Stream Classification Techniques
Introduction
To the untrained eye streams and river systems may appear to be a simple network of
natural open channels. In reality, these networks are a complex system consisting of multiple
parameters used to represent dimension, flow, sediment, etc. Stream classification is the process
of trying to analyze these parameters and label them in a descriptive manner. The ultimate goal
is to be able to take these parameters and classify the river system as a whole. There is not
currently a single stream classification system used to classify streams worldwide. There are a
number of different methods currently in use. The Leopold and Wolman and the Montgomery
and Buffington are two common methods along with arguably the most used Rosgen method
(Ward et al., 2008). This paper will take an in-depth look at these three methods. Stream
classification plays an important role in river restoration. In order to restore an ecosystem to a
more natural setting it is necessary to be able to accurately classify the open channel to the
desired setting.
Engineers, geomorphologists, and biologists have been trying to classify streams for the
past century. The first recorded method of stream classification is a system set up by Davis in
1899 (Harman, 1999). There are a number of different reasons for classifying a stream. In order
to avoid oversimplifying the process of classification Rosgen listed four reasons for classifying
streams. The first reason is to be able to predict the behavior of the river in regard to its physical
aesthetics. The next purpose of classifying a stream is to develop relationships for given stream
types in regard to hydraulics and sediment. The third reason is to develop a technique to
extrapolate data specific to the site and apply them to similar rivers. The final reason to classify
a river is to be able to provide a consistent reference for describing the river’s morphology for
those working in various disciplines (Rosgen, 1994).
The purpose of developing a consistent classification reference plays a significant role in
restoration. River restoration is a communal effort and therefore can only be achieved through
the work of professionals across a multitude of disciplines. In this rational, Rosgen’s fourth
reason for classification becomes key. In order to perform the specific engineering steps to
restore the river to its previous state, classification is required. Classification can aid in
describing a specific parameter of the river in which restoration is desired. Prior to restoration
certain aspects of the river have been changed to unnatural settings in regard to its natural state.
Proper classification allows a river that is under restoration to be compared to a river that has
retained its natural setting. This enables the parties involved to execute the restoration project
which better emulates the natural setting of the river.
A current challenge in classifying a river is deciding which classification system to use.
Currently Rosgen is arguably the most commonly used classification system (Ward et al., 2008).
This paper will focus on this method along with another frequently used system developed by
David Montgomery and John Buffington in 1997. In addition to these methods this paper will
also briefly analyze techniques developed by Strahler, Leopold and Wolman, and Schumm. This
paper provides comparison and contrast of the featured methods.
Rosgen’s method
Analysis of Rosgen will be considered first. For the purpose of easily classifying rivers,
Rosgen has broken the process into four levels. A river starts by being classified using Level I.
The river is then further classified in Level II, by describing the river in the next sub-genre of
classification. The river is then further classified in Levels III and IV. Each level deals with a
different topic of characterization. Level I begin with geomorphic characterization. Level II
deals with morphological descriptions. Level III characterizes the steams state. Finally, Level
IV addresses validation of process characteristics (Rosgen, 1994). For the purpose of clarity,
Rosgen primarily describes Level I and II in detail, and only briefly describes Level III and IV.
Therefore, this paper will also focus mainly on Level I and II.
Level I provides a broad geomorphic characterization to start the classification process.
Landform and fluvial characteristics are described and combine channel relief, shape, and
dimension profiles (Rosgen, 1994). There are 8 categories that a stream can be classified as in
Rosgen’s method. Streams are broadly classified as“A,” “B,” “C,” “D,” “DA,” “E,” “F,” or “G.”
These categories are used to describe a variety of characteristics.
The first distinguishable characteristics in Level I are the longitudinal profiles used to
represent slope. The slopes start with Aa+ being very steep at >10% gradually decreasing to DA
at <0.5% and then increasing in slope to 4% at G. Slope can be related to bed features and can
be described as pools, riffles, rapids, cascades, and steps (Rosgen, 1994). Riffle/pool streams are
represented with CE, and F streams. Rapids are found in B and G streams, while steps and
cascades are found in A and Aa+ streams.
