U.S. Department of the Interior

U.S. Geological Survey

Turbidity as a management 

tool to constrain Rainbow 

Trout downstream of the Paria 

River

David Ward, Ken Sheehan, Ben Vaage

U.S. Geological Survey, Southwest Biological Science Center, 

Grand Canyon Monitoring and Research Center
Turbidity

NTU vs FNU vs TSS

Nephlometric Turbidity Units Formazin Turbidity Units Total Suspended Solids ppm or mg/L

Clear Muddy 
100

1,000

10,000

100,000

0 50 100 150 200 250 300 350

COLORADO RIVER AT LEES FERRY

Mean daily value

Mean+1SE daily value 

TURBIDITY, IN FNUs

JULIAN DAY

200 FNUs

Pre-dam

Post-dam

Day of Year

Prior to GCD the 

Colorado River was 

very different

Voichick and Topping 2014, Scientific Investigations Report 2014-5097

Turbidity shaped the unique 

endemic fishes
Even prior to Glen Canyon Dam the 

Colorado River was impacted by 

invasive species

But only by species adapted

To live in turbid water!
Fish are physiologically adapted to their environment 

and anthropogenic changes to those environments 

have consequences for species survival. 
Conservation of imperiled native fishes requires an 

understanding of the ecological and evolutionary 

processes that have shaped predator-prey interactions 

and how those interactions are mediated by specific 

features of habitat. To be effective, management of 

native fishes must include elements of maintaining or 

enhancing features of habitat that reduce predation 

vulnerability 

(Matter and Mannan 2005, How do prey persist, The Journal of Wildlife Management 69: 1315-1320). 

Physiology – Morphology - Behavior
Previous Experiments

• How does turbidity 

effect the ability of 

rainbow and brown 

trout to capture 

juvenile chub?
Turbidity (FNU)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Probability of Survival (%)

0

20

40

60

80

100

Mean

95 % C.I.

Percent probability of survival for juvenile humpback chub (60 mm 

TL) exposed to predation by rainbow trout (285 mm TL) as 

turbidity increases from 0 – 150 FNU at 15° C. 

Rainbow trout

Ward et al. 2016. Effects of turbidity on predation vulnerability of Juvenile Humpback chub

to Rainbow and Brown Trout. Journal of Fish and Wildlife Management 7: 1-8
Rainbow Trout Summary

25 NTU 50 NTU 75 NTU 100 

NTU

46 % 

survival

62 %

survival

60 %

survival

75 %

survival

Compared to trials in clear water where survival = 0

(For 60 mm Humpback chub and 285 mm Rainbow Trout)
Brown trout

Turbidity (FNU)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Probability of Survival (%)

0

10

20

30

40

50

Mean

95 % C.I.

Percent probability of survival for juvenile humpback chub (60 mm 

TL) exposed to predation by brown trout (285 mm TL) as turbidity 

increases from 0 – 150 FNU at 15° C. 

Brown trout

Ward et al. 2016. Effects of turbidity on predation vulnerability of Juvenile Humpback chub

to Rainbow and Brown Trout. Journal of Fish and Wildlife Management 7: 1-8
Brown Trout Summary

25 NTU 50 NTU 75 NTU 100

NTU

.05 % 

survival

1 %

survival

20 %

survival

35 %

survival

Compared to trials in clear water where survival = 0

(For 60 mm Humpback chub and 285 mm Rainbow Trout)
This experiment

• How does turbidity 

effect growth and 

survival of rainbow 

trout?
Methods
Large Rainbow Trout

Days

0 5 10 15 20 25 30 35

Turbidity (NTU)

0

20

40

60

80

100

120

140

160

180

Tank 1 Control

Tank 2 Treatment

Tank 3 Treatment

Tank 4 Control

Preliminary data, do not cite
Large Rainbow Trout (329 mm mean TL)

Water Clarity

Clear (<10 NTU) Turbid (80 NTU)

Change in Weight (g)

-60

-40

-20

0

20

40

60

p>0.0783

N = 16

Preliminary data, do not cite
Small Rainbow Trout

Days

5 10 15 20 25 30 35 40

Turbidity (NTU)

0

20

40

60

80

100

120

140

160

180

Tank 1 Control

Tank 2 Treatment

Tank 3 Treatment

Tank 4 Control

Preliminary data, do not cite
Small Rainbow Trout (100 mm TL)

Treatment

Clear (2 NTU) Turbid (90 NTU)

Change in Weight (g)

1

2

3

4

5

6

7

N = 32

Preliminary data, do not cite
So we know extended turbidity 

causes reduced trout condition –

What does that mean for 

management?

 Low condition increases natural mortality

 Lowers reproductive potential

 Makes fish more susceptible to disease
Rainbow Trout 

200020022004200620082010201220142016

0

20

40

60

80

100

CPUE (fish/hr)

Can we see a turbidity effect in the field data?

