1 

Proposal to Amend the High-Flow Experiment Protocol and Other 

Considerations

Developed by the Flow Ad Hoc Group, through the Technical Work Group of the 

Glen Canyon Dam Adaptive Management Program in partnership with the Grand 

Canyon Monitoring and Research Center and the Bureau of Reclamation. 

Accepted by the Technical Work Group on August 9, 2023 and forwarded to the 

Adaptive Management Work Groupon August 10, 2023 for consideration.

Map derived from the GCDAMP Wiki and modified by Mel Fegler, Wyoming State Engineer’s Office
2 

TABLE OF CONTENTS

1. Introduction .................................................................................................................. 4

2. Problem Statement and Objectives............................................................................... 5

3. Background ................................................................................................................... 6

System History .............................................................................................................. 6

Declining Reservoir Elevations....................................................................................... 7

Current Sandbar Conditions.......................................................................................... 8

4. HFE Elements for Consideration.................................................................................... 8

5. Proposed Revisions to the HFE Protocol........................................................................ 9

The Original 2011 HFE Protocol..................................................................................... 9

Current Sediment Accounting Periods and HFE Implementation Windows.................. 9

Proposed changes to the modeling component and analysis phases: Revision to 

Section “1.2 MODELING” of the HFE Protocol.............................................................10

HFE Protocol Revisions in Relation to LTEMP Resource Goals.....................................12

6. Spring HFEs and Potential Impacts to Other LTEMP Resources...................................14

Figures.............................................................................................................................17

Tables..............................................................................................................................23

References ......................................................................................................................25

LIST OF FIGURES AND TABLES

Figure 1. Modeled sandbar volume from 2002 to 2019 

Figure 2. Sandbar types

Figure 3. Mean normalized sandbar volumes

Figure 4. Cumulative changes in sandbar volume between October 2012 and October 2019 and 

between October 2019 and October 2022

Figure 5. Example of gully erosion at Tatahatso Beach

Figure 6. Average monthly sand loads from the Paria River and Little Colorado River and existing 

LTEMP Accounting and Implementation windows

Figure 7. Post-hoc modeling for HFE Implementation considerations

Figure 8. Predicted resource effects from Spring Disturbance Flow, Spring HFE and Fall HFE

Table 1. Technical alternatives for 16 different HFE scenarios

Table 2. HFEs described in LTEMP
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ACKNOWLEDGEMENTS

The Technical Work Group (TWG) would like to acknowledge Larry Stevens, Craig Ellsworth and 

Paul Grams for their initial data compilation, writing, and fact checking efforts. Also wish to 

thank Laura Dye, Emily Young, Seth Shanahan, Shana Rapoport, Mel Fegler, Colleen 

Cunningham, Christina Noftsker, Leslie James, Rob Billerbeck, Ted Kennedy, and Betsy Morgan 

for all their writing and editing work.

Additionally, the TWG acknowledges Andrew Schultz, Ben Reeder, Bill Persons, Bill Stewart, 

Clarence Fuller, Cliff Barrett, Dan Leavitt, David Topping, Emily Young, Laura Dye, Helen Fairley, 

Jeremy Hammen, Jim Strogen, Kelly Burke, Kristen Johnson, Michelle Garrison, Nicole Gibney, 

Ryan Mann, Shane Capron, and Ted Rampton for collaborative participation and guidance.

EXECUTIVE SUMMARY

TBD after acceptance of Draft Document
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1. INTRODUCTION

High-flow experiments (HFEs) from Glen Canyon Dam (GCD) are part of the Long-Term 

Experimental Management Plan (LTEMP) to determine whether periodic high releases from 

GCD can be used to sustainably manage sandbars and shoreline habitats in the Colorado River 

Ecosystem (CRE) downstream, particularly as Fall and Spring-timed HFEs. The sediment 

resource goal as defined in the GCD LTEMP is to: “Increase and retain fine sediment volume, 

area, and distribution in the Glen, Marble, and Grand Canyon reaches above the elevation of 

the average base flow for ecological, cultural, and recreational purposes.” Over the past 25 

years, scientific information on the use and timing of HFEs has improved understanding of how 

best to manage tributary-derived sediment supplies below the dam for these purposes. Refined 

evaluation of opportunities and impediments for HFEs over the past decade under significantly 

lower Lake Powell reservoir levels warrants review of the HFE implementation protocols. In 

particular, re-evaluation of the HFE sediment Accounting Period and Implementation Window is 

indicated as scientifically supportable, to potentially more fully and adaptively achieve the 

LTEMP goals as they relate to using HFEs.

At the February 15, 2023 Glen Canyon Dam Adaptive Management Program’s 

(GCDAMP) Adaptive Management Work Group (AMWG) meeting, AMWG directed the 

Technical Work Group (TWG) to review the sediment Accounting Period and provide feedback 

to the Bureau of Reclamation (Reclamation). Consequently, the TWG charged the Flow Ad Hoc 

Group (FLAHG) with the following: 

The FLAHG is charged with working with Grand Canyon Monitoring and 

Research Center (GCMRC) to draft an outline and a full proposal for amending 

the HFE protocol. In doing so, the FLAHG should consider the information 

developed by GCMRC in response to the Secretary’s Designee’s August 18, 

2022 request to evaluate high-flow experiments under low-elevations/lowflows and any other pertinent information to evaluate the accounting window.

