United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-128

May 2004

Quantifying Channel Maintenance 

Instream Flows:

An Approach for Gravel-Bed Streams in the 

Western United States

Larry J. Schmidt and John P. Potyondy

Phase 1 Transport

(sand & finer gravel)

typically

supply-limited

Bedload

transport

rate

(tons/day)

Flow

frequency

(days)

Total bedload

transport

(tons)

RANGE OF CHANNEL MAINTENANCE FLOWS

Moves 100% of bedload mass

Qthreshold Qtrigger Qeffective Qcap

Phase 2 Transport

(gravel and larger sizes) typically transport-limited
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Fort Collins, CO 80526

Schmidt, Larry J.; Potyondy, John P. 2004. Quantifying channel maintenance instream flows: an approach 

for gravel-bed streams in the Western United States. Gen. Tech. Rep. RMRS-GTR-128. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 33 p.

Abstract

This paper discusses one approach for quantifying channel maintenance instream flow necessary to achieve the 

Forest Service Organic Act purpose of securing favorable conditions of water flows. The approach is appropriate 

for quantifying channel maintenance flows on perennial, unregulated, snowmelt-dominated, gravel-bed streams 

with alluvial reaches. The approach identifies the minimum essential regime of streamflows necessary for the 

channel and its floodplain to remain fully functioning with respect to sediment and flow conveyance. The paper 

discusses the role of water, sediment, and vegetation in maintaining a channel and provides methodologies for 

estimating the upper and lower limits of the required sediment transporting flows. Conceptually, these flows range 

from intermediate flows associated with initial coarse sediment movement from the coarse surface layer of gravel-bed streams (Phase 2 transport) up to the 25-year flow event. The paper also provides suggestions for analyzing and displaying results, implementing studies at the watershed scale, determining data needs, and post-project management and evaluation. Best application of the approach occurs at sites having long-term bedload data 

and streamflow records.

Keywords: gravel-bed streams, channel maintenance instream flows, bedload transport

The Authors

Larry L. Schmidt, Program Manager and Physical Scientist (retired), USDA Forest Service, Rocky Mountain 

Research Station, Stream Systems Technology Center, 2150A Centre Avenue, Fort Collins, CO 80526-1891.

John P. Potyondy, Program Manager and Physical Scientist, USDA Forest Service, Rocky Mountain Research 

Station, Stream Systems Technology Center, 2150A Centre Avenue, Fort Collins, CO 80526-1891.
Contents

Author’s Note . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

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

Benefits of Channel Maintenance Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Consequences of Failing to Provide Necessary Flows . . . . . . . . . . . . . . . . . . . . . . . . 1

Scientific Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

The Role of Water and Sediment in Maintaining a Stream Channel . . . . . . . . . . . . . . . 3

The Importance of Bedload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The Role of Streamside Vegetation in Maintaining a Stream Channel . . . . . . . . . . . . . . . 6

Adverse Effects of a Lack of Channel-Maintaining Flows . . . . . . . . . . . . . . . . . . . . . . . . 8

Quantification Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Typical Channel Maintenance Hydrographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Quantifying Flows Needed for Sediment Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Estimating the Lower Limit of Sediment Transporting Flows . . . . . . . . . . . . . . . . . . . . . 13

Estimating the Upper Limit of Sediment Transporting Flows . . . . . . . . . . . . . . . . . . . . . 17

Quantifying Flows Needed for Streamside Vegetation . . . . . . . . . . . . . . . . . . . . . . . . 18

Analyzing and Displaying Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Cumulative Sediment Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Cumulative Sediment-Water-Time Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Long-Term Water Use Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Implementation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Scales of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Setting the Context of the Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Selecting Specific Quantification Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

General Data Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Post-Project Management and Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Acknowledgments

The Stream Systems Technology Center has been working on stream channel maintenance issues for over 10 

years. During that time, we have consulted with a wide array of scientists in the Forest Service, other agencies, and universities and a large number of National Forest specialists, expert witnesses, and lawyers who have 

worked with us. We sincerely appreciate the constructive, frank, and thoughtful suggestions and comments provided over the years and have tried our best to develop an approach that is both scientifically sound and readily 

explainable to non-technical audiences, such as managers and judges. In developing an approach, we take full 

responsibility for the interpretations we have made and recognize the divergent viewpoints and the many different 

ways to solve this problem. Helpful review and comments were provided on the most recent draft by Jack King, 

Tom Lisle, John Buffington, Chuck Troendle, Sandra Ryan, William Emmett, Lois Witte, and Steve Glasser. Their 

comments greatly improved the quality of the final product. We also wish to acknowledge earlier reviews and 

helpful thought-provoking comments and discussions provided by Bob Beschta, Luna Leopold, Dave Dawdy, Bill 

Jackson, Jon Nelson, Ned Andrews, Robert Milhous, John Pitlick, Bill Dietrich, Matt Kondolf, Dave Montgomery, 

Peter Wilcock, Gary Parker, Peter Whiting, Bob Ziemer, Gordon Grant, Kristin Bunte, Mike Collette, Tim Sullivan, 

and numerous other individuals with whom we have discussed this issue over the years.
The Legal Context of Forest Service Channel 

Maintenance Instream Flows

This channel maintenance quantification approach identifies an essential regime of water flows representing the minimum quantity and timing of flows needed for the self-maintenance of stream channels 

on the National Forests. Channel maintenance flows are non-consumptive instream flows specifically 

designed to maintain the physical characteristics of the stream channel. Effective channel maintenance 

flow regimes have naturally variable flows of sufficient duration, magnitude, frequency, and timing to 

maintain channel morphology and streamside vegetation so that the capacity of the channel to convey 

natural flows is unimpaired over the long-term.

Providing channel maintenance flows responds in part to a stated purpose of the reservation act for 

National Forests, found in the Organic Administration Act of June 4, 1897 (16 USC 475):

“... No National Forest shall be established except to improve and protect the forest 

within the boundaries or for the purpose of securing favorable conditions of 

water flows and to furnish a continuous supply of timber for the use and necessities 

of citizens of the United States. ...”

The Organic Act of 1897 first defined the purposes for which forest reserves could be withdrawn from 

the public domain and managed. The Act recognizes the importance of watershed protection to the establishment of National Forests and provides the Forest Service with the authority to administer these 

lands and to protect and improve valuable water resources. The Act expressly gave the federal government jurisdiction over water usage on National Forest System lands by requiring that all waters within the 

boundaries of the National Forest be used under the rules and regulations of the United States as well 

as under the laws of the states (Witte 2001).

Over the years, the federal government has made many attempts to secure federal reserved water rights for 

instream purposes (Gillilan and Brown 1997). A 1978 U.S. Supreme Court decision, known as United States 

v. New Mexico (438 U.S. 696), has had the most pronounced effect on the ability of the Forest Service to acquire instream flows for the National Forests. In U.S. v. New Mexico, the Supreme Court narrowly construed 

the primary purposes for the National Forests to be twofold: maintaining favorable conditions of water flows 

and production of timber. The Court also concluded that the implied reservation theory could not be used to 

obtain instream flows to protect recreation, fish, and wildlife.

The stringent test articulated by the U.S. Supreme Court states that an implied reserve water right is 

found only if water is necessary for the purposes of the reserve, that is, to secure favorable conditions of 

water flows. Furthermore, the water claimed must be the minimum amount of water necessary to fulfill 

the purposes of the reservation and no more (Witte 2001).

The term “minimum” has been interpreted to mean the least total amount of water in the long term. Thus, 

the “minimum” amount necessary varies from year to year depending on the natural flows available to 

accomplish the purpose in any year. In high flow years, more flow is required, albeit nonconsumptively. 

In low flow years, little to none of the water is useful for channel maintenance. The net effect of this produces the minimum amount of flow needed in total in the long term.

Within the confines of court imposed constraints, the Forest Service has made instream flows claims 

for channel maintenance in several Western water adjudications. The claims have necessarily had to 

be based on the legislative history of the Organic Act and the Agency’s subsequent interpretation of the 

phrase “favorable conditions of water flows” in legal proceedings. As a consequence, channel maintenance claims have been narrowly limited to physical channel maintenance, excluding fisheries and 

aquatic ecosystem considerations.

The United States has typically asserted that amounts of water fully capable of transporting all of the 

sediment loads from the headwaters to valleys downstream of forest boundaries in alluvial channels, 

maintaining channel conveyance capacity, and maintaining protective watershed vegetation are essential parts of “securing favorable conditions of water flows” consistent with the Organic Act. While some 

courts have agreed that stream integrity is a favorable condition of water flows, they have ruled against 

the United States based on other factual and legal considerations. The Forest Service continues to develop approaches that support reserved water right claims based on the primary purposes of the Organic 

Act, the “securing of favorable conditions of water flows.”

The approach described in this document has been specifically developed within this limiting framework. 

Application in cases involving the reservation principle must necessarily be constrained to the primary 

purposes related to the establishment of National Forests. The limitations imposed on the Forest Service 

have little influence on the technical merits of the approach and the fundamental concepts can be adapted and applied as part of a broader ecosystem context for addressing instream flow needs by others.

Authors’ Note:

ii
iii

Executive Summary

This channel maintenance quantification approach identifies the scientific basis and a methodology for estimating essential water flow regimes needed for the self-maintenance of stream channels. 

The Forest Service must estimate essential instream flows because increasing demand for future 

off-stream uses such as irrigation and municipal water supply threaten to deplete critical portions of 

instream flows on the National Forests.

We hypothesize that channel maintenance flows, coupled with proper management of upland watersheds, will provide for favorable conditions of water flows. We recognize, however, that successful 

watershed management requires a holistic approach involving management of the uplands as well 

as stream channels and their flow regimes. Favorable flows occur when the stream channel is able 

to pass sediment in equilibrium so that aggradation doesn’t significantly increase flood peaks. To provide for favorable flows, it may be necessary to legally or administratively secure essential portions of 

the natural streamflow in National Forest streams. These non-consumptive flows, known as channel 

maintenance instream flows, are designed to maintain the physical characteristics of the stream 

channel critical to unimpaired flow and sediment conveyance. Because they are nonconsumptive, 

except for losses due to evaporation and transpiration, the water flows through the protected reach 

and is available for other uses, both instream and offstream.

The approach herein describes one way to estimate the minimum essential regime of streamflows 

necessary for the channel and its floodplain to remain fully functioning with respect to sediment and 

flow conveyance. A stream channel in a fully functioning condition conveys water and sediment without aggradation or degradation, dissipates energy, reduces flood peaks, sustains flows, and otherwise acts as a natural stream.

Appropriate Scope of Application

This approach is appropriate for quantifying channel maintenance flows on perennial, unregulated, 

snowmelt-dominated, gravel-bed streams with alluvial reaches. Many of the concepts and principles 

presented have potential application to rain-dominated and other systems, but they must be extrapolated carefully to these systems. The approach is unlikely to work in arid environments with ephemeral channels where hydrographs are flashy.

This proposed channel maintenance flow methodology is primarily intended to evaluate run-of-theriver projects that pass most or all bedload sediment in the natural stream channel. Channel maintenance flow regimes below large storage reservoirs or hydropower facilities, while they include management objectives of moving sediment and maintaining streamside vegetation, require different 

analysis techniques to address the wide variety of possible ecological or management conditions 

that may exist.

Reliance on Natural Processes

This approach recognizes the fundamental importance of natural processes in maintaining channels. From a practical viewpoint, retaining essential portions of the natural runoff processes provides 

the most practical and cost effective means of achieving natural self-maintaining stream channels 

on National Forest System lands. Artificially maintaining channel capacity and riparian vegetation by 

periodic channel dredging, concrete lining, or irrigation is technically, economically, and environmentally infeasible to protect extensive channel networks.

Channel maintenance flows are intended to maintain channel function. Remedying upstream disturbances or protecting the channel from cumulative upstream effects of such disturbances requires 

more than channel maintenance flows. Channel maintenance flows seldom, if ever, reverse past 

channel disturbances and impacted systems may, in some circumstances, fail to fully attain the “favorable flows” condition without additional intervention on the landscape or in the stream channel.

Alternative Channel Maintenance Flow Approaches

Many approaches that deal with maintaining channel form and processes have addressed channel 

maintenance flows in natural channels (Reiser and others 1985, 1989; Milhous 1986; Rosgen and 

others 1986; Andrews and Nankervis 1995; Leaf 1998; Emmett 1999; McNamara and others 2000). 

Other approaches with broad geomorphic and ecosystems-based perspectives have also been presented (Hill and others 1991; Jackson and Beschta 1992; Ligon and others 1995; Whiting 2002). 

None of these provide a comprehensive methodology for estimating the essential flow regime needed for channel maintenance on National Forest streams.
The approach presented here differs from the above approaches in that it specifically addresses 

Forest Service channel maintenance needs focusing on transporting all supplied sediment with a 

minimum amount of streamflow. It also includes maintaining streamside vegetation as an integral part 

of the required flow regime since naturally stable channels always have abundant streamside vegetation. The approach is based on current scientific knowledge and represents an operating hypothesis that will need to be tailored to specific reach and site characteristics. The methodology will continue to be refined as knowledge about flows and their relationship to sediment transport and streamside vegetation improves.

Summary of Water Needed for Channel Maintenance

Channel maintenance in gravel-bed streams requires a range of instream flows that transport bedload sediments through the channel network. This flow regime prevents stream constriction by inchannel sediment deposition and/or in-channel vegetation establishment. The range of flows also 

provides sufficient high and low flows to sustain vital channel bank and streambank and floodplain 

vegetation.

Our analysis indicates that in coarse-grained gravel-bed streams of the Rocky Mountains, low flows 

that occur most of the year transport insignificant bedload sediment. However, when higher flows 

begin to fill the channel near its capacity, coarse bedload transport and scour of vegetation within 

the channel combine to help maintain channel morphology. In addition, these high flows periodically inundate the floodplain, helping to sustain and regenerate streambank and floodplain vegetation. 

Consequently, a range of intermediate to high flows that typically occurs for a limited time during the 

year provides for the necessary stream channel maintenance.

Our analysis further shows that over the long term, approximately 20 percent of the annual streamflow is necessary for channel maintenance in gravel-bed channels located within Western snowmeltdominated watersheds. During extremely wet years, when much of the sediment transport occurs, 

up to 50 percent of the annual stream flow may be needed. However, during many low flow years, 

as little as zero percent may be needed, because flows capable of transporting bedload sediment 

fail to occur. Each site has unique requirements and results may differ from this range of numbers. 

Interestingly, satisfying the channel maintenance needs results in a roughly even flow being available 

to off stream uses in any given year.

