Long Term Resource Monitoring Program

Technical Report

2010–T001

2010

Evaluation of Light Penetration on Navigation 

Pools 8 and 13 of the Upper Mississippi River
Long Term Resource Monitoring Program Technical Reports 

provide Long Term Resource Monitoring Program partners 

with scientific and technical support.

All reports in this series receive anonymous peer review.

Cover graphic by Mi Ae Lipe-Butterbrodt 
Evaluation of Light Penetration on 

Navigation Pools 8 and 13 of the 

Upper Mississippi River 

By Shawn Giblin, Kraig Hoff, Jim Fischer, and Terry Dukerschein

Long Term Resource Monitoring Program

Technical Report 2010–T001 

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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Suggested citation:

Giblin, Shawn, Hoff, Kraig, Fischer, Jim, and Dukerschein, Terry, 2010, Evaluation of light penetration on Navigation 

Pools 8 and 13 of the Upper Mississippi River: U.S. Geological Survey Long Term Resource Monitoring Program Technical Report 2010–T001, 16 p. 
iii

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 

Upper Midwest Environmental Sciences 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 multiuse character. The long-term 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 supports Task 2.2.3 as specified in Goal 2, Monitor Resource Change, of the LTRMP 

Operating Plan (U.S. Fish and Wildlife Service, 1993). This report was developed with funding 

provided by the LTRMP. 
iv

Contents

Preface ...........................................................................................................................................................iii

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

Background.....................................................................................................................................................1

Methods...........................................................................................................................................................2

Results and Discussion.................................................................................................................................2

Conditions During the Study Period...................................................................................................2

Historical Comparisons........................................................................................................................5

Comparisons of Water-Quality Parameters to Light Extinction ....................................................5

Comparison of Results to Proposed Water-Quality Criteria ..........................................................7

Effects of Tributaries on Light Extinction .................................................................................................11

Effects of Composition of Suspended Solids on Light Extinction ........................................................15

References Cited..........................................................................................................................................16 

Figures

1. Map showing Navigation Pool 8 of the Upper Mississippi River .........................................3

2. Map showing Navigation Pool 13 of the Upper Mississippi River .......................................4

3–12. Graphs showing:

3. Relation between light-extinction coefficient and total suspended solids 

from Pools 8 and 13 (pool and tributary sites combined), Upper Mississipppi 

River, May 6 to July 16, 2003 ..............................................................................................7

4. Relation between light-extinction coefficient and transparency tube 

from Pools 8 and 13 (pool and tributary sites combined), Upper Mississipppi 

River, May 6 to July 16, 2003 ..............................................................................................8

5. Relation between light-extinction coefficient and turbidity from Pools 8 and 

13 (pool and tributary sites combined), Upper Mississipppi River, May 6 to 

July 16, 2003..........................................................................................................................8

6. Relation between light-extinction coefficient and Secchi depth from Pools 8 

and 13 (pool and tributary sites combined), Upper Mississipppi River, May 6 to 

July 16, 2003..........................................................................................................................9

7. Relation between 1 percent of surface light and Secchi depth from Pools 8 

and 13 (pool and tributary sites combined), Upper Mississipppi River, May 6 to 

July 16, 2003........................................................................................................................10

8. Relation between 1 percent of surface light and transparency tube from 

Pools 8 and 13 (pool and tributary sites combined), Upper Mississipppi River, 

May 6 to July 16, 2003 .......................................................................................................10

9. Pool 8 light-extinction summary......................................................................................12

10. Pool 13 light-extinction summary....................................................................................12

11. Mississippi River Pool 8 average light-extinction coefficient by transect .............13

12. Mississippi River Pool 8 average turbidity by transect...............................................14 
v

Tables

1. Average discharge for Dams 8 and 13 for 2003 compared to recent averages 

based on U.S. Army Corps of Engineers data ..........................................................................5

2. Average total suspended solids and turbidity for the study period compared to 

1994–2002. The average for 1994–2002 is from Long Term Resource Monitoring 

Program sampling in the main channel during late July and early August of 

each year .......................................................................................................................................5

3. Summary of average water-quality and light-extinction variables for 2003.......................5

4. Regression between light-extinction coefficient (m-1) (dependent variable) and 

water-quality parameters (independent variable). Data are combined for Pool 8 

and tributaries and Pool 13 and tributaries ..............................................................................6

5. Regression between light-extinction coefficient (m-1) (dependent variable) and 

water-quality parameters (independent variable). Data are segregated for Pool 8 

and Pool 13 tributary and main channel ...................................................................................6

6. Relation among water-quality parameters for 2003 study using data combined 

from Pool 8, Pool 13, and tributaries..........................................................................................9

7. Comparison of Upper Mississippi River Conservation Committee recommended 

water-quality criteria to main channel data from Pools 8 and 13 for May 6 to 

July 16, 2003.................................................................................................................................11

8. Comparison of average water-quality characteristics of the Black River upstream 

and downstream of Lake Onalaska compared to the main channel of the 

Mississippi River in Pool 8. Data are from May 6 to July 16 from 2000 to 2005 ................13

9. Average discharge for the Mississippi River and tributaries for 1970–2000 based 

on U.S. Geological Survey data................................................................................................14

10. Comparison of total suspended solids, volatile suspended solids, nonvolatile 

suspended solids, and percentage volatile suspended solids for Pools 8 and 13...........15 

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

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

concentrations of turbidity are given in nephelometric turbidity units (NTU). 

