2017, and for concentrated releases in

September and February of each year in the study period. .................................................9

4. Graph showing daily average total phosphorus concentrations in Little St. Germain

Creek, Wisconsin, during September 1, 2014, through February 28, 2017. .......................10

5. Graph showing monthly total phosphorus loads exported out of Little St. Germain

Lake, Wisconsin, for current-release, September, and February release scenarios .....12

Tables

1. Measured phosphorus loads entering and leaving Little St. Germain Lake, 

Wisconsin, and phosphorus loads leaving the lake under various release

 scenarios.......................................................................................................................................7

2. Phosphorus export from Little St. Germain Lake, Wisconsin, from 2015 to 2017 

during the winter drawdown (September through February) and under modified 

flow-release scenarios..............................................................................................................11
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

mile (mi) 1.609 kilometer (km)

acre 0.004047 square kilometer (km2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

acre-foot (acre-ft) 1,233 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Supplemental Information

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

Abbreviations

CY calendar year

GCLAS Graphical Constituent Loading Analysis System

PY phosphorus year [from September 1 of the defined year to

 August 31 of the next year]

USGS U.S. Geological Survey

WVIC Wisconsin Valley Improvement Company
Evaluation of the Effects of Changes in the Timing of 

Water-Level Drawdowns on the Export of Phosphorus 

from Little St. Germain Lake, Wisconsin 

By Eric D. Dantoin and Dale M. Robertson

Abstract

Little St. Germain Lake is a 978-acre, multibasin lake 

in Vilas County, Wisconsin. In the interest of improving its 

water quality, the Little St. Germain Lake Protection and 

Rehabilitation District initiated a cooperative study with the 

U.S. Geological Survey to describe the current phosphorus 

input into and export from Little St. Germain Lake and evaluate how water releases at different times of the year could 

affect the amount of phosphorus being released from the lake, 

potentially affecting water quality in the lake. Approximately 

780 pounds per year of phosphorus are exported out of the 

lake, which is about 80 percent of that contributed to the lake 

by Muskellunge Creek. By focusing the release of water to 

times when phosphorus concentrations near the outlet of the 

lake are highest, export of phosphorus from the lake could increase by about 60 to 80 pounds annually. This is equivalent to 

reducing phosphorus contributions from Muskellunge Creek 

by about 7 to 9 percent. Increasing phosphorus export from the 

lake should improve the long-term water-quality of the lake, 

especially in the southern basins of the lake.

Introduction

Little St. Germain Lake, located just northeast of the 

town of St. Germain in Vilas County, Wisconsin (fig. 1), is one 

of 21 impoundments operated by Wisconsin Valley Improvement Company (WVIC). Little St. Germain Lake is managed 

as part of the Wisconsin River Reservoir System in compliance with Federal Energy Regulatory Commission (FERC) 

license P–2113 to maintain as uniform a flow as practicable 

in the Wisconsin River and a reasonable balance among the 

benefits the water resource provides, including water conservation, flood control, low flow augmentation, hydroelectric 

generation, water quality, wildlife, and recreation. The level 

of the lake, which was originally dammed in 1882, has been 

maintained by WVIC at about 2.25 feet (ft) above its natural 

level (also referred to as natural run) since 1908 using a manually regulated sluice gate; this maximum level was established in 1908 and never changed (Stewart, 1911). The lake is 

annually drawn down by about 1.5 ft from December through 

March.

In the interest of protecting and improving the water 

quality of the lake, the Little St. Germain Lake Protection and 

Rehabilitation District has collaborated with the Wisconsin 

Department of Natural Resources and the U.S. Geological 

Survey (USGS) to study the water quality of the lake. Results 

of these studies indicated that relatively high loading (material transported per unit of time) of phosphorus into the lake 

from the relatively pristine surrounding watershed has caused 

most of Little St. Germain Lake to become eutrophic, with 

the possible exception of the West Bay. In an attempt to find 

management strategies to improve the water quality of Little 

St. Germain Lake, the USGS, in cooperation with the Little 

St. Germain Lake Protection and Rehabilitation District, 

investigated how changes in the timing of winter drawdown 

could increase the release of phosphorus from Little St. 

Germain Lake. In this report, we describe the current load of 

phosphorus input into and exported from Little St. Germain 

Lake during 2014 to 2016, the effects of water being released 

at different times of the year on the amount of phosphorus 

leaving Little St. Germain Lake, and the potential effects of 

changing the timing of water releases on water quality in Little 

St. Germain Lake.

Description of the Study Area

Little St. Germain Lake (fig. 1) is a multibasin lake with 

a total surface area of 978 acres and a volume of 11,500 acrefeet. Little St. Germain Lake has six basins: Upper East Bay 

(119 acres), East Bay (336 acres), No Fish Bay (69 acres), 

West Bay (213 acres), South Bay (122 acres), and Second 

South Bay (119 acres). The major tributary to the lake is 

Muskellunge Creek, which flows about 3 miles (mi) from 

shallow, relatively pristine but eutrophic Muskellunge Lake 

into the northern end of the East Bay. Muskellunge Lake 

was extensively studied by the USGS during 2000 to 2001 

as part of a cooperative study with the Muskellunge Lake 

Association (Robertson and others, 2003). Results from that 

study were used to demonstrate that groundwater and other 

natural sources were the dominant sources of phosphorus to 
2 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

St. Germain

70

155

BIRCHWOOD 

DRIVE

Muskellunge

Creek

Muskellunge

Lake

Little St. Germain Lake

West Bay

East Bay

Upper

East Bay

No Fish

Bay

South

Bay

Second

South

Bay

0 1 2 MILES

0 1 2 KILOMETERS

Little St. Germain Creek

Base from U.S. Geological Survey 30-meter

resolution land-cover data, 2017

Land use/land cover from WISCLAND geographic 

information coverage (Lillesand and others, 1998)

WISCONSIN

Study area

Forest

Wetland

Cranberry bog

Urban

Agriculture

Grassland

Water

Barren and shrubland

EXPLANATION

Land use or land cover

Drainage-basin boundary

Stream-sampling site

45°57'30"

45°55'

89°27'30" 89°22'30'

Figure 1. The watershed of Little St. Germain Lake, Wisconsin; water-quality sampling locations are also identified.

