Long Term Resource Monitoring Program

Special Report

97-S002

Water Quality Characteristics

in Navigation Pool 4

of the Mississippi River,1990

December 1997
LTRMP Special Reports provide Long Term Resource Monitoring Program partners

with scientific and technical support. The opinions and conclusions

in LTRMP Special Reports are those of the author(s) and do not

necessarily reflect those of the Environmental Management Technical Center.

All reports in this series receive anonymous peer review.

Environmental Management

Technical Center

CENTER DIRECTOR

Robert L. Delaney

APPLIED RIVER SCIENCE

ACTING DIRECTOR

Ken Lubinski

PROGRAM OPERATIONS

ACTING DIRECTOR

Linda Leake

REPORT EDITOR

Deborah K. Harris

Cover graphic by Mi Ae Lipe-Butterbrodt

Mention of trade names or commercial products does not constitute endorsement

or recommendation for use by the U.S. Geological Survey, U.S. Department of the Interior.

Printed on recycled paper 
Water Quality Characteristics

in Navigation Pool 4

of the Mississippi River, 1990

by

Robert M. Burdis

Minnesota Department of Natural Resources

Mississippi Monitoring Station

1801 S. Oak Street

Lake City, Minnesota 55041

Project Leader:

Dave Soballe

December 1997

Prepared for

U.S. Geological Survey

Environmental Management Technical Center

575 Lester Avenue

Onalaska, Wisconsin 54650

This PDF file may appear different from the printed report

because of slight variations incurred by electronic transmission.

The substance of the report remains unchanged.
Suggested citation:

Burdis, R. M. 1997. Water quality characteristics in Navigation Pool 4 of the Mississippi River, 1990. U.S. Geological Survey,

Environmental Management Technical Center, Onalaska, Wisconsin, December 1997. LTRMP 97-S002. 25 pp.

Additional copies of this report may be obtained from the National Technical Information Service, 5285 Port Royal Road,

Springfield, VA 22161 (1-800-553-6847 or 703-487-4650). Also available to registered users from the Defense Technical

Information, Center, Attn: Help Desk, 8725 Kingman Road, Suite 0944, Fort Belvoir, VA 22060-6218 (1-800-225-3842 or

703-767-9050).
iii

Contents

Page

Preface ......................................................................... v

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

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

Study Area ...................................................................... 1

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

Results and Discussion ............................................................. 6

Tributaries .................................................................... 7

Lake Pepin ................................................................... 10

Upper Pool 4 ................................................................. 16

Lower Pool 4 ................................................................. 20

Summary ....................................................................... 25

References ...................................................................... 25

Table

Table. Long Term Resource Monitoring Program water quality sampling sites for Mississippi River 

Pool 4 ................................................................... 4

Figures

Figure 1. Location of Long Term Resource Monitoring Program water quality sites in Pool 4 of the

Mississippi River ........................................................ 2

Figure 2. Regression of natural log of Secchi transparency on natural log of turbidity during

open water ............................................................. 5

Figure 3. Regression of natural log of Secchi transparency on natural log of turbidity during

ice cover ............................................................... 5

Figure 4. Comparison of 1990 and 30-year mean monthly ambient temperature for the Red Wing,

Minnesota, area ......................................................... 6

Figure 5. Comparison of 1990 and 30-year mean monthly precipitation for the Red Wing,

Minnesota, area ......................................................... 6

Figure 6. Mean weekly discharge (1990) at Lock and Dams 3 and 4 ......................... 7

Figure 7. Surface current velocity at tributary mouths .................................... 7

Figure 8. Surface dissolved oxygen (DO) at tributary mouths .............................. 8

Figure 9. Surface water temperature at tributary mouths .................................. 8

Figure 10. Surface specific conductance at tributary mouths ................................ 9

Figure 11. Surface nephelometric turbidity at tributary mouths .............................. 9
iv

Figure 12. Secchi transparency at tributary mouths ...................................... 10

Figure 13. Mean surface nephelometric turbidity along three transects in Lake Pepin ............ 11

Figure 14. Mean and range of surface nephelometric turbidity in Lake Pepin (Bay City is treated 

separately) ............................................................ 11

Figure 15. Mean and range of Secchi transparency in Lake Pepin (Bay City is treated separately) ... 12

