Simulation of Flow Regimes to Reduce Habitat 

for T. tubifex 

By Robert T. Milhous 

Open-File Report 2008–1107 

U.S. Department of the Interior 

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

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2008 

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Suggested citation: 

Milhous, R.T., 2008, Simulation of flow regimes to reduce habitat for T. Tubifex: 

Fort Collins, Colo., U.S. Geological Survey, Fort Collins Science Center, Open-File 

Report 2008–1107, 16p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the 

individual copyright owners to reproduce any copyrighted material contained within 

this report. 

ii
Contents 

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

Willow Creek Case Study..............................................................................................................3

Step 1. Determining size of the sediment to be removed..........................................................7

Step 2. Determine location of the sediment to be flushed from the river ...................................9

Step 3. Determine flushing flows needed to remove the sediment..........................................10

Discussion and Conclusions .......................................................................................................15

Acknowledgements .....................................................................................................................16

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

Figures 

 1. The life cycle of whirling disease.............................................................................................2

 2. Location of Willow Creek, Grand County, Colo., and the study site ........................................3

 3. Picture of the pool section of the Willow Creek study site .......................................................4

 4. Map of the Willow Creek study reach......................................................................................5

 5. Picture of the surface of the pool used for the case study.......................................................6

 6. Number of oligochaete worms as related to the median size of the sediment in Willow Creek

.................................................................................................................................................7

 7. The number of oligochaete worms as related to median size of sediment, for a median size 

of 3 mm 

or less ......................................................................................................................................8

 8. Number of worms in each sample as related to the percent of the sediment that is less than 

1.35 mm...................................................................................................................................8

 9. Particle size distribution of the fine sediment at two locations in the backwater area of the 

Willow Creek study reach ........................................................................................................9

10. Grain size distribution of three samples of fine sediment in the pool....................................10

11. The daily discharge exceeded 10 percent of the days in each water year in Willow Creek 

below 

Willow Creek Reservoir..........................................................................................................12

12. Daily streamflows in Willow Creek below Willow Creek Reservoir during water year 1982..13

13. Percent of the base flow pool cross-sectional area flushed as related to stream discharge 14

14. Number of days in each water year that the discharge was greater than or equal to the 

discharge required to move fines from 90 percent of the base flow pool cross-sectional area

...............................................................................................................................................14

15. Percent of the base flow pool cross-sectional area flushed as related to stream discharge, 

for two different estimates for the size of sediment to be flushed from the river. ...................15

iii
Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

Inch (in.) 2.54 centimeter (cm) 

Inch (in.) 25.4 millimeter (mm) 

Foot (ft) 0.3048 meter (m) 

Area 

Square foot (ft2) 929.0 square centimeter (cm2) 

Square foot (ft2) 0.09290 square meter (m2) 

Volume 

Cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate 

Foot per second (ft/s) 0.3048 meter per second (m/s) 

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

Density 

Pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3) 

Pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter 

(g/cm3

) 

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

°F=(1.8×°C)+32 

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

°C=(°F-32)/1.8 

Vertical coordinate information is referenced to the insert datum name (and abbreviation) here for instance, “North 

American Vertical Datum of 1988 (NAVD 88).” 

Horizontal coordinate information is referenced to the insert datum name (and abbreviation) here for instance, 

“North American Datum of 1983 (NAD 83).” 

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

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). 

iv
SI to Inch/Pound 

Multiply By To obtain 

Length 

Centimeter (cm) 0.3937 inch (in.) 

Millimeter (mm) 0.03937 inch (in.) 