Cross section morphology is also described in level I of Rosgen’s method. The cross
sections differ greatly in the 9 categories ranging from deep and narrow to wide and shallow.
The cross section morphology also describes the flood plain ranging from well-developed flood
plains to virtually no flood plain.
Finally Level I discusses plan view morphology. The 9 categories describe the sinuosity
of the river system in question. River A types represent relatively straight streams, B represents
low sinuosity, C represents meandering streams, E represents high meandering, and D/DA
represent complex braided systems (Rosgen, 1994). This form of classification often uses the
meander width ratio to describe the sinuosity. Plan view morphology is also very important for
proper river restoration. Rosgen’s method can be used for “describing the most probable state of
channel pattern in stream restoration work,” (Rosgen, 1994).
Level II represents the morphological description of the channel. The next level of
classification further describes the stream system in a more specific manner. This level breaks
the channel into discreet slope ranges and introduces particle sizes of channel material. The
stream types are given numbers to represent particle size diameter of the material with 1
representing bedrock, 2 is boulder, 3 is cobble, 4 is gravel, 5 is sand, and 6 is silt/clay (Rosgen,
1994). This generates 42 major stream types. The morphological description can only be
applied to a limited length of river channel. This is due to the fact that morphology of stream
systems often changes in a relatively short distance. Level II is therefore applied to only
individual reaches, as opposed to being averaged over the entire basin (Rosgen, 1994).
The continuum concept is also applied to Level II. As stated before, stream systems are
often changing throughout its length. Some parameters change while others stay the same and
therefore only one or two of the variables that define a stream classification will be outside of the
presented values. “This level recognizes and describes a continuum of river morphology within
and between stream types,” (Rosgen, 1994). This application allows stream parameters such as
slope to be sorted in sub-categories as opposed to slope. For example, if the majority of
variables of a stream fit in the classification of C4 but has a slope of less than 0.001, the stream
can be classified as C4c- (Rosgen, 1994).
Other variables considered at this level are entrenchment, width/depth, and sinuosity. For
entrenchment, the entrenchment ratio can be defined as “the width of the flood-prone area to the
bankfull surface width of the channel,” (Rosgen, 1994). The entrenchment ratios are given
numbers for classification where 1 to 1.4 are significantly entrenched streams, 1.41 to 2.2 can be
described as moderately entrenched, and greater than 2.2 are slightly entrenched. Width/depth
ratio can be described as “the ratio of bankfull channel width to mean depth,” (Rosgen, 1994). A
small ratio can be considered less than 12 while a moderate to high ratio is considered greater
than 12. Sinuosity is defined as “the ratio of stream length to valley length,” (Rosgen, 1994).
Sinuosity is often linked to slope and particle size of the channel and leads into our next topic of
consideration.
Level II also addresses channel materials and slope. Channel materials play important
roles in sediment transport as well as the development of the form, plan, and profile of the
channel (Rosgen, 1994). Channel materials are classified using the pebble count method. Water
surface slope plays an important role in channel morphology. Slopes, like other variables, can
delineate from the expected values of the channels classification and therefore can be addressed
with the continuum concept.
Level III describes the state of streams and helps measure existing conditions in response
to channel change. This level acts as a method to propose prediction methodologies and can be
used to aid in restoration efforts. Important variables in order to apply Level III include riparian
vegetation, depositional patterns, meander patterns, confinement features, fish habitat indices,
flow regime, river size category, debris occurrence, channel stability index, and bank erodibility
(Rosgen, 1994).
The last level of classification in the Rosgen method is Level IV, describing verification.
This level provides specific information on stream processes used to verify various parameters.
This level helps “provide sediment, hydraulic and biological information related to specific
stream types,” (Rosgen, 1994). Classification at this level requires measurements and
observations of sediment transport, bank erosion, channel geometry, biological data, and riparian
vegetation data (Rosgen, 1994).
Rosgen’s method is currently the most used classification system. Rosgen also talks
about applying the system to restoration efforts. Historical data has shown that streams have
been changing character due to imposed man-made alterations in order to provide things like
flood control, hydro-electric power, and allocation of water rights. These variables used to
classify a river are often changed due to these alterations. Therefore, “to restore the “disturbed”
river, the natural stable tendencies must be understood to predict the most probable form,”
(Rosgen, 1994). Stream classification aids in providing the restoration team with knowledge of
how a system’s variables naturally behave. Restoration efforts are typically more successful in
conjunction with proper classification.