Figure 16. Dissolved oxygen isopleths for upper Lake Pepin site M781.2O .................... 13

Figure 17. Dissolved oxygen isopleths for middle Lake Pepin site M775.6Q ................... 13

Figure 18. Dissolved oxygen isopleths for lower Lake Pepin site M766.0l ..................... 14

Figure 19. Mean and range of surface dissolved oxygen (DO) in Lake Pepin (Bay City is treated 

separately) ............................................................ 14

Figure 20. Mean and range of surface water temperature in Lake Pepin (Bay City is treated

separately) ............................................................ 15

Figure 21. Mean and range of surface specific conductance in Lake Pepin (Bay City is treated 

separately) ............................................................ 15

Figure 22. Mean surface dissolved oxygen (DO) for each habitat in upper Pool 4 ............... 16

Figure 23. Mean surface water temperature for each habitat in upper Pool 4 ................... 17

Figure 24. Mean surface specific conductance for each habitat in upper Pool 4 ................. 17

Figure 25. Mean surface nephelometric turbidity for each habitat in upper Pool 4 ............... 18

Figure 26. Mean Secchi transparency for each habitat in upper Pool 4 ....................... 19

Figure 27. Mean surface current velocity for each habitat in upper Pool 4 ..................... 19

Figure 28. Mean Secchi transparency for each habitat in lower Pool 4. ....................... 20

Figure 29. Comparison of surface turbidity at unstructured main channel border sites in upper 

and lower Pool 4 ....................................................... 21

Figure 30. Mean surface water temperature for each habitat in lower Pool 4 ................... 22

Figure 31. Mean surface dissolved oxygen (DO) for each habitat in lower Pool 4 ............... 22

Figure 32. Mean surface specific conductance for each habitat in lower Pool 4 ................. 23

Figure 33. Mean surface nephelometric turbidity for each habitat in lower Pool 4 ............... 24

Figure 34. Mean surface current velocity for each habitat in lower Pool 4 ..................... 24
v

Preface

The Long Term Resource Monitoring Program (LTRMP) was authorized under the Water Resources

Development Act of 1986 (Public Law 99-662) as an element of the U.S. Army Corps of Engineers'

Environmental Management Program. The LTRMP is being implemented by the Environmental Management

Technical Center, a U.S. Geological Survey science center, in cooperation with the five Upper Mississippi

River System (UMRS) States of Illinois, Iowa, Minnesota, Missouri, and Wisconsin. The U.S. Army Corps

of Engineers provides guidance and has overall Program responsibility. The mode of operation and respective

roles of the agencies are outlined in a 1988 Memorandum of Agreement.

The UMRS encompasses the commercially navigable reaches of the Upper Mississippi River, as well

as the Illinois River and navigable portions of the Kaskaskia, Black, St. Croix, and Minnesota Rivers.

Congress has declared the UMRS to be both a nationally significant ecosystem and a nationally significant

commercial navigation system. The mission of the LTRMP is to provide decision makers with information

for maintaining the UMRS as a sustainable large river ecosystem given its multiple-use character. The longterm goals of the Program are to understand the system, determine resource trends and effects, develop

management alternatives, manage information, and develop useful products.

This report was prepared under Task 2.2.3.6, Evaluate and Summarize Current Monitoring Results

as specified in Goal 2, Monitor and Evaluate the Condition of the Upper Mississippi River Ecosystem,

of the Operating Plan (USFWS 1993). This report was developed with funding provided by the Long Term

Resource Monitoring Program.
Water Quality Characteristics

in Navigation Pool 4

of the Mississippi River, 1990

by

Robert M. Burdis

Abstract

Resource Trend Analysis water quality monitoring was initiated in Navigation Pool 4 of the Upper Mississippi

River in January 1990 as part of the Long Term Resource Monitoring Program. Water quality sampling was

conducted in different habitats of the Upper Mississippi River System to interpret and predict short-term

variability and long-term trends.

Introduction

Water quality plays a primary role in controlling the flora, fauna, and habitat found in the Upper Mississippi

River System (UMRS). Many biotic and abiotic factors influence water quality, including land use practices,

commercial and recreational navigation, municipal and industrial discharges, navigation control structures, and

vegetation. An understanding of water quality trends and their mechanisms is rudimentary in developing a

better understanding of the ecology of the UMRS.

