In cooperation with the U.S. Army Joint Readiness Training Center and Fort Polk 

Effects of Hardened Low-Water Crossings on Periphyton 

and Water Quality in Selected Streams at the Fort Polk 

Military Reservation, Louisiana, 1998–99 and 2003–04 

Scientific Investigations Report 2007–5279 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover: Low-water crossing at Little Brushy Creek at the Fort Polk Military Reservation, Louisiana, May 

2003 (photograph by Dennis J. Jeffrey, U.S. Geological Survey). 
Effects of Hardened Low-Water Crossings 

on Periphyton and Water Quality in 

Selected Streams at the Fort Polk Military 

Reservation, Louisiana, 1998–99 and 2003–04

By Barbara W. Bryan, C. Frederick Bryan, John K. Lovelace, and Roland W. Tollett 

In cooperation with the U.S. Army Joint Readiness Training Center and Fort Polk 

Scientific Investigations Report 2007–5279

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Bryan, B.W., Bryan, C.F., Lovelace, J.K., and Tollett, R.W., 2007, Effects of hardened low-water crossings on 

periphyton and water quality in selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 

2003–04: U.S. Geological Survey Scientific Investigations Report 2007–5279, 36 p. 
iii

Contents

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

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

Purpose and Scope .............................................................................................................................1

Description of Study Area ..................................................................................................................3

Methods ..........................................................................................................................................................3

Periphyton .............................................................................................................................................3

Water Quality ........................................................................................................................................6

Statistical Analysis ..............................................................................................................................6

Quality Control and Quality Assurance ............................................................................................6

Effects of Hardened Low-Water Crossings on Periphyton ....................................................................7

Effects of Hardened Low-Water Crossings on Water Quality .............................................................17

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

References ...................................................................................................................................................35

Figures

1. Map showing location of study area, low-water crossings, and data-collection 

sites at the Fort Polk Military Reservation, Vernon Parish, Louisiana ................................2

2–9. Graphs showing: 

2. Monthly average cell density of blue-green, green, and diatom algae from 

sites upstream and downstream from hardened low-water crossings on 

selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04 ..........14

3. Calcium concentrations in water from sites upstream and downstream from 

hardened low-water crossings on selected streams at the Fort Polk Military 

Reservation, Louisiana, 1998–99 and 2003–04 .............................................................26

4. Magnesium concentrations in water from sites upstream and downstream 

from hardened low-water crossings on selected streams at the Fort Polk 

Military Reservation, Louisiana, 1998–99 and 2003–04 ...............................................27

5. Concentration of total dissolved solids in water from sites upstream and 

downstream from hardened low-water crossings on selected streams at 

the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04 .......................28

6. Specific conductance in water from sites upstream and downstream from 

hardened low-water crossings on selected streams at the Fort Polk Military 

Reservation, Louisiana, 1998–99 and 2003–04 .............................................................29

7. Alkalinity concentrations in water from sites upstream and downstream from 

hardened low-water crossings on selected streams at the Fort Polk Military 

Reservation, Louisiana, 1998–99 and 2003–04 .............................................................30

8. Manganese concentrations in water from sites upstream and downstream 

from hardened low-water crossings on selected streams at the Fort Polk 

Military Reservation, Louisiana, 1998–99 and 2003–04 ...............................................31

9. Monthly rainfall at Leesville, Louisiana, during periphyton and water-quality 

data collection at selected streams at the Fort Polk Military Reservation, 

Louisiana, October 1998 through September 1999 and April 2003 through 

April 2004 ............................................................................................................................34
iv

Tables

1. Periphyton and water-quality sampling dates and hardened low-water crossing 

installation and modification dates for selected streams at the Fort Polk Military 

Reservation, Louisiana, 2003–04 ...............................................................................................4

2. Percent frequency of occurrence and average cell density for periphyton taxa on 

wood cores and glass slides collected at sites upstream and downstream from 

low-water crossings on selected streams at the Fort Polk Military Reservation, 

Louisiana, 2003–04 .......................................................................................................................8

3. Percent frequency of occurrence of major algal divisions in selected streams at 

the Fort Polk Military Reservation, Louisiana, 2003–04 .......................................................12

4. Percent frequency of occurrence (top row) and average cell density (bottom row) 

for the 10 most frequently occurring periphyton taxa in all samples collected at 

upstream and downstream sites on selected streams at the Fort Polk Military 

Reservation, Louisiana, 2003–04 .............................................................................................13

5. Percent frequency of occurrence (top row) and average cell density (bottom row) 

of the 10 most frequently occurring periphyton taxa in all samples collected before 

and after modification or installation of hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 2003–04 ...................................15

6. Percent frequency of occurrence (top row) and average cell density (bottom row) 

of the seven most frequently occurring periphyton taxa collected on glass slides 

from sites upstream and downstream from hardened low-water crossings on 

selected streams at the Fort Polk Military Reservation, Louisiana, during September 

through October 1999 and September through November 2003 ........................................16

7. Water-quality data for sites upstream and downstream from hardened low-water 

crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 

1998–99 and 2003–04 .................................................................................................................18

8. Average water-quality properties at sites upstream and downstream from 

hardened low-water crossings on Big Brushy Creek, Tributary to East Fork of 

Sixmile Creek, Tributary to Birds Creek, Tributary to Big Brushy Creek, and Little 

Brushy Creek at the Fort Polk Military Reservation, Louisiana, 1998–99 and 

2003–04 ........................................................................................................................................32
v

Conversion Factors, Datums, and Abbreviated Periphyton 

and Water-Quality Units

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

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

°F=(1.8×°C)+32

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

Horizontal coordinate information is referenced to the North American Datum of 1983.

Abbreviated Periphyton and Water-Quality Units

cells per square millimeter (cells/mm2)

micrograms per liter (μg/L)

micrometer (μm)

microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C)

milligrams per liter (mg/L)

milliliter (mL)

millimeter (mm)

square millimeter (mm2)
Blank Page
Abstract

In 2003, the U.S. Geological Survey (USGS), at the 

request of the U.S. Army Joint Readiness Training Center 

and Fort Polk, began a follow-up study to determine whether 

installation and modification of hardened low-water crossings 

had short-term (less than 1 year) or long-term (greater than 

1 year) effects on periphyton or water quality in five streams at 

the Fort Polk Military Reservation, Louisiana. Periphyton data 

were statistically analyzed for possible differences between 

samples collected at upstream and downstream sites and 

before and after low-water crossings were modified on three 

streams, Big Brushy Creek, Tributary to East Fork of Sixmile 

Creek, and Tributary to Birds Creek, during 2003–04. Periphyton data also were analyzed for possible differences between 

samples collected at upstream and downstream sites on two 

streams, Tributary to Big Brushy Creek and Little Brushy 

Creek, during 1998–99 and 2003. Variations in periphyton 

communities could not be conclusively attributed to the modifications. Most of the significant changes in percent frequency 

of occurrence and average cell density of the 10 most frequently occurring periphyton taxa were increases at downstream sites after the hardened low-water crossing installations 

or modifications. However, these changes in the periphyton 

community are not necessarily deleterious to the community 

structure.

Water-quality data collected from upstream and downstream sites on the five streams during 2003–04 were analyzed 

for possible differences caused by the hardened crossings. 

Generally, average water-quality values and concentrations 

were similar at upstream and downstream sites. When average 

water-quality values or concentrations changed significantly, 

they almost always changed significantly at both the upstream 

and downstream sites. It is probable that observed variations 

in water quality at both upstream and downstream sites are 

related to differences in rainfall and streamflow during the 

sample-collection periods rather than an effect of the hardened 

low-water crossing installations or modifications, but additional study is needed.

Introduction

Streams in training areas at the Fort Polk Military 

Reservation, La. (hereinafter referred to as the Reservation) 

(fig. 1), may be affected by the installation, modification, 

and use of hardened low-water crossings. During 1998–99, 

periphyton, water quality, and stream habitat were monitored 

in four streams, Big Brushy Creek (BB), Tributary to East 

Fork of Sixmile Creek (TEF), Tributary to Big Brushy Creek 

(TBB), and Little Brushy Creek (LB), to determine possible 

effects of low-water crossings. Upstream and downstream sites 

were monitored before and after the crossings were hardened 

with interlocking concrete blocks laid over an aggregate base. 

Results of the study are presented in Tollett and others (2002). 

No substantial effects were observed during the 3-month sampling period after the hardened crossings were installed. The 

absence of any indication of substantial short-term (less than 

1 year) effects from hardened crossing installations was, in 

part, due to significant effects of natural seasonal variation in 

the periphyton and water quality. In 2003, the U.S. Geological 

Survey (USGS), at the request of the U.S. Army Joint Readiness Training Center and Fort Polk, began a follow-up study 

to determine whether installation and modification of hardened low-water crossings had short-term (less than 1 year) or 

long-term (greater than 1 year) effects on periphyton or water 

quality in the same four streams and one additional stream, 

Tributary to Birds Creek (TBC), at the Reservation.

Purpose and Scope

This report documents the effects of the installation and 

modification of hardened low-water crossings on periphyton 

communities and water quality in five streams at the Reservation. Periphyton and water-quality data collected from 

sites located upstream and downstream from the low-water 

crossings during 1998–99 and 2003–04 are presented and 

Effects of Hardened Low-Water Crossings on Periphyton 

and Water Quality in Selected Streams at the Fort Polk 

Military Reservation, Louisiana, 1998–99 and 2003–04 

By Barbara W. Bryan1, C. Frederick Bryan1, John K. Lovelace2, and Roland W. Tollett2 

1

 Bryan and Associates.

2

 U.S. Geological Survey.
Figure 1. Location of study area, low-water crossings, and data-collection sites at the Fort Polk Military Reservation, Vernon Parish, 

Louisiana.

Vernon Parish

INDEX MAP

LOUISIANA

TEXAS

GULF OF MEXICO

ARKANSAS

M ISSISSIPPI

Fort Polk

Military

Reservation

Hicks

R.11W.

R.12W. R.10W. R.9W. R.8W. R.7W. R.6W. R.5W.

T.4N.

T.3N.

T.2N.

T.1N.

T.1S.

T.2S.

93° 30 ́

93° 15 ́

93° 00 ́

31° 15 ́

31° 00 ́

Hornbeck Kurthwood

Simpson

Lacamp

Fullerton

Knight Rosepine SABINE

R

IVER

Evans

Pitkin

Leander

Anacoco

New Llano

T E X A 

S

Slagle

10

28

RAPIDES

171

171

8

8

117

NATCHITOCHES

SABINE

BEAUREGARD ALLEN

LEESVILLE

APPROXIMATE BOUNDARY

 OF STUDY AREA

EXPLANATION

10 KILOMETERS

0 10 MILES

0

463

Map credit: Modified from Louisiana Oil Spill Coordinator, Office of the

Governor, Louisiana GIS Digital Map, May 2007 Compilation DVD

LOW-WATER CROSSING

 LOCATION AND DATA-

 COLLECTION SITE

 (see table 1 for site names)

Ea st

Cre ek

Bird s

Fo rk Sixm ile

Little

Brushy

Creek

Cre ek

Cre ek Big

Brushy

BBUP,

BBDN

LBUP,

LBDN

TBBUP,

TBBDN

TBCUP,

TBCDN

TEFUP,

TEFDN

TBCUP,

TBCDN

2 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
summarized. Periphyton data were statistically analyzed for 

possible differences between samples collected at upstream 

and downstream sites and before and after low-water crossings were modified on three streams, BB, TEF, and TBC, 

during 2003–04. Periphyton data also were analyzed for 

possible differences between samples collected at upstream 

and downstream sites on two streams, TBB and LB, during 1998–1999 and 2003. Water-quality data collected from 

upstream and downstream sites on the five streams during 

2003–04 were analyzed for possible differences caused by 

the hardened crossings. Water-quality data collected from two 

streams, BB and TEF, were analyzed for possible differences 

between samples collected at upstream and downstream sites, 

and differences between samples collected before and after the 

low-water crossings were installed in 1999 and later in 2003. 

Data and results are presented in tables and graphs. Information presented in this report will help the U.S. Army assess 

the potential effects of current and future hardened low-water 

crossings on periphyton communities and water quality in 

streams draining the Reservation.

Description of Study Area

The study area is located in east-central Vernon Parish, 

in west-central Louisiana, on the eastern half of the Reservation (fig. 1). The land is used primarily for military training 

and forestry. The streams in the study area are typically small 

(first- and second-order) streams that drain hilly, densely 

forested, piney uplands. Altitudes in the study area range 

from about 300 to 350 ft above the North American Vertical 

Datum of 1988. Drainage basins in this area are characterized by loamy soils, with rapid changes in stream runoff and 

infiltration during heavy rainfall (Tollett and others, 2002). 

Basin characteristics and stream habitat for the study area are 

described in Tollett and others (2002).

The climate in west-central Louisiana is humid subtropical. The average annual rainfall for west-central Louisiana 

is 56 in., and the average annual temperature is 18 °C (Betty 

Wall, Louisiana Office of State Climatology, written commun., 

2002).

Hardened low-water crossings installed at BB, TEF, 

TBB, and LB in the study area are flexible pads consisting of 

cabled or interlocking concrete blocks that form a hardened 

surface path approximately 12-ft wide. Each pad was laid 

along the stream bottom at right angles to the channel and on 

each side of the channel within the approaching roadways to 

a distance sufficient to prevent erosion of soil into the stream 

channel. Each pad was laid on a stabilized base, 12- to 24-in. 

thick, made of aggregate no larger than 1.5 in. in diameter. The 

crossings were excavated to an appropriate depth so that the 

top of the pad was at the natural height of the stream bottom. 

Dolomitic limestone was originally used as the base material 

for the cabled or interlocking concrete pads, and washed pea 

gravel was used as the interstitial filler between the individual 

concrete blocks (Tollett and others, 2002, p. 5). Findings by 

Tollett and others (2002, p. 46) indicated that dissolution of 

the dolomitic limestone may have altered water quality in 

LB, and the base material was subsequently replaced by a 

calcium-free aggregate. The dolomitic limestone at the lowwater crossings at BB and TEF was replaced in 2003 during 

this study.

The hardened low-water crossing installed at TBC consists of a reinforced concrete roadway laid over a stabilized 

aggregate base. The crossing was excavated to an appropriate 

depth so that the top of the roadway is at the natural height of 

the stream bottom.

Methods

To determine whether additional modifications at 

two existing sites on BB and TEF and the installation of a 

hardened low-water crossing at one new site on TBC had 

any effects on periphyton and water quality in the streams, 

monthly periphyton and water-quality samples were collected 

upstream and downstream from each low-water crossing for 

a minimum of 2 months during 2003–04 before and after the 

modification or installation of the hardened crossing (table 1). 

Periphyton and water-quality samples were collected during 

normal flow conditions at upstream and downstream sample 

locations typically within 30 ft of the hardened low-water 

crossing. Sample locations were selected to have available 

submerged woody debris and similar light penetration, stream 

velocity, and stream geometry to facilitate comparison of 

results. To determine whether the modifications had any 

long-term effects on periphyton and water quality at two 

existing sites on TBB and LB, monthly periphyton and waterquality samples were collected upstream and downstream 

from each crossing for a minimum of 3 months during 2003 

(table 1). The resulting data were analyzed and compared to 

data collected at the same sites during 1998–99, and analyzed 

for differences between upstream and downstream sites before 

and after modifications. Data-collection methods used during 

2003–04 were consistent with those used during 1998–99, 

which were documented in Tollett and others (2002).

Periphyton

Periphyton samples were obtained from shallow corings 

made into submerged logs using a sharpened pipe, 19 mm 

in diameter. Periphyton samples also were obtained from 

glass slides (75 mm by 25 mm) that had been suspended 

just below the water surface in diatometers, each containing 

eight slides, placed near the submerged logs for a 30-day 

colonization period. A minimum of four cores and four 

slides usually were collected. Each sample was placed in 

a separate sample jar containing ambient stream water and 

preserved in 4- to 6-percent formalin solution. The samples 

were then sent to Bryan and Associates in Baton Rouge, La., 

for analysis.

