U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5102

Prepared in cooperation with Pikes Peak Area Council of Governments, Colorado 

Water Conservation Board, Colorado Springs City Engineering, and Lower Arkansas 

Valley Water Conservancy District

Prediction of Suspended-Sediment Concentrations 

at Selected Sites in the Fountain Creek Watershed, 

Colorado, 2008–09
Cover. The photograph was taken using a waterproof camera setting on the bed of Fountain 

Creek. The image shows grains of sand and gravel on the streambed of Fountain Creek and turbid 

streamflow caused by suspended sediment. Photograph was taken by Robert W. Stogner, 

U.S. Geological Survey.
Prediction of Suspended-Sediment 

Concentrations at Selected Sites in the 

Fountain Creek Watershed, Colorado, 

2008–09

By Robert W. Stogner, Sr., Jonathan M. Nelson, Richard R. McDonald, 

Paul J. Kinzel, and David P. Mau

Scientific Investigations Report 2012–5102

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with Pikes Peak Area Council of Governments, 

Colorado Water Conservation Board, Colorado Springs City Engineering, 

and Lower Arkansas Valley Water Conservancy District
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Stogner, R.W., Sr., Nelson, J.M., McDonald, R.R., Kinzel, P.J., and Mau, D.P., 2013, Prediction of suspended-sediment 

concentrations at selected sites in the Fountain Creek watershed, Colorado, 2008–09: U.S. Geological Survey 

Scientific Investigations Report 2012–5102, 36 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Description of Fountain Creek Watershed .......................................................................................3

Description of Study Sites...................................................................................................................5

Monument Creek .........................................................................................................................5

Sand Creek....................................................................................................................................7

Methods of Investigation..............................................................................................................................7

Measurement of Stream Stage, Velocity, and Discharge..............................................................7

Measurements of Channel Topography............................................................................................7

Measurement of Suspended-Sediment Concentration and Particle Size ..................................9

Measurement of Bed-Material Particle Size ...................................................................................9

Computational Methods for Flow and Suspended Sediment......................................................10

Suspended-Sediment Modeling.......................................................................................................10

Rouse Method with Smith/McLean Reference Concentration..........................................10

Rouse Method with Reference Concentration Equation Calibrated with Field Data .....11

Advection-Diffusion Solution...................................................................................................11

Statistical Analysis ....................................................................................................................11

Flow and Suspended-Sediment Model Development ...........................................................................11

Field Data Integration.........................................................................................................................12

Bed Material Particle-Size Data .............................................................................................12

Suspended-Sediment Data ......................................................................................................12

Flow Modeling..............................................................................................................................................20

Flow-Model Calibration......................................................................................................................20

Monument Creek .......................................................................................................................20

Sand Creek..................................................................................................................................20

Flow-Model Results............................................................................................................................20

Predicted Suspended-Sediment Concentrations...................................................................................23

Implications for Future Work .....................................................................................................................31

Summary........................................................................................................................................................34

References Cited..........................................................................................................................................34

Figures 

1. Location and generalized surficial geology of the Fountain Creek watershed and 

general location of Palmer Divide, Colorado ...........................................................................2

2. Annual hydrograph of mean-daily streamflow at selected locations in the Fountain 

Creek watershed, Colorado ........................................................................................................4

3. Location of the Monument Creek study site in the vicinity of Monument Creek near 

Woodmen Road (U.S. Geological Survey (USGS) site 07103970) with location of 

benchmarks, reference points, transducers, auto-sampler intakes, and survey 

points ..............................................................................................................................................6
iv

4. Location of the Sand Creek study site in the vicinity of Sand Creek near Las Vegas 

Street (U.S. Geological Survey (USGS) site 07105600) with location of benchmarks, 

reference points, transducers, auto-sampler intakes, and survey points ..........................8

5. Particle-size distribution data for bed-sediment samples collected in Monument 

Creek near U.S. Geological Survey gage site 07103970. (Sampling locations are 

shown in figure 3.) ......................................................................................................................13

6. Particle-size distribution data for bed-sediment samples collected in Sand Creek 

 near U.S. Geological Survey gage site 07105600. (Sampling locations are shown in 

figure 4.)........................................................................................................................................14

7. Particle-size distribution for bed sediments for composite of all samples collected 

 in Monument Creek near U.S. Geological Survey gage site 07103970 and Sand 

 Creek near U.S. Geological Survey gage site 07105600 .......................................................15

8. Streamflow, suspended-sediment, and sand concentrations in samples 

collected from the Monument Creek site during August 2008............................................16

9. Particle-size distribution data for suspended-sediment samples collected at a 

single point in the vertical in Monument Creek near U.S. Geological Survey gage 

site 07103970 (location is shown in figure 3)..........................................................................17

10. Particle-size distribution data for suspended-sediment samples collected at a 

single point in the vertical in Sand Creek near U.S. Geological Survey gage site 

07105600 (location is shown in figure 4)..................................................................................17

11. Measured suspended-sediment concentrations of samples collected at a single

 vertical location in Monument Creek near U.S. Geological Survey gage site 

07103970 as a function of streamflow .....................................................................................18

12. Measured suspended-sediment concentrations of samples collected at a single 

vertical location in Sand Creek near U.S. Geological Survey gage site 07105600 

as a function of streamflow ......................................................................................................19

13. Root-mean-squared error between the measured and predicted water-surface 

 elevation as a function of roughness at (A) the upper Monument Creek reach and 

 (B) the lower Monument Creek reach.....................................................................................21

14. Measured and predicted water surface as a function of streamwise distance in 

the downstream direction for the calibrated value of roughness at (A) the upper 

Monument Creek reach and (B) the lower Monument Creek reach .................................22

15. Predicted stream velocities in the upper Monument Creek study reach at 

 (A) 2.52 cubic meters per second and (B) 11.52 cubic meters per second.......................24

16. Predicted bed shear stress at the upper Monument Creek study reach at (A) 2.52 

 cubic meters per second and (B) 11.52 cubic meters per second.....................................25

17. The measured suspended-sediment concentrations in the (A) upper reach of the 

Monument Creek site and (B) lower reach of the Monument Creek site .........................26

18. Predicted sand concentration at the (A) upper reach of the Monument Creek site 

 using the average calibrated gamma (0.003) from the lower reach and (B) lower 

 reach of the Monument Creek site using the average calibrated gamma (0.0035) 

 from the upper reach .................................................................................................................28

19. Mean suspended-sand concentrations at the upper Monument Creek reach for 

 (A) a storm-flow event at 2.52 cubic meters per second and (B) a stormflow event 

 at 11.52 cubic meters per second ............................................................................................29

20. Suspended-sand load computed at each section in the modeling domain for the 

 upper Monument Creek reach for two sampled storm-flow events..................................30
v

21. Total load computed at each section in the modeling domain for the upper 

 Monument Creek reach for two sampled storm-flow events .............................................30

22. The measured and predicted sand fraction as a function of particle size for each 

 of two sampled storm events....................................................................................................32

23. (A) The measured total suspended-sediment concentration using the Equal Width 

Increment (EWI) method for all samples collected at the upper Monument Creek 

 reach from 2001–08 and the mean predicted suspended-sand concentration 

(particle-size distributions 1-5), and (B) Particle-size fractions for the predicted 

particle-size distributions 1–4 described in (A).....................................................................33

Tables

1. Suspended-sediment analyses and size fraction data provided by each analysis.........10

2. Calculated gamma for each measured sand concentration in the upper subreach 

of the Monument Creek site......................................................................................................27

3. Calculated gamma for each measured sand concentration in the lower subreach 

of the Monument Creek site......................................................................................................27
vi

Conversion Factors

SI to Inch/Pound 

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

micron (mm) 0.00003937 inch (in.) 

meter (m) 3.281 foot (ft) 

Area

square kilometer (km2) 0.3861 square mile (mi2

)

square meter (m2) 0.0002471 acre

square meter (m2) 10.76 square foot (ft2

) 

Volume

cubic meter (m3) 264.2 gallon (gal) 

cubic meter (m3) 35.31 cubic foot (ft3

)

milliliter (mL) 0.03381 ounce (fl. oz)

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3

/s)

Pressure: 1 Pascal (Pa) = 1 Newton(N)/m2

6.895 kilopascals (kPa) = 1 pound per square inch 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the “North American Vertical Datum of 1988 

(NAVD 88)”

Horizontal coordinate information is referenced to the “North American Datum of 1983 (NAD 

83)”

Elevation, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
vii

Abbreviations and acronyms used in this report

DCP data collection platform 

EWI equal-width increment

FaSTMECH Flow and Sediment Transport with Morphologic Evolution of Channels

GCS grade-control structure

GPS Global Positioning System

MD-SWMS Multi-Dimensional Surface-Water Modeling System

RMSE root mean squared error 

RTK real-time kinematic

USGS U.S. Geological Survey

Prediction of Suspended-Sediment Concentrations at 

Selected Sites in the Fountain Creek Watershed, Colorado, 

2008–09

By Robert W. Stogner, Sr., Jonathan M. Nelson, Richard R. McDonald, Paul J. Kinzel, and David P. Mau

Abstract

In 2008, the U.S. Geological Survey (USGS), in cooperation with Pikes Peak Area Council of Governments, Colorado 

Water Conservation Board, Colorado Springs City Engineering, and the Lower Arkansas Valley Water Conservancy 

District, began a small-scale pilot study to evaluate the effectiveness of the use of a computational model of streamflow 

and suspended-sediment transport for predicting suspendedsediment concentrations and loads in the Fountain Creek 

watershed in Colorado. Increased erosion and sedimentation 

damage have been identified by the Fountain Creek Watershed 

Plan as key problems within the watershed. A recommendation 

in the Fountain Creek Watershed plan for management of the 

basin is to establish measurable criteria to determine if progress in reducing erosion and sedimentation damage is being 

made. The major objective of this study was to test a computational method to predict local suspended-sediment loads at 

two sites with different geomorphic characteristics in order to 

evaluate the feasibility of using such an approach to predict 

local suspended-sediment loads throughout the entire watershed. Detailed topographic surveys, particle-size data, and suspended-sediment samples were collected at two gaged sites: 

Monument Creek above Woodmen Road at Colorado Springs, 

Colorado (USGS gage 07103970), and Sand Creek above 

mouth at Colorado Springs, Colorado (USGS gage 07105600). 

These data were used to construct three-dimensional computational models of relatively short channel reaches at each 

site. The streamflow component of these models predicted a 

spatially distributed field of water-surface elevation, water 

velocity, and bed shear stress for a range of stream discharges. 