The proposal should consider the science, approach, and compliance elements 

that may be needed to make changes to the protocol. An outline of the 

proposal is requested by the Spring [2023] TWG meeting and the full proposal 

by the Summer [2023] TWG meeting.

To meet this charge, the FLAHG developed an outline for the April 2023 TWG meeting. 

The draft document presented here expands on that outline and provides guidance on how to 

potentially refine the HFE Implementation Windows and sediment Accounting Periods, as well 

as other elements of the HFE protocol, such as forecasted inflows, Paria River sand mass 

balance modeling, and compliance. Also included herein is a consideration of impacts to highrisk non-native fish, hydropower resources, cultural considerations, etc., for future HFEs, 

especially in relation to sustained low Lake Powell reservoir elevations. The proposed changes 

identified in this document are based on initial considerations and are subject to change as 

discussions and review of implications for resources continue. 
5 

2. PROBLEM STATEMENT AND OBJECTIVES

 Aridification and overallocation of water resources in the western U.S. has led to lower 

reservoir elevations in Lake Powell. This has resulted in various problems including increased 

temperatures of water releases, entrainment of warm-water non-native fish, and risks to power 

production at GCD. These and other problems resulted in the Secretary of the Interior deciding 

to not implement Fall HFEs in 2015, 2021, and 2022, despite reaching the appropriate input 

triggers for sediment HFEs. The absence of Spring HFEs during the first 10 years of the HFE 

protocol, coupled with analyses documenting reduced transport of fine sediments in years with 

low release volumes and low Lake Powell elevations, have prompted the GCDAMP to reassess 

the HFE protocol. The objective of this document is to inform decision makers as to whether 

adjustments in the sediment accounting and HFE implementation procedures are warranted. 

Provided below is some of the information considered in developing this proposal: 

● Data on sediment input and mainstream transport are now monitored with 

sufficient resolution to predict HFE effects; 

● The Fall HFE Implementation Window, October-November, occurs before the 

spring runoff, therefore extra water released during that time may increase the 

risk of Lake Powell approaching or falling below minimum power pool elevation;

● A Fall HFE may increase the potential risk of entrainment and dispersal of warmwater non-native fish into the Grand Canyon when reservoir elevations are low 

(below 3530’ +/-), and fish density in the forebay is high. Risk of non-native fish 

entrainment through the dam is of heightened concern without a fish barrier in 

place; 

● Sandbar volume resulting from Fall HFEs may be largely eroded before the highuse recreation season begins and before the peak season for wind transport;

● Spring HFEs under the existing HFE protocol have been triggered less frequently 

than predicted in LTEMP;

● High flows historically occurred more frequently during the spring than the fall;

● Spring HFEs may be more reliably planned and implemented than Fall HFEs 

because the runoff forecast is less uncertain in the spring than the fall;

● Reduced releases from GCD as a result of recent low reservoir elevations has 

resulted in extended residence time for new sediment supplies in the 

mainstream channel; and 

● Proactive Spring HFEs are limited to years when annual releases are 10.0 million 

acre-ft (MAF) or greater. These high-volume years are unlikely to occur under 

low reservoir elevations and recent hydrologic trends, and therefore 

adjustments to the HFE implementation triggers and duration of proactive Spring 

HFEs may be warranted. This topic has not been fully discussed within the FLAHG 

and is not integrated into this document. 

If HFEs are infrequently implemented, sandbars will be infrequently rebuilt; and existing 

sandbar deposits will erode relatively quickly. With more frequent HFEs, sandbars will sustain 
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their size longer and may progressively increase in size (Figure 1). The HFE protocol was 

designed to test if “sandbar building during HFEs could exceed sandbar erosion between HFEs 

such that sandbar size can be increased and maintained over several years” (LTEMP ROD page 

B-19). Modeling indicated Fall HFEs would be triggered in about 77% of the years in the LTEMP 

period, or 15 in 20 years (LTEMP ROD page B-20). Modeling also indicated that there may be 

sufficient sediment input for Spring HFEs in about 26% of the years in the LTEMP period, or 5 in 

20 years (LTEMP ROD page B-19). Although it was possible for there to be some years with two 

HFEs (one in fall and one in spring), and some years with no HFEs, the experimental design to 

test the hypothesis was intended for an average of one sediment-triggered HFE per year to 

occur.

While this proposed protocol revision is a direct response to the February 15, 2023

AMWG request, the GCDAMP has been considering the feasibility of increased opportunities for 

Spring HFEs since 2019. The Department of the Interior (DOI) released a Memorandum on 

August 14, 2019 that states: “In response to stakeholder input at recent AMWG meetings, the 

feasibility of conducting Spring High Flow Experiments (HFE), along with modeling for 

improvements and efficiencies that benefit resources including natural, cultural, recreational, 

and hydropower should be explored. As a potential starting point, I [Dr. Tim Petty, who was 

serving as the Secretary’s Designee] encourage you to consider opportunities to conduct higher 

spring releases within power plant capacity, along with Spring HFEs that may be triggered under 

the current LTEMP Protocol.” 

