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Watershed Morphology of Highland and Mountain Ecoregions in Eastern
Oklahoma
Dale K. Splintera; Daniel C. Dauwalterb; Richard A. Marstonc; William L. Fisherb
a
University of Wisconsin-Whitewater, b U.S. Geological Survey, Oklahoma Cooperative Fish and
Wildlife Research Unit, c Kansas State University,
First published on: 13 December 2010
To cite this Article Splinter, Dale K. , Dauwalter, Daniel C. , Marston, Richard A. and Fisher, William L.(2010) 'Watershed
Morphology of Highland and Mountain Ecoregions in Eastern Oklahoma', The Professional Geographer,, First published
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Watershed Morphology of Highland and Mountain
Ecoregions in Eastern Oklahoma
Dale K. Splinter
University of Wisconsin–Whitewater
Daniel C. Dauwalter
U.S. Geological Survey, Oklahoma Cooperative Fish and Wildlife Research Unit
Downloaded By: [Marston, Richard A.] At: 01:11 14 December 2010
Richard A. Marston
Kansas State University
William L. Fisher
U.S. Geological Survey, Oklahoma Cooperative Fish and Wildlife Research Unit
The fluvial system represents a nested hierarchy that reflects the relationship among different spatial and
temporal scales. Within the hierarchy, larger scale variables influence the characteristics of the next lower
nested scale. Ecoregions represent one of the largest scales in the fluvial hierarchy and are defined by recurring
patterns of geology, climate, land use, soils, and potential natural vegetation. Watersheds, the next largest
scale, are often nested into a single ecoregion and therefore have properties that are indicative of a given
ecoregion. Differences in watershed morphology (relief, drainage density, circularity ratio, relief ratio, and
ruggedness number) were evaluated among three ecoregions in eastern Oklahoma: Ozark Highlands, Boston
Mountains, and Ouachita Mountains. These ecoregions were selected because of their high-quality stream resources and diverse aquatic communities and are of special management interest to the Oklahoma Department
of Wildlife Conservation. One hundred thirty-four watersheds in first- through fourth-order streams were
compared. Using a nonparametric, two-factor analysis of variance (α = 0.05) we concluded that the relief,
drainage density, relief ratio, and ruggedness number all changed among ecoregion and stream order, whereas
circularity ratio only changed with stream order. Our study shows that ecoregions can be used as a broad-scale
framework for watershed management. Key Words: ecoregions, Oklahoma, streams, watershed morphology.
C Copyright 2011 by Association of American Geographers.
The Professional Geographer, 63(1) 2011, pages 1–13 Initial submission, July2007; revised submissions, January and August 2008, May and September 2009; final acceptance,
November 2009.
Published by Taylor & Francis Group, LLC.
2 Volume 63, Number 1, February 2011
Downloaded By: [Marston, Richard A.] At: 01:11 14 December 2010
El sistema fluvial representa una jerarquı́a anidada que refleja la relación entre diferentes escalas espaciales
y temporales. Dentro de la jerarquı́a, las escalas variables más grandes influyen sobre las caracterı́sticas de la
siguiente escala anidada de menor valor. Las eco-regiones representan una de las escalas más grandes en la
jerarquı́a fluvial y se definen por medio de patrones recurrentes de geologı́a, clima, uso de la tierra, suelos y
vegetación natural potencial. Las cuencas, que son la siguiente escala en importancia, a menudo se albergan
en una sola eco-región y por tanto exhiben las propiedades indicativas de una eco-región dada. Las diferencias
en la morfologı́a de las cuencas (relieve, densidad de drenaje, razón de circularidad, razón de relieve y número
de escabrosidad) fueron evaluadas entre tres eco-regiones del oriente de Oklahoma: los Altos de las Ozark,
las Montañas de Boston, y las Montañas Ouachita. Se seleccionaron estas eco-regiones debido a su dotación
de corrientes fluviales de alta caliudad y diversas comunidades acuáticas, y porque son de especial interés de
manejo para el Departamento de Conservación de Vida Silvestre de Oklahoma. Se compararon ciento treinta
y cuatro cuencas con corrientes del primero al cuarto orden. Utilizando un análisis de varianza de dos factores
(α = 0.05), no-paramétrico, concluimos que el relieve, la densidad del drenaje, la razón de relieve y el número
de escabrosidad, en conjunto, cambiaron entre la eco-región y el orden de las corrientes, en tanto que la razón
de circularidad solo cambió con el orden de las corrientes. Nuestro estudio muestra que las eco-regiones
pueden utilizarse como un marco de escala amplia para el manejo de cuencas. Palabras clave: eco-regiones,
Oklahoma, corrientes, morfologı́a de cuencas.