A nanometer (nm) is equal to 10-9 m (a billionth of a meter).

Conversion Factors and Abbreviations 
vi

Abbreviations used in this report

LTRMP Long Term Resource Monitoring Program

NTRU nephelometric turbidity ratio units

NTU nephelometric turbidity units

NVSS nonvolatile suspended solids

PAR photosynthetically active radiation

RM river mile

R2	 coefficient of determination

SAV submersed aquatic vegetation

TSS total suspended solids

UMESC Upper Midwest Environmental Sciences Center

UMRCC Upper Mississippi River Conservation Committee

UMRS Upper Mississippi River System

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VSS volatile suspended solids 
Evaluation of Light Penetration on Navigation Pools 8 and 

13 of the Upper Mississippi River 

By Shawn Giblin, Kraig Hoff, Jim Fischer, and Terry Dukerschein 

Abstract

The availability of light can have a dramatic affect on 

macrophyte and phytoplankton abundance in virtually all 

aquatic ecosystems. The Long Term Resource Monitoring 

Program and other monitoring programs often measure factors that affect light extinction (nonvolatile suspended solids, 

volatile suspended solids, and chlorophyll) and correlates of 

light extinction (turbidity and Secchi depth), but rarely do they 

directly measure light extinction. Data on light extinction, 

Secchi depth, transparency tube, turbidity, total suspended 

solids, and volatile suspended solids were collected during 

summer 2003 on Pools 8 and 13 of the Upper Mississippi 

River. Regressions were developed to predict light extinction 

based upon Secchi depth, transparency tube, turbidity, and 

total suspended solids. Transparency tube, Secchi depth, and 

turbidity all showed strong relations with light extinction and 

can effectively predict light extinction. Total suspended solids 

did not show as strong a relation to light extinction. Volatile 

suspended solids had a greater affect on light extinction than 

nonvolatile suspended solids. The data were compared to 

recommended criteria established for light extinction, Secchi 

depth, total suspended solids, and turbidity by the Upper Mississippi River Conservation Committee to sustain submersed 

aquatic vegetation in the Upper Mississippi River. During the 

study period, the average condition in Pool 8 met or exceeded 

all of the criteria whereas the average condition in Pool 13 

failed to meet any of the criteria. This report provides river 

managers with an effective tool to predict light extinction 

based upon readily available data. 

Background 

Solar radiation affects the productivity and metabolism 

of aquatic ecosystems (Wetzel, 2001). A large portion of the 

biological energy production in lakes and rivers is a result 

of energy derived from solar radiation used in photosynthesis. The extinction of light in aquatic ecosystems is a function of the properties of water, particles suspended in the 

water, and dissolved and colored compounds in the water. In 

unproductive systems dissolved compounds play an important 

role in light extinction. Turbidity and phytoplankton abundance play a larger role in light extinction in more productive 

aquatic systems (Wetzel, 2001). Light extinction can exhibit 

considerable spatial and temporal variation within a given 

system. Weather, season, water quality, biological activity, ice 

cover, snow cover, flow regime, and vegetation density all can

affect the amount of light available for photosynthetic activity. 

The availability of light can have a dramatic affect on 

macrophyte and phytoplankton abundance and distribution 

in virtually all aquatic ecosystems. In turn, the abundance 

of macrophytes can affect the amount of nursery habitat for 

fish, invertebrate abundance, and water quality (Korschgen

and others, 1997; Janecek, 1988). Vallisneria americana

Michx. is considered an important resource for waterfowl 

and fish within the Upper Mississippi River System (UMRS)

(Korschgen and others, 1997). Periodic declines of Vallisneria

within the UMRS have had a negative effect on ecosystem 

health (Kimber and others, 1995). Light availability is a major

factor affecting Vallisneria abundance, growth, and reproduction on the UMRS (Kimber and others, 1995; Doyle, 2000;

Kreiling and others, 2007). Recent emphasis on the effect of

light regime on submersed aquatic vegetation (SAV) in the 

UMRS has prompted researchers and river managers to relate 

commonly available light-penetration indicators (e.g., turbidity, Secchi depth, and total suspended solids (TSS)) to light 

extinction (Upper Mississippi River Conservation Committee, 

2003). 

The purpose of this study was threefold: (1) to develop 

relations between frequently measured water-quality parameters and light extinction, (2) to gage how tributary water quality affects light extinction in the UMRS, and (3) to determine 

what fraction of the suspended load, volatile versus nonvolatile, contributes more to light extinction in the river. Monitoring programs, including the Long Term Resource Monitoring 

Program (LTRMP), often measure factors that affect light 

extinction (nonvolatile suspended solids, volatile suspended 

solids, and chlorophyll) and other correlates of light extinction (turbidity and Secchi depth), but rarely do they directly 

measure light extinction. Developing relations between waterquality parameters and light extinction will provide river 

managers with additional tools to identify problems within the 

system. 
2 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Methods 

In Pool 8 of the Upper Mississippi River, five transects

consisting of three sites each were established on the main 

channel. The Black River, La Crosse River, Root River, and 

Coon Creek each were sampled at one location to monitor the 

tributaries to Pool 8. Transects were established at Minnesota 

Island (River Mile (RM) 701.1), Riverside Park immediately 

downstream of the La Crosse River (RM 698), immediately 

downstream of the Root River (RM 693.5), Horseshoe Island 

(RM 687.8), and immediately upstream of Lock and Dam 8 

at Genoa, Wisconsin (RM 679.5, fig. 1). In Pool 13, four

transects consisting of three to five sites each were established on main- and side-channel sites. The Maquoketa River, 

