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<br />\ ~~!!..!,', /./:' ".
<br />\ \ -~ ~----.$'/ ."
<br />_,.~ ............ /.... ....::::....e,; .................
<br />~i ,..... ~.._....... /1, ....""-,....~~.
<br />.....Joe ,_.. "'y / ........,~;..,-,.....
<br />......'" - - .,.,~( .F:,";'~ '.!~!."..
<br />_-::!:'--\ ......... ~!.......
<br />...........-
<br />
<br />20
<br />
<br />'<:l"
<br />~-4
<br />~g. 2.
<br />
<br />RIVER MEANDERING
<br />
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<br />
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<br />
<br />Fig. 1. Meander shift as shown by 1938, 1956 and 1968 aeri.l
<br />photographs, CilIlarron River ne.r Perklns, Cltlahana
<br />(from Shen and SChumm, 1981).
<br />
<br />ZONE I
<br />
<br />'i5J' ----------------- .,
<br />., r.
<br />
<br />. "
<br />----------------
<br />
<br />
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<br />~ - . ------~.--- .'
<br />.., I..
<br />~n______'1 ..
<br />. I .
<br />. I I
<br />I I
<br />I I
<br />I I
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<br />ZONE " I :
<br />I I
<br />J. .. _~-_-----~...,
<br />. ~=:::_~--~ ~
<br />
<br />lONE zJ:
<br />
<br />\
<br />: "---
<br />
<br />ldealised cross sections shc:7dng chanqes in Zones 1, 2 r 3
<br />during one season in the Kraft Badlands. Diagram to right
<br />of each cross sectJ.on illustrates channel behaviour in
<br />each zone (from Bergstrom and SChumm, 1971). See text for
<br />discussion.
<br />
<br />,.
<br />. '.,
<br />. .
<br />
<br />RIVER MORPHOLOGY AND BEHAViOR
<br />
<br />21
<br />
<br />Zone 1 aggradation coincides with Zone 2 degradation and vfce versa.
<br />
<br />Zone 3: Only durIng the l.rgest storms did runoff occur In all
<br />three zones. During small storms. when Zone 1 and/or Zone 2 were
<br />actively producing and transporting sediment, Zone 3 received little
<br />flow capable of modifying the valley fill (Fig. 2c (1)). Therefore,
<br />sediment was stored in Zone 3 (1, 2, 3a) in fan.like channel con.
<br />vexities. When runoff occurred in Zone 3. these deposits were incised.
<br />and sediment was moved out of the basin (3b). As sediment continued to
<br />be contributed from Zone 2, a particular cross section in Zone 3 would
<br />alternately aggrade and de9rade (4a, 4b), depending on sediment loads,
<br />The~efore. reaches of channel in Zone 3 were frequently out of phase at
<br />any given time. as the convexities were formed and destroyed.
<br />
<br />In summary, predictions based upon data from one location may not
<br />be useful elsewhere. Extrapolation in space is as hazardous as extra-
<br />polation in time.
<br />
<br />Convergence:
<br />
<br />Convergence refers to the condition when different processes or
<br />causes produce similar effects. For example, braided streams result
<br />from aggradation, but they also can be "stableU with the braided
<br />morphology being the effect of high bedload transport on steep
<br />gradient,. Under similar conditions flashy discharge may also maintain
<br />a braided channel. when elsewhere, with more uniform hydrologic con-
<br />d1tions, the channels meander. In fact, both meandering and braided
<br />patterns can be the result of either sediment load, hydrology or valley
<br />slope characteristics (Fig, 3), Therefore, 1t Is difficult to Infer
<br />process from form (6, 7, 19), and attempts to do so have been termed
<br />the genetic fallacy (12).
<br />
<br />This type of prOblem became very obvious during the early days of
<br />planetary studies when the origin of channels on the moon (sinuous
<br />rills) were attrlbted to flowing water. Eventually the channels were
<br />determined to be volcanic and fluidization features (21). In fact, it
<br />can be difficult to determine . from the appearance of .the landform if 1t
<br />is depositional or erosional, for example, an alluvial fan, a rock fan,
<br />or a pediment.
<br />
<br />In addition to similar land forms. similar effects may also be
<br />produced by very different causes. For example, incision of a stream
<br />may be due to b.se-level lowering, tectonic uplift or climate change.
<br />Obviously under these conditions extrapolation must rest upon a careful
<br />study of the system because the response cannot be understood or con-
<br />trolled without information on causation. For example, If a base-level
<br />lowering has caused incision (e.g., channelization, dredging), then
<br />grade-control is required. but if the incision is related to hydrologiC
<br />factors t~en runoff controls would be appropriate.
<br />
<br />D1verqence:
<br />
<br />Divergence ;s the antonym of convergence. and it refers to similar
<br />causes or processes producing different effects. For example, a
<br />climate fluctuation may trigger massive landslides in one area.
<br />
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