T
he fluvial system is spatially and temporally hierarchical (Schumm and Lichty
1965; Frissell et al. 1986; Kondolf et al. 2003).
Schumm and Lichty (1965) explained that an
integrated set of independent and dependent
variables shape and control watershed characteristics over time and space. They argued
that over a long duration (i.e., an erosional
period), time, initial relief, geology, and climate are independent variables that influence
vegetation, sediment yield, hillslope morphology, and hydrology. In accordance with the
work by Schumm and Lichty, Omernik (1987)
stated that the causal factors of climate, soil and
geology, vegetation, and physiography define
ecosystems in a regional framework. In turn,
ecoregion delineations for the United States
were created by examining the factors that
cause regional variation or those factors that
integrate causal factors (Omernik 1987). Kondolf et al. (2003) stated that similarities in climate, geomorphology, lithology, and land-use
history will lead to stream channel characteristics that are inherently similar within a given region. In a nested hierarchical order, the unique
combination of geology, climate, vegetation,
and land-use has and continues to influence watershed morphology spatially and temporally.
Our objective was to determine whether
watershed morphology differed among three
ecoregions (Ozark Highlands, Boston Mountains, and Ouachita Mountains) in eastern
Oklahoma. We hypothesize that watershed
morphology differs (α = 0.05) among ecoregions because watershed evolution and the
current geomorphic processes acting in the
watershed result from the interplay of the resisting framework and driving forces applied to
geomorphic systems over time (Ritter, Kochel,
and Miller 2002). Because the resisting framework (geologic structure and lithology) and
driving forces (climate and land use) differ
among ecoregions, watershed evolution and the
geomorphic processes acting at multiple spatial
scales are initiated at the ecoregion level. If our
hypothesis is accepted: (1) watershed morphology differs by ecoregion because the mosaic
of natural and human forces that affect watershed morphology are more similar within
than between ecoregions; and (2) watershed
planners and managers will be able to evaluate management options by ecoregion rather
than on a watershed-by-watershed basis, which
will permit more efficient use of resources and
more timely responses to needed management
changes.
We studied 134 watersheds to examine
whether watershed morphology, measured as
relief, drainage density, circularity ratio, relief
ratio, and ruggedness, differed among three
physically contrasting ecoregions in eastern
Oklahoma. The morphological metrics previously listed were used because they are often used to describe watershed morphology
(Patton and Baker 1976; Harlin 1984; Lièbault
et al. 2002).
Ecoregions and Watershed
Management
Ecoregions were originally developed to provide a geographic framework for ecosystem
management (Omernik 1987). Omernik (1987)
stated that ecoregions will allow managers,
planners, and scientists to (1) compare similarities and differences of land–water relationships;
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Watershed Morphology of Ecoregions in Eastern Oklahoma 3
(2) establish water quality standards that are acceptable for a given region; (3) locate places to
serve as monitoring, demonstration, and reference sites; (4) extrapolate from empirical data
collected at other locations; and (5) predict the
effect of land-use change. Omernik and Bailey
(1997) reiterated the importance of ecoregions
in providing a spatial framework for ecosystem assessment, research, inventory, monitoring, and management. Extrapolation of sitespecific data across an ecoregion allows for the
prediction of system function at unsampled locations (Omernik and Bailey 1997).
Hundreds of millions of dollars are invested
annually to manage and restore watersheds in
North America (Roni 2005). Comprehension
of how landscape controls (i.e., geology, climate, land use, soils, and vegetation) influence
watershed processes is important for the successful management and restoration of watersheds. Watershed processes influence stream
habitat, which is critical for ecosystem function at smaller scales (Roni 2005). Ecoregions
encompass the broad-scale landscape controls
that watershed managers need to understand
before management plans can be developed and
initiated. Failure to understand linkages between scales in the fluvial hierarchy can result
in unsuccessful watershed management plans
(Frissell and Ralph 1998).