3. BACKGROUND

 

System History 

Historically, the Colorado River could be characterized as a high-elevation snowpackdriven, bi-modal (meaning large inflows from both spring runoff and monsoonal storms) river 

system that featured large springtime floods as snowpack melted, transitioning into lower-flow 

conditions through summer, fall, and winter. Streamflow in the Colorado River between Lake 

Powell and Lake Mead has been controlled by GCD since March 1963 and operations have 

largely eliminated snowmelt-driven spring floods and elevated base flows while at the same 

time, created diurnal flow fluctuations (Topping et al. 2003). Lake Powell traps upstream 

sediment and associated inputs from Colorado River tributaries downstream of GCD are limited 

(Topping et al. 2000). This results in clear water releases that may continue downstream long 

distances (Stevens et al. 1997). One of the most visible impacts of GCD installation was the 

decline in the number and size of alluvial sandbars (Dolan et al. 1974; Schmidt and Graf 1990). 

Although narrow strips of fine sediment line the banks throughout much of Grand Canyon, the 

largest sandbars occur in zones of recirculating flow. These eddies can be dozens of meters 

wide and hundreds of meters long (Figure 2). Because sandbars are composed of 

unconsolidated sediment, they inevitably erode through several processes including cutbank 

retreat, seepage processes, gullying, and wind deflation (Bauer and Schmidt 1993; Budhu and 

Gobin 1994). Prior to completion of GCD, sediment-rich spring floods replenished sandbars 

annually. Without those floods, progressive sandbar erosion occurs. 
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Since 1996, HFEs have been the primary mechanism used by Reclamation to determine 

if and how the sediment resource goal, now defined in LTEMP, can be achieved. Declining 

reservoir elevations as well as the interaction among low reservoir conditions and the threats 

posed by warm-water non-native species have disrupted the frequency of HFE implementation 

during the LTEMP period. This proposal presents a process for refining the protocol for 

implementing HFEs, primarily through adjustments of the HFE sediment Accounting Periods and 

Spring Implementation Window to provide greater opportunity for HFE implementation under 

varying/uncertain hydrologies, including low reservoir conditions observed in current times. 

HFEs have been conducted at GCD on an experimental basis since 1996 in an effort to 

rebuild sandbars and shoreline habitats with settled sand along the Colorado River in the lower 

Glen Canyon National Recreation Area and through Grand Canyon National Park. Findings from 

the initial experimental “controlled floods” resulted in the design of HFEs with the following 

characteristics: 1) peak magnitude as high as possible within facilities constraints, 2) duration as 

long as possible without creating a deficit in the available supply of sand, and 3) frequent 

implementation to offset intervening erosion. These factors are incorporated in the “HFE 

Protocol” (US Department of the Interior 2016). 

Between 2012 and 2018, five HFEs were conducted under the HFE Protocol, all resulting 

in beneficial sandbar deposition and improved beach quality for camping. Since the last Fall 

HFE1 in 2018, sandbars have significantly eroded, and the condition of sandbars have 

deteriorated. 

Continued conditions of low runoff, low reservoir levels, and other resource concerns 

have created risks that have prevented implementation of HFEs for four consecutive years. 

Risks identified during the consideration of past HFEs included the concerns of warm-water 

non-native fish entrainment and dispersal, as well as impacts to hydropower production and 

water delivery. If reservoir levels do not recover or deteriorate further, such risks are likely to 

persist. Several of these risks could be alleviated by adjusting the HFE Protocol to allow 

increased likelihood of triggering HFEs in the spring when hydrologic conditions and reservoir 

levels are receiving spring runoff. 

 

Declining Reservoir Elevations 

The Grand Canyon Protection Act of 1992 directs the Secretary of the Interior to operate 

GCD “in such a manner as to protect, mitigate adverse impacts to, and improve the values for 

which Grand Canyon National Park and Glen Canyon National Recreation Area were 

established.” The operation and actions of GCD must be fully consistent with and subject to the 

Law of the River. Declining reservoir elevations have made this dual mandate difficult. For 

example, if elevations at GCD cannot be maintained above 3490’, the ability to deliver water 

downstream is affected (or impacted) and there may be implications to critical infrastructure 

and power grid reliability. 

In recent years, releases of lower monthly volumes during the winter have allowed less 

sediment to be transported downstream. Low reservoir levels and warmer releases have 

1 The last HFE was implemented in Spring 2023, but the most recent Fall HFE was implemented in 2018. Given 

extraordinary conditions, Reclamation conducted a one-time experimental spring flow in April of 2023 for 72 

hours. The results from this experimental flow have not been fully analyzed at the time of this report.
8 

increased the potential for entrainment and downstream distribution of predatory non-native 

fish. Evaluating these complex trade-offs requires timely and high-quality monitoring data and 

must allow for continuous informed application of adaptive ecosystem management practices. 

