U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Field measures of gravel quality in the South Platte River

north of Denver, Colorado: a pilot study

by

David A. Lindsey1 and John F. Shary1

Open-File Report 97-451

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey

editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or

firm names is for descriptive purposes only and does not imply endorsement by the U. S.

Government.

This report can be downloaded via ‘anonymous ftp’ from a USGS system named

greenwood.cr.usgs.gov (136.177.21.122). The file is located in a directory named /pub/open-filereports/ofr-97-0451.

1USGS Denver CO 80225
CONTENTS

Summary 1 Summary 

Purpose and limitations of study 1 Purpose and limitations of study 

Description of gravel bars and relationship to terrace deposits 3 Description of gravel bars and relationship to terrace deposits 

Sampling and methods 4 Sampling and methods 

Results 6 Results 

Analysis of variance 14 Analysis of variance 

Suggestions for future work 17 Suggestions for future work 

References cited 18 References cited 

LIST OF FIGURES

Figure 1.—Location of study area and valley of South Platte River north of Denver, Colorado, showing sample

sites and principal terrace levels (terraces modified from Colton, 1978, and Trimble and Machette, 1979).

Figure 2.—Sketches illustrating gravel bars in the South Platte River. A) Cross-section of a gravel bar; B) Plan

view of successive gravel bars. Not to scale.

Figure 3.—Sampling design for gravel bars at two sites on the South Platte River near Welby and Ft. Lupton,

Colorado.

Figure 4.—Cumulative frequency distributions showing particle size for gravel bars at A) station 30, Welby, B)

station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado.

Figure 5.—Pie diagrams showing pebble lithology (0.75-2 inch sieve fraction) for gravel bars at A) station 30,

Welby, B) station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each pie

combines counts of 50 pebbles each for 3 samples (total of 150 pebbles).

Figure 6.—Zingg diagrams showing pebble shape (0.75-2 inch sieve fraction) for gravel bars at A) station 30,

Welby, B) station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each diagram

combines measurements of 50 pebbles each for 3 samples (total of 150 pebbles).

Figure 7.—Histograms showing pebble sphericity (Ψ) for gravel bars at A) station 30, Welby, B) station 31,

Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each histogram combines

calculations on measurements of 50 pebbles each for 3 samples (total of 150 pebbles).

Figure 8.—Histograms showing distribution of pebble roundness for gravel bars at A) station 30, Welby, B)

station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each histogram

combines roundness of 50 pebbles each for 3 samples (total of 150 pebbles).

i
LIST OF TABLES

Table 1.—Location and lengths of gravel bars sampled in the South Platte River north of Denver, Colorado.

Table 2.—Sieve sizes used for particle-size analysis of gravel.

Table 3.—Criteria used for assigning pebbles to lithologic categories.

Table 4.—Particle-size parameters for gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado.

Table 5.—Lithology of pebbles in gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado.

Table 6.—Mean shape of pebbles in gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado.

Table 7.—Roundness of pebbles in gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado.

Table 8.—Four-level analysis of variance of particle-size parameters for gravel bars of the South Platte River at

sites near Welby and Ft. Lupton, Colorado.

Table 9.—Four-level analysis of variance of pebble lithology in gravel bars of the South Platte River at sites

near Welby and Ft. Lupton, Colorado.

Table 10.—Four-level analysis of variance of pebble shape parameters in gravel bars of the South Platte River

at sites near Welby and Ft. Lupton, Colorado.

Table 11.—Four-level analysis of variance of pebble roundness in gravel bars of the South Platte River at sites

near Welby and Ft. Lupton, Colorado.

ii
U. S. GEOLOGICAL SURVEY OPEN-FILE REPORT

FIELD MEASURES OF GRAVEL QUALITY IN THE SOUTH PLATTE RIVER

NORTH OF DENVER, COLORADO: A PILOT STUDY

by David A. Lindsey and John F. Shary

Summary

This pilot study of gravel in the bed of the South Platte River north of Denver was undertaken to identify

field measures of aggregate quality and sampling methods that would be useful in modeling and assessing

alluvial gravel deposits in the Front Range Urban Corridor. Gravel bars in the streambed were selected for initial

study because they represent a uniform depositional environment that is readily accessible for sampling.

Although gravel is not mined from the bed of the South Platte, the terraces that serve as the source of gravel

for the streambed are mined extensively. Two sites, one upstream near Welby (78th Avenue) and one 18

miles downstream near Ft. Lupton, were selected to study the downstream range of gravel aggregate quality

in the segment of the South Platte valley that is currently being mined north of Denver. A four-level sampling

design was used to allow measurement of variation over various distances within the river gravel. Particle size

was determined by sieving, the lithology of pebbles was identified, and shape and rounding of pebbles was

measured.

 Analysis of variance revealed measures of particle size, shape, and roundness that have downstream

(between-site) variation in the South Platte and could be used to map gravel quality. Among measures of

particle size, the cumulative weight percent of particles >4 mm shows a higher proportion of downstream

variation than any other parameter. Percent pegmatite is the only measure of pebble lithology that probably

could be mapped downstream. Additional sampling, using an optimal sampling plan, would be required to

confirm indications of the pilot study, that large particle size diminishes downstream, the percentage of

pegmatite among pebbles increases, the proportion of spherical (equidimensional) pebbles increases, and

pebble roundness decreases downstream. Many other measures have high variation between successive gravel

bars (spaced about 600-900 feet apart); with proper sampling design, they could be measured and mapped at

the scale of individual gravel pits but not regionally within a drainage. A few measures, including particle

sorting and pebble lithology, have mostly local (between-sample) variance and cannot be mapped at any useful

scale.

Purpose and

limitations of study

The South Platte River valley

north of Denver is the last major

source of gravel aggregate

remaining in the metropolitan area

(Schwochow, 1980); in 1995-96

the valley north of Denver was the

site of 15 active permitted mine

operations for sand and gravel

(Hemborg, 1996). As production

moves down the valley, the quality

of gravel produced from the valley

will change. Downstream changes

in gravel aggregate quality along

the South Platte River are not only

of interest to gravel producers in

the area but are also relevant to

urban planning. An understanding

of these changes may enable

better prediction of the potential

downstream limit of gravel

production and of post-mining land

uses.

This pilot study of gravel in

the bed of the South Platte River

north of Denver (Figure 1) was

undertaken to identify field

measures of aggregate quality and

sampling methods that would be

useful in modeling and assessing

alluvial gravel deposits of the

South Platte River and other

streams of the Front Range Urban

Corridor. Field study methods for

gravel aggregate quality are timeconsuming; efficient measures of

quality and optimal sampling

designs were sought to facilitate

rapid study. A secondary

objective of the study was to

gather data that could be

compared with gravel aggregate

quality in adjacent stream terraces

and assess the utility of using

stream gravel as a sampling

medium for estimating quality of

aggregate in terrace deposits.

Although gravel is not mined

from the bed of the South Platte,

the terraces that serve as the

source of gravel for the stream

bed are mined extensively. The

gravel bars of the river offer some

advantages for study, in that they

represent a uniform, observable

depositional environment that is

readily accessible. In contrast,

study of terrace deposits requires

excavations, which are not

everywhere present, and the

deposits themselves require

stratigraphic study before they
can be sampled properly.

Additionally, the approach of

sampling streambed gravel for

assessing aggregate quality could be

used where no gravel is currently

mined. The principal disadvantage of

sampling streambed gravel as a proxy

for terrace gravel is the possibility of

sampling bias. Sampling bias in

streambed gravel may be caused by

the confinement of sampling to layers

near the surface and by the

possibility that the characteristics of

stream gravel have been affected by

human activities and structures or by

natural environmental change. If

gravel in adjacent terraces was

deposited long ago, under different

environmental conditions of climate

and stream size, then it may not

share all of the quality characteristics

of modern stream gravel.