Methods 3
Table 1. Periphyton and water-quality sampling dates and hardened low-water crossing installation and modification dates for 

selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04—Continued. 

[I, installation; M, modification]

Site name

Site 

abbreviation 

(fig. 1)

Latitude Longitude

Hardened low-water 

crossing installation or 

modification date

Sampling date

Periphyton Water

quality 

Big Brushy Creek upstream BBUP 31° 02' 31" 92° 54' 44" 05-26-1999 (I) 104-10-2003 04-10-2003

06-17-2003 (M) 05-08-2003 05-07-2003

06-04-2003 06-04-2003

07-01-2003 07-01-2003

08-05-2003 08-06-2003

09-11-2003 09-10-2003

11-06-2003 10-07-2003

11-05-2003

Big Brushy Creek downstreamBBDN 31° 02' 30" 92° 54' 44" 05-26-1999 (I) 104-10-2003 04-10-2003

06-17-2003 (M) 05-08-2003 05-07-2003

06-04-2003 06-04-2003

07-01-2003 07-01-2003

08-05-2003 08-06-2003

09-11-2003 09-10-2003

10-09-2003 10-07-2003

11-06-2003 11-05-2003

Tributary to East Fork of 

Sixmile Creek upstream

TEFUP 31° 02' 20" 92° 57' 53" 06-14-1999 (I) 104-10-2003 04-10-2003

08-08-2003 (M) 05-08-2003 05-07-2003

06-04-2003 06-04-2003

1

07-01-2003 07-01-2003

08-05-2003 08-06-2003

09-11-2003 09-10-2003

10-09-2003 10-07-2003

11-06-2003 11-05-2003

Tributary to East Fork of 

Sixmile Creek downstream

TEFDN 31° 02' 19" 92° 57' 53" 06-14-1999 (I) 104-10-2003 04-10-2003

08-08-2003 (M) 05-08-2003 05-07-2003

06-04-2003 06-04-2003

07-01-2003 07-01-2003

08-05-2003 08-06-2003

09-11-2003 09-10-2003

10-09-2003 10-07-2003

11-06-2003 11-05-2003

Tributary to Big Brushy Creek 

upstream

TBBUP 31° 03' 13" 92° 55' 22" 05-26-1999 (I) 09-11-2003 08-06-2003

10-09-2003 09-10-2003

11-06-2003 10-07-2003

11-05-2003

Tributary to Big Brushy Creek 

downstream

TBBDN 31° 03' 13" 92° 55' 21" 05-26-1999 (I) 09-11-2003 08-06-2003

10-09-2003 09-10-2003

11-06-2003 10-07-2003

11-05-2003

4 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams

Table 1. Periphyton and water-quality sampling dates and hardened low-water crossing installation and modification dates for 

selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04. 

[I, installation; M, modification]
Three samples of each substrate from each site were analyzed. Periphyton samples from wood cores were brushed with 

a modified short-bristled toothbrush and rinsed back into their 

sample jars. Glass slides were scraped with a razor blade and 

rinsed into their sample jars. Samples were increased to known 

volumes (usually 200 mL) using deionized water, subsampled with a wide-mouthed pipette into a modified Utermöhl 

chamber, and allowed to settle overnight before microscopic 

examination. Periphyton counts and identifications were made 

at 1,000 times magnification using Zeiss inverted microscopes. 

Only chloroplast-bearing algal cells and diatom frustules were 

counted; empty cells and frustules were omitted. Cells were 

counted along an average of three transects across the approximate center of the slip covering the bottom of the chamber. 

Cell densities were reported to the nearest 0.1 cell/mm2 (cell 

per square millimeter). Fractions of cells resulted from the 

conversion of actual cell counts per square micrometer of 

substrate, determined using a microscope, to cells per square 

millimeter of the substrate, using the equation detailed in 

Tollett and others (2002, p. 10).

References used for identification and nomenclature 

were Prescott (1951), Patrick and Reimer (1966, 1975), 

Whitford and Schumacher (1973), Krammer and LangBertalot (1986, 1988, 1991a, 1991b), Dillard (1989a, 1989b, 

1990, 1991a, 1991b, 1993), and Cox (1996). To confirm 

identifications, aliquots from some of the more diverse 

samples were burned to remove organic matter (Clesceri and 

others, 1998) and sealed in Hyrax mounting medium on 

slides. Some taxa could not be identified to the species level, 

as sufficient diagnostic features were not seen; these were 

counted as sp. (1 morph) or spp. (greater than or equal to 

2 morphs). Cells of the filamentous colonies (for example, 

Oscillatoria spp., Mougeotia spp., and Ulothrix spp.) were 

counted, but cell densities (cells/mm2) of the globular colonies 

(for example, Microcystis spp. and Gomphosphaeria spp.) 

were estimated by counting cells in several large squares 

Table 1. Periphyton and water-quality sampling dates and hardened low-water crossing installation and modification dates for 

selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04—Continued. 

[I, installation; M, modification]

Site name

Site 

abbreviation 

(fig. 1)

Latitude Longitude

Hardened low-water 

crossing installation or 

modification date

Sampling date

Periphyton Water

quality 

Tributary to Birds Creek 

upstream

TBCUP 31° 02' 05" 93° 03' 19" 12-23-2003 (I) 104-11-2003 04-11-2003

05-08-2003 05-07-2003

06-04-2003 06-04-2003

07-01-2003 07-01-2003

08-05-2003 08-06-2003

05-10-2004 05-10-2004

06-14-2004 06-14-2004

Tributary to Birds Creek 

downstream

TBCDN 31° 02' 04" 93° 03' 19" 12-23-2003 (I) 104-11-2003 04-11-2003

05-08-2003 05-07-2003

06-04-2003 06-04-2003

07-01-2003 07-01-2003

08-05-2003 08-06-2003

05-10-2004 05-10-2004

06-14-2004 06-14-2004

Little Brushy Creek upstream LBUP 31° 02' 48" 92° 56' 29" 10-30-1998 (I) 09-11-2003 08-06-2003

10-09-2003 09-10-2003

11-05-2003 10-07-2003

11-05-2003

Little Brushy Creek downstreamLBDN 31° 02' 47" 92° 56' 29" 10-30-1998 (I) 09-11-2003 08-06-2003

10-09-2003 09-10-2003

11-05-2003 10-07-2003

11-05-2003

1

 No slides collected. 

Methods 5
within the Whipple grid (the grid of a calibrated Whipple 

reticle), while focusing through the colony, and expanding that 

count by the number of squares in the grid occupied by the 

colony.

Methods for the calculation of percent frequency of 

occurrence and average cell density were from Clesceri 

and others (1998). The percent frequency of occurrence for 

each taxon for each site for each month or longer period 

was derived by dividing the number of times a taxon occurred 

by the number of samples collected at the site during the 

period. The overall percent frequency of occurrence of each 

taxon (table 2) was obtained by summing the number of 

times each taxon appeared in samples collected during the 

2003–04 sampling period and dividing by the total number 

of samples. The average cell density for each taxon for each 

site for each month or longer period was derived by summing 

the cell densities of each taxon that appeared in samples 

collected at a site during the period and dividing the sum by 

the total number of samples collected at the site during the 

period in which the taxon appeared. The overall average 

cell density for each taxon (table 2) was obtained by summing the cell densities of each taxon that appeared in samples 

collected during the 2003–04 sampling period and dividing 

the sum by the total number of samples in which the taxon 

appeared.

Water Quality

Water-quality samples were collected at each site concurrently or within 2 days of periphyton collection using USGS 

field methods (Wilde and Radtke, 1998). Water samples were 

collected from the shallow, slow-moving streams by dipping 

the sample bottle just below the surface of the water near the 

center of flow. The mouth of the bottle was pointed upstream, 

as near to the center of flow as possible without disturbing the 

bottom sediments. Filtered samples were obtained by passing 

whole water through a 0.45-μm cellulose nitrate filter capsule 

and pouring into appropriate bottles. Filtered major inorganic 

ion samples were treated with nitric acid to a pH less than 

2 standard units. All bottles were chilled and shipped to the 

USGS National Water Quality Laboratory in Lakewood, Colo.

A multiparameter water-quality meter was suspended 

about 1 ft below the water surface at the center of the stream 

for collection of water-quality data, which included specific conductance, pH, temperature, turbidity, and dissolved 

oxygen. Alkalinity also was determined in the field using 

the inflection point titration method (Wilde and Radtke, 

1998). Concentrations of nutrients, major inorganic ions, 

total dissolved solids, iron, and manganese were determined 

from laboratory analysis of water samples by using methods 

described by Fishman and Friedman (1989).

Nutrients included dissolved ammonia (as nitrogen), dissolved nitrite (as nitrogen), dissolved ammonia plus organic 

nitrogen (as nitrogen), nitrite plus nitrate (as nitrogen), and 

dissolved phosphorus. Major inorganic ions included calcium, 

magnesium, sodium, potassium, sulfate, chloride, fluoride, 

bicarbonate, and silica. Bicarbonate was calculated from alkalinity (Wilde and Radtke, 1998).

Statistical Analysis

Data from downstream sites were first analyzed to 

determine changes that occurred after modifications. Data 

from upstream sites were then analyzed to determine whether 

changes at downstream sites were accompanied by statistically 

significant changes at the upstream site.

Percent frequency of occurrence and average cell density 

of 10 of the most frequently occurring periphyton taxa were 

used to compare streams, upstream and downstream sites, 

and conditions before and after low-water crossing modifications. Selected water-quality data from the streams also were 

used to evaluate differences in water quality. For statistical 

analyses, less than (<) values were replaced with values equal 

to one-half of the less-than value. Periphyton and waterquality data were inspected for normality. Data that were not 

normally distributed (Shapiro-Wilk Test statistic “W” < 0.05), 

were transformed using a natural logarithmic transformation 

(loge), and percentages were transformed using an inverse 

trigonometric sine transformation (ARSIN) prior to statistical 

analyses. Summary statistics and all linear models (One-Way 

Analysis of Variance [ANOVA]), with accompanying pairwise 

comparisons (Tukey’s Honestly Significant Differences [HSD] 

test), were performed using SAS 8.2 (Statistical Analysis 

System, 2001) and Statistix®8 (Analytical Software, 2003). 

Statistical analyses were considered significant when p-values 

were less than or equal to 0.05. Because the sample sizes were 

sometimes small or unequal, it is possible that average differences were not always large enough to be detected by statistical analyses.

Quality Control and Quality Assurance

Quality-control procedures for periphyton included 

analyses of three samples from each substrate at each site during each sampling event to provide an estimate of variance for 

each site and sampling event. One sample from each substrate, 

site, and sampling event was archived in original condition for 

future reference. For quality assurance, about one percent of 

the periphyton samples were sent to Michael Hein, Water and 

Air Research, Inc., Gainesville, Fla., to verify analyses. Photographs were taken of some problematic taxa and were sent to 

Michael Hein to confirm identification. 

Quality-control water samples were collected to ensure 

sample-collection, sample-processing, and laboratory analytical procedures did not introduce bias into results and to 

determine the variability associated with collection and 

analysis of samples. Quality-control procedures included 

calibration of instruments for field measurements of pH, 

specific conductance, water temperature, and turbidity. The 

pH and specific conductance probes on the multiparameter 

6 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
water-quality meter and a separate pH meter (used for alkalinity titrations) and turbidity meter were checked with laboratory-grade standard solutions each day before use, following 

manufacturer’s procedures and USGS methods (Wilde and 

Radtke, 1998). The temperature probe was checked prior to 

each sampling period against a thermometer traceable to the 

National Institute of Standards and Technology.

Quality-control procedures also included the collection 

of seven sequential replicate samples and two field-blank 

samples. All water-quality data collected for this report are 

stored in the USGS National Water Information System and 

are on file at the USGS office in Baton Rouge, La.

Replicate samples were collected to quality assure 

sample-collection methods and laboratory-analytical 

procedures. The relative percent difference (RPD) between 

constituent concentrations in a sample and the corresponding 

sequential replicate were calculated by multiplying 100 by 

the absolute value of the difference between the sample and 

sequential replicate concentration divided by the average of 

the sample and sequential replicate concentrations. The RPDs 

were not calculated when concentrations were estimated in 

the laboratory or were not detected for either the environmental or replicate sample. The RPDs between the samples and 

replicates for major inorganic ions in water typically were 

<5 percent (<, less than or equal to). The RPDs for calcium, 

residue on evaporation, ammonia, nitrite, nitrite plus nitrate, 

phosphorus, and iron exceeded 5 percent for one or more 

sample set. The highest RPDs were for phosphorus (86 percent), nitrite (67 percent), iron (65 percent), and residue on 

evaporation (58 percent). These high RPDs were primarily due 

to relatively low concentrations of constituents in the samples 

and sequential replicates and did not reflect substantial waterquality differences between the samples and sequential 

replicates.

Field-blank samples collected at two sites were analyzed 

to assure that collection, preparation, processing, and analytical procedures did not contaminate the samples. Samples of 

inorganic-free blank water were poured at the site or passed 

through processing equipment at the site into appropriate 

bottles. The blank samples were analyzed for major inorganic 

ions and selected nutrients. Concentrations of major inorganic 

ions were less than analytical reporting limits. All nutrient 

concentrations were less than or equal to analytical reporting 

limits.

Effects of Hardened Low-Water 

Crossings on Periphyton

A total of 362 periphyton taxa were identified from 

312 samples analyzed (table 2). Diatoms (Chrysophyta) were 

the most diverse group of algae, with 144 taxa identified. 

Green algae (Chlorophyta) ranked second, with 136 taxa. 

Blue-green algae (Cyanophyta) ranked third with 62 taxa 

(table 3). The remaining 20 taxa identified were distributed 

among four other plant groups. Diatom taxa outnumbered the 

other major divisions by at least 2.5 times in 90 percent of the 

samples. Green algae taxa outnumbered diatoms in <5 percent 

of the samples. The percent frequency of occurrence of major 

algal divisions in presented in table 3.

There was no significant difference in percent frequency 

of occurrence or cell density of taxa between substrates 

(slides and wood cores). Consequently, percent frequency of 

occurrence and cell density of all monthly samples from all 

substrate types for each of the 10 most frequently occurring 

taxa on both substrates were summed and averaged (table 4), 

and ANOVA’s and pairwise comparisons (Tukey’s HSD) were 

performed to check for possible differences between upstream 

and downstream sites. The only taxon in which the percent 

frequency of occurrence and cell density varied between 

upstream and downstream sites was Eunotia flexuosa, which 

occurred in a significantly (p<0.05) lower percentage of the 

samples at BB upstream (BBUP) (table 4). Although some 

differences in percent frequency of occurrence or average 

cell density for other taxa at other upstream and downstream 

sites may have been larger than Eunotia flexuosa at BB, the 

wide variation in monthly values, relative to the degrees of 

freedom (a maximum of six samples each month) at each site 

precluded the finding of significant variations in the periphyton communities.

