Channel types
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A wide variety of stream channels exist and these can be divided into two groups: low gradient streams (less than a couple of percent in gradient) and high gradient streams.
Low gradient streams can be divided into Braided rivers, Wandering rivers, Single Thread Sinous rivers and Anastomosing rivers. Which channel type occurs depends on stream gradient, vegetation and sediment supply. Braided rivers tend to occur on steeper gradients where there is a large supply of sediment while single thread sinous channels occur where there is a lower sediment supply. Anastomosing channels are multithreaded, but are much more stable than braided channels and commonly have thick clay and silt banks and occur at lower gradients. Wandering Rivers fall between sinuous single thread and braided streams and are relatively stable multi-channel gravel bed Rivers.
High gradient streams have been divided into riffle-pool (which to some extent covers all of the low gradient stream morphologies discussed), rapid/plane bed, step-pool and cascade morphologies. Riffle-pools are composed of migrating pools and transfers bars called riffles and occur on gradients less than 1-2 percent. Rapids (also called plane bed, but not to be confused with the plane beds described in sand bed rivers) lack distinct pools and bars but commonly have stone cells or clusters and occur on gradients in the range of 1-5 percent. Step-pools are comprised of channel spanning pools and boulder/cobble steps that cause subcritical flow in the pool and supercritical flow over the steps. They occur in gradients in the range of 5 and 20%. At steeper gradients (approx > 10-15 percent, but not strictly known) Cascade units exist where the channel is dominated by boulders and cobbles and channel spanning pools do not exist. Pocket pools are common. In all four channel types large woody debris (LWD) may strongly influence the channel type.
See Church (1992) for more details on low gradient streams and Grant et al. (1990) and Buffington and Montgomery (1997) for more details regarding high gradient streams.
Buffington, J. M., and D. R. Montgomery (1997), A systematic analysis of eight decades of incipient motion studies, with special reference to gravel-bedded rivers, Water Resources Research, 33, 1993-2029. Church, M. (1992), Channel morphology and topology, in The River Handbook, edited by P. Calow and G. E. Petts, pp. 126-143, Blackwell. Grant, G. E., F. J. Swanson, and M. G. Wolman (1990), Pattern and origin of stepped-bed morphology in high gradient streams, western Cascades, Oregon, Geological Society of America, Bulletin, 102, 340-352.
