U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Bed-Material Entrainment Potential, 

Roaring Fork River at Basalt, Colorado

By John G. Elliott

Denver, Colorado

2002

02–4223

Prepared in cooperation with the

TOWN OF BASALT, COLORADO
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225–0046
Cover photograph:

Roaring Fork River at Basalt, Colorado

Aerial photograph taken November 8, 2000

Discharge approximately 290 cubic feet per second

Aerial photograph supplied by the Matrix Design Group, Denver, Colorado
CONTENTS III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 2

Purpose and Scope....................................................................................................................................................... 4

Acknowledgments ....................................................................................................................................................... 4

Study Area ................................................................................................................................................................... 4

Previous Investigations ................................................................................................................................................ 5

Study Methods ............................................................................................................................................................. 5

Bed-Material Entrainment Estimation................................................................................................................................... 11

Water-Surface Profiles and Hydraulic Geometry ........................................................................................................ 11

HEC–RAS Calculations..................................................................................................................................... 11

Additional Water-Surface Profiles..................................................................................................................... 12

Hydraulic Geometry .......................................................................................................................................... 15

Bed-Material Characteristics ....................................................................................................................................... 15

Sampling Sites ................................................................................................................................................... 15

Size Analysis ..................................................................................................................................................... 15

Flood-Generated Shear Stress and Entrainment Potential........................................................................................... 15

Shear Stress Estimation ..................................................................................................................................... 15

Critical Shear Stress........................................................................................................................................... 19

Entrainment Potential ........................................................................................................................................ 21

Analysis ....................................................................................................................................................................... 21

Summary................................................................................................................................................................................ 28

References Cited.................................................................................................................................................................... 31

FIGURES

1. Map showing location of study area and the Roaring Fork River study reach at Basalt, Colorado.......................... 3

2A–2D. Aerial photographs showing the:

2A. Study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 

290 cubic feet per second, cross-section and sediment-sampling sites, and the edge of water at 

1,260 cubic feet per second ............................................................................................................................... 6

2B. Study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 

290 cubic feet per second, cross-section and sediment-sampling sites, and the edge of water at 

1,260 cubic feet per second ............................................................................................................................... 7

2C. Study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 

290 cubic feet per second, cross-section and sediment-sampling sites, and the edge of water at 

1,260 cubic feet per second ............................................................................................................................... 8

2D. Study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 

290 cubic feet per second, cross-section and sediment-sampling sites, and the edge of water at 

1,260 cubic feet per second ............................................................................................................................... 9

3. Cross section 6, approximately 675 feet downstream from Highway 82 Upper 

Bypass Bridge, showing surveyed and simulated water-surface elevations for several streamflows ....................... 10

4A–4D. Longitudinal profiles of the:

4A. Roaring Fork River at Basalt, Colorado, showing simulated water-surface profiles for various floods, 

the water-surface profile surveyed June 2001, and cross-section locations. ..................................................... 13

4B. Roaring Fork River at Basalt, Colorado, showing simulated water-surface profiles for various floods, 

the water-surface profile surveyed June 2001, and cross-section locations. ..................................................... 13

4C. Roaring Fork River at Basalt, Colorado, showing simulated water-surface profiles for various floods, 

the water-surface profile surveyed June 2001, and cross-section locations. ..................................................... 14

4D. Roaring Fork River at Basalt, Colorado, showing simulated water-surface profiles for various floods, 

the water-surface profile surveyed June 2001, and cross-section locations. ..................................................... 14
IV CONTENTS

5. Cross section showing cross-channel shear-stress distribution and mean shear stress for the 100-year flood 

and the bar-inundating flow, Roaring Fork River at Basalt, cross section 6............................................................. 19

6. Graph showing downstream mean shear-stress distribution on the streambed at various discharges and 

critical shear stress for sediment on the streambed at cross sections 2 through 19 .................................................. 20

7. Graphs showing entrainment potential of the streambed, thalweg, and alluvial bars at the (A) bar-inundating

discharge, (B) 10-year flood, (C) 50-year flood, and (D) 100-year flood, Roaring Fork River at Basalt ................ 22

TABLES

1. Hydraulic geometry from selected cross sections in the Roaring Fork River study reach ..................................... 16

2. Sediment-size characteristics of the streambed, alluvial bars, and streambanks of the Roaring Fork River 

at Basalt................................................................................................................................................................... 25

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929—a 

geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929

Multiply By To obtain

millimeter (mm) 0.03937 inch

meter (m) 3.281 foot (ft)

kilograms per square meter (kg/m2) 0.2048 pounds per square foot (lb/ft2)

newtons per square meter (N/m2) 0.02088 pounds per square foot (lb/ft2)

newtons per cubic meter (N/m3) 0.006365 pounds per cubic foot (lb/ft3)

foot (ft) 0.3048 meter (m)

mile 1.609 kilometers (km)

acre 4,047 square meter (m2)

square mile (mi2) 2.590 square kilometers (km2)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

acre-foot (acre-ft) 1,233 cubic meter (m3)

ton, short 0.9072 megagram (Mg)
Abstract 1

Bed-Material Entrainment Potential, Roaring Fork 

River at Basalt, Colorado

By John G. Elliott

Abstract

The Roaring Fork River at Basalt, Colorado, has a frequently mobile streambed 

composed of gravel, cobbles, and boulders. 

Recent urban and highway development on the 

flood plain, earlier attempts to realign and confine 

the channel, and flow obstructions such as bridge 

openings and piers have altered the hydrology, 

hydraulics, sediment transport, and sediment 

deposition areas of the Roaring Fork. Entrainment 

and deposition of coarse sediment on the 

streambed and in large alluvial bars have reduced 

the flood-conveying capacity of the river. 

Previous engineering studies have identified 

flood-prone areas and hazards related to inundation and high streamflow velocity, but those 

studies have not evaluated the potential response 

of the channel to discharges that entrain the coarse 

streambed. This study builds upon the results of 

earlier flood studies and identifies some potential 

areas of concern associated with bed-material 

entrainment. 

Cross-section surveys and simulated watersurface elevations from a previously run HEC–

RAS model were used to calculate the boundary 

shear stress on the mean streambed, in the 

thalweg, and on the tops of adjacent alluvial bars 

for four reference streamflows. Sediment-size 

characteristics were determined for surficial material on the streambed, on large alluvial bars, and 

on a streambank. The median particle size (d50) 

for the streambed samples was 165 millimeters 

and for the alluvial bars and bank samples was 

107 millimeters.

Shear stresses generated by the 10-, 50-, 

and 100-year floods, and by a more common flow 

that just inundated most of the alluvial bars in the 

study reach were calculated at 14 of the cross 

sections used in the Roaring Fork River HEC–

RAS model. The Shields equation was used with 

a Shields parameter of 0.030 to estimate the critical shear stress for entrainment of the median 

sediment particle size on the mean streambed, in 

the thalweg, and on adjacent alluvial bar surfaces 

at the 14 cross sections. 

Sediment-entrainment potential for a 

specific geomorphic surface was expressed as the 

ratio of the flood-generated boundary shear stress 

to the critical shear stress (τ o/τ c) with respect to 

two threshold conditions. The partial entrainment 

threshold (τ o/τ c=1) is the condition where the 

mean boundary shear stress (τ o) equals the critical shear stress for the median particle size (τ c) at 

that cross section. At this threshold discharge, the 

d50 particle size becomes entrained, but movement of d50-size particles may be limited to a few 

individual particles or in a small area of the streambed surface. The complete entrainment 

threshold (τ o/τ c=2) is the condition where τ o is 

twice the critical shear stress for the median 

particle size, the condition where complete or 

widespread mobilization of the d50 particle-size 

fraction is anticipated. 

Entrainment potential for a specific reference streamflow varied greatly in the downstream 

direction. At some cross sections, the bed or bar 

material was mobile, whereas at other cross 

sections, the bed or bar material was immobile for 

the same discharge. The significance of downstream variability is that sediment entrained at one 

cross section may be transported into, but not 

through, a cross section farther downstream, a 
2 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

situation resulting in sediment deposition and 

possibly progressive aggradation and loss of 

channel conveyance. 

Little or no sediment in the d50-size range is 

likely to be entrained or transported through much 

of the study reach by the bar-inundating streamflow. However, the entrainment potential at this 

discharge increases abruptly to more than twice 

the critical value, then decreases abruptly, at a 

series of cross sections located downstream from 

the Emma and Midland Avenue Bridges. Median 

particle-size sediment is mobile at most cross 

sections in the study reach during the 10-year 

flood; however, the bed material is immobile at 

cross sections just upstream from the Upper 

Bypass and Midland Avenue Bridges. A similar 

situation exists upstream from all three bridges in 

the study reach for the 50- and 100-year floods. 

Anecdotal evidence and aerial photographs from 

1987, 1997, and 2000 indicate streambed aggradation or alluvial bar formation upstream from 

each bridge. 

The reach downstream from the Upper 

Bypass Bridge was characterized by a consistently moderate to high entrainment potential at 

the 10-, 50-, and 100-year floods. Moderate to 

high entrainment potential in this reach may be a 

relict condition from a late-19th century effort to 

straighten the channel, an action that consequently steepened the channel as well. The potential for bed-material entrainment at all simulated 

flood discharges in this reach could indicate that 

this reach is an efficient transporter of supplied 

sediment. The river reach between the confluence 

of the Fryingpan River and the Midland Avenue 

Bridge is hydraulically complex, geomorphically 

dynamic, and not well represented by the HEC–

RAS one-dimensional streamflow model used in 

this study. Aerial photography and anecdotal 

evidence indicate this reach recently has been an 

area of streambed deposition. 

Entrainment estimates in this report are 

limited by the precision and availability of data 

that were used to calculate shear stresses under 

the discharge scenarios. The location and spacing 

of cross sections, the resolution of channel geometry from widely spaced surveyed points, and the 

model-generated water-surface slopes from the 

HEC–RAS model are relatively insensitive to the 

scale and scope of this investigation, the main 

flood-conveying channel where entrainable sediment is stored. Additional sediment measurements or onsite observation of sediment 

entrainment would allow calculation of the 

Shields parameter, rather than an estimate. More 

topographic detail of the streambed and channel 

in some reaches would allow more precise estimates of shear stress and sediment-entrainment 

potential. 

INTRODUCTION

The town of Basalt, Colorado, is located at the 

confluence of the Fryingpan River with the Roaring 

Fork River in Eagle and Pitkin Counties (fig. 1). The 

Roaring Fork River is a single-thread, high-gradient, 

cobble/boulder-bed river that presents a natural hazard 

to the town, nearby residential structures, several 

bridges and roadways, and the municipal waste-treatment facility. The hazard originates from high velocity 

snowmelt runoff and a high bedload-transport rate that 

cause intermittent channel realignment as well as 

scour and deposition of coarse-grain bars and islands. 

Recent urban, highway, and recreational development 

on the flood plain, earlier attempts to realign and 

confine the Roaring Fork channel with retaining walls 

and levees, and flow obstructions such as bridge openings and piers have altered the hydraulics, sediment 

transport, and sediment deposition in the river at 

Basalt. 

Gravel-, cobble-, and boulder-size bed material 

is transported almost annually in the Roaring Fork 

River. Deposition of this coarse sediment on the 

streambed and in large alluvial bars has reduced the 

flood-conveying capacity of the river at the Upper 

Bypass Bridge, through Basalt, and downstream from 

the Fryingpan River confluence. In addition, sedimentation may have diminished fish habitat in the study 

reach and affected the function of an irrigation intake 

structure (Dave Konchan, Town of Basalt, oral 

commun., 2000). Sediment must be mechanically 

removed from the Upper Bypass Bridge and other 

reaches annually to maintain channel conveyance. 
INTRODUCTION 3

Fryingpan

Roaring

Fork

River

River

Fork

Roaring

Sopris

Creek

River

SUPPLY

HOME

HOME

SUPPLY

DITCH

DITCH

Treatment

Pond

Cedar Brook

Lake

1/2 0

0 .5 1 KILOMETER

1 MILE

NATIONAL GEODETIC VERTICAL DATUM OF 1929

107°00' 107°02'30" 107°05'

39°22'30"

39°20'

Base from U.S. Geological SurveyBasalt 1:24000, 1961, photorevised 1987Additional revision fromMatrix Design Group, Denver, Colorado

Highway 82

Emma Road

Midland Avenue Bridge

Emma Bridge

Old Highway 82

Highway 82

Highway 82

Upper Bypass

Bridge

Lower Bypass

Bridge

New

Begin Study Reach

End Study Reach

Basalt

Wingo

Emma

EAGLE COPITKIN CO

EAGLE COPITKIN CO

6800

6800

6800

6800

6800

Arkansas

South

Platte

River

River

Rio

Grande

Colorado

Gunnison

River

River

Denver

Pueblo

Study

area

Grand Junction

Figure 1. Location of study area and the Roaring Fork River study reach at Basalt, Colorado.
4 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

Purpose and Scope

The Town of Basalt is considering several strategies to mitigate or accommodate high water and sediment discharges, including continued dredging, 

revised flood-plain zoning, and reconfiguring the 

channel cross section and sinuosity. Previous studies 

have identified flood-prone areas and hazards related 

to inundation and high streamflow velocity, but the 

studies have not evaluated response of the channel and 

the streambed to discharges that entrain, or result in 

movement of, coarse streambed sediment. This report 

quantifies the potential for sediment scour or deposition at specific locations in the Roaring Fork River and 

will enable the Town of Basalt to better assess hazards 

related to changes in floodflow conveyance and allocate resources in response to immediate or long-term 

channel degradation or aggradation. This report also 

addresses potential flood-conveyance problems and 

hazards to life and property associated with bedmaterial movement. 