Apple River, Plum River, and Elk River each were sampled at 

one location to monitor the tributaries to Pool 13. The transect locations for Pool 13 were directly downstream of each 

tributary at RM 548.5, 545.1, 536.6, and 528.3 (fig. 2). All

transects were perpendicular to the main channel. Transects 

were selected to measure the effect of tributaries and to detect 

whether lateral or longitudinal light gradients exist within 

the pools. Sites were sampled weekly from May 6 to July 16, 

2003. 

Data were collected at every site for turbidity, Secchi 

depth, transparency tube, and underwater photosynthetically 

active radiation (400 to 700 nm). Turbidity and Secchi depth 

information was obtained using standard LTRMP protocols 

(Soballe and Fischer, 2004). Turbidity was analyzed with a 

Hach 2100P turbidimeter (Hach Company, 1995) and reported 

in nephelometric turbidity units (NTU). The 2100P uses a 

tungsten-filament lamp light source, 90-degree detection

angle, and multiple detectors with ratio compensation. The 

2100P does not have the option of turning the ratio compensation off. With ratio compensation on, the instrument’s microprocessor calculates a ratio of signals from each detector. For 

this reason, 2100P values are sometimes reported as nephelometric turbidity ratio units (NTRU). However, turbidity 

values are expressed as NTU for this report. The Hach 2100P 

was checked daily with low, medium, and high NTU Gelex 

Secondary Standards. The Hach 2100P turbidimeter was 

calibrated quarterly using Hach StablCal Stabilized Formazin 

Turbidity Standards. 

The transparency tube was a clear, plastic tube 120 cm 

long marked in 1-cm increments with a small Secchi pattern 

painted on the bottom. Water was collected from 0.20 m below 

the surface and poured into the tube until the pattern disappeared. Readings were taken in the shade and recorded to the 

nearest centimeter (U.S. Environmental Protection Agency, 

2006). 

Samples for TSS and volatile suspended solids (VSS) 

were collected at one site per transect per sampling episode 

and at all tributaries according to LTRMP standard procedures 

(Soballe and Fischer, 2004). Suspended solids were determined gravimetrically following standard methods (Greenburg 

and others, 1992). TSS and VSS laboratory analysis was done 

at the U.S. Geological Survey Upper Midwest Environmental Sciences Center (UMESC) Water Quality Laboratory in 

La Crosse, Wisconsin. Nonvolatile suspended solid (NVSS) 

values were calculated by subtracting VSS from TSS. 

Photosynthetically active radiation (PAR) was measured in micromoles s-1 m-2 using two LI-192SA Underwater 

Quantum Light Sensors and an LI-1000 datalogger (LI-COR, 

Inc., 2006). Calibration was done before and after sampling by 

holding the sensors side by side outdoors and recording three 

10-second averages. A correction factor was applied to one 

cell to ensure both cells yielded the same response under identical light exposure. All underwater-light measurements were 

done between 1000 and 1500 hours. Both sensors were placed 

on a single pole and were positioned 90 degrees apart. The 

sensors were deployed over the side of the boat or from shore 

so that the upper sensor was approximately 0.25 m below 

the water surface and the lower sensor was 0.75 m below 

the water surface. The lower sensor was placed at 0.5 m in 

La Crosse River and Coon Creek when depth was insufficient.

Sensors were held as close to horizontal as possible and placed 

to avoid shadows. The sensors were allowed to stabilize for 20 

to 30 seconds and three 10-second readings were recorded for 

each site and later averaged. Light-extinction coefficient was

calculated as 

k = [ln(Io)- ln(Iz)]/z, 

where 

	 k	 is light-extinction coefficient (1/m),

Io is surface or upper light measurement, 

Iz is light measurement at depth z, and

z is depth interval between Io and Iz.

The depth of 1 percent of surface light (zI1%) was calculated as 

(zI1%) = ln(100)/k.

Results and Discussion 

Conditions During the Study Period 

In 2003, discharge at Dam 8 was slightly above the recent 

average for May and July and slightly below average for June 

(table 1). Discharge at Dam 13 was slightly above average 

for May, below average for June, and near average for July 

(table 1). Based on LTRMP data for 1994–2002, main channel 

turbidity and TSS in 2003 were near average for Pool 8 and 

above average for Pool 13 (table 2). Substantial differences 

in water quality between Pools 8 and 13 (table 2) translated 

into differences in light-extinction measurements between the 

pools (table 3). 
Results and Discussion 3

Minnesota Island 

Transect

Black River

La Crosse River Riverside Park

Transect

La Crosse, Wisconsin

Root River Root River

Transect

Horseshoe Island 

Transect

Coon Creek

Genoa Transect

Lock and Dam 8

0 0.5 1 2

Kilometers

Figure 1. Navigation Pool 8 of the Upper Mississippi River. (Location of sampling sites for 

the light penetration study in red.) 
4 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Maquoketa River

Transect

Maquoketa River

Apple River

Transect

Apple River

Plum River

Plum River

Transect

Elk River

Transect

Elk River

Savanna, Illinois

Bellevue, 

Iowa

Lock and Dam 13

0 1 2 4

Kilometers

Figure 2. Navigation Pool 13 of the Upper Mississippi River. (Location of sampling sites for the light penetration study in red.) 
Results and Discussion 5

Table 1. Average discharge for Dams 8 and 13 for 2003 

compared to recent averages based on U.S. Army Corps of 

Engineers data. 