The data gathered for this pilot

study are limited to twelve samples

taken from four gravel bars at two

sites. This level of sampling was

sufficient to provide useful measures

of the general quality of gravel

aggregate in the area studied, to

identify useful measures for field

studies of gravel aggregate, and to

guide optimization of field sampling

design. However, the small number

of samples studied severely limits the

precision of estimates of mean values

and tests for differences among

sampling sites. Also, for the small

number of samples taken in the pilot

study, correlations among various

measures are prone to random

effects; hence, no detailed analysis of

correlation was undertaken.

105° 104° 52' 30" 104° 45'

40°

07'

30"

40°

39°

52'

30"

40°

15'

0 5 MILES

0 5 KILOMETERS

Welby

Brighton

Ft. Lupton

Stations

32, 33

Stations

30, 31

Platteville

St. Vrain Creek

Sand Creek

Clear Creek

South

Platte River

EXPLANATION

Low terraces

High terraces

Figure 1.—Location of study area and

valley of South Platte River north of

Denver, Colorado, showing sample

sites and principal terrace levels

(terraces modified from Colton,

1978, and Trimble and Machette,

1979).

2
Description of gravel bars

and relationship to

terrace deposits

Gravel bars in the South Platte

River comprise a sample of coarse

sediment in the low terraces of the

South Platte valley. The river

continually reworks terrace

sediment as its channel migrates

laterally across the valley (Figure

2). Since the end of the Pinedale

ice age about 11,000 years ago

(Wayne and others, 1991), the

South Platte has eroded and

reworked the Broadway, Louviers,

and older terrace deposits. By this

process of erosion and

redeposition, the river has formed

a suite of low terraces that are

generally referred to pre-Piney

Creek, Piney Creek, and post-Piney

Creek levels (Scott, 1965). Also,

the low terraces may themselves

be reworked by the river. The

depth of reworking probably varies

locally, but based on bank heights

and exposures in gravel pits, it

appears to be in the range of 5-10

ft in the area studied.

Gravel pits in the low terraces

reveal two ages of alluvial

sediment: 1) older, lower sand and

gravel thought to have been

deposited prior to about 11,000

years ago, and 2) younger,

overlying sand and gravel formed

by post-Pinedale channel

migration, erosion, and

redeposition of sand and gravel.

Commonly, the two ages of alluvial

sediment can be distinguished by

discontinuities in color and grain

size in the walls of gravel pits.

Locally, the younger sand and

gravel beds contain abundant logs.

Radiocarbon dates of logs from the

younger sand and gravel indicate

that some of it was deposited as

recently as 300 years ago or less.

Gravel bars in the river bed

represent the first stage of

redeposition of alluvial sediment

eroded from the river banks

(Figure 2). They form only during

flood stage, when the river leaves

its sinuous low-flow channel and

flows directly across the bars.

During flood stage, the river

carries a coarse bedload of sand,

pebbles, and cobbles, which it

deposits on the bar surface as the

flood ends and the water level

drops. When the channel moves

to another location, the gravel

bars are reincorporated into the

floodplain, where they appear as

beds of gravel in the younger

alluvium. The gravel bars

themselves contain a range of

particle sizes and vary locally from

gravelly to sandy. After a flood,

the surface of the bar may be

covered with coarse gravel, but

sand occupies the interstices

beneath the surface. Pebbles on

the bar surface lie in imbricate

position and are arranged in

patterns of varying particle size

that reflect the direction of flow.

Debris, including logs, may be left

on the surface also. Foresets,

exposed in the eroded flanks of

the gravel bar, represent migration

of the bar as it follows and

backfills channel meanders. All of

these features are seen in gravel

beds of the terraces.

The gravel bars in the South

Platte sampled for this study are

lateral bars (for a discussion of bar

terminology, see Church and

Jones, 1982). Lateral bars are

characteristically attached to

alternate banks of the river at low

flow. A typical lateral bar in the

South Platte consists of a broad

gravel surface separated from the

bank by a small back-channel or

chute that becomes active during

flood stage. Bushes and other

vegetation may grow along the

bank side of the chute, and in

some cases have encroached on

the main part of the bar. At low

flow, the river may undercut the

flank and lower end of the gravel

bar, but modification of the bar is

minor until the next flood. Bars

are also common in the middle of

the channel where the South Platte

assumes a braided pattern; these

mid-channel bars were not sampled

for this study.

Figure 2.--Sketches illustrating gravel bars in the South Platte River.

A) Cross-section of gravel bar; B) Plan view of successive gravel bars.

Not to scale.

3

A) Cross section of gravel bar and river banks

B) Plan view of gravel bars and river channel

GRAVEL BAR

 BANK

EROSION

 BANK

ACCRETION

 BANK

EROSION

CHUTE

CHANNEL

 BANK

ACCRETION CHUTE

GRAVEL

SAND

SOIL

CHANNEL

GRAVEL

 BAR CHUTE

BANK EROSION

OF OLD DEPOSITS

BANK ACCRETION OF

NEW SAND AND GRAVEL

NEW SOIL
Sampling and methods

Two sites that are readily

accessible by road were selected

for sampling (Table 1; Figure 3).

The southernmost, upstream site

is near Welby (78th Avenue, City

of Thornton, Commerce City 7 1/

2’ topographic map), where a

railroad bridge crosses the South

Platte River. The site is

immediately downstream from the

confluence of Clear Creek and the

South Platte. The northernmost,

downstream site is near Ft.

Lupton, one mile north of State

Highway 52, where a Weld County

road crosses the river (Ft. Lupton

7 1/2’ topographic map). The two

sites are about 18 miles (mi)

apart, and were chosen to

encompass the present extent of

gravel mining in terraces of the

South Platte. No major tributary

with access to a source of coarse

gravel enters the South Platte

between the two sampled sites.

Modifications made to the river

channel and its banks are a

possible source of variation in

characteristics of gravel deposited

by the river. Such modifications

are common along the South

Platte because the floodplain is

farmed and mined extensively and

contains numerous structures and

facilities. The river below the

upstream site near Welby has been

straightened, and levees protect

the adjacent floodplain. In

addition, the river makes a sharp

bend and a railroad crosses the

river. The railroad bridge is wide

and does not appear to be capable

of obstructing high water except

during rare large floods. Gravel

bars immediately upstream and

downstream from the bend in the

COUNTY

 ROAD

South Platte River

Station 31, Welby site

(78th Avenue)

A

C D

Station 33, Ft. Lupton site

(one mile north of State route 52)

Station 32, Ft. Lupton site

(one mile north of State route 52)

10 FT

300 FT

A

D

B

Station 30, Welby site (78th Avenue)

A

C

D

120 FT

B

C

D

270 FT

240 FT

10 FT

10 FT

APPROX. 600 FT

BETWEEN BAR

MIDPOINTS

10 FT

APPROX. 900 FT

BETWEEN BAR

MIDPOINTS

 18 MILES

BETWEEN

 SITES

RAILROAD

Figure 3.--Sampling design for

gravel bars at two sites on the

South Platte River near Welby and

Ft. Lupton, Colorado.

4
river at the railroad crossing were

selected for sampling. The river at

the downstream site appears to be

less modified than at the upstream

locality; it has not been

straightened, but it has two

channels south (upstream) of the

county road crossing. During low

flow the river follows a meander;

however, during high flow the river

uses a cutoff about 1/4 mi west

of the meander. High flow through

the cutoff is responsible for

deposition of the gravel bars found

within it and immediately

downstream. The bridge at the

site is wide and does not appear to

present a major obstruction to

flow except during rare large

floods. A low levee west of the

river is about 100 ft or more from

the channel. The first gravel bars

above and below the bridge were

selected for sampling. Vegetation

was minimal on all of the bars

sampled.