Study Area

Navigation Pool 4 (Figure 1) of the Mississippi River is 71.1 km (44.2 miles) long, starting at Lock and

Dam 3 about 8.8 km (5.5 miles) upriver of Red Wing, Minnesota, and ending at Lock and Dam 4 at Alma,

Wisconsin. Pool 4 is unique to the Mississippi River in that it contains Lake Pepin, a natural riverine lake

formed by the delta of the Chippewa River. Spanning almost half the distance of the pool, Lake Pepin is 33.4

km (20.8 miles) long, varies in width from about 1.6 to 3.2 km (1 to 2 miles), and has a surface area of 10,282

ha (25,408 acres) and a mean depth of 5.3 m (17.4 feet; Minnesota Pollution Control Agency 1989).

The mouths of three major tributaries empty into Pool 4. The Chippewa River, which has distinct water

quality characteristics from those of the Mississippi, drains a 24,553 km (9,480 square miles) area of

2

Wisconsin and enters the Mississippi River just below Lake Pepin. The Cannon and Vermillion Rivers have

drainage areas in Minnesota of 3,841 and 557 km (1,483 and 215 square miles), respectively. Their

2

confluence enters the Mississippi River about 1.1 km (0.7 mile) below Lock and Dam 3. Although not direct

tributaries of Pool 4, the Minnesota and St. Croix Rivers influence water quality within Pool 4. The Minnesota

River enters the Mississippi River 76 km (47 miles) above Lock and Dam 3, drains a 42,864-km (16,550- 2

square-mile) area in Minnesota, and carries a high suspended sediment load. The St. Croix River enters the

Mississippi River from Wisconsin about 23 km (14 miles) above Lock and Dam 3 and has a drainage area

of 20,098 km (7,760 square miles). 2
2

Figure 1. Location of Long Term Resource Monitoring Program water quality sites in Pool 4 of

the Mississippi River.
3

Methods

Twenty-six water quality sites were established in selected habitats in Navigation Pool 4 of the Mississippi

River (Table). Sites were chosen on the basis of winter accessibility and their representation of habitats found

in the UMRS. Sites were selected along the length of the pool to determine if longitudinal changes in water

quality occur within the same habitats. Where possible, both vegetated and nonvegetated sites were placed

in backwaters to determine the effects of aquatic macrophyte beds on water quality. Three transects were

monitored in Lake Pepin to determine both longitudinal and latitudinal differences within the lake. The mouths

of three major tributaries were also monitored to determine their effects on water quality in Pool 4.

Water quality sites were sampled weekly except for two isolated backwaters that were sampled monthly.

Water quality parameters were measured by techniques and equipment outlined in the Long Term Resource

Monitoring Program (LTRMP) Procedures Manual (Lubinski and Rasmussen 1988). The following

parameters were measured:

Water temperature Dissolved oxygen

Specific conductance Turbidity

Current velocity and direction Secchi transparency

Water depth Wind speed and direction

Wave height Percentage of ice and snow cover

Ice and snow depth

Report codes were used to indicate confidence in the accuracy of the data collected. A report code of 5

was assigned to data believed to be accurate and representative of a particular site. Report codes of

1 through 4 indicate disturbances or equipment malfunctions resulting in data that may not accurately

represent a site. Only data with a report code of 5 were used in the present report.

Dissolved oxygen and temperature were measured with a Yellow Springs Instrument Model 57 dissolved

oxygen meter (Yellow Springs Instrument Co., Inc., Yellow Springs, Ohio). Air calibration was made at the

start and end of each sampling day; additional calibrations were made throughout the day when necessary.

Precision and accuracy were checked monthly against Winkler's method for the determination of dissolved

oxygen (DO). Because of the limitations of the Model 57, dissolved oxygen concentrations of >20.0 mg/L

were arbitrarily given a value of 21.0 mg/L.

Secchi disk transparency was measured in centimeters with a disk mounted on a calibrated pole. A value

of !9 was recorded when the disk could be seen to the bottom. Estimates of Secchi transparencies were later

predicted from a regression of the natural log of turbidity on the natural log of Secchi transparency

(Figures 2 and 3).