Muskellunge Lake. Outflow from Little St. Germain Lake is to 

Little St. Germain Creek, which leaves the southern side of the 

Second South Bay and flows about 1 mi before draining into 

the Rainbow Flowage of the Wisconsin River.

The total area of the Little St. Germain Lake watershed 

is 10 square miles (mi2), not including the area of the lake 

itself. The watershed is predominantly forest (68 percent), 

wetland (17 percent), and water (8 percent), although areas of 

low-density residential development exist immediately around 

the lake (fig. 1). The soils in the watershed consist mainly of 

well-drained sand and sandy loams. These soils are thought to 

be naturally high in phosphorus content (Wisconsin Department of Natural Resources, 1985), which may lead to more 

phosphorus entering lakes in this area than entering lakes in 

other areas.

Previous Studies

In 1983, the Wisconsin Department of Natural Resources 

and the Little St. Germain Lake Protection and Rehabilitation 

District studied the water quality in Little St. Germain Lake 

and examined management alternatives (Wisconsin Department of Natural Resources, 1985). Results of that study indicated relatively high phosphorus loading to Little St. Germain 

Lake and proposed that monitoring should continue. Water 

quality in Little St. Germain Lake was monitored in depth 

from 1991 to 1994 by the USGS as part of a cooperative study 

with the Little St. Germain Lake Protection and Rehabilitation 

District (U.S. Geological Survey, 2017). The study demonstrated variation in water quality among the various basins of 

the lake and found extensive areas of winter anoxia (absence 

of oxygen).

Additional studies by the USGS from 1994 to 2000 were 

conducted to refine the water and phosphorus budgets of Little 

St. Germain Lake, quantify the effects of annual drawdowns, 

define the extent of winter anoxia, and provide information needed to develop a comprehensive lake-management 

plan (Robertson and Rose, 2000). Results of those studies 

indicated that Muskellunge Creek, which drains a relatively 

pristine watershed including eutrophic Muskellunge Lake, 

was the dominant source of phosphorus to Little St. Germain 

Lake; however, groundwater contributions (based on limited 
Methods 3

information) appeared to be important and is an area of study 

that would benefit from further quantification. Results of those 

studies also indicated extensive areas of anoxia during winter 

in the Upper East, South, and Second South Bays (fig. 1). 

Based on results of those studies, aerators were placed in 

Upper East Bay, northern end of East Bay, and South Bay and 

operated throughout winter. From 2000 to 2005, the USGS 

evaluated the success of the aerators at eliminating winter 

anoxia, developed detailed phosphorus budgets for the lake, 

and assessed how the water quality in Little St. Germain Lake 

should respond to changes in phosphorus loading (Robertson and others, 2005). The evaluation demonstrated that the 

aerators were successful at preventing winter anoxia and that 

groundwater input was an important driver of the eutrophic 

condition of Little St. Germain Lake. From 2010 to 2014, the 

USGS used all the water-quality and phosphorus loading data 

collected in the lake to describe the interannual and long-term 

changes in the trophic state of Little St. Germain Lake and 

examine how morphology, climate, winter aeration, and beaver activity affect the water quality in Little St. Germain Lake 

(Robertson and others, 2016).

Methods

Purpose and Approach

The purpose of the study detailed in this report was to 

investigate the potential effects of changing the timing of winter drawdown to coincide with highest total phosphorus concentrations in Little St. Germain Lake and determine a strategy 

to maximize the release of phosphorus from Little St. Germain 

Lake. It is believed that increasing the removal of phosphorus 

from Little St. Germain Lake should improve water quality 

in the lake, especially in the southern basins. Increasing the 

removal of phosphorus in this manner is analogous to using 

hypolimnetic withdrawals to remove phosphorus from a lake 

to improve its water quality (Cooke and others, 1993). Previous studies have shown that removing deep water (hypolimnetic water) with elevated total phosphorus concentrations can 

be an efficient and valuable tool in reducing near-surface total 

phosphorus concentrations in deep reservoirs (Nürnberg, 1987; 

Nürnberg and others, 1987). Continual removal of phosphorus 

from a reservoir or lake should gradually reduce the phosphorus stored in the bottom sediments of the lake and reduce 

the release of phosphorus back into the water column. In the 

case of Little St. Germain Lake, hypolimnetic withdrawal 

would not be effective in removing phosphorus because of the 

morphology of the lake, which results in little stratification in 

Second South Bay near its outlet.

The hypothesis being examined in this study focuses 

on maximizing flow releases from the lake during periods 

of highest total phosphorus concentrations, which generally 

occur during early fall because of phosphorus being released 

from the sediments throughout summer and during fall 

turnover, and during mid to late winter because of phosphorus 

being released during periods of anoxia near the sediment 

surface. Phosphorus concentrations and loads in the inflow and 

outflow from Little St. Germain Lake were determined for the 

study period (2014–2017; referred to as current conditions) to 

establish the baseline conditions for water-release operations. 