A four-level (also called

“hierarchical” or “nested”)

sampling design was used to allow

measurement of variation over

various distances within the river

gravel (Figure 3). Multilevel

sampling designs like the one used

here are discussed in detail by

Miesch (1976). From highest to

lowest, the four levels are defined

by distance between sites,

between gravel bars, between

ends of gravel bars, and between

samples (Figure 3). At each site,

two gravel bars, located in

sequence and on opposite sides of

the river, were selected as primary

sampling stations and assigned

unique station numbers. The

distance between the midpoints of

the gravel bars sampled was about

900 feet (ft) at the Welby site and

about 600 ft at the Ft. Lupton

site. Inspection of other bars at

the sampled sites showed that the

distances between bars ranges

from about 600 to 1,000 ft at

each site. For each gravel bar

(stations 30-33), both upstream

and downstream ends of the bar

were sampled. The distance

between upstream and

downstream samples on each bar

was set at 80 percent of the

length of the bar, which yielded

distances of 120, 300, 270, and

240 ft. At each end of the gravel

bar, two samples were located ten

feet apart, yielding a total of four

sample locations (A, B, C, and D)

from each bar. To reduce the

workload of sample processing,

one of the four sample locations

from each bar was eliminated by

flipping a coin twice. The total

number of samples collected and

processed was twelve.

Gravel samples weighing 30-

50 pounds (lbs) were dug from a 2

X 2 foot area in the top 10 inches

(in) of the bar. Samples ranged in

weight from about 30 to 50 lbs;

average sample weight was about

40 lbs. Each sample was

processed in the field by dry

sieving and the weight of each

fraction recorded with a hand

scale. Sieve mesh sizes used are

given in Table 2. The 0.75-2.00

inch pebble fraction was reserved

for lithologic identification, shape,

and roundness determinations.

The first 50 pebbles (selected

blind from a bag) from the

reserved size fraction were

classified by lithology and

roundness and measured with a

ruler to determine the long (A),

intermediate (B), and short (C)

dimensions. Assignment of

pebbles to lithologic categories

was simplified and systematized to

assure consistency (Table 3).

Table 1.--Location and lengths of gravel bars sampled in the South Platte

River north of Denver, Colorado. Locations are for bar midpoints,

determined from 1:24,000 USGS topographic maps of the Commerce

City (Welby site), and Ft. Lupton (Ft. Lupton site) 7 1/2’ quadrangles.

FT, feet.

SITE

GRAVEL

BAR

(STATION)

LATITUDE LONGITUDE

BAR

LENGTH

(FT)

Welby 3 0 39° 50' 16" N. 104° 56' 54" W. 150

-do- 3 1 39° 50' 23" N. 104° 56' 46" W. 375

Ft. Lupton 3 2 40° 5' 37" N. 104° 49' 14" W. 340

-do- 3 3 40° 5' 43" N. 104° 49' 18" W. 300

Table 2.—Sieve sizes used for

particle-size analysis of gravel.

SIEVE

MESH OR

SIZE

METRIC

SIZE

2 inches 50.8 mm

3/4 inch 18.85 mm

No. 5 4 mm

No. 12 1.65 mm

No. 35 0.5 mm

No. 60 0.25 mm

5
Thus lithology, shape, and

roundness were determined for a

total of 600 pebbles, distributed

300/site, 150/gravel bar, and 50/

sample. Study methods, statistical

analysis, and interpretation of

particle size, shape, and roundness

are described by Pettijohn (1975),

and additional details on methods

of measuring and statistical

analysis of particle size are

provided by Krumbein and

Pettijohn (1938).

Results

Sampling of the gravel bars at

the Welby and Ft. Lupton sites

shows that the channel of the

South Platte is depositing gravel of

good quality throughout the 18

miles between sites. Although

sampling was not sufficient to

distinguish differences between

the mean values at each site with

a high level of statistical

significance, some major

differences are evident from

examining the data. The

coarseness of gravel diminishes

greatly in this distance. The major

variation in pebble lithology is a

relative increase in abundance of

pegmatite, accompanied by minor

decreases in other rock types,

most notably in mafic rocks.

Spherical pebbles are more

abundant downstream. In contrast

to results of studies of gravels in

other regions, roundness

decreases downstream.

Confirmation of downstream

changes in gravel characteristics

identified here would require

adjustments in the sampling design

in order to discriminate between

site means. This topic is discussed

under “Analysis of variance.”

Interpretation of cumulative

frequency plots shows that

maximum particle size may exceed

100 mm sieve diameter at the

Welby site (stations 30 and 31,

Figure 4A,B) but probably does

not exceed 50 mm at the Ft.

Lupton site (stations 32 and 33,

Figure 4C,D), a decrease of 50

percent (pct) or more in 18 mi.

Cumulative weight percents for

individual sieve sizes, such as

P>1.65 mm and P>4 mm, decrease

by 16 and 32 pct, respectively.

Percentile (except P10) and

quartile measures of particle size,

derived from cumulative frequency

curves (Figure 4), show

downstream decreases of 75-83

pct (Table 4). Particle size of the

two gravel bars at the Welby site

differs considerably, and size on

the lower bar (station 31) exhibits

more variability than on the upper

bar (station 30). Particle size is

relatively uniform within gravel

bars at the Ft. Lupton site.

Sorting is variable with no

discernible pattern, skewness of

particle size distributions is

positive and tends to increase

downstream, and kurtosis is

slightly positive and remarkably

uniform (Table 4).

Pebble lithology is dominated

by rocks derived from the

Precambrian granite-gneiss terrane

Table 3.—Criteria used for assigning pebbles to lithologic categories.

LITHOLOGIC

CATEGORY COMPOSITION TEXTURE ORFABRIC

GRAIN

SIZE

Granite Quartz + feldspar +/- mica and

amphibole No fabric < 10 mm

Pegmatite Quartz + feldspar No fabric > 10 mm

Quartz Quartz No fabric > 10 mm

Gneiss Quartz + feldspar +mica or

amphibole

Foliated +/-

layered

< 10 mm

> 1 mm

Schist Quartz + feldspar + mica or

amphibole

Foliated +/-

layered < 1 mm

Amphibolite Mainly amphibole Foliated > 1 mm

Diabase Pyroxene or amphibole + feldspar Subophitic

texture > 1 mm

Mafic porphyry

Phenocrysts: feldspar + Fe-Mg

minerals

Matrix: dark-colored

Porphyritic

texture

Phenocrysts >

0.5 mm

Intermediate

porphyry

Phenocrysts: feldspar + Fe-Mg

minerals

Matrix: light-colored

Porphyritic

texture

Phenocrysts >

0.5 mm

Felsic porphyry Phenocrysts: feldspar +/- quartz

Matrix: light-colored

Porphyritic

texture

Phenocrysts >

0.5 mm

Sandstone Quartz > feldspar Rounded grains < 2 mm

Quartzite Quartz grains and cement Metamorphic

fabric < 2 mm

6
Figure 4.—Cumulative frequency distributions showing particle size for gravel bars at A) station 30, Welby, B)

station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado.

