Critical Evaluation of How the Rosgen Classification and Associated ‘‘natural Channel Design’’ Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response

Over the past 10 years the Rosgen classification system and its associated methods of ‘‘natural channel design’’ have become synonymous to some with the term ‘‘stream restoration’’ and the science of fluvial geomorphology. Since the mid 1990s, this classification approach has become widely adopted by governmental agencies, particularly those funding restoration projects. The purposes of this article are to present a critical review, highlight inconsistencies and identify technical problems of Rosgen’s ‘‘natural channel design’’ approach to stream restoration. This paper’s primary thesis is that alluvial streams are open systems that adjust to altered inputs of energy and materials, and that a form-based system largely ignores this critical component. Problems with the use of the classification are encountered with identifying bankfull dimensions, particularly in incising channels and with the mixing of bed and bank sediment into a single population. Its use for engineering design and restoration may be flawed by ignoring some processes governed by force and resistance, and the imbalance between sediment supply and transporting power in unstable systems. An example of how C5 channels composed of different bank sediments adjust differently and to different equilibrium morphologies in response to an identical disturbance is shown. This contradicts the fundamental underpinning of ‘‘natural channel design’’ and the ‘‘reference-reach approach.’’ The Rosgen classification is probably best applied as a communication tool to describe channel form but, in combination with ‘‘natural channel design’’ techniques, are not diagnostic of how to mitigate channel instability or predict equilibrium morphologies. For this, physically based, mechanistic approaches that rely on quantifying the driving and resisting forces that control active processes and ultimate channel morphology are better suited as the physics of erosion, transport, and deposition are the same regardless of the hydro-physiographic province or stream type because of the uniformity of physical laws. (KEY TERMS: geomorphology; fluvial processes; rivers ⁄ streams; restoration; sediment; Rosgen classification.) Simon, A., M. Doyle, M. Kondolf, F.D. Shields Jr., B. Rhoads, and M. McPhillips, 2007. Critical Evaluation of How the Rosgen Classification and Associated ‘‘Natural Channel Design’’ Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response. Journal of the American Water Resources Association (JAWRA) 43(5):1117-1131. DOI: 10.1111 ⁄ j.1752-1688.2007.00091.x Paper No. J05210 of the Journal of the American Water Resources Association (JAWRA). Received December 13, 2005; accepted January 9, 2007. a 2007 American Water Resources Association. No claim to original U.S. government works. Discussions are open until April 1, 2008. Respectively, Research Geologist, USDA-ARS, National Sedimentation Laboratory, Oxford, Mississippi 38655; Associate Professor, Department of Geography, University of North Carolina, Chapel Hill, North Carolina 27599; Associate Professor, Department of Landscape Architecture and Environmental Planning, University of California at Berkeley, Berkeley, California 94720; Research Hydraulic Engineer, USDA-ARS National Sedimentation Laboratory, Oxford, Mississippi 38655; Professor, Department of Geography, University of Illinois, Urbana-Champaign, Illinois 61801; and Principal Scientist, Intuition and Logic Inc., Amelia Island, Florida (E-Mail ⁄Simon: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1117 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 43, No. 5 AMERICAN WATER RESOURCES ASSOCIATION October 2007