Different flow-release scenarios were evaluated to determine 

the feasibility of increasing the amount of phosphorus released 

from the lake. In each scenario, daily total phosphorus concentrations leaving the lake were unchanged from the baseline 

conditions, but the flows were modified. Outflow from the lake 

when it was not being drawn down was maintained at baseline 

conditions; however, flows from September through February 

(associated with winter drawdown) were modified. The magnitude of changes in phosphorus removal were compared with 

the total baseline phosphorus leaving the lake and compared 

with the total phosphorus input to the lake from Muskellunge 

Creek, which is the major tributary to the lake (fig. 1).

Data Collection

Daily loads of phosphorus entering and leaving Little 

St. Germain Lake from November 2014 to December 2016 

were determined from flow measurements and water sample 

concentrations collected in Muskellunge Creek at Birchwood 

Drive (USGS streamgage 05390685, Muskellunge Creek near 

St. Germain, WI) and Little St. Germain Creek at State Highway 70 (USGS streamgage 05390701, Little Saint Germain 

Creek near Eagle River, WI), respectively (fig. 1). Continuous 

(15-minute-interval) water elevations were measured at Muskellunge Creek, from which continuous flow was calculated 

using standard USGS measurement and computation procedures (Rantz and others, 1982). Daily outflows into Little St. 

Germain Creek, which were calculated from lake elevations 

and gate-opening settings, were provided by WVIC. Discharge 

measurements were also made by the USGS at Little St. 

Germain Creek using a midsection method at approximately 

monthly intervals. These measurements were used to validate 

the WVIC flow calculations and make adjustments to WVICestimated flow if needed.

Biweekly water samples were collected by the USGS and 

a local observer at the two USGS streamgages from October 2014 to December 2016. In general, USGS researchers 

collected water samples near the middle of each month, and 

a local observer collected water samples early in the month. 

Water samples were collected using simple grab sampling 

techniques with a 250-milliliter bottle. The well-mixed, 

shallow sampling locations allowed a single grab to be an 

acceptable collection method at both sites (U.S. Geological 

Survey, 2012). All water samples were preserved and shipped 

on ice to the Wisconsin State Laboratory of Hygiene for total 

phosphorus analysis in accordance with standard analytical 

procedures described in the “Manual of Analytical Methods, 

Inorganic Chemistry Unit” (Wisconsin State Laboratory of 

Hygiene, 1993).
4 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

Load and Daily Average Concentration 

Computations

Daily phosphorus loads in Muskellunge Creek and 

Little St. Germain Creek were calculated using the Graphical 

Constituent Loading Analysis System (GCLAS) computer 

program developed by Koltun and others (2006) that is used 

to estimate loads of water-quality constituents from continuous streamflow data and instantaneous constituent concentrations. To estimate total phosphorus concentrations during 

unsampled periods, phosphorus concentrations were linearly 

interpolated between measurements except at the beginning 

and end of each large change in flow (referred to as an event). 

Estimated concentrations were added to the time series to 

better describe concentrations just prior to an event and just 

following an event or to describe concentrations during events 

that lacked measured concentrations. Concentrations at the 

beginning of each large change in flow (event) were estimated 

based on measured sample concentrations during preceding 

base-flow periods. Concentrations at the end of the events 

were estimated from concentrations measured shortly after the 

end of an event. GCLAS was also used to estimate concentrations during unsampled high-flow events using relations 

of individual water sample concentrations with associated 

discharges. These additional estimated concentrations increase 

the confidence in calculated loads by accounting for the effects 

of changing flows between known sampled concentrations. 

From the daily loads, daily average total phosphorus concentrations for September 2014 to December 2016 in Little St. 

Germain Creek were calculated by dividing the daily total 

load by the total daily flow. All streamflow and daily estimated 

total phosphorus concentrations and loads from September 

2014 to December 2016 are available from the National Water 

Information System (U.S. Geological Survey, 2017). Daily 

flows and concentrations were not measured for January 

and February 2017; therefore, the flows and concentrations 

for this period were assumed to be the same as in January to 

February 2016.

Quantifying Effects of Changes in the Timing of 

Water Releases

The effects of changing the timing of the water release 

were quantified by comparing phosphorus loading exported 

from Little St. Germain Lake during the study period (Scenario 1; baseline) with loadings for eight additional release 

scenarios. It was thought that phosphorus export from the lake 

could be increased if the flows leaving the lake were concentrated when total phosphorus concentrations were highest (late 

summer near September and middle to late winter near February). To develop the scenarios for flow and total phosphorus 

concentrations, the measured baseline daily loads were first 

separated into daily flows and daily total phosphorus concentrations. Daily flows for September through February were 

then adjusted to reflect the flow for each of the eight nonbaseline release scenarios. 

The flows from the actual dam operation from September 

1, 2014, to December 31, 2016 were extrapolated to February 

28, 2017 (the full study period). This full period is referred to 

as the current release in Scenario 1 and provided a baseline 

to compare the effects of drawing down water levels in three 

distinct drawdown periods (September through February in 3 

consecutive years). Flow and phosphorus releases were simulated for eight nonbaseline scenarios. In the first six scenarios, 

total winter drawdown was simulated to occur during specific 

one-month periods (Scenario 2, September; Scenario 3, October; Scenario 4, November; Scenario 5, December; Scenario 

6, January; and Scenario 7, February). In two additional 

scenarios, drawdown occurred over the 2 months of January 

and February (Scenario 8) or was split between September and 

February (Scenario 9).

Daily flows for each scenario were constructed by first 

calculating the total flow during September through February for each year from 2014 through 2017. For each year, the 

total minimum flow rate for the months without simulated 

drawdown during the September through February period 

was set to 5.6 ft3/s (minimum allowable flow; Wisconsin 

Valley Improvement Company, 2017). The flow rates for the 

simulated drawdown month(s) were then computed from the 

differences in the total flow for September through February 

and the total flow during the remaining months at minimum 

flow divided into the number of days for the specific month(s). 