Although Rosgen’s method is currently the most widely used, it doesn’t go without
criticism. (Simon et al, 2007) takes a critical look at the Rosgen method of classification and
addresses what could be considered a number of critical flaws. Simon primarily addresses the
issue of Rosgen’s analysis of bankfull dimensions. When considering sediments for
classification, Rosgen suggests that particle counts should be considered from one bankfull level
to the opposite bankfull level. (Simon et al, 2007) suggests that this mixes two different alluvial
materials requiring different forces and processes while depositing at different times.
Classification related to this issue can be seen when trying to describe two channels classified as
C. One channel can have gravel bed and silt-clay banks, while the other containing a sand bed
and sandbanks. These two channels could have the same median diameters of particle size. This
would put both channels in the C5 type even though they would have completely different
sediment transport regimes (Simon et al, 2007).
Montgomery and Buffington
An alternative form of stream classification was developed by David Montgomery and
John Buffington in 1997. This method differs from Rosgen’s in that it primarily addresses
mountain stream systems with a steeper gradient. This method recognizes three primary
channel-reach substrates consisting of bedrock, alluvium, and colluvium (Montgomery et al,
1997). Alluvium substrates can be broken into 5 additional categories consisting of dune ripple,
pool riffle, plane bed, step pool, and cascade. This creates a total of 7 different classification
categories in which any mountainous river can be described. Montgomery and Buffington do
not apply the continuum concept to their method. (Montgomery et al, 1997) states that for
simplification of using a classification system, the multiple channel types between each category
shall not be considered.
Cascade channels are the first alluvial channels to be considered. Cascade flow is usually
described as tumbling or rolling flow dictated by large clasts. These channels typically exhibit
steep slopes and are typically laid with cobbles and large boulders for bed material, creating a
somewhat aggressive flow (Montgomery et al, 1997). Cascade channels are typically confined
by valley walls and are narrow in nature. Cascade channels transport sediment at high rates with
great efficiency and deliver transport to channels with smaller gradient.
Step-pool channels can be described as longitudinal steps separating multiple pools and
reaches of the river as the flow travels downstream. The channels alter from supercritical to
subcritical flow and contain pool spacing of approximately one to every four channel widths
(Montgomery et al, 1997). This type of channel can be associated with steep gradients and are
considered deep in comparison with depth. They are usually confined by valley (walls?) which
keep them deep in nature. Like cascade channels, they consist of cobble and boulder bed
material. They differ from cascade channels in that they exhibit a cyclic process of a steep step
to relatively calm flow, while cascade channels are defined by their constant tumbling flow.
Plane-bed channels differ from the previous channels in that they typically have long
stretches of beds that contain little to no features (Montgomery et al, 1997). Low width to depth
ratios are typically found in these channels and can be confined or unconfined. These channels
usually exhibit moderate to steep gradients. Plane-bed channels typically are dominated by
gravel or cobble bed materials. Plane-bed channels act as both a supply and transport channel
for sediments.
Pool-riffle channels exhibit a wave-like bed floor that helps define a sequence of bars,
pools, and riffles (Montgomery et al, 1997). These channels typically occur at moderate to small
slopes. These channels have large flood plains due to unconfined embankments with a width to
depth ratio that varies from small to moderate. Gravel is the typical bed material found in these
channels. Like plane-bed channels, Pool-riffle channels can act as both the supply and transport
for sediment.
Dune ripple channels differ from the other categories of channels in that they are
typically low gradient with slower velocity. Small dunes of sand make up the bed floor and have
a tendency to create small ripple like waves on the water surface. Dune ripple channels are
classified as transport channels in regards to sediment. Bed particle size and the efficient ability
to transport sediment separate this from the similar pool-riffle channel (Montgomery et al, 1997).
Colluvial channels consist of small headwater streams that contain little to ephemeral
fluvial transport and flow over a colluvial valley fill (Montgomery et al, 1997). Research
involving colluvial channels is lacking and therefore is more difficult to classify. Bed material
varies in these channels and cannot be specifically classified. Vegetation, debris, and bedrock
steps all reduce the energy available for sediment transport in these channels.