To better manage sediment in the Colorado River below Glen Canyon Dam, the review of the 

adequacy of the HFE Implementation Windows and sediment Accounting Periods under low 

reservoir conditions is warranted.

 

Current Sandbar Conditions 

A total of 10 experimental releases at or above power plant capacity of various 

durations have been conducted since the first experimental release in 1996. In a recent 

summary of sandbar monitoring data, Hazel et al. (2022) showed that sand volume increased at 

86% of the monitoring sites between 2004 and 2020 due to the seven HFEs that occurred 

during that period (Figure 3). In contrast, net erosion occurred at 61% of the monitoring sites 

between 1990 and 2003 – a period that included only one HFE (Hazel et al. 2022). 

No HFEs occurred between 2019 and 2022. Since the 2018 HFE, sandbar volume has 

declined (Figure 4, lower) due to gradual erosion resulting from “normal” GCD operations that 

would be expected following any individual HFE (Hazel et al. 2010), as well as dramatic episodes 

of non-GCD related local erosion in 2021 and 2022 during summer thunderstorms2

. Hillslope 

runoff during these events resulted in the formation of gullies and deposition of debris on 

sandbar campsites (Figure 5). Similar damage caused during previous monsoon seasons was 

repaired by HFEs, which bury debris and fill gullies (Grams et al. 2018).

4. HFE ELEMENTS FOR CONSIDERATION

 High flow experiments have overall shown trends in sand deposition resulting in the 

building of sandbars and beaches. High flow releases rebuild sandbars by entraining sediment 

from the riverbed and redepositing that sediment along the banks and in eddies. Because the 

locations of eddies are controlled primarily by channel constrictions at tributary debris fans, the 

zones of deposition are relatively stable and deposition usually occurs on existing sandbars or 

at the sites of former sandbars that have eroded. HFEs are achieved using GCD power plant 

capacity releases (<31,500 cubic feet per second (cfs) with all power generating units and full 

reservoir conditions) combined with power plant bypass through the “jet tubes” (<15,000 cfs 

with all four tubes) not to exceed a total of 45,000 cfs.

Four essential components have been identified for HFEs to be successful in the long-term:

1. Sufficient sand, or sand load, in the mainstream river channel to build sandbars without 

progressively depleting the sand supply; 

2. The sand grain size is sufficiently fine to create conditions of high sand concentration in 

eddies; 

2 This is supported by observational data presented by Grand Canyon Monitoring and Research Center (GCMRC) 

staff at the January 2023 Annual Reporting Meeting of the GCDAMP . 
9 

3. Sustained flow magnitude is sufficiently high and of sufficient duration to deposit sand 

at high-elevation sections of sandbars without creating a negative deficit in the sand 

supply; and

4. HFEs must be frequent enough to offset erosion and to achieve increasing or neutral 

sandbar size (Wright and Kennedy 2011). 

With the exception of the first component, these elements were implemented in the HFE 

Protocol Environmental Assessment (U.S. Department of the Interior 2012) and were 

subsequently included in the HFE Protocol in the LTEMP ROD (U.S. Department of the Interior 

2016). Background literature and supplemental materials are available in detail within the 

GCDAMP Wiki HFE Page. 

5. PROPOSED REVISIONS TO THE HFE PROTOCOL

 

The Original 2011 HFE Protocol 

The HFE Protocol established release magnitudes for HFEs that could range from 31,500 

cfs to 45,000 cfs, and potential durations ranging from one hour to 96 hours (Table 1). The 

LTEMP extended the duration of Fall HFEs for up to 250 hours (with specifications listed in 

Table 2) and established a Proactive Spring HFE that could be triggered by an equalization 

release of ≥10.0 MAF (see Table 2). 

The original HFE Protocol was intended to be experimental in nature and was designed 

to learn how to incorporate high releases into future GCD operations in a manner that 

effectively conserves sediment and sediment-dependent resources in the long term. The new 

proposed hypothesis [under the LTEMP] is intended to test if recurring HFEs that provided 

sandbar building would exceed sandbar erosion in periods between HFEs, such that sandbar 

size could be maintained and increased over time. The HFE Protocol was originally envisioned 

to test releases over the 10-year period from 2011-2020. Environmental compliance was later 

extended for the duration of the LTEMP EIS (2016-2036). The Environmental Assessment for the 

HFE Protocol can be found on Reclamation’s website. Supporting documents and information 

collected from past HFEs is available at GCDAMP Wiki HFE Page, Reclamation’s GCDAMP 

website, and GCMRC’s Sandbar Monitoring site. 