The flows for the remaining part of the year (March through 

August) remained at the gaged daily discharge. Daily total 

phosphorus loads for each scenario were then calculated by 

multiplying simulated daily flows by the total phosphorus 

concentrations determined for the baseline period.

Results

Streamflow

Daily average streamflows at the outlet and inlet to Little 

St. Germain Lake are shown in figure 2. WVIC-computed 

discharges were compared to USGS discharge measurements 

to evaluate the accuracy of the WVIC-computed discharges. In 

general, the USGS discharge measurements matched the data 

provided by WVIC reasonably well, except for flows less than 

8 ft3/s. WVIC-computed flows less than 8 ft3/s, on average, 

were 9 percent higher than the USGS discharge measurements. 

Therefore, all supplied flows less than 8 ft3/s were decreased 

by 9 percent.

The average daily flow rate in Little St. Germain Creek 

from September 2014 through February 2017 was 14.7 ft3/s. 

The minimum and maximum daily average flow rates were 

5.11 ft3/s (February 27, 2016) and 84.5 ft3/s (June 10, 2015), 

respectively. Flow out of the lake was typically highest during late fall and early winter because of the lake being drawn 
Results 5

0

20

40

60

80

100

Daily discharge, in cubic feet per second

A. Discharge

0

2

4

6

8

10

12

14

Sept. Dec. Mar. June Sept. Dec. Mar. June Sept. Dec.

Daily total phosphorus load, in pounds

C. Daily load

0.00

0.02

0.04

0.06

0.08

0.10

Concentration, in milligram per liter

B. Total phosphorus concentration

Little St. Germain Creek

Muskellunge Creek

EXPLANATION

Little St. Germain Creek

Muskellunge Creek

EXPLANATION

Little St. Germain Creek

Muskellunge Creek

EXPLANATION

Date

 2014 2015 2016

Figure 2. A, streamflow, B, total phosphorus concentrations, and C, daily phosphorus loads in Muskellunge 

Creek and Little St. Germain Creek into and out from Little St. Germain Lake, Wisconsin. 
6 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

down and lowest when the lake was being refilled in early 

spring. An anomaly in flow at the outlet occurred during June 

10–11, 2015, when flow rates exceeded 80 ft3/s. This highflow event was caused by extensive rain during the preceding 

weeks, which caused lake elevation to be greater than the 

FERC authorized maximum water elevation. Increased outflows from the lake were needed to reduce the water level and 

comply with the FERC license requirements.

The average daily flow rate into the lake from Muskellunge Creek from November 2014 through December 2016 

was 8.3 ft3/s. The minimum and maximum daily average flow 

rates were 4.9 ft3/s (March 3, 2014) and 20.7 ft3/s (August 

21, 2015), respectively. In general, flow rates were highest in 

Muskellunge Creek during spring and summer and lowest in 

midsummer. There were more high-flow events during April 

through October 2016 than during other periods of this study.

Stream Water Quality

During October 2014 through December 2016, total 

phosphorus concentrations in Little St. Germain Creek ranged 

from 0.017 to 0.043 milligram per liter (mg/L), with an average concentration of 0.03 mg/L (fig. 2). All total phosphorus 

concentrations were below 0.075 mg/L, which is the Wisconsin total phosphorus water-quality criterion for wadeable streams (Wisconsin Department of Natural Resources, 

2017). Concentrations remained relatively constant (0.02 to 

0.03 mg/L) for most of the study period; however, concentrations increased in early fall and late winter in each of the 3 

years of the study period. The average total phosphorus concentration in Little St. Germain Creek was similar to average 

concentrations measured in previous studies (Robertson and 

Rose, 2000, Robertson and others, 2005).

Total phosphorus concentrations in Muskellunge Creek 

during the study period ranged from 0.03 mg/L in spring 2016 

to 0.08 mg/L in fall 2015 (fig. 2), with an overall average 

concentration of 0.06 mg/L. Except for a few outliers, total 

phosphorus concentrations remained around 0.05 to 0.07 mg/L 

regardless of flow or seasonality. The average total phosphorus 

concentration was similar to average concentrations measured 

in previous studies (Robertson and Rose 2000, Robertson and 

others, 2005). Total phosphorus concentrations in the outflow 

at Little St. Germain Creek were about 50 percent of those 

entering the lake from Muskellunge Creek.

Output of Phosphorus from Little St. Germain 

Lake

The total output of phosphorus (load) from Little St. Germain Lake to Little St. Germain Creek during September 2014 

to December 2016 is shown in figure 2. The average daily load 

for this entire period was 2.3 pounds per day (lb/d), but ranged 

from 0.8 lb/d to the highest daily loads of 13.0 lb/d on June 

10–11, 2015. The increased flow on these days magnified the 

loads of phosphorus during this period even with relatively 

low total phosphorus concentrations in the discharge water. 

Monthly loads are summarized in table 1.

To determine how important the phosphorus load 

exported from the lake during winter drawdown was to the 

total annual load leaving the lake, annual phosphorus loads 

were computed from September 1 through August 31 using 

the actual flows and concentrations. This 12-month period is 

referred to as a phosphorus year (PY) for the year in which 

the period ends. During the 2015 phosphorus year (PY2015) 

and the 2016 phosphorus year (PY2016), the total loads of 

phosphorus leaving the lake were 756 pounds (lb) and 758 lb, 

respectively. During the drawdown period of these years 

(September through February), the total loads of phosphorus 

leaving the lake were 503 lb and 420 lb, respectively. Therefore, the drawdown periods represented 67 and 55 percent, 

respectively, of the total phosphorus loads leaving the lake, 

which demonstrates that the export of phosphorus during the 

drawdown period is a substantial part of the total annual phosphorus load exported from the lake.