In this report:

1. A previously surveyed reach of the Roaring Fork 

River is reevaluated for the purpose of estimating 

bed-material entrainment potential. The reach 

begins just upstream from the Upper Bypass 

Bridge and extends approximately 1 mile downstream. The study reach includes the confluence 

of the Fryingpan River and three bridges. 

2. Bed-material particle-size distributions at several 

locations in the study reaches where entrainment 

or deposition potentially could occur are determined.

3. Bed-material entrainment potential from the 

channel and adjacent alluvial bars, if present, are 

estimated at several previously surveyed cross 

sections for several flood-discharge scenarios. 

Acknowledgments

The author wishes to acknowledge Dave 

Konchan (formerly of the Town of Basalt) for his overview of the problem and for assistance in sediment 

measurements. Mark Beckler (Sopris Engineering) 

supplied earlier digital survey files of the Roaring Fork 

River and provided survey control-point coordinates 

used to rectify the recent U.S. Geological Survey 

(USGS) channel survey. Robert Krehbiel (Matrix 

Design Group, Inc.) provided the rectified 2000 aerial 

photography and the HEC–RAS model output that was 

a critical component of the entrainment potential analysis. Rick McLaughlin (McLaughlin Water Engineers, 

Inc.) provided results of previous hydrologic investigations and the 1987 and 1997 aerial photographs. Paul 

Kinzel and Stevan Gyetvai (USGS) assisted with the 

2001 water-surface survey, and Stevan Gyetvai used a 

geographical information system (GIS) to prepare 

composite images of the aerial photography and 

survey and sample-location information. Finally, the 

author sincerely appreciates the interest and cooperation of numerous landowners along the Roaring Fork 

River and their permission for access to the river. 

Study Area

The Roaring Fork River emerges from a 

confined valley at Wingo onto a 1-mile-wide valley, 

approximately 2 miles upstream from Basalt. The 

valley width decreases to about 0.3 mile near the old 

town of Emma, approximately 1 mile downstream 

from the confluence of the Fryingpan River (fig. 1). 

Aerial photographs reveal several paleochannel positions, or that the river formerly had a multithread 

channel across the wide valley floor near Basalt. Some 

of these paleochannels are used as irrigation ditches, 

and others periodically convey floodflows not 

contained by the main channel. The gradient of the 

Roaring Fork River increases from about 0.008 ft/ft in 

the confined valley upstream from Wingo to about 

0.014 ft/ft through most of the wide valley and into the 

town of Basalt. The Roaring Fork gradient then 

decreases to about 0.008 ft/ft immediately downstream 

from the Fryingpan River confluence in Basalt. The 

Fryingpan River gradient is about 0.015 ft/ft at its 

mouth in Basalt. 

Streamflow in the Roaring Fork River upstream 

from the Fryingpan River confluence is largely unregulated except for a transmountain diversion through 

the Twin Lakes tunnel above Aspen and local irrigation diversions. Daily discharge records have been 

collected on the Roaring Fork at two nearby USGS 

streamflow-gaging stations: station 09073400 Roaring 

Fork River near Aspen (1964 to present, drainage area 

108 mi2), approximately 17 miles upstream from 

Basalt, and station 09081000 Roaring Fork River near 

Emma (1998 to present, drainage area approximately 

853 mi2), approximately 2.5 miles downstream from 

Basalt. Daily discharge records have been collected on 
INTRODUCTION 5

the Fryingpan River at station 09080400 Fryingpan 

River near Ruedi (1964 to present, drainage area 

238 mi2), 12.5 miles upstream from Basalt (Crowfoot 

and others, 2000). 

The Fryingpan River drainage area at the 

confluence is approximately twice the Roaring Fork 

drainage area upstream from the Fryingpan River 

confluence at Basalt; however, flood peaks on the 

Fryingpan River have been attenuated by Ruedi Reservoir since 1968. Streamflow regulation on the 

Fryingpan River may have affected sediment transport 

and deposition at and downstream from the confluence 

with the Roaring Fork River in Basalt. A segment of 

the Roaring Fork River in Basalt was straightened 

during railroad construction in the 19th century. This 

reduction in sinuosity and the resulting steeper river 

gradient facilitate the entrainment and transport of 

coarse streambed material. Areas of sediment deposition in the form of large alluvial bars composed of 

gravel, cobbles, and some boulders are present 

throughout the study reach (fig. 2). 

Several feet of vertical aggradation by deposition of cobble- and boulder-size material have 

occurred under the Upper Bypass Bridge since 1995 

(Dave Konchan, Town of Basalt, oral commun., 2000). 

The streambed elevation immediately downstream 

from the Fryingpan River confluence appears to be at 

the approximate elevation of the former flood plain, a 

surface now occupied by mobile homes, businesses, 

and multistory residences. A levee and a gabion 

retaining wall have been constructed to help prevent 

flooding in these areas. These and another levee 

upstream from the Upper Bypass Bridge complicate 

estimations of flood hydraulics. 

Previous Investigations

Several engineering studies have been done by 

Federal agencies or commissioned by the Town of 

Basalt to determine flood-plain hydraulic conditions. 

Some of the previous studies have acknowledged the 

significance of floods with respect to streambed scour, 

aggradation, and channel realignment; however, none 

of the studies quantified sediment entrainment, transport, and deposition processes. 

The first detailed hydraulic studies with 

surveyed cross sections were done in 1976 and 1978 

by the consulting firm of Wright-McLaughlin Engineers for the Colorado Water Conservation Board. The 

Federal Emergency Management Agency (FEMA) 

studied and mapped flood-prone areas along the 

Roaring Fork River in Basalt in 1979, 1980, and 1987 

(McLaughlin Water Engineers, Inc., 2000). 

Numerous changes occurred in the Roaring Fork 

River channel between the late 1980's and the middle 

1990's, including ongoing urban development, channel 

adjustments from a large flood in 1995, and the 

construction of the Highway 82 Upper Bypass Bridge 

beginning in 1987 and the Midland Avenue Bridge 

beginning in 1997. The July 1995 flood was estimated 

to have had a recurrence of 14 years at the Roaring 

Fork River near Aspen streamflow gage (09073400) 

and a recurrence of 28 years at the Roaring Fork at 

Glenwood Springs gage (09085000). Recognizing the 

potential effect of these changes on flood-prone areas, 

FEMA commissioned another study in 1997 to redefine the 100-year flood plain. The 1997 study used 

several channel cross sections in the study reach and 

the HEC–RAS one-dimensional hydraulic model, and 

recalculated flood-hazard areas for the Roaring Fork 

River channel and flood plain. Results of the 1997 

study, with additional modifications, are summarized 

in a report by McLaughlin Water Engineers, Inc. 

(2000). Output from the HEC–RAS model and subsequent flood studies, including cross-section locations, 

flood-plain and channel topography, and water-surface 

profiles for floods of 10-, 50-, 100-, and 500-year 

recurrence intervals, were compiled by Matrix Design 

Group, Inc. (2000). 

One noteworthy observation from earlier studies 

indicated that some flood discharge might be carried 

by a paleochannel that bifurcated just upstream from 

the Upper Bypass Bridge (figs. 1 and 2). This streamflow divergence occurred during the flood of 1957, a 

flood estimated to have a recurrence interval of 

approximately 60 years at the Glenwood Springs 

streamflow-gaging station (09085000) (U.S. Interagency Advisory Committee on Water Data, 1982). 

Floodwater carried by the paleochannel in 1957 inundated agricultural land south of Basalt. This area 

subsequently has been developed with retail and residential structures south of the realigned Highway 82. 

Study Methods

Onsite evaluations were made of 18 channel 

cross sections (cross sections 2 through 19) used in the 

HEC–RAS model (Matrix Design Group, Inc., 2000) 
6 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

XS–2

98) (

)

XS– 3 (96)

XS– 4 (94)

XS 5 (90) – 

Sediment measurements

Cross section

Waters edge surveyed June 7, 2001.

Discharge approximately 1,260

Roaring Fork River at R

o

a

r

i

n

g F

o

r

k R

i

v

e

r a

t

Basalt, Colorado B

a

s

a

l

t

, C

o

l

o

r

a

d

o

Aerial Photo A

e

rial P

h

otograph taken November 8, 2000 ta

k

e

n N

o

v

e

m

b

e

r 8, 2

0

0

0

Discharge approximately Disc

h

a

r

g

e a

p

p

ro

xim

ately

290 cubic feet per second 2

9

0 c

u

bic fe

et p

e

r se

c

o

n

d

EXPLANATION

cubic feet per second

Roaring Fork River

00 50 100 METERS

400 FEET 200 300 100

Figure 2A. Aerial photographs of the study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 290 cubic feet per second, crosssection and sediment-sampling sites, and the edge of water at 1,260 cubic feet per second.
INTRODUCTION

7

XS– 6 (88.6)

XS –7 (88.5) XS–

 

8 

 

(86.5)

Sediment measurements

Cross section

Waters edge surveyed June 7, 2001.

Discharge approximately 1,260

Roaring Fork River at Roaring Fork River at

Basalt, Colorado Basalt, Colorado

Aerial Photo Aerial Photograph taken November 8, 2000 taken November 8, 2000

Discharge approximately Discharge approximately

290 cubic feet per second 290 cubic feet per second

EXPLANATION

cubic feet per second

Roaring Fork River Roaring Fork River

00 50 100 METERS

400 FEET 200 300 100

Figure 2B. Aerial photographs of the study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 290 cubic feet per second, crosssection and sediment-sampling sites, and the edge of water at 1,260 cubic feet per second.
8 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, ColoradoXS 9– ( 86.4) XS– 10 (85.6) XS– 11 (83.6) Roaring Fork River at Roaring Fork River at Basalt, Colorado Basalt, Colorado Aerial Photograph taken November 8, 2000 Aerial Photograph taken November 8, 2000 Discharge approximately Discharge approximately 290 cubic feet per second 290 cubic feet per second

Sediment measurements

Cross section

Waters edge surveyed June 7, 2001.

Discharge approximately 1,260

EXPLANATION

cubic feet per second

Roaring Fork River Roaring Fork River

00 50 100 METERS

400 FEET 200 300 100

Figure 2C. Aerial photographs of the study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 290 cubic feet per second, crosssection and sediment-sampling sites, and the edge of water at 1,260 cubic feet per second.
INTRODUCTION 9

XS–14 (80.5)

S X 15 – 78. ( ) 0

XS–16 76. ( 65) .

XS– 17 (

(

76.38)

XS–18 76.2)

XS– 19 76. (

1) .

S X 13 – 81. (

) 0

S X 12 – 83. (

) 0

Roaring Fork River at R

o

a

r

i

n

g F

o

r

k R

i

v

e

r a

t

Basalt, Colorado B

a

s

a

l

t

, C

o

l

o

r

a

d

o

Aerial Photo A

e

rial P

h

otograph taken November 8, 2000 ta

k

e

n N

o

v

e

m

b

e

r 8, 2

0

0

0

Discharge approximately Disc

h

a

r

g

e a

p

p

ro

xim

ately

290 cubic feet per second 2

9

0 c

u

bic fe

et p

e

r se

c

o

n

d

Roaring Fork RiverRoaring Fork River

Sediment measurements

Cross section

Waters edge surveyed June 7, 2001.