Using the model predictions, along with measured particle 

sizes, the sediment-transport component of the model predicted the suspended-sediment concentration throughout the 

reach of interest. These computed concentrations were used 

with predicted flow patterns and channel morphology to determine fluxes of suspended sediment for the median particle size 

and for the measured range of particle sizes in the channel. 

Three different techniques were investigated for making the 

suspended-sediment predictions; these techniques have varying degrees of reliance on measured data and also have greatly 

differing degrees of complexity. Based on these data, the 

calibrated Rouse method provided the best balance between 

accuracy and both computational and data collection costs; 

the presence of substantial washload was the primary factor 

in eliminating the simpler and the more complex techniques. 

Based on this work, using the selected technique at additional 

sites in the watershed to determine relative loads and source 

areas appears plausible. However, to ensure that the methodology presented in this report yields reasonable results at other 

selected sites in the basin, it is necessary to collect additional 

verification data sets at those locations.

Introduction

The Fountain Creek watershed drainage network currently is a highly dynamic system arising from (1) the potential for very high rainfall rates (orographic induced along the 

Palmer Divide and the Rampart Range (fig. 1) (Doesken and 

others, 2003; Meister, 2010), (2) easily eroded alluvium along 

the valley bottoms east of the foothills (Fountain Creek Watershed Flood Control and Greenway District, 2011), and (3) very 

rapid urban development. Very rapid urban development has 

the potential to exacerbate the rainfall runoff rates, increase 

storm runoff peaks, and undermine the natural geomorphic 

protections against uncontrolled incision provided by bed 

armoring and floodplain vegetation and the resulting sedimentation in the lower reaches of the watershed (Fountain Creek 

Watershed Flood Control and Greenway District, 2011). 

The Fountain Creek Watershed Plan (U.S. Army Corps 

of Engineers, 2009) identified both erosion and sedimentation 

as key issues in the watershed. The plan also recommended 

the development of measureable criteria to evaluate both the 

magnitude of erosion in the watershed and the effectiveness of 

management actions to reduce erosion. Therefore a thorough 

evaluation of sediment transport mass balance throughout the 

stream network was important. Such an evaluation required 

examining both of the fundamental modes of sediment 

transport, namely bedload and suspended load (Middleton 

and Southard, 1984). Bedload transport in Fountain Creek has 

been studied in detail (U.S. Army Corps of Engineers, 2009). 
2 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

07106500

07105600

07103970

Pueblo

Woodland Park

Palmer Lake

Monument

Manitou Springs

Green Mountain Falls

Fountain Creek

Sand Creek

Monument Creek Pikes Peak

105°00’ 104°45' 104°30'

39°00’

38°45'

38°30'

38°15'

Colorado Piedmont

Base from U.S. Geological Survey

Albers Equal-area projection

Standard parallels 37°30'N and 40°30'N

Central meridian 105°30'W Front Range of Southern Rocky Mountains Rampart Range

Colorado Springs

Arkansas River

Arkansas River

Palmer Divide

TELLER COUNTY

EL PASO COUNTY

PUEBLO COUNTY

FREMONT COUNTY

EXPLANATION

Municipal area

Watershed boundary

Generalized surficial geology

Igneous

Metamorphic

Sedimentary

Sedimentary--Alluvium

Sampling site and number

07103985

Sand Creek

watershed

Monument Creek site watershed

COLORADO

Denver

Study 

area

0 2.5 5 MILES

0 2.5 5 KILOMETERS

Figure 1. Location and generalized surficial geology of the Fountain Creek watershed and general 

location of Palmer Divide, Colorado.
Introduction 3

In contrast, relatively little work has been done to evaluate the 

transport of suspended sediment throughout the watershed by 

particle-size range despite the fact that parts of the Fountain 

Creek network have the potential to export large volumes of 

suspended sediment, including sand-size particles, during high 

flows. 

The most reliable way to determine the dominant 

sources of water in a drainage network for particular rainfall events is to gage the water discharge, hereafter referred 

to as “streamflow,” in that network densely and accurately. 

Similarly, the most reliable way to determine the sources of 

various types and sizes of sediment in a drainage network is 

to gage suspended-sediment discharge, hereafter referred to as 

“sediment-transport rate,” in that network densely and accurately. Unfortunately, in a complex watershed like Fountain 

Creek, this field-based methodology is relatively time consuming and expensive. Thus, the high cost of making suspendedsediment measurements has been a barrier to addressing this 

dominant component of the sediment transport problem in 

the Fountain Creek watershed. However, models of flow and 

suspended-sediment transport applied at critical reaches using 

detailed local data, when used together with a relatively small 

set of field measurements of suspended-sediment transport, 

offer a less expensive alternative to detailed field data collection efforts that would be required to directly gage suspendedsediment transport. These models can potentially predict both 

flow patterns and sediment fluxes based on measured topography and particle sizes of bed material and suspended sediment. 

Using this technique at several locations would provide spatial 

information about the relative contributions of different source 

areas and provide a sediment load for various tributaries.

In 2008, the U.S. Geological Survey (USGS), in cooperation with Pikes Peak Area Council of Governments, 

Colorado Water Conservation Board, Colorado Springs City 

Engineering, and the Lower Arkansas Valley Water Conservancy District, began a small-scale pilot study to evaluate the 

effectiveness of the use of a computational model of flow and 

suspended-sediment transport for predicting suspended-sediment concentrations and loads in the Fountain Creek watershed in southeastern Colorado. 

Purpose and Scope

The purpose of this report is to present the results of a 

pilot study to develop, calibrate, and validate a multidimensional model as an inexpensive tool to predict suspendedsediment concentrations and to calculate suspended-sediment 

loads at various locations within the Fountain Creek watershed 

in southeastern Colorado. Two sites with different geomorphic 

characteristics were selected, and three different suspendedsediment transport techniques were evaluated in order to 

assess the feasibility of using these techniques to predict 

local suspended-sediment loads throughout the watershed as 

a lower cost alternative to traditional gaging methods. This 

report describes (1) data requirements of the hydraulic and 

sediment transport models, (2) data collection methods, (3) 

hydraulic and sediment transport model calibration, and (4) 

an evaluation of each of the three techniques for predicting 

suspended-sediment concentrations at the two sites. Comparisons of accuracy and cost associated with each technique 

provide information that can be used to select a methodology 

that best fits suspended-sediment monitoring objectives within 

the Fountain Creek watershed. Ideally, this will lead to the 

application of a singular method on a watershed-wide selection of sites. 

Description of Fountain Creek Watershed

The Fountain Creek watershed, in and along the eastern 

slope of the Front Range section of the southern Rocky Mountains, drains approximately 2,410 square kilometers (km2

) of 

parts of Teller, El Paso, and Pueblo Counties in eastern Colorado (fig. 1). Land-surface elevation ranges from 4,300 meters 

(m) at the summit of Pikes Peak to 1,414 m at the confluence 

of Fountain Creek and the Arkansas River. Surficial geology is 

dominated by various igneous rock units within the Rampart 

Range and by various sedimentary rock units in the Colorado Piedmont. More detailed geologic and soil information 

is available in Larsen (1981), von Guerard (1989), and U.S. 

Army Corps of Engineers (2009).

Streamflow can vary hourly, daily, and seasonally and is 

driven by several natural and anthropogenic processes. Snowmelt runoff from the Rampart Range generally occurs between 

mid-April and mid-June (Stogner, 2000) and is characterized 

by diurnal variations in streamflow in response to cycles of 

heating and cooling over the course of a day. Duration and 

magnitude of snowmelt runoff varies annually depending on 

extent and depth of winter and spring snowfall and on the 

rate of spring heating. Storm runoff generally occurs between 

mid-June and early October and is characterized by rapid 

increases in streamflow in response to afternoon and evening convectional storms. Duration and magnitude of storm 

runoff varies depending on extent, intensity, and duration of 

storms. Flashfloods are common during summer storms and 

can produce extreme changes in stream-channel geometry. 

Water use and water management cause hourly, daily, and 

seasonal changes in streamflow through importation of nonnative water, increased return flows from increased water use, 

water diversions, wastewater-treatment-plant discharges, and 

changes in water management and operations (Stogner, 2000). 

Figure 2 shows mean-daily streamflow for U.S. Geological 

Survey (USGS) streamflow gage 07106500 (Fountain Creek 

at Pueblo, Colo.) near the confluence with the Arkansas River. 

The hydrograph represents the annual streamflow characteristics for the Fountain Creek watershed, which is dominated by 

spring snowmelt and summer stormflow.

Land use within the Fountain Creek watershed has 

substantially changed since 1900 as a result of increased 

population. The population of El Paso County was approximately 31,600 in 1900, approximately 74,500 in 1950, and 
4 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1

10

0.01

0.1

1

10

Mean-daily streamflow, in cubic meters per second

0.1

1

10

07103970, Monument Creek above Woodmen Road at Colorado Springs, Colo. 1997 to 2010

07105600, Sand Creek above mouth at Colorado Springs, Colo. 2003 to 2010

07106500, Fountain Creek at Pueblo, Colo. 1922 to 2010

Baseflow Snowmelt Stormflow Baseflow

Figure 2. Annual hydrograph of mean-daily streamflow at selected locations in the Fountain Creek watershed, 

Colorado.
Introduction 5

approximately 517,000 in 2000 (U.S. Census Bureau, 2010a; 

U.S. Census Bureau, 2010b). As population increased, changes 

in land use have facilitated additional changes in streamflow 

characteristics, specifically through urbanization and increased 

impervious land cover (Stogner, 2000). Estimated impervious 

land cover in El Paso County increased by approximately 10 

percent between 1997 and 2001 and by approximately 6 percent between 2001 and 2005 (Mau and others, 2007). During 

this time, the population of El Paso County increased by about 

3 percent annually (Mau and others, 2007). Initial effects on 

streamflow characteristics may be subtle, but as the percentage of impervious land cover increases with time, substantial 

change can occur in storm-runoff timing and intensity resulting in larger peak streamflows (Dunne and Leopold, 1978). 

Streambed and streambank material within Fountain 

Creek and its tributaries is predominantly sand- and gravelsize (0.0623–64 millimeters (mm)) alluvium, with some materials that are cobble- and boulder-size (greater than 64 mm) in 

the upland and mountainous reaches. Minimum streamflows 

in Fountain Creek and its tributaries have the capacity to transport sand-size particles; higher streamflows ranging from 0.28 

cubic meters per second (m3/s) to 17 m3/s have the capacity to 

transport gravel-size particles (Mau and others, 2007). Erosion 

and sedimentation within the Fountain Creek watershed can 

contribute to financial losses associated with damage to public 

and private property, as well as to transportation and municipal 

infrastructure near Fountain Creek and its tributaries.

