The Type DA or anastomosed stream is a multiple-thread channel system with a very low stream gradient and the bankfull width of each individual channel noted as highly variable. Stream banks are often constructed with fine grained cohesive bank material, supporting dense-rooted vegetation species, and are extremely stable. Channel slopes are very gentle, commonly found to be at or less than 0.0001%. Lateral migration rates of the individual channels are very low except for infrequent avulsion. Relative to the Type D streams, Type DA streams are considered as a stable system composed of multiple channels. Channel width/depth ratios and sinuosities may vary from very low to very high. The related valley morphology is seen as a series of broad, gently sloping wetland features developed on or within lacustrine deposits. Type DA streams make up a very small number of observed stream types, but are unique both in the process of their creation and maintenance. The geologic processes responsible for the development of anastomosed rivers include subsidence of sedimentary basins in tectonically active forelands, valley base level rises at the basin outlet, regional basin tilting derived from glacial-induced differential isostatic rebound, and the uplifting of sea or lake bed levels. The bedform features of the Type DA stream are riffle/pool, similar to Type C or E streams. The streambanks and island surfaces between channels are well vegetated and constructed with either fine grained alluvium or fine, cohesive depositional material. The ratio of bedload to total sediment load is very low for these very stable stream types.
References
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. http://www.epa.gov/watertrain/stream_class/index.htm 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.