Discharge approximately 1,260

EXPLANATION

cubic feet per second

00 50 100 METERS

400 FEET 200 300 100

Figure 2D. Aerial photographs of the study reach taken November 8, 2000, showing the Roaring Fork River at a flow of approximately 290 cubic feet per second, crosssection and sediment-sampling sites, and the edge of water at 1,260 cubic feet per second.
10 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

6,610

6,615

6,620

6,625

6,630

6,635

6,640

100-year flood, 9,400 cubic feet per second

50-year flood, 8,500 cubic feet per second

10-year flood, 6,100 cubic feet per second

Bar-inundating flow

2001 water surface, 1,260 cubic feet per second

1998 streambed surface

Bar 8860

South Terrace

HORIZONTAL DISTANCE, IN FEET

ELEVATION ABOVE SEA LEVEL, IN FEET

Old Highway 82

980 1,000 1,020 1,040 1,060 1,080 1,100 1,120 1,140 1,160 1,180 1,200 1,220 1,240

EXPLANATION

of the study reach (fig. 2). Most of these cross sections 

were surveyed in October 1998 and April 1999 (Mark 

Beckler, Sopris Engineering, oral commun., 2001); 

however, some cross sections were extrapolated from a 

digital topographic model (DTM). Four cross sections 

were excluded from the entrainment analyses because 

it was determined that the hydraulics at those locations 

were complicated by bridge contractions or piers or 

that channel geometry extrapolation from the DTM 

was unreliable (cross sections 4, 5, 12, and 17). 

Water-surface elevations from the HEC–RAS 

model output were used to compute water-surface 

slope between the cross sections and to compute the 

hydraulic geometry at each cross section for four reference streamflows: the 10-, 50-, and 100-year floods 

and a more common streamflow that just inundated the 

high point of alluvial bars (fig. 3). Earlier flood-plain 

studies had suggested that up to about 43 percent of 

the 100-year flood discharge might be carried by a 

paleochannel that bifurcated just upstream from the 

Upper Bypass Bridge (fig. 2A) (Dave Konchan, Town 

of Basalt, oral commun., 2000). However, the 

HEC-RAS scenario used in this study assumed that all 

floodflows up to and including the 100-year flood 

were prevented from entering the paleochannel by a 

levee and were contained in the main channel and on 

the adjacent alluvial bars, terraces, and flood plain. 

The maximum elevation of alluvial bars were 

identified in some of the Matrix Design Group, Inc., 

cross-section surveys made in 1998 and 1999; at other 

cross sections, the maximum elevations of alluvial 

bars were estimated by visual interpretation of the 

plotted streambed-surface profile. Alluvial bars were 

absent at three cross sections at the time cross sections 

were surveyed in 1998–99 (cross sections 2, 10, and 

13). At cross section 14, alluvial bar 8050 had been 

reshaped considerably and shifted leftward between 

the time of the survey and the onsite assessment in 

2000 (fig. 2). Consequently, entrainment-potential 

assessments were not made for alluvial bars at cross 

sections 2, 10, 13, and 14. 

Figure 3. Cross section 6, approximately 675 feet downstream from Highway 82 Upper Bypass Bridge, 

showing surveyed and simulated water-surface elevations for several streamflows.
BED-MATERIAL ENTRAINMENT ESTIMATION 11

Bed-material measurements were made by the 

USGS using standard methods to determine particlesize characteristics of the channel and, if present, of 

the alluvial bars (Wolman, 1954) at 14 cross sections 

in the Roaring Fork River. Wolman "pebble counts" 

were made in a linear traverse of the channel cross 

section where the channel was wadeable in late 

November 2000. In unwadeable sections, pebble 

counts of the streambed were made in a random 

manner at one-footstep intervals in shallower areas 

where it was safe to wade. Pebble counts on alluvial 

bars and along one bank were made in a linear traverse 

parallel to the edge of water. Bed-material subsurface 

measurements were not made. Sediment-size characteristics calculated from the bed-material measurements were used to determine the critical shear stress 

for sediment entrainment. 

The critical shear stress for entrainment of the 

existing sediment at 14 cross sections was compared to 

the boundary shear stress generated by the 10-, 50-, 

and 100-year floods and by a more common streamflow that just inundated the high point of the alluvial 

bars in the study reach. Critical shear stress and 

entrainment for the 500-year flood were not evaluated 

because the incremental increase in water-surface 

elevation above that of the 100-year flood was small. 

Boundary shear stress in the mean cross section and at 

specific points along the cross section were computed 

with depth and slope from the cross-section surveys 

and HEC–RAS simulated water-surface elevations for 

each of the four reference streamflows.

Entrainment estimates were made for the 

Roaring Fork River by using the method developed by 

Elliott and Hammack (1999 and 2000) for the 

Gunnison River in Black Canyon National Park. In the 

Black Canyon study, existing sediment-size characteristics were used with HEC–RAS streamflow model 

output to determine the flood discharges necessary to 

entrain surficial sediment from alluvial surfaces where 

it was desirable to scour and obliterate encroaching 

riparian vegetation. In the Roaring Fork study, existing 

sediment-size characteristics and HEC–RAS streamflow model output were used to evaluate the potential 

for existing alluvial deposits to become mobile under a 

range of flood-discharge scenarios. 

This study used the output from the earlier 

HEC–RAS one-dimensional streamflow model 

(Matrix Design Group, Inc., 2000). No new floodflow 

routing calculations were made. The study examined 

floodflow conditions only in the existing main channel 

between the laterally confining terraces and levees. It 

assumed all floodflows were conveyed by the main 

channel through downtown Basalt and that no flow 

occurred through the paleochannel bifurcation just 

upstream from the Upper Bypass Bridge. Also, the 

study looked only at free-flowing subreaches of the 

river and excluded cross sections where the hydraulics 

computations were excessively complicated by bridge 

contractions and piers. 

BED-MATERIAL ENTRAINMENT 

ESTIMATION

Bed-material entrainment estimates were made 

on the basis of existing sediment properties at several 

locations in the study area and on the anticipated local 

hydraulic conditions associated with the four reference 

streamflows. 

Water-Surface Profiles and Hydraulic 

Geometry

The local hydraulic conditions for each reference streamflow were estimated with surveyed or 

extrapolated channel cross sections and with simulated 

water-surface elevations generated with the HEC–

RAS one-dimensional streamflow model. 

HEC-RAS Calculations

One-dimensional water-surface profile models 

such as HEC–RAS (Hydrologic Engineering Center, 

1997) are used to estimate water-surface elevations, 

streamflow depths, and hydraulic conditions in river 

channels and adjacent flood plains. The HEC–RAS 

model uses surveyed cross sections of the river 

channel and the adjacent flood plain and terraces. Onedimensional modeling is appropriate where streamflow is uniform (velocity is relatively constant in both 

magnitude and direction between adjacent cross 

sections), but one-dimensional model output becomes 

less reliable in the vicinity of bridge piers and contractions if streamflow becomes nonuniform and critical. 

A two- or multi-dimensional water-surface profile 

model is more appropriate for complex hydraulic 

conditions such as when streamflow becomes nonuniform and critical; however, two- and multi-dimensional modeling was beyond the scope of this study. 
12 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

HEC–RAS model output was provided by 

Matrix Design Group, Inc., (2000) and is used in this 

study to calculate water-surface slopes and hydraulic 

geometry variables for the 10-year, 50-year, and 100-

year floods. The 500-year flood was modeled but was 

not used in subsequent analyses because the incremental increase in water-surface elevation above that 

of the 100-year flood was small. The HEC–RAS simulations assumed all flood discharge passed through the 

study reach. Earlier flood-inundation studies had 

determined that the discharges associated with the 10-

year, 50-year, and 100-year floods had the following 

magnitudes in the study reach (Robert Krehbiel, 

Matrix Design Group, Inc., oral commun., 2001):

Regional flood-frequency equations derived 

from an analysis of the magnitude and frequency of 

Colorado floods indicate that these discharge estimates 

for the 10-year, 50-year, and 100-year floods may be 

too large (Vaill, 2000); however, computation of 

revised flood magnitudes is beyond the scope of this 

report. Consequently, the flood magnitudes provided 

by Matrix Design Group, Inc. (2000) will be used 

hereafter. 

Water-surface slopes for the 10-year, 50-year, 

and 100-year floods were calculated from the watersurface-elevation differences between adjacent 

surveyed or extrapolated cross sections (fig. 4). Slopes 

assigned to the various cross sections were the slopes 

of the subreach immediately downstream from the 

cross section. 

Additional Water-Surface Profiles

The HEC–RAS model simulated water-surface 

elevations only for streamflow greater than or equal to 

the 10-year flood. Anecdotal and onsite evidence indicate that bedload transport commonly occurs at 

streamflows with less than a 10-year recurrence. 

Therefore, lesser streamflows may be significant in 

redistributing bed-material sediments in the study 

reach. The upper surface of point bars or other alluvial 

bars often correlates with the water surface of the 

bankfull discharge (Williams, 1978) or of a discharge 

that is significant in transporting bed-material sediment. The bankfull discharge is the most significant 

discharge in maintenance of the channel form in many 

streams (Wolman and Miller, 1960; Andrews, 1984). 

The study reach has several alluvial bars that are indicative of recent sediment entrainment (fig. 2A–D). 

Many of these bars are inundated simultaneously by a 

streamflow of approximately 3,000 ft3

/s, measured at 

the gaging station downstream from the Fryingpan 

River confluence (Dave Konchan, Town of Basalt, oral 

commun., 2000). The "bar-inundating discharge" 

upstream from the Fryingpan River confluence is 

approximately 2,500 ft3

/s. Based on this anecdotal 

observation and other studies (Jarrett and England, 

2002), the elevation of the alluvial bar tops may be 

considered a surrogate for the water surface of a 

specific discharge, with an associated recurrence, 

common to several subreaches in the study area. A 

water-surface profile for the bar-inundating discharge 

was estimated from alluvial-bar surfaces expressed in 

the cross-section surveys (fig. 4A–D). A recurrence 

interval could not be estimated for the 3,000-ft3

/s 

discharge because the streamflow record at the Emma 

gaging station, established in 1998, is too brief to 

calculate floodflow frequency statistics. 

In addition to the water-surface elevations simulated by the HEC–RAS model and the estimated barinundating water-surface profile, another survey was 

made by the USGS in June 2001 at a discharge of 

approximately 1,260 ft3

/s to determine the watersurface elevation representative of a low- to intermediate-streamflow condition (fig. 4A–D). A comparison 

of water-surface profiles in figure 4D reveals that the 

surveyed 1,260-ft3/s water-surface elevation is lower 

than the estimated bar-top elevations at cross sections 

12, 13, 17, 18, and 19. This relation is logical if the 

assumption is correct that the alluvial bars in the study 

reach were formed by a discharge of 3,000 ft3/s or 

greater. However, the 1,260-ft3

/s water-surface elevation is higher than the estimated bar-top elevations at 

cross sections 14, 15, and 16 (Bar 8050, fig. 2D). 

Possible explanations for this apparent contradiction 

could be that (1) local channel-geometry adjustments 

may have occurred in this subreach between the dates 

of the cross-section surveys (April 1999) and the 

water-surface survey (June 2001), or (2) alluvial bars 

Recurrence 

interval

Discharge 

upstream from 

Fryingpan River 

(cubic feet per 

second)

Discharge

downstream from 

Fryingpan River 

(cubic feet per 

second)

10-year flood 6,100 7,100

50-year flood 8,500 9,400

100-year flood 9,400 10,400
BED-MATERIAL ENTRAINMENT ESTIMATION 13

6,615

6,620

6,630

6,625

6,635

6,640

6,645

6,650

32,400 32,600 32,800 33,000 33,200 33,400 33,600 33,800 34,000

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

ELEVATION ABOVE SEA LEVEL, IN FEET

100-Year flood

50-Year flood

10-Year flood

Bar-inundating flow

Surveyed 1,260 cubic feet per second

Streambed

 Cross section with entrainment estimate 

 Cross section without entrainment estimate

Upper

Bypass

Bridge

EXPLANATION

2

5 4

4

3

2

EXPLANATION

6,600

6,605

6,610

6,615

6,620

6,625

6,630

6,635

31,000 31,200 31,400 31,600 31,800 32,000 32,200 32,400 32,600

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

7

9

6 Cross section with entrainment estimate 

4 Cross section without entrainment estimate

8

6

ELEVATION ABOVE SEA LEVEL, IN FEET

100-Year flood

50-Year flood

10-Year flood

Bar-inundating flow

Surveyed 1,260 cubic feet per second

Streambed

Figure 4A. Longitudinal profile of the Roaring Fork River at Basalt, Colorado, showing simulated 

water-surface profiles for various floods, the water-surface profile surveyed June 2001, and crosssection locations. Corresponds to stream segment shown in figure 2A.