.01 .1 1 10 100

PARTICLE SIZE IN MM

0

10

20

30

40

50

60

70

80

90

100

33D

33C

33B

SAMPLE

.01 .1 1 10 100

PARTICLE SIZE IN MM

0

10

20

30

40

50

60

70

80

90

100

32D

32C

32A

SAMPLE

.01 .1 1 10 100

PARTICLE SIZE IN MM

0

10

20

30

40

50

60

70

80

90

100

31D

31B

31A

SAMPLE

.01 .1 1 10 100

PARTICLE SIZE IN MM

0

10

20

30

40

50

60

70

80

90

100

30D

30C

30A

SAMPLE

CUMULATIVE WEIGHT PERCENT

CUMULATIVE WEIGHT PERCENT

CUMULATIVE WEIGHT PERCENTCUMULATIVE WEIGHT PERCENT

A: Station 30, Welby site

B: Station 31, Welby site

C: Station 32, Ft. Lupton site

D: Station 33, Ft. Lupton site

7
SAMPLE

NUMBER

P> 1.65 MM

(PCT)

P> 4 MM

(PCT)

P10

(MM)

P50

(MM)

P90

(MM)

Q1

(MM)

Q3

(MM) SORTING SKEWNESS KURTOSIS

Welby site

30A 84.0 80.7 1 30 100 6 70 3.42 0.68 0.32

30C 84.5 78.8 1 30 200 6 90 3.87 0.77 0.21

30D 88.4 80.5 1 50 400 7 200 5.35 0.75 0.24

Station 30

Mean 85.7 80.0 1 37 233 6 120 4.21 0.73 0.26

31A 63.6 54.8 0.6 5 30 1 10 3.16 0.63 0.15

31B 72.0 66.0 0.7 9 40 1 30 5.48 0.61 0.37

31D 88.6 73.7 1 20 90 3 40 3.65 0.55 0.21

Station 31

Mean 74.7 64.8 0.8 8 53 2 27 4.10 0.60 0.24

Welby Mean 80.2 72.4 0.9 23 143 4 73 4.15 0.67 0.25

Ft. Lupton site

32A 64.5 54.4 0.6 5 30 1 10 3.16 0.63 0.15

32C 72.1 58.2 0.7 6 40 1 20 4.47 0.75 0.24

32D 62.8 46.2 0.6 3 30 1 20 4.47 1.49 0.32

Station 32

Mean 66.5 52.9 0.6 5 33 1 17 4.03 0.96 0.24

33B 60.1 43.3 0.6 3 20 1 8 2.83 0.94 0.18

33C 62.5 37.0 0.6 3 10 1 7 2.65 0.88 0.32

33D 82.7 56.4 0.8 5 20 2 10 2.24 0.89 0.21

Station 33

Mean 68.4 45.6 0.7 4 17 1 8 2.57 0.90 0.24

Ft. Lupton

Mean 67.5 49.3 0.7 4 25 1 13 3.30 0.93 0.24

Net downstream change between sites (in percent)

Change -16 -32 -22 -83 -83 -75 -82 -20 +39 -4

8

Table 4.—Particle-size parameters for gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado. P>, cumulative weight percent for stated particle size; P10, 10th percentile; P50, 50th percentile;

P90, 90th percentile; Q1, 1st quartile (= 25th percentile); Q3, third quartile (= 75th percentile). Sorting

(geometric quartile deviation) = √ (Q3 / Q1); skewness (geometric quartile skewness) = √ ((Q1Q3) / (P50)2);

kurtosis (Kelley’s quartile kurtosis) = (Q3 - Q1) / (2(P90 - P10)). MM, millimeters; PCT, percent.
of the Front Range (Figure 5).

Granite, pegmatite, vein quartz,

and gneiss are the principal rocks

identified from the Front Range

terrane. Much of the granite is

coarse, making the distinction

between granite and pegmatite in

the gravel somewhat arbitrary.

Schist and mafic rocks (mainly

diabase and amphibolite) are

common minor constituents.

Volcanic porphyry, including

intermediate to silicic types from

both the Front Range and the

plains, commonly comprises about

6 percent (more at the Welby site)

of the pebbles; some of the

volcanic rocks may be deleterious

to aggregate quality (Langer and

Knepper, 1995). Two types of

sandstone, derived from the Lower

Cretaceous Dakota Sandstone and

the Pennsylvanian and Permian

Fountain Formation along the

range front, are common also

(Table 5); they are not considered

deleterious constituents. Pebbles

of Dakota Sandstone are fairly

common and always appear to be

well-indurated and hard. Quartzite

(of Precambrian? origin) and

petrified wood (probably derived

locally from the Late Cretaceous

and Paleocene Denver Formation)

are rare constituents of the gravel.

Petrified wood was not

encountered in the samples but

was observed on the gravel bars.

Overall, pebble lithology of the

gravel bars compares closely with

that recorded for gravel in terraces

nearby (Colton and Fitch, 1974).

Downstream changes in pebble

lithology are limited principally to a

major increase in pct pegmatite, a

small increase in pct quartz, and

small decreases in pct schist, mafic

rocks, and volcanic porphyry

(Table 5). Although minor in

absolute values, pct mafic rocks

declines by 73 pct of original

abundance in 18 mi; the observed

decrease probably reflects weak

resistance to abrasion during

stream transport. Although

pegmatite appears to become an

important constituent in the

pebble size fraction downstream,

the overall content of pegmatite in

the gravel would be expected to

decrease as particle size decreases

and pebbles of coarse-grained

rocks are reduced to individual

mineral grains.

Pebble shape is dominated by

disc shapes (oblate spheroids)

Figure 5.—Pie diagrams showing pebble lithology (0.75-2 inch sieve

fraction) for gravel bars at A) station 30, Welby, B) station 31, Welby,

C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each

pie combines counts of 50 pebbles each for 3 samples (total of 150

pebbles).

D: Station 33, Ft. Lupton site

A: Station 30, Welby site C: Station 32, Ft. Lupton site

B: Station 31, Welby site

4

2 3

1

5

6

7

8

9

11

12

4

2 1

5

6 7

8

9

11

12

4

3

1

5

6 7

8

9

10

12

4

2 3 1

5

6

7

8

9

11 12 11

LITHOLOGIC CATEGORY

6 INTERMEDIATE PORPHYRY

5 GRANITE

4 GNEISS

3 FELSIC PORPHYRY

2 DIABASE

1 AMPHIBOLITE

12 SCHIST

11 SANDSTONE

9 QUARTZ

8 PEGMATITE

7 MAFIC PORPHYRY

10 QUARTZITE

9
Table 5.--Lithology of pebbles in gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado. Lithologic identification determined for 50 pebbles per sample, 18.85-50.8 mm (0.75-2.00 in) size

fraction. PCT, percent.

