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<br /> <br />5 art 2?jver <br /> <br />in the water, or bounced along the bed. The river's competence to pluck larger and larger <br />particles from its bed is a direct function of water velocity. as well as the shape of the <br />channel where a particular sand grain. cobble. or boulder happens to lie. <br />We think of rivers as the cutting edge of erosion. When viewed through the long lens of <br />a geologic perspective. it is true: rivers do carve their valleys. But when viewed through <br />the shorter focal length of an historical perspective, a river typically is in dynamic equilib- <br />rium with its valley. Sometimes the river scours its bed during floods or high flows of long <br /> <br /> <br />The Salt River at Canyan C,eek <br /> <br />duration; sometimes it builds up (aggrades) its bed during lower. <br />flow periods as sidestreams continue to deliver large quantities of <br />sediment. For a particular segment of river channel. the instanta. <br />neous elements of this equilibrium include discharge. sediment <br />load. and channel slope. shape. and roughness (Pickup. 1976). At <br />any instant. a river will adjust its channel to these elements. Each <br />river system displays a unique response to the inputs of these <br />elements (Baker. 1977). Confronted by floods of equal magnitude. a <br />segment of river confined to a bedrock channel will behave quite <br />differently than another segment flowing across an alluvial plain. <br />Riparian vegetation plays an essential role in the evolution of a <br />river corridor. Streamside trees and brush take advantage of peren- <br />nial surface water and fine'grained substrate for growth. The plants <br />may be native - willows. cottonwoods. and mesquite - or exotics. <br /> <br />13 <br />