 

Current Sediment Accounting Periods and HFE Implementation Windows 

The current HFE Protocol (U.S. Department of the Interior 2016) uses a sediment 

Accounting Period and Implementation Window which occurs semi-annually (described below; 

Figure 6). The Accounting Periods are based on the expected timing of sediment delivery and 

on typical GCD operations. The sediment Accounting Periods are the periods over which sand 

inputs and exports are measured in order to evaluate whether or not conditions have been met 

to trigger an HFE. In determining whether or not to conduct an HFE, the Sediment Accounting 

Model (Wright et al. 2010) is utilized and incorporates monitoring data (reservoir elevations, 

GCD releases, and sediment) with the goal of maintaining a positive sand mass balance at the 

end of the Accounting Period. Evaluation of sufficient sediment input to trigger an HFE 
10

considers sand input from the Paria River during sediment Accounting Periods to achieve a 

positive sand mass balance in Marble Canyon with implementation of an HFE. 

The current Spring HFE Implementation Window is from March 1 to April 30, with a 

December 1 through June 30 sediment Accounting Period. The Fall HFE Implementation 

Window is from October 1 to November 30, with a July 1 through November 30 sediment 

Accounting Period (Figure 6). Regardless of whether or not an HFE is conducted, the sediment 

Accounting Periods do not roll-over accumulated sand from the prior Accounting Period in 

order to trigger a potential HFE during the following Implementation Window (Figure 6). After 

the Accounting Period is over, the sediment baseline is reset for the next Accounting Period. 

Only sand received during the current Accounting Period can be included in the model as input 

sediment to trigger an HFE.

A recent example of needed flexibility in the HFE sediment Accounting Period was 

encountered in the spring of 2023. During the 2023 spring sediment Accounting Period, much 

of the sediment remained from the previous Fall Accounting Period due to low annual release 

volumes and shifted water delivery schedules. Thus, the positive sediment mass balance in 

Marble Canyon in spring 2023 did not accumulate in the 2023 Spring Accounting Period, but 

remained from prior Accounting Periods. Given the positive sediment mass balance that 

persisted into spring 2023, as well as the condition of beaches, the lack of HFEs since 2018, 

exceptional hydrologic circumstances and the need to release 9.5 MAF in the water year 

(including the need to release 0.523 MAF of water temporarily held back earlier in the water 

year), a one-time spring experiment was conducted with the goal of benefitting multiple 

resources. A Supplemental Information Report was provided by Reclamation.

There are currently four key analysis phases to the HFE modeling component associated 

with the two sediment budget Accounting Periods and the two HFE Implementation Windows. 

This process is conducted prior to each HFE Implementation Window and continued throughout 

the Implementation Window to monitor the potential for considering the implementation of an 

HFE (LTEMP HFE Protocol, pg C-5). The four key analysis phases are: Phase 1: Fall Accounting 

Period, Phase 2: October-November HFE Window, Phase 3: Spring Accounting Period, and 

Phase 4: March-April sediment-triggered HFE Window.

 

Proposed changes to the modeling component and analysis phases: Revision to Section “1.2 

MODELING” of the HFE Protocol 

The primary proposed revision to the HFE Protocol involves combining the two sediment 

Accounting Periods into a single, annual, Accounting Period (Figure 7). The two HFE 

Implementation Windows, one in the fall and one in the spring, would be preserved with some 

small modifications. The following is a step-by-step proposed revision of the HFE key analysis 

components described above and found on page C-5 to C-6 of the LTEMP ROD, with the 

combination of two of the original phases for a total of three phases. The text that is provided 

below in italics is proposed to replace the text found in the HFE Protocol. Additional nonitalicized text is provided for context. 

● [Replace paragraph starting with “Phase 1:...” on LTEMP ROD page C-5 with] Revised 

Phase 1: Annual Accounting Period. The annual Accounting Period is from July 1 to June 
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30. Beginning on July 1 of each year, monitoring data will be used to track the sand 

storage from Paria River inputs in Marble Canyon. 

The revised annual Accounting Period becomes a combination of the Fall and 

Spring Accounting Periods in the original HFE protocol (Figure 7). The revised annual 

Accounting Period is extended to one year beginning on July 1 and ending on June 30 of 

the following year. Beginning on July 1 each year, sediment monitoring data will be used 

to track the sand storage from Paria River inputs into Marble Canyon. HFEs will be 

triggered based on maintaining a positive sand mass balance from the start of the 

Accounting Period to the end of the HFE (not to the end of the sediment Accounting 

Period as is currently done) using the lower bound of the estimated sand supply. 

Resetting the sediment mass balance starting value on an annual basis has some 

advantages over the current procedures. Any sand that may remain because a Fall HFE 

was not implemented would also accrue into the sediment mass balance. This situation 

increases the likelihood of triggering a Spring HFE. This proposed revision does not 

mean that more than one Fall HFE should be implemented in the Fall Implementation 

Window or more than one Spring HFE should be implemented in the Spring 

Implementation Window.

Resetting the sediment account mass balance start value on an annual basis 

means that any sand that accrued into the balance during the first year, but was not 

used in that year, is unavailable for use in future years. This potentially abandoned 

sediment is referred to here as “rollover” sediment. 

The FLAHG did not specify how to use rollover sediment because more analyses 

are needed. However, the FLAHG generally agreed that it was important to consider. 