Figure 4B. Longitudinal profile of the Roaring Fork River at Basalt, Colorado, showing simulated watersurface profiles for various floods, the water-surface profile surveyed June 2001, and cross-section 

locations. Corresponds to stream segment shown in figure 2B.
14 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

EXPLANATION

6,585

6,590

6,595

6,600

6,605

6,610

6,615

6,620

29,800 30,000 30,200 30,400 30,600 30,800 31,000 31,200 31,400

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

10

9

9 Cross section with entrainment estimate 

4 Cross section without entrainment estimate

11

ELEVATION ABOVE SEA LEVEL, IN FEET

100-Year flood

50-Year flood

10-Year flood

Bar-inundating flow

Surveyed 1,260 cubic feet per second

Streambed

EXPLANATION

6,570

6,575

6,580

6,585

6,590

6,595

6,600

6,605

28,600 28,800 29,000 29,200 29,400 29,600 29,800 30,000 30,200

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

13

14 15

16

17

19

12

18

11

ELEVATION ABOVE SEA LEVEL, IN FEET

100-Year flood

50-Year flood

10-Year flood

Bar-inundating flow

Surveyed 1,260 cubic feet per second

Streambed

11 Cross section with entrainment estimate 

12 Cross section without entrainment estimate Midland Bridge

Emma Bridge

Figure 4C. Longitudinal profile of the Roaring Fork River at Basalt, Colorado, showing simulated 

water-surface profiles for various floods, the water-surface profile surveyed June 2001, and crosssection locations. Corresponds to stream segment shown in figure 2C.

Figure 4D. Longitudinal profile of the Roaring Fork River at Basalt, Colorado, showing simulated watersurface profiles for various floods, the water-surface profile surveyed June 2001, and cross-section 

locations. Corresponds to stream segment shown in figure 2D.
BED-MATERIAL ENTRAINMENT ESTIMATION 15

in the study reach are formed by multiple discharge 

magnitudes rather than by a single discharge common 

to all. 

Hydraulic Geometry

Hydraulic geometry variables (cross-section 

area, mean depth, mean shear stress, and shear stress at 

a point) were calculated from the surveyed or extrapolated cross sections and the HEC–RAS water-surface 

elevations (fig. 3). These values and other information 

are presented in table 1. 

Bed-Material Characteristics

Sediment-entrainment potential for a specific 

flood or runoff event is partly determined by the 

size characteristics of the material composing the 

streambed, alluvial bars, and streambanks. Bedmaterial characteristics were determined from onsite 

measurements at several locations in the study reach. 

Sampling Sites

Sediment-size characteristics, including the 

median particle size (d50), were determined for surficial streambed material, large alluvial bars adjacent to 

the low-flow channel, and one streambank using the 

Wolman (1954) method. Subsurface sediment size 

was not determined. The median sediment-particle 

size (d50), the standard deviation of the d50, and the 

geometric standard deviation were determined at 

16 sites in the study reach (table 2). 

Measurements of the streambed and one streambank were made in November 2000 when streamflow 

was very low. Measurements of alluvial bars were 

made in May and October 2001. Streambed measurements were made by traversing the low-flow channel 

from the left edge of water to the right edge of water 

and generally represent the coarsest material in the 

river channel. This material is inundated year round. 

The alluvial bars and banks are deposits that are inundated by relatively frequently occurring streamflows 

but are subaerially exposed for most of the year. The 

streambed in the low-flow channel at most cross 

sections in the study reach is composed of cobble-size 

(64- to 256-mm diameter) and boulder-size (greater 

than 256-mm diameter) material. The alluvial bars and 

banks are composed of slightly finer material in the 

gravel-size (4- to 64-mm diameter), cobble-size, and 

occasionally boulder-size range. 

Size Analysis

The median sediment-particle size of the 

streambed samples ranged from 128 to 202 mm and 

averaged 165 mm (table 2). The geometric standard 

deviation is a nondimensional measure of the degree 

of sorting and is defined as the square root of the ratio 

of the 84th-percentile particle size to the 16th-percentile particle size (Vanoni, 1975, p. 36). For streambed 

samples, the geometric standard deviation ranged from 

1.78 to 2.62. 

The median sediment-particle size of the alluvial bars and one bank ranged from 32 to 182 mm and 

averaged 107 mm. The geometric standard deviation 

for most alluvial bar deposits ranged from 2.08 to 

2.93. Two samples had very high geometric standard 

deviations. Bar 8650 had a geometric standard deviation of 24.13, indicating a very poorly sorted deposit. 

Bar 8650 was largely composed of cobble-size material (64- to 256-mm diameter) but had an extensive 

veneer of sandy material (less than 2-mm diameter), 

possibly deposited by a flow subsequent to the flow 

that deposited the cobble-size material. The one bank 

sample, Bank.7800, had a geometric standard deviation of 7.24, indicating poor sorting. The sorting characteristics of this bank might reflect a recent 

mechanical reworking of the bank material during 

channel stabilization activity. 

Flood-Generated Shear Stress and 

Entrainment Potential

Sediment entrainment, or movement, in stream 

channels is partly a function of the boundary shear 

stress, a tangential stress created by flowing water 

acting on sediment particles resting on the streambed 

or other inundated alluvial surfaces. Entrainment 

potential for sediment on a specific geomorphic 

surface is estimated by the relation between floodgenerated shear stress and the critical shear stress of 

the sediment particles, the shear stress at which 

general movement of sediment begins. 

Shear Stress Estimation

Shear stress is proportional to the square of 

streamflow velocity and is most accurately determined 
16 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

Table 1. Hydraulic geometry from selected cross sections in the Roaring Fork River study reach

[Upstream distance measured in channel from Garfield–Eagle County line; Matrix reference number from previously published report (Matrix Design 

Group, Inc., 2000); na, no alluvial bar, not applicable; cross sections 6, 7, 8, 9, 10, 11, 14, 15, 16, 18, surveyed; cross sections 2, 3, 13, 19 extrapolated from 

digital topographic model; lb/ft2, pound per square foot]

Crosssection 

number

Matrix

reference 

number

Upstream 

distance

(ft)

Crosssection area 

(ft2)

Mean

depth

(ft)

Watersurface 

slope

(ft/ft)

Mean

streambed 

shear stress 

(lb/ft2)

Thalweg 

shear stress 

(lb/ft2)

Bar-top 

shear stress 

(lb/ft2)

Bar-inundating discharge

2 98 33,910 297.20 1.97 0.0197 2.42 4.14 na

3 96 33,259 170.14 0.99 0.0077 0.48 1.77 0.00

6 88.6 32,382 420.95 2.79 0.0099 1.72 3.58 0.00

7 88.5 31,997 403.43 2.56 0.0111 1.77 4.84 0.00

8 86.5 31,507 364.83 2.17 0.0139 1.88 6.09 0.00

9 86.4 31,045 335.15 2.39 0.0157 2.35 3.63 0.00

10 85.6 30,824 150.99 1.50 0.0105 0.98 2.06 na

11 83.6 30,144 210.39 2.35 0.0107 1.57 2.73 0.00

13 81 29,709 293.10 2.76 0.0218 3.75 5.60 na

14 80.5 29,449 226.00 1.50 0.0078 0.73 1.85 0.00

15 78 29,359 215.63 1.69 0.0056 0.59 1.27 0.00

16 76.65 29,219 269.23 2.28 0.0025 0.36 0.70 0.00

18 76.2 28,885 573.45 4.16 0.0121 3.14 4.27 na

19 76.1 28,778 320.02 2.36 0.0131 1.93 4.10 0.00

10-year flood

2 98 33,910 717.74 2.97 0.0155 2.87 4.82 na

3 96 33,259 1,031.97 4.59 0.0039 1.12 1.57 1.05

6 88.6 32,382 579.15 3.35 0.0117 2.44 5.82 1.59

7 88.5 31,997 752.87 2.97 0.0115 2.13 6.09 1.08

8 86.5 31,507 588.26 3.42 0.0128 2.73 6.65 1.05

9 86.4 31,045 602.56 3.67 0.0097 2.22 3.35 1.10

10 85.6 30,824 492.01 4.35 0.0111 3.02 4.36 na

11 83.6 30,144 465.67 4.95 0.0060 1.85 2.57 1.04

13 81 29,709 518.40 4.66 0.0117 3.40 4.53 na

14 80.5 29,449 1,022.27 5.66 0.0078 2.76 4.15 2.29

15 78 29,359 913.50 5.74 0.0074 2.65 3.85 2.17

16 76.65 29,219 847.25 6.01 0.0002 0.082 0.12 0.06

18 76.2 28,885 623.60 4.42 0.0082 2.26 3.55 na

19 76.1 28,778 562.29 3.85 0.0095 2.28 3.98 1.01
BED-MATERIAL ENTRAINMENT ESTIMATION 17

50-year flood

2 98 33,910 858.57 3.52 0.0149 3.28 5.17 na

3 96 33,259 1,264.81 5.29 0.0031 1.02 1.44 1.03

6 88.6 32,382 729.07 4.10 0.0124 3.17 6.82 2.34

7 88.5 31,997 900.63 3.54 0.0108 2.38 6.11 1.40

8 86.5 31,507 751.33 4.31 0.0128 3.44 7.40 1.80

9 86.4 31,045 762.78 4.47 0.0107 2.99 4.33 1.86

10 85.6 30,824 575.90 5.05 0.0110 3.47 4.83 na

11 83.6 30,144 542.92 5.71 0.0043 1.53 2.06 0.97

13 81 29,709 753.99 6.32 0.0159 6.27 8.16 na

14 80.5 29,449 1,190.19 6.46 0.0086 3.47 5.06 3.02

15 78 29,359 1,048.04 6.50 0.0100 4.06 5.73 3.46

16 76.65 29,219 912.99 6.43 0.0004 0.18 0.26 0.14

18 76.2 28,885 769.18 4.37 0.0107 2.92 5.19 na

19 76.1 28,778 644.34 4.34 0.0097 2.63 4.41 1.37

100-year flood

2 98 33,910 897.90 3.67 0.0145 3.32 5.18 na

3 96 33,259 1,362.66 5.52 0.0028 0.96 1.37 1.00

6 88.6 32,382 782.95 4.34 0.0128 3.46 7.28 2.66

7 88.5 31,997 938.87 3.68 0.0103 2.37 5.93 1.43

8 86.5 31,507 812.98 4.63 0.0129 3.73 7.74 2.09

9 86.4 31,045 811.19 4.70 0.0109 3.20 4.60 2.08

10 85.6 30,824 603.53 5.29 0.0109 3.60 4.96 na

11 83.6 30,144 569.82 5.94 0.0042 1.56 2.09 1.01

13 81 29,709 797.62 6.66 0.0159 6.61 8.52 na

14 80.5 29,449 1,257.81 6.81 0.0088 3.74 5.38 3.29

15 78 29,359 1,105.03 6.81 0.0111 4.72 6.60 4.09

16 76.65 29,219 941.16 6.58 0.0006 0.23 0.34 0.18

18 76.2 28,885 874.16 4.86 0.0147 4.45 7.67 na

19 76.1 28,778 669.79 4.48 0.0097 2.71 4.51 1.48

Table 1. Hydraulic geometry from selected cross sections in the Roaring Fork River study reach—Continued

[Upstream distance measured in channel from Garfield–Eagle County line; Matrix reference number from previously published report (Matrix Design 

Group, Inc., 2000); na, no alluvial bar, not applicable; cross sections 6, 7, 8, 9, 10, 11, 14, 15, 16, 18, surveyed; cross sections 2, 3, 13, 19 extrapolated from 

digital topographic model; lb/ft2, pound per square foot]

Crosssection 

number

Matrix

reference 

number

Upstream 

distance

(ft)

Crosssection area 

(ft2)

Mean

depth

(ft)

Watersurface 

slope

(ft/ft)

Mean

streambed 

shear stress 

(lb/ft2)

Thalweg 

shear stress 

(lb/ft2)