Meter (m) 3.281 foot (ft) 

Area 

Square meter (m2) 10.76 square foot (ft2) 

Square centimeter (cm2) 0.1550 square inch (ft2) 

Volume 

Cubic meter (m3) 35.31 cubic foot (ft3) 

Flow rate 

Meter per second (m/s) 3.281 foot per second (ft/s) 

Cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s) 

Density 

Kilogram per cubic meter (kg/m3) 0.06242 pound per cubic foot (lb/ft3) 

Gram per cubic centimeter 

(g/cm3

) 

62.4220 pound per cubic foot (lb/ft3

) 

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

°F=(1.8×°C)+32 

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

°C=(°F-32)/1.8 

Vertical coordinate information is referenced to the insert datum name (and abbreviation) here, for instance, “North 

American Vertical Datum of 1988 (NAVD 88)” 

Horizontal coordinate information is referenced to the insert datum name (and abbreviation) here, for instance, 

“North American Datum of 1983 (NAD 83)” 

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

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

liter (μg/L). 

v
Simulation of Flow Regimes to Reduce 

Habitat for T. tubifex 

By Robert T. Milhous1

Introduction 

Whirling disease has had a significant impact on trout fisheries of the American west by 

reducing the numbers and quality of rainbow trout in infected streams. The life cycle of whirling 

disease is presented in figure 1. The figure is from a New Mexico Fish and Game publication 

distributed to fishermen. Of interest is that items 1 through 4 relate to the river bottom (sediment) 

or to worms that live in the sediment. A critical factor in the life cycle of the whirling disease 

parasite is the fine sediment that provides the optimum habitat for Tubifex tubifex, an oligochaete 

worm that acts as an intermediate host for the disease (Sauter and Gude, 1996; and Krueger, 2002). 

Oligochaete worms are used herein as an indicator of probability of the existence of T. tubifex. The 

assumption is that if habitat conditions are good for oligochaete worms in general, the habitat is 

also good for T. tubifex. 

This report presents a model for the simulation of flushing flows required to remove 

undesirable fines and sand from a pool. Undesirable fines may also need to be flushed from runs, 

the surface layer (armour), and backwater areas. Well-defined links of specific particle sizes to 

oligochaete worm abundance is needed to justify the use of flushing flows to move sediment. 

An analytical method for estimating the streamflows needed to remove the fine sediment is 

demonstrated herein. The overall steps to follow in removing fines from a stream are: 

Step 1. Determine size of the sediment that is the habitat for oligochaete worms. 

Step 2. Determine location of the sediment that is the habitat for oligochaete worms. 

Step 3. Determine streamflows needed to flush (remove) the sediment that is the habitat for 

oligochaete worms. 

The case study approach is used to present the method and to demonstrate its application. 

The case is derived from the sediment and oligochaete worm habitat of Willow Creek. Willow 

Creek is a tributary of the Upper Colorado River located in Grand County, Colo. Willow Creek 

Reservoir (an element of the Colorado–Big Thompson Project) controls the streamflows of the 

creek and is just above the study site. 

 

1

 Hydrologist, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, Colo. 

1
Figure 1. The life cycle of whirling disease (New Mexico Department of Fish and Game, 

2003). 

2
Willow Creek Case Study 

Willow Creek, above Willow Creek Reservoir, where the watershed area is 329 km2, had an 

average annual discharge of 2.0 m3

/s for water years 1955–1960. For the same period, the 

discharge below the reservoir (watershed area of 347 km2

) was 0.7 m3/s. A map showing the 

location of the creek and the study reach is presented in figure 2. The study reach of Willow Creek 

is a gravel and cobble bed stream with runs, riffles, and pools. There is a section of bank where the 

creek is eroding a bank of fine sand, clays, and silt. Downstream of this bank is a pool with a deep 

deposit (about 0.3 m) of fine sand, silt, and clay on top of gravel and cobbles. 

The pool analyzed in this case study is in the middle of the photograph presented in 

figure 3, and the bank on the left of the picture is the location of some of the deposited fine 

sediment. 

A map of the study reach is presented in figure 4. Water flows from the upper part of the 

map to the lower part. Only the pool in the lower part of the study reach is considered in this 

report. A picture of the surface of the pool is given in figure 5. The plants on the surface are 

rooted in the fine sediments which should be removed from the pool in order to reduce the quality 

of the oligochaete habitat. 

A discussion of the three steps in the analysis of environmental flows required to remove 

sediment in order to reduce the habitat for oligochaete worms follows. 

