Altered and degraded riparian areas have adversely impacted streams throughout the
West. Montana river systems exemplify the transformation of natural fluvial ecosystems to
unstable and "simplified" aquatic environments. Riparian habitat alterations
contribute to widespread declines of inland native fishes and often favor exotic species.
Natural channel design philosophy (NCDP) aims to restore natural channel stability, or
dynamic equilibrium, and habitat to impaired streams. Streams in dynamic equilibrium are
generally more biologically productive, providing higher quality and more complex habitat
than altered or unstable streams. This philosophy requires a multidisciplinary approach to
stream restoration along with an understanding of historical riparian land use. As a basis
for habitat restoration, the Rosgen Stream Classification System (RSCS) and NCDP methods
strive to reinstate natural channel form, function and dynamic equilibrium (Rosgen 1994;
Rosgen 1996). Although Rosgen methods are often criticized as overly-simplistic, this
interpretation is often based solely on the RCSC. When properly applied, NCDP methods
provide a robust, widely tested, and well-accepted approach to the design of natural
channels that can successfully meet habitat restoration objectives and survive extreme
flood events (Schmetterling and Pierce 1999). In addition to meeting natural channel and
fisheries objectives, this process requires compatible riparian land-use practices.
NCDP is the foundation for developing a naturally stable channel design and meeting
habitat restoration objectives. The RSCS reach characterization is core to this
methodology and in its rudimentary form, categorizes streams into one of eight primary
stream types (Rosgen 1996; Bain and Stevenson 1999). However, the RSCS is only an initial
step to a complex protocol for temporally evaluating bank stability, sediment availability
and transport, and riparian condition. Geomorphic indicators (bankfull channel),
prediction (reference reaches and dimensionless ratios), and method validation (regional
curves) define naturally functioning channels. NCDP focuses on restoring geomorphic
characteristics while incorporating fish habitat structures composed of native materials
in natural arrays that better replicate native salmonid habitat as necessary for restoring
inland fish populations.
Natural Channel Design Philosophy Methodology
|Assessment and Evaluation
Assessing the stream at multiple scales is the initial step of
the NCDP. Describing stream valley morphology and channel patterns from time-series aerial
photographs provides watershed characterization. The watershed's natural condition and the
anthropogenic impacts on the stream can be quantified by measuring flows, sediment
transport, and debris movement. Anthropogenic impacts (e.g. silvicuture) alter the natural
watershed condition. A broad-scale quantitative watershed-level investigation is essential
for predicting a restoration project's limitations, risks, and potential.
At the reach level, stream geomorphology and fish habitat are quantified by field data
collection in both project and reference reaches. The reference reach should be naturally
functioning, provide optimal fish habitat, and serve as a model for the design channel.
"Bankfull" indicators and other geomorphic variables are measured in both
reaches and are fundamental to the NCDP. Bankfull elevation, a geomorphic indicator
signifying the point of incipient flooding, coincides with the stage above which the
stream accesses its floodplain or flood-prone area (Rosgen 1996). By doing the work that
creates the average morphologic channel characteristics, bankfull discharge forms and
maintains the channel over time (Dunne and Leopold 1978).
NCDP begins with a rigorous multi-scale data collection that quantifies channel shape,
pattern, and gradient (Rosgen 1996). Riparian health, instream habitat, and fish
population surveys, along with measurements of discharge, sediment, and bed and bank
stability, permit the assessment and evaluation of existing and potential channel
conditions as well as biological attributes of the project.
|Data Synthesis and Design
Successful natural channel design must include all aspects of
dynamic equilibrium. Channel pattern (plan view characteristics), dimension (channel size
and shape), and profile (longitudinal elevations and gradients) are measured. Appropriate
designs may include creating aquatic habitat, prescribing a revegetation plan, and
constructing an appropriate floodplain.
Synthesizing reference reach field data and incorporating regional
stream information helps identify design channel parameters. Regional data and
dimensionless coefficients collected by Rosgen (1994) and others, help predict channel
attributes relative to the watershed area and bankfull characteristics. Watershed
discharge, sediment entrainment, and bankfull channel cross sections are then
hydraulically modeled to validate bankfull discharge. Design dimensions are developed
relative to bankfull discharge. Comparing design dimensions to dimensionless coefficients
and a reference reach database further validates the design.
The final restoration design developed with NCDP seeks to mimic a
stream in dynamic equilibrium with its watershed, and provides a diverse and complex
channel capable of conveying flows, transporting sediment, and integrates essential
habitat features related to native fish recovery goals.
Restoration design implementation is critical, yet often overlooked. A
comprehensive design is only successful if accurately constructed with acute attention to
detail and elevational control. Whole trees, rootwads, rock, shrubs, and other woody
debris should be collected locally from the floodplain or a nearby source to reduce
project costs and importation of foreign species. An experienced practitioner is
responsible for determining the types of structures required to meet restoration goals.
Vegetation colonization through mature shrub and sod mat transplanting, as well as
other revegetation efforts, along with woody materials and rock provide immediate fish
habitat and temporary bank stability. These structures allow for shrub colonization which,
when established, provides for long-term dynamic equilibrium and habitat complexity.
Discussion and Conclusion
Historic channel modifications simplify and degrade natural channels and habitat
necessary for sustaining native species. While Rosgen's methods provide a template for
restoring natural channel features to impaired streams, stream restoration based on NCDP
must include site-specific habitat and fisheries considerations. Reference and project
reach data may provide at treatment and control for riparian health, habitat, and fish
population studies related to a restoration project. Inclusion of this information helps
identify limiting factors and define fisheries objectives. A competent practitioner must
recognize the physical and biological potential of a stream and the environmental
constrains to properly develop and implement a restoration plan.
Other design methods have and will be promoted in the future. Typically, these methods
rarely use appropriate native materials and are often over-engineered (hardened with rock)
or are based on computer models. It is unlikely these techniques would withstand critical
multi-disciplinary scientific review.
NCDP techniques are available to the scientific community, and continue to evolve
through professional discussion, data sharing, and project monitoring. Restoring dynamic
equilibrium to impaired streams while mimicking natural processes and habitat features is
a successful method for restoring inland native fish habitat in altered and degraded
Bain, M, B., and N. J. Stevenson. 1999. Aquatic habitat assessment: common Methods.
American Fisheries Society, Bethesda, Maryland.
Dunne, T. and L.B. Leopold.1978. Water in Environmental Planning. W.H. Freeman and Co.,
San Francisco, CA. 818pp.
Rosgen, D.L. 1994. A classification of natural rivers. Catena 22: 169-199.
Rosgen, D.L. 1996. Applied River Morphology. Wildland Hydrology.
Schmetterling, D.A. and R.W. Pierce, 1999. Success of instream habitat structures after
a 5-year flood in Gold Creek, Montana. Restoration Ecology 7(4), pp. 369-375.