[Data are reported in cubic feet per second]

Time period

Dam 8 Dam 13

1983–2002 2003 1986–2002 2003

May 6–May 31 64,922 81,392 89,732 109,986

June 52,603 46,743 75,614 60,828

July 1–July 16 55,273 62,650 76,098 76,366

Table 2. Average total suspended solids and turbidity for the 

study period compared to 1994–2002. The average for 1994–2002 

is from Long Term Resource Monitoring Program sampling in the 

main channel during late July and early August of each year. 

[TSS, total suspended solids; mg/L, milligrams per liter; NTU, nephelometric 

turbidity units]

Location and parameter 1994–2002 

average

2003 study 

average

Pool 8 TSS (mg/L) 21.1 23.4

Pool 8 turbidity (NTU) 15.1 14.9

Pool 13 TSS (mg/L) 35.1 56.3

Pool 13 turbidity (NTU) 25.7 37.2

Table 3. Summary of average water-quality and light-extinction 

variables for 2003. 

[cm, centimeter; NTU, nephelometric turbidity units; TSS, total suspended 

solids; mg/L, milligrams per liter; VSS, volatile suspended solids; m, meter]

Measurement Pool 8 Pool 8 

tributaries Pool 13 Pool 13 tributaries

Secchi depth (cm) 64.10 57.79 36.44 40.67

Transparency 

tube (cm)

42.88 38.33 22.48 26.53

Turbidity (NTU) 14.89 22.83 37.22 27.11

TSS (mg/L) 23.42 49.75 56.26 32.13

VSS (mg/L) 5.82 8.74 11.41 7.31

Light extinction 

coefficient (m-1)

2.85 3.50 4.84 3.65

1 percent of 

surface light (m)

1.66 1.51 1.02 1.44

Historical Comparisons 

Light-extinction data collected by the Wisconsin Department of Natural Resources at Lock and Dams 8 and 9 for 

1988–98, that did not account for surface reflection, yielded

an average light-extinction coefficient of 4.21 m-1 (J. Sullivan, unpub. data, 2007). Sullivan also collected data from 

Lock and Dams 8 and 9 during 2003–06 that yielded average 

light-extinction coefficients of 2.79 and 2.84 m-1, respectively. 

An evaluation of light extinction in Pool 8 during 1983–84 

yielded an average light-extinction coefficient of 4.08 m-1

(Korschgen and others, 1997). Another evaluation of light

extinction (not accounting for surface reflection), conducted

on Lake Onalaska in Pool 7 during the summer of 1990, 

showed an average light-extinction coefficient of 4.64 m-1

(Kimber and others, 1995). Examination of the data indicated

increased light penetration in the UMRS in recent years. 

LTRMP data show that average concentrations of suspended 

solids in Pool 8 decreased appreciably during 1994–2002 

(Johnson and Hagerty, 2008). 

Comparisons of Water-Quality Parameters to 

Light Extinction 

The small amount of light-extinction data available for 

the Upper Mississippi River have motivated researchers to 

develop regression models to predict light extinction based 

upon commonly collected water-quality parameters. In the 

majority of the regression analyses we conducted, nonlinear

regression achieved higher R2 values than linear regression. 

Data from Pool 8, Pool 13, and selected tributaries were combined to predict light extinction from TSS, transparency tube, 

turbidity, and Secchi depth (table 4). Data also were analyzed 

only from sites where all variables were measured to determine which of the four variables showed the highest proportion of variability in light extinction. Turbidity explained the 

highest proportion of variability among the four variables, 

whereas TSS explained the least proportion of variability 

(table 4). Data from Pools 8 and 13 also were segregated by 

pool and into tributary and main channel groups to compare 

regressions (table 5). These analyses revealed a greater correlation for the main channel sites than the tributary sites when 

data from both pools were combined. 
6 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Table 4. Regression between light-extinction coefficient (m-1) 

(dependent variable) and water-quality parameters (independent 

variable). Data are combined for Pool 8 and tributaries and Pool 13 

and tributaries. 

[N, number of measurements; R2, coefficient of determination;

TSS, total suspended solids; mg/L, milligrams per liter; cm, centimeter; 

NTU, nephelometric turbidity units]

Independent variable N Equation R2

TSS (mg/L) 157 y= 0.794x0.4241 0.676

Transparency tube (cm) 360 y= 45.85x-0.7502 .823

Turbidity (NTU) 360 y= 0.6973x0.5336 .822

Secchi depth (cm) 370 y= 72.84x-0.7849 .788

Transparency tube (cm)a 157 y= 42.95x-0.7375 .752

Turbidity (NTU)a 157 y= 0.7152x0.5163 .769

Secchi depth (cm)a 157 y= 63.44x-0.7557 .720

a

 Only sites with TSS data are included. 