Percent frequency of occurrence and cell density (table 2) 

for all samples varied similarly, and 17 taxa occurred in more 

than 40 percent of the samples. As in the earlier study (Tollett 

and others, 2002), diatoms were the most frequently occurring 

algae (table 3), and filamentous green algae and colonial bluegreen forms were numerically dominant when they occurred 

(fig. 2, table 2). The colonial blue-green alga, Microcystis 

incerta, was present in 60.2 percent of the samples, and the 

filamentous green alga, Oedogonium spp., was present in 43.3 

percent of the samples. Although the average cell density of 

the four most frequently occurring diatoms ranged from 37 to 

100 cells/mm2, the combined average cell densities of the 20 

most frequently occurring diatoms totaled 674.8 cells/mm2, 

which was less than 8 percent of the average cell density of 

the most abundant blue-green alga, Microcystis incerta, 

which was 5,300 cells/mm2. Growth habits of different algal 

divisions tended to be different. Diatoms typically appeared 

singly or in small, loosely connected groups, whereas green 

algae typically were species (for example, Oedogonium spp. 

and Ulothrix sp.) that develop long filaments. Blue-green 

algae were species that develop large globular (for example, 

Microcystis incerta), plate-like (for example, Merismopedia

spp.), or filamentous (for example, Oscillatoria spp.) colonies. The average cell densities of blue-green algae and green 

algae also tended to fluctuate from month-to-month, while the 

diatoms occurred more consistently at most sites during the 

study (fig. 2).

Periphyton data from BB, TEF, and TBC collected 

before and after the low-water crossings were modified during 

2003 were analyzed to determine whether changes may have 

occurred. Variations in periphyton communities could not be 

Effects of Hardened Low-Water Crossings on Periphyton 7
Table 2. Percent frequency of occurrence and average cell density for periphyton taxa on wood cores and glass slides collected at 

sites upstream and downstream from low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04.

[sp., one morph; spp., two or more morphs; var., variety] 

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Cyanophyta

Anabaena spp. 13.4 267

Aphanocapsa delicatissima 5.1 418

Aphanocapsa elachista var. conferta 4.9 786

Aphanocapsa elachista var. planctonica .6 130

Aphanocapsa endophytica .3 2,160

Aphanocapsa spp. 1.9 151

Aphanothece Castagnei .3 3,040

Aphanothece clathrata 1.0 791

Aphanothece nidulans 2.8 459

Aphanothece nidulans var. endophytica .3 3.5

Aphanothece saxicola 19.5 415

Aphanothece spp. 1.6 348

Calothrix sp. .6 68.5

Chamaesiphon spp. 1.0 2,070

Chroococcus dispersus 39.1 104

Chroococcus dispersus var. minor 27.9 191

Chroococcus limneticus 1.3 123

Chroococcus limneticus var. distans .6 48.7

Chroococcus Prescottii .3 100

Chroococcus turgidus 1.3 47.0

Coelosphaerium pallidum 3.5 117

Coelosphaerium spp. 17.6 1,250

Cylindrospermum spp. 1.6 149

Dactylococcopsis acicularis .3 6.7

Dactylococcopsis sp. .3 2.5

Fischerella spp. .6 1,080

Gloeocapsa spp. 1.6 1,190

Gloeothece linearis 1.3 124

Gloeothece rupestris .3 9.4

Gloeothece spp. 3.2 231

Gomphosphaeria lacustris 3.5 362

Gomphosphaeria spp. 21.5 1,560

Hapalosiphon aureus .6 162

Hapalosiphon flexuosus .3 202

Hapalosiphon hibernicus .6 40.3

Hapalosiphon spp. 5.1 1,520

Johannesbaptistia pellucida 2.6 85.0

Lyngbya spp. 1.3 196

Merismopedia tenuissima 1.0 2,570

Microcystis aeruginosa 1.6 152

Microcystis incerta 60.2 5,300

Microcystis spp. 5.1 7,010

Nodularia spp. .6 224

Nostoc spp. 3.2 1,420

Oscillatoria geminata 8.6 180

Oscillatoria lacustris .3 19.0

Oscillatoria prolifica .6 48.0

Oscillatoria splendida .3 15.0

Oscillatoria spp. 24.7 384

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Cyanophyta—Continued

Oscillatoria subtilissima 0.3 47.6

Oscillatoria tenuis 1.3 456

Phormidium inundatum .3 58.0

Phormidium spp. 19.6 184

Phormidium tenue 1.6 661

Plectonema spp. .6 75.0

Rhabdoderma Gorskii .6 221

Rhabdoderma sigmoidea .3 162

Rhabdoderma sp. 1.3 135

Schizothrix spp. 1.0 1,110

Scytonema spp. 1.3 2,050

Stigonema mamillosum .3 1,180

Symploca muscorum .6 724

Chlorophyta

Actinotaenium clevei .3 .7

Actinotaenium cucurbitinum var. longum .6 .2

Ankistrodesmus falcatus .3 462

Aphanochaete repens .3 106

Aphanochaete sp. .3 25.0

Basal cells 9.0 5.2

Bulbochaete spp. 17.0 37.3

Carteria sp. .3 4.0

Characium obtusum .3 23.1

Characium Rabenhorstii .3 .2

Characium spp. 2.6 4.5

Chlamydomonas spp. 11.2 7.6

Chlorella spp. 1.3 16.0

Chlorococcum humicola 1.0 10.3

Chlorococcum spp. .6 28.2

Closterium acerosum .6 .5

Closterium angustatum .3 .4

Closterium archerianum form grande .3 .7

Closterium cornu .3 1.0

Closterium gracile var. elongatum .3 2.6

Closterium intermedium 1.0 6.8

Closterium jenneri 1.0 2.0

Closterium juncidum .6 .7

Closterium lineatum .3 11.5

Closterium macilentum .3 3.8

Closterium moniliferum 1.6 1.3

Closterium parvulum 1.3 8.8

Closterium parvulum var. angustatum 2.6 3.5

Closterium planum .3 6.3

Closterium praelongum .3 3.8

Closterium ralfsii .3 .8

Closterium setaceum 1.3 .7

Closterium setaceum form sigmoideum .6 3.8

Closterium spp. 7.4 2.8

Closterium toxon 1.0 6.3

8 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chlorophyta—Continued

Coelosphaerium pallidum 0.3 290

Coleochaete spp. 1.9 46.8

Cosmarium angulosum .3 9.6

Cosmarium crenatum .3 .7

Cosmarium cucurbitinum .3 .3

Cosmarium exiguum 1.0 10.6

Cosmarium exiguum var. subrectangulum 1.0 6.8

Cosmarium margaritatum 1.0 .7

Cosmarium minimum .9 5.8

Cosmarium minutissimum .3 9.4

Cosmarium minutissimum var. depressum .6 25.0

Cosmarium punctulatum 2.2 6.9

Cosmarium punctulatum var.

subpunctulatum

.3 2.0

Cosmarium raciborskii .3 2.0

Cosmarium regnellii .3 .8

Cosmarium spp. 11.5 5.8

Cosmarium subtumidum 1.6 2.2

Cosmarium tenue .6 27.0

Cosmarium tenue var. minus 1.6 7.0

Cosmarium tinctum .3 19.0

Cosmarium tumidum .3 .5

Cosmarium turpinii .3 .7

Crucigenia sp. .3 26.8

Cylindrocapsa conferta 1.0 143

Cylindrocapsa geminella 1.6 694

Cylindrocapsa spp. 8.3 115

Cylindrocystis brebissonii var. minor .3 1.2

Desmidium sp. 1.0 3.3

Dicranochaete reniformis 11.2 4.0

Euastrum affine 1.0 3.0

Euastrum ansatum .3 7.3

Euastrum binale .3 1.1

Euastrum denticulatum .3 .7

Euastrum didelta .3 7.7

Euastrum insulare .3 2.6

Euastrum insulare var. lacustre .3 .2

Euastrum spp. 1.3 9.0

Euastrum trigiberrum .3 11.2

Franceia spp. .6 3.8

Geminella interrupta .3 34.1

Geminella minor 1.0 18.4

Gloeocystis gigas .3 1.7

Gloeocystis major .3 1.8

Gloeocystis spp. 4.5 54.7

Gloeocystis vesiculosa .6 2,200

Golenkenia sp. .3 3.1

Gonatozygon pilosum .3 1.0

Hormidium spp. 4.8 65.0

Hyalotheca dissiliens .3 11.4

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chlorophyta—Continued

Hyalotheca spp. 0.6 8.9

Kirchneriella contorta .3 19.2

Kirchneriella lunaris .3 3.1

Micrasterias denticulata var. intermedia .6 5.2

Micrasterias spp. 1.3 27.0

Microspora pachyderma .6 21.4

Microspora spp. 9.6 68.5

Mougeotia micropora .3 26.0

Mougeotia spp. 25.0 73.0

Netrium digitus .3 1.3

Netrium digitus var. naegelii 1.0 1.4

Netrium interruptum .3 9.0

Netrium oblongum var. cylindricum .3 .8

Netrium spp. 1.9 2.6

Oedogonium spp. 43.3 31.6

Oocystis spp. 9.6 24.9

Penium exiguum .6 2.1

Penium margaritaceum .6 2.1

Penium spp. 1.0 .7

Phymatodocis nordstedtiana form minor .3 2.5

Pleodorina spp. .3 256

Pleurotaenium spp. .6 2.4

Protococcus spp. 5.1 39.1

Protoderma viride 8.0 77.3

Rhizoclonium spp. 5.4 54.9

Schroederia spp. 2.6 3.6

Selenastrum sp. .3 19.0

Sorastrum spinulosum .3 14.0

Sphaerocystis Schroeteri .6 149

Sphaerocystis spp. 2.6 112

Spirogyra spp. 7.7 16.4

Spondylosium planum 4.8 9.4

Spondylosium spp. 4.8 5.3

Staurastrum alternans 6.4 11.3

Staurastrum brebissonii .3 1.0

Staurastrum dilatatum 1.6 2.6

Staurastrum hexacerum .3 10.2

Staurastrum margaritaceum .3 8.0

Staurastrum punctulatum .3 5.2

Staurastrum spp. 2.6 5.6

Stigeoclonium sp. 1.3 234

Stigonema mamillosum .3 2,300

Tetmemorus brebissonii 2.2 7.4

Tetmemorus laevis 1.0 2.7

Tetmemorus spp. 1.0 1.1

Tetraedron regulare .3 7.7

Ulothrix spp. 2.5 13.4

Ulothrix subconstricta .3 55.4

Ulothrix subtilissima 1.0 25.8

Ulothrix variabilis 5.8 41.6

Table 2. Percent frequency of occurrence and average cell density for periphyton taxa on wood cores and glass slides collected at 

sites upstream and downstream from low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–

04—Continued. 

[sp., one morph; spp., two or more morphs; var., variety]

Effects of Hardened Low-Water Crossings on Periphyton 9
Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chlorophyta—Continued

Volvox spp. 0.6 145

Zygnema sp. .3 114

Chrysophyta

(Class Bacillariophyceae)

Achnanthes clevei 1.3 15.1

Achnanthes delicatula .6 7.0

Achnanthes exigua 2.2 21.8

Achnanthes lanceolata 8.3 15.5

Achnanthes lanceolata var. dubia 1.3 38.2

Achnanthes lanceolata var. rostrata .3 18.8

Achnanthes linearis .6 7.1

Achnanthes marginulata 4.8 16.3

Achnanthes minutissima 48.0 16.3

Achnanthes rosenstockii 3.8 20.2

Achnanthes sp. .3 1.0

Amphora ovalis .6 .3

Aulacoseira granulata .6 .8

Aulocoseira herzogii .3 47.0

Aulocoseira sp. .6 7.4

Brachysira serians var. brachysira 78.8 100

Brachysira vitrea 41.3 13.5

Cocconeis sp. .3 8.2

Cymbella affinis 1.6 3.4

Cymbella amphicephala .3 .7

Cymbella cuspidata 3.5 7.2

Cymbella gracilis 1.0 7.6

Cymbella helvetica .3 9.4

Cymbella lunata 1.0 15.0

Cymbella microcephala .3 4.0

Cymbella minuta 1.9 14.6

Cymbella minuta var. pseudogracilis .6 19.5

Cymbella spp. 3.2 6.3

Diatoma anceps .3 1.1

Diploneis parma .3 2.6

Encyonema gaeumannii 22.1 8.6

Encyonema minutum 1.3 1.3

Encyonema neogracile 44.6 26.9

Eunotia bidentula 2.9 9.0

Eunotia bilunaris 93.9 93.2

Eunotia diodon 55.4 15.0

Eunotia exigua 59.9 15.2

Eunotia flexuosa 76.0 45.2

Eunotia formica 39.7 24.4

Eunotia incisa 40.1 27.8

Eunotia minor 1.0 9.0

Eunotia monodon 48.7 18.8

Eunotia muscicola 6.1 4.3

Eunotia muscicola var. tridentula 9.0 6.0

Eunotia naegelii 73.4 37.2

Eunotia paludosa 66.3 29.0

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chrysophyta—Continued

Eunotia pectinalis 1.0 2.8

Eunotia pectinalis var. minor 27.9 14.4

Eunotia pectinalis var. undulata 1.6 23.9

Eunotia praerupta 8.3 10.4

Eunotia praerupta var. bidens .6 3.2

Eunotia serra var. diadema .3 5.7

Eunotia serra var. tetraodon 1.6 5.7

Eunotia soleirolii .3 2.7

Eunotia spp. 1.9 16.4

Eunotia sudetica .3 14.2

Eunotia tenella .3 4.8

Eunotia triodon .3 .5

Eunotia zygodon .3 18.9

Fragilaria capucina 11.8 20.9

Fragilaria crotonensis .3 .2

Fragilaria delicatissima 3.5 20.6

Fragilaria exigua .6 5.0

Fragilaria spp. 13.8 6.6

Fragilaria tenera 20.5 16.5

Fragilaria ulna 10.6 18.8

Fragilaria vaucheriae .6 19.4

Frustulia rhomboides 66.7 48.1

Frustulia rhomboides var.

amphipleuroides

15.4 17.5

Frustulia rhomboides var. capitata 53.5 65.1

Frustulia rhomboides var. saxonica 48.7 45.3

Frustulia vulgaris 3.2 9.9

Gomphonema affine 5.8 11.3

Gomphonema angustatum 12.5 13.0

Gomphonema angustum 32.0 24.4

Gomphonema brasiliense 8.0 16.0

Gomphonema gracile 27.6 19.9

Gomphonema minutum 2.9 18.8

Gomphonema parvulum 10.9 13.6

Gomphonema spp. 8.3 4.9

Hantzschia amphioxys .6 12.5

Navicula angusta 11.8 16.7

Navicula bryophila .3 184

Navicula capitata 1.0 22.6

Navicula cincta 4.2 17.2

Navicula cryptocephala 29.2 15.9

Navicula cryptotenella 21.2 8.1

Navicula exigua .3 3.8

Navicula gottlandica 34.9 13.5

Navicula hungarica .3 .7

Navicula ingenua 1.3 3.9

Navicula jaernefeltii .6 5.9

Navicula lateropunctata 10.2 10.3

Navicula minima 2.6 3.7

Navicula notha 4.2 10.3

Table 2. Percent frequency of occurrence and average cell density for periphyton taxa on wood cores and glass slides collected at 

sites upstream and downstream from low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–

04—Continued. 

[sp., one morph; spp., two or more morphs; var., variety]

10 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
conclusively attributed to the modifications. Data analysis 

indicated no significant seasonal variation, a dominant factor in a previous study (Tollett and others, 2002), probably 

because no periphyton samples were collected during winter 

months.