Current velocity was measured in meters per second with a Marsh-McBirney Model 201D velocity meter

(Marsh-McBirney, Inc., Frederick, Maryland). An internal calibration check was performed at each site.

Turbidity was measured in nephelometric turbidity units (NTU) with a Hach Model 16800 PortaLab

Turbidimeter (Hach Company, Loveland, Colorado). The meter was field-calibrated at each site to Gelex

standards. Formazine calibrations were performed at least every 3 months, and precision and accuracy were

checked monthly.

Specific conductance was measured in microsiemens (FS) at 25 EC with a LabComp Model SCT-100

conductivity meter (Cole-Palmer, Niles, Illinois). Calibrations were made at least weekly using a 1,000 FS/cm

standard, and precision and accuracy were checked monthly.
4

Table. Long Term Resource Monitoring Program water quality sampling sites for Mississippi River Pool 4.

____________________________________________________________________________________________

Location Location Habitat Depth Sample Vegetation

code name code (m) frequency code a b c

____________________________________________________________________________________________

M796.9N Tailwater TWB 2.44 WK01 OP

M796.7M Tailwater wing dam TWW 2.19 WK01 OP

M795.8L Isolated backwater BWI-O 0.70 MT01 OP

M795.7K Vermillion River TRBM 2.74 WK01 OP

M795.7J Cannon River TRBM 1.75 WK01 OP

M793.9P Channel border unstructured CBU 1.93 WK01 OP

M792.5N Wisconsin channel SCU 1.05 WK01 OP

M790.3F Mud Lake BWC-O 1.09 WK01 OP

M786.1S Bay City Flats IMP-B 1.45 WK01 OP

M781.2X L. Pepin - Wis. side IMP-L 2.20 WK01 OP

M781.2O L. Pepin - middle IMP-L 5.35 WK01 OP

M781.2E L. Pepin - Minn. side IMP-L 7.13 WK01 OP

M775.6Y L. Pepin - Wis. side IMP-L 3.37 WK01 OP

M775.6Q L. Pepin - middle IMP-L 8.48 WK01 OP

M775.6L L. Pepin - Minn. side IMP-L 3.60 WK01 OP

M766.0O L. Pepin - Wis. side IMP-L 2.20 WK01 OP

M766.0I L. Pepin - middle IMP-L 5.83 WK01 OP

M766.0B L. Pepin - Minn. side IMP-L 11.41 WK01 OP

M764.8G Isolated backwater BWI-V 1.02 MT01 FL/SB

M763.5B Chippewa River TRBM 1.31 WK01 OP

M761.5E Channel border unstructured CBU 2.95 WK01 OP

M760.7O Channel border wing dam CBW 1.61 WK01 OP

M758.6X Big Lake BWC-V 1.13 WK01 FL/SB

M758.6Y Big Lake BWC-O 1.61 WK01 OP

M753.2V Peterson Lake IMP-V 1.16 WK01 SB

M753.2S Peterson Lake IMP-O 3.56 WK01 OP

__________________________________________________________________________________________________________

TWB = Tailwater channel border; TWW = Tailwater wing dam; BWC-O = Backwater contiguous—open water; BWC-V = a

Backwater contiguous—vegetated water; BWI-O = Backwater isolated—Open water; BWI-V = Backwater isolated—vegetated

water; CBU = Main channel border unstructured; CBW = Main channel border wing dam; IMP-B = Impounded—bay; IMP-L =

Impounded—lake characteristics; IMP-O = Impounded—open water; IMP-V = Impounded—vegetated water; SCU = Side

channel unstructured border; TRBM = Tributary mouth.

WK01 = Weekly; MT01 = Monthly. b

OP = Open water; FL = Floating-leaf rooted vegetation; SB = Submergent rooted vegetation. c
5

Figure 2. Regression of natural log of Secchi transparency on natural log of turbidity during open

water.

Figure 3. Regression of natural log of Secchi transparency on natural log of turbidity during ice

cover.
6

Figure 4. Comparison of 1990 and 30-year mean monthly ambient temperature for the Red

Wing, Minnesota, area.

Figure 5. Comparison of 1990 and 30-year mean monthly precipitation for the Red Wing,

Minnesota, area.