Bar-top 

shear stress 

(lb/ft2)
18 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado Table 2. Sediment-size characteristics of the streambed, alluvial bars, and streambanks of the Roaring Fork River at Basalt [XChan; entire streambed sampled; RChan, right portion of streambed sampled; LChan, left portion of streambed sampled; location is upstream distance from Garfield–Eagle County line; geometric stan- dard deviation = square root of the ratio of the 84th percentile size to the 16th percentile size; critical shear stress for the median sediment size calculated with equation 2 and a Shields parameter of 0.030; November 30, 2000, discharge approximately 280 ft3/s; May 16–17, 2001, discharge approximately 1,200 ft3/s; October 8, 2001, discharge approximately 395 ft3/s; ft, feet; mm, millimeter; lb/ft2, pound per square foot; mmddyy, month/day/year; ft3/s, cubic feet per second; m, meter; %, percent] Sample code Location, upstream distance (ft) Applicable to cross section, bar, or bank 16th percentile size (mm) Median sediment size (mm) 84th percentile size (mm) Standard deviation (mm) Geometric standard deviation Critical shear stress (lb/ft2) Date sampled (mmddyy) Remarks Streambed XChan.33450 33,450 XS–2 80 202 376 135 2.17 2.05 11/30/00 Sampled left edge of water to right edge of water XChan.9600 33,259 XS–3 75 152 329 115 2.09 1.55 11/30/00 Sampled left edge of water to right edge of water RChan.33100 33,100 XS–6 50 200 340 129 2.62 2.03 11/30/00 Channel unwadeable on left, sampled right XChan.8360 30,200 XS–11 91 162 287 106 1.78 1.64 11/30/00 Sampled left edge of water to right edge of water XChan.29650 29,650 XS–13, 14 57 145 280 104 2.21 1.47 11/29/00 Sampled left edge of water to right edge of water LChan.7800 29,359 XS–15 81 168 270 90 1.82 1.70 11/29/00 Channel unwadeable on right, sampled left LChan.7665 29,219 XS–16, 18, 19 52 128 230 81 2.09 1.30 11/29/00 Channel unwadeable on right, sampled left Mean of streambed 165 1.68

Alluvial bars and banks

Bar.9600 33,500 Bar.9600, XS–3 56 138 260 101 2.16 1.39 05/16/01 1–2 m from right edge of waterBar.9400 33,100 Bar.9400 24 66 150 76 2.48 0.66 05/16/01 2 m from right edge of water, 

above Upper Bypass Bridge

Bar.8860 32,400 Bar.8860, XS–6 30 89 241 95 2.84 0.90 10/08/01 0.5–1 m from right edge of waterBar.8850 32,000 Bar.8850, XS–7 8 32 69 59 2.93 0.33 05/17/01 1–5 m from right edge of waterBar.8650 31,500 Bar.8650, XS–8 0.3 48 146 76 24.13 0.49 05/17/01 3–6 m from left edge of water, 

21% of observations sand

Bar.8640 31,100 Bar.8640, XS–9,10 56 160 296 124 2.31 1.62 10/08/01 1 m from right edge of waterBar.8300 29,950 Bar.8300, XS–11 70 182 322 122 2.14 1.85 05/17/01 2–4 m from right edge of water, 

above Emma Bridge

Bank.7800 29,350 Bank.7800, 

XS–14,15

5 172 273 142 7.24 1.75 11/30/00 1 m from right edge of water, 

above Midland Avenue Bridge

Bar.7610 28,800 Bar.7610, XS–19 30 76 130 56 2.08 0.77 05/17/01 1–3 m from left edge of waterMean of alluvial bars and banks 107 1.08
BED-MATERIAL ENTRAINMENT ESTIMATION 19

by measurements of velocity vectors in downstream, 

lateral, and vertical directions. When velocity data are 

unavailable, mean boundary shear stress in a channel 

cross section commonly is approximated by the relation between boundary shear stress, flow depth, and 

energy gradient, given by the duBoys equation (Chow, 

1959, p. 168): 

(1)

where

τ o is the mean boundary shear stress, in newtons per 

square meter; 

γ is the specific weight of water (9,807 N/m3); 

D is the mean flow depth, in meters; and 

S is the energy gradient, in meter per meter; watersurface slope is often used as a substitute. 

Mean boundary shear stress in equation 1 

(newtons per square meter) is converted to English 

units (pounds per square foot) by multiplying by 

0.02088. 

Because flow depth and water-surface slope 

vary with increasing or decreasing discharge, so too 

does the mean boundary shear stress. The assumptions 

for using equation 1 are (1) the channel cross section 

has a regular, or trapezoidal, shape and width at least 

10 times greater than its depth, (2) streamflow is 

steady (there is a continuity of discharge from cross 

section to cross section in the reach), and (3) streamflow is uniform (velocity is constant in both magnitude 

and direction through the reach). Application of equation 1 is inappropriate in channel sections where there 

is a strong lateral variation in acceleration or where 

abrupt, local changes in streambed gradient occur, 

such as at bridge contractions and around bridge piers. 

The channel in the study reach was not trapezoidal, although at most cross sections a single, dominant channel conveyed most of the streamflow of 

nonflood events. All cross sections examined in this 

study had an active channel width at least 16 times the 

mean flow depth of the active channel. Alluvial point 

bars and alternate bars were common in the study 

reach (fig. 2). The presence of alternate bars in the 

channel caused some diversion of flow around the bars 

in small chute channels, but this was considered to be 

insignificant in the shear stress calculations. The barinundating streamflow was the discharge at which the 

associated water surface just inundated the highest 

surface on the alternate bars. The bar-inundating 

streamflow was entirely conveyed by the active 

channel. 

Streamflow of the 10-, 50-, and 100-year floods 

was conveyed by the active channel, adjacent floodplain surfaces (fig. 3), and, at some cross sections, by 

formerly active channel segments known as paleochannels (fig. 2C, between XS–10 and XS-11). Shear 

stress for this analysis was calculated only for the 

active channel and adjacent flood-plain surfaces. 

Although paleochannels and some flood-plain areas 

on the distal side of levees were inundated by floodflows, these areas were not included in the shear-stress 

or bed-material entrainment potential analyses. 

Streamflow upstream and downstream from the 

Fryingpan River confluence was assumed to be steady 

and, other than at the confluence of the Fryingpan 

River, there were no significant local inflows to 

either reach. Infiltration losses were assumed to be 

insignificant. 

Most natural streams do not completely satisfy 

the assumptions for equation 1, and the boundary 

shear stress associated with any specific discharge is 

nonuniformly distributed across the channel. Lateral 

and downstream variations in cross-section 

morphology and variations in slope and energy 

gradient with discharge result in a wide range of 

boundary shear stresses and, consequently, produce 

spatially variable conditions for sediment entrainment, 

sorting, and deposition. Streamflow depths at specific 

points along each cross section were substituted for 

the cross-section mean flow depth (D) in equation 1 to 

examine the relative effects of different discharges at 

deeper and shallower locations within the study reach 

(Elliott and Hammack, 1999). Using water-surface 

elevation output from the HEC–RAS model, shear 

stresses associated with the 10-year, 50-year, and 

100-year floods were calculated for the mean 

streambed (using the mean depth in the channel) and 

for the thalweg and alluvial bar tops (using point 

depths). In addition, shear stress associated with the 

bar-inundating discharge was calculated for the mean 

streambed and the thalweg. Cross-section geometry, 

the variation in cross-channel shear-stress distribution, 

and the mean shear stress for the 100-year flood and 

the bar-inundating discharge are shown for a typical 

cross section in figure 5. 

Shear stress on the streambed also is highly variable from cross section to cross section in the Roaring 

τ o = γ DS
20 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

EXPLANATION

0

1

2

3

4

5

6

7

8

980 1,000 1,020 1,040 1,060 1,080 1,100 1,120 1,140 1,160 1,180 1,200 1,220 1,240

HORIZONTAL DISTANCE, IN FEET

6,615

6,625

6,635

Cross-channel shear stress, 100-year flood

Mean shear stress, 100-year flood

Cross-channel shear stress, bar-inundating flow

Mean shear stress, bar-inundating flow

Streambed surface

Bar 8860

Terrace

Thalweg

Old Highway 82

SHEAR STRESS, IN

POUNDS PER SQUARE FOOT

ELEVATION ABOVE 

SEA LEVEL, IN FEET

Fork River. The downstream variations in mean shear 

stress at the bar-inundating discharge and the 10-year, 

50-year, and 100-year floods are shown in figure 6. 

Critical Shear Stress

The periodic breakup and entrainment of the 

relatively coarse streambed surface in a gravel- and 

cobble-bed stream is the process by which a river 

maintains its channel dimensions and transports the 

water and sediment supplied to it. Periodic bedmaterial entrainment also is an important component 

of aquatic and riparian ecology (Milhous, 1982; 

Friedman and Auble, 1999). The critical shear stress 

(τ c) is the shear stress at which general movement, or 

entrainment, of streambed sediment begins and has 

been related to sediment-size characteristics (Shields, 

1936; Lane, 1955; Fahnestock, 1963; Carling, 1983; 

Komar, 1987; Wilcock, 1992). Elliott and Parker 

(1997) and Elliott and Hammack (2000) used the 

Shields (1936) equation to estimate the critical shear 

stress for entrainment of sediment d50 on a variety of 

alluvial surfaces: 

(2)

where

τ c is the critical shear stress, in newtons per square 

meter; 

τ *c is the dimensionless critical shear stress or 

Shields parameter; 

γ s is the specific weight of sediment (assumed to be 

2.65 times the specific weight of water); 

γ is the specific weight of water (9,807 N/m3); and 

d50 is the median particle size, in meters. 

Critical shear stress in equation 2 (newtons per 

square meter) is converted to English units (pounds 

per square foot) by multiplying by 0.02088. 

Use of equation 2 requires an estimated or 

calculated dimensionless critical shear stress, or 

τ c τ *

c γ s = ( ) – γ d50

Figure 5. Cross-channel shear-stress distribution and mean shear stress for the 100-year flood and the 

bar-inundating flow, Roaring Fork River at Basalt, cross section 6.
BED-MATERIAL ENTRAINMENT ESTIMATION 21

EXPLANATION

0

1

2

3

4

5

6

7

28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 33,000 33,500 34,000

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

SHEAR STRESS, IN POUNDS PER SQUARE FOOT

100-year flood

50-year flood

10-year flood

Bar-inundating flow

Critical shear for streambed

2

3

7

8

11

13

14

15

16

18

19

6 9

10

Midland Bridge

Emma Bridge

Upper Bypass Bridge

Shields parameter, τ *c. Neill (1968) recommended a 

τ *c of 0.030 for streambeds composed of coarse materials; however, other investigators have demonstrated a 

variable τ *c in channels with mixed-size sediments 

(Komar, 1987) and in channels where the bed-surface 

size is significantly larger than the subsurface size 

(Parker and others, 1982; Andrews, 1983). Powell and 

Ashworth (1995) found τ *c varied with the strength of 

bed-surface structure. Tightly structured beds (those 

with sheltered particles, interlocked grains, or strong 

imbrication) had τ *c between 0.055 and 0.067, 

whereas loosely structured beds (those with an openparticle framework) had τ *c between 0.0096 and 

0.011. 

Elliott and Hammack (2000) determined from 

onsite observations that a τ *c value of 0.030 was 

appropriate for the coarse alluvial bar material in the 

Gunnison River in Black Canyon. The Roaring Fork 

River investigation did not include onsite observations 

of sediment entrainment or measurement of actual 

hydraulic conditions at the moment of sediment 

entrainment; therefore, the Shields parameter could 

not be precisely determined in this study as it could be 

in some flume and instrumented field studies. Critical 

shear stresses for each sediment measurement in the 

Roaring Fork study area were calculated with Shields 

τ *c values of 0.030, a conservative value for sediment 

entrainment, but one that may be appropriate for a 

coarse-bed channel like the Roaring Fork River and 

for situations where hazard recognition is an objective 

for resource managers. Entrainment estimates using 

a conservative τ *c value of 0.030 imply that the 

streambed becomes mobile at a shallower flow depth 

and lesser discharge. If a larger value of τ *c was determined to be more appropriate for the Roaring Fork 

River, then deeper flows and greater discharge would 

be required to initiate movement of the streambed. 

The experiments of Lane (1955) and Wilcock 

(1992, p. 297) indicate that equation 2 and d50 can be 

used to provide a minimum estimate of the shear stress 

necessary to initiate general movement of a mixed-size 

sediment. Streamflows or floods that generate a 

boundary shear in excess of the critical shear should 

therefore cause the initiation of bed-material moveFigure 6. Downstream mean shear-stress distribution on the streambed at various discharges and critical 

shear stress for sediment on the streambed at cross sections 2 through 19.
22 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

ment. Over a period of time, and if great enough, the 

excess shear stress will result in bedload transport and, 

possibly, channel adjustments. 

When the boundary shear stresses associated 

with a range of discharges, τ o, are compared to the 

critical shear stress, τ c, for the d50 of the sediment, it is 

possible to evaluate the sediment-entrainment potential of each discharge with respect to the streambed in 

general or of a particular geomorphic surface or location on a cross section (Elliott and Hammack, 1999 

and 2000). The critical shear stress associated with 

sediment entrainment (eq. 2) is, at best, a minimum 

estimate of the critical discharge because only a small 

area of the entire surface or only a few particles of the 

d50 size may be entrained by the critical discharge 

(Lisle and others, 1993; Milhous, 1982). Wilcock and 

McArdell (1993) observed that complete mobilization 

of a size fraction, such as d50 , occurred at roughly 

twice the shear stress necessary to initiate movement 

of individual particles in that size fraction. 