Comparison (Tukey’s HSD) of percent frequency of 

occurrence of the 10 most frequently occurring taxa showed 

that Eunotia flexuosa and Frustulia rhomboides at BB indicated significant interaction between both upstream and downstream sites, and before and after the crossing modification 

on BB (table 5). Comparison of average cell density of 

the 10 most frequently occurring taxa showed Eunotia exigua 

at BB and Eunotia bilunaris at TEF indicated significant 

interaction between both upstream and downstream sites, 

and before and after crossing modifications on those streams 

(table 5). Because of these interactions, it was not possible 

to conclude that the hardened low-water crossing modifications caused the differences in those taxa. For example, 

even though the average cell densities of Eunotia bilunaris

at TEF were significantly lower at both upstream and 

downstream sites after the low-water crossings were 

modified, the difference cannot be assigned to the crossing 

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chrysophyta—Continued

Navicula obscura 0.3 28.4

Navicula radiosa 1.9 11.2

Navicula rhynchocephala 1.6 10.5

Navicula schroeteri var. escambia 1.0 20.7

Navicula spp. 15.1 11.4

Neidium apiculatum 1.6 6.2

Neidium hercynicum form substratum .3 3.8

Neidium iridis .3 1.9

Neidium ladogense var. densestriatum 2.9 4.7

Neidium spp. .6 .7

Nitzschia acicularis .6 5.4

Nitzschia angustata .3 1.0

Nitzschia filiformis 1.0 18.4

Nitzschia flexoides .6 2.7

Nitzschia frustulum 1.0 5.4

Nitzschia gracilis 1.6 16.1

Nitzschia levidensis 1.0 6.0

Nitzschia palea 3.5 10.2

Nitzschia paleacea 14.1 10.8

Nitzschia paleaeformis 1.6 21.7

Nitzschia sigmoidea 1.9 10.4

Nitzschia spp. 5.4 4.0

Pinnularia abaujensis 14.7 9.4

Pinnularia biceps 23.7 8.7

Pinnularia maior .3 7.1

Pinnularia maior var. transversa .3 .7

Pinnularia mesolepta .6 2.1

Navicula parvula .3 11.8

Pinnularia spp. 13.1 7.4

Pinnularia streptoraphe .6 7.6

Pinnularia subcapitata 1.0 4.5

Rhopalodia gibberula 7.6 100

Sellophora pupula var. rectangularis 7.4 8.2

Stauroneis anceps .3 1.3

Stenopterobia curvula 13.8 9.0

Surirella amphyoxis .3 4.8

Surirella angusta 24.0 9.4

Surirella brebissonii 1.0 9.7

Surirella linearis 1.0 8.1

Taxon

Percent

frequency

of 

occurrence

Average

cell

density

(cells per 

square

millimeter)

Chrysophyta—Continued

Surirella minuta 1.3 2.4

Surirella roba 1.3 7.2

Surirella robusta 1.0 8.8

Surirella spp. 7.0 7.8

Surirella tenera 4.5 4.9

Synedra acus 1.6 5.0

Synedra spp. 3.5 3.5

Synedra ulna 2.9 6.1

Tabellaria flocculosa .3 .7

Tabellaria sp. .3 .4

Chrysophyta

(Class Chrysophyceae)

Derepyxis amphora 3.2 3.0

Derepysix spp. 2.6 6.0

Dinobryon spp. 2.9 2.8

Lagynion Scherffelii 5.1 12.9

(Class Cryptophyceae)

Cryptomonas sp. .3 3.2

(Class Xanthophyceae)

Characiopsis sp. 1.9 .8

Chlorochromonas sp. .6 .6

Ophiocytium desertum var. minor .6 6.2

Ophiocytium spp. 1.6 9.0

Peroniella planctonica .3 5.2

Peroniella sp. 1.0 5.6

Stipitococcus apiculatus .3 14.4

Stipitococcus spp. 1.3 3.9

Stipitococcus vasiformis .3 .8

Euglenophyta

Phacus sp. .3 .7

Trachelomonas spp. 1.0 2.9

Rhodophyta

Batrachospermum turfosum 12.5 453

Colonial Bacterium

Sphaerotilus sp. 17.6 96.6

Ciliata

Unknown ciliates 21.2 5.9

Vorticella spp. 3.8 3.8

Table 2. Percent frequency of occurrence and average cell density for periphyton taxa on wood cores and glass slides collected at 

sites upstream and downstream from low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–

04—Continued.

[sp., one morph; spp., two or more morphs; var., variety]

Effects of Hardened Low-Water Crossings on Periphyton 11
modification because of the interaction with the differences 

due to location (table 5). Although average cell densities of 

Eunotia bilunaris at TEF downstream (TEFDN) were significantly different before and after the modification, the average 

cell densities also were significantly different from average 

cell densities at TEF upstream (TEFUP) before and after 

modification.

Most of the significant changes in percent frequency of 

occurrence and average cell density of the 10 most frequently 

occurring periphyton taxa were increases at downstream sites 

after the hardened low-water crossing installations or modifications. However, these changes in the periphyton community 

are not necessarily deleterious to the community structure.

At BB, the reasons for significant differences in percent 

frequency of occurrence of Eunotia flexuosa and Frustulia 

rhomboides, as well as cell density of Eunotia exigua, also 

were inconclusive due to the interactions (table 5). However, average cell densities for Achnanthes minutissima were 

significantly low at the BB downstream (BBDN) before 

crossing modifications and significantly high at BBUP after 

the modification. Yet, average cell densities of Achnanthes 

minutissima at BBUP before the modification were similar to 

those at BBDN after the modification. Although the increase 

in average cell densities of Achnanthes minutissima at BBUP 

due to the crossing modification downstream is unlikely, it 

could be argued that if the canopy around the crossing was 

opened up because of the low-water crossing modification, 

then an increase in ambient sunlight could result in an increase 

in cell densities of the periphyton upstream, but not downstream from the crossing. Cell densities were higher, although 

not significantly higher, for only two other taxa (Brachysira 

serians var. brachysira and Eunotia naegelii) at BBDN after 

the modification. Average cell densities of the other six taxa 

either remained the same or were lower in density downstream 

after the modification (table 5).

At TEF, the highest average cell density for Eunotia 

flexuosa was at the TEFDN before crossing modifications, 

yet densities were similar at TEFUP and TEFDN after 

the stream-crossing modification (table 5). Conversely, 

Frustulia rhomboides var. crassinervia was significantly low 

in frequency of occurrence at TEFDN before the crossing was 

modified, although this taxon was similar in percent frequency 

of occurrence at TEFUP, before and after the modification, 

as well as at TEFDN after the modification. The percent 

frequency of occurrence for Microcystis incerta was lowest 

at TEFUP before the modification and appeared with similar 

frequency in samples from TEFDN after the modification; the 

percent frequency of occurrence at TEFUP after the modification was similar to that at TEFDN both before and after the 

crossing was modified.

At TBC, percent frequency of occurrence of Eunotia 

flexuosa was highest at TBC upstream (TBCUP) both before 

and after the crossing modification, and the cell density of this 

taxon was significantly low at TBC downstream (TBCDN) 

both before and after modification (table 5). Similarly, the 

average cell density of Eunotia naegelii was significantly low 

at TBCDN before the crossing modification and significantly 

high at TBCUP before the modification. Although the average cell density of Eunotia naegelii was significantly higher 

at TBCUP than at TBCDN before the modification, average 

cell densities for this taxon at TBCUP and TBCDN were 

similar after the crossing modification. Average cell densities 

of Eunotia exigua at both TBCUP and TBCDN were similar 

before and after the modification, but were both significantly 

higher after crossing modification. Conversely, the percent frequency of occurrence of Achnanthes minutissima was significantly lower at TBCDN than at TBCUP, both before and after 

modification, suggesting that the crossing modification had no 

effect on the taxon’s frequency of occurrence.

Periphyton data collected from LB and TBB after the 

hardened low-water crossings were installed in 1999 and 

periphyton data collected in 2003 were analyzed to determine 

whether long-term changes may have occurred after the installation (table 6). Although periphyton samples from submerged 

vegetation and gravel also were collected during 1998–99, 

only samples from glass slides and wood cores collected 

during 1998–99 were used in the comparisons to 2003–04 

data. Pairwise comparisons were performed on averages for 

percentage frequency of occurrences and cell densities of 

seven taxa common in both 1999 and 2003 data sets (table 6). 

The percent frequency of occurrence of Eunotia bilunaris was 

significantly lower at TBB downstream (TBBDN) than at TBB 

upstream (TBBUP) during 1999; but the percent frequency 

of occurrence at TBBUP during 1999 was similar to that at 

TBBUP and TBBDN during 2003. However, the percent frequency of occurrence Frustulia rhomboides var. crassinervia 

was not different at TBBUP and TBBDN for each study 

period, but the percent frequency of occurrences at both sites 

were significantly lower during 1999 than during 2003.

At LB, there was significant interaction among the factors (upstream and downstream and before and after) in the 

ANOVA analyses of the average percent frequency of occurrences and average cell densities of Eunotia paludosa. This 

indicates that it is inconclusive whether or not the hardened 

crossing modification affected either attribute of this taxon. 

During 1999, average cell densities of Frustulia rhomboides 

Table 3. Percent frequency of occurrence of major algal 

divisions in selected streams at the Fort Polk Military Reservation, 

Louisiana, 2003–04. 

Percent frequency

of occurrence

Number of taxa

DiatomsGreen 

algae

Blue-green 

algae Other

Greater than or equal to 80 1 0 0 0

70–79 3 0 0 0

60–69 2 0 1 0

50–59 3 0 0 0

40–49 6 1 0 0

30–39 3 0 1 0

20–29 8 1 3 1

10–19 12 4 4 2

Less than or equal to 10 106 130 53 17

Total number of taxa 144 136 62 20

12 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
Table 4. Percent frequency of occurrence (top row) and average cell density (bottom row) for the 10 most frequently occurring periphyton taxa in all samples collected at 

upstream and downstream sites on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04.[See fig. 1 for location of site. Tributary to Big Brushy Creek and Little Brushy Creek were sampled only during autumn 2004; the remaining sites were sampled during spring, summer, and autumn 2003. Cell density is in cells per square millimeter. Analysis of variance and pairwise comparisons were done on data from samples collected within the same month. Sites with different superscripts are significantly different (p less than or equal to 0.05). var., variety]

Taxon

Big Brushy Creek

(BB)

Tributary to East Fork of 

Sixmile Creek (TEF)

Tributary to Big Brushy 

Creek (TBB)

Tributary to Birds Creek 

(TBC)

Little Brushy Creek 

(LB)

Upstream Down- stream Upstream Down- stream Upstream Down- stream Upstream Down- stream Upstream Down- stream

Eunotia bilunaris 94.4 82.2 100.0 97.8 100.0 94.4 94.4 97.4 87.5 94.4

117.8 78.9 94.7 105.4 63.5 41.3 181.0 63.0 23.9 70.7

Brachysira serians brachysira 72.2 62.2 85.7 84.4 77.8 83.3 83.3 57.9 75.0 88.9

38.0 100.0 66.2 81.0 180.8 20.5 317.0 68.8 15.5 70.8

Eunotia flexuosa 53.6b 69.8ab 100.0a 93.4ab 83.3a 50.0a 100.0a 71.2ab 81.2a 66.7a

29.9 12.2 23.8 51.3 10.3 5.0 157.0 15.8 18.5 25.2

Eunotia naegelii 66.7 60.0 85.7 80.0 83.3 61.1 86.3 65.8 93.8 50.0

40.4 17.5 41.1 49.2 20.7 10.8 73.3 24.3 20.4 23.1

Frustulia rhomboides 41.7 42.2 76.2 77.8 94.4 94.4 72.2 36.8 93.8 100.0

30.2 14.3 13.8 87.5 108.9 30.6 75.0 9.0 26.0 53.4

Frustulia rhomboides var. crassinervia 55.6 24.4 71.4 62.2 66.7 38.9 69.4 63.2 50.0 50.0

59.0 17.4 27.0 77.1 71.8 7.2 186.0 45.1 9.9 35.2

Eunotia paludosa 75.0 75.6 80.9 75.6 61.1 33.3 66.7 55.3 37.5 61.1

35.4 44.3 22.3 27.8 11.9 30.9 35.6 17.1 4.1 28.5

Eunotia exigua 72.2 48.9 50.0 53.3 55.6 66.7 69.4 65.8 62.5 61.1

15.3 17.1 8.7 7.9 8.2 7.8 24.2 18.1 7.3 33.6

Microcystis incerta 66.7 64.4 61.9 66.7 33.3 44.4 58.3 72.0 37.5 50.0

2,371.0 3,517.0 1,423.0 4,828.0 1,198.0 475.3 2,789.0 3,854.0 557.0 53,201.0

Achnanthes minutissima 66.7 40.0 40.5 53.3 33.3 55.6 52.8 32.4 50.0 72.2

18.3 7.0 14.0 11.8 12.6 5.0 38.9 12.4 20.0 15.2

Effects of Hardened Low-Water Crossings on Periphyton 13
Figure 2. Monthly average cell density of blue-green, green, and diatom algae from sites upstream and downstream from hardened 

low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04. (See fig. 1 for location of site.) 

14 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams MONTHLY 

AVERAGE CELL DENSITY, IN CELLS PER SQUARE MILLIMETER

Big Brushy Creek (BB)

2003

APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. 1

10

100

1,000

10,000

100,000

1,000,000

No dataNo dataNo data

Tributary to East Fork Sixmile Creek (TEF)

2003

APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. 1

10

100

1,000

10,000

100,000

1,000,000

Tributary to Birds Creek (TBC)

2003

APR. MAY JUNE JULY AUG. MAY JUNE

1

10

100

1,000

10,000

100,000

1,000,000

2004

Blue-green algae, upstream Blue-green algae, downstream

Green algae, upstream Green algae, downstream

Diatom algae, upstream Diatom algae, downstream

EXPLANATION

Low-water crossing

 modification date

Low-water crossing

 installation date
Table 5. Percent frequency of occurrence (top row) and average cell density (bottom row) of the 10 most frequently occurring periphyton taxa in all samples collected before 

and after modification or installation of hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 2003–04.[See fig. 1 for location of site. Cell density is in cells per square millimeter. Averages with different superscripts are significantly different (p less than or equal to 0.05). var., variety]

Taxon

Big Brushy Creek (BB) Tributary to East Fork Creek (TEF) Tributary to Birds Creek (TBC)

Upstream Downstream Upstream Downstream Upstream Downstream

Before After Before After Before After Before After Before After Before After

Eunotia bilunaris 89.0 91.8 83.3 80.2 100.0 100.0 96.7 100.0 93.4 100.0 96.0 100.0

104.8 81.1 127.5 33.5 1125.5 132.8 1120.8 176.6 126.1 226.1 153.6 18.0

Brachysira serians brachysira 55.3 70.5 44.7 73.2 80.2 94.3 73.4 94.3 50.6 91.5 51.4 83.5

21.9 22.0 2.4 91.8 61.0 58.1 73.8 60.3 179.9 388.7 47.0 20.3

Eunotia flexuosa 167.0 154.2 177.7 143.0 100.0 89.0 93.4 78.0 100.0 100.0 71.2 67.0

17.6 14.1 10.1 5.1 24.4b 16.2b 68.9a 7.8b 86.3b 171.0ab 8.8c 14.3c

Eunotia naegelii 61.0 62.5 61.0 56.6 86.6 77.7 80.0 77.7 90.0 83.5 56.0 75.0

31.4 18.0 8.3 11.6 42.8 18.0 54.1 17.2 62.3a 49.1ab 2.9c 44.2b

Frustulia rhomboides 133.0 141.8 139.0 146.6 77.7 80.2 70.0 94.3 73.4 75.0 40.0 33.0

7.6 11.3 5.4 5.4 8.1 12.4 61.1 78.6 17.7 121.8 3.3 3.2

Frustulia rhomboides var. 

crassinervia

39.0 29.0 16.7 26.8 73.6a 72.3a 53.4b 66.7a 56.8 100.0 46.6 91.0

7.6 19.2 4.6 4.1 39.4 18.6 44.2 53.6 31.0 319.1 25.2 19.2

Eunotia paludosa 94.3 54.2 83.3 63.6 83.4 77.7 76.8 72.0 76.6 58.0 61.4 33.0

28.9 17.4 49.9 23.8 20.7 11.0 7.4 30.0 8.3 28.0 10.2 4.2

Eunotia exigua 50.0 79.2 39.0 49.8 36.8 66.7 53.4 55.7 49.8 91.5 57.4 83.5

14.7 111.0 112.5 16.7 2.1 6.9 3.9 4.7 8.5b 30.7a 5.2b 26.0a

Microcystis incerta 77.7 50.0 89.0 33.2 46.4b 72.3a 69.8a 55.3ab 46.6 83.5 67.8 75.0

2,738.0 527.0 2,284.0 2,258.0 1,159.0 316.0 5,047.0 476.0 1,477.0 1,530.0 698.0 7,102.0

Achnanthes minutissima 39.0 74.8 27.7 46.8 53.4 33.3 46.6 61.0 60.0a 58.5a 36.8b 25.0b

3.5b 12.9a .8c 3.8b 4.8 5.7 8.5 3.0 23.9 7.9 3.9 4.7

1 Significant interaction between location (upstream and downstream) and treatment (before and after low-water crossing modification); differences in averages could not be assigned to either location or 

treatment.