Loveland and Merchant (2004, S1) wrote
that “ecoregions fuse the concept of ecosystems
with the geographic concept of regions.” In
doing so, they underscored the importance
among ecology, geography, and geomorphology. Studies involving fish, macroinvertebrates,
and geomorphology have utilized ecoregions
as a spatial framework for study (Larsen
et al. 1986; Rohm, Giese, and Bennett 1987;
Lyons 1989; Newell and Magnuson 1999;
McCormick, Peck, and Larsen 2000; Pan
et al. 2000; Rabeni and Doisy 2000; Dauwalter
et al. 2007; Dauwalter et al. 2008). The
dynamic relationship bridging ecology and
geomorphology was portrayed at the 36th
International Geomorphology Binghamton
Symposium in 2005 and the 2004 Association
of American Geographers annual meeting in
Philadelphia, Pennsylvania (Renschler, Doyle,
and Thoms et al. 2007; Urban and Daniels
2006). Aquatic scientists recognize that habitat
dictates the richness and abundance of species,
which is partly influenced by the characteristics
of the ecoregion (Dauwalter et al. 2008).
Watershed Morphology
Geomorphologists use morphometric analysis
to investigate watershed morphology quantitatively (Chorley, Schumm, and Sugden 1984).
Horton (1932) introduced watershed analysis
to explain watershed function (Gregory and
Walling 1973). This quantitative morphometric analysis of watersheds was continued by a
series of methodological and theoretical papers
spanning more than a quarter century (Horton
1945; Langbein 1947; Strahler 1952, 1958,
1964; Schumm 1956). These papers helped
establish how morphometric analyses could
be used to differentiate geomorphological
processes in contrasting regions.
Morisawa (1962) investigated whether the
watersheds of the Allegheny Plateau, Allegheny
Mountains, and Cumberland Plateau regions of
the Appalachian Plateau were morphologically
different. She found that watershed morphology differed among these regions. Morisawa
stated that these findings support separating
each of the three regions into distinct geomorphic sections. Lewis (1969) used similar
watershed characteristics to classify Indiana
into contrasting morphometric regions.
Morphometric analyses have recently
been used in process-based studies and for
environmental management. Jamieson et al.
(2004) showed that tectonic zones in the
Indus Valley of Ladakh, in north India, can be
differentiated using morphometric analyses of
longitudinal valleys. Watersheds draining one
of the tectonic zones were shorter, narrower,
and had lower hypsometric integrals than
the other two. These watersheds have been
influenced by thrust propagation that has led to
erosion and increased sediment delivery to the
main stem of the river and elevated local base
levels. Morphometric analyses have also been
conducted on paleodrainages in the deserts
of Kuwait to understand the genesis and hydrological implications of runoff (Al-Sulaimi,
Khalaf, and Mukhopadhyay 1997).
Watershed morphology influences the response of a flood hydrograph for a given basin.
The shape of the flood hydrograph is dictated
by the routing of water through the watershed
(Ritter, Kochel, and Miller 2002). Patton and
Baker (1976) reported that drainage density
and stream frequency are good measures
to predict peak discharge for watersheds in
regions with unlike characteristics. Drainage
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4 Volume 63, Number 1, February 2011
density is an areal morphometric variable that
is often a function of climate, lithology, and
relief (Chorley, Schumm, and Sugden 1984).
Semiarid watersheds generally have higher
drainage densities than humid watersheds
because less precipitation decreases vegetation,
which accelerates overland flow and erosion
in arid regions (Ritter, Kochel, and Miller
2002). In regions with similar climate and
precipitation regimes, lithology and relief (resisting framework) are the dominant controls
on drainage density (Chorley, Schumm, and
Sugden 1984). Watershed circularity also plays
a prominent role in the characteristics of the
flood hydrograph. Assuming that watersheds
have similar patterns of stream networks, circular watersheds will supply flow to the outlet
more quickly than elongated watersheds (Singh
1992). This is particularly true in watersheds
with high relief ratios and ruggedness numbers.
Watershed morphology also affects aquatic
organisms. Potter et al. (2004) found that
aquatic biodiversity in North Carolina is
most at risk in agricultural lands draining
watersheds with high circularity because
circular watersheds have short delivery times
of maximum flow. This decreases the amount
of time available for pollutants to settle out of
the water, which increases water quality degradation and decreases biodiversity of aquatic
macroinvertebrates. Relationships among
morphometric variables, stream habitat, and
fish abundance have been documented in small
Rocky Mountain streams (Lanka, Hubert, and
Wesche 1987). Lanka, Hubert, and Wesche
(1987) found that low basin relief, low relief
ratio, and relatively low drainage density produced the better trout habitat and concluded
that measures of drainage basin morphology
could be useful for predicting trout habitat in
streams via simple morphometric calculations.
Study Area
We investigated to what extent the resisting
framework and driving forces acting within
ecoregions (Ozark Highlands, Boston Mountains, and Ouachita Mountains) had an effect
on watersheds in eastern Oklahoma (Figure 1).
