Effects of drought-related low phosphorous 

concentrations on the aquatic ecosystem 

downstream of Glen Canyon Dam, and potential to 

improve productivity via nutrient fertilization

Josh Korman

Ecometric Research

Amy Ker

University of British Columbia

Ken Ashley

British Columbia Institute of Technology

with contributions from:

Bridget Deemer and Charles Yackulic

US Geological Survey

Grand Canyon Monitoring & Research Center

Funded by Western Area Power Administration 1
GCD AMP management actions (water)

Experimental Flow Actions

(Quantity)

Water Quality ‘In-Actions’

(Quality)

Spring HFE’s (‘96, ’08)* High water temperature*

Fall HFE’s (‘04, ’12, ‘13, ’14, ’16, ’18) Low dissolved oxygen*

Bug Flows (‘18, ‘19, ‘20, ‘22)

Low phosphorous concentrations*

Low steady summer flow (‘00)*

Fall steady flows (‘09, ‘10, ‘11)

Equalization flows (’11)*

* affect on bugs or fish

Bugs

Low steady summer flow (‘00)

Spring HFEs (‘96, ‘08) +

Equalization (2011)

High water temperature

Low dissolved oxygen

Low phosphorous -

Trout Chub Bass

+ -

+ -

+  - -

- +/- +

-

- -(?)

2
Could water quality, rather than 

water quantity, be the dominant 

influence on fish populations 

downstream of GCD under current 

and future conditions?

Or has water quality always been 

the dominant influence? Existing growth and 

survival models for Humpback chub (Gila cypha) in the mainstem 

include effects of water temperature, turbidity, and rainbow trout 

abundance. They do not include flow effects.

3
Soluble Reactive Phosphorous (SRP) concentration

at Glen Canyon Dam

Factors influencing SRP in GCD Releases

4

1. Inflows (amount & source)

2. Processes in reservoir

3. Reservoir elevation (depth of 

withdrawal)

2002-2004 

drought

higher inflows 

(‘07-’09 & ‘11 -

equalization)

2012+

drought

2018-2022

drought

Data provided by B. Deemer, USGS
Observed effects of SRP on Colorado River 

aquatic ecosystem

1. Phosphorous limiting element for phytoplankton production in Lake Powell 

(Gloss 1977)

2. SRP positively correlated with invertebrate drift in Glen and Marble Canyons, 

2012-2016 (Korman et al. 2021) and secondary production in Glen Canyon 

(Yard et al. 2022)

3. SRP positively correlated with rainbow trout growth in Glen Canyon, 2012-

2021 (Korman et al. in review)

4. SRP was a better predictor of rainbow trout recruitment in Glen Canyon than 

flow (Yackulic, unpublished data)

5. Interannual trends in condition factor of humpback chub, flannelmouth

sucker (Catostomus latipinnis), and bluehead sucker (Catostomus discobolus)

in Grand Canyon were similar, and broadly correlated with the trend in 

phosphorous concentration (Yard and Yackulic, unpublished data)

5
Nutrient fertilization is a common and well-tested 

method for enhancing productivity in freshwater 

ecosystems that are nutrient-limited

• Fertilization projects have focused on addressing 2 limitations:

• Mitigating impacts of phosphorous losses due to impoundment (dams)

• Mitigating impacts of reduced salmon returns due to overfishing and low marine 

survival

• Fertilization literature survey reviewed 166 papers

• 111 separate nutrient enhancement projects

• 80% of projects have been conducted in North America, with most others in Europe

• Majority of studies (58%) from British Columbia (43), Alaska (41), and Idaho (21)

• 86% of fish species assessed were salmonids (but some chub and suckers)

• A number of efforts conducted in large lakes and large rivers (e.g., Libby Dam on 

Kootenai River in Montana)

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Nutrient Fertilization Studies

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Ecosystem Responses

Component Measured # of 

Studies

% of 

studies

% of positive 

responses

Nutrients 74 44% 72%

Algae 122 74% 58%

Macrophytes 9 5% 58%

Invertebrates 97 59% 62%

Fish 89 54% 68%

Fish Population Responses

Component Negative Neutral Positive

Fish growth 3% 25% 72%

Fish abundance 10% 26% 64%
Preliminary Calculations for Glen Canyon 

Fertilization

• Target phosphorous concentration of 3 ug/L at release point (Glen 

Canyon Dam)

• March 1 – August 31 (spring-summer)

• Requires ~21,000 gallons of 10-34-0 liquid fertilizer dripped in at 

ratio of 70,000,000 (river flow) : 1 (liquid fertilizer)

• Cost of ~$65,000/yr at $570/metric ton 

• Current cost about 2-fold higher due to war in Ukraine.

8
Is phosphorous fertilization a viable GCD AMP action 

to mitigate effects of drought and impoundment?

• Does not require water 

• Does not impair hydropower 

• Relatively low cost 

• Makin’ life, not takin’ life 

• Extensive support in literature that it can be effective 

• Indications in GCD AMP data it could be effective 

• Relatively easy to manipulate/study 

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Potential next steps

• WAPA will distribute fertilization literature review to AMP

• GCMRC could include a fertilization component in their upcoming 3-year 

workplan

• Baby step: nutrient-algae bioassays to better evaluate P limitation

• Big-boy step: experimental fertilization in Glen Canyon

• Simpler logistics/lower costs

• Extensive baseline of drift, trout growth and recruitment

• Might help offset current impacts of high temperature and low oxygen on 

rainbow trout

• Could be implemented at downstream locations for native fish (near LCR)

10
At lower reservoir elevations we are drawing 

increasingly from the reservoir surface waters

In summer: 

Lower dissolved oxygen

warmer water

less SRP rich water

Figure from Vernieu et al. 2005 

(second version manually 

modified) 

12
Lower water levels (and warmer 

temperatures) will generally mean 

less SRP

Average SRP gradient of 2 

μg L-1 per 10 m drop below 

the penstock 

Mechanisms:

Plankton in surface water 

take up phosphorus so less 

is available

Surface waters have higher 

pH/temp so free SRP is 

bound to calcite

Preliminary Data, Do Not Cite 13
Depth-related differences in SRP 

are biologically meaningful

2 μg SRP L-1

14
Phosphorous (P) additions produce twice the growth benefit

for rainbow trout relative to bug flows if 

baseline P is low (drought)

no bug flows for

both scenarios

drought SRP (2018-2021) 

for both scenarios

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Potential Impediments for Experimental 

Implementation of Phosphorous Fertilization

• Historically, GCD AMP actions have focused on changes in flow or 

mechanical removal

• fertilization is an unfamiliar action

• Does not fit in with natural flow paradigm

• Administrative concerns 

• phosphorous can be a pollutant (but in systems with too much phosphorous)

• May produce benefits to rainbow trout population in Glen Canyon

• Reduce probability of collapse due to high temperature and low oxygen

• But high temperature and low oxygen may be overwhelming

• Uncertain response for Colorado River Ecosystem, as some of its 

characteristics are rare in fertilization studies

• Abundant macrophytes (Glen)

• Sandy river bottom and periods of high turbidity (Marble, Little Colorado River inflow reach)

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