Implications for Water Users

The proposed channel maintenance flow allocation has a negligible effect on most existing water 

rights, particularly senior water rights downstream of National Forests. In snowmelt-dominated systems, most water needed to maintain the channel occurs during spring runoff, and a large share of 

channel maintenance needs can be met during wetter years. Channel maintenance flows may limit 

future opportunities for diversions and storage within the National Forest.

Forest Service Application of This Methodology

This channel maintenance methodology can be used to analyze instream flow requirements for water rights adjudications, water rights negotiations, Forest Planning applications, or evaluating water 

diversion or storage projects that might adversely alter natural flow regimes. The Forest Service generally applies this methodology in two settings:

1. Water adjudications, water negotiations, and Forest Planning where the approach is used to 

estimate the instream flow needed to fulfill the Forest Service’s Organic Act mandate, and

2. Special use permits, easement stipulations, and licenses for authorized water diversions and 

impoundments on National Forest System lands where the approach is used to estimate the 

minimum streamflows that must remain in the channel as a term and condition during the grant of 

a special use authorization.

iv

Quantifying Channel Maintenance 

Instream Flows:

An Approach for Gravel-Bed Streams in the 

Western United States

Larry J. Schmidt and John P. Potyondy

vi
USDA Forest Service RMRS-GTR-128. 2004. 1

Leaving a portion of the natural streamflow in the 

channel satisfies numerous physical and ecological 

functions. Instream flow regimes are commonly quantified to provide instream water essential for recreational 

opportunities and aesthetics, to maintain habitat features 

for fish and other aquatic biota, or to maintain floodplains, riparian vegetation, and the channel itself.

This channel maintenance flow quantification approach is designed for gravel-bed rivers. It identifies 

essential water flows needed for the self-maintenance of 

stream channels with the smallest amount of water necessary to achieve the Organic Act purpose of providing 

for favorable conditions of water flows. Failure to provide channel maintenance flows adversely affects the 

stream channel, floodplains, and streamside vegetation 

within years or decades.

Benefits of Channel Maintenance Flows

While everyone appreciates the benefits of clean 

drinking water and food grown with irrigated water, the 

benefits of channel maintenance flows to environmental 

systems are less readily apparent but equally important.

Channel maintenance flows provide these benefits:

• Convey water and erosion products from tributary areas through the stream system without aggradation or 

degradation,

• Maintain the relationship between the channel and the 

floodplain by temporarily storing flood flows on the 

floodplain,

• Maintain the ability of the stream to dissipate energy 

on the floodplain,

• Maintain essential channel capacity to avoid increasing flood risk to adjacent and downstream facilities,

• Maintain pools, riffles, meanders, and other physical 

habitats necessary to sustain aquatic ecosystems,

• Provide pools as sources of water for fire suppression 

and escape areas for firefighters, and

• Provide navigation conduits on larger streams and 

rivers to provide for recreational floating and power boating.

Healthy streambank and floodplain vegetation, commonly referred to as riparian areas, are a key factor in 

maintaining channels by providing:

• Roots to bind and support banks and reduce bank erosion,

• A vegetation filter that removes and stabilizes sediments and nutrients moving toward the stream from 

adjacent slopes,

• Surface roughness on the floodplain favorable for recharging groundwater systems,

• Effective floodplain soil conditions to detain flood water for later release to sustain low flows,

• Moist corridors that act as natural fuel-breaks, fire line 

anchor points, and safety zones,

• Large woody debris which creates log-steps and other 

structural features that form pools and bars, and

• Shade to the stream that maintains cool water temperatures necessary to sustain cold-water aquatic life.

Consequences of Failing to Provide 

Necessary Flows

Natural self-maintaining channels are the goal of this 

channel maintenance approach. Desired channel maintenance occurs when the flow regime can transport the 

quantity and sizes of sediment imposed on the channel without aggrading or degrading the channel over the long term.

Failure to convey the water and erosion products from 

tributary areas through the stream system causes sediment 

to accumulate in the channel. Accumulations are most severe in low gradient adjustable channels and those with high 

sediment input from tributary channels or valley walls. The 

accumulated sediment reduces channel capacity and provides sites favorable for vegetation establishment within 

the channel. We hypothesize that the greater the extent 

of the accumulation and vegetation encroachment in the 

channel, the higher the likelihood of increasing the elevation of future floods and thereby increasing flood damage 

to streamside resources or property.

In some cases, accumulations of sediment and debris 

may block the channel resulting in high flood surges of 

debris and sediment. Natural disturbance events (landslides, fires, etc.) occasionally deliver large quantities of 

sediment and wood to the channel, and these hillslope 

processes continue irrespective of the condition of the 

downstream channel. In the absence of natural stream 

flows, channel blockages may develop and be breached 

with catastrophic downstream consequences when infrequent large floods occur. These large floods may scour 

the channel at one time, deliver undesirable quantities 

of sediment downstream, and produce channel incision 

that drains groundwater.

Failure to maintain streamside vegetation can change 

vegetation composition and reduce plant density. 

Decreased streamside vegetation can lead to increased 

bank erosion and accelerated delivery of sediment to 

the channel since the ability of roots to bind soil in 

the streambanks is reduced. Streamside and floodplain 

vegetation also temporarily detains flood flows on the 

floodplain. Gradual release during the low-flow period 

stretches out water availability for downstream users.

Introduction
2 USDA Forest Service RMRS-GTR-128. 2004. 

Premises

Scientists have observed and investigated natural alluvial channels for many years. 

From these studies and body of literature, a series of premises have evolved that 

describe channels and the processes responsible for their behavior.

Channels and streams exist in nature. They have a form that varies across the landscape 

and characteristics, such as pools, riffles, and meanders, which repeat themselves within 

the channel. These channels have formed in alluvium without human interference; from this 

we conclude that natural streams are self-formed.

Channels are a vessel for conveying water and erosion products. This vessel has sides 

(banks) and a bottom (bed). Channels are part of a natural watershed system that supplies 

both water and sediment to the stream and carries it downstream.

The flow sequences that forms channels vary in timing and quantity over the long-term.

The variable nature of flows, geology, climate, and vegetation results in a consistency of 

self-formed channel features that repeat themselves given similar conditions. From this 

we conclude that common processes are involved in creating and maintaining the channel 

forms observed in nature.

The water confined in channels periodically moves the sediments that form the bed 

and banks of channels. The water in the channel also flows over the adjacent floodplain. 

Therefore, the floodplain is an extension of the channel.

The floodplain supports typical streamside vegetation. Abundant streamside vegetation 

along streams is a common feature of many natural systems. The streamside and floodplain 

vegetation co-evolved with the current physical environment and water regime.

A certain quantity of water is necessary to transport the bed material and erosion 

products entering the channel network and thereby maintain channel form. These 

self-formed channels have certain characteristics regarding width, depth, and bank height 

that developed under the current climatic regime.

We understand alluvial channels to some degree. For example, it is evident that the 

channel adjusts its width and depth in response to changes in flow and sediment regime. 

Commonly accepted generalizations about alluvial channels include:

• The general form of the channel persists in the current climatic regime.

• Flows reaching or exceeding the top of the bank (bankfull) are attained with moderate 

frequency—on average every year or two.

• Bankfull flows completely fill the principal channel to the elevation of the floodplain.

• Bankfull flows move some of the material on the bed of the channel.

We understand vegetation-flow relationships to some degree. A complete understanding 

of the effects of streamflow regimes on specific vegetation communities is lacking. 

However, in general all streamside vegetation depends on the hydrologic flow regime to 

some significant extent.

• Streamflows provide a source of abundant moisture.

• Streamflows transport seed and propagules.

• High flows deposit sediment and scour areas of the floodplain creating and maintaining 

regeneration sites.

• Flows in the main channel suppress vegetation growth in the channel by several 

mechanisms including scour and inundation.

The flow regime needed to maintain the channel depends upon the amount and sizes 

of sediment entering the channel over the long-term. To exist, a channel reach must 

convey all the mass and sizes of alluvial sediment supplied over time. Otherwise, the 

unconveyed incoming sediment load will accumulate. The best evidence of the largest 

sizes of sediment periodically moved is the size of particles on the surface of active inchannel bars. Discharges needed to move these larger particles often occur at flows near 

bankfull discharge. Because bedload transport rates increase rapidly with greater but less 

frequent discharges, in the long-term a relatively small volume of water moves a high 

percentage of the bedload.

Source: A conversation with M. Gordon 

“Reds” Wolman, 

Dept. of Geography 

and Environmental 

Engineering, 

Johns Hopkins 

University (personal 

communication 1996).
USDA Forest Service RMRS-GTR-128. 2004. 3

Scientific Basis

This section discusses what science tells us about the 

role of water and sediment in forming and maintaining 

gravel-bed streams, the importance of streamside vegetation to these processes, and the adverse consequences 

likely to result from a lack of channel maintaining flows.

We define channel maintenance flows as non-consumptive instream flows specifically designed to 

maintain the physical characteristics of the stream 

channel. We hypothesize that stream channels function properly when they support vegetation typically 

adapted for life along the stream and have naturally variable flows of sufficient duration, magnitude, frequency, 

and timing to maintain channel morphology so that the 

capacity of the channel to convey natural flows is unimpaired over the long term.

Channel maintenance defined in this manner involves 

more than maintaining the channel as a conduit for sediment. Essential attributes of a properly-functioning, 

self-maintaining channel include:

• Moving all the mass and sizes of alluvial sediment 

supplied to the channel,

• Maintaining channel capacity by preventing vegetative growth in the bed of the channel,

• Scouring the channel bed to reshape alluvial features,

• Providing for lateral migration within the stream corridor,

• Periodically inundating the floodplain, and

• Protecting and sustaining channel banks and the floodplain by maintaining healthy streamside vegetation.

The Role of Water and Sediment in 

Maintaining a Stream Channel

Fluvial geomorphic processes are fundamental for 

determining the structure and function of river ecosystems and maintaining their dynamic morphology (Ligon 

et al. 1995; McBain and Trush 1997; Trush et al. 2000). 

A basic premise of fluvial geomorphology is that stream 

channels adjust to transport the sediment and water supplied to the channel by the drainage basin (Leopold 

1962). The shape of the channel is controlled by a 

number of factors including the magnitude and duration of formative flows, the character of the transported 

sediment, and the composition of the bed and bank, including vegetation (Leopold et al. 1964).

Gravel-Bed Rivers

Alluvial rivers can be broadly classified as gravelbedded or sand-bedded (Simons and Simons 1987). 

Gravel-bed streams and rivers have unconsolidated particles, with median sizes larger than sand (>2 mm) and 

a coarse channel-bed surface layer that overlays a finer 

subsurface layer.

Like all alluvial rivers, gravel-bed rivers continually adjust their geometry, surface texture, pattern, and 

slope in response to the imposed particle sizes, sediment 

load, and discharge to make transport rates match input 

rates. Although natural streamflow varies considerably, 

a channel’s average morphology varies considerably 

from section to section but stays predictably the same 

even while migrating laterally within the river corridor. Significant bedload transport in gravel bed-rivers 

typically begins at intermediate discharges approaching 

bankfull flow (Carling 1995). Over a period of years, 

the sediment load supplied to a channel is transported 

by the available discharge. For a channel to maintain its 

morphology over moderate time scales (10 - 100 years), 

sediment inflow must equal sediment outflow.

The consistency in the behavior of alluvial channels 

was first demonstrated by Leopold and Maddock (1953) 

in terms of average hydraulic geometry. Downstream 

increases in depth, width, and velocity relative to increases in discharge were similar for rivers of different 

size and in different settings, demonstrating a remarkably consistent channel form, from one river to another 

and from one cross-section to another (Leopold 1994). 

From this it follows that the form of a channel is the result of predictable channel-forming processes and that 

the channel is self-maintained.

Channel Form and Maintenance: 

The Bankfull Concept

Wolman and Leopold (1957) concluded that flows 

near bankfull discharge (the discharge which just begins 

to inundate the floodplain) largely control the form of alluvial channels. Dunne and Leopold (1978) summarized 

this viewpoint:

“Bankfull stage corresponds to the discharge at which 

channel maintenance is most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and 

generally doing work that results in the average morphologic characteristics of channels.”

Although bankfull flows commonly move the largest single increment of sediment, flows below and above 

bankfull are also needed to convey all of the mass and 

sizes of sediment supplied to a gravel-bed river.

A bankfull channel continually migrates across its 

floodplain by eroding material from the outside bank 

of meander bends and depositing material on the inside 

of meander bends (Leopold et al. 1964). An interrelated 
4 USDA Forest Service RMRS-GTR-128. 2004. 

feedback mechanism involving bankfull discharge, a 

bankfull channel, and its floodplain creates and maintains this dynamic channel form (Parker 1978; Andrews 

1984). Flows at or near bankfull are confined by channel 

banks sufficiently to mobilize and transport bed material 

frequently. However, as flows begin to exceed bankfull 

stage, they are no longer confined and flow depth and 

mean velocity increase less rapidly, thereby reducing the 

potential for high discharge to cause catastrophic channel damage. This feedback process helps to maintain 

equilibrium in the channel form.

Leopold and others (1964) observed that bankfull 

discharge occurs frequently (on average, two out of every three years). Wolman and Leopold (1957) suggested 

a common recurrence interval of one to two years for 

bankfull discharge in humid temperate streams but acknowledge that recurrence intervals for some sample 

sites diverge from these values. The concept of a universal recurrence interval for bankfull discharge has been 

questioned by numerous investigators (Kilpatrick and 

Barnes 1964; Harvey 1969; Gregory and Walling 1973; 

Hey 1975; Williams 1978; Richards 1982; Knighton 

1998). Thus, the users of this approach must be aware 

that there are exceptions to the general rule.

Channel Form and Maintenance: 

Magnitude-Frequency Concepts

Wolman and Miller (1960) hypothesized that a 

range of intermediate high flows, rather than bankfull 

discharge alone, forms the channel and effectively transports the most sediment. They recognized that although 

the highest floods transport the most sediment per event, 

the more frequent, lower magnitude floods transport 

more sediment over the long-term. Discharge frequency 

times the amount of sediment transported by each discharge defines the long-term sediment transport curve 

over the entire flow regime (figure 1).

A range of intermediate flows near the peak of Curve 

C (“effective discharge”) transports more sediment over 

the long-term than either higher or lower discharges. 