Effects of Hardened Low-Water Crossings on Periphyton 15
Table 6. Percent frequency of occurrence (top row) and average cell density (bottom row) of the seven most frequently occurring periphyton taxa collected on glass slides 

from sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, during September through October 1999 and September through November 2003.[See fig. 1 for location of site. Averages with different superscripts are significantly different (p less than or equal to 0.05). Cell density is in cells per square millimeter. var., variety]

Taxon

Tributary to Big Brushy Creek (TBB) Little Brushy Creek (LB)

Upstream Downstream Upstream Downstream

1999 2003 1999 2003 1999 2003 1999 2003

Eunotia bilunaris 84.0a 100.0a 34.0b 100.0a 34.0 94.0 67.0 84.0

10.6 20.0 1.6 32.0 20.1 7.2 6.2 39.3

Brachysira serians brachysira 16.5 56.0 .0 78.0 66.0 67.0 84.0 100.0

2.5 .6 .0 1.8 34.8 2.4 28.5 14.3

Frustulia rhomboides var. crassinervia 50.0b 89.0a 50.0b 100.0a 83.5 100.0 16.5 100.0

6.9 1.5 12.5 2.3 15.4b 29.0a 27.9a 11.4b

Eunotia flexuosa 100.0 78.0 83.5 89.0 100.0 77.7 100.0 77.7

50.5 2.7 466.5 4.7 188.5 12.9 113.8 13.9

Eunotia paludosa 100.0 55.7 100.0 33.3 1100.0 144.3 183.5 166.7

221.7 1.7 175.6 6.5 151.0 12.3 130.0 120.7

Eunotia naegelii 66.5 89.0 66.5 100.0 83.5 66.7 100.0 66.7

554.2 14.0 46.4 9.4 72.6 4.6 21.4 11.8

Achnanthes minutissima 16.5 22.0 .0 44.3 16.5 58.5 100.0 66.5

.3 2.4 .0 2.0 11.7b 1.9b 76.4a 3.2b

1Significant interaction between location (upstream and downstream) and treatment (1999 and 2003) for percent frequency of occurrence and cell density: percent frequency of occurrence at upstream and 

downstream sites was significantly different in 1999, but similar in 2003; cell density was significantly different upstream and downstream and in 1999 and 2003. Differences in averages could not be assigned to either location or treatment.

16 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
var. crassinervia were significantly (p<0.05) higher at LB 

downstream (LBDN), and during 2003, the average cell densities were significantly higher at LB upstream (LBUP). In contrast, the average cell density for Achnanthes minutissima was 

significantly higher at LBDN during 1999; all other averages 

for this taxon were similar during 1999 and 2003 at LBUP and 

LBDN (table 6).

Effects of Hardened Low-Water 

Crossings on Water Quality

Water-quality data (table 7) indicate that the sampled 

streams were typical of small streams in this area (Tollett 

and Fendick, 1998). Water-quality data from upstream and 

downstream sites are listed in table 7. The streams are acidic 

and poorly buffered, and pH ranged from 5.0 to 6.8 standard 

units. Field measurements of alkalinity ranged from 0.1 to 

6.7 mg/L, and field measurements of specific conductance 

ranged from 13 to 36 μS/cm at 25 °C. Nutrient concentrations 

were small. Dissolved (filtered) ammonia (as nitrogen) ranged 

from <0.002 to 0.056 mg/L, and many results were less than 

detection limits. Dissolved ammonia plus organic nitrogen (as 

nitrogen) ranged from <0.1 to 0.4 mg/L. Dissolved nitrite plus 

nitrate (as nitrogen) ranged from <0.002 to 0.079 mg/L, and 

many results, especially from samples collected during 1999, 

were less than detection limits. Dissolved phosphorus (as P) 

ranged from <0.002 to 0.009 mg/L, and most results were 

less than detection limits. Calcium, magnesium, and sodium 

concentrations were small. Calcium ranged from 0.28 to 

2.06 mg/L, magnesium ranged from 0.31 to 0.94 mg/L, and 

sodium ranged from 1.5 to 3.0 mg/L. Silica concentrations 

were large relative to other constituents measured and ranged 

from 7.3 to 13 mg/L. 

Water-quality changes that may have resulted from 

original installation of the low-water crossings on BB, TEF, 

TBB, and LB, generally disappeared over time (figs. 3 through 

8, table 8). Tollett and others (2002, p. 38–39) reported that 

the most pronounced changes occurred at LB, where, in 1998, 

calcium, magnesium, manganese, total dissolved solids, 

specific conductance, and alkalinity were all significantly 

(p<0.05) higher at the downstream site compared with the 

upstream site after the low-water crossing installation in 1998. 

These higher values were attributed to changes in stream 

geometry, which caused a pool to form at LBUP, and possible 

dissolution of dolomitic limestone used as a base material 

in the low-water crossing. Tollett and others (2002) further 

reported that the crossing was modified in 2000, shortly after 

these changes were noted, to create a more natural channel, 

which reduced the pooling, and the limestone base material 

was replaced by inert gravel. By 2003, calcium concentrations 

at LBDN were again consistent with those at LBUP (fig. 3, 

table 8). Similarly to LB, increased calcium concentrations 

at TEFDN after the low-water crossing installation in 1999 

may have resulted from the use of the dolomitic limestone. 

However, figure 3 shows that calcium concentrations at 

TEFDN were again similar to TEFUP by the time the limestone was replaced at the crossing in 2003, indicating that dissolution of the limestone probably had already slowed down or 

stopped.

All water-quality data were summed and averaged 

in 4-month groups (November, December, January, and 

February; March, April, May, and June; and July, August, 

September, and October), then statistically analyzed for 

seasonal differences. The only significant seasonal differences 

indicated were in water temperature, which was, as expected, 

significantly lower during the sampling period before modifications, because samples were collected only during winter 

months before the modifications at TEF and BB.

Water-quality data also were analyzed for significant 

differences between upstream and downstream sites before 

and after the installation of the hardened low-water crossings 

during 1998–99, and later, during 2003–04 (table 8). The 

data were grouped, summed, and averaged by upstream and 

downstream sites for periods referred to as before, after, and 

later in the text and in table 8. Average values for specific 

conductance and pH, and average concentrations of dissolved 

oxygen, dissolved solids, alkalinity, calcium, magnesium, 

sodium, potassium, iron, manganese, silica, sulfate, and 

chloride in samples were compared. Generally, average waterquality values and concentrations were similar at upstream and 

downstream sites. Notable exceptions were average values of 

specific conductance and pH, and concentrations of dissolved 

oxygen, alkalinity, and calcium, which often were higher at 

downstream sites than at upstream sites. Average iron concentrations often were higher at upstream sites than at downstream sites.

When average water-quality values or concentrations 

changed significantly, they almost always changed significantly at both the upstream and downstream sites. Average 

concentrations of alkalinity were significantly (p<0.05) 

higher after the low-water crossing installation and later at 

upstream and downstream sites on BB and TEF. Average 

concentrations of iron were significantly higher after the 

installation and later at upstream and downstream sites on 

TEF and TBB. Average concentrations of silica and chloride 

also were significantly higher after the installation and later 

at upstream and downstream sites on TEF. Average values 

for specific conductance and concentrations of dissolved 

solids, calcium, sodium, potassium, silica, and sulfate 

increased significantly after the low-water crossing installations at TEF and TBB, but decreased significantly later 

to averages similar to averages before the installations. Average concentrations of dissolved oxygen were significantly 

lower after the installation and later at both upstream and 

downstream sites on TBB. Average concentrations of sodium, 

sulfate, and chloride were significantly lower after the installation and later at upstream and downstream sites on BB 

(table 8).

Monthly rainfall at Leesville, La. (fig. 1), ranged from 

0.5 to 8.9 in. during the sample-collection periods (fig. 9). 

Effects of Hardened Low-Water Crossings on Water Quality 17
Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 

1998–99 and 2003–04—Continued.

Date Time

Specific 

conductance, 

field

(μS/cm at 

25 degrees 

Celsius)

pH, water, 

whole, 

field 

(standard 

units)

Temperature, 

air

(degrees 

Celsius)

Temperature,

water 

(degrees 

Celsius)

Turbidity, 

field

(NTU)

Oxygen, 

dissolved

Nitrogen, 

ammonia, 

filtered

(as N)

Nitrogen, 

nitrite, 

filtered

(as N)

Nitrogen, 

ammonia 

plus 

organic, 

filtered

(as N)

Nitrogen, 

nitrite plus 

nitrate, 

filtered

(as N)

Phosphorus, 

filtered

(as P)

Big Brushy Creek upstream (BBUP)

12-22-1998 1100 23 6.0 2.0 14.3 ----- 8.6 0.023 <0.010 0.114 0.079 <0.050

01-26-1999 1220 25 6.2 19.0 12.5 ----- 9.3 <.020 .012 .132 <.050 <.00402-24-1999 1000 21 6.1 9.5 10.1 ----- 8.1 .044 <.010 .202 <.050 .00403-23-1999 1220 22 5.6 25.0 16.0 ----- 7.3 .056 <.010 .125 <.050 <.004

07-08-1999 1030 18 6.1 ----- 24.1 ----- 6.2 <.020 <.010 .195 .053 <.00408-27-1999 1000 21 6.0 ----- 26.9 ----- 4.4 <.020 <.010 .256 <.050 <.004

10-01-1999 0915 27 5.6 22.5 17.0 ----- 6.4 <.020 <.010 .259 <.050 E.003

04-10-2003 1630 22 6.2 ----- 13.4 ----- 7.5 .015 E.001 <.200 .005 <.002

05-07-2003 1600 21 6.1 ----- 23.8 ----- ----- .021 .001 <.200 .021 <.00206-04-2003 1430 21 6.0 29.5 23.7 20.0 6.4 .026 .002 <.200 .019 .00607-01-2003 1200 18 5.8 31.5 23.3 15.0 7.1 .016 .002 <.200 .002 <.00208-06-2003 0930 17 6.0 32.0 25.7 6.7 7.0 .019 <.001 <.200 .026 <.002

09-10-2003 1230 16 5.9 ----- 23.6 13.0 7.5 .013 .002 <.200 .015 .00410-07-2003 1230 16 6.0 ----- 19.6 5.4 8.1 .004 <.001 <.200 .006 .00311-05-2003 1200 16 6.0 ----- 17.2 4.7 11.1 .004 .001 .300 <.002 .003

Big Brushy Creek downstream (BBDN)

12-22-1998 1030 23 6.0 2.0 14.3 ----- 8.6 <.020 <.010 .106 .075 <.050

01-26-1999 1140 26 5.7 19.0 12.5 ----- 9.0 .022 <.010 .112 <.050 <.004

02-24-1999 0900 21 6.4 9.5 10.1 ----- 8.4 <.020 <.010 .154 <.050 <.004

03-23-1999 1200 22 5.9 25.0 16.0 ----- 7.7 .029 <.010 .119 <.050 <.004

07-08-1999 1015 18 5.5 ----- 24.0 ----- 6.5 <.020 <.010 .238 .051 .00508-27-1999 0900 21 5.8 ----- 27.0 ----- 4.6 <.020 <.010 .239 <.050 <.004

10-01-1999 0900 27 5.5 22.5 16.9 ----- 6.5 <.020 <.010 .191 <.050 <.006

04-10-2003 1600 23 6.8 ----- 13.4 ----- 7.7 .015 <.001 <.200 .003 <.002

05-07-2003 1530 21 6.2 ----- 23.8 ----- ----- .026 .001 .200 .014 <.002

06-04-2003 1415 21 6.1 29.5 23.7 22.0 6.6 .026 .002 <.200 .024 <.002

07-01-2003 1130 18 5.8 31.5 23.3 15.0 7.2 .014 <.001 <.200 .004 <.002

08-06-2003 0900 18 6.2 32.0 25.7 7.2 7.3 .016 <.001 <.200 .026 <.002

09-10-2003 1230 16 6.0 33.0 23.7 14.0 7.8 .009 .002 .200 .012 <.002

10-07-2003 1200 17 6.1 ----- 19.6 5.2 8.4 .006 <.001 <.200 .006 .003

11-05-2003 1130 17 6.1 ----- 17.3 4.9 10.8 .005 .001 <.200 <.002 .003

Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04.[See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value]

18 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
Effects of Hardened Low-Water Crossings on Water Quality 19

Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued.[See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value].

Date Time

Specific 

conductance, 

field

(μS/cm at 

25 degrees 

Celsius)

pH, water, 

whole, 

field 

(standard 

units)

Temperature, 

air

(degrees 

Celsius)

Temperature,

water 

(degrees 

Celsius)

Turbidity, 

field

(NTU)

Oxygen, 

dissolved

Nitrogen, 

ammonia, 

filtered

(as N)

Nitrogen, 

nitrite, 

filtered

(as N)

Nitrogen, 

ammonia 

plus 

organic, 

filtered

(as N)

Nitrogen, 

nitrite plus 

nitrate, 

filtered

(as N)

Phosphorus, 

filtered

(as P)

Tributary to East Fork of Sixmile Creek upstream (TEFUP)

12-22-1998 1030 21 5.5 1.0 ----- ----- ----- <0.020 <0.010 <0.100 0.076 <0.050

01-26-1999 1140 21 5.7 17.0 13.7 ----- 9.6 <.020 <.010 .116 <.050 <.004

02-24-1999 0900 18 5.8 8.5 10.6 ----- 6.0 .022 <.010 .126 <.050 <.004

03-23-1999 1200 18 5.5 23.0 16.0 ----- 9.6 .034 <.010 E.091 <.050 <.004

07-08-1999 1015 19 5.7 ----- 24.2 ----- 5.9 <.020 <.010 .221 <.050 .004

08-27-1999 0900 18 5.6 32.0 25.3 ----- 3.1 <.020 <.010 .188 <.050 <.00410-01-1999 0900 36 5.5 22.0 18.7 ----- 6.4 <.020 <.010 .339 <.050 E.003

04-10-2003 1600 18 5.5 22.0 12.2 ----- 8.6 .013 <.001 .200 <.002 <.002

05-07-2003 1530 18 5.7 ----- 23.2 ----- ----- <.002 <.001 <.200 .003 <.002

06-04-2003 1415 20 5.5 29.0 22.8 3.9 7.2 .016 .002 <.200 .003 <.002

07-01-2003 1130 18 5.3 27.0 22.9 4.4 7.6 .009 .002 .200 <.002 <.002

08-06-2003 0900 17 5.3 33.0 25.8 2.5 5.8 .009 <.001 <.200 .003 <.002

09-10-2003 1230 17 5.3 ----- 23.2 3.7 6.5 .014 .002 .200 .004 .00410-07-2003 1200 17 5.4 ----- 19.7 2.6 6.8 .004 <.001 <.200 <.002 <.002

11-05-2003 1130 17 5.5 ----- 17.2 1.8 9.2 .003 .002 <.200 <.002 .002

Tributary to East Fork of Sixmile Creek downstream (TEFDN)