Description of Study Sites

Two sites with different geomorphic characteristics were 

selected within the Fountain Creek watershed to provide case 

examples of typical stream types within the system. Selection 

of the field sites was based on availability of historical streamflow and sediment data, and site characteristics represented 

by streams originating in the Rampart Range and Colorado 

Piedmont. Monument Creek above Woodmen Road, USGS 

site 07103970 (hereafter referred to as “the Monument Creek 

site”), was selected to represent the streams originating in the 

Rampart Range. This site is dominated by snowmelt runoff 

(figs. 1 and 2). Sand Creek near the confluence with Fountain 

Creek, USGS site 07105600 (hereafter referred to as “the Sand 

Creek site”), was selected to represent the streams originating 

in the Colorado Piedmont. This site is dominated by storm 

runoff (figs. 1 and 2). 

Monument Creek

Monument Creek, a major tributary to Fountain Creek, 

originates in the northwestern part of the Fountain Creek 

watershed in the Rampart Range. The creek flows northeasterly toward the community of Palmer Lake and then southernly through the city of Colorado Springs to the confluence 

with Fountain Creek. The Monument Creek watershed above 

USGS site 07103970 (fig. 1) encompasses about 469 km2

(U.S. Geological Survey, 2012a). Land use in 2005 was 

predominantly undeveloped (45.8 percent), residential (19.0 

percent), and military (16.0 percent) (Mau and others, 2007). 

Mau and others (2007) estimated that 11.3 percent of land in 

the watershed was impervious to precipitation. 

Monument Creek is a perennial stream with a singlethread, meandering channel with eroded banks (cutbanks) and 

point bars. The cutbanks expose fine sediments and mixtures 

of relatively fine sands with coarser gravels. The channel 

cross section is typically incised (Pikes Peak Area Council of 

Governments, 2009) with floodplains and terraces covered 

in grasses and woody vegetation. The riparian area varies in 

width from about 30 to 120 m within the rural corridor and 

is generally constrained by natural terraces. Upper terraces 

generally are not immediately impacted by urban or rural 

encroachment. Within the urban corridor of Colorado Springs, 

the riparian area adjacent to Monument Creek generally is 

narrower and constrained by natural terraces or concrete floodretaining walls. Urban development often extends to margins 

created by the concrete walls. 

Data collection locations at the Monument Creek site 

are shown in figure 3. The site is divided into an upper and a 

lower subreach by a concrete grade-control structure (GCS). 

The upper subreach is about 240 m in length and has a gradient of about 0.0048. The USGS streamflow gage at the Monument Creek site is located on the right bank near the GCS. The 

site was instrumented with a Sutron Accububble and Sutron 

Model 8210 Data Collection Platform (DCP) to measure and 

record height of water surface above an established datum 

plane, hereafter referred to as “stage,” and an ISCO Model 

3700 automatic pumping sampler, hereafter referred to as 

an “autosampler,” to collect water samples for analysis of 

suspended-sediment concentration. The intake tubing for 

the ISCO autosampler at this site has an internal diameter of 

9.53 mm, is about 10-m long, and generally slopes downward 

toward the stream for the entire length of the tubing, toward 

the right bank. The riparian area of the upper subreach has a 

relatively wide floodplain constrained by natural terraces and 

some urban development. Below the GCS, the bed elevation decreased by approximately 1 m. The lower subreach 

is about 250 m in length and has a gradient of about 0.0037. 

A gage station was installed on the left bank at the terminus 

of the reach and was instrumented in a similar manner with 

respect to the upper reach. The intake tubing for the ISCO 

autosampler at this site has an internal diameter of 9.53 mm, 

is about 3 m long, and generally slopes downward toward the 

stream for the entire length of the tubing; the intake is about 

0.5 m from the left bank. The lower subreach is constrained 

by natural terraces through much of its length, transitioning to 

concrete flood walls and a narrow riparian area at the terminus 

of the reach. Tubing intakes were located at a fixed point in 

the water column. Bed material at the Monument Creek site 

is dominated by particles in the sand- and gravel-size fraction 

(0.0623–64 mm); however, bed scour within the upper subreach may seasonally expose the underlying bedrock.
6 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

M18

M11

M09

M05

M03

M01

M06

M19

M16

M17

M14

M15 M13

M12

M10

M08

M07

M04

M02

104°49' 104°49'05"

38°56'05"

38°56'

BM01

BM02

RP01

T01

T02

(385557104485801)

Colorado 

Springs

Pueblo

Denver

Colorado 

Springs

COLORADO

07103970

07105600

(07103970)

Upper subreach

Lower subreach

EXPLANATION

Location and number of bed-sediment sample

Benchmarks and reference points

Transducer

Gage (USGS site identifier) and autosampler locations

Survey points in or near channel

Elevation (meters)

1,906.38-1,906.76

1,906.77-1,908.34

1,908.35-1,909.13

1,909.14-1,909.89

1,909.90-1,911.35

1,911.36-1,915.80

1,915.81-1,920.15

M16

Imagery: USGS orthophotography, 1-meter pixels, 2011 photography. 

Projection: UTM zone 13, North American 

Datum of 1983

0 20 40 80 METERS

0 100 200 300 FEET

60

Figure 3. Location of the Monument Creek study site in the vicinity of Monument Creek near Woodmen Road (U.S. 

Geological Survey (USGS) site 07103970) with location of benchmarks, reference points, transducers, auto-sampler 

intakes, and survey points.
Methods of Investigation 7

Sand Creek

Sand Creek, a tributary to Fountain Creek, originates 

in the northeastern part of the Fountain Creek watershed. 

The creek flows southwesterly through the city of Colorado 

Springs to the confluence with Fountain Creek. The Sand 

Creek watershed above USGS site 07105600 (fig. 1) encompasses about 136 km2

 (U.S. Geological Survey, 2012b). Land 

use in 2005 was predominantly agriculture (34.0 percent), 

undeveloped (18.9 percent), residential (25.9 percent), and 

commercial (4.9 percent) (Mau and others, 2007). Mau and 

others (2007) estimated that 10.9 percent of land in the watershed was impervious to precipitation. Sand Creek has been 

identified as one of the largest contributors of sediment load 

to Fountain Creek (von Guerard, 1989; U.S. Army Corps of 

Engineers, 2009). 

Sand Creek is an ephemeral, braided stream that is sand 

material dominated. The channel cross section is approximately trapezoidal with rip-rapped banks interspersed with 

grasses and woody vegetation (Pikes Peak Area Council of 

Governments, 2009). The riparian area adjacent to Sand Creek 

is narrow, less than 30 m, through most of the urban corridor 

due to channelization, channel widening, and (or) channel 

constriction associated with urban encroachment. Transportation, sewer, and utility infrastructure at several Sand Creek 

crossings are considered threatened due to lateral migration 

or downcutting (U.S. Army Corps of Engineers, 2009). Upper 

terraces generally are developed for commercial, industrial, or 

residential uses. 

Data location of the Sand Creek site are shown in figure 4. 

The site is divided into an upper and a lower subreach by a 

concrete GCS. The upper subreach is about 150 m in length 

and has a gradient of about 0.0097. The USGS streamflow 

gage at the Sand Creek site is located on the left bank near the 

GCS. The site was instrumented with a Sutron Accububble 

and Sutron Model 8210 DCP to measure and record stage and 

an ISCO Model 3700 autosampler to collect water samples for 

analysis of suspended-sediment concentration. The intake tubing for the ISCO autosampler at this site had an internal diameter of 9.53 mm, was about 25 m long, and initially sloped 

downward toward the stream. However, approximately 13 m 

of tubing lies relatively flat across the top of the GCS before 

opening near mid-channel. Below the GCS, the bed elevation decreased by approximately 0.2 m. The lower subreach 

is about 280 m in length and has a gradient of about 0.0098. 

A gage station was installed at the terminus of the reach and 

similarly instrumented with a Sutron Accububble and Model 

8200A DCP to measure and record stream stage and an ISCO 

Model 3700 autosampler to collect water samples for analysis of suspended-sediment concentration. The intake tubing 

for the ISCO autosampler at this site is about 10 m long and 

slopes downward toward the stream for the entire length of 

the tubing; the intake is approximately 2 m streamward of the 

left bank. Tubing intakes at Sand Creek were located at a fixed 

point in the water column. The riparian area is narrow and 

constrained by rip-rapped terraces. Bed material is dominated 

by particles in the sand- and gravel-size fraction (0.0623–

64 mm). 

Methods of Investigation

Field work at the two sites consisted of continuation of 

existing USGS streamflow-gaging-station operations as well 

as data collection specifically needed for model application 

and verification. Initial geomorphic surveys, including channel 

topography, water-surface elevations, and bed-material particle-size distributions, were conducted during June 2007 with 

additional surveys conducted through May 2008. At the Sand 

Creek site, there was insufficient water present in the channel 

when the channel was surveyed to define water-surface elevations. In order to get water-surface elevations for calibration, 

two pressure transducers were installed to measure stage 

during periods of flow. Streamflow measurements and total 

suspended-sediment samples were collected between August 

2007 and October 2008. A subset of the suspended-sediment 

samples was analyzed for sand concentrations and silt-andclay concentrations, and a smaller subset was analyzed for 

the full particle-size distribution of the sand and silt-and-clay 

fraction. 

Measurement of Stream Stage, Velocity, and 

Discharge

Routine discharge measurements were made at the 

Monument Creek and Sand Creek sites. Discharge measurements were made using standard USGS methods to define 

stage-discharge relation curves, hereafter referred to as “rating 

curves,” at the Monument Creek and Sand Creek sites (Carter 

and Davidian, 1968; Buchanan and Somers, 1969). Two pressure transducers were installed at each site to supplement the 

stage data collected at the stream gages. Instantaneous stage, 

velocity, and streamflow data were obtained from discharge 

measurements of Monument Creek and Sand Creek.

Measurements of Channel Topography

Reaches were surveyed with a Trimble Model R8 Global 

Navigation Satellite System survey-grade global positioning 

system (GPS) operated in real-time kinematic (RTK) mode. 

