Hydrology, Water Quality, and Response to Changes in 

Phosphorus Loading of Minocqua and Kawaguesaga Lakes, 

Oneida County, Wisconsin, With Special Emphasis on 

Effects of Urbanization

Scientific Investigations Report 2010–5196

Prepared in cooperation with the Minocqua/Kawaguesaga Lakes Protection Association 

through the Town of Minocqua, Wisconsin

U.S. Department of the Interior

U.S. Geological Survey
Cover: Aerial photo of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin.
Hydrology, Water Quality, and Response to 

Changes in Phosphorus Loading of 

Minocqua and Kawaguesaga Lakes, 

Oneida County, Wisconsin, With Special 

Emphasis on Effects of Urbanization

By Herbert S. Garn, Dale M. Robertson, William J. Rose, and David A. Saad

Prepared in cooperation with the Minocqua/Kawaguesaga Lakes Protection 

Association through the Town of Minocqua, Wisconsin

Scientific Investigations Report 2010–5196

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Garn, H.S., Robertson, D.M., Rose, W.J., and Saad, D.A., 2010, Hydrology, water quality, and response to changes 

in phosphorus loading of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, with special emphasis on 

effects of urbanization: U.S. Geological Survey Scientific Investigations Report 2010-5196, 54 p.
iii

Contents

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

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

Minocqua and Kawaguesaga Lakes and Their Watersheds ........................................................8

 Purpose and Scope ...........................................................................................................................10

Data-Collection Methods and Sites..........................................................................................................10

Lake-Stage and Water-Quality Monitoring.....................................................................................10

Lake Classification..............................................................................................................................12

Stream Monitoring..............................................................................................................................12

Groundwater Monitoring...................................................................................................................12

Lake Water Quality ......................................................................................................................................13

Water Chemistry and Trophic Conditions .......................................................................................13

Inferences about Changes in Water Quality from Lake-Sediment Cores .................................21

Hydrology and Water Budget ....................................................................................................................24

Change in Storage ..............................................................................................................................24

Precipitation.........................................................................................................................................24

Evaporation..........................................................................................................................................24

Surface-Water Inflow ........................................................................................................................25

Gaged Sites.................................................................................................................................25

Nearshore Ungaged Area (Including Urban Areas) ............................................................25

Surface-Water Outflow......................................................................................................................28

Groundwater Inflow and Outflow.....................................................................................................28

Water-Budget Summary....................................................................................................................34

Sources of Phosphorus...............................................................................................................................34

Precipitation.........................................................................................................................................34

Surface-Water Inflow .......................................................................................................................37

Gaged Inlets................................................................................................................................37

Urban and Other Ungaged Near-Lake Drainage Areas ......................................................37

Groundwater........................................................................................................................................38

Septic Systems....................................................................................................................................38

Surface-Water Outflow .....................................................................................................................40

Phosphorus-Budget Summary .........................................................................................................40

Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading .................43

Modeling Approach............................................................................................................................43

Data Requirements.............................................................................................................................43

Algorithms and Calibration................................................................................................................45

Response in Water Quality to General Changes in Phosphorus Loading .................................47

Changes in Phosphorus Concentrations ...............................................................................47

Changes in Chlorophyll a Concentrations .............................................................................47

Changes in Secchi Depths (Water Clarity)............................................................................47

Changes in Trophic Status........................................................................................................47

Importance of Internal Phosphorus Loading .................................................................................47
iv

Figures

Figure 1. Map showing surface-water and topographic watersheds of Minocqua and

Kawaguesaga Lakes, Oneida County, Wisconsin ....................................... 3

Figure 2. Map showing subbasins and land use/land cover of the near-lake drainage

area of Minocqua and Kawaguesaga Lakes, Wisconsin, from land-use

classifications defined in the 2001 National Land Cover Data ........................ 5

Figure 3. Graph showing population of Minocqua, Wisconsin, 1910–2006 ..................... 5

Figure 4. Photographs showing typical lakeshore residential development prior to the

1970s and around 2000 at Minocqua Lake, Wisconsin ................................. 6

Figure 5. Map showing locations and types of data-collection sites at Minocqua and

Kawaguesaga Lakes, Oneida County, Wisconsin ....................................... 11

Figure 6. Graphs showing water temperature and dissolved oxygen profiles on

4 selected days during summer 2007 at the deep holes in the main East Basin,

Northwest Basin of Minocqua Lake, and Kawaguesaga Lake, Oneida County,

Wisconsin .................................................................................... 14

Figure 7. Graphs showing near-surface total phosphorus concentration, chlorophyll a

concentration, and Secchi depths at the deep-hole site in the East Basin of

Minocqua Lake, Oneida County, Wisconsin, from 1973 to 2009........................ 16

Figure 8. Graphs showing near-surface total phosphorus concentration, chlorophyll a

concentration, and Secchi depths at the deep-hole site of Kawaguesaga Lake,

Oneida County, Wisconsin, from 1973 to 2007 .......................................... 18

Figure 9. Graphs showing near-bottom total phosphorus concentrations at the

deep-hole sites in the East Basin of Minocqua Lake, and Kawaguesaga Lake,

Oneida County, Wisconsin.................................................................. 20

Figure 10. Graphs showing summer average near-surface total phosphorus

concentrations, chlorophyll a concentrations, and Secchi depths at deep-hole

sites in the basins of Minocqua and Kawaguesaga Lakes, Oneida County,

Wisconsin, from 1973 to 2009............................................................... 22

Simulated Changes in Water Quality in Response to Changes in Phosphorus 

Loading —Continued.....................................................................................................................43

Response in Water Quality to Specific Urbanization/Residential Development Scenarios...49

Response Under Average 2006–07 Conditions .....................................................................49

Response Under Average Hydrologic Conditions ................................................................49

Response in Water Quality to a Combination of Changes in the Watershed ...........................50

Summary and Conclusions.........................................................................................................................50

References....................................................................................................................................................52

Contents—Continued
v

Figure 11. Graphs showing trophic state index (TSI) values based on surface

concentrations of total phosphorus, chlorophyll a, and Secchi depths at the

deep-hole sites in the East Basin of Minocqua Lake, and Kawaguesaga Lake,

Oneida County, Wisconsin, 1973 to 2009 ................................................ 23

Figure 12. Graph showing precipitation at Minocqua Lake during November 2005 to

October 2007 relative to long-term average precipitation, measured at

Minocqua Dam, Oneida County, Wisconsin ............................................. 25

Figure 13. Graph showing daily discharge, phosphorus concentrations, and phosphorus

loads at Link Creek near Woodruff, Tomahawk River at Thoroughfare Road

near Minocqua, and Tomahawk River at Minocqua Dam near Minocqua,

Wisconsin, November 2005 through October 2007 .................................... 27

Figure 14. Graphs showing hydrologic features simulated with GFLOW analytic elements

near Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin ............... 29

Figure 15. Map showing measured and simulated head, lake stage, and streamflow for

the calibrated GFLOW model............................................................... 31

Figure 16. Graphs showing simulated groundwater contributing area and flow direction

for Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin ............... 32

Figure 17. Map showing simulated groundwater-flow direction and flux (flow) rates for

shoreline segments of Minocqua and Kawaguesaga Lakes, Oneida County,

Wisconsin .................................................................................... 33

Figure 18. Charts showing water budgets for Minocqua and Kawaguesaga Lakes,

Wisconsin, for monitoring years 2006 and 2007.......................................... 35

Figure 19. Graphs showing monthly accuracy of the water budget based on comparison

of total water inputs and outputs plus storage change for Minocqua and

Kawaguesaga Lakes, Wisconsin, November 2005 to October 2007 .................. 36

Figure 20. Map showing area served by sanitary sewer within the Lakeland Sanitary

District No. 1, Minocqua, Wisconsin ...................................................... 40

Figure 21. Charts showing phosphorus budgets for Minocqua and Kawaguesaga Lakes,

Oneida County, Wisconsin, combined, for monitoring years 2006 and 2007 ......... 42

Figure 22. Graphs showing measured and simulated average summer near-surface total

phosphorus concentrations, chlorophyll a concentrations, and Secchi depths

in the basins of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin... 46

Figure 23. Graphs showing simulated changes in near-surfacetotal phosphorus

concentrations, chlorophyll a concentrations, and Secchi depths in the basins

of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, in response

to various phosphorus-loading scenarios ................................................ 48

Figures—Continued
vi

Tables

Table 2. Land use/land cover in the contributing areas to Minocqua and Kawaguesaga

Lakes, from 2001 National Land Cover Data ............................................. 9

Table 1. Morphometric characteristics of Minocqua and Kawaguesaga Lakes, Oneida

County, Wisconsin........................................................................... 9

Table 3. Near-surface summer average (June through September) water-quality and

trophic state index (TSI) values for Minocqua Lake, Oneida County, Wisconsin ... 17

Table 4. Near-surface summer average (June through September) water-quality and

trophic state index (TSI) values for Kawaguesaga Lake, Oneida County, 

Wisconsin .................................................................................... 19

Table 5. Summary of water-budget components for Minocqua and Kawaguesaga

Lakes, Oneida County, Wisconsin, during monitoring years 2006–07, during

average hydrologic conditions, and with future development in the watershed ... 26

Table 6. Summary of phosphorus-budget components for Minocqua and Kawaguesaga

Lakes, Oneida County, Wisconsin, for monitoring years 2006–07, during

average hydrologic conditions, and with future development in the watershed ... 36

Table 7. Concentrations of dissolved phosphorus in groundwater from piezometers

around Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin ............ 39

Table 8. Predicted near-surface, summer average total phosphorus and chlorophyll a

concentrations and Secchi depths in Minocqua and Kawaguesaga Lakes,

Oneida County, Wisconsin, for various scenarios based on simulations with

the BATHTUB model ........................................................................ 44

Table 9. Algorithms used within BATHTUB to simulate water quality in the basins in

Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin ..................... 45
vii

Conversion Factors and Datum

Conversion Factors

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

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

cubic ft (ft3) 0.02832 cubic meter (m3)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Deposition, application, and yield rate

pound per square mile (lb/mi2) 0.175133 kilogram per square kilometer (kg/km2)

Soil permeability rate

inch per hour (in/h) 25.4 millimeter per hour (mm/h)

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

°F=(1.8 ×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C).

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

or micrograms per liter (μg/L).

Datum

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

(NAVD 88).

Water-level elevations in this report refer to distance above the vertical datum.
viii

Acknowledgments

Technical Reviewers

Paul F. Juckem, Hydrologist, U.S. Geological Survey, Middleton, Wis.

Kevin Gauthier, Sr., Water Resources Management Specialist, Wisconsin Department of Natural 

Resources, Rhinelander, Wis.

Local Project Coordinators

Sally Murwin, Minocqua/Kawaguesaga Lakes Protection Assoc., Minocqua, Wis.

Richard Garrett, Minocqua/Kawaguesaga Lakes Protection Assoc., Minocqua, Wis.

Editorial and Graphics

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

James L. Kennedy, Geographer, U.S. Geological Survey, Middleton, Wis.

Marie C. Peppler, Physical Scientist, U.S. Geological Survey, Middleton, Wis.

C. Michael Eberle, Technical Writer-Editor, U.S. Geological Survey, Columbus Publishing Service 

Center, Columbus, Ohio

Data Collection and Analysis

Judy A. Horwatich, WinSLAMM modeling, Hydraulic Engineer, U.S. Geological Survey, 

Middleton, Wis.

Paul C. Reneau, Hydrologic Technician, U.S. Geological Survey, Rhinelander, Wis.

Brent W. Olson, Hydrologic Technician, U.S. Geological Survey, Rhinelander, Wis.

Lindsay L. Sutton, local observer, Minocqua, Wis.

Approving Official

Kevin J. Breen, Bureau Approving Official, U.S. Geological Survey, New Cumberland, Penn.
Hydrology, Water Quality, and Response to Changes in 

Phosphorus Loading of Minocqua and Kawaguesaga 

Lakes, Oneida County, Wisconsin, With Special 

Emphasis on Effects of Urbanization

By Herbert S. Garn, Dale M. Robertson, William J. Rose, and David A. Saad

Abstract

Minocqua and Kawaguesaga Lakes are 1,318- and 

690-acre interconnected lakes in the popular recreation area 

of north-central Wisconsin. The lakes are the lower end of a 

complex chain of lakes in Oneida and Vilas Counties, Wis. 

There is concern that increased stormwater runoff from 

rapidly growing residential/commercial developments and 

impervious surfaces from the urbanized areas of the Town 

of Minocqua and Woodruff, as well as increased effluent 

from septic systems around their heavily developed shoreline 

has increased nutrient loading to the lakes. Maintaining the 

quality of the lakes to sustain the tourist-based economy of 

the towns and the area was a concern raised by the Minocqua/

Kawaguesaga Lakes Protection Association. Following 

several small studies, a detailed study during 2006 and 2007 

was done by the U.S. Geological Survey, in cooperation with 

the Minocqua/Kawaguesaga Lakes Protection Association 

through the Town of Minocqua to describe the hydrology and 

water quality of the lakes, quantify the sources of phosphorus 

including those associated with urban development and to 

better understand the present and future effects of phosphorus 

loading on the water quality of the lakes. 

The water quality of Minocqua and Kawaguesaga 

Lakes appears to have improved since 1963, when a new 

sewage-treatment plant was constructed and its discharge 

was bypassed around the lakes, resulting in a decrease in 

phosphorus loading to the lakes. Since the mid-1980s, the 

water quality of the lakes has changed little in response 

to fluctuations in phosphorus loading from the watershed. 

From 1986 to 2009, summer average concentrations of 

near-surface total phosphorus in the main East Basin of 

Minocqua Lake fluctuated from 0.009 mg/L to 0.027 mg/L 

but generally remained less than 0.022 mg/L, indicating that 

the lake is mesotrophic. Phosphorus concentrations from 

1988 through 1996, however, were lower than the longterm average, possibly the result of an extended drought in 

the area. Water‐quality data for Kawaguesaga Lake had a

similar pattern to that of Minocqua Lake. Summer average 

chlorophyll a concentrations and Secchi depths also indicate 

that the lakes generally are mesotrophic but occasionally 

borderline eutrophic, with no long-term trends.

During the study, major water and phosphorus sources 

were measured directly, and minor sources were estimated to 

construct detailed water and phosphorus budgets for the lakes 

for monitoring years (MY) 2006 and 2007. During these years, 

the Minocqua Thoroughfare contributed about 38 percent of 

the total inflow to the lakes, and Tomahawk Thoroughfare 

contributed 34 percent; near-lake inflow, precipitation, 

and groundwater contributed about 1, 16, and 11 percent 

of the total inflow, respectively. Water leaves the lakes 

primarily through the Tomahawk River outlet (83 percent) 

or by evaporation (14 percent), with minor outflow to 

groundwater. Total input of phosphorus to both lakes was 

about 3,440 pounds in MY 2006 and 2,200 pounds in MY 

2007. The largest sources of phosphorus entering the lakes 

were the Minocqua and Tomahawk Thoroughfares, which 

delivered about 39 and 26 percent of the total, respectively. 

The near-lake drainage area, containing most of the urban 

and residential developments, disproportionately accounted 

for about 12 percent of the total phosphorus input but only 

about 1 percent of the total water input (estimated with 

WinSLAMM). The next largest contributions were from septic 

systems and precipitation, each contributing about 10 percent, 

whereas groundwater delivered about 4 percent of the total 

phosphorus input. 

Empirical lake water-quality models within BATHTUB 

were used to simulate the response of Minocqua and 

Kawaguesaga Lakes to 19 phosphorus-loading scenarios. 
23 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

These scenarios included the current base years (2006–07) 

for which lake water quality and loading were known, nine 

general increases or decreases in phosphorus loading from 

controllable external sources (inputs from the tributaries 

and nearshore areas around the lakes and input from septic 

systems), and nine scenarios corresponding to future 

changes in phosphorus loading from residential and urban 

development, referred to as “2030 buildout,” and removal of 

septic system inputs. The 2030 buildout scenario with existing 

stormwater controls resulted in a degradation in water quality: 

phosphorus concentrations increased by about 0.001 mg/L, 

chlorophyll a concentrations increased by 0.2–0.8 μg/L, and 

Secchi depths decreased slightly. The largest degradation 

in water quality was estimated to occur in Kawaguesaga 

Lake. If 2030 buildout occurred with implementation of best 

management practices to achieve a 50-percent reduction in 

loading from near-lake drainages, it is possible that water 

quality would change very little from existing conditions. 

Numerous noncontributing areas exist within the watershed 

that help minimize surface runoff and nutrient loading to the 

lakes; however, if future development included extending or 

connecting drainage from these areas into the lakes, loading 

to the lakes could greatly increase and cause a degradation 

in the water quality of the lakes. Simulations of removal of 

phosphorus loading from septic systems around Minocqua 

Lake improved the water quality of the lakes: in simulations 

for that scenario, phosphorus concentrations decreased by 

about 0.001 mg/L, chlorophyll a concentrations decreased by 

0.5–0.7 μg/L, and Secchi depths increased by 0.3–0.7 ft. If 

all controllable external phosphorus loading could be reduced 

by 50 percent, the lakes would become oligotrophic with 

respect to phosphorus concentration but would still remain 

mesotrophic with respect to chlorophyll a concentration and 

Secchi depth. Improvements in the water quality of the lakes 

are likely only with a combination of management actions that 

decrease inputs from the developed near-lake drainage areas 

and from septic systems.

Introduction

Minocqua and Kawaguesaga Lakes (fig. 1) are the 

lower lakes in a complex chain of lakes in the Northern Lake 

and Forests ecoregion of north-central Wisconsin (Omernik 

and others, 2000). Water levels of both lakes are controlled 

by the Minocqua Dam at the outlet of Kawaguesaga Lake. 

Water flows from the lakes into the Tomahawk River and 

eventually into the Wisconsin River. These lakes are popular 

for fishing, swimming, and other outdoor activities. The 

lakes are adjacent to the Towns of Minocqua and Woodruff 

and about 65 mi north of Wausau, Wis. Because of the many 

uses for the lakes, the area has become a major vacation and 

retirement destination, a tourist-oriented commercial center, 

and an important resource for Oneida County. This popularity, 

however, has resulted in rapid urbanization in Minocqua and 

Woodruff and the areas around the lakes. This urbanization 

may affect the hydrology and nutrient loading to the lakes 

and ultimately the water quality of the lakes. Therefore, 

understanding the factors affecting the water quality of 

Minocqua and Kawaguesaga Lakes, especially those factors 

associated with the increased pressure from urbanization, is 

very important and is addressed in this report. 

In the early 1800s, a Chippewa Indian Village was 

located between Minocqua and Kawaguesaga Lakes. By 1886, 

the village had been moved to the peninsula extending out 

into Minocqua Lake, referred to as the “Island” (P. Garrison 

and J. Hurley, Wisconsin Department of Natural Resources, 

written commun., 1992). Growth in the area, other than the 

Indian Village, began with the extension of the railway from 

Merrill, Wis., to Minocqua in 1887, and shortly thereafter the 

area was logged. The Town of Minocqua was platted in 1888. 

Water levels have been controlled on the chain of lakes since 

1889, when the Minocqua Dam (fig. 1) was constructed as part 

of logging operations to transport logs downstream. The dam 

increased lake levels by about 4 ft (Minocqua/Kawaguesaga 

Lakes Protection Association, 2003). The original earthen dam 

washed out and was rebuilt several times. In 1907, the dam 

was acquired by the Wisconsin Valley Improvement Company 

(WVIC), a private corporation, to enable the lakes to be 

operated as a headwater reservoir. In 1917, the existing earth 

and wood-frame dam was replaced by a concrete dam with 

wood slide gates. The wood outlet structure was then replaced 

in 1954 with steel tainter gates and steel gate sills (D. Coon, 

Wisconsin Valley Improvement Company, written commun., 

2007). The dam and lake levels continue to be managed by 

WVIC for storage to augment the flow of the Wisconsin 

River that benefits hydroelectric generation, flood control, 

water quality, and recreation. The dam generally is operated 

to store runoff during spring and to release water during 

winter (January to March), with exceptions for dry years. The 

maximum water level is set at 1585.05 ft year-round, and the 

minimum is set at 1582.72 ft during winter and 1584.05 ft 

during summer (Wisconsin Valley Improvement Company, 

2007). The lake level is maintained at a relatively stable level 

throughout summer.

The water quality of the lakes has been a public concern 

for many years. Water-quality data have been collected by the 

Wisconsin Department of Natural Resources (WDNR) as early 

as 1973. The Minocqua Area Lake Improvement Association 

was established in 1994 to preserve and protect the welfare of 

the Minocqua area lakes. In 1997, the organization changed 

its name to the Minocqua/Kawaguesaga Lakes Protection 

Association (referred to hereafter as “Lakes Association” 

for brevity). In 2000, the Lakes Association revived a lakes 

protection program and began participation in the WDNR 

lake data-collection program. The WDNR and Lakes 

Association have done various studies of the two lakes and 

their watersheds over the past 15 years. Much of the technical 

information available about the system is generally not in a 

form that is easily used by the Lakes Association, towns, and 

other local governments for making management decisions. 
Introduction 3

Figure 1. Surface-water and topographic watersheds of Minocqua and Kawaguesaga Lakes, Oneida County, 

Wisconsin. The topographic watershed includes areas that do not contribute to overland flow to the lakes.

89°45' 89°36'

45°57' 45°48'

EXPLANATION

Topographic watershed

Surface-water watershed

Nearshore subwatershed

0 2 4 1 KILOMETERS

0 1 2 4 MILES

Aerial image from U.S. Department of Agriculture, 2006

Study site

Arbor Vitae

Woodruff

Little

Arbor

Vitae

Lake

Big

Arbor

Vitae

Lake

Minocqua Dam

Carrol Lake

Madeline

Lake

Mid Lake

Kawaguesaga

Lake

Minocqua

Lake

Baker

Lake

Tomahawk

Lake

Minocqua

Tomahawk 

Thoroughfare

Minocqua 

Thoroughfare

(Link Creek)

Fish Hatchery

Tomahawk River
45 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Lake-bottom sediment cores were collected in 1991 and 

1992 from Minocqua Lake by the WDNR for analysis of 

historical sedimentation (Young, 1994; P. Garrison, Wisconsin 

Department of Natural Resources, written commun., 2003). 

Cores were collected from the East, Northwest, and Southwest 

Basins of Minocqua Lake (fig. 2). That study found that 

development since about 1890 had caused an increase in the 

rates of sediment and phosphorus accumulation, with largest 

increases in rates occurring after about 1960. The study also 

concluded that water quality in the lake had degraded at a 

slow rate since the late 1800s but that the rate of degradation 

accelerated around 1950 (P. Garrison and J. Hurley, Wisconsin 

Department of Natural Resources, written commun., 1992). 