Low discharges (Curve B) cannot transport significant 

amounts of bedload sediment even though they occur 

much of the time. Higher flows become more efficient at 

transporting bedload because doubling discharge more 

than doubles sediment transport rate (Curve A) in most 

streams. However, high flows rarely occur, so the amount 

of bedload moved in the long term is small. Wolman and 

Miller (1960) further noted that bankfull discharge and 

effective discharge usually have similar values.

The conclusion that effective discharge is generally approximated by bankfull discharge is supported by 

studies of gravel-bed rivers in the western United States 

and elsewhere (Emmett 1975; Andrews 1980; Webb and 

Walling 1982; Emmett and Averett 1989; Leopold 1992; 

Andrews 1994; Troendle and Olsen 1994; Battalla and 

Sala 1995; Troendle et al. 1996) although there remains 

disagreement over the universality of this relationship 

(Pickup and Warner 1976; Richards 1982; Ashmore 

and Day 1988; Nash 1994). Considerable ambiguity 

in the literature arises from variability in how effective 

discharge is computed, limited flow records, and difficulties associated with field identification of bankfull 

stage (Knighton 1998). Recent work by Emmett and 

Wolman (2001) suggests that the steeper the bedload 

transport curve, the more important are large flows in 

transporting bedload, and the larger effective discharge 

becomes.

The Importance of Bedload

Although suspended sediment usually constitutes 

more of the total sediment load than bedload, suspended sediment has a less important role in structuring 

the morphology of gravel-bed channels Gomez 1991; 

Leopold 1992). Because of the critical role bedload 

transport has on channel morphology in gravel-bed rivers, this approach uses only bedload transport curves in 

recommending maintenance flows.

Bedload transport mechanisms are incompletely understood in spite of their importance to channel form. 

Bedload movement is episodic and discontinuous, fluctuating rapidly within several hours or minutes (Custer 

et al. 1987; Gomez et al. 1989; Bunte 1996; Leopold and 

Figure 1—The magnitude-frequency concept applied to bedload transport and discharge (adapted from Wolman and 

Miller 1960). The concept suggests that a range of intermediate flows transports more sediment over the long-term 

than either high or low discharges. Qthreshold refers to a 

threshold discharge at which bedload transport begins.
USDA Forest Service RMRS-GTR-128. 2004. 5

Emmett 1997). Highly variable transport is influenced 

by numerous factors including variations in stream 

power along a reach, sporadic sediment input from outside the channel, local bank erosion, turbulent velocity 

fluctuations, and other temporary imbalances in the sediment budget over short spatial and temporal scales.

In general, smaller particles are moved more frequently than the larger particles comprising the channel 

bed. The frequency depends on the hydrologic regime 

and the relative abundance of fines. Slope and grain 

size are related and both decrease in larger drainages 

as discharge increases. One reason upland channels are 

so resistant to change is that they have a wider range 

of particle sizes in the bed and most of the load is selectively transported over the coarse surface with little 

change in storage. As a result, larger magnitude, less frequent flows, are needed to rework the bed of mountain 

rivers with coarse substrates (Baker 1977; Lisle 1987; 

Carling 1988; Grant et al. 1990; Lisle 1990; Rhoads 

1992; Knighton 1998).

The sediment load of gravel-bed rivers typically contains fine sediment that is mobile over a large range 

of flows. This fine load is often supply-limited, i.e., 

transport capacity is greater than supply. Fine particles 

comprise a significant portion of the annual bedload and 

must be moved through the system to maintain channel 

capacity and equilibrium (Leopold 1992). Gravel-bed 

rivers also contain coarse sediment mobilized only 

during higher flows. This coarse load is often transport limited, i.e., transport capacity is less than supply. 

Most particles in the channel bed of gravel-rivers are 

entrained by discharges equal to or less than bankfull 

stage although the transport rate of some sizes is low 

and sporadic (Andrews 1983; Pitlick 1988; Leopold and 

Rosgen 1991; Leopold 1992; Buffington 1995; Leopold 

and Emmett 1997).

Phases of Bedload Transport

Emmett (1976) first suggested two distinct phases 

of bedload transport in armored channels. This concept 

of phases has subsequently been expanded to describe 

bedload transport in gravel-bed rivers (Jackson 1981; 

Jackson and Beschta 1982; Beschta 1987; Ashworth 

and Ferguson 1989; Warburton 1992; Andrews and 

Smith 1992). Recent studies have confirmed that intermediate discharges transport mainly sand whereas 

discharges near or above bankfull transport coarser bedload (Church et al. 1991; Komar and Shih 1992; Kuhnle 

1993; Carling 1995; Lisle 1995; Wathen et al. 1995; 

Petts and Maddock 1996; Wilcock and McArdell 1997; 

Whiting et al. 1999; Ryan and Emmett 2002; Church 

and Hassan 2002).

Phase 1 sediments are surface deposits of sand sized 

particles or fine gravel located primarily in pools, along 

channel edges, and behind obstructions in riffles. Phase 

1 Transport signals the initial mobilization and transport of fine bedload over the relatively coarse channel 

bed surface. Ryan and Troendle (1996) concluded that 

Phase 1 Transport begins at flows between 0.33 and 0.5 

of bankfull discharge in coarse-grained channels on the 

Faser Experimental Watershed in Colorado. Similarly 

Emmett (1975) found transport begins at the minimum 

of annual peak flows (about 0.4 bankfull) for streams in 

the Salmon River drainage in Idaho.

Phase 2 Transport is associated with the initial coarse 

sediment movement from the coarse surface layer and 

underlying channel bed. As flow increases, bedload becomes progressively coarser as a greater proportion of 

the material is mobilized intermittently and non-uniformly from the armor layer. This has significance 

to channel morphology. As the armor layer becomes 

mobilized, finer substrate is also made available, rapidly increasing both the rate of bedload transport and 

the sizes of bed material in transport (Emmett 1984; 

Gomez 1983; Sidle 1988; Ashworth and Ferguson 1989; 

Warburton 1992; Ryan et al. 2002). Phase 2 Transport 

appears to occur once every one to two years (requiring 

flows approaching bankfull discharge).

Phase 2 Transport initiates nonuniformly throughout 

the channel and may be a localized phenomenon as evidenced by the entrainment of bed material from riffles 

(Jackson 1981) and the nonuniformity of particle sizes 

and local shear stress (Seal and Paola 1995; Lisle et al. 

2000). Conceptually, Phase 2 transport is similar to partial transport (Wilcock and McArdell 1993, 1997) which 

argues that as flows increase, first selective transport occurs, followed by partial transport when all sizes, but not 

all particles, are moved. This is followed by full mobility whereby all rocks are moved during the event, not all 

at once or uniformly, but at different times from different 

patches. At flows between 50 and 70 percent of bankfull discharge Haschenburger and Wilcock (2003) found 

about 60 percent of the bed in a state of partial transport, 

with full entrainment of nearly all grain sizes occurring at 

flows approximately equal to the bankfull discharge.

Although the beginning of Phase 2 Transport occurs 

over a narrow range of discharges, precise identification of the discharge at initiation is difficult. Buffington 

(1995) suggested using flows near bankfull as a first approximation for the onset of gravel transport. Others 

have found that Phase 2 Transport generally begins at 

discharges between 60 to 100 percent of bankfull discharge (Jackson 1981; Pitlick 1994; Carling 1995; Petts 

and Maddock 1996; Ryan and Troendle 1996; Whitaker 
6 USDA Forest Service RMRS-GTR-128. 2004. 

1997; Ryan et al. 2002; Trush et al. 2000; Ryan et al. 

submitted).

The beginning of Phase 2 Transport in gravel-bed 

rivers is significant to channel maintenance because it 

begins the important process of significant sediment 

movement of the sizes of material making up the bed 

and banks of the channel. Consequently, we hypothesize 

that processes occurring during Phase 2 Transport dominate channel morphology and function in gravel-bed 

rivers. Retaining flows adequate to achieve and exceed 

Phase 2 Transport begins to provide the minimum flows 

needed to achieve channel maintenance objectives.

The Role of Streamside Vegetation in 

Maintaining a Stream Channel

A properly functioning channel requires a flow regime that not only conveys water and sediment, but also 

maintains adequate streamside vegetation to protect the 

integrity of channel banks and floodplain while at the 

same time keeping the channel proper free of perennial vegetation. This seemingly contradictory role occurs 

because a natural tension exists between vegetation 

and flows in the channel. Plants continually attempt to 

establish and occupy any viable moist surface. Success 

results in dense vegetation and abundant roots that serve 

to protect channel banks from the forces of flowing water. Water has a positive influence on vegetation because 

it provides the moisture plants need to survive, but water 

also inundates and scours vegetation that lies within the 

active channel. The negative side of plant growth from 

the viewpoint of channel maintenance is that vegetation 

may become established on sediment deposits within the 

channel during periods of low flow, reducing the ability 

of the channel to convey sediment and flood waters.

Important vegetation-related physical factors for 

maintaining the channel are the role plant root systems 

play in binding soil in the banks and the way floodplain 

vegetation slows water velocity during overbank flows, 

thereby trapping sediment and maintaining the floodplain elevation. Streamside vegetation along the active 

boundary of the channel has the most impact on bank 

stability (Thorne 1990) and therefore is particularly important to channel maintenance of rivers and streams.

Keeping the Channel Free of Perennial 

Vegetation

Vegetation constriction refers to the narrowing of a 

stream channel by perennial woody vegetation within 

the bed of the channel that is normally devoid of plants. 

Channels experiencing the normal range of streamflows 

are rarely constricted by vegetation, but channels with 

significant diversions may undergo vegetation constriction. Vegetation constriction becomes undesirable when 

the extent of the vegetation impairs flood capacity and 

elevates floodwater levels.

Prevention of vegetation encroachment into stream 

channels involves a combination of hydrologic, geochemical, and fluvial processes. The maintenance of 

active unvegetated channels involves all or some combination of the following: (1) prevention of germination of 

seeds and the establishment of seedlings in the channel, 

(2) the elimination of individuals that do get established 

in the channel, and (3) the maintenance of conditions 

that inhibit plant growth and prevent vegetative spread 

(rhizomes, root sprouts, vegetative propagules) of 

existing riparian plants into the channel. Because of the 

range of requirements and tolerances of riparian plant 

species, it is difficult to generalize or to identify specific conditions that maintain channels free of vegetation, 

but the primary processes that prevent encroachment are 

well-understood and are briefly summarized below.

Germination of seeds and the establishment of seedlings in the channel is inhibited by submergence of 

available seedbed sites at the time when the seeds of 

likely colonizers are dispersing. Seeds are prevented 

Summary of the Scientific Basis for Channel 

Maintenance Sediment Transport Flows:

• Inflow of sediment must equal the outflow of sediment for 

the channel to be maintained over the long term.

• Bedload transport is of greater importance to channel 

formation and channel maintenance than suspended 

sediment in gravel-bed streams.

• All flows capable of transporting bedload sediment are important to channel morphology.

• The magnitude-frequency concept shows that intermediate 

discharges (for reference, flows near bankfull/effective 

discharge) transport more sediment overall than other 

flows in gravel-bed channels.

• The flow threshold of coarse bedload sediment transport 

begins at less than bankfull/effective flow when a significant portion of the gravel bed mobilizes.

• Because higher flows transport more tons of bedload sediment per unit volume of water, they are more efficient 

at transporting bedload sediment.

• To transport all bedload over the long-term using natural 

flows, the natural duration of a range of intermediate to 

high flows are needed to assure transport of all of the 

mass and sizes of bedload (i.e., high flows by themselves lack sufficient duration and frequency to transport the entire load).
USDA Forest Service RMRS-GTR-128. 2004. 7

from reaching sites suitable for germination and growth 

when the surfaces suited to germination and growth are 

inundated. Conversely, seeds do land on sites that are 

suited to germination and growth, but conditions rapidly 

change following seed deposition (i.e., the site rapidly 

dries out, becomes too saturated, or is inundated) and recruitment is unlikely to occur. For example, if substrate 

is coarse and seedlings require a slow rate of groundwater decline to survive, a rapid stage decline would be 

likely to eliminate those seedlings through rapidly draining substrates. This would cause desiccation of those 

seedlings with high water requirements resulting in high 

seedling mortality (Mahoney and Rood 1998).

If seeds are deposited on sites suitable for germination, the conditions of such sites must also be suitable 

for seedling survival for recruitment to occur. If such an 

area is subsequently re-submerged, scoured, or buried 

by sediment, the likelihood of seedling survival diminishes (Van Der Sman and others 1993). Also, if other 

factors such as soil chemistry, texture, light quality, the 

length of time between flooding (or flow duration of the 

surface) are inappropriate for survival of seedlings, then 

even germination and seedling establishment is unlikely to result in long-term survival of an individual on a 

site. The wider the range of tolerances of a particular 

species, the higher likelihood it is that it will be able to 

persist through the seasonally changing conditions along 

an active stream channel. This is one reason annuals 

and generalists (ruderal species, annuals, and invasive 

weeds) are more common in streamside habitats than in 

upland areas.

Conditions that inhibit plant growth and prevent 

vegetative spread (rhizomes, root sprouts, vegetative 

propagules) of existing riparian plants into the channel include: rapid stage changes that include prolonged 

inundation of streamside area followed by drought conditions (common along arid-land streams); prolonged 

inundation that results in anoxic conditions in stream 

channel substrates; and flooding at more frequent intervals than those required for successful establishment of 

individuals in the channel (Keddy and Ellis 1985; Cronk 

and Fennessy 2001). In addition, mechanical disturbances in the channel that are of a magnitude sufficient to 

either mobilize bed material or to remove above ground 

biomass (stems, leaves) and uproot individuals are also 

key factors in maintaining channels free of vegetation 

(Friedman and Auble 1999).

Maintaining Streamside Vegetation Along the 

Channel

The composition and structure of streamside vegetation is dependent on the occasional occurrence of 

seasonal flood flows and subsequent water level declines that maintain moist substrates (Jackson et al. 

1987; Stromberg et al. 1991; Mahoney and Rood 1993). 

The magnitude, duration, timing, and frequency of 

streamflow can have a profound effect on streamside 

vegetation, in turn substantially influencing channel dynamics (Gebhardt et al. 1989; Gregory et al. 1991; Hill 

et al. 1991; Jackson and Beschta 1992). The relationship is complex because riparian vegetation depends 

on combinations of surface water, shallow groundwater, and precipitation. These factors vary depending on 

the geologic and geomorphic setting, the species and 

its characteristics, and the position of the plant on the 

floodplain (Stromberg and Patten 1996). Flows above 

bankfull provide for streamside vegetation needs by inundating the floodplain, encouraging floodplain scour 

and sediment deposition, providing for water storage 

and nutrient cycling, and the periodic disturbance required by streamside plant communities.