These ecoregions have high-quality stream resources that support diverse aquatic communities (Dauwalter et al. 2008). Black bass (Micropterus spp.) are popular sport fishes in these
streams, and recreational fishing provides im-
portant economic revenue in this portion of
Oklahoma (Fisher et al. 2002). As a result,
the Stream Management Program of the Oklahoma Department of Wildlife Conservation
is active in managing stream resources in eastern Oklahoma (Hyler et al. 2004). Black bass
populations have been shown to differ among
ecoregions in eastern Oklahoma (Balkenbush and Fisher 2001; Dauwalter and Fisher
2008), and fisheries management has been
regionalized to reflect these differences (Fisher,
Tejan, and Balkenbush 2004). Although populations are known to differ, stream habitat
management is based on the physical characteristics of stream channels nested within the
hierarchy of the fluvial system. Addressing how
variables making up ecoregions influence watershed morphology is a critical step in determining whether resource management focused
on the physical aspects of the fluvial system can
also be regionalized.
Woods et al. (2005) described the Ozark
Highlands as being composed of watersheds
that are high to moderately dissected. Lithology is mostly limestone and dolostone with
interbedded chert. Karst features, such as sinkholes and caves, are common. Cool, spring-fed
perennial streams are also common; however,
during the summer many first- and secondorder streams are dry and third- and fourthorder streams become intermittent (Splinter
2006). Precipitation is approximately 100 cm
to 125 cm annually. Prior to the nineteenth
century, the plateau region consisted of oakhickory forests and grasslands; today, agricultural land and increased residential areas have
replaced native vegetation (Woods et al. 2005).
Rapid suburbanization of the region, along
with intensive grazing and poultry farms, has
greatly decreased water quality in some streams
(Peterson et al. 1998; Woods et al. 2005). Soil
orders on uplands consist of Ultisols, Alfisols,
and Mollisols.
The Boston Mountains are immediately
south and west of the Ozark Highlands. Like
the Ozark Highlands, this region is highly
dissected (Woods et al. 2005). A difference in
lithology between the Boston Mountains and
Ozark Highlands is the main characteristic that
distinguishes these two regions. Lithology of
the Boston Mountains is primarily sandstone
and shale, with minor amounts of limestone.
Streams in this region tend to be cool water
but less influenced by springs than streams
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Watershed Morphology of Ecoregions in Eastern Oklahoma 5
Figure 1 Randomly selected pour points on streams (stream orders 1–4) in the Ozark Highlands, Boston
Mountains, and Ouachita Mountains ecoregions in eastern Oklahoma. Contributing watersheds above
each pour point were delineated and used in morphometric analyses.
in the Ozark Highlands (Woods et al. 2005).
Channel substrate tends to be larger than the
cherty gravel existing in streams of the Ozark
Highlands (Splinter 2006). Precipitation is
approximately 110 cm to 130 cm annually.
Land use consists of forest and woodland, with
flatter areas used for ranching and farming.
The potential natural vegetation includes
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6 Volume 63, Number 1, February 2011
mostly oak-hickory forest (Woods et al. 2005).
Soil orders on uplands consist of Ultisols,
Inceptisols, and Entisols.
The Ouachita Mountains are south of the
Boston Mountains. The Ouachita Mountains
ecoregion is a mosaic of low mountains and
high hills (150–750 m of local relief) of folded
Paleozoic rocks (Woods et al. 2005). Lithology,
highly variable across the ecoregion, consists
mostly of sandstone, shale, and novaculite.
Streams in this region are often confined
by geologic structure and large substrates,
lack springs, and have a reduced summer
flow (Splinter 2006). Maximum mean annual
precipitation occurs on south-facing ridges,
increases to the east, and is 110 cm to 145 cm
annually. Much of the Ouachita Mountains
are forested, with larger valleys used for pastureland (Woods et al. 2005). Specific land use
includes forestry, logging, ranching, woodland
grazing, and recreation. Commercial pine
plantations are scattered across the ecoregion
(Woods et al. 2005). The potential natural
vegetation includes oak-hickory-pine forest
(Woods et al. 2005). Soil orders consist of
Ultisols, Alfisols, and Inceptisols.
