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<br />Horizontal instability of a stream may take two forms: .!!!!.~ and
<br />pattern movement. Avulsion is the sudden sllift of tht~ channel from one
<br />part of the valley floodplain to another by developme[lt of s new course
<br />(Figure 15). This sometimes happens during floods of higher magnItude.
<br />Pattern movement can be due to cutoffs that reduce loe:al channel sinuosity
<br />or to the growth of meanders. The same stt'eam may ha\'e patterns .ranging
<br />from essentially straight, to very meander1,ng and evert braided. 1_ateral
<br />IIligration of meander bends may be due to bank caving clr slumping. However,
<br />the process of bank caving does not occur at maximum ['ates during flood
<br />flows. This is because the water in the stream helps to support the
<br />channel-bank materials. However, following the rapid decrease in water
<br />level after a flood event, streambanks on the outside of bends may fail
<br />quite suddenly. This occurs at a threshold value when, the shear lltrength
<br />of the bank material is exceeded.
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<br />By definition, progressive deposition both within the channel and on
<br />the floodplain is aggradation (Leopold et 81. 1964). ,An aggrad1q: channel
<br />is one that tends to fill with sediment 88 a result of imbalance between
<br />the sediment load contributed to the channel and ability of the flow to
<br />transport the load through the channel (ASCE 1975). C~annel aggradation is
<br />generally a long term process existing over fairly ext,ens1ve reaches of
<br />stream systems. Aggradation may be slow at first and Isubject to periodic
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