Soil moisture regimes and water table fluctuations 

are often strongly tied to streamflow fluctuations, especially in alluvial valleys where surface water is often 

hydraulically connected to shallow groundwater in the 

floodplain aquifer (Castro and Hornberger 1991; Busch 

et al. 1992). In some cases, groundwater may sustain 

streamside vegetation by tapping groundwater sources. However, high temporal and spatial variability in 

stream-groundwater interactions are common.

The dependence of streamside vegetation on instream 

flows rather than groundwater may be intermittent 

rather than continuous along a stream. Water entering 

floodplain aquifers from direct precipitation, colluvial 

aquifers, up-gradient valley aquifers, fault leakage, or 

other geological features may be locally more important 

than water from the channel in supporting streamside vegetation. In arid environments, the association 

between riparian vegetation and streamflows is particularly evident (Stromberg and Patten 1996). Because 

of the complexity of factors, site-specific data (observations of water table elevations and stream stage over 

time) are needed to determine the nature of the dependence of alluvial water tables upon streamflow (Harris 

et al. 1987).

Year-round baseflows may be needed in some situations to maintain streamside vegetation, but baseflow 

has a minor direct role in maintaining the physical character of the channel. For example, vegetation along 

streams in steep alpine environments is less dependent 

on instream baseflow because lateral inflow from hillslopes supplies adequate moisture to the rooting zone 

(Stromberg and Patten 1996). In many high elevation or 

humid regions typical of National Forests, the prevailing 
8 USDA Forest Service RMRS-GTR-128. 2004. 

climatic regime is moist enough to sustain vegetation. 

Also, return flows and tributary inflows below irrigation 

diversions may replenish groundwater and soil moisture sufficiently to maintain vegetation. Furthermore, in 

many situations where fisheries instream flows are provided, these baseflows are adequate to sustain riparian 

vegetation communities during low flow periods.

Reaches that lose water to the subsurface, in wide, 

lowland, unconfined valleys in arid and semi-arid regions are most likely to require a seasonal baseflow to 

maintain the water table and dependent riparian vegetation. In contrast, streamside vegetation along upland 

streams containing reaches that seasonally gain water 

from subsurface flow can typically be sustained without 

in-channel flows that supplement the high flows required 

for sediment transport. These gaining streams are typically found in humid regions and mountain canyons that 

often have adequate year-round soil moisture and subsurface input from hillslopes. Site-specific investigation 

will be required to define the nature of stream-subsurface flow connectivity and the seasonal needs for flows 

to maintain vegetation. A good guideline for maintaining 

riparian vegetation in a natural state is to preserve the 

natural phases of the annual hydrograph to the extent 

feasible (Poff et al. 1997).

Adverse Effects of a Lack of ChannelMaintaining Flows

Streams need to retain essential channel maintenance 

instream flows that convey sediment because future offstream diversions for uses such as irrigation and municipal 

water supply may deplete critical portions of these flows. 

In the absence of these essential instream flows, sediment 

may accumulate in the channel and the channel may respond by altering its size, morphology, meander pattern, 

rate of migration, streambed elevation, bed-material 

composition, floodplain morphology, and/or streamside 

vegetation. These channel alterations are frequently detrimental to favorable flow and sediment conveyance.

The response of fluvial systems to river regulation 

has been well documented (Schumm 1971; Bray and 

Kellerhalls 1979; Petts 1980; Williams and Wolman 

1984; Andrews 1986). Most of these studies are based on 

the morphological effects of dams on large river systems 

with low gradient channels. Typical responses to reduced 

flows include a decrease in channel size and capacity, 

aggradation at the confluence of tributaries, an increase 

in riparian vegetation, and narrowing of the channel with 

effects often extending far downstream. Reduced channel capacity, through narrowing of channel width, is the 

most common adjustment to flow depletion.

Flows from headwater streams commonly found on 

National Forest System lands are generally reduced 

by irrigation diversions rather than dams. Dams affect 

sediment and flow regimes differently than irrigation diversions. Thus, study results downstream of large dams 

are difficult to extrapolate directly to diversions. Large 

dams typically trap all bedload sediment, dramatically 

reduce flood peaks, and often increase low flows while 

keeping annual water volumes unchanged. In contrast, 

diversions are typically low-head structures that reduce 

the magnitude and duration of flows during a part of the 

year (typically the growing season), have minor effects 

on intermediate and large flood peaks, and are generally 

designed to trap or divert a small proportion of total annual bedload.

Channel Response Below Diversions

The extent of channel alteration from small-scale irrigation diversions is incompletely understood. Channel 

response is complex and depends on the amount of water 

diverted (flow volume); changes in flow rates and flow 

duration; physical characteristics of the channel below 

the diversion with respect to bed mobility thresholds and 

Summary of the Scientific Basis for Channel 

Maintenance Streamside Vegetation Flows:

• The flow regime, including magnitude, duration, timing, 

and frequency of flow, has a profound effect on streamside vegetation.

• Changes to streamside vegetation at the boundary of the 

channel have the most impact on stream bank stability 

and maintaining the channel.

• Flows above bankfull help to scour vegetation from the 

channel and provide for periodic floodplain disturbance 

and groundwater recharge that many species require 

for regeneration.

• Seasonal groundwater levels and the ability of roots to access soil moisture often sustain streamside vegetation 

without the need for in-channel flow. Therefore, periodic 

overbank flows are often adequate to meet streamside 

vegetation needs for regeneration.

• Year-round baseflows may be needed to sustain protective streambank and floodplain vegetation in some situations, typically losing reaches in arid and semi-arid 

regions.

• Because of the complexity of streamflow, water table, 

and soil moisture interactions, and their relation to 

the needs of riparian species, site-specific analysis is 

required to estimate maintenance flows needed to sustain riparian vegetation.
USDA Forest Service RMRS-GTR-128. 2004. 9

bank materials; timing, amount, and size of sediment 

supplied to the channel as well as that removed by the 

diversion structure; the length of time the diversion has 

been in place; and the presence and species composition 

of riparian vegetation within the downstream channel. 

Because of this complexity, extrapolation of diversion 

studies requires caution.

Few researchers have studied channel response to 

diversion in small, mountainous watersheds. Wesche 

(1991) studied 20 channels in southern Wyoming and 

northern Colorado. Flow depletion ranged from 17 to 

90 percent and had been going on from 12 to 106 years. 

Comparison of channel properties above and below diversions showed that in low gradient channels (less than 

1.5 percent slope), streams narrowed, vegetation encroached, and flood conveyance capacity diminished. 

Changes to steep gradient channels (slopes greater than 

4 percent) and moderate gradient channels (slopes 1.5 

to 4 percent), however, lacked statistically significant 

change. Wesche concluded that high elevation, steep 

gradient channels with high stream energy, low sediment loadings, and short growing seasons can better 

maintain channel dimensions with reduced flow regimes 

over long periods of time than low gradient reaches.

Bohn and King (2000) studied 21 small diversion 

sites in Idaho. Although hydrologic records were unavailable at these sites, small diversion structures 

apparently failed to divert a significant portion of high 

springtime peak flows—implying that channel-forming 

flows passed down the channels. Estimates of edgeof-vegetation flow conveyance below diversions were

significantly smaller than those above headgates by 

about 25 to 30 percent, indicating that flow reduction 

due to small diversions leaves discernable indicators in 

the channel.

Another study on several subalpine coarse-grained 

channels in the Rocky Mountains in Colorado was 

unable to detect significant changes in channel capacity between diverted and undiverted reaches and 

attributed this to the stability of subalpine channels and 

the large variance in channel dimensions (Ryan 1994; 

Ryan 1997). Total annual discharge had been reduced 

between 20 and 60 percent. The percentage of water 

diverted varied annually; however, the magnitude and 

frequency of bankfull and higher flows remained nearly 

the same. Observed reductions in channel width, where 

they occurred, were due to vegetation establishment on 

formerly active surfaces. Width reduction ranged from 

35 to 50 percent but was limited to low gradient (1 to 

2 percent slopes) and pool-riffle channels with gravel 

bars. Step-pool channels, dominated by large boulders 

with slopes greater than 3 to 4 percent, lacked evidence 

of notable decline in their width—supporting the conclusions of Wesche (1991).

Because of the systematic, long-term nature of flow 

diversions, long-term effects are probable. Centuries 

may be needed, however, to detect significant reductions 

in flow capacity in steep gradient channels (Ryan 1994). 

While some flows are needed in all channels to maintain 

proper function, adverse effects are most easily detected 

and more likely to occur in low gradient, self-formed alluvial channels.

Quantification Approach

At the simplest level of abstraction, a regime of natural flows is essential for the maintenance of natural 

stream channels. This flow regime must provide for sediment transport of the particles supplied to the channel 

and making up the bed of the channel and provide for 

streamside vegetation to protect and bind the channel 

banks. In fulfilling these roles, sediment and vegetation 

have both separate and combined influences on channel 

maintenance processes.

The proposed channel maintenance approach therefore consists of bedload sediment transporting flows 

and streamside vegetation sustaining flows. Flows 

to transport bedload sediment are a critical part of every channel maintenance instream flow. Intermediate to 

high sediment transporting flows provide some benefits 

to vegetation and may be adequate to sustain streamside 

vegetation in many geographic circumstances where adequate moisture from non-stream sources is available year 

round. In other situations, flows to sustain streamside vegetation will need to be evaluated on a site specific basis.

Typical Channel Maintenance Hydrographs

This section provides an overview of typical channel 

maintenance hydrographs using data in a hypothetical 

example from Halfmoon Creek, a snowmelt-dominated gravel-bed stream located in Colorado. This section 

is intended to help the reader understand the nature of 

a channel maintenance instream flow regime, including the lower and the upper limit of the claimed flows 

and the amounts of water typically required for channel maintenance from wet through dry years. Procedures 

and data needed to estimate the lower and upper limits 

are explained in a subsequent section.

A typical sediment-transporting channel maintenance 

hydrograph includes a range of discharges making up a 

portion of the rising and falling limbs of the annual hydrograph, excluding discharges in excess of the 25-year 
10 USDA Forest Service RMRS-GTR-128. 2004. 

event. Conceptually, the required maintenance flow regime begins at a discharge at which transport-limited 

gravels making up the bed of the channel begin to move 

(Qtrigger) and includes all flows up to and including the 

instantaneous 25-year flow (Qcap). This range of flows 

moves all bedload sediment, scours vegetation from 

the channel, partially inundates the floodplain, and provides high flow functions needed to sustain streamside 

vegetation. These essential channel maintenance flows 

are claimed for the entire duration during which they 

naturally occur. Figures 2a, b, and c show examples of 

typical hydrographs for maintaining bedload sediment 

transporting at Halfmoon Creek, Colorado, for average, 

high, and low runoff years, respectively using an average lower limit flow initiation value.

The channel maintenance flow regime in the example was specifically selected to begin at 0.8 of bankfull 

discharge to illustrate the effect on flow volumes derived 

from using an average lower limit value.

Regardless of flow ignition values, most of the water required for channel maintenance occurs during high 

runoff years (figure 2b). During typical low runoff years, 

none of the annual water yield is needed for channel 

maintenance because flows exceeding the 0.8 of bankfull initiation flow fail to occur in dry years (figure 2c).

The volume of water available to accomplish channel 

maintenance varies significantly from year to year, averaging about 16 percent of the annual water yield over 

the long-term for Halfmoon Creek, Colorado, and ranging from zero to 60 percent for individual years. Figure 

3 illustrates this sequence of channel maintenance flow 

needs from wet to dry runoff years defined in terms of annual water yield exceedence. Since channel maintaining 

flows rely on flow near and above bankfull, a high portion 

of channel maintenance water needs are satisfied during 

wet years. For example, in the highest flow year (exceeded 5 percent of years, 1984) 46 percent of the annual 

water yield is used for channel maintenance while during progressively lower runoff years smaller portions of 

the annual yield are used for channel maintenance. In an 

average runoff year (exceeded 50 percent of years, 1972) 

only 19 percent of the yield is used for channel maintenance. During nearly one-third of the low flow years, 

channel maintenance flows are unnecessary because 

gravel-transporting discharges are seldom achieved. 

The decreasing pattern of water used for channel maintenance progressing from wet to dry years is illustrated in 

figure 3. While this is one example from Colorado, other 

streams follow the same general pattern.

The selection of the lower limit flow initiation value 

influences both the volume of water needed for channel 

maintenance and the number of years that flows exceeding 

the threshold occur. As discussed previously, Phase 2 

transport flow initiation typically begins at discharges 

between 60 to 100 percent of bankfull discharge. For 

the Halfmoon Creek example, a lower flow initiation 

value of 0.6 of bankfull increases the average volume 

needed for channel maintenance to 22 percent of annual 

water yield with a range of zero to 75 percent for individual years. During nearly one-quarter of the low flow 

years, channel maintenance flows would be unnecessary because gravel-transporting discharges exceeding 

0.6 of bankfull fail to occur. In contrast, a higher flow 

initiation value set at bankfull discharge decreases the 

average volume needed for channel maintenance to 7 

percent of annual water yield with a range of zero to 

47 percent for individual years. During almost 60 percent of the years, flows exceeding bankfull would fail to 

occur and no water would be needed to satisfy channel 

maintenance purposes.

Additional streamside vegetation flows may be required to supplement the bedload transporting flows in 

some circumstances. These additional baseflows or recession flows may be necessary to prevent vegetation 

establishment in the channel or to provide for streamside 

vegetation maintenance and regeneration.

Quantifying Flows Needed for Sediment 

Transport

The bedload sediment transporting flows are primarily designed to route the total mass and sizes of alluvial 

bedload supplied to the channel through the channel 

system in order to maintain channel capacity with the 

minimum amount of water. This requires defining the 

beginning of Phase 2 sediment transport (lower limit) 

that dominates channel morphology and function and 

some high flow (upper limit) above which undesirable 

flooding may occur and which fails to transport significant amounts of sediment.

Estimating the Lower Limit of Sediment 

Transporting Flows

Conceptual Basis of the Lower Limit

The onset of measurable bedload transport happens over a narrow range of discharges and is thus 

practically and conceptually initiated by a threshold 

discharge. Because of the heterogeneous particle size 

distribution of the channel-bed surface in gravel-bed 

rivers, finer particles are initially mobilized by low 

flows. These often supply-limited fine particle sizes 

mobilize earlier than the transport-limited larger sizes. 