Method
Watershed Selection
We randomly selected watersheds in the Ozark
Highlands, Boston Mountains, and Ouachita
Mountains ecoregions (Figure 1). To select watersheds, we randomly selected 149 pour points
on a stream network and delineated the watersheds for each pour point. The stream network
c using a 30-m
was delineated in ArcView 3.3
Table 1
Digital Elevation Model from the USGS National Elevation Dataset. We used a flow accumulation threshold of 1.35 km2 that matched
the extent of the stream network from 1:24,000
topographic maps and accurately depicted firstorder stream initiation. The number of watersheds selected per ecoregion was approximately
proportional to the area of each ecoregion:
twenty-five in the Ozark Highlands, thirtyone in the Boston Mountains, and seventyeight in the Ouachita Mountains. Watersheds
within each ecoregion were equally distributed
among stream orders one through four. This allowed for comparable sampling coverage across
all three ecoregions and ensured that watersheds of both small and large streams were
sampled. Watersheds that were not at least 90
percent within one ecoregion were excluded.
Only a limited number of different fourthorder streams could be selected in the Ozark
Highlands and Boston Mountains because of
ecoregion size. Of the 149 watersheds originally selected, only 15 (10.1 percent) failed to
meet the 90 percent confinement criteria and
the remaining 134 were used for the analysis.
Morphometric Variables
Five morphometric variables were measured
c and ArcGIS 9.1
c (ESRI,
using ArcView 3.3
Redlands, CA; Table 1). Drainage density was
calculated by dividing the sum of stream lengths
in the watershed by the watershed area (Horton
1945). Circularity ratio is the area of the
watershed divided by the area of a circle with
the same perimeter as the basin (Miller 1953).
This variable expresses the overall shape of
the watersheds. A value of one represents a
Watershed variables used to discriminate morphology difference among ecoregions
Variable
Source
Calculation
Drainage density
(km/km2 )
Circularity ratio
Horton (1945)
stream length/Watershed area
Miller (1953)
Area of watershed/Area of circle
Relief (m)
Strahler (1952),
Schumm (1956)
Schumm (1956)
High elevation – Low elevation
Relief ratio
Ruggedness number
Patton and Baker
(1976)
Note: Table modified after Strahler (1958).
Watershed relief/Watershed
length
Drainage density × Basin relief
Purpose
Expresses the overall
dissection of the watershed
Represents how quickly water
enters and exits the stream
Influences the erosion potential
of the watershed
Represents the overall
steepness of the watershed
Used to measure the flash
flood potential of streams
Watershed Morphology of Ecoregions in Eastern Oklahoma 7
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perfect circle. Relief is the highest elevation
in the watershed minus the lowest elevation in
the watershed. Relief ratio was calculated by
dividing the total basin relief (outlet to summit of watershed) by the basin length (Schumm
1956). The basin length used to calculate the relief ratio was a straight line from the watershed
outlet to the summit, unless the straight line
would have crossed the watershed boundary.
Where this occurred, the line was bent along
the channel and continued until the watershed
and the valley were parallel. Ruggedness number is basin relief multiplied by drainage density.
Statistical Analysis
Although our primary interest was in how watershed morphology varied among ecoregions
and stream orders, we used Spearman rank correlations to show the interrelationships among
watershed morphology variables. Differences
in watershed morphology among ecoregions
and stream orders were determined using a
nonparametric, two-factor analysis of variance
with watershed morphology as the response
variable, and the ecoregion and stream order
as the main effects. Analyses were done on
ranked data because variances were different
among stream orders for some variables. Linear contrasts were used to determine pairwise
differences in watershed morphology among
ecoregions when an ecoregion main effect was
evident (Kuehl 2000). Polynomial contrasts
were used to test for trends in watershed morphology with stream order (Kuehl 2000). Type
I error rate was set at α = 0.05. Analyses were
done using SAS version 9.1 statistical software
(SAS Institute, Inc., Cary, NC). Rejection of
the statistical null hypothesis (H0 ) would support our scientific hypothesis (H1 ):
H0 : Watershed morphology is not different
(α ≥ 0.05) among ecoregions.
H1 : Watershed morphology is different (α ≤
0.05) among ecoregions.
If H1 is accepted, the resisting framework
(lithology and structure) and driving forces
(land use and climate) that influence watershed
morphology can be differentiated by ecoregion.
For example, changes to driving forces (i.e.,
land use) impact ecological and geomorphic
processes occurring at the watershed scale.
Results and Discussion
Watershed morphology differed among ecoregions in eastern Oklahoma (Table 2), and
morphologic variables within ecoregions were
highly correlated (Table 3). Although the high
correlations were not surprising given that
some watershed morphology variables were
used to calculate others, the differences in watershed morphology among ecoregions while
simultaneously accounting for stream orders
supports our hypothesis that ecoregions represent broad-scale composite variables that control the development of the fluvial hierarchy.