Entrainment Potential

Once the critical shear stress (τ c) has been identified for the sediment on a specific alluvial surface, 

the critical discharge (the minimum streamflow 

required to entrain sediment d50) can be estimated by 

equating τ c with the boundary shear stress (τ o) and 

using the relation between shear stress and discharge 

at a specific cross-section location. Sediment-entrainment potential for a specific geomorphic surface can 

be expressed as the ratio of flood-generated boundary 

shear stress to the critical shear stress (τ o/τ c). The 

τ o/τ c ratio integrates several geomorphic and sediment variables (flow depth, energy gradient or watersurface slope, median sediment-particle size, critical 

shear stress) and is applicable over a wide range of 

variable values. When calculated for a range of 

discharges at several cross sections in a river reach, the 

τ o/τ c ratio facilitates site-to-site comparison of 

entrainment potential and provides a method to evaluate the relative effects of different hypothetical flood 

discharges (Elliott and Hammack, 2000). 

Entrainment potential was calculated for the 

four reference streamflows at 14 of the 18 cross 

sections in the 1-mile study reach of the Roaring Fork 

River (fig. 2). The entrainment potential at each cross 

section for a specific reference streamflow can be 

viewed in a downstream perspective. Figure 7A–D 

shows the entrainment potential for the channel 

thalweg (calculated from the greatest depth at each 

cross section), the mean streambed (calculated with 

the mean depth at each cross section), and the tops of 

the alluvial bars, if present and if inundated (calculated 

with the flow depth over the highest point on the bar). 

Two entrainment threshold lines are plotted in 

figure 7A–D. The partial entrainment threshold 

(τ o/τ c=1) (solid horizontal line in fig. 7) is the condition where the mean boundary, or at-a-point, shear 

stress (τ o) just equals the critical shear stress for the 

median particle size (τ c) at that cross section. At this 

threshold streamflow, the d50 particle size becomes 

entrained, but movement of d50-size particles may be 

limited to a few particles and in a limited area of the 

streambed. The complete entrainment threshold 

(τ o/τ c=2) (dashed horizontal line in fig. 7) is the 

condition where τ o is twice the critical shear stress for 

the median particle size, the condition where Wilcock 

and McArdell (1993) observed complete or widespread mobilization of a particle-size fraction such as 

the d50. In most locations, τ o increases with discharge 

(eq. 1), whereas τ c is relatively constant for a given 

sediment d50 (fig. 6). The ratio of τ o/τ c generally 

increases with discharge, although the rate of change 

and the value of τ o/τ c vary with channel configuration, local energy gradient, and local sediment size. 

Entrainment is anticipated and the critical discharge is 

attained when τ o/τ c exceeds a threshold value of 1.0 

(or 2.0 for complete mobilization); but as figure 7A–D 

illustrates, the critical discharge is not identical for 

similar geomorphic surfaces at different cross sections. 

 Analysis

This study investigates the entrainment, or 

movement, of sediment on the streambed and assumes 

that it is an indicator of the tendency for a river reach 

to transport or deposit sediment under a specific 

streamflow scenario. Entrainment estimates such as 

these are not surrogates for bedload-transport 

measurements or estimates; however, entrainment estimates can be used to assess the anticipated behavior of 

a river during floods of different magnitudes. 

A stream channel in a "steady state" of equilibrium has a relatively constant cross-section geometry, 

downstream gradient, and planform pattern over time 

(Schumm, 1977, p. 96–100). Such a channel is considered stable because, even though the channel will 

gradually shift its location through the flood plain, it 
BED-MATERIAL ENTRAINMENT ESTIMATION 23

EXPLANATION

0

1

2

3

4

5

6

28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 33,000 33,500 34,000

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

Thalweg with bar-inundating flow 

Mean streambed with bar-inundating flow 

Bar tops are at the water surface

Complete entrainment threshold ( =2 )

Partial entrainment threshold 

Cross section

2

3

7

8

8

11

13

14

16 15

18

19

RATIO OF FLOOD SHEAR TO CRITICAL SHEAR

FOR d ( / ) 50 o c

( / ) o c

( / ) o c ( = ) o

o

c

c

A

Midland Bridge

Emma Bridge

Upper Bypass Bridge

Some sediment moving

No sediment moving

Much

sediment moving

6

9

10

EXPLANATION

0

1

2

3

4

5

6

28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 33,000 33,500 34,000

Thalweg with 10-year flood 

Mean streambed with 10-year flood 

Bar tops with 10-year flood 

2

3

7

8

11

13

14

15

16

18

19

Much 

6

9

10

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

8

RATIO OF FLOOD SHEAR TO CRITICAL SHEAR

FOR d ( / ) 50 o c

B

Complete entrainment threshold ( =2 )

Partial entrainment threshold 

Cross section

( = ) o

o

c

c ( / ) o c

( / ) o c

( / ) o c

Midland Bridge

Emma Bridge

Upper Bypass Bridge

sediment moving

Some sediment moving

No sediment moving

Figure 7. Entrainment potential (τ o/τ c) of the streambed, thalweg, and alluvial bars at the 

(A) bar-inundating discharge, (B) 10-year flood, (C) 50-year flood, and (D) 100-year flood, 

Roaring Fork River at Basalt.
24 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

EXPLANATION

0

1

2

3

4

5

6

28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 33,000 33,500 34,000

Thalweg with 50-year flood 

Mean streambed with 50-year flood 

Bar tops with 50-year flood 

2

3

7

8

11

13

14

15

16

18

19

Upper Bypass Bridge

Emma Bridge

Midland Bridge

Much sediment moving

Some sediment moving

No sediment moving

6

9

10

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

8

RATIO OF FLOOD SHEAR TO CRITICAL SHEAR

FOR d ( / ) 50 o c

C

Complete entrainment threshold ( =2 )

Partial entrainment threshold 

Cross section

( = ) o

o

c

c ( / ) o c

( / ) o c

( / ) o c

EXPLANATION

0

1

2

3

4

5

6

28,500 29,000 29,500 30,000 30,500 31,000 31,500 32,000 32,500 33,000 33,500 34,000

Thalweg with 100-year flood 

Mean streambed with 100-year flood 

Bar tops with 100-year flood 

2

3

7

8

11

13

14

15

16

18

19

Upper Bypass Bridge

Emma Bridge

Midland Bridge

Much sediment moving

Some sediment moving

No sediment moving

6

9

10

UPSTREAM CHANNEL DISTANCE FROM GARFIELD/EAGLE COUNTY LINE, IN FEET

8

RATIO OF FLOOD SHEAR TO CRITICAL SHEAR

FOR d ( / ) 50 o c

D

Complete entrainment threshold ( =2 )

Partial entrainment threshold 

Cross section

( = ) o

o

c

c ( / ) o c

( / ) o c

( / ) o c

Figure 7. Entrainment potential (τ o/τ c) of the streambed, thalweg, and alluvial bars at the 

(A) bar-inundating discharge, (B) 10-year flood, (C) 50-year flood, and (D) 100-year flood, 

Roaring Fork River at Basalt—Continued.
BED-MATERIAL ENTRAINMENT ESTIMATION 25

will maintain its average cross-section area, width, 

depth, and pattern (sinuosity and bend radius of curvature). An equilibrium, or stable, channel has a form 

(fluvial geomorphology) that enables it to transport the 

water and sediment supplied to it without progressive, 

unidirectional aggradation or degradation (Leopold 

and others, 1964, p. 266–268). Channel segments in 

disequilibrium tend to have a fluvial geomorphology 

that reflects an imbalance between streamflow and 

sediment transport. Excess shear stress in alluvial 

channels can be associated with vertical or horizontal 

erosion (streambed scour or bank erosion). Deficient 

shear stress can be associated with channel aggradation. 

Channel modifications by humans can cause a 

relatively stable channel to become unstable. For 

example, channel straightening by cutting off or shortening meander bends will steepen the flow path and 

increase shear stress on the streambed, often resulting 

in a degraded or incised channel. Constriction of the 

channel width, such as at a bridge crossing, can 

impound floodflows and create backwater areas. Backwater areas upstream from channel constrictions are 

associated with decreased flow velocity, water-surface 

slope, and shear stress, conditions that contribute to 

channel aggradation. Streamflow velocity and bed 

shear stress can increase at, and downstream from, a 

constriction and cause streambed scour in the channel. 

Large-scale eddies also can form downstream from a 

constriction, possibly resulting in lateral erosion of 

nonresistant streambanks and redeposition of sediment 

in the low-velocity areas of the eddy. 

The mean shear stress on the Roaring Fork 

streambed at any reference streamflow is highly variable from cross section to cross section (fig. 6); 

however, the median sediment size of the streambed 

ranged from 128 to 202 mm (table 2), and the critical 

shear stress necessary to entrain the bed material fell 

within the relatively narrow range of 1.30 to 2.05 lb/ft2

(fig. 6). Consequently, the plots in figure 7A–D show 

great downstream variability in entrainment potential 

at any reference streamflow. At some cross sections, 

the bed or bar material is mobile (τ o/τ c plots above the 

threshold line), whereas at other cross sections, the 

bed or bar material is immobile (τ o/τ c plots below the 

threshold line) for the same discharge. The significance of the downstream variability is that sediment 

entrained at one cross section may be unable to move 

at a cross section farther downstream, a situation 

resulting in deposition and possibly aggradation over a 

period of time. 

The entrainment plot for the mean streambed 

during a bar-inundating streamflow (fig. 7A, solid 

line) indicates that little or no sediment in the d50-size 

range is likely to be entrained or transported through 

much of the study reach. However, the entrainment 

potential increases abruptly to more than twice the 

critical value (τ o/τ c greater than 2.0) at cross sections 

13 and 18, then decreases just downstream. Cross 

sections 13 and 18 are located on the downstream 

faces of the Emma and Midland Avenue Bridges 

(fig. 2D), where the HEC–RAS-simulated watersurface slope steepens significantly. The abrupt 

increase in entrainment potential indicates that 

d50-size material is moving at cross sections 13 and 18 

(d50 = 145 and 128 mm, respectively, table 2). Of 

significance in this channel subreach is the indication 

that d50-size material is not being entrained immediately downstream from both bridges at cross sections 

14, 15, 16, and 19 (d50-range 128 to 168 mm) by the 

same discharge. This spatial variation could indicate 

that sediment removed from the streambed at cross 

sections 13 and 18 is being deposited at the downstream cross sections over time, possibly aggrading 

the channel and reducing flood conveyance by 

decreasing the channel area. Sediment is more mobile 

in the thalweg (fig. 7A, short-dashed red line) than on 

the mean streambed (solid red line) because flow 

depths are greatest over the thalweg. No sediment is 

moving on the highest alluvial bar surfaces (square 

symbols) because, by definition, they are not inundated by this discharge. 

The entrainment plot for the mean streambed 

during a 10-year flood (fig. 7B, solid line) indicates 

that some sediment is moving at most cross sections in 

the study reach. Exceptions are at cross sections 3 and 

16, where mean streambed shear stress is less than the 

critical shear stress for entrainment of the d50-size 

material. Because these cross sections are downstream 

from cross sections where some entrainment is occurring, there is a potential for aggradation over time. One 

noteworthy change in entrainment potential between 

the bar-inundating discharge and the 10-year flood is 

that the entrainment potential decreased at cross 

section 18. This decrease in the entrainment potential 

is a reflection of the decrease in water-surface slope 

downstream from cross section 18 as discharge 

increased (fig. 4, table 1). As with the bar-inundating 

discharge scenario, sediment is more mobile in the 
26 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

thalweg (short-dashed red line) than on the mean 

streambed (solid red line), and entrainment-potential 

trends tend to mirror those of the mean streambed. All 

of the large, unvegetated alluvial bars visible in 

figure 2 are inundated by the 10-year flood (fig. 3), 

and sediment composing some of those bars is 

becoming entrained at this discharge (fig. 7B, square 

symbols). 

The entrainment plot for the mean streambed 

during a 50-year flood (fig. 7C, solid black line) indicates that some sediment is moving at most cross 

sections in the study reach. Exceptions are at cross 

sections 3, 16, and possibly 11, where mean streambed 

shear stress is less than the critical shear stress for 

entrainment of the d50-size material. The entrainment 

potential at cross section 11 is slightly less for the 

50-year flood than for the 10-year flood because the 

net effect of a decrease in water-surface slope 

outweighs the effect of an increase in mean flow depth 

(table 1). The entrainment potential downstream from 

the Emma Bridge (cross sections 13, 14, and 15) is 

great because of an increase in water-surface slope. 