Therefore, as flows rise, sand and finer gravels on the 

channel-bed surface mobilize first (Phase 1 Transport) 

followed by the transport of coarser particles (Phase 

2 Transport).
USDA Forest Service RMRS-GTR-128. 2004. 11

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Figure 2a—A typical channel maintenance flow hydrograph applied to 

an average runoff year (exceeded 50% of years). The discharge 

data is from Halfmoon Creek, 

Colorado in 1972. The lower limit 

(Qtrigger) used to initiate the channel maintenance flow regime is 0.8 

of the 1.5-year flow and the upper 

limit (Qcap) is the 25-year flow. For 

the average year shown, channel 

maintenance flows require 3,851 

acre-feet of water of the 20,023 

acre-feet of total natural water 

yield, or 19% of the annual flow in 

Figure 2b—A typical channel maintenance flow hydrograph applied to a 

high runoff year (exceeded 20% of 

years). The discharge data is from 

Halfmoon Creek, Colorado in 1970. 

The lower limit (Qtrigger) used to initiate the channel maintenance flow 

regime is 0.8 of the 1.5-year flow 

and the upper limit (Qcap) is the 25-

year flow. For the high runoff year 

shown, channel maintenance flows 

require 6,938 acre-feet of water of 

the 27,070 acre-feet of total natural 

water yield, or 26% of the annual 

flow in this year. Most of the channel maintenance work takes place 

during these high runoff years. �

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Figure 2c—A typical channel maintenance flow hydrograph applied to a 

low runoff year (exceeded 80% of 

years). The discharge data is from 

Halfmoon Creek, Colorado in1991. 

The lower limit (Qtrigger) used to initiate the channel maintenance flow 

regime is 0.8 of the 1.5-year flow 

and the upper limit (Qcap) is the 25-

year flow. No channel-maintaining 

sediment transport occurs during 

this dry year because low flows are 

unable to move coarse sediment. 

During the low runoff year shown, 

none of the 16,094 acre-feet of total natural water yield is needed for 

channel maintenance.
12 USDA Forest Service RMRS-GTR-128. 2004. 

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Figure 3—A sequence of channel maintenance flows from wet (exceeded 5% of years) to dry (exceeded 90% of years) years. 

The volume of water needed for channel maintenance (shown in black in the figures) is highly variable from year to year and 

the percent of annual water yield used for channel maintenance decreases from wet to dry years in a regular manner. During 

many low flow years (about one-third of all the years), channel maintenance flows are unnecessary because gravel-moving 

discharges fail to occur.
USDA Forest Service RMRS-GTR-128. 2004. 13

In gravel-bed rivers, the channel-bed is commonly 

coarser than the subsurface (Church et al. 1987; Dietrich 

et al. 1989). This relatively coarse surface layer limits entrainment and transport of finer subsurface particles until 

discharge is sufficiently great to mobilize the coarser surface layer (not just moving sand size particles over an 

immobile bed as in Phase 1 Transport) making the finer 

subsurface bed material also available for transport. The 

discharge necessary to begin to mobilize the coarse surface layer is the threshold discharge for Phase 2 Transport.

A general long-term bedload transport model that 

incorporates these transport initiation concepts within 

the context of magnitude-frequency analysis (figure 4) 

serves as the template for estimating the specific range 

of natural flows necessary for channel maintenance. The 

threshold concepts of Phase 1 Transport (Qthreshold) and 

Phase 2 Transport (Qtrigger) are used to estimate the starting point of the channel maintenance flow regime.

Total bedload transport in figure 4 is computed as the 

long-term product of flow frequency (days) and bedload 

transport rate (tons/day) as proposed by the magnitudefrequency concept of Wolman and Miller (1960). The 

area under the total bedload transport curve represents 

100 percent of the bedload transport. We hypothesize 

that all discharges greater than the threshold discharge 

for bedload transport (Qtrigger) will move bedload in 

its entirety because these streams are supply limited, 

meaning that the streams can move more sediment, especially in the finer classes, than they presently do.

To estimate long-term total bedload transport, the 

following data and analyses are required: (1) historical 

daily mean discharge to estimate flow frequency and 

amount; (2) a bedload transport rating curve derived 

from measured bedload sampling; and (3) total bedload 

transport computed by combining daily mean discharge 

and bedload transport rate over a long-term flow record. 

Typically, existing or synthesized flow records of 20 

years or longer adequately represent the long-term with 

respect to discharge. Bedload measurements made over 

a full range of flows including those exceeding bankfull 

stage adequately represent long-term sediment transport 

provided the channel is in quasi-equilibrium and lacks 

substantial sediment pulses or waves that may change 

the transport curve.

The model illustrates that low flows, even though 

they occur most of the time, transport relatively minor 

amounts of bedload sediment because the bedload transport rate is near zero. In contrast, higher flows become 

more efficient at transporting bedload (figure 4). For example, doubling the flow rate more than doubles the 

bedload transport rate. Very large flood flows, although 

having the highest transport rates, account for relatively 

minor amounts of bedload sediment because high flows 

occur infrequently and are generally of short duration. 

Consequently, the largest proportion of the total bedload is transported by flows around the peak of the total 

bedload transport curve (i.e., the effective discharge). 

In many gravel-bed rivers, bankfull discharge approximates effective discharge.

Channel maintenance instream flows reserve the 

highest flows (those most efficient at transporting bedload sediment) and then progressively include lower 

flows. This assures transport of the highest percentage 

of total bedload mass with the least volume of water.

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Figure 4—Schematic of long-term total 

bedload transport model for gravelbed rivers based on Wolman and 

Miller’s magnitude-frequency concepts. Bedload movement begins at 

Qthreshold, flows between Qthreshold and 

Qtrigger are Phase 1 transport, Qtrigger

is the beginning of Phase 2 transport, 

flows exceeding Qtrigger are Phase 2 

and move the majority of the coarse 

sizes, and Qcap is the upper limit of the 

required channel maintenance flow 

regime. Flows from Qtrigger to Qcap constitute the range of flows necessary to 

maintain the channel.
14 USDA Forest Service RMRS-GTR-128. 2004. 

The total bedload transport model (figure 4) implies 

that all flows greater than Qthreshold are needed to transport 

100 percent of the total bedload mass. However, due to 

the supply-limited nature of gravel-bed rivers, meaning 

that the streams are capable of moving more sediment 

than they presently carry, a narrower range of high flow, 

beginning at Qtrigger, can still move almost 100 percent of 

the bedload sediment over the long-term without compromising channel maintenance objectives (Wolman et 

al. 1997). While some temporary increase in fine sand 

content may occur in the channel due to loss of some of 

the natural hydrograph (Parker et al. 2003), the deviation of the proposed channel maintenance hydrograph 

from the natural hydrograph is small and the temporary 

increase in surface fines is judged to be insignificant to 

channel processes.

While a temporary deposition of fines is generally undesirable for ecological reasons, we hypothesize that the 

accumulated fines will be moved by larger flows with 

excess stream energy that occur later in the year or in 

subsequent years. Improved efficiency of gravel transport occurs because the rate of gravel transport has been 

shown to increase with increasing sand content (Wilcock 

1998). Wilcock demonstrated that an increase in sand 

content of approximately 15 to 30 percent increases the 

efficiency with which the stream transports gravel-sized 

material. Shifting channel maintenance flows from the 

onset of sand and finer gravel movement (Qthreshold) to 

the onset of medium gravel movement (Qtrigger), thus delaying the initiation point of the channel maintenance 

hydrograph, is desirable because it minimizes the longterm volume of water needed for channel maintenance.

Phase 1 flows between the onset of sand movement 

and gravel movement can only be given up without 

sacrificing the aim of channel maintenance flows when 

streams are armored and the sand and fine gravel are 

supply-limited. In streams with abundant supply of 

all grain sizes (e.g., transport-limited streams, such as 

sand-bed streams), flow is transporting bedload at or 

near its capacity most of the time. In these instances, 

all flows capable of transporting bedload are essential to 

assure conveying all of the bedload, and claimed channel maintenance flows may need to start with the critical 

conditions for sand (Qthreshold). Direct bedload measurements or an analysis of bed mobility may be required 

in some circumstances to determine the applicability 

of the model proposed here for channels impacted by 

excessive fine sediment such that the channel is out of 

equilibrium.

The discharge associated with the “threshold” shift 

from Phase 1 Transport to Phase 2 Transport has been estimated from size analysis of measured bedload samples 

(Ryan et al. 2002; Emmett 1999), from measured 

bedload samples from in-channel sediment traps (Bunte 

et al. 2001), or from analysis of bed material composition (McNamara et al. 2000). Discharges sufficient to 

move the coarse surface layer typically occur only during a small percentage of the time. Obtaining sufficient 

bedload measurements to detect the threshold flow is a 

difficult task requiring a large number of well-timed samples. Site-specific measured data is always preferred, but 

may be infeasible. In these cases, it may be appropriate to 

extrapolate values of trigger flows from published literature (Jackson 1981; Pitlick 1994; Carling 1995; Petts and 

Maddock 1996; Ryan and Troendle 1996; Whitaker 1997; 

Ryan et al. 2002; Trush et al. 2000; Ryan et al., submitted) 

and verify these with measurements at local sites.

Methodology for Estimating the Lower Limit

Precise estimation of the lower limit starting point 

of the channel maintenance flow regime is challenging. 

While the existence of different phases of transport in 

gravel-bed rivers is widely acknowledged, the threshold 

between phases often lacks a precise value due to the 

nature of bedload transport phenomena and sampling 

difficulties. All estimations require some degree of professional judgment applied to the data available.

The starting point is best estimated from a combination of considering the particle size distributions of the 

bedload, the size distribution of the bed material, and 

bedload transport rates. In our judgment, the starting 

point of the required flows for channel maintenance is 

best estimated by examining direct empirical bedload 

measurements. Direct measurement provides tangible 

evidence of the particle sizes and the amount of material moving at different discharges without reliance on 

computational techniques or theoretical constructs of 

sediment transport. Indirect extrapolation approaches 

based on research studies may be necessary where data 

are unavailable.

Direct Approaches

The following direct approaches using measured bedload data may be used to estimate the starting point of 

channel maintenance flows. Measured bedload data can 

be obtained using either Helley-Smith samplers or bedload traps (Bunte et al. 2001). The starting point can 

be inferred using visual estimation or the more rigorous piece-wise regression analysis, which also provides 

confidence limits on the estimate.

(1) Visual Estimation

Data from Little Granite Creek in Wyoming are used 

to illustrate visual estimation. Figure 5 shows bedload 
USDA Forest Service RMRS-GTR-128. 2004. 15

discharge plotted as a function of water discharge for 

Little Granite Creek in Wyoming using logarithmic 

scales commonly used to plot bedload rating curves 

(Emmett 1999). A logarithmic plot of bedload transport, 

however, obscures the sudden increase in transport rates 

associated with the starting point of channel maintenance. The data in figure 5 are plotted arithmetically by 

particle-size class in figure 6 (Ryan and Emmett 2002) 

to show a dramatic increase in bedload transport at about 

220 cfs.

Dramatic, well-defined shifts as seen in figure 6 are 

seldom evident in bedload transport data sets. Visual 

identification of the starting point by its nature tends 

to be subjective. To minimize this subjectivity, collect 

sufficient bedload samples such that when the data are arranged by particle size class, at least 50–100 data points 

constitute the transport rate data on the graph. Make sure 

that the plot includes a large number of samples, welldistributed over the range of flows, and that it includes 

flows near bankfull discharge and higher. To aid visual 

estimation, look for abrupt increases in both the size and 

amount of material being moved in the range of 0.6 - 1.0 

bankfull discharge, which has been shown to represent 

the onset of Phase 2 Transport. For Little Granite Creek, 

it is apparent that both the sizes and amount of bedload 

discharge increase dramatically at about 220 cfs, 96 percent of the field determined bankfull discharge of 230 

cfs, suggesting a shift from Phase 1 Transport to Phase 

2 Transport.

(2) Piecewise Regression

To overcome the subjectivity and limitation of visual estimation, Ryan et al. (2002) apply a piecewise 

linear regression model to total bedload transport data. 

Piecewise linear regression fits one or more linear functions to ranges of data (Neter et al. 1989). Ryan et al. 

(2002) hypothesize the existence of different linear relationships for bedload transport over different ranges of 

discharge. One linear function is fitted to predominately 

sand-sized material representative of Phase 1 Transport, 

while a second linear function is fitted to higher flows 

representative of Phase 2 Transport that include coarser materials. The model is written so that the function is 

continuous at all points, including the breakpoint. The 

inflection point, where the fitted functions intersect, is 

termed the “breakpoint” and is interpreted to represent 

the flow at which a substantial change in the rate and nature of coarse-grained transport occurs (figure 7). The 

breakpoint generated by the piecewise linear regression 

model provides a statistically based, objective way of 

estimating the channel maintenance starting point.

The visual estimation and piecewise regression methods may produce different results, but in other analyses, 

they are quite similar (Ryan et al. 2002). Because 

breakpoint analysis fits a line that must have a slope 

for the upper portion of the data associated with coarser 

material, breakpoint analysis typically results in a 

starting point discharge lower than would be selected 

visually. For Little Granite Creek, Emmett using visual 

indicators, identified the starting point at about 220 cfs 

while breakpoint analysis estimates the starting point at 

171 cfs (Ryan and Emmett 2002). Both estimates are 

within the confidence limits of the piecewise regression 

breakpoint analysis and suggest that Phase 2 transport 

begins as flows approach bankfull discharge.

While piecewise regression provides an objective, statistical procedure for estimating the threshold discharge, 

judgment is still essential to properly interpret the data. 

A robust analysis requires a high number of well-distributed data points spaced over a wide range of flows, 

including bankfull or greater discharges. Data gaps over 

the range of flows need to be avoided, particularly near 

the breakpoint. A minimum number of about 80 bedload 

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Figure 5—Logarithmic plot of bedload transport (Gb) as a 

function of water discharge (Q) for Little Granite Creek 

(from Emmett 1999). A logarithmic plot of bedload transport 

tends to obscure the sudden increase in transport rates associated with the starting point of channel maintenance.
16 USDA Forest Service RMRS-GTR-128. 2004. 

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Figure 6—Arithmetic plot of bedload 

discharge and water discharge by particle-size classes for Little Granite Creek 

(from Ryan and Emmett 2002). Lines indicate values of the breakpoint discharge 

(171 cfs) and the 95% confidence limits 

on the breakpoint estimate. The discharge 

at which coarse gravel begins to appear in 

the samples coincides more or less with 

the value of the breakpoint. The rate of 

bedload transport increases dramatically 

at about 220 cfs -- a value within the confidence limits of the breakpoint estimate 

from the piecewise regression procedure. 