Input of Phosphorus from Muskellunge Creek to 

Little St. Germain Lake

The total phosphorus input into Little St. Germain Lake 

from Muskellunge Creek was measured from November 1, 

2014, to November 30, 2016 and amounted to 1,920 lb. Daily 

loadings into Little St. Germain Lake were more constant than 

those leaving the lake because concentrations of total phosphorus and streamflows at the inlet were more constant than 

at the outlet. The daily phosphorus loads ranged from 1.5 lb/d 

on March 3, 2015, to 7.8 lb/d on August 21, 2016. The lowest daily load on March 3, 2015, resulted from a low winter 

concentration of total phosphorus and a decreased late-winter 

flow. The highest daily load on August 21, 2016, resulted from 

a relatively high late summer concentration of total phosphorus and increased flow following substantial rainfall. Monthly 

phosphorus loads from Muskellunge Creek are listed in 

table 1. These results indicate that there is less seasonal variation in the loading at the inlet than at the outlet from Little St. 

Germain Lake.

Effects Of Changes in the Timing of the Water 

Releases from Little St. Germain Lake

To quantify the effects of focusing the release of water 

during winter drawdown to specific months on the total 

phosphorus load exported from Little St. Germain Lake, eight 

different scenarios were simulated and compared to the actual 

current baseline loading (Scenario 1). The experimental flow 

rates during specified drawdowns months (Scenarios 2–7) 

ranged between 71.1 and 86.9 ft3/s. The daily flows for three 

of the scenarios (Scenarios 1, 2, and 7) are shown in figure 3. 

By drawing down the water levels in the lake over 2 months 

under Scenarios 8 and 9, flow rates for the specified monthly 
Results 7

Table 1. Measured phosphorus loads entering and leaving Little St. Germain Lake, Wisconsin, and phosphorus loads leaving the lake 

under various release scenarios.

[Phosphorus loads enter Little St. Germain Lake from Muskellunge Creek. lb, pound; NA, not available; CY, calendar year; PY, phosphorus year (period from 

September 1 to August 31, ending in the year specified)]

Year Month

Phosphorus 

load 

entering 

lake, in lb

Phosphorus load leaving lake, in lb, under scenario

1 2 3 4 5 6 7 8 9

Baseline

Release

September

Release

October

Release

November

Release

December

Release

January

Release

February

Release

January 

and

February

Release

September 

and 

February

Release

2014 September NA 104 442 28 28 28 28 28 28 242

October NA 65 23 361 23 23 23 23 23 23

November 82 75 20 20 313 20 20 20 20 20

December 69 74 20 20 20 310 20 20 20 20

2015 January 70 96 24 24 24 24 380 24 211 24

February 54 89 31 31 31 31 31 528 267 271

March 82 57 57 57 57 57 57 57 57 57

April 107 36 36 36 36 36 36 36 36 36

May 83 30 30 30 30 30 30 30 30 30

June 109 67 67 67 67 67 67 67 67 67

July 72 29 29 29 29 29 29 29 29 29

August 60 34 34 34 34 34 34 34 34 34

September 68 34 369 28 28 28 28 28 28 204

October 73 32 24 305 24 24 24 24 24 24

November 90 81 21 21 269 21 21 21 21 21

December 83 83 20 20 20 257 20 20 20 20

2016 January 67 99 25 25 25 25 319 25 180 25

February 57 91 27 27 27 27 27 360 185 187

March 87 30 30 30 30 30 30 30 30 30

April 101 35 35 35 35 35 35 35 35 35

May 91 64 64 64 64 64 64 64 64 64

June 92 60 60 60 60 60 60 60 60 60

July 66 51 51 51 51 51 51 51 51 51

August 108 99 99 99 99 99 99 99 99 99

September 92 111 562 38 38 38 38 38 38 309

October 80 78 33 484 33 33 33 33 33 33

November 70 65 22 22 335 22 22 22 22 22

December NA 125 30 30 30 436 30 30 30 30

2017 January NA 99 25 25 25 25 365 25 204 25

February NA 90 26 26 26 26 26 412 209 212

Summary periods (12 months)

CY2015 938* 668 742 683 650 638 755 897 823 816

CY2016 994* 907 1038 963 825 918 756 898 824 817

PY2015 NA 756 813 736 692 689 754 896 821 853

PY2016 983 758 823 764 731 719 777 816 795 820
8 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

Table 1. Measured phosphorus loads entering and leaving Little St. Germain Lake, Wisconsin, and phosphorus loads leaving the lake 

under various release scenarios.—Continued

[Phosphorus loads enter Little St. Germain Lake from Muskellunge Creek. lb, pound; NA, not available; CY, calendar year; PY, phosphorus year (period from 

September 1 to August 31, ending in the year specified)]

Year Month

Phosphorus 

load entering lake, 

in lb

Phosphorus load leaving lake, in lb, under scenario

1 2 3 4 5 6 7 8 9

Baseline

Release

September

Release

October

Release

November

Release

December

Release

January

Release

February

Release

January 

and

February

Release

September 

and 

February

Release

Drawdown period (September–February)

2015 drawdown NA 503 560 483 439 436 501 643 568 600

2016 drawdown 437 420 485 426 393 381 439 478 457 482

2017 drawdown NA 568 699 624 487 580 514 560 536 632

*Full year of loading was obtained using loads from December to November of the specified year.

releases were reduced from approximately 75 ft3/s to 40 ft3/s, 

depending on the year.