The low value for entrainment potential at cross 

section 16 indicates potential aggradation during the 

50-year flood. Sediment on most alluvial bars is potentially entrainable by the 50-year flood; however, sediment on the alluvial bars at cross sections 3, 11, and 16 

does not appear to be mobile (fig. 7C, square 

symbols). Depending on the size of sediment transported from upstream and onto these surfaces, the alluvial bars at cross sections 3, 11, and 16 could be 

depositional sites during a 50-year flood. 

The entrainment plots for the mean streambed 

(solid blue line) and for alluvial bars (square symbols) 

during a 100-year flood (fig. 7D) are similar to those 

of the 50-year flood (fig. 7C). At most cross sections, 

there is little increase in flow depth between the 50- 

and 100-year floods (fig. 3), and the water-surface 

slopes show little change (fig. 4, table 1). One change 

from the 50-year flood to the 100-year flood is an 

abrupt increase in entrainment potential at cross 

section 18, downstream from the Midland Avenue 

Bridge, where the HEC–RAS-simulated slope has 

increased. 

An examination of the entrainment-potential 

plots for all four discharge scenarios (fig. 7A–D) 

reveals some characteristics or trends common to all 

scenarios. The ratio of τ o/τ c at cross section 3 never 

exceeded the partial entrainment threshold of 1.0, indicating no movement of d50-size sediment (152 mm) at 

this location. Because the streambed d50 upstream at 

cross section 2 (202 mm) is partially mobile (1.0 < 

τ o/τ c< 2.0) for all discharge scenarios, it might be 

expected from this analysis that material entrained 

from the vicinity of cross section 2 would be deposited in the vicinity of cross section 3. Anecdotal 

evidence (Dave Konchan, Town of Basalt, oral 

commun., 2000), onsite observation, and the aerial 

photograph taken in November 2000 (fig. 2A) indicate 

that the channel segment between cross section 3 and 

the Upper Bypass Bridge is a location of periodic 

streambed aggradation. The alluvial bar (Bar 9600) in 

figure 2A has become larger since an aerial photograph was taken in 1987 (McLaughlin Water Engineers, Inc., 2000). From a channel maintenance and 

management perspective, the river segment upstream 

from the Upper Bypass Bridge has been, and could 

continue to be, an area of concern due to aggradation 

and loss of channel conveyance. Aggradation here 

likely is due to the backwater effects of the Upper 

Bypass Bridge, which causes a decrease in watersurface slope upstream, and partly is due to the 

channel curvature at Bar 9600 (fig. 2A), which dissipates streamflow energy and contributes to a loss of 

competence (that is, a decrease in the maximum 

particle size the channel can transport). 

The ratio of τ o/τ c for the mean streambed at 

cross sections 6, 7, 8, and 9 is near to, or exceeds, the 

partial mobility threshold (τ o/τ c = 1.0) for all simulated streamflows (fig. 7A–D). The entrainment potential for the deeper thalweg part of the channel is well 

above the complete entrainment threshold (τ o/τ c = 

2.0) at these cross sections also. The reach including 

cross sections 6, 7, 8, and 9 is slightly sinuous (fig. 

2B), but this reach was artificially straightened during 

railroad construction in the late 19th century (Dave 

Konchan, Town of Basalt, oral commun., 2000). The 

consistently moderate to high entrainment potential in 

this reach may be a relict condition from the earlier 

effort to straighten the channel, an action that consequently steepened the channel as well. The probability 

of bed-material entrainment at all simulated discharges 

in this formerly straightened reach could indicate that 

the reach is an efficient transporter of supplied sediment, assuming that the supplied sediment has a 

particle size comparable to the sediment measured at 

these cross sections and alluvial bars (table 2). 

A channel avulsion, or cutoff, of a meander 

between cross sections 10 and 11 occurred sometime 

between 1987 and 1997 (fig. 2C). The effect of this 
BED-MATERIAL ENTRAINMENT ESTIMATION 27

cutoff was to shorten the flow path and steepen the 

slope between cross sections 10 and 11. All simulated 

reference streamflows between these cross sections are 

laterally confined by a terrace on the north; however, 

the abandoned channel segment on the south has a 

sufficiently low relief to convey a portion of the 

discharge of the 10-, 50-, and 100-year floods. The 

significance of this condition is that bed material of the 

main channel is potentially entrainable from cross 

section 10 at the flows equal to or greater than the 10-

year flood (figs. 7B, 7C, and 7D), and the main 

channel here may function as an efficient transporter 

of supplied bedload similar to the reach immediately 

upstream (cross sections 6 through 9). The channel 

segment in the meander bend abandoned after 1987 is 

filled with gravel- and cobble-size sediment, indicating that flood discharges may transport coarse sediment into but not through this side channel. The 

entrainment potential at cross section 11 is much less 

than at cross section 10 for the 10-, 50-, and 100-year 

floods (figs. 7B, 7C, and 7D) because the streambed 

sediment is slightly coarser at cross section 11 than 

cross section 10 (table 2) and because the watersurface slope also is less (table 1, fig. 4D) due to the 

backwater effects of the Emma Bridge. Deposition of 

sediment in the vicinity of Bar 8300 may be a result of 

the decrease in shear stress downstream from cross 

section 11 (fig. 2C).

The potential effect of channel constrictions 

caused by the three bridges in Basalt is apparent in the 

longitudinal water-surface profiles (figs. 4A and 4D), 

the plot of downstream mean shear stress (fig. 6), and 

the entrainment potential plots (figs. 7A, 7B, 7C, and 

7D). The narrow bridge openings and the abutments 

create a backwater area for flows equal to and larger 

than the 10-year flood. The alignment of piers acute to 

the flow direction and trapped woody debris beneath 

the Upper Bypass Bridge also may contribute to the 

backwater effect here, but this is not accounted for in 

the HEC-RAS one-dimensional flow model. Based on 

the estimated bar-inundating water-surface profile and 

the 1,260 ft3

/s surveyed profile, the backwater effects 

of lower discharges may be less significant (figs. 4A 

and 4D). The entrainment-potential plots suggest that 

the streambed is immobile for all reference streamflows at cross sections 3 and 16, upstream from the 

Upper Bypass and Midland Avenue Bridges, and 

immobile to only partly mobile at cross section 11, 

upstream from the Emma Bridge (fig. 7A–D). Bar 

9600, downstream from cross section 3 (fig. 2A), and 

Bar 8300, downstream from cross section 11, are 

depositional features that may have been enhanced by 

the backwater effects of the Upper Bypass and Emma 

Bridges. 

Aerial photographs, surveyed channel cross 

sections, and anecdotal evidence indicate the river 

reach between the confluence of the Fryingpan River 

and the Midland Avenue Bridge recently has been an 

area of streambed deposition and reworking. This 

reach is hydraulically complex, geomorphically 

dynamic, and not well represented by the HEC–RAS 

one-dimensional streamflow model; however, aerial 

photographs from 1987, 1997, and 2000 indicate 

active sediment reworking in the reach. Bar 8050, now 

connected to the left bank at cross sections 14 and 15 

(fig. 2D), was a midchannel bar in 1997. This bar was 

attached to the right bank upstream from the 

Fryingpan confluence and extended downstream from 

the present location of cross section 16 (fig. 2D) in the 

1987 aerial photograph (McLaughlin Water Engineers, 

Inc., 2000). 

The analysis of bed-material entrainment 

potential in this study is dependent on the resolution 

of previous surveys and the representativeness of 

streamflow modeling. The HEC–RAS one-dimensional model output from the 1997 FEMA study 

(McLaughlin Water Engineers, Inc., 2000) is adequate 

for flood-plain mapping and an understanding of 

general flood-related hazards over a broad area 

including the Town of Basalt. However, the location 

and spacing of cross sections, the resolution of channel 

geometry from surveyed points, and the model-generated water-surface slopes are relatively insensitive to 

the scale and scope of this investigation, the main 

flood-conveying channel where much entrainable 

sediment is stored. More topographic detail of the 

streambed and channel and more closely spaced cross 

sections in some subreaches would allow more precise 

estimates of variation in shear stress and sediment 

entrainment potential. The 1,260-ft3

/s water-surface 

profile from 2001 (fig. 4) reveals subtle variations in 

flow depth and water-surface slope, whereas the HEC–

RAS water-surface profiles derived from single points 

at widely spaced cross sections do not. 

The water-surface profile for the bar-inundating 

discharge was extrapolated from cross-section survey 

data that sometimes were not annotated. In these situations, the bar tops were identified by a visual interpretation of the cross-section plot. When compared to the 

surveyed water surface at 1,260 ft3/s, a discharge that 
28 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

did not inundate the major alluvial bars (fig. 2), the 

extrapolated bar tops sometimes were lower than the 

surveyed water surface (fig. 4). This could be because 

some of the surveyed surfaces were incorrectly identified as bar tops. Other possible explanations for this 

apparent contradiction could be that local channelgeometry and bar-topography adjustments have 

occurred between the time of the cross-section surveys 

(October 1998 and April 1999) and the time of the 

water-surface survey (June 2001) or that alluvial bars 

in the study reach are formed by multiple discharges 

rather than by a single discharge common to all. 

Regional flood-frequency equations derived 

from an analysis of the magnitude and frequency of 

Colorado floods (Vaill, 2000) indicate that the FEMA 

discharge estimates (Matrix Design Group, Inc., 2000) 

used in this study for the 10-, 50-, and 100-year floods 

may be too large. Published magnitude-frequency estimates for Colorado streams have been overestimated 

by as much as 35 percent when compared to Log 

Pearson III analysis of long-term gaging-station 

records (Jarrett, 1993). The significance of overestimating the discharge and the water-surface elevation, 

for example, for a flood of a 10-year recurrence, is that 

the entrainment estimates for the 10-year flood 

presented in this report may occur less frequently than 

an average of once in 10 years. The computation of 

revised flood magnitudes is beyond the scope of this 

report but could improve the accuracy of inundation 

and sediment-transport estimates in future studies. 

Another shortcoming of this study is that 

streamflows between the bar-inundating discharge 

(approximately 3,000 ft3/s downstream from the 

Fryingpan River) and the 10-year flood (7,100 ft3

/s 

downstream from the Fryingpan River) were not 

surveyed or simulated. This broad range of discharges 

is relatively common in the Roaring Fork River and 

includes the bankfull discharge, a frequently occurring 

streamflow shown by Andrews (1984) to be a significant transporter of bedload in Colorado mountain 

gravel-bed rivers. Frequent and widespread entrainment and transportation of bedload can result in 

adjustments in channel cross-section properties. 

Therefore, a comprehensive assessment of bed material entrainment potential should include additional 

computations within this important discharge range. 

This study identifies cross sections or channel 

subreaches where bed-material entrainment potential 

is highly variable relative to the entrainment potential 

immediately upstream or downstream—for example, 

in the subreaches just upstream from each of the three 

bridges. A persistent tendency for entrainment at all 

reference streamflows might indicate a similar 

tendency to transport sediment supplied from 

upstream or to scour the local bed material. 

Conversely, a persistent tendency for there to be no 

entrainment at any reference streamflow might indicate a higher probability for the bed to become 

armored, or immobile, or for the bed to aggrade and 

for flood conveyance to be reduced. Understanding the 

likely response of the streambed to a range of 

discharges will allow resource managers to identify 

channel subreaches where erosion-mitigation efforts 

or channel-clearing activities may be needed during or 

immediately after the recession of a flood. Identification of the geomorphic factors that lead to a high or 

low probability of sediment entrainment (slope, flow 

depth, sediment-size characteristics, and so forth) also 

is critical to proper design of channel modifications, 

such as cross-section changes, sinuosity and gradient 

changes, and the like. 

Future studies to improve understanding of sediment-related issues in the Roaring Fork could include 

refinement of shear stress calculations through application of a multidimensional streamflow model 

(McDonald and others, 2001). Such a model would 

produce more accurate shear stress estimates and 

would compute shear stress over a greater area of the 

streambed, as compared to the calculations of this 

study, which are limited to only 14 widely spaced 

cross sections. Sediment-entrainment-potential calculations give only an indication of what the geomorphic 

response to floods could be. More accurate forecasts 

of channel adjustments resulting from sediment redistribution could be made through use of sedimenttransport functions linked to a multidimensional flow 

model. 

SUMMARY

The Roaring Fork River is a single-thread, highgradient, cobble/boulder-bed river that presents a 

natural hazard from high-velocity snowmelt runoff, 

flood inundation, and a high bedload-transport rate 

that cause intermittent channel realignment, streambed 

scour, and deposition of coarse-grain bars and islands. 