SAMPLE

NUMBER

PCT

GRANITE

PCT

PEGMATITE

PCT

QUARTZ

PCT

GNEISS

PCT

SCHIST

PCT

MAFIC

ROCKS

PCT

PORPHYRY

PCT

SANDSTONE

PCT

QUARTZITE

Welby site

30A 28 2 10 28 6 2 12 12 0

30C 26 4 10 26 10 12 6 6 0

30D 42 10 6 16 4 4 6 12 0

Station 30

Mean 32 5.3 8.7 23.3 7 6 8 10 0

31A 30 20 4 32 0 0 10 4 0

31B 40 22 6 24 0 0 0 8 0

31D 40 16 2 20 4 4 6 8 0

Station 31

Mean 36.7 19.3 4 25.3 1.3 1.3 5.3 6.7 0

Welby

Mean 34.3 12.3 6.3 24.3 4 3.7 6.7 8.3 0

Ft. Lupton site

32A 34 24 6 24 4 0 4 4 0

32C 36 22 8 12 4 2 6 10 0

32D 24 26 10 22 4 2 4 8 0

Station 32

Mean 31.3 24 8 19.3 4 1.3 4.7 7.3 0

33B 30 32 8 22 0 0 4 4 0

33C 26 22 6 24 2 2 6 12 0

33D 42 22 6 16 2 0 6 4 2

Station 33

Mean 32.7 25.3 6.7 20.7 1.3 0.7 5.3 6.7 0.7

Ft. Lupton

Mean 32 24.7 7.3 20 2.7 1 5 7 .3

Net downstream change between sites (in percent)

Change -7 +100 +16 -18 -32 -73 -25 -16 --

10
and, except for station 30 at the

Welby site, by spherical particles

(Figure 6). Shapes were classified

according to the method of Zingg

(Pettijohn, 1975, p. 54) and the

sphericity (tendency toward

equidimensional shape) was

calculated according to the

method of Krumbein (1941). The

percentage of spherical pebbles is

much greater at the downstream

site, but the difference between

gravel bars at Welby is also striking

(Table 6). Rod- and blade-shapes

are also common (Table 6), but

axial ratios below 0.5 are rare for

individual pebbles (Figure 6), so

that even these shapes should not

yield weak particles. In general,

differences among sites and

stations for calculated values of

A: Station 30, Welby site

B: Station 31, Welby site

C: Station 32, Ft. Lupton site

D: Station 33, Ft. Lupton site

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

B/A

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

C/B

ROD-SHAPED

DISC-SHAPED SPHERICAL

BLADE-SHAPED

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

B/A

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

C/B

ROD-SHAPED

SPHERICAL

DISC-SHAPED

BLADE-SHAPED

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

B/A

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

C/B

ROD-SHAPED

DISC-SHAPED SPHERICAL

BLADE-SHAPED

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

B/A

.1 .2 .3 .4 .5 .6 .7 .8 .9 1

C/B

SCHIST

QUARTZ SANDSTONE

MAFIC PORPHYRY PEGMATITE

GRANITE INTERMEDIATE PORPHYRY

FELSIC PORPHYRY GNEISS

AMPHIBOLITE DIABASE

LITHOLOGIC CATEGORY

QUARTZITE

ROD-SHAPED

SPHERICAL

DISC-SHAPED

BLADE-SHAPED

Figure 6.—Zingg diagrams showing pebble shape (0.75-2 inch sieve fraction) for gravel bars at A) station 30,

Welby, B) station 31, Welby, C) station 32, Ft. Lupton, and D) station 33, Ft. Lupton, Colorado. Each diagram

combines measurements of 50 pebbles each for 3 samples (total of 150 pebbles).

11
SAMPLE

NUMBER MEAN B/A MEAN C/B MEAN Ψ PCTSPHERICAL

PCT DISCSHAPED

PCT

RODSHAPED

PCT BLADESHAPED

Welby site

30A 0.760 0.507 0.654 8 70 4 18

30C 0.729 0.540 0.648 8 54 14 24

30D 0.742 0.500 0.644 4 66 6 24

Station 30

Mean 0.743 0.516 0.649 7 63 8 22

31A 0.748 0.649 0.709 30 52 12 6

31B 0.715 0.652 0.686 24 40 22 14

31D 0.724 0.605 0.672 18 44 18 20

Station 31

Mean 0.729 0.635 0.689 24 45 17 14

Welby

Mean 0.736 0.576 0.669 15 54 13 18

Ft. Lupton site

32A 0.716 0.671 0.694 28 36 26 10

32C 0.783 0.598 0.709 26 60 8 6

32D 0.739 0.598 0.681 16 48 18 18

Station 32

Mean 0.746 0.622 0.694 23 48 17 12

33B 0.756 0.664 0.721 32 44 12 12

33C 0.765 0.592 0.689 36 46 8 10

33D 0.790 0.642 0.730 38 46 8 8

Station 33

Mean 0.770 0.633 0.713 35 45 10 10

Ft. Lupton

Mean 0.758 0.627 0.704 29 47 13 11

Net downstream change between sites (in percent)

Change +3 +9 +5 +93 -13 0 -39

12

Table 6.—Mean shape of pebbles in gravel bars of the South Platte River at sites near Welby and Ft. Lupton,

Colorado. Mean shape calculated from measured lengths of long (A), intermediate (B), and short (C) pebble

axes (Pettijohn, 1975, p. 54), 50 pebbles per sample, 18.85-50.8 mm (0.75-2.00 in) size fraction. B/A is

inversely proportional to flatness; C/B is inversely proportional to elongation. Sphericity (Ψ) = 3√ (BC/A2)

measures degree of equidimensional shape; when A=B=C, Ψ = 1. Shape classified as spherical, disc-, rod-, and

blade-shaped by criteria of Zingg (Pettijohn, 1975, fig. 3-18). PCT, percent.
subjected to breakage during

transport in the stream.

In summary, the desirable

feature of large particle size

appears to vary inversely with the

desirable qualities of abundant

0

10

20

30

40

.4 .5 .6 .7 .8 .9 1

SPHERICITY

.3

NUMBER OF PEBBLES

0

10

20

30

40

.4 .5 .6 .7 .8 .9 1

SPHERICITY

.3

NUMBER OF PEBBLES

0

10

20

30

40

.4 .5 .6 .7 .8 .9 1

SPHERICITY

NUMBER OF PEBBLES

.3

0

10

20

30

40

.4 .5 .6 .7 .8 .9 1

SPHERICITY

NUMBER OF PEBBLES

.3

D: Station 33,

Ft. Lupton site

C: Station 32,

Ft. Lupton site

B: Station 31,

Welby site

A: Station 30,

Welby site

Figure 7.—Histograms showing

pebble sphericity (Ψ) for gravel

bars at A) station 30, Welby, B)

station 31, Welby, C) station 32,

Ft. Lupton, and D) station 33, Ft.

Lupton, Colorado. Each histogram

combines calculations on

measurements of 50 pebbles each

for 3 samples (total of 150

pebbles).

flatness (B/A), elongation (C/B),

and sphericity (Ψ) appear small

(Table 6), and frequency

distributions of sphericity values

for individual pebbles from each

gravel bar show almost complete

overlap (Figure 7). However, as

will be discussed under analysis of

variance, the modest downstream

increase in mean sphericity is

sufficiently consistent that, with

additional sampling, a distinction

might be possible. No relation

between pebble shape and

lithology is apparent. Gneiss,

granite and pegmatite all appear to

plot in the same shape field on the

Zingg diagrams (Figure 6).

Pebble roundness is the only

quality characteristic selected for

this study that depends on

subjective estimates. The 50

pebbles selected for lithologic

identification and shape

measurement were classified using

the visual comparison chart of

Pettijohn (1975, p. Fig. 3-24).

Rounded and well-rounded pebbles

dominate at the Welby site, but

subrounded and subangular

pebbles are abundant at the Ft.

Lupton site (Figure 8). Average

roundness also decreases slightly

downstream, but the reliability of

this calculated parameter is in

doubt because average roundness

varies greatly among stations

(Table 7). The downstream

decrease in well-rounded pebbles

and corresponding increase in

subrounded and subangular

pebbles observed in the South

Platte is unusual in that roundness

generally increases downstream

and correlates positively with

sphericity (Plumley, 1948). The

difference between rounding in

pebbles of the South Platte and

streams studied elsewhere may

stem from differences in pebble

lithology, such as the abundance

of pegmatite, which tends to break

into smaller pebbles along intergranular surfaces and feldspar

cleavages. The inference may be

drawn (for further testing) that

pebble roundness decreases with

particle size downstream in the

South Platte because the larger,

rounded pebbles of pegmatite are

Figure 8.—Histograms showing

distribution of pebble roundness

for gravel bars at A) station 30,

Welby, B) station 31, Welby, C)

station 32, Ft. Lupton, and D)

station 33, Ft. Lupton, Colorado.