The RTK GPS equipment uses a radio to transmit data from 

a stationary GPS receiver (base) to one or more mobile GPS 

receivers (rovers) where topographic measurements are collected and computed in real time (Trimble Navigation Limited, 

1998). During each topographic survey, the base receiver 

was positioned over a permanent benchmark located in close 

proximity to the reach surveyed (figs. 3 and 4). Benchmarks 

were placed by USGS personnel, and the horizontal and vertical coordinates were determined by a static GPS survey. The 

data were post-processed by the National Geodetic Survey’s 
Figure 4. Location of the Sand Creek study site in the vicinity of Sand Creek near Las Vegas Street (U.S. Geological Survey (USGS) sof benchmarks, reference points, transducers, auto-sampler intakes, and survey points.

S10

S11

S08

S03

S05

S04

S06

S07

S09

S12

104°46'35" 104°46'30" 104°46'25" 

38°47'15"38°47'10"

Colorado 

Springs

07103970

07105600

Pueblo

Denver

Colorado 

Springs

COLORADO

E

Location and Benchmarks aTransducerGage (USGS s

Survey points in or neElevation (meters)

1,768.7-1,770.81,770.9-1,772.71,772.8-1,773.91,774.0-1,775.01,775.1-1,776.11,776.2-1,777.51,777.6-1,781.7

T03

BM03

BM04

(384713104463301)

Lower subreach

(07105600)

RP01

Imagery: USGS orthophotograph, 1-meter pixels, 2011 photography. Projection: UTM zone 13, North American Datum of 1983

S07

0 20 40 80 METERS0 50 100 200 300 FEET

60
Methods of Investigation 9

On-line User Positioning Service (http://www.ngs.noaa.

gov/OPUS/). Topographic measurements collected with the 

survey-grade GPS in RTK mode have a horizontal accuracy of 

± 1 centimeter (cm) and a vertical accuracy of ± 2 cm (Trimble 

Navigation Limited, 2009). Topographic data collected with 

the GPS were primarily along transects oriented parallel to the 

principal flow direction in the reaches; additional topographic 

data were collected as shown in figures 3 and 4, which show 

the planform locations of the raw survey points color-coded 

for elevation at the two sites. The positions surveyed with the 

roving receivers were stored in the data collectors and later 

downloaded and integrated to make elevation grids of the two 

study sites. 

Topographic measurements at each site were made over a 

reach length of approximately 10–20 stream widths (figs. 3 and 

4). This minimized the potential for errors in model predictions near the gage and the autosamplers that were due to 

unquantified lateral distributions of flow at the upstream end 

of the reach. The Monument Creek and Sand Creek models 

were separated into two reaches because of the presence of the 

GCS.

Measurement of Suspended-Sediment 

Concentration and Particle Size

In response to the signal from the DCP, the ISCO autosamplers used a peristaltic pump to purge the sample tubing 

prior to and after sample collection by pumping air through 

the tubing out to the stream. To collect a sample of water, 

the pump reversed direction creating a vacuum in the sample 

tubing. The resulting suction drew water from the stream, and 

twenty-four 500-milliliter (mL) bottles were sequentially filled 

to a calibrated volume. These samplers provided relatively 

infrequent (daily) samples during low flow and more frequent 

samples (typically hourly) based on stage and rate-of-change 

in stage during storm runoff events. 

The downstream samplers provided critical verification 

information for the model results and calibration information 

for some of the suspended-sediment methods. Both the verification and calibration information are described in the section 

“Suspended-Sediment Modeling.” In addition, suspendedsediment data were used to separate wash load and suspended 

load fluxes for the different particle sizes, which is required for 

constraining model calculations of suspended-sediment load. 

Wash load is a special case of suspended load and consists of 

the smallest particles, silt and clay, which usually are absent 

on the streambed (Ritter and others, 2002). Like the dissolved load of a chemical constituent, these small particles are 

distributed uniformly throughout the flow, so their sedimenttransport rate can be determined from a single sample at any 

location within a gaged cross section. Once the wash load was 

separated from the total suspended-sediment load, the remaining information on coarser material concentration and size 

distribution was used to determine the size distribution of sand 

on the riverbed. Sediment-size information from these samples 

was determined through analysis of sand and silt-and-clay 

concentrations, hereafter referred to as “sand-fine samples.” 

A more detailed nine-point particle-size, hereafter referred to 

as “9-point samples,” analysis was carried out for a subset of 

all the samples collected. All samples were analyzed at the 

USGS Iowa Water Science Center Sediment Laboratory using 

standard methods (Guy, 1969).

Programmable DCPs were co-located with autosamplers 

at monitoring locations. DCPs were programmed to measure and record stage at 5- or 15-minute intervals and record 

autosampler activation. During storms, DCPs evaluated stage 

and rate-of-change in stage and signaled autosamplers to 

collect a sample based on programmed minimum stage and 

rate-of-change in stage thresholds. Autosamplers logged the 

dates and times of sample collections. During rapid changes in 

stage, DCPs signaled automatic samplers to collect samples at 

5-minute intervals. As the rate-of-change in stage decreased, 

frequency of sample collection by the automatic samplers 

decreased to 30- and 60-minute intervals. When stage dropped 

below stage thresholds, DCPs no longer signaled automatic 

samplers to collect a sample. Different laboratory methods 

were employed to evaluate sediment concentration in the 

water samples collected because the size of suspended sediment is not static and can increase and decrease with change in 

streamflow (Leopold and others, 1964; Knighton, 1998; Ritter 

and others, 2002). Based on stage during time of collection, 

sediment samples were analyzed for suspended-sediment 

concentration, sand-fine concentration, or 9-point analysis 

(table 1). Nine-point analyses were limited to samples collected at or near the peak streamflow for each sampled event 

when stream energy is greatest and may transport larger size 

sediments. Nine-point analyses provided detailed data on size 

of sediments in suspension during peak streamflow. Analyses 

for sand-fine concentrations were generally associated with 

samples collected immediately preceding and (or) following 

the peak streamflow. The sand-fine analyses provided concentration data for sand-sized particles and silt-and-clay sized 

particles. The remaining samples (collected after peak streamflow) were analyzed for suspended-sediment concentration, 

and no distinction was made between sand and silt-and-clay 

sized particles (table 1).

Measurement of Bed-Material Particle Size

Bed-material samples were collected using a US BMH53 sampler (Edwards and Glysson, 1999) at 19 locations at 

the Monument Creek site (fig. 3) and 12 locations at the Sand 

Creek site (fig. 4) to characterize the size distribution and 

the spatial variation of particle sizes on the bed. Sample sites 

were selected randomly. This information was used in the flow 

model to estimate channel roughness and in the suspendedsediment computations. Particle-size analysis (Guy, 1969) was 

used to determine the detailed distribution of sand sizes. 
10 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Table 1. Suspended-sediment analyses and size fraction data provided by each analysis.

[mm, millimeters; C, computed; X, analyzed; --, not analyzed]

Analysis Total concentration

Sand,

percent greater 

than 0.0625 mm

Silt and clay,

percent finer 

than 0.0625 mm

Particle size, in millimeters, percent finer than

0.5 0.25 0.125 0.0625 0.031 0.016 0.008 0.004 0.002

Suspendedsediment 

concentration

X -- -- -- -- -- -- -- -- -- -- --

Sand-fine 

concentration X C X -- -- -- -- -- -- -- -- --

Nine-point X C X X X X X X X X X X

Computational Methods for Flow and 

Suspended Sediment

This section presents a brief description of the flowmodeling application and a brief description of the suspendedsediment modeling both in terms of application and verification. The foundation of the detailed suspended-sediment 

transport modeling in this report is to provide an accurate 

prediction of the flow field at the study sites. To accomplish 

this, the USGS Multi-Dimensional Surface-Water Modeling 

System (MD_SWMS) was used with measured hydrographs 

and the topographic data described in the section “Measurement of Channel Topography.” The modeling system, 

MD_SWMS, is a USGS developed public-domain pre- and 

post-processing application for computational models of 

surface-water hydraulics and sediment transport. In addition 

to computational models, the system includes a graphical 

user interface (McDonald and others, 2001) that allows the 

modeler to build and edit data sets of the modeling system’s 

computational surface-water models. For the modeling at the 

two sites in the Fountain Creek watershed, three-dimensional 

surface-water modeling was accomplished using Flow and 

Sediment Transport with Morphologic Evolution of Channels 

(FaSTMECH); this model is one of the computational models 

within MD_SWMS and was developed by the USGS (Nelson 

and McDonald, 1997; Nelson and others, 2003). FaSTMECH 

includes a 2-dimensional, vertically averaged model and a 

submodel that calculates vertical distribution of the primary 

velocity and the secondary flow about the vertically averaged 

flow. This method, commonly referred to as quasi-3-dimensional, has been shown to simulate the velocity field, bed shear 

stress, and resulting patterns of erosion and deposition where 

secondary flows are significant, such as meander bends, without the complexity of a 3-dimensional model. This model has 

been used and verified extensively on a wide variety of rivers 

(Conaway and Moran, 2004; Kinzel and others, 2009; Logan 

and others, 2011; McDonald and others, 2006; McDonald and 

others, 2010).

Simulations of flow for Monument Creek and Sand Creek 

were developed using the surveyed topographic and roughness 

information from each reach. For each discharge of interest 

(high-streamflow events with pumped suspended-sediment 

samples), stage for that discharge is used as a boundary condition, and the bed roughness was set by a combination of the 

measured particle sizes on the bed and calibration. FaSTMECH model simulations of predicted stage were compared 

to measured stage. Differences between the predicted stage 

value and the measured stage value for various channel 

roughness values were evaluated by computing the root mean 

squared error (RMSE) (Helsel and Hirsch, 1992). The flowmodel results provide the information, including boundary 

shear stress, needed for computation of suspended-sediment 

transport during various streamflow events at each site. 

Suspended-Sediment Modeling

With the measured particle sizes of bed material and both 

velocities and boundary shear stresses from the flow modeling, 

there is enough information available to compute suspendedsediment concentrations and loads over a hydrograph or for 

specific steady flow conditions. Three different methods of 

suspended-sediment concentration estimates were evaluated in this study. In each case, the particle sizes were used 

to determine settling velocities based on relations given by 

Dietrich (1982). A comparison of the different methods and 

a relative cost effectiveness and evaluation of accuracy was 

done in the context of a broader watershed-wide application of 

these techniques. Model predictions for suspended sediment at 

the location of the autosampler nozzle for the first two methods discussed below were calibrated for the best-fit value of 

gamma, the resuspension parameter, which is a dimensionless 

parameter in the reference concentration equations relating 

bed shear stress to near-bed suspended-sediment concentration for each size class present in the bed (Smith and McLean, 

1977). The details of the three methods are described in the 

following sections.