Colorado

N

Study Area

Willow Cr.

Colorado River

Granby

Lake

Granby

34

Figure 2. Location of Willow Creek, Grand County, Colo., and the study site which is just 

below Willow Creek Reservoir. Map courtesy of Terry Waddle, Fort Collins Science 

Center. 

3
Pool

Figure 3. Picture of the pool section of the Willow Creek study site. 

4
Figure 4. Map of the Willow Creek study reach. Bed elevations are in meters. Map 

courtesy of Terry Waddle, Fort Collins Science Center. 

5
Figure 5. Picture of the surface of the pool used for the case study. 

6
Step 1. Determining size of the sediment to be removed 

The first step in the process of calculating the streamflows needed to reduce the habitat for 

oligochaete worms is to determine the size of the sediment that makes a habitat for the worms. A 

straightforward measure of the sediment characteristics of a stream is the median size (d50). The 

number of oligochaete worms in a sample of the substrate is used here as an index to the quality of 

the sediment for the worms. A relation between the median size of the sediment and the number of 

oligochaete worms in a sediment core is presented in figure 6. An expansion of the left side of 

figure 6 (d50 < 3 mm) is presented in figure 7. The objective of the expansions is to show the 

relation of the sizes of substrate that are high quality habitats for the worms. The diagram (figure 

7) suggests median substrate sediment size of less than 1.35 mm produces good oligochaete habitat. 

Another way of approaching the problem is to look at how the number of worms is related 

to the percentage of the substrate that is less than 1.35 mm. The relation between the percent of the 

sediment less than 1.35 mm and the number of worms in each sample is presented in figure 8. 

We now have met the objective of knowing the size of sediment that needs to be flushed 

from Willow Creek in order to reduce oligochaete worm habitat. The flushing flow objective is to 

remove sediment deposits from Willow Creek where more than 50 percent of the substrate 

sediment is less than 1.35 mm. 

0

50

100

150

200

250

300

0 5 10 15 20 25

TOTAL OLIGOCHAETES

MEDIAN SIZE OF SEDIMENT, IN MILLIMETERS

Figure 6. Number of oligochaete worms as related to the median size of the sediment in 

Willow Creek. 

7
0

50

100

150

200

250

300

0.0 0.5 1.0 1.5 2.0 2.5 3.0

TOTAL OLIGOCHAETES

MEDIAN SIZE OF SEDIMENT, IN MILLIMETERS

Figure 7. The number of oligochaete worms as related to median size of sediment, for a 

median size of 3 mm or less (expansion of the left side of figure 6). 

0

50

100

150

200

250

300

0 10 20 30 40 50 60 70 80 90 100

TOTAL OLIGOCHAETES

PERCENT SEDIMENT LESS THAN 1.35 MILLIMETERS

Figure 8. Number of worms in each sample as related to the percent of the sediment that 

is less than 1.35 mm. 

8
Step 2. Determine location of the sediment to be flushed from the river 

There are two locations within the study area with fine sediment: a backwater at the upper 

end of the study reach and a pool downstream of the eroding banks, shown in figure 3. Particle size 

distributions of the fine sediment found at two locations in the backwater area are presented in 

figure 9. The sample labeled “backwater” was not on one of the cross sections used in the study 

but was taken at a location that appeared to be representative of the sediment deposited in the 

backwater. The sample labeled C/S 16.5 was obtained at one of the cross sections used in the 

study. The vertical line on figure 9 marks a particle size of 1.35 mm. The vertical line intersects the 

relation for the backwater sample at the point were about 65 percent of the particles are finer, and 

the line for C/S 16.5 intersects that relation at the point where about 90 percent of the particles are 

finer. The results from step 1 indicate that the backwater area is prime habitat for oligochaete 

worms. The backwater area is probably not a natural riverine feature of the creek but caused by 

construction activities related to the construction of Willow Creek dam. A design for isolating the 

backwater is presented in Waddle and Terrell (2003). A berm isolating the backwater area was 

constructed in November of 2004. 