Daily total phosphorus concentrations for September 

2014 to December 2016 in Little St. Germain Creek for all 

the scenarios were calculated by dividing the daily total loads 

for Scenario 1 (baseline conditions) by the measured daily 

average flows (fig. 4). Because the daily concentrations were 

not measured for the last 2 months of the simulation period 

(January and February 2017), they were assumed to be the 

same as those measured in January and February 2016. Daily 

total phosphorus loads for each scenario were then obtained by 

multiplying the daily average flows (fig. 3) by the daily average total phosphorus concentrations (fig. 4).

Phosphorus loads exported out of Little St. Germain 

Lake during winter drawdown (September through February) during each of the nine scenarios are listed in table 2. 

The effects of focusing flow to a specific time period varied 

by year, but the scenarios that focused the removal of water 

in September (Scenario 2), February (Scenario 7), and split 

between September and February (Scenario 9) on average 

had the largest increase in phosphorus loads exported from 

the lake from 63 to 84 lb/yr (table 2). Results for Scenarios 

2 and 7 are compared with current (baseline) phosphorus 

release in figure 5. Scenarios that increased releases during 

November (Scenario 4) and December (Scenario 5) generally decreased the load of phosphorus exported from the lake 

by an average of 58 lb/yr and 32 lb/yr, respectively (table 2). 

Focused releases during October (Scenario 3) and January 

(Scenario 6) had little effect on the amount of phosphorus 

exported from the lake. Release spread out during January and 

February (Scenario 8) resulted in an average increased export 

of 23 lb/yr of phosphorus. Scenario 9 split the focused release 

between September and February and resulted in an average 

increase in export of 84 lb/yr of phosphorus. Focused flow 

releases resulted in a change in the average annual phosphorus 

exported from the lake ranging from, on average, an 11 percent decrease (focusing the release in November; Scenario 4) 

to a 17 percent increase (focusing the release in September; 

Scenario 2) in phosphorus export compared with the phosphorus load typically exported from the lake for the entire period 

from September through February (current-release conditions).

Relative Importance of Focused Water 

Withdrawals

During 2015 to 2016, input of phosphorus from Muskellunge Creek into Little St. Germain Lake was 962 lb/yr 

(table 1). By focusing the release of water to a specific month 

or months (September, Scenario 2; February, Scenario 7; and 

September and February, Scenario 9), it would have been possible to remove an additional 63 to 84 lb/yr, on average, from 

Little St. Germain Lake (table 2). This is equivalent to reducing the contributions of phosphorus from Muskellunge Creek 

by 7 to 9 percent, annually.

Discussion

Factors Causing Focused Drawdowns To Affect 

Phosphorus Export

External phosphorus input to a lake drives in-lake 

phosphorus concentrations and lake productivity (plant and 

algal growth). The phosphorus is then either exported out of 

the lake or deposited in the lake sediments. The phosphorus 

that is deposited in the bottom sediments, however, does not 

all remain there; some of the phosphorus is released back into 

the water column, referred to as “internal phosphorus loading.” Mortimer (1941) demonstrated that phosphorus release 

rates increase dramatically when dissolved oxygen near the 

sediment-water interface is completely depleted (a condition 

known as anoxia). Typically, in deep dimictic lakes (lakes that 
Discussion 9

0

25

50

75

100

Daily average flow,

in cubic feet per second

A. Baseline release

0

25

50

75

100

0

25

50

75

100

B. Release in September

C. Release in

 February 

Sept. Dec. Mar. June Sept. Dec. Mar. June Sept. Dec.

Date

 2014 2015 2016

Daily average flow,

in cubic feet per second

Daily average flow,

in cubic feet per second

Mar.

2017

Figure 3. Flow releases from Little St. Germain Lake, Wisconsin, during September 1, 2014 through February 28, 2017, 

and for concentrated releases in September and February of each year in the study period. Scenario 1 is the baseline 

scenario for current releases for the period of study and Scenarios 2 and 7 are releases of all excess water from the 

lake in September and February, respectively.
10 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

0.000

0.010

0.020

0.030

0.040

0.050

Total phosphorus concentration,

in milligram per liter

Sept. Dec. Mar. June Sept. Dec. Mar. June Sept. Dec.

Date

 2014 2015 2016

Mar.

2017

Figure 4. Daily average total phosphorus concentrations in 

Little St. Germain Creek, Wisconsin, during September 1, 2014, 

through February 28, 2017.

stratify throughout summer), phosphorus released from the 

deep sediments is retained in the deep (hypolimnetic) water 

and primarily released to the near-surface (epilimnetic) water 

during spring and fall turnover. Therefore, near-surface total 

phosphorus concentrations in dimictic lakes usually remain 

stable or decrease as summer progresses (Welch and Cooke, 

1995). In shallow lakes, as are most of the basins in Little 

St. Germain Lake, microzones of anaerobic conditions can 

develop at the sediment/water interface during short periods 

of stratification, especially during summer when biological 

activity is high. Then, the frequent mixing events that occur in 

shallow lakes transport this phosphorus to the surface waters, 

which results in near-surface total phosphorus concentrations 

and productivity increasing throughout summer (Welch and 

Cooke, 1995; Robertson and others, 2016). In shallow lakes, 

total phosphorus concentrations usually reach the highest 

concentrations of the year in fall (September or October). 

Therefore, increasing the export of water from the lake late in 

fall may increase the export of phosphorus from shallow lakes, 

such as observed in Scenarios 2 (September release) and 9 

(September and February release).

In shallow lakes, as are most of the basins in Little St. 