The following elements should be considered by Reclamation when considering the 

impacts of a future contemplated action resulting from this recommendation: 

 

● Element 1: Reclamation should analyze adding rollover sediment from 

one or more years to the current July 1 starting value if a sedimenttriggered HFE was not conducted in the previous annual Accounting 

Period. 

● Element 2: Reclamation should acknowledge that the communication and 

consultation procedures established in Section 1.4 of the LTEMP ROD will 

be used to make recommendations on using any available rollover 

sediment. 

● [Replace paragraph starting with “Phase 2:...” on LTEMP ROD page C-5 with] Revised 

Phase 2: October-November HFE Implementation Window. Beginning July 1, sand 

storage and forecast hydrology will be evaluated using the sediment budget model to 

determine whether conditions are suitable for a Fall HFE or if conditions warrant 

delaying considerations until Phase 3 (i.e., the Spring HFE Implementation Window). The 
12

model determines what magnitude and duration of a Fall HFE, if any, would produce a 

positive sand mass balance at the end of a Fall HFE. If the model produces a positive 

result, the largest Fall HFE that would result in a positive sand mass balance or delayed 

consideration of a Spring HFE is forwarded to the decision and implementation 

component (see below), which also allows for other factors to be considered in the 

planning process (see Section 1.1). During the decision process, sediment input will 

continue to be measured, the model will continue to be run and results or output will be 

forwarded to decision-makers to allow for refinement of the previously recommended 

magnitude and duration of either a Fall HFE or deferred to a Spring HFE. If the model 

produces a negative sand mass balance, the model will continue to be rerun using more 

recent sediment input to determine whether a positive sand mass balance will be 

reached in time to have a Fall HFE in the release window. 

 

● [Replace paragraph starting with “Phase 3:...” and paragraph starting with “Phase 4:...” 

on LTEMP ROD page C-6 with] Revised Phase 3: March-June sediment-triggered HFE 

Implementation Window. The evaluation in this phase is the same for the OctoberNovember HFE window (see Revised Phase 2) with the model output being forwarded to 

the decision and implementation component. The model output will be used in the same 

way as for the October-November determination. The annual Accounting Period will be 

used to determine if there are suitable conditions to consider implementation of a Spring 

HFE from March 1 to June 30. Whether or not a Spring HFE is scheduled, sediment inputs 

will continue to be monitored through the end of the annual Accounting Period. Note 

that Proactive Spring HFEs (see Section 3.2), which are triggered by water volume and 

not sediment inputs, could occur in April, May, or June. In addition, Spring HFEs will not 

be tested in years when there has been an extended-duration Fall HFE earlier in the 

same water year. 

Reclamation proposed an action to prevent the establishment of Smallmouth Bass 

below Glen Canyon Dam in the Glen Canyon Dam/Smallmouth Bass Flow Options Draft 

Environmental Assessment (Draft EA). However, Reclamation decided to pursue an 

Environmental Impact Statement based on comments received on the Draft EA. The 

FLAHG did not resolve a potential future scenario where multiple flow actions (e.g. HFE, 

Smallmouth Bass flow action) are triggered in the same water year. However, some 

FLAHG members have already identified impacts to resources that may not be 

acceptable from their viewpoint. Additional analyses are needed to understand the 

impacts and tradeoffs of these actions potentially occurring in the same water year. The 

FLAHG recommends Reclamation analyze such impacts and tradeoffs. 

HFE Protocol Revisions in Relation to LTEMP Resource Goals 

The objectives of this proposed revision to the HFE protocol (as described above) are to 

increase and retain fine sediment volume, area, and distribution in the Glen, Marble, and Grand 

Canyon reaches above the elevation of the average base flow to help attain the resource goals

as currently described by the LTEMP. The proposed revisions to the sediment Accounting 
13

Periods and Implementation Windows, and sediment rollover into subsequent HFE Accounting 

Periods achieves these goals by:

● Expanding opportunities to retain sediment resources in periods where HFEs may 

provide additional benefits to multiple LTEMP resources while balancing impacts among 

other resources;

● Implementing and evaluating how HFEs might be conducted during low reservoir 

conditions; 

● Increasing the likelihood that HFEs are implemented with a frequency that is consistent 

with the original LTEMP experimental design; and

● Adapting the HFE Protocol to continue to test and evaluate the LTEMP hypothesis, 

including the importance and effectiveness of springtime HFEs, under the new and 

continuously changing conditions of the Colorado River ecosystem. 

To achieve sediment resource objectives intended by LTEMP, it is necessary to amend 

the modeling component and key analysis phases as described above. Implementation of the 

proposed HFE protocol revisions will require the continued assessment of sediment input and 

storage in relation to projected releases, dam maintenance schedules, impacts to other 

resources, and timing with other experiments. In addition to the consolidation of the sediment 

Accounting Periods, consideration should be given to extending the Spring HFE Implementation 

Window such that it would end on June 30, as opposed to April 30. This will increase the 

likelihood of triggering an HFE by capturing late spring runoff and sediment inputs from the 

Paria River. The FLAHG encourages analyses of both extending and not extending to June 30 

when Reclamation analyzes the impacts of a future contemplated action. Evaluating and 

documenting this analysis will help determine whether or not HFEs are feasible with 

consideration to hydropower and other resources during June, and ultimately, will help set 

expectations and streamline the LTEMP decision framework.