Results and Discussion

For the sampling period of January through December 1990, temperatures were above normal from January

through March and about normal for the rest of the year (Figure 4). Precipitation was above normal the first

half of the year (particularly in June) and below normal later in the year (Figure 5). Discharge at Lock and

Dams 3 and 4 (Figure 6) peaked during spring runoff and again in June because of heavy rainfall.
7

Figure 6. Mean weekly discharge (1990) at Lock and Dams 3 and 4.

Figure 7. Surface current velocity at tributary mouths.

Tributaries

The Vermillion River receives input from the Mississippi River through sloughs and from a number of

backwater lakes before it empties into Pool 4. In its lower reaches, the Vermillion River has a low gradient

and, consequently, relatively low current velocities (Figure 7) that allowed for some temperature and DO

stratification. The supersaturated DO concentrations observed in the Vermillion River (Figure 8) were due to

high densities of phytoplankton during low discharge. Surface water temperatures tended to be higher than in

other tributaries (Figure 9), which can also be attributed to the lower gradient. Specific conductance

(Figure 10) tended to be slightly higher than that of the main channel of the Mississippi River, indicating some

differences in water quality. Turbidity and water clarity (Figures 11 and 12) did not seem greatly affected by

heavy rainfall in the Vermillion watershed.
8

Figure 8. Surface dissolved oxygen (DO) at tributary mouths.

Figure 9. Surface water temperature at tributary mouths.
9

Figure 10. Surface specific conductance at tributary mouths.

Figure 11. Surface nephelometric turbidity at tributary mouths.
10

Figure 12. Secchi transparency at tributary mouths.

The Cannon River drains an area of steep slopes and highly erodible soils. This river is much more

susceptible to episodic events than the Vermillion River, exhibiting much higher velocity and turbidity during

periods of heavy rainfall (Figures 7 and 11). The Cannon River has a high concentration of dissolved salts and

the highest specific conductance of all sites monitored (Figure 10). In summer, high numbers of blue-green

algae (primarily Aphanizomenon sp.), which may have originated from upstream reservoirs, were at times

observed at the mouth of the Cannon River.

The Chippewa River, the largest tributary of Pool 4, is characterized by low conductivity (Figure 10) and

a high concentration of dissolved organic matter. Although the river transports a large bed load of sand, it does

not typically carry a high concentration of suspended solids; hence, turbidity levels are relatively low (Figure

11). The blue-green algae Microcystis sp. was present in high numbers in summer. During high flows in the

Chippewa River, large areas of lower Pool 4 (particularly Big Lake, a contiguous backwater) are influenced

by this tributary.

Lake Pepin

As the Mississippi River enters Lake Pepin in the middle of Pool 4, the current slows, allowing Lake Pepin

to act as a giant settling basin for suspended material. This settling effect greatly reduces turbidity and

increases water clarity in a downstream direction (Figure 13), resulting in a wide range of turbidity and Secchi

transparency values from upstream to downstream (Figures 14 and 15). Wind and wave action along the

relatively shallow basin, with a maximum fetch of 19 km (11.8 miles), often cause resuspension of sediments

and increased turbidity levels, particularly in the lake's upper end and in shallow bays such as Bay City Flats.

Sediments in most areas of Lake Pepin are typically silt, and turbidity levels near the bottom are often higher

than those at the surface.
11

Figure 13. Mean surface nephelometric turbidity along three transects in Lake Pepin.

Figure 14. Mean and range of surface nephelometric turbidity in Lake Pepin (Bay City is treated

separately).
12

Figure 15. Mean and range of Secchi transparency in Lake Pepin (Bay City is treated separately).

Dissolved oxygen stratification occurred in Lake Pepin during winter and occasionally during summer.

Although complete turnover of the water column occurred only in spring and fall, Lake Pepin fluctuated in

summer between stratified and well mixed depending on wind conditions. This process is shown in dissolved

oxygen isopleths for the middle sites along three transects in Lake Pepin (Figures 16–18). Stratification

occurred less often and to a lesser degree at sites with higher flows along the Minnesota shoreline. During

stratification, when winds blew from one shore to another, warmer, well-oxygenated water from the surface

accumulated on the windward shore while cooler, less-oxygenated water from the hypolimnion predominated

on the leeward shore, resulting in a wide range of surface DO concentrations and temperatures in the lake

(Figures 19 and 20). During winter and summer stratification, supersaturated levels of DO occurred in the

epilimnion because of high algae production. The DO levels in the hypolimnion quickly dropped below 5.0

mg/L during summer stratification (Figures 16–18).