12-22-1998 1500 20 5.5 1.0 ----- ----- ----- <.020 <.010 <.100 .071 <.05001-27-1999 0900 21 5.6 17.0 13.7 ----- 9.5 .020 <.010 .142 <.050 <.004

02-25-1999 0620 18 5.6 8.5 10.6 ----- 9.7 <.020 <.010 E.095 <.050 <.004

03-24-1999 0950 18 5.5 23.0 16.0 ----- 9.7 .051 <.010 .108 <.050 <.004

07-07-1999 1120 18 5.9 ----- 24.2 ----- 5.8 <.020 <.010 .251 <.050 <.004

08-19-1999 1000 23 5.9 32.0 25.7 ----- 4.3 <.020 <.010 .200 <.050 <.004

09-30-1999 1415 36 5.5 22.0 18.8 ----- 6.6 <.020 <.010 .321 <.050 E.004

04-10-2003 1400 19 5.8 ----- 12.0 ----- 8.9 .011 <.001 <.200 <.002 <.002

05-07-2003 1630 19 5.9 ----- 23.2 ----- ----- .025 .002 <.200 .019 .00206-04-2003 1300 20 5.6 29.0 22.9 4.4 7.0 .009 .002 .200 .004 <.002

07-01-2003 0930 18 5.4 27.0 22.9 4.3 7.6 .006 .002 .300 <.002 <.002

08-06-2003 1330 18 5.5 31.0 26.3 2.8 6.2 .014 <.001 <.200 .003 <.002

09-10-2003 0930 17 5.5 ----- 23.2 4.1 6.4 .018 .002 .400 .004 .009

10-07-2003 0900 16 5.5 ----- 20.0 1.2 8.0 <.002 <.001 <.200 <.002 <.002

11-05-2003 0830 17 5.5 ----- 17.2 2.0 8.3 .003 .002 .200 <.002 <.002
20 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value] Date Time Specific conductance, field (μS/cm at 25 degrees Celsius) pH, water, whole, field (standard units) Temperature, air (degrees Celsius) Temperature, water (degrees Celsius) Turbidity, field (NTU) Oxygen, dissolved Nitrogen, ammonia, filtered (as N) Nitrogen, nitrite, filtered (as N) Nitrogen, ammonia plus organic, filtered (as N) Nitrogen, nitrite plus nitrate, filtered (as N) Phosphorus, filtered (as P) Tributary to Big Brushy Creek upstream (TBBUP) 12-22-1998 1240 18 5.2 2.0 13.1 ----- 8.7 <0.020 <0.010 0.112 0.071 <0.050 01-26-1999 1430 19 5.2 ----- 14.2 ----- 9.1 .031 .011 E.083 <.050 <.004 02-24-1999 1515 18 5.5 21.5 13.4 ----- 8.5 <.020 <.010 E.077 <.050 <.004 03-23-1999 1425 18 5.4 25.0 16.8 ----- 8.3 .043 <.010 E.082 <.050 <.004 07-07-1999 0900 19 5.3 25.8 23.6 ----- 6.4 <.020 <.010 .220 <.050 <.004 08-26-1999 1300 24 5.2 ----- 26.6 ----- ----- .038 <.010 .272 <.050 .004 10-01-1999 1145 30 5.7 20.5 16.9 ----- 5.0 <.020 <.010 .208 <.050 E.003 08-06-2003 1100 17 5.3 32.5 25.3 2.2 6.8 .009 <.001 <.200 .003 <.002 09-10-2003 1200 17 5.1 ----- 23.3 4.9 4.0 .008 <.001 <.200 <.002 .003 10-07-2003 1130 17 5.3 ----- 20.2 1.0 5.8 .004 <.001 <.200 <.002 <.002 11-05-2003 1100 17 5.4 ----- 17.5 3.5 7.7 .003 .002 .200 <.002 <.002 Tributary to Big Brushy Creek downstream (TBBDN) 12-22-1998 1215 19 5.5 2.0 12.9 ----- 7.5 <.020 <.010 .101 .072 <.050 01-26-1999 1445 19 5.2 ----- 14.4 ----- 8.8 <.020 <.010 E.093 <.050 <.004 02-24-1999 1500 18 5.6 21.5 13.7 ----- 8.2 <.020 <.010 E.083 <.050 <.004 03-23-1999 1410 18 5.4 25.0 16.8 ----- 8.1 .032 <.010 E.059 <.050 <.004 07-07-1999 0830 19 5.0 25.8 23.6 ----- 6.4 <.020 <.010 .221 <.050 <.004 08-26-1999 1230 27 5.4 ----- 26.9 ----- ----- .031 <.010 .239 <.050 .004 10-01-1999 1115 31 6.5 20.5 16.9 ----- 5.6 .031 <.010 .327 <.050 E.005 08-06-2003 1030 18 5.2 33.0 25.2 2.2 6.1 .009 <.001 <.200 .004 <.002 09-10-2003 1130 17 5.2 ----- 23.5 2.9 5.2 .009 .001 .200 .002 .004 10-07-2003 1100 18 5.3 ----- 20.4 .5 6.4 <.002 <.001 .200 <.002 <.002 11-05-2003 1030 18 5.4 ----- 17.7 3.3 8.6 .005 .001 <.200 <.002 <.002

Tributary to Birds Creek upstream (TBCUP)

04-11-2003 1000 22 5.9 ----- 12.4 ----- 6.3 ----- ----- ----- ----- -----

05-07-2003 1400 24 6.0 ----- 22.8 ----- ----- .023 .001 <.200 .016 <.00206-04-2003 1100 24 5.8 28.5 22.5 15.0 5.9 .032 .002 .200 .024 <.002

07-01-2003 1400 19 5.7 ----- 23.4 16.0 6.8 .022 <.001 <.200 .018 <.002

01-07-2004 0950 21 5.8 ----- 6.9 4.7 11.6 .013 .001 <.200 .004 <.020

05-10-2004 1045 18 5.6 ----- 19.3 8.8 7.5 .016 <.001 <.200 .007 <.002

06-14-2004 1030 20 5.3 ----- 22.3 30.0 7.7 .008 .002 .400 .058 <.002
Effects of Hardened Low-Water Crossings on Water Quality 21

Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued.[See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value]

Date Time

Specific 

conductance, 

field

(μS/cm at 

25 degrees 

Celsius)

pH, water, 

whole, 

field 

(standard 

units)

Temperature, 

air

(degrees 

Celsius)

Temperature,

water 

(degrees 

Celsius)

Turbidity, 

field

(NTU)

Oxygen, 

dissolved

Nitrogen, 

ammonia, 

filtered

(as N)

Nitrogen, 

nitrite, 

filtered

(as N)

Nitrogen, 

ammonia 

plus 

organic, 

filtered

(as N)

Nitrogen, 

nitrite plus 

nitrate, 

filtered

(as N)

Phosphorus, 

filtered

(as P)

Tributary to Birds Creek downstream (TBCDN)

04-11-2003 0930 21 5.8 ----- 12.3 ----- 6.4 0.008 E0.001 <0.020 0.008 <0.002

05-07-2003 1330 24 6.3 ----- 22.8 ----- ----- .021 .001 <.020 .021 <.00206-04-2003 1130 23 5.8 28.5 22.5 18.0 6.0 .025 .002 <.020 .024 <.00207-01-2003 1330 19 5.6 30.5 23.4 16.0 6.8 .021 <.001 .300 .018 <.00201-07-2004 0920 22 5.9 ----- 6.6 6.4 12.0 .015 .001 .300 .004 <.02005-10-2004 1030 18 5.7 ----- 20.1 10.0 8.0 .004 <.001 <.200 .007 <.002

06-14-2004 1000 20 5.3 ----- 22.4 30.0 7.8 .012 .002 .400 .063 .004

Little Brushy Creek upstream (LBUP)

12-22-1998 1410 18 5.6 2.0 ----- ----- ----- <.020 <.010 <.100 .070 <.050

01-27-1999 1145 18 5.5 22.0 14.9 ----- 7.8 <.020 <.010 E.099 <.050 <.004

02-25-1999 0845 16 5.8 16.0 11.8 ----- 8.3 .052 <.010 .120 <.050 <.004

03-24-1999 1320 17 5.7 29.0 18.3 ----- 10.2 .039 <.010 E.081 <.050 <.004

07-08-1999 0845 18 5.8 ----- 22.9 ----- 6.2 <.020 <.010 .215 <.050 <.004

08-26-1999 1045 17 5.6 ----- ----- ----- ----- <.020 <.010 E.092 <.050 <.004

10-01-1999 1315 22 ----- 20.5 18.3 ----- 7.1 <.020 <.010 .199 <.050 E.003

08-06-2003 1230 15 5.8 31.5 24.7 .8 7.1 .005 <.001 <.200 <.002 <.002

09-10-2003 1100 13 5.7 ----- 22.8 2.6 7.1 .010 .002 <.200 .002 .005

10-07-2003 1030 16 5.9 ----- 19.5 11.0 7.1 .012 <.001 <.200 <.002 .00311-05-2003 1000 14 5.9 ----- 17.5 2.0 12.6 .003 .001 <.200 <.002 .003

Little Brushy Creek downstream (LBDN)

12-22-1998 1345 18 6.0 2.0 ----- ----- ----- <.020 <.010 <.100 .070 <.050

01-27-1999 1115 19 5.6 22.0 14.6 ----- 8.9 <.020 <.010 E.086 <.050 <.004

02-25-1999 0815 18 6.0 16.0 11.8 ----- 9.7 <.020 <.010 E.09 <.050 <.004

03-24-1999 1300 18 5.5 29.0 17.6 ----- 10.2 .044 <.010 .100 <.050 <.004

07-08-1999 0815 19 6.1 ----- 22.8 ----- 6.3 <.020 <.010 .189 <.050 <.004

08-26-1999 0945 22 5.6 ----- 25.9 ----- 5.1 <.020 <.010 .188 <.050 <.004

10-01-1999 1245 25 5.4 20.5 18.4 ----- 7.1 <.020 <.010 .178 <.050 E.005

08-06-2003 1200 15 5.9 31.5 25.7 ----- 6.8 <.002 <.001 <.200 <.002 <.002

09-10-2003 1030 14 5.8 ----- 23.3 2.8 7.2 .003 .001 <.200 <.002 .004

10-07-2003 1000 15 6.0 ----- 20.4 1.8 8.6 <.002 <.001 <.200 <.002 <.00211-05-2003 0930 15 6.0 ----- 17.8 1.5 12.6 .002 .002 <.200 <.002 .003
22 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value] Date Time Calcium, filtered (as Ca) Mag- nesium, filtered (as Mg) Sodium, filtered (as Na) Potas- sium, filtered (as K) Bicar- bonate, calcu- lated (as HCO3

)

Alkalinity, 

field,

(as CaCO3

)

Sulfate, 

filtered

(as SO4

)

Chloride, 

filtered

(as Cl)

Fluoride, 

filtered

 (as F)

Silica, 

filtered

(as SiO2

)

Residue on 

evaporation

at 180 

degrees 

Celsius, 

filtered

Iron, 

filtered

(as Fe)

Manganese, 

filtered

(as Mn)

Big Brushy Creek upstream (BBUP)

12-22-1998 1100 0.90 0.56 2.10 0.48 4.2 3.4 1.90 3.9 <0.1 9.8 27 0.175 0.04501-26-1999 1220 .87 .56 2.30 .41 4.0 3.3 1.90 3.9 <.1 9.6 29 .146 .03802-24-1999 1000 .68 .43 1.88 .29 ----- ----- 1.00 3.2 <.1 9.1 29 .175 .02603-23-1999 1220 .79 .51 2.05 .30 4.9 4.0 .80 3.5 <.1 9.3 29 .325 .04007-08-1999 1030 .85 .46 1.51 .50 5.5 4.5 .80 2.5 <.1 7.9 28 .208 .03508-27-1999 1000 .93 .48 1.79 .35 5.0 4.1 .20 3.1 <.1 8.2 29 .211 .06710-01-1999 0915 1.04 .69 2.21 .86 5.6 4.6 2.20 3.4 <.1 8.8 38 .248 .05004-10-2003 1630 .94 .55 2.00 .30 5.1 3.3 .80 3.1 <.1 9.8 27 .132 .02405-07-2003 1600 .83 .50 1.80 .30 4.1 4.1 .40 3.1 <.1 9.1 28 .375 .05806-04-2003 1430 .84 .49 1.80 .30 3.4 4.2 .60 3.1 <.1 8.5 35 .254 .04607-01-2003 1200 .93 .49 1.70 .30 3.5 2.8 .80 2.8 <.1 11.0 41 .291 .04108-06-2003 0930 .78 .47 1.80 .20 5.2 4.3 .30 3.1 <.1 9.1 24 .294 .04109-10-2003 1230 .86 .48 1.60 .40 4.1 3.3 .79 2.9 <.1 8.9 25 .164 .04210-07-2003 1230 .57 .38 1.80 .30 .9 .8 .47 3.2 <.1 9.5 19 .160 .02011-05-2003 1200 .62 .43 1.80 .30 2.1 1.4 .40 3.1 <.1 9.8 15 .183 .031

Big Brushy Creek downstream (BBDN)

12-22-1998 1030 .90 .56 2.10 .49 4.2 3.4 1.90 3.9 <.1 9.8 27 .192 .04401-26-1999 1140 .88 .56 2.27 .42 4.3 3.5 1.80 4.0 <.1 9.6 29 .146 .03802-24-1999 0900 1.02 .46 1.93 .23 ----- ----- 1.10 3.3 <.1 9.5 31 .256 .02903-23-1999 1200 .82 .51 2.06 .30 5.1 4.2 .70 3.4 <.1 9.3 23 .232 .04007-08-1999 1015 .90 .46 1.46 .44 4.8 3.9 .80 2.4 <.1 7.7 26 .238 .03508-27-1999 0900 .94 .47 1.72 .28 7.0 5.7 .10 2.9 <.1 8.1 27 .124 .06010-01-1999 0900 1.10 .70 2.25 .80 4.0 3.3 2.20 3.5 <.1 8.8 37 .182 .05004-10-2003 1600 .95 .53 2.00 .30 6.4 5.3 .90 3.2 <.1 9.8 27 .162 .02405-07-2003 1530 .86 .50 1.90 .30 6.0 4.9 .40 3.1 <.1 8.9 28 .196 .05206-04-2003 1415 .90 .48 1.80 .30 3.5 3.0 .60 3.1 <.1 8.6 29 .176 .04307-01-2003 1130 .94 .49 1.70 .30 3.7 3.0 .80 2.7 <.1 11.0 48 .265 .04008-06-2003 0900 .98 .48 1.80 .30 2.7 2.2 .40 3.1 <.1 9.3 25 .269 .03909-10-2003 1230 .94 .47 1.60 .40 3.8 3.1 .80 2.9 <.1 9.0 24 .168 .04110-07-2003 1200 .67 .39 1.80 .30 1.6 1.3 .53 3.2 <.1 9.4 18 .136 .01911-05-2003 1130 .76 .42 1.70 .30 3.5 2.1 .41 3.2 <.1 9.8 17 .196 .031
Effects of Hardened Low-Water Crossings on Water Quality 23

Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value]

Date Time

Calcium, 

filtered

(as Ca)

Magnesium, 

filtered

(as Mg)

Sodium, 

filtered

(as Na)

Potassium, 

filtered

(as K)

Bicarbonate, 

calculated

(as HCO3

)

Alkalinity, 

field,

(as CaCO3

)

Sulfate, 

filtered

(as SO4

)

Chloride, 

filtered

(as Cl)

Fluoride, 

filtered

 (as F)

Silica, 

filtered

(as SiO2

)

Residue on 

evaporation

at 180 

degrees 

Celsius, 

filtered

Iron, 

filtered

(as Fe)

Manganese, 

filtered

(as Mn)