Table 5. Regression between light-extinction coefficient (m-1) (dependent variable) 

and water-quality parameters (independent variable). Data are segregated for Pool 8 

and Pool 13 tributary and main channel. 

[R2, coefficient of determination; TSS, total suspended solids; mg/L, milligrams per liter;

cm, centimeter; NTU, nephelometric turbidity units]

Independent variable Equation R2

Pool 8 and 13 tributaries TSS (mg/L) y= 0.9954x0.3524 0.564

Pool 8 and 13 main channel TSS (mg/L) y= 0.5149x0.5534 .879

Pool 8 with tributaries TSS (mg/L) y= 1.0444x0.3197 .620

Pool 13 with tributaries TSS (mg/L) y= 0.6023x0.5231 .786

Pool 8 main channel TSS (mg/L) y= 0.9512x0.3491 .629

Pool 13 main and side channel TSS (mg/L) y= 0.4402x0.5989 .804

Pool 8 and 13 tributaries transparency tube (cm) y= 43.811x-0.7567 .643

Pool 8 and 13 main channel transparency tube (cm) y= 46.157x-0.7471 .893

Pool 8 with tributaries transparency tube (cm) y= 49.425x-0.765 .701

Pool 13 with tributaries transparency tube (cm) y= 63.275x-0.8602 .758

Pool 8 main channel transparency tube (cm) y= 29.345x-0.6267 .515

Pool 13 main and side channel transparency tube (cm) y= 50.488x-0.7757 .846

Pool 8 and 13 tributaries turbidity (NTU) y= 0.762x0.4855 .653

Pool 8 and 13 main channel turbidity (NTU) y= 0.6821x0.5456 .888

Pool 8 with tributaries turbidity (NTU) y= 0.8085x0.4742 .674

Pool 13 with tributaries turbidity (NTU) y= 0.7096x0.5325 .765

Pool 8 main channel turbidity (NTU) y= 0.812x0.4713 .571

Pool 13 main and side channel turbidity (NTU) y= 0.8579x0.4862 .831

Pool 8 and 13 tributaries Secchi depth (cm) y= 55.799x-0.7358 .587

Pool 8 and 13 main channel Secchi depth (cm) y= 78.21x-0.7984 .867

Pool 8 with tributaries Secchi depth (cm) y= 117.49x-0.8985 .721

Pool 13 with tributaries Secchi depth (cm) y= 69.37x-0.7735 .653

Pool 8 main channel Secchi depth (cm) y= 64.626x-0.7561 .560

Pool 13 main and side channel Secchi depth (cm) y= 59.305x-0.7162 .750
Results and Discussion 7

Regressions also were developed relating light-extinction 

coefficient to TSS, transparency tube, turbidity, and Secchi depth data for each pool, including tributaries (table 5; 

figs. 3–6). The regressions for transparency tube, turbidity, and

Secchi depth versus light extinction for Pools 8 and 13 were 

strong and similar during the study period. The regression for 

TSS versus light extinction, however, revealed substantial 

differences between Pools 8 and 13 when the tributaries were 

included. In addition, regressions were developed among 

all water-quality variables to estimate what the value of the 

dependent variable would be based on an independent variable value (e.g. estimated TSS based upon known turbidity; 

table 6). The regressions for TSS versus transparency tube and 

Secchi depth were weaker than the other regressions. 

Comparison of various relations can reveal the value in 

predicting real-time environmental conditions within the river. 

Cole (1979) found that multiplying the Secchi depth by 2.7 to 

3.0 provides a good estimate of the depth at which 1 percent of 

surface light penetrates (compensation point), delimiting the 

lower depth of the photic zone. A factor of 2.66 was derived 

using the data collected during this study (fig. 7). A similar

relation was developed for transparency tube readings (fig. 8).

In this case, a factor of 4.03 times the transparency tube reading provided an estimate of the compensation point. Nonlinear 

regression revealed an even stronger correlation for both of 

these relations. The equation for 1 percent of surface light 

versus Secchi depth is 

(y=2.3484x0.7849, r 2= 0.788). 

The equation for 1 percent of surface light versus transparency 

tube is 

(y=3.1788x0.7502, r 2= 0.823). 

Comparison of Results to Proposed WaterQuality Criteria 

The Water Quality Section of the Upper Mississippi 

River Conservation Committee (UMRCC) has proposed 

water-quality criteria to sustain submersed aquatic vegetation 

in the UMRS (Upper Mississippi River Conservation Committee, 2003; table 7). During the study period the average 

condition in Pool 8 met all the criteria, whereas the average 

condition in Pool 13 failed to meet any of the criteria (table 7). 

Using regressions from table 4, the value at which each of 

the water-quality parameters met the UMRCC recommended 

light-extinction coefficient of 3.42 m-1 was 49.2 cm for Secchi 

depth, 31.3 milligrams per liter (mg/L) for TSS, 19.7 NTU for 

turbidity, and 31.8 cm for transparency tube. This indicates 

that the recommended TSS criteria of 25 mg/L may be too 

low, and a value nearer 30 mg/L may be more appropriate. The 

UMRCC light criteria did not include transparency tube data 

as a potential metric. Using regression data from table 4, a 

transparency tube measurement of roughly 32 cm corresponds 

to the recommended light-extinction coefficient of 3.42 m-1. 