Difficult – need something more proximate – Fish Condition

Arizona Game and Fish Long-term Monitoring Data, Electrofishing

Date
Prel

Date

Mainstem Colorado River Turbidity, USGS 30-m

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

Sep 

M

ar 

FNU 

0 50 100 200 300 

2005 2006 2007 2008 2009 2010 20 11 20 12 20 13 20 14 20 15 
Date 

Mar 

Sep 

Mar 

0 

50 

FNU 

100 150 200 250 300 

r 
Preliminary 

0 50 150 250 0 50 150 250 

I l l I l I 

Mar Apr 

Apr May 

May Jun 

Jun Jul 

Jul N Aug N Aug 0 Sep ~ 0 Sep 

- Oct 

-

N 

Oct Nov 

Nov 

Dec 

Dec Jan 

Jan Feb 

Feb Mar 

I I I I 

0.7 

0.8 

0.9 1.0 0.7 

0.8 

0.9 1.0 

Fi sh Condition Fi 

sh Condition 

FNU 

FNU 

0 50 150 250 0 50 150 250 

',_ I I I I I Mar 

Mar 

Apr Apr 

May 

May 

Jun Jun 

Jul N Jul~ .=::b. J N Aug Aug I I 0 0 Sep 

-

(II Sep I I -c.., 

Oct Oct 

Nov Nov 
Preliminary data, do not cite

0 

M 

I() 

N 

0 

N 

I() 

~ 

0 

~ 

I() 

0 

Rainbow Trout Fish Cond ition: Weight in the CR 

Small CR Trout Clear(S)to Turbid (E) Change in Fish Condition by Weight 

0.0 0.5 1.0 1.5 

Small Trout Fish Condition 

0 

Large CR Trout Clear (S) to Turbid (E) Change in Fish Condition by Weight 

CD-.----------------~-------~ N 

0 

CD 

N 

0 

0 

N 

0 

(X) 

..-

0 

CD 

..-

0.0 0.5 1.0 1.5 

Large Trout Fish Condition 
Yearly Days Above 100 FNU & RBT Condition

Preliminary data, do not cite

=> 

z 

LL 

0 

0 50 

T""9 

~ 0 1.00 

..c 

ro 

Z' 40 0.98 

""C 

..c 

s.... 0.96 

::::l 

+,I 

..c 

+,I 30 .$ 0.94 

~ ro 

0 

'+--

0 20 

0.92 C 

0 

:j:i 

0.90 ""C 

C 

0 

s.... 

Q) 0.88 () 

..c ..c 

E (/) 

·- ::::l 

z 

0.86 LL 

10 ---~~~~~~~~~~~~~~~~~~~~~~~ 

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 

Year 
~USGS 

Spatial Trends in RBT Condition Factor 

Condition Factor 

• {0.84,0.86] 

• {0.86,0.88] 

• {0.88,0.9] 

• {0.9,0.92] 

• {0.92,0.94] 

• {0.94,0.96] 

) 

f( 

I 
Manipulation of turbidity from the Paria River to benefit downstream 

native fish (without impacting Lees Ferry Rainbow trout) 

warrants further evaluation
Sediment augmentation was considered as part 

of 1995 GC Dam EIS

Randle 2007 Study – Sediment Augmentation

 Target -

 500 ppm TSS (500-600 NTU) 

 1 million tons of sand

 8 month period from May – Dec

 We may only need 1/5th of the silt/Clay and no sand 

 May only need it from April to July
https://www.gcmrc.gov/discharge_qw_sediment/station/GCDAMP/09382000

USGS 09382000 PRRIR RIVER RT LEES FERRY, RZ 

2000.0 

"Cl 1000. 0 

C 

0 

(.) 

Q) 

I,') 

'-

Q) 

Q. 

-- -_ :t=====t==== : == - -------- ~ _v ~~

,= ;= 

10. e i,..,..'="° b I ~- ~ i::_::::::::::t:\ , 

I 

, 

----__ 

-

-

=~ =====t=====t===== ======i::::===t===== 

_..,. --------

-

-- - ,--r 

1.0 Jan ---------------------------------------------------------- 01F eh Olar 01Rpr Ol1ay 01Jun 01.Jul 0:1.Rug 01Sep O:Dct 01Nov O:Dec -....01 --oiiiiiioll 

2017 2017 2017 2017 2017 2017 2017 2017 2017 2017 2017 2017 

- Discharge -- Period of approved data 

- Estinated 
Conclusion

 Relatively low turbidity (100 FNU) not only impacts 

predation dynamics but is also likely to negatively 

impact rainbow trout populations downstream of the 

Paria.

 Higher turbidities may be necessary to effect 

predation dynamics of brown trout. 

 Laboratory, field data and literature all suggest that 

rainbow trout populations can be negatively 

impacted by low level turbidity – but duration is 

important
Instead of always reaching for Mechanical Removal

Why not experiment with other methods of

Making the river more conducive to native fish?

Turbidity manipulation ?

Caveat – not my expertise !
A Sediment Retention Basin

Sluice gates to allow controlled delivery of turbidity

How about some small scale turbidity manipulation

Experiments using a small pump back system

with diesel water pumps
Example: Sediment Retention Basin
Rental equipment to experiment ?
Doesn’t need to look like this
But if it looked like this –

it would be way better for native fish

Colorado River near Moab
1. Presentation name:

Turbidity as a potential tool to constrain Rainbow Trout populations and reduce

predation/competition on juvenile humpback chub

2. Where this work fits in the work plan?

Project 9.6 in FY 15-17 workplan

3. How this work relates to GCDAMP goals and needs and the LTEMP EIS?

Goals to manage both a recreational trout fishery and protect native fish downstream

4. When the project started, where is at now, when will it end?

FY 15-17 project - completed

5. How was the project funded?

GCDAMP

6. How will the project results be used for GCDAMP management decisions and actions?

These results indicate methods to manage turbidity to meet fish objectives may

need further investigation

7. What adjustments to the project should be considered based on work to date?

None – project completed

Requested Elements