Germain Lake, dissolved oxygen concentrations throughout 

the water column can drop below critical concentrations during winter and cause increased internal phosphorus loading 

and a gradual increase in phosphorus concentrations under the 

ice (Wetzel, 1983; Hudson and Kirschner, 1997; Robertson 

and Rose, 2000). Therefore, increasing the export of water in 

late winter may increase the export of phosphorus from shallow lakes, such as observed in Scenarios 7 (February release) 

and 9 (September and February release).

Because of the release of phosphorus from the bottom 

sediments, there is distinct seasonality in total phosphorus 

concentrations in Little St. Germain Lake. The purpose of 

focusing water releases during late summer and middle to late 

winter is to take advantage of this seasonality in inlake total 

phosphorus concentrations and release water from the lake 

when the total phosphorus concentrations are greatest.

Effects of Drawdowns on Lake Water Quality

By focusing winter drawdown to a specific month or 

months, it might be possible to remove an additional 60 to 

80 lb/yr of phosphorus from Little St. Germain Lake (table 2). 

Over several years, this would be equivalent to reducing the 

contributions of phosphorus from Muskellunge Creek by 6 

to 9 percent annually. It is not expected that modifying the 

timing of winter drawdown will immediately improve the 

water quality of Little St. Germain Lake, but by continually 

removing an additional 60 to 80 lb of phosphorus each year 

through optimizing the timing of the release, total phosphorus 

concentrations stored in the sediments of Little St. Germain 

Lake are expected to gradually decline, similar to what occurs 

with hypolimnetic withdrawal systems (Nurnberg, 1987; 

Nurnberg and others, 1987), and reduce the release of phosphorus back into the water column, resulting in lower total 

phosphorus concentrations in the lake. The decreases in total 

phosphorus concentrations are expected to primarily occur 

in the most downstream areas of the lake, such as the South 

and Second South Bays. It was estimated that a 25 percent 

decrease in phosphorus loading to Little St. Germain Lake 

should decrease summer average total phosphorus concentrations by about 0.004 to 0.007 mg/L in the East Bay of Little 

St. Germain Lake (Robertson and others, 2005). If the South 

and Second South Bays of Little St. Germain Lake respond in 

a similar way as the East Bay, it is expected that a 7 to 9 percent increase in phosphorus removal from the lake (or about 

7/25ths to 9/25ths of the changes simulated for the East Bay) 

should gradually result in about a 0.001 to 0.003 mg/L reduction in total phosphorus concentrations in the South Bays. 

Decreases in total phosphorus concentrations should then 

decrease algae (chlorophyll a) concentrations and improve 

water clarity (Secchi depth) in the downstream bays.

When dissolved oxygen concentrations throughout the 

water column drop below critical concentrations under the 

ice during winter, it can cause fish kills (referred to as “winterkill”), increased internal phosphorus loading, and odor 

problems where water leaves the lake (Wetzel, 1983; Hudson 

and Kirschner, 1997). To try to eliminate these issues, aeration 

systems have been installed in Little St. Germain Lake and 

operated throughout winter. Winter aeration has been shown 

to have measurable effects on the water quality in Little St. 

Germain Lake (Robertson and others, 2005) and other lakes 

(Hudson and Kirschner, 1997) in winter and has eliminated 

winterkill in Little St. Germain Lake (S. Gilbert, Wisconsin 

Department of Natural Resources, pers. commun., 2014).

By focusing water withdrawal from the lake to a specific month or months, it may be possible to reduce the 

extent of winter anoxia in specific areas of Little St. Germain 

Lake. When water levels are drawndown in late fall or early 

winter, much of the water that has high dissolved oxygen 
Discussion 11

Table 2. Phosphorus export from Little St. Germain Lake, Wisconsin, from 2015 to 2017 during the winter drawdown (September 

through February) and under modified flow-release scenarios.

Year

Scenario

1 2 3 4 5 6 7 8 9

Baseline

Release

Sept. 

Release

Oct.

Release

Nov.

Release

Dec.

Release

Jan. 

Release

Feb.

Release

Jan./Feb. 

Release

Sept./Feb. 

Release

Phosphorus loading into Little St. Germain Creek for September through February, in pounds

2015 503 560 483 439 436 501 643 568 600

2016 419 484 425 392 381 438 477 456 481

2017 568 699 624 487 580 514 560 536 632

Increase in phosphorus removed from the lake, in pounds

2015 0 57 -20 -64 -68 -2 140 65 97

2016 0 66 6 -27 -38 19 58 38 62

2017 0 130 56 -82 11 -54 -9 -33 63

Average 84 14 -58 -32 -12 63 23 74

Percent increase in drawdown (September–February) phosphorus removal

2015 0 11 -4 -13 -13 -0 28 13 19

2016 0 16 2 -6 -9 5 14 9 15

2017 0 23 10 -14 2 -10 -2 -6 11

Average 0 17 2 -11 -7 -2 13 5 15

Percent increase in the annual phosphorus removal

2015 0 8 -3 -8 -9 -0 18 9 13

2016 0 9 1 -4 -5 3 8 5 8

2017 0 17 7 -11 1 -7 -1 -4 8

Average 0 11 2 -8 -4 -2 8 3 10

Percentage of increase in phosphorus removal compared to phosphorus inputs in Muskellunge Creek1

2015 0 6 -2 -7 -7 -0 14 7 10

2016 0 7 1 -3 -4 2 6 4 6

2017 0 13 6 -8 1 -6 -1 -3 7

Average 0 9 1 -6 -3 -1 6 2 8

1

Average annual load for Muskellunge Creek is based on loads from November, 2015 through October, 2016. 

concentrations is released before ice covers the surface of the 

lake. This results in an increased rate of dissolved oxygen 

depletion as winter progresses because oxygen in the sediment 

is consumed in a smaller volume of lake water. However, if 

water is released later in the winter, in February (Scenarios 

7 and 9), much of the oxygen consumption would be from a 

larger volume of water, and dissolved oxygen concentrations 

would not be reduced so greatly. However, maintaining higher 

water levels early in the winter may reduce the effectiveness 

of a mid- to late-winter (February) withdrawal in terms of 

phosphorus load reduction because higher water levels should 

result in increased dissolved oxygen concentrations, decreased 

phosphorus release from sediments, and decreased total phosphorus concentrations in the lake.