Rouse Method with Smith/McLean Reference 

Concentration 

The first method for computing suspended-sediment 

concentrations and fluxes by size class is based on the method 

described by Rouse, and empirically calibrated by Smith and 
Flow and Suspended-Sediment Model Development 11

McLean (1977) and McLean (1992). This method predicts the 

vertical profile of suspended-sediment concentration as a function of the local (at a point) bed shear stress, the critical shear 

stress for motion of the particles, the settling velocities of particles in suspension, and a single empirical coefficient relating 

the concentration of particle sizes in the bed with the concentration in suspension. An assumption in this method is that 

the flow transporting the sediment is steady and horizontally 

uniform, where horizontally uniform means that there are no 

spatial accelerations in the flow. The strength of this method 

is that it is easy to apply particle size; the weakness of the 

method is that real river flows are not steady and uniform. In 

some locations, suspended-sediment concentrations are poorly 

correlated with the local bed shear stress; this is especially true 

in regions with strong lateral shear. In some cases, this issue 

is avoided by dealing with a spatially averaged view of the 

suspended sediment, but there is no guarantee that a spatially 

averaged view is at all appropriate. In most nonlinear problems, the solution for average conditions is a poor approximation to the average of the solution for real conditions. Furthermore, various studies suggest that the empirical coefficient 

used for the reference concentration varies widely (Harris 

and Wiberg, 2001); even the functional form of the reference 

concentration equation is not universally accepted (Garcia 

and Parker, 1991). Nevertheless, this is the simplest method 

requiring the fewest field data. This method yields concentration throughout the flow. Subsequently determining the fluxes 

(loads) requires multiplying the concentration at each point by 

the computed downstream component of velocity and stage 

and averaging over each cross section.

Rouse Method with Reference Concentration 

Equation Calibrated with Field Data 

This method is essentially the same as the first method 

described above, with the important exception that the reference concentration for the Rouse solution is set using field 

data. Thus, errors associated with the form of the reference concentration equation and its empirical parameters 

are avoided. However, the method still avoids dealing with 

real spatial nonuniformity caused by local spatial accelerations and decelerations. The results from this method should 

be more accurate because they are more closely tied to the 

observations. Seeing as how this method requires more data, 

it is important to weigh the improved accuracy in lieu of the 

additional costs of a more detailed data collection strategy. In 

most cases the increased accuracy may not be substantial. This 

method requires more than one autosampler at each of the two 

sites. At one location, the autosampler collects the calibration 

data set. At the second location, the autosampler collects the 

verification data set. To ensure the verification data set is independent of the calibration data set, the two samplers should be 

located as far apart as feasible at each of the sites.

Advection-Diffusion Solution 

The third method for computing suspended-sediment 

concentration and flux involved using the computational 

solution of the equation expressing mass conservation for the 

suspended sediment (Fischer and others, 1979). This equation explicitly includes horizontal variation in streamflow 

and is called the advection-diffusion equation for suspended 

sediment. Using this method, the local concentration is not 

dependent only on the local bed shear stress, but also depends 

on the spatial advection of suspended material in the stream 

(even areas with low bed shear stresses can have high concentrations if the flux of sediment from upstream is large and 

the settling velocity of the material is relatively small). The 

strength of this method is that it addresses spatial nonuniformity in the flow. The weakness is that it requires a much more 

complicated numerical model and requires boundary conditions in the form of either a measurement or computation of 

the suspended-sediment concentration at the upstream end 

of the reach and a so-called “reference flux” condition at the 

bed. For these boundary conditions, the model will use both 

of the Rouse methods. In many, but not all cases, the solution 

becomes somewhat independent of the upstream boundary 

condition if the reach is of sufficient length; hence, the desire 

to space autosamplers as far apart in each reach as is practical. 

As part of the numerical solution process, this method yields 

both concentration and sediment flux for each particle size 

throughout the computational domain. The average of these 

concentrations can be used to predict total suspended load.

Statistical Analysis

Simple linear regression was used to determine the best 

straight-line fit between two variables (one dependent and one 

independent variable) and obtain a predictive equation. The 

coefficient of determination, r-squared (r2), is a statistical measure of how well the predictive equation (regression line) fits 

the real data points. It varies between 0 and 1 and indicates the 

amount of variability in the dependent variable described by 

the independent variable (that is, an r2 value of 0 indicates that 

there is no relation between the dependent and independent 

variables; and an r2 value of 1 indicates that there is a perfect 

relation between the dependent and independent variables) 

(Helsel and Hirsch, 1992).

Flow and Suspended-Sediment Model 

Development

The following sections discuss the results of the field data 

collections for channel particle size and suspended-sediment 

concentration and grain size, and the application of that data 

to the flow and suspended-sediment model development and 

calibration. 
12 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Field Data Integration

The basic field data used for the modeling effort consisted 

of the survey data shown in figures 3 and 4, the bed material 

particle-size data used for roughness and suspended-sediment 

modeling, and gage information that provided a relation 

between stage and streamflow (discharge) at a single location. In addition to these basic data required for setting up the 

model, suspended-sediment-concentration data were collected 

at the autosampler locations to provide calibration and verification data. Water-surface elevations were measured to verify 

flow-model results, and about 3 percent of the suspendedsediment samples were analyzed for particle-size distributions 

in order to compare them to model predictions and to ascertain 

how large the washload component of suspended-sediment 

load was at the field sites. Washload refers to the portion of 

the suspended material that is not present in the bed, or at 

least is present in such small amounts that the total amount of 

washload in suspension is determined primarily by upstream 

supply, not availability in the bed.

Bed Material Particle-Size Data

Bed-sediment particle sizes at both sites (figs. 5 and 6) 

were dominated by sand-size (0.063 to 2 mm) and gravelsize (2 to 64 mm) particles. At the Monument Creek site, the 

median (50-percent finer) bed-sediment particle size varied 

from about 1 mm (coarse sand) to 4 mm (very fine gravel), 

with the exception of a single sample (sample M06, fig. 5), 

that was collected at the top of a bar near the upstream end of 

the reach. Particle size of bed sediments of sample M06 was 

predominately (74 percent) finer than 0.25 mm (fine sand) 

and indicate the presence of an upstream source of fine sediments that were not evident in other bed-sediment samples 

collected at the Monument Creek site. The apparent presence 

of an upstream source of fine sediments and the small percentage of fine sediments in other samples collected suggests that 

particles less than 0.25 mm are readily transported through the 

reach under the current hydrologic regime. Figure 7 shows the 

distribution of bed sediments constructed from the composite 

of the individual sample results. Generally, the particle-size 

variation through the reach was small enough that it appeared 

reasonable to use the single composite bed-sediment distribution to represent the reach. This composite sample for Monument Creek, shown as a solid line in figure 7, had a median 

size of about 2 mm and a 16th and 84th percentiles (percent 

finer) of about 0.6 and 7 mm, respectively. In Sand Creek, the 

bed sediment was slightly finer in size, with the median particle size varying spatially between about 0.5 and 2 mm (fig. 

6). The presence of a substantial upstream source of fine sediments was not evident in samples collected at the Sand Creek 

site as sample M06 suggested at the Monument Creek site. 

This would suggest that such a source does not exist (unlikely 

given the characteristics of the Colorado Piedmont through 

which Sand Creek flows) or that, similar to the Monument 

Creek site, fine particles are readily transported through the 

reach. As in the case of Monument Creek, the distribution of 

the composite sample was assumed to be suitable to represent 

the bed-sediment distribution of the reach because the differences between individual samples were not large, especially 

considering the wide variation in size at any single point. 

The bed-sediment distribution based on the composite of all 

samples collected at Sand Creek had a median size of about 

1.4 mm with 16th and 84th percentiles of about 0.4 and 3.0 

mm, respectively. The general distribution of bed sediments at 

the Monument and Sand Creek sites were similar (fig. 7), with 

Sand Creek trending toward a slightly larger percent of particles in the very coarse sand (1 to 2 mm) to very fine gravel 

(2 to 4 mm) size classes.

Suspended-Sediment Data

Concentrations of suspended sediment and sand in samples collected from Monument Creek during a series of storms 

in August 2008 varied temporally in response to changes in 

streamflow (fig. 8). Suspended-sediment concentrations during 

storms were largest at or near peak streamflow. 

The particle-size distributions of the suspended-sediment 

samples for two pumped samples at the Monument Creek 

site (fig. 9) and three pumped samples at the Sand Creek site 

(fig. 10) were collected from a single point in the vertical by 

autosamplers. These sizes are dramatically finer than the bed 

material, reflecting the fact that larger particles cannot generally be put into suspension even at very high streamflows in 

these systems. At the Monument Creek site, a proportion of 

the sediment-particle sizes in suspension are present in the 

bed, so this system is not completely dominated by washload. 

Therefore, a model based on measured bed material should 

have some predictive value. This relation is less evident at the 

Sand Creek site where suspended-sediment size distributions 

were much finer than essentially all of the bed material. This 

may be associated with the fact that the peak streamflows at 

which suspended-sediment size distributions were measured at 

the Sand Creek site was greater than the peak streamflows at 

which suspended-sediment size distributions were measured 

at the Monument Creek site. However, there may be also an 

issue with these measurements related to the length of the 

tubing used on the autosampler (Bent and others, 2001) and 

the sampling efficiency as a function of flow velocity and 

orientation of the autosampler intake nozzles (Winterstein, 

1986). Some researchers have suggested that long tubing runs 

and intake orientation inhibit the collection of sand-size material. As addressed in more detail in the section “Flow Model 

Calibration,” other issues at the Sand Creek site rendered this 

question of little importance, at least for this initial study.

In Monument Creek, the suspended-sediment concentration increased with streamflow (fig. 11). Equation 1 describes 

this log-log relation between streamflow (x) and suspendedsediment concentration (y). This relation is less evident in 

Sand Creek (fig. 12), which appeared to show some increase in 

suspended-sediment concentration with streamflow (eq. 2), but 
Flow and Suspended-Sediment Model Development 13

100

80

60

40

20

0

0 0.063 0.125 0.25 0.5 1 2 4 8 16

Percent finer

Median

EXPLANATION

Bed-sediment sample number

Sieve size, in millimeters

MO1

MO3

MO4

MO5

MO6

MO7

MO8

MO9

M10

M11

M12

MO2

M13

M15

M16

M17

M18

M19

M14

Figure 5. Particle-size distribution data for bed-sediment samples collected in Monument Creek near 

U.S. Geological Survey gage site 07103970. (Sampling locations are shown in figure 3.) 
14 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 6. Particle-size distribution data for bed-sediment samples collected in Sand Creek near U.S. Geological Survey gage 

site 07105600. (Sampling locations are shown in figure 4.) 