As a result, ecoregions can provide a framework
to regionalize watershed management and the
management of stream resources.
Table 2 Summary data for watershed morphology: Means and standard deviation (in parentheses) for
each of the variables by stream orders are reported
Region and order
Boston Mountains (1)
Ozark Highlands (1)
Ouachita Mountains (1)
Boston Mountains (2)
Ozark Highlands (2)
Ouachita Mountains (2)
Boston Mountains (3)
Ozark Highlands (3)
Ouachita Mountains (3)
Boston Mountains (4)
Ozark Highlands (4)
Ouachita Mountains (4)
Number
Drainage
density
(km/km2 )
Circularity
ratio
6
7
19
9
6
22
9
7
19
7
5
18
0.33 (0.27)
0.42 (0.25)
0.50 (0.26)
0.49 (0.06)
0.70 (0.16)
0.64 (0.13)
0.63 (0.04)
0.68 (0.08)
0.71 (0.11)
0.65 (0.02)
0.71 (0.04)
0.72 (0.06)
0.64 (0.05)
0.60 (0.11)
0.55 (0.14)
0.53 (0.06)
0.45 (0.10)
0.48 (0.11)
0.41 (0.09)
0.42 (0.11)
0.40 (0.10)
0.32 (0.05)
0.40 (0.05)
0.41 (0.06)
Relief
(m)
118.81 (71.48)
58.77 (23.36)
157.06 (78.14)
175.94 (49.80)
79.64 (20.26)
223.50 (136.29)
256.10 (41.52)
134.29 (41.24)
320.20 (124.83)
379.88 (76.37)
152.80 (10.77)
379.41 (181.26)
Relief
ratio
Ruggedness
number
0.06 (0.03)
0.02 (0.01)
0.06 (0.04)
0.04 (0.02)
0.02 (0.01)
0.03 (0.01)
0.02 (0.01)
0.01 (0.01)
0.02 (0.01)
0.01 (0.00)
0.01 (0.00)
0.01 (0.01)
0.05 (0.05)
0.03 (0.02)
0.07 (0.05)
0.09 (0.03)
0.06 (0.02)
0.15 (0.11)
0.16 (0.03)
0.09 (0.02)
0.22 (0.09)
0.25 (0.05)
0.11 (0.01)
0.27 (0.11)
8 Volume 63, Number 1, February 2011
Table 3 Spearman rank correlations (rs) between watershed morphology variables and stream order
by ecoregion
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Boston Mountains (n = 31)
Circularity ratio
Relief (m)
Relief ratio
Ruggedness
Ozark Highlands (n = 26)
Circularity ratio
Relief (m)
Relief ratio
Ruggedness
Ouachita Mountains (n = 78)
Circularity ratio
Relief (m)
Relief ratio
Ruggedness
Drainage density (km/km2 )
Circularity ratio
Relief (m)
Relief ratio
−0.590
0.694
−0.601
0.784
−0.791
0.853
−0.806
−0.703
0.974
−0.762
−0.707
0.411
−0.456
0.601
−0.690
0.558
−0.579
−0.456
0.942
−0.581
−0.561
0.247
−0.563
0.529
−0.372
0.600
−0.549
−0.013∗
0.663
−0.243
∗
All correlations are significant ( p < 0.05) except those with an asterisk ( ).
Relief
Relief differed among ecoregions, F (2, 128) =
34.12, p < 0.001, and stream order, F (3, 128)
= 30.75, p < 0.001. Relief was lower in
the Ozark Highlands than in the Ouachita
Mountains, F (128) = 66.36, p = 0.001, and
Boston Mountains, F (128) = 39.96, p = 0.001.
No difference in relief existed between the
Ouachita Mountains and Boston Mountains,
F (128) = 0.65, p = 0.420. Polynomial contrasts showed that relief increased with stream
order in all ecoregions, F (128) = 88.20,
p < 0.001.
Watershed relief in the Ozark Highlands was
lower in all stream orders than the relief in the
Ouachita Mountains and the Boston Mountains (Figure 2). The Ozark Highlands are
more closely associated with plateau-like characteristics (i.e., Springfield Plateau) than the
more rugged Ouachita Mountains and Boston
Mountains. The watersheds of the Ozark
Highlands, however, tend to be moderately to
highly dissected, with well-established stream
networks. Maximum elevations in the Ozark
Highlands are approximately 450 m, and minimum elevations are less than 120 m in the valley bottoms (Woods et al. 2005). The Boston
Mountains consist of low mountains and rolling
hills with higher maximum and minimum elevations than the Ozark Highlands. Maximum
elevations are approximately 520 m, with minimum elevations of approximately 140 m. The
Ouachita Mountains have both the highest and
lowest elevations among the three ecoregions.