Extending the Spring HFE Implementation Window may provide additional resource 

benefits, to be tested as part of the HFE hypothesis, such as disrupting Brown Trout and 

Smallmouth Bass spawning, mimicking historical spring floods, improving the natural processes 

resource goal, increasing the potential for aeolian transport of fluvial sediment to rebury and 

protect archaeological sites, and improving the summertime recreation camping experience. 

The impacts associated with extending (or not) the Implementation Window should be 

analyzed and more fully considered during an environmental compliance process. The 

extension of the Spring HFE Implementation Window would provide opportunities for improved 

understanding of these potential positive or adverse impacts. If the decision to perform a 

Spring HFE is made later in the Implementation Window due to the proposed extension, there 

may be potential negative impacts. The later implementation of an HFE may provide less 

opportunities to shift water volumes between months that are important for hydropower and 

other resources. The potential positive and negative effects to multiple resources will continue 

to be considered in the LTEMP decision framework. 
14

6. SPRING HFES AND POTENTIAL IMPACTS TO OTHER LTEMP RESOURCES 

Overview

The relationships between GCDAMP goals and the role of HFEs is complex, as some 

resources benefit from HFEs, some remain unaffected, and some are negatively affected (Figure 

8). The GCDAMP has acknowledged the potential additional benefits to resources other than 

sediment through the implementation of Spring HFEs. 

Below are the anticipated potential benefits of a Spring HFE, as well as potential adverse 

impacts to be considered during any process for revising the HFE Protocol. This is not an 

exhaustive list, and it is anticipated that additional resource impacts will be determined and 

considered in future analyses. As is the current practice, a number of these potential impacts 

may already be evaluated through the LTEMP decision framework as part of the normal process 

to recommend HFEs. Some of these concepts may have greater weight than others, but are 

listed here for further evaluation: 

● Lake Powell reservoir elevations – identification of critical reservoir elevations may 

change through the post-2026 Colorado River operations development process, 

therefore no specific elevation is identified here. However, sufficient reservoir 

elevations will continue to be essential for “normal” operations at GCD, including when 

an HFE is considered by the LTEMP decision framework. For example, there may be a 

greater risk of falling below minimum reservoir elevation thresholds (i.e., minimum 

power pool) during a Fall or Spring HFE that may be triggered prior to spring runoff. 

● Non-native warm-water fish – There is likely a greater risk of entrainment and dispersal 

during a Fall HFE (e.g., Smallmouth Bass, Green Sunfish).

● Fish migration and spawning – The timing of an HFE may provide benefits or adverse 

impacts based on native and non-native fish life histories (e.g., Humpback Chub, 

Razorback Sucker, and Flannelmouth Sucker may benefit from a Spring HFE while Brown 

Trout may benefit from a Fall HFE).

● Improved planning process – A Spring HFE that can utilize fall sediment allows more 

time to plan for power replacement, potential shifts in monthly water volumes, and 

monitoring trips.

● Recreation season (April-September) – A Spring HFE is in closer proximity to, and allows 

more suitable timing, for sandbar persistence during the high-use recreation season. 

Sandbars deposited by a Fall HFE are subject to erosion at high-priority areas 

(campsites, etc.) before the arrival of the high-use recreation season. 

● Aeolian sand transport – A Spring HFE would coincide with the windy season (April-June) 

and provide a greater potential for aeolian sand transport. 

● Hydropower impacts – Implementing a Fall or Spring HFE during 'peak power months' 

(i.e., June-August or December-February), could have significant negative impacts on 

energy grid reliability and replacement power costs. Hydropower impacts, along with 

other resources, will continue to be considered in the LTEMP decision framework. The 

following considerations (below) should be included in the NEPA LTEMP SEIS analysis. 
15

o Impacts to hydropower can be reduced by taking water for HFEs from non-peak 

shoulder months instead of peak power months as described in the HFE 

Protocol. 

o A notice of at least two weeks prior to implementing an experiment that 

substantially deviates from normal operations is required by Western Area 

Power Administration (WAPA).

o Advanced decision-making of HFEs has the potential to reduce impacts to 

hydropower and allows time for resource planning. Making a decision to conduct 

an HFE at least three months in advance provides the most flexibility for 

hydropower rate and contract notifications. Establishing reliable protocols allows 

utilities to accurately assess their 5-year resources planning outlook for 

construction and acquisition needs.

o Effects analysis of hydropower rate structure changes that have occurred since 

LTEMP ROD should be included (e.g., WAPA-199). 

o Concern about multiple bypass actions in a single Implementation Window.

● Basin Fund impacts – Depending on the timing and duration of an HFE, as well as 

current power market costs and resource availability, WAPA’s assessment of the Basin 

Fund will be impacted by a multitude of factors. In turn, the status of the Basin Fund has 

direct implications for funding the statutory obligations assigned to it, which include 

Reclamation and WAPA operation and maintenance costs, repayment of investment 

plus interest, irrigation assistance and non-power programs such as the Colorado River 

Basin Salinity Control Program. Basin Fund levels also have a direct impact on the 

potential application of a cost recovery charge to Colorado River Storage Project 

customers. 