Figure 3. Relation between light-extinction coefficient and total suspended solids from Pools 8 and 13 (pool and 

tributary sites combined), Upper Mississipppi River, May 6 to July 16, 2003. 

Pool 8 y = 1.0444x0.3197

r2 = 0.6197

Pool 13 y = 0.6023x0.5231

r2 = 0.7862

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120 140 160

LIGHT EXTINCTION COEFFICIENT (m-1)

TOTAL SUSPENDED SOLIDS, IN MILLIGRAMS PER LITER

Pool 8

Pool 13

Pool 8

Pool 13
8 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Figure 4. Relation between light-extinction coefficient and transparency tube from Pools 8 and 13 (pool and tributary 

sites combined), Upper Mississipppi River, May 6 to July 16, 2003. 

Figure 5. Relation between light-extinction coefficient and turbidity from Pools 8 and 13 (pool and tributary sites 

combined), Upper Mississipppi River, May 6 to July 16, 2003. 

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80

TRANSPARENCY TUBE, IN CENTIMETERS

LIGHT EXTINCTION COEFFICIENT (m-1)

Pool 8

Pool 13

Pool 8

Pool 13

Pool 13 y = 63.275x-0.8602

r2 = 0.7577

Pool 8 y = 49.425x-0.765

r2 = 0.7006

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120

TURBIDITY, IN NEPHELOMETRIC TURBIDITY UNITS

LIGHT EXTINCTION COEFFICIENT (m-1)

Pool 8

Pool 13

Pool 8

Pool 13

Pool 13 y = 0.7096x0.5325

r2 = 0.7647

Pool 8 y = 0.8085x0.4742

r2 = 0.6738
Results and Discussion 9

Figure 6. Relation between light-extinction coefficient and Secchi depth from Pools 8 and 13 (pool and tributary 

sites combined), Upper Mississipppi River, May 6 to July 16, 2003. 

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120

SECCHI DEPTH, IN CENTIMETERS

Pool 8

Pool 13

Pool 8

Pool 13

LIGHT EXTINCTION COEFFICIENT (m-1)

Pool 13 y = 69.37x-0.7735

r2 = 0.6526

Pool 8 y = 117.49x-0.8985

r2 = 0.7209

Table 6. Relation between water-quality parameters for 2003 study using data combined from 

Pool 8, Pool 13, and tributaries. 

[R2, coefficient of determination; N, number of measurements; cm, centimeter; TSS, total suspended solids;

mg/L, milligrams per liter; NTU, nephelometric turbidity units]

Independent variable Dependent variable Equation R2 N

Transparency tube (cm) Secchi depth (cm) y= 1.3361x+6.6764 0.898 360

Transparency tube (cm) TSS (mg/L) y= 2627x-1.29 .613 157

Transparency tube (cm) Turbidity (NTU) y= 2073.5x-1.3458 .894 360

Secchi depth (cm) Transparency tube (cm) y= 0.6722x-1.1867 .898 360

Secchi depth (cm) TSS (mg/L) y= 5179.1x-1.3225 .575 157

Secchi depth (cm) Turbidity (NTU) y= 4900.4x-1.4149 .876 360

TSS (mg/L) Transparency tube (cm) y= 158.32x-0.4749 .613 157

TSS (mg/L) Secchi depth (cm) y= 211.54x-0.4344 .575 157

TSS (mg/L) Turbidity (NTU) y= 1.3159x0.8006 .834 157

Turbidity (NTU) Transparency tube (cm) y= 228.69x-0.6646 .894 360

Turbidity (NTU) Secchi depth (cm) y= 309.84x-0.6193 .876 360

Turbidity (NTU) TSS (mg/L) y= 1.332x1.0421 .834 157
10 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Figure 7. Relation between 1 percent of surface light and Secchi depth from Pools 8 and 13 (pool and tributary sites 

combined), Upper Mississipppi River, May 6 to July 16, 2003. 

Figure 8. Relation between 1 percent of surface light and transparency tube from Pools 8 and 13 (pool and tributary 

sites combined), Upper Mississipppi River, May 6 to July 16, 2003. 

Linear regression 

y = 2.6615x

r2

 = 0.692

Nonlinear regression

y = 2.3484x0.7849

r2 = 0.788

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.2 0.4 0.6 0.8 1 1.2

1 PERCENT OF SURFACE LIGHT, IN METERS

SECCHI DEPTH, IN METERS

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

TRANSPARENCY TUBE, IN METERS

1 PERCENT OF SURFACE LIGHT, IN METERS

Linear regression 

y = 4.0347x

r2

 = 0.6486

Nonlinear regression

y = 3.1788x0.7502

r2 = 0.823
Effects of Tributaries on Light Extinction 11

Effects of Tributaries on Light 

Extinction 

Water quality within the Upper Mississippi River is 

dependent upon the water quality of the tributaries feeding 

the system (Wasley, 2000). The degree that water quality is 

affected is a function of the discharge and concentration of 

suspended sediment of the tributary relative to the UMRS. The 

LTRMP monitored water quality in the tributaries of Pools 8 

and 13, including data on TSS, turbidity, and Secchi depth. 