A decrease in specific conductance in Lake Pepin and in other Pool 4 habitats in March and April

(Figure 21) can be attributed to the dilution effect of spring runoff. Conductivity also decreased in fall, but its

causes are uncertain. 

Lake Pepin is a eutrophic lake known for its occasionally large algal blooms. The blue-green algae

Aphanizomenon sp. and Microcystis sp. were observed throughout summer. High densities of the

zooplankton Daphnia sp. were also observed in the lower end of Lake Pepin in early June. 
13

Figure 16. Dissolved oxygen isopleths for upper Lake Pepin site M781.2O.

Figure 17. Dissolved oxygen isopleths for middle Lake Pepin site M775.6Q.
14

Figure 18. Dissolved oxygen isopleths for lower Lake Pepin site M766.0l.

Figure 19. Mean and range of surface dissolved oxygen (DO) in Lake Pepin (Bay City is

treated separately).
15

Figure 20. Mean and range of surface water temperature in Lake Pepin (Bay City is treated

separately).

Figure 21. Mean and range of surface specific conductance in Lake Pepin (Bay City is

treated separately).
16

Figure 22. Mean surface dissolved oxygen (DO) for each habitat in upper Pool 4.

Upper Pool 4

Upper Pool 4 is the section of river running from Lock and Dam 3 to the head of Lake Pepin. The main

channel of upper Pool 4 is virtually ice-free in winter because of warmwater discharge from a nuclear power

plant just upriver of Lock and Dam 3. The backwaters in this stretch of river contain only scattered, sparse

beds of aquatic macrophytes. Because of the lack of vegetation, comparison between vegetated and

nonvegetated sites was not possible.

Habitats monitored in upper Pool 4 included unstructured channel border (non-wing dam sites), tailwater,

tailwater wing dam, side channel, and contiguous and isolated backwaters (Table). The tailwater and side

channel sites were not established until November 1990 and are not included in the analysis.

Dissolved oxygen concentrations and water temperatures were similar among all habitats during most of

the year (Figures 22 and 23). In winter, temperatures tended to be slightly higher in the main channel habitats

because of the thermal effluent from the nuclear power plant; DO concentrations were slightly lower in the

main channel than those in the contiguous backwater. The isolated backwater, which is a small, shallow body

of water fed by the Vermillion River during high flows, was very productive, with high densities of

phytoplankton and supersaturated DO levels.

Specific conductance was also similar among all habitats of upper Pool 4 (Figure 24). As discussed

previously, a decrease in conductivity was observed in spring and fall. The channel border unstructured site

tended to have slightly higher conductivity than the tailwater wing dam and backwater sites, which may be

attributed to this site's location downstream of the confluence of the Cannon and Vermillion Rivers.
17

Figure 23. Mean surface water temperature for each habitat in upper Pool 4.

Figure 24. Mean surface specific conductance for each habitat in upper Pool 4.
18

Figure 25. Mean surface nephelometric turbidity for each habitat in upper Pool 4.

Turbidity was similar among all habitats except on a few occasions (Figure 25). The peak in turbidity

observed in early June at the main channel border and contiguous backwater sites resulted from the load of

suspended sediment transported by the Cannon River. Heavy rainfall in the Cannon River watershed in June

resulted in high flows and extremely turbid conditions in the Cannon River. The elevated turbidity observed

in August through October in the contiguous backwater may be a result of wind and wave action resuspending

sediments from the bottom. In winter, turbidity was lower and water clarity greater (Figure 26) for all habitats.

Current velocity in upper Pool 4 (Figure 27) varied with the amount of discharge entering the pool at Lock

and Dam 3 and, to a lesser extent, from the Cannon and Vermillion Rivers. The tailwater wing dam site was

located just behind a wing dam that is exposed during low flows. During low discharges, an eddy was created

and the current ran parallel to the wing dam at relatively slow velocities compared with those of the main

channel. During high discharges, water flowed over the wing dam, significantly increasing current velocities.