● Foodbase – A Spring HFE could provide benefits to the natural processes goal. For 

example, a national synthesis of flow and biological data from over 700 streams and 

rivers in the lower 48 states found that healthy communities of native aquatic 

invertebrates and fish were most often present where flood disturbance still occurred, 

and where flood timing was seasonally appropriate (i.e., similar to the natural condition; 

Carlisle et al. 2017). 

● High-risk non-native fish spawning – Smallmouth Bass spawning occurs when water 

temperatures rise to 16C (typically June in Lees Ferry). Analysis should include the 

consideration of an HFE that may also be beneficial in limiting invasive fish spawning 

where possible to benefit the most resources.

● Cultural Resources – There could be potential adverse impact to cultural sites in Glen 

Canyon from future HFEs due to erosion, as well as the potential to increase protection 

and improve the condition of some cultural resources in the Grand Canyon. Tribal 

engagement is needed to fully assess potential impacts to cultural resources. The 

assessment of individual effects and their relevance will likely vary by Tribe.

● Riparian vegetation and habitat value – Dispersal and growth of native and non-native 

(i.e., salt cedar) riparian plant species may vary depending on implementation timing of 

HFEs. An extended Spring HFE Implementation Window may enhance plant growth with 

greater habitat value in restoration areas.
16

● Aquatic vegetation – HFEs are likely beneficial in scouring/impacting undesirable aquatic 

vegetation, with spring HFEs likely to have longer-lasting effects/benefits compared to 

fall HFEs (Torn et al. 2010).

● Trout fishery – Although acute disruption of spawning activity and angling opportunities

may occur during HFEs, Spring HFEs also have the potential to promote spawning. 

Rainbow trout typically spawn in the spring and are adapted to spring flood 

disturbances. Benefits to other resources are likely to indirectly benefit Rainbow Trout

in the long-term (e.g., increased foodbase productivity and spawning substrate 

scouring/cleaning).

● Additional sediment inputs – A Spring HFE could mobilize additional sediment inputs 

from the Little Colorado River floods that occur in January-March, while large Paria River 

floods typically occur in August-September. 

● Adaptive management – The opportunity to learn from these experiments, including 

their benefits and impacts, will provide crucial information to inform future 

recommendations within LTEMP. 
17

FIGURES

Figure 1. Modeled sandbar volume from 2002 to 2019 showing effects of HFE frequency (a) 

Time series and (b) exceedance probability of bar volume for different frequencies of controlled 

floods during the period 2012–2019. From Mueller and Grams (2021).

Figure 2. Sandbar types (from Schmidt and Graf 1990 and Mueller et al. 2018).
18

Figure 3. Mean normalized sandbar volumes in narrow-medium reattachment, wide 

reattachment, and separation or undifferentiated sandbars from 1992-2022 in above-normaloperations elevations (upper graph) versus within-normal-operations elevations (lower graph). 

Based on Hazel et. al (2022), with figures derived from GCDAMP AMWG February 2023 

Overview of Projects A, B, and L and Evaluation of High-Flow Experiments During Aridification

presentation. 
19

Figure 4. Cumulative changes in sandbar volume between October 2012 and October 2019 

(upper graph) and between October 2019 and October 2022 (lower graph) at 44 long-term 

monitoring sites. Based on Hazel et. al (2022), with figures derived from GCDAMP AMWG 

February 2023 Overview of Projects A, B, and L and Evaluation of High-Flow Experiments During 

Aridification presentation. 
20

Figure 5. Example of gully erosion at Tatahatso Beach (Colorado River Mile 37L) between 

October 2020 and October 2022. Photos by P. Grams, USGS.

Figure 6. Average monthly sand loads from the Paria River and Little Colorado River showing 

the existing Fall and Spring HFE Accounting Periods and Implementation Windows defined in 

the 2011 HFE Protocol.
21

Figure 7. Post-hoc modeling for HFE Implementation comparing the current HFE 

Implementation Window to an annual HFE Implementation Window. Table courtesy Paul 

Grams, USGS
22

Figure 8. Graphs showing the lowest (left) and highest (right) predicted effects of a Spring 

Disturbance Flow, a Spring HFE, and a Fall HFE on LTEMP resources, as derived from the Spring 

Disturbance Flow - Predicted Effects document. Open symbols indicate resources where the 

direction of effect and/or the strength of the effect was unknown. Low weight of evidence 

indicates greater levels of critical uncertainty whereas high weight of evidence indicates lower 

levels of critical uncertainty. 

Please note that these graphs do not fully correspond to the ideas proposed in this document, 

but rather serve as a general guideline for potential effects to various resources and should be 

considered in further analysis. 
23

TABLES

Table 1. Technical alternatives for 16 different HFE scenarios. 
24

Table 2. HFEs described in LTEMP. 
25

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