Based on this study, the average light-extinction coefficient

of the tributaries to Pool 8 was higher than the Pool 8 main 

channel, whereas the average light-extinction coefficient of

the tributaries to Pool 13 was lower than the Pool 13 main 

channel. Within Pool 8, the Black River had a lower lightextinction coefficient, and the La Crosse River, Root River,

and Coon Creek had a higher light-extinction coefficient than

the main channel (fig. 9). Scheffe’s multiple-comparison

procedure (Zar, 1984) indicated that light extinction in the 

main channel was significantly different than the La Crosse

River and Coon Creek at the 0.05 level during the study 

period. Within Pool 13, the Elk, Apple, and Plum Rivers had 

lower light-extinction coefficients, and the Maquoketa River

had a higher light-extinction coefficient than the main channel

(fig. 10). Scheffe’s multiple-comparison procedure indicated

that light extinction in the main channel was significantly different than the Apple and Elk Rivers at the 0.05 level during 

the study period. 

Tributaries can affect water quality laterally across the 

Mississippi River (Houser, 2005). Data collected during this 

study showed an east to west gradient of light penetration in 

Pool 8 with light penetration being slightly deeper on the east 

side of the main channel (fig. 11). The data point for the Root

River Transect, Site 1, is suspect owing to two light-penetration observations that were extremely low for observed TSS, 

turbidity, and transparency tube values from the same site 

visit. The east to west gradient can be partially explained by 

the incomplete mixing of water from the Black River. Analysis 

of LTRMP fixed-site sampling data for 2000–05 (during the

same months as the study period) indicated that a high proportion of NVSS settled out upstream in Lake Onalaska as the 

Black River traveled through the lake from Pool 7 into Pool 8 

(fig. 1). The concentration of VSS also declined (at a slower

rate) and remained lower downstream of Lake Onalaska than 

that found in the main channel in Pool 8 (table 8). The loss of 

NVSS at a faster rate than VSS likely was the result of faster 

sinking rates of NVSS, which is consistent with observations at Lake Pepin (Megard, 2006a). Although the La Crosse 

River had a high light-extinction coefficient, the Black River

discharge was many times greater than the La Crosse River, 

resulting in lower light extinction on the east side of the Mississippi River downstream of the La Crosse and Black Rivers 

(table 9). Turbidity generally was higher in the west side of the 

river relative to the east side with the Root River and Genoa 

Transects showing the most pronounced gradient (fig. 12). In

Pool 13, lateral gradients in turbidity were less pronounced. 

The only pattern was an increase in main channel turbidity on 

the west side of the main channel where the turbid Maquoketa 

River empties into the Upper Mississippi River. The lack of 

pattern likely was the result of the relatively small flow contribution of three of the four tributaries entering Pool 13 relative 

to the Upper Mississippi River (table 9). 

A weak longitudinal light-penetration gradient was 

observed for Pool 8 with an increase in light-extinction 

coefficient downstream. Although this trend was statistically

insignificant, the lack of significance likely can be attributed to

small sample size. High light-extinction values of the tributaries discharging to lower Pool 8 and wave-induced sediment 

resuspension owing to long wind fetch likely contributed to 

this trend. The poorest area for light penetration in Pool 8 

(west side of the Genoa Transect) is scheduled for rehabilitation to reduce sediment resuspension. Data collected during 

this study indicate that management efforts are being directed 

effectively to an area in the pool with high concentrations of 

suspended sediment. 

Table 7. Comparison of Upper Mississippi River Conservation Committee recommended 

water-quality criteria (UMRCC, 2003) to main channel data from Pools 8 and 13 for May 6 to 

July 16, 2003. 

[cm, centimeter; TSS, total suspended solids; mg/L, milligrams per liter; NTU, nephelometric turbidity units; 

≤, less than or equal to; ≥, greater than or equal to]

Light extinction 

coefficient (m-1)

Secchi depth 

(cm)

TSS 

(mg/L)

Turbidity 

(NTU)

UMRCC recommendation ≤ 3.42 ≥ 50 ≤ 25 ≤ 20

Pool 8—main channel average 2.85 64.1 23.4 14.9

Pool 13—main channel average 4.84 36.4 56.3 37.2
12 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Figure 9. Pool 8 light-extinction summary. The solid line inside the box is the median. The upper and lower ends of the box are the 

25th and 75th percentiles. The whiskers denote the 10th and 90th percentiles. The dotted line is the average for each site. 

Figure 10. Pool 13 light-extinction summary. The solid line inside the box is the median. The upper and lower ends of the box are the 

25th and 75th percentiles. The whiskers denote the 10th and 90th percentiles. The dotted line is the average for each site. 
Effects of Tributaries on Light Extinction 13

2.6

2.7

2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

Site 1 Site 2 Site 3

LIGHT EXTINCTION COEFFICIENT (m-1)

WEST TO EAST

Riverside Transect

Root River Transect

Horseshoe Transect

Genoa Transect

Average by site

?

Minnesota Island Transect

Figure 11. Mississippi River Pool 8 average light-extinction coefficient by transect. The question mark indicates this data point 

is suspect. 

Table 8. Comparison of average water-quality characteristics of the Black River 

upstream and downstream of Lake Onalaska compared to the main channel of the 

Mississippi River in Pool 8. Data are from May 6 to July 16 from 2000 to 2005. 