This region of folded mountains and open hills
has maximum elevations that exceed 800 m, and
valley elevations are less than 20 m (Woods
et al. 2005). The northern boundary of the
Ouachita Mountains consists of east to west
trending watersheds that have the highest relief
in the region.
Drainage Density
Drainage density differed among ecoregions,
F (2, 128) = 11.88, p < 0.001, and stream order,
F (3, 128) = 17.27, p < 0.001. Drainage density was lower in the Boston Mountains than
in the Ouachita Mountains, F (128) = 22.70,
p = 0.001, and Ozark Highlands, F (128) =
12.54, p = 0.001. No difference in drainage
density existed between the Ouachita Mountains and Ozark Highlands, F (128) = 0.07,
p = 0.799. Polynomial contrasts showed that
drainage density increased with stream order
in all ecoregions, F (128) = 48.64, p < 0.001.
Drainage density is often a function of relief
(Schumm 1956; Mosley 1974; Montgomery
and Dietrich 1989). Watersheds with high
relief have erosion potentials greater than
watersheds with lower relief, which allows high
relief streams to downcut and migrate upslope
in a headward direction (Chorley, Schumm,
and Sugden 1984). In the analysis of drainage
density, this relationship was not verified in
all ecoregions. The highest drainage densities
occurred in the Ozark Highlands and the Ouachita Mountains (Figure 2). The relationship
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Watershed Morphology of Ecoregions in Eastern Oklahoma 9
Figure 2 Watershed characteristics by ecoregion and stream order. Mean values are shown, and error
bars are ± 1 SE. Ecoregions with different letters were significantly different (α = 0.05) as determined
by linear contrasts.
between watershed relief and drainage density
holds true when examining the Ouachita
Mountains. The Ozark Highlands, however,
have a higher drainage density with a much
lower relief than either the Ouachita Mountains or the Boston Mountains (Figure 2).
Watersheds in the Boston Mountains have
the lowest drainage density but the second
highest relief. These results suggest that
another variable or combination of variables is
responsible for controlling drainage density in
the Ozark Highlands.
Previous studies have found that lithology,
a resisting framework variable, plays a significant role in the drainage density of streams
(Ray and Fisher 1960; Hadely and Schumm
1961; Lièbault et al. 2002). The lithology of
the Ozark Highlands is comprised primarily
of chert and limestone (e.g., cherty limestone)
that weathers and erodes more easily than the
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10 Volume 63, Number 1, February 2011
sandstones in the Boston Mountains and sandstones and novaculites of the Ouachita Mountains. The Ozark Highlands consist of well to
excessively drained soils that form in colluvium
and the underling clay residuum from cherty
limestone. During high-intensity rainfall, infiltration is low and sheet erosion is common.
Where surface limestone has been dissolved,
the headward migration of a stream channel
has been intensified. The jointed and fractured
limestone (i.e., lithology and structure) serves
as a catalyst for rill development and the headward migration that initiates stream channels.
It is possible that the highly dissolvable cherty
limestone of the Ozark Highlands has promoted the initiation of stream channels and influenced drainage densities in these watersheds.
In addition to lithology and underlying geologic structure, land use (i.e., driving
framework) might have played a role in the high
drainage density of the Ozark Highlands. Studies of the Ozark Highlands in Missouri report
that changes in land use between the mid-1800s
and mid-1900s influenced the headward migration of streams and indirectly increased the
drainage density of these watersheds (Jacobson
and Primm 1997). Missouri Ozark streams are
like those in the Oklahoma Ozarks; streams in
both regions contain large amounts of gravel
that is being redistributed throughout the
system during mid- to high-magnitude floods
(Jacobson 1995; Remshardt and Fisher 2009).
Jacobson and Primm (1997) proposed that the
increase in gravel resulted from the extension
of the stream network. If gravel is coming
from the headward migration of channels, then
the erosion from land use change probably
plays a role in the higher drainage density of
the Oklahoma Ozark Highlands, which were
extensively logged in the late 1800s and the
mid-1900s (Rice and Penfound 1959) and later
became open-range grazing land.
