Evaluating the Frequency of Triggered Spring High 

Flow Experiments (HFE’s) Assumed in the LongTerm Experimental and Management Plan

Presentation to Glen Canyon Dam Adaptive Management Program, 

Technical Working Group

June 24, 2020

Paul Grams and David Topping

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

Monitoring and Research Center
The HFE Protocol: 

• Track sand inputs from Paria

River and model sand budget 

during designated accounting 

periods

• July 1 – Dec. 1

• Dec. 1 – Jun. 30

• Find the magnitude and 

duration of HFE that “fits” the 

amount of sand available

• Schedule HFE

Wright and Kennedy (2011)

Glen Canyon Dam Hydrograph, USBR

Preliminary data, do not cite
Sand accounting periods • The objective of the HFE Protocol in the LTEMP is to achieve sandbar building while retaining a positive 

sand mass balance. 

• Sand mass balance is a relative measure and 

depends on the period over which it is computed. • For LTEMP alternative analysis, sand mass 

balance was evaluated over the duration of 20

-

year simulations. • This long-term evaluation will be the 

ultimate test of the HFE Protocol and is 

being monitored in Project B.2

• For HFE implementation, we evaluate the mass 

balance over short accounting periods. • These accounting periods were chosen to coincide with the periods of most likely 

sediment inputs from the Paria River

Example of long-term 

sand mass balance 

for evaluation of dam 

operations over 

periods of many 

years (Grams and 

others, 2019)

Example of short-

term sand mass 

balance over fall 

accounting period for 

HFE implementation
Sand accounting periods, cont.

Short-term mass balance 

accounting windows:

• Distinct Spring and Fall accounting 

periods:

• Can design HFE to “use” only 

recent sand inputs.

• HFEs are implemented when 

storage in Upper Marble Canyon 

is highest

• Simple decision process

Plot from LTEMP EIS, Appendix P
Frequency of Spring HFEs 

As estimated in LTEMP

• Simulations designed to represent 

the full range of historical 

conditions:

• 21 hydrologic traces

• 3 sediment traces (low, 

median, high)

 May be sufficient sediment input 

to trigger Spring HFEs in “26% of 

the years in the LTEMP period”

Estimated number of HFEs to occur during 20-year 

implementation of LTEMP 

(“D” was selected alternative)

Plot from LTEMP EIS, Appendix E
Frequency of Spring HFEs, 

cont. 

Based on observations 

of past 20 years:

• Compare December – April Paria

sand inputs with December to 

April sand export from Marble 

Canyon

 May have been sufficient 

sediment input to trigger Spring 

HFE: “Once since 1998”

Annual Paria River sand input (black) compared with Marble 

Canyon sand export (red)

Data from: https://www.gcmrc.gov/discharge_qw_sediment
Frequency of Spring HFEs, , cont. 2

Why the difference between the 

two analyses?

• The LTEMP simulations considered 

Paria River sand inputs since 1963

• Fall (summer) sand inputs from 

Paria have been relatively 

consistent

• Spring (winter) sand inputs were at 

least 3 times greater between 1964 

and 1997 than between 1998 and 

present

 Summer sand inputs have been 

consistent, but winter sand inputs 

have not been consistent

Maybe there will be a return to larger 

winter floods, or maybe there has been a 

shift towards less winter precipitation.

• Black circles (summer/fall inputs) and red diamonds 

(winter/spring inputs) are data used in LTEMP

• Blue “+” are 1998 – 2018 data we looked at (same)

Data from: https://www.gcmrc.gov/discharge_qw_sediment
How to trigger a Spring HFE: • Merge the separate fall and spring accounting 

periods to one annual accounting period: • Could design to implement HFE in fall or 

spring

• Would need a process for deciding 

whether to implement HFE in Fall or 

Spring

• Would likely end up with more sand export before implementing spring HFE • Lower dam releases in winter would result in more sand left for spring 

HFE

• Can evaluate this with sand mass 

balance model

• Although a single annual accounting period is 

not the process that was described in LTEMP, 

it is fully consistent with the scientific basis for 

the “store-and-release” HFE approach that 

was adopted in LTEMP.

Plot from LTEMP EIS, Appendix P
How to trigger a Spring HFE, cont:

Looking back at sand budgets for Marble Canyon 

from 2002 to 2017:

• 9 years with fall sediment triggers: 

(> 300,000 metric tons of accumulation)

• 1 year with winter inputs resulted in 

greater sand enrichment for the next 

spring

• 5 years where winter inputs meant 

fall and spring sand enrichment in 

Marble Canyon were about the same

• 2 years where sand enrichment in fall 

was much larger than in spring

• 1 year where there was sand 

enrichment in fall and no sand 

enrichment in spring Plot from LTEMP EIS, Appendix P

Data from: https://www.gcmrc.gov/discharge_qw_sediment