Recognizing and understanding the potential effects of 

changing the timing of winter drawdown to an optimized 

period is important. To better evaluate implementation of a 

specific drawdown strategy, the potential positive and negative effects, whether speculative or known, should also be 

considered. The most positive effect of Scenarios 2, 7, and 9 

is increased removal of phosphorus from Little St. Germain 

Lake, which could potentially improve its long-term water 

quality, especially in the most downstream South and Second 

South Bays of the lake. The anticipated increased phosphorus 

export out of Little St. Germain Lake is similar for all three 

of these scenarios; therefore, the potential negative effects of 

each of these three scenarios were examined.
12 Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin

0

100

200

300

400

500

600

Phosphorus load, in pounds per month

A. Baseline release

B. Release in September

C. Release in February

Sept. Jan. May Sept. Jan. May Sept. Jan.

Date

 2014 2015 2016

0

100

200

300

400

500

600

Phosphorus load, in pounds per month

0

100

200

300

400

500

600

Phosphorus load, in pounds per month

2017

Figure 5. Monthly total phosphorus loads exported out of Little St. Germain Lake, Wisconsin, for baselinerelease, September, and February release scenarios.
References Cited 13

A concentrated September release appears to have the 

most potential negative effects of the three scenarios. Drawing down Little St. Germain Lake in September could affect 

open-water recreation and shoreline structure use, such as 

access to boat docks. Drawing down Little St. Germain Lake 

early in fall could also increase the risk of more widespread 

anoxia issues because of the smaller volume of oxygen-rich 

water in the lake before the lake freezes. Focusing the release 

to a 1-month period should be achievable given the design of 

the dam except during very dry conditions, but could result 

in flows that may be greater than the morphology of Little St. 

Germain Lake Creek can handle and cause bank and channel 

erosion. In addition, fall turnover is often weather dependent and may not have always occurred prior to a September 

release and possibly result in lower total phosphorus concentrations in the lake. Lowering the lake elevations earlier than 

current-release operations, such as in September, may increase 

groundwater flow into the lake by increasing the gradient 

between the surface of the lake and that of the surrounding 

groundwater for a longer period of time. Increased flow of 

groundwater with relatively high phosphorus concentrations into the lake could potentially increase the phosphorus 

loading to the lake. Further investigation and modeling may 

be required to determine the significance of these types of 

groundwater/lake interactions.

With a concentrated February release, Little St. Germain 

Lake would maintain a greater volume of water during more 

of the winter months and reduce the anoxic conditions in the 

lake. Maintaining a higher lake elevation during the winter 

months may also decrease the groundwater flow by maintaining a weaker gradient between the lake and the groundwater. 

However, with lake levels remaining higher than currentrelease conditions, shoreline structures could be damaged 

by ice. Focused drawdown during the winter months could 

potentially cause ice jams downstream and affect the two 

recreational bridges downstream of the lake where Little St. 

Germain Creek flows into the Rainbow Flowage. In addition, 

the unnatural flows during midwinter could cause negative ecological effects that should be considered (Wisconsin 

Department of Natural Resources, 2011).

A split September and February drawdown (Scenario 

9) may minimize the negative effects that could occur in 

Scenarios 2 and 7 by reducing the flow rates compared with 

focused single-month releases. With the flow rates leaving 

the lake cut in half during each of the 2 months, the effects 

on channel morphology and ecological concerns should be 

reduced. The overall effectiveness of this scenario may be 

slightly more weather dependent than a full drawdown in 

February but more effective than only focusing the drawdown 

to September. A split January and February drawdown would 

minimize the negative effects of increased flow but would not 

remove as much phosphorus as Scenarios 2, 7, and 9 because 

dissolved oxygen concentrations have not yet been depleted 

in January and phosphorus concentrations in the lake have 

not yet increased.

Summary

In the interest of improving the water quality of Little 

St. Germain Lake, the U.S. Geological Survey studied 

phosphorus loading into and out of the lake and evaluated 

how focusing winter drawdown to different times of the year 

should affect the amount of phosphorus being released from 

the lake, potentially influencing the water quality in the lake. 

During 2015 and 2016, an average of about 780 pounds per 

year (lb/yr) of phosphorus was exported out of the lake, which 

is about 80 percent of that contributed to the lake by Muskellunge Creek. By optimizing the timing of winter drawdown 

to when concentrations near the outlet of the lake are highest 

(September and February), there could be an increased export 

of phosphorus from the lake by about 60 to 80 lb/yr, which is 

equivalent to reducing the contributions of total phosphorus 

from Muskellunge Creek by about 7 to 9 percent. Increasing 

phosphorus export from the lake should gradually improve the 

water quality of the lake, especially in the South and Second South Bays of the lake. The study focused drawdown to 

specific months, but in practice, the phosphorus concentrations 

in the South and Second South Bays of Little St. Germain 

Lake should drive the withdrawal strategy. To optimize the 

phosphorus release, the timing of the drawdown would be best 

served by coinciding with the highest concentrations of phosphorus in the lake, typically in fall and late winter.

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Dantoin and Robertson—Effects of Changes in Timing of Water-Level Drawdowns on Phosphorus Export, Little St. Germain Lake, Wisconsin—SIR 2018–5078

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20185078