The second location of fine sediment is the pool in the lower part of the study reach. The 

particle size distribution relation for three samples from the pool is presented in figure 10. The 

median size of the sediment is 0.96 mm. The lowest relation (shown by the dashed line) shows that 

60 percent of the substrate in the pool is less than 1.35 mm––prime habitat for oligochaete worms. 

There are two ways to reduce the fines in the pool: (1) reduce the transport of fines to the pool by 

reducing the erosion from the banks, and (2) have streamflows remove the sediment from the pool. 

A combination of the two techniques would likely be the most successful for reduction of the 

pool’s worm habitat. 

Both the backwater area and the pool area have ample quantities of sediment that meet the 

worm habitat criteria. The objective pursued below is to remove fine sediment from the pool area. 

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

PERCENT SMALLER

PARTICLE SIZE, MILLIMETERS

C/S 16.5

Backwater

1.35 mm

Figure 9. Particle size distribution of the fine sediment at two locations in the backwater 

area of the Willow Creek study reach. The dashed line shows the percent finer for a 

particle size of 1.35 mm. 

9
0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

PERCENT OF SUBSTRATE FINER

PARTICLE SIZE, MILLIMETERS

Figure 10. Grain size distribution of three samples of fine sediment in the pool. 

 

Step 3. Determine flushing flows needed to remove the sediment 

The equation used to calculate the shear stress on the stream bed required to move 

sediment is: 

τ ( −= 1)γ Gdk spscrt (1) 

where τcrt is the shear stress required to move sediment particles of size dp , Gs

 is the specific gravity 

of the particles, γ is the unit weight of water, and ks

 is an empirical coefficient relating the 

downward force caused by the weight of the particles to the horizontal and uplift forces needed to 

move the particles. The logic (model) behind the equation is presented below. 

The (hydraulic) driving force is down slope: 

 

( ) 2

d Π= dRSF p γ (2) 

where Fd is the driving force, R is the hydraulic radius, S is the energy slope, and the other terms are 

as defined above. 

10
The resisting Force (sediment) is the weight of the particle acting downward: 

( ) ( ) ( ) ( )

6

1

6

1 3 2

s spp p

r

dGddG

F

Π− = Π− = γ γ (3) 

where Gs

 is the specific gravity of the sediment particles, Fr is the resisting force, and the other 

terms are as defined above. 

The ratio of the driving force to the resisting force is: 

( ) − 61 =

spr

d

Gd

RS

F

F (4) 

Typically the 1/6 factor is not used and the ratio 

( −1) =

sp

s Gd

RS k (5) 

is called the dimensionless shear stress. 

For any value of ks, a corresponding value of shear stress can be calculated using the equation: 

= ( ( −1)) Gdk sps τ (6) 

At the start of movement (critical): 

( ) τ = ( ( ) −1 γ ) Gdk spcrtscrt (7) 

and for continued movement of the sediment the required shear stress (τd) is: 

( ) τ τ >> ( ( ) −1 γ ) Gdk spcrtscrtd (8) 

As a reminder, ks

 is the empirical coefficient converting the resisting downward stress to a resisting 

horizontal stress. 

The selection of the empirical parameter (ks

) for the beginning of substrate movement must 

be selected (on a case by case basis) using judgment, as there is a wide range of values proposed in 

the literature. The range of values from the literature for ks

 is between 0.021 and 0.047. There is 

also a required time dimension to the value of ks

 required––the longer the flushing flow, the lower 

the acceptable value. Another way of looking at this is that the higher the flow, the more rapid the 

removal of the fines. In Willow Creek the duration is expected to be relatively short; hence, the 

selected ks ks

 should be relatively large (the upper limit). Once the material begins to move, it 

needs to be transported from the reach; for that reason, a value of 0.055 was selected for ks

. 