[NTU, nephelometric turbidity units; TSS, total suspended solids; mg/L, milligrams per liter; 

VSS, volatile suspended solids]

Turbidity 

(NTU)

TSS 

(mg/L)

VSS

(mg/L)

Black River upstream of Lake Onalaska (BK 14.2M) 16.4 23.1 5.9

Black River downstream of Lake Onalaska (BK 01.0M) 6.1 7.4 4.4

Mississippi River in upper Pool 8 (M 701.1D) 16.8 23.9 5.8
14 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

Table 9. Average discharge for the Mississippi River and tributaries for 1970–2000 based on 

U.S. Geological Survey data. 

[UMR, Upper Mississippi River; data are in cubic feet per second]

River Discharge River Discharge

Black Rivera 2,111 Maquoketa Rivera 1,259

La Crosse Rivera 469 Apple Rivera 217

Root Rivera 1,047 Plum Rivera 235

Coon Creeka 60 Elk Riverc 42

UMR at Winona, Minnesota (Pool 6)b 34,290 UMR at Clinton, Iowa (Pool 14)b 54,000

a

 Data from Wasley (2000).

b

 U.S. Geological Survey (USGS) gaging station data for 1970–2000.

c

 USGS gaging station data for 1995–97.

10

11

12

13

14

15

16

17

18

19

20

Site 1 Site 2 Site 3

TURBIDITY, IN NEPHELOMETRIC TURBIDITY UNITS

WEST TO EAST

Riverside Transect

Root River Transect

Horseshoe Transect

Genoa Transect

Average by site

Minnesota Island Transect

Figure 12. Mississippi River Pool 8 average turbidity by transect. 
Effects of Composition of Suspended Solids on Light Extinction 15

Effects of Composition of Suspended 

Solids on Light Extinction 

The light-scattering properties of suspensoids in lentic systems tend to vary depending upon the proportion of 

organic (VSS) and inorganic (NVSS) particles in suspension 

(Megard, 2006a). Generally, organic suspensoids scatter light 

more strongly than inorganic suspensoids (Megard, 2006b). 

Multiple linear regression of data collected during this study 

resulted in the following equation: 

Light-Extinction Coefficient (m-1) = 1.0961 

+ 0.2497 (VSS mg/L) + 0.0167 (NVSS mg/L) 

R2= 0.808. 

This equation supports the theory that VSS is a greater 

light attenuator proportionally than NVSS; however, NVSS 

tends to occur at higher concentrations in the UMRS. The 

average concentration of VSS was 8.2 mg/L, and the average concentration of NVSS was 31.5 mg/L during the study 

period. Based on the regression equation above, on average, 

the concentration of VSS accounts for 56 percent of light 

extinction and NVSS accounts for 14 percent. The intercept 

estimates background extinction owing to unmeasured variables, including dissolved organic carbon, which on average 

accounts for 30 percent of light extinction. 

The different effects of the contribution of NVSS and 

VSS to light extinction may illustrate the differences between 

Pools 8 and 13 in the relation of TSS to light extinction 

(fig. 3). During the study period, Pool 13 had a higher average

concentration of VSS (a greater light attenuator) and therefore, 

a higher light-extinction coefficient. This response was more

pronounced at higher concentrations of TSS and may account 

for some of the disparity between Pools 8 and 13 at concentrations of TSS exceeding 80 mg/L (table 10). It also is likely that 

variability in the proportion of TSS that is comprised of VSS 

is a cause for the weak relation between TSS and light extinction. All of the Pool 8 values greater than 80 mg/L were from 

Coon Creek and Root River, and five of the seven Pool 13

observations were main channel sites. This indicates that there 

may be important differences in TSS makeup between tributaries and the main channel that are affecting light extinction. 

The effects of seasonal light penetration on vegetation 

and fish within the Upper Mississippi River remain poorly

understood; however, this report presents river managers with 

tools to predict light extinction based upon commonly collected water-quality variables, thereby providing opportunities 

to further investigate these unknown effects. Transparency 

tube, Secchi depth, and turbidity all showed strong relations 

with light extinction and can be used to effectively predict 

light extinction. TSS did not show as strong a relation to 

light extinction. This report also provides some insight into 

the effect that VSS and NVSS are having on light penetration in the Upper Mississippi River and its tributaries. We 

expect the general relations and principals presented in this 

report to apply to other parts of the UMRS as well as other 

river systems. Utilizing relations presented in this report in 

conjunction with biological indicators of light penetration,

such as SAV, represents important tools in understanding light 

dynamics within the UMRS (Sullivan and others, 2009). The 

light regime on the Upper Mississippi River has wide-ranging 

ramifications that affect the overall health of the ecosystem,

and we have illustrated that readily available data can be used 

to predict light extinction 

Table 10. Comparison of total suspended solids, volatile suspended solids, nonvolatile 

suspended solids, and percentage volatile suspended solids for Pools 8 and 13.

[N, number of measurements; TSS, total suspended solids; mg/L, milligrams per liter; VSS, volatile suspended 

solids; NVSS, nonvolatile suspended solids; k, light extinction coefficient (m-1); tributary data included; 

only values greater than 80 mg/L TSS are included]

N TSS

(mg/L)

VSS

(mg/L)

NVSS

(mg/L)

Percentage 

VSS k

Pool 8 average 9 107.04 12.34 94.7 11.42 5.05

Pool 13 average 7 102.76 16.51 86.24 16.26 7.97
16 Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

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Long Term Resource Monitoring Program—Evaluation of Light Penetration on Navigation Pools 8 and 13 of the Upper Mississippi River

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.