175 



■ LOW-FLOW CHANNELS, FLOOD 

 PLAINS, POOLS, RIFFLES, AND 

 MEANDER SEQUENCES 



Natural streams have an equilibrium 

 in which the components of the fluvial 

 system including the watershed, 

 length, slope, width and depth of the 

 channel, floodplain, and channel bed- 

 forms evolve in relationship with each 

 other. This equilibrium determines the 

 nature of the eroding, transporting, 

 sorting, and deposirional processes of 

 streams. The equilibrium can be upset 

 by land-use activities, channelization 

 or other modifications. It is possible 

 to help design more stable streams 

 In incorporating these fluvial 

 characteristics ol low-flow channels. 

 floodplains, meandering channels 

 ■iiid pool and i illle sequences into the 

 design of channel modification pro- 

 jects. Meanders provide a process of 

 transporting sediments and building 

 floodplains; pools and riffles maintain 

 a natural sorting of bed-load materials, 

 facilitate a diversity of stream bank 

 vegetation and provide habitats 

 necessary for the feeding, breeding, 

 and cover for in-stream life. The 

 stability and environmental values of 

 a stream can be improved by return- 

 ing these diverse components to a 



BANK 



-5BPIM&NT 



M EAMPER SEQUENCE- 



PEFLECTOl^S ANPROCK 



clusters fop, peveloplng" 

 pools and riffles 



channelized or damaged stream. 



In situations in which a channel 

 must be modified to accommodate in- 

 creased flows from urbanization, a 

 channel design which includes the 

 components of a natural fluvial system 

 (in their proper relationship to one 

 another) will be better able to develop 

 a new equilibrium. Additionally, it 

 will provide a stability and reduction 

 in the maintenance problems inherent 

 in channelization projects. Channel 

 enlargement projects can be designed 

 which retain a more natural low-flow 

 channel, and which include ap- 

 propriately spaced meanders. Some 

 projects have used a single bank 

 modification design in which only one 

 bank is disturbed by widening and the 

 widening is done in a way to retain 

 the existing meander sequence. Chan- 

 nel capacity should be designed to ac- 

 commodate natural riparian growth 

 along both sides of a meandering low- 

 flow channel. Another option for 

 accommodating increased urban run- 

 off is to direct excess flood flows into 