In order to remove the sediment in the pool that has a median size of less than 1.3 mm, it is 

necessary to move the total mass of fines and sand found in the pool. The target size of the 

sediment to be removed from the creek bed is the size at which 90 percent is smaller than that size 

(d90). For Willow Creek, this is 2.79 mm (the average d90 for the three samples of the sediment 

found in the pool; see figure 10). The measured specific gravity of the deposit is 2.65. 

11
Not all of the stream bed needs to be cleaned of fine sediment––just the bed area underwater 

during the portion of the year that the small fish can be infected by whirling disease. For Willow 

Creek, the period from May 1 to September 30 was selected. For the purposes of reducing the 

infection of fish by whirling disease, the streamflows during this period are used to determine a 

base flow. The selected base flow for each year used in this analysis is the discharge exceeded by 

10 percent of the days. This discharge for each water year is presented in figure 11. In the Willow 

Creek streambed area, a streamflow on the order of 1.4 m3/s would be a reasonable target area for 

the flushing of fines. During the winter and late fall the streamflow is on the order of 0.2 m3

/s, 

which would be within the stream area flushed if the streambed area at a discharge of 1.4 m3/s was 

flushed of sand and fines. For the present analysis, the base flow was assumed to be 1.4 m3

/s, and 

the objective was to estimate the discharge required to remove the fine sediment in the part of the 

pool that is underwater at a discharge of 1.4 m3/s. An example of the discharge in the river for 

water year 1982 is shown in figure 12. The streamflow pattern is probably typical of the variation 

during a typical water year, with long periods of nearly constant discharge and some periods with 

very low streamflows. In the case of water year 1982 the 10 percent discharge is 1.29 m3/s. (In 

water year 1982, May 1 is day 241 and September 30 is day 365.) 

0

2

4

6

8

10

12

14

16

1954 1959 1964 1969 1974 1979

DISCHARGE EXCEEDED 10% OF DAYS, 

IN CUBIC METERS PER SECOND 

WATER YEAR

Figure 11. The daily discharge exceeded 10 percent of the days in each water year in 

Willow Creek below Willow Creek Reservoir, water years 1954–1982. The horizontal line 

marks a discharge of 1.4 m3/s. 

12
0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 50 100 150 200 250 300 350 400

DAILY DISCHARGE, 

IN CUBIC METERS PER SECOND

DAY IN WATER YEAR 1982

Figure 12. Daily streamflows in Willow Creek below Willow Creek Reservoir during water 

year 1982. 

The shear stress is not uniform across a cross section for any discharge. There is a flushing 

discharge that would remove sediment from the area that is wet at the base flow, but for discharges 

less than that flow only part of the stream bed will be flushed. The Physical Habitat Simulation 

System (Milhous and others, 1989) was used to make the hydraulic calculations of the shear stress 

on the stream bed and the areas of the pool where flushing of the fines and sand will occur. The 

results of the flushing flow calculations are presented in figure 13. 

The selection of a flushing flow using the information in figure 13 requires judgment. If, in 

the judgment of aquatic biologists, the river is adequately flushed of fines when 90 percent of the 

base flow stream bed is flushed, than the required flushing flow for Willow Creek is 7.32 m3/s (the 

dashed line in figure 13). 

The frequency of flushing flows is also important. The period of record available to the 

public is from 1954 through 1982. Using this data, the number of days in each water year that the 

discharge exceeds the flushing flow discharge of 7.32 m3/s was determined for each water year. 

The results are presented in figure 14. A review of the data for the Colorado River above Grand 

Lake suggests the 29-yr period (1954–82) is probably representative of the last 60 years. 

13
0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

PERCENT BASE AREA FLUSHED

DISCHARGE IN CUBIC METERS PER SECOND

Figure 13. Percent of the base flow pool cross-sectional area flushed as related to 

stream discharge. The dashed line shows the flushing flow needed if 90 percent of the 

bed surface needs to be flushed. 