Tributary to East Fork of Sixmile Creek upstream (TEFUP)

12-22-1998 1030 0.51 0.44 2.00 0.16 2.7 2.2 1.50 3.7 <0.1 10.5 25 0.114 0.03701-26-1999 1140 .45 .42 2.16 .12 2.9 2.4 1.50 3.6 <.1 10.5 25 .114 .02802-24-1999 0900 .42 .33 1.96 .14 2.4 2.0 1.10 3.2 <.1 9.5 20 .078 .01503-23-1999 1200 .39 .37 1.98 .11 3.0 2.5 .80 3.6 <.1 10.2 27 .131 .02107-08-1999 1015 .42 .36 2.02 .17 3.3 2.7 .40 3.5 <.1 9.8 25 .277 .03308-27-1999 0900 .41 .37 2.02 .17 3.5 2.9 .20 3.4 <.1 9.0 28 .121 .07510-01-1999 0900 .92 .92 3.00 .73 2.9 2.4 3.70 4.6 <.1 11.0 46 .191 .09104-10-2003 1600 .37 .36 2.00 .20 2.0 1.5 .80 3.3 <.1 10.0 27 .097 .01905-07-2003 1530 .38 .37 2.00 .20 1.5 1.1 .40 3.4 <.1 9.5 23 .147 .02606-04-2003 1415 .43 .41 2.10 .20 1.7 1.2 .60 3.7 <.1 9.6 33 .171 .02807-01-2003 1130 .61 .43 2.00 .20 .8 .5 .80 3.4 <.1 12.0 38 .277 .04308-06-2003 0900 .37 .35 2.00 .10 2.4 1.7 .30 3.6 <.1 9.9 25 .161 .03509-10-2003 1230 .44 .39 2.00 .20 1.6 1.2 .30 3.7 <.1 11.0 27 .202 .03510-07-2003 1200 .28 .33 2.00 .10 .4 .1 .36 3.7 <.1 11.0 18 .208 .02211-05-2003 1130 .33 .40 2.00 .20 3.1 .6 .34 3.7 <.1 11.0 15 .301 .030

Tributary to East Fork of Sixmile Creek downstream (TEFDN)

12-22-1998 1500 .51 .44 2.01 .16 3.4 2.2 1.50 3.7 <.1 10.5 24 .116 .03801-27-1999 0900 .45 .42 2.16 .15 3.2 2.4 1.50 3.8 <.1 10.4 24 .107 .02802-25-1999 0620 .38 .34 1.85 .10 2.4 2.0 1.10 3.3 <.1 9.4 20 .081 .01603-24-1999 0950 .39 .37 1.98 .12 2.9 2.5 .80 3.5 <.1 10.2 26 .131 .02207-07-1999 1120 .69 .38 2.06 .22 4.0 2.7 .40 3.3 <.1 9.9 29 .158 .03808-19-1999 1000 2.06 .39 2.06 .27 7.8 2.9 .30 3.4 <.1 9.1 32 .380 .07709-30-1999 1415 1.23 .94 3.02 .96 3.8 2.4 3.80 4.7 <.1 11.0 48 .208 .09604-10-2003 1400 .41 .35 2.00 .20 2.7 2.2 .80 3.3 <.1 10.0 23 .132 .01805-07-2003 1630 .52 .38 2.00 .20 1.9 1.5 .40 3.4 <.1 9.5 24 .175 .02706-04-2003 1300 .61 .44 2.00 .20 2.7 2.1 .70 3.7 <.1 9.6 47 .156 .03107-01-2003 0930 .64 .44 2.00 .20 .4 .3 .80 3.3 <.1 12.0 33 .265 .04408-06-2003 1330 .47 .36 2.00 .20 5.9 4.8 .30 3.6 <.1 10.0 27 .202 .03909-10-2003 0930 .52 .40 2.00 .20 1.1 .8 .40 3.8 <.1 11.0 26 .205 .03910-07-2003 0900 .32 .33 2.00 .10 1.0 .7 .35 3.6 <.1 11.0 29 .135 .02511-05-2003 0830 .36 .40 2.00 .20 6.2 1.7 .34 3.6 <.1 11.0 16 .299 .034
24 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value] Date Time Calcium, filtered (as Ca) Mag- nesium, filtered (as Mg) Sodium, filtered (as Na) Potas- sium, filtered (as K) Bicar- bonate, calcu- lated (as HCO3

)

Alkalinity, 

field,

(as CaCO3

)

Sulfate, 

filtered

(as SO4

)

Chloride, 

filtered

(as Cl)

Fluoride, 

filtered

 (as F)

Silica, 

filtered

(as SiO2

)

Residue on 

evaporation

at 180 

degrees 

Celsius, 

filtered

Iron, 

filtered

(as Fe)

Manganese, 

filtered

(as Mn)

Tributary to Big Brushy Creek upstream (TBBUP)

12-22-1998 1240 0.42 0.39 1.91 0.17 2.5 2.0 1.20 3.5 <0.1 8.8 23 0.105 0.07301-26-1999 1430 .36 .38 2.00 .13 2.0 1.6 1.10 3.2 <.1 9.2 21 .087 .05002-24-1999 1515 .34 .34 1.79 .13 3.4 2.8 1.00 3.0 <.1 8.7 21 .053 .03703-23-1999 1425 .34 .35 1.84 .13 3.9 3.2 .70 3.2 <.1 8.9 23 .100 .04607-07-1999 0900 .38 .37 2.00 .14 2.3 1.9 .20 3.3 <.1 11.1 29 .194 .09608-26-1999 1300 .56 .47 2.05 .34 4.8 3.9 <.1 3.7 <.1 12.9 23 .598 .58710-01-1999 1145 .60 .66 2.75 .33 4.4 3.6 2.90 4.4 <.1 12.6 34 .185 .18508-06-2003 1100 .33 .37 2.00 .10 .5 .2 .20 3.6 <.1 12.0 25 .361 .08709-10-2003 1200 .38 .38 2.00 .10 .5 .1 .30 3.8 <.1 12.0 25 .208 .09110-07-2003 1130 .31 .38 2.00 .10 .3 .3 .38 3.8 <.1 13.0 27 .235 .08111-05-2003 1100 .35 .44 2.00 .20 4.4 3.4 .32 3.6 <.1 13.0 16 .309 .103

Tributary to Big Brushy Creek downstream (TBBDN)

12-22-1998 1215 .43 .39 1.92 .17 3.4 2.8 1.20 3.4 <.1 8.8 23 .113 .07301-26-1999 1445 .36 .37 1.99 .14 2.7 2.2 1.10 3.3 <.1 9.2 23 .098 .05302-24-1999 1500 .36 .35 1.83 .10 3.4 2.8 1.00 3.0 <.1 8.8 21 .058 .03703-23-1999 1410 .34 .35 1.85 .12 3.0 2.5 .60 3.1 <.1 9.0 24 .091 .04707-07-1999 0830 .41 .37 1.96 .15 3.4 2.8 .30 3.5 <.1 11.0 28 .234 .10108-26-1999 1230 1.03 .50 2.08 .45 8.0 6.6 <.10 3.7 <.1 12.9 30 .561 .56410-01-1999 1115 .72 .67 3.00 .46 3.7 3.0 2.70 4.4 <.1 12.8 34 .175 .18708-06-2003 1030 .36 .37 2.00 .10 .5 .2 .20 3.6 <.1 12.0 27 .352 .08709-10-2003 1130 .41 .38 2.01 .16 1.7 1.1 .28 3.8 .0 12.5 25 .197 .08810-07-2003 1100 .34 .38 2.00 .10 .4 .3 .39 3.8 <.1 12.0 18 .162 .07211-05-2003 1030 .47 .45 2.00 .20 6.0 3.6 .32 3.6 <.1 13.0 16 .284 .100

Tributary to Birds Creek upstream (TBCUP)

04-11-2003 1000 ----- ----- ----- ----- 2.9 2.3 ----- ----- ----- ----- ----- ----- -----05-07-2003 1400 .83 .59 2.10 .50 4.9 4.0 .40 3.6 <.1 10.0 36 .156 .05506-04-2003 1100 .90 .57 2.10 .60 4.5 3.6 .60 3.6 <.1 11.0 44 .309 .04307-01-2003 1400 .76 .49 1.90 .40 3.3 2.6 .70 3.2 <.1 10.0 54 .226 .03201-07-2004 0950 .73 .57 2.40 .40 .8 .5 1.20 4.4 <.1 12.0 22 .194 .04905-10-2004 1045 .56 .41 2.10 .40 2.3 1.8 .55 3.6 <.1 10.0 22 .198 .02606-14-2004 1030 .94 .54 1.50 .70 1.0 .6 1.20 2.6 <.1 7.3 ----- .362 .074
Table 7. Water-quality data for sites upstream and downstream from hardened low-water crossings on selected streams at the Fort Polk Military Reservation, Louisiana, 

1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Values are in milligrams per liter, except as noted. μS/cm, microsiemens per centimeter; NTU, nephelometric turbidity units; -----, no data; <, less than indicated value; E, estimated value]

Date Time

Calcium, 

filtered

(as Ca)

Magnesium, 

filtered

(as Mg)

Sodium, 

filtered

(as Na)

Potassium, 

filtered

(as K)

Bicarbonate, 

calculated

(as HCO3

)

Alkalinity, 

field,

(as CaCO3

)

Sulfate, 

filtered

(as SO4

)

Chloride, 

filtered

(as Cl)

Fluoride, 

filtered

 (as F)

Silica, 

filtered

(as SiO2

)

Residue on 

evaporation

at 180 

degrees 

Celsius, 

filtered

Iron, 

filtered

(as Fe)

Manganese, 

filtered

(as Mn)

Tributary to Birds Creek downstream (TBCDN)

04-11-2003 0930 0.69 0.50 2.10 0.40 3.9 3.2 0.70 3.6 <0.10 11.0 27 0.240 0.02505-07-2003 1330 .83 .59 2.10 .50 4.0 3.3 .40 3.6 <.10 10.0 31 .234 .05506-04-2003 1130 .86 .57 2.10 .50 4.0 3.2 .60 3.6 <.10 11.0 39 .150 .04107-01-2003 1330 .76 .49 1.90 .40 2.4 1.9 .70 3.2 <.10 10.0 46 .224 .03101-07-2004 0920 .75 .55 2.40 .40 .6 .5 1.20 4.3 <.10 12.0 27 .204 .05105-10-2004 1030 .59 .42 1.90 .40 5.0 4.0 .54 3.4 <.10 10.0 20 .160 .02906-14-2004 1000 .99 .54 1.50 .70 1.4 1.0 1.30 2.6 <.10 7.3 ----- .368 .078

Little Brushy Creek upstream (LBUP)

12-22-1998 1410 .40 .38 1.89 .18 3.7 3.0 .90 3.3 <.1 9.2 19 .146 .03501-27-1999 1145 .33 .36 1.97 <.10 2.2 1.8 .90 3.3 <.1 9.1 21 .104 .01702-25-1999 0845 .33 .31 1.82 .11 3.7 3.0 .70 3.1 <.1 8.6 21 .073 .01103-24-1999 1320 .31 .33 1.80 .13 3.2 2.6 .50 3.1 <.1 8.8 24 .100 .01207-08-1999 0845 .39 .40 1.93 .17 3.9 3.2 .40 3.1 <.1 9.4 25 .170 .02108-26-1999 1045 .29 .36 1.80 .25 3.3 2.7 .20 2.9 <.1 8.8 11 .119 .01510-01-1999 1315 .46 .48 2.22 .33 3.8 3.1 1.30 3.5 <.1 10.0 29 .171 .02608-06-2003 1230 .31 .37 1.80 .20 2.0 1.5 .30 3.0 <.1 9.4 23 .152 .01209-10-2003 1100 .29 .32 1.80 .20 1.6 1.3 .40 3.1 <.1 9.6 19 .111 .01410-07-2003 1030 .28 .36 1.90 .20 .3 .3 .41 3.2 <.1 9.7 22 .099 .00811-05-2003 1000 .30 .39 1.80 .20 1.2 1.0 .39 3.0 <.1 9.8 15 .130 .010

Little Brushy Creek downstream (LBDN)

12-22-1998 1345 .68 .39 1.87 .18 4.4 3.6 .90 3.2 <.1 9.2 23 .147 .03201-27-1999 1115 .61 .37 1.98 .14 3.8 3.1 .90 3.1 <.1 9.1 22 .116 .01702-25-1999 0815 .59 .33 1.84 .11 4.9 4.0 .70 3.2 <.1 8.6 21 .071 .01103-24-1999 1300 .68 .37 1.84 .16 4.8 3.9 .40 3.1 <.1 9.1 24 .106 .01207-08-1999 0815 1.05 .46 1.94 .17 5.7 4.7 .40 3.1 <.1 9.4 28 .177 .02008-26-1999 0945 1.23 .44 1.81 .20 7.1 5.8 .20 3.0 <.1 8.9 27 .099 .01310-01-1999 1245 1.19 .52 2.22 .33 8.2 6.7 1.20 3.4 <.1 10.1 32 .161 .02308-06-2003 1200 .36 .36 1.80 .20 3.5 2.8 .30 3.0 <.1 9.3 20 .106 .01109-10-2003 1030 .33 .33 1.80 .20 3.4 2.5 .40 3.1 <.1 9.6 18 .094 .01310-07-2003 1000 .32 .36 1.90 .20 2.1 1.8 .41 3.1 <.1 9.8 23 .102 .00911-05-2003 0930 .36 .39 1.80 .20 3.5 2.4 .38 3.0 <.1 9.8 15 .134 .011

Effects of Hardened Low-Water Crossings on Water Quality 25
Figure 3. Calcium concentrations in water from sites upstream and downstream from hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

26 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams

0.0

0.5

1.0

1.5

2.0

2.5

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

 

0.0

0.5

1.0

1.5

2.0

2.5

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

 

0.0

0.5

1.0

1.5

2.0

2.5

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

 

0.0

0.5

1.0

1.5

2.0

2.5

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

 

0.0

0.5

1.0

1.5

2.0

2.5

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

CALCIUM CONCENTRATION, IN MILLIGRAMS PER LITE

R

YEAR
Figure 4. Magnesium concentrations in water from sites upstream and downstream from hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

Effects of Hardened Low-Water Crossings on Water Quality 27

0.0

0.5

1.0

1.5

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

 

0.0

0.5

1.0

1.5

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

 

0.0

0.5

1.0

1.5

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

 

0.0

0.5

1.0

1.5

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

 

0.0

0.5

1.0

1.5

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

MAGNESIUM CONCENTRATION, IN MILLIGRAMS PER LITE

R

YEAR
Figure 5. Concentration of total dissolved solids in water from sites upstream and downstream from hardened low-water crossings on 

selected streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

28 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams

10

20

30

40

50

60

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

 

10

20

30

40

50

60

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

10

20

30

40

50

60

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

10

20

30

40

50

60

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

10

20

30

40

50

60

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

TOTAL DISSOLVED SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITE

R

YEAR
Figure 6. Specific conductance in water from sites upstream and downstream from hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

Effects of Hardened Low-Water Crossings on Water Quality 29

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIU

S

10

20

30

40

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

 

10

20

30

40

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

 

10

20

30

40

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

 

10

20

30

40

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

 

10

20

30

40

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

YEAR
Figure 7. Alkalinity concentrations in water from sites upstream and downstream from hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

30 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams

0

1

2

3

4

5

6

7

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

 

0

1

2

3

4

5

6

7

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

 

0

1

2

3

4

5

6

7

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

0

1

2

3

4

5

6

7

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

0

1

2

3

4

5

6

7

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

ALKALINITY CONCENTRATION, IN MILLIGRAMS PER LITE

R

YEAR
Figure 8. Manganese concentrations in water from sites upstream and downstream from hardened low-water crossings on selected 

streams at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. (See fig. 1 for location of site.)