Changes in land use have also occurred
in the Boston Mountains and the Ouachita
Mountains. The drainage densities in these
regions apparently have been impacted less
by changes in land use than in the Ozark
Highlands. These differences might be attributed to the more resistant lithology and
structure of the Boston Mountains and the
Ouachita Mountains. Ridgetops of the Boston
Mountains are primarily resistant sandstone
with sideslopes of interbedded sandstone and
shale (Woods et al. 2005). The Ouachita
Mountains consist of sandstone, shale, chert,
and novaculite. Hillslopes are more resistant to
erosion in the Boston Mountains and Ouachita
Mountains and are impacted less by changes in
land use than those in the Ozark Highlands.
Circularity Ratio
Circularity ratio changed with stream order,
F (2, 128) = 24.79, p < 0.001, but did not differ among ecoregions, F (2, 128) = 0.33, p =
0.718). Circularity decreased with stream order
in all ecoregions, F (128) = 70.07, p ≤ 0.001
(Figure 2). These results show that basin shape
does not differ among the three ecoregions.
Relief Ratio
Relief ratio differed among ecoregions, F (2,
128) = 18.17, p < 0.001, and stream order, F (3,
128) = 41.89, p < 0.001. Relief ratio was lower
in the Ozark Highlands than in the Ouachita
Mountains, F (128) = 32.48, p ≤ 0.001, and
Boston Mountains, F (128) = 26.94, p ≤ 0.001.
No difference in relief ratio existed between
the Ouachita Mountains and Boston Mountains, F (128) = 0.17, p = 0.685. Polynomial
contrasts suggested that relief ratio decreases
with stream order in all ecoregions, F (128) =
119.26, p < 0.001.
The relief ratio of the Ozark Highlands was
lower than in the other ecoregions, which is
a result of the overall low relief of the region
(Figure 2). Less change exists in relief ratio by
watershed size in the Ozark Highlands than the
Boston Mountains or the Ouachita Mountains.
Relief ratio in all three ecoregions decreased as watershed size increased but decreased slower in the Ozark Highlands than in
the Boston Mountains or Ouachita Mountains.
Ruggedness Number
Ruggedness number differed among ecoregions, F (2, 128) = 26.67, p < 0.001, and
stream order, F (3, 128) = 50.84, p < 0.001.
Ruggedness number was lower in the Ozark
Highlands than in the Ouachita Mountains,
F (128) = 53.03, p ≤ 0.001, and Boston Mountains, F (128) = 18.48, p ≤ 0.001. Ruggedness
number for the Boston Mountains was less
than the Ouachita Mountains, F (128) = 5.93,
p = 0.016. Polynomial contrasts suggested that
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Watershed Morphology of Ecoregions in Eastern Oklahoma 11
ruggedness number increases with stream order
in all ecoregions, F (128) = 146.42, p < 0.001.
Ruggedness number increased as watershed
size increased (Figure 2). This occurs because
drainage density and relief increase as watershed size increases, both of which are multiplied
together to calculate ruggedness number.
on Ozark Highland streams needs to be better
understood. In addition, Level IV ecoregions
have been established for much of the United
States. Level IV ecoregions are more detailed, which might allow for more detailed
assessment of watershed morphology and the
associated cascading fluvial hierarchy. Conclusion
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DALE K. SPLINTER is an Assistant Professor in the
Department of Geography and Geology at the University of Wisconsin–Whitewater, Whitewater, WI
53190. E-mail: splinted@uww.edu. His research interests include geomorphology, water resources, and
hydroecology.
DANIEL C. DAUWALTER was a Graduate Research Associate in the Oklahoma Cooperative
Fish and Wildlife Research Unit and Department of Zoology, Stillwater, OK 74078. E-mail:
DDauwalter@tu.org. He is currently a Fisheries Scientist with Trout Unlimited, and his research interests include the spatial analysis of stream habitats and
fishes, stream restoration, and conservation planning.
RICHARD A. MARSTON is a University Distinguished Professor and Head in the Department of
Geography at Kansas State University, Manhattan,
KS 66506. E-mail: rmarston@ksu.edu. His research
interests include geomorphology, hydrology, and
mountain geography.
WILLIAM L. FISHER is Leader of the United
States Geological Survey, New York Cooperative
Fish and Wildlife Research Unit and an Associate
Professor in the Department of Natural Resources
at Cornell University, Ithaca, NY 14853. E-mail:
wlf9@cornell.edu. His research interests include fisheries science, stream ecology, and geographic information system applications in natural resource
management.