Both estimates indicate a substantial increase in the amount of bedload transport 

at flows approaching bankfull (230 cfs).

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Figure 7—Arithmetic plot of total bedload 

transport and water discharge for Little 

Granite Creek (from Ryan and Emmett 

2002). The figure illustrates three fits 

(linear, power, and piecewise linear) to 

total bedload transport data on arithmetic scale and shows the breakpoint 

of 171 cfs. Bars represent the 95% 

confidence limits on the estimate of 

the breakpoint; bar height is arbitrary.
USDA Forest Service RMRS-GTR-128. 2004. 17

transport samples are generally thought necessary for a 

robust analysis. A minimum of about 30 points is needed 

to estimate each of the piecewise regression functions. 

Conclusions drawn from smaller samples are difficult to 

interpret and typically have broader confidence interval 

bands (Personal communication, S. Ryan, 2003, Rocky 

Mountain Research Station).

Indirect Approaches

Channel maintenance flow quantifications may need 

to be quantified without benefit of detailed field data. In 

these instances, it may be necessary to rely on existing 

research studies (Jackson 1981; Pitlick 1994; Carling 

1995; Petts and Maddock 1996; Ryan and Troendle 

1996; Whitaker 1997; Ryan et al. 2002; Trush et al. 

2000; Ryan et al. submitted) and extrapolate basic concepts regarding the starting point.

Sufficient research information exists at this time for 

selected regions to predict a starting point for broadscale application. For example, studies by Ryan et 

al. (2002, submitted) of coarse-grained channels in 

Colorado and Wyoming concluded that the transition 

from Phase 1 Transport to Phase 2 Transport occurs between 60 to 100 percent of the 1.5-year discharge, with 

an average of 80 percent. An average starting point of 80 

percent of the 1.5-year discharge provides a good first 

approximation for general application while a selection 

of 60 percent of bankfull provides a more conservative estimate for more mobile systems with higher sand 

loads. A limited number of sites should be measured in 

the geographic area of interest to verify that the selected 

starting point applies locally.

Estimating the Upper Limit of Sediment 

Transporting Flows

Conceptual Basis for the Upper Limit

Prevention of flood damage was a primary reason why 

Congress wanted “favorable conditions of water flows” 

from the National Forests. Numerous government agencies actively work to protect the public from the adverse 

impacts of catastrophic flooding, such as 100-year flood 

events, through regulations prohibiting occupancy of 

floodplains, flood insurance and flood control programs, and structural flood protection and floodproofing. 

Society generally considers property damage-causing 

flows as undesirable or “unfavorable.” Therefore, as a 

practical matter, we propose placing an upper limit (Qcap) 

on channel maintenance instream flows.

A 25-year flow upper cap for channel maintenance 

instream flows balances the need to limit flood damage 

while still transporting sufficient sediment, particularly 

the larger sizes, to fulfill channel maintenance and riparian regeneration functions. Because the 25-year event 

is equaled or exceeded infrequently, on average in 4 

percent of years, flows in excess of the 25-year event 

transport relatively small amounts of sediment over the 

long term. Cumulative sediment transport computations 

of long-term flow records in Idaho and Colorado indicate 

that flows exceeding the 25-year flow generally transport about 1 percent or less of total bedload (King 1998; 

Schmidt 1998). This amount of forgone sediment transport will likely have an insignificant impact on channel 

capacity. Our analyses of several sites in Colorado indicate that limiting flows to the 5-year event risks leaving 

almost 10 percent of the sediment in the channel.

Rare, extremely high flows are part of the natural 

flow regime and have geomorphic and ecological importance over extended time scales (Junk et al. 1989). 

Scientific literature suggests that intermediate flows primarily affect channels and the floodplain, while large 

hydrologic events increasingly affect broader landscape 

features including terraces and valley sides (Petts and 

Maddock 1996). Extreme high flows may excessively 

widen the channel, damage property, structures, aquatic 

and riparian resources, and even threaten lives. Extreme 

floods may have catastrophic impacts on some channels (Newson 1980; Stewart and LaMarche 1967) while 

leaving others virtually unchanged by dispersing flood 

water over the valley floodplain and terraces (Pitlick 

1993; Miller 1990; Costa 1974). Most of the distinctive 

morphology of the valley bottom, especially in lowgradient alluvial streams, is from gradual reworking of 

fluvial sediments by intermediate flows whereas in low 

order tributary mountain streams, bedrock control, hillslope process, tributary input, and extreme high flows 

take a more dominant role (Grant and Swanson 1995).

Hill et al. (1991) identify valley-forming flows as 

peak discharges that exceed the 25-year recurrence 

event. The ratios of average water depth at mean annual discharge divided by mean bankfull depth for flow 

events of various recurrence intervals for alluvial rivers 

in four regions of the United States show remarkable 

consistency (Leopold et al. 1964; Emmett 1975). For a 

25-year event, the average flow depth is 1.5 times the 

bankfull depth and it ranges from 1.3 in Idaho to 1.7 in 

the Cascades. This implies that a 25-year flow typically 

inundates the floodplain to an average depth of about 

1.5 times the mean bankfull depth. This depth of overbank flow periodically inundates, transports sediment, 

and creates new surfaces for streamside vegetation regeneration and floodplain maintenance.
18 USDA Forest Service RMRS-GTR-128. 2004. 

With respect to vegetation, Hill et al. (1991) estimated 

that riparian maintenance flows (i.e., flows that inundate 

the riparian zone) occur at frequencies between 1.5 and 

10 years. Trush et al. (2000) concluded that large flows, 

those generally exceeding 10- to 20-year recurrence intervals, are necessary to sustain floodplain morphology 

and complexity. These overbank flows reshape meander sequences, form and maintain side channels, avulse 

main stem channels, rejuvenate riparian stands, scour 

floodplains, and perpetuate off-channel wetlands. Based 

on this information, the 25-year event appears to be a 

conservative estimator of flows adequate to periodically inundate riparian vegetation and maintain floodplain 

complexity; accordingly, we chose to use it as a reasonable upper limit

Methodology for Estimating the Upper Limit

The upper limit of the channel maintenance instream 

flow is the 25-year instantaneous peak flow. It can be directly estimated using standard flood frequency analysis 

if streamflow data are available and the period of record is adequate (U.S. Water Resources Council 1982). 

Where flow data is lacking, regional flood frequency 

analyses techniques of the U.S. Geological Survey may 

be used (Jennings et al. 1994).

Deviation from the 25-year upper limit, that is, selecting a lower return period flood event, should only 

be done after careful consideration of the consequences 

of having a lower magnitude cap. Decreasing the upper 

flow limit increases the risk of sediment accumulation 

in the channel and may have other consequences of failing to maintain or regenerate riparian vegetation. Any 

downward adjustments should be supported by scientific 

study and analysis.

Quantifying Flows Needed for 

Streamside Vegetation

Streamside vegetation flows may be needed in addition to bedload sediment transporting flows to maintain 

vegetation and protect channel banks. Because bedload 

transporting flows exceed bankfull stage, they scour 

vegetation from the channel and periodically inundate, 

deposit sediment, and scour portions of the floodplain 

in snowmelt streams, often when viable seeds are being released, thus preventing germination on active bars. 

At the same time, overbank flows recharge local aquifers and provide disturbance to maintain and regenerate 

other vegetation communities. Generally, in temperate 

mountainous environments, adequate soil moisture during the growing season is available to sustain streamside 

and floodplain vegetation because subsurface flows contribute moisture to riparian soils, shallow groundwater 

aquifers recharge from overbank flows, and mountainous sideslopes contribute water to maintain soil 

moisture and baseflow.

A streamside vegetation flow component in addition 

to bedload transporting flows may need to be considered 

on a site specific basis. For example, specific baseflows 

for vegetation may be needed where riparian vegetation 

directly depends on year-long baseflows or groundwater 

derived from channel flows, or where significant growth 

of woody vegetation might occur in the channel without 

streamflow. Streamside vegetation maintenance flows 

may also include high flows for successful regeneration 

of a particular species (e.g., the recruitment box model for cottonwood regeneration) (Mahoney and Rood 

1993).

An interdisciplinary team consisting of a riparian 

ecologist, hydrologist, and geomorphologist as a minimum is recommended to evaluate streamside vegetation 

flow needs and design site-specific studies to understand 

vegetation, streamflow, and groundwater interactions. 

Site-specific linkages between in-channel flow regimes 

and the maintenance requirements of streamside vegetation (Auble et al. 1994) need to be developed by the 

interdisciplinary team because universally applicable 

and generally accepted methods for estimating the flow 

regime necessary to maintain riparian vegetation and 

surrounding floodplains are presently lacking (Hill et 

al. 1991).

Analyzing and Displaying 

Results

This section provides guidance on analysis and display 

techniques that may be used to address questions such as:

• How much sediment is moved by the channel maintenance flow regime?

• What parts of the hydrograph move the most sediment?

• How efficient is the flow regime at moving sediment; 

that is, does the flow regime move the most sediment 

with the least water?

• Will flow fluctuations resulting from a channel maintenance claim adversely affect channel stability?

• How much water is needed for channel maintenance 

and how much remains for off-stream uses?

• How does the amount of water needed for channel 

maintenance vary from year to year?

• What geomorphic surfaces and vegetation are inundated by sediment transporting flows?
USDA Forest Service RMRS-GTR-128. 2004. 19

Cumulative Sediment Analysis

Cumulative sediment analysis provides one way to 

compute the total quantity of bedload sediment transported by the discharges in the channel. The results 

of the computation are used to show the amounts 

of sediment moved by various ranges of discharge 

(magnitude-frequency histogram) and the relationship 

between water, sediment, and the time required to 

move bedload sediment (cumulative sediment-watertime curve).

A cumulative sediment analysis requires a long-term 

flow record and a bedload rating curve. To do the cumulative sediment analysis, rank daily mean discharges 

for the period of record from low to high. For each 

mean daily discharge value, determine the frequency 

of that flow in days and then compute the cumulative 

frequency in percent. For each mean daily discharge 

value, determine the water volume in acre-feet and as a 

cumulative percentage. Assign a bedload transport rate 

in tons per day to each mean daily discharge from a 

bedload rating curve for the site. For each mean daily 

discharge value, compute total bedload in tons and as 

a cumulative percentage. A small portion of a sample 

computation is illustrated and explained in table 1.

The results of the computation are best examined graphically. Two types of data display are useful 

to understand sediment-discharge relationships: (1) 

Magnitude-Frequency Histograms, and (2) Cumulative 

Sediment-Water-Time Curves.

Magnitude-Frequency Histograms

The magnitude-frequency histogram (figure 8) shows 

the frequency of discharge and the total bedload transported by uniform increments of water discharge (cfs). 

The purpose of this analysis is to illustrate that over the 

long-term a majority of the bedload is typically moved 

by a range of intermediate discharges and that this range 

includes bankfull discharge.

To construct the histogram, select 20 to 30 equal interval discharge bins and sum the previously computed 

total bedload transport for each bin. Express bin totals 

as percentages and plot them versus discharge bin midpoints. Alternatively, magnitude-frequency histograms 

can also be developed by breaking a flow duration curve 

into 20 to 30 equal-interval discharge bins and applying 

Table 1—Example cumulative sediment analysis worksheet illustrating how to compute the values in each of the columns.

(1) (2) (3) (4) (5) (6) (7) (8)

 Cumulative Bedload

 Discharge Cumulative Water water transport Total Cumulative

Discharge frequency frequency volume volume rate bedload bedload

(cfs) (days) (%) (acre-feet) (%) (tons/day) (tons) (%)

2 0 0.0 0 0.0 0.000 0.00 0.00

3 42 0.5 250 0.0 0.000 0.01 0.00

4 48 1.1 381 0.1 0.001 0.03 0.00

5 125 2.6 1,239 0.2 0.001 0.1 0.00

6 302 6.2 3,593 0.5 0.001 0.4 0.01

7 237 9.0 3,290 0.8 0.002 0.5 0.01

8 512 15.1 8,122 1.6 0.003 1.4 0.02

9 481 20.8 8,584 2.4 0.004 1.7 0.04

10 549 27.3 10,887 3.4 0.005 2.5 0.06

11 418 32.3 9,118 4.3 0.006 2.3 0.08

12 356 36.5 8,471 5.1 0.007 2.4 0.11

13 192 38.8 4,950 5.6 0.008 1.5 0.12

14 333 42.8 9,245 6.4 0.009 3.2 0.15

15 180 44.9 5,354 7.0 0.011 2.0 0.17

etc. etc. etc. etc. etc. etc. etc. etc.

Description of computations:

Column 1: Discharge (cubic feet per second). Daily mean discharge values sorted from low to high for each distinct discharge for the entire 

period of record.

Column 2: Frequency (days). The total number of days each discharge occurred.

Column 3: Cumulative frequency (percent). The cumulative number of days expressed as a percent of total days for the period of record.

Column 4: Water volume (acre-feet). Discharge (column 1) multiplied by days (column 2) multiplied by a 1.983 conversion factor (1 secondfoot-day = 1.983 acre-feet).

Column 5: Cumulative water volume (percent). The cumulative water volume expressed as a percent of total water volume.

Column 6: Bedload transport rate (tons/day). Multiply each discharge by the bedload transport equation (Qs = f(Qw)) estimated for the stream. 

In this example, (Qs = 0.000028Qw

2.21); column 1 raised to the 2.21 power multiplied by 0.000028.

Column 7: Total bedload (tons). Tons per day of bedload (volumn 6) multiplied by the frequency or days (Column 2) that a specific discharge 

occurred during the period of record.

Column 8: Cumulative bedload (percent). The cumulative bedload tons expressed as a percent of total tons of bedload.
20 USDA Forest Service RMRS-GTR-128. 2004. 

a bedload rating curve to discharge intervals to compute 

effective discharge and the magnitude-frequency histogram (Andrews 1980; Andrews and Nankervis 1995).

In this example (figure 8), 78 percent of total bedload is transported by the range of flow between 150 and 

650 cubic feet per second, which encompasses bankfull 

discharge. Extreme high flows tend to move a relatively small percentage of total bedload compared to the 

amount of sediment moved by intermediate flows surrounding bankfull discharge. The most frequent flow bin 

(25 cfs) that occurs 70 percent of the time transports little sediment (1 percent). By contrast, the least frequent 

flow bin (975 cfs) occurs only 0.01 percent of the time 

and transports a similarly small amount (about 1 percent) of the sediment.