Each histogram combines

roundness of 50 pebbles each for

3 samples (total of 150 pebbles).

A B CD E

ROUNDNESS CLASS

0

10

20

30

40

50

60

70

80

90

100

NUMBER OF PEBBLES

A B CD E

ROUNDNESS CLASS

0

10

20

30

40

50

60

70

80

90

100

NUMBER OF PEBBLES

A B CD E

ROUNDNESS CLASS

0

10

20

30

40

50

60

70

80

90

100

NUMBER OF PEBBLES

A B CD E

ROUNDNESS CLASS

0

10

20

30

40

50

60

70

80

90

100

NUMBER OF PEBBLES

A. Station 30,

Welby site

B. Station 31,

Welby site

C. Station 32,

Ft. Lupton site

D. Station 33,

Ft. Lupton site

13
Analysis of variance

The following discussion and

application of analysis of variance

is based on a detailed discussion of

multi-level sampling designs and

analysis of variance by Miesch

(1976).

The model of variation

(Miesch, 1976) for a measurement

in the four-level sampling plan used

in this study is given by:

Xijkm = μ + αi + βij + γijk + δijkm

where Xijkm is a single

measurement on the mth sample

for the kth bar-end on the jth

gravel bar at the ith site. Site i =

1 or 2 (Welby or Ft. Lupton), bar j

= 1 or 2 (upstream or

downstream), bar end k = 1 or 2

(upstream or downstream), and

sample m = 1 or 2. Each

measurement is considered as the

sum of the grand mean (μ) of all

sample values and the deviations

introduced by each site (αi), gravel

bar (βij), bar end (γijk), and sample

(δijkm). Only single measurements

were made because measurement

error was deemed insignificant for

dimensions and shapes of gravel

particles.

The model defined by the

sampling design was used to

estimate the amount of variation

(variance) associated with each

level and thereby guide designs for

future studies. If αi, βij, γijk, and

δijkm are random variables with

mean zero and variances of sα2,

sβ2, sγ

2, and sδ2, and the variances

are additive, then:

ST2 = sα2 + sβ2 + sγ

2 + sδ2

where ST2 is the estimate of total

variance and sα2 is an estimate of

variance between sites, sβ2 is an

estimate of variance between

gravel bars, sγ

2 is an estimate of

variance between ends of bars,

SAMPLE

NUMBER

MEAN

ROUNDNESS PCT A PCT B PCT C PCT D PCT E

Welby site

30A 0.70 0 0 0 32 68

30C 0.57 2 6 26 18 48

30D 0.63 2 2 12 28 56

Station 30

Mean 0.63 1.3 2.7 12.7 26 57.3

31A 0.48 8 10 22 32 28

31B 0.47 8 14 20 30 28

31D 0.57 8 6 16 22 48

Station 31

Mean 0.51 8 10 19.3 28 34.7

Welby

Mean 0.57 4.7 6.3 16 27 46

Ft. Lupton site

32A 0.51 2 12 14 46 26

32C 0.58 2 10 10 32 46

32D 0.51 2 10 16 46 26

Station 32

Mean 0.53 2 10.7 13.3 41.3 32.7

33B 0.39 12 18 28 30 12

33C 0.41 4 18 38 22 18

33D 0.39 10 26 24 22 18

Station 33

Mean 0.40 8.7 20.7 30 24.7 16

Ft. Lupton

Mean 0.47 5.3 15.7 21.7 33 24.3

Net downstream change between sites (in percent)

Change -18 +13 +149 +36 +22 -47

Table 7.--Roundness of pebbles in gravel bars of the South Platte River

at sites near Welby and Ft. Lupton, Colorado. Estimated by visual

comparison with roundness classes (Pettijohn, 1975, fig. 3-24), 50

pebbles per sample, 18.85-50.8 mm (0.75-2.00 in) size fraction. Mean

roundness = class abundance X mid-point values of Pettijohn (1975,

table 3-9). Classes defined as follows: A, angular; B, subangular; C,

subrounded; D, rounded; E, well rounded. PCT, percent.

gravel upstream and 2) fine,

spherical, angular gravel

downstream. A downstream

increase in the proportion of

pegmatite pebbles may affect

gravel quality adversely.

spherical shapes and angular

surfaces (poorly-rounded

particles). The gravel deposits of

the South Platte can be expected

to occupy a continuum between 1)

coarse, disc-shaped, rounded

14
and sδ2 is an estimate of variance

between samples. Estimates for

variance components at the four

levels in the sampling plan were

calculated for a selection of

measures and calculated

parameters for particle size and

pebble characteristics in the gravel

bars (Tables 8-11). In the present

pilot study, based on only 12

samples, total variance (ST2) is

overestimated by the method (of

Anderson and Bancroft, 1952, p.

325-330) for calculation of

variance components in

unbalanced sampling designs. In

the tables, the total variance

shown was calculated by the

standard formula for variance. The

variance components were then

calculated by the method of

Anderson and Bancroft (1952) and

the components expressed in

terms of percent of total variance.

One general conclusion drawn

from inspection of variance

components is that separate

sampling of upstream and

downstream ends of gravel bars is

unnecessary. Variance

components for the bar-end level

of sampling are nil for many

measures, including particle size

(Tables 8-11). Separate sampling

of bar-ends was included in the

design because downstream

decrease in particle size was seen

on some surfaces of gravel bars in

the South Platte. Also,

downstream decreases in particle

size have been described from

individual channel bars of braided

streams (Bluck, 1982). For the

lateral bars of the South Platte,

downstream variation of particle

size is evidently local and limited

to visually obvious coarse

particles.

The analysis shows that, for

measures of particle size, P>4 mm

shows a higher proportion of

regional variation (between sites)

than any other measure or

calculated parameter (Table 8). In

fact, for P>4 mm, the sampling

design was sufficient to distinguish

site means at the 0.11 level of

significance (in other words, there

is only an 11 pct probability that

the means are the same). Other

measures (P>1.65) and derived

parameters (P10 and P50) also

show strong regional variation. All

of these could be used to study

regional variations in particle size,

but the value P>4 mm may be the

most powerful measure, and it has

the additional advantage of being

directly acquired during sieving by

PARTICLE

SIZE

PARAMETER

VARIANCE

SAMPLING LEVEL VARIANCE RATIO

BETWEEN

SITES

BETWEEN

GRAVEL

BARS

BETWEEN

BAR

ENDS

BETWEEN

SAMPLES SITES GRAVEL

BARS

Percent of total variance

P>1.65 MM 124 41 0 13 46 0.69 0

P>4 MM 226 62 17 0 21 1.63 0.81

P10 0.033 46 12 25 17 0.85 0.29

P50 231 37 46* 0 17 0.59 2.71

P90 12,700 19 37 11 33 0.23 0.84

Q1 5.54 18 73* 5 4 0.22 8.11

Q3 3,150 18 45 0 37 0.22 1.22

Sorting 1.07 7 25 0 68 0.08 0.37

Skewness 0.063 32 0 0 68 0.47 0

Kurtosis 0.005 1 0 0 99 0.01 0

15

Table 8.—Four-level analysis of variance of particle-size parameters for gravel bars of the South Platte River

at sites near Welby and Ft. Lupton, Colorado. P>, cumulative weight percent for stated particle size; P10,

10th percentile; P50, 50th percentile; P90, 90th percentile; Q1, 1st quartile; Q3, third quartile. Asterisks (*)

indicate significant differences among mean values (≤0.05 probability that means are equal). Site variance

ratio = between-site variance/total within-site variance. Bar variance ratio = between-bar variance/total

within-bar variance. MM, millimeters.
simply adding the weights of

gravel on the sieves of 4 mm and

larger mesh openings. Fortunately,

P>4 mm has been widely reported

in previous studies of gravel in the

Front Range Urban Corridor (e.g.,

Colton and Fitch, 1974; Trimble

and Fitch, 1974a,b), making it a

good choice for regional study.