Effects of Hardened Low-Water Crossings on Water Quality 31

0.0

0.1

0.2

0.3

Big Brushy Creek, upstream (BBUP)

Big Brushy Creek, downstream (BBDN)

Crossing installation date

Crossing modification date

0.0

0.1

0.2

0.3

Little Brushy Creek, upstream (LBUP)

Little Brushy Creek, downstream (LBDN)

Crossing installation date

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Tributary to Big Brushy Creek, upstream (TBBUP)

Tributary to Big Brushy Creek, downstream (TBBDN)

Crossing installation date

0.0

0.1

0.2

0.3

Tributary to Birds Creek, upstream (TBCUP)

Tributary to Birds Creek, downstream (TBCDN)

Crossing installation date

1998 1999 2000 2001 2002 2003 2004

0.0

0.1

0.2

0.3

Tributary to East Fork Creek, upstream (TEFUP)

Tributary to East Fork Creek, downstream (TEFDN)

Crossing installation date

Crossing modification date

MANGANESE CONCENTRATION, IN MILLIGRAMS PER LITE

R

YEAR
Table 8. Average water-quality properties at sites upstream and downstream from hardened low-water crossings on Big Brushy Creek, Tributary to East Fork of Sixmile Creek, 

Tributary to Birds Creek, Tributary to Big Brushy Creek, and Little Brushy Creek at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04. [See fig. 1 for location of site. Before, samples collected before low-water crossing installation; After, samples collected within 1 year after hardened low-water crossing installation; Later, samples collected 5 years after hardened low-water crossing installation. Averages with different superscripts are statistically different (p less than or equal to 0.05). Units are milligrams per liter, unless indicated otherwise. μS/cm, microsiemens per centimeter]

Property or 

constituent

Big Brushy Creek (BB) Tributary to East Fork Creek (TEF) Tributary to Big Brushy Creek (TBB)

Upstream Downstream Upstream Downstream Upstream Downstream

Before After Later Before After Later Before After Later Before After Later Before After Later Before After Later

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Apr. 

2003–

Nov. 

2003

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Apr. 

2003–

Nov. 

2003

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Apr. 

2003–

Nov. 

2003

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Apr. 

2003–

Nov. 

2003

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Aug. 

2003–

Nov. 

2003

Dec. 

1998–

Mar. 

1999

July 

1999–

Oct. 

1999

Aug. 

2003–

Nov. 

2003

Temperature, water 

(degrees Celsius)

13.2b 22.7a 21.3a 13.2b 22.6a 21.3a 13.4b 22.7a 20.9a 13.4b 22.9a 20.9a 14.4b 22.3a 21.6a 14.4b 22.4a 21.7a

Specific conductance, field (μS/

cm at 25 degrees Celsius)

22.8a 22.0a 18.4b 23.0a 22.0a 18.9b 19.5b 24.3a 17.8b 19.2b 25.7a 18.0b 18.2b 24.3a 17.0b 18.5b 25.7a 17.8b

pH, whole water, 

field

6.0ab 5.9b 6.0a 6.0ab 5.6b 6.2a 5.6 5.6 5.4 5.5 5.8 5.6 5.3 5.4 5.3 5.4 5.6 5.3

Oxygen, dissolved 8.3a 5.7b 7.8a 8.4a 5.9b 7.9a 8.4a 5.1c 7.3b 9.6a 5.6c 7.5b 8.6a 5.7b 6.1b 8.2a 6.0b 6.6bResidue on evaporation at 180 

degrees Celsius, filtered

29 32 27 28 30 27 24b 33a 26ab 24b 36a 28ab 22b 29a 23b 23b 31a 22b

Alkalinity, field 

(as CaCO3

)

3.3b 4.3ab 4.9a 3.7b 4.3ab 5.1a 2.0b 2.7a 3.2a 2.5b 4.0a 3.2a 2.2b 2.7b 1.0a 2.2b 2.5b 1.3a

Calcium, filtered 

(as Ca)

.81 .94 .79 .90 .98 .88 .44ab .58a .40b .43ab 1.33a .48b .36b .51a .34b .37b .72a .40b

Magnesium, filtered (as Mg) .52 .54 .47 .52 .54 .47 .39 .55 .38 .39 .57 .39 .36b .50a .39b .37b .51a .40bSodium, filtered 

(as Na)

2.10a 1.80b 1.80b 2.10a 1.80b 1.80b 2.00b 2.30a 2.00b 2.00b 2.40a 2.00b 1.90b 2.30a 2.00b 1.90b 2.30a 2.00b

Potassium, filtered 

(as K)

.37b .57a .30b .36b .51a .31b .13b .36a .18b .13b .48a .19b .14b .27a .12b .13b .35a .14b

Iron, filtered (as Fe) 205 222 232 206 196 181 109b 196a 196a 109b 249a 196a 86b 325a 278a 90b 323a 266aManganese, filtered 

(as Mn) 

37 51 38 38 49 36 25b 66a 30b 26b 70a 32b 51b 289a 90b 53b 284a 86b

Silica, filtered 

(as SiO2

)

9.4a 8.3b 9.6a 9.6a 8.2b 9.5a 10.2 10.0 10.0 10.1 10.0 10.5 8.9b 12.2a 12.5a 8.9b 12.3a 12.4a

Sulfate, filtered 

(as SO4

)

1.40a 1.10ab .60b 1.40a 1.00ab .60b 1.20ab 1.40a .50b 1.20ab 1.50a .50b 1.00b 1.60a .30b 1.00ab 1.50a .30b

Chloride, filtered 

(as Cl)

3.6a 3.0b 3.0b 3.6a 2.9b 3.1b 3.5 3.8 3.6 3.6 3.8 3.5 3.2b 3.8a 3.7a 3.2b 3.8a 3.7a

32 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
Table 8. Average water-quality properties at sites upstream and downstream from hardened low-water crossings on Big Brushy Creek, Tributary to East Fork of Sixmile Creek, 

Tributary to Birds Creek, Tributary to Big Brushy Creek, and Little Brushy Creek at the Fort Polk Military Reservation, Louisiana, 1998–99 and 2003–04—Continued. [See fig. 1 for location of site. Before, samples collected before low-water crossing installation; After, samples collected within 1 year after hardened low-water crossing installation; Later, samples collected 5 years after hardened low-water crossing installation. Averages with different superscripts are statistically different (p less than or equal to 0.05). Units are milligrams per liter, unless indicated otherwise. μS/cm, microsiemens per centimeter]

Property or 

constituent

Tributary to Birds Creek (TBC) Little Brushy Creek (LB)

Upstream Downstream Upstream Downstream

Before After Before After After Later After Later

May 

2003–

July 

2003

Jan. 

2004–

June 

2004

May 

2003–

July 

2003

Jan. 

2004–

June 

2004

Dec. 

1998–

Oct. 

1999

Sept. 

2003–

Nov. 

2003

Dec. 

1998–

Oct. 

1999

Sept. 

2003–

Nov. 

2003

Temperature, water (degrees 

Celsius)

20.3a 16.2b 19.2a 18.1b 17.4a 21.8a 17.4a 21.8a

Specific conductance, field 

(μS/cm at 25 degrees Celsius)

22.2 19.7 22.7 19.8 18.0a 14.5b 19.8a 14.8b

pH, whole water, field 5.8 5.6 6.0 5.6 5.7 5.8 5.7 5.9Oxygen, dissolved 6.7b 8.9a 6.6b 8.6a 8.2 8.8 7.8 8.8Residue on evaporation 

at 180 degrees Celsius, filtered

45a 20b 32a 27b 21 20 25 19

Alkalinity, field (as CaCO3) 5.3a 3.7b 5.0a 3.8b 2.8b 3.8a 4.7a 3.8aCalcium, filtered (as Ca) .83 .74 .79 .77 .36b .30b .86a .34bMagnesium, filtered 

(as Mg)

.55 .51 .55 .50 .37 .36 .41 .36

Sodium, filtered (as Na) 2.00 2.00 2.10 1.90 1.90 1.80 1.90 1.80Potassium, filtered (as K) .50 .50 .47 .48 .17 .20 1.80 2.00Iron, filtered (as Fe) 230 251 208 239 126 123 125 109Manganese, filtered (as Mn) 43 50 40 47 20a 11b 18a 11bSilica, filtered (as SiO2) 10.3 9.8 10.7 9.8 9.1a 9.6b 9.2a 9.6bSulfate, filtered (as SO4) .57b .98a .57b .94a .70a .38b .67a .37bChloride, filtered (as Cl) 3.5 3.5 3.6 3.4 3.2 3.1 3.2 3.0

Effects of Hardened Low-Water Crossings on Water Quality 33
It is probable that observed variations in water quality at 

both upstream and downstream sites are related to differences 

in rainfall and streamflow during the sample-collection 

periods rather than an effect of the hardened low-water 

crossing installations or modifications, but additional study is 

needed.

Summary

In 2003, the U.S. Geological Survey (USGS), at the 

request of the U.S. Army Joint Readiness Training Center 

and Fort Polk, began a follow-up study to determine whether 

installation and modification of hardened low-water crossings 

had short-term (less than 1 year) or long-term (greater than 

1 year) effects on periphyton or water quality in five streams 

at the Fort Polk Military Reservation, Louisiana. Periphyton data were statistically analyzed for possible differences 

between samples collected at upstream and downstream sites 

and before and after low-water crossings were modified on 

three streams, Big Brushy Creek (BB), Tributary to East Fork 

of Sixmile Creek (TEF), and Tributary to Birds Creek (TBC), 

during 2003–04. Periphyton data also were analyzed for possible differences between samples collected at upstream and 

downstream sites on two streams, Tributary to Big Brushy 

Creek (TBB) and Little Brushy Creek (LB), during 1998–1999 

and 2003. Water-quality data collected from upstream and 

downstream sites on all five streams during 2003–04 were analyzed for possible differences caused by the hardened crossings. Water-quality data collected from two streams, BB and 

TEF, were analyzed for possible differences between samples 

collected at upstream and downstream sites, and differences 

between samples collected before and after the low-water 

crossings were installed in 1999 and later, in 2003.

A total of 362 periphyton taxa were identified from 

312 samples analyzed. Diatoms (Chrysophyta) were the most 

diverse group of algae, with 144 taxa identified. Green algae 

(Chlorophyta) ranked second, with 136 taxa. Blue-green 

algae (Cyanophyta) ranked third with 62 taxa. The remaining 20 taxa identified were distributed amongst four other 

plant groups, including red algae (Rhodophyta), filamentous 

bacteria, and other golden brown algae (within three Classes 

of the Division, Chrysophyta). Diatom taxa outnumbered the 

other major divisions by at least 2.5 times in 90 percent of the 

samples. Green algae taxa outnumbered diatoms in less than 

5 percent of the samples.

Percent frequency of occurrence and cell density across 

all samples varied similarly and 17 taxa occurred in more than 

40 percent of the samples. Diatoms were the most frequently 

occurring algae, and filamentous green algae and colonial 

blue-green forms were numerically dominant when they 

occurred. The colonial blue-green alga, Microcystis incerta, 

was present in 60.2 percent of the samples, and the filamentous green alga, Oedogonium spp., was present in 43.3 percent 

of the samples. The average cell densities of blue-green algae 

and green algae tended to fluctuate from month-to-month, 

while the diatoms occurred more consistently at most sites 

during the study. 

Variations in periphyton communities in samples collected before and after the low-water crossings were modified during 2003 could not be conclusively attributed to the 

modifications. Data analysis indicated no significant seasonal 

variation, a dominant factor in a previous study, probably 

because no periphyton samples were collected during winter 

months. In addition, there was no significant difference in percent frequency of occurrence or cell density of taxa between 

substrates (slides and wood cores).

Comparison of percent frequency of occurrence and 

average cell density of the 10 most frequently occurring taxa 

showed that four diatom taxa, Eunotia binularis, Eunotia 

flexuosa, Frustulia rhomboides, and Eunotia exigua, indicated 

significant interaction between upstream and downstream 

sites, and before and after crossing modifications at BB, TEF, 

Figure 9. Monthly rainfall at Leesville, Louisiana, during periphyton and water-quality data collection at selected streams at the Fort 

Polk Military Reservation, Louisiana, October 1998 through September 1999 and April 2003 through April 2004 (Betty Wall, Louisiana 

Office of State Climatology, written commun., 2004). 

34 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams

1998 1999 2000 2001 2002 2003 2004

0

2

4

6

8

10

No data No data No data

RAINFALL, IN INCHE

S

YEAR
and TBC. Because of these interactions, it was not possible to 

conclude that the hardened low-water crossing modifications 

caused the differences in those taxa. Periphyton data collected from TBB and LB during 1998–99 and 2003 also were 

analyzed to determine whether long-term changes may have 

occurred after the installation of the hardened low-water 

crossings during 1998–99.

Most of the significant changes in percent frequency 

of occurrence and average cell density of the 10 most frequently occurring periphyton taxa were increases at downstream sites after the hardened low-water crossing installations 

or modifications. However, these changes in the periphyton 

community are not necessarily deleterious to the community 

structure.

Water-quality data collected from upstream and downstream sites on the five streams during 2003–04 were analyzed 

for possible differences caused by the hardened crossings. 

Water-quality data collected from two streams, BB and TEF, 

were analyzed for possible differences between samples 

collected at upstream and downstream sites and differences 

between samples collected before and after the low-water 

crossings were installed in 1999 and later, in 2003.

The streams are acidic and poorly buffered, and pH 

ranged from 5.0 to 6.8 standard units. Field measurements of 

alkalinity ranged from 0.1 to 6.7 mg/L, and field measurements of specific conductance ranged from 13 to 36 μS/cm. 

Nutrient concentrations were small. Dissolved ammonia (as 

nitrogen) ranged from less than 0.002 to 0.056 mg/L, and 

many results were below detection limits. Dissolved ammonia plus organic nitrogen (as nitrogen) ranged from less than 

0.1 to 0.4 mg/L. Dissolved nitrite plus nitrate (as nitrogen) 

ranged from less than 0.002 to 0.079 mg/L, and many results, 

especially from samples collected during 1999, were below 

detection limits. Dissolved phosphorus (as P) ranged from less 

than 0.002 to 0.009 mg/L, and most results were below detection limits. Calcium, magnesium, and sodium concentrations 

were small. Calcium ranged from 0.28 to 2.06 mg/L, magnesium ranged from 0.31 to 0.94 mg/L, and sodium ranged from 

1.5 to 3.0 mg/L. Silica concentrations were large relative to 

other constituents measured and ranged from 7.3 to 13 mg/L. 

All analyzed water-quality properties and constituents were 

typical of small streams in this area.

Water-quality changes that may have resulted from original installation of the low-water crossings on BB, TEF, TBB, 

and LB, generally disappeared over time. Increased calcium 

concentrations at the downstream site on TEF after the lowwater crossing installation in 1999 may have resulted from the 

use of dolomitic limestone. However, calcium concentrations 

at the downstream site on TEF were consistent with those at 

the upstream site by the time the limestone was replaced at the 

crossing in 2003, indicating that dissolution of the limestone 

probably had already slowed down or stopped.

Water-quality data also were analyzed for significant 

differences between upstream and downstream sites before 

and after the installation of the hardened low-water crossings 

during 1998–99, and later, during 2003–04. Generally, average water-quality values and concentrations were similar at 

upstream and downstream sites. When average water-quality 

values or concentrations changed significantly, they almost 

always changed significantly at both the upstream and downstream sites. It is probable that observed variations in water 

quality at both upstream and downstream sites are related to 

differences in rainfall and stream flow during the samplecollection periods rather than an effect of the hardened lowwater crossing installations or modifications, but additional 

study is needed.

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36 Effects of Hardened Low-Water Crossings on Periphyton and Water Quality in Selected Streams
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