Cumulative Sediment-Water-Time Curves

A cumulative sediment, water, and time curve (figure 

9) illustrates the relationship between bedload transport 

and the amount of water volume and time needed to accomplish the bedload transport. The curve displays the 

cumulative time, water, and sediment associated with 

various levels of discharge. The purpose of this analysis 

is to illustrate that all of the bedload can be moved with a 

relatively small portion of the annual water yield during 

a small part of the year. Data for the curves come from 

the cumulative sediment analysis.

Figure 9 illustrates an example channel maintenance 

flow regime starting at 260 cfs and going up to 1,000 

cfs. The starting discharge (260 cfs) has been selected 

as described previously assuming supply-limitation of 

fines. The bedload making up 30 percent of the total is 

assumed to be Phase 1 fines that will be moved by flows 

of 260 cfs and higher. The upper limit is selected as the 

25-year flow (1,000 cfs). This flow regime from 260 cfs 

to 1,000 cfs uses 35 percent of the water (100 percent 

minus 65 percent) and occurs on average 6 percent of 

the time (100 percent minus 94 percent). The remaining 65 percent of the water that occurs 94 percent of the 

time, 243 days of the year, is available for other purposes.

Analysis of Ramping Rates

The channel maintenance instream flow regime follows natural flow patterns between the upper and lower 

limits once the channel maintenance flow is initiated. 

Concern often arises that excessive ramping rates, that 

is, rapid increases in flow at the beginning of the channel maintenance hydrograph or rapid decreases in flow 

at the end of the required flows, may result in damage to 

the channel and aquatic species. Since channel maintenance flow regimes normally begin and end at discharges 

less than bankfull, a concern exists that rapid short-term 

flow increases or drawdown between baseflows and the 

starting point may adversely impact channel banks. The 

concern is greatest where the flow changes affect sparsely vegetated, steep, sandy or silty erosive banks, or clay 

banks that may slump due to water retention and the 

build-up of positive pore pressures during drawdown 

(Thorne and Tovey 1981; Springer et al. 1985). In addition, the change in stage over time is often perceived 

to be substantially greater than that experienced under 

unregulated conditions. Some of this perceived concern 

may result from vertically exaggerated cross-section 

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Figure 8—Example of a sediment 

magnitude-water frequency histogram. Much (78%) of the total 

bedload is transported by an intermediate range of flow between 

150 and 650 cfs. Extreme high 

flows and the most frequent low 

flows tend to move a relatively 

small percentage of total bedload 

in the long-term compared to intermediate flows.
USDA Forest Service RMRS-GTR-128. 2004. 21

plots that suggest greater relative drawndown than what 

occurs naturally in channel systems.

In most gravel-bed rivers, the area of potential adverse 

impact due to rapid flow changes is the channel perimeter between the bottom of the channel (the stage of 

zero flow or low baseflows) and the stage of the starting 

point of the channel maintenance flow regime (normally a value around 0.8 bankfull discharge). This region 

is typically part of the channel bottom that slopes gently 

toward the stream banks, and for many gravel-bed rivers 

it is armored by coarse bed materials. Banks of gravel-bed rivers in the Interior West are frequently well 

vegetated and often composed of well drained non-cohesive gravel or coarser materials. The physical setting 

of gravel-bed rivers in this region therefore mitigates 

against much of the adverse channel damage that might 

result from rapid flow changes.

Several additional factors work to minimize adverse ramping effects due to the initiation of a channel 

maintenance flow regime. In most cases where irrigators manage water withdrawals, water is turned into the 

channel over a one day period of time, rather than as 

an instantaneous “wall of water.” Typically, the channel maintenance hydrograph is triggered during the 

spring runoff when diversion is minimal, runoff is at 

a maximum, and a significant amount of natural runoff is already in the stream channel. In many instances, 

diverters either lack physical capacity or a water right to 

divert significant flow from the stream. In other cases, 

instream flow requirements for fisheries or other purposes may require leaving some water in the channel.

All of these factors work to reduce the stage difference between existing flows and the stage of the channel 

maintenance starting point discharge, reducing potential 

ramping impact. Flows above the starting point follow 

natural ramping rates and are therefore unaffected by the 

nature of the channel maintenance flow regime.

A stage-discharge relation for the cross-section where 

the analysis of ramping takes place can be used to evaluate ramping. One can examine the historical flow record, 

use channel cross-sections, and use a stage-discharge 

relationship to determine the maximum daily natural 

changes in stage and discharge and compare it to stage 

changes expected from implementing the instream flow 

regime. Direct the analysis toward the effect changes of 

stage have on banks and the channel rather than looking 

at changes in flow volumes or rates alone.

Fluctuations in stage from the proposed flow regime 

that exceed natural stage fluctuations indicate an area 

of concern. The degree of armoring of the channel, the 

nature of bank materials, vegetation rooting characteristics, and the elevation of side channels, if fisheries are a 

concern, may also be considered in an analysis of bank 

and channel resilience to the effects of ramping.

If the analysis indicates a need for gradual increases 

or decreases, it may be possible to negotiate acceptable 

rates of stage change and operating procedures with affected water users on a case-by-case basis. The goal 

of these negotiated agreements would be to turn water 

into and out of the channel gradually over several days. 

Arrangements of this sort may have benefits for water 

users because gradually turning water into irrigation 

conveyance facilities avoids damage. Giving up some 

water from the prescribed channel maintenance flow to 

achieve a mutually agreeable operating procedure may 

avoid undue complexity in the claim structure and may 

benefit both parties.

Long-Term Water Use Trends

Since the channel maintenance flow regime varies from year to year depending on flow conditions, a 

simple statement of the average percentage of the mean 

annual flow essential to satisfy instream flow needs is 

inadequate to properly characterize the nature of longterm water use. An analysis of the historical flow record 

is useful to gain insight into how much of the natural 

flow regime needs to remain in the stream for channel 

maintenance and how much is likely to be available for 

other off-stream uses.

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Figure 9—Cumulative sediment-water-time curves showing cumulative percents of sediment, water, and time for 

a channel maintenance regime from 260 to 1,000 cfs. The 

channel maintenance flow regime from 260 to 1,000 cfs 

uses 35% of the water (100% - 65%), and occurs on average 6% of the time (100% - 94%). The remaining 65% of 

the water occurs 94% of the time and is available for other 

purposes.
22 USDA Forest Service RMRS-GTR-128. 2004. 

The analysis can be conducted by obtaining a longterm historical flow record at or near the site, estimating 

the appropriate channel maintenance flow regime for 

the site, superimposing the channel maintenance flow 

regime on each year of the historical record, and tabulating the natural and instream flow water volumes for 

each year. Figure 10 displays the partitioning of total 

annual water yield between that required for channel 

maintenance and that available for others for Halfmoon 

Creek, CO, a stream with a 50-year period of record. 

During some years, no water is needed for channel 

maintenance while over 20,000 acre-feet are needed in 

other years. On average, about 17,100 acre-feet of water is available for other uses including fish habitat or 

other instream flow needs. In most years, a firm yield 

of about 15,000 acre-feet of water will be available and 

the excess is never less than 10,000 acre-feet.

The nature of the channel maintenance flow regime is 

such that in drought years significant water in excess of 

channel maintenance needs remains available for appropriation by water users. In fact, during the worst drought 

years, all the water is available because starting point 

flows for channel maintenance are never reached. This 

analysis should help respond to misplaced concerns of 

water users that channel maintenance instream flow 

needs will deprive them of water.

Displaying Geomorphic Implications

A channel/floodplain inundation analysis is useful to 

evaluate how well the computed channel maintenance 

flows fit the channel and the floodplain and provides 

a preliminary assessment of the degree to which 

streamside vegetation may be inundated. Analysts can 

develop graphs from cross-section data, stage-discharge 

rating curves, and hydraulic modeling if necessary. The 

analysis serves to verify the extent to which the 25-year 

recurrence flow or other flow frequencies inundate the 

floodplain to provide for important riparian vegetation 

maintenance function. The cross-section in figure 11 illustrates how the channel maintenance flow regime fills 

the channel and inundates the floodplain.

Implementation Strategies

This section describes strategies for implementing 

studies following the previously described scientific 

channel maintenance concepts and principles. Analysts 

need to recognize that they may find situations where 

short time frames and limited flow and sediment data 

will require modifications or substitutions in the approach described.

Scales of Application

Two general categories of analysis are recognized:

• Site-specific application appropriate for projects; 

and

• Broad-scale application such as basin-wide collaborative watershed planning.

Quantitatively estimating channel maintenance instream flows is a complex endeavor best conducted at 

the site-specific level of application. Accordingly, these 

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Figure 10—A display of the longterm partitioning of annual water 

yield between that needed for 

channel maintenance and that 

excess to channel maintenance 

needs. During some years, 

no water is needed for channel maintenance while a firm 

yield of about 15,000 acre-feet 

of water in excess to channel 

maintenance needs is annually 

available except for one drought 

when water yield is reduced to 

about 10,000 acre-feet.
USDA Forest Service RMRS-GTR-128. 2004. 23

guidelines focus on site-specific application. Broadscale application at the watershed or basin level must 

often rely on less detailed field data. In these instances, 

the scientific concepts contained in this approach will 

need to be extrapolated to particular geographic areas 

and streams of interest.

If extrapolation is necessary, site-specific analysis at 

a limited number of representative sites is highly recommended to test if the extrapolated concepts apply to 

the geographic area of interest. The extent of local verification needs to be determined in each instance. For 

example, extensive bedload data sets useful for supporting the extrapolation of channel maintenance concepts 

exist for National Forest sites in Idaho, Oregon, and 

Colorado.

Setting the Context of the Analysis

Developing a study strategy is an important first step 

in applying this approach. The scope and detail of the 

strategy may vary depending on the application, but 

strategic thinking, analysis, and planning are key to success.

Detailed instream flow studies require significant investments of time and money and may require years to 

complete. While this might be necessary in special cases involving judicial proceedings, it is not necessary in 

all cases. Obtain commitment from management for the 

duration of required studies so that resources match expectations and focus data collection on answering the 

question at hand.

The value that organizations, individuals, government 

agencies, and society place on individual streams varies. 

Stream designations such as Wild and Scenic Rivers, 

Outstanding Resource Waters, Blue Ribbon Trout 

Streams, those containing threatened or endangered 

species, and other designations are of great concern to 

the public. In many cases, guidance contained in Forest 

Plans prioritizes stream reaches or watersheds according to resource value. While differing approaches are 

used by individual Forests, all prioritizing schemes address pertinent local resource issues, are the result of a 

large amount of public input, and have been formalized 

in the planning process. Use this information base and 

build upon it if necessary. Typically, streams with high 

resource value should receive the highest priority for instream flow protection.

The competition between instream and off-stream 

consumptive uses of water is more intense in some 

streams than in others. Conflicting demands may occur 

in streams identified for water resource development, 

watersheds with private inholdings, and streams desirable for hydropower development. Other streams, such 

as those that are remote from population centers, unsuitable for water development, located within Wilderness 

areas or completely within National Forest boundaries where the agency controls access and rights of way, 

have less potential for conflict.

Large dams, diversions, or withdrawals that significantly alter the natural flow regime are more likely to 

alter channel morphology and function than run-of-theriver hydropower projects or minor diversions. Give 

priority to sites where a lack of channel maintenance 

flows may produce a potential significant downstream 

risk to life, damage to property, or environmental values. 

Alluvial rivers and streams are at greater risk of channel 

change as a result of modified flows than steep reaches 

with bedrock channel characteristics. Rivers and streams 

where riparian vegetation depends on water from the 

stream (losing reaches and arid western streams) are 

often at greater risk than streams that gain water from 

Figure 11—Channel and floodplain cross-section plot of the South Fork Cache La Poudre River, Colorado. 

Near to true scale plots can be used to evaluate the relationship between channel maintenance flow 

stages, flow frequencies, and the degree of inundation provided by the channel maintenance flow regime for riparian vegetation maintenance. The plot above has a 2:1 vertical exaggeration meaning that 

the channel appears deeper than it actually is. The stick figure fisherman is scaled to be 6 feet tall to 

provide true scale perspective.

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24 USDA Forest Service RMRS-GTR-128. 2004. 

adjacent sideslopes and where riparian vegetation is less 

dependent on instream water for survival.

Selecting Specific Quantification Sites

Hydrology, geology, climate, vegetation, and land 

use within the watershed influence water and sediment 

sources, and the interaction of these factors combine 

to determine the nature, distribution, and condition of 

stream channels.

Stream classification, such as the system proposed by 

Rosgen (1996) and Montgomery and Buffington (1997, 

1998), can be a valuable tool for understanding and assessing stream adjustability. In mountain watersheds, 

alluvial valleys and alluvial reaches are commonly interspersed with bedrock-controlled segments and it is 

essential to view the entire drainage network as a whole 

system. At the simplest level, two major channel types, 

strongly linked to geology and valley form, are important to channel maintenance.

The first of these, unconstrained channels, flow 

through the alluvial deposits of wide valley floors 

allowing the channel to adjust to changing flows, sediment transport regimes, and major modifications of 

riparian vegetation. Floodplains are usually present. 

Unconstrained reaches are common on low to moderate gradient rivers and may be an important local 

component of valley floors in mountainous topography. 

Because unconstrained channels are highly adjustable, 

channel maintenance concepts are most appropriately 

applied to them.

A second major channel type, constrained channels, 

flow in narrow valley floors, tightly bounded by valley walls with limited opportunity for lateral movement 

or alteration of the channel due to boulder or bedrock 

channel materials. Constrained channels typically have 

slopes greater than 3 to 4 percent and step-pool, cascade, 

or bedrock morphology. Floodplains, if present, tend to 

be discontinuous and poorly developed. The position 

of the constrained reach is often controlled by major 

geologic controls including landslides, moraines, outwash terraces, and tributary fans. Because constrained 

channels are relatively non-adjustable, a lack of channel maintenance flows has less severe consequences and 

these channels may be assigned a lower priority.

In addition to inherent characteristics, an evaluation 

of changes to the stream resulting from watershed disturbance and land use is useful. Begin by determining:

• Watershed Condition. Evaluate the degree to which 

the watershed has been altered by management activities (logging, grazing, road construction, fire history, etc.) and try to estimate the effect this has had 

on streams. Reid and Dunne (1996) provide one 

approach for evaluating sediment budgets and management impacts. Evaluate streams for evidence of 

recent aggradation or degradation. If evident, attempt 

to identify the cause. Causes may be natural (e.g., 

catastrophic flood, geologic uplift, climate change) 

or human-induced (overgrazing, dam failure, etc.), or 

some combination. The objective is to verify whether or not the stream is in a state of quasi-equilibrium. 