Comparable power for regional

studies would be expected using

P>3/16 inch (4.76 mm), a sieve

size commonly used by the gravel

industry. In contrast, calculated

parameters such as sorting and

kurtosis show little regional

(between-site) variation and a high

degree of local (between-sample)

variation, making them unsuitable

for regional comparison.

The major component of

variance between gravel bars

indicates that future sampling for

regional studies should be adjusted

to reduce the effect of this source

of variation. Adjustment is

determined by calculating the ratio

of between-site variance to withinsite variance (Table 8). From the

variance ratio for P>4 mm at sites

(v = 1.63, Table 8), the number of

random samples required within a

site (nr = 5) is determined by

consulting the graph given by

Miesch (1976, fig. 2A). Sampling

among levels can be guided by

calculating the following (Miesch,

1976):

Er =

(sβ2 + sγ

2 + sδ2)/nr = 17.18

for nr = 5 where Er is the

maximum acceptable error

variance; and:

Es =

sβ2/nβ + sγ

2/nβnγ + sδ2/nβnγnδ

where nβ, nγ, and nδ are the

number of gravel bars, bar ends,

and samples, respectively, within a

site. According to Miesch (1976,

p. A9), samples must be sufficient

in number and distributed so that

Es does not exceed Er . If nβ = 5,

nγ = 1, and nδ = 1, then Es =

17.18. Thus, only 10 samples are

necessary to discriminate P>4 mm

between two sites 18 mi apart if a

single random sample is selected

from each of five bars at each site.

This is so because the major

sources of within-site variance

(among bars and among samples)

can both be minimized by

increasing the number of bars

sampled from two to five. In

contrast, sampling for the pilot

study was not adequate to

discriminate P>4 mm between

sites 18 mi apart because nβ = 2

and nγ + nδ = 3, yielding Es = 27.

If the sites were closer together

(and thus less distinct), more

gravel bars might have to be

sampled.

Variance ratios were also

calculated for discriminating

between gravel bars, for possible

guidance in designing studies

scaled to individual gravel pits.

Suppose that gravel beds were

deposited by bars spaced 600-900

ft apart, as is the case for the bars

sampled in the river, and the

measure of particle size P>4 mm

PEBBLE

LITHOLOGY VARIANCE

SAMPLING LEVEL VARIANCE RATIO

BETWEEN

SITES

BETWEEN

GRAVEL

BARS

BETWEEN

BAR ENDS

BETWEEN

SAMPLES SITES GRAVEL

BARS

Percent of total variance

PCT GRANITE 44.7 0 7 0 93 0 0.08

PCT PEGMATITE 80.1 46 36 12 6 0.85 2.00

PCT QUARTZ 6.2 0 52 12 36 0 1.08

PCT GNEISS 30.9 17 0 0 83 0.20 0

PCT SCHIST 8.2 0 63* 0 37 0 1.70

PCT MAFIC ROCKS 11.5 6 19 6 69 0.06 0.26

PCT PORPHYRY 9.1 3 1 0 96 0.03 0.01

PCT SANDSTONE 10.8 0 3 0 97 0 0.03

Table 9.--Four-level analysis of variance of pebble lithology in gravel bars of the South Platte River at sites near

Welby and Ft. Lupton, Colorado. Lithology represented by percentage rock type for the18.85-50.8 mm (0.75-

2.00 in) size fraction. Asterisks (*) indicate significant differences among mean values (≤0.05 probability that

means are equal). PCT, percent.

16
was to be mapped at that scale in

a bed of gravel in the pit. The

number of samples required within

600-900 foot segments of the

same gravel bed would be nr = 6 if

v = 0.81. Of course, a final

sampling design for a specific

gravel pit should be designed from

a pilot study of the pit and the

design should be tailored to

produce the level of detail required

by the operators.

The method of sampling

design and optimization described

for P>4 mm can be applied to

other measures of gravel quality.

Among the best candidates for

study of regional variation of

gravel quality in the South Platte

valley are pct pegmatite (Table 9),

mean sphericity (Table 10), and

pct well-rounded pebbles (Table

11). For the variance ratios of

these measures, 6-8 gravel bars

should be sampled at each site to

distinguish sites 18 mi apart. In

contrast, many measures have

high variation among gravel bars

(spaced 600-900 ft apart); with

proper sampling design, they could

be mapped with confidence at the

scale of individual gravel pits. A

few measures, including kurtosis,

pct granite, pct gneiss, pct

porphyry, and pct sandstone, have

mostly between-sample variance

and cannot be mapped at any

useful scale. Sampling in young

terrace gravels could be guided by

the same criteria.

Suggestions

for future work

Based on results of the pilot

study, the following suggestions

apply for sampling gravel quality:

1) Concentrate on

characteristics that are easy to

measure and show variation

appropriate to the scale of study.

For studies of downstream

variation of gravel in a single

drainage such as the South Platte,

particle size measures such as P>4

mm are best.

2) The number of sieves (and

time spent sieving) can be reduced

by using only coarse meshes; a

series of sieves having mesh

openings of 1 1/2, 3/4, 3/8, and

3/16 inches, commonly employed

in the gravel industry, is better

suited to gathering the required

data on particle size than a more

wide-ranging series extending to

fine sand, commonly employed in

sedimentologic studies and used

for the pilot study. Depending

upon the distribution of particle

size, a wide range of sieve sizes

might be required to determine

sorting, skewness, and kurtosis,

but these parameters are not

generally mappable because they

show high variation among closely

spaced samples.

3) Sampling of 6-8 successive

gravel bars or, in terrace gravels,

PEBBLE SHAPE VARIANCE

SAMPLING LEVEL VARIANCE RATIO

BETWEEN

SITES

BETWEEN

GRAVEL

BARS

BETWEEN

BAR ENDS

BETWEEN

SAMPLES SITES GRAVEL

BARS

Percent of total variance

MEAN B/A 0.00060 20 4 5 71 0.25 0.05

MEAN C/B 0.00348 0 69* 19 12 0 2.23

MEAN SPHERICITY (Ψ) 0.00079 31 38* 0 31 0.45 1.22

PCT SPHERICAL 131 25 60* 3 12 0.33 4.00

PCT DISC-SHAPED 107 0 46 11 43 0 0.85

PCT ROD-SHAPED 46.2 0 41 5 54 O 0.69

PCT BLADE-SHAPED 42.5 28 21 0 51 0.39 0.41

Table 10.--Four-level analysis of variance of pebble shape parameters in gravel bars of the South Platte River

at sites near Welby and Ft. Lupton, Colorado. Shape is represented by the mean values of B/A and C/B, where

A is the pebble long axis, B is the intermediate axis, and C is the short axis, by mean sphericity (Ψ), and by the

percentage of spherical, disc-, rod-, and blade shaped pebbles for 50 pebbles in the18.85-50.8 mm (0.75-

2.00 in) size fraction. Asterisks (*) indicate significant differences among mean values (≤0.05 probability that

means are equal). Site variance ratio = between-site variance/total within-site variance. Bar variance ratio =

between-bar variance/total within-bar variance. PCT, percent.