100

80

60

40

20

0

0 0.063 0.125 0.25 0.5 1 2 4 8 16

Percent finer

Median

EXPLANATION

Bed-sediment sample number

Sieve size, in millimeters

SO1

SO3

SO4

SO5

SO6

SO7

SO8

SO9

S10

S11

S12

SO2
Flow and Suspended-Sediment Model Development 15

Figure 7. Particle-size distribution for bed sediments for composite of all samples collected in 

Monument Creek near U.S. Geological Survey gage site 07103970 and Sand Creek near U.S. Geologcial 

Survey gage site 07105600.

100

80

60

40

20

0

0 0.063 0.125 0.25 0.5 1 2 4 8 16

Sieve size, in millimeters

Percent finer

Median

84th percentile

 16th percentile

EXPLANATION

Monument Creek

Sand Creek
16 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 8. Streamflow, suspended-sediment, and sand concentrations in samples collected from the Monument 

Creek site during August 2008.

08/06/2008 08/10/2008 08/14/2008 08/18/2008

Streamflow, in cubic meters per second

0

5

10

15

20

25

30

Suspended-sediment concentration,

in milligrams per liter 

0

1,000

2,000

3,000

4,000

5,000

6,000

Streamflow

Upstream suspended-sediment concentration

Downstream suspended-sediment concentration

08/06/2008 08/10/2008 08/14/2008 08/18/2008

Streamflow, in cubic meters per second

0

5

10

15

20

25

30

Sand concentration, in milligrams per liter

0

500

1,000

1,500

2,000

Streamflow

Upstream sand concentration

Downstream sand concentration

EXPLANATION

EXPLANATION
Flow and Suspended-Sediment Model Development 17

Figure 9. Particle-size distribution data for suspended-sediment samples collected at a single point 

in the vertical in Monument Creek near U.S. Geological Survey gage site 07103970 (location is shown in 

figure 3.)

Particle size, in millimeters

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Percent finer

0

20

40

60

80

100

08/05/2008

08/16/2008

EXPLANATION

Figure 10. Particle-size distribution data for suspended-sediment samples collected at a single point in 

the vertical in Sand Creek near U.S. Geological Survey gage site 07105600 (location is shown in figure 4.)

Particle size, in millimeters

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Percent finer

0

20

40

60

80

100

08/14/2008

09/12/2008

09/12/2008

EXPLANATION
18 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 11. Measured suspended-sediment concentrations of samples collected at a single vertical 

location in Monument Creek near U.S. Geological Survey gage site 07103970 as a function of streamflow.

Stream discharge, in cubic meters per second

0.01 0.1 1 10 100

Suspended-sediment concentration, in milligrams per liter

1

10

100

1,000

10,000

log10 suspended-sediment concentration = 2.361 + 1.119 (log10 streamflow),

Coefficient of determination (r2) = 0.675
Flow and Suspended-Sediment Model Development 19

Figure 12. Measured suspended-sediment concentrations of samples collected at a single vertical 

location in Sand Creek near U.S. Geological Survey gage site 07105600 as a function of streamflow.

Stream discharge, in cubic meters per second

1 10 100

Suspended-sediment concentration, in milligrams per liter

1,000

10,000

100,000

log10 suspended-sediment concentration = 3.645 + 0.2115 (log10 streamflow),

coefficient of determination (r2) = 0.131
20 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

also showed similar concentrations across the range of measured streamflows. Agreement with the measured suspendedsediment particle-size distributions suggested that washload 

may be dominant at Sand Creek, at least for the streamflows 

measured. If this is the case for the range of streamflows 

at Sand Creek, it is doubtful that the proposed method will 

be successful for that situation. However, at higher streamflows, the proposed method may be appropriate where more 

of the actual bed material can be put into suspension by the 

streamflow. 

 

 log10y = 2.361 + 1.119(log10 x), r2 = 0.675 (1) 

 

 log10y = 3.645 + 0.2115(log10 x), r2 =0.131 (2) 

Flow Modeling

Flow-model development and results are described in this 

section. For each streamflow of interest (high-flow events with 

pumped suspended-sediment samples), stage for that streamflow is used as a boundary condition, and the bed roughness 

was set by a combination of the measured particle sizes on the 

bed and calibration. 

Flow-Model Calibration

Typically, in cases where channel roughness is dominated 

by grain roughness rather than bedforms or other bed features, 

roughness is set in terms of the roughness length, z0 (Middleton and Southard, 1984). This parameter is directly related to 

particle size using empirical results. For very well sorted, level 

beds, the value is typically set as 0.033 times the median particle size (Nikuradse, 1933). For more natural, poorly sorted 

beds with some microscale topography, z0 is usually found 

empirically to lie in the range of 0.05 to 0.5 times the median 

particle size. In some cases, researchers (Whiting and Dietrich, 

1991) relate the roughness length to the 84th percentile of the 

bed particle-size distribution, with multipliers ranging from 

0.03 to 0.2 for typical beds.

Monument Creek

For the case of Monument Creek, predictions for z0 

ranged from 0.0001 to 0.001 m based on median particle size, 

or 0.0002 to 0.0014 m based on the 84th percentile of the bed 

particle size. In order to set z0 more precisely, the FaSTMECH 

model was run with a range of roughnesses for the streamflow 

measured during the ground-surface survey (which included 

water-surface elevations, primarily at edge-of-water). The 

predicted water-surface elevations were compared to the measured ones and the RMSE (departure of predicted value from 

true value) (Helsel and Hirsch, 1992) was computed for each 

choice of roughness. The result of this process is shown in 

figures 13A and 13B, which shows the RMSE as a function of 

the choice of roughness length, z0. The minimum error (best 

prediction) was found for a choice of z0 of 0.0003 m for the 

upper reach and 0.004 m for the lower reach. The calibrated 

value for the upper reach lies well within the literature values. 

The higher roughness length of the lower reach is a result of 

coarse riffle area composed of large cobbles and small boulders, not sampled as part of the mobile bed, near the downstream end of the reach. Model predictions for water-surface 

elevation compared to those measured are shown in figures 

14A and 14B.

Sand Creek

There was no value of roughness that gave an acceptable 

fit at the Sand Creek site. As previously mentioned in the 

section “Methods of Investigation,” there was insufficient 

water present in the channel at the Sand Creek site when the 

channel was surveyed to define water-surface elevations. In 

order to get water-surface elevations for calibration, two 

pressure transducers were installed to measure stage during 

periods of stormflow. However, the morphological characteristics of the bed were altered during the first substantial 

stormflow event during the first year of the study; a maximum 

discharge of 39 m3/s occurred on August 16, 2008. Additionally, the downstream transducer was lost during a substantial 

streamflow event (maximum discharge of 101 m3/s), which 

occurred on September 12, 2008. Streamflows during the 

September 11–12, 2008, storm further modified the morphological characteristics of the bed. During these two flow 

events, large areas of the bed scoured by more than 0.3 m, 

whereas in other areas of the channel, deposition raised the 

bed by more than 0.3 m. Transducers were reinstalled for the 

second year, and some stage data were collected. However, 

when the calibration process was attempted, no reasonable 

choice of roughness gave an acceptable fit to the measured 

water-surface elevations. It appears that this was associated 

with changes in the bed morphology, so that the original 

surveyed elevations were not representative of the channel 

bed when the water-surface elevations were measured. Further 

analysis at this site was discontinued for this study because of 

the problem in obtaining representative water-surface elevations, changes in channel geometry, the loss of the downstream 

transducer, and potential issues with the measured suspendedsediment concentrations due to the length of the intake tube. 

Future study at the Sand Creek site and (or) sites similar to 

Sand Creek with the objective of developing streamflow and 

sediment transport models may require pre- and post-storm 

channel surveys in order to account for dynamic changes in 

bed morphology over the study period.

Flow-Model Results

A complete data set for the analysis presented in this 

report was collected at the Monument Creek site and is the 

focus of the remaining report. The Monument Creek site was 

split into a lower and upper reach, with the division at the 
Flow and Suspended-Sediment Model Development 21

Figure 13. Root-mean-squared error between the measured and predicted 

water-surface elevation as a function of roughness at (A) the upper Monument 

Creek reach and (B) the lower Monument Creek reach. 

Roughness length, meters

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016

Root-mean-squared error

0.020

0.025

0.030

0.035

0.040

Roughness length, meters

0.000 0.004 0.008 0.012 0.016 0.020

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

A

B

Root-mean-squared error
22 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 14. Measured and predicted water surface as a function of streamwise 

distance in the downstream direction for the calibrated value of roughness at (A) the 

upper Monument Creek reach and (B) the lower Monument Creek reach.

Downstream distance, in meters

0 50 100 150 200 250 300

Water-surface elevation, in meters

1907.0

1907.5

1908.0

1908.5

1909.0

1909.5

Downstream distance, in meters

0 50 100 150 200 250 300

1907.0

1907.5

1908.0

1908.5

1909.0

1909.5

A

B

Observed water-surface elevation

Predicted water-surface elevation

EXPLANATION

Water-surface elevation, in meters
Predicted Suspended-Sediment Concentrations 23

GCS present in the channel near the midpoint of the survey 

(fig. 3). An example of the model predictions for vertically 

averaged velocity and bed shear stress at the upper Monument 

Creek site are shown in figures 15 and 16 for the two stormevent discharges during which pumped samples of suspended 

sediment were collected (August 5 and 16, 2000). These 

predictions were computed using the measured stage for these 

events and the calibrated roughness value discussed in section 

“Flow Model Calibration.” Velocity and bed shear stress are 

the principal outputs of the model used to drive the suspended 

sediment computations. The bed shear stress is used to compute the reference concentration for the Rouse model, and the 

predicted velocities (including their vertical structure) are used 

with the predicted concentration profiles to compute loads in 

the channel. 

Predicted Suspended-Sediment 

Concentrations

As previously noted, a substantial amount of the suspended material in samples collected from the upper and lower 

reach of the Monument Creek site was finer than the particle 

sizes found in the bed. Thus, a Rouse prediction based only 

on the bed material sizes and local bed shear stress cannot 

accurately predict the total suspended-sediment concentration. 