Streams that are significantly out of equilibrium may 

require modified analysis procedures.

• Water Management Infrastructure. Determine the 

number of dams, reservoirs, and diversions within 

the watershed and schematically outline the effect 

they have on the flow regime. Evaluate the degree 

to which the structures alter peak flows, annual volumes, low flows, or combinations of flows, and evaluate their significance to resource values and channel maintenance. Be especially aware of long-term 

impacts, such as channels that may have adjusted to 

new flow regimes below dams or diversion, or channels that have expanded as a result of increased runoff due to management activities (e.g., transbasin diversions, road construction, ski area development, 

urban expansion) or changed land-use practices. The 

approach proposed here needs to be modified for application below major dams, especially those that retain all bedload sediment. Application of channel 

maintenance flows are unlikely to return severely impacted systems to pre-disturbance conditions. These 

systems will require more complex and comprehensive flow management schemes and different analysis techniques.

• Stream Modification. Evaluate past and present 

channel clearing, bank protection work, and channelization. Examine the effect these activities have had 

on channel form and the ability of the channel to adjust in the future. Effects may be positive or negative, 

permanent or transitory, depending on the nature of 

the activity. Recognize that simply imposing a natural flow regime into these systems will seldom “turn 

back the clock” and restore system to pre-disturbance 

conditions.

Vegetation plays an important role in channel dynamics. The effect of riparian vegetation on channel form is 

scale-dependent and typically more important on small 

streams than larger river systems. Evaluate the ability 

of streamside vegetation to be maintained and to reproduce under reduced flow regimes. Attempt to determine 

whether streamside vegetation depends on in-channel flows or groundwater connectivity for survival. 

Evaluate the general hydrologic relationship between 
USDA Forest Service RMRS-GTR-128. 2004. 25

surface and groundwater with an eye toward determining if the reach is a gaining (the local groundwater flow 

regime tends to feed the stream) or a losing reach (water generally moves from the stream to support the local 

groundwater table). Losing reaches are more likely to 

need instream flows to maintain streamside vegetation 

especially in arid or semiarid environments.

Based on an integrated evaluation of watershed characteristics, channel adjustability, and management impacts, 

select channel maintenance instream flow quantification 

sites having the following physical characteristics:

• Select perennial, alluvial, unconstrained, gravel-bed 

channel reaches.

• Avoid bedrock and bedrock-controlled reaches.

• Avoid reaches with gradients greater than 3 to 4 percent.

• Avoid braided reaches.

• Select reaches that are in relative quasi-equilibrium (i.e., avoid reaches that are actively aggrading/

degrading).

• Select sites with evident natural alluvial features 

(floodplains, bars, vegetation).

• Avoid reaches directly impacted by roads, bridges, buildings, diversions, dams, channel structures, 

heavy livestock use, or any kind of human-induced 

alteration.

• Avoid reaches immediately below dams and reservoirs.

• Locate study transects within reaches on relatively 

straight segments between meander bends.

• Select reaches with reasonable access for repeated 

measurements.

• Give preference to sites with existing streamflow and 

bedload data.

General Data Needs

The channel maintenance quantification approach is 

facilitated by site-specific hydrologic, sediment, channel geometry, and vegetation data. Table 2 shows useful 

data and its application in the analysis.

Hydrologic and Sediment Data

Streamflow and bedload transport data at the quantification site greatly facilitates application of this 

approach. Where measured flow or sediment data are 

unavailable or unattainable in a reasonable time, models or other extrapolation techniques may be required. 

Although numerous techniques are available to estimate 

discharge and model bedload transport at ungaged sites, 

site-specific sediment and flow data minimize many of 

the technical objections that may be raised about a channel maintenance quantification.

Measured bedload data provide direct physical evidence of the sizes and amounts of bedload sediment 

transported through the reach under various flow conditions. Collecting quality data and adherence to standard 

data collection and analysis protocols are essential.

Bedload sediment transport is extremely variable. 

Even with careful site selection and rigorous field technique, inherent natural and sampling variability is high. 

Table 2—Types of hydrologic, sediment, channel geometry, and streamside vegetation and the uses of that data in a channel 

maintenance instream flow analysis.

Data category Type of data Use of data

Hydrologic Long-term daily mean discharges Compute cumulative bedload sediment transport;

 (flow duration) magnitude-frequency analysis; compute long-term

 water allocation time series

Peak flows Estimate upper limit of the channel maintenance

 (flow frequency) hydrograph

Sediment Bedload transport data Compute cumulative bedload sediment transport; 

 conduct magnitude-frequency analysis

Bedload and bed-material particle Estimate lower limit of the channel maintenance 

 size distribution hydrograph

Channel geometry Site map, photographs, channel Characterize the quantification site, estimate 

 cross-sections, longitudinal profile, bankfull bankfull discharge, provide a data base for 

 stage (discharge), stage-discharge rating monitoring channel change over time, evaluate 

 curve, bed-material size distribution how flows fit the physical dimensions of the 

 channel, and support various aspects of the 

 analysis

Streamside vegetation Specific parameters determined by Determine the relationship between discharge and 

 study team streamside vegetation
26 USDA Forest Service RMRS-GTR-128. 2004. 

Ideally, the data should span several years and represent a range of flow conditions from wet through dry 

years since rating curves may shift from year to year. 

Sampled rates of bedload transport must cover the full 

range of flows capable of transporting sediment in the 

stream. As a practical matter, technically supportable 

sediment rating curves require a minimum of 20 data 

points well dispersed over a range of flows and may be 

obtained in as little as one runoff season depending on 

flow conditions. In most cases, the ability to collect data 

at flow levels exceeding bankfull stage is a constraining factor.

Sampling Bedload Transport

The Helley-Smith bedload sampler (Helley and 

Smith 1971; Emmett 1980) is commonly used for 

bedload measurements in the United States. The U.S. 

Geological Survey recommends using the recently developed Federal Interagency Sedimentation Project US 

BLH-84 bedload sampler for new projects. For consistency, use the same sampler throughout the course of the 

study at the same site. Regardless of device, sampling 

is generally carried out at uniform cross-sections similar to sections for discharge measurement or suspended 

sediment sampling. The recommended procedure is to 

conduct two traverses of the stream, sampling at least 

20 equally spaced cross-channel locations on each traverse with a sampling duration of 30 or 60 seconds at 

each vertical (Emmett 1981). Ryan and Troendle (1997) 

provide specific guidance for measuring bedload in 

coarse-grained mountain channels.

Details of field and computation techniques for 

bedload and suspended sediment measurements are 

available in the U.S. Forest Service publication Methods 

for Collecting and Analyzing Fluvial Sediment Data

(USDA Forest Service 1988) and the U.S. Geological 

Survey publication Field Methods for Measurement of 

Fluvial Sediment (Edwards et al. 1999).

It may be advantageous to use bedload traps in lieu 

of Helley-Smith samplers in wadeable channels with 

coarse bed material (Bunte et al. 2001; Bunte et al. 

Submitted). Bedload traps are particularly useful where 

the initiation of gravel transport is all that is required 

or initiation of gravel motion information is needed at a 

large number of sites.

Streamflow Data

The preferred choice for any hydrologic analysis is to 

have a long-term discharge record at the quantification 

site. Records exceeding 20 years are more representative of the long-term and reduce the influence of drought 

or wet years on the record. In many cases records of less 

than 20 years will need to be extended and correlated using hydrologically similar long-term stations.

In the absence of a stream gaging station near the 

quantification site, initiating a program to collect continuous flow data for a short period of time at the site 

is often advantageous. Short-term concurrent flow measurements at the quantification site can be used to extend 

its record to that from an existing long-term gage (Moog 

et al. 1999). Discharge at ungaged sites can also be estimated using the index station method (Searcy 1959), 

station correlation techniques (Searcy 1960; Hirsch 

1982; Gordon et al. 1992), or regional dimensionless 

flow duration curves (Emmett 1975).

Federal agencies have adopted the Log-Pearson Type 

III distribution to analyze peak flow frequency data at 

locations where a systematic record of peak flows is 

available. The United States Water Resource Council 

(1982) Bulletin No. 17B, Guidelines for Determining 

Flood Frequency, presents the currently accepted methods for analyzing peak flow frequency data at gaging 

stations. A variety of regional techniques and procedures 

have been published by the U.S. Geological Survey and 

others for estimating peak discharges at ungaged sites 

either by river basin (for example, Thomas and others 

1963 Snake River Basin), specific states (for example, 

Thomas and Lindskov 1983 state of Utah), or geographic regions (for example Hedman and Osterkamp 1982, 

western United States).

Channel Geometry Data

Procedures for conducting a channel geometry survey to characterize the nature of a study stream reach 

are discussed in Stream Channel Reference Sites: An 

Illustrated Guide to Field Technique (Harrelson et al. 

1994). Examples of channel geometry data analysis 

techniques are illustrated in The Channels and Waters of 

the Upper Salmon River Area, Idaho (Emmett 1975) and 

reported in standard texts such as Dunne and Leopold 

(1978).

An important part of any channel geometry survey includes a detailed site map of the study reach showing a 

plan view of the reach with sufficient accuracy to clearly 

depict channel features, floodplains, terraces, data collection sites, and other features. The site map should use 

surveying and mapping techniques such as plane table 

mapping, total station surveys, or aerial photographs. A 

good set of aerial or ground photographs can be a valuable supplement to the site map.

Normally, cross-sections are located in straight reaches between meanders in sufficient number to characterize 

the variability of the study reach. Tie all cross-sections 

to permanent benchmarks and extend them across the 
USDA Forest Service RMRS-GTR-128. 2004. 27

channel well above bankfull stage to at least the predicted 25-year flow level or about 1.5 times bankfull 

depth.

The survey needs to include a longitudinal profile 

of the stream that establishes the elevation of the existing water surface, channel bottom, and the floodplain 

(bankfull stage). Normally, longitudinal profiles extend 

a length of approximately 20 times bankfull channel 

width along the channel. Data from the longitudinal 

profile survey is used to identify bankfull stage at crosssections.

The best indicator of bankfull stage is the level of the 

active floodplain as identified by the presence of depositional surfaces (USDA Forest Service 1995, 2002, 

2003). A reach averaging approach, based on the identification of bankfull stage at multiple locations along 

a longitudinal profile, provides a better estimate of 

bankfull stage than selecting bankfull stage at a single 

cross-section (Dunne and Leopold 1978).

Discharge Rating Curves

The best way to determine flow quantities for various levels of water in a stream is to make a number of 

discharge measurements with a current meter and develop a stage-discharge relation, commonly called a rating 

curve. Once the curve is constructed, discharges can be 

accurately estimated from the water level, or stage, in 

a stream (assuming the bed remains stable). A rating 

curve is essential for estimating discharge associated 

with bankfull stage and to quickly determine discharge 

when bedload measurements are made. Discharge 

measurement procedures and the development of stagedischarge relations and their adjustment are described 

in detail in U.S. Geological Survey, Techniques of 

Water-Resources Investigations (Buchanan and 

Somers 1969; Kennedy 1984) and summarized in 

the U.S. Geological Survey’s National Handbook of 

Recommended Methods for Water-data Acquisition

(U.S. Geological Survey 1977). A tutorial on measuring stream discharge is available on CD-ROM from the 

USGS (Nolan and Shields 2000).

Particle Size Data

Wolman pebble counts (Wolman 1954) are useful to 

characterize the surface particle size distribution of the 

bed material making up the study stream. The particle 

size distributions of the subsurface material, bars, and 

bank materials are often also useful to understand sediment transport processes. Procedures for bed-material 

sampling of coarse material in gravel-bed rivers are discussed in the U.S. Geological Survey publication Field 

Methods for Measurement of Fluvial Sediment (Edwards 

et al. 1999) and other publications, for example Yuzyk 

(1986) and Bunte and Abt (2001).

Streamside Vegetation Data

Vegetation data is collected to determine if streamside 

vegetation depends on soil water (or water-influenced 

environmental conditions) derived from channel flows. 

Due to high variability in stream-groundwater interactions among drainages and along stream channels and 

the diverse response of streamside vegetation, data-intensive site-specific analysis is required. The linkage 

between the stream and alluvial aquifer is best determined by observation and inference based upon 

groundwater wells, or piezometers, and river stage data, 

or if indicated, soil water chemistry, including isotopes.

Post-Project Management 

and Evaluation

Administration of channel maintenance instream 

flows becomes important once an instream flow allocation is acquired. A key element of post-project evaluation 

is verifying that the instream flows obtained are actually flowing down the channel in the manner specified. 

Stream gages established during the study can be especially valuable to assist with post-project evaluations.

Follow-up measurements are essential for determining if the established channel maintenance instream flow 

regime is actually present and adequate to maintain the 

capacity of the channel and maintain streamside vegetation. If the channel is adjusting to the imposed flow 

regime in an unacceptable manner, either causing increases or decreases to channel dimensions or streamside 

vegetation, corrective measures may be needed. This is 

best done in the context of adaptive management.

Channel maintenance post-project evaluations should 

as a minimum address the following key questions:

• Is the channel receiving the range of flows determined 

necessary in Forest Plans, special-use permits, or by 

State permit or decree?

• Are the dimensions of the channel stable compared to 

undiverted reference reaches?

• Is sediment, both in size and volume, being distributed and transported in such a way as to prevent either 

long-term aggradation or degradation?

• Is there any change in channel capacity over time due 

to vegetative ingrowth or sediment deposition?

• Is streamside vegetation in a maintaining or a declining trend?
28 USDA Forest Service RMRS-GTR-128. 2004. 

The following list identifies critical data elements 

useful to support post-project management and evaluations:

• Discharge information (preferably from a continuously recording stream gage).

• A permanent record of stream cross-section dimensions and elevations, referenced to a permanent 

benchmark.

• Channel geometry data at permanent sites to observe 

any shifts in channel width, depth, or slope.

• Bedload sediment measurements, including particle 

size distribution and stream discharge over a wide 

range of flows.

• Channel material size distribution in the reach.

• Longitudinal profile to document changes to slope and 

pool spacing.

• Monumented photo-points to document channel and 

vegetation changes over time.

• Riparian vegetation transects to document species 

composition and distribution changes.

• A record of flow diversions and natural flows.

Procedures for establishing permanent reference sites 

to measure change to the physical character of stream 

channels and to monitor trends in fluvial and geomorphic conditions are discussed in Stream Channel 

Reference Sites: An Illustrated Guide to Field Technique

(Harrelson et al. 1994).

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