17
of 6-8 successive locations spaced

the same distance as gravel bars in

the modern stream, are probably

required to yield sufficiently

precise estimates for regional

study of gravel in a single drainage

like the South Platte. If the gravel

deposits in terraces were

deposited recently, under the

same conditions of climate and

streamflow as the present, sample

spacing in terrace gravels might be

guided by a survey of gravel bar

spacing in the streambed.

4) Sampling at a few sites

should be sufficient to

characterize pebble shape and

lithology in a drainage. Studies of

pebble shape and lithology are

time-consuming, and shape and

lithology show only minor

downstream variation. However,

pebble lithology is important for

predicting quality of gravel

aggregate in drainages having

different sources; such predictions

could probably be made with

modest sampling across a number

of drainages.

5) The relationship of rounding

to gravel aggregate quality needs

investigation. Rounding is easy to

determine by visual estimation and

shows promise of downstream

change.

6) Studies using different

types of samples and sampling

patterns, as discussed by Otto

(1938) and Griffiths and Ondrick

(1968), should be tried to assure

that estimates of gravel quality are

unbiased and to identify ways to

minimize between-sample variance.

7) Further studies should be

directed toward gravel deposits in

terraces of the South Platte, to

test the application of suggestions

made here. Such studies are

underway.

References cited

Anderson, R. L., and Bancroft, T. A.,

1952, Statistical theory in

research: New York, McGraw-Hill,

399 p.

Bluck, G. J.,1982, Texture of gravel

bars in braided streams, in Hey, R.

D., Bathurst, J. C., and Thorne, C.

R., Gravel-bed rivers: fluvial

processes, engineering and

management: Chichester, U. K.,

Wiley, p. 339-355.

Church, M., and Jones, D., 1982,

Channel bars in gravel-bed rivers,

in Hey, R. D., Bathurst, J. C., and

Thorne, C. R., Gravel-bed rivers:

fluvial processes, engineering and

management: Chichester, U. K.,

Wiley, p. 291-338.

Colton, R. B., 1978, Geologic map of

the Boulder-Fort Collins-Greeley

area, Front Range Urban Corridor,

Colorado: U. S. Geological Survey

Miscellaneous Investigations Map I855-G, scale 1:100,000.

Colton, R. B., and Fitch, H. R., 1974,

Map showing potential sources of

gravel and crushed-rock aggregate

in the Boulder-Fort Collins-Greeley

area, Front Range Urban Corridor,

Colorado: U. S. Geological Survey

Miscellaneous Investigations Series

Map I-855-D, scale 1:100,000.

Griffiths, J. C., and Ondrick, C. W.,

1968, Sampling a geological

population: Kansas Geologial

Survey Computer Contribution 30,

53 p.

PEBBLE

ROUNDNESS VARIANCE

SAMPLING LEVEL VARIANCE RATIO

BETWEEN

SITES

BETWEEN

GRAVEL

BARS

BETWEEN

BAR ENDS

BETWEEN

SAMPLES SITES GRAVEL

BARS

Percent of total variance

MEAN ROUNDNESS 0.00943 10 65* 15 10 0.11 2.60

PCT A 15.6 0 81* 1 18 0 4.26

PCT B 53.5 34 49* 0 17 0.52 2.88

PCT C 96.3 0 48 7 45 0 0.92

PCT D 78.5 0 55 6 38 0 1.25

PCT E 305 34 37 14 15 0.52 1.28

Table 11.--Four-level analysis of variance of pebble roundness in gravel bars of the South Platte River at sites

near Welby and Ft. Lupton, Colorado. Roundness is represented by average roundness (calculated from class

mid-point values of Pettijohn, 1975, table 3-9) and by the percentage of pebbles in classes A (angular), B

(subangular), C (subrounded), D (rounded), and E (well rounded) for the18.85-50.8 mm (0.75-2.00 in) size

fraction. Asterisks (*) indicate significant differences among mean values (≤0.05 probability that means are

equal). PCT, percent.

18
Hemborg, H. T., 1996, Active

permitted mine operations in

Colorado, 1995-1996: Colorado

Geological Survey Information

Series 41, 52 p.

Langer, W. H., and Knepper, D. H., Jr.,

1995, Geologic characterization of

natural aggregate: a field

geologist’s guide to natural

aggregate resource assessment:

U. S. Geological Survey Open-File

Report 95-582, 32 p.

Krumbein, W. C., 1941, Measurement

and geological significance of

shape and roundness of

sedimentary particles: Journal of

Sedimentary Petrology, v. 11, no.

2, p. 64-72.

Krumbein, W. C., and Pettijohn, F. J.,

1938, Manual of sedimentary

petrography: New York, AppletonCentury-Crofts, 549 p.

Miesch, A. T., 1976, Geochemical

survey of Missouri—methods of

sampling, laboratory analysis, and

statistical reduction of data, with

sections on Laboratory Methods

by P. R. Barnett, A. J. Bartel, J. I.

Dinnin, G. L. Feder, T. F. Harms, C.

Huffman, Jr., V. J. Janzer, H. T.

Millard, Jr., H. G. Neiman, M. W.

Skougstad, and J. S. Wahlberg: U.

S. Geological Survey Professional

Paper 954-A, 39 p.

Otto, G. H., 1938, The sedimentation

unit and its use in field sampling:

Journal of Geology, v. 46, no. 4, p.

569-582.

Pettijohn, F. J., 1975, Sedimentary

rocks (3rd ed.): New York, Harper

and Row, 628 p.

Plumley, W. J., 1948, Black Hills terrace

gravels: a study in sediment

transport: Journal of Geology, v.

56, no. 6, p. 526-577.

Scott, G. R., 1965, Nonglacial

Quaternary geology of the

southern and middle Rocky

Mountains, in Wright, H. E., Jr., and

Frey, D. G., eds., the Quaternary of

the United States: Princeton, N.

J., Princeton University Press, p.

243-254.

Schwochow, S. D., 1980, The effects

of mineral conservation legislation

on Colorado’s aggregate industry,

in Schwochow, S. D., ed.,

Proceedings of the Fifteenth

Forum on Geology of Industrial

Minerals, Golden, Colorado, June

13-15, 1979, Industrial minerals in

Colorado and the Rocky Mountain

Region: Colorado Geological

Survey Resource Series 8, p. 29-

41.

Trimble, D. E., and Fitch, H. R., 1974a,

Map showing potential gravel

sources and crushed-rock

aggregate in the Colorado SpringsCastle Rock area, Front Range

Urban Corridor, Colorado: U. S.

Geological Survey Miscellaneous

Investigations Series Map I-857-A,

scale 1:100,000.

Trimble, D. E., and Fitch, H. R., 1974b,

Map showing potential gravel

sources and crushed rock

aggregate in the greater Denver

area, Front Range Urban Corridor,

Colorado: U. S. Geological Survey

Miscellaneous Investigations Series

Map I-856-A, scale 1:100,000.

Trimble, D. E., and Machette, M. N.,

1979, Geological map of the

greater Denver area, Front Range

Urban Corridor, Colorado: U. S.

Geological Survey Miscellaneous

Investigations Map I-856-H, scale

1:100,000.

Wayne, W. J., Aber, J. S., Agard, S. S.,

Bergantino, R. N., Bluemle, J. P.,

Coates, D. A., Cooley, M. E.,

Madole, R. F., Martin, J. E., Mears,

B., Jr., Morrison, R. B., and

Sutherland, W. M., 1991,

Quaternary geology of the

northern Great Plains, in Morrison,

R. B., ed., Quaternary nonglacial

Geology: Conterminous U. S.:

Boulder, Geological Society of

America, The Geology of North

America, v. K-12, p. 441-476.

19