Recent urban, highway, and recreational development 

on the flood plain, earlier attempts to realign and 

confine the Roaring Fork channel with retaining walls 
SUMMARY 29

and levees, and flow obstructions such as bridge openings and piers have altered the hydraulics, sediment 

entrainment, sediment transport, and sediment deposition in the river at Basalt. Gravel, cobble, and boulder 

bed material is transported almost annually in the 

Roaring Fork River, and deposition of this coarse sediment on the streambed and in large bars has reduced 

the flood-conveying capacity of the river. In addition, 

sedimentation may have diminished fish habitat in the 

reach and affected the function of an irrigation intake 

structure. Previous studies have identified flood-prone 

areas and hazards related to inundation and high 

streamflow velocity, but those studies have not evaluated response of the channel to discharges that entrain 

the coarse streambed. This study builds upon the 

results of earlier flood studies and addresses potential 

problems associated with bed-material entrainment. 

Sediment entrainment estimates were made at 

several locations along the Roaring Fork River for four 

reference streamflows: the bar-inundating streamflow, 

and the 10-, 50-, and 100-year floods. The bar-inundating streamflow is approximately 2,500 ft3

/s 

upstream from the Fryingpan River confluence and 

approximately 3,000 ft3

/s downstream from the 

confluence. The 10-year flood is approximately 

6,100 ft3/s upstream from the confluence and approximately 7,100 ft3

/s downstream from the confluence. 

The 50-year flood is approximately 8,500 ft3/s 

upstream from the confluence and approximately 

9,400 ft3

/s downstream from the confluence. The 

100-year flood is approximately 9,400 ft3/s upstream 

from the confluence and approximately 10,400 ft3

/s 

downstream from the confluence. Regional floodfrequency equations derived from an analysis of the 

magnitude and frequency of Colorado floods indicate 

that the discharge estimates for the 10-year, 50-year, 

and 100-year floods may be too large; however, 

computation of revised flood magnitudes is beyond the 

scope of this report. Consequently, the flood magnitudes provided by Matrix Design Group, Inc. (2001) 

were used. 

Sediment entrainment in stream channels is 

partly a function of the boundary shear stress (τ o) 

acting on sediment particles resting on or in the 

streambed or other inundated alluvial surfaces, and 

partly a function of the critical shear stress (τ c) of the 

sediment. Shear stress in the Roaring Fork River was 

calculated at 14 previously surveyed cross sections 

using the duBoys equation for the four reference 

streamflows. Shear stress was calculated for the active 

channel using the mean streamflow depth, and for the 

thalweg and adjacent alluvial-bar surfaces using at-apoint depths determined from cross-section surveys 

and simulated water-surface elevations used in a 

previous HEC–RAS model. 

The critical shear stress (τ c) is the shear stress 

at which general movement, or entrainment, of 

streambed sediment begins and has been related to 

sediment-size characteristics. Sediment-size characteristics, including the median particle size (d50), were 

determined for surficial material on the streambed of 

the low-flow channel, on large alluvial bars adjacent to 

the low-flow channel, and on one streambank using 

the Wolman method. Subsurface-sediment size was 

not determined. The mean of the d50 for seven 

streambed samples was 165 mm, the mean of the d50

for nine alluvial bar and bank samples was 107 mm. 

The bed and most bar deposits were relatively well 

sorted, a result of fluvial reworking. 

The Shields equation was used to estimate the 

critical shear stress for entrainment of sediment d50 on 

the mean streambed, as well as on the thalweg or 

deepest point in the channel, and on adjacent alluvial 

bar surfaces, if present. The critical shear stresses for 

each sediment measurement in the Roaring Fork study 

area were calculated with Shields τ *c values of 0.030, 

a conservative value for sediment entrainment but one 

that may be appropriate for a coarse-bed channel like 

the Roaring Fork River and for situations where 

hazard recognition is an objective for resource 

managers. Entrainment estimates using a conservative 

τ *c value of 0.030 imply that the streambed becomes 

mobile at a shallower flow depth and lesser discharge 

than at a larger τ *c value. If a larger value of τ *c was 

determined to be more appropriate for the Roaring 

Fork River, then deeper flows and greater discharge 

would be required to initiate movement of the 

streambed. 

Sediment-entrainment potential for a specific 

geomorphic surface can be expressed as the ratio of 

boundary shear stress to the critical shear stress 

(τ o/τ c) of sediment on that surface. The partial 

entrainment threshold (τ o/τ c=1) is the condition 

where the boundary shear stress (τ o) just equals the 

critical shear stress for the median particle size (τ c). 

At this threshold streamflow, the d50 particle size 

becomes entrained, but movement of d50-size particles 

is limited to a few individual particles and to a limited 

area of the cross section. The complete entrainment 

threshold (τ o/τ c=2) is the condition where τ o is twice 
30 Bed-Material Entrainment Potential, Roaring Fork River at Basalt, Colorado

the critical shear stress for the median particle size, the 

condition where complete or widespread mobilization 

of the d50 particle-size fraction occurs. 

The τ o/τ c ratio integrates several geomorphic 

and sediment variables (flow depth, energy gradient or 

water-surface slope, median sediment-particle size, 

critical shear stress) and is applicable over a wide 

range of variable values. When calculated for a range 

of discharges at several cross sections in a river reach, 

the τ o/τ c ratio facilitates site-to-site comparison of 

entrainment potential and provides a method to evaluate the relative effects of different hypothetical flood 

discharges. Downstream variability in τ o/τ c might 

indicate that sediment entrained at one cross section 

may be unable to move at a cross section farther downstream, a situation resulting in deposition and possibly 

aggradation and loss of channel conveyance over a 

period of time. 

The entrainment plot for the mean streambed 

during a bar-inundating streamflow indicates that little 

or no sediment in the d50-size range is likely to be 

entrained or transported through much of the study 

reach. However, the entrainment potential increases 

abruptly to more than twice the critical value (τ o/τ c

greater than 2) at cross sections 13 and 18 (the downstream faces of the Emma and Midland Avenue 

Bridges), then decreases just downstream. The abrupt 

increase and decrease in entrainment potential indicates that d50-size material is moving at cross sections 

13 and 18 but that the d50-size material is not being 

entrained immediately downstream from both bridges 

at cross sections 14, 15, 16, and 19. This spatial variation could indicate that sediment removed from the 

streambed at cross sections 13 and 18 is being deposited at the downstream cross sections over time, 

possibly aggrading the channel and reducing floodconveyance by decreasing the channel area. 

The entrainment plot for the mean streambed 

during a 10-year flood indicates that some sediment is 

moving at most cross sections in the study reach. 

Exceptions are at cross sections 3 and 16, where mean 

streambed shear stress is less than the critical shear 

stress for entrainment of the d50-size material. Because 

these cross sections are downstream from cross 

sections where some entrainment is occurring, there is 

a potential for aggradation over time. 

The entrainment plots for the mean streambed 

during a 50- or 100-year flood indicate that some sediment is moving at most cross sections in the study 

reach. Exceptions are at cross sections 3, 16, and 

possibly 11, where mean streambed shear stresses are 

less than the critical shear stress for entrainment of the 

d50-size material. The entrainment potential downstream from the Emma Bridge (at cross sections 13, 

14, and 15) is great because of an increase in depth and 

in water-surface slope. The low value for entrainment 

potential at cross section 16 indicates potential aggradation during the 50- and 100-year floods. Sediment 

on most alluvial bars is potentially entrainable by the 

50- and 100-year floods; however, sediment on the 

alluvial bars at cross sections 3, 11, and 16 does not 

appear to be mobile. Depending on the size of sediment transported from upstream and onto these 

surfaces, the alluvial bars at cross sections 3, 11, and 

16 could be depositional sites even during the largest 

of the simulated floods. 

The ratio of τ o/τ c at cross section 3 never 

exceeds the partial entrainment threshold of 1.0, 

indicating no probable movement of d50-size sediment 

at this location. Because the streambed d50 upstream at 

cross section 2 is coarser and is partially mobile 

(1.0 < τ o/τ c< 2.0) for all discharge scenarios, it might 

be expected from this analysis that material entrained 

from the vicinity of cross section 2 would be deposited 

in the vicinity of cross section 3. Anecdotal evidence, 

onsite observation, and the aerial photograph taken in 

November 2000 indicate that the channel segment 

between cross section 3 and the Upper Bypass Bridge 

is a location of periodic streambed aggradation and 

could continue to be an area of concern due to aggradation and loss of channel conveyance. Aggradation 

here likely is due to the backwater effects of the Upper 

Bypass Bridge and partly due to the channel curvature 

at Bar 9600. 

The consistently moderate to high entrainment 

potential in the reach including cross sections 6, 7, 8, 

and 9 may be a relict condition from a 19th century 

effort to straighten the channel, an action that consequently steepened the channel as well. The probability 

of bed-material entrainment at all simulated discharges 

in this reach could indicate that the reach is an efficient 

transporter of supplied sediment. 

A channel avulsion of the meander between 

cross sections 10 and 11 occurred sometime between 

1987 and 1997. The effect of this cutoff was to shorten 

the flow path and steepen the slope between cross 

sections 10 and 11. Bed material in the main channel is 

potentially entrainable from cross section 10 at the 

flows equal to or greater than the 10-year flood, and 

the main channel here may function as an efficient 
REFERENCES CITED 31

transporter of supplied bedload similar to the reach 

immediately upstream (cross sections 6 through 9). 

The channel segment in the meander bend abandoned 

after 1987 is filled with gravel- and cobble-size sediment, indicating that flood discharges may transport 

coarse sediment into but not through this side channel. 

The entrainment potential at cross section 11 is 

much less than at cross section 10 for the 10-, 50-, and 

100-year floods because the streambed sediment is 

slightly coarser at cross section 11 than at cross 

section 10 and because the water-surface slope is less 

due to the backwater effects of the Emma Bridge. 

Deposition of sediment in the vicinity of Bar 8300 

may be a result of the decrease in water-surface slope 

and shear stress downstream from cross section 11. 

Channel constrictions at the three bridges in 

Basalt create backwater areas of impounded water 

during the simulated 10-, 50-, and 100-year floods. 

The alignment of piers acute to the flow direction and 

trapped woody debris beneath the Upper Bypass 

Bridge also may contribute to the backwater effect 

here, but this is not accounted for in the HEC–RAS 

one-dimensional flow model. The entrainmentpotential plots indicate that the streambed is immobile 

for all simulated streamflows at cross sections 3 and 

16, upstream from the Upper Bypass and Midland 

Avenue Bridges, and immobile to only partly mobile 

at cross section 11, upstream from the Emma Bridge. 

Bar 9600, downstream from cross section 3, and Bar 

8300, downstream from cross section 11, are depositional features that may have been enhanced by the 

backwater effects of the Upper Bypass and Emma 

Bridges. 

 The entrainment estimates of this study are 

limited by the precision and availability of data that 

were used to calculate shear stress under the discharge 

scenarios. The location and spacing of cross sections, 

the resolution of channel geometry from surveyed 

points, and the model-generated water-surface slopes 

are relatively insensitive to the scale and scope of this 

investigation, the main flood-conveying channel where 

entrainable sediment is stored. More topographic 

detail of the streambed and channel and more closely 

spaced cross sections in some subreaches would allow 

more precise estimates of shear stress and sedimententrainment potential. The river reach between the 

confluence of the Fryingpan River and the Midland 

Avenue Bridge is hydraulically complex, geomorphically dynamic, and not well represented by the HEC–

RAS one-dimensional streamflow model. Anecdotal 

evidence and aerial photography indicate this reach 

recently has been an area of streambed deposition. Bar 

8050 now is connected to the left bank at cross 

sections 14 and 15 but was a midchannel bar in 1997 

and was attached to the right bank upstream from the 

Fryingpan River confluence in the 1987 aerial photograph. 

The HEC–RAS-generated water-surface profiles 

used in this study were simulated using channel topography surveyed in 1998 and 1999, whereas sediment 

characteristics were measured in 2000 and 2001. It is 

possible that local channel-geometry adjustments 

occurred at some locations between the dates of 

surveying and sediment measurements. The discharges 

between the bar-inundating flow (approximately 

3,000 ft3/s) and the 10-year flood (6,100 or 7,100 ft3/s) 

were not surveyed or simulated. This broad range of 

discharges is relatively common in the Roaring Fork 

River and includes the bankfull discharge, shown to be 

a significant transporter of bedload in Colorado mountain gravel-bed rivers. 

The FEMA discharge estimates used in this 

study for the 10-, 50-, and 100-year floods may be too 

large. The significance of overestimating the discharge 

and the water-surface elevation for a flood of specific 

recurrence is that the entrainment estimates in this 

report may occur less frequently than the stated flood 

recurrences imply. Sediment-entrainment potential 

calculations give only an indication of what the 

geomorphic response to floods could be. More accurate forecasts of channel adjustments resulting from 

sediment redistribution could be made by using a 

multidimensional streamflow model and sedimenttransport functions. 

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