Figures 17A and 17B show sand concentration and silt concentration as a function of discharge for the pumped samples 

collected during storm-flow events at the Monument Creek 

site (fig. 8). The sand-silt demarcation is taken at 63 microns 

(0.063 mm) and corresponds well to the size division between 

suspended sediment absent from and present in the bed (see 

figs. 5 and 8). Figures 17A and 17B show that sand concentrations in both the upper and lower Monument Creek reaches 

tend to increase with streamflow, but the silt concentration is 

essentially independent of streamflow over these storm-flow 

events, with an average concentration of 880 milligrams per 

liter (mg/L) at the upstream site and 1,062 mg/L at the downstream site.

The tendency for sand concentrations to increase with 

streamflow, whereas silt-and-clay concentrations are independent of streamflow, has significant implications on choosing 

which of the three methods for computing the suspendedsediment concentrations are most appropriate. The first Rouse 

method has the advantage that it does not require calibration 

data; however, suspended sediment must be measured because 

washload cannot be estimated from the bed material. Thus, 

one of the critical conclusions is that measurement of suspended sediment is required because of the quantity of washload in these systems. Therefore, there is no particular advantage in using this first Rouse method over the second Rouse 

method (with reference concentration equation calibrated 

with field data). At least a minimal field data set must be collected to determine washload, which also indicates that data 

are available for calibration of the reference concentration of 

larger sizes. Furthermore, because of the uncertainty in washload specification (a range of about a factor of two; figs. 17A

and 17B), the additional accuracy and modeling complexity of 

the advection-diffusion solution method is of little value. This 

is true because any additional accuracy in the sand concentration obtained through an advection-diffusion method will be 

overwhelmed by the uncertainty in washload. Therefore, of the 

three methods used in this study, the most reasonable approach 

is to use the second Rouse method for computing sand 

concentrations (calibrated by a few measurements or as many 

measurements as required to get a stable calibration) and add 

in washload as measured by those same field measurements. 

This provides the best balance between desired accuracy and 

the feasibility of carrying out the approach across the Fountain 

Creek watershed.

The Rouse method with reference concentration equation 

calibrated with field data was carried out for both the upper 

and the lower reaches of the Monument Creek site. The calibrated values and the average for the various pumped samples 

are shown in tables 2 and 3 for the upper and lower reaches, 

respectively. At the upper reach (table 2), the average calibrated gamma value was found to be 0.00347, the median was 

0.00055, and gamma values ranged from 0.00022 to 0.015. At 

the lower reach (table 3), the average value was found to be 

0.00296, the median was 0.00275, and gamma values ranged 

from 0.0014 to 0.006. Although this is a very broad range, it is 

in agreement with previous studies. For example, Harris and 

Wiberg (2001) found values of gamma ranging from 10–5 to 

10–3, where the lower values (10–5 to 10–4) are associated with 

continental shelf sediments composed of fine sand and silt, 

and the higher values (10–4 to 10–3) are associated with sandbedded rivers and flumes. Using a very limited data set, Smith 

and McLean (1977) found a gamma of 0.0024. The average 

value of gamma from both the upper and lower reaches of the 

Monument Creek site of 0.0032 is consistent with the range 

of literature values, especially the higher end values associated with sand-bedded rivers. The roughly similar values of 

gamma between the upper and lower reaches provides a good 

test of the methodology. The predictions of sand concentration 

as measured at the autosampler nozzle at both the upper and 

lower reaches using the calibrated average gamma from the 

lower reach to predict the upper reach and similarly, the upper 

reach to predict the lower reach (fig. 18) yields satisfactory 

results. 

The predicted mean sand concentrations throughout the 

study reach using the calibrated gamma are shown in figure 

19 for two of the storms in Monument Creek during the study 

period. In addition, because the concentration is predicted at 

each spatial node in the FaSTMECH model grid, it is possible 

to integrate these concentrations with local velocities in order 

to compute loads. In figure 20 the sand load computed at each 

model cross section is presented, whereas the total load (incorporating the measured washload material in a simple additive 

form) is shown in figure 21. There is some spatial variability 

in the predicted load, from upstream to downstream, suggesting that the bed may scour around the middle of the reach 
24 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 15. Predicted stream velocities in the upper Monument Creek study reach at (A) 2.52 cubic 

meters per second and (B) 11.52 cubic meters per second.

A

B

Velocity, in

meters per

second

Velocity, in 

meters per 

second

Imagery: USGS orthophotography, 0.3 meter pixels, 2008 photography. 

Projection: UTM zone 13, North American Datum of 1983

3.39

3.16

2.94

2.71

2.49

2.26

2.03

1.81

1.58

1.36

1.13

0.90

0.68

0.45

0.23

0.00

3.39

3.16

2.94

2.71

2.49

2.26

2.03

1.81

1.58

1.36

1.13

0.90

0.68

0.45

0.23

0.00

0

0

20 40 80 METERS

100 200 300 FEET

60

N

EXPLANATION

EXPLANATION
Predicted Suspended-Sediment Concentrations 25

Figure 16. Predicted bed shear stress at the upper Monument Creek study reach at (A) 2.52 cubic 

meters per second and (B) 11.52 cubic meters per second.

Shear stress, in 

Newtons per 

square meter 

Shear stress, in 

Newtons per 

square meter 

Imagery: USGS orthophotography, 0.3 meter pixels, 2008 photography. 

Projection: UTM zone 13, North American Datum of 1983

A

B

0

0

20 40 80 METERS

100 200 300 FEET

60

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

EXPLANATION

N

EXPLANATION
26 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 17. The measured suspended-sediment concentrations in the (A) upper reach of the Monument 

Creek site and (B) lower reach of the Monument Creek site.

B

Stream discharge, in cubic meters per second

0.1 1 10 100

1

10

100

1,000

10,000

A

Suspended-sediment concentration, in milligrams per liter

1

10

100

1,000

10,000

Sand concentration

Silt/clay concentration

Average silt/clay concentration

EXPLANATION
Predicted Suspended-Sediment Concentrations 27

[m3/s, cubic meters per second; mg/L, milligrams per liter]

Stream 

discharge

(m3/s)

Total concentration (mg/L)Sand concentration (mg/L)Silt and clay concentration (mg/L) Calibrated gamma

Predicted sand 

concentration, 

average gamma 

(mg/L)

Predicted sand 

concentration 

using the average gamma for 

the lower reach 

of the Monument 

Creek site 

(mg/L)

0.79 872 64.7 807 0.015 14.8 13.0

1.95 472 12.2 460 0.00023 176 153

1.98 570 53 517 0.001 159 152

2.52 1,580 906 676 0.0126 254 221

2.72 498 23.9 474 0.00041 196 170

3.54 496 27.6 468 0.00022 416 363

3.94 957 79.2 877 0.00031 827 721

4.19 2,410 273 2,140 0.004 1,320 1,150

4.42 908 50.9 858 0.00026 644 562

11.8 2,360 843 1,520 0.00067 3,900 933

 Average 879.7 0.00347

 Median 741.5 0.00055

Table 2. Calculated gamma for each measured sand concentration in the upper subreach of the Monument Creek site.

[m3/s, cubic meters per second; mg/L, milligrams per liter]

Stream 

discharge

(m3/s)

Total concentration (mg/L)Sand concentration (mg/L)Silt and clay concentration (mg/L) Calibrated gamma

Predicted sand 

concentration, 

average gamma 

(mg/L)

Predicted sand 

concentration 

using the average 

gamma for the 

upper reach of the 

Monument Creek 

site 

(mg/L)

 1.95 610 159 451 0.0029 163 190

 1.98 822 265 557 0.006 135 157

 2.72 928 345 583 0.0031 318 370

 3.54 1,170 583 583 0.0016 1,050 1,220

 3.94 2,200 954 1,250 0.0025 1,230 1,420

 4.19 5,040 1,830 3,210 0.0042 1,340 1,550

 4.42 1,540 689 848 0.0014 1,460 1,680

 4.63 2,040 1,030 1,010 0.0019 1,550 1,800

Average 1,061.5 0.00296

Median 715.5 0.00275

Table 3. Calculated gamma for each measured sand concentration in the lower subreach of the Monument Creek site.
28 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

1

10

100

1,000

10,000

Stream discharge, in cubic meters per second

0.1 1 10 100

Suspended-sand concentration, in milligrams per liter

1

10

100

1,000

10,000

Sand concentration

Predicted sand concentration

EXPLANATION

Suspended-sand concentration, in milligrams per liter

A

B

Figure 18. Predicted sand concentration at the (A) upper reach of the Monument Creek site using the average 

calibrated gamma (0.003) from the lower reach and (B) lower reach of the Monument Creek site using the 

average calibrated gamma (0.0035) from the upper reach.
Predicted Suspended-Sediment Concentrations 29

Figure 19. Mean suspended-sand concentrations at the upper Monument Creek reach for 

(A) a storm-flow event at 2.52 cubic meters per second and (B) a stormflow event at 11.52 

cubic meters per second.

Imagery: USGS orthophotography, 0.3 meter pixels, 2008 photography. 

Projection: UTM zone 13, North American Datum of 1983

7.19

6.71

6.22

5.73

5.27

4.79

4.30

3.84

3.35

2.87

2.41

1.92

1.43

0.95

0.49

0.00

7.7

7.2

6.7

6.1

5.6

5.1

4.6

4.1

3.6

3.1

2.6

2.0

1.5

1.0

0.5

0. 0

Suspended-sand 

concentration in 

milligrams per liter (x103

)

Suspended-sand 

concentration in 

milligrams per liter (x103

)

A

B

0

0

20 40 80 METERS

100 200 300 FEET

60

7.7

7.2

6.7

6.1

5.6

5.1

4.6

4.1

3.6

3.1

2.6

2.0

1.5

1.0

0.5

0.0

7.7

7.2

6.7

6.1

5.6

5.1

4.6

4.1

3.6

3.1

2.6

2.0

1.5

1.0

0.5

0.0

7.19

6.71

6.22

5.73

5.27

4.79

4.30

3.84

3.35

2.87

1.92

1.43

0.95

0.49

0.00

2.41

EXPLANATION

EXPLANATION

N
30 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Figure 20. Suspended-sand load computed at each section in the modeling domain for the upper Monument Creek reach 

for two sampled storm-flow events.