The WDNR surveyed macrophyte occurrence and 

distribution in 1989, 1993, and 1996 as part of the longterm-trend lake monitoring program (Wisconsin Department 

of Natural Resources, written commun., 1996). The most 

common species of macrophyte was coontail (Ceratophyllum 

demersum L.), which had increased in the frequency of 

occurrence since 1989. The percentage of littoral area 

vegetated also increased from 87 percent in 1989 to 92 

percent in 1996. The macrophyte community is characterized 

by high diversity and primarily submergent species. Soil 

fertility of the littoral zone sediments was surveyed in 2002 

(S. McComas, Blue Water Science, written commun., 2002) 

to evaluate the potential to support growth of the nuisance 

Eurasian watermilfoil (Myriophyllum spicatum L.). Eurasian 

watermilfoil was found only in several small patches 

in Minocqua Lake covering only a few acres. Nitrogen 

concentrations in littoral sediments were low to moderate, and 

the potential for supporting nuisance growth was limited to 

only a few small areas totaling less than 20 acres.

The impacts from septic systems on groundwater and 

surface water were evaluated in 1996 by the University of 

Wisconsin-Stevens Point (Lindemann and others, 1997). The 

study evaluated about half of the 160 homes on Minocqua 

Lake that had onsite sewage-disposal systems. The researchers 

found that high groundwater levels and highly permeable 

sandy soils can cause incomplete onsite wastewater treatment 

and loading of nutrients to be a concern. They also found 

high concentrations of ammonium, nitrate, phosphorus, and 

chloride in groundwater entering the lake at several sites 

with septic systems. The study did not, however, attempt 

to quantify the nutrients contributed to the lake by septic 

systems. The majority of septic systems studied were more 

than 25 years old (installed during the 1970s or earlier), and 

any problems associated with them were likely to increase in 

the future as they aged further. 

Stormwater runoff from the residential/commercial 

developments and impervious surfaces in the urbanized area 

on the Island (fig. 2), as well as septic systems in the heavily 

developed shoreline area of Minocqua, may be sources of 

increased nutrient loading to the lakes. Prior to 1935, the 

sewage system for the Island discharged raw sewage into 

Minocqua Lake from multiple outfalls. In 1935, the first 

treatment facility was constructed on the south side of the 

pond on the Island, but it still discharged directly into the 

Northwest Basin of Minocqua Lake. In 1963, a new treatment 

plant was built west of Minocqua, discharging effluent into 

the Tomahawk River downstream and bypassing the lakes 

(Minocqua/Kawaguesaga Lakes Protection Association, 

2003). The treatment plant started receiving wastewater from 

the Town of Woodruff in 1965. The consolidated Lakeland 

Sanitary District No. 1 was formed in 1975 and currently 

provides sanitary-sewer service to Minocqua/Woodruff and 

areas to the north and south of the Island. The Town of Arbor 

Vitae was added to the sanitary district in 1991. The areas 

served have expanded since 1965, and in 1996 the district 

served about 40 percent of properties on Minocqua Lake 

(Lindemann and others, 1997). In 2001, the sanitary district 

was reported to be nearing its treatment capacity (Minocqua/

Kawaguesaga Lakes Protection Association, 2003). The 

sanitary district began construction of an upgraded wastewater 

facility in 2008 to provide additional treatment capacity for 

the next 20 years. As part of that design process, a 20-year 

planned area of service was identified that includes almost 

all of the shoreline of Minocqua Lake and the north and 

northwest shore of Kawaguesaga Lake (R. Groth, Lakeland 

Sanitary District No. 1, written commun., 2009). 

Another point source of nutrient loading identified 

in the watershed (Blake, 1996; Minocqua/Kawaguesaga 

Lakes Protection Association, 2003) is the WDNR-operated 

Woodruff Fish Hatchery on Minocqua Thoroughfare below 

Madeline Lake (fig. 1). The hatchery and rearing ponds were 

established in 1901 and discharged effluent into the Minocqua 

Thoroughfare on a seasonal basis. In 1991, the hatchery was 

estimated to contribute about 80 lb of phosphorus per year 

(about 2 percent of the total annual load) to Minocqua Lake 

(Blake, 1996). The fish hatchery was enlarged and modernized 

in 1993; improvements included construction of a facility 

to treat and infiltrate water before it is discharged to the 

Thoroughfare.

Like communities around many northern Wisconsin 

lakes undergoing urbanization, Minocqua has experienced a 

population boom that doubled from 1960 to 1980 and grew 

by an additional 38 percent from 1990 to 2000 (fig. 3; U.S. 

Census Bureau, 2003). Vacant lands on lakes in the northern 

lakes region are disappearing—the number of lakefront homes 

has more than doubled since the 1960s. By 1994, vacant 

lots around the lakes were a rarity (Minocqua/Kawaguesaga 

Lakes Protection Association, 2003). More than 80 percent of 

northern Wisconsin’s lakeshore frontage is privately owned, 

and the WDNR estimated that all of the remaining privately 

owned shoreland would be developed by 2015 (Wisconsin 

State Journal, 2005). In addition, in many cases, small summer 

cottages have been replaced by large urban-style homes 

accompanied by larger impervious surfaces, clearing of natural 

vegetation from shorelines, and creation of lawns down to the 

edge of water (fig. 4).
Introduction 5

Figure 2. Subbasins and land use/land cover of the near-lake drainage area of Minocqua and 

Kawaguesaga Lakes, Wisconsin, from land-use classifications defined in the 2001 National Land 

Cover Data (Multi-Resolution Land Characteristics Consortium, 2001). Noncontributing areas are 

areas with internal drainages that do not drain to the lake.

89°45' 89°42' 89°39'

45°54' 45°51'

0 0.5 1 2 MILES

0 0.5 1 2 KILOMETERS

EXPLANATION

Forest

Open water

Low intensity residential 

Urban

Grassland

Wetland

Noncontributing areas

Near shore basins

Base map from U.S. Geological 

Survey digital data, 1:24,000

Minocqua

Woodruff

Tomahawk Thoroughfare

Minocqua Thoroughfare

(Link Creek) Northwest

Basin

Southwest

Basin

East

Basin

Minocqua Dam

Kawaguesaga

Lake

Baker 

Lake

Clawson 

Lake

Minocqua

Lake

Mid Lake

YEAR

MINOCQUA POPULATION

0

1,000

2,000

3,000

4,000

5,000

6,000

1900 1920 1940 1960 1980 2000 2020

Figure 3. Population of Minocqua, 

Wisconsin, 1910–2006 (U.S. Census 

Bureau, 2003; J. Kumbera, Oneida 

County Economic Development 

Corporation, written comun., 2007).
67 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

A

B

Figure 4. (A) Typical lakeshore residential development prior to the 1970s and (B) 

around 2000 at Minocqua Lake, Wisconsin.
Introduction 7

Urbanization commonly affects the hydrology of the 

area. It not only affects the type and the magnitude of runoff 

but also stream-channel stability, sedimentation/erosion, and 

water quality. These changes result from initial clearing of 

vegetation, soil disturbance and compaction, and ditching and 

draining, followed by covering parts of the land surface with 

impervious roofs, parking lots, and roads. Gutters, drains and 

storm sewers convey runoff rapidly to stream channels, which 

are often extended, straightened or lined to make them more 

efficient at transferring water. Natural storage areas, swales, 

drainageways, and wetlands may be altered or eliminated by 

urban development. These actions can increase downstream 

flooding by forcing more water into drainage networks. 

Each of these changes can result in a substantial increase in 

peak discharge and duration of a given flow magnitude—

transmitting the floodwave downstream faster and with less 

retardation, thereby destabilizing channels and increasing 

sediment and other pollutant loads (Booth and Jackson, 1997; 

Tang and others, 2005). As a result of these changes, channels 

become unstable, and their widths and depths typically 

increase in response to urban development. Booth and Jackson 

(1997), in a study of various watersheds in Kings County, 

Wash., found that at low levels of watershed urbanization—

beyond about 8–10 percent of contributing effective 

impervious area—channels generally became unstable, and 

downstream aquatic systems were degraded.

Graczyk and others (2003) monitored and compared 

nutrient yields from developed (homes with lawns) and 

forested sites in nearshore areas of four northern Wisconsin 

lakes. For all constituents, median nutrient yields in surface 

runoff from developed sites were greater than those from 

wooded sites. Annual median total phosphorus yield from 

lawns (0.026 lb/acre) was greater than that from wooded 

catchments (0.003 lb/acre) by about a factor of 10. Median 

surface runoff from lawns was generally about a factor of 

10 greater than that from the wooded catchments. Thus, 

the increased water volumes from lawns resulted in greater 

nutrient loads and annual yields from the developed sites.

Various best management practices (BMPs) are 

commonly used to control stormwater runoff; among these are 

wet and dry detention ponds, created wetlands, grassed swales, 

filtration practices, and infiltration practices. Pollutant-removal 

efficiencies, expressed as a percentage, are commonly used 

to describe the ability of a practice to reduce pollutant levels 

between the inflow and the outflow of the control structure. 

Median pollutant-removal percentages by these BMPs 

for total phosphorus generally range from 0 to 70 percent 

(Pennington and others, 2003; U.S. Environmental Protection 

Agency, 2009). Use of these efficiencies as a measure may 

be misleading, however, because it does not take into account 

the importance of runoff-volume reduction and its effect on 

total load. Land-use-planning approaches (commonly referred 

to as “smart growth”) designed to minimize the increase in 

runoff from urbanization by optimally selecting the placement 

and pattern of development may reduce the increase in runoff 

from projected development by as much as 20 percent (Tang 

and others, 2005). To assist in the selection of effective BMPs, 

comparative information about the performance of BMP 

categories may be obtained from a Web-based tool: the Urban 

BMP Performance Tool created by the U.S. Environmental 

Protection Agency (2009), which provides access to studies 

and data pertinent to pollutant removal and stormwatervolume reduction.

The long-term management and protection of Minocqua 

and Kawaguesaga Lakes in the face of increased development 

and urbanization is a major concern to local and state 

government. Maintaining the quality of area lakes is key to 

sustaining the highly successful tourist-based economy of the 

towns. In 1994, the WDNR approved the Minocqua–Woodruff 

Priority Lakes Project Plan, prepared under the provisions 

of the Wisconsin Nonpoint Source Pollution Abatement 

Program, which recognized the lakes as high-quality resource 

values because of their water quality, trophy fishery, and 

quality multiuse recreation (Blake, 1996). In the plan, it 

was concluded that the lakes have experienced an increase 

in nonpoint-source pollution over the last several decades, 

including construction-site erosion, urban stormwater runoff, 

and impacts from lakeside development. It was also concluded 

that the water quality of the lakes was declining, as indicated 

by the perception that populations of algae and aquatic plants 

were increasing. The plan presented watershed-management 

recommendations to reduce the delivery phosphorus and 

sediment to the lakes and provided financial support by 

WDNR to achieve water-quality objectives for the lakes. The 

plan included estimates of phosphorus input quantities from 

the various sources, including septic systems (as of 1991), 

to target management efforts. The accuracy and reliability of 

these estimates, however, were questionable and based upon 

limited data and the methods available at that time. For various 

reasons, the implementation of the plan was terminated by the 

Town of Minocqua in 1996 (Minocqua/Kawaguesaga Lakes 

Protection Association, 2003). 

Degradation in the water quality of lakes may have an 

economic impact on an area. What is the economic value of 

a high-quality lake? Monies are commonly allocated for lake 

management and protection with little or no information about 

the economic effects of lake water quality. Several studies 

have investigated the effects of accelerated eutrophication, and 

resulting decrease in lake water quality, on local economies 

and property values (Dodds and others, 2009). In addition to 

biological effects, reduced water clarity and increased algae 

and plant growth can reduce a lake’s aesthetic appeal, lower 

recreational benefits, and lower prices of lakefront properties 

around the lake. Studies in Maine estimated the effect of 

water clarity on lakefront property prices for a variety of lakes 

(Michael and others, 1996; Boyle and others, 1998). The 

authors found that part of the property value can be attributed 

to water clarity. An improvement of about 3 ft in lake water 

clarity as measured with a Secchi disk resulted in increased 

property prices ranging from $11 to $200 per frontage foot 
89 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

(1990–94 Maine prices). These increases, when aggregated 

over all properties around a lake, could equate to several 

million dollars in increased property values per lake (Michael 

and others, 1996). The authors also found that the change in 

property value was greater when water clarity declined than 

when it improved, thus suggesting that it may be better to 

protect existing water clarity than to try to improve it after 

it has declined (Boyle and others, 1998). A study of lakes in 

Minnesota found a 15 percent decrease in property values for 

every 3 ft loss in clarity (Dodds and others, 2009). 

Eiswerth and others (2005) investigated the economic 

effects of restoring degraded Delavan Lake in Walworth 

County, Wis. More than $7 million was spent on restoring the 

quality of the 2,000-acre lake in the late 1980s to early 1990s. 

Property values increased 350 percent between 1987 and 1995, 

compared to 280 percent for nearby lakes. The Delavan Lake 

restoration was estimated to increase the average lake property 

by $177,000 from 1987 to 2003, an aggregated increase in 

valuation of over $99 million. In addition, the study found 

that degradation of water quality reduced regional economic 

activity by 8 to13 percent, or $5 million to $6 million per year. 

In summary, the studies clearly showed that lake-water clarity 

significantly affects the local economy and property prices and 

that lakefront property owners place a high economic value on 

good water clarity. 

The Town of Minocqua, the Lakes Association, and 

area residents are concerned about water quality of the 

lakes, frequency of algal blooms, excessive weed growth, 

and effects of additional development and urbanization. The 

sensitivity of the lakes’ water quality to changes in nutrient 

loading associated with additional development within the 

watershed is unknown. Quantitative information needs to be 

refined and updated with regard to actual rates of phosphorus 

loading from the various sources so that the significance of 

stormwater or septic-system loading as a percentage of total 

loading to the lakes can be evaluated. To establish realistic 

water-quality goals for the lakes, historical water-quality data 

need to be assembled, and accurate nutrient loading estimates 

are needed to enable water-quality models to be calibrated 

and used to simulate the future response of the lakes. With 

calibrated models, the lakes’ response to incremental increases 

or decreases in phosphorus loading can be evaluated and be 

useful for determining practices or actions to help protect and 

improve the lakes as part of an updated lake-management 

plan.

To provide a better understanding of the factors that 

affect the water quality of Minocqua and Kawaguesaga 

Lakes, a detailed study of the lakes and their watersheds was 

begun in 2005 by the U.S. Geological Survey (USGS). This 

study, in cooperation with the Minocqua/Kawaguesaga Lakes 

Protection Association through the Town of Minocqua, was 

partially funded through the Lake Protection Grant Program of 

the WDNR and the Cooperative Water Program of the USGS.

The study had the following objectives:

• Describe the hydrology and water quality of the lakes.

• Quantify the major inflows and determine a detailed 

phosphorus budget by quantifying the phosphorus 

loading associated with each source, including loading 

from urban areas and septic systems.

• Provide a better understanding of the problems and 

sources of phosphorus for developing future lakemanagement actions.

• Evaluate and relate the measured water and phosphorus 

loads to observed and modeled water-quality responses 

within the lakes by using available lake water-quality 

models.

• Assess how the water quality of the lakes should 

respond to increases or decreases in phosphorus 

loading from the watershed and changes in loading 

associated with specific future management actions, 

including some that are aimed at reducing phosphorus 

loads to the lakes.

Minocqua and Kawaguesaga Lakes and Their 

Watersheds

The Minocqua–Woodruff Priority Lakes Project Plan 

(Blake, 1996) includes a description of the lakes, their 

watersheds and water-resource conditions, and an estimate of 

the phosphorus budgets (sources of phosphorus loading) for 

the lakes based on unit-area loading data and simple models 

available at the time (1991). Rapid land-use changes, however, 

have occurred since the analysis in 1993–94, and new data, 

information, and techniques are now available.

Minocqua and Kawaguesaga Lakes are natural-drainage 

lakes, with water level controlled by a low-head dam at their 

outlet, in the headwaters of the Tomahawk River. Minocqua 

Lake is about 2.7 mi long and has a maximum width of 

about 2.0 mi; Kawaguesaga Lake is about 1.8 mi long and 

has a maximum width of about 1.7 mi. Both Minocqua and 

Kawaguesaga Lakes have complex, varied shorelines with 

multiple basins and islands that add to their scenic beauty. 

Minocqua Lake is divided into three distinct basins, having a 

maximum depth of 60 ft in the main East Basin, 30 ft in the 

Northwest Basin, and 40 ft in the Southwest Basin (fig. 2). 

Kawaguesaga Lake, whose maximum depth is 44 ft, has a 

less developed shoreline than Minocqua Lake. Most of the 

undeveloped shoreline around Minocqua Lake is due to 

adjacent wetlands. Kawaguesaga Lake also has a 120-acre 

pine forest—a State Natural Area—on the southwest side of 

the lake. The physical characteristics of the lakes, by basin, are 

summarized in table 1. 
Introduction 9

The area and 

volume of Minocqua 

Lake are 1,360 acres 

and 31,765 acre-ft 

and of Kawaguesaga 

Lake are 670 acres and 

11,881 acre-ft, as given 

on the 1972 WDNR 

lake survey maps. 

Mean depths of the two 

lakes are 23 ft and 18 ft 

(Wisconsin Department 

of Natural Resources, 

2001). In this study, the 

morphometries of the 

lakes were reevaluated 

on the basis of an aerial image obtained from the 2005 

National Agricultural Imagery Program (U.S. Department of 

Agriculture, 2006). By using geographic information system 

(GIS) techniques, the resulting surface areas of Minocqua 

and Kawaguesaga Lakes were determined to be 1,318 and 

690 acres and the volumes to be 30,473 and 11,538 acre-ft, 

respectively. The surface delineation of adjacent wetland 

areas for this study may have been different from the previous 

delineations and may account for the slight differences in 

the two determinations of area and volume. The area and 

volume values determined in this study were used for all of the 

computations in this report. 

Minocqua Lake receives inflows from two major 

tributaries: Minocqua Thoroughfare, which drains an upstream 

chain of lakes from the northeast including Big Arbor Vitae, 

through Link Creek to Little Arbor Vitae, Carrol and Madeline 

Lakes; and Tomahawk Thoroughfare, which drains the 

area from the southeast, including Tomahawk Lake (fig. 1). 

Table 1. Morphometric characteristics of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin.

Basin Area

(acres)

Shoreline 

length 

(miles)*

Developed 

shoreline 

(percent)

Depth

(feet) Volume

(acre-feet)

Maximum Mean

Minocqua

East Basin 888 10.37 83 60 24 21,716

Northwest Basin 209 3.24 86 30 17 3,573

Southwest Basin 221 3.65 100 40 23 5,184

  Entire lake 1,318 17.26 87 60 23 30,473

Kawaguesaga

  Entire lake 690 9.72 71 44 17 11,538

*

 Excluding islands.

Table 2. Land use/land cover in the contributing areas to Minocqua and Kawaguesaga Lakes, from 2001 National Land Cover Data.

[Data from the Multi-Resolution Land Characteristics Consortium, 2001]

Basin/drainage* Area 

(acres)

Agriculture 

(percent)

Forest 

(percent)

Shrub/ 

grassland 

(percent)

Wetland 

(percent)

Low-density 

residential 

(percent)

Urban 

(percent)

Water 

(percent)

Gaged watersheds

Tomahawk Thoroughfare 18,700 0.2 56.4 2.0 6.7 5.1 1.2 28.4

Minocqua Thoroughfare 15,200 0.0 62.5 1.0 10.1 5.2 0.3 20.9

Subwatersheds of individual lakes and basins

Mid Lake 598 0.0 38.6 1.7 29.9 20.8 1.6 7.3

Minocqua Lake 1,370 0.0 43.3 3.1 17.1 12.6 17.9 5.9

East Basin 818 0.0 50.3 1.7 14.0 14.2 12.1 7.6

Northwest Basin 355 0.0 30.3 3.3 21.6 9.4 30.1 5.4

Southwest Basin 199 0.0 37.8 8.8 21.6 11.8 20.0 0.0

Kawaguesaga Lake 1,580 0.0 51.8 1.0 34.3 6.4 0.0 6.5

Entire watershed

Entire drainage above dam 37,400 0.1 57.9 1.6 10.0 5.8 1.4 23.2

*

 Not including areas of Mid, Minocqua, and Kawaguesaga Lakes.

An adjacent groundwater-discharge lake, Mid Lake, also 

discharges into the Tomahawk Thoroughfare channel upstream 

from its mouth with Minocqua Lake (fig. 1). Baker Lake, on 

the south side of Kawaguesaga Lake, has a small outlet and is 

the only tributary that flows into Kawaguesaga Lake (fig. 2). 

The total contributing area upstream from the outlet dam is 

58.5 mi2 (37,400 acres).

The surface-water contributing area of the Minocqua 

Thoroughfare at State Highway 47 at the inlet to Minocqua 

Lake is 23.8 mi2 (15,200 acres; table 2). The second tributary, 

Tomahawk Thoroughfare, including flows from Tomahawk 

and Mid Lakes (fig. 2), has a surface-water contributing area 

above its mouth of 30.1 mi2 (19,300 acres). The ungaged 

near-lake area, lacking any distinct stream channels, has a 

combined contributing area of 2,950 acres: 1,370 acres around 

Minocqua Lake and 1,580 acres around Kawaguesaga Lake. 

Surface-water contributing areas to each of the basins in 

Minocqua and Kawaguesaga Lakes are described in table 2.
101 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Land use/land cover in the area contributing 

surface water to the lakes is primarily a mixture of forest 

(57.9 percent), open water (23.2 percent), and wetland 

(10.0 percent), with areas of low-density residential 

(5.8 percent, including the golf course) and smaller areas of 

urban (1.4 percent), shrub and grassland (1.6 percent), and 

agriculture (0.1 percent) (table 2). Most of the headwaters 

are within the Northern Highland-American Legion State 

Forest. Land-cover data were retrieved from the 2001 National 

Land Cover Data (Multi-Resolution Land Characteristics 

Consortium, 2001), and rural residential development was 

updated from 2005 aerial photographs. Agricultural uses are 

not a factor in this area, but urban and low-density residential 

uses take on greater significance, especially in the drainage 

areas of the Northwest and Southwest Basins of Minocqua 

Lake. Land uses in subwatersheds draining into each of the 

basins are given in table 2.

The contributing watershed consists of surficial glacial 

deposits, including pitted outwash and other ice-contact 

deposits and end moraines composed of sand and gravel. The 

mostly hilly, rolling terrain has flat to steep slopes. Depth of 

the surficial materials may be as much as 150–300 ft over 

igneous and metamorphic bedrock (Oakes and Cotter, 1975; 

Mudrey and others, 1982). The stratified glacial drift is the 

only important source of groundwater, yielding moderate to 

large quantities of water. Soils of the rolling to undulating 

uplands are primarily loamy sand and sand with permeabilities 

of 2.5 to 5 in/h or more, and muck or peat soils occur in 

the wetlands (Hole and others, 1968; Oakes and Cotter, 

1975). Scattered throughout the area are many small closed 

depressions (internally drained areas (fig. 2), commonly with 

ponds or wetlands but without outlets) that do not contribute 

surface runoff to the lakes. Clawson Lake is an example 

of larger closed basin adjacent to the main lakes. Several 

smaller depressions are within the urban areas of Minocqua 

and Woodruff. The total noncontributing area within the 

entire upstream watershed was not determined, but closed 

basins (noncontributing areas) were taken into account in 

runoff estimates from the urban and other nearshore areas of 

Kawaguesaga and Minocqua Lakes (fig. 2).

 Purpose and Scope

This report describes the water quality of Minocqua and 

Kawaguesaga Lakes, quantifies the water and phosphorus 

budgets for the lakes on the basis of data collected from 

November 2005 through October 2007, gives estimates of 

the phosphorus loading from the urbanized areas and septic 

systems, and presents the results of model simulations that 

demonstrate the potential effects that changes in phosphorus 

inputs may have on the water quality of the lakes.

Water-quality data and detailed water and phosphorus 

budgets described in this report improve the understanding of 

the hydrologic system of the lakes and aid in the understanding 

of how lakes in general respond to changes in nutrient inputs. 

The data and interpretations herein should be useful to the 

lake organizations and to county and local officials in the 

preparation of a comprehensive lake-management plan. In 

addition, this report adds to the Wisconsin literature on lakes 

for which detailed hydrologic, phosphorus-loading, and 

water‐quality information are available to compare with other

lakes in the region.

Data-Collection Methods and Sites

USGS personnel collected stream and lake data from 

November 2005 through October 2007 as part of this study 

and had collected detailed lake water-quality data previously 

in 2003. In addition, earlier water-quality data for the lakes 

were collected by the WVIC, WDNR, and volunteers from the 

Lakes Association as part of the WDNR’s Wisconsin Citizen 

Lake Monitoring Program (Wisconsin Department of Natural 

Resources, 2008). Data collected by the WDNR were supplied 

by J. Vennie (Wisconsin Department of Natural Resources, 

written commun., 2003), and WVIC data were supplied by 

C. Wendt (Wisconsin Valley Improvement Company, written 

commun., 2003). More recent Citizen Lake Monitoring data 

were obtained from the WDNR’s World Wide Web page 

(Wisconsin Department of Natural Resources, 2008). All 

available data, but primarily near-surface concentrations of 

total phosphorus and chlorophyll a, plus Secchi depths, were 

used to characterize long-term changes in the water quality 

of the lakes; however, only data collected from November 

1, 2005, to October 31, 2007, were used to describe the 

hydrology and phosphorus inputs to the lakes. This latter 

period was divided into two monitoring years (MY): 

November 2005 through October 2006 (hereafter referred to 

as “MY 2006”) and November 2006 through October 2007 

(hereafter referred to as “MY 2007”).

Lake-Stage and Water-Quality Monitoring

A continuously recording (at 15-minute intervals) 

lake‐stage gage was installed as part of the streamflow-gaging

station operated on Minocqua Thoroughfare (Link Creek) at 

State Highway 47 on the east side of Minocqua Lake (fig. 5). 

WVIC also collected daily lake-stage data manually at the 

Minocqua Dam on Kawaguesaga Lake; these data were 

supplied by S. Morgan (Wisconsin Valley Improvement 

Company, written commun., 2007). 
Data-Collection Methods and Sites 11

#

89°45 89°42' 89°39

45°54'

45°51'

EXPLANATION

Lake-sampling site

Lake-stage gage

Miscellaneous sampling site

Piezometer

Streamflow-gaging station

0 0.5 1 MILE

0 0.5 1 KILOMETER

Well 7

Well 9

Well 8

Well 6

Well 5

Well 4

Well 3

Well 2

Well 15

Well 14

Well 13 Well 12

Well 11

Well 10

Well 1

#

"

Aerial image from U.S. Department of Agriculture, 2006

Kawaguesaga

Lake

Minocqua Lake

East Basin

Northwest

Basin

Southwest

Basin

Figure 5. Locations and types of data-collection sites at Minocqua and Kawaguesaga Lakes, Oneida County, 

Wisconsin.

Water-quality data for the lakes were collected by the 

WDNR from 1973 to 1974 as part of a statewide assessment 

and from 1986 to 1999 as part of their Long-Term Trend 

monitoring program, by the WVIC from 1979 to 1983, by 

volunteers from the local community and Lakes Association 

(Citizen Monitoring) from 1993 to 2009, and by the USGS 

in 2003 and from 2006 to 2007 as part of this study. In 

2003, the USGS collected data in spring and monthly from 

May through September at the deep-hole site in the East 

Basin of Minocqua Lake, near the deepest location in the 

Southwest Basin of Minocqua Lake, and at the deep-hole 

site in Kawaguesaga Lake (fig. 5). In 2006 and 2007, USGS 

sampling supplemented the Citizen Monitoring and was done 

at the deep-hole sites in both lakes and near the deepest part 

of the Northwest Basin in spring and monthly from May 

through September. USGS protocols were similar to those 

of the WDNR Long-Term Trend monitoring program. These 

protocols involved collecting profiles of water temperature, 

dissolved oxygen, specific conductance, and pH with a 

multiparameter meter, and water clarity (Secchi depth) with 

a standard 8-in.-diameter black and white Secchi disk during 

each visit. Near-surface samples were collected with a Van 

Dorn sampler and were analyzed for total phosphorus and 

chlorophyll a concentrations and, during midsummer, for 

various nitrogen species (nitrite plus nitrate and Kjeldahl 

nitrogen). Near-bottom samples were collected about 1 m 

above the sediment-water interface and analyzed for total 

phosphorus concentration. In addition, water samples collected 

at the deep-hole sites in each lake during spring turnover in 

2003 were analyzed for common ions and other characteristics 

such as color, turbidity, alkalinity, total dissolved solids, and 

silica.

All of the lake-stage, water-quality, and quality-assurance 

data collected by the USGS were published in the USGS 

annual data report series “Water Quality and Lake-Stage Data 

for Wisconsin Lakes,” water years 2003, 2006, and 2007 (U.S. 

Geological Survey, Wisconsin Water Science Center LakeStudies Team, 2004, 2007, 2008).
123 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Lake Classification

One method of classifying the water quality of a lake 

is with trophic state index (TSI) values based on near-surface 

concentrations of total phosphorus and chlorophyll a, plus 

Secchi depths, as developed by Carlson (1977). The indices 

were developed to place these three characteristics on similar 

scales to allow comparison of different lakes. TSI values 

based on total phosphorus concentrations (TSIP), chlorophyll a

concentrations (TSIC), and Secchi depths (TSISD) were 

computed for each sampling by use of equations 1–3. The 

individual index values were averaged monthly, and the 

monthly average values were then used to compute summer 

(June through September) average TSI values:

TSIP = 4.15 + 14.42 * ln (total phosphorus) (1)

[total phosphorus in micrograms per liter * 1,000]

TSIC = 30.6 + 9.81 * ln (chlorophyll a) (2)

[chlorophyll a in micrograms per liter]

TSISD = 77.12 – 14.41* ln (Secchi depth). (3)

[Secchi depth in feet]

Oligotrophic lakes (TSI values less than 40) have a 

limited supply of nutrients; typically have low phosphorus 

concentrations, low algal populations, and high water clarity; 

and contain oxygen throughout the year in their deepest 

zones (Wisconsin Department of Natural Resources, 1992). 

Mesotrophic lakes (TSI values between 40 and 50) have a 

moderate supply of nutrients, a tendency to produce moderate 

algal blooms, and moderate clarity: occasional oxygen 

depletions in the deepest zones of the lake are possible. 

Eutrophic lakes (TSI values greater than 50) are nutrient rich 

and have correspondingly severe water-quality problems, such 

as frequent seasonal algal blooms and poor clarity; oxygen 

depletion is common throughout the deeper zones of the lake. 

Eutrophic lakes with TSI values greater than 60 are often 

further classified as hypereutrophic lakes, and they typically 

have even more severe water-quality problems, with frequent 

extensive algal blooms. 

Stream Monitoring

Three stream sites (fig. 5) were equipped with 

instrumentation to continuously monitor flow (at 15-minute 

intervals). At one of the sites, Tomahawk River at State 

Highway 70 (about 0.5 mi downstream from the Minocqua 

dam), water level was measured to determine flow by use of 

standard stage-discharge relations (Rantz and others, 1982). 

The other two sites, Tomahawk Thoroughfare at Thoroughfare 

Road and Minocqua Thoroughfare (Link Creek) at State 

Highway 47 at the inlets to Minocqua Lake, were not suited 

to flow determination by standard stage-discharge methods 

because of backwater conditions, slow velocities, and periods 

of negative flow. These sites were equipped with acoustic 

Doppler velocity meters and water-level sensors and were 

operated as velocity-index stations (Sauer, 2002). From 

the 15-minute data, daily average flows for each site were 

computed. The data are stored and maintained in the USGS 

National Water Information System (NWIS) database. 

Samples were collected by USGS staff or a local 

observer approximately monthly and during flow events by 

use of the Equal-Width-Increment (EWI) method (Edwards 

and Glysson, 1999) or by grab methods. Tomahawk River 

at the lake outlet was sampled by the EWI method below 

the dam on Kawaguesaga Lake rather than at the gaging 

station farther downstream. Water samples were analyzed 

for concentrations of total phosphorus. Phosphorus loads 

for the continuously monitored sites at two inlets and for the 

Tomahawk River outlet were computed by use of techniques 

described by Porterfield (1972) for integrating streamflow and 

concentration. 

Intermittent discharge measurements and water samples 

were collected at several miscellaneous sites. Discharge 

measurements were made with an acoustic Doppler current 

profiler, and samples were collected at the mouth of 

Tomahawk Thoroughfare with Minocqua Lake, about 1.3 mi 

downstream from the gaging station, which included the 

outflow from Mid Lake. Discharge measurements and water 

samples were also collected during periods of runoff at four 

storm-sewer outlets from the urbanized area of the Island 

that discharge into Minocqua Lake (fig. 5). Measurements 

and water samples were also collected at an outfall on 

the southeast part of the Island that discharges pumped 

groundwater into Minocqua Lake; this groundwater is pumped 

to the surface and aerated by flowing over a step cascade as 

part of a remediation project to clean up dry-cleaning solvent 

contamination in the shallow aquifer. Other measurements 

and water samples were collected upstream and downstream 

from the WDNR fish hatchery on Minocqua Thoroughfare. All 

water samples were analyzed for total phosphorus. Additional 

water-quality data collected by the Woodruff Fish Hatchery 

were supplied by S. Ohm (Wisconsin Department of Natural 

Resources, written commun., 2007).

All water-quality data and daily flows and loads were 

published in the annual USGS water-data report series 

(Waschbusch and others, 2007, 2008) in compact-disk format. 

Groundwater Monitoring

Fifteen shallow, small-diameter piezometers 

(1/2-in.‐diameter observation wells) were installed around

the periphery of both lakes (fig. 5) to help define areas 

contributing groundwater to each lake, to determine the 

phosphorus concentration in the groundwater entering 
Lake Water Quality 13

the lakes, and to quantify the phosphorus loading from 

groundwater. The piezometers were installed approximately 

2–3 ft below the water table, to depths of 5–8 ft. Ten 

piezometers were located along the shoreline of Minocqua 

Lake, and five along Kawaguesaga Lake. Groundwater 

gradients (determined from the differences in water elevation 

in the piezometers and elevation of the lake surface) and 

dissolved phosphorus concentrations were measured three 

times each year in 2006 and 2007 for each piezometer. 

Additional analyses for dissolved chloride and field 

measurements of water temperature, dissolved oxygen, 

specific conductance, and pH were also obtained with a 

flowthrough multiparameter meter during some of these 

sampling visits.

Water-surface elevations of selected nearby lakes and 

streams were surveyed in October 2006 with a Real-Time 

Kinematic Global Positioning System (RTK-GPS) for 

calibration of a local groundwater model (described in later 

sections). The elevations surveyed have an expected accuracy 

of about 0.1 ft. Concurrent base-flow discharge measurements 

were made at stream sites during the survey.

All lake, stream, and groundwater samples were analyzed 

by the Wisconsin State Laboratory of Hygiene in accordance 

with standard analytical procedures described in its “Manual 

of Analytical Methods, Inorganic Chemistry Unit” (Wisconsin 

State Laboratory of Hygiene, 1993). 

Lake Water Quality

Most of the basins in Minocqua and Kawaguesaga 

Lakes are dimictic, meaning that they typically undergo two 

periods of extensive mixing (often referred to as “turnover”) 

during spring and fall, and thermally stratify during summer 

and winter. During summer, a thermocline (the depth interval 

where the temperature decreases abruptly) develops in late 

June and remains well established through September. The 

thermocline usually develops from about 13 to 30 ft below 

the surface in Minocqua Lake and from 16 to 30 ft below 

the surface in Kawaguesaga Lake (fig. 6). Bottom water 

temperatures were from about 8 to 10°C in Minocqua Lake 

and from about 11 to 13°C in Kawaguesaga Lake through the 

summer; surface water temperatures can approach or exceed 

25°C. Dissolved oxygen concentrations are near saturation 

throughout the lakes just after the ice melts, but oxygen is 

slowly depleted below the thermocline in both lakes during 

June and July. By mid-August to mid-September, dissolved 

oxygen concentrations below the thermocline are near zero 

(less than 0.5 mg/L) and unable to support fish. Oxygen 

depletion in the hypolimnion of Kawaguesaga Lake is 

more rapid and occurs earlier than in Minocqua Lake. This 

anaerobic condition in bottom waters is fairly common and 

similar to what occurs in other mesotrophic lakes. During 

late fall, the lakes completely mix, and dissolved oxygen 

concentrations at all depths are near saturation again. The 

Northwest Basin of Minocqua Lake was polymictic, meaning 

it frequently mixes during all seasons except under the ice. 

This results in its deep water warming throughout summer and 

bottom dissolved oxygen concentrations also varying between 

samplings (fig. 6B). The Northwest Basin occasionally mixed 

to the bottom because of its shallow depth. 

Water Chemistry and Trophic Conditions

The water chemistry of Minocqua and Kawaguesaga 

Lakes is similar and fairly typical for lakes in north-central 

Wisconsin, reflecting the igneous bedrock and glacial 

deposits in the area. The hardness of the lake water is 

about 45 to 50 mg/L as CaCO3, which is classified as soft 

water (Hem, 1985). Hardness is caused primarily by the 

presence of calcium and magnesium. According to Lillie 

and Mason (1983), relatively low concentrations of calcium, 

magnesium, and alkalinity characterize lakes in this region. 

They found that for 189 lakes sampled in this region, average 

concentrations of calcium, magnesium, and alkalinity were 10, 

5, and 37 mg/L, respectively. During this study, concentrations 

of calcium, magnesium, and alkalinity in Kawaguesaga and 

Minocqua Lakes were 13, 4, and 45 mg/L, respectively.

The specific conductance throughout the lakes during 

spring and fall overturns and near the surface during other 

periods is about 110–140 μS/cm but increases to greater than 

250 μS/cm near the bottom of the lakes when these areas 

are anaerobic during late summer. The pH in the upper part 

of the lakes is typically between 7 and 8 standard units; it is 

about 6.5 to 7.5 near the bottom when the lakes are stratified. 

Specific conductance commonly increases and pH decreases in 

the deep anoxic zones during late summer.

Concentrations of chloride increased in Minocqua Lake 

from 1.0–2.0 mg/L in 1973 to 6.1 mg/L in 2003, more than a 

threefold increase. The median concentration of chloride in 

lakes in this area reported by Lillie and Mason (1983) was 

2 mg/L. Concentrations of chloride and sodium are indicators 

of increasing urban development in the watershed. Increasing 

use of salt for deicing roads combined with increasing use of 

water softeners is the likely cause of this increase in Minocqua 

Lake and other developed/urbanized lakes. The changes in 

sodium and chloride concentration in Minocqua Lake are 

similar to the trends observed for many other lakes in the more 

populated areas of southeastern Wisconsin (Garn and others, 

2006). Concentrations of chloride, however, are still very low 

compared to those in southeastern Wisconsin lakes. According 

to U.S. Environmental Protection Agency (1988) water-quality 

criteria, freshwater aquatic organisms should not be affected 

unacceptably if the 4-day average concentration of dissolved 

chloride does not exceed 230 mg/L.
145 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

0 10 20 0 10 20 0 10 20

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DISSOLVED OXYGEN (D.O.), IN MILLIGRAMS PER LITER

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MINOCQUA LAKE, EAST BASIN

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WATER TEMPERATURE (W.T.), IN DEGREES CELSIUS

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06-18-07 07-18-07

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DISSOLVED OXYGEN (D.O.), IN MILLIGRAMS PER LITER

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D.O.

W.T.

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5

0

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0

A

B

C

Figure 6. Water 

temperature and 

dissolved oxygen 

profiles on 4 selected 

days during summer 

2007 at the deep holes in 

(A) the main East Basin, 

(B) Northwest Basin of 

Minocqua Lake, and 

(C) Kawaguesaga 

Lake, Oneida County, 

Wisconsin.
Lake Water Quality 15

Phosphorus and nitrogen are essential nutrients for plant 

growth and are the nutrients that usually limit algal growth 

in most lakes. High nutrient concentrations can cause high 

algal populations (blooms); therefore, increased nutrient 

loading can be the cause of accelerated eutrophication (that 

is, aging and increased productivity) of lakes. Near-surface 

total phosphorus concentrations in the East Basin of Minocqua 

Lake have ranged from 0.006 mg/L to 0.07 mg/L (fig. 7). 

In the early 1970s and 1980s, reported total phosphorus 

concentrations were as high as 0.07 mg/L during fall turnover 

and ranged from 0.02 to 0.03 mg/L during spring turnover. 

The high concentrations occurred after 1965, when the 

new sewage‐treatment plant for Minocqua and Woodruff

discharged effluent into the Tomahawk River downstream 

from the lakes. The probable causes for these higher 

concentrations after diversion are not clear, but the lake may 

not have recovered fully by that time to reach equilibrium, and 

the effects of the diversion may have been limited because the 

initial sewered service area was smaller than at present. 

From 1972 to 2009, summer average or growingseason concentrations (June through September) of nearsurface total phosphorus in the East Basin fluctuated from 

0.009 mg/L in 1989 to a maximum of 0.03 mg/L in 1984, but 

generally remained less than 0.022 mg/L after 1986 (table 3; 

fig. 7). There appears to be a gradual increase in phosphorus 

concentration since 1988; however, the lower concentrations 

from 1988 through 1996 may have been lower than the 

long-term average, possibly because of an extended drought 

that occurred in the late 1980s. During the 2-year study 

period 2006–07, the average summer concentration of total 

phosphorus was 0.015 mg/L, and the average concentration 

during spring overturn was 0.020 mg/L. Total phosphorus 

concentrations in the Southwest Basin were similar to those 

of the East Basin, but those of the Northwest Basin were 

generally greater. From 1991 to 2007, near-surface total 

phosphorus in the Northwest Basin ranged from 0.007 to a 

maximum of 0.043 mg/L; summer average concentrations 

ranged from 0.011 mg/L in 1991 to 0.022 mg/L in 2003, and 

the average summer concentration was 0.018 mg/L (table 3). 

During 2006–07, the average summer concentration of nearsurface total phosphorus was 0.021 mg/L. Total phosphorus 

concentrations from 0.012 to 0.024 mg/L indicate mesotrophic 

conditions; therefore, the summer average total phosphorus 

concentrations indicate that since 1986, Minocqua Lake has 

generally been mesotrophic.

Available nutrient data for Kawaguesaga Lake are 

intermittent over time but exhibit similar patterns in water 

quality to that of Minocqua Lake. Near-surface concentrations 

of total phosphorus in Kawaguesaga Lake have ranged 

from 0.006 mg/L to 0.06 mg/L (fig. 8). In the early 1970s 

and 1980s, total phosphorus concentrations were as high as 

0.06 mg/L in fall and winter, ranged from 0.006 to 0.033 mg/L 

during spring turnover, and ranged from 0.013 to 0.040 mg/L 

in summer (table 4; fig 8). From 1991 to 2007, summer 

average concentrations ranged from 0.009 mg/L in 1991 

to a maximum of 0.026 mg/L in 2001 but were less than 

0.024 mg/L after 2001. During 2006–07, the average summer 

concentration was 0.018 mg/L. Therefore, the summer average 

total phosphorus concentrations indicate that the lake was also 

generally mesotrophic since 1991.

Concentrations of near-surface total nitrogen (computed 

as the sum of Kjeldahl nitrogen and dissolved nitrite plus 

nitrate) ranged from 0.31 to 0.94 mg/L during the 1970s 

to 1980s in Minocqua Lake and from 0.27 to 1.05 mg/L 

in Kawaguesaga Lake. During summer 2003, near-surface 

total nitrogen concentrations were 0.38 mg/L in Minocqua 

Lake and 0.28 mg/L in Kawaguesaga Lake; relatively low 

concentrations compared to the earlier data. 

The ratio of the near-surface concentrations of total 

nitrogen to total phosphorus (N:P ratio) is often used to 

determine the potential limiting nutrient in a lake. The specific 

value of this ratio that determines which nutrient potentially 

is limiting differs under varied conditions such as water 

temperature, light intensity, and nutrient deficiencies (Correll, 

1998); however, a ratio greater than about 16:1 by weight 

usually indicates that phosphorus should be the potentially 

limiting nutrient. Ratios less than 10:1 are generally 

considered to indicate nitrogen limitation. The N:P ratios for 

the 1970s through 1980s summer data for Minocqua Lake 

ranged from 15:1 to 94:1, with an average of 35:1. The N:P 

ratios for the 1970s and 1991 summer data for Kawaguesaga 

Lake ranged from 11:1 to 52:1, with an average of 29:1. This 

indicates that phosphorus was the potentially limiting nutrient. 

In 2003, the N:P ratios during spring turnover for Minocqua 

and Kawaguesaga Lakes were 15:1 and 13:1; which indicate 

that algal productivity may be colimited by nitrogen and 

phosphorus. Reductions in phosphorus concentrations should 

not only increase the N:P ratio but also favor the growth of 

green algae over the growth of blue-green algae. Blue-green 

algae usually are not limited by nitrogen because they can 

fix nitrogen from the atmosphere. Blue-green algae are the 

least desirable type of algae because they commonly form 

extensive blooms, are potentially toxic, and are usually the 

least preferred by grazing zooplankton. Therefore, phosphorus 

should be the nutrient of concern when considering 

management efforts to improve the water quality of these 

lakes.
167 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 7. Near-surface (A) total phosphorus concentration, (B) chlorophyll a concentration, 

and (C) Secchi depths at the deep-hole site in the East Basin of Minocqua Lake, Oneida County, 

Wisconsin, from 1973 to 2009.

0

5

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1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

SECCHI DEPTH, IN FEET

0

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CHLOROPHYLL 

A CONCENTRATION, 

IN MICROGRAMS PER LITER

A

B

C

YEAR

0.00

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0.03

0.04

0.05

0.06

0.07

0.08

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER

U.S. Geological Survey

Citizen Monitoring

Wisconsin Department of Natural Resources

Wisconsin Valley Improvement Company

Summer average total phosphorus conc.

Summer average chlorophyll a conc. 

Summer average Secchi depth

EXPLANATION
Lake Water Quality 17

Table 3. Near-surface summer average (June through September) water-quality and trophic state index (TSI) values 

for Minocqua Lake, Oneida County, Wisconsin.

[Abbreviations: mg/L, milligrams per liter; μg/L, micrograms per liter; ft, foot; –, not measured]

Year

Total

phosphorus

(mg/L)

Chlorophyll a

(μg/L)

Secchi

depth

(ft)

Trophic state index values

Phosphorus Chlorophyll a

Secchi

depth

East Basin (Deep-hole site)

1973 0.010 – 8.5 37 – 46

1974 0.010 – 8.0 37 – 47

1979 0.026 6.6 11.1 51 49 43

1980 0.019 – 9.5 47 – 45

1981 0.000 – 16.4 0 – 37

1982 0.024 – 14.4 50 – 39

1983 0.030 – 14.4 53 – 39

1986 0.017 7.3 – 45 50 –

1987 0.016 6.0 – 44 47 –

1988 0.011 9.3 – 38 50 –

1989 0.009 5.0 13.6 35 46 40

1990 0.010 5.0 – 37 46 –

1991 0.011 3.7 11.4 38 43 42

1992 0.011 7.6 13.4 38 49 40

1993 0.011 5.6 13.6 37 47 40

1994 0.017 5.0 11.4 43 45 42

1995 0.010 3.3 13.0 36 42 40

1996 0.012 5.0 11.8 39 46 42

1997 0.027 4.3 12.3 50 44 41

1998 0.014 4.4 13.2 41 43 40

1999 0.016 4.6 11.6 44 45 42

2000 0.020 2.9 12.1 47 41 42

2001 0.020 5.5 11.4 47 46 42

2002 0.017 4.2 – 45 44 –

2003 0.020 5.1 12.0 47 46 42

2004 0.021 4.8 11.8 48 45 42

2005 0.017 4.7 13.2 45 45 40

2006 0.017 5.1 10.6 45 46 43

2007 0.014 5.3 12.5 41 45 41

2008 0.028 3.5 12.5 52 40 41

2009 0.018 3.1 12.5 46 41 41

Average: 1973–2009 0.016 5.1 12.2 42 45 41

  Average: 2006–07 0.015 5.2 11.5 43 46 42

Northwest Basin

1991 0.011 5.3 8.5 38 47 46

1992 – – 12.8 – – 40

1993 0.018 9.8 10.8 46 53 43

1994 – – 9.6 – – 44

1995 – – 9.7 – – 44

1996 – – 14.2 – – 39

1997 – – 11.6 – – 42

1998 – – 9.4 – – 45

1999 – – 9.2 – – 45

2000 – – 8.9 – – 46

2001 – – 9.0 – – 45

2003 0.022 10.4 9.5 49 54 45

2006 0.021 11.7 8.4 48 55 47

2007 0.020 9.9 10.9 47 53 43

Average: 1991–2007 0.018 9.4 10.2 46 52 44

  Average: 2006–07 0.021 10.8 9.6 48 54 45
189 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

SECCHI DEPTH, IN FEET

CHLOROPHYLL 

A CONCENTRATION, 

IN MICROGRAMS PER LITER

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER

A

B

C

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YEAR

U.S. Geological Survey

Citizen Monitoring

Wisconsin Department of Natural Resources

Wisconsin Valley Improvement Company

Summer average total phosphorus conc.

Summer average chlorophyll a conc. 

Summer average Secchi depth 

EXPLANATION

Figure 8. Near-surface (A) total phosphorus concentration, (B) chlorophyll a concentration, and (C) 

Secchi depths at the deep-hole site of Kawaguesaga Lake, Oneida County, Wisconsin, from 1973 to 2007.
Lake Water Quality 19

Table 4. Near-surface summer average (June through September) water-quality and trophic state index (TSI) 

values for Kawaguesaga Lake, Oneida County, Wisconsin.

[Abbreviations: mg/L, milligrams per liter; μg/L, micrograms per liter; ft, feet; –, not measured]

Year

Total

phosphorus 

(mg/L)

Chlorophyll a

(μg/L)

Secchi

depth

(ft)

Trophic state index values

Phosphorus Chlorophyll a

Secchi

depth

1973 0.020 – 10.5 47 – 43

1974 0.020 – 7.2 47 – 49

1975 0.000 – – – – –

1979 0.021 – – 48 – –

1980 0.025 – 10.5 51 – 43

1981 0.013 – 13.1 41 – 40

1982 0.034 – 11.8 55 – 42

1983 0.040 – 15.1 57 – 38

1991 0.009 4.3 11.2 36 45 42

2001 0.026 5.7 10.1 51 46 44

2002 0.019 4.7 11.9 46 40 42

2003 0.018 8.3 9.9 46 50 45

2004 – – 12.0 – – 41

2006 0.016 7.9 9.3 44 47 47

2007 0.021 8.1 9.3 48 48 46

Average: 1973–2008 0.020 6.5 10.9 47 46 43

  Average: 2006–07 0.018 8.0 9.3 46 47 46

Total phosphorus concentrations just above the bottom 

at the deep-hole sites ranged from less than 0.01 to 0.57 mg/L 

for Minocqua Lake and from less than 0.01 to 0.23 mg/L for 

Kawaguesaga Lake (fig. 9). In the 1970s, late-summer nearbottom concentrations measured in Minocqua Lake were as 

high as 0.17 mg/L; in the 1980s, bottom concentrations were 

as high as 0.256 mg/L; and during 2003–07 were as high 

as 0.273 to 0.465 mg/L after the onset of anoxic conditions 

(fig. 9). During 2003-07, near-bottom concentrations in 

the Northwest and Southwest Basins reached maximum 

concentrations of 0.349 and 0.359 mg/L, respectively, during 

anoxic conditions. In Kawaguesaga Lake, late-summer 

near-bottom concentrations measured in the 1970s through 

1980s reached only 0.11 mg/L. During 2003–07, near-bottom 

concentrations in Kawaguesaga Lake reached a maximum of 

0.100 to 0.231 mg/L. It appears that the bottom concentrations 

in both the East Basin of Minocqua Lake and Kawaguesaga 

Lake have increased since the 1970s; however, the apparent 

increase was primarily caused by a change in sampling 

protocols with more intense sampling in late summer (August) 

occurring after 1982. 

Total phosphorus concentrations observed during anoxic 

periods are indicative of phosphorus release from the bottom 

sediments (referred to as “internal loading”). Because of the 

shallow average depth of parts of Minocqua Lake—especially 

in the Northwest Basin—and Kawaguesaga Lake, it is 

probable that mixing events during summer distribute some 

of the phosphorus released from the sediment throughout the 

water column, increasing near-surface concentrations and 

decreasing the buildup of phosphorus in bottom water as 

summer progresses.

Chlorophyll a is a photosynthetic pigment found in 

algae and other green plants. Its concentration is commonly 

used as a measure of the density of the algal population in 

a lake. Concentrations between about 2 and 7 μg/L indicate 

mesotrophic conditions, between 7 and 20 μg/L indicate 

eutrophic conditions, and greater than 20 μg/L indicate 

hypereutrophic conditions and are usually associated 

with frequent algal blooms. Few chlorophyll a data were 

available for Minocqua Lake prior to 1980; the one value 

available was 6.59 μg/L measured in 1979. Since 1986, 

near-surface chlorophyll a concentrations have ranged 

from less than 1 to 19 μg/L (fig. 7; table 3). In a number 

of years, highest concentrations were measured in spring. 

Summer average concentrations for the period of record 

ranged from 2.9 μg/L in 2000 to 9.33 μg/L in 1988, with an 

overall average of summer concentrations of 5.2 μg/L. The 

average summer concentration during 2006–07 was also 

5.2 μg/L. Chlorophyll a concentrations remained relatively 

unchanged since the early 1990s. Therefore, summer average 

concentrations of chlorophyll a since 1986 indicate that 

the lake typically is mesotrophic to occasionally borderline 

eutrophic. 
201 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

B Kawaguesaga Lake

A East Basin Minocqua Lake

0.0

0.1

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1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER

YEAR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER

U.S. Geological Survey

Wisconsin Department of Natural Resources

Wisconsin Valley Improvement Company

EXPLANATION

Figure 9. Near-bottom total phosphorus concentrations at the deep-hole sites in the (A) East Basin of Minocqua 

Lake, and (B) Kawaguesaga Lake, Oneida County, Wisconsin.
Lake Water Quality 21

Chlorophyll a data were available for the Northwest 

Basin of Minocqua Lake for 1991, 1993 2003, and 2006–07. 

From 1991 to 2007, chlorophyll a in the Northwest Basin 

ranged from 1.73 to 23.2 μg/L, and summer average 

concentrations ranged from 5.33 μg/L in 1991 to 11.72 μg/L in 

2006. During 2006–07, the average summer concentration of 

chlorophyll a was 10.8 μg/L.

Chlorophyll a data were available for Kawaguesaga 

Lake for 1991 and 2001–07 (fig 8; table 4). Concentrations 

of chlorophyll a ranged from less than 1 μg/L (2002) to 

21.1 μg/L (2006). Summer average concentrations ranged 

from 4.33 μg/L in 1991 to 8.33 μg/L in 2003, with an overall 

average of summer concentrations of 6.50 μg/L. Therefore, 

summer average concentrations of chlorophyll a indicate that 

Kawaguesaga Lake typically is mesotrophic and occasionally 

is borderline eutrophic.

The average summer water clarity in the East Basin of 

Minocqua Lake, as measured with a Secchi disk, is 12.2 ft, 

but individual measurements have ranged from about 6.5 ft 

in several years to 20.3 ft in 1981 (fig. 7; table 3). Summer 

average readings from 1973 to 2009 ranged from 8.0 ft in 

1974 to 16.4 ft in 1981, with no apparent trend since 1989. 

During 2006–07, the average summer Secchi depth was 

11.5 ft. In the Northwest Basin, from 1991 to 2007, Secchi 

depths ranged from 4.9 ft in 1993 and 2006 to 22.7 ft in 1992; 

summer average readings ranged from 8.4 ft in 2006 to 14.2 ft 

in 1996 and averaged 10.2 ft overall. During 2006–07, the 

average summer Secchi depth was 9.6 ft in the Northwest 

Basin, which is 0.3 ft less than in the East Basin. Secchi 

depths less than 6 ft indicate eutrophic conditions, from 6 to 

13 ft indicate mesotrophic conditions, and greater than 13 ft 

indicate oligotrophic conditions. Therefore, summer average 

Secchi depths also indicate that Minocqua Lake is typically 

mesotrophic.

Secchi-depth data for Kawaguesaga Lake were 

intermittent over time but exhibit a similar pattern to that for 

Minocqua Lake. Secchi depths in Kawaguesaga Lake have 

ranged from 4.9 ft in 2006 to 23.3 ft in 1981 (fig. 8). From 

1973 to 2007, summer average Secchi depths ranged from 

7.2 ft in 1974 to 15.1 ft in 1983, with an average of 10.9 ft. 

During 2006–07, the average of summer depths was 9.3 ft 

(fig. 8; table 4). Therefore, the summer average Secchi depths 

also indicate that the lake was generally mesotrophic.

Summer average concentrations of total phosphorus and 

chlorophyll a and Secchi depths of all four lake sampling 

sites are compared in figure 10. Kawaguesaga Lake and 

the Northwest Basin of Minocqua Lake usually had higher 

concentrations of total phosphorus and chlorophyll a, and 

lower Secchi depths, than did the East and Southwest Basins.

One method to describe the trophic status of a lake is 

to use Carlson’s (1977) trophic state index (TSI). Figure 11

illustrates the variation in summer average trophic state 

indices for Minocqua and Kawaguesaga Lakes from 1973 to 

2009. According to all three trophic state indices, the lakes are 

generally mesotrophic but can be classified from oligotrophic 

to eutrophic during individual years. 

The index values based on total phosphorus and 

chlorophyll a concentrations, plus Secchi depths, indicate 

different trophic conditions in the lakes. From 1986 to 1996, 

there was more chlorophyll in the lakes than would have been 

expected given the measured phosphorus concentrations and 

water clarity. From 1997 to 2007, the amount of chlorophyll 

was similar to what would have been expected given the 

measured phosphorus concentrations, whereas the water was 

slightly clearer than expected from the other two parameters. 

In 2008 and 2009, the amount of chlorophyll was less and 

clarity was better than what would have been expected given 

the measured phosphorus concentrations. The reasons for 

the indices not being consistent are likely due to biological 

factors. The index values based on Secchi depths have 

not varied as dramatically as those based on the other two 

parameters.

The water quality of Minocqua and Kawaguesaga Lakes 

fluctuated during the period of record (1973 to 2009). The 

indices indicate that the trophic state of the lakes has changed 

very little since the 1970s; except for a period from the late 

1980s to mid 1990s when there were lower phosphorus 

concentrations in the lakes. 

Inferences about Changes in Water Quality from 

Lake-Sediment Cores

Analyses of sediment cores extracted from the East, 

Southwest, and Northwest Basins of Minocqua Lake by 

the WDNR in August 1991 were used to quantify historical 

changes in sedimentation rates and describe changes in the 

water quality of Minocqua Lake (P. Garrison and J. Hurley, 

written commun., 1992, 2003; Blake, 1996). That study found 

that development since about 1890 has caused an increase 

in rates of sediment and phosphorus accumulation, with 

largest increases in rates occurring after about 1960. The 

cores indicated that the Southwest Basin was more strongly 

affected by the development of the Town of Minocqua than 

the other basins. Analysis of the diatom communities in the 

cores indicated a progressive but gradual decline in the water 

quality (increased eutrophication) throughout the lake until 

about 1950 and an acceleration of that decline after 1950. The 

nutrient input from raw sewage until 1935 and treated effluent 

until 1965 into the Northwest Basin was not reflected in the 

sediment cores. Degradation in water quality appeared to have 

changed the least in the deeper East Basin of the lake. 
223 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 10. Summer average near-surface (A) total phosphorus concentrations, (B) chlorophyll a

concentrations, and (C) Secchi depths at deep-hole sites in the basins of Minocqua and Kawaguesaga Lakes, 

Oneida County, Wisconsin, from 1973 to 2009.

A

B

C

0

2

4

6

8

10

12

14

16

18

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

SECCHI DEPTH, IN FEET

East Basin of Minocqua Lake

Explanation

Northwest Basin of Minocqua Lake

Southwest Basin of Minocqua Lake

Kawaguesaga Lake 

0

2

4

6

8

10

12

14

CHLOROPHYLL 

A CONCENTRATION, 

IN MICROGRAMS PER LITER

0.00

0.01

0.02

0.03

0.04

0.05

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER

YEAR
Lake Water Quality 23

Figure 11. Trophic state index (TSI) values based on surface concentrations of total phosphorus, 

chlorophyll a, and Secchi depths at the deep-hole sites in (A) the East Basin of Minocqua Lake, and (B) 

Kawaguesaga Lake, Oneida County, Wisconsin, 1973 to 2009.

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

A

B

30

40

50

60

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008

TROPHIC STATE INDEX

30

40

50

60

TROPHIC STATE INDEX

YEAR

Minocqua Lake - East Basin 

Kawaguesaga Lake

Total phosphorus

Chlorophyll a

Secchi depth

EXPLANATION

Trophic State Index based on:
245 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Hydrology and Water Budget

Because the productivity in Minocqua and Kawaguesaga 

Lakes is limited or colimited by phosphorus (based on N:P 

ratios) and because reductions in phosphorus concentrations 

should favor production of green algae over blue-green algae, 

reductions in the input of phosphorus to the lakes should 

reduce phosphorus concentrations in the lakes and thus 

improve water quality. Therefore, it is important to understand 

the origins of the phosphorus reaching the lake. Almost all 

of the phosphorus entering the lakes is transported by water 

inputs; therefore, to quantify phosphorus inputs, it is necessary 

to first quantify the water inputs. Water budgets for the lakes 

were quantified for each of the two monitoring years, MY 

2006 (November 1, 2005, to October 31, 2006) and MY 2007 

(November 1, 2006, to October 31, 2007).

The hydrology of the lakes can be described in terms of 

components of its water budget. The water budget for a period 

of interest may be represented as follows:

	 ∆S = (P + SWIn + GWIn) – (E + SWOut + GWOut), (4)

where ∆S is change in the volume of water stored in the lakes 

and is equal to the sum of the volumes of water entering the 

lakes minus the sum of the volumes leaving the lakes. Water 

enters the lakes as precipitation (P), surface-water inflow 

(SWIn), and groundwater inflow (GWIn). Water leaves the 

lakes through evaporation (E), surface-water outflow (SWOut), 

and groundwater outflow (GWOut). Each term of the water 

budget was estimated on a daily basis.

Change in Storage

Changes in the volume of the lakes were determined 

from water elevations measured continuously at the lake-stage 

gage on the northeast side of Minocqua Lake at the Minocqua 

Thoroughfare gaging site rather than the less frequent manual 

readings at the WVIC staff gage at the Minocqua Dam 

(fig. 5). Water is released from about mid-November to late 

February and stored in spring and summer. In MY 2006, water 

level fluctuated from 1584.39 ft (November) to 1,583.15 ft 

(February), and in MY 2007, water level fluctuated from 

1,583.76 ft (November) to 1,584.80 ft (May) (Waschbusch 

and others, 2007; 2008). Because of dry conditions, no 

drawdown occurred in MY 2007, and the lakes gradually lost 

storage over the summer. To simplify computations of the 

change in storage, the areas of Minocqua and Kawaguesaga 

and Lakes were assumed to be constant (1,318 and 690 acres, 

respectively) during the study. For MY 2006, the net change 

in lake level from the beginning to end of the period was 

−0.45 ft, representing a loss in storage of 593 acre-ft in 

Minocqua Lake and 315 acre-ft in Kawaguesaga Lake. For 

MY 2007, there was a gain in stage of 0.75 ft, representing 

a gain in storage of 988 acre-ft in Minocqua Lake and 

517 acre-ft in Kawaguesaga Lake.

Precipitation

During the 2-year study period, daily precipitation 

was measured with a recording gage at the Tomahawk 

Thoroughfare gaging station near the southeastern end 

of Minocqua Lake during nonfreezing periods and at the 

Minocqua Dam, reported by the National Weather Service, 

during freezing periods. During winter and other periods of 

missing record, precipitation was estimated by averaging the 

daily data from two other National Weather Service stations at 

Willow Reservoir (about 13 mi southwest of Minocqua) and 

at St. Germain (about 11 mi northeast of Minocqua; National 

Climatic Data Center, 2006, 2007). Precipitation on the lake 

surface during MY 2006 was 25.67 in. (2,820 acre-ft for 

Minocqua Lake and 1,480 acre-ft for Kawaguesaga Lake); 

during MY 2007, precipitation totaled 28.22 in. (3,100 acre-ft 

for Minocqua Lake and 1,620 acre-ft for Kawaguesaga 

Lake). Precipitation was 80 percent of the long-term average 

(1971–2000) in 2006 and 88 percent in 2007, reflecting an 

extended dry period in the region. Monthly precipitation 

during each study year is compared to the 1971–2000 average 

monthly precipitation in figure 12. In MY 2006, precipitation 

totals for November through March and for July were above 

or near the long-term average; all other monthly totals were 

much below average. In MY 2007, only December, March, 

May, and October totals were above or near the long-term 

average, and all other months were below average. 

Evaporation

Evaporation from the lake surface was estimated 

from pan evaporation measured at Marshfield, Wis., during 

June–September (National Climatic Data Center, 2006, 

2007) and long-term monthly estimates of pan evaporation 

at Green Bay, Wis., from Farnsworth and Thompson (1982). 

Pan evaporation at Marshfield for months without data was 

estimated from the ratio of Marshfield to Green Bay pan 

evaporation. Estimated pan data for Marshfield were then 

adjusted for the difference in location to Minocqua Lake. 

Monthly lake/pan coefficients computed from long-term panand lake-evaporation data for Rainbow Reservoir furnished by 

S. Morgan (Wisconsin Valley Improvement Company, written 

commun., 2008) were then used to adjust the pan-evaporation 

rates to obtain monthly lake evaporation for the lakes. These 

coefficients ranged from 0.73 in May to 1.03 in September. It 

was assumed that evaporation from the lakes was zero during 

periods of ice cover and that the lakes were ice covered from 

December through March. Annual evaporation was estimated 

to be 23.93 in. in MY 2006 (2,630 acre-ft from Minocqua 

Lake and 1,380 acre-ft from Kawaguesaga Lake) and 

23.22 in. in MY 2007 (2,550 acre-ft from Minocqua Lake and 

1,340 acre-ft from Kawaguesaga Lake; table 5).
Hydrology and Water Budget 25

Figure 12. Precipitation at 

Minocqua Lake during November 

2005 to October 2007 relative to 

long-term average precipitation, 

measured at Minocqua Dam, Oneida 

County, Wisconsin. 0

1

2

3

4

5

6

Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct.

PRECIPITATION, IN INCHES

Nov. 2005 − Oct. 2006

Nov. 2006 − Oct. 2007

Long-term average, 1971−2000 Minocqua Dam

Explanation

MONTH

Surface-Water Inflow

Annual runoff for rivers in the region reflected the dry 

conditions in north-central Wisconsin during both monitoring 

years. In both 2006 and 2007, the dry summers prompted the 

Governor to declare counties in northern Wisconsin in a state 

of emergency because of drought conditions. In MY 2006, 

runoff at nearby long-term USGS gaging stations ranged 

from 60 percent of the long-term average annual runoff 

(1991–2006) at Bear River near Manitowish Waters, Wis., 

to 81 percent of the average annual runoff (1991–2006) at 

Trout River at Trout Lake near Boulder Junction, Wis., and 

85 percent of the average annual runoff (1936–2006) at the 

Wisconsin River at Rainbow Lake near Lake Tomahawk, Wis. 

(Waschbusch and others, 2007). In MY 2007, runoff ranged 

from 32 percent of the average annual runoff (1991–2007) 

at the Bear River station to 58 percent of the average annual 

runoff at Trout River at Trout Lake and 62 percent of the 

average annual runoff at the Wisconsin River at Rainbow Lake 

(Waschbusch and others, 2008).

Gaged Sites

Daily inflow from Minocqua Thoroughfare (Link Creek), 

one of the main tributaries to the lakes, was measured at 

the gaging station at State Highway 47 (fig. 5). The annual 

average flow at this site was 16.76 ft3/s (12,100 acre-ft) in 

MY 2006 and 12.76 ft3/s (9,240 acre-ft) in MY 2007 (table 5, 

fig. 13A). 

Daily inflow from Tomahawk Thoroughfare, the other 

main tributary that includes Tomahawk Lake, was measured at 

the gaging station at Thoroughfare Road (fig. 5). The annual 

average flow at this site was 17.5 ft3/s (12,700 acre-ft) in MY 

2006 and 5.68 ft3/s (4,110 acre-ft) in MY 2007. This site is 

characterized by not only extended periods (up to several 

days) of negative flow caused by west winds but also changes 

in gate openings and water releases at the dam (fig. 13B). 

This station is not directly at the mouth to Minocqua Lake, so 

there is a resulting ungaged area of 598 acres(fig. 5). The flow 

from Mid Lake and the intervening area between the mouth of 

Tomahawk Thoroughfare and the gage was estimated with the 

following equation, which was applied on a daily time step.

,

where

is the flow from the intervening area,

is precipitation,

is evaporation,

is groundwater discharge to Mid Lake, and

is change in storage in the channel and Mid

Lake from the previo

i

i

Q P E GW S

Q

P

E

GW

S

= − + − ∆

∆

us day.

(5)

It was assumed that the water level in the channel 

and Mid Lake is the same as the water level in Minocqua 

Lake. The estimated flow from this additional Mid Lake 

drainage area was added to the measured streamflow at 

Thoroughfare Road. Thus, the total annual flow from 

Tomahawk Thoroughfare at its mouth with Minocqua Lake 

was an estimated 19.69 ft3/s (14,300 acre-ft) in MY 2006 and 

7.06 ft3/s (5,110 acre-ft) in MY 2007 (table 5). 

Nearshore Ungaged Area (Including Urban 

Areas)

Smaller subwatersheds making up the near-lake drainage 

area (referred to herein as “near-lake drainage”; fig. 2) also 

contribute runoff to the lakes, especially from the more 

developed urbanized areas with impervious surfaces. These 

were subdivided into the areas contributing runoff to each 

of the basins of Minocqua Lake and to Kawaguesaga Lake. 

Numerous noncontributing drainage areas exist within the 

near-lake drainage that effect surface runoff to the lakes.
267 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Table 5. Summary of water-budget components for Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, during monitoring 

years 2006–07, during average hydrologic conditions, and with future development in the watershed.

[Abbreviations: MY, monitoring year November–October; NA, not applicable]

Budget component MY 2006 MY 2007 2-year 

average

Percent of 

2-year total

Average 

hydrology

2030 buildout

MY 2006–07 

hydrology

Average 

hydrology

Change in storage (acre-feet) -908 1,500 592 NA 0 0 0

Inputs to lake (acre-feet)

Precipitation

Minocqua Lake 2,820 3,100 2,960 10.5 3,520 2,960 3,520

Kawaguesaga Lake 1,480 1,620 1,550 5.5 1,850 1,550 1,850

Tributaries

 Minocqua Thoroughfare (inlet) 12,100 9,240 10,700 37.8 14,300 10,700 14,300

 Tomahawk Thoroughfare (inlet) 14,300 5,110 9,680 34.3 16,400 9,680 16,400

Near-lake drainage

(including urbanized areas)

Minocqua Lake

East Basin 108 133 121 0.4 140 191 2

Northwest Basin 84 104 94 0.3 98 127 1

Southwest Basin 32 41 36.5 0.1 45 60 

Kawaguesaga Lake 19 23 21 0.1 23 38 42

Groundwater

Minocqua Lake 2,880 2,200 2,540 9.0 3,000 2,540 3,000

Kawaguesaga Lake 638 486 562 2.0 650 562 650

TOTAL INPUT 34,400 22,100 28,300 100 40,000 28,400 40,200

Outputs from lake (acre-feet)

Evaporation

Minocqua Lake 2,630 2,550 2,590 9.4

Kawaguesaga Lake 1,380 1,340 1,360 4.9

Groundwater

Minocqua Lake 0 0 0 0.0

Kawaguesaga Lake 667 667 667 2.4

Tomahawk River (outlet) 30,300 15,600 22,900 83.3

TOTAL OUTPUT 35,000 20,100 27,600 100
Hydrology and Water Budget 27

Figure 13. Daily discharge, phosphorus concentrations, and phosphorus loads 

at (A) Link Creek near Woodruff (Minocqua Thoroughfare), (B) Tomahawk River at 

Thoroughfare Road near Minocqua (Tomahawk Thoroughfare), and (C) Tomahawk River 

at Minocqua Dam near Minocqua, Wisconsin, November 2005 through October 2007.

0

10

20

30

40

50

60

70

80

90

0

1

2

3

4

5

6

7

8

9

Water discharge

Phosphorus concentration

Phosphorus load

DISCHARGE, IN CUBIC FEET PER SECOND, ANDCONCENTRATION, IN MICROGRAMS PER LITER

LOAD, IN POUNDS PER DAY

-60

-40

-20

0

20

40

60

80

100

-6

-4

-2

0

2

4

6

8

10

LOAD, IN POUNDS PER DAY

DISCHARGE, IN CUBIC FEET PER SECOND, ANDCONCENTRATION, IN MICROGRAMS PER LITER

0

20

40

60

80

100

120

140

0

2

4

6

8

10

12

14

DISCHARGE, IN CUBIC FEET PER SECOND, AND CONCENTRATION, IN MICROGRAMS PER LITER

LOAD, IN POUNDS PER DAY

A

B

C

NOV. JAN. MAR. MAY JULY SEP. NOV. JAN. MAR. MAY JULY SEP. NOV.

2005 2006 2007

EXPLANATION
289 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Runoff from the ungaged area was estimated by using 

the Windows Source Loading and Management Model 

(WinSLAMM), which is commonly used to generate runoff 

volumes and loads from urban areas (Pitt and Voorhees, 

1993). The latest standard parameter files and coefficients 

(May 2009) were used in WinSLAMM to estimate runoff 

(available on the USGS Web page http://wi.water.usgs.gov/

slamm/index.html). Runoff coefficients for forest on sandy 

soils were from those estimated by Graczyk and others (2003) 

for northern Wisconsin. Runoff was estimated on the basis 

of actual precipitation measured in MY 2006 and MY 2007 

(less than average rainfall); urban land uses (commercial/

industrial and high-density residential) were assumed to have 

streets with curb-and-gutter drainage, whereas low-density 

rural residential uses were assumed to have roads drained 

by shallow roadside swales. Total runoff estimated for the 

ungaged contributing areas was 243 acre-ft in MY 2006 

and 301 acre-ft in MY 2007. Runoff to the East Basin was 

108 acre-ft in MY 2006 and 133 acre-ft in MY 2007, to the 

Southwest Basin was 32 acre-ft in MY 2006 and 41 acre-ft in 

MY 2007, and to the Northwest Basin was 84 acre-ft in MY 

2006 and 104 acre-ft in MY 2007. The remaining ungaged 

area surrounding Kawaguesaga Lake has a contributing area of 

about 1,581 acres and contributed only about 19 acre-ft in MY 

2006 and 23 acre-ft in MY 2007.

Surface-Water Outflow

Daily outflow from the lakes was measured at the 

gaging station at the Tomahawk River at State Highway 70 

(about 0.5 mi downstream from Minocqua Dam; fig. 5). The 

measured outflow was adjusted for the estimated groundwater 

discharge between the dam and the gaging station (0.7 ft3/s) 

to determine the outflow at Minocqua Dam. Outflow at the 

dam ranged from 6 to 116 ft3/s (fig. 13C). The total annual 

surface-water outflow was 30,300 acre-ft in MY 2006 and 

15,600 acre-ft in MY 2007 (table 5). 

Groundwater Inflow and Outflow

The two-dimensional, analytic-element, steady-state, 

groundwater-flow model GFLOW (Haitjema, 1995) was used 

to define groundwater-source areas around Minocqua and 

Kawaguesaga Lakes and to allocate groundwater discharge to 

shoreline segments represented by the installed piezometers 

around the lakes. A complete description of analytic-element 

modeling is beyond the scope of this report; however, a brief 

description is given below. A thorough discussion of the 

methods for applying the model can be found in Strack (1989) 

and Haitjema (1995).

An infinite aquifer is assumed in analytic-element 

modeling. In GFLOW, the study area (domain) does not 

require a grid or involve interpolation between cells. To 

construct an analytic-element model, features that affect 

groundwater flow (such as surface-water bodies, aquifer 

characteristics, and recharge) are entered as mathematical 

elements or strings of elements. The amount of detail specified 

for the features depends on the distance from the area of 

interest. Each element is represented by an analytic solution. 

The effects of these individual solutions are added together to 

arrive at a solution for the groundwater-flow system. Because 

the solution is not confined to a grid, groundwater levels 

(heads) and flows can be computed anywhere in the study 

area without averaging values at specific locations in the 

model (nodal averaging). In the GFLOW model, the analytic 

elements are two dimensional and are used to simulate only 

steady-state conditions. Groundwater-flow systems are three 

dimensional; however, two-dimensional models can provide 

reasonable approximations of groundwater flowlines when 

the lengths of the flowlines are long compared to the aquifer 

thickness (Haitjema, 1995, p. 23). In the study area, most 

groundwater was assumed to move through unconsolidated 

deposits that have a maximum saturated thickness of 250 ft or 

less. The lengths of flowlines from recharge areas to discharge 

areas are typically several thousand feet or more. 

The GFLOW model for the study area covers an area 

extending approximately 10 to 15 mi around Minocqua and 

Kawaguesaga Lakes (fig. 14). The geometry of the singlelayer model includes a bottom elevation set at 1,450 ft and 

an average aquifer thickness of 250 ft, which were based 

on well logs for the study area and additional information 

from Patterson (1989). Recharge was applied uniformly 

across the entire simulated area. Surface-water features were 

simulated by using several types of linesink elements. The 

model includes “farfield” and “nearfield” sources and sinks 

of water (collectively referred to as linesinks). The farfield 

area surrounds the nearfield area of interest. In the farfield, 

streams and lakes are simulated as coarse linesinks having 

little or no resistance between the surface-water feature and 

the groundwater-flow system. The purpose of simulating the 

farfield is to have the model explicitly define the regional 

groundwater-flow field near the area of interest. The nearfield 

represents the area of interest and includes Minocqua and 

Kawaguesaga Lakes, all upstream lakes and streams, plus 

most adjacent lakes and streams. 

Nearfield lakes and streams, which are in good 

connection with the groundwater-flow system, were included 

as linesinks with resistance. In analytic-element modeling, 

resistance is computed by dividing the bed-sediment thickness 

by the vertical hydraulic conductivity. Nearfield streams were 

simulated by using head-specified linesinks with resistance of 

0.3 day and widths ranging from 5 to 100 ft. Nearfield lakes 

were simulated as head-specified or lake linesinks, both with a 

resistance of 0.5 day. Lake widths and depths were determined 

from USGS topographic maps, and lake stages were measured 

with a GPS-RTK survey instrument. Resistance values used 

in this model are similar to values used in other GFLOW 

models for areas nearby (Hunt and others, 1998; Graczyk 

and others, 2003, Robertson and others, 2005). Minocqua, 
Hydrology and Water Budget 29

Figure 14. Hydrologic features simulated with GFLOW analytic elements near Minocqua and 

Kawaguesaga Lakes, Oneida County, Wisconsin.

Kawaguesaga, Tomahawk, Carrol/Madeline, Little Arbor 

Vitae, Big Arbor Vitae, Little Musky, Arrowhead, Brandy, and 

Johnson Lakes were simulated as lake linesinks, in accordance 

with the methodology described by Hunt and others (2003). 

The lake linesink solves for lake stage on the basis of 

simulated surface–water inflows and estimated surface-water 

outflows from the lake. Use of the lake linesink requires an 

estimate of the lake stage/outflow and lake stage/surface-area 

relationships. Surface outflows from six lakes were estimated 

from nearby, downstream streamflow measurements, made 

under base-flow conditions during October 19–20, 2006. 

Where applicable, lake stage was measured concurrently 

with streamflow (lake outflow) measurements. The model 

includes one pumping well that represents average daily 

pumping in 2006 (0.71 ft3/s) from Minocqua municipal wells 

3 and 4. Domestic pumping wells were not included in the 

model because the withdrawals from these wells are widely 

distributed and the withdrawals are relatively small (especially 

where balanced by return flow from septic systems).

The GFLOW model was calibrated by using the 

parameter estimation code PEST (Doherty, 2004). Measured 

streamflows and lake stages were used as flux and head targets 

0 2.5 5 MILES

0 2.5 5 KILOMETERS

Farfield element (linesink without resistance, streams and lakes)

Nearfield element (linesink with resistance, lakes)

Nearfield element (linesink with resistance, streams)

Nearfield element (lake element)

Recharge element

Roads and highways

Pumping well

Flux target

Head target

Lake-stage target

EXPLANATION

89°55' 89°30'

45°58'

45°45'

Kawaguesaga

Lake

Minocqua Lake
301 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

during model calibration. Water-level elevations estimated 

from 81 driller’s well logs (by using measured depth to water 

and corresponding topographic quadrangle map elevations) 

also were used as head targets. Recharge and horizontal 

hydraulic conductivity were automatically varied during 

model calibration until differences between measured and 

simulated water levels, lake stages, and streamflows were 

minimized. Measured lake stages and streamflows were given 

relatively similar weight during model calibration. To do this, 

weights for lake stages were set to 10; weights for streamflows 

ranged from 0.00001 to 0.0004 (because model streamflow 

units were in cubic feet per day). Because groundwater 

levels were measured over a period of years and estimated 

from topographic maps, these targets were given less weight 

(set to 1) during calibration than those for lake stages and 

streamflows. Groundwater gradients from the shoreline 

piezometers were not included in the PEST calibration, but 

were manually compared with the simulated gaining and 

losing shoreline segments. 

The calibrated model has a recharge rate of 8.4 in/yr and 

hydraulic conductivity of 41.8 ft/d. These are similar to values 

used in previous models near the study area (Hunt and others, 

1998; Graczyk and others, 2003; Robertson and others, 2005). 

Simulated heads and streamflows were reasonably close to 

measured values. The mean and absolute mean difference 

between simulated and measured groundwater levels were 

−0.5 and 4.4 ft, respectively (fig. 15). The mean and absolute 

mean differences were 0.09 and 0.54 ft3/s, respectively, 

for streamflow. For lake stages, mean and absolute mean 

differences were 0 and 0.3 ft, respectively. Simulated gaining 

and losing shoreline segments were reasonably similar to those 

indicated by gradients measured in shoreline piezometers.

On the basis of similarity between measured and 

simulated groundwater levels, lakes stages, and streamflows, 

the model appears to accurately simulate the groundwater flow 

near Minocqua and Kawaguesaga Lakes during October 2006. 

The model represents one set of steady-state, base-flow 

conditions; however, in reality, there are seasonal variations 

in groundwater levels, lake stage, and streamflow. It was 

assumed that seasonal variations in groundwater levels and 

natural lake stages would be relatively uniform throughout the 

model area, thus having minimal effect on the extent of the 

groundwater-source areas and on the net gains and losses to 

the groundwater-flow system immediately adjacent to nearfield 

lakes. Additionally, it was assumed that human‐induced

seasonal changes in lake stages (dams) would cause only 

short-term effects on gains and losses and that those effects 

would be nearly zero on an annual basis. Given these 

assumptions, the model can be used to define groundwatersource areas around Minocqua and Kawaguesaga Lakes and 

to allocate groundwater gains and losses on an annual basis to 

shoreline segments represented by the piezometers installed 

around the lakes. 

The groundwater contributing area for the lakes was 

delineated by using particle-tracking techniques in GFLOW. 

The groundwater contributing area covers about 59 mi2 and 

represents an area that is slightly different than the surfacewater contributing area (fig. 16). Based on measured and 

simulated information, groundwater discharges to most 

of the shoreline surrounding Kawaguesaga and Minocqua 

Lakes, except for the northern shore of Kawaguesaga Lake 

near the dam, which loses water, and the western shore near 

piezometers 10 and 11, which seasonally may gain or lose 

water. Net groundwater flow to the lakes via direct shoreline 

segments is 3.46 ft3/s (2,510 acre-ft per year). The area of 

greatest groundwater discharge to Minocqua Lake is along the 

east and northeast shorelines (fig. 17). Although much of the 

Kawaguesaga shoreline has groundwater discharge to the lake, 

the net discharge is negative due to losses from the lake to the 

groundwater system near the dam and the outlet stream.

In order to match groundwater inflows and outflows 

from the steady-state model with the monitoring period during 

which data were collected to compute nutrient loading, the 

simulated groundwater to the lakes needed to be adjusted. 

This adjustment was necessary because the 2-year study was 

done during a period of continuing below-normal precipitation 

and below-normal groundwater recharge. It is reasonable to 

assume that groundwater discharge was declining through the 

2-year study period. The steady-state model was calibrated 

on the basis of conditions near the end of the first year of the 

study. Simulated groundwater discharges to the lakes were 

adjusted proportionately to the declining flow at the Minocqua 

Thoroughfare gage from MY 2006 to MY 2007. The average 

flow at the Minocqua Thoroughfare gaging station during 

MY 2007 was 76 percent of that in MY 2006. The key 

assumption is that groundwater discharge to Minocqua and 

Kawaguesaga Lakes is proportional to flow in Link Creek at 

the Minocqua Thoroughfare gaging station and is based on the 

likelihood that most of the flow in Link Creek originates as 

groundwater discharge. The adjustment factor for the midpoint 

of the second year was assumed to be 0.76 of that for MY 

2006. Adjusted groundwater discharges to Kawaguesaga and 

Minocqua Lakes are given in table 5. 

Although this model was considered reasonably 

calibrated and was sufficient for estimates of groundwater 

inflow to and outflow from Minocqua and Kawaguesaga Lake, 

local heterogeneities that were not simulated with the model 

may locally alter the contributing area to a small degree. The 

accuracy of simulated groundwater level, lake stage, and 

streamflow is only as good as the accuracy of the measured 

data used for calibration. Groundwater levels based on driller’s 

logs have an accuracy of ±5 feet (one half of the topographic 

quadrangle map contour interval). Lake stages measured using 

RTK-GPS have an accuracy of ±0.1 ft. Measured streamflows 

were generally rated as fair to good with an accuracy of ±5 to 

8 percent. Use of the model for evaluations beyond the stated 

purpose (to estimate groundwater-contributing areas to the 

lakes and groundwater exchanges with the lakes) or accuracy 

may require reevaluation of the model and may require 

recalibration of the model to match new or existing data that 

correspond to the new purpose. 
Hydrology and Water Budget 31

Figure 15. Measured and simulated (A) head (groundwater levels), (B) lake stage, and (C) streamflow for the calibrated 

GFLOW model.

1,570

1,580

1,590

1,600

1,610

1,620

1,630

1,640

1,570 1,580 1,590 1,600 1,610 1,620 1,630 1,640

MEASURED HEAD, FEET ABOVE SEA LEVEL

SIMULATED HEAD, FEET ABOVE NGVD 88

1,580

1,585

1,590

1,595

1,600

1,605

1,610

1,615

1,580 1,585 1,590 1,595 1,600 1,605 1,610 1,615

MEASURED LAKE STAGE, FEET ABOVE SEA LEVEL

SIMULATED LAKE STAGE, FEET ABOVE NGVD 88

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

MEASURED STREAMFLOW, 

IN CUBIC FEET PER SECOND

SIMULATED STREAMFLOW, IN CUBIC FEET PER SECOND

A B

C
323 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 16. Simulated groundwater contributing area and flow direction for Minocqua and 

Kawaguesaga Lakes, Oneida County, Wisconsin.

Groundwater-flow path (arrow indicates direction of flow)

Minocqua/Kawaguesaga groundwater-contributing area

89°48' 89°32'

45°58'

45°48'

EXPLANATION 0 2 4 MILES

0 2 4 KILOMETERS

Highway

Kawaguesaga

Lake

Minocqua Lake
Hydrology and Water Budget 33

Figure 17. Simulated groundwater-flow direction and flux (flow) rates for shoreline 

segments of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin.

0 0.5 1 MILE

0 0.5 1 KILOMETER

89°44'

45°53'

45°51'

Groundwater flux, in cubic feet per day per foot of shoreline

-8.21 to -0.01

0 to 2.55

2.56 to 3.76

3.77 to 7.47

7.48 to 12.7

Road

Highway

Piezometer and identifier

Groundwater-flow path (arrow indicates direction of flow)

EXPLANATION

Kawaguesaga

Lake

Minocqua Lake

89°41'
345 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Water-Budget Summary

Minocqua and Tomahawk Thoroughfares are the 

largest sources of water to the lakes, supplying about 38 

and 34 percent, respectively, of the total input during the 

2-year study period (table 5; fig. 18). Other sources were 

precipitation (16 percent), groundwater (11 percent), and the 

ungaged near-lake drainage that includes the urbanized area 

(less than 1 percent). The total input of water in MY 2006 

(34,400 acre-ft) was more than in MY 2007 (22,100 acre-ft) 

because of the prolonged dry period and below-normal 

precipitation, even though precipitation was somewhat greater 

in MY 2007. The residence time—that is, the length of time 

required for water entering the lakes to completely replace 

the volume of water in the lakes—was 1.2 and 2.1 years, 

respectively, or 1.5 years on the basis of the average inflow for 

the 2 years. 

The total annual output of water from the lakes was more 

than the total input in MY 2006, resulting in a decrease in lake 

stage of 0.45 ft or a decrease of 908 acre-ft in storage from 

the beginning to the end of the study year. Annual output of 

water was less than the total input in MY 2007, resulting in an 

increase in lake stage of 0.75 ft or an increase of 1,500 acre-ft 

in storage for the year. This amounted to an overall increase 

in lake stage of 0.30 ft in the lakes during the 2-year study 

period. On average, 83 percent of the total water leaving the 

lakes left through the Tomahawk River outlet at the Minocqua 

Dam, evaporation accounted for 14 percent, and outflow to 

groundwater accounted for the remaining 2 percent (table 5; 

fig. 18).

The quality or accuracy of the water budget was 

evaluated by comparing the monthly sum of all inputs with 

the sum of all outputs plus the change in storage for the lakes 

(fig. 19). The differences between the calculated inputs and 

outputs reflect the cumulative errors in the estimates of all 

of the components in the water budget. The largest monthly 

differences in 2006 were in June and July, which when 

expressed as percentages of the total measured output plus 

change in storage were 13.4 and 12.8 percent, respectively. 

In 2007, the largest monthly differences were in April and 

September, which when expressed as percentages were about 

22 and 30 percent. On an annual basis, the percentage error 

was about 1 percent for MY 2006 and about 2 percent for MY 

2007.

The hydrology documented in MY 2006–07 represents 

conditions occurring during a series of dry years. To determine 

how the hydrologic loading to the lakes would change under 

more typical hydrologic conditions, flows were calculated 

for each of the budget components (table 5) during longterm average hydrologic conditions. During MY 2006–07, 

precipitation was 84 percent of average; runoff volumes at 

the Minocqua Thoroughfare and Tomahawk Thoroughfare 

gaging stations were assumed to be 75 percent and 59 percent 

of average, respectively, on the basis of record from nearby 

gaging stations; groundwater discharge was assumed to be 

about 85 percent of average. Runoff volumes for the near‐lake

drainage areas were computed with WinSLAMM by using 

1975 precipitation values at Duluth, which represented 

average conditions. Allocation of the hydrologic inputs during 

average hydrologic conditions is given in table 5.

Total hydrologic inputs to the lakes were also estimated 

for the two 2030 buildout scenarios: 2030 buildout with 2006–

07 hydrology and 2030 buildout with average hydrology. All 

of the hydrologic inputs except nearshore runoff were assumed 

to remain unchanged from average MY 2006–07 and average 

hydrologic conditions. However, runoff from the ungaged area 

was estimated by using WinSLAMM with precipitation from 

MY 2006–07 and 1975 (table 5).

Sources of Phosphorus

To help define where the phosphorus in Minocqua 

and Kawaguesaga Lakes originates and how much may be 

controllable, a detailed phosphorus budget was computed for 

each monitoring year. External sources of phosphorus to the 

lakes include precipitation, surface-water and groundwater 

inflow, and contributions from septic systems. In addition 

to these external sources, phosphorus can be released from 

the bottom sediments of the lakes, which is considered an 

internal source of phosphorus (internal loading). Phosphorus 

is removed from the lakes through surface-water outflow and 

deposition to the lake sediments. 

Precipitation

Atmospheric deposition of phosphorus on Minocqua 

and Kawaguesaga Lakes was determined from phosphorus 

concentrations measured in wetfall (rain and snow) and 

phosphorus-deposition rates measured in dryfall at a weather 

station operated at Whitefish Lake in Douglas County, Wis. 

(Robertson and others, 2009). Total monthly wetfall deposition 

was computed by multiplying the average estimated 

phosphorus concentration of 0.016 mg/L by the monthly 

precipitation on the lakes. Phosphorus in dryfall was estimated 

from monthly deposition rates measured at Whitefish Lake, 

which were adjusted for lakes with few conifers in the 

surrounding area (approximately 26 lb/mi2/yr and applied 

equally over May through October). Total phosphorus inputs 

to the lakes from precipitation were 269 lb in MY 2006 and 

288 lb in MY 2007 (table 6). 
Hydrology and Water Budget 35

Figure 18. Charts showing water budgets for Minocqua and Kawaguesaga Lakes, Wisconsin, for monitoring years 2006 

and 2007. (%, percent).

Water inputs and outputs for Minocqua/Kawaguesaga Lakes, November 2005–October 2006

WATER INPUTS

[34,400 acre-feet]

WATER OUTPUT

[35,000 acre-feet]

WATER OUTPUTS

[20,100 acre-feet]

WATER INPUTS

[22,100 acre-feet]

Near-lake drainage 

(including urbanized areas) 

0.7%

Water inputs and outputs for Minocqua/Kawaguesaga Lakes, November 2006–October 2007

Minocqua

Thoroughfare

35.2%

Groundwater

10.2%

Groundwater

outflow

2%

Evaporation

11%

Outlet

87%

Groundwater

12% Precipitation

21%

Minocqua

Thoroughfare

43%

Outlet

78%

Evaporation

19%

Groundwater

outflow

3%

Near-lake drainage 

(including urbanized areas)

1.3%

Tomahawk 

Thoroughfare

23%

Precipation

Tomahawk 12.5%

Thoroughfare

41.4%

Precipitation

69%
367 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 19. Monthly accuracy of the water budget based on comparison of total water inputs and outputs plus 

storage change for Minocqua and Kawaguesaga Lakes, Wisconsin, November 2005 to October 2007.

Table 6. Summary of phosphorus-budget components for Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, for 

monitoring years 2006–07, during average hydrologic conditions, and with future development in the watershed.

[Abbreviations: MY, monitoring year November–October]

Budget component MY 2006 MY 2007 2-year

average

Percent of 

2-year total

Average 

hydrology

2030 buildout

 MY 2006–07 

hydrology

Average 

hydrology

Inputs to lake (pounds)

Precipitation

Minocqua Lake 177 189 183 6.5 218 183 218

Kawaguesaga Lake 92 99 96 3.4 114 96 114

Tributaries

Minocqua Thoroughfare (inlet) 1,460 720 1,090 38.7 1,460 1,090 1,460

Tomahawk Thoroughfare (inlet) 995 442 719 25.5 1,220 719 1,220

Near-lake drainage

(including urbanized areas)

Minocqua Lake

East Basin 132 155 144 5.1 160 227 2

Northwest Basin 88 106 97 3.4 110 130 1

Southwest Basin 41 51 46 1.6 55 61 

Kawaguesaga Lake 33 46 40 1.4 45 64 67

Groundwater

Minocqua Lake 108 82 95 3.4 106 95 106

Kawaguesaga Lake 17 13 15 0.5 18 15 18

Septic systems

Minocqua Lake 183 183 183 6.5 183 183 183

Kawaguesaga Lake 113 113 113 4.0 113 186 186

TOTAL INPUT 3,440 2,200 2,820 100.0 3,800 3,050 4,010

Outputs from lake and storage (pounds)

Retained in lake * 1,910 1,320 1,610 57.2

Tomahawk River (outlet) 1,530 878 1,210 42.7

TOTAL OUTPUT AND STORAGE 3,440 2,200 2,820 100.0

*

 Assumed that groundwater output was negligible from lake and was included with storage.

0

1,000

2,000

3,000

4,000

5,000

6,000

NOV. JAN. MAR. MAY JULY SEP. NOV. JAN. MAR. MAY JULY SEP.

Inputs

Outputs plus storage change

VOLUME, IN ACRE-FEET

2005 2006 2007

EXPLANATION
Sources of Phosphorus 37

Surface-Water Inflow 

Phosphorus loads were computed for the continuously 

monitored sites at the Minocqua Thoroughfare inlet and the 

Tomahawk Thoroughfare inlet to Minocqua Lake, and at the 

Kawaguesaga Lake outlet to the Tomahawk River. Phosphorus 

loads were estimated for the ungaged near-lake drainage areas 

for each basin in Minocqua and Kawaguesaga Lakes.

Gaged Inlets

Phosphorus in Minocqua Thoroughfare inlet at 

State Highway 47 (fig. 5) originates from the upper 

watershed, including four upstream lakes. Total phosphorus 

concentrations for 42 samples collected during the 2-year 

study ranged from 0.019 to 0.077 mg/L (fig. 13). The median 

and flow-weighted-average concentrations were 0.036 and 

0.038 mg/L, respectively. On average, about 50 percent of 

the phosphorus was in the dissolved form, based on seven 

samples. The total quantity of phosphorus delivered from 

Minocqua Thoroughfare was 1,460 lb in MY 2006 and 

720 lb in MY 2007 (table 6). Yields from the Minocqua 

Thoroughfare watershed were 52.2 lb/mi2 in MY 2006 and 

25.7 lb/mi2 in MY 2007. These yields are among the lowest 

of rural watersheds that were monitored in Wisconsin and 

similar to that measured for the Bear River near Manitowish 

Waters in the Northern Lakes and Forests ecoregion (Corsi 

and others, 1997). The yields are likely lower than long-term 

average yields because the two study years were characterized 

by below-average precipitation and runoff. Yields are also 

expected to be low from this watershed because of the 

trapping effect of upstream lakes.

The WDNR fish hatchery was not an important point 

source of phosphorus. Most of the effluent is pumped into 

a settling pond and then into an infiltration pond before the 

overflow is discharged into the Minocqua Thoroughfare. 

Discharge from the hatchery ranged from about 0.1 to 

2.0 ft3/s, and phosphorus concentrations ranged from 0.027 

to 0.068 mg/L (S. Ohm, Wisconsin Department of Natural 

Resources, written commun., 2007). With few exceptions, 

phosphorus concentrations of the hatchery discharge were 

near or less than the stream concentrations measured at the 

gaging station at State Highway 47; therefore, the hatchery 

is no longer an important source of phosphorus loading to 

the lakes. Any loading from the hatchery would be included 

in the total quantity of phosphorus delivered from Minocqua 

Thoroughfare measured at the gaging station.

Phosphorus in Tomahawk Thoroughfare at the gaging 

station originates from the upstream watershed that includes 

3,400-acre Tomahawk Lake. Total phosphorus concentrations 

for 40 samples collected during the 2-year study ranged from 

0.011 to 0.040 mg/L (fig. 13). The median and flow-weightedaverage concentrations of total phosphorus were 0.018 and 

0.017 mg/L, respectively. On average, about 40 percent of 

the phosphorus was in the dissolved form, based on seven 

samples. The total quantity of phosphorus measured at 

Tomahawk Thoroughfare at Thoroughfare Road was 570 lb in 

MY 2006 and 200 lb in MY 2007.

Estimated phosphorus loading from the additional 

Mid Lake drainage area was added to the measured load 

at Thoroughfare Road. Phosphorus input from Mid Lake 

and the intervening area between the mouth of Tomahawk 

Thoroughfare and the gaging station at Thoroughfare 

Road was estimated by multiplying the estimated flow by 

the estimated average phosphorus concentration from the 

intervening area. The average phosphorus concentration from 

the intervening area was determined on the basis of nine 

samples collected at the mouth, which had a higher average 

concentration of 0.029 mg/L, compared with an average 

of 0.017 mg/L measured at the gaging station. In order to 

increase the average concentration by 0.012 mg/L and only 

increase the water volume by 13 to 25 percent requires the 

average phosphorus concentration for the water added in this 

area to be 0.109 mg/L. The total quantity of phosphorus added 

in this intervening area was 425 lb in MY 2006 and 242 lb in 

MY 2007. The intervening channel area and Mid Lake were 

important sources of phosphorus.

The total quantity of phosphorus delivered from 

Tomahawk Thoroughfare at the inlet to Minocqua Lake was 

995 lb in MY 2006 and 442 lb in MY 2007 (table 6); on 

average, about 50 percent of this total load was contributed 

by the Mid Lake intervening area. The flow-weightedaverage concentration of total phosphorus at the mouth 

was 0.027 mg/L. Yields from the Tomahawk Thoroughfare 

watershed were 32.2 lb/mi2 in MY 2006 and 14.3 lb/mi2 in 

MY 2007; these are very low yields, reflecting the nature of 

the watershed (size of Tomahawk Lake and low watershed/

lake area ratio) and the dry period during the study.

Urban and Other Ungaged Near-Lake Drainage 

Areas

Phosphorus loading to the lakes from the near-lake 

drainage was divided into contributions to the East, Northwest, 

and Southwest Basins, which contain the urbanized and 

residential land-use areas, and that for Kawaguesaga Lake.

During the 2-year study, 16 water samples were obtained 

from stormwater outfalls in the urban area of the Island. 

Total phosphorus concentrations in these samples ranged 

from 0.031 to 0.281 mg/L, with an average concentration 

of 0.113 mg/L. The WDNR had collected stormwater grab 

samples at five sites in 1991 that ranged from 0.1 to 1.6 mg/L, 

averaging 0.49 mg/L (Blake, 1996). Water and phosphorus 

loads from the near-lake drainage areas were simulated 

with WinSLAMM. WinSLAMM was applied with all of the 

standard phosphorus concentrations except those for forested 

areas: which were obtained from Graczyk and others (2003). 

Average flow-weighted concentrations of total phosphorus for 

each basin resulting from the modeling were 0.44 mg/L (East 

Basin), 0.38 mg/L (Northwest Basin), 0.46 mg/L (Southwest 

Basin), and 0.70 mg/L (Kawaguesaga contributing area). 
389 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

On the basis of the WinSLAMM model that included the 

drainage controls that exist in the watershed, the estimated 

load of phosphorus from the near-lake drainage area of the 

East Basin was 132 lb in MY 2006 and 155 lb in MY 2007; 

from the near-lake drainage area of the Northwest Basin was 

88 lb in MY 2006 and 106 lb in MY 2007; and the near-lake 

drainage area of the Southwest Basin was 41 lb in MY 2006 

and 51 lb in MY 2007 (table 6). The average annual yields 

and flow-weighted average total phosphorus concentration 

were highest from the more urbanized areas in watersheds 

of the Northwest and Southwest Basins (30 and 20 percent 

urban land use, respectively): about 165 lb/mi2 and 0.40 mg/L, 

respectively. The median yield from monitored urban 

watersheds reported by Corsi and others (1997) was 318 lb/

mi2, with a range from 133 to 1,210 lb/mi2. The load estimated 

by WinSLAMM was near the lower end of this range but 

considered reasonable. The estimated load of phosphorus by 

WinSLAMM from the contributing areas of the near-lake 

drainage area of Kawaguesaga Lake was 33 to 46 lb/yr, a low 

yield resulting from the small amount of runoff produced by 

the sandy soils from the 2.5-mi2 contributing area during the 

2 years of below-average rainfall. The contributing area to 

Kawaguesaga Lake also has a high percentage of wetlands, 

which is not included in loading estimates by WinSLAMM. 

For comparison, if unit-area loads similar to those measured at 

the Bear River and Tomahawk Thoroughfare gaging stations 

were applied (about 20 lb/mi2 phosphorus in MY 2006 and 

15 lb/mi2 in MY 2007), the resulting estimated load from 

the Kawaguesaga Lake contributing area would be about 

49 lb in MY 2006 and 37 lb in MY 2007. The total estimated 

phosphorus load from the contributing areas of the entire nearlake drainage was 294 lb in MY 2006 and 358 lb in MY 2007, 

for an average annual yield of about 71 lb/mi2. 

Groundwater

Estimates of loading from natural background 

groundwater inflow and contributions from septic systems 

into the groundwater were made separately. To determine 

the natural groundwater contributions, piezometers were 

installed around the lakeshore and sampled to determine 

the concentrations of dissolved phosphorus in groundwater 

entering the lakes. Average dissolved phosphorus 

concentrations in piezometers ranged from less than 0.007 

to 0.098 mg/L (table 7). Phosphorus concentrations in some 

piezometers appeared elevated above background levels 

and likely affected by septic-tank effluent and urban or 

wetland sources. Phosphorus concentrations in 3 out of the 

14 piezometers (piezometers 6, 7, and 13) were much greater 

than concentrations in the other piezometers. Hence, it was 

assumed that these piezometers may have been influenced by a 

local factor, such as septic-system effluent, and they were not 

used to estimate background concentrations. Concentrations 

from the remaining piezometers, 0.008 to 0.027 mg/L, 

were used for different shoreline segments to estimate the 

phosphorus load from natural groundwater inflow for the 

areas where gradients were into the lake (fig. 17). The annual 

phosphorus load contributed by groundwater was estimated 

to be 108 lb in MY 2006 and 82 lb in MY 2007 for Minocqua 

Lake and 17 lb in MY 2006 and 13 lb in MY 2007 for 

Kawaguesaga Lake (table 6).

Septic Systems

Lakeshore septic systems may represent an important 

source of phosphorus to some lakes. Phosphorus loading from 

nearshore septic systems to Minocqua and Kawaguesaga 

Lakes was estimated separately from background groundwater 

inflow by applying per capita export coefficients from 

households to onsite septic systems. Input from septic 

systems was only determined for areas where groundwater 

enters the lakes. These areas were determined from the 

nearshore piezometers with positive groundwater gradients 

to the lake and the groundwater model. The latest population 

and occupancy information, site-specific soil information, 

and septic-system characteristics were evaluated and used 

in calculations to estimate the quantity of phosphorus from 

near‐lake systems

The input of phosphorus from septic systems (M) was 

estimated by use of equation 6 (Reckhow and others, 1980):

M = Es * (number of capita years) * (1 - SR), (6)

where M is a function of a phosphorus export coefficient, 

Es, a soil retention coefficient, SR, and the number of people 

using the septic systems annually (number of capita years). In 

applying equation 6, it was assumed that the most likely value 

for Es was 1.5 lb of phosphorus per capita per year. Typical 

export coefficients range from about 1 lb per capita per year 

(Reckhow and others, 1980; Panuska and Kreider, 2002) to 

more than 2 lb per capita per year (Garn and others, 1996). 

This approach is described in more detail by Garn and others 

(1996). 

Population and occupancy information were obtained 

from 2000 census data and local sources. About 40 percent 

of the total number of housing units around Minocqua 

and Kawaguesaga Lakes were assumed to be year-round 

residences according to the 2002 Town of Minocqua Land Use 

Plan (Lindemann and others, 1997; A. Faust, North Central 

Wisconsin Regional Planning Commission, written commun., 

2007). Of the remaining seasonal residences, 60 percent were 

assumed to have occupancy during the summer and weekends 

(0.5 year) and 40 percent to be occupied during summer only 

for 0.3 year (J. Beckwith, Minocqua /Kawaguesaga Lakes 

Protection Association, written commun., 2007). The average 

number of persons per household for Minocqua was assumed 

to be 2.4 on the basis of 2000 Oneida County census data 

(ePodunk, Inc., 2007).
Sources of Phosphorus 39

All households and facilities around the lakes are not 

likely to affect the lakes because part of the area and shoreline, 

including the Island, is served by sanitary sewer (fig. 20), and 

part of the shoreline of Kawaguesaga Lake has groundwater 

gradients that are away from the lake (fig. 17). In this study, 

the number of residences potentially affecting the lakes 

was determined for the area within 250 ft of the unsewered 

portion of the lakeshore and having groundwater gradients 

into the lakes. Residences with septic drain fields greater than 

250 ft from the shoreline were assumed to have no effect on 

the lakes. The number of parcels of land with houses within 

this 250-ft zone was determined from aerial photos and land 

records available from Oneida County Department of Planning 

and Zoning (2007). The total number of residences in this 

250‐ft zone was 235 (145 seasonal) for Minocqua Lake and

169 (101 seasonal) for Kawaguesaga Lake. Heavy seasonal 

visitor use at the Minocqua Country Club on Minocqua 

Lake and at Camp Kawaga on Kawaguesaga Lake was also 

included. Number of capita-years was then calculated by 

multiplying the number of residences by number of persons 

per household by fraction of the year occupied. The Northwest 

and Southwest Basins of Minocqua Lake have few residences 

with onsite septic systems because much of that shoreline is 

served by sewer. The west shoreline of Kawaguesaga Lake 

that is in a neutral or transition area having groundwater 

outflow part of the time was assumed to contribute septic input 

50 percent of the time.

A soil has a finite phosphorus adsorption capacity, which 

may be reduced by drain-field effluent over time; hence, new 

systems provide more soil retention than old systems do. The 

septic survey by Lindemann and others (1997) found that 

about 58 percent of the systems around Minocqua Lake were 

installed during the 1970s or earlier (42 percent in the 1980s 

or more recent)—an age near or exceeding the normal life 

expectancy of the system. About 15 percent of the systems 

were found to be failing. In addition to age, maintenance of 

septic systems also influences the effectiveness of the system. 

If scum and sludge are allowed to build up in the tank to a 

point where solids are carried out into the drain field, soil 

spaces may become plugged and result in system failure. 

Septic-system failures and water contamination are most 

likely to occur in shallow soils with high or perched water 

tables, in shallow soils over bedrock, on steep slopes that 

allow effluent to surface, and in very coarse textured soils low 

in organic matter that have high permeability rates and low 

phosphorus‐retention capacity

Soils of the nearshore area consist predominantly of Vilas 

loamy sand, Croswell sand and loamy sand, Sayner loamy 

sand, and Keweenaw-Vilas or Keweenaw-Sayner Complex 

(Natural Resources Conservation Service, 2006). These soils 

that are underlain by sand and gravel are excessively drained, 

have high subsoil permeabilities, and are classified as a poor 

filter and very limited application for septic-tank absorption 

fields. A SR coefficient of 0.5 was used for these soils, plus an 

Table 7. Concentrations of dissolved phosphorus in groundwater from piezometers 

around Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin.

[All concentrations are given in milligrams per liter (mg/L). Symbols: <, less than; –, not available] 

Piezometer 

number

Date of measurement

Average 

concentration 2006 2007

09-07 07-18 10-19 06-04 07-25 09-19

1 0.022 0.006 0.009 0.005 <0.005 <0.005 <0.009

2 0.009 0.008 0.012 <0.005 <0.005 <0.005 <0.007

3 0.016 0.006 0.014 0.005 <0.005 <0.005 <0.009

4 0.030 0.018 0.019 0.010 0.012 0.012 0.017

5 0.018 0.017 0.013 0.009 0.010 0.020 0.015

6 0.038 0.025 0.025 0.019 0.023 0.021 0.025

7 0.119 0.123 0.072 0.066 0.141 0.068 0.098

8 0.012 0.009 0.018 <0.005 <0.005 <0.005 < 0.009

9 – 0.017 0.020 0.012 0.013 0.014 0.015

10* 0.013 0.017 0.018 0.009 0.006 0.013 0.013

11* 0.014 0.014 0.016 0.010 0.012 0.011 0.013

12* 0.032 0.028 0.015 – – 0.03 0.026

13 – 0.033 0.020 0.026 0.029 0.027 0.027

14 – – 0.015 0.011 0.013 0.015 0.014

15 0.019 0.020 0.028 0.013 0.014 0.015 0.018

*

 Piezometers were in areas with neutral flow or groundwater outflow.
401 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

89°45' 89°42' 

45°54' 45°51'

0 0.5 1 MILE

0 0.5 1 KILOMETER

EXPLANATION

Sewer-service area

Nearshore drainage area boundary

Aerial image from U.S. Department of Agriculture, 2006

Southwest

Basin

Northwest

Basin

East

Basin

Kawaguesaga

Lake

Minocqua

Lake

SR of 0.25 for the fraction of the phosphorus that is removed 

by storage of solids in the septic tank. For septic systems that 

were determined to be failing within the 250-ft zone, SR = 0.20 

was used. Therefore, as with the selection of an Es value, a 

most likely SR value of 0.75 (75 percent retention) was used in 

the calculations for that case, and a low value of 0.60 and high 

value of 0.85 were also evaluated for best-case and worst-case 

estimates. 

On the basis of the above assumptions, the annual 

estimated “most likely” input of phosphorus from near-lake 

septic systems was 183 lb to Minocqua Lake and 113 lb to 

Kawaguesaga Lake (table 6). Because inputs from septic 

systems were not directly measured, low and high estimates 

for septic inputs were obtained by applying high and low 

estimates for Es

 and SR in equation 6. The low and high 

estimates for phosphorus inputs from septic systems were 77 

to 345 lb into Minocqua and 45 to 220 lb into Kawaguesaga 

Lake, respectively. Oneida County has a septic-system 

inspection (repeated at 3-year intervals), pumping, and 

corrective-action program; but this program applies only to 

those systems constructed after July 1, 1980, to identify those 

systems in need of replacement or repairs (K. Jennrich, Oneida 

County Planning and Zoning Department, oral commun., 

2009). 

Surface-Water Outflow 

Phosphorus is discharged from Kawaguesaga Lake 

into the Tomahawk River through the Minocqua Dam. Total 

phosphorus concentrations in the 40 samples collected at the 

outlet during the 2-year study period ranged from 0.012 to 

0.034 mg/L, and the flow-weighted-average concentration was 

0.019 mg/L. The total phosphorus output from the lakes to the 

Tomahawk River was 1,530 lb in MY 2006 and 878 lb in MY 

2007 (table 6).

Phosphorus-Budget Summary

The average annual load of phosphorus to Minocqua 

and Kawaguesaga Lakes during MY 2006 and 2007 was 

2,820 lb, of which 1,610 lb (57 percent) was retained within 

Figure 20. Area served by sanitary sewer within the Lakeland Sanitary District No. 1, Minocqua, Wisconsin.
Sources of Phosphorus 41

the lake system (table 6). The total load was 3,440 lb in 

MY 2006 and 2,200 lb in MY 2007. The largest external 

source of phosphorus entering the lakes was from Minocqua 

Thoroughfare, which delivered about 39 percent of the 

total load (fig. 21) compared with about 38 percent of the 

water input (fig. 18). The next largest contribution was from 

Tomahawk Thoroughfare, which delivered about 26 percent 

of the total phosphorus load (34 percent of the water input). 

The ungaged near-lake area containing the town and rural 

residential areas contributed 11.5 percent of the total load, 

compared with only about 1 percent of the water input. 

Septic systems accounted for 10.5 percent of the total load. 

Contributions from precipitation and groundwater accounted 

for 9.9 and 3.9 percent of the total load, respectively. If the 

high estimates for phosphorus contributions from septic 

systems (345 lb to Minocqua Lake and 220 lb to Kawaguesaga 

Lake) were used in the budget, septic systems would increase 

to about 18 percent of the total external phosphorus load. 

Phosphorus loading in MY 2006 was about 1.6 times that in 

MY 2007 because the continuing drought during 2007 reduced 

stream and groundwater inputs, even though precipitation was 

greater in 2007.

The average annual surface-water discharge of 

phosphorus to the Tomahawk River from the lakes was 

1,210 lb: 1,530 lb in MY 2006 and 878 lb in MY 2007. 

Therefore, on average, about 43 percent of the total 

phosphorus load to the lakes is lost through the Minocqua 

Dam. More than 50 percent of the external phosphorus 

entering the lakes is retained in the lake sediments, part of 

which may be potentially released to the water column in the 

future. Lakes normally accumulate phosphorus in their bottom 

sediment; therefore, phosphorus inputs exceed phosphorus 

losses in most lakes (Graneli, 1999). A prolonged period with 

phosphorus exports exceeding inputs would indicate that there 

has been a reduction in phosphorus inputs from the watershed 

and that phosphorus concentrations in the lakes should be 

decreasing. 

The phosphorus loading documented in MY 2006–07 

represents what occurs during a series of dry years. To 

determine how the phosphorus loading to the lakes would 

change under more typical or average hydrologic conditions, 

loads were calculated for each component of the phosphorus 

budget (tables 5 and 6) on the basis of long-term average 

precipitation and flow values (described earlier) and with 

the same phosphorus concentrations as estimated for MY 

2006–07. Loads for the near-lake drainage areas were 

computed with WinSLAMM, with flows estimated by use 

of average precipitation. No adjustments were made to 

septic‐system inputs. Thus, for average hydrologic conditions,

the total annual phosphorus load to the lakes is estimated to 

increase to about 3,800 lb (table 6).

The total load of phosphorus to the lakes in MY 2006 

and MY 2007 and that estimated for more typical hydrologic 

conditions is probably less than it was prior to 1965, when 

sewage effluent was discharged into Minocqua Lake. After 

1965, however, the loading to the lakes may have gradually 

increased because of development in the watershed. The 

WDNR estimated that the total external load of phosphorus 

to Minocqua Lake during 1991 was about 4,500 lb, with 

1,480 lb (33 percent) contributed by the two inlets, 1,050 lb 

(23 percent) by precipitation, 760 lb (17 percent) by urban and 

residential development from the Town of Minocqua, 700 lb 

(15 percent) by groundwater, and 335 lb (7 percent) from 

septic systems (Blake, 1996). This total estimate is more than 

1.5 times that in MY 2006–07 and about 1.2 times estimated 

for typical or average hydrologic conditions. The 1991 loading 

estimates for individual sources to Minocqua Lake differ 

greatly from those estimated in this study: precipitation, 

1,050 lb in 1991 compared to 183 lb in MY 2006–07 and 

218 lb with average hydrology; groundwater, 700 lb in 1991 

compared to 95 lb in MY 2006–07 and average hydrology; 

and urban development, 760 lb in 1991 compared to 287 lb in 

MY 2006–07 and 325 lb with average hydrology. It is believed 

that the values found in this study are more accurate than 

those estimated for 1991 (Blake, 1996) because of additional 

atmospheric deposition studies (Robertson and others, 2009), 

and the additional groundwater measurements and modeling 

(groundwater and urban) done as part of this study. Input from 

precipitation, groundwater, and urban areas should not have 

changed significantly over the past 20 years; they may have 

possibly even increased. Loading from urban and residential 

development may have increased over the past recent years, 

although inputs from septic systems may have decreased by 

expansion of the sewered area. Therefore, the phosphorus 

loading to the lakes should not have decreased since 1991. 

The refined budget from this study provides a different 

interpretation of the relative importance of human-related 

sources of phosphorus than would be inferred from the 1991 

estimates (Blake, 1996).

According to information in the Towns of Minocqua and 

Woodruff Zoning Maps and Land Use Plan developed by the 

North Central Wisconsin Regional Planning Commission, 

much of the watershed around the lakes may be further 

developed in the near future. Future residential and urban/

commercial development throughout the watershed may affect 

the phosphorus loading and potentially the water quality of 

the lakes. Therefore, the loading for this future condition was 

estimated. In the absence of estimated acreages for specific 

land uses in the future, it was assumed that by 2030 (referred 

to as “2030 buildout”) there would be an additional 506 acres 

of residential and urban development (a growth of 60 percent 

in each land-use category was assumed). Of the 506 acres, 

134 acres would be in the Kawaguesaga Lake contributing 

watershed area, and 372 acres would be in the Minocqua Lake 

contributing area. For the Kawaguesaga Lake watershed, 

forested acres would be converted to an additional 124 acres 

of low-density and 10 acres of high-density residential land. 
423 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 21. Phosphorus budgets for Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, 

combined, for monitoring years 2006 and 2007. (%, percent).

Monitoring year November 2005–October 2006

PHOSPHORUS INPUTS, MY 2006 

[3,440 pounds]

PHOSPHORUS OUTPUTS, MY 2006 

PHOSPHORUS INPUTS, MY 2007 PHOSPHORUS OUTPUTS, MY 2007 

[2,200 pounds]

Monitoring year November 2006–October 2007

Groundwater

4%

Groundwater 

4.3%

Precipitation

8%

Minocqua

Thoroughfare

42%

Retained 

in lakes

55%

Retained 

in lakes

60%

Outlet

45%

Outlet

40%

Minocqua

Thoroughfare

32.7%

Tomahawk 

Thoroughfare

29%

Tomahawk 

Thoroughfare

39.4%

Septic

9%

Precipitation

13.1%

Septic

13.5%

Near-lake drainage 

(including urbanized areas)

16.3%

Near-lake drainage 

(including urbanized areas)

8%
Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading 43

This development was assumed to take place on lakeshore 

properties and increase septic-system inputs to 186 lb 

(table 6). For the Minocqua Lake watershed, forested acres 

would be converted to an additional 234 acres of low-density 

residential (151 acres in the area around the East Basin, 

41 acres in the area around the Northwest Basin, and 42 acres 

in the area around Southwest Basin) and 138 acres of urban/

commercial land (51 acres in the area around East Basin, 

57 acres in the area around Northwest Basin, and 30 acres in 

the area around Southwest Basin). These developments on 

Minocqua Lake were assumed to be in sewered areas or away 

from the lake and not to affect septic-system inputs. 

With changes in land use for the 2030 buildout without 

additional stormwater controls and hydrology similar to MY 

2006–07, the total annual phosphorus load from the nearshore 

areas of the lakes was estimated with WinSLAMM to increase 

by about 160 lb (or an overall increase in loading of about 

5 percent). The total loading to the lakes would increase 

from 2,820 lb to 3,050 lb (table 6). If average hydrology is 

used, the total loading to the lakes would increase by 150 lb 

(from 3,800 to 4,010 lb) or an overall increase in loading 

of about 4 percent. Another possible future condition that 

was examined assumed the changes in phosphorus loading 

associated with 2030 buildout occurred, but with stormwater 

and nonpoint controls added in subwatersheds to achieve 

a 50-percent reduction in loading from these areas. With 

these changes, the annual phosphorus loading is estimated 

to slightly decrease to about 2,810 lb for hydrology similar 

to MY 2006–07 and decrease to about 3,760 lb with average 

hydrology (slightly less than current conditions).

Simulated Changes in Water Quality in 

Response to Changes in Phosphorus 

Loading 

Empirical models that relate phosphorus loading 

to measured water-quality characteristics can be used to 

determine how changes in phosphorus loading to Minocqua 

and Kawaguesaga Lakes, including that from urban sources, 

could either improve or degrade the water quality of the lakes. 

These models were developed on the basis of comparisons of 

hydrologic and phosphorus loading estimates determined for 

many different lake systems with specific measures describing 

lake water quality, such as near-surface phosphorus and 

chlorophyll a concentrations and Secchi depth. Some of these 

empirical models are contained within the BATHTUB model 

(Walker, 1996). The BATHTUB model includes hydrologic 

transport algorithms and is, therefore, capable of simulating 

changes in water quality in complex lakes (multibasin lakes 

or lakes connected by short channels). To predict changes 

in the water quality of Minocqua and Kawaguesaga Lakes 

that should occur with changes in phosphorus loading, 

the BATHTUB model was applied to the three basins of 

Minocqua Lake and the main basin of Kawaguesaga Lake. 

Modeling Approach

To estimate the expected changes in water quality in 

Minocqua and Kawaguesaga Lakes in response to changes 

in phosphorus loading, 19 scenarios were simulated with the 

BATHTUB model (table 8). Scenario 1 simulated the average 

conditions for MY 2006–07 and was used to calibrate the 

BATHTUB model and establish a base case to which other 

simulations can be compared. Three basinwide scenarios 

(2–4) simulated decreases in controllable external phosphorus 

loading by 75, 50, and 25 percent, and six scenarios (5–10) 

simulated increases in controllable external phosphorus 

loading by 25, 50, 75, 100, 150, and 200 percent. Controllable 

external loading includes phosphorus from all of the tributaries 

and nearshore areas around the lakes and input from septic 

systems. Scenarios 11–14 simulate possible anthropogenic 

changes in the near-lake watershed with hydrology similar to 

that of MY 2006–07: 

• scenario 11, projected development in the near-lake 

watershed (2030 buildout); 

• scenario 12, projected 2030 buildout with controls;

• scenario 13, removing septic effluent from around both 

lakes; and

• scenario 14, removing septic effluent from just around 

Minocqua Lake. 

Because MY 2006–07 were dry years, scenarios 15–19 are 

similar to scenarios 1 and 11–14, respectively, but simulated 

how the anthropogenic modeling results would change if 

the simulations were performed with long-term average 

hydrologic conditions. Scenario 1 (MY 2006-07 hydrology) 

and scenario 15 (average hydrology) establish base cases to 

compare the other simulations under two different hydrologic 

regimes.

Data Requirements

Four types of data are required as input into BATHTUB: 

morphometric data (table 1), water-quality data (tables 3

and 4), hydrologic data (table 5), and nutrient-loading data 

(table 6). In addition to total phosphorus concentrations 

(computed by dividing the input of phosphorus by the 

input of water for each source), dissolved orthophosphorus 

concentrations are needed. Dissolved orthophosphorus 

concentrations were obtained by assuming that the dissolved 

form represented 35 percent of the total phosphorus in 

Minocqua and Tomahawk Thoroughfares, 50 percent of 

the phosphorus in nearshore runoff, and 100 percent of the 

phosphorus in septic and groundwater inputs. The time period 

for the hydrologic and nutrient loading used in the BATHTUB 

model is dependent on the phosphorus-turnover ratio (number 

of times the phosphorus mass in the lakes is replaced in a 

specific time period). The model should be run for a period 

that results in a phosphorus-turnover ratio of 2 or larger. 
445 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin Table 8. Predicted near-surface, summer average total phosphorus and chlorophyll a concentrations and Secchi depths in Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, for various scenarios based on simulations with the BATHTUB model. Scenario number Scenario description Total annual external phosphorus loading (pounds) Simulated total phosphorus (milligrams per liter) Simulated chlorophyll a (micrograms per liter) Simulated Secchi depth (feet) East Basin South- west Basin North- west Basin Kawaguesaga East Basin South- west Basin North- west Basin Kawaguesaga East Basin South- west Basin North- west Basin Kawaguesaga General response scenarios

1 Average of MY 2006–07

(base scenario)

2,820 0.015 0.016 0.021 0.017 5.6 6.0 8.6 7.0 10.8 10.5 8.9 9.8

Change in controllable phosphorus loading:

2 75-percent decrease 997 0.009 0.009 0.014 0.011 2.9 3.3 5.7 4.0 14.1 13.5 10.8 12.53 50-percent decrease 1,610 0.011 0.012 0.017 0.013 4.0 4.3 6.8 5.2 12.8 12.1 9.8 11.54 25-percent decrease 2,210 0.013 0.014 0.019 0.015 4.8 5.2 7.7 6.2 11.8 11.5 9.2 10.55 25-percent increase 3,420 0.017 0.017 0.023 0.019 6.2 6.7 9.3 7.8 10.5 10.2 8.2 9.26 50-percent increase 4,040 0.018 0.019 0.025 0.021 6.8 7.3 10.0 8.5 9.8 9.5 7.9 8.97 75-percent increase 4,640 0.020 0.020 0.026 0.022 7.4 7.8 10.6 9.2 9.5 9.2 7.9 8.58 100-percent increase 5,250 0.021 0.022 0.028 0.024 7.9 8.4 11.2 9.7 9.2 8.9 7.5 8.29 150-percent increase 6,470 0.024 0.024 0.030 0.026 8.7 9.3 12.2 10.8 8.5 8.5 7.2 7.5

10 200-percent increase 7,680 0.026 0.027 0.033 0.029 9.5 10.1 13.0 11.7 8.2 7.9 6.9 7.2

Specific scenarios

MY 2006–07 hydrology:

11 Proposed 2030 buildout 3,050 0.016 0.016 0.022 0.019 5.8 6.2 9.1 7.8 10.8 10.5 8.5 9.2

12 Proposed 2030 buildout 

with controls

2,810 0.015 0.016 0.020 0.018 5.5 5.9 8.1 7.4 11.2 10.5 9.2 9.5

13 Septic effluent removed 

around both lakes

2,520 0.014 0.014 0.019 0.015 5.0 5.3 7.8 5.8 11.5 11.2 9.2 10.8

14 Septic effluent removed 

from around Minocqua Lake

2,640 0.014 0.015 0.019 0.017 5.1 5.5 7.9 6.7 11.5 11.2 9.2 10.2

Average hydrology:

15 Base scenario 3,800 0.015 0.015 0.020 0.016 5.5 5.8 8.1 6.6 11.2 10.8 9.2 10.216 Proposed 2030 buildout 4,010 0.015 0.016 0.021 0.017 5.6 6.0 8.7 7.1 10.8 10.5 8.5 9.817 Proposed 2030 buildout 

with controls

3,760 0.015 0.015 0.019 0.017 5.4 5.7 7.6 6.8 11.2 10.8 9.2 9.8

18 Septic effluent removed 

from around both lakes

3,500 0.014 0.014 0.019 0.014 5.1 5.3 7.5 5.7 11.5 11.2 9.5 10.8

19 Septic effluent removed 

from around Minocqua Lake

3,610 0.014 0.014 0.019 0.016 5.1 5.4 7.5 6.3 11.5 11.2 9.5 10.5
Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading 45

Annual simulations resulted in a phosphorus turnover ratio 

of 2 for the lakes as a whole; therefore, the average annual 

loadings in tables 5 and 6 were used in the model. Even 

though loading data are summarized for the entire year, the 

model simulates water quality only for the growing season 

(May through September). 

Phosphorus from internal loading (sediment release) is 

not typically included as a phosphorus source for BATHTUB, 

because most empirical models already incorporate this 

source. Additional internal loading, therefore, should be 

added only when a lake/reservoir has abnormally high internal 

loading (Walker, 1996). It was believed that Northwest Basin, 

being shallower than the other basins and probably polymictic 

(that is, mixing multiple times throughout the summer), has 

abnormally high internal loading; therefore, an additional 

internal loading of 0.075 mg/m2/d was applied in BATHTUB 

simulations, which equates to about 50 pounds per year. It was 

assumed that internal phosphorus loading to the Northwest 

Basin remained constant in all simulations.

Algorithms and Calibration

When applying BATHTUB, dispersion coefficients 

between each pair of basins (exchange of water with 

adjacent upstream basins/lakes) and the specific algorithms 

used to simulate each water-quality characteristic must be 

defined. A dispersion coefficient of 1.0 represents very little 

constriction between adjacent basins, and a value near 0.0 

represents almost no mixing with the upstream adjacent 

basin. Because of the constrictions between the Northwest 

and Southwest Basins and between the Southwest Basin and 

Kawaguesaga Lake, a dispersion coefficient of 0.5 was applied 

between these basins. The dispersion coefficient between 

the other adjacent basins was set to the default value of 1.0. 

The algorithms chosen to simulate total phosphorus and 

chlorophyll a concentrations are listed in table 9; all are the 

default algorithms and have been shown to usually be the best 

models for most lakes and reservoirs (Walker, 1996). Nonalgal 

turbidity factors (or the reduction in clarity caused by factors 

other than algae) were computed within BATHTUB. 

With the average annual loading data from MY 2006–07, 

BATHTUB was used to simulate the summer average water 

quality (May through September) in each basin. The capability 

of the model to simulate changes in the water quality of 

the lakes was evaluated by inputting the morphometry of 

the lakes and the hydrologic and phosphorus loadings into 

the model and simulating near-surface total phosphorus 

and chlorophyll a concentrations and Secchi depth. The 

simulated concentrations and Secchi depth were close to 

those measured in the lakes. This similarity confirms that the 

model is expected to accurately simulate changes in the lakes 

in response to changes in phosphorus loading. Calibration 

coefficients for each water-quality constituent may be applied 

for each lake/basin to improve the accuracy of the model. 

The only calibration coefficients applied in the simulations 

were those for phosphorus in the East Basin (3.0) and in 

Kawaguesaga Lake (0.4). No other calibrations were required. 

The simulated (before and after calibration) and measured 

values for MY 2006–07 are compared in figure 22. The 

coefficient of variation for each measured value was computed 

on the basis of average summer data from 2001 to 2008 to 

provide an estimate of the certainty of the measured data 

(plus and minus 1 standard error around the average value). 

After model calibration, BATHTUB accurately simulated total 

phosphorus, chlorophyll a, and Secchi depth. 

Table 9. Algorithms used within BATHTUB to simulate water quality in the basins in Minocqua 

and Kawaguesaga Lakes, Oneida County, Wisconsin.

[Abbreviations: –, not simulated]

Process Model Number Algorithm description

Total phosphorus 1 Second order, available phosphorus

Nitrogen balance – None

Chlorophyll a concentration 2 Phosphorus, light, and residence time

Secchi depth 1 Chlorophyll a and turbidity

Dispersion 1 Fisher numeric

Phosphorus calibration 1 Decay rates

Nitrogen calibration – None
467 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Figure 22. Measured and simulated (before and after calibration with the BATHTUB model) average 

summer (May through September) near-surface (A) total phosphorus concentrations, (B) chlorophyll a

concentrations, and (C) Secchi depths in the basins of Minocqua and Kawaguesaga Lakes, Oneida 

County, Wisconsin.

 EAST

BASIN

MINOCQUA 

LAKE

NORTHWEST

BASIN

MINOCQUA 

LAKE

SOUTHWEST

BASIN

MINOCQUA 

LAKE

SECCHI DEPTH, IN FEET

15

10

5

0

A

KAWAGUESAGA

LAKE

CHLOROPHYLL 

A CONCENTRATION, 

IN MICROGRAMS PER LITER

12

9

6

3

0

TOTAL PHOSPHORUS CONCENTRATION, 

IN MILLIGRAMS PER LITER 

Measured

Precalibration

Postcalibration

Measured

Precalibration

Postcalibration

0.03

0.02

0.01

0.00

B

C

Measured

Precalibration

Postcalibration
Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading 47

Response in Water Quality to General Changes 

in Phosphorus Loading

To predict the expected response in water quality in 

Minocqua and Kawaguesaga Lakes to watershed-wide 

changes in external phosphorus loading, nine scenarios (2–10) 

were simulated with the BATHTUB model. The volumeweighted-average total and dissolved orthophosphorus 

concentrations (total nutrient load divided by total water 

load) for each of the sources were decreased by 75, 50, 

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

200 percent. Nitrogen concentrations were not altered in the 

various scenarios. Because the lakes were often phosphorus 

limited, altering nitrogen concentrations was expected and 

shown to have little effect on the simulated results. 

Changes in Phosphorus Concentrations

On the basis of BATHTUB model simulations, 

phosphorus concentrations in each basin should have a 

relatively linear response to a linear change in controllable 

external phosphorus loading (fig. 23A and table 8). The 

changes in in-lake phosphorus concentrations, on a percentage 

basis, are smaller than the changes in external phosphorus 

loadings. Changes in in-lake phosphorus concentrations 

are only about 40–50 percent of the changes in phosphorus 

loading. For example, a 50-percent reduction in controllable 

external phosphorus loading causes about a 26-percent 

decrease in in-lake phosphorus concentrations (a decrease 

from 0.015 mg/L to 0.011 mg/L in the Minocqua East Basin); 

a 50-percent increase in external loading causes a 20-percent 

increase in concentrations (from 0.015 to 0.018 mg/L; 

table 8). The estimated percent changes in in-lake phosphorus 

concentrations, being smaller than the percent reductions 

in controllable external phosphorus loadings, are partly 

the result of not simulating reductions in the contributions 

from groundwater and precipitation, which represent about 

15 percent of the total phosphorus input to the lake.

Changes in Chlorophyll a Concentrations

Simulated average summer chlorophyll a concentrations 

have a nonlinear response to a linear change in controllable 

external phosphorus loading (fig. 23B and table 8). The 

response in chlorophyll a concentrations is larger (on a 

percentage basis) for decreases in loading than for similar 

increases in loading. A 50-percent reduction in controllable 

external phosphorus loading in the East Basin results in a 

29-percent reduction in chlorophyll a concentrations (from 5.6 

to 4.0 μg/L), whereas a 50-percent increase in loading results 

in an increase by only 21 percent, and a 200-percent increase 

in loading results in an increase by only 70 percent. The 

estimated percent changes in chlorophyll a concentrations, 

being smaller than the reductions in controllable external 

phosphorus loadings, are again partly the result of not 

simulating reductions in the phosphorus input from groundwater 

and precipitation.

Changes in Secchi Depths (Water Clarity)

Simulated Secchi depths are more responsive to decreases 

in controllable external phosphorus loading than to increases 

in loading (fig. 23C and table 8). When external phosphorus 

loading to the East Basin is decreased by 50 percent, Secchi 

depths increase by 2 ft (18 percent); however, when loading 

is increased by 50 percent, Secchi depths decrease by only 

about 1ft (10 percent). With a 200-percent increase in external 

phosphorus loading, Secchi depths are estimated to decrease by 

2.6 ft (24 percent). 

Changes in Trophic Status

TSI values indicate that Minocqua and Kawaguesaga Lakes 

are typically mesotrophic (TSI values between 40 and 50; fig. 7

and tables 3 and 4). On the basis of the simulations with the 

BATHTUB model, if controllable external phosphorus loading 

could be reduced by 50 percent from that measured in MY 

2006–07, all of the basins in the lakes, except the Northwest 

Basin, would become oligotrophic with respect to phosphorus 

concentration (less than 0.012 mg/L); however, all basins would 

remain mesotrophic with respect to chlorophyll a concentration 

and Secchi depth (fig. 23). If, however, controllable external 

phosphorus loading increased by 150 percent from that 

measured in MY 2006–07, all areas of both lakes would become 

eutrophic with respect to total phosphorus and chlorophyll a

concentrations. 

Importance of Internal Phosphorus Loading

When deep water of productive lakes becomes anaerobic, 

the rate of phosphorus release from the deep sediments is often 

greatly accelerated. Typically, in deep dimictic lakes (lakes that 

remain stratified throughout summer, such as the East Basin), 

phosphorus from internal loading is trapped in the hypolimnion 

during most of the summer and released into the shallow water 

primarily at fall overturn. But in polymictic lakes, such as the 

Northwest Basin, phosphorus released from the sediments 

is not trapped in the bottom of the lake but is intermittently 

released to the shallow water throughout summer. These mixing 

events resulted in near‐surface phosphorus concentrations in

the Northwest Basin increasing as summer progressed, and 

they further resulted in a higher overall near-surface, summer 

average phosphorus concentration than that of the rest of the 

lake. This is the primary reason why the additional internal 

phosphorus load had to be added to Northwest Basin in the 

BATHTUB model for accurate simulations. It was estimated 

that internal loading added about 50 lb of phosphorus during 

summer, or about 23 percent of the annual phosphorus loading 

to the Northwest Basin.
489 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin Figure 23. Simulated changes in near-surface (A) total phosphorus concentrations, (B) chlorophyll a concentrations, and (C) Secchi depths in the basins of Minocqua and Kawaguesaga Lakes, Oneida County, Wisconsin, in response to various phosphorus-loading scenarios. Controlled external loading includes phosphorus from all of the tributaries and nearshore areas around the lakes and input from septic systems. Specific scenarios are described in table 9. Eutrophic Eutrophic Eutrophic Mesotrophic Mesotrophic Mesotrophic Oligotrophic Oligotrophic Oligotrophic 100% 50%100%

50

353025201510

-100 0-50 10050 150 -50 50 150 200

PERCENT CHANGE (EAST BASIN)

TOTAL PHOSPHORUS CONCENTRATION, 

IN MICROGRAMS PER LITER

Phosphorus response

86420

16141210

100 0 100 200

SECCHI DEPTH, IN FEET

SCENARIOS - 

AVERAGE HYDROLOGY

SCENARIOS - 

AVERAGE HYDROLOGY

Secchi depth response

PERCENT CHANGE IN CONTROLLABLE 

PHOSPHORUS LOADINGS

40%20%0

-20%-40%-60%-80%-100%

PERCENT CHANGE (EAST BASIN)

15 16 17 18 19

50%0

--

-

C

A

15 16 17 18 19

-50 50 150 SCENARIOS - 

15 16 17 18 19

AVERAGE HYDROLOGY PERCENT CHANGE IN CONTROLLABLE 

PHOSPHORUS LOADINGS

PERCENT CHANGE IN CONTROLLABLE 

PHOSPHORUS LOADINGS

150%100%50%0

-50%-100%

86420

141210

-100 0 100 200

PERCENT CHANGE (EAST BASIN)

 CONCENTRATION, A CHLOROPHYLL 

IN MICROGRAMS PER LITER

Chlorophyll a response

B

15 Base scenario

16 Proposed 2030 buildout17 Proposed 2030 buildout with controls18 Septic effluent removed from around both lakes19 Septic effluent removed from around Minocqua Lake

East Basin of Minocqua LakeSouthwest Basin of Minocqua LakeNorthwest Basin of Minocqua LakeKawaguesaga Lake

EXPLANATION

Scenarios with average hydrologic conditions
Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading 49

If external phosphorus loading to the Northwest Basin is 

reduced in the future, the results from the BATHTUB model 

(with internal loading included) should accurately predict 

the short-term changes in water quality that would occur in 

this basin. However, if the reduction in external load were 

to persist for many years, the internal phosphorus release 

rate should gradually decrease and come to equilibrium with 

the new external loading. This should ultimately result in a 

slightly greater improvement in water quality than predicted in 

figure 23. 

Response in Water Quality to Specific 

Urbanization/Residential Development 

Scenarios

Responses in water quality to anthropogenic changes in 

the near-lake drainage area were simulated with hydrology 

similar to that for average MY 2006–07 conditions, as well as 

for long-term average hydrologic conditions.

Response Under Average 2006–07 Conditions

Future development of the near-lake drainage area 

around the lakes was estimated and described previously for 

the 2030 buildout condition. To estimate how these changes 

would affect water quality, scenario 11 was used to simulate 

the effects of future changes in phosphorus loading from 

residential and urban/commercial development throughout 

the watershed. With these changes in land use, but without 

new or additional stormwater controls, the total annual 

phosphorus load to the lakes would increase by about 230 lb 

(or an overall increase in loading of about 8.2 percent), from 

about 2,820 lb to about 3,050 lb (tables 6 and 8). Scenario 

12 represents the changes in phosphorus loading associated 

with 2030 buildout but with stormwater and nonpoint controls 

added (implementation of best management practices) in 

subwatersheds to achieve a 50-percent reduction in loading 

from these areas. With these changes, the annual phosphorus 

loading is estimated to decrease to about 2,810 lb (an overall 

decrease in loading of 0.4 percent). 

Future 2030 buildout in the watershed without controls 

(scenario 11) resulted in a slight degradation in water quality: 

phosphorus concentrations increased by about 0.001 mg/L, 

chlorophyll a concentrations increased by about 0.2 to 

0.8 μg/L, and Secchi depths decreased slightly (table 8). The 

largest degradation in water quality was estimated to occur 

in Kawaguesaga Lake, where chlorophyll a concentrations 

were estimated to increase by 0.8 μg/L and Secchi depth 

was estimated to decrease by 0.6 ft. If, however, the buildout 

occurs with additional stormwater and nonpoint controls 

(installation of various best management practices) for a 

50-percent reduction in loading from these near-lake areas 

(scenario 12), it is possible that water quality will change very 

little. In 2009, statewide legislation was passed to include 

additional point and nonpoint controls such as banning the use 

of phosphorus in lawn fertilizer and household dishwasher 

detergents to control phosphorus inputs from these sources 

(Wisconsin Association of Lakes, written commun., 2009). 

Buildout with controls (scenario 12) resulted in a slight 

improvement in water quality in the Northwest Basin because 

of decreases in near-lake loading and slight degradation in 

Kawaguesaga Lake because of increases in loading from 

new septic systems. In the Northwest Basin, phosphorus 

concentrations decreased by about 0.001 mg/L, chlorophyll a

concentrations decreased by 0.5 μg/L, and Secchi depths 

increased by 0.3 ft. In Kawaguesaga Lake, phosphorus 

concentrations increased by about 0.001 mg/L , chlorophyll a

concentrations increased by 0.4 μg/L, and Secchi depths 

decreased by 0.3 ft.

If it were possible to completely remove the phosphorus 

loading from septic systems around both lakes (scenario 13), 

the total phosphorus loading to the lakes would decrease from 

2,820 lb to 2,520 lb: a reduction of about 300 lb or a reduction 

of total loading of 10 percent. This reduction in phosphorus 

loading is expected to improve water quality: phosphorus 

concentrations decreased by about 0.001–0.002 mg/L, 

chlorophyll a concentrations decreased by 0.6–1.2 μg/L, 

and Secchi depths increased by 0.7 to 1.0 ft (table 8). The 

largest improvements in water quality are expected to occur 

in Kawaguesaga and the Northwest and Southwest Basins of 

Minocqua Lake. If it were only possible to remove phosphorus 

loading from septic systems around Minocqua Lake (scenario 

14), the total phosphorus loading to the lakes would decrease 

from 2,820 lb to 2,640 lb: a reduction of about 180 lb or a 

reduction in total loading of about 6 percent. This reduction in 

phosphorus loading is expected to improve the water quality 

in Minocqua Lake. Phosphorus concentrations would decrease 

by about 0.001 mg/L, chlorophyll a concentrations would 

decrease by 0.5–0.7 μg/L, and Secchi depths would increase 

by 0.3 to 0.7 ft (table 8). A slight improvement in water quality 

is also expected to occur in Kawaguesaga Lake because of the 

movement of water from Minocqua Lake into Kawaguesaga 

Lake.

Response Under Average Hydrologic Conditions

Because MY 2006–07 were dry years, scenarios 

15–19 simulated how the modeling results would change 

if the simulations were performed for years with average 

hydrologic conditions. Scenario 15 was used to determine 

what the water quality in the lakes would have been if average 

hydrological conditions had occurred during the study period. 

During average hydrological conditions, the total loading 

to the lakes is estimated to increase from 2,820 lb to about 

3,800 lb (tables 6 and 8: an increase of about 1,000 lb or about 

35 percent). This increase in loading is expected to occur 

with an increase in water input by about 42 percent (table 5). 
501 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Results from scenario 15 indicate that the increased loading of 

water and phosphorus should result in a slight improvement in 

water quality in most basins. Total phosphorus concentrations 

would decrease by less than 0.001 mg/L, chlorophyll a

concentrations would decrease by 0.1–0.5 μg/L, and Secchi 

depths would increase by 0.3–0.4 ft in comparison to the 

MY 2006–07 hydrologic conditions (table 8 and fig. 23). The 

largest improvement in water quality occurs in the Northwest 

Basin, where the relative importance of internal loading 

decreased. It is expected that the general response curves for 

each basin in figure 23 would remain relatively similar but be 

offset by these small changes; therefore, the general response 

curves would still be applicable. 

Scenarios 16–19 simulated the effects of each of the 

anthropogenic modifications to the watershed during average 

hydrologic conditions (only the results for scenarios 15–19 

for the average, more typical, hydrologic conditions are 

shown in fig. 23). The changes in water quality from the 

base scenario with MY 2006–07 hydrology (scenario 1) 

to the base scenario with average hydrologic conditions 

(scenario 15) were found to be the same for each of the other 

anthropogenic modifications (scenarios 16–19). Therefore, 

each anthropogenic modification (2030 buildout or septic 

removal) has similar impacts on water quality as described in 

the previous section whether the hydrology from MY 2006–07 

or average hydrologic conditions is considered. 

Response in Water Quality to a Combination of 

Changes in the Watershed

In the previous section, BATHTUB was used to simulate 

the effects of specific scenarios; however, in reality multiple 

changes can take place in the watershed. Once the additive 

effects of these potential actions are quantified, either with the 

approaches used in this study or with other numerical models, 

the results in figure 23 can be used to evaluate how the lakes 

should respond. For example, if septic effluent from the entire 

near-lake drainages were removed and 2030 buildout occurred 

with additional stormwater and nonpoint controls, then the 

total loading to the lakes should be reduced by approximately 

310 to 340 lbs. This represents a total reduction of about 

10 percent. The general response curves on the left side of 

each graph in figure 23 can then be used to determine how 

these changes affect water quality.

Other changes in the watershed could have much more 

dramatic effects than the scenarios evaluated in table 8 and 

figure 23. For example, if the large internally drained areas in 

figure 2 were modified to drain to the lakes, especially those 

on the Island or in Woodruff, then the effects of urbanization 

would be much larger than previously described. Presently, 

the urban areas represent only about 1 percent of the water 

added to the lakes, but represent about 10–12 percent of the 

phosphorus loading to the lakes. If the internally drained areas 

are removed, additional urbanization could occur and then the 

runoff and the associated phosphorus would drain to the lakes. 

Additional modeling would be needed to determine how this 

would affect the phosphorus loading.

Presently internal loading of phosphorus from the 

sediments appears to have only a small effect on the water 

quality of the lakes during summer, except for the Northwest 

Basin. However, if phosphorus loading to the lakes increased 

and remained higher than presently measured, then the internal 

phosphorus loading from the sediments would also increase. 

This should ultimately result in larger degradations in water 

quality than predicted in figure 23.

Summary and Conclusions

Minocqua and Kawaguesaga Lakes (1,318 and 

690 acres, respectively) are the lowermost lakes of a chain 

in the lakes region of north-central Wisconsin. Minocqua 

Lake receives inflows from two major tributaries: Minocqua 

Thoroughfare and Tomahawk Thoroughfare. Minocqua Lake 

is interconnected with and flows directly into Kawaguesaga 

Lake. Water levels of both lakes are controlled by the 

Minocqua Dam, whose outlet discharges to the Tomahawk 

River. 

Like many northern lake settlements, the two local 

towns, Minocqua and Woodruff, have experienced a boom in 

population and land development. Stormwater runoff from 

the residential/commercial developments and impervious 

surfaces on the Island, as well as septic systems in the 

heavily developed shoreline area of the lakes, are a concern 

as a source of increased nutrient loading to the lakes. The 

long‐term management and protection of these lakes in the

face of increased development and urbanization is a major 

concern to local and state governments. 

To provide a better understanding of the factors that 

affect the water quality of Minocqua and Kawaguesaga Lakes, 

a detailed study of the lakes and their watersheds was done 

in 2006–07 by the USGS in cooperation with the Minocqua/

Kawaguesaga Lakes Protection Association through the Town 

of Minocqua. The purposes of the study were to describe the 

hydrology and water quality of the lakes; quantify the sources 

of phosphorus loading to the lakes, including those associated 

with urban development; and evaluate the effects of future 

changes in phosphorus inputs on the water quality of the lakes. 

Water-quality data were collected in the lakes during 2003 and 

2006–07 and compared with historical data. Major water and 

phosphorus sources were measured directly, and minor sources 

were estimated to construct detailed water and phosphorus 

budgets for the lakes during monitoring years (MY) 2006 

and 2007. The lake water quality model BATHTUB was 

then used to simulate how the water quality of the lakes 

currently responds to nutrient loading and would respond to 

future changes. Results from model simulations will aid in 

decisionmaking for future lake management by predicting how 

the lakes should respond to specific land-development and 

watershed-management scenarios. 
Summary and Conclusions 51

The water quality of Minocqua and Kawaguesaga 

Lakes appears to have improved since 1963, when a new 

sewage-treatment plant was constructed and its discharge was 

bypassed around the lakes, resulting in decreased loading of 

phosphorus to the lakes compared to former times. Since the 

mid-1980s, the water quality of the lakes has changed very 

little. Phosphorus concentrations from 1988 through 1996, 

however, were lower than the long-term average, possibly 

the result of an extended drought that occurred in the late 

1980s. From 1986 to 2007, summer average concentrations 

of near-surface total phosphorus in the Minocqua main East 

Basin fluctuated from 0.009 mg/L to 0.027 mg/L but generally 

remained less than 0.022 mg/L (mesotrophic); the average 

summer concentration was 0.016 mg/L. Total phosphorus 

concentrations in the Southwest Basin generally were similar 

to those of the East Basin, but those of the Northwest Basin 

were generally greater. From 1991 to 2007, near-surface total 

phosphorus in the Northwest Basin ranged from 0.007 to a 

maximum of 0.043 mg/L; the average summer concentration 

was 0.018 mg/L. Nutrient concentrations in Kawaguesaga 

Lake exhibits a similar pattern to that of Minocqua Lake. From 

1991 to 2007, summer average concentrations of near-surface 

total phosphorus have ranged from 0.009 mg/L to 0.026 mg/L; 

the overall average for summer was 0.018 μg/L. Since 1986, 

summer average chlorophyll a concentrations and Secchi 

depths indicate that both lakes generally are mesotrophic but 

occasionally borderline eutrophic, with no long-term trends.

During the 2-year detailed monitoring period, the 

Minocqua Thoroughfare contributed about 38 percent of the 

water inflow, Tomahawk Thoroughfare contributed 34 percent, 

and near-lake surface inflow, precipitation, and groundwater 

contributed about 1, 16, and 11 percent of the total inflow, 

respectively. The average residence time of water in the 

lakes was 1.5 years. Water leaves the lakes primarily through 

the Tomahawk River outlet (83 percent) or by evaporation 

(14 percent), with minor outflow to groundwater. 

Total input of phosphorus to both lakes was 3,440 pounds 

in MY 2006 and 2,200 pounds in MY 2007. The largest 

sources of phosphorus entering the lakes were the Minocqua 

and Tomahawk Thoroughfares, which delivered about 39 and 

26 percent of the total phosphorus input, respectively. A large 

fraction of the input from Tomahawk Thoroughfare originates 

from Mid Lake and the adjacent wetlands from Thoroughfare 

Road to the mouth. The next largest contribution came from 

the near-lake drainage area containing the adjacent urban and 

residential developments, which disproportionately accounted 

for about 12 percent of the total phosphorus input but only 

about 1 percent of the total water input. Septic systems and 

precipitation contributed about 11 and 10 percent of the total 

phosphorus input, whereas groundwater delivered about 

4 percent of the total phosphorus.

 To predict the changes in water quality of Minocqua 

and Kawaguesaga Lakes that should occur with changes in 

phosphorus loading, the BATHTUB model was applied to 

the three basins of Minocqua Lake and the main basin of 

Kawaguesaga Lake. A relatively linear response was found 

between phosphorus concentrations and external phosphorus 

loading. The changes in in-lake phosphorus concentrations, 

on a percentage basis, were smaller than the changes in 

external phosphorus loadings—representing only about 40–50 

percent of the changes in phosphorus loading. Simulated 

average summer chlorophyll a concentrations and Secchi 

depths (water clarity) showed a nonlinear response to a linear 

change in controllable external phosphorus loading (inputs 

from the tributaries and nearshore areas around the lakes and 

input from septic systems). The response of chlorophyll a

concentrations is larger (on a percentage basis) for decreases 

in loading than for similar increases in loading. Simulated 

chlorophyll a concentrations and Secchi depths were more 

responsive to decreases in controllable external phosphorus 

loading than to increases in loading. If controllable external 

phosphorus loading were reduced by 50 percent from that 

measured in MY 2006–07, all of the basins in the lakes 

except the Northwest Basin should become oligotrophic with 

respect to phosphorus concentration; however, all basins 

would still remain mesotrophic with respect to chlorophyll a

concentration and Secchi depth.

Responses in water quality of the lakes to anthropogenic 

changes in the near-lake watershed were simulated for several 

scenarios, including the effects of changes in phosphorus 

loading from residential and urban development—referred 

to as “2030 buildout”—and removal of septic-system inputs. 

The 2030 buildout scenario with existing stormwater controls 

resulted in a slight degradation in water quality: phosphorus 

concentrations increased by about 0.001 mg/L, chlorophyll a

concentrations increased by 0.2 to 0.8 μg/L, and Secchi depths 

decreased slightly. The largest degradation in water quality 

was estimated to occur in Kawaguesaga Lake. If 2030 buildout 

occurred with additional stormwater and nonpoint controls 

added in near-lake subwatersheds to achieve a 50-percent 

reduction in loading from these areas, it is possible that 

water quality may change very little. Buildout with controls 

resulted in a slight improvement in water quality in the 

Northwest Basin because of decreases in nearshore loading 

and a slight degradation in Kawaguesaga Lake because of 

increases in loading from new septic systems. The removal 

of phosphorus loading from septic systems around Minocqua 

Lake is expected to improve the water quality in Minocqua 

Lake. Simulated phosphorus concentrations decreased by 

about 0.001 mg/L, chlorophyll a concentrations decreased 

by 0.5–0.7 μg/L, and Secchi depths increased by 0.3 to 

0.7 ft. If it were possible to remove septic-system inputs 

from around both Minocqua and Kawaguesaga Lakes, even 

greater improvements would be likely: simulated phosphorus 

concentrations decreased by about 0.001–0.002 mg/L, 

chlorophyll a concentrations decreased by 0.6–1.2 μg/L, and 

Secchi depths increased by 0.7 to 1.0 ft for that scenario. 

The largest improvements in water quality were expected to 

occur in Kawaguesaga Lake and the Northwest and Southwest 

basins of Minocqua Lake.
523 Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin

Future development in the Minocqua/Kawaguesaga 

watersheds with existing stormwater controls was shown to 

have a detrimental effect on the water quality of the lakes; 

therefore, management actions to minimize future phosphorus 

input into these lakes are important for maintaining the water 

quality of the lakes. Preservation of environmental corridors 

and open areas will also contribute to protecting the lakes’ 

natural drainage systems and their water quality by reducing 

nonpoint-source pollution and by providing buffers for 

filtration and infiltration. In addition to protecting natural 

drainage systems in urban areas, environmental corridors can 

protect and preserve sensitive natural areas such as wetlands, 

flood plains, steep slopes, and other areas that would impair 

surface or groundwater quality if disturbed or developed. 

Numerous noncontributing (internally drained) areas within 

the watershed, such as the large noncontributing area on the 

Island, are of key importance in minimizing surface runoff 

and nutrient loading to Minocqua and Kawaguesaga Lakes. 

Development scenarios assumed that these noncontributing 

areas were maintained. Future development of these areas that 

included extending or connecting drainage from these areas 

into the lakes would increase the contributing watershed area 

and greatly increase loading to the lakes. Phosphorus loading 

from upstream tributary sources is presently (2007) at very 

low levels; therefore, reductions in loading and improvements 

in water quality may be most likely achieved by decreasing 

inputs from the developed near-lake drainage areas and from 

septic systems around the lakes.

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Publishing support provided by the U.S. Geological Survey

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For more information concerning the research in this report, contact the

 Director, Wisconsin Water Science Center

U.S. Geological Survey

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Middleton, Wisconsin 53562

http://wi.water.usgs.gov/
Garn and others— Hydrology, Water Quality, and Response to Changes in Phosphorus Loading of Minocqua and Kawaguesaga Lakes, Wisconsin—SIR 2010–5196