0

10

20

30

40

50

60

1954 1959 1964 1969 1974 1979

NUMBER OF DAYS

WATER YEAR

Figure 14. Number of days in each water year that the discharge was greater than or 

equal to the discharge required to move fines from 90 percent of the base flow pool crosssectional area (7.32 m3

/s). Period of record was water years 1954–1982; the station is 

Willow Creek below Willow Creek Reservoir (09021000) and is just upstream of the study 

area. Data is from the records of the U.S. Geological Survey. 

14
There is concern about the higher discharges eroding the banks just above the pool. 

Sediment is falling off the nearly vertical banks and forming a deposit of sediment at the bank toe 

just above the 1.4 m3/s waterline. This deposit is easily eroded and may fill the pool as fast as the 

higher discharge removes sediment from the pool. For this reason, a design for bank protection 

works, which would protect those basks upstream of the pool at risk for erosion, was developed at 

the same time as the design to isolate the backwater (Waddle and Terrell, 2003). 

The selection of a target sediment size for removal from the stream is critical. The analysis 

herein used the size at which 90 percent is smaller (2.79 mm). If the median size (1.1 mm) was 

used as the target size, the flushing flow analysis would yield different results. Results for a median 

sediment size of 1.1 mm are added to the diagram for the Willow Creek results to illustrate the 

difference (figure 15). The target discharge for removal of 1.1 mm sediment is 2.73 m3

/s compared 

to the 7.32 m3

/s used in the analysis above. 

PERCENT BASE FLOW 

AREA FLUSHED

DISCHARGE (m^3/s)

0 2 4 6 8 10 12 14 16

10

20

30

40

50

60

70

80

90

100

0

1.1 mm

2.79 mm

Figure 15. Percent of the base flow pool cross-sectional area flushed as related to 

stream discharge, for two different estimates for the size of sediment to be flushed from 

the river. 

Discussion and Conclusions 

The hypothesis of this paper is that one can determine a flushing flow that removes the sand 

and fines that are good habitat for oligochaete worms. Standard hydraulic calculations were used to 

accomplish this task. There are many issues associated with flushing flows other than determining 

the quantity of the streamflow. Among these other issues are the use of the water for other 

purposes and the impacts of the sediment flushing flows on other instream resources. 

Key to the determination of a flushing flow is the knowledge of the relation between the 

quality of the sediment for oligochaete worms and the characteristics of the sediment. 

15
Acknowledgements 

The maps presented as figures 2 and 4 were created by Terry Waddle. The samples of 

worms and sediment used in the analysis for figures 6, 7, and 8 were collected by Jim Terrell, who 

also developed the protocol for the sampling. Sediment sample analysis was done by Julie Roth. 

Surveying and bed material sampling in the field was done by a team of Waddle, Roth, Terrell, and 

Milhous. This open file report is completely based on a presentation made at the 9th Annual 

Whirling Disease Symposium held in February 2003 in Seattle, Washington. 

References Cited 

Krueger, R.C., 2002, Correlations among environmental features, Myxobolus cerebralis infection 

prevalence in oligochaetes, and salmonid infection risk in the Madison River, Montana: 

Bozeman, Mont, Montana State University, Master of Science thesis. 

Milhous, R.T., Updike, M.A., and Schneider, D.M., 1989, Physical Habitat Simulation System 

reference manual––Version II: U.S. Fish and Wildlife Service, Instream Flow Information Paper 

No. 26, Biological Report 89(16), variously paged. 

New Mexico Department of Fish and Game, 2003, New Mexico fishing tules and information. 

2003–2004 license year: Santa Fe, N. Mex., p. 15. 

Sauter, G., and Gude, H., 1996, Influence of grain size on the distribution of tubificid oligocharte 

species: Hydrobioiogia, v. 334, p. 97–101. 

Waddle, T.J., and Terrell, J.W., 2003, Modeling channel modifications to reduce habitat for 

T.tubifex, in 9th Annual Whirling Disease Symposium, Seattle, Wash., February 6–7, 2003, 

“Managing the Risk”, Proceedings: Bozeman, Mont., Whirling Disease Foundation, p. 59-60 

Available at http://whirlingdisease.montana.edu/research/symposium.htm.

16