Distance downstream from beginning of reach, in meters

0 50 100 150 200 250

Suspended-sand load, in kilograms per second

0.1

1

10

100

11.52 cubic meters per second

2.52 cubic meters per second

EXPLANATION

Distance downstream from beginning of reach, in meters

0 50 100 150 200 250

Total load, in kilograms per second

1

10

100

11.52 cubic meters per second

2.52 cubic meters per second

EXPLANATION

Figure 21. Total load computed at each section in the modeling domain for the upper Monument Creek 

reach for two sampled storm-flow events.
Implications for Future Work 31

during storms, but generally a simple streamwise average of 

these cross-sectional computed loads can be used to construct 

load-discharge curves. 

Another test of the second Rouse method is to verify that 

the model predicts the distribution of size classes in suspension accurately. Figure 22 shows a comparison between the 

predicted and measured particle-size distributions in suspension for the two sampled storms. Given the simplicity of the 

method and the variability in the bed particle sizes (fig. 5), the 

predictions provide reasonable predictions of both the fraction 

and particle size.

A final test of the method is a comparison of both predicted cross-sectionally averaged suspended sand and total 

concentration in the upper Monument Creek site with measured total suspended-sediment concentrations collected using 

the Equal Width Increment (EWI) (U.S. Geological Survey, 

2006) method between 2001–2008. Figure 23A shows the 

EWI sample suspended-sediment concentrations collected as 

a function of streamflow. There is an upward trend in total 

suspended-sediment concentration with increase in streamflow, although there is at least an order of magnitude variation 

in the concentration for any given value of the streamflow. 

The predicted sand concentrations using the Rouse method 

described in the “Suspended Sediment Modeling” section 

above, and using the calibrated gamma value for the upper 

Monument Creek site, are also shown for a set of variable 

particle-size fractions of the bed sediments (fig. 23B) based on 

five particle-size distributions (1–5 in fig. 23A). The predicted 

sand concentrations fall within the higher end of the range of 

measured total suspended-sediment concentrations using the 

average particle-size distribution calculated from all 10 of the 

measured particle-size samples at the upper Monument Creek 

reach (distribution 1 in fig. 23). The prediction was for sand 

concentration, and the EWI suspended-sediment sample measurements report total concentration, and it is expected that 

the prediction will fall within the lower end of the measured 

range of total concentrations for each streamflow because 

the silt-and-clay fraction is not predicted. To investigate the 

sensitivity of the calculation to the measured particle size, 

three other particle-size distributions (distributions 2–4 in fig. 

23A) were calculated as follows: (2) the average particle-size 

distribution calculated from all nine main channel samples 

(the one sample collected on the top of the island in the upper 

Monument Creek reach that had a much higher percentage of 

smaller particle sizes compared with the other nine samples 

collected in the main channel was not used to calculate the 

average), (3) the coarsest sample of the 10 collected bed 

samples, and (4) the finest grain-size sample of the 10 collected bed samples. Using (2) and (3) described above results 

in only a small change to the particle-size distribution used to 

predict the suspended-sand concentration (fig. 23B) but results 

in predicted suspended-sand concentrations that fall within 

the lower range of the measured concentrations as would be 

expected. Using (3), the finest fraction, results in prediction 

of unrealistically high sand concentrations. The prediction is 

sensitive to the specification of the particle-size distribution of 

the bed material and suggests that care should be taken to collect bed material samples that are representative of the active 

bed during the likely flow events to be simulated. A final 

calculation (distribution 5 in fig. 23) was made by predicting 

the suspended-sand concentration using the average particlesize distribution of the collected main channel sediments and 

adding the average measured silt-and-clay concentration as a 

constant value throughout the water column. The resulting predicted total suspended-sediment concentration falls within the 

bounds of the measured data above a streamflow of 2 m3/s, but 

approaches the value of the constant silt-and-clay concentration below 2 m3

/s. An inspection of the streamflow history during the study (fig. 8) shows that base-flow conditions generally 

exist at less than 1 or 2 m3/s. Assuming that the source of the 

silt-and-clay fraction of the total suspended-sediment load is 

associated with overland flow from rainfall events or access to 

fine sediment deposited during a previous high-flow event on 

the floodplain or high banks, some accommodation should be 

made for the lack of suspended silt-and-clay during base-flow 

conditions.

Implications for Future Work

Based on what has been learned from this preliminary 

study, the recommended procedure for using a model-based 

methodology to estimate suspended-sediment loads in the 

Fountain Creek watershed is a calibrated Rouse method in 

conjunction with the FaSTMECH flow model. Detailed surveys would be made at each field site and a single autosampler 

would be installed. Additionally, bed-material samples would 

be collected throughout the reach (approximately 10 samples 

depending on observed spatial variability). If there is a 

streamgage nearby, this could be used for stage-discharge data, 

otherwise, a temporary pressure transducer and some streamflow measurements would be required. The autosampler would 

be used to collect suspended-sediment samples. The data from 

the suspended-sediment samples would be used to calibrate 

the reference concentration at the location of the sampler, 

and then that value would be used in the model to estimate 

suspended-sediment loads through the reach. These estimates 

could also be used to develop load-discharge curves. [A simple 

streamwise average of these cross-sectional computed loads 

should be used to construct load-discharge curves. Fine-tuning 

a curve for the suspended silt-clay fraction would benefit from 

a more complete set of samples collected through the autosampler over a wider range of streamflows.] Washload would 

be estimated as a function of streamflow (if warranted) based 

on the data from samples collected by the autosampler used 

to calibrate the reference concentrations for sand. If this basic 

methodology was followed at most significant junctions in 

the watershed, an overall view of the sources of sediment and 

potential effects of focused remediation could be developed.
32 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

0.00001 0.0001 0.001

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Fraction of total concentration

Grain size, in millimeters

Fraction of total concentration

Measured

Predicted

EXPLANATION

2.52 cubic meters per second

11.52 cubic meters per second

Figure 22. The measured and predicted sand fraction as a function of particle size for each of two 

sampled storm events.
Implications for Future Work 33

Stream discharge, in cubic meters per second

0.01 0.1 1 10 100

Suspended-sand concentration, in milligrams per liter

0.1

1

10

100

1,000

10,000

100,000

1,000,000

Grain size, in millimeters

0.01 0.1 1 10

Fraction of sample

0.0

0.1

0.2

0.3

0.4

0.5

0.6

B

A

All samples fraction by size class (1) 

All main channel samples fraction by size class (2)

Coarsest sediment sample (3) 

Finest sediment sample (4) 

EXPLANATION

Equal-width-increment suspended-sediment concentration

(1) the mean particle-size distribution of all 10 measured bed-sediment samples

(2) the mean particle-size distribution of all nine main channel bed-sediment samples

(3) the coarsest particle-size distribution 

(4) the finest particle-size distribution

(5) the particle-size distribution of the nine main channel bed-sediment samples and adding a 

EXPLANATION

constant (844.8 milligrams per liter) silt-and-clay fraction based on the average silt-and-clay 

fraction measured.

Figure 23. (A) The measured total suspended-sediment concentration using the Equal Width Increment (EWI) method 

for all samples collected at the upper Monument Creek reach from 2001–08 and the mean predicted suspendedsand concentration (particle-size distributions 1–5), and (B) Particle-size fractions for the predicted particle-size 

distributions 1–4 described in (A).
34 Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09

Summary

In 1998, the USGS began a small-scale pilot study to 

evaluate the effectiveness of a computational model of flow 

and suspended-sediment transport for predicting suspendedsediment concentrations and loads in the Fountain Creek 

watershed in Colorado. This study was conducted in cooperation with Pikes Peak Area Council of Governments, Colorado 

Water Conservation Board, Colorado Springs City Engineering, and the Lower Arkansas Valley Water Conservancy 

District. The Fountain Creek Corridor Restoration Master Plan 

identified erosion and sedimentation damage as key problems 

within the basin, and a recommendation for management of 

the basin from the plan is to establish measurable criteria to 

determine if progress in reducing erosion and sedimentation damage is being made. This report describes efforts to 

develop, calibrate, and validate a multidimensional model 

as an inexpensive tool to predict suspended-sediment loads 

at various locations within the Fountain Creek watershed in 

southeastern Colorado. Two locations in the Fountain Creek 

watershed, Monument Creek above Woodmen Road and Sand 

Creek near the mouth, were selected as study sites. 

Suspended-sediment samples were collected using 

automatic-pumping samplers located at USGS gages in each 

study reach and at the downstream end of each study reach. 

Stage was measured at USGS gages, and pressure transducers 

were located in the study reaches. The geometry of the study 

reaches were surveyed to a horizontal accuracy of ±1 cm and a 

vertical accuracy of ±2 cm using RTK GPS equipment. 

The USGS MultiDimensional Surface-Water Modeling 

System was used to evaluate cost of development and accuracy of three models: (1) Rouse method with Smith/McLean 

reference concentration, (2) Rouse method with referenceconcentration equation calibrated with field data, and (3) 

advection-diffusion solution. 

Bed-sediment samples were collected to characterize 

the size distribution and spatial variation of particle sizes of 

sediment on the beds in the two study reaches. These data 

were used to develop and (or) calibrate the flow and sediment 

transport models.

Bed-sediment particle sizes at both locations were 

dominated by sand- and gravel-size particles. Changes in bed 

geometry as a result of scour and fill of bed sediments of ±0.3 

m in the Sand Creek reach during storm flows altered the initial boundary conditions of the model. Subsequently, calibration of the flow model was not possible; therefore, no further 

analyses were attempted for this reach. Attempts to develop a 

model for the Sand Creek site and (or) similar sites may necessitate pre- and post-storm channel surveys given the dynamic 

nature of the bed and the implications for model calibration. 

At the Monument Creek site, the presence of a grade-control 

structure near the center of the study reach necessitated the 

splitting of the reach into two subreaches, an upper and lower 

reach. 

At the Monument Creek site, a portion of the suspended 

particles were finer than particle sizes found in bed sediments. 

Therefore, the Rouse method with Smith/McLean reference 

concentration cannot accurately predict the total suspendedsediment concentration based only on bed-material sizes and 

local bed shear stress. Therefore, because a minimal amount of 

field data would be required to determine washload, the Rouse 

method with reference-concentration equation calibrated 

with field data would have some predictive power. Also, the 

additional accuracy and model complexity of the advectiondiffusion model is of little value due to the uncertainty in the 

washload concentration. Data collected in the lower reach 

were used to validate the model developed from data collected in the upper reach. Results of model predictions of flow 

velocity and sediment transport are compared to velocity and 

sediment data for two storm streamflows, 2.52 m3/s and 11.52 

m3/s, for which suspended-sediment data were collected.

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For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Stogner and others— Suspended-Sediment Concentrations, Fountain Creek Watershed, Colorado, 2008–09—Scientific Investigations Report 2012–5102