Rosgen Type Aa+ Streams

January 26, 2012 Print Friendly and PDF
An "Aa+" Channel Type.
Photo courtesy of the National Riparian Service Team

Type Aa+ streams are very steep (greater than 10% grade), well entrenched, have a low width-depth ratio, and are totally confined. The streambed is typically a step/pool morphology with chutes, debris flows, and waterfalls. The Aa+ stream types often occur in debris avalanche terrain, zones of deep depositions such as glacial tills and outwash terraces, or landforms that are structurally controlled or influenced by faults, joints, or other structural contact zones. Streamflow at the bankfull stage in the Aa+ stream type is generally observed as a torrent or waterfall. These stream types can be associated with bedrock and zones of deep deposition and/or be deeply incised in residual soils. The Aa+ can often be described as high-energy/high-sediment supply systems due to their inherently steep channel slopes and narrow/deep channel cross sections.


Montana Stream Management Guide for Landowners, Managers, and Stream Users. 1998. Montana Dept. of Environmental Quality. 34 pp.

Rosgen, D. L. (1994). A classification of natural rivers. Catena, 22, 169-199.

Rosgen, D.L. and H.L. Silvey. 1996. Applied River Morphology. Wildland Hydrology Books, Fort Collins, CO.

Surber, Gene and Bob Ehrhart. 1998. Stream and Riparian Area Management. Montana State University Cooperative Extension Service.

United States Environmental Protection Agency. 2008. Fundamentals of the Rosgen Stream Classification System. USDA EPA Watershed Academy. Accessed on 17 May 2010.

Ward, A., J. L. D’Ambrosio, and D. Mecklenburg. 2008. Stream Classification. The Ohio State University Extension. Agriculture and Natural Resources Fact Sheet AEX-445-01.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.