At times, wind may have influenced current velocity in the contiguous backwater, whereas velocity at the main

channel border site was dependent primarily on discharge.
19

Figure 26. Mean Secchi transparency for each habitat in upper Pool 4.

Figure 27. Mean surface current velocity for each habitat in upper Pool 4.
20

Figure 28. Mean Secchi transparency for each habitat in lower Pool 4.

Lower Pool 4

Lower Pool 4 is the section of river running from the lower end of Lake Pepin to Lock and Dam 4. Lower

Pool 4 differs considerably from upper Pool 4. Water clarity is greater (Figure 28) and turbidity levels lower

(Figure 29) in lower Pool 4 because of the settling of suspended material in Lake Pepin. The backwaters in

this stretch of river had abundant aquatic macrophytes as opposed to upper Pool 4, where aquatic vegetation

was sparse. Unlike upper Pool 4, the main channel in lower Pool 4 tends to freeze in early winter. As Lake

Pepin stratifies, warm water from the hypolimnion and a high current velocity caused by the narrowing of the

river at the mouth of Lake Pepin cause the channel to reopen.

Habitats monitored in lower Pool 4 include vegetated and nonvegetated contiguous backwater, vegetated

and nonvegetated impounded area, structured and unstructured channel border, and an isolated backwater

(Table).
21

Figure 29. Comparison of surface turbidity at unstructured main channel border sites in upper

and lower Pool 4.

Water temperature and DO concentrations were similar among all habitats of lower Pool 4 (Figures 30

and 31) with a few exceptions. Supersaturated DO levels occurred at the vegetated site in the impounded

habitat from June through August. The elevated DO levels were caused by a dense bed of macrophytes,

consisting primarily of coontail (Ceratophyllum demersum) and American wildcelery (Vallisneria

americana). The vegetated site in the contiguous backwater, consisting primarily of American wildcelery and

American lotus (Nelumbo lutea), is somewhat of an anomaly in that DO levels in the summer months were

often lower than those at the nonvegetated site about 100 m away. Conductivity differences of 100 FS/cm

occasionally occurred between the two sites. Unlike the vegetated site in the impounded area (which

frequently had saturated and even supersaturated DO levels under the ice), the vegetated backwater site had

DO concentrations below saturation in winter. The site may be affected by nearby springs along the shore or

a wastewater treatment settling pond approximately 2.4 km (1.5 miles) upstream.

Specific conductance measured at the contiguous and isolated backwater sites was consistently lower than

conductance in other habitats of lower Pool 4 (Figure 32). The network of sloughs occurring in the lower

reaches of the Chippewa River influenced both backwaters. During high flows in the Chippewa River, these

backwaters were inundated with low conductivity water from the Chippewa River. The decrease in

conductivity observed in all habitats in August resulted from high flows in the Chippewa River, which affected

most of lower Pool 4. The impounded and main channel habitats had similar conductivity to that found in upper

Pool 4 and Lake Pepin.
22

Figure 30. Mean surface water temperature for each habitat in lower Pool 4.

Figure 31. Mean surface dissolved oxygen (DO) for each habitat in lower Pool 4.
23

Figure 32. Mean surface specific conductance for each habitat in lower Pool 4.

Turbidity in lower Pool 4 was similar and relatively stable among all habitats except in the contiguous

backwater (Figure 33). The contiguous backwater is a large, shallow expanse of water affected by wind and

wave action. The peaks in turbidity that occurred in the backwater in spring and fall resulted from high winds

that resuspended sediments. High flows in June caused turbidity to increase in all habitats of lower Pool 4.

The elevated turbidities were more pronounced in the contiguous backwater because of the unusually high

turbidity in the Chippewa River during this period.

Current velocities increased with increased discharge in all habitats (Figure 34). Current velocity in the

contiguous backwater and impounded habitats was considerably less than that in the main channel habitats,

particularly during high discharge. Current velocity did not differ much between the unstructured and wing

dam habitats of the main channel. Unlike the wing dam in upper Pool 4, which is exposed during low flows,

the wing dam in lower Pool 4 is always submerged and does not have much effect on current velocity.