Bedrock channels are the last considered category in this classification system. These
channels are defined by their lack of an alluvial bed. These channels are usually confined by
some sort of valley wall. These channels consist of rock beds and are typically steeper in nature
than alluvial channels. Deep flow and steep channel gradients prevent these channels from
having an alluvial bed (Montgomery et al, 1997).
In comparison to Rosgen’s method, this system differs greatly. Rosgen’s system allows
hundreds of options for classifying a river while Montgomery’s method only allows you seven
options for describing a river. Rosgen’s method has a broader application to all climates and
geographical regions while Montgomery’s method was developed specifically for mountainous
regions of steep gradient. (Rosgen, 1994) suggests that his classification method can aid in
restoration efforts. This is not the case for Montgomery’s method as stated by (Montgomery et
al, 1997), “restoration designs requires further information on reach-specific characteristics.”
This suggests that Montgomery’s method may be lacking in substance to properly classify a river
to establish design criteria. This may be why Rosgen’s method is more commonly used.
Stream Classification with GIS
The classification of steams into perennial, intermittent, and ephemeral classes is an
important part of stream classification and can be applied to many different fields. Currently,
field techniques are primarily used to establish a stream being perennial, intermittent, or
ephemeral. While field techniques can be a reasonable way of obtaining data they are often
riddled with measurement errors and can also be economically inefficient (Restrepo et al).
Research in stream classification using GIS is currently ongoing and appearing to be more
accurate and significantly cheaper.
The use of GIS to classify streams as perennial, intermittent, and ephemeral is currently
being considered to replace field techniques. (Restrepo et al) is currently researching the
effectiveness of using GIS in place of field techniques. Restrepo primarily compares the GIS
technique with commonly used field technique developed by the North Carolina Division of
Water Quality (NCDWQ). NCDWQ technique requires field technicians to keep records of field
observations and measurements using a thorough form to be filled out to report data. The form
contains information to present on alluvial deposits, bankfull bench, active or relic flood plain, a
continuous bed and bank, levees, sinuosity, and biological indicators. Field techniques such as
these require a significant amount of experience and often exhibit a large amount of subjectivity.
The most important part of GIS stream classification is the allocation of accurate data.
Digital data used for GIS classification is currently available for soil texture and elevation. The
National Hydrography Dataset (NHD) from the EPA and Elevation Derivatives for National
Applications (EDNA) from the USGS are two datasets that can be easily accessed for stream
delineation (Restrepo et al). The majority of the biological parameters that are measured using
the field technique are unable to be portrayed using GIS; however, a number of other important
parameters are accurately measured including: hydrologic soils, sinuosity, land use, groundwater,
and baseflow. (Restrepo et al) performed a case study in order to compare GIS analysis with
field techniques.
(Restrepo et al) performed a case study of the Upper Neuse River Basin to compare to
field techniques. The study utilized high resolution LIDAR data. A shaded relief was projected
using Albers spatial reference. Land use land cover data was then obtained from the USGS
dataset. High quality soil texture information was also obtained from the USGS. Manual editing
was performed to create the DEM data. GIS information was then generated and compared to
the LIDAR data. The streamlines correlated well with the LIDAR data. Physical field data was
not available at the time the paper was written. The study is still in progress.
Other classification techniques (would prefer two or three more brief explanations of other
classification techniques)
Application to classification to S.W. systems
Stream Classification and Restoration
Conclusion
Stream classification acts as the first step in proper stream management. To properly
classify a stream, parameters such as dimension, flow, sediment, etc must be understood.
Rosgen’s method is currently the most used classification technique in the scientific community.
Using four levels of classification, Rosgen’s method yields a total of 94 possible types (Simon et
al, 2007). In contrast to Rosgen’s method, Montgomery and Buffington’s method calls for only
7 classification classes and is used primarily to describe mountainous systems with steep
gradients. Stream classification plays an important role in river restoration. In order to properly
restore a system to its natural setting, every parameter must be understood and classified to
predict changes either naturally or manmade.