Current velocities in the main channel of lower Pool 4 tended to be higher than those in the main channel of

the upper pool, probably because of the higher discharge in the lower pool caused by the Chippewa River.
24

Figure 33. Mean surface nephelometric turbidity for each habitat in lower Pool 4.

Figure 34. Mean surface current velocity for each habitat in lower Pool 4.
25

Summary

Perhaps the most salient feature of Pool 4 is Lake Pepin, a eutrophic riverine lake. Lake Pepin acts as a

settling basin for suspended material, consequently improving water clarity in the lower pool. In summer,

stratification occurred intermittently in Lake Pepin, depending on wind conditions. During summer stratification

periods, DO in the hypolimnion decreased to concentrations of 5.0 mg/L and lower. Supersaturated

concentrations of DO caused by phytoplankton blooms were frequently measured under the ice and

occasionally in the epilimnion in summer.

The backwaters of lower Pool 4 had abundant aquatic macrophytes, whereas the aquatic vegetation in

upper Pool 4 backwaters was sparse. Beds of aquatic macrophytes increased dissolved oxygen concentration

and slowed current velocity during peak density.

The flow regime of the Chippewa River influences a large area of lower Pool 4 with its highly stained, lowconductivity water. The Cannon and Vermillion Rivers also have distinct water quality characteristics that

affect areas of upper Pool 4, but to a much lesser degree than the Chippewa River influences the lower pool.

References

Lubinski, K. S., and J. L. Rasmussen. 1988. Procedures Manual of the Long Term Resource Monitoring

Program for the Upper Mississippi River System. U.S. Fish and Wildlife Service, Environmental

Management Technical Center, Onalaska, Wisconsin. EMTC 88-03. 216 pp.

Minnesota Pollution Control Agency. 1989. Review of water quality conditions in Lake Pepin for the summer

of 1988 (Working Paper). Division of Water Quality, St. Paul, Minnesota. 51 pp.

U.S. Fish and Wildlife Service. 1993. Operating Plan for the Upper Mississippi River System Long Term

Resource Monitoring Program. Environmental Management Technical Center, Onalaska, Wisconsin,

Revised September 1993. EMTC 91-P002R. 179 pp. (NTIS # PB94-160199)
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 1997 1990

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Water quality characteristics at selected habitat sites in navigation Pool 4 of the Mississippi River, 1990

6. AUTHOR(S)

Robert M. Burdis

7. PERFORMING ORGANIZATION NAME AND ADDRESS 8. PERFORMING ORGANIZATION

Minnesota Department of Natural Resources

Mississippi Monitoring Station

1801 S. Oak Street

Lake City, Minnesota 55041

 REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

U.S. Geological Survey 

Environmental Management Technical Center 97-S002

575 Lester Avenue

Onalaska, Wisconsin 54650

 AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Release unlimited. Available from National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161 

(1-800-553-6847 or 703-487-4650). Also available to registered users from the Defense Technical Information, Center,

Attn: Help Desk, 8725 Kingman Road, Suite 0944, Fort Belvoir, VA 22060-6218 (1-800-225-3842 or 703-767-9050).

13. ABSTRACT (Maximum 200 words)

Resource Trend Analysis water quality monitoring was initiated in Navigation Pool 4 of the Upper Mississippi River in January 1990 as part of the Long Term

Resource Monitoring Program. Water quality sampling was conducted in different habitats of the Upper Mississippi River System to interpret and predict short-term

variability and long-term trends.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Dissolved oxygen, Environmental Management Program, Lake Pepin, Long Term Resource Monitoring Program, Minnesota, 25 pp.

monitoring, Wisconsin

16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

 OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified
The Long Term Resource Monitoring Program (LTRMP) for the Upper Mississippi

River System was authorized under the Water Resources Development Act of 1986 as 

an element of the Environmental Management Program. The mission of the LTRMP

is to provide river managers with information for maintaining the Upper Mississippi 

River System as a sustainable large river ecosystem given its multiple-use character.

The LTRMP is a cooperative effort by the U.S. Geological Survey, the U.S. Army Corps

of Engineers, and the States of Illinois, Iowa